Pillared metal-organic frameworks: properties and applications [1st ed.] 9781119460374, 1119460379, 9781119460398, 1119460395

Table of contents :
Title page......Page 8
Copyright page......Page 9
Preface......Page 10
Abbreviations......Page 11
1.1 What are the Metal-Organic Frameworks?......Page 23
1.2 Synthesis of Metal-Organic Frameworks......Page 25
1.3 Structural Highlights of Metal-Organic Frameworks......Page 26
1.5 High Thermal and Chemical Stability......Page 29
1.6 Applications of Metal-Organic Frameworks......Page 30
1.7 Conclusion......Page 47
References......Page 48
2.1 Introduction......Page 52
2.2 Topology and Diversity in Pillar-Layered MOFs......Page 54
2.3 Synthesis Methods in Pillar-Layered MOFs......Page 56
2.4 Linkers in Pillar-Layered MOFs......Page 58
References......Page 59
3.1 Introduction......Page 62
References......Page 69
4.1 Introduction......Page 74
4.2 Bipyridine......Page 171
4.3 Dabco......Page 173
4.4 Imidazole and Pyrazole......Page 175
4.5 Triazole and Tetrazole......Page 178
4.6 Pyrazine and Pipyrazine......Page 180
4.7 Amide, Imide, Amin and Azine/Azo Spacer......Page 181
References......Page 186
5.1 Introduction......Page 192
5.2 Non-Interpentrated Frameworks......Page 195
5.3 Frameworks with Interpenetration......Page 197
5.4 Control over Interpenetration......Page 198
References......Page 200
6.1 Introduction......Page 204
References......Page 300
7.1 Stability in Pillar-Layer MOFs......Page 303
7.2 Interpenetration in Pillar-Layer MOFs......Page 307
References......Page 313
8.2 Gas Storage and Separation in Pillar-Layer MOFs......Page 317
8.3 Catalysis in Pillar-Layer MOFs......Page 325
8.4 Adsorptive Removal and Separation of Chemicals in Pillar-Layer MOFs......Page 331
8.5 Sensing in Pillar-Layer MOFs......Page 346
References......Page 351
Glossary......Page 360
Subject Index......Page 365
End User License Agreement......Page 367

Citation preview

Contents Cover Title page Copyright page Preface Abbreviations Chapter 1: Introduction to Metal-Organic Frameworks 1.1 What are the Metal-Organic Frameworks? 1.2 Synthesis of Metal-Organic Frameworks 1.3 Structural Highlights of Metal-Organic Frameworks 1.4 Expansion of Metal-Organic Frameworks Structures 1.5 High Thermal and Chemical Stability 1.6 Applications of Metal-Organic Frameworks 1.7 Conclusion References Chapter 2: Pillar-Layer Metal-Organic Frameworks 2.1 Introduction 2.2 Topology and Diversity in Pillar-Layered MOFs 2.3 Synthesis Methods in Pillar-Layered MOFs 2.4 Linkers in Pillar-Layered MOFs 2.5 Conclusion References Chapter 3: Rigid and Flexible Pillars 3.1 Introduction 3.2 Conclusion References Chapter 4: Introduction to N-donor Pillars 4.1 Introduction 4.2 Bipyridine 4.3 Dabco 4.4 Imidazole and Pyrazole 4.5 Triazole and Tetrazole

4.6 Pyrazine and Pipyrazine 4.7 Amide, Imide, Amin and Azine/Azo Spacer 4.8 Conclusion References Chapter 5: Introduction to Aromatic and Aliphatic Pillars 5.1 Introduction 5.2 Non-Interpentrated Frameworks 5.3 Frameworks with Interpenetration 5.4 Control over Interpenetration 5.5 Conclusion References Chapter 6: Introduction to O-Donor Pillars 6.1 Introduction 6.2 Conclusion References Chapter 7: Stability and Interpenetration in Pillar-Layer MOFs 7.1 Stability in Pillar-Layer MOFs 7.2 Interpenetration in Pillar-Layer MOFs 7.3 Conclusion References Chapter 8: Properties and Applications of Pillar-Layer MOFs 8.1 Introduction 8.2 Gas Storage and Separation in Pillar-Layer MOFs 8.3 Catalysis in Pillar-Layer MOFs 8.4 Adsorptive Removal and Separation of Chemicals in Pillar-Layer MOFs 8.5 Sensing in Pillar-Layer MOFs 8.6 Conclusion References Glossary Subject Index End User License Agreement

List of Illustrations Chapter 1

Figure 1.1 Some inorganic secondary building units (A) and organic linkers (B) [1]. Figure 1.2 Crystal structures of MOF-5 and MOF-2 with different MBB. Figure 1.3 Crystal structures of Zn4O(BTB)2 prepared in different sol... Figure 1.4 CO2 adsorption isotherms for Mg2(dobdc) collected over a t... Figure 1.5 Basic schemes showing the types of CO2 capture. The processes for pos... Chapter 2 Figure 2.1 Simple representation of pillar linker used for improving dimensionality. Figure 2.2 Number of pillar MOFs reported each year. Figure 2.3 (a) Representation of azine functional group only on pillar [3], (b) Representat... Figure 2.4 The rational percentage of main reported RCSR topologies in pillarlayered MOFs... Figure 2.5 The number of different synthetic methods in pillar-layered MOFs. Figure 2.6 Schematic representation of pillared-layered exchange in a 3D MOF. Figure 2.7 Comparison between reaction routes of EASY-MOFs and SALE/SMILE (blue and green r... Chapter 3 Figure 3.1 Showing several cases of guest accommodation (a) shrinking (b) expanding (c) sha... Figure 3.2 Schematic demonstration of changing pillar length. Figure 3.3 Library of fu-bdc linkers used in the preparation of (Zn2(fu-bdc)... Figure 3.4 Structural representations of the different forms of (Zn2(2,5-BME-bdc... Chapter 4 Figure 4.1 View of the 3D framework (a) along the c-axis showing oval shaped channels decor... Figure 4.2 Reaction Schemes toward (Ni(HBTC)(dabco)) via random ligand exchange and toward... Figure 4.3 (a) Structures of TBAPy (left) and dabco (right); (b) Crystal Structure of NU-50... Figure 4.4 The PL intensities of MOF toward selective aromatic molecules.

Figure 4.5 The structurally related ancillary ligands. Figure 4.6 3D structure of (Co8.5(µ4-O)(bpdc)3 (bpz)... Figure 4.7 (a) Comparison of adsorption isotherms of H2O on (Cu2(pzdc... Figure 4.8 The binding character of 2,6-diaminopurine in reported complexes. Figure 4.9 Views of TMU-4. (a) Representation showing the pore channels and that the networ... Figure 4.10 The interaction between the azine moiety of 4-bpdh and picric acid is highlighted (bottom). Color code: O: red; N: blue; C: black; and Zn: blue polyhedra. Chapter 5 Figure 5.1 Design of pore space via the introduction of a rotational module as a molecular... Chapter 6 Figure 6.1 (a) The 63 layer formed by tipb ligands and nickel atoms. (b) The (3,... Figure 6.2 ((Zn2(TRZ)2)8(dicarboxylate)4) build... Chapter 7 Figure 7.1 Water adsorption isotherms as a function of relative humidity at 298 K in air fo... Figure 7.2 Synthetic rout and structures of three MOFs. Figure 7.3 Structures of (a) (Ni(bpea)(L1)(H2O))n, (b) (Ni(bpea)(L2))... Figure 7.4 (a) Synthesis method of TMU-25 and TMU-26, (b) Chemical structure of the organic... Figure 7.5 Coordination environments of Cd and ligands in (a) (Cd(L)(bpy)), (b) (Cd(... Chapter 8 Figure 8.1 Yield-versus-time profile of aldol-type condensation reaction of (a) 2cyclopent... Figure 8.2 Kinetic breakthrough curves of SO2 (A) and NH3 (B) contami... Figure 8.3 (top) Luminescence of the powdered [Zn2(bdc)2(dpNDI)]... Figure 8.4 (A) Crystal structure of highly hydrophobic [Ni8(OH)4(H... Figure 8.5 The PL intensities of compound 3 introduced to various pure solvents (a) and dif...

Figure 8.6 The PL intensities of 3 toward selective aromatic molecules with concentr...

List of Tables Chapter 1 Table 1.1 CO2 and N2 uptake in selected metal-organic frameworks at... Chapter 4 Table 4.1 N, O and S-donor-based ligands (Pillars) as building blocks for MOFs. Chapter 6 Table 6.1 O-donor-based ligands as layers for MOFs. Chapter 7 Table 7.1 Summary of the compositions and water stability characteristics for the some pi... Chapter 8 Table 8.1 Coupling reaction of malononitrile and carbonyl compounds with the prepared MOF... Table 8.2 Kinetic equation of CR removal.

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Pillared Metal-Organic Frameworks Properties and Applications Lida Hashemi Ali Morsali Department of Chemistry, Tarbiat Modares University, Tehran, Islamic Republic of Iran

This edition first published 2019 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2019 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley prod-ucts visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchant-ability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 978-1-119-46024-4

Preface Metal-organic Frameworks (MOFs) are the class of promising materials which have attracted a tremendous amount of attention in the recent decades. MOFs are regarded as a subclass of coordination polymers (CP) which are constructed by self-assembly of metal ions or metal clusters linked together by organic ligands containing multiple binding sites oriented with specific angularity generating structures with permanent porosity, high specific surface area and tunable topology. Possessing the merits of alternative structural tailorability and functionality, make them appropriate for different potential applications such as gas adsorption and separation, catalysis, biomedical applications, sensing, host guest induced separation and etc. Pillar-layered MOFs are among the most studied research area related to inorganic polymers. Investigating their properties and application as a multi donor 3-D porous frameworks is of interest. Modifying pillar moieties as a third building blocks of pillar-layered MOFs, together with metal nodes and oxygen donor linkers, can enhance controlling structure assembly and lead to specific properties into obtained structures. The structure of a pillar can be easily modified which causes better design of desired structural topology and pore environment. An overview on structural properties of this branch of MOFs is done in this book which can be useful for better understanding their properties and fabrication of new advanced structures. Lida Hashemi and Ali Morsali Department of Chemistry, Tarbiat Modares University, Tehran, Islamic Republic of Iran January 2019

Abbreviations 1,2-H2BDC

1,2- benzenedicarboxylic acid

1,3-bib 1,4-bdc-NH2, NH2BDC, H2atpa

1,3-bis(1H-imidazol-1-yl)benzene 2-minoterephthalic acid, 2-amino-1,4-benzenedicarboxylic acid

1,4-bib (1,4-bbi) (bimb)(biim-4)

1,4-bis(imidazol)butane or 1,1′(1,4-butanediyl)bis(imidazole) or 1,4di(1H-imidazol-1-yl)butane or 1,4-bis(imidazol-1′-yl)butane or 1,1′(1,4-butanediyl)bis(imidazole) 1,4-bis(1H-imidazol-1-yl)benzene or 4,4′-bis(imidazol-1-yl)phenyl

1,4-bib (dib) (dip) (BIP) 1,4-ndc 1,4-pdaa 1,6-bih (bimh)

1,4-naphthalenedicarboxylate 1,4-phenylenediacetic acid 1,6-bis(imidazol-1-yl)hexane

2,2/,3,3/-odpda

2,2/,3,3/-oxydiphthalic dianhydride

2,2/,3,3/-tdpda 2,2′-bipy 2,5 -bptz (bpt) 2,5-pydc 2,6-H2bptp

2,2/,3,3/-thiodiphthalic dianhydride 2,2′-bpyridine 2,5 -bis(pyrid-4-yl)1,3,4-thiadiazole pyridine-2,5-dicarboxylic acid 2,6-bis(3-(pyrid-4-yl)-1,2,4-triazolyl)pyridine

2,6-H2pydc

2,6-pyridine dicarboxylic acid

2F-4spy 2-PyBIm 3 -abpt (3-bpt) 3 –bpcb 3 –PYTZ 3,3′-bpt 3,3′-dmbpy 3,3′-tmbpt 3,4′-bpdc 3,4′-tmbpt

2′-fluoro-4-styrylpyridine 2-(2-pyridyl)benzimidazole 4 -amino-3,5-bis(3-pyridyl)-1,2,4-triazole N,N′-bis(3-pyridinecarboxamide)-1,4-benzene 3,6 -bis(pyridin-3-yl)-1,2,4,5-tetrazine 3-(5-phenyl-1H-1,2,4-triazol-3-yl) pyridine 3,3′- dimethyl-4,4′-bipyridine 1-((1H-1,2,4-triazol-1-yl) methyl)-3,5- bis(3-pyridyl)-1,2,4-triazole biphenyl-3,4′-dicarboxylate 1-((1H-1,2,4-triazol-1-yl) methyl)-3-(3- pyridyl)-5-(4-pyridyl)-1,2,4triazole)

3,5-daba 3,5-PDC 3-bpah 3-bpdb (3-bpmh) (3bpd) 3-bpdh

3,5-diaminobenzoate 3,5-pyridinecarboxylate N,N′-bis(3-pyridinecarboxamide)-1,2-cyclohexane 1,4-bis(3-pyridyl)-2,3- diaza-1,3-butadiene

3-bpmp 3-dpba 3-dpha

3-bis(3-pyridylmethyl)piperazine N,N′-di(3-pyridyl)butanediamide N,N′-bis(3-pyridyl)adipamide or N,N′-di(3-pyridyl) hexanedioicdiamide N,N′-bis(3-pyridyl)malonamide or N,N′-di(3pyridyl)propanediamide N,N′-bis(3-pyridyl) heptandiamide or N,N′-di(3pyridyl(pimelicdiamide N,N′-di(3-pyridyl)sebacicdiamide N,N′-di(3-pyridinecarboxamide)-1,6-hexane N-(3-pyridyl)-isonicotinamide N-(pyridin-3-yl)nicotinamide 3-pyridyltetrazoles azobenzene- 4,4′-dicarboxylica acid, 4,4′-(diazene-1,2-diyl) dibenzoic acid 4,4′-bis(benzimidazol)propane 4,4′-bpyridine 4,4′-(1H-1,2,4-triazole-3,5-diyl) dipyridine or 1H-3,5-bis(4pyridyl)-1,2,4-triazole 4,4′-di (1H-imidazole-1-yl)-1,1′-biphenyl

3-dppa 3-dppia 3-dpsea 3-dpyh 3-pina 3-pna 3-ptz 4,4′-ADB, AzDC 4,4′-bibp 4,4′-bipy 4,4′-bpt (bpt) 4,4′-DIB (bibm) (bpim)(4,4′-bimbp) (bimb) (bibp)(dibp) 4,4′-tmbpt 4-bmbpd 4-bpah 4-bpdh 4-bpmb

2,5-bis-(3-pyridyl)-3,4-diaza-2,4-hexadiene

1-((1H-1,2,4-triazol-1-yl)methyl)-3,5-bis(4-pyridyl)-1,2,4-triazole N,N′-bis(4-methylenepyridin-4-yl)-1,4-benzenedicarboxamide N,N′-bis(4-pyridinecarboxamide)-1,2-cyclohexane 2,5-bis-(4-pyridyl)-3,4-diaza-2,4-hexadiene N1,N4-bis-((pyridin-4-yl)methylene) benzene-1,4-diamine or N,N′bis-(4-pyridylmethylene)-1,4-benzenediamine

4-bpmh (4-bpdb) (bphz)(azpy) 4-Br-H2ip

trans 4,4′-azobispyridine or N,N-bis-pyridine-4-ylmethylenehydrazin, 1,2-bis-(4-pyridylmethylene)hydrazine 4-bromobromoisophthalic acid

4-nbpy (4-bpmn) 4-pina 4-pna

Bis-pyridin-4-ylmethylene-naphtalene-1,5-diamine N-(4-pyridyl)-isonicotinamide N-(4-pyridyl)-nicotinamide

4-ptz (4-H-ptz) 5-Br-H2ip

4-pyridyltetrazole 5-bromoisophthalate

5-iipa

5-iodo-isophthalic acid

5-OH-H2bdc, H2hip

5-hydroxyisophthalic acid

9Meade Abimb Adb Apyr Asp azpy (abpy) (azbpy) (azopy) (dpa) (pdp) (dpd) Atz Bbp Bdc BDC-Br BDC-Cl2

9-methyladenine 2-amine-4,4′-bis(1-imidazolyl)-bibenzene anthracene-1,5-dicarboxylic acid 2-amino-pyrazine Aspartic acid 4,4′-azopyridine or 4,4′-azobis (pyridine) or 1,2-di(pyridin-4-yl) diazene or 4- ((E) - 4 - pyridinylazo) pyridine or dipyridine-4-yldiazen aminotetrazole or 5-amino-1H-tetrazole 1,3-bis(benzimidazol)propane terephthalic acid, 1,4-benzenedicarboxylic acid 2-bromo-1,4- benzenedicarboxylic acid 2,5 dichloro-1,4-benzenedicarboxylic acid

BDC-OH Beb Betib Bhep bib (dmib)

2-hydroxy-1,4-benzenedicarboxylic acid 1,4-bis(2-ethylbenzimidazol-1-ylmethyl) benzene 1,4-bis(2-ethyl-1H-imidazol-1-yl) butane 1,4-bis(2-hydroxyethyl)piperazine 1,4-bis(1-imidazol-yl)-2,5-dimethyl benzene

BIDPE biim-2 (bie) Bimx Bip Bisopib

4,4/-bis(imidazol-1-yl)diphenyl ether 1,1′-(1,2-ethyl)bis(imidazole) or 1,2-bis(imidazol-1-yl)ethane 1,4-bis(imidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene 1,5-bis(imidazole-1-yl)pentane 1,4-bis(2-isopropyl-1Himidazol-1-yl)butane

Bmb BME-bdc

1,4-bis(2-methylbenzimidazol-1-ylmethyl) benzene 2,5-bis(2-methoxyethoxy)-1,4-benzene-dicarboxylate

bmib (bib) (bmeib)

1,4-bis(2-methyl-1H-imidazol-1-yl)butane or bis(2-methyl-1Himidazol-1-yl)butane 1,4-bis(2-methylimidazol-1-ylmethyl)-benzene or 4,4′-bis (2methylimidazol- 1-yl)benzene Bicycle)2,2,2(octane-1,4-dicarboxylate N,N′-bis(picolinamide)azine

bmix (4,4′-bmib) (pbmeix) BODC Bpa bpb (bpbenz) (pbyb) (dpb) bpbix (bimb) (bmb) (bpim) Bpbp bpda (HBPPA) (bpta) Bpdab Bpdc Bpeb Bpfb Bpfn Bphy

1,4 -bis(4-pyridyl)benzene or 1,4-di(pyridine-4-yl)benzene 4,4′-bis((1H-imidazol-1-yl)methyl) biphenyl or 4,4′-bis(imidazol-1ylmethyl)bibenzene or 4,4′-di(1H-imidazol-1-yl)biphenyl 5,5′-bis(4-pyridyl)-2,2′-bithiophene N,N′-bis(4-pyridinyl)-1,4benzenedicarboxamide or N,N′-bis-(4-pyridyl)phthalamide 1,2-bis(pyridin-4-ylmethyl)diazene 4,4′-biphenyldicarboxylic acid 1,4-bis)2-(4-pyridyl)etheny) benzene N,N′-bis(4-pyridylformamide)-1,4-benzene or N,N′-(1,4-phenylene) diisonicotinamide N,N -bis-(4-pyridylformamide)-1,5-naphthalenediamine 1,2-bis(4-pyridyl)hydrazine

BPTC bpydbH2

benzophenone 4,4/-dicarboxylic acid 4 -amino-3,5-bis(4-pyridyl)-1,2,4-triazole or 3,5-di(pyridin-4yl)-4H-1,2,4-triazol-4-amine 3,3′,4,4′-benzophenone tetracarboxylate 4,4′-(4,4′-bipyridine-2,6-diyl) dibenzoic acid

bpz (Me4bpz)

3,3′-5,5′-tetramethyl-4,4′-bipyrazole

Br-iptH2

5-(bromomethyl)-isophthalic acid

bta3Btb

benzene-1,3,5-triacetate benzene-1,3,5-tribenzoate

BPnDC bpt (dpta) (4-abpt)

btb (btab) 1,4-bis(1,2,4-triazol-1-yl)butane btbp (btp) (btmb) 4,4′-bis(1,2,4-triazol-1-ylmethyl) biphenyl Btec, H4pyro, H4pma benzene-1,2,4,5-tetracarboxylate, pyromellitic acid

Btmx BTPA

1,4-bis(1,2,4-triazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene bis(4-(1H- 1,2,4-triazol-1-yl)phenyl) amine

btre (bte) Btx btx (bbtz) (btb) (btrb) Ceta

1,2-bis(1,2,4-triazol-4-yl)ethane 1,4-bis(1,2,4-triazol-4-yl)benzene 1,4-bis(1,2,4-triazol-1-ylmethyl) benzene Bicycle)2.2.2(oct-7-ene-2,3,5,6-tetracarboxylate

dabco (Ted) (DBO) DA-H2P

1,4- diazabicyclo(2.2.2)octane 5,15-di(4-pyridylacetyl)-10,20-diphyenyl) porphyrinato

datz (datrz)

3,5-damino-1,2,4-triazole

Dbds DBrBDC DFD Dia dipytz (bpt) (bpta) (DPT)(4-PYTZ) DMBDC DMBDC dmbpy (2,2′-dmbpy) Dmpz Dmpzb Dmpzh Dmtrz

4,4′-dipyridyldisulfide 2,5-dibromo-1,4-benzenedicarboxylate 9,9-dipropylfluorene-2,7-dicarboxylate 9,10-bis (1H-imidazol-1-yl) anthracene di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) or 3,6-di(4-pyridyl)-1,2,4,5tetrazine or 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine 2,5-dimethyl-1,4-benzenedicarboxylic acid 2,5-dimethoxy-1,4-benzenedicarboxylate 2,2′-dimethyl-4,4′-bipyriine N,N′-dimethylpiperazine 1,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)butane 1,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)hexane 3,5-dimethyl-1H, 1,2,4-triazolate

Dnpdc Doa Dpa DPBT DPEA (bpe) (bpea) (bpa) DPEE (bpe) (dpe) (bpee) (dipy) DPG (dpyg) DPMNI (DPNDI) (DPNI)(PNMI) DPP (bpp) (dpp)

2, 2′-dinitrobiphenyl-4,4′-dicarboxylate 1,4-dioxane di(pyridin-4-yl)amine or 4,4′-dipyridylamine 4,7di(4-pyridyl)-2,1,3-benzothiadiazole 1,2-di(pyridin-4-yl)ethane or 1,2 bis(4-pyridyl)ethane 1,2-di(pyridin-4-yl)ethylene or 1,2-di(4-pyridyl)ethylene or 1,2-di(4pyridyl)ethylene, 1,2-trans-bis(4-pyridyl) ethane 1,2-di(4-pyridyl)-1,2-ethanediol or 1,2-di(4-pyridyl)-glycol N,N′-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide 1,3-di (pyridine-4-yl) propane or 1,3-bis (4-pyridyl)-propane, 4,4′-

(tmdpy) (tbpy) dps (dpys)

trimethlyene dipyridine 4,4′-dipyridylsulfide

Dstc Dtb FDA F-H2P

3,3/,4,4/-diphenylsulfonetetracarboxylate 1,3-di-(1,2,4-triazole-4-yl)benzene furan-2,5-dicarboxylic acid 5,15-dipyridyl-10,20-bis-(pentafluorophenyl)porphyrin

FMA, H2fum

fumaric acid

fu-bdc

Functionalized bdc

H2ADA

1,3-adamantanediacetic acid

H2ADC

1,3-adamantanedicarboxylic acid

H2adc

anthrancene-9,10-dicarboxylic acid

H2adi, H2adip

adipic acid

H2AIP

5-aminoisophthalic acid

H2aza

azelaic acid

H2bcp

1,3-bis(4-carboxy-phenoxy) propane

H2bcpb

3,5-bis(3-carboxyphenyl)pyridine

H2BDA

(R)- 1,1/-binaphthyl-2,2/-dihydroxy-5,5/-dicarboxylic acid

H2BDC, 1,4-bdc,

terephthalic acid, 1,4-

H2tp, tpa

benzenedicarboxylic acid

H2bpdado

2,2′-bipyridine-3,3′-dicarboxylate-1,1′-dioxide

H2bpdc

biphenyl- 2,4-dicarboxylate

H2bpea

biphenylethene-4/,4-dicarboxylic acid

H2btca

benzotriazole-5-carboxylate acid

H2btdc

2,2/- -5,5/-dicarboxylic acid

H2camph

(+)-camphoric acid, D-camphor acid

H2cca, CNC

4-carboxycinnamic acid

H2cpds

nicotinate 6,6/-dithiodinicotinic acid

H2cpfa

(R)-4-(4-(1-carboxyethoxy) phenoxy)-3-fluorobenzoic acid

H2CPNA

5-(4/-carboxylphenyl) nicotinic acid

H2cpoa

4-carboxyphenoxy acetic acid

H2cpp

3-(4-carboxyphenyl)propionic acid

H2cys

L-cysteic acid

H2dcpy

3-(2′,5′-dicarboxylphenyl)pyridine acid

H2epda

5-ethyl-pyridine- 2,3-dicarboxylic acid

H2FDC

9-fluorenone-2,7-dicarboxylic acid

H2glu

glutaric acid

H2hfipbb

4,4′(hexa-fluoroisopropylidene) bis-(benzoicacid)

H2ipa-CH3, H2mip

5-methyl-isophthalic acid

H2ma

Malic acid

H2mbdc, H2ipa, 1,3H2bdc, m-H2BDC

isophthalic acid, 1,3-benzenedicarboxylic Acid, m-phthalic acid, benzene-1,3-dicarboxylic acid

H2mal

Malonic acid

H2MBP

4,4′-methylene-bispyrazole

H2mbpdc

2-methyl-4,4/-biphenyldicarboxylicacid

H2MDP

methylene bis(3,5-dimethylpyrazole)

H2mta

2-(methoxycabonyl)terephthalic acid

H2-muco

muconic acid

H2nbpdc

2-nitrobiphenyl- 4,4′-dicarboxylic acid

H2NDC

2,6-naphthalenedicarboxylic acid

H2oba

4,4′-oxybis(benzoic acid)

H2ox

oxalic acid

H2PA

pamoic acid

H2pbda

3-pyridin-3-yloxy)benzene-1,2-dicarboxylic acid

H2pdac, o-H2pda,

1,2-phenylenediacetic acid,

H2pda

o-phenylenediacetic acid

H2pim

pimelic acid

H2psa

phenylsuccinic acid

H2pyip

5-(pyridine-4-yl)- isophthalic acid

H2sba

suberic acid

H2sdba

4,4′-sulfonyldibenzoic acid

H2SDBA

sulfone-4,4/-biphenyldicarboxylate

H2sea

sebacylic acid

H2suc, H2SA

succinic acid

H2tbip

5-tert-butylisophthalic acid

H2tdc

thiophene-2,5-dicarboxylic acid

H2TDC

triptycenedicarboxylic acid

H3ATTCA

2-amino-(1,1:3,1-terphenyl)-4,4,5-tricarboxylic acid

H3BCPBA

3,5-bi(4-carboxyphenoxy)-benzoic acid

H3BCPBA

3,5-bi(4-carboxy-phenoxy)-benzoic acid

H3bcta

4,4′,4″-(1,3,5-benzenetriyltris (carbonylimino))trisbenzoate acid

H3bidc

1H-benzimidazole-5,6-dicarboxylic acid

H3bpta

3,4′,5-biphenyltricarboxylic acid

H3BTC

1,3,5-benzenetricarboxylic acid, trimesic acid

H3bta

1,2,4-benzenetricarboxylic acid

H3CAM

4-hydroxypyridine-2,6-dicarboxylic acid

H3CmdcpBr

N-carboxymethyl-3,5-dicarboxylpyridinium bromide

H3cpia H3CPIP

5-(4-carboxyphenoxy)- isophthalic acid 5-(4-carboxyphenoxy)isophthalic acid

H3cpop

4-(4-carboxyphenoxy)phthalate acid

H3CTC

cis,cis-1,3,5-cyclohexanetricarboxylic acid

H3dpob

3-(2′,3′-dicarboxylphenoxy)-benzonic acid

H3DPPA

3-(4-hydroxyl pyridinium-1-yl) phthalic acid

H3IDC, Himdc

imidazole-4,5-dicarboxylic acid, 4,5-imidazole dicarboxylate

H3EIDC, H3eimda

2-ethyl-1H-imidazole-4,5-dicarboxylic acid

H3mimda

1H-2-methyl-4,5-imidazole-Dicarboxylic acid

H3nbta

5-nitro-1,2,3-benzenetricarboxylic acid

H3OAIP

5-oxyacetate isophthalic acid

H3PIA

5-(2-carboxypyrrolidine-1-carbonyl) isophthalic acid

H3ppat

phosphonoacetic acid

H3SIP, H3sipa

5-Sulfoisophthalic Acid

H3tca

tricarballylic acid

H3tci

tris(2-carboxyethyl)isocyanurate

H3TMTA

4,4′,4″-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoic cid

H4abtc

3,3′,5,5′-azobenzenetetracarboxylic acid

H4adip

5-aminodiacetic isophthalic acid

H4ata H4bpt

2,3,6,7-anthracenetetracarboxylic acid 3,3′,5,5′-biphenyltetracarboxylic acid

H4bpta

1,1/-biphenyl- 2,2/,6,6/-tetracarboxylic acid

H4bptc, H4odpa

3,3/,4,4/-biphenyltetracarboxylic acid

H4btca

1,2,3,4-butanetetracarboxylic acid

H4dcpp

4,5-di(4′-carboxylphenyl)phthalic acid

H4dht

2,5-dihydroxyterephthalic acid

H4dpstc

3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride

H4DSBDC

2,5-disulfonylterephthalate acid

H4obda

1,4-bis(4- oxy-1,2-benzene dicarboxylic acid)benzene

H4ODPT

4,4/-oxidiphthalic acid

H4pbta

5,5′-phenylenebis(methylene)-1,1′-3,3′-(benzene-tetracarboxylic acid)

H4tptc H5bpbc

terphenyl-3,2//,5//,3/-tetracarboxylic acid biphenyl-2,4,6,3′,5′-pentacarboxylic acid

H6BTP

1,3,5-Benzenetriphosphonic acid

H6dccpa

3,4-di(3,5-dicarboxyl phenyl) phthalic acid

H6TTHA

1,3,5-triazine-2,4,6-triamine hexaacetic acid

Hade (ad) Hatz (artz)(atz) Hbimtz HBTA (btz) (BTAH)

9H-purin-6-amine or adenine 1H-1,2,4-triazol-3-amine or 3-amino-1,2,4-triazole 2-(1H-tetrazol-5-ylmethyl)-1H-benzoimidazole 1,2,3-benzenetriazole or 1H-benzotriazole or benzotriazol

Hcnp-H2ipa

5-(4-carboxy-2- nitrophenoxy) isophthalic acid

Hdap Hdmpp Hem HET

2,6-diaminopurine 3,5-dimethyl-4-(4′-pyridyl)pyrazole 4-(2-hydroxyethyl)- morpholine 1,4,5,6,7,7-hexachlorobicyclo) 2.2.1(hept-5-ene-2,3- dicarboxylic acid

Hibpa Hin HIPT Hmt HPBA Hpptp or Hpptpd Hpypz Hpyt HPyz Hti (TBZ) Htrb HTZ Impy IP ipO iso-nia

4-isobutyl-α-methylphenylacetic acid isonicotinic acid 5-(4-(1H-imidazol-1-yl)phenyl)-1H-tetrazolate hexamethylenetetramine (methenamine) 4-(4-pyridyl) benzoic acid 2-(3-(4-(pyridin-4-yl)phenyl)-1H-1,2,4-triazol-5-yl)pyridine 4-(1H-pyrazol-4-yl)pyridine 5 -(4-pyridyl)-1,3,4-oxadiazole-2-thiol 1H-pyrazole 4-(1H-benzo(d)imidazol-2-yl) thiazole or thiabendazole hexakis(1,2,4-triazol-ylmethy1) benzene 1H-tetrazole 2,6-di(1H-imidazol-1-yl)pyridine 1-H-imidazo(4,5-f) (1,10)-phenanthroline 2-hydroxyisophthalic acid Isonicotinamide

Mbim m-bimb (1,3-bimb) (mbix) Mbtz Mfd

N,N′-(1,1-methyl)-bis(imidazole) 1,3-bis(1H-imidazol-1-yl)methyl)-benzene or 1,3-bis(imidazol-1ylmethyl)benzene 1,3-bis(1,2,4-triazol-1-ylmethyl) benzene 9,9-dimethylfluorene-2,7-dicarboxylate

m-H4bptc

biphenyl-2,3/,3,4/-tetracarboxylic acid

MMBDC M pbatb Mpda MTAZ NBzG

2-monomethyl 1,4-benzenedicarboxylic acid 4,4′,4″,4″′-(1,3-phenylenebis (azanetriyl)tetrabenzoate) 1,3-phenylenediacetate 5-methyl-tetrazole N-benzoyl-Lglutamate

NCG nIm

N-carbamyl-L-glutamate 2-nitroimidazole

Niox NO2-bdc

1,2- cyclohexanedionedioxime 2-nitro-1,4-benzenedicarboxylate, 2- nitroterephthalic acid

NO2-bdc, H2nip

5-nitro-1,3-benzenedicarboxylate, 5-nitroisophthalic acid

Obix (1,2-Bimb) OH-iptH2 OPY

1,2-bis(imidazol-1-ylmethyl)benzene 5-(hydroxymethyl)isophthalic acid 4,4′-(oxybis(4,1-phenylene)) dipyridine

Ox p-bimb (bix) (BIYB) pbib (1,4-bimb) (bimx) (1,4-dimb) Pbetix Pbisopix Pbmb p-bpmp (4-bpmp)

oxalic acid 1,4-bis(1H-imidazol-1-yl)methyl)-benzene or 4,4′-bis(imidazol-1ylmethyl)benzene

p-BrPhH3IDC

1,4-bis((2-ethyl-1H-imidazol-1-yl) methyl)benzene 1,4-bis((2-isopropyl-1H-imidazol-1-yl)methyl)benzene 1,1′-(1,3-propane)bis-(2-methylbenzimidazole) 4- bis(4-pyridinylmethyl)piperazine or 1,4-bis(4-pyridinylmethyl) piperazine 2-(p-bromophenyl)-1H-imidazole-4,5-dicarboxylic acid

p-cbiaH3 PCIH

5-(4-carboxybenzyloxy)isophthalic acid 4-pyridinecarbaldehyde isonicotinoyl hydrazine

Pcp

P,P/-diphenyl-diphosphinate

Pdoa

2,2/-(1,3-phenylenedioxy) bis(acetate)

Pdpy Phen PPA (pi)

piperazine-1,4-diylbis(pyridme-4-ylmethanone 1,10-phenanthroline Piperazine

PTA px2ampy

1,3,5-triaza-7-phosphaadamantane 1,4-bis(2-pyridylaminomethyl) benzene

px3ampy

1,4-bis(3-pyridylaminomethyl) benzene

py3T (tpt) (4-ptz)

tris(4-pyridyl)-triazine or 2,4,6-tris(4-pyridyl)-1,3,5-triazine

pydcH2

pyridine-2,4-dicarboxylic acid

Pyrdc, H2pydc

pyridine-2,3-dicarboxylate

Pyz

Pyrazine

Pzdc Pzta

pyrazine-2,3-dicarboxylate 5-(2-pyrazinyl) tetrazole

R-4-bpmb

N1,N4-bis(pyridin-4-ylmethyl) benzene-1,4-diamine

R-4-bpmn R-GLA-Me

N1,N5-bis(pyridin-4-ylmethyl) naphthalene-1,5-diamine R-2-methylglutarate

S-nia Stp Tadp TCMBT

Thionicotinamide 2-sulfoterephthalate 4,4′-(2H-1,2,3-triazole-2,4-diyl) dipyridine N,N′,N″-tris(carboxymethyl)-1,3,5-benzenetricarboxamide

TCPB TCPP Tfbdc, tftpa

TPB Tpcb trans-H2chdc, CDC

1,2,4,5-tetrakis(4-carboxyphenyl)-benzene tetrakis(4-carboxyphenyl) porphyrin Tetrafluoroterephthalate, tetrafluorobenzene-1,4-dicarboxylate, 2,3,5,6- tetrafluoro-1,4-benzenedicarboxylic acid Tetrafluoroisophthalate Tetrafluorosuccinate 1,3,5-tris(1-imidazolyl)benzene tris(4-(1H-imidazol- 1-yl)phenyl) amine 1,3,5-tris(p-imidazolylphenyl) benzene Tetramethylterephthalate, 2,3,5,6-tetramethyl-1,4Benzenedicarboxylic acid 1,2,4,5-tetra(4-pyridyl)benzene tetrakis(4-pyridyl)cyclobutane trans-1,4-cyclohexanedicarboxylic acid

Trz Tt

1,2,4-triazole tris(triazolyl)borate

TTPA

tris(4-(1H-1,2,4-triazol-1- yl) phenyl)amine

TTPBDA

N4,N4,N4′,N4′-tetrakis(4-(1H-1,2,4-triazol-1-yl)phenyl)-(1,1′biphenyl)-4,4′-diamine 4,4’-(carbonylbis(azanediyl)) dibenzoic acid

Tfipa Tfsuc Tib Tipa Tipb Tmbdc

Ubl

Chapter 1 Introduction to Metal-Organic Frameworks 1.1 What are the Metal-Organic Frameworks? Metal-organic frameworks (MOFs) are made by linking inorganic and organic units by strong bonds (reticular synthesis). The flexibility with which the constituents’ geometry, size, and functionality can be varied has led to more than 20,000 different MOFs being reported and studied within the past decade. The organic units are ditopic or polytopic organic carboxylates (and other similar negatively charged molecules), which, when linked to metalcontaining units, yield architecturally robust crystalline MOF structures with a typical porosity of greater than 50% of the MOF crystal volume. The surface area values of such MOFs typically range from 1000 to 10,000 m2/g, thus exceeding those of traditional porous materials such as zeolites and carbons. To date, MOFs with permanent porosity are more extensive in their variety and multiplicity than any other class of porous materials. Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology has not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties [1]. The building blocks of a MOF (SBU) are carefully chosen such that their properties are retained and exhibited by the product material. Whereas the nature and concentration of the monomers in an organic polymer determine its processability, physical and optical characteristics, it is the network connectivity of the building units that largely determines the properties of a MOF. These may include magnetic exchange, acentricity for non-linear optical (NLO) applications, or the definition of large channels available for the passage of molecules. The inclusions of chiral centers or reactive sites within an open framework are also active goals for generating functional materials. Consequently, MOF synthesis not only requires the selection and/or preparation of desired modules, but also some foresight as to how they will be assembled in the final solid. In order to aid the process of structure prediction, the concept of secondary building units (SBUs) as structural entities was adopted from zeolite structure analysis [2]. These are simple geometric figures representing the inorganic clusters or coordination spheres that are linked together by the (typically linear) organic components to form the product framework. Examples of some SBUs that are

commonly encountered in MOFs are illustrated in Figure 1.1.

Figure 1.1 Some inorganic secondary building units (A) and organic linkers (B) [1]. Although many of these units have been observed in molecular species [3,4], they are generally not introduced directly, but are formed in situ under specific synthetic conditions. Conversely, branched organic links with greater than two coordinating functionalities constitute preformed SBU s. The success of an SBU in the design of open frameworks relies both on its rigidity and directionality of bonding, which must be reliably maintained during the assembly process. The conceptual approach by which a metal–organic framework is designed and assembled is termed reticular synthesis and is based upon identification of how building blocks come

together to form a net, or reticulate. It is hypothesized, and indeed observed for a large number of compounds, that the various network topologies adopted by MOFs are represented by only a small number of simple, high symmetry structures [5]. These have been likened to the nets underlying simple inorganic compounds, such as diamond, graphite, SrSi2, and PtS. Foreknowledge as to which topology will be adopted by a given set of building blocks is particularly relevant to the development of porous materials, as it is precisely the expansion of these simple nets by the organic links that defines voids within the solid. Knowledge may also be gleaned about the likelihood of catenation, where two or more identical frameworks are intergrown at the expense of pore volume. This may take the form of interpenetration [6], where the networks are maximally displaced from each other, or inter weaving, where they are minimally displaced and exhibit close contacts that may result in mutual reinforcement [7]. The former is commonly cited as one of the major obstacles that must be overcome in the development of a porous MOF. The possibility of either of these events is directly dependent on the network topology [8] or distortions and the nature of metal, ligand or counter-ions [9,10]. To prepare MOFs with even higher surface area (ultrahigh porosity) requires an increase in storage space per weight of the material. Longer organic linkers provide larger storage space and a greater number of adsorption sites within a given material. However, the large space within the crystal framework makes it prone to form interpenetrating structures (two or more frameworks grow and mutually intertwine together). The most effective way to prevent interpenetration is by making MOFs whose topology inhibits interpenetration because it would require the second framework to have a different topology. Additionally, it is important to keep the pore diameter in the micropore range (below 2 nm) by judicious selection of organic linkers in order to maximize the BET surface area of the framework, because it is known that BET surface areas obtained from isotherms are similar to the geometric surface areas derived from the crystal structure.

1.2 Synthesis of Metal-Organic Frameworks MOFs are typically synthesized by combining organic ligands and metal salts in solvothermal reactions at relatively low temperatures (below 300 °C). The characteristics of the ligand (bond angles, ligand length, bulkiness, chirality, etc.) play a crucial role in dictating what the resultant framework will be. Additionally, the tendency of metal ions to adopt certain geometries also influences the structure of the MOF. The reactants are mixed in high boiling, polar solvents such as water, dialkyl formamides, dimethyl sulfoxide or acetonitrile. The most important parameters of solvothermal MOF synthesis are temperature, the concentrations of metal salt and ligand (which can be varied across a large range), the extent of solubility of the reactants in the solvent, and the pH of the solution. Although experience often dictates the best conditions for growing these crystalline frameworks, experimentation and trial-and-error methods are still often necessary [11] (Scheme 1.1).

Scheme 1.1 Schematics showing the synthesis and applications of MOFs. In addition to this standard method, several other synthetic methodologies are described in the literature including the mixture of non-miscible solvents, an electrochemical route, and a high-throughput approach. One of the most promising alternatives is microwave irradiation which allows access to a wide range of temperatures and can be used to shorten crystallization times while controlling face morphology and particle size distribution. A serious limitation of this approach is the general lack of formation of crystals large enough to obtain good structural data.

1.3 Structural Highlights of Metal-Organic Frameworks When considering the structure of MOFs, it is helpful to recognize the secondary building units (SBUs), which dictate the final topology of a framework. While the organic linkers are also important SBUs, their structure seldom changes during MOF assembly. Recently, the geometry of the SBU has been proven to be dependent on not only the structure of the ligand and type of metal utilized, but also the metal to ligand ratio, the solvent, and the source of anions to balance the charge of the metal ion. Several publications discussed the topic of SBU formation and structure in depth. Pores are the void spaces formed within MOFs (or any porous materials) upon the removal of guest molecules. In general, large pores are advantageous for conducting host–guest chemistry such as catalysis, therefore mesoporous (openings between 20 and 500Å) or even macroporous (openings greater than 500Å) materials are attractive. Microporous materials have pores less than 20Å which result in strong interactions between gas molecules and the pore walls making them good candidates for gas storage and gas separation applications. In all cases, measurements of these openings are done from atom to atom while subtracting the van der Waals radii to give

the space available for access by guest molecules. The pores of MOFs are usually occupied by solvent molecules that must be removed for most applications. Structural collapse can occur and, in general, the larger the pore, the more likely the collapse. Permanent porosity results when the framework remains intact and is more difficult to achieve in mesoporous MOFs than in microporous analogues. Although MOFs can be constructed with ligands designed to generate large pores, frameworks will often interpenetrate one another to maximize packing efficiency. In such cases, the pores sizes are greatly reduced, but this may be beneficial for some applications. Indeed interpenetrated frameworks have been intentionally formed and found to lead to improved performance, for example, in H2 storage. Following the synthesis, MOFs, like other coordination polymers, may participate in further chemical reactions to decorate the frameworks with molecules or functional groups in what is known as post-synthetic modification (PSM). Sometimes the presence of a certain functional group on a ligand prevents the formation of the targeted MOF. In this situation, it is necessary to first form a MOF with the desired topology, and then add the functional group to the framework. This may be applied to MOFs that are designed for catalysis and gas storage, as these applications require functional groups to modify the surface property and pore geometry. It is important to keep in mind that the two most important factors in PSM are making sure that the reagent used to enhance the functionality is small enough to fit inside the cavity of the MOF and that the reaction conditions will not destroy the framework. If the reagent is too small to enter the cavity or the framework is destroyed by the reaction, the modification will be useless. The assembly of metal ions and organic ligands from solution into a solid-state phase can be accomplished in various ways, and may give rise to different products. The factors determining the assembly pathway can be relatively subtle. To date, little explicit connection between preparative conditions and the exact structure of the resultant product has been revealed. A wide variety of structures may arise from small differences in synthesis conditions. In general, factors such as identity of solvent, solvent concentration, nature of counterion, metal–ligand ratios, metal coordination geometries, pH values, temperature, and nature of guest molecules are thought to play important roles in formation of thermodynamically favoured products. For example, combination of zinc nitrate and terephthalic acid (H2BDC) produces MOF-5 from N,N-diethylformamide (DEF), but this combination can also yield another phase, MOF2, by simply changing the solvent to N,N-dimethylformanide (DMF). Although the two MOFs are synthesized from similar starting materials, their molecular building blocks are quite different: an octahedral basic Zn4O (O2C–)6 cluster in MOF-5 and a square planar paddlewheel Zn2(O2C–)4 in MOF-2 (Figure 1.2).

Figure 1.2 Crystal structures of MOF-5 and MOF-2 with different MBB. In general, the structural differences between isomeric MOFs having identical metal and ligand components lie in their distinct SBBs, even when the same starting materials are employed to synthesize these MOFs. However, the same SBBs and organic linkers can also assemble into different phases by changing synthetic conditions. For example, the recently reported two-fold interpenetrated Zn/BTB-ant and Zn/BTB-tsx species show the same Zn4O(O2C–)6 SBBs and organic linkers as those in MOF-177 but with different topological structures. The topological differences between these three phases result from differences in their preparative methods: MOF-177 being synthesized in DEF, whereas Zn/ BTB-ant and Zn/BTB-tsx are synthesized in DMF and DMF–H2O, respectively (Figure 1.3).

Figure 1.3 Crystal structures of Zn4O(BTB)2 prepared in different solvothermal reactions.

1.4 Expansion of Metal-Organic Frameworks Structures An x-ray diffraction study performed on a single crystal of MOF-5 dosed with nitrogen or argon gas identified the adsorption sites within the pores. The zinc oxide SBU, the faces, and, surprisingly, the edges of the BDC2– linker serve as adsorption sites. This study uncovered the origin of the high porosity and has enabled the design of MOFs with even higher porosities. Moreover, it has been reported that expanded tritopic linkers based on alkyne rather than phenylene units should increase the number of adsorption sites and increase the surface area. For many practical purposes, such as storing gases, calculating the surface area per volume is more relevant. By this standard, the value for MOF-5, 2200 m2/cm3, is among the very best reported for MOFs. Note that the external surface area of a nanocube with edges measuring 3nm would be 2000m2/cm3. However, nanocrystallites on this scale with “clean” surfaces would immediately aggregate, ultimately leaving their potential high surface area inaccessible. A family of 16 cubic MOFs-IRMOF-1 [also known as MOF-5, which is the parent MOF of the isoreticular (IR) series] to IRMOF-16 with the same underlying topology (isoreticular) was made with expanded and variously functionalized organic linkers. This development heralded the potential for expanding and functionalizing MOFs for applications in gas storage and separations. The same work demonstrated that a large number of topologically identical but functionally distinctive structures can be made. Note that the topology of these isoreticular MOFs is typically represented with a three-letter code, pcu, which refers to its primitive cubic net. One of the smallest isoreticular structures of MOF-5 is Zn4O(fumarate)3; one of the largest is IRMOF-16 [Zn4O(TPDC)3; TPDC2– = terphenyl-4,4”-dicarboxylate]. In this expansion, the unit cell edge is doubled and its volume is increased by a factor of 8. The degree of interpenetration, and thus the porosity and density of thesematerials, can be controlled by changing the concentration of reactants, temperature, or other experimental conditions (5). The concept of the isoreticular expansion is not simply limited to cubic (pcu) structures, as illustrated by the expansion of MOF-177 to give MOF-180 [Zn4O(BTE)2] and MOF-200, which use larger triangular organic linkers. Contrary to the MOF-5 type of expanded framework, expanded structures of MOF-177 are noninterpenetrating despite the high porosity of these MOFs (89% and 90% for MOF-180 and MOF-200, respectively). These results highlight the critical role of selecting topology.

1.5 High Thermal and Chemical Stability Because MOFs are composed entirely of strong bonds (e.g., C-C, C-H, C-O, and M-O), they show high thermal stability ranging from 250° to 500°C [19]. It has been a challenge to make chemically stable MOFs because of their susceptibility to link-displacement reactions when

treated with solvents over extended periods of time (days). The first example of a MOF with exceptional chemical stability is zeolitic imidazolate framework–8 [ZIF-8, Zn(MIm)2; MIm– = 2-methylimidazolate], which was reported in 2006 [19a]. ZIF-8 is unaltered after immersion in boiling methanol, benzene, and water for up to 7 days, and in concentrated sodium hydroxide at 100°C for 24 hours. MOFs based on the Zr(IV) cuboctahedral SBU also show high chemical stability; UiO-66 [Zr6O4(OH)4(BDC)6] and its NO2- and Brfunctionalized derivatives demonstrated high acid (HCl, pH = 1) and base resistance (NaOH, pH = 14). The stability also remains when tetratopic organic linkers are used; both MOF-525 [Zr6O4(OH)4(TpCPP-H2)3; TpCPP = tetra-para-carboxyphenylporphyrin] and 545 [Zr6O8(TpCPP-H2)2] are chemically stable in methanol, water, and acidic conditions for 12 hours [19d]. Furthermore, a pyrazolate-bridged MOF [Ni3(BTP)2; BTP3– = 4,4,4-(benzene1,3,5-triyl)tris(pyrazol-1-ide)] is stable for 2 weeks in a wide range of aqueous solutions (pH = 2 to 14) at 100°C (60). The high chemical stability observed in these MOFs is expected to enhance their performance in the capture of carbon dioxide from humid flue gas and extend MOFs’ applications to water-containing processes.

1.6 Applications of Metal-Organic Frameworks From the very beginnings of MOF research, it was recognized that not only would the framework components be alterable, but also the contents of the cavities they would define. To provide evidence of the accessibility of these void regions, ion and solvent molecule exchanges were studied. These analyses are useful as a preliminary demonstration of the integrity of an open framework when coupled with PXRD, as long as the crystallite integrity and composition of the exchange solvent are monitored to exclude the possibility of a dissolution/recrystallization mechanism. Quantitative exchange studies with a variety of small molecules have also identified MOFs with specificity towards guest shape or functionality. These observations have inspired beliefs that with proper tailoring, MOFs may be produced to act as highly selective molecular sieves, sensors, or catalysts. Sensor capabilities become realizable when the optical, electronic, or magnetic properties of the framework are altered by guest interactions. This phenomenon has been demonstrated in MOFs containing luminescent lanthanides or paramagnetic transition metals. Catalytic behavior has been reported in only a few instances and this area deserves much more attention. In other hand the most promising applications of metal–organic frameworks is gas storage and selective gas adsorption (Scheme 1.1).

1.6.1 Gas (Hydrogen and Methane) Storage in MOFs A tank charged with a porous adsorbent enables a gas to be stored at a much lower pressure than an identical tank without an adsorbent. Thus, high pressure tanks and multi-stage compressors can be avoided providing a safer and more economical gas storage method. Many gas storage studies have been conducted on porous adsorbents such as activated carbon, carbon nanotubes, and zeolites. MOFs have received growing attention as such

adsorbents due to their tunable pore geometries and flexible frameworks. The need to reduce global reliance on fossil fuels by the use of alternative technologies has pushed hydrogen and methane gases to the forefront of gas storage applications. This section will review the stateof-the-art study of hydrogen and methane storage in MOFs. 1.6.1.1 Hydrogen Storage in MOFs Hydrogen is an ideal energy carrier. It almost triples the gravimetric heat of combustion of gasoline (120 MJ/kg vs. 44.5 MJ/kg), and the main byproduct after energy release is water. This makes hydrogen a leading candidate for on-board fuel. However, hydrogen exists in a gaseous state at ambient temperature and pressure with a density of 0.08 kg/m3. Even in its liquid state, which requires pressurizing at a very low temperature (20.27 K), the density can only reach 70.8 kg/m3, one tenth of that of gasoline(~700 kg/m3).This extremely low volumetric storage density presents a hurdle for the practical usage of hydrogen as a fuel. In order to guide the research into hydrogen storage, the U.S. Department of Energy (DOE) set gravimetric and volumetric storage targets for on-board hydrogen storage for 2010 (6 wt%, 45 g/L) and 2015 (9 wt%, 81 g/L). Current hydrogen storage techniques involve the use of high pressure tanks, cryogenic tanks, chemisorption, and physisorption. Pure tank-based hydrogen storage suffers from safety and economic issues. The chemisorption approach allows the formation of chemical bonds between adsorbed hydrogen and the storage materials, leading to greater hydrogen storage density. However, the kinetics, reversibility and heat management still remain a problem [12]. Physisorption, on the other hand, is based on weak interactions (mainly van der Waals interactions) between the adsorbed hydrogen and the adsorbent, leading to fast kinetics, full reversibility, and manageable heat during hydrogen fueling. However, the promising data from physisorption-based hydrogen storage are all obtained at a cryogenic state (normally 77 K), and the adsorption becomes insignificant at ambient temperature. Hydrogen storage in MOFs is based on physisorption. It has been well established that under physisorption mode, the saturation hydrogen uptake at 77K has a positive correlation with the surface area of the materials. This is not surprising since increasing the surface area enhances the contact between hydrogen and the adsorbent resulting in an increased hydrogen uptake. Compared to other porous materials, some MOFs have higher surface areas and subsequently higher hydrogen uptake capacity. One advantage of using MOFs over other porous materials is that in some MOFs, unsaturated metal centers (UMCs) can be generated by the removal of the coordinated solvent molecules under vacuum. The interaction between hydrogen and the UMCs is much higher than that with pure carbon materials, and the isosteric heat of adsorption can sometimes go as high as 12–13 kJ/mol [13], very close to the projected optimum 15.1 kJ/mol. Theoretical studies revealed that the binding energy between hydrogen and transition metals can be tuned from about 10 to 50 kJ mol-1 by using different transition metals in the MOF system. This was partially confirmed by a recent experiment in which a series of isostructural MOFs were prepared from a variety of metal ions (Mg, Mn, Co, Ni, Zn) [14]. The hydrogen sorption data showed that within this series the zinc MOF has the lowest heat of adsorption (~8.5 kJ/mol) while the nickel one has the highest (~12.9 kJ/mol),

and the order of the heat of adsorption matches the Irving–Williams sequence reasonably well. This confirmed calculations indicating that the major interaction between the UMCs and hydrogen molecules is Coulombic attractions. Results indicated that UMCs can increase the binding between hydrogen and MOFs, but their effect in the hydrogen uptake is almost completely hidden in the high pressure range where surface area and pore volume play primary roles. The efficiency of hydrogen storage by physisorption should increase as the surface density of UMCs is increased. Many theoretical calculations support the idea that doping MOFs with metal ions could enhance the hydrogen uptake capacity. This enhancement is proposed to originate from the strong interactions between hydrogen and the doped metal ions. 1.6.1.2 Methane Storage in MOFs Natural gas (NG) is another good candidate for on-board fuel. The main component of NG is methane (>95%), while the rest is a mixture of ethane, other hydrocarbons, nitrogen, and carbon dioxide. Unlike for hydrogen, the heat of adsorption for methane (about 20 kJ/mol) is already within the ideal scope for practical usage. DOE has set a methane storage target: 180 v/v at ambient temperature and pressure no more than 35 bars. Some of the carbon materials have already reached this target, but they have limited packing density. Thus, the focus has been on increasing the surface area of the porous sorbent. In 1997, Kondo et al. reported the first methane sorption study using MOFs. The breakthrough result obtained by Ma et al. showed that the methane uptake in a MOF can exceed the DOE target. One point of concern is that uptake data calculations are based on the MOFs crystallographic density, which is higher than the packing density due to the void generated by particle packing. More methane uptake data needs to be calculated from the MOFs real packing density from MOFs in order to evaluate the potential of MOFs in methane storage.

1.6.2 Carbon Dioxide Capture in MOFs Here, we address a number of aspects related to the adsorption of CO2 within metal-organic frameworks that are important considerations when evaluating new materials for CO2 capture applications, such as the adsorption capacity and enthalpy of adsorption. Single-component gas adsorption isotherm data can further be used to estimate the adsorption selectivity for CO2 over other gases, which is a crucial parameter that determines the purity of the captured CO2. Detailed knowledge of the binding environment of CO2 within the pores of the framework can give vital information regarding the structural and chemical features contributing to the observed material performance, and in situ vibrational spectroscopy and crystallographic methods have recently emerged as invaluable tools for probing the adsorption phenomena. In this section, we present the most significant results that have been reported in this regard. 1.6.2.1 Capacity for CO2 The adsorptive capacity is a critical parameter for consideration when evaluating metal-

organic frameworks for CO2 capture. The gravimetric CO2 uptake, which refers to the quantity of CO2 adsorbed within a unit mass of the material, dictates the mass of the metalorganic framework required to form the adsorbent bed. Meanwhile, the volumetric capacity refers to how densely the CO2 can be stored within the material and is an equally crucial parameter, since it has a significant influence on the volume of the adsorbent bed. Both parameters also have an important role in determining the heating efficiency of the metalorganic framework, which directly impacts the energy penalty required for material regeneration and desorption of the captured CO2. The high internal surface areas of metalorganic frameworks provide an opportunity for large CO2 adsorption capacities to be achieved, owing to the efficient packing and close approach of the guest molecules on the pore surface. For example, at 35 bar, the volumetric CO2 adsorption capacity for MOF-177 reaches a storage density of 320 cm3(STP)/cm3, which is approximately 9 times higher than the quantity stored at this pressure in a container without the metal-organic framework and is higher than conventional materials used for such an application, namely, zeolite 13X and MAXSORB. Note that, the volumetric adsorption capacity of activated carbon materials at high pressures has been demonstrated to be competitive with metal-organic frameworks. 1.6.2.2 Enthalpy of Adsorption The enthalpy of adsorption of CO2 is a critical parameter that has a significant influence over the performance of a given material for CO2 capture applications. The magnitude of the enthalpy of adsorption dictates the affinity of the pore surface toward CO2, which in turn plays a crucial role in determining the adsorptive selectivity and the energy required to release the CO2 molecules during regeneration. Precise control of the binding strength of CO2 is essential if metal-organic frameworks are to be optimized such that they can lower the energy requirements of the capture process. Specifically, the use of a material that binds CO2 too strongly would increase the regeneration cost owing to the large quantity of energy required in order to break the framework … CO2 interactions. Meanwhile, if the enthalpy of adsorption is too low, although the material would be more readily regenerated, the purity of the captured CO2 would be lowered owing to the decreased adsorption selectivity, and the volume of the adsorbent beds would also be increased due to the lower density of CO2 adsorption. Owing to the presence of a variety of binding environments for CO2 within the pores of metal-organic frameworks, the enthalpy of CO2 adsorption is frequently expressed as an isosteric heat of adsorption (Qst) as a function of the quantity of CO2 adsorbed. The Qst value is a parameter that describes the average enthalpy of adsorption for an adsorbing gas molecule at a specific surface coverage and is usually evaluated using two or more CO2 adsorption isotherms collected at similar temperatures (usually within 10 K of each other). Usually the temperature- dependent isotherms are first fit to a high-order polynomial equation to obtain an expression for the pressure (P) in terms of the quantity of CO2 adsorbed (N), and the Qst values are subsequently computed using the Clausius-Clapeyron equation.

1.6.2.3 Selectivity for CO2 In CO2 capture applications, a high selectivity for CO2 over the other components of the gas mixture is essential. This selectivity can originate from two main mechanisms. In size based selectivity (kinetic separation), a metal-organic framework with small pore size may permit molecules only up to a certain kinetic diameter to diffuse into the pores, allowing the molecules to be separated based on size. For CO2/N2 and CO2/H2 separations, the relatively similar kinetic diameters of the molecules would require materials operating on a sizeselective mechanism to possess very small pores, which maylimit the diffusion of gases throughout the material. While some metal-organic frameworks do exhibit pore apertures in this size regime, almost all materials that exhibit high surface areas and high adsorption capacities for CO2 possess pore openings that are significantly larger than the sizes of the molecules. Thus, most studies of metal-organic frameworks rely on the separation of the molecules based on adsorptive phenomena. The adsorptive selectivity (thermodynamic separation) arises owing to the difference in affinity of the various components of the gas mixture to be adsorbed on the pore surface of the metal-organic framework. For selectivity based upon a physisorptive adsorption mechanism, the separation relies on the gas molecules having different physical properties, such as the polarizability or the quadrupole moment, resulting in a higher enthalpy of adsorption of certain molecules over others. For example, for the CO2/N2 separation relevant to post-combustion CO2 captures, the higher polarizability (CO2, 29.1×10-25 cm-3; N2, 17.4×10-25 cm-3) and quadrupole moment (CO2, 13.4×10-40 C.m2; N2, 4.7×10-40 C.m2) of CO2 compared with N2 results in a higher affinity of the surface of the material for CO2. The selectivity can be further enhanced by installing highly charged groups, such as polar organic substituents or exposed metal cation sites, which take greater advantage of this difference in the polarizability of the molecules. Alternatively, the adsorptive selectivity can arise due to chemical interactions between certain components of the gas mixture and surface functionalities of the metal-organic framework. Functionalities that recognize certain molecules based on their propensity for participating in specific chemistry can result in much higher selectivities than those obtained from purely physisorptive mechanisms. For example, in the case of CO2/N2 separations, the susceptibility of the carbon atom in CO2 to attack by nucleophiles has led to the investigation of materials possessing strong Lewis bases, such as amines. The interaction of CO2 with an amine can result in a C … N bond as observed in the aqueous amine solutions, resulting in highly selective adsorption of CO2 over N2. For O2/N2 separations in oxy-fuel combustion, the ability for O2 to participate in electron transfer reactions has led to the investigation of materials constructed from redox active metal centers.

1.6.3 Post-Combustion Capture The combustion of coal in air generates flue gas with a relatively low CO2 concentration (1516%), while the bulk of the effluent is composed of N2 and other minor components, such as

H2O, O2, CO, NOx, and SOx. The gas stream is released at a total pressure of approximately 1 bar. Since SOx removal would precede CO2 capture, the flue gas would be expected to enter the CO2 scrubber at temperatures between 40 and 60 °C. An ideal adsorbent for capturing CO2 from post-combustion flue gas would exhibit a high selectivity for CO2 over the other flue gas components, high gravimetric and volumetric CO2 adsorption capacities, minimal energy penalty for regeneration, long-term stability under the operating conditions, and rapid diffusion of the gas through the adsorbent material. The preparation of nextgeneration metal-organic frameworks that satisfy all of these requirements is currently a difficult synthetic challenge, although significant progress has been made in recent years. 1.6.3.1 Ideal Adsorbed Solution Theory (IAST) In practice, it is challenging to measure directly the adsorption selectivity of an adsorbent for gas mixtures, such as those encountered in CO2 capture applications. However, the performance can be conveniently predicted from the single-component adsorption isotherms of the constituents of the mixed gas via modeling techniques, such as ideal adsorbed solution theory (IAST). In this method, the isotherms are collected at the same temperatures, and IAST is applied in order to predict the expected selectivity of the material. 1.6.3.2 Metal-Organic Frameworks for CO2/N2 Separation Many of the frameworks exhibit large CO2 adsorption capacities at pressures at and above 1 bar, owing to their high surface areas. However, these compounds are generally not well suited for post-combustion capture, since the adsorption capacity at lower pressures is a more relevant consideration due to the low partial pressure of CO2. The selectivity calculation for CO2 over N2 is best performed using the adsorption capacities at pressures of approximately 0.15 bar for CO2 and 0.75 bar for N2. Since the total pressure of a flue gas is approximately 1 bar, selectivity calculations based upon the quantity of both CO2 and N2 adsorbed at 1 bar drastically overestimate the fraction of CO2 in post-combustion flue gas and the total pressure of the gas. It is important to emphasize that selectivity factors are based on purecomponent adsorption isotherms and do not necessarily represent the true selectivity of the material in a CO2/N2 mixture. As such, the direct measurement of multicomponent isotherms, which has been recently performed for CO2/CH4 mixtures, is necessary in order to evaluate the accuracy of selectivity factors. As a result, calculated selectivity factors are useful for preliminary evaluations of different materials. Note that, although gas adsorption measurements have most commonly been made at or below 298 K, it would be of benefit for a more realistic evaluation of post-combustion CO2 capture performance if researchers were to begin reporting adsorption data in the range of 313-333 K. In Table 1.1, the materials displaying the highest selectivities are generally those bearing functionalized pore surfaces. Surface functionalities that interact strongly with CO2 (and to a lesser extent N2) frequently increase adsorbent capacity at low pressures. Ideally, for high adsorption selectivities, the

CO2 adsorption should be maximized at pressures near 0.15 bar. Note that for metal-organic frameworks with strongly polarizing sites, the selectivity values listed in Table 1.1 likely underestimate the true adsorptive selectivity of the adsorbent. This is because in a realistic flue gas mixture, the strongest binding sites would be predominantly occupied by CO2 owing to its greater polarizability and quadrupole moment. Thus, single-component isotherms overestimate the adsorption of N2 and consequently reduce the calculated selectivity. Utilization of the IAST method (see section 1.6.3.1) for selectivity calculations minimizes this effect and enables more accurate selectivity values to be calculated. Since using IAST to predict selectivity for a gas mixture requires accurate fits of the pure-component isotherms, selectivities are instead reported in Table 1.1 using the adsorption values obtained directly from the pure-component isotherms. Indeed, IAST values at the appropriate pressures have been reported for only a handful of frameworks, but values for certain adsorbents are mentioned in the discussion that follows. It should be noted that the relevance of selectivity values calculated for flexible frameworks via the aforementioned method has not been fully established, although it is likely that the quantities calculated by this methodology would overestimate the true adsorptive selectivity. This is because flexible frameworks generally adsorb little N2 at 0.75 bar because the apertures of the framework are not opened by N2 in single component gas adsorption experiments. However, if CO2 affords the necessary gate opening pressure, the quantity of N2 adsorbed is likely to be significantly higher than that observed during the single-component experiment. Since IAST cannot account for such effects, direct selectivity measurements of binary mixtures are likely the only accurate gauge of selectivity in flexible metal-organic frameworks. Thus, Table 1.1 omits selectivity values for flexible frameworks, although single-component CO2 and N2 capacities at the relevant pressures are tabulated.

Table 1.1 CO2 and N2 uptake in selected metal-organic frameworks at pressures relevant to post-combustion CO2 capture. Chemical formula

CO2 uptake at 0.15 bar (wt %) Mg-MOF-74, 20.6 Mg-CPO-27 Ni-MOF-74, 16.9 CPO-27-Ni

N2 uptake at 0.75 bar (wt %) 1.83

Selectivity Temp (K) 44

303

2.14

30

298

----

----

298

Cu3(BTC)2

Co-MOF-74, 14.2 CPO-27-Co HKUST-1 11.6

0.41

101

293

Cu3(TATB)2

CuTATB-60 5.8

0.82

24

298

Co2(adenine)2(CO2CH3)2 bio-MOF-11 5.4

0.28

65

298

Zn(nbIm)(nIm) H3[(Cu4Cl)3(BTTri)8]

ZIF-78 Cu-BTTri

3.3 2.9

0.36 0.49

30 19

298 298

Zn(MeIM)2

ZIF-8

0.6

----

----

298

Zn4O(BDC-NH2)3 Zn4O(BTB)2 Zn4O(BDC)(BTB)4/3 Zn4O(BDC)3

IRMOF-3 MOF-177 UMCM-1 MOF-5, IRMOF-1

0.6 0.6 0.5 0.5

---0.39 -------

---4 -------

298 298 298 298

Mg2(dobdc) Ni2(dobdc) Co2(dobdc)

Common names

More recently, Mg2(dobdc) and MOF-177 were evaluated for use in a temperature swing adsorption-based process by collecting single-component CO2 and N2 adsorption isotherms across a temperature range of interest for such a process (25-200 °C (Figure 1.4)[15]. The working capacity of the materials was calculated as a function of the desorption temperature as the difference between the quantity of CO2 adsorbed at a partial pressure of 0.15 bar at the flue gas temperature (40 °C), and the corresponding value at 1 bar at the desorption temperature. For Mg2- (dobdc), it was demonstrated that the working capacity for CO2 reaches 17.6 wt % for a 200 °C desorption temperature. The increase in working capacity with desorption temperature arises from the fact that the quantity of CO2 adsorbed is reduced at higher temperatures, leading to more complete evacuation of the pores during regeneration. A similar analysis performed on MOF-177 resulted in a negative CO2 working capacity across the entire temperature range. This is ascribed to the relatively limited CO2 adsorption capacity at low pressures due to the lack of strong binding sites within the pores,

which affords a near-linear isotherm that steadily increases up to the desorption pressure. Even at the higher temperatures, a relatively high quantity of CO2 is retained at 1 bar, leading to a poor working capacity even at a desorption temperature of 200 °C. The results highlight the importance of selecting a structure containing a high density of strong binding sites within the pores, making Mg2(dobdc) a suitable candidate for further testing.

Figure 1.4 CO2 adsorption isotherms for Mg2(dobdc) collected over a temperature range of 20-200 °C [15].

1.6.4 Pre-Combustion Capture Pre-combustion CO2 capture is a process in which fuel is decarbonated prior to combustion, resulting in zero carbon dioxide production during the combustion step (Figure 1.5). Here, coal is gasified, generally at high temperature and pressure, to produce synthesis gas or “syn gas”, which is a mixture of mostly H2, CO, CO2, and H2O. This gas mixture is then run through the water-gas shift reaction to produce H2 and CO2 (“shifted syn gas”) at high pressure and slightly elevated temperature (5-40 bar and 40 °C, depending on the production plant). Pre-combustion CO2 capture, which refers to the separation of CO2 from H2 within this gas mixture, can then be performed to afford pure H2, which is subsequently combusted in a power plant to generate electricity.

Figure 1.5 Basic schemes showing the types of CO2 capture. The processes for postcombustion capture, pre-combustion capture, and oxy-fuel combustion. The main separation required for each type of process is indicated next to each of the headings in parentheses. 1.6.4.1 Metal-Organic Frameworks as Adsorbents Metal-organic frameworks have recently been investigated as potential next-generation adsorbents for pressure-swing adsorption-based separation of CO2 from H2. Their high surface areas afford enhanced gas adsorption capacities compared with porous solids conventionally employed in multilayer beds within current PSA systems, namely, activated carbons and zeolites, and their tunable surface chemistry is anticipated to facilitate further optimization of the material properties. Here, optimization refers to creating adsorption characteristics that are ideal for the CO2/H2 separation with the aim of reducing the regeneration cost of the PSA adsorbent, while maintaining a high gas adsorption working capacity and selectivity for CO2 over H2. Despite the opportunity for the development of metal- organic frameworks as pre-combustion CO2 capture adsorbents (and H2 purification adsorbents), relatively few reports have emerged in this regard. Although evaluation of the performance of candidate frameworks can be well-approximated via the collection of highpressure, single-component CO2 and H2 isotherms at near-ambient temperature, there are only a small number of examples where such experiments have been performed. In cases where they have been reported, the isotherms have seldom been discussed and analyzed in

the context of precombustion capture. Note that in this section, we consider CO2/H2 separations only in the context of pressure-swing adsorption based processes in which the separation of the gases is achieved by a thermodynamic equilibrium that results from the bulk adsorptive properties of the material. An alternative strategy for achieving the separation would be to make use of the difference in the kinetic diameters or diffusion properties of the two molecules in a kinetic-based separation using metal-organic framework membranes. It should further be noted that, although the present discussion predominantly addresses CO2/H2 separations, CO2/CO separation is also highly relevant to pre-combustion CO2 capture and has been investigated in metal-organic frameworks such as ZIF-100 and mixedlinker MOF-5 variants (multivariate “MTV” metal-organic frameworks). The separation of CH4 and H2 is also highly relevant to hydrogen purification and has been investigated in metal-organic frameworks. The selectivity of the material is also a crucial parameter when considering a material for precombustion CO2 capture. There can be a trade-off between the working capacity and the selectivity, since more selective materials will tend to have a steeper initial portion in the CO2 isotherms (P < 1 bar). This leads to a lower working capacity because the CO2 adsorbed at the lowest pressures will not be removed at the purge pressure. Thus, since the working capacity is highly sensitive to the lower pressure region of the adsorption isotherm, a single high-pressure CO2 isotherm is not sufficient to evaluate a material for precombustion CO2 capture. For example, among the more promising Metal-organic frameworks the MOF-74 structure types (Mg- and Ni-MOF-74), which possess exposed metal cation sites? These materials do not have the highest CO2 saturation capacity and also have a relatively steep CO2 adsorption isotherm, but the balance between selectivity and working capacity renders them extremely promising for pre-combustion CO2 capture. In contrast, MOF-177 has a high CO2 capacity and a shallow initial rise in its CO2 adsorption isotherm. However, its internal surface imparts little CO2/H2 selectivity and after further investigation, MOF-177 is shown to be a poor pre-combustion CO2 capture material. These two examples serve to stress that while metal-organic frameworks adsorb large amounts of CO2 at high pressures, they need to be carefully evaluated in order to elucidate the most promising candidates for pre-combustion CO2 capture.

1.6.5 Selective Gas Adsorption in MOFs With the ever-increasing demand for cheaper and more environmentally friendly industrial applications, new means for selective gas adsorption and separation are being examined. Currently, Industrial methods for selective gas adsorption rely heavily on cryogenic as well as membrane- and adsorption-based techniques. In adsorption-based separation, commonly used adsorbents include zeolites, molecular sieves, carbon nanotubes, aluminosilicates, and silica gel. However, all materials for selective gas adsorption are chosen based on two main criteria: (1) the adsorption capacity of the adsorbent; and (2) the selectivity of the adsorbent for an adsorbate. These properties are dictated by the chemical composition and structure of

the adsorbent, as well as the equilibrium pressure and temperature during the adsorption. MOFs are very promising candidates for selective gas adsorption, which can lead to gas separation. The principal mechanisms based on which selective gas adsorption is achieved in MOFs are adsorbate–surface interactions and size-exclusion (molecular sieving effect). The former involves the chemical and/or physical interaction between the adsorbent and the adsorbate while the latter depends on the dimension and shape of the framework pores. It is important to keep in mind that the two effects are capable of working independently as well as cooperatively.

1.6.6 Useful Gas Separations in MOFs The separation of alkane isomers from natural gas is of primary concern for industrial applications. It has been demonstrated that MOF-508 may be utilized to separate such alkanes using gas chromatographic techniques. The 3D pillared layer provides the necessary framework to selectively separate linear and branched alkanes by adsorbing the linear ones while allowing the branched alkanes to pass because of the difference in van der Waals interactions between the isomers and the frameworks pores. Additionally, this MOF proved effective in separating natural gas mixtures through gas chromatography. Recently, MOFs have been incorporated into thin-film materials to aid in gas separation. The copper net supported Cu3(btc)2 MOF thin film developed by Guo et al. demonstrated the successful separation of H2 from H2/N2, H2/CO2, and H2/CH4 mixtures. This MOF demonstrated excellent permeation selectivity for H2 and possessed separation factors much higher than traditional zeolites. The recyclability of this MOF further enhances its potential for application in H2 separation and purification. Couck et al. demonstrated the successful separation of CO2 and CH4 gases using amino-functionalized MIL-53(Al). Because CO2 possesses a quadrupole moment, the CO2 molecules have a high affinity for the amino groups. Breakthrough experiments demonstrated that the weakly adsorbed CH4 is able to pass through a MOF-packed column while CO2 is adsorbed. Thus, efficient separation of the two gases can be performed at ambient conditions.

1.6.7 Catalysis in MOFs As porous materials, MOFs may prove to be very useful in catalysis. Theoretically, the pores of MOFs can be tailored in a systematicway allowing optimization for specific catalytic applications. Besides the high metal content of MOFs, one of their greatest advantages is that the active sites are rarely different because of the highly crystalline nature of the material. Although catalysis is one of the most promising applications of such materials, only a few examples have been reported to date. In these MOFs, size- and shape-selective catalytic applications depend on porosity and the presence of catalytically active transition-metal centers.

1.6.8 Magnetic Properties of MOFs Magnets are very important materials with an ever-increasing number of uses. Thus, an important goal of the research of magnetic materials is the improvement of the properties of magnets as well as exploring new functions, in particular in combination with other useful phenomena. The magnetic properties such as ferromagnetism, antiferromagnetism, and ferrimagnetism of polymetallic systems derive from the cooperative exchange interactions between the paramagnetic metal ions or organic radicals through diamagnetic bridging entities. Therefore, their magnetic behaviors depend on the intrinsic nature of both the metal and the organic ligand as well as the particular level of organization created by the metal– ligand coordination interaction. As a result, in pursuing the magnetism of MOFs, the ligand design is crucial both to organize the paramagnetic metal ions in a desired topology and to efficiently transmit exchange interactions between the metal ions in a controlled manner. The formation of a bulk material with nonzero spin requires a framework that allows for parallel coupling of the spins of neighboring paramagnetic spin carriers or antiparallel coupling of unequal spins. Canted spin orientations may also result. It should be pointed out that there is always a tendency for antiparallel coupling of spins because the state of low-spin multiplicity is often more stable than the state of high-spin multiplicity [16]. Magnetic studies of MOFs are embedded in the area of molecular magnets and the design of low-dimensional magnetic materials, magnetic sensors, and multifunctional materials. Indeed, closed shell organic ligands that are typically used in MOFs mostly give rise to only weak magnetic interactions. In order to achieve a strong coupling between the metal centers, short oxo, cyano, or azido bridges are needed [17]. Alternatively, polymeric metal cyanide compounds are frequently encountered in magnetic investigations but fall outside the scope of this review. Furthermore, the porosity of MOFs provides additional interesting phenomena in regards to magnetic properties. The use of chemical coordination or crystal engineering techniques allows for the systematic design of MOFs with adjustable magnetic properties.

1.6.9 Luminescence and Sensors in MOFs The potential use of MOFs as luminescent materials has spurred much interest in the area. These materials can be prepared by combining the luminescent metal ions or clusters and organic ligands, as well as special guest molecules. 1.6.9.1 Luminescence in MOFs Metal-organic Frameworks (MOFs) have been rapidly developed as a new type of multifunctional luminescent materials [18, 19]. Such organic–inorganic hybrid luminescence, which can take advantage of both traditional inorganic and organic photoluminescent materials, can provide unique luminescent properties and thus lead to some novel functional materials. In fact, the luminescent properties of MOF materials can not only be generated from the metal ions/clusters and organic linkers but can also be tuned by guest molecules/ions and the interplay/interactions among these different components [20]. Due to the structural diversity of metal–organic frameworks, which are complex systems potentially

including multiple types of ligand molecules, inorganic ions and clusters, and guest molecules or ions, this photoluminescence can arise from a variety of mechanisms. In ligand-centered emission, both photon absorption and emission processes occur on the same organic ligand. However, absorption and emission events also commonly occur in separate locations within the framework, with non-radiative energy and electron transfer processes described by Förster–Dexterntheory bridging the gap. Among these mechanisms are ligand-to-ligand charge transfer (LLCT), ligand-to-metal charge transfer (LMCT), metalto-ligand charge transfer (MLCT), and metal-to-metal charge transfer (MMCT), as well as processes that involve guest molecules in the LMOF pores, such as guestcentered emission and guest-sensitization (Scheme 1.2).

Scheme 1.2 (a) Schematic illustration of the various photophysical processes, (b) schematic representation of the various possibilities contributing to the emission of MOFs. 1.6.9.1.1 Luminescence in MOFs Based on Metal Centers Lanthanide metal ions have been widely used in MOF syntheses due to their coordination diversity and luminescent properties. Chandler et al. [21] reported a stepwise approach to synthesizing a MOF with photophysical properties through incorporation of lanthanide metal complexes in the framework, namely [Ba2(H2O)4[LnL3(H2O)2](H2O)nCl]∞ (L = 4,4’-disulfo2,2’- bipyridine-N,N’-dioxide, Ln = Sm, Eu, Gd, Tb, Dy). A stepwise approach was used and allowed for an examination of the significance of the ratio of metal to organic building units in the preparation of a MOF containing lanthanide metal ions. By decreasing the ligand to metal ratio, a less dense, porous framework was prepared that maintained its luminescent characteristics. Additionally, metalloligands were referred to as building units for the development of lanthanide containing MOFs for use as sensing devices. Proper selection of the linker is necessary to adequately shield the metal to prevent quenching of its luminescent properties. De Lill et al. [22] reported in 2007 the synthesis and photoluminescent properties of a Eu MOF and a Eu/Tb mixed system MOF. In the mixed system, both Eu and Tb emissions were observed along with an increase in Eu emission intensity relative to the Eu MOF. This has been attributed to the Eu being sensitized by both the organic linkers and the Tb. 1.6.9.1.2 Luminescence in MOFs Based on Organic Ligands Two luminescent stilbene-based MOFs were prepared based on trans-4,4’-stilbene

dicarboxylic acid (LH2) and zinc nitrate in two different solvents. A 2D network structure Zn3L3(DMF)2 was obtained in DMF, while a 3D porous framework structure Zn4OL3 resulted from DEF [23]. The optical properties of both demonstrate that the LH2 organic ligand serves as the chromophore. In both cases, the rigidity of the stilbene ligand increases upon coordination to the metal center, resulting in increased emission lifetimes for the MOF crystals as compared to solutions of trans stilbene. Studies have also been conducted with luminescent organic linkers, such as H2hfipbb as reported by Gandara et al. in 2007 [24]. As a ligand, hfipbb emits a blue-white color under UV light, which is slightly modified with the addition of different lanthanide metals, leading to the possible uses of these MOFs as diodes. 1.6.9.1.3 Luminescence Based on Guest Molecules in MOFs Huang et al. [25] reported a 3D porous MOF, [Cd3L6] (BF4)2(SiF6)(OH)2.13.5H2O (L = 2,6di(4-triazolyl)pyridine), in which the guest species in the open channels can be removed and reintroduced reversibly without destroying the porous framework. Heating in air at different temperatures (180, 200, 225, 250 °C) for 1 day generated a series of dehydrated products. One of the dehydrated complexes was rehydrated when exposed to H2O vapor for 1 or 2 days. The solid-state luminescence spectra of these complexes revealed that guest molecules removed/rehydrated from the MOF undoubtedly influence the weak interactions between a ligand and a metal center. The work provides a convenient and effective route for tuning emissions between UV and visible wavelengths by controlling the number of guest molecules and may be useful for the design and fabrication of multifunctional luminescent materials. 1.6.9.2 Sensors in MOFs The Metal-Organic Frameworks (MOFs), that possess luminescent properties together with size- or shape-selective sorption properties can be used as sensing devices. The MOFs which are assembled from metal ions and organic bridging ligands, are promising as modified electrode materials than to other coordination compounds, owing to their possess high surface area, tunable structures, porosity and the condensation. Scientists have developed all kinds of MOFs sensors in various application fields [26-28]. 1.6.9.2.1 Sensors for Selective Ion Monitoring Recently, Chen et al. [29] reported a prototype luminescent MOF, Tb(BTC)·G(MOF-76,G= guest solvent). The luminescence properties of the anion incorporated Tb(BTC)·G (MOF76b,G=methanol) microcrystalline solids were studied by immersing Tb(BTC) in methanolic solutions of different concentrations of NaX (X=F–, Cl–, and Br–) and Na2X (X=CO32– and SO42–). The results show that the luminescence intensity of the anion incorporated MOF-76b is significantly increased, particularly for the F– incorporated MOF. The special properties underlying the potential of MOF-76 for the recognition and sensing of anions exhibit a highsensitivity sensing function with respect to the fluoride anion. Chen et al. [30] recently reported the synthesis of luminescent MOFs, [Eu(pdc)1.5(dmf)].(DMF)0.5(H2O)0.5 (pdc =

pyridine-3,5-dicarboxylate), with Lewis basic pyridyl sites for metal ion sensing. The luminescence characteristics of this MOF were studied as the pyridyl sites were coordinated to additional metal ions introduced in a DMF solution. It was found that the identity of the additional metal ion is very significant to the luminescence capability of the complex, with alkali and alkaline earth metals having little effect on the luminescence while other metal ions, such as Cu2+, cause significant quenching. 1.6.9.2.2 Sensors for the Presence and/or Types of Guest/Solvent Molecules Chen et al. [31] reported a rare earth microporous MOF of Eu(BTC) (with open Eu3+ metal sites). Within the MOF, ethanol, acetone, dimethyl formamide, and other small molecules exhibit different enhancing and quenching effects on the luminescence intensity. This MOF is suitable for the binding and sensing of small molecules by using the specific properties of luminescent open Eu3+ sites. Using ultrasonic methods, Qiu et al. [32] obtained nanocrystals of a fluorescent microporous MOF, Zn3(BTC)2.12H2O, and quantitatively analyzed the signals of organoamines in an acetonitrile solution using fluorescence spectrophotometric titrations. Remarkable changes of emission intensity (fluorescence quenching) were observed when the volume of ethylamine added into acetonitrile was changed. This fluorescence quenching suggests a high sensitivity to ethylamine and may lead to some highly sensitive sensors for organoamines. 1.6.9.2.3 Sensors for Stress-Induced Chemical Detection Besides sensors for anions and guest molecules, MOFs can also be used for stress-induced chemical detection. Recent work by Allendorf et al. [33] demonstrated that the energy of molecular adsorption can be converted to mechanical energy to create a highly responsive, reversible, and selective sensor by integrating a thin film of MOF HKUST-1 with a microcantilever surface. This sensor responds to water, methanol, and ethanol vapors, but yields no response to either N2 or O2. This is the first report of the use of surface-enhanced Raman spectroscopy to characterize the structure of a MOF film. 1.6.9.2.4 Sensors for Anisotropic Photoluminescence Probes Harbuzaru et al. [34] presented new microporous Ln3+-based materials, ITQMOF-1 and ITQMOF-2, based on the highly hydrophobicorganic ligand Hfipbb. These MOFs show no quenching of photoluminescence in the presence of water, but a sharp decrease in photoluminescence in the presence of ethanol enables them to be used to sense ethanol in both air and water. It was also observed that the choice of Ln3+ metal is significant to the luminescent and magnetic properties of the MOFs. A ferromagnetic interaction between the Tb3+ ions and a green emission in UV light was observed for ITQMOF-1-Tb while ITQMOF-1-(5Eu–95Gd) showed an antiferromagnetic interaction between Gd3+ and a red emission in UV light.

1.6.10 Drug Storage and Delivery in MOFs

The inability of conventional orally administered drugs to deliver medication at a controlled release rate has spawned much interest and research in novel methods for drug delivery. Developed delivery systems include polymeric-based systems, liposome based systems, microporous zeolites, mesoporous silicon, and other mesoporous materials. Essentially, these different delivery routes are classified into organic systems and inorganic systems. Organic systems benefit from a wide array of biocompatibility, the ability to uptake many drugs, yet lack a controlled release mechanism. The inorganic delivery materials are able to deliver the adsorbed drugs at a controlled rate due to their ordered porous network, but have a decreased loading capacity. 1.6.10.1 Drug-Delivery Methods Uhrich et al. provide a complete review of the organic polymeric drug-delivery materials [35]. Most inorganic delivery materials have a mesoporous structure to allow for optimal uptake and delivery of drugs (microporous materials usually do not possess large enough apertures for useful drug delivery). Recently, Salonen et al. provided a review on the state of mesoporous silicon in drug delivery. Zeolites in the mesoporous size range have also been studied for their application in drug delivery. 1.6.10.1.1 Inorganic Drug-Delivery Materials MCM-41 is an inorganic material composed of siloxane bridges with silanol groups available for interaction with guest species and for better control of the size of the pore. It has been one of the most intensely studied inorganic compounds for drug delivery because the pore walls allow for direct adsorption with no need for organic functionalization. Munoz et al. provided the necessary background research for drug delivery in mesoporous silicate with their study of the release rate of ibuprofen in MCM-41 modified with aminopropyl groups. Their findings established that the release rate of ibuprofen in MCM-41 without the addition of functional groups is independent of pore size so long as the mesoporous opening is larger than the delivered drug. Complete delivery of the drug occurs within two days. In addition, the functionalization of MCM-41 with organic silane groups can impact the amount of drug adsorbed, and therefore, the delivery rate. 1.6.10.1.2 MOF Drug-Delivery Materials As hybrid organic–inorganic compounds, MOFs present themselves as optimal drug-delivery materials due to the adjustability of the framework’s functional groups and the tunable pore size. With MOFs, the benefits of using organic materials (biocompatibility and the ability to uptake large amounts of drugs) and inorganic materials (controlled release) may both be utilized. However, the main drawback of using MOFs is that the small pore size, with diameters that usually fall in the microporous range, limits the uptake and/or number of drug molecules that can be stored within the framework [36]. To solve this problem, MOFs containing pores in the mesoporous range must be synthesized. Two such MOFs have been prepared by Horcajada et al. as MIL-100 and MIL-101 [36]. These structures proved suitable for drug delivery due to their well-defined, ordered porosity. MIL-100 contains pore

diameters of 25–29Å with pentagonal window openings of 4.8Å and hexagonal windows of 8.6Å while MIL-101 possesses pore sizes of 29–34Å with a very large window opening of 12Å for the pentagonal and 16Å for the hexagonal windows [36]. MIL-100 was found to uptake 0.35 g ibuprofen/g dehydrated MIL-100 whereas MIL-101 was able to uptake 1.4 g ibuprofen/g dehydrated MIL-101. The difference can be explained by the size of ibuprofen (6A×10.3 Å) which is able to fit in both the pentagonal and hexagonal windows of MIL-101, but not into the smaller pentagonal window of MIL-100 [36]. Desorption rates for MIL-101 exceeded that of MIL-100 by six days vs. three days, respectively. The initial delivery mechanism consists of simple diffusion of weakly bonded molecules, whereas π-π interactions between the aromatic rings and the ibuprofen are responsible for the elongated delivery times [36].

1.6.11 MOFs; Precursors for Preparation of Nano-Materials Metal-organic frameworks may be suitable precursors for the preparation of desirable nanoscale materials. Use of MOCPs as precursors for preparation of inorganic nano-materials such as metal oxides is a method with interesting advantages. Some of these advantages are simplicity of processing and forgoing the need for special instruments; relationship between structures of the target products and raw materials; being more appropriate for phase control of products; better control over process conditions, particle size, particle crystal structure, and purity; reducing the chance of interparticle collisions; simplicity, cost effectiveness and the potential for large-scale fabrication. In addition, through presence of capping ligand, the undesired aggregation of nanoproducts will be hindered. Another important advantage of this method is that elemental components can be controlled as desired which is accomplished by mixing the selected metal ions and the appropriate organic bridging ligands. Also one of the unique features of this method is formation of nano-materials that could not be obtained by other methods. For instance, Cd3OSO4 nanostructure may only be produced via this method. However, the process entails complicated steps involving preparation of polymer-based precursors. Most likely the mechanism of final morphology formation in nano-materials depends on various intermediates controlled by inner and external forces during the formation process. Crystal structure and the interactions in MOCP (such as covalence, coordination, hydrogen, and van der Waals forces) are inner forces affecting intermediates and solvent–MOCP interactions, electrostatic and dipolar fields as well as hydrophilic or hydrophobic interaction give rise to external forces controlling system morphology [37-44].

1.7 Conclusion In this section describes structure and applies of Metal-Organic Frameworks (MOFs). Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in many applications. MOFs may be produced to act as highly selective molecular sieves, sensors and catalyst. Sensor capabilities become realizable when the optical, electronic, or magnetic properties of the framework are altered by guest

interactions. Catalytic behavior has been reported in only a few instances and this area deserves much more attention. In other hand the most promising applications of metal– organic frameworks is gas storage and selective gas adsorption.

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Chapter 2 Pillar-Layer Metal-Organic Frameworks 2.1 Introduction Pillar-layered Metal-Organic Frameworks (MOF) is among the most striking research area in crystalline materials. Due to great number of reports related to this area, investigating their structure diversity, properties and applicability as multi donor 3-D porous frameworks is of great interests. Modifying pillar moieties as a third building blocks of pillar-layered MOFs, together with metal nodes and oxygen donor linkers can enhance controlling structure assembly and led to specific properties into obtained structures. The structure of pillar can be easily modified which cause better designing of desired structural topology and pore environment. Although the past decade has witnessed remarkable advances in this vibrant research area, pillar-layered MOF have never been reviewed as an independent research area until now. Therefore, a review summarizing their performance is highly expected. This review covers important issues related to pillar-layered MOFs, from their structure properties, synthesis and stability, to their diversity in linkers and their improved applicability. Finally, the pillar linkers have been studied separately according to diverse donor atoms and functional groups existed in each linker. A great deal of research effort has been devoted to the design and explicit control of pore structure and sizes at the molecular level in the frameworks. To this end, introduction of mixed functionalized organic ligands containing N and/or O donors to coordinate with suitable metal or metal clusters is a promising way for synthesizing desirable structures. Mixed-ligand strategy has been proved to be a feasible method for enhancing tunability in frameworks and achieve unusual MOFs refrence review. Only mixed ligand pillar MOFs are investigated in this content. Compared to in situ generated inorganic building blocks, bridging ligands are more important building blocks to construct MOFs. Attaining 3-D porous structures is a significant scientific goal owing to vibrant applicability of these frameworks in adsorption of guest molecules. Pillaring strategy is one effective and straight forward way for the construction of three-dimensional (3D) porous frameworks from 2D layers (Figure 2.1).

Figure 2.1 Simple representation of pillar linker used for improving dimensionality. Introducing pillars into MOFs opened new possibilities for researchers to better control on the building blocks that will assemble into frameworks with desired network properties. First reports on pillar linkers were utilizing simple nitrogen donor ligands such as bpy [1] or Dabco [2]. Increasing number of investigations in this field led to more complex and tunable structures. The growing trend of studies is demonstrated in Figure 2.2, up to date.

Figure 2.2 Number of pillar MOFs reported each year. Design and synthesis of MOFs with improved surface area, selectivity, stability and functionality by inducing little changes in skeleton of the pillars is indeed an attractive scientific goal. The layer structural motif is usually not affected by modifying pillars; however, these adjustments will lead to different pore size and shape as well as functionality. In order to reach this goal vast numbers of researches have been done on IRMOFs with little difference in their pillar architecture. Generally, introducing pillars into MOFs can affect various structural properties of MOFs. For instance, in mixed linker pillar-layered MOFs tuning of framework backbone is facilitated due to existence of dual linkers. Introducing functional groups on both linker and

pillar moieties can led to better design of pore environment which make pillar MOFs a good candidate for a desired application. The diversity of functional group location in pore walls can be well tuned in pillared frameworks. Functions can locate whether on pillar (Figure 2.3a) or on linker (Figure 2.3d). Also same function can locate on both linker and pillar (Figure 2.3c). Different functional groups on linker and pillar can also exist in one pore (Figure 2.3b).

Figure 2.3 (a) Representation of azine functional group only on pillar [3], (b) Representation of different urine functional group on pillar and carboxylate group on linker [4], (c) Representation of same urine functional group on pillar and linker [4], (d) Representation of carboxylate functional group only on linker [5]. Although many pillar-layered MOFs have been reported till now, but some beliefs such as low porosity and stability of their structures have limited more specific investigations in this vibrant area and the pillar containing frameworks have been neglected to be used for advanced applications. But in this book, we have emphasized on high tunability of pillarlayered MOFs which can help scientist for constructing high stable frameworks for desired application. Considering these reports, the effect of rigidity and flexibility, length and functionality of pillars on topology, stability and degree of interpenetration and application was studied separately here.

2.2 Topology and Diversity in Pillar-Layered MOFs Increasing number of publications is found in the area of metal–organic frameworks (MOFs) with topological considerations. To enhance the description of crystal structures of MOFs and better classification of reported frameworks, notification of network topology is recommended. Topology is actually a branch of mathematics, which is attributed to bending, stretching and squeezing, but not to bond breaking [6]. Through investigating structure topologies, well information on the manner in which atoms organize themselves can be attained. Such

knowledge is also helpful for designed synthesis of MOFs and related materials from component parts. The suggested method for network topologies is to use the three letter codes from the Reticular Chemistry Structure Resource, RCSR [7]. Other kind of symbols are also used to report network topologies (Vertex symbol and point symbol) but RCSR work as better identifiers for nets. The compounds with same underlying topologies which are called isoreticular (IR) structures have same name in RCSR. Investigating the topological issues in pillar-layered MOFs can lead scientist toward better understanding of resulted structures which help better designed synthesis. Many reported pillar MOFs are categorized as IRMOFs because the layer structural motif is usually not affected by modifying pillars leading to same topology. Pillar containing networks mostly show typical topologies as dia, pcu, CdSO4 and α-Po topology. Figure 2.4 show the rational percentage of main reported RCSR topologies in pillar-layered MOFs. Mostly, pcu or α-Po topology as a simple cubic network with six equivalent vertices of an octahedron, have been seen in pillar layered MOFs. As pillars link two consecutive layers, mostly they occupy axial site in metal coordination sphere which can led to simple pcu topology.

Figure 2.4 The rational percentage of main reported RCSR topologies in pillar-layered MOFs (a) pcu net [8], (b) dia net [4], (c) tfz net [9], (d) Cds net [10] and (e) fsc net [11]. In addition to typical networks in pillar MOFs, reports on some examples of rare topologies indicates that inducing pillars lead to beautiful novel topologies and enhance topological diversity [12]. An unusual topology was reported in Cd based framework (Cd1.5(BTC)(BPE) (H2O)2) in which metal-BTC layers were pillared by the disordered BPE ligands leading to (9, ) topology with two different types of three-connected and one kind of four-connected node, all characterized by the shortest topological circuits enclosing nine nodes [13]. Another report on new topological type within MOFs is in a helical pillar-layered complex (Mn(tptc)0.5 (bpp)) with the Schlafli symbol of (43.66.8)2(46.67.82) in which Mn(II) ion is

connected to five tpcp ligands, and in turn each tpcp ligand links six Mn(II) ions and each tpcp ligand is coplanar with six Mn(II) ions, and thus adopts a planar hexagonal topological neighbors of a vertex [14]. Through number of other rare topologies in this class of MOFs we can conclude that pillaring approach can serve as an effective way to understand and further manipulate some unprecedented networks in pillar-layered MOFs.

2.3 Synthesis Methods in Pillar-Layered MOFs The last two decades have seen great advances in the field of pillar-layered metal organic frameworks both in the discovery of new structures and the development of new functional properties of them. However, less attention was paid to inform new synthetic strategies with higher efficiency. Till now, most reported pillar-layered MOFs were produced through conventional methods such as solvothermal method which was firstly introduced by Yaghi [15-17]. Figure 2.5 displays the number of different synthetic methods used for pillar-layered MOFs assembly.

Figure 2.5 The number of different synthetic methods in pillar-layered MOFs. Utilizing new synthetic methods for pillar-layered MOF assembly such as spray drying, SolGel [18] or other cost effective methods have not been studied up to now which can be a vast rout for scientists to work on. Although pillar MOFs were mostly obtained through self-assembly of building blocks in appropriate conditions, however not all desired structures are constructed through simple one step process. Post synthesis modification (PSM) act as an alternative strategy for building unattainable frameworks. Topology, applicability and functionality of synthesized MOFs can be modified toward chemical alternation of organic moieties. New structures can be assembled by replacement of coordinated ligands (solvent-assisted linker exchange (SALE))

[19] or replacing non-coordinated ligands (solvent-assisted ligand incorporation (SALI)) in pillar-layered MOFs. These methods are fast growing leading to new structures with better functionality. A PSM was done on a new MOF (Zn2(Cam)2(apyr).2DMF) with amino functionalized pillars by Kenneth W. Henderson, to investigate the transformation of amino groups in the pores into imine functionalities via acetaldehyde [20]. Inaccessible structures can be synthesized through pillar exchange process. First example of pillar exchange was reported by Wonyoung Choe et al. [21]. DPNI bridging linker was successfully exchanged by pyridine in two porphyrin based MOFs. bpy linker with porphyrin based sheets is hardly assembled through one step process, thus PSM is one promising way to produce such structures and control applicability. Figure 2.6 exhibits the schematic representation of pillared-layered exchange in a 3D MOF.

Figure 2.6 Schematic representation of pillared-layered exchange in a 3D MOF. Another similar report was done by Joseph T. Hupp and Omar K. Farha group on porphyrin based MOF films with bpy pillars which were obtained through layer-by-layer (LbL) strategy [22]. Two new structures were fabricated via SALE and post-metalation reaction. The linker exchange of bpy with dabco was easily preceded owing to higher basicity of dabco [22]. Single crystal to crystal (SC-SC) linker exchange was also studied with different pillar groups. The length of the entering pillar affects the pore structure of the resulted MOFs. The longer bridging pillar is preferred when diverse linkers are exposed to parent structure. The effect of linker and pore aperture on post-metalation on resulted crystals was investigated successively. Bulkier solvated metal ions cause adjustment of the smallest linker in the framework [23]. The adsorption properties, stability and pore size of MOF materials can be tuned through PSM method. For instance, selective adsorption of CO2 over N2 was improved by PSM (2+2) cycloaddition of bpee pillar in ((Co3(bpee)3(H2O)4) (Cr(CN)6)2·2(bpee)·2(C2H5OH)·2(H2O))n by using light [24]. PSM is a fast growing issue in pillar layered MOFs and new substitute methods are introduced recently for easier MOF assembly. Using acid–solvent synergy for metal-organic

framework synthesis (EASY-MOF) is one novel study introduced by Paul A. Webley group which is used for assembly of MOFs that cannot be obtained by conventional methods (Figure 2.7) [25]. The dabco pillar was exchanged in (Zn2(BDC)2(DABCO)) as a parent structure with four different linkers by adding acid along with linker solution to MOF crystals. Presence of acid, type of applied acid and its concentration is necessary for complete exchange process.

Figure 2.7 Comparison between reaction routes of EASY-MOFs and SALE/SMILE (blue and green rectangles represent N-containing and carboxylic ligands, respectively). EASY-MOF along with other novel strategies will pave the way for structural conversions to explore new structures with high purity.

2.4 Linkers in Pillar-Layered MOFs The mixed-ligand systems with different donor atoms introduce even more diversity to MOFs and could lead to pillared-layer structures. Using diverse donor atoms in linkers for programming coordination frameworks is a promising approach. As ligands mostly act as an electron donor lewis bases, the nature of donor atom can be so important in coordination system. The oxygen donor atoms in linkers are hard bases and coordinate efficiently with harder metals. Besides, nitrogen donor linkers are better candidates for softer linkers. The acidic groups mostly contain two oxygen donor atoms which prefer to coordinate in equatorial site of the metal, while only one nitrogen of pyridine containing ligands coordinates axially to one metal [26]. By combination of diverse carboxylic acids and neutral pyridine auxiliary ligands higher dimensional networks with specific topology are assembled. N-coordinated pillars have higher basicity which may have a positive effect in their stability and applications. Although there are some examples of pillars using different atoms such as oxygen or sulfur

as donors in reported researches, but literatures are focused mostly on N-coordinated pillars.

2.5 Conclusion In the past decade pillar-layered MOF have never been reviewed as an independent research area until now. Therefore, in this section we summarize their performance is highly expected. This section covers important issues related to pillar-layered MOFs, from their structure properties, synthesis and stability, to their diversity in linkers and their improved applicability. Finally, the pillar linkers have been studied separately according to diverse donor atoms and functional groups existed in each linker.

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Chapter 3 Rigid and Flexible Pillars 3.1 Introduction The flexibility of MOFs can originate from the breaking or making of bonds as well as bond rearrangements driven by various external stimuli like heat, light, pressure, etc.; [1-3] whereas the structural transformations are generally associated with the removal or exchange of guest molecules, changes of the coordination number of the metal atoms or conformational changes in the flexible organic ligand [4,5]. It has been observed that due to the presence of dynamic behavior, such MOFs exhibit unique selective, stepwise adsorption which might be useful for the separation of gases and solvent vapors. Sometimes dynamic MOFs also exhibit nice molecular recognition and/or sensing applications, which could not be achieved with rigid MOFs. The close interactions of the dynamic structures, was developed in crystal engineering as a concept in coordination polymers of ‘supramolecular isomerism’ which involved the production of different types of superstructure by changing the reaction parameters [6,7]. With time, this field gained enormous attention to study the details of the self-assembly and crystal growth processes with respect to the structure–property relationship associated with the change of a framework’s structure and/or superstructure [8-10]. However, the objective underlying the study of the structural transformation between the frameworks, with different dimensionalities, lay in gaining an understanding of the chemistry of the systematic cleavage and formation of the metal–ligand bonds in dynamic PCPs [1-5]. This challenging goal of relating the structures and properties of dynamic frameworks motivated us to investigate the intrinsic mechanism of the formation of dynamic metal–organic frameworks and their interconversion. Structural rigidity arises from strong metal–ligand binding, whereas the flexibility stems from flexible linkers, variable coordination geometry of the metal ions, or guest induced movement of the framework. The flexible framework structures can change reversibly upon removal of guest and their introduction [11]. Flexibility can cause high porosity and thermal stability through contraction/expansion of the framework [12]. Usage of long flexible pillars over rigid ones for achieving porous frameworks facilitates the degree of conformational isomerism and affects their porosity control and selectivity [13,14]. Interexchange between rigidity and flexibility of organic ligands can lead to innovative new ideas. Flexibility of the framework is depended on both carboxylate and pillar moieties. Rigid pillars such as bpy together with flexible carboxylate groups can induce elasticity into structures [15,11]. Difference in gas adsorption of two isostructural MOFs with bpe and bpa pillars can be a proof for the effect of pillar flexibility in MOFs functionality. The rigid bpe pillar caused the framework more stable, so the structure could not easily change when the gas entered the cavities. However, the bpa induced stretchiness into the framework which

enhanced the gas sorption capacity [16]. Utilizing rigid bpy instead of flexible bix pillar in series of novel complexes reported by Hong-Jie Zhang et al. can cause sterically hindered change. However bpy ligand can be not only instead of linear bix ligand but also the aqua ligand to generate a new 3D MOF [17]. As is referred, organic ligands play important roles in directing the final structural properties [18]. For constructing appropriate coordination polymers, the key factor is to design of organic ligands which favors structure-specific self-assembly. Polycarboxylic acids, as a crucial family of multidentate O-donor ligands, have been extensively used due to their versatile coordination conformations and strong coordination ability [19]. Up to now, plentiful research have been focused on the use of rigid carboxylate ligands, such as 1,4benzenedicarboxylic acid (p-H2BDC), 4,4’-biphenyldicarboxylic acid (H2BPDC), 1,3,5tris(4-carboxyphenyl)benzene (H3BTB) [20]. MOFs built from the rigid carboxylate ligands generally exhibit exceptional porosity and surface area, which can be used in adsorption and separation processes. At the same time, a lot of intriguing metal-organic frameworks based on flexible carboxylate ligands, for instance, 1,4- cyclohexanedicarboxylate and tetrahydrofurantetercarboxylate with flexible core have also been extensively employed for the preparation of functional MOFs [21]. Compared with rigid and flexible carboxylate ligands mentioned above, a semirigid V-shaped [22] tricarboxylate ligand named 5-(4carboxy-2-nitrophenoxy) isophthalic acid (H3L), featuring two benzene rings connected by a nonmetallic O atom, captured attention. It could exhibit irregular orientations when connecting to metals, thereby affording abundant structural motifs. Furthermore, though the nitro group is not tangled in coordination, it may influence on the constructing processes, the structures and even the properties of MOFs for considering its electronic and geometric effects [23]. To the best of our knowledge, there are rarely MOFs assembled from H3L ligand reported by Hong-Jie Zhang and other groups [24]. Hong-Jie Zhang group got a set of lanthanide coordination polymers exhibiting the same 3D architecture, which is built up from binuclear paddle-wheel building blocks. Liu and coworkers synthesized two novel Co metalorganic frameworks possessing 3-nodal networks with new topological prototypes. Gu and coworkers reported a fascinating Co polythreaded coordination network formed by 1D crankshaft shaped chains threading into a 2D undulated sheet. Taking their work into account, there are still lots of opportunities worth to exploring. In Hong-Jie Zhang report not only prepared another two new Co 3D frameworks but also got four different transitional metals 3D frameworks. In addition, the ancillary ligands containing N-donor have been used widely to extend the metal carboxylate system to a higher dimensional network [25]. Hong-Jie Zhang et al. have successfully synthesized six metal-organic frameworks namely, {[Zn1.5(L)(bix)1.5(H2O)]· H2O}n (1), {[Co1.5(L)(bix)1.5(H2O)]·H2O}n (2), {[Co2(L)2(4,4bpy)3].5H2O}n (3), {[Cd3(L)2(4,4’-bpy)3].4.5H2O}n (4), {[Cu3 (L)2(4,4’-bpy)3].3H2O}n (5), {[Ni1.5(L)(4,4’-bpy)1.5(H2O)3]. 2H2O}n (6) assembling by the semirigid V-shaped ligand, two neutral ligands, and different metal ions under suitable hydrothermal conditions. Structure analysis shows that the 4,4’-bpy ligand can be treated as linear linker like bix ligand and terminal ligand like aqua ligand. The bix ligand displays a Z-type since it is more

flexible, otherwise the 4,4’-bpy ligand is a linear and rigid ligand. Considering the difference between two neutral ligands, we speculate that Z-type bix ligand hinders the further reaction with Co(II) ion to develop into a 3D structure, however 4,4’-bpy ligand can be not only instead of bix ligand but also the aqua ligand to generate a new 3D MOF owing to its smaller steric hindrance. Secondly, the Zn3 core cluster is symmetrical and linear, however Co3 core cluster is antisymmetrical and nonlinear. This can be presumed to the sterically hindered change caused by different neutral ligands. In complexes 1-5, the tricarboxylate ligands adopt η1, μ2-η2, μ2-η2:η1 and syn-syn-μ2-η1:η1 coordination modes, which links metal atoms to form trinuclear metal units. In complex 6, the tricarboxylate ligands adopt η1 and η1:η1 coordination modes with only two carboxylate groups, which lead the metal atoms to form isolated mononuclear motif. Therefore, the coordination modes of the carboxylate ligands are primarily responsible for the structural diversity of the complexes. Furthermore, the Ncontaining auxiliary ligands varied in the assembly process are also important for the terminal structures of the complexes. Interestingly, for complexes 1-5, all of them are constructed through the trinuclear SBUs and L3- to form a 2D layer, then pillared by 4,4’bpy and bix ligands to complete the 3D frameworks. In this regard, the semirigid V-shaped tricarboxylate ligand is a good candidate to construct varieties of 3D frameworks. Finally, results illustrate that the coordination modes of the carboxylate ligands and the coordination behaviors of metal ions play important roles in the construction of coordination polymers, meanwhile indicate that the neutral ligand could affect the formation of the final architecture. It is believed that the initiatory research assembled in crystal engineering of functional metal-organic frameworks based on semirigid V-shaped tricarboxylate ligands would lead to novel coordination polymers with interesting structures and properties. The direct effect of pillar flexibility in structure of four novel MOFs has been investigated. Flexible pillars such as bpy prefer to co-assemble with longer carboxylate ligands. As reported, less flexible succinate prefers to co assemble with pyrazine instead of bpy in the framework. The reaction of metal ions, flexible aliphatic dicarboxylates and rigid bidentate linear ligands under mild conditions afford four novel metal–organic coordination polymers, [Cd(µ-mal) (µ-pyz)0.5(H2O)]n 1 (mal = malonate dianion, pyz = pyrazine), [Cd2(µ-suc)2(µpyz)(H2O)2]n 2 (suc = succinate dianion), and {[M(µ-bipy)(H2O)4][suc].4H2O}n (M = Co, 3, M = Zn, 4, bipy = 4,4’-bipyridine). Two of them contain either succinate or malonate and pyrazine bridging ligands and exhibit 2D and 3D structures. Compound 1 is a new member of a relatively small group of bridging malonate–pyz systems. 2 represent the first example of flexible self-assembled succinate–pyz mixed bridging ligand coordination network. The other two structures exhibit linear chain structures with 4,4’-bipyridine bridging ligands and noncoordinated succinate anions, which play a templating and charge-compensating role in the structure. Therefore, Hongyun Zhang deduce that the carbon aliphatic backbone number of dicarboxylates and the length of linear diimines play an important role in crystal structure construction and have some influence on the properties of the resultant compounds [26]. In 3D structures, in presence of layers which are connected by hydrogen bonding and hydrophobic interactions, dynamic behavior toward guest adsorption known as breathing

effect is common. Mostly in flexible pillar-layered structures, the breathing effect influences the gas adsorption positively [27]. The hysteretic gas desorption behavior is useful in sensing and selective storage, because it helps to store the guest molecules in the structure even after decreasing the gas pressure. CPL-2 ((Cu2(pzdc)2(bpy)).G) showed dynamic response toward water and benzene adsorption [28]. This reversible structural change on guest molecule adsorption/desorption leading to expansion, shrinkage or “shape-responsive fitting” are showed in Figure 3.1.

Figure 3.1 Showing several cases of guest accommodation (a) shrinking (b) expanding (c) shape-responsive fitting. By adjusting the pillar lengths, open isostructural frameworks with tunable pore volume and higher surface area are resulted which can have distinct gas adsorption properties. The expected amount of gas adsorb in structures mostly is different from real values because of flexibility. As reported, bpy was replaced by longer flexible pillars to construct a Zn based MOF which result in larger interlayer distance and bigger channels (Figure 3.2). Consequently the structure turned from non-porous to porous material with stepwise N2 adsorption due to pore opening process caused by flexible azpy pillar.

Figure 3.2 Schematic demonstration of changing pillar length. Xian-He Bu et al. systemically synthesized and characterized a series of Pillar-layer structure porous MOFs {[Zn4(bpta)2(4,4’-bipy)2(H2O)2].(DMF)3.H2O (1) (DMF = N, N’dimethylformamide and 4,4’-bipy = 4,4’-bipyridine), [Zn4(bpta)2(azpy)2(H2O)2].(DMF)4. (H2O)3 (2) and [Zn4(bpta)2(dipytz)2(H2O)2].(DMF)4.H2O (3) (azpy =4,4’ -azopyridine, dipytz = di-3,6-(4-pyridyl)-1,2,4,5-tetrazine)} in which [Zn4(bpta)2(H2O)2] (H4bpta = 1,1’biphenyl-2,2’,6,6’-tetracarboxylic acid) layers are connected by length-controllable bipyridine pillars. The synthetic strategy allows a systematic variation of the pillar to construct a series of open frameworks with similar structure. Pore volume and window size could be adjusted by selection of pillar ligands, and the resulted three MOFs show different gas adsorption properties depending on their pore structures. In addition, hydrogen adsorption properties of the resulted MOFs were studied. Investigation of this pillar-layer system provides a nice example of controllable construction of porous MOFs, and utilization of longer and bulky pillars is expected to be more profitable for gas storage [29]. The experimental and theoretical efforts have recently been focused on the rational design and controlled synthesis of metal-organic frameworks (MOFs) due to not only their potential applications but also their intriguing versatile architectures. One of the most effective ways is the appropriate choice of well-designed molecular building blocks (MBB) and metal ions or clusters to achieve the targeting assemblies of such extended crystalline systems with desired structural features and/or physicochemical properties. Although MBBs with 2-fold linear, 3fold and 4-fold symmetries are popular choices for obtaining the well-defined MOFs, 2-fold non-linear MBB is rarely studied. This lies in two aspects: bigger steric hindrance and necessity of the auxiliary ligand known as pillar to sustain the architecture. Compared with BDC, non-linear FDA {furan-2,5-dicarboxyl acid} possesses two advantages as the MBB: the high openness net with the flexible bond angle of two carboxyl (~126°) and the polarity

owing to the oxygen atom of ring, which is potentially applied for gas-storage and gasseparation. Due to the non-linear FDA as the MBB, the choice of the pillarwould effect the structural formations–porosity and interpenetration. With respect to the steric hindrance and electronic effect, the flexible and rigid pillars are employed to the construction of structure, respectively. Two novel MOFs have been synthesized by Ya-feng Li et al. group, in which the flexible bpp results in the robust porous [Zn3·(FDA)3·bpp·H2O]·2H2O (1), and the rigid bipy in 3-/3-D interpenetrated [Zn·(FDA)·bipy]·2H2O (2). The flexible bpp pillar used instead of rigid bpy pillar resulted in production of stable framework with permanent porosity and medium CO2 uptake. The structural difference is owing to the properties of ligand-to-ligand pillars (flexible bpp for 1 and rigid bipy for 2). As a result, 1 possesses the permanent and stable pore which is able to uptake CO2, and 2 forms the 3-/3-D interpenetrated net [30]. Structural rigidity arises from strong metal–ligand binding, which uses linkers having constitutional stiffness or metal clusters as the building units, whereas the flexibility stems from flexible linkers, variable coordination geometry of the metal ions, or guestinduced cooperative movement of the framework on a periodic scale [31]. In contrast to rigid frameworks, flexible or soft porous frameworks offer ideal platforms for molecular recognition properties, which enhance the selectivity by responding tospecific guest molecules in a particular way [32]. Although during the past few years there are reports on supramolecular isomerism for a particular metal ion and ligand pair system, to the best of our knowledge, different functional supramolecular isomerisms and their structure–property relationships based on the rigid and flexible structures have yet to be properly accounted for. Kitagawa et al. report the synthesis, structure, and adsorption properties of two new 3D PCPs, {[Cu(bipy)0.5(pyrdc)]. 3H2O} (1) and {[Cu(bipy)0.5(pyrdc)]·0.5bipy·3H2O} (2), derived from pyridine- 2,3-dicarboxylate (pyrdc) and 4,4’-bipyridyl (bipy) with CuII. The use of pyrdc is limited in the construction of PCPs, and its different binding mode of the 3carboxy group in two different solvent systems resulted in two different framework structures, related as structural supramolecular isomers. The rigidity and flexibility associated with the supramolecular isomers ensues unprecedented adsorption behavior, as observed in the CO2 and MeOH adsorption studies. The two frameworks are related by structural isomerism and exhibit different adsorption behaviors with small molecules. The different gas adsorption profiles have been observed in CO2 adsorption of two structural isomers reported by Kitagawa et al. with bpy pillar. Changing the solvent induced diversity into framework flexibility and caused hysteresis behaviour just in one of them [33]. In addition to afore mentioned case, breathing phenomenon has been studied in series of (Zn2(fu-bdc)2(dabco)) MOFs with different functionalized linkers shown in Figure 3.3 [34]. The obtained crystals exhibit shrinking toward guest removal and successive enlargement through guest adsorption that is directly depended on the dangling side of the linker (Figure 3.4). The void space decreases as the size of the substituted groups are larger. In 2 and 5 substituted rings, all the pores incorporate the dangling side which led to more flexible structure. Generally, the position and size of side chains on linkers are attributed to framework flexibility.

Figure 3.3 Library of fu-bdc linkers used in the preparation of (Zn2(fu-bdc)2(dabco))n MOFs (Compounds 1–9).

Figure 3.4 Structural representations of the different forms of (Zn2(2,5-BME-bdc)2(dabco))n along the crystallographic c-axis. Sebastian Henke et al. demonstrate the first systematic approach for the implementation and tuning of network dynamics and responsiveness in metal–organic frameworks. The responsive behavior of pillared-layered MOFs of the type [Zn2(fu- bdc)2(dabco)]n can be tailored extensively, by the utilization of a wide variety of functionalized linkers. Importantly, the parent framework [Zn2(bdc)2(dabco)]n exhibits intrinsic structural flexibility due to the integrated paddle-wheel building unit. Via implementation of bdc-type linkers, which are substituted with flexible side chains of a specific size regime, a “breathing” behavior in response to adsorbed guest molecules (DMF, CO2) can be triggered in this type of networks, while other molecules such as N2 are barely adsorbed and do not trigger the structural transitions. Upon guest removal the frameworks contract and transfer from a large pore to a narrow pore form, exhibiting a drastically reduced cell volume. Remarkably, the substitution pattern of the linker and also the bulkiness and chemical nature of the substituents affect the structure of the narrow pore phase and thus the responsive behavior of the framework. If the linker is substituted in position 2 and 3 the structural flexibility of the respective pillaredlayered MOF is drastically reduced, when compared to the corresponding 2,5-disubstituted derivatives. Substituents that are sterically less demanding and/or possess functionalities with a stronger interaction potential provoke a more drastic contraction of the framework. Hence, the degree of contraction in the responsive pillared-layered MOFs can be systematically varied from 86% to 72% of the initial volume depending on the utilized substituents. These structural differences of the contracted, guest-free materials drastically influence their gas sorption properties. The networks hardly adsorb N2, whereas CO2 is adsorbed in a stepwise fashion. Notably, the detailed shape of the CO2 isotherm (e.g., transition pressure, width of the hysteresis) is substantially affected by the nature of the implemented substituent, pointing out the huge potential for a fine-tuning of such properties for desired applications in sensing and separation by specific design of the linker. Moreover, combining our variably

functionalized linkers with the concept of MOF solid solutions offers very rich additional dimensions of tailoring the structural dynamics and responsiveness. Implementation of two differently functionalized linkers in varying ratios next to each other yields MOF “copolymers” of increased inherent complexity, which show a non-linear dependence of their gas sorption properties on the applied ratio of components. They think that their methodology is in principle transferable to a wide variety of other MOF systems and may allow to implement and to tune such responsive properties in other network families. In addition, this concept may be of great value for the precise design of functional MOF thin films [35] and membranes [36] for applications in highly selective gas separation or chromatography [37, 38].

3.2 Conclusion Flexibility of the framework is depended on both carboxylate and pillar moieties. Rigid pillars such as bpy together with flexible carboxylate groups can induce elasticity into structures. The rigid pillar caused the framework more stable, so the structure could not easily change when the gas entered the cavities, so, utilizing rigid pillars instead of flexible pillar in complexes can cause sterically hindered change and flexible pillars prefer to coassemble with longer carboxylate ligands.

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Chapter 4 Introduction to N-donor Pillars 4.1 Introduction The heart of purposive MOF synthesis is design of organic linkers with convenient functional groups. The effects of functional groups on linkers are more designable compared with the in situ generated inorganic building blocks. In theory, the introduction of different functional groups into MOFs should be more straightforward when compared to that of other inorganic solids because MOFs possess an organic component (the pillars) suitable for installing any number of chemical moieties. By combination of diverse carboxylic acids and neutral pyridine auxiliary ligands higher dimensional networks with specific topology are assembled. N-coordinated pillars have higher basicity which may have a positive effect in their stability and applications. Although there are some examples of pillars using different atoms such as oxygen or sulfur as donors in reported researches, but literatures are focused mostly on N-coordinated pillars. Table 4.1 summarizes the N, O and S-donor pillars up to date. Table 4.1 N, O and S-donor-based ligands (Pillars) as building blocks for MOFs. Ligands The names of ligands

The abbreviated ligands

P1

1,2- cyclohexanedionedioxime

Niox

P2

Isonicotinamide

iso-nia

P3

Thionicotinamide

S-nia

P4

1,4- diazabicyclo(2.2.2)octane

dabco (Ted) (DBO)

P5

Pyrazine

Pyz

P6

2-amino-pyrazine

Apyr

P7

1,4-dioxane

Doa

Structure of ligands

P8

1,3,5-triaza-7-phosphaadamantane

PTA

P9

hexamethylenetetramine (methenamine)

Hmt

P10

1H-pyrazole

HPyz

P11

5-methyl-tetrazole

MTAZ

P12

2-nitroimidazole

nIm

P13

3,6-divinylpyrazine-2,5dicarbaldehyde

-

P14

1,4-bis(2-hydroxyethyl)piperazine

Bhep

P15

4-(2-hydroxyethyl)- morpholine

Hem

P16

1,2,4-triazole

Trz

P17

N,N′-dimethylpiperazine

Dmpz

P18

3,5-dimethyl-1H, 1,2,4-triazolate

Dmtrz

P19

3,5-damino-1,2,4-triazole

datz (datrz)

P20

1H-1,2,4-triazol-3-amine or 3-amino- Hatz (artz)(atz)

1,2,4-triazole

P21

1H-tetrazole

HTZ

P22

aminotetrazole or 5-amino-1H-

Atz

tetrazole

P23

1,2-di(pyridin-4-yl)ethylene or 1,2di(4-pyridyl)ethylene or 1,2-di(4pyridyl) ethylene, 1,2-trans-bis(4pyridyl) ethane

DPEE (bpe) (dpe) (bpee) (dipy)

P24

1,2-di(pyridin-4-yl)ethane or 1,2 bis(4-pyridyl)ethane

DPEA (bpe) (bpea) (bpa)

P25

2,2′-bpyridine

2,2′-bipy

P26

1,3-di (pyridine-4-yl) propane or 1,3- DPP (bpp) (dpp) bis (4-pyridyl)-propane, 4,4′(tmdpy) (tbpy) trimethlyenedipyridine

P27

trans 4,4′-azobispyridine or N,Nbispyridine-4-ylmethylene-hydrazin, 1,2-bis-(4pyridylmethylene)hydrazine

4-bpmh (4-bpdb) (bphz) (azpy)

P28

3,3′- dimethyl-4,4′-bipyridine

3,3′-dmbpy

P29

2,5-bis-(4-pyridyl)-3,4-diaza-2,4hexadiene

4-bpdh

P30

2,5-bis-(3-pyridyl)-3,4-diaza-2,4hexadiene

3-bpdh

P31

2,2′-dimethyl-4,4′-bipyriine

dmbpy (2,2′-dmbpy)

P32

4,4′-azopyridine or 4,4′-azobis (pyridine) or 1,2-di(pyridin-4-yl) diazene or 4- ((E) - 4 - pyridinylazo) pyridine or dipyridine-4-yl-diazen

azpy (abpy) (azbpy) (azopy) (dpa) (pdp) (dpd)

P33

1,4-bis(3-pyridyl)-2,3- diaza-1,3butadiene

3-bpdb (3-bpmh) (3bpd)

P34

1,2-bis(pyridin-4-ylmethyl)diazene

Bpdab

P35

1,2-di(4-pyridyl)-1,2-ethanediol or 1,2-di(4-pyridyl)-glycol

DPG (dpyg)

P36

1,4-bis(2-(4-pyridyl)etheny) benzene Bpeb

P37

1,2-bis(4-pyridyl)hydrazine

Bphy

P38

5 -(4-pyridyl)-1,3,4-oxadiazole-2thiol

Hpyt

P39

N,N′-bis(picolinamide)azine

Bpa

P40

4,4′-dipyridyldisulfide

Dbds

P41

2-amino-4,4′-bipyridme

-

P42

4,4′-bpyridine

4,4′-bipy

P43

1,2-bis(4′-pyridylmethylamino)ethane -

P44

4,4′-(1H-1,2,4-triazole-3,5-diyl) dipyridine or 1H-3,5-bis(4pyridyl)-1,2,4-triazole

4,4′-bpt (bpt)

P45

2′-fluoro-4-styrylpyridine

2F-4spy

P46

4,4′-dipyridylsulfide

dps (dpys)

P47

4-pyridinecarbaldehyde isonicotinoyl PCIH hydrazone

P48

N,N′-di(3-pyridinecarboxamide)-1, 6- 3-dpyh hexane

P49

N,N′-bis(3-pyridyl)oxamide

-

P50

N,N′-bis(3-pyridyl)malonamide or N,N′-di(3-pyridyl)propanediamide

3-dppa

P51

N,N′-di(3-pyridyl)butanediamide

3-dpba

P52

N,N′-di(3-pyridyl) succinamide

-

P53

N,N′-bis(3-pyridyl)adipamide or N,N 3-dpha ′-di(3-pyridyl)hexanedioicdiamide

P54

N,N′-bis(3-pyridyl)heptandiamide or N,N′-di(3-pyridyl)pimelicdiamide

3-dppia

P55

N,N′-bis(3-pyridyl)sebacicdiamide

-

P56

N,N′-di(3-pyridyl)sebacicdiamide

3-dpsea

P57

4-(1H-pyrazol-4-yl)pyridine

Hpypz

P58

N,N′-(1,1-methyl)-bis(imidazole)

Mbim

P59

1,1′-(1,2-ethyl)bis(imidazole) or 1,2bis(imidazol-1-yl)ethane

biim-2 (bie)

P60

9-methyladenine

9Meade

P61

9H-purin-6-amine or adenine

Hade (ad)

P62

1,6-bis(imidazol-1-yl)hexane

1,6-bih (bimh)

P63

3-pyridyltetrazoles

3-ptz

P64

methylene bis(3,5-dimethylpyrazole) H2MDP

P65

1,4-bis(2-isopropyl-1Himidazol-1-yl) Bisopib butane

P66

1,4-bis(2-ethyl-1H-imidazol-1yl)butane

Betib

P67

4,4′-methylene-bispyrazole

H2MBP

P68

1,4-bis(2-methyl-1H-imidazol-1-yl) bmib (bib) (bmeib) butane or bis(2-methyl-1H-imidazol1-yl)butane

P69

1,4-bis(3,5-dimethyl-1H-pyrazol-1-yl) Dmpzb butane

P70

1,6-bis(3,5-dimethyl-1H-pyrazol-1-yl) Dmpzh hexane

P71

3,5-dimethyl-4-(4′-pyridyl)pyrazole

Hdmpp

P72

3,3′-5,5′-tetramethyl-4,4′-bipyrazole

bpz (Me4bpz)

P73

1,4-bis(imidazol)butane or 1.1′(1,41,4-bib (1,4-bbi) butanediyl)bis(imidazole) or 1,4(bimb) (biim-4) di(1H-imidazol-1-yl)butane or 1,4bis(imidazol-1′-yl)butane or 1,1′-(1,4-

butanediyl)bis(imidazole)

P74

2,6-diaminopurine

Hdap

P75

1,2-bis(1,2,4-triazol-4-yl)ethane

btre (bte)

P76

1,4-bis(1,2,4-triazol-1-yl)butane

btb (btab)

P77

1,2,3-benzenetriazole or 1Hbenzotriazole or benzotriazol

HBTA (btz) (BTAH)

P78

5-(2-pyrazinyl) tetrazole

Pzta

P79

4-pyridyltetrazole

4-ptz (4-H-ptz)

P80

di(pyridin-4-yl)amine or 4,4′dipyridylamine

Dpa

P81

N-(pyridin-3-yl)nicotinamide

3-pna

P82

N-(4-pyridyl)-isonicotinamide

4-pina

P83

N-(3-pyridyl)-isonicotinamide

3-pina

P84

N-(4-pyridyl)-nicotinamide

4-pna

P85

N,N′-bis(4-pyridinyl)-1,4benzenedicarboxamide or N,N′-bis(4-pyridyl)phthalamide

bpda (HBPPA) (bpta)

P86

N1,N3-di(pyridin-4-yl)isophthalamide or N,N′-bis-(4-pyridyl)isophthalamide

P87

N,N′-bis(4-pyridylformamide)-1,4benzene or N,N′-(1,4-phenylene) diisonicotinamide

Bpfb

P88

N1,N4-di(pyridin-3yl)terephthalamide

-

P89

N1,N4-bis(pyridin-3-ylmethyl)

-

cyclohexane-1,4-dicarboxamide

P90

N,N′-bis(3-pyridinecarboxamide)-1,4- 3 –bpcb benzene

P91

2,5 -bis(pyrid-4-yl)1,3,4-thiadiazole

2,5 -bptz (bpt)

P92

4 -amino-3,5-bis(4-pyridyl)-1,2,4triazole or 3,5-di(pyridin-4-yl)-4H1,2,4-triazol-4-amine

bpt (dpta) (4-abpt)

P93

4 -amino-3,5-bis(3-pyridyl)-1,2,4triazole

3 -abpt (3-bpt)

P94

1,4 -bis(4-pyridyl)benzene or 1,4-

bpb (bpbenz) (pbyb)

di(pyridine-4-yl)benzene

(dpb)

P95

di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) or dipytz (bpt) (bpta) 3,6-di(4-pyridyl)-1,2,4,5-tetrazine or (DPT) (4-PYTZ) 3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine

P96

3,6 -bis(pyridin-3-yl)-1,2,4,5-tetrazine 3 –PYTZ

P97

4,4′-(2,3,5,6-tetramethyl-1,4phenylene) dipyridine

-

P98

1,4 -bis(pyridin-4-ylethynyl)benzene

-

P99

N1,N4-bis(pyridin-4-ylmethyl) benzene-1,4-diamine

R-4-bpmb

P100

N1,N4-bis-((pyridin-4-yl)methylene) benzene-1,4-diamine or N,N′-bis-(4pyridylmethylene)-1,4benzenediamine

4-bpmb

P101

3-(5-phenyl-1H-1,2,4-triazol-3-yl) pyridine

3,3′-bpt

P102

4,4′-(2H-1,2,3-triazole-2,4-diyl) dipyridine

Tadp

P103

4,4′-(2,3,5,6-Tetramethoxy-1,4phenylene)dipyridine

-

P104

4- bis(4-pyridinylmethyl)piperazine or p-bpmp (4-bpmp) 1,4-bis(4-pyridinylmethyl)piperazine

P105

3-bis(3-pyridylmethyl)piperazine

3-bpmp

P106

2,3-difluoro-1,4-bis(4pyridyl)benzene

-

P107

piperazine-1,4-diylbis(pyridine-4ylmethanone

Pdpy

P108

2,5-bis(2-hydroxyethoxy)-1,4-bis(4pyridyl)benzene

-

P109

1,10-phenanthroline

Phen

P110

1,4-bis(2-

px2ampy

pyridylaminomethyl)benzene

P111

1,4-bis(3pyridylaminomethyl)benzene

px3ampy

P112

N,N′-bis(4-pyridinecarboxamide)-1, 2-cyclohexane

4-bpah

P113

N,N′-bis(3-pyridinecarboxamide)-1, 2-cyclohexane

3-bpah

P114

N3,N5-di(pyridin-3-yl) pyridine-3,5dicarboxamide

P115

N,N′-bis(4-methylenepyridin-4-yl)-1, 4-bmbpd 4-benzenedicarboxamide

-

P116

2,6-bis(pyrazin-2-ylthio)pyrazine

-

P117

1,1′-((perfluoro-1,4-phenylene) bis(methylene))bis(1H-imidazole)

-

P118

2-(2-pyridyl)benzimidazole

2-PyBIm

P119

1,3-bis(1H-imidazol-1-yl)methyl)benzene or 1,3-bis(imidazol-1ylmethyl)benzene

m-bimb (1,3-bimb) (mbix)

P120

1,4-bis(1H-imidazol-1-yl)methyl)benzene or 4,4′-bis(imidazol-1ylmethyl)benzene

p-bimb (bix) (BIYB) pbib (1,4-bimb) (bimx) (1,4-dimb)

P121

1,4-bis-(imidazol-1-yl-methylene) benzene

-

P122

4-(1H-benzo(d)imidazol-2-yl)thiazole Hti (TBZ) or thiabendazole

P123

2,6-di(1H-imidazol-1-yl)pyridine

Impy

P124

1,4-bis(2-methylimidazol-1ylmethyl)-benzene or 4,4′-bis(2methylimidazol-1-yl)benzene

bmix (4,4′-bmib) (pbmeix)

P125

1,4-bis((2-isopropyl-1H-imidazol-1yl) methyl)benzene

Pbisopix

P126

1,4-bis((2-ethyl-1H-imidazol-1-yl) methyl)benzene

Pbetix

P127

1,2-bis(imidazol-1-ylmethyl)benzene Obix (1,2-Bimb)

P128

1,4-bis(1-imidazol-yl)-2,5-dimethyl benzene

bib (dmib)

P129

1,3-bis(1H-imidazol-1-yl)benzene

1,3-bib

P130

1,4-bis(1H-imidazol-1-yl)benzene or 4,4′-bis(imidazol-1-yl)phenyl

1,4-bib (dib) (dip) (BIP)

P131

(E)-2-(2-(pyridin-3-yl)vinyl)-1Hbenzo) d)imidazole

-

P132

(E)-2-(2-(pyridin-4-yl)vinyl)-1Hbenzo) d)imidazole

-

P133

1,4-bis(imidazol-1-ylmethyl)-2,3,5,6- Bimx tetramethylbenzene

P134

1,4-bis(1,2,4-triazol-1Btmx ylmethyl)-2,3,5,6-tetramethylbenzene

P135

1,4-bis(1,2,4-triazol-4-yl)benzene

Btx

P136

1,4-bis(1,2,4-triazol-1ylmethyl)benzene

btx (bbtz) (btb) (btrb)

P137

1,3-bis(1,2,4-triazol-1ylmethyl)benzene

Mbtz

P138

tris(triazolyl)borate

Tt

P139

1,3-di-(1,2,4-triazole-4-yl)benzene

Dtb

P140

1-(1H-imidazol-4-yl)-4-(4H-tetrazol- 5-yl)benzene

P141

2-(1H-tetrazol-5-ylmethyl)-1Hbenzoimidazole

Hbimtz

P142

4,7di(4-pyridyl)-2,1,3benzothiadiazole

DPBT

P143

5,5’-bis(4-pyridyl)-2,2’-bithiophene

Bpbp

P144

2,6-di(pyridin-4-yl)naphthalene

-

P145

N1,N5-bis(pyridin-4-ylmethyl) naphthalene-1,5-diamine

R-4-bpmn

P146

4,4′-di(pyridin-4-yl)-1,1′-biphenyl

-

P147

Bis-pyridin-4-ylmethylenenaphtalene-1,5-diamine

4-nbpy (4-bpmn)

P148

4,4′-(oxybis(4,1phenylene))dipyridine

OPY

P149

2-(3-(4-(pyridin-4-yl)phenyl)-1H1,2,4-triazol-5-yl)pyridine

Hpptp or Hpptpd

P150

2-amine-4,4′-bis(1-imidazolyl)bibenzene

Abimb

P151

4,4′-di(1H-imidazole-1-yl)-1,1′biphenyl

4,4′-DIB (bibm) (bpim) (4,4′-bimbp) (bimb) (bibp)(dibp)

P152

1,1′-(1,3-propane)bis-(2-

Pbmb

methylbenzimidazole)

P153

1,2,4,5-tetra(4-pyridyl)benzene

TPB

P154

tris(4-pyridyl)-triazine or 2,4,6-tris(4- py3T (tpt) (4-ptz) pyridyl)-1,3,5-triazine

P155

1-H-imidazo(4,5-f) (1,10)phenanthroline

IP

P156

N,N′-bis-(4-pyridylformamide)-1,5naphthalenediamine

Bpfn

P157

4,4′-oxybis(N-(pyridine-4yl)benzamide

-

P158

4,4′-bis((1H-imidazol-1-yl)methyl) biphenyl or 4,4′-bis(imidazol-1ylmethyl)bibenzene or 4,4′di(1Himidazol-1-yl)biphenyl

bpbix (bimb) (bmb) (bpim)

P159

1,3,5-tris(1-imidazolyl)benzene

Tib

P160

4,4′-bis(benzimidazol)propane

4,4′-bibp

P161

2,7-bis(ethoxyimidazole)-naphthalene -

P162

1,3-bis(benzimidazol)propane

Bbp

P163

1,2-bis(2-(4H- 1,2,4-triazol-4-yl) phenoxy)ethane

-

P164

1-((1H-1,2,4-triazol-1-yl)methyl)-3(3-pyridyl)-5-(4-pyridyl)-1,2,4triazole)

3,4′-tmbpt

P165

1-((1H-1,2,4-triazol-1-yl)methyl)-3,5- 4,4′-tmbpt bis(4-pyridyl)-1,2,4-triazole

P166

1-((1H-1,2,4-triazol-1-yl)methyl)-3,5- 3,3′-tmbpt bis(3-pyridyl)-1,2,4-triazole

P167

bis(4-(1H- 1,2,4-triazol-1-yl)phenyl) amine

BTPA

P168

1,2-bis)3-(4H-1,2,4- triazol-4-yl) phenoxy)ethane

-

P169

1,2-bis)4-(4H-1,2,4-triazol- 4-yl) phenoxy)ethane

-

P170

4,4′-bis(1,2,4-triazol-1-ylmethyl) biphenyl

btbp (btp) (btmb)

P171

bis(4-(4H-1,2,4-triazol-4-yl)phenyl) methane

-

P172

bis(4-(4H-1,2,4-triazol-4-yl)phenyl) ethane

-

P173

N4,N4′-bis(pyridin-4-ylmethylene)biphenyl-4,4′-diamine

P174

4,4′-((1-ethyl-1H-imidazole-4,5-diyl) bis(4,1-phenylene))dipyridine

-

P175

tetrakis(4-pyridyl)cyclobutane

Tpcb

P176

1,2,4,5-tetrakis(imidazol-1-ylmethyl) benzene

P177

9,10-bis (1H-imidazol-1yl)anthracene

Dia

P178

1,4-Bis)2-(2-pyridyl)imidazol-1ylmethyl)benzene

-

P179

1,4-bis(2-ethylbenzimidazol-1ylmethyl) benzene

Beb

P180

1,4-bis(2-methylbenzimidazol-1ylmethyl) benzene

Bmb

P181

1,2-Bis)2-(2-pyridyl)imidazol-1ylmethyl)benzene

-

P182

2,6-bis(3-(pyrid-4-yl)-1,2,4-triazolyl) 2,6-H2bptp pyridine

P183

tris(4-(1H-1,2,4-triazol-1- yl)phenyl) amine

TTPA

P184

N,N′-di(4-pyridyl)-1,4,5,8naphthalenetetracarboxydiimide

DPMNI (DPNDI) (DPNI) (PNMI)

P185

tris(4-(1H-imidazol- 1yl)phenyl)amine

Tipa

P186

3,3′,5,5′-tetra(imidazol-1-yl)-1,1′biphenyl

-

P187

hexakis(1,2,4-triazolylmethy1)benzene

Htrb

P188

N4,N4,N4′,N4′-tetrakis(4-(1H-1,2,4triazol-1-yl) phenyl)-(1,1′biphenyl)-4,4′-diamine

TTPBDA

P189

Dipyridyldibenzotetraaza)14)annulene -

P190

5,15-di(4-pyridylacetyl)-10,20diphyenyl) porphyrinato

Da-H2P

P191

5,15-dipyridyl-10,20-bis(pentafluorophenyl)porphyrin

F-H2P

P192

5-(4-(1H-imidazol-1-yl) phenyl)-1H- HIPT tetrazolate

P193

Piperazine

PPA (pi)

P194

1,3,5-tris(p-imidazolylphenyl)benzene Tipb

P195

1,5-bis(imidazole-1-yl)pentane

Bip

P196

4,4/-bis(imidazol-1-yl)diphenyl ether BIDPE

P197

terephthalic acid, 1,4benzenedicarboxylic acid

Bdc

P198

oxalic acid

Ox

P199

5-(4-carboxyphenoxy)isophthalic acid H3cpip

The functional groups on the organic pillars not only could bring new functionalities into the frameworks but also guide the formation of the resulting structures through adjusting molecular symmetry and supramolecular interactions. For chemists that are interested in developing nanometer-sized pores that provides novel guest assembly, functionalization of the pore surface through pillars is a very attractive idea. Overall, the data show that the species of multifunctional pillars play a vital role in crystal structure construction and have direct influence on pore size and properties of the resultant compounds compare with their parent frameworks. Combined tuning of functional groups and pore size may be a promising method to develop high-performance MOF materials. Therefore, a number of attempts have been made to introduce specific functional groups (acid-base sites, ion-exchange sites, chemical interaction sites, hydrogen-bonding sites, chiral sites and so on) onto pillars for fine tuning of the pore environment to adjust hydrophilicity/hydrophobicity, polarity and acidity/basicity. Among reported literatures, it is worth noting that the functional groups seem to be generally oriented toward the inner side of the pore. The effect of each functional spacer presented in pillar linkers on structure topologies and applications is investigated here, dividedly.

4.2 Bipyridine Bpy is widely used for designing and constructing different kinds of porous compounds because of its π-stacking ability and bridging coordination property. Tailoring of the length and the chemical environment of bpyridyl based linkers have stimulated great interest. Whether the N atoms in pyridyl groups are located in para, Meta or ortho position, the structural topology and functionality is influenced. Modifying the backbone structure by introducing methyl, ethyl, ethylene or propyl between two pyridyl groups can also change framework properties. Coordinated bpy presents interesting electronic and photonic properties. Bpy and its subsets are able to play different roles in formation of the frameworks when it adopts variant coordination mode. Although bpy mostly adopt two donors bridging mode to extend 2D sheets into 3D supramolecular frameworks, it also can act as terminal monodentate ligand inside the frameworks. The guest bpy can also exist in channels and acts as template causing

stability improvement in various structures. For instance, the bpy ligand had different designation in six d10 complexes as a bridging ligand, terminal ligand and as a guest in lattice. Yuan-Gen Yao and coworkers reported three novel frameworks, which bpy acted specifically as a template and deprotonating reagent. The species of H2bpy2+, bridging bpy and monodentate bpy are existed in building blocks of three reported MOFs. Introduction of bpy into the structure of the metal-organic architecture can change the structure dimensionality wisely, for example by employing bpy into the structure of 2D honey comb network of ((Zn2(bpydb)2(H2O)2)(DMA)3(H2O))n, two new 3D MOFs were obtained. Applying higher reaction temperature has brought diversity in networks by removing their coordinated ethanol molecules. Direct reaction of bpa pillar and Hcnp-H2ipa acid with Co(II) can also improve the 2D structure of ((Co2(OH)(cnp-ipa)(EtOH)(H2O)))n to 3D interpenetrated network [1]. By introducing bpy coligand to FDC based MOFs the overall structure and luminescent property was influenced. It is also possible to use bpy to cross-link metal phosphonate layers into pillared layered architectures. Another work reported by T. K. Maji and co-workers demonstrate that using bpy together with Himdc acid can form a 3D rigid framework with polar pore surfaces (Figure 4.1). The aromatic π cloud and polar N atoms of bpy along with hetero atoms (N, O) of the applied acid interact more effectively with methanol solvent guest and hydrogen gas molecules [2].

Figure 4.1 View of the 3D framework (a) along the c-axis showing oval shaped channels decorated with pendent carboxylate oxygen atoms and pyridyl moieties; (b) square shaped channels along perpendicular to b-axis decorated with pendent carboxylate oxygen atoms. Applying bpy together with TCNQ which is an interesting N-donor active ligand lead to new pillar layered structure with highly electron-rich surface for guest molecules such as benzene [3]. Bilayer packings were also reported in bpy pillar MOFs which these double-pillar fashions can improve thermal stability [4, 5]. Using a suitable bridging ligand and pillaring bpy, led to first structurally characterized pillared-bilayer complex with a three-dimensional open framework [6]. BPP (1,3-bis(4-pridyl)propane) was applied as a flexible linker owing to the three methylene groups between pyridyl rings to produce porous architectures [7, 8]. As reported, the bpp pillar can adopt different conformations such as TT, TG, GG or GG’ (T: trans; G: gauch),

which produce specific porous polymers with different N atoms separations. Another important bpy subsets are bpe and dpe molecules which are extensively used as pillars in fabrication of 3D frameworks. When alkyl groups, are selectively introduced between the two 4-pyridyl rings of bpy, the ligand acquires variable flexibility and functionality which can direct specific framework properties. Bpe is a flexible ligand which can adopt the gauche and anti-conformations and causes a variety of network topologies. The space between two nitrogen atoms in anti bpe is longer than that of bpy. Many applications have been investigated in bpy-pillar MOFs, such as water and polar solvents or n-alkanes selective adsorption, magnetic measurements, luminescent and drug loading [9]. Bpy has been used as a very favorable linker in MOFs for practical gas adsorption investigation. Another interesting aspect of bpy containing MOFs are applying them in Gas-Chromatogaraphy column for effective separation.

4.3 Dabco Another common applied pillar in MOF structures is 1,4-diazabicyclo(2.2.2)octane, a bidentate diaza ligand which can be used as a second ligand and improves framework dimensionality [10]. Dabco pillar insertion into a two dimensional structure produced a 3D (Cu(tfbdc)(dabco)0.5). This insertion made 2D planes slide along each other which led to higher pore volume [11]. Dabco as an aliphatic pillar is shorter than bpy which can affect the resulted structure via distortion in coordination sites. As reported, bpy can be replaced by dabco through post synthetic ligand exchange. (Ni(HBTC)(bpy)) was synthesized by solvothermal method. Then replacing of its pillar linkers completely ((Ni(HBTC)(dabco)) or alternating with dabco (Ni2(HBTC)2(bpy)0.6(dabco)1.4) caused to reduction of the pore volume in obtained MOFs (Figure 4.2). This phenomenon leads to pore hydrophobicity and polarity alternation which affect their N2, CO2 and CH4 adsorption. Pore volume of (Ni(HBTC)(bpy)) is greater than (Ni2(HBTC)2(bpy)0.6(dabco)14) and (Ni(HBTC) (dabco)) respectively due to linker lengths which led to higher N2 and CH4 uptake capacity for (Ni(HBTC) (bpy)). However higher CO2 adsorption was reported for (Ni2(HBTC)2(bpy)0.6(dabco)1.4) because of pore polarity.

Figure 4.2 Reaction Schemes toward (Ni(HBTC)(dabco)) via random ligand exchange and toward (Ni2(HBTC)2(bpy)0.6(dabco)1.4), via selective ligand exchange. In comparison with ligands in 2D layers, the pillar moieties are loosely bond to the metal nodes. For instance, the dabco pillars can be replaced by water molecules reversibly in a 3D framework NU-505-Ni. The Ni nodes were used instead of Zn in order to produce stable structure with permanent porosity. Exchanging dabco pillars with water can be considered as a novel method to produce mixed pillar MOFs with different N-donor pillars and striking functionalities (Figure 4.3) [12].

Figure 4.3 (a) Structures of TBAPy (left) and dabco (right); (b) Crystal Structure of NU-505Zn Viewed along the a-axis (left) and c-axis (right); and (c) Transmetalation of NU-505-Zn with Ni(II), Followed by Water Treatment and Reinsertion of dabco Pillars. Modulators have been introduced as promising media to achieve crystals with specific dimensions. A study reported on dabco pillar layered MOF (Cu2(ndc)2(dabco)) showed that

dabco’s nitrogen atom interacts with amine containing the similar lone pair located at nitrogen atom which tends to the crystals growth in the other orientation. A new investigation on the mentioned MOF with dabco pillars demonstrated that different dimensions in applied crystals can lead to distinct adsorption behavior due to varied pore environment. In (Cu2(ndc)2(dabco)) crystallographic face containing Cu and dabco had much larger pores in comparison with that including two-dimensional Cu and ndc layers. Discriminate adsorption behavior was emerged in different pore openings for target MOFs. A report by Kumar Maji and workers has been done to compare dabco as an aliphatic pillar with pirazine and pipirazine [13]. Application of these different spacers having different size and structure had direct effect on the magnetic and porous properties of obtained 3D frameworks. Dabco as a bulkier pillar reduces pore size resulting in non-porous MOF. In the open literatures, MOFs containing dabco exhibit special adsorption properties due to their flexibility. DUT-8(Ni), a MOF with (Ni2(2,6-NDC)2(DABCO)), demonstrated special breathing effect determined by 129Xe NMR spectroscopy. The pore opening occurs in presence of Xe through Xe-MOF interaction that overcame the π-π interactions of the linkers in the non-porous structure. The idea was further improved by the authors to compare flexibility in series of DUT-8(M) structures with diverse metal ions toward 129Xe NMR and 13C NMR spectroscopy along with several gas adsorptions. The results showed that breathing was correlated with the nature of metal ion as the DUT-8(Cu) showed no transformation toward solvent or gas removal or adsorption while Co and Ni were flexible by introduction of external stimuli. One new stable and hydrophobic MOF reported containing dabco pillar is (Zn2(adb)2(dabco)) with striking structural transformation through desolvation, resolvation and following hydration [14]. This structure also showed phase transformation during gas sorption with high BET surface area. The dabco pillar-layered MOFs are extensively used for adsorption, separation, catalysis, fluorescent or as dielectric hybrids.

4.4 Imidazole and Pyrazole Producing mixed ligand MOFs through application of pillars with imidazole is an ongoing effort. Nitrogen in imidazole can coordinate to metal or be as a spacer in linker backbone. The free rotation of imidazole rings can provide the requirements for self-assembly. The aromaticity of this group along with nitrogen lone pairs can have influence in MOF functionality. The π-π* transitions in aromatic groups of bpbix and bpim pillars was seen in solid state diffuse reflectance spectra of four reported MOFs. Generally, π-π interactions are common through imidazole rings in structures. Combination of bimb, D-camphor acid led to assembly of three new structures (M2(D-

cam)2(bimb)2)n.3.5nH2O (M= Mn, Co) and (Cd8(D-cam)2(bimb)2)n [15]. The authors proposed that inter ligand interactions were facilitated through bimb aromatic rings which has direct effect on luminescent property of resulted MOFs. Figure 4.4 shows the PL intensities of Cd based MOF for diverse aromatic molecules. The selective adsorption of methyl orange over methylene blue dyes were also correlated to positive metal ions and π-π* interactions of benzene rings in pillar linker.

Figure 4.4 The PL intensities of MOF toward selective aromatic molecules. The reported florescence in ((Zn2(odpa)(IP))·4H2O)n MOF were also attributed to ip pillar with extended aromatic system. In addition to aforementioned, two novel MOFs with bmib pillar linker were reported [16]. Stability of these frameworks is related to supramolecular interactions between bmib and anionic frameworks. The authors also claim that these structures are good candidates for proton conduction which is ascribed to protonation of pillar and its extensive hydrogen bands with surrounding molecules.

The imidazole group introduces rigidity into the structure while the spacers such as CH2 groups improve linker flexibility so the structures show interesting properties [17]. The degree of rigidness or flexibility was mostly studied in imidazole pillared MOFs. A v-shaped imidazole linker BIDPE takes as a half flexible pillar for generation of 6 new MOFs with new topologies. The configuration of BIDPE affected the resulted structures through adjusting the angle for better coordination. The pillar angles were exchanging from 114.206° to 138.853° depending on applied acidic ligand [18]. This pillar was previously used by this group as well [19]. A similar study on flexibility was done using four different imidazole pillars generating nine new MOFs. Auxiliary N-donor ligands backbone is important in resulted MOF topology which is demonstrated in Figure 4.5. The longer pillars generate bigger voids that produce higher degree of interpenetration and improving flexibility in linkers lead to more complicated structures [20].

Figure 4.5 The structurally related ancillary ligands. Photocatalyst activity was also investigated in imidazole based MOFs. Photocatalytic activities for the degradation of methyl orange were measured in six reported structures by Guang-hua Cui [21]. The pbmb pillar has –(CH2)n– backbone and the 2-position substituent methyl group which help the pillar for better coordination to metal ions. Similar to imidazole, plethora of examples were reported for producing pyrazole based new MOFs. Existence of azole group in such MOFs enhances stability and resistance of these structures. As reported, BPz was introduced into the structure of six new pyrazole based MOFs and their gas uptake and vapor sorption were studied. BPz is a flexible pillar that induces hydrophobicity into the cages toward its methyl groups which can affect the vapor sorption and cause dynamics into the structure. The polar NH group in linker can also form hydrogen bonds inside the framework and effect on higher CO2 uptake due to relatively high quadropole moment of CO2. The same BPz pillar was used for construction of uncommon structure with metal–carboxylate–pyrazolate clusters (Figure 4.6). The pryazole group enhances water stability, pore polarity and also make pores narrower which as a result improves CO2/N2 selectivity [22].

Figure 4.6 3D structure of (Co8.5(µ4-O)(bpdc)3 (bpz)3(Hbpz)3)·6(DMF)·9(CH3OH)·15(H2O) viewed along the c axis (a) and (111) direction (b), respectively, forming an intersected 3D porous system. Zn4(HMe4bpz)2(bpdc)3)n, based Me4bpz ligands shows high thermal and chemical stability and possesses first example of uninodal five-connected with zfy topology. The Me4bpz is a neutral bridging ligand with methyl groups that orient toward the pore surface and acts as a hydrogen acceptor. The structure is triply interpenetrated and a lot of supramolecular intractions stabilizes the whole framework [23]. Photodegradation of congo red was also estimated for eight MOFs with bispyrazole pillar H2MBP. The MOFs exhibit different catalytic activities due to their metal centers, functional groups and pores environment [24]. As aforementioned, photodegradation of toxic organic dyes were investigated in three Cu pyrazole based MOFs with a potential application in wastewater purification [25]. Their magnetic behavior was also depended on Cu-pyrazole SBUs.

4.5 Triazole and Tetrazole To date, triazolate groups were introduced into the ligands as a functional group or as a linker to improve frameworks dimensionality. Triazole can act as a three-donor ligand and produces

2D layers with metals. As a result, it does not act as a pillar to link planes. Although, there are many examples on pillars containing triazole groups. Employing triazolate based btb pillar, led to construction of (4,6)-connected self-catenated H-bonding framework based on the cross-linking of 2D→3D inclined polycatenate motifs. Interesting report on two structures using imidazole and triazole based pillars demonstrated the profound effect of auxiliary N-donor spacers on topology and properties of resulted coordination frameworks. Application of imidazole based bix pillar led to Co5 clusters that are extended into an uncommon two fold self-penetrating ile net. While replacing bix by longer ptb ligand generated Co8 clusters with 8-connected self-penetrating (420.68) framework, which is the highest-connected uninodal self-penetrating motif. In this sense, changing the auxiliary N-donor ligands has direct effect on the assembled supramolecular structures. Very similar study on 12 new MOFs using imidazole and triazole based pillars also indicated that obtaining versatile coordination geometries toward utilizing different flexible N-donor pillars is a promising synthetic route. The final structures were good candidates as photoluminescent materials. Isomeric structures with same metal, ligand and 1,2,4-triazol-3amine pillar was reported by authors to emphasize on the effect of synthesis condition specially speed of crystallization. A flexible bis triazole spacer btmb improved dimentionality of six metal carboxylate sheets to 3D framework through two exo-N atoms of the triazole units coordinating to metal centers. The obtained structures were used as heterogeneous for the oxidative coupling of 2,6-dimethylphenol (DMP) to poly(1,4-phenylene ether) (PPE) and diphenoquinone (DPQ). The authors proposed that catalytic activities of these structures were depended on structure versatility [26]. The triazole spacers can improve framework functionality. Given that, replacing bpy with triazolate based pillar 4,4′-(2H-1,2,3-triazole-2,4-diyl)dipyridine was done in order to study the effect of triazole group on CO2 capacity. The resulted MOF named MTAF-3, was isostructural to MOF-508 while two nitrogen atoms of 1,2,3- triazolate group was oriented toward the channels. This framework adsorb a substantial amount of CO2 with an uptake capacity of 10.0 wt% at 273 K under 1 atm of pressure, showing higher capacity by a factor of 2.7 compared to MOF-508 under the same conditions owing to the stronger CO2– framework interactions caused by the Lewis base nitrogen atoms [27]. Confirming the pillars mentioned formerly, whether tetrazole group coordinates directly to the nodes or be as a spacer, its specific nature can affect framework properties. Four nitrogen electron-donating atoms in the tetrazolyl ring provide diverse coordination modes for this group. The structures can also stabilize through hydrogen bands formed by amino group and nitrogen atoms of present in tetrazolate rings [28]. The tetrazine-based compounds also form electroconductive charge-transfer complexes. The high order of such ligands in MOF crystals, combined with the microporosity, would be an additional advantage for such applications [28].

A report by Guanghua Li and Zhan Shi group used both tetrazole and tetrazine groups for constructing new MOFs. Different dimensionalities in pillar backbones as well as versatile degree of rigidity induce diverse topologies and interpenetrations into the resulted frameworks along with luminescent property [29]. Application of long tetrazine based DPT linker led to formation of robust triply interpenetrated structure with permanent porosity. The DPT linker can stabilize the framework through weak supramolecular interactions as forming hydrogen bonds. The activated framework takes up 1.28 wt % hydrogen gas and exhibits high hydrogen storage density of 95.2% at 1 atm and 77 K.

4.6 Pyrazine and Pipyrazine This molecule links effectively to metal center within electron donating nitrogen atoms [30]. Replacing bpy with pzy in a stable MOF Ni(HBTC)(bpy).3DMF led to formation of Ni(HBTC)(pyz).3DMF where honeycomb grid layers constructed by Ni-BTC groups are interlinked by pyz pillars. The exchange in pillar lengths is promising method to tune the shape and size of the channels that affect gas adsorption. Non porous pyrazine based MOF was improved to porous structure with selective gas adsorption ability by enlargement of pore apertures owing to pyrazine exchange with longer pillar linker, bpy and bpe pillar groups [31]. Water adsorption isotherms were also studied in these three MOF by Hirofumi Kanoh group [32]. The pores in 2-D sheet (Cu2(pzdc)2) are not big enough for passing water molecules. All the structures show similar type I isotherm for water adsorption (Figure 4.7a). The absence of hysteresis indicated that the frameworks are stable upon water. In larger voids, first water molecules were connected to hydrophilic sites of the walls and then water molecules are absorbed on the pre-adsorbed water molecules because of the presence of free space in the pores (Figure 4.7b).

Figure 4.7 (a) Comparison of adsorption isotherms of H2O on (Cu2(pzdc)2(pyz)) (F), on (Cu2 (pzdc)2(bpy)) (E), on ((Cu2(pzdc)2(bpe)) (Q) at 303 K. (b) Schematic representations of water adsorption on small or large porous compounds. Pyrazine group was used as a rotational organic part in ((CdNa(2-stp)(pyz)0.5(H2O))(H2O))n structure and its dynamic was studied by 2HNMR. This group can rotate along the N-N axes and take four angles of 0, 76.4, 180, and 256.48. Replacing pyrazine with bulkier dabco pillar

produced another MOF with same dynamic motion but higher energy barrier. The guest adsorption also reduces the pillar rotation [33]. CPL-1 was initially reported by Susumu Kitagawa as a flexible framework with pyrazine pillar layers and its O2 adsorption was studied [34]. The molecular O2 forms ladder in 1D channels and the structure shows shrinkage toward removal of H2O guest molecules and expansion by introducing O2 molecules. The magnetic properties of O2 adsorbed in CPL-1 have also been investigated [35]. The authors developed their investigation on CPL-1 by studying the molecular motion of methanol molecules adsorbed in CPL-1 voids through 2HNMR. Only two motions were observed for methanol, rotation and wobbling [36]. Further study on acetylene adsorption of CPL-1 also showed that acidic hydrogen molecules of C2H2 interact with basic oxygen atoms exist in CPL-1 pores causing higher adsorption of acetylene. Controlling the orientation of crystals through adding a reagent was also studied in pyrazine based MOFs which caused interesting size-dependent adsorption properties. Piperazine was rarely used as a pillar for MOF fabrication. The simple structure of piperazine provides the ability of functionalizing this pillar by different organic groups. Piperazine can act as H donor molecule to form weak H-bondings.

4.7 Amide, Imide, Amin and Azine/Azo Spacer Amide functional group is widely used in pillar-layered MOFs due to simple fabrication, amazing polarity and high hydrophilic nature. Amides contain both hydrogen donor and hydrogen acceptor molecules causing supramolecular interactions into the structure. Amide based isomers were introduced for MOF assembly as pillars with same metal and ligand to understand the structure directing effects of nitrogen donor disposition. Different cis and trans conformations of applied dipridylamide pillars along with diverse nitrogen positions produced MOFs with special versatile topologies and porosities. Hydrogen bonds formed between amide pillar and carboxylic groups provided ancillary structural stabilization. Same study was done using three N-donor pillars with amide spacer. Pillars show different backbone orientation such as linear ligating and angular topologies. Amide groups can effectively enhance functionalities. Amide group presents in pillar backbone of ((Zn4(BDC)4(BPDA)4)·5DMF·3H2O) involved in efficient intermolecular interactions with the adsorbed CO2 molecules which led to selective adsorption of CO2/N2. The major source of attraction should be from a Lewis acid–Lewis base interaction between the CO2 carbon atom and the amide oxygen atom in one channel while amide hydrogen atom interacts with a CO2 oxygen atom in next channel. CO2 amide bindings also improve CO2·CO2 binding. Very similar study showed CO2/CH4 selectivity of amide based pillar MOFs through polar acylamide groups. Although amide pillar is flexible, there was no obvious change of the pattern after the solvent exchange which might be attributed to the N–H···O interactions

between the DMF molecules and the amide groups. CO2 adsorption of flexible ECUT-15 was also studied showing dynamic change via UV irradiation [37]. Selectivity of amide functionalized pillar MOFs was also studied by our group. The MOF with more accessible amide groups shows a higher CO2/N2 selectivity value imcoparison with the other amidecontaining and imine-containing MOFs. DPNI pillar with extended aromatic group is among the widely used pillars for MOF generation which can be categorize in imide based pillar MOFs [38]. MOFs containing DPNI show dipole moment aligned with the long axis of the molecule and might be redox active because this pillar is able to accept electrons and it is possible to form charge-transfer (CT) complexes with electron-donating guest molecules incorporated into the pores. One promising candidate for light adsorbing MOFs is (Zn2(NDC)2(DPNI)). DPNI can form π stacking interactions with applied di-acid ligands forming interpenetrated structure and its CT interaction was also investigated [39]. Very similar study on optical properties of the MOF (Zn(DMF)-NO3)2(NDC)(DPMNI) showed that spectral features in the visible region of the electronic spectra was caused by monoradical anion and dianion states of the DPMNI forming upon one- and two-electron reduction [40]. The extended π stacking presented in the framework caused a long lived excited state. Application of this pillar along with DBT pillar in a porphyrin based MOFs led to characterization of five structures with two new structural topologies, bilayer and interpenetrated AA stacking pattern. Bilayer topology was attained by linking two porphyrin planes by pillar molecule through connecting the central Zn metal in porphyrin sheet and one of the axial sites of the Zn SBUs. The pillar can only link Zn SUBs which produced interpenetrated AA stacking pattern. NH2 functional group is among the most reported organic moieties used in MOF assembly [41]. The amine group generally exists as a spacer in pillar backbone and orient toward the pores, forming H bonds with water or other guest molecules. Same interactions between amine and acid ligands were reported. The amine tetrazolate pillar was generated in situ and induced stability into the MOFs through supramolecular interactions. Amine polarity also affects properties and functionalities of porous coordination polymers [42]. Increasing CO2 uptake by applying polar NH2 in linker was reported by Na Xu and Peng Cheng groups [43]. Datz linker with amino and triazole functionalized pores showed enhanced CO2 sorption capacity in comparison with its analogue using bpy pillar. High quadrupole moment of CO2 caused stronger interaction with pore surface and higher adsorption and selective CO2 adsorption over N2 and CH4 [44]. Apyr pillar in meso helical (Zn2(Cam)2(apyr).2DMF) also gives amino functionality into the pores and its ability for adsorption of enantioselective alcohols was studied [45]. An extraordinary behavior was reported for Hdap pillar molecule as an amine containing pillar. Whether it acts as mono, di or tri dentated molecule, its electronic nature would change to positive, neutral or negative linker respectively [46] (Figure 4.8). Six new coordination

polymers were synthesized using Hdap and their structure were influenced directly by this linker, amine group along with imino group involve in supramolecular H-bonding interactions.

Figure 4.8 The binding character of 2,6-diaminopurine in reported complexes. Whether double or single bonds existed between two nitrogen atoms in diaza groups, the overall effect of these spacers on the structure is the same. Many pillars containing diazo have been introduced into frameworks to date, leading to striking properties. The Azo group within the walls can orient toward the channels and improve structures property and applicability. A report by Kitagawa et al. proposed that using azpy pillar layer in activated ((Cd(pzdc)(azpy)))n (1a) framework produced larger pore voids in comparison to non-activated sample due to lone pair–lone pair electronic repulsion between the pendant oxygen atom of the carboxylate and nitrogen atom of the azo group. The same isostructural framework ((Cd(pzdc) (bpee)))n (2a) exhibited shrinkage after activation due to H-bonding interactions between hydrogen atoms of ethylene in bpee and oxygen atoms of pzdc [47]. Azo group can affect the structure adsorption properties owing to its basic nature, polarity and Trans and cis transformation [48]. An azo based pillar MOF using 4,4′-azobis(prydine) was reported as a good candidate for hydrogen adsorption through forming strong interactions between H2 molecules and pore walls. Also a change in C-C-N bending movements of azo group in reported pillar-layered MOF caused decreasing CO2 adsorption. 4-bpdb and 4-bpdh as a diaza based pillars can facilitate the 3D supramolecular arrangements through forming π-π and C-H … π interactions [49]. Our group also reported a lot of diaza

based structures using 4-bpdb and 4-bpdh, and deeply investigated their effect on MOF functionality [50]. Detailed gas sorption studies on TMU-4 (Zn2(oba)2(4-bpdb))·(DMF)x and TMU-5 (Zn2(oba)2(4-bpdh))·(DMF)y stated the effect of azine function on adsorption. The narrow pores of TMU-5 adsorbed higher amount of CO2 than TMU-4 owing to better attraction of CO2 molecules with azine groups on the pore walls [51]. The basicity of pillar linkers of this two MOFs along with TMU-6 ((Zn(oba) (4-bpmb)0.5)n (DMF)z) was also studied profoundly by Morsali’s group [52]. To this end, Knoevenagel condensation reaction using these three MOFs was performed. In TMU-6 the introduction of a phenyl ring into the pillar ligand creates two imine groups instead of an azine group. The pore structures of three MOFs are stated in Figure 4.9 highlighting their azine function on the bridging pillar. TMU-5 shows enhanced catalytic activity due to pore architecture which causes the reactants to attract more efficiently with azine function as a Lewis base with noticeable alpha effect, while in TMU-6 azine linkers have lowest basicity along of absence of alpha effect. Morphological diversity also led to change in catalytic activity of TMU-5 [53]. Difference in pillar basicity of these three azine-based MOFs along with void space diversity were also confirmed by detailed study on removal and extraction of some heavy-metal ions from aqueous samples in Morsali’s group [54].

Figure 4.9 Views of TMU-4. (a) Representation showing the pore channels and that the network is doubly interpenetrated (in red and green). (b) Representation of the pores, highlighting the azine groups (blue balls). Views of TMU-5. (c) Representation of the pores, highlighting the azine groups (in blue). Views of TMU-6. (d) Binuclear Zn2 cluster (O: red; N: blue; C: grey; and Zn: pink). (e) Representation of the pores, highlighting the azine groups (in blue). (f) Representation showing the pore channels and that the network is threefold interpenetrated (in red, green and yellow). Hydrogen atoms and DMF molecules are omitted for clarity. The basicity activity of azine group in TMU-4 was also influenced the sensing properties and selectivity of this MOF [55]. Adsorptive removal of MO using TMU-5 was also enhanced through acid–base type reaction or hydrogen bonding interactions between the dye and the free carboxylic acid and azine groups [56]. Hydrogen bonding also can correlate the basic azine group in TMU-5 as a sensor to picric acid (Figure 4.10) resulting in selective detection of low concentrations of picric acid in presence of nitroaromatic compounds [57].

Figure 4.10 The interaction between the azine moiety of 4-bpdh and picric acid is highlighted (bottom). Color code: O: red; N: blue; C: black; and Zn: blue polyhedra. Other MOFs using diaza based pillars were also reported in Morsali’s group [58]. And used as catalysts [59] or sorbents [60-62].

4.8 Conclusion The functional groups on the organic pillars not only could bring new functionalities into the frameworks but also guide the formation of the resulting structures through adjusting molecular symmetry and supramolecular interactions. The effect of each functional spacer presented in pillar linkers on structure topologies and applications is investigated in this section. The coordination mode of N-donor pillars such as bpy, dabco, Imidazole, pyrazole, triazole, tetrazole, pyrazine, pipyrazine, Amide, imide, Amin and azine/azo spacer have been studied too.

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Chapter 5 Introduction to Aromatic and Aliphatic Pillars 5.1 Introduction The effects of aromatic and aliphatic spacers in pillar-layered MOFs assembly have been widely investigated. Applying aromatic spacers in pillar backbone can influence frameworks properties and application which lead researchers toward designing potential architecture. Supramolecular interactions induced by aromatic groups not only improve structural stability and rigidity which guide to specific topologies, but also influence the MOF efficiency in adsorption, sensing, magnetic, catalytic properties and so on. Aromatic system along with extended π electron spacer in 3,5-bis-(2-(pyridin-4-yl)vinyl)pyridine (bpvp) led to high sensing ability of nitroaromatics and ketones in novel MOF (Zn4(Hbpvp)2(BTC)3(HCOO)(H2O)2)·4H2O. π-π* and π-π* transitions related to pillar linker caused specific sensing ability with low detection limit. A fascinating study on the effect of pillar π-electron system on sensing in MOFs was reported by our group. Two IRMOFs TMU-6 and (Zn(oba)(4-nbpy)0.5)n.(DMF)1.5 (TMU-21) sensing efficiency was compared [1]. Results show that extending the π-system of pillar ligand affects the pore polarity and hydrophobicity which improve the interactions with aromatic compounds. Similar study on catalytic activity of TMU-6 and TMU-21 and their reduced analogous also demonstrated the effect of aromaticity on hydrophobicity-polarity of pore environment. Aliphatic spacers such as ethyl or propyl in bpe and bpp or side alkyl chains induced flexibility into pillar linkers which can be useful as mentioned before. Also aliphatic alkyls can be used as functional groups to modify pore size and shape as well as MOF chemical comportment, like hydrophobicity. Incorporation of methyl group on pillar linker 2,2′dimethyl-4,4′-bpyridine increased the isosteric heats of CO2 uptake (Qst) of resulted MOFs, due to H-bond-like interactions between CO2 and electron donating methyl group. Presence of methyl also prevented the interpenetration to occur between pore. Furthermore, the stability of obtained structures is improved due to increasing hydrophobicity of the pores which prevent water molecules to inter the void space [2]. Our group also investigated the effect of electron donating methyl groups on higher basicity of pillar which result in improving catalytic activity in Knoevenagel condensation reaction. Porous metal–organic frameworks (MOFs) built up from organic linkers and inorganic connectors have recently begun to be explored as heterogeneous catalysts, owing to their well ordered porous structures, flexible and dynamic behaviours in response to guest molecules and designable channel surface functionalities [3]. Although the low thermal and chemical

stability of MOFs as compared to their inorganic counterparts have restricted their use only under mild conditions, there have been several reports already that showed MOFs can be excellent heterogeneous catalysts for Knoevenagel condensation [4-11]. In study of our group, the MOFs [Zn2(oba)2(4-bpdb)]n.(DMF)x, TMU-4, and [Zn(oba)(4-bpdh)0.5]n.(DMF)y, TMU-5 [12] (H2oba = 4,4-oxybisbenzoic acid, 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3butadiene and 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene, Scheme 5.1), have shown selective CO2 adsorption due to presence of two internal Lewis basic sites per ligand in the pore walls. Considering the CO2 adsorption of these compounds, here their performance as heterogeneous catalysts for the Knoevenagel condensation reaction is investigated. To the best of our knowledge, it is the good report that knoevenagel condensation is performed using MOFs that contain azine functionalized pores acting as base catalyst. A new 3D MOF, [Zn(oba)(4-bpmb)0.5]n(DMF)z TMU-6 (4-bpmb = N1,N4-bis((pyridin-4-yl)methylene) benzene-1,4-diamine, Scheme 5.1), was synthesized by introduction of the phenyl ring in the pillar ligand due to the examination of the basicity of the N-donor ligand and its effect on the considered reaction. Functionalization of MOF pore walls with azine groups enables greater interaction between the walls and substrate molecules, thereby increasing catalytic activity of the MOFs.

Scheme 5.1 Chemical structure of H2oba, 4-bpdb, 4-bpdh and 4-bpmb. This study demonstrated that the basicity of azine functionalized pores is probably an important factor in the catalytic performance. TMU-5 with narrow and interconnected pores in three dimensions has higher catalytic activity upon increasing basicity of its N-donor ligand [13]. Aliphatic or aromatic dicarboxylates can form 1D, 2D or 3D structures based on stoichiometry and reaction conditions, although with some dicarboxylates dimensionalities can be modulated by exploiting the geometric constrain. Like cis-1,4cyclohexanedicarboxylic acid (chdcH2), 1,3-adamantane dicarboxylic acid (1,3-adc), 2,5-

dihydroxybenzoic acid (dhbc), 2,4-pyridinedicarboxylic acid (2,4-pyrdc) linkers have different type of binding angle due the conformation or positions of the carboxylate groups which leads to low dimensional structures. These low dimensional chains may be further extended to two dimensions by a second neutral spacer (Scheme 5.2).

Scheme 5.2 Schematic of design of 2D networks: Stacking of the 2D layers in AA, AB and ABC fashion; Interdigitation of 2D layers. Versatile bindings of such dicarboxylates have been exploited in haldar’s laboratory and various types of 2D structures were obtained. They have reported a 2D square grid network {[Cu2(cis-chdc)2(bpee).3H2O)}n [14] composed of cis-(a,e)-cyclohexanedicarboxylate (cischdc) and 1,2-bis(4-pyridyl) ethylene (bpee). chdcH2 shows three possible conformations a,a-trans, e,e-trans and a,e-cis based on two carboxylate groups and variety of structures can be achieved based on conformation dependent binding of carboxylate. In the structure a paddle-wheel binuclear unit of Cu2(CO2)4 are linked by the four cis-chdc to form a 1D chain. The cis configuration of chdc directs a 1D chain formation which are connected by the bpee linkers resulting in a 2D rectangular grid type of structure with the dimensions of 11 × 5.5 Å2. These 2D grids are further stacked in a ABAB fashion through π … π interactions between the pyridine rings of bpee which ensues a one dimensional channel with dimensions of 5.5 × 2.75 Å2. After removal of the guest molecules, the framework undergoes an expansion due to sliding between the 2D grids as realized by the powder X-ray diffraction (PXRD) pattern. The expanded framework shows permanent microporosity as revealed by selective type I CO2 uptake profile and different solvent vapour adsorption studies. It is worth mentioning that trans-chdc imparts a 2D sheet based on similar Cu2(CO2)4 core and further associating with 4,4′-bipy or bpee resulting in 3D α-Po type

framework. Numerous 3D MOFs solely constructed by different types of aliphatic or aromatic polycarboxylate linkers have been reported by several groups. Aliphatic dicarboxylate like fumarate, malate, glutarate, succinate, adipate have been used in the construction of MOFs along with second organic spacers. Such 3D mixed linker frameworks show different type of network topologies based on the binding modes of the aliphatic carboxylates. It is worth to mention that very few frameworks show permanent porosity due to impenetrable structure. Several pyridyl dicarboxylate linkers also has been used for the construction of 2D layers which are pillared by the pyridyl based neutral linkers. Aromatic dicarboxylate linkers like Pyrazine-2,3-dicarboxylate, Pyridine-2,3/2,4/2,5-dicarboxylate in general construct a dense 2D layers which does not allow the growth of another net and hence a non-interpenetrated structure is obtained.

5.2 Non-Interpentrated Frameworks Kitagawa et al. reported a series of pillared-layer 3D frameworks (CPL-Coordination polymers with pillared layer structures) of Cu(II) and Cd(II) using pyrazine-2,3dicarboxylate (pyzdc) and several other organic pillars of different lengths and functionalization [15, 16]. Here Cu(II) or Cd(II) form a 2D layers with pyzdc and then pillars by exobidentate organic linkers to form 3D frameworks (Scheme 5.3). These frameworks are non-interpenetrated, and show tunable pore size, surface area and modifiable pore surface based on length and chemicalfunctionalization of the pillars. In this series CPL-1 {[Cu2(2,3pyzdc)2(pyz)].2H2O]} where Cupyzdc layers extended by pyrazine linkers extensively studied for gas adsorption. Here one carboxylate group acts as a chelating and the other one remains as mono-dentate and pyrazine only coordinates to axial position of Cu(II). CPL-1 adsorbs N2, O2, Ar, C2H2, CH4 and CO2 with a type-I profile at different temperature. CPL-1 is a good host for oxygen molecules and structure determination with O2 dosed sample suggest a 1D array of O2 molecules inside the pores and they interact antiferromagnetically [17]. A detailed study of C2H2 and CO2 adsorption at 298 K with CPL-1 shows the binding with C2H2 is stronger compared to CO2. At a fractional loading of 0.2 the Qst value obtained for C2H2 is 42.5 kJ/mol which is much higher than that of CO2 (Qst ~ 31.9 kJ/mol). Structure determined by synchrotron X-ray powder diffraction data (XRPD) showed one molecule of C2H2 is present per pore of CPL-1 and the intermolecular distance between C2H2 is 4.8 Å suggesting a close pack organization inside the pores but separation is enough to avoid any explosion. Moreover, one end of C2H2 is oriented towards noncoordinated oxygen atoms of pyzdc and connected through strong O···H-C hydrogen bond. Such sort of interactions and confinement makes a stable accommodation of C2H2 in the periodic pores. CPL-2 [{[Cu2(2,3-pyzdc)2(4,4′-bipy)].guest} having a bipyridyl pillar shows higher uptake of N2, O2 and Xe compared to that of CPL-1 due to longer pillar but in an incommensurate manner. CPL-2 also showed interesting guest responsive structural changes when benzene is

incorporated in the framework [18]. The other most studied compound is CPL-7 [Cu2(pzdc)2(dpyg)]n which is having –OH functionalized pillar dpyg (1,2-Di(4-pyridyl)glycol). These free –OH groups which are oriented towards the pore channels undergo hydrogen bonding interactions with the guest water molecules. Interestingly, after removal of guest water molecules the interlayer distance is shortened which resulted to a contracted framework. The adsorption of MeOH vapour and CH4 shows almost no uptake at low pressures. Even at high pressure for CH4 no uptake was observed but for MeOH a sharp uptake was observed at P/P0 ~ 0.23 and the final uptake amount is 6.2 mmol/g. The desorption pathway is quite different from the adsorption path and till P/P0 ~ 0.1 almost all adsorbed MeOH retains inside the pore. Such sudden uptake is related to the structural change of the framework and the –OH groups in the pore surface is the driving force. These – OH groups interact with the incoming MeOH solvent vapor through hydrogen bonding but this is not possible for CH4 and hence no uptake for CH4 was observed. Thus only by incorporating a –OH group in the pillar adsorption properties are altered. Later with similar precursors that are used for CPL-4 and 5, two new pillared layer frameworks of Cd(II); {[Cd(pyzdc)(azpy)].2H2O}n and {[Cd(pyzdc)(bpee)].1.5H2O} were reported [19]. In these structures each of the pyrazine linker connect to three Cd(II) centers through three oxygen and one nitrogen atoms to form a 2D corrugated layer and these layers are further pillared by azpy/bpee to form 3D structures. Importantly the channel surfaces are decorated with oxygen atoms of carboxylate groups which also form hydrogen bonds with guest water molecules.

Scheme 5.3 Formation of 2D layer of {Cu2(pyzdc)2} and these layers are pillared by neutral linkers to form pillared layer frameworks. Interesting observations were the expansion and shrinkage of the structures after removal of guest water molecules. In case of azpy pillared framework after guest removal an expansion of the framework was observed by single crystal X-ray diffraction analysis. The reason of the expansion is the repulsion between the lone pairs of pendent oxygen atoms of carboxylate group and the nitrogen atoms of azpy linkers. But in case of bpee pillared compound such repulsive interaction is absent rather an attractive C-H···O interaction directs shrinkage of the

framework after guest solvent removal. Such expansion and shrinkage in the framework is also reflected in adsorption properties. Another flexible CPL type analogous compound {[Cd2(pyzdc)2L(H2O)2].5(H2O).(CH3CH2OH)}n having the pillar as 2,5-bis(2hydroxyethoxy)-1,4-bis(4-pyridyl)benzene (L) [20]. Here the pillar flexibility plays an important role towards the flexible nature of the framework. The guest removed framework is a nonporous phase and at a particular pressure of CO2 the porosity is regained which is reflected in the gate opening type uptake profile of CO2 at 195 K. Moreover, the H2O vapour adsorption profile also showed a three step uptake indicating structural transformation in each step.

5.3 Frameworks with Interpenetration Interpenetrated frameworks are spontaneously formed by increasing lengths of the organic spacers based on filling of the large spaces by self-catenation. Framework interpenetration reduces the pore size and corresponding surface area and sometime it leads to the highly condensed nonporous structure with high thermal stability. However small micropores based on self-catenation are beneficial for storage, separation and purification of small molecules. Furthermore, entangled framework shows dynamic structural transformation through shearing or displacement of the nets and such structural flexibility has been exploited for storage, separation and sensory applications. A series of microporous interpenetrated 3D primitive cubic MOFs with general formula {[M(CO2)2R]2(L)}n (M2+ = Cu, Ni, Co, Zn, R(CO2)2 = dicarboxylate and L = organic pillar linker) based on mixed linkers have been reported. Here paddle-wheel M2(CO2)4 cores are connected by the dicarboxylate linkers to form a 2D grid which is further extended to 3D by different organic pillars. Based on different aliphatic or aromatic dicarboxylates and varieties of pillar linkers with different functionalities several 3D interpenetrated frameworks have been reported by groups of Zaworotko, Kitagawa, Chen, Kim, Hupp et al. and also by Maji group [21, 22, 23, 24]. Two such Cu(II) based frameworks, [Cu3(bipy)1.5(2,6-ndc)3]n and {[Cu(bpe)0.5(2,6ndc)].0.5H2O}n were reported by our group [23]. The 2D grids are formed by the linking of paddle-wheel units Cu2(COO)4 through 2,6-ndc. Two different pillars, bipy and bpe connect the grids to form two different 3D three-fold interpenetrated frameworks. Further the nature of the pillar also affects the framework properties. Bipy pillared framework was found to be rigid but bpe which is considered to be flexible due to the ethane linkage imposes flexibility into the framework. The Langmuir surface area was 337 m2/g for flexible framework, found to be more compared to rigid bipy framework (113 m2/g). Both the frameworks showed appreciable amount of CO2 and H2 uptake at 195 and 77 K, respectively. Most importantly, the density of H2 in the rigid framework (0.0801 g/cc) was found to be higher than liquid hydrogen (0.0708 g/cc) which is due to the strong confinement of H2 in small pores in the interpenetrated framework.

5.4 Control over Interpenetration Control over interpenetration is of paramount importance for synthesizing MOFs with large surface area. Several elegant strategies like rational design of organic linkers, use of large template or solvent molecules, liquid phase epitaxy etc have been reported to fabricate MOFs without catenation. Interpenetration in 3D frameworks based on mixed linkers can be controlled judiciously by design of the linker length, size and functionalities. Kim et al. shows excellent examples where the pore sizes are rationally tuned in [Zn2(1,4bdc)2(dabco)], [Zn2(1,4- bdc)(tmbdc) (dabco)], [Zn2(tmbdc)2(dabco)], [Zn2(1,4ndc)2(dabco)], [Zn2(tfbdc)2(dabco)], and [Zn2(tmbdc)2(bpy)] (dabco = 1,4diazabicyclo[2.2.2]octane, tmbdc = tetramethylterepthalate; tfbdc = tetrafluoroterepthalate) by controlling the framework interpenetration through the linker modulation [9g]. All these frameworks were non-interpenetrated with α-Po type 3D pillared layer nets and showed good amount of H2 uptake properties. But increasing the pillar length in [Zn2(1,4-bdc)2(bpy)] and [Zn2(2,6-ndc)2(bpy)] resulted two and three-fold interpenetration, respectively. Similarly the two-fold interpenetration in [Zn2(bdc)2(dpNDI)] can also be controlled by increasing the size of the linker and this has been achieved by changing the bdc by 9,10-anthracene dicarboxylic acid (adc) in {[Zn2(adc)2(dpNDI)].guest} which is noninterpenetrated α-Po type 3D pillared layer network. Similarly, {[Zn2(adc)2(dabco)](DMF)3.6(MeOH)1.8(H2O)1.8}n is a noninterpenetrating framework having Zn2(COO)4 paddle-wheel and adc in the 2D plane to form square net and these nets are pillared by dabco [25]. Using the same pillar dabco which is also bulky in size Kim et al. reported a noninterpenetrating {[Zn2(bdc)2(dabco)] (DMF)4(H2O)0.5}n framework [26]. Here the dabco must be driving force which does not allow the secondary net to grow. The 2D square grid formed by Zn2(COO)4 paddle-wheel and bdc is distorted and the framework shows flexible behaviour. This flexible nature was utilized by Kitagawa et al. for CO2 sensing purpose by incorporation of a emissive guest in the nano channels [22b]. From the above mentioned examples it is clear that to achieve noninterpenetrated structures the size and shape of the linkers play the vital role [27]. The design of pore properties utilizing flexible motifs and functional groups is of importance to obtain porous coordination polymers with desirable functions. We have prepared a 3D pillared-layer coordination polymer, {[Cd2(pzdc)2L(H2O)2].5(H2O).(CH3CH2OH)}n (1, H2pzdc) 2.3-pyrazinedicarboxylic acid; L) 2,5-bis(2-hydroxyethoxy)-1.4-bis(4pyridyl)benzene) showing (i) a rotatable pillar bearing ethylene glycol side chains acting as a molecular gate with locking/unlocking interactions triggered by guest inclusion between the side chains, (ii) framework flexibility with slippage of the layers, and (iii) coordinatively unsaturated metal centers as guest accessible sites through the removal of the water coligands. The framework clearly shows reversible single-crystal-to-single-crystal transformations in response to the removal and rebinding of guest molecules, the observation of these processes has provided fundamental clues to the understanding of the sorption profiles. The X-ray structures indicate that the 3D host framework is retained during the

transformations, involving mainly rotation of the pillars and slippage of the layers. The structure of dried form 2, [Cd2(pzdc)2L]n, has no void volume and no water coligands. Interestingly, the adsorption isotherm of water for 2 at 298 K exhibits three distinct steps coinciding with the framework functions. Compound 2 favors the uptake of CO2 (195 K) over N2 (77 K) and O2 (77 K). Above all, they report on a molecular gate with a rotational module exhibiting a locking/unlocking system which accounts for gate-opening type sorption profiles. These dynamic responses either showing expansion or shrinkage of the pores are allowed by various structural motifs which fall into three main categories: (i) mechanical motifs such as interpenetration [28, 29]; and stacking [30-34]; (ii) chemical motifs requiring the addition, reorientation, or cleavage of a coordination bond [35-38]; and (iii) rotational modules involving bridging ligands containing parts that can undergo free rotation [39, 40]. While the first two entail a complete overhaul of the frameworks and sometimes a change in dimensionality to display a change in pore geometry and volume, the latter only involves the rotation of a restricted part of the bridging ligands. In this regard, rotational modules are intrinsically different from the others as only a subtle change in the orientation of a part of the ligand can trigger drastic changes in the characteristics of the pores, not only in geometry and volume, but also in terms of accessibility; that is, rotational modules can act as local molecular-gates for guest inclusion. Reports of such materials are still scarce, whereas the others are well-documented [41, 42]. They designed a 4-pyridine terminated rodlike ligand containing ethylene glycol side chains (L) as pillar (Scheme 5.4) that plays important roles in the resulting framework. First, the pore properties can be tuned via rotation of L, thereby inducing gate-opening type adsorption. Second, the guest-free framework can be stabilized by allowing ethylene glycol side chains to occupy the empty pore as guest molecules. The paired pillars (L) in 1 are stabilized by aromatic interactions (distance from edge of the pyridine ring to center of the neighboring pyridine ring) 3.44-3.76 Å) as well as hydrogen bonding (C · · ·O) 2.76-3.23 Å) among the ethylene glycol side chains. The ethylene glycol side chains are oriented on the crystallographic ac plane without severe disorder to form two types of channels about 10.12 × 6.01 Å2 along the a axis and channels of 2.83 × 1.62 Å2 along the b axis (the channel size is measured by considering van der Waals radii for constituting atoms.).

Scheme 5.4 2,5-Bis(2-hydroxyethoxy)-1,4-bis(4-pyridyl)benzene (L). Finally, the making of a molecular gate with a rotational module exhibiting a locking/unlocking system can be regarded as an approach for the generation of previously undeveloped advanced porous materials (Figure 5.1) [43].

Figure 5.1 Design of pore space via the introduction of a rotational module as a molecular gate with locking/unlocking interactions triggered by guest inclusion.

5.5 Conclusion The effects of aromatic and aliphatic spacers in pillar-layered MOFs assembly have been widely investigated in this part. Applying aromatic spacers in pillar backbone can influence frameworks properties and application which lead researchers toward designing potential architecture. Aliphatic spacers such as ethyl or propyl in bpe and bpp or side alkyl chains induced flexibility into pillar linkers which can be useful. Also aliphatic alkyls can be used as functional groups to modify pore size and shape as well as MOF chemical comportment, like hydrophobicity. Aliphatic dicarboxylate like fumarate, malate, glutarate, succinate, adipate have been used in the construction of MOFs along with second organic spacers. Such 3D mixed linker frameworks show different type of network topologies based on the binding modes of the aliphatic carboxylatesAromatic dicarboxylate linkers like Pyrazine-2,3dicarboxylate, Pyridine-2,3/2,4/2,5-dicarboxylate in general construct a dense 2D layers which does not allow the growth of another net and hence a non-interpenetrated structure is obtained.

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material. Science, 298, p.1762-1765, 2002. 29. Maji, T.K., Matsuda, R., Kitagawa, S., A flexible interpenetrating coordination framework with a bimodal porous functionality. Nat. Mater., 6, p.142-148, 2007. 30. Biradha, K., Hongo, Y., Fujita, M., Crystal-to-Crystal Sliding of 2D Coordination Layers Triggered by Guest Exchange. Angew. Chem., Int. Ed., 41, p. 3395-3398, 2002. 31. Navarro, J.A.R. et al., Guest-Induced Modification of a Magnetically Active Ultramicroporous, Gismondine-like, Copper(II) Coordination Network. J. Am. Chem. Soc., 130, p.3978-3984, 2008. 32. Kitaura, R. et al., Porous coordination-polymer crystals with gated channels specific for supercritical gases. Angew. Chem., Int. Ed., 42, p.428-431, 2003. 33. Kondo, A. et al., Novel Expansion/Shrinkage Modulation of 2D Layered MOF Triggered by Clathrate Formation with CO2 Molecules. Nano Lett., 6, p.2581-2584, 2006. 34. Maji, T.K. et al., Expanding and Shrinking Porous Modulation Based on Pillared-Layer Coordination Polymers Showing Selective Guest Adsorption. Angew. Chem., Int. Ed., 43, p.3269-3272, 2004. 35. Bradshaw, D., Warren, J.E., Rosseinsky, M.J., Reversible Concerted Ligand Substitution at Alternating Metal Sites in an Extended Solid. Science, 315, p.977-980, 2007. 36. Matsuda, R. et al., Guest Shape-Responsive Fitting of Porous Coordination Polymer with Shrinkable Framework. J. Am. Chem. Soc., 126, p.14063-14070, 2004. 37. Ghosh, S.K., Zhang, J.-P., Kitagawa, S., Reversible Topochemical Transformation of a Soft Crystal of a Coordination Polymer. Angew. Chem., Int. Ed., 46, p.7965-7968, 2007. 38. Kaneko, W., Ohba, M., Kitagawa, S., A Flexible Coordination Polymer Crystal Providing Reversible Structural and Magnetic Conversions. J. Am. Chem. Soc., 129, p.13706-13712, 2007. 39. Horike, S. et al., Dynamic Motion of Building Blocks in Porous Coordination Polymers. Angew. Chem., Int. Ed., 45, p.7226-7230, 2006. 40. Lee, E.Y., Jang, S.Y., Suh, M.P., Multifunctionality and Crystal Dynamics of a Highly Stable, Porous Metal–Organic Framework [Zn4O(NTB)2]. J. Am. Chem. Soc., 127, p.63746381, 2005. 41. Kitagawa, S. and Matsuda, R., Chemistry of coordination space of porous coordination polymers. Coord. Chem. Rev., 251, p.2490-2509, 2007. 42. Férey, G. and Serre, C., Large breathing effects in three-dimensional porous hybrid matter: facts, analyses, rules and consequences. Chem. Soc. Rev., 38, p.1380-1399, 2009. 43. Seo, J. et al., A Pillared-Layer Coordination Polymer with a Rotatable Pillar Acting as a Molecular Gate for Guest Molecules. J. Am. Chem. Soc., 131, p.12792-12800, 2009.

Chapter 6 Introduction to O-Donor Pillars 6.1 Introduction Along with Nitrogen donor pillars, reports on oxygen donor pillars show growing trend. These oxygen donating pillars can bridge 2D sheets which are formed by connection of metal nodes with whether oxygen-donor or nitrogen-donor as main ligands. Table 6.1 summarizes the reported oxygen donor ligands presented in layer motifs of pillar layered MOFs. Most of these linkers had two coordinating carboxylate groups in their backbone, which formed 2D layers. Multi-donor linkers such as tri, tetra, panta, hexa and octatopics, are also widely used in pillar-layered MOFs. Table 6.1 O-donor-based ligands as layers for MOFs. Ligands The names of ligands

The abbreviated ligands

O1

oxalic acid

H2OX

O2

Malonic acid

H2mal

O3

Malic acid

H2ma

O4

succinic acid

H2suc, H2SA

O5

fumaric acid

FMA, H2fum

O6

glutaric acid

H2glu

O7

R-2-methylglutarate

R-GLA-Me

Structure of ligands

O8

adipic acid

H2adi, H2adip

O9

muconic acid

H2-muco

O10

Aspartic acid

Asp

O11

L-cysteic acid

H2cys

O12

pimelic acid

H2pim

O13

Tetrafluorosuccinate

Tfsuc

O14

suberic acid

H2sba

O15

azelaic acid

H2aza

O16

sebacylic acid

H2sea

O17

N-carbamyl-L-glutamate

NCG

O18

N-benzoyl-Lglutamate

NBzG

O19

phosphonoacetic acid

H3PPat

O20

tricarballylic acid

H3tca

O21

1,2,3,4-butanetetracarboxylic acid H4btca

O22

terephthalic acid, 1,4benzenedicarboxylic acid

O23

2-minoterephthalic acid,2-amino- 1,4-bdc-NH2, 1,4-benzenedicarboxylic acid NH2-BDC, H2atpa

O24

Tetramethylterephthalate, 2,3,5,6- Tmbdc tetramethyl-1,4Benzenedicarboxylic acid

O25

Functionalized bdc

O26

Tetrafluoroterephthalate, Tfbdc, tftpa tetrafluorobenzene-1,4dicarboxylate, 2,3,5,6- tetrafluoro1,4-benzenedicarboxylic acid

O27

2-monomethyl 1,4benzenedicarboxylic acid

H2BDC, 1,4bdc, H2tp, tpa

fu-bdc

MMBDC

O28

2,5-dimethyl-1,4benzenedicarboxylic acid

DMBDC

O29

2-nitro-1,4benzenedicarboxylate,2nitroterephthalic acid

NO2-bdc

O30

2-hydroxy-1,4benzenedicarboxylic acid

BDC-OH

O31

2-bromo-1,4- benzenedicarboxylic BDC-Br acid

O32

2,5 dichloro-1,4benzenedicarboxylic acid

BDC-Cl2

O33

alkoxy functionalized bdc

-

O34

2,5-disulfonylterephthalate acid

H4DSBDC

O35

2,5-dihydroxyterephthalic acid

H4dht

O36

2,5-dibromo-1,4benzenedicarboxylate

DBrBDC

O37

2-sulfoterephthalate

Stp

O38

1,2,4-benzenetricarboxylic acid

H3bta

O39

2-(methoxycabonyl)terephthalic acid

H2mta

O40

3-(4-carboxyphenyl)propionic acid H2cpp

O41

1,4-phenylenediacetic acid

O42

trans-1,4-cydohexanedicarboxylic trans-H2chdc, acid CDC

1,4-pdaa

O43

4-carboxycinnamic acid

H2cca, CNC

O44

4-carboxyphenoxy acetic acid

H2cpoa

O45

(+)-camphoric acid, D-camphor acid

H2camph

O46

(1S,3R)-1,2,2-trimethyl-1,3cyclopentanedicarboxylic acid

-

O47

pyridine-2,5-dicarboxylic acid

2,5-pydc

O48

Bicycle(2,2,2)octane-1,4dicarboxylate

BODC

O49

o-,m-, and p-nitrobenzoates

-

O50

isonicotinic acid

Hin

O51

p-xylylenediphosphonic acid

-

O52

4-isobutyl-α-methylphenylacetic acid

Hibpa

O53

1,3-adamantanediacetic acid

H2ADA

O54

1,3-adamantanedicarboxylic acid

H2ADC

O55

isophthalic acid, 1,3benzenedicarboxylic Acid, mphthalic acid, benzene-1,3dicarboxylic acid

H2mbdc, H2ipa, 1,3-H2bdc, mH2BDC

O56

5-hydroxyisophthalic acid

5-OH-H2bdc, H2hip

O57

5-nitro-1,3-benzenedicarboxylate, NO2-bdc, H2nip 5-nitroisophthalic acid

O58

5-aminoisophthalic acid

H2AIP

O59

5-methyl-isophthalic acid

H2ipa-CH3, H2mip

O60

5-tert-butylisophthalic acid

H2tbip

O61

2-hydroxyisophthalic acid

ipO

O62

5-bromoisophthalate

5-Br-H2ip 4-Br-H2ip

O63

4-bromobromoisophthalic acid

O64

Tetrafluoroisophthalate

Tfipa

O65

2,2/-(1,3-phenylenedioxy) bis(acetate)

Pdoa

O66

5-Sulfoisophthalic Acid

H3SIP, H3sipa

O67

3,5-pyridinecarboxylate

3,5-PDC

O68

thiophene-2,5-dicarboxylic acid

H2tdc

O69

2,6-pyridine dicarboxylic acid

2,6-H2pydc

O70

pyridine-2,4-dicarboxylic acid

pydcH2

O71

1,3-phenylenediacetate

Mpda

O72

5-iodo-isophthalic acid

5-iipa

O73

Methoxyisophthalate

-

O74

furan-2,5-dicarboxylic acid

FDA

O75

2,5-bis(2-methoxyethoxy)-1,4benzene-dicarboxylate

BME-bdc

O76

3,5-diaminobenzoate

3,5-daba

O77

5-oxyacetate isophthalic acid

H3OAIP

O78

4-hydroxypyridine-2,6dicarboxylic acid

H3CAM

O79

benzene-1,2,4,5-tetracarboxylate, pyromellitic acid

Btec, H4pyro, H4pma

O80

1,3,5-benzenetricarboxylic acid, trimesic acid

H3BTC

O81

cis,cis-1,3,5cyclohexanetricarboxylic acid

H3CTC

bta3-

O82

benzene-1,3,5-triacetate

O83

5-(bromomethyl)-isophthalic acid Br-iptH2

O84

5-(hydroxymethyl)isophthalic acid OH-iptH2

O85

cyclohexane-1,2,4,5tetracarboxylic acid

-

O86

N-carboxymethyl-3, 5dicarboxylpyridinium bromide

H3CmdcpBr

O87

1,2,3-benzenetricarboxylic acid

H3bta

O88

5-nitro-1,2,3-benzenetricarboxylic H3nbta acid

O89

benzene-1,2,3-triyldioxy-triacetic acid

O90

5-aminodiacetic isophthalic acid

H4adip

O91

m-sulfophenylphosphonic acid

-

O92

1,3,5-Benzenetriphosphonic acid

H6BTP

O93

cis,cis,cis,cis-1,2,3,4cyclopentaneteracarboxylic acid

-

O94

tris(2-carboxyethyl)isocyanurate

H3tci

O95

1,2,3,4,5-Benzenepentacarboxylic acid

O96

N,N′,N″tris(carboxymethyl)-1,3,5benzenetricarboxamide

TCMBT

O97

1,2,3,4-benzenetetracarboxylic acid

-

O98

1,2- benzenedicarboxylic acid

1,2-H2BDC

O99

1,2-phenylenediacetic acid, ophenylenediacetic acid

H2pdac, oH2pda, H2pda

O100

Bicycle(2.2.2)oct-7-ene-2,3,5,6tetracarboxylate

Ceta

O101

2,5-bis(2-methoxyethoxy)benzene dicarboxylic acid

O102

2,5-bis(3methoxypropoxy)benzene dicarboxylic acid

-

O103

2,5-bis(4-methoxybutoxy)benzene dicarboxylic acid

O104

2-(2-methoxyethoxy)benzene dicarboxylic acid

-

O105

5-ethyl-pyridine- 2,3-dicarboxylic H2epda acid

O106

pyrazine-2,3-dicarboxylate

Pzdc

O107

pyridine-2,3-dicarboxylate

Pyrdc, H2pydc

O108

2,5-dimethoxy-1,4benzenedicarboxylate

DMBDC

O109

imidazole-4,5-dicarboxylic acid,4,5-imidazole dicarboxylate

H3IDC, Himdc

O110

2-ethyl-1H-imidazole-4,5dicarboxylic acid

H3EIDC, H3eimda

O111

1H-2-methyl-4,5-imidazoleDicarboxylic acid

H3mimda

O112

1,3,5-triazine-2,4,6-triamine hexaacetic acid

H6TTHA

O113

phenylsuccinic acid

H2psa

O114

1,4,5,6,7,7-

HET

hexachlorobicyclo(2.2.1) hept-5ene-2,3- dicarboxylic acid

O115

2,6-naphthalenedicarboxylic acid

H2NDC

O116

4,4′-biphenyldicarboxylic acid

Bpdc

O117

2,2′-biphenyldicarboxylic acid

-

O118

1,1/-biphenyl- 2,2/,6,6/tetracarboxylic acid

H4bpta

O119

biphenylethene-4/,4-dicarboxylic acid

H2bpea

O120

2,2′-bipyridine-3,3′-dicarboxylate- H2bpdado 1,1′-dioxide

O121

1,4-naphthalenedicarboxylate

1,4-ndc

O122

biphenyl-2,4,6,3′,5′pentacarboxylic acid

H5bpbc

O123

2-nitrobiphenyl- 4,4′-dicarboxylic H2nbpdc acid

O124

biphenyl-2,3/,3,4/-tetracarboxylic acid

m-H4bptc

O125

3,3/,4,4/-biphenyltetracarboxylic acid

H4bptc, H4odpa

O126

biphenyl-4,4/-diphosphonic acid

-

O127

2,6,2′,6′-tetranitro-biphenyl-4,4′dicarboxylic acid

-

O128

3,4′,5-biphenyltricarboxylic acid

H3bpta

O129

biphenyl- 2,4-dicarboxylate

H2bpdc

O130

biphenyl-3,4′-dicarboxylate

3,4′-bpdc

O131

biphenyl-3,3′,5-tricarboxylic acid -

O132

2-methyl-4,4/biphenyldicarboxylic acid

H2mbpdc

O133

3-(4-carboxyphenoxy)phthalic

-

acid

O134

2, 2′-dinitrobiphenyl-4,4′dicarboxylate

Dnpdc

O135

3,3′,5,5′-biphenyltetracarboxylic acid

H4bpt

O136

5-(pyridine-4-yl)- isophthalic acid H2pyip

O137

4-(4-pyridyl) benzoic acid

HPBA

O138

1H-benzimidazole-5,6dicarboxylic acid

H3bidc

O139

3-(4-hydroxyl pyridinium-1-yl) phthalic acid

H3DPPA

O140

2-(p-bromophenyl)-1H-imidazole- p-BrPhH3IDC 4,5-dicarboxylic acid

O141

5-(6-Carboxypyridin-3yl)isophthalic acid

-

O142

benzotriazole-5-carboxylate acid

H2btca

O143

5-(4/-carboxylphenyl) nicotinic

H2CPNA

acid

O144

3-(2′,5′-dicarboxylphenyl)pyridine H2dcpy acid

O145

4-carboxy-1-(3,5dicarboxybenzyl)-pyridinium chloride

-

O146

4-(5-carboxypyridin-2yl)isophthalic acid

-

O147

4,4′(hexa-fluoroisopropylidene) bis-(benzoicacid)

H2hfipbb

O148

4,4′-oxybis(benzoic acid)

H2oba

O149

4,4/-oxidiphthalic acid

O150

5-(4-carboxyphenoxy)-isophthalic H3cpia acid

O151

5-(4-carboxy-2- nitrophenoxy) isophthalic acid

H4ODPT

Hcnp-H2ipa

O152

6-(3′,4′- dicarboxylphenoxy)isophthalic acid

-

O153

4-(4-carboxyphenoxy)phthalate acid

H3cpop

O154

4-(3′,5′-icarboxylphenoxy)phthalic acid

O155

3-(3′,5′-icarboxylphenoxy)phthalic acid

O156

3-(2-carboxyphenoxy)phthalic acid

-

O157

4-(2-carboxyphenoxy)phthalic acid

-

O158

3-(4-carboxyphenoxy)phthalic acid

-

O159

3-(2′,3′-dicarboxylphenoxy)benzonic acid

H3dpob

O160

2-(2′,4′dicarboxylphenoxy)benzoic acid

-

O161

(R)-4-(4-(1carboxyethoxy)phenoxy)-3fluorobenzoic acid

H2cpfa

O162

5-(4p-cbiaH3 carboxybenzyloxy)isophthalic acid

O163

3-pyridin-3-yloxy)benzene-1,2dicarboxylic acid

H2pbda

O164

5-(pyridin-2-ylmethoxy)isophthalic acid

-

O165

4,4′-dicarboxydiphenylamine

-

O166

2-(4-carboxybenzylamino) benzoic acid

-

O167

azobenzene- 4,4′-dicarboxylica acid, 4,4′-(diazene-1,2diyl)dibenzoic acid

4,4’-ADB, AzDC

O168

3,3′,5,5′azobenzenetetracarboxylic acid

H4abtc

O169

2-(phenyldiazenyl)terephthalate

-

O170

2,2′-azodibenzoic acid

-

O171

4-(4-carboxybenzamido)benzoic acid

-

O172

nicotinate 6,6/-dithiodinicotinic acid

H2cpds

O173

2,2/- -5,5/-dicarboxylic acid

H2btdc

O174

P,P/-diphenyl-diphosphinate

Pcp

O175

dimethylthio-tetrathiafulvalenebicarboxylate

-

O176

bis(4-nitrophenyl)phosphoric acid -

O177

2,2′-phosphinico-dibenzoic acid

-

O178

4,4′-sulfonyldibenzoic acid

H2sdba

O179

3,3/,4,4/diphenylsulfonetetracarboxylate

Dstc

O180

sulfone-4,4/-biphenyldicarboxylate H2SDBA

O181

3,3′,4,4′diphenylsulfonetetracarboxylic dianhydride

H4dpstc

O182

4,4′Ubl (carbonylbis(azanediyl))dibenzoic acid

O183

5,5′-(carbonylbis(azanediyl)) diisophthalic acid

-

O184

5-(2-carboxypyrrolidine-1carbonyl) isophthalic acid

H3PIA

O185

1,3-bis(4-carboxyphenoxy)propane

H2bcp

O186

5,5′-(hexane-1,6-diyl)-bis (oxy) diisophthalic acid

-

O187

benzophenone 4,4/-dicarboxylic acid

BPnDC

O188

3,3′,4,4′-benzophenone tetracarboxylate

BPTC

O189

9,9-dimethylfluorene-2,7dicarboxylate

Mfd

O190

9,9-dipropylfluorene-2,7dicarboxylate

DFD

O191

1,4-bis(4- oxy-1,2-benzene dicarboxylic acid)benzene

H4obda

O192

9-fluorenone-2,7-dicarboxylic acid H2FDC

O193

3,5-bis(3-carboxyphenyl)pyridine H2bcpb

O194

3-carboxyl-phenyl-(4-(2′carboxyl-phenyl)-benzyl) ether

-

O195

1,1’:3’,1”-Terphenyl-4,4’,4”,6’tetracarboxylic acid

-

O196

1,1′:3′,1″-terphenyl-4,4″,5′tricarboxylic acid

-

O197

2-amino-(1,1:3,1-terphenyl)-4,4,5- H3ATTCA tricarboxylic acid

O198

4,5-di(4′-carboxylphenyl)phthalic H4dcpp acid

O199

anthrancene-9,10-dicarboxylic acid

H2adc

O200

5,5′-(p-xylylenediamino)-1,1′, 3,3′-(benzenetetra-carboxylic acid)

O201

3,5-bi(4-carboxyphenoxy)-benzoic H3BCPBA acid

O202

4,4′-((1,2phenylenebis(methylene)) bis(oxy))dibenzoic acid

-

O203

5,5′H4pbta phenylenebis(methylene)-1,1′-3,3′(benzene-tetracarboxylic acid)

O204

3,5-bi(4-carboxy-phenoxy)benzoic acid

H3BCPBA

O205

anthracene-1,5-dicarboxylic acid

Adb

O206

terphenyl-3,2//,5//,3/tetracarboxylic acid

H4tptc

O207

2,3,6,7-anthracenetetracarboxylic H4ata acid

O208

2,2/,3,3/-oxydiphthalic dianhydride 2,2/,3,3/-odpda

O209

2,2/,3,3/-thiodiphthalic dianhydride

2,2/,3,3/-tdpda

O210

3,4-bi(4-carboxyphenyl)- benzoic acid

O211

3,4-di(3,5-dicarboxyl phenyl) phthalic acid

H6dccpa

O212

N-(3-propanoic acid)-1,8naphthalimide

-

O213

N-(4-butanoic acid)-1,8naphthalimide

-

O214

p-terphenyl-4,4″-dicarboxylic acid, 2′,5′-dimethyl

-

O215

4,4′-(4,4′-bipyridine-2,6-diyl) dibenzoic acid

bpydbH2

O216

4,4′-((4-(pyridin-4yl)phenyl)azanediyl) dibenzoic acid

-

O217

1,2,4,5-tetrakis(4-carboxyphenyl)- TCPB benzene

O218

1,4-dibromo-2,3,5,6-tetrakis (4carboxyphenyl)benzene



O219

tetrakis(4carboxyphenyl)porphyrin

TCPP

O220

1,3-bis(3,5-dicarboxyphenyl) imidazolium

-

O221

9,10-anthracene dibenzoate

-

O222

Tetrakis)4carboxyphenyl)oxamethyl) methane

-

O223

tetrakis[(3,5dicarboxyphenoxy)methyl

-

O224

benzene-1,3,5-tribenzoate

Btb

O225

pamoic acid

H2PA

O226

4,4’,4”-(2,4,6-trimethylbenzene1,3,5-triyl)tribenzoiccid

H3TMTA

O227

4,4′,4″-(1,3,5-benzenetriyltris H3bcta (carbonylimino))trisbenzoate acid

O228

tris(4′-carboxybiphenyl)amine

-

O229

2,6-dicarboxyl-1,3,5,7tetramethyl-8-phenyl-4,4difluoroboradiazaindacene

-

O230

(R)- 1,1/-binaphthyl-2,2/dihydroxy-5,5/-dicarboxylic acid

H2BDA

O231

triptycenedicarboxylic acid

H2TDC

O232

4,4′,4″,4′″-(1,3phenylenebis(azanetriyl) tetrabenzoate)

Mpbatb

Utilizing Tipb as a three dentate imidazole-containing ligand, led to formation of 63 layers which are further connected by versatile carboxylate pillars as stated in Figure 6.1 [1]. Detailed study on porosity of resulted frameworks revealed the direct effect of size, geometry, and functional groups of the applied pillars. By enlarging the length of carboxylate linker the degree of interpenetration is increased and results in distinctive decreasing of available voids. The overall topology and adsorption properties were also affected through increasing pillar length [1].

Figure 6.1 (a) The 63 layer formed by tipb ligands and nickel atoms. (b) The (3,5)-connected 2-fold interpenetrating gra topology of 1 and 2. (c) The (3,5)-connected 4-fold interpenetrating hms topology of 3 and 4. Another related research is rebuilt the effect of o-donor pillar length and functionality on sorption of six different gases. Building blocks of resulted MOFs are demonstrated in Figure 6.2. Selective CO2/CH4 and C2 Hydrocarbons/CH4 adsorption in these materials is obtained through contemporary use of pore size and functionality [2]. Also the adsorption profile of three MOFs were compared through tuning the number of connected amino groups on 1,2,4triazolate together with pillar modification [3]. The effective pore volume used for CO2 adsorption is highest in (Zn2(atz)2(oba)) which is attributed to longer v-shaped pillar and well oriented amino group.

Figure 6.2 ((Zn2(TRZ)2)8(dicarboxylate)4) building blocks in MOFs 1–7. 1,2,4-triazole together with ATPA (2-aminoterephthalic acid) pillar construct a three dimensional mixed ligand MOF with amino functionalized pores which show the highest enthalpy for CO2 and also greatest selective adsorption of CO2/N2 compared with the reported pillar layer porous materials [4]. 4-(1H-pyrazol-4-yl)pyridine as a triazole type ligand can also form 2-D structures with Zn cores which further was extended by carboxylate pillar to form novel 3-D structure with high methane storage capacity [5]. Applying two carboxylate ligands is a novel promising method in pillar-layered MOF assembly which fabricates stable structures with designable topologies and tunable properties. The impressions of each spacer group on general structure are variable so that slight conversion in special pillar segment can change whole chemistry of structure. Although, general outcome of this part demonstrated that some spacers like imidazole, triazole or aromatic groups led to higher stability or increasing performance of the structure owing to supramolecular interactions such as C-H…π, π…π stacking and hydrogen bondings. Whereas other functional groups such as Amid, Amin and Imine affect the surface chemistry through their polarity and forming H-bondings. Also presence of nonpolar functional groups as alkyls effect the framework performance by electron donating or hydrophobicity tendency and changing the size of the pores can be done by applying bulkier functional groups which can optimize guest-host interactions. Although the methodology to tune the pores within porous MOFs has been conceptually established, subtle control of pore sizes has been rarely fulfilled. This is mainly because there will typically be a big pore expansion when one shorter organic linker has been replaced by another longer organic linker in the non-interpenetrated reticular MOFs. In this regard, framework interpenetration can not only enforce the framework and thus the pore structure,

but can also deliberately control the pore size/curvature. Over the past several years, Chen’s group have successfully made use of paddle-wheel secondary building units (SBUs) [M2(COO)4] (M= Zn2+, Cu2+, Co2+, and Ni2+), bicarboxylate linker R-(COO)2, and bidentate pillar linker L to assemble doubly and triply interpenetrated MOFs [M2(COO)2)2(L).Gx] (Scheme 6.1a, G=guest molecules) of primitive cubic nets for their selective gas sorption [6]. With this in mind, They speculated that such primitive cubic nets can be also fulfilled by the incorporation of pyridylcarboxylate linker (K-COO) and bicarboxylate linker R(COO)2 with paddle-wheel [M2 (COO)4] (M=Zn2+, Cu2+, Co2+, and Ni2+) into [M2(K(COO))2(R(COO)2).Gx] (Scheme 6.1b, G=guest molecules). Although, this approach was used to construct cubic nets, the frameworks obtained were condensed because of a shorter pyridylcarboxylate linker [7].

Scheme 6.1 Comparison of two approaches to construct microporous MOFs of primitive cubic nets. A new approach has been realized to construct a three-dimensional doubly interpenetrated cubic metal–organic framework Zn2 (PBA)2 (BDC).(DMF)3 (H2O)4 (UTSA-36, HPBA=4(4-pyridyl) benzoic acid, H2BDC=1,4- benzenedicarboxylic acid) through the self-assembly of the pyridylcarboxylate linker 4-(4-pyridyl) benzoate and bicarboxylate linker 1,4benzenedicarxylate with paddle-wheel [Zn2(COO)4]. The activated UTSA-36 a exhibits highly selective gas sorption of C2H6, C2H4 and C2H2 over CH4 with the Henry law’s selectivities of 11 to 25 in the temperature range of 273 to 296 K attributed to the unique 3D intersected pore structure of about 3.1 to 4.8 Å within the framework, indicating that UTSA36a is a potentially very useful and promising microporous material for such industrially important separation of C2 hydrocarbons over methane [8]. Organic ligands are often carboxylates or N-donor ligands, such as amines or pyridines, which contain a number of different connection modes. One of the most popular choices of

the O-donor ligand is 1,4-benzenedicarboxylic acid (BDC) because its dicarboxyl groups manifest various bonding modes and its phenyl ring provides structural rigidity [9-15]. In some cases of MOFs, more than one type of ligand is applied to build a framework to expand the structural diversity as well as to enhance physical properties. In the recent work by Kim and co-workers, a number of Zn-based MOFs have been prepared using both aromatic dicarboxylates and diamines as ligands. These frameworks have been also studied for their gas sorption properties. In an effort to discover new materials with interesting threedimensional topologies and physical properties, they have synthesized new MOFs using Zn2+ ion as a metal center and 1,2,4-triazole and BDC as ligands under hydrothermal conditions. In this report, we present the synthesis, thermogravimetric analysis (TGA), and structure determination of three novel Zn-triazole-BDC frameworks, namely, Zn5(H2O)2(C2H2N3)4(C8H4O4)3.9H2O, Zn2(C2H2N3)2(C2H3N3)(C8H4O4).2H2O, and Zn4(H2O)2(C2H2N3)4(C8H4O4)2.14H2O. These materials represent new MOF structure types which are built from isolated metal polyhedra with two different organic linkers. Structure 1 containing tetrahedra and octahedra, while structures 2 and 3 show additional five-coordinated trigonal bipyramidal geometry. All structures contain eight membered rings built from Zn polyhedra and triazole. These rings are subsequently connected by BDC [16]. Many similar three-dimensional (3D) Zn-triazolate-dicarboxylate pillared-layered frameworks have been investigated. For example, we got a series of non-porous structures with the Zn(atz) layers pillared by long and flexible aliphatic dicarboxylates, while a microporous material was obtained with the very short carbonate [17]. Li et al. synthesized three porous frameworks with different pore sizes and shapes, by using linear aromatic dicarboxylates to connect Zn-triazolate layers, [18] while Gao et al. used sulfate and 3,5diamino-1,2,4-triazolate (datz–) to construct a porous framework [19]. However, CO2 adsorption was not investigated for these compounds, probably because of their very small pore sizes and/or lack of the amino group. Three Zn-triazolate-dicarboxylate frameworks, namely [Zn2(atz)2(ipa)] (1, Hatz = 3-amino-1,2,4-triazole, H2ipa = isophthalic acid), [Zn2(datz)2 (ipa)] (2, Hdatz = 3,5-diamino-1,2,4-triazole), and [Zn2(atz)2 (oba)] (3, H2oba = 4,4’-oxobisbenzoic acid), have been synthesized by solvothermal reactions. Crystal-structure analyses demonstrated that 1-3 are isoreticular three-dimensional porous structures consisting of Zn-triazolate layers and dicarboxylate pillars. The major differences of three compounds are the lengths and orientations of the pillars. Thermogravimetry analyses and powder X-ray diffraction measurements showed that 1-3 have high thermal stability and good water stability. The adsorption properties of 1-3 were investigated by N2, CO2 and CH4 sorption experiments. Because 3 have exposed amino groups and distinct pore shape, it has the highest CO2 uptake and CO2/N2 and CO2/CH4 selectivities at 273 K [20]. In other work, selection of 3-(4-hydroxyl pyridinium-1-yl) phthalic acid (H3DPPA) ligand as the bifunctional linker to synthesize lanthanide MOFs was based on the following considerations: (i) it lacks of symmetry in the positioning of carboxylate groups, which

benefit for structural tenability. Two adjacent carboxyl groups in benzene ring avoid the steric hindrance; (ii) owning a rigid aromatic ring and a flexible –COOH spacer, and having three abstractable hydrogen atoms, it is a good bridging ligand for constructing variable configurations; [21] (iii) the cooperativity of pyridine and phthalate functions may afford the formation of MOFs [22]. Meanwhile, oxalate ligand plays an important role in coordination chemistry, which may adopt remarkable versatile binding modes or configurations, and can facilitate the formation of extended structures by connecting metal centers [23]. Whereas, lanthanide compounds containing both aromatic multi-donor and rigid oxalate as the auxiliary ligand have been seldom documented [24]. As a continuation of previous investigations and in order to better understand the influence exerted by the second ligand on structures and the properties in these systems, here, has been presented the syntheses, structures, photoluminescence and magnetic properties of four new lanthanide–organic polymers based on the H3DPPA ligand. Four new lanthanide coordination polymers with three dimensional (3D) frameworks, namely, {[Ln2(DPPA)2 (µ2-C2O4)(H2O)2]·2H2O}n (Ln = Nd (1), Sm (2)) and [Ln2(HDPPA)2(DPPA)2]n (Ln = Tb (3), Yb (4)), (H3DPPA = 3-(4-hydroxyl pyridinium-1-yl) phthalic acid and H2C2O4 = oxalic acid) has been synthesized by Feng and et al. this investigation represents the first example of a series of new lanthanide–organic frameworks containingboth the aromatic multi-dentate HDPPA/DPPA and oxalate linkers. Introduction of the rigid oxalate auxiliary ligand does not alter the crystal system or multi-dimension, but results in different topology networks. Compound 3 displays the strong characteristic emission in the visible region by removing O–H variation bonding to Ln(III) ion. Primary magnetic studies indicate that their magnetic properties are different, though the lanthanide magnetic centers mediated through the same carboxylato bridging. The depopulation of the Stark levels orpossibly antiferromagnetic coupling dominates the magnetic properties in Sm(III), Nd(III) and Tb(III) compounds. Two series of compounds also displayed differences in thermal stability property [25].

6.2 Conclusion The impressions of each spacer group on general structure are variable so that slight conversion in special pillar segment can change whole chemistry of structure. Along with Nitrogen donor pillars, reports on oxygen donor pillars show growing trend [26]. These oxygen donating pillars can bridge 2D sheets which are formed by connection of metal nodes with whether oxygen-donor or nitrogen-donor as main ligands.

References 1. Chen, Q. et al., Pillared Metal–Organic Frameworks Based on 63 Layers: Structure Modulation and Sorption Properties. Cryst. Growth Des., 14(10), p.5189-5195, 2014. 2. Zhai, Q.-G. et al., Design of Pore Size and Functionality in Pillar-Layered Zn-Triazolate-

Dicarboxylate Frameworks and Their High CO2/CH4 and C2 Hydrocarbons/CH4 Selectivity. Inorg. Chem., 54(20), p.9862-9868, 2015. 3. Chen, K.-J. et al., New Zn-Aminotriazolate-Dicarboxylate Frameworks: Synthesis, Structures, and Adsorption Properties. Cryst. Growth Des., 13(5), p.2118-2123, 2013. 4. Yun, R. et al., Formation of a metal-organic framework with high gas uptakes based upon amino-decorated polyhedral cages. RSC Adv., 5(4), p.2374-2377, 2015. 5. Lin, J.-M. et al., A novel pillared-layer-type porous coordination polymer featuring threedimensional pore system and high methane storage capacity. Sci. China Chem., 59(8), p.970974, 2016. 6. (a) Chen, B. et al., A Microporous Metal–Organic Framework for Gas-Chromatographic Separation of Alkanes. Angew. Chem., 118, p.1418-1421, 2006; (b) Chen, B. et al., Hydrogen Adsorption in an Interpenetrated Dynamic Metal–Organic Framework. Inorg. Chem., 45, p.5718-5720, 2006; (c) Barcia, P.S. et al., Kinetic Separation of Hexane Isomers by FixedBed Adsorption with a Microporous Metal–Organic Framework. J. Phys. Chem. B, 111, p.3101-6103, 2007. 7. Zeng, M.-H. et al., Twofold interpenetrating zinc carboxylates framework with isonicotinic acid, and succinate or fumarate as coligands featuring new mixed carboxylate-bridged [Zn2(O2CR)4] dimer as subunits. J. Mol. Struct., 828, p.75-79, 2007. 8. Das, M.C. et al., A New Approach to Construct a Doubly Interpenetrated Microporous Metal–Organic Framework of Primitive Cubic Net for Highly Selective Sorption of Small Hydrocarbon Molecules. Chem. Eur. J., 17, p.7817-7822, 2011. 9. Li, H. et al., Coordinatively Unsaturated Metal Centers in the Extended Porous Framework of Zn3(BDC)3·6CH3OH (BDC = 1,4-Benzenedicarboxylate). J. Am. Chem. Soc., 120, p.2186-2187, 1998. 10. Rossi, N. et al., Hydrogen storage in microporous metal-organic frameworks. Science, 300, p.1127-1129, 2003. 11. Serre, C. et al., Very Large Breathing Effect in the First Nanoporous Chromium(III)Based Solids: MIL-53 or CrIII(OH)·{O2C–C6H4–CO2}·{HO2C–C6H4–CO2H}x·H2Oy. J. Am. Chem. Soc., 124, p.13519-13526, 2002. 12. Loiseau, T. et al., Synthesis and structural characterization of a new open-framework zinc terephthalate Zn3(OH)2(bdc)2·2DEF, with infinite Zn–(µ3-OH)–Zn chains. J. Solid State Chem., 178, p.621-628, 2005. 13. Loiseau, T. et al., Hydrothermal synthesis and structural characterization of a gallium pyromellitate Ga(OH)(btec)·0.5H2O, with infinite Ga-(µ2-OH)-Ga chains (MIL-61). Solid State Sci., 7, p.603-609, 2005. 14. Choi, E.Y. et al., Benzene-Templated Hydrothermal Synthesis of Metal–Organic Frameworks with Selective Sorption Properties. Chem. Eur. J., 10, p.5535-5540, 2004.

15. Chun, H. et al., Synthesis, X-ray Crystal Structures, and Gas Sorption Properties of Pillared Square Grid Nets Based on Paddle-Wheel Motifs: Implications for Hydrogen Storage in Porous Materials. Chem. Eur. J., 11, p.3521-3529, 2005. 16. Park, H. et al., Hydrothermal Synthesis and Structural Characterization of Novel Zn– Triazole–Benzenedicarboxylate Frameworks. Chem. Mater., 18, p.525-531, 2006. 17. Lin, Y.-Y. et al., Pillaring Zn-Triazolate Layers with Flexible Aliphatic Dicarboxylates into Three-Dimensional Metal–Organic Frameworks. Cryst. Growth Des., 8, p.3673-3679, 2008. 18. Park, H. et al., Synthesis, Structure Determination, and Hydrogen Sorption Studies of New Metal–Organic Frameworks Using Triazole and Naphthalenedicarboxylic Acid. Chem. Mater., 19, p.1302-1308, 2007. 19. Zhang, Y.-L., Chen, S.-P., Gao, S.-L., Synthesis and Characterization of New M-Triazole Complexes (M = Co, Cu, Zn), Z. Anorg. Allg. Chem., 635, p.537-543, 2009. 20. Chen, K.-J. et al., New Zn-Aminotriazolate-Dicarboxylate Frameworks: Synthesis, Structures, and Adsorption Properties. Cryst. Growth Des., 13(5), p.2118-2123, 2013. 21. Yao, Y., Che, Y., Zheng, J., The Coordination Chemistry of Benzimidazole-5,6dicarboxylic Acid with Mn(II), Ni(II), and Ln(III) Complexes (Ln = Tb, Ho, Er, Lu). Cryst. Growth Des., 8, p.2299-2306, 2008. 22. Han, Y.F. et al., Syntheses, structures, photoluminescence, and magnetic properties of nanoporous 3D lanthanide coordination polymers with 4,4′-biphenyldicarboxylate ligand. CrystEngComm, 10, p.1237-1242, 2008. 23. Luo, F. et al., Ln2(C2O4)(O2CCH2OH)4 (Ln = Gd(III), Tb(III)): Unprecedented Interpenetrating 4,6-Connected Net Built on Unusual Rectangular and Distorted Hexagonal Lanthanide Nodes. Cryst. Growth Des., 8(10), p.3511-3513, 2008. 24. Sun, Y.Q., Zhang, J., Yang, G.Y., Two novel luminescent lanthanide sulfate–carboxylates with an unusual 2-D bamboo-raft-like structure based on the linkages of left- and righthanded helical tubes involving in situ decarboxylation. Chem. Commun., 0, p.1947-1949, 2006. 25. Feng, X. et al., A series of 3D lanthanide frameworks constructed from aromatic multicarboxylate ligand: Structural diversity, luminescence and magnetic properties. Dalton Trans., 42, p.10292-10303, 2013. 26. Zare Karizi, F., Joharian, M., Morsali, A., Pillar-layered MOFs: Functionality, Interpenetration, Felexibility and Applications. J. Mater. Chem. A, 6, p.19288-19329, 2018.

Chapter 7 Stability and Interpenetration in Pillar-Layer MOFs 7.1 Stability in Pillar-Layer MOFs Permanent porosity is considered as an important issue in MOF assembly. The framework stability must be well tuned in order to produce potentially useful MOFs. Materials must exhibit long-term stability under the operating conditions of one process. Stable frameworks can resist degradation under wild conditions such as heat, humid, acidic or basic solutions and guest elimination [1]. Improving thermal and chemical stability of MOFs has been studied widely and various investigations have been done in this area. Applying pillars or modifying pillar backbone like changing pillar rigidity, hydrophobicity or functional groups can affect framework stability. To be considered viable candidates for real world applications, materials must beyond possessing the intrinsic functionality needed to meet the performance requirements of the process. In addition, materials must also exhibit long-term stability under the operating conditions of the process. Depending on the application, the stability requirements will vary. For industrially relevant heterogeneous catalysis applications, high thermal stability may be required, whereas in applications requiring MOF powders to be processed into thin-films or pellets, high mechanical stability may be necessary. At a minimum, most adsorption applications require that the material maintain structural stability in the presence of air. Beyond this, more stringent chemical stability requirements may exist for processes such as postcombustion CO2 captureand air pollution control where stability in the presence high humidity and acidic or basic conditions may also be needed [2, 3]. Relative to inorganic porous solids such as activated carbons and zeolites, MOFs tend to have lower chemical stability due to their metal–ligand coordination bonds. However, in recent years, a growing number of MOFs with high-valence metal ions such as Fe3+, Zr4+, and Hf4+ that exhibit stability even in the presence of acidic and basic conditions have emerged [4]. Such metals allow high nuclearity among metal clusters along with strong electrostatic interactions in the metal–ligand bonds in order to form more hydrolytically stable coordination complexes. Here, has been tried to develop a more diverse array of design strategies that can be used to improve MOF chemical stability without altering the identity of the metal in the metal cluster. To achieve this, our approach was to explore how functional groups and steric factors within the pore space can be used to tune stability properties. Such understanding would open further possibilities for simultaneously adjusting the stability and adsorption characteristics of MOFs that are of interest for target applications via careful choice of appropriate pore functionality. Table 7.1 summarizes the composition and relative stability of the structures explored in our past studies. The two strategies has been exploited to increase the stability of the isostructural frameworks in Table 7.1 were to (i) incorporate bulky, nonpolar functional groups near the corner of each metal site in the pore space and (ii) introduce catenation, or interpenetration, within the frameworks. The structural

characteristics of the zinc-based MOFs listed in Table 7.1 are quite diverse. These frameworks vary primarily in the type (polar versus nonpolar) and number of functional groups grafted on the BDC ligand but also include instances where the number of rings on the layered or pillaring ligands are extended in either the axial or lateral direction. Within these structures, there are also frameworks that possess 50:50 mixtures of BDC with different mono-, di-, and tetramethyl functionalized BDC ligands. The stability of all frameworks in Table 7.1 were determined via a comparison of the BET areas derived from N2 adsorption at 77 K and the powder X-ray diffraction (PXRD) patterns obtained before and after sample exposure to 80% RH at 298 K in air. Table 7.1 Summary of the compositions and water stability characteristics for the some pillared structures. Structure Zn-DMOF

Composition Zn2(BDC)2(DABCO)

Stability at 80% RH Low

Zn-DMOF-Br

Zn2(BDC-Br)2(DABCO)

Low

Zn-DMOF-OH

Zn2(BDC-OH)2(DABCO)

Low

Zn-DMOF-NO2

Zn2(BDC-NO2)2(DABCO)

Low

Zn-DMOF-NH2

Zn2(BDC-NH2)2(DABCO)

Low

Zn-DMOF-MM1/2 Zn2(BDC-MM)(BDC) (DABCO) Low Zn-DMOF-MM

Zn2(BDC-MM)2(DABCO)

Low

Zn-DMOF-Cl2

Zn2(BDC-Cl2)2(DABCO)

Low

Zn-DMOF-DM1/2 Zn2(BDC-DM)(BDC) (DABCO) Low Zn-DMOF-DM

Zn2(BDC-DM)2(DABCO)

Low

Zn-DMOF-TF

Zn2(BDC-TF)2(DABCO)

Partial

Zn-DMOF-TM1/2 Zn2(BDC-TM)(BDC) (DABCO) Partial Zn-DMOF-NDC Zn2(NDC)2(DABCO)

High

Zn-DMOF-ADC Zn2(ADC)2(DABCO)

High

Zn-DMOF-TM

Zn2(BDC-TM)2(DABCO)

High

Co-DMOF-TM

Co2(BDC-TM)2(DABCO)

High

Cu-DMOF-TM

Cu2(BDC-TM)2(DABCO)

High

Ni-DMOF-TM

Ni2(BDC-TM)2(DABCO)

High

Zn-MOF-508

Zn2(BDC)2(BPY)

High

It is necessary that, at a very minimum, a combination of BET and PXRD pattern analysis

before and after water exposure be considered in making such stability characterizations in order to avoid incorrect conclusions regarding the sample’s stability after water exposure [4]. This is because PXRD only provides spatially averaged, long-range information about the crystalline of the framework and can thus neglect critical information evident from BET analysis regarding local pore collapse at the crystal’s surface that may render the internal surface area of the structure inaccessible. Beyond this, measuring the adsorption isotherm during water exposure is also a useful practice because it allows one to understand whether the stability differences between materials can be attributed to an increased resiliency for the structure toward water or if the stability is merely a consequence of an increased hydrophobicity that prevents water from adsorbing into the pores. Figure 7.1 shows the water adsorption isotherms as a function of relative humidity at 298 K for all structures in Table 7.1 that exhibit high stability under humid conditions. For the unstable materials, structural breakdown during water adsorption results in isotherms with saturation uptakes that are well below what is expected based on their available porosity. However, among stable structures, the water loadings more closely correlate with porosity and can also be used to understand the inherent hydrophobicity of the structure. For the highly stable Zn-DMOF-TM and ZnDMOF-ADC structures, the isotherms in Figure 7.1a show that the structures adsorb significant amounts of water (greater than all of the less stable Zn-DMOF frameworks on a mole per kilogram basis [5-7]) at their saturation loadings. This indicates that their improvement in stability relative to the less stable structures is not a result of excluding water from the pore space and instead arises from an increased resiliency within the structures toward water. Furthermore, because this high level of stability is only exhibited in structures possessing the BDC-TM and ADC ligands but not those structures with the BDC-DM or NDC ligands, we can also conclude that it is critically important that the nonpolar groups be present near the corners of each metal cluster in the structure for this high level of stability to be present.

Figure 7.1 Water adsorption isotherms as a function of relative humidity at 298 K in air for all structures in Table 1 that exhibit high stability under humid conditions. (a) Zinc-based MOFs with different ligands and (b) DMOF-TM variants with different metal identities. The other strategy that has been exploited for increasing structural stability is utilizing a pillaring ligand that enables framework catenation to occur during the synthesis process. In this case, replacing the DABCO ligand in Zn-DMOF with the longer BPY ligand results in the formation of an isostructural, 2-fold interpenetrated Zn-MOF-508 structure. Despite DABCO having a higher basicity, which one would expect to increase the strength of the resulting coordination bond in the structure, Zn-DMOF is the less stable variant and exhibits a near complete loss of its surface area after exposure to 80% RH at 298 K. On the other hand, Zn-MOF-508 retains its complete surface area under these same conditions. However, the origin of this structure’s greater water stability is not as clear as in the former cases because of its much lower water uptake during adsorption. As a result, one cannot determine whether its stability improvement is due to catenation bringing an increased water resiliency to the structure or instead is simply a result of its lower water uptake [8]. Introducing pillars can develop the thermal stability of MOFs by linking 2D sheets and producing 3D frameworks [9, 10-12]. Applying Bipy as a pillar together with Cd nodes and

5-(pyridin-2-ylmethoxy)-iso phthalic acid (L) led to formation of new 3-D pillar layered framework with higher thermal stability in comparison to 2-D layered (Cd2(L)2)n·n(H2O) [9]. Modifying pillars for higher stability is a promising method. As reported, symmetry and rigidity of linkers can facilitate crystallization and improve stability. Using more rigid bpe pillar instead of bpa led to formation of more stable framework [13]. Functional spacers also affect structure stability through forming supramolecular interactions with oxygen-donor moieties. As reported, the bulkier non-polar functional groups on the linker can improve the stability through protecting the metals from other guest molecules to coordinate them [14]. As mentioned previously, another effective way for achieving more stable pillared layered frameworks is increasing interpenetration degree. By controlling the degree of interpenetration, the stability and porosity can be optimized for the best response [15, 16]. A 3-fold interpenetrated structure was achieved through applying longer 4-bpdb linker in (Zn2(cca)2(4-bpdb))n·(DMF)2n (H2cca=4-carboxycinnamic acid, 4-bpdb=1,4-bis(4pyridyl)-2,3-diaza-1,3-butadiene) which led to higher stability degree [17]. While there are a number of reports on water stable MOFs, MOFs mainly exhibit poor stability under watery conditions that limit their applications. Stability in humid condition is crucial for Bio functionality of MOFs. The framework will be collapsed if the water molecules coordinate to metal centers and substitute the organic ligand or pillar places [18]. Therefore, the hydrothermal stability of the MOF is directly related to metal-ligand bond strength. An insight for improving MOF stability in humid environments is applying hydrophobic functional groups on the pillar groups. Functionalizing bpy pillar by methyl groups can enhance water stability of the resulted structures over the parent bpy pillar frameworks. These methyl groups can prevent the water molecules to inter the pores and coordinate to the metal centers. As a negative point of view, mostly pillar layered MOFs are known as an unstable structure. But there are many reports that confirm the high stability of pillar induced frameworks which make them good candidate for advanced applications such as catalysis [19], electrodes or electron transfer [20].

7.2 Interpenetration in Pillar-Layer MOFs Many interesting examples of interpenetrated pillar-layered metal-organic frameworks have been reported [21]. During the formation of porous materials, the empty voids tend to become filled by different moieties [22], however in specific conditions, enlarging the open spaces may lead to framework catenation in which the voids will be accommodated by one or more networks [23]. By adjusting the spacer, a wide variety of MOFs with different degrees of interpenetration were obtained [24]. This phenomenon cause reduction in accessible space and lead to close packing. Although interpenetration is undesirable due to deduction of the voids and limits framework efficiency for applied applications, it can cause excellent thermal

stability for the structure upon guest removal. Also with ingenious design and controlled degree of interpenetrating networks, give novel topologies and useful properties can be achieved. Mostly, adding pillars can cause interpenetration in MOFs. A report by Wei Shi et al. [25] was focused on reforming the interpenetration in structure through applying pillars. The 63 honeycomb layer structure of ((Zn2(bpydb)2(H2O)2) (DMA)3(H2O))n will reform into two new 3D polycatenated and two-fold interpenetrated frameworks by adding bpy pillar at 80 °C and 120 °C, respectively. Figure 7.2 shows the synthetic rout and structures of three MOFs.

Figure 7.2 Synthetic rout and structures of three MOFs. As mentioned, interpenetration can improve structural flexibility [26], selectivity and stability. On the other hand, it reduces the pore volume and surface area. Controlling the interpenetration degree for optimizing framework desired properties and functionality has been widely investigated [27]. Controlling the interpenetration fold through adjusting the bridging pillars is one promising approach. An effective way to assemble the high-fold interpenetration is extending the length of the pillar. Applying longer pillars, led to producing bigger voids which are not favorable and tend to form close packed structures through catenation. As shown in Figure 7.3 using long heterocyclic aromatic pillars instead of flexible 1,3-di(4-pyridyl)propane produced new interpenetrated structures (Ni(bpea)(L1) (H2O))n. The more rigid and longer 1,4-bis-(imidazol-1-yl-methylene) benzene pillar modified the structure to produce an interlocked structure (Ni(bpea)(L2))n. By applying more rigid 1,4-bis(1-imidazolyl)benzene), interpenetration folds were increased and the free spaces were minimized led to formation of ((Ni(bpea)(L3)1.5(H2O)). (L3))n [28].

Figure 7.3 Structures of (a) (Ni(bpea)(L1)(H2O))n, (b) (Ni(bpea)(L2))n, and (c) ((M(bpea) (L3)1.5(H2O)). (L3))n. Systematic adjustments of pillars length were also investigated in series of pyridyl-based MOFs [29]. Using longer pillars instead of bpy changed the interpenetration fold from three in bpy-based MOF to 6 fold in isostructural bpe and bpa-based isostructural MOFs. Introducing bpp as a longer and more flexible linker instead of bpy also formed a 2D-3D polycatenated architecture. Our group also reported two novel MOFs (Zn(NH2-bdc) (L)).2DMF (TMU-25) and (Cd(NH2-bdc)(L)).2DMF (TMU-26) by introducing long N4′N4’-bis(pyridin-4-ylmethylene)biphenyl-4,4′-diamine (L) as pillar [30]. The interpenetration occurs in structures framework (Figure 7.4a) which reduces voids despite increasing length of linker in comparison to previously reported linkers (Figure 7.4b).

Figure 7.4 (a) Synthesis method of TMU-25 and TMU-26, (b) Chemical structure of the organic ligands, with different lengths, used as pillars. However, some exceptions were reported which indicated that not always the longer pillars cause higher interpenetration. As reported [31], although the longer tetrazine pillar was used instead of bpy to produce novel framework, the well-knit layers of Zn and bpta ligand, limited the free space and prevent interpenetration phenomena. In the pillar MOFs, interpenetration may induce flexible behavior into structures through guest adsorption. The pillar properties can affect the interpenetration and bring flexibility into the whole framework. The dynamic behavior increases if the interpenetrated networks have high agility related to each other. Employing bulky linkers is a promising method for controlling interpenetration. Introducing functional groups on pillar can change the interpenetration fold. As an example, utilizing methyl functionalized bpy instead of simple bpy pillar in two novel structures prevent the high degree interpenetration formation [32]. The methyl groups were oriented toward the framework pores and reduced the pore volume which could prevent the catenation. This reduction, had a positive effect on CO2 uptake capacity in Zn(BDC) (DMBPY)0.5·(DMF)0.5(H2O)0.5 by optimizing the pore volume for better adsorption. Another systematic study on controlling interpenetration fold showed that alkyl substitutions on ligands together with changing pillar lengths (Scheme 7.1) can reduce the pore size and inherit the interpenetration to occur [33].

Scheme 7.1 Linkers (L) and pillars (P) in paddle-wheel-based nets 1-8. A similar approach for controlling interpenetration is applying bipillar units into MOFs which strong π interactions are existed between the bilayers which can stabilized the framework efficiently. As an example, three non-interpenetrated bipillar MOFs have been reported using diverse pillars (bpy for ((Zn2(L)(bpy)2)·(NO3)·(DMF)6·(H2O)9)n, dpe for ((Zn2(L)(dpe)2)·(NO3)·(DMF)3· (H2O)2)n, and bpb for ((Zn2(L) (bpb)2)·(NO3)·(DMF)3·(H2O)4)n (Scheme 7.2) [34]. The modification of length of applied pillars has effect their electronic environments, pore sizes and consequently their selective gas adsorption. The bilayer pillar structures together with orientation of the imidazolium ring in the linker have reduced the voids and prevent the interpenetration.

Scheme 7.2 Representation of the bipillar-layer frameworks for the systematic modulation of the pores.

Interpenetration control can also happen through modifying reaction conditions such as solvent, temperature and reactants concentrations. One of the first reported works in this area in pillar MOFs was reported by Qiang Xu [35], which enlarged the nano porous structures to mesoporous by reducing interpenetration. Reducing the reaction temperature which was used to assemble two-fold interpenetrated (Cd(L)(bpy)) (2-amino-1,4-benzenedicarboxylic acid (L)), led to production of (Cd(L)(bpy)·4H2O·2.5DMF) with much higher free volume. The little difference in structure of (Cd(L)(bpy)·4H2O·2.5DMF), avoided the new chain to interpenetrate through the pores. In (Cd(L) (bpy)·4.5H2O·3DMF) the free volume was enlarged much higher through decreasing the reaction time and concentration of the reactants (Figure 7.5).

Figure 7.5 Coordination environments of Cd and ligands in (a) (Cd(L)(bpy)), (b) (Cd(L) (bpy)·4H2O·2.5DMF), and (c) (Cd(L) (bpy)·4.5H2O·3DMF). Removing the guest solvents in the pores upon activation process also can affect the interpenetration folds. As reported, a two-fold interpenetrated bpy pillar layered MOF (Zn2(ndc)2(bpy)) converts to triply interpenetrated structure by desolvation. This transformation occurs in room temperature and there is no need to change the interpenetration degree through heating the materials [36]. Generally speaking, because interpenetration is a common phenomenon in pillar-layered MOFs, investigating the possible approaches for better control on its degree through pillar

modification is of interest.

7.3 Conclusion The framework stability must be well tuned in order to produce potentially useful MOFs. Materials must exhibit long-term stability under the operating conditions of one process. Stable frameworks can resist degradation under wild conditions such as heat, humid, acidic or basic solutions and guest elimination Applying pillars or modifying pillar backbone like changing pillar rigidity, hydrophobicity or functional groups can affect framework stability. The two strategies has been exploited to increase the stability of frameworks: (i) incorporate bulky, nonpolar functional groups near the corner of each metal site in the pore space and (ii) introduce catenation, or interpenetration, within the frameworks. By adjusting the spacer, wide variety of MOFs with different degrees of interpenetration were obtained that cause reduction in accessible space and lead to close packing. Although interpenetration is undesirable due to deduction of the voids and limits framework efficiency for applied applications, it can cause excellent thermal stability for the structure upon guest removal. Also with ingenious design and controlled degree of interpenetrating networks, give novel topologies and useful properties can be achieved. Interpenetration can improve structural flexibility, selectivity and stability. On the other hand, it reduces the pore volume and surface area. Controlling the interpenetration degree for optimizing framework desired properties and functionality has been widely investigated.

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Chapter 8 Properties and Applications of Pillar-Layer MOFs 8.1 Introduction MOFs composed of mixed linkers provide greater flexibility in terms of surface area, modifiable pore size and chemical environment. In general, one anionic linker and one neutral linker connect with the metal ion/cluster to generate mixed linker framework. Depending on the binding mode of the anionic linker framework extends in different dimensions; A V-shaped anionic linker would not grow in two-dimension rather a 1D chain will form. The neutral linkers mostly serve as pillar and further increase the dimensionality. Depending on the length of the linkers porosity can be achieved and a systematic control is possible. The extent of entanglement in a 3D framework can also be tuned by altering the neutral or anionic linker. Moreover the linkers can be functionalized extensively to meet the aimed applications such as gas separation, catalysis, magnetism and molecular sensing. Such sort of modulation over functionality and porosity is not possible with a single linker system. In this part tried to discuss mixed linkers based framework structures, their versatile topologies and tunable porous properties and applications. There are several advantages in mixed linkers based MOFs compared to the MOFs synthesized from a single linker. First and foremost, it is conceivable to tune the pore size/surface area of the MOFs in a greater extent by opting the linkers of different sizes and lengths. MOFs with two linkers consent much more design ability. The linker backbone can be tuned by introducing the different functional groups, which would offer the prospect to tune the chemical environment of the pore surface. The mixed linkers MOFs are found to be highly flexible which would be realized by variable metal geometry, versatile linker flexibility and weak metal-ligand binding. These flexible MOFs divulge many interesting properties like gated and stepwise adsorption, selective adsorption and molecular switching applications [1]. In particular such materials showed applications in sensing and separation of gases [2].

8.2 Gas Storage and Separation in Pillar-Layer MOFs Porous MOP or MOFs, owing to their adjustable pore sizes and controllable pore-surface properties, present a diversity of applications in clean energy, such as hydrogen and methane storage and CO2 capture. Particularly, given the increasingly serious global warming, the effective capture and removal of CO2 from industrial flue gas is becoming an important environmental issue [3]. In view of the problems existing in CO2 capture, recent studies have demonstrated that MOFs could be a promising physical adsorbent for CO2, being an alternative to the existing benchmark materials for CO2 capture at low concentration and

moderate temperature [4]. Gas storage and separation are closely associated with the alleviation of greenhouse effect, the widespread use of clean energy, the control of toxic gases, and various other aspects in human society. In this section, we highlight the recent advances in gas storage and separation using metal-organic frameworks (MOFs). In addition to summarizing the gas uptakes of some benchmark MOFs, we emphasize on the desired chemical properties of MOFs for different gas storage/separation scenarios. Greenhouse gases (CO2), energy-related gases (H2 and CH4), and toxic gases (CO and NH3) are covered in this part (Scheme 8.1).

Scheme 8.1 Gas storage and separation in metal-organic frameworks. Discussion over the mixed linker framework systems will be continued with categorization of such systems as follows: Two dimensional networks based on mixed linkers Three dimensional frameworks with mixed linkers This can be subcategorized as: a. Anionic linker (other than carboxylate) based frameworks b. Carboxylate linker based frameworks: Interpenetrated/Non-interpenetrated Such categorization is made on the basis of dimensionality and the final topology of the framework which ultimately dictates the porous properties of the frameworks.

8.2.1 Two Dimensional Networks Based on Mixed Linkers Kitagawa et al. documented a series of 2D networks of layered topology with formula {[Co(NCS)2(3-pia/3-pna)2].xsolvent} 2D (3-pia = N-3-pyridylisonicotinamide, 3-pna = N-3pyridylnicotinamide) [5]. Each octahedral Co(II) is connected with four bridging 3-pia/3-pna spacers and two thiocyanate (NCS–) anions fill other two remaining sites. The layers stack in either edge-to-edge eclipsed or staggered manner regulated by the amide-amide or amideguest hydrogen bonding interactions and show guest induced dynamic behavior based on sliding of the networks. Recently Noro et al. reported a 2D network {[Cu(PF6)(4,4’bpy)2(MeOH)].PF6.3MeOH}n based on mixed linkers where a 2D layer is formed by 4,4’bpy and the axial sites are occupied by the by one methanol and one PF6– [6]. The other uncoordinated –PF6– anions and guest methanol are sandwiched between the 2D undulating layers. In desolvated framework interlayer separation is decreased as suggested by the PXRD measurements and exhibits stepwise uptake of N2, O2, CO2 and Ar indicating the flexible nature of the framework. Further it shows selective uptake of CO2 at 278 K over other gases like N2, O2, CH4 and Ar which has been corroborated to the presence of –F atoms which interacts strongly with CO2 molecules and also supported by density functional theory (DFT) based calculations. Another analogous framework {[Cu(4,4’-bpy)2(OTf)2].2EtOH.H2O} (OTf = trifluoromethanesulfonate) shows similar flexibility and stepwise adsorption [7]. In this structure the bpy connects the octahedral centers Cu(II) to form the 2D square grid like structure. The axial sites of Cu(II) are occupied by the OTf anions. Here also removal of the guest molecules creates shrinkage of the framework. N2 adsorption isotherm at 77 K shows a double step uptake which reveals the shrinkage of the framework. The first step (P/P0 = 0 0.11) shows the filling of the micropores in the contracted structure and the second step hints the structural expansion. 8.2.1.1 2D Interdigitated Networks 2D networks can also be interdigitated with each other supported through weak molecular forces such as hydrogen bonds and π-π interactions resulting in higher dimensional frameworks. The interlayer space is occupied by the guest solvent molecules and several interesting guest responsive structural contraction and expansion have been observed. Such framework flexibility has been realized by the gated and stepwise adsorption of gases and solvent vapors. Kitagawa et al. first reported a interdigitated flexible framework of Cu(II) {[Cu(dhbc)2(4,4’-bpy)].H2O} [8] (dhbc = 2,5-dihydroxybenzoic acid) where 1D {Cu(dhbc)} chains are linked by 4,4’-bpy to form 2D corrugated sheet. The 2D network undergoes interdigitation supported by face-to-face π-π interactions between dhbc to form a 3D extended structure with 1D channels of dimensions 3.4 × 3.4 Å2 along c-axis occupied by water molecules. Removal of guest water molecules shows structural contraction (27% reduction in cell volume) due to the glide motions of the π-stacked dhbc moieties along the caxis as revealed by the PXRD measurements. Due to the decrease in interlayer separation

type I gas adsorption profile (with CO2 or N2) was not observed, rather at 298 K a gated hysteretic sorption profile was obtained with CO2 and various supercritical gases (N2, CH4, O2) at high pressure. The abrupt rise in uptake at particular pressure referred as gate opening pressure and for N2, O2 and CO2 are 50, 35 and 0.4 atm respectively. Such gate opening behavior corresponds to structural expansion at particular pressure and can be modulated by interactions between adsorbates and host framework. 8.2.1.2 2D Pillared-Bilayer Networks Apart from such interdigitated or stacked layer systems, we can also achieve more complicated and structurally fascinating 2D pillared-bilayer framework utilizing slightly different type of organic mixed linker system. A Cu(II) based framework {[Cu(pyrdc)(bpp)] (5H2O)}n (pyrdc = pyridine-2,3-dicarboxylate; bpp = 1,3-bis(4-pyridyl)-propane) was reported having pillared-bilayer type of structure, [9] where Cu(II)-pyrdc form a 2D undulated layers. The most interesting aspect of the structure is the binding of bpp spacer; the pyridine-(CH2)3- portion of bpp is a part of 2D layer {Cu(pyrdc)} and remaining pyridine part of the bpp connect the other layer, resulting in a pillared-bilayer structure. Here conformational flexibility of bpp along the alkyl chain (–CH2-CH2-CH2-) resulting in an unusual bent conformation which directs to the formation of pillared-bilayer structure. The available channels are of dimensions 5.5 × 3.7 Å2 and 2.3 × 2.1 Å2 along c and b-axis and these are occupied by guest water molecules. Dehydrated framework shows single-crystal-tosingle crystal structural transformation and a new non-porous phase of molecular formula {[Cu(pyrdc)(bpp)]2}n is obtained where detached one pyridine part from Cu(II) occupies the void space. As expected, the non-porous compound does not uptake N2 or O2 but it shows an unusual high uptake of CO2 at 195 K. The adsorption profile shows a gate opening type profile; the isotherm suddenly increases at relative pressure P/P0 ~ 0.23 and attains a saturated level at P/P0 ~ 0.41 with an uptake amount of 100 mL/g (1.96 molecules per CuII in the pore). Furthermore determination of CO2 included single crystal structure shows similar structure as of as-synthesized framework with the regeneration of Cu(II)-N bond with bpp. This also provides an insight of gate opening behavior and corresponding structural transformation with CO2. The CO2 molecules are adsorbed into the micropores and undergo various C-H···O hydrogen bonding interactions that drive CO2 inclusion and such interactions are not possible for N2 or O2 therefore no uptake was observed. Hence the selectivity is driven by the interactions with the pillar modules. Recently, a Cd(II) based pillared-bilayer type framework {[Cd4(azpy)2(pyrdc)4(H2O)2].9H2O}n have been reported [10]. The unusual binding mode of pyrdc forms a 2D [Cd4(pyrdc)4]n corrugated sheet along bc plane. These sheets are then pillared along a direction by azpy in a criss-cross and canted fashion to form a pillaredbilayer network. Such arrangements lead to a convex type 1D channel along c-axis with dimensions 3.5 × 8.8 Å2. The extension to 3D inhibits by the coordination of water

molecules. Between the bilayer networks there is a 2D coordination space occupied by the guest water molecules which extend the networks to a 3D dimension through hydrogen bonding. Existence of two different void spaces; 1D channels and 2D coordination space was realized through solvent vapour adsorption and PXRD analysis. It was observed that due to smaller kinetic diameter H2O/MeOH can be accommodated into the both the coordination spaces and this is reflected in the double step hysteretic uptake profiles of H2O and MeOH solvent vapors. Bulky EtOH can go only into the 1D channel, thereby showing only single step adsorption. This framework also shows selective adsorption of CO2 over Ar, CH4 and H2 at 77 K. The typical type-I uptake profile for CO2 is corroborated to the presence of – N=N- and –COO– groups on the pore surface which reduces the diffusion barrier.

8.2.2 Three Dimensional Frameworks with Mixed Linkers The mixed linker based 3D frameworks are well known and their versatile structure and porous properties are substantially explored compared to the 2D structures. The synthesis of 2D network is quite challenging as control over the binding modes of two different linkers is a difficult task. In contrast construction of 3D frameworks is rather straightforward and versatile network topologies can be obtained by controlling the stoichiometry or other reaction conditions like temperature, solvent and pH of the medium. Use of long linkers in construction of 3D framework may result in framework interpenetration which significantly reduces porosity in the framework. Therefore control of interpenetration is of paramount importance in the design of 3D porous framework in mixed linkers system. 8.2.2.1 Anionic Linker (Other Than Carboxylate) Based Frameworks There are several anionic linkers which can yield pillared-layer type motifs. In pillared layer type structures a 2D sheet or grid is further connected by a second spacer to form a 3D structure. Earliest of such reports include a fluoro-functionalized microporous framework, [{Zn(4,4’-bipy)2SiF6].xguest} reported by Zaworotko et al. [11]. Structure determination revealed that the 2D grid of {Zn(4,4’-bipy)2} pillared by -SiF62– anionic linker resulting in a 3D ramework. Later on Kita gawa et al. reported isostructural Cu(II) analogue [{CuSiF6(4,4’-bipyridine)2.8H2O}]n [12]. The Cu(II) structure contains two dimensional channels with pore dimensions of 8 × 8 and 6 × 2 Å2. At 298 K and 36 atm pressure this framework showed 6 mmol/g of CH4 uptake which is higher than zeolite 5A at same condition. Moreover the density of methane adsorbed in the micropores is 0.21 g/mL which is comparable to compressed methane at 300 K and 280 atm. This suggests a very compact micropore filling by CH4 and this must be attributed to the fluorine enriched pore surface. This is historically the first MOF with CH4 uptake characteristic. Followed by this work, two new pillared layer structures were synthesized based on pyrazine pillar. {[Cu(SiF6) (pyz)3].2H2O}n and {[Zn(SiF6)(pyz)2].2MeOH}n both have the same pore surface but the pore dimensions are less; 2.5 × 2.2 and 4.5 × 4.5 Å2, respectively [13]. The micropore of

{[Zn(SiF6)(pyz)2].2MeOH}n compound showed a very sharp uptake of H2 at 77 K and the uptake amount is 0.65 and 1.09 wt% at 1 and 100 bar, respectively. More important features are the density of H2 inside the pore (0.18 molecules/Å3) is comparable to liquid H2 (0.21 molecules/Å3) and high isosteric heat of adsorption at zero coverage (8.2 kJ/mol). Presence of such ultramicroporosity also revealed unprecedented CO2 adsorption profiles at 298 and 195 K and has been found that at 298 K uptake of CO2 is higher than 195 K which is contrary to the usual phenomenon (48 mL/g and 35 mL/g at 298 and 195 K, respectively) [13]. The observed unusual phenomenon was explained using computational studies and Raman spectroscopy. Raman spectra taken at two different temperatures (173 and 323 K) exhibited changes in the –C-H stretching frequencies of pyrazine suggesting nondegenerate state of pyrazine rings at 173 K. Computational studies revealed that pyrazine rings are not collinear at low temperature rather slanted with an angle of 19°. This framework has also been studied by Zaworotko et al. for separation of CO2/CH4 and CO2/N2 gas mixture at room temperature and shows good kinetics and thermodynamics as a separating material [14]. In the same report two new analogous pillared layer -SiF6 functionalized frameworks; {[Cu(dpa)2(SiF6)]}n and its two-fold interpenetrated phase {[Cu(dpa)2(SiF6)]}n-i were studied (dpa = 4,4’-dipyridylacetylene). The void space as well as the pore window dimension is less in case of interpenetrated phase but the CO2 uptake amount at 298 K till 1 atm pressure are 41.4 and 121.2 mL/g. At zero loading the enthalpy of adsorption is also higher for interpenetrated structure (22 and 31.9 kJ/mol for non-interpenetrated and interpenetrated, respectively). 8.2.2.2 Carboxylate Linker Based Frameworks Numerous 3D MOFs solely constructed by different types of aliphatic or aromatic polycarboxylate linkers have been reported by several groups. Aliphatic dicarboxylate like fumarate, malate, glutarate, succinate, adipate have been used in the construction of MOFs along with second organic spacers. Such 3D mixed linker frameworks show different type of network topologies based on the binding modes of the aliphatic carboxylates. It is worth to mention that very few frameworks show permanent porosity due to impenetrable structure. Several pyridyl dicarboxylate linkers also has been used for the construction of 2D layers which are pillared by the pyridyl based neutral linkers. Aromatic dicarboxylate linkers like Pyrazine-2,3-dicarboxylate, Pyridine-2,3/2,4/2,5-dicarboxylate in general construct a dense 2D layers which does not allow the growth of another net and hence a non-interpenetrated structure is obtained. I) Non-Interpenetrated Frameworks Kitagawa et al. reported a series of pillared-layer 3D frameworks (CPL-Coordination polymers with pillared layer structures) of Cu(II) and Cd(II) using pyrazine-2,3dicarboxylate (pyzdc) and several other organic pillars of different lengths and functionalization [15]. Here Cu(II) or Cd(II) form a 2D layers with pyzdc and then pillars by exobidentate organic linkers to form 3D frameworks. These frameworks are non-

interpenetrated, and show tunable pore size, surface area and modifiable pore surface based on length and chemical functionalization of the pillars. In this series CPL-1 {[Cu2(2,3pyzdc)2(pyz)].2H2O]} where Cupyzdc layers extended by pyrazine linkers extensively studied for gas adsorption. Here one carboxylate group acts as a chelating and the other one remains as mono-dentate and pyrazine only coordinates to axial position of Cu(II). CPL-1 adsorbs N2, O2, Ar, C2H2, CH4 and CO2 with a type-I profile at different temperature. CPL-1 is a good host for oxygen molecules and structure determination with O2 dosed sample suggest a 1D array of O2 molecules inside the pores and they interact antiferromagnetically [16]. A detailed study of C2H2 and CO2 adsorption at 298 K with CPL-1 shows the binding with C2H2 is stronger compared to CO2. At a fractional loading of 0.2 the Qst value obtained for C2H2 is 42.5 kJ/mol which is much higher than that of CO2 (Qst ~ 31.9 kJ/mol). Structure determined by synchrotron X-ray powder diffraction data (XRPD) showed one molecule of C2H2 is present per pore of CPL-1 and the intermolecular distance between C2H2 is 4.8 Å suggesting a close pack organization inside the pores but separation is enough to avoid any explosion. Moreover, one end of C2H2 is oriented towards noncoordinated oxygen atoms of pyzdc and connected through strong O···H-C hydrogen bond. Such sort of interactions and confinement makes a stable accommodation of C2H2 in the periodic pores. CPL-2 [{[Cu2(2,3-pyzdc)2(4,4’-bipy)].guest} having a bipyridyl pillar shows higher uptake of N2, O2 and Xe compared to that of CPL-1 due to longer pillar but in an incommensurate manner. CPL-2 also showed interesting guest responsive structural changes when benzene is incorporated in the framework [17]. The other most studied compound is CPL-7 [Cu2(pzdc)2(dpyg)]n which is having –OH functionalized pillar dpyg (1,2-Di(4-pyridyl)glycol). These free –OH groups which are oriented towards the pore channels undergo hydrogen bonding interactions with the guest water molecules. Interestingly, after removal of guest water molecules the interlayer distance is shortened which resulted to a contracted framework. The adsorption of MeOH vapour and CH4 shows almost no uptake at low pressures. Even at high pressure for CH4 no uptake was observed but for MeOH a sharp uptake was observed at P/P0 ~ 0.23 and the final uptake amount is 6.2 mmol/g. The desorption pathway is quite different from the adsorption path and till P/P0 ~ 0.1 almost all adsorbed MeOH retains inside the pore. Such sudden uptake is related to the structural change of the framework and the –OH groups in the pore surface is the driving force. These – OH groups interact with the incoming MeOH solvent vapor through hydrogen bonding but this is not possible for CH4 and hence no uptake for CH4 was observed. Thus only by incorporating a –OH group in the pillar adsorption properties are altered. Multiple binding sites; like –OH, -COOH groups or –NH2, -COOH in same linker can generate a condensed structure. These binding sites simultaneously can bind to more than one metal centres and generate different framework structures along with another linker. In this case also the framework will be non-interpenetrating. A pillared layer framework structure [Cd(bipy)0.5(Himdc)](DMF)]n (Himdc = 4,5-imidazoledicarboxylate) with highly polar pore

surface was reported recently from Maji group [18]. The 2D layer is formed by the imdc and Cd(II) metal center. The Himdc acts as a chelator and also through oxo-bridge it connects Cd(II) centers to form a corrugated layer along bc plane and these are connected by bipy to afford a 3D structure with three dimensional channels. The channel surfaces are decorated with the pendent oxygen atoms of carboxylate groups of Himdc. Due such arrangement the pore surface is highly polar and that is reflected in the adsorption studies. The BET surface area was calculated to be 498 m2/g and the CO2 uptake profile was found to be double step with distinct hysteresis. Such type of uptake profile is expected for a flexible framework but this particular framework was found to be rigid enough upon desolvation. This unusual phenomenon was referred to the different adsorption sites in the pore channels. At 298 K this framework exhibits appreciable amount of CO2 and CH4 uptake (11.4 and 6.5 wt % respectively). The impact of pendent carboxylate oxygens on the pore surface was most prominently observed with steep uptake of H2 at 77 K in high pressure as well as in low pressure. At high pressure the final uptake amount was 1.23 wt % and the heat of adsorption value calculated using Clausius Clapeyron equation was found to be 13.3 kJ/mol, which is one of the highest values reported so far in MOFs. Again the theoretical studies confirmed that the adsorption sites are the pendent oxygen atoms from the carboxylate groups and the aromatic π electrons of pyridine ring. Recently Maji group have reported 3D homochiral MOFs based on L-malate dianion in combination with pyridyl based linker, {[M(L-mal) (azpy)0.5].2H2O}n and {[M(Lmal) (bpee)0.5].H2O}n (M = Co/Ni); (L-mal = L-malate dianion, azpy = 4,4’-azobipyridine and bpee = 1,2-bis(4-pyridyl)ethylene) [19]. All the frameworks are isostructural and nonporous to N2. {[Co(L-mal)(azpy)0.5].2H2O}n and {[Co(L-mal) (bpee)0.5]. H2O}n do not adsorb N2, O2, H2, N2 or Ar at 195 K but show typical type I uptake profile for CO2 gas suggesting microporosity. The CO2 selectivity can be attributed to the undulating 1D channel system, which provide diffusion barrier for other gases. For, CO2 having large quadrupole moment which can interact efficiently with the polar pore surface which is decorated with the carboxylate oxygens and π electron clouds from the pillars. It is worth to mention that polarity of the surface modulates the CO2 uptake properties. {[Co(Lmal)(azpy)0.5].2H2O}n shows higher CO2 uptake (100 mL/g) compared to {[Co(L-mal) (bpee)0.5].H2O}n (40 mL/g) as the former bearing a much polar pore surface due to the presence of –N=N- groups. II) Frameworks with interpenetration Interpenetrated frameworks are spontaneously formed by increasing lengths of the organic spacers based on filling of the large spaces by self-catenation. Framework interpenetration reduces the pore size and corresponding surface area and sometime it leads to the highly condensed nonporous structure with high thermal stability. However small micropores based on self-catenation are beneficial for storage, separation and purification of small molecules. Furthermore, entangled framework shows dynamic structural transformation through shearing or displacement of the nets and such structural flexibility has been exploited for storage, separation and sensory applications. A series of microporous interpenetrated 3D

primitive cubic MOFs with general formula {[M(CO2)2R]2(L)}n (M2+ = Cu, Ni, Co, Zn, R(CO2)2 = dicarboxylate and L = organic pillar linker) based on mixed linkers have been reported. Here paddle-wheel M2(CO2)4 cores are connected by the dicarboxylate linkers to form a 2D grid which is further extended to 3D by different organic pillars. Based on different aliphatic or aromatic dicarboxylates and varieties of pillar linkers with different functionalities several 3D interpenetrated frameworks have been reported by groups of Zaworotko, Kitagawa, Chen, Kim, Hupp et al. and also by Maji group [20, 21, 22, 23]. Two such Cu(II) based frameworks, [Cu3(bipy)1.5(2,6-ndc)3]n and {[Cu(bpe)0.5(2,6ndc)].0.5H2O}n were reported by Maji group [20]. The 2D grids are formed by the linking of paddle-wheel units Cu2(COO)4 through 2,6-ndc. Two different pillars, bipy and bpe connect the grids to form two different 3D three-fold interpenetrated frameworks. Further the nature of the pillar also affects the framework properties. Bipy pillared framework was found to be rigid but bpe which is considered to be flexible due to the ethane linkage imposes flexibility into the framework. The Langmuir surface area was 337 m2/g for flexible framework, found to be more compared to rigid bipy framework (113 m2/g). Both the frameworks showed appreciable amount of CO2 and H2 uptake at 195 and 77 K, respectively. Most importantly, the density of H2 in the rigid framework (0.0801 g/cc) was found to be higher than liquid hydrogen (0.0708 g/cc) which is due to the strong confinement of H2 in small pores in the interpenetrated framework [24].

8.3 Catalysis in Pillar-Layer MOFs Because of the high percentage of metal content of MOFs, catalysis is one field that draws attention of people from all facets, be it organic synthesis or material chemistry. One of the important aspects of designing MOFs, which can be potentially useful in the field of catalysis, is by post-synthetic modification, resulting in chemical alterations in the framework [25-28]. Thus, by judiciously choosing an appropriate MOF capable of such versatile behavior, a route to design catalytically active MOPs can be achieved, particularly in porous structures due to their high surface areas. Designing heterogeneous MOF embedded with catalytic active sites is generally performed depending on the final catalytic activity of interest. Two general catalytically active groups can be divided as (I) MOFs containing catalytic metal Lewis acid active sites, and (II) MOFs including catalytic active functional organic sites. In the former subclasses of these catalysts that are the most popular cases, MOF structure has intrinsically unsaturated metal nodes or employed as a support for stabilizing transition metal nanoparticles [29]. In the latter subclass of heterogeneous MOF catalysts, which is less developed, the actual catalytic sites are the organic functional groups that are the organic linkers themselves used for MOF construction or can be incorporated within the framework by post-synthesis modifications [30]. Among common Lewis acid based MOF catalysts, those bearing basic functional groups provide deserving heterogeneous catalysts for organic transformations. It has been indicated that

NH2-funtionalized linkers, resulted in improved basicity and catalytic activity of these MOF structures. Actually, overall catalysis reactions proceeded via synergic effect of the Lewis acid metal nodes placed at neighbouring basic sites on the pore surface of MOF structure eventually make bifunctional acid-base heterogeneous catalysts [31,32]. On the other side, accommodation of the supramolecular organocatalysts especially Brønsted-acidic groups such as urea, thioura and squaramide into MOF have attracted specific attentions. The improved activities of MOFs containing these potential organocatalytic groups have been proved for Friedel-Crafts [33], Henry [34], acetalization, Morita-Baylis-Hillman, reactions [35] and ring opening of epoxides [36]. Application of MOFs as heterogeneous organocatalyst with pure Brønsted-basic moieties has only been scarcely explored. Functionalization of the organic linkers, either by pre- or postsynthetic modification within the MOF structures including saturated metal ion nodes can reveal the role of these types of solid catalysts that are more important analogous for pharmaceutical synthetic applications. The Knoevenagel condensation is a typical test reaction for the evaluation of basic catalysts which is the most common used reaction for investigation of MOF containing basic functionalized structures due to its amenable reaction conditions and analysis. Therefore, more competent catalytic systems with high efficiency for sluggish organic reactions are desirable in this immature area. Structural tunability of MOF materials provides the possibility of designing the framework with potential capability toward catalyzing organic reactions. A feature that has rarely been taken into consideration for improving catalytic activity of MOFs, is hydrophobicity character of the pore surface. Controlling the hydrophobicity character has been usually performed to reduce the wettability of the pore surfaces to eventually create moisture-stable MOF structures [37]. In this regard, introduction of the organic moieties inside the pores of MOFs has been substantially developed upon post-synthetic modification [38]. It has been known that this physical property of MOFs could improve through introduction of hydrophobic alkyl chains. Accordingly, increasing the hydrophobic nature of these MOFs can mainly change their capability for gas storage and water adsorption properties [39]. Recently, Farrusseng and co-workers studied the effect of hydrophilic–hydrophobic balance environment surrounding the catalytic sites using ZIF-type materials in Knoevenogel condensation [40,41]. In this contribution, for the first time Morsali and co-workers demonstrated that tuning the hydrophobicity properties of the MOF walls can adjust the permeation characteristics of substrates and a type of subtle substrate selectivity can be directed toward their potential catalytic activities. Four MOF catalysts possess imine and/or amine basic N-donor pillars bearing phenyl or naphtyl cores owing different hydrophobic character around the basic reaction center were prepared via simple mechanochemical synthesis. They were characterized thoroughly using TG, IR and PXRD analysis. For the first time, aldol-type condensation reaction of malononitrile with ketone-functionalized carbonyl substrates developed in the presence of the basic MOF organocatalysts. Moreover, it has been successfully showed that a subtle substrate selectivity can be addressed during the reaction of three slightly different α,β-unsaturated carbonyl compounds in contrast the effect of size control barriers that commonly direct heterogeneous reaction pathway. Therefore, a series of

various basic imine and amine- functionalized MOFs were synthesized which are different in terms of structural hydrophobicity (Scheme 8.2).

Scheme 8.2 Synthesis of a series of IRMOFs using four pillar ligands; according to the estimated Pka, their basicity is in the order of RL1> L1> RL2 >L2. Images of the catalysts in the mixture of H2O and PhCH3 right. Regarding to the weak Lewis basicity of these prepared MOF series, in addition to numerous Knoevenogel reaction developed for investigation of basic MOFs, herein we chose aldol-type coupling reaction of malononitrile with α,β-unsaturated carbonyl compounds. In contrast with Knoevenogel reaction, replacing of aromatic aldehydes with less reactive ketone function has led to more difficult coupling reaction in this case. While Knoevenogel reaction of malononitrile and benzaldehyde in the absence of any catalyst produced 19% of the corresponding coupling product at room temperature, no detectable product was formed during the reaction of malononitrile and 1-cyclohexen-2-one in the same reaction conditions, even at elevated temperature 60 C, in the absence of catalyst and in MeOH as solvent within 24 h). At preliminary stage of the catalytic reaction investigations, during the optimization of the reaction solvent, the reaction mixtures proceeded in water in the presence of all prepared MOFs were subjected to toluene in order to extracting the organic phase and analysing the reaction progress with gas chromatography. The interaction of each of these immersed catalysts in water after addition of non-polar solvent was very interesting. As illustrated in Scheme 8.2, while the relatively hydrophobic catalyst, TMU-21(RL2) lay on the solvents border, more hydrophobic MOF, TMU-21(L2) is completely assembled and tends to move away from the polar aqueous environment. In comparison, the phenyl-core containing catalysts, i.e. TMU-6(L1) and particularly TMU-6(RL1) spread on the border of the solvents. The behaviour of four MOFs was also checked with THF and DCM as extracting solvent. In the presence of THF, no clear separated phase is detected. Further interesting results were observed in the presence of DCM that is immiscible with water and places under that. MOFs containing amine base pillars, TMU-6 (RL1) and TMU-21 (RL2) escape from the organic phase while the imine based MOFs specially TMU-21 (L2) with naphtyl-core pillar tend to penetrate into DCM phase. Subsequently, during the reaction survey of 2-cyclohexen-1-one as electrophilic substrate, with all the solid base MOF catalysts, best results were obtained in

the presence of 5 mol of the catalysts, and in MeOH as solvent at 60 C. As summarized in Table 8.1, after 24 h of reaction 60% of the corresponding addition product was formed in the presence of TMU-21(L2) that can catalyze the reaction better that other MOF compounds. To shed light, two other substrates i.e. 2-cyclopenten-1-one as less hydrophobic substrate and 4,4-dimethyl-2-cyclohexen-1-one as more hydrophobic case were chosen and separately reacted in the presence of the four prepared MOF catalysts in the same reaction conditions. All catalysts are arranged in the order of their hydrophobicity character in Table 8.1. A quick look at these data surprisingly indicates that in the case of less hydrophobic substrate, 2cyclopenten-1-one, TMU-6(RL1) which is the less hydrophobic and more basic catalyst proceeded the reaction with high efficiency (93.7% yield). Quite the opposite, the best conversion for the more hydrophobic substrate, i.e. 4,4-dimethyl-2-cyclohexen-1-one was obtained in the presence of the more hydrophobic MOF catalyst, TMU-21(L2). This impressive result encourages them to investigate more precisely about their catalyst system. Table 8.1 Coupling reaction of malononitrile and carbonyl compounds with the prepared MOFs within 24 h.

1 2 3 4

Catalyst TMU-6(RL1) TMU-6(L1) TMU-21(RL2) TMU-21(L2)

Yield(%)a 93.7 89 60 76.3

Yield(%) 40 45 36 60

Yield(%) 48 53.4 31.3 72.7

5 TMU-6(RL1) 73 6 TMU-21(L2) —

— 98

aGC yield based on internal standard method, reaction conditions; substrates (0.2 mmol). Malononitrile (0.3 mmol), catalyst (5 mol%, 15 mg), MeOH (3ml), 60°C.

Very recently, Wang et al. systematically investigated the catalytic properties of Zn, Zr, Al and Cr derived MOFs tagged with primary alkyl amino group via post-synthetic modifications [42]. In the size-selectivity studies performed using various sizes of electrophilic aldehydes; Cr-MIL-101-NH-RNH2 showed higher catalytic reactivity because of its large pore and cage sizes in comparison of the other selected MOFs. It should be noted that, according to single crystal data reported for TMU-21(L2) and TMU-6(L1), within their MOFs series, the former containing naphthalene cores possesses smaller channels and aperture size relative to the latter including phenyl core structures. Considering the undeniable effects of structural parameters such as pore size and topology, the most and least catalytic activities of their system that observed in the case of smallest and largest substrates, (2-cyclopenten-1-one and 4,4-dimethyl-2-cyclohexen-1-one, respectively) logically related to

their ability to get closer to the basic reaction center. Although, decreasing the catalytic activity during the substrates series in the presence of more basic amine containing catalysts, TMU-6 (RL1) and TMU-21(RL2), can be ascribed to increasing the substrates size, however improving the catalytic activity in the cases of two larger substrates in the presence of TMU21(L2) indicates the subtle role of the catalyst structural hydrophobicity. As Table 8.1 show, the catalytic activity of less basic, more hydrophobic structure, TMU-21(L2), is clearly more than its amine-functionalized analogous MOF, TMU-21(RL2). More importantly, during the substrates series, (rows 3 and 4, Table 1), the mentioned differences in catalytic activity become more obvious with the increase of the substrate lipophilicity in order of 16, 24 and 41%. For more precise investigation, the performance comparison of all catalysts for the less and more hydrophilic substrates was respectively plotted in Figure 8.1a and b. These figures obviously show that regardless of imine or amine functions of the MOFs, using phenyl-core including catalysts, TMU-6(L1) and TMU-6(RL1), 2-cyclopenten-1-one reacts more effective than 4,4-Dimethyl-2-cyclohexen-1-one, cause of its smaller size that facilitates its proximity process to the basic reaction centers in the MOF structure. As Figure 8.1a shows, despite the increase in catalytic activity in the case of 2-cyclopenten-1-one generally follow from the basic character of the MOFs, TMU-21(L2) interrupt this process and indicates more reactivity than the more basic analogous structure, TMU-21(RL2). On the other side, the reactivity of the catalysts changes remarkably for 4,4-dimethyl-2-cyclohexen-1-one. Whereas, TMU-21(L2) is the most reactive catalyst, TMU-21(RL2) is the weaker one (Figure 8.1b). Taking into account that both of these catalysts have naphthalene-core ligands as well as different basic centers, the results prove the vital role of hydrophobic character of the former catalyst on the reaction process. It seems that the mass transport limitations cause by pore diffusion has been fixed by increased hydrophobicity nature around the reaction center. Moreover, decreasing the catalyst reactivity of TMU-6(RL1) even less than its iminecontaining analogous structure, TMU-6(L1), points out the meaningful and prominent effect of this structural specificity that has not been reported yet. results shows, while TMU-6(RL1) is the most reactive catalyst in the case of less hydrophobic substrate, cyclopenten-1-one, however TMU-21(L2) is the most effective ones for less hydrophilic one, 4,4-dimethyl-2cyclohexen-1-one. In addition, these plots demonstrates that although TMU-21(RL2) contains the amine-function, it is less reactive catalyst for all substrates especially those have larger size. These data along with the moderate results of 2-cyclohexen-1-one (Table 8.1) explicitly represent the close and break-even competition between the effective reaction parameters. However, a clear change in the catalytic behaviour of TMU-21(L2) in the presence of the bulkier substrate indicates that steric effect has been suppressed by the hydrophobicity induced within the walls of the catalyst structure.

Figure 8.1 Yield-versus-time profile of aldol-type condensation reaction of (a) 2cyclopenten-1-one and (b) 4,4-dimethyl-2-cyclohexen-1-one catalyzed by four MOF structures in the reaction conditions indicated in Table 8.1. Further proof of this claim is the substrate selectivity test, which was implemented for the most reactive catalysts, namely TMU-6(RL1) and TMU-21(L2). In two separated catalytic runs, the solution mixture of two compounds, 2-cyclopenten-1-one and 4,4-dimethyl-2cyclohexen-1-one led to react in a reaction vessel in the presence of the equal amount of the catalysts and twice amount of malononitrile under the same reaction conditions. As illustrated in Scheme 8.3, after 24h of the reaction with TMU-6(RL1), 96% of the corresponding product was formed from the former, whereas the latter substrate reacts moderately to produce 62% yield. Interestingly, during the same reaction time with TMU21(L2), more hindered substrate, 4,4-dimethyl-2-cyclohexen-1-one beats the competition to give 58% yield, while less hindered substrate, 2-cyclopenten-1-one reacts more slowly and produces 45% yield.

Scheme 8.3 Illustrative substrate selectivity observed within the catalytic aldol reaction of malononitrile in mixture solutions of both substrates in the presence of more basis MOF left side and less basic, more hydrophobic MOF right side; conditions: substrates 0.2 mmol, malononitrile 0.6 mmol, catalysts 5 mol, MeOH 4 ml, 60 C. Finally, TMU-6(RL1) and TMU-21(RL2) bearing the reduced analogous pillar ligands of TMU-6(L1) and TMU-21(L2), respectively. The structure-activity relationship, induced in this way, cause different hydrophobicity-polarity character around the pore environment.

Consequently, this subtle structural diversity developed a heterogeneous catalyst system containing solid base organocatalyst MOFs that represented subtle substrate selectivity through aldol-type condensation reaction. It has been indicated that a slight change in hydrophobicity environment around the basic reaction center in MOF structure could interestingly change the preference of the substrates to be reacted while they have very little structure diversity [43].

8.4 Adsorptive Removal and Separation of Chemicals in Pillar-Layer MOFs Efficient removal and separation of chemicals from the environment has become a vital issue from a biological and environmental point of view (Scheme 8.4). Currently, adsorptive removal/separation is one of the most promising approaches for cleaning purposes. Selective adsorption/removal of various sulfur- and nitrogen-containing compounds, olefins, and πelectron-rich gases via π-complex formation between an adsorbent and adsorbate molecules is very competitive. Porous metal-organic framework (MOF) materials are very promising in the adsorption/separation of various liquids and gases owing to their distinct characteristics.

Scheme 8.4 Harmful gas removal in metal-organic frameworks. This part summarizes the literature on the adsorptive removal/separation of various πelectronrich compounds mainly from fuel and gases using MOF materials containing metal ions that are active for π-complexation. Details of the π-complexation, including mechanism, pros/cons, applications, and efficient ways to form the complex, are discussed systematically. Adsorption can be superior to other techniques for removal/separation of chemicals because of its comparatively low cost, variety of applications, simplicity of design, easy operation, low amount of harmful secondary products, and the simple regeneration of the adsorbents. Since the invention of various porous materials and the huge development in adsorptive separation processes, adsorption is the key separation technique in industries. Porous materials including activated carbons, zeolites, and metal-organic frameworks (MOFs) were studied comprehensively for the adsorption/separation of various molecules from liquids,

vapors, or gases. For efficient adsorptive removal, porosity, pore architecture, and favorable adsorption sites are required. In addition, some active species such as functional groups (acidic or basic), metal ions, metal oxides, metal salts, ionic liquids, and polyoxometalates are generally introduced to the porous sorbents for selective uptake of adsorbates through some common interactions such as acid-base interactions, π-complexation, π-π interactions, and H-bonding. MOFs are principally composed of two key components: a metal ion or a cluster of metal ions and an organic molecule known as a linker. The organic units are usually di-, tri-, or tetradentate ligands. The attention given to MOFs is due to the huge porosity and facile tunability of their pore size/shape from the microporous to the mesoporous scale via alteration of the connectivity of the inorganic moiety and the organic linkers [44, 45]. As well, several analogous MOFs [44] can be built from (i) the same ligand and different metal ions, (ii) identical metal ions and different ligands, and (iii) MOFs with functionality in the linkers. Furthermore, MOFs have numerous potential applications including adsorption/separation and storage [46, 47]. Because of the potential applications of MOFs in various fields, not only research on MOFs but also the number of publications (per year) related to MOFs has increased rapidly. MOFs are promising solids for adsorption/separation-related applications because of the easy modification of pore surfaces, which leads to selective adsorption of specific guest molecules with particular functionalities. In addition, the central metal ions, coordinatively unsaturated sites (CUSs), and functional groups on the organic linkers were employed effectively for further interactions between the adsorbate molecules and MOF-based adsorbents; this makes MOF-type materials superior to other porous sorbents in the efficient adsorptive removal/separation of hazardous molecules [48] (Scheme 8.5).

Scheme 8.5 Adsorptive removal and separation of harmful chemicals in pillar-layer MOFs. Sulfur- or nitrogen-containing organics are naturally occurring species that exist in fossil fuels such as crude oil, diesel, gasoline, jet fuel, and heating oil. Currently, around 85% of global energy originates from fossil fuels. This massive combustion of fossil fuel is the key source of poisonous emissions that account for hazardous air pollution, the greenhouse effect, and global warming and an unsafe impact on living organisms. Moreover, sulfurand nitrogencontaining compounds (SCCs and NCCs, respectively) poison the catalysts of various catalytic processes in emission control systems and decrease their efficiencies in different ways. Thus, it is very important to remove SCCs and NCCs before exposing the fuel or its combustion products to the atmosphere. Recently, removal of these unsafe organic

compounds from crude oils before operation has become a big challenge. For example, based on EU and US strategies, the sulfur level in fuels should not be more than 10 and 15 ppmw, respectively [49]. Pollutants originating from NCCs have also become alarming in recent years. The World Health Organization (WHO) suggested a guideline to limit the exposure of NO2 to the air. According to the WHO, yearly emissions should be limited to 40 µg m-3, and the exposure should not exceed 200 µg m-3 within any hour. Vehicle-derived pollutants such as SO2, NO2, CH4, CO, and black carbon contribute to global warming. In addition, huge amounts of CO are produced in various oxidation processes as a byproduct and mix with other gases in the environment. Atmospheric CO is extremely poisonous to human beings as it can attach quickly to hemoglobin and hinder circulation. Currently, the carbon balance of the world is one of the most vital environmental issues; hence, reducing anthropogenic emissions of CO has become a great challenge for mankind. The effective capture of harmful chemicals is of great importance both for the protection of the environment and for those who are at risk of being exposed to such substances. In this context, the use of MOFs with adequate pore size/shape is not enough for an efficient capture of unsafe gases/vapors and other more specific interactions between the hazardous adsorbates and the host are desirable. For example, the presence of open metal sites (coordinatively unsaturated metal centres) or certain functionalizations on the pore surface may enhance the adsorption selectivity/efficiency of MOFs towards certain toxic compounds via coordination bonds, acid–base/electrostatic interactions, π-complex/H-bonding formation, etc. 8.4.1 Adsorptive Removal of Harmful Gases The possible use of MOFs for the removal of gaseous sulfur compounds has been recently investigated. The presence of open metal sites and H-bonding donor–acceptor groups in these adsorbents has been proposed as suitable features for the efficient removal of S-compounds. In this context, two different classes of MIL systems (MIL stands for Materials of the Institute Lavoisier), the series MIL-53(Al, Cr) and MIL-47(V) [M(X)(bdc)] (bdc = 1,4terephthalate; X = O for M = V4+, OH for M = Al3+, Cr3+) and the series MIL-100(Cr) [Cr3F(H2O)2O(btc)2] (btc = 1,3,5-tricarboxylate) and MIL-101(Cr) [Cr3F(H2O)2O(bdc)3] have been explored for the adsorption of H2S [50,51]. Small-pore MIL-47(V) and MIL53(Al, Cr) exhibit reversible adsorption behaviour under H2S pressure. Moreover, IR measurements and molecular simulations reveal that the MIL-47(V) framework remains rigid upon H2S adsorption up to a pressure of 1.8 MPa, while MIL-53(Cr) shows a flexible behaviour undertaking two structural transitions: the first one from a large pore form (LP) to its narrow pore version (NP) at very low pressure and the second one from the NP to the LP form at higher pressure. These structural transitions explain the two-step shape of the adsorption isotherm of MIL-53(Cr). Moreover, the arrangements of H2S molecules within the pores of these MIL species have been investigated. At an initial stage, H2S molecules form hydrogen bonds, either as a hydrogen donor (in MIL-47(V)) or hydrogen acceptor (in MIL53(Cr)) with the µ2-O and µ2-OH groups, respectively, at the pore surface. In the case of MIL47(V), the interaction of acidic H2S is also reinforced through other basic centers of the

porous matrix, such as the oxygens of the carboxylate groups and the p electrons of the benzene rings. At higher pressures (1.8 MPa), the adsorbates arrange within the channel to form dimers and the strength of the hydrogen bonds becomes weaker. It should be noted that the strength of H2S–adsorbent interactions follow the sequence: MIL-53(Cr) NP › MIL-47(V) › MIL-53(Cr) LP. This means that MIL-47(V) would be the most promising candidate for purification of gases containing H2S as a pollutant as H2S is more easily desorbed from MIL47(V) than from MIL-53(Cr) NP. Preliminary simulations performed on MIL-47(V) suggest a high selectivity for H2S over CH4 in the range of pressures relevant for Pressure Swing Adsorption (PSA). On the other hand, large-pore MIL-100(Cr) and MIL-101(Cr) also show a high uptake of H2S exhibiting type-I adsorption isotherms (16.7 and 38.4 mmol g-1 at 2 MPa, respectively). However, the desorption appears to be irreversible, either because of a partial destruction of the framework or because of the strong interactions between the host and the H2S molecules. Their lack of ability to regenerate hampers the use of these mesoporous MOFs for the removal of H2S from a practical application point of view. M-CPO-27 also known as M-MOF-74 [M2(2,5-dhbdc)(H2O)2], (2,5-dhbdc = 2,5-dihydroxyterephthalate M = Ni2+, Zn2+) can bind H2S relatively strongly, allowing the storage of the gas for several months [52]. It should be noted that in spite of the rather aggressive chemical nature of H2S, Ni-CPO-27 only shows slight loss in crystallinity after storage and release whereas Zn-CPO27 was amorphized by prolonged storage over a year. The study of the interaction of NiCPO-27 with H2S using powder X-ray diffraction and X-ray pair distribution function analysis reveals that H2S is clearly coordinated to coordinatively unsaturated metal sites. The release of H2S from these MOFs has also been studied for biological applications taking into account the signal properties of this molecule. It has been demonstrated that a proportion of the stored hydrogen sulfide can be released by exposing these materials to moisture as the water molecules replace the adsorbate at the metal site. Thus, the contact of the H2S loaded materials with a moist atmosphere gives rise to the release of around one third of the chemisorbed H2S from Ni-CPO-27 (1.8 mmol g-1) and Zn-CPO-27 (0.5 mmol g-1) after 1 h. The lower adsorptive and delivery potential of Zn-CPO-27 has been attributed to difficulties in the full activation of the sample [53] and to the softer nature of Zn metal centres which gives rise to stronger and irreversible sulfur binding. FMOF-2 [Zn2OH(2,2’-bis(4carboxyphenyl) hexafluoropropane)1.5] is a breathing MOF, which has been tested for potential acid gas removal applications [54]. Indeed, it remains structurally stable upon adsorption of SO2 and H2S reaching weight capacities of 14% and 8.3%, respectively, at 1 bar. Moreover, both gas adsorption–desorption isotherms show hysteresis confirming the breathing motion of FMOF-2 after gas adsorption–desorption. Six isoreticular MOFs, namely MOF-5 ([Zn4O(bdc)3], bdc = 1,4-terephthalate), IRMOF-3 ([Zn4O(abdc)3], abdc = 2-amino1,4-benzene dicarboxylate), MOF-177 ([Zn4O(btb)3], btb = 1,3,5- benzenetribenzoate), IRMOF-62 ([Zn4O(bdb)3], bdb = butadiynedibenzoate), ZnCPO-27, and [Cu3(btc)2] (btc = 1,3,5-tricarboxylate), prove to be very effective in removing

gas and vapor contaminants, such as sulfur dioxide, ammonia, chlorine tetrahydrothiophene, benzene, dichloromethane and ethylene oxide (Figure 8.2) [55]. The behaviour of each MOF has been compared to that of BPL carbon. In this section, only the behaviour related to the adsorption of toxic gases will be discussed (for the discussion of the capture of volatile organic compounds, please see below). Noteworthy, it has been demonstrated that the presence of coordinatively unsaturated metal sites (i.e. Zn-CPO-27 and [Cu3(btc)2]) and an amino functionality in IRMOF-3 prove to be effective in adsorbing ammonia and sulfur dioxide highly outperforming the behaviour of BPL carbon by a factor of at least 59 in the case of ammonia or performing equally well, in the case of sulfur dioxide (Figure 8.2).

Figure 8.2 Kinetic breakthrough curves of SO2 (A) and NH3 (B) contaminants over various MOFs. The improvement of ammonia adsorption in Zn-CPO-27 and [Cu3(btc)2] is explained because of the presence of Zn2+ and Cu2+ open metal sites, which behave as Lewis acids to coordinate NH3 Lewis bases, which is not possible in the activated carbon. Deeper studies on the interaction of NH3 with [Cu3(btc)2] have postulated that [Cu3(btc)2] reacts with ammonia to form a presumed diammine–copper(II) complex, under dry conditions [56]. Under humid

conditions, initially, water enhances NH3 adsorption on [Cu3(btc)2] via dissolution of NH3 in the water film [57] but in a latter step Cu(OH)2 and (NH4)3BTC species are formed thereby inducing an irreversible loss of structure and porosityreflected in a significant decrease in available sites and capacity over multiple dynamic adsorption cycles of breakthrough curves [56]. In the case of IRMOF-3, amine groups constitute reactive electron-rich groups available for hydrogen bonding with NH3 molecules, improving the adsorption behavior in comparison with MOF-5 and BPL carbon. MOF-5, MOF-177 and IRMOF-62 lack any reactive functionality and show worse dynamic adsorption capacity for ammonia than the previously mentioned functionalized MOFs (Figure 8.2). In spite of this, these MOFs show a better performance than BPL carbon towards this gas. However, they show little or no capacity for SO2. Regarding chlorine adsorption, open metal sites in [Cu3(btc)2] are ineffective in this case as Cl2 does not typically act as a ligand. However, halogen molecules can give rise to oxidative addition reactions to Pt redox active metal centers as demonstrated in the spin cross-over (SCO) {Fe(pyz)[PtX2n(CN)4]} (n = 0 to 1) materials as reported by Kitagawa, Real and colleagues [58]. Moreover, the halogen addition reaction gives rise to a precise control of the SCO transition temperature of the resulting {Fe(pyz)[PtX2n(CN)4]} materials. On the other hand, IRMOF-3 and IRMOF-62 have demonstrated considerable capacity for chlorine adsorption. Indeed, IRMOF-3 outperforms BPL carbon by a factor of 1.76 and the adsorption capacity of IRMOF-62 is in the same order of magnitude than the activated carbon. These results outline that despite the high capacities for thermodynamic gas adsorption, MOFs lacking reactive adsorption sites are ineffective in kinetic gas adsorption. In contrast, it should be noted that, in the case of MOFs with open metal sites (OMS) the presence of humidity may hamper the adsorption of the adsorbate of interest as a consequence of the irreversible coordination of water molecules to the open metal sites (see above). This feature is a main drawback for the application of MOFs containing OMS in competition with classical hydrophobic adsorbents like activated carbons (see also the section on the capture of volatile organic compounds). Mavrandonakis and colleagues [59] have recently carried out a computational study of the interaction of a series of toxic gases CO, OCS, SO2, NO, NO2, N2O, NH3, PH3 with the open metal sites of [Cu3(btc)2]. The calculated adsorption energies of the toxic gases and common adsorbate molecules follow the trend NH3 › H2O › PH3 › H2S › SO2 › CO ≈ OCS ≈ CO2 ≈ NyOx › N2 › O2. Only ammonia is adsorbed more strongly on the metal sites than water, exceeding the water–framework interaction by 30 kJ mol-1. Noteworthy, although hydrogen sulfide and phosphane are less strongly bound by 10 kJ mol-1 compared to water, the interactions are significantly stronger than physisorption falling in the regime of chemisorption. These findings are in agreement with the experimental observations that ammonia, water and hydrogen sulfide decompose the framework [56]. The M-CPO-27 (M-MOF-74) (M = Zn2+, Co2+, Ni2+, Mg2+) series have been tested for removing NH3, CNCl and SO2 from air in dry and humid conditions and dynamic breakthrough capacities were compared to 13X zeolite and BPL activated carbon [60]. The

results reveal that in the presence of a humid atmosphere at 25 °C CPO-27 analogues adsorb an appreciable amount of water accounting at least for 0.1 g of water per gram of adsorbent. In dry conditions, the dynamic loadings of NH3 on all the CPO-27 analogues are generally near to or exceed the maximum loadings of traditional materials, such as 13X zeolite, which adsorbs 2.86 mol kg-1 under the same conditions, and BPL activated carbon, which has no capacity for dry ammonia. The best performing material, Mg-CPO-27 loads three times the capacity of 13X zeolite and the worst performing analog, Ni-CPO-27 loads 80% of the capacity of 13X. It should be noted that when ammonia is loaded in dry conditions, a significant portion of the adsorbate is retained when the MOF materials are exposed to dry air during desorption. However, the 13X zeolite did not retain any of the adsorbed ammonia under the same breakthrough conditions. As expected, the adsorptive capacity of these materials is reduced in a humid gas stream, but they maintain significant adsorption capacity for NH3. Indeed, Mg-CPO-27, the worst performing material under humid conditions, shows 6 times the capacity of BPL activated carbon. On the other hand, CPO-27 structures adsorb dry cyanogen chloride in the same order of magnitude as classical materials, reaching 70% of the retention by Co-CPO-27 even after desorption. However, under humid conditions, none of these MOFs adsorb an appreciable quantity of CNCl resembling the behaviour of 13X zeolite in both dry and humid conditions. Regarding SO2 removal, only Mg-CPO-27 loads this acid gas. In general, Co and Mg analogues show better adsorption of the four gases in both dry and humid conditions. Furthermore, it is clear that the presence of water diminished the adsorption capacity of these MOFs towards the studied gases as a consequence of the competition of the water molecules with the toxic gases for the adsorption at the metal centres leading to diminished breakthrough times and total capacities. The presence of open metal sites is also highly beneficial for the capture of NO. A series of MOFs have been assayed for the capture and release of NO for biological applications. This is the case for [Cu3(btc)2] and CPO-27 structures, [61] which adsorb up to 3 and 8 mmol of NO per gram of MOF, respectively, considerably more than other similar nanoporous solids, such as zeolites. However, the adsorption of NO on [Cu3(btc)2] can be considered nearly irreversible as only 1 mmol of NO per gram of material is released upon exposure to moisture while CPO-27 materials completely desorb all the stored NO under similar conditions. Other biodegradable materials, such as the Fe-based MIL-88A [Fe3O(CH3CO2)(fumarate)3], MIL-88B [Fe3OX(1,4-bdc)3], 1,4-bdc = 1,4-benzenedicarboxylate (X = F, Cl, OH), MIL-88B-NO2 [Fe3OX(2-NO2-bdc)3], 2-NO2-bdc = 2-nitroterephthalate (X = F, Cl, OH) and MIL-88B-2OH [Fe3OX(2,5-dhbdc)3], 2,5-dhbdc = 2,5-dihydroxyterephthalate (X = F, Cl, OH), show loading capacities lying within the 1–2.5 mmol g-1 range [62]. In this case, the low accessibility to the pores is due to the absence of a breathing mechanism of the structure with the pores remaining closed and preventing the easy diffusion of NO to bind the accessible iron metal sites. The released amount of NO from these structures in the presence of a wet gas is even lower, a few tenths of mmol g-1, which is attributed to the desorption of the physisorbed NO prior to the desorption tests, but it is still enough to ensure a significant release at the biological level over prolonged periods of time. A similar behaviour is observed in BioMIL-3

[Ca2(azbz-TC)- (H2O)(DMF)], azbz-TC = 3,3’,5,5’-azobenzenetetracarboxylate, which adsorbs 0.8 mmol g-1 of NO at 1 atm, a quantity which is much lower than the theoretical unsaturated Ca2+ metal sites [63]. The absence of a plateau in the NO adsorption branch suggests apossible higher adsorption at higher pressures. However, residual coordinated DMF, steric hindrance and diffusion constraints make coordination on the unsaturated metal sites more difficult. These factors also contribute to the slow and low delivery of NO from this material (5 mmol g-1 overnight). Other methodologies have been developed to store NO in MOFs. This is the case for the postsynthetic functionalization of the open metal sites in [Cu3(btc)2] with the bifunctional 4-(methylamino)-pyridine to produce secondary amines, which are able to react with NO to yield the diazen-1-ium-1,2-diolate (NONOate) derivative [64]. A similar strategy consists in the functionalization of the organic linkers with amines available to interact with NO yielding NONOate compounds. CO adsorption has also been studied in MOFs with openmetal sites. It has been demonstrated that [Cu3(btc)2] preferentially adsorbs CO towards H2 and N2 at 298 K. This can be explained because of the electrostatic interactions between the partial charge of the Cu2+ sites and the CO dipole [65]. On the other hand, Ni-CPO-27 strongly coordinates CO at room temperature [66]. Most of the Ni2+ sites are involved in the interaction (about 80%), forming 1:1 linear adducts whose interaction enthalpy is slightly above 50 kJ mol-1. However, the adsorption is completely reversible at room temperature after prolonged evacuation. As well as for NO, the biocompatible series MIL-88B and MIL-88B-NH2 [Fe3OX(2-NO2-bdc)3], 2-NH2-bdc = 2aminoterephthalate (X = F, Cl, OH), have been studied for the capture and release of CO for therapeutic applications showing that CO binding occurs to unsaturated FeII/FeIII coordination sites generated by the activation procedure [67].

8.4.2 Capture of Volatile Organic Compounds The characteristics of adsorbent materials for the capture of volatile organic compounds can be significantly different from the ones for gases. In this regard, the diffusion kinetics of vapor molecules might be very slow in narrow pore materials and, on the other hand, the presence of open metal sites can be sometimes disadvantageous for air/gas purification purposes in the presence of moisture. As mentioned in the previous section, six isoreticular MOFs, namely MOF-5, IRMOF-3, MOF-177, IRMOF-62, Zn-CPO-27, and [Cu3(btc)2] are shown as selective adsorbents of tetrahydrothiophene, benzene, dichloromethane and ethylene oxide. In the same line as the results of breakthrough experiments on gaseous contaminants, MOF-5 and MOF-177 do not perform well as kinetic adsorption media. IRMOF-62 is largely outclassed by BPL carbon except in the case of ethylene oxide adsorption, for which both materials are equally ineffective. IRMOF-3 is a poor adsorbent for the vapors chosen, because none of them behave as good Lewis acids. MOFs containing open metal sites are found to be the most effective in removing vapors from the gas stream. ZnCPO-27 and [Cu3(btc)2] outperform BPL carbon by an order of magnitude in ethylene oxide adsorption. However, Zn-CPO-27 is not effective against the entire range of vapors, whereas

[Cu3(btc)2] is effective. [Cu3(btc)2] outperforms BPL carbon in tetrahydrothiophene adsorption by a factor of 3 although there is essentially no difference in performance between the activated carbon and [Cu3(btc)2] in dichloromethane and benzene adsorption. It should be noted, however, that the presence of moisture can significantly diminish the performance of [Cu3(btc)2] as a consequence of the blockage of the open metal sites by water molecules. MIL-101 ([Cr3F(H2O)2O(bdc)]) has proved to be effective in the adsorption of a wide range of VOCs with various functional groups and polarities, i.e. acetone, benzene, toluene, ethylbenzene, xylenes, n-hexane, methanol, butanone, dichloromethane and n-butylamine. Yan et al. demonstrated that MIL-101 has higher affinity for VOCs containing heteroatoms or aromatic rings, especially the amines [68]. In addition, MIL-101 shows higher adsorption capacities for acetone, toluene, ethylbenzene and xylenes than other commonly used adsorbents, such as zeolites, resins, activated carbon together with their derivatives [69]. For example, MIL-101 adsorbs 16.7 mmol g-1 of benzene at 288 K and 56 mbar, which is almost twice the values for zeolites (silicalite-1 and SBA-15) and 3–5 times larger than that of activated carbons (Ajax and ACF). In this case, the profile of the thermally programmed desorption curve shows two separated peaks corresponding to two major sites for benzene adsorption. The strong interactions would occur between the Cr3+ metal centers and the adsorbate molecules and the slightly weaker interactions take place between the space of the pores and the adsorbate. Consecutive cycles of adsorption–desorption show that fast desorption kinetics, high desorption efficiency and stable adsorption capacity over five cycles are available. The efficiency of benzene desorption can reach 97% exhibiting a high reversibility of benzene adsorption–desorption [70]. On the other hand, the modulation of the porous network of the anionic MOF NH4[Cu3(OH) (capz)] (capz = 4-carboxypyrazolato) by means of ion-exchange processes of the extra-framework NH4+ cations leads to profound changes in the textural properties of its porous surface and in the adsorption selectivity towards benzene–cyclohexane mixtures [71]. Indeed, exposure of the A[Cu3(OH) (capz)] materials (A = NH4+, Li+, Na+, K+, Me3NH+, Et3NH+) to benzene–cyclohexane 1 : 1 vapor mixtures shows significant enrichment of the adsorbate phase in the benzene component. This behaviour is explained taking into account that although the increasing bulk of the exchanged cations still permit access of molecules to the porous structure, a concomitant increase in size-exclusion selectivity takes place. The flexible MIL-53(Al) framework adsorbs xylene isomers and ethylbenzene [72]. Adsorption isotherms at 110 °C show two well-defined steps and hysteresis, corresponding to the opening or breathing of the framework, as induced by the presence of adsorbing molecules. At low pressures, adsorption leads to framework contraction. In the limited space of the closed form of the MIL-53(Al) framework, only a single-file arrangement of molecules can be accommodated, adsorbed along the length of the pores. At much higher pressures, the pores are reopened. In the open form, there is space enough for the xylene isomers to be adsorbed in pairs along the length of the pores, leading to a doubling of the amount adsorbed [72, 73]. Due to differences in the efficiencies with which the different isomers can be packed inside the pores, the adsorption capacity is highly affected. This phenomenon is known as

commensurate adsorption and occurs when the molecular size and shape of the adsorbate lead to an orientation and adsorbed amount that is compatible and self-consistent with the crystal symmetry and pore structure of the adsorbent [73]. A modified MOF, CuCl2-loaded MIL-47(V) shows remarkable adsorption capacity of benzothiophene [74]. MIL-47(V) shows an unexpected reduction ability to form CuI ions from loaded CuII ions probably because of the presence of VIII in the MOF. The obtained CuI ions show a beneficial effect on the adsorption of benzothiophene probably through π-complexation. It has been demonstrated that the adsorption capacity increases with increasing CuCl2 loading up to a specific content (Cu/V = 0.05 mol mol-1) and decreases with further increase in the CuCl2 content, showing that there is an optimum CuCl2 concentration. This is due to the contribution of both the porosity/acidity of MIL-47(V) and a CuI site derived from CuCl2. On the other hand, the development of sensor materials is also a very active field of research. In this regard, [Zn2(bdc)2- (dpNDI)]n (H2bdc = benzene-1,4-dicarboxylic acid; dpNDI = N,N0-di(4pyridyl)-1,4,5,8-naphthalenediimide) gives rise to strong stacking interactions with VOC molecules with a concomitant strong change in the emission color sensitive to the ionization potential of each molecule (Figure 8.3) [75].

Figure 8.3 (top) Luminescence of the powdered [Zn2(bdc)2(dpNDI)]n material suspended in the liquid exposed to different VOC molecules after excitation at 365 nm using a commercial ultraviolet lamp. (bottom) Collected normalized spectra of the [Zn2(bdc)2(dpNDI)]n@ VOC compounds excited at 370 nm. In this context, the {Fe(pyz)[Pt(CN)4]} system can incorporate a variety of vapor guest molecules, namely benzene and CS2, which modify the pore size and optical and magnetic properties of the materials and might be useful for sensing purposes [76]. As mentioned before, MOFs with coordinatively unsaturated metal sites, such as [Cu3(btc)2], are ineffective for the capture of VOCs in the presence of ambient moisture. With the aim of overcoming this problem, we have reported a novel flexible MOF, namely [Ni(bpb)] [77] (H2bpb = 1,4(4-bispyrazolyl)benzene), which is efficient for the adsorption of benzene and cyclohexane. In addition, this material captures tetrahydrothiophene (up to 0.34 g of tetrahydrothiophene

per gram of material) from CH4–CO2 mixtures in dynamic conditions even in the presence of humidity (60%), thus overcoming the problems raised by prototypic MOF-5 and [Cu3(btc)2] MOF materials in practical applications. As previously mentioned in the introduction, a special family of VOCs is the warfare agents. The search for efficient adsorbents of these harmful molecules in practical applications is of great interest. It should be noted that in real conditions, humidity will always be present and water molecules will constitute an important competitor in the adsorption processof these toxic molecules. In this context, we have demonstrated that the use of highly hydrophobic MOFs for the capture of warfare agents is an adequate strategy for the selective adsorption of these molecules even in extreme humid conditions. The basis of this strategy remains in the high affinity of the porous matrix towards the non-polar warfare agents in contrast with its low affinity for polar water molecules. For example, the MOF-5 analog [Zn4O(dmcapz)3] (dmcapz = 3,5-dimethyl-4carboxypyrazolato) is suitable for the capture of diisopropylfluorophosphate (DIFP, model of sarin nerve gas) and diethylsulfide (DES, model of vesicant mustard gas) [78]. This compound shows remarkable chemical, mechanical and thermal stability granted by the nature of the M–N,O(carboxypyrazolato) coordinative bonds. In addition, it is highly hydrophobic, which results in large VOC–H2O partition coefficients. Under ambient conditions, [Zn4O(dmcapz)3] clearly outperforms the behavior of [Cu3(btc)2] which, when hydrated, does not retain either DES or DIFP. However, the performance of [Zn4O(dmcapz)3] approaches that of molecular sieve activated carbon Carboxen suggesting similar adsorption processes dominated by the small size and apolar nature of the pores in both materials. Then, it has been clearly demonstrated the necessity of stabilizing MOFs against ambient humidity to turn these porous materials more suitable for specialized and industrial applications. In this context, preor postsynthetic modification approaches have been exploredin order to enhance the hydrophobicity of MOFs. One of the most successful approaches is the introduction of alkane or fluoroalkane [79] residues in the organic linkers. In this context, Omary and colleagues reported the synthesis of totally fluorinated MOFs, coined FMOF-1 and FMOF-2, from the combination of Ag+ with 3,5-bis(trifluoromethyl)-1,2,4-triazolate. The hydrophobic character of FMOF-1 is mainly responsible for the observed efficient selective adsorption ability for aliphatic and aromatic oil components (benzene, toluene, p-xylene, cyclohexane and n-hexane), while preventing the entrance of water molecules into their pores. Likewise, the isoreticular series [Ni8(OH)4(H2O)2(L)6] (H2L1 = 1H-pyrazole-4-carboxylic acid; H2L2 = 4-(1H-pyrazol-4-yl)benzoic acid, H2L3 = 4,4’-benzene-1,4-diylbis- (1H-pyrazole), H2L4 = 4,4’-buta-1,3-diyne-1,4-diylbis(1H-pyrazole), H2L5 = 4,4’-(benzene-1,4-diyldiethyne-2,1diyl)bis(1H-pyrazole), H2L5-R (R = methyl, trifluoromethyl)) proves that the use of metal– azolate coordinative bonds gives rise to materials with enhanced stability towards hydrolysis [79]. Moreover, the length and functionalization of the linkers impact on the pore size as well as on the surface polarity (Figure 8.4). The most significant results regarding the capture of warfare agents were achieved under a strongly competitive moist atmosphere (up to 80% RH) and have been compared with the hydrophobic active carbon Blücher-101408, which is the active phase of Saratoga filtering systems. Indeed, 1 H NMR analysis of the adsorbate

phase after the adsorption process of DES in 80% RH moist streams reveals that only [Ni8(OH)4(H2O)2(L5-CF3)6] and Blücher-101408 efficiently capture DES under these conditions, witnessing that the adsorption of DES in this MOF is not affected by the presence of humidity. This isoreticular series might be considered as the first step towards the rational design of MOFs for the capture of harmful VOCs under highly demanding environmental conditions.

Figure 8.4 (A) Crystal structure of highly hydrophobic [Ni8(OH)4(H2O)2 (L5)6] (H2L5 = 4,4’-(benzene-2,5-trifluoromethyl-1,4-diyldiethyne-2,1-diyl)bis- (1H-pyrazole)). (B) Impact of benzene functionalization with hydrophobic residues on the water adsorption isotherms at 298 K [79].

8.4.3 Adsorptive Removal of Iodine The research of iodine for use in the life sciences, marine atmosphere, materials science and nuclear industries has become a growing active field. So, the study of enrichment of iodine is

more valuable [80]. Driven by the recent successful encapsulation of functional species, such as drugs, dyes, light emitters, explosives, etc., into cavities, Morsali group and others successfully used metal–organic frameworks for the removal and recovery of iodine [81-83]. Herein, has been report a detailed investigation that demonstrates the influence of an amine group on the ad/desorption rates of iodine in two novel isoreticular two-fold interpenetrated pillared-layer microporous Zn(II) metal–organic frameworks (Scheme 8.6).

Scheme 8.6 Schematic view of the comparative synthesis of TMU-16 and TMU-16-NH2. There are a considerable number of functional porous coordination polymers that have been synthesized using 4-bpdh as a neutral azine or azo chromophore containing bipyridyl ligand, in combination with dicarboxylate, using the pillar-layer technique. The pyridyl based spacer can act as a pillar to link the metal–carboxylate layer to produce higher dimensional crystalline novel topological frameworks containing channels or cavities. Furthermore, to investigate the influence of an amine group on the adsorption and desorption rate of iodine, the pores in the MOF were successfully functionalized with potentially reactive groups (– NH2) without changing the SBU or the underlying framework topology. The aminefunctionalized isoreticular framework, [Zn2(NH2-BDC)2(4-bpdh)]·3DMF (TMU-16-NH2) was synthesized using an amine substituted BDC ligand. To explore the absorption ability of the compounds to I2, fresh samples of TMU-16 and TMU-16-NH2 (100 mg) were immersed in a hexane (3 mL) solution of I2 (0.005 mol L-1) and were monitored in real time with a camera. For the amine containing network, TMU-16-NH2, the color of the crystals intensified quickly from orange to dark brown (30 min) and the dark brown solution of I2 faded quickly to colorless in less than 30 minutes. While in the non-functionalized analogue, TMU-16, the dark brown solution of I2 faded slower to colorless in about 2 hours. The gravimetric analysis and TGA curves show that each formula unit in TMU-16 and TMU-16-NH2 can adsorb about 0.6 I2 (4.8 I2 molecules per each unit cell). The entry of I2 into the TMU-16 and TMU-16NH2 host frameworks leads to a distinct decrease in intensity of the adsorption band at 520 nm, which corresponds to the concentration of I2. The rapid decline of the UV/vis peak also proves that TMU-16-NH2 can adsorb iodine 1.4 times faster than the non-functionalized analogue. The adsorption amount of I2 was the same for both networks (~45%). The exceptional affinity of both TMU-16 and TMU-16-NH2 for I2 may be attributed to the structural character of the regular π-electron walls made of 4-bpdh.

That is, there is a striking difference compared to conventional adsorbent materials that are lacking an accessible interaction between I2 and the host. Results further support the idea that a judicious selection of building blocks in the assembly process can not only give a predictable new structure but also incorporate interesting properties. So, assembling MOFs with an NH2 group tagged on the linker of the framework, TMU-16-NH2, as a hydrogen bond donor, can find more applications for the encapsulation of iodine. While the amine-functionalized network shows a faster adsorption rate, the delivery of iodine from the non-functionalized homolog is faster. However, the iodine contents were very similar for both networks [83]. The use of synthetic dyes such as azo dyes especially in the textile industry and the discharge of waste material containing these compounds with intensive color and toxicity into the aquatic systems are considered as environmental threats. Several methods including physical, chemical and biological methods have been investigated for removal of dye from waste water. Among them, removal of dye by adsorption technologies is regarded as one of the most competitive methods due to high efficiency, economic feasibility and simplicity of operation. Here we reported two new CdIJII)-based MOFs that has been synthesized by choosing the oba oxygen donor ligand and investigating the effect of N-donor ligands 1,4-bisIJ4pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb) and 4,4’-bipyridine (4,4’-bipy) as pillars. Interestingly, both MOFs were synthesized easily and rapidly via mechanosynthesis. Also, we demonstrate the capability of two MOFs in removal of Congo red dye. Two new MOFs, [Cd2(oba)2(4-bpdb)2]n.(DMF)x (TMU-8) and [Cd(oba)(4,4’-bipy)]n·DMF)y (TMU-9), were successfully synthesized via mechanochemical grinding of N-donor ligands with Cd(II) and H2oba and then analysed by X-ray crystallography. Single-crystal X-ray data show that TMU-8 and TMU-9 possess different structural topologies and different pore sizes. These two new MOFs can be synthesized easily, rapidly and in an environmentally friendly manner within 25 min via mechanosynthesis. These MOFs show high removal efficiency and first-order reaction kinetics in the presence of 50 ppm Congo red solution. The removal efficiency of these MOFs in the presence of 50 ppm CR aqueous solution was investigated. The absorption spectra show that the maximum percent removal of CR is roughly 97.3% (97.3 mg g-1, 112.7 g mol-1) and 92% (92 mg g-1, 48.3 g mol-1) for 25 and 60 min in TMU-8 and TMU-9, respectively. On the other hand, there is a large change in the CR concentration in a shorter time with the TMU-8 MOF. This observation can be attributed to the larger pore size and void space of TMU-8. In order to determine the kinetics of the removal of CR in the solutions suspended on TMU-8 and TMU-9, different kinds of kinetic orders are attempted in expressing the reaction kinetics. Each correlation coefficient was calculated from the kinetic equation, where R0, R1 and R2 represent the correlation coefficients of zero-, first and second-order rate equations, respectively (Table 8.2). The comparison between these correlation coefficients shows that R1 has the best correlation for two MOFs. Therefore, it is suggested that the removal of CR in the solutions suspended on MOFs belongs to first-order

reaction kinetics. Also, the K1 value for TMU-8 is higher than that for TMU-9 so this MOF has higher activity for removing CR [84]. Table 8.2 Kinetic equation of CR removal. MOFs Order(s)

K1(min-1) R0

R1

R2

TMU-8 ln(C0/C) = 0.1271t-0.3981 0.1271

0.7281 0.953 0.7287

TMU-9 ln(C0/C) = 0.0365t-0.0353 0.0365

0.8142 0.9858 0.842

In the petrochemical industry, separation of low research octane number (RON) alkanes by adsorption is a very important method to boost RON as a more environmentally friendly and economically viable process. Recently, selective adsorption of alkane isomers has been investigated on MOFs, which opens a new perspective potential application in the very important petrochemical industry [85]. The pcu (a-Po) net of [Zn2(BDC)2(dabco)]n, which was constructed by [Zn(BDC)] as the layer and dabco as the pillar, shows different adsorption properties for n-hexane (n-HEX, RON: 24.8), 3-methylpentane (3MP, 74.5) and 2,2-dimethylbutane (22DMB, 94.0). A theoretical study on it shows that the alkane isomers are adsorbed along walls defined by linkers, greatly different from the gases that are adsorbed near metal clusters preferentially [85b]. This result suggests the different purposes of separation may be achieved by adjusting the layer distance of similar structures. In order to study whether or not the layer distance of such pillar–layer structure can lead to different adsorption behavior of alkane isomers, a new three-dimensional (3D) structure of {[Zn2(HBDC)2(dmtrz)2].guest}n (1, H2BDC = 1,4benzenedicarboxylate acid, Hdmtrz = 3,5-dimethyl-1H, 1, 2, 4-triazole) was solvothermally synthesized. The adsorption properties of n-HEX, 3MP and 22DMB were studied on 1, showing a selective adsorption of n-HEX and 3MP (low RON compounds) over 22DMB (high RON compound). Furthermore, the selectivities were confirmed by gas chromatography (GC) separation results [86]. Study of structure effect on morphine, methyl orange and other chemicals adsorption affinity on metal-organic framework had been reviewed too [87].

8.5 Sensing in Pillar-Layer MOFs Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) are open, crystalline supramolecular coordination architectures with porous facets. These chemically tailorable framework materials are the subject of intense and expansive research, and are particularly relevant in the fields of sensory materials and device engineering. As the subfield of MOF-based sensing has developed, much diverse chemical functionality have been carefully and rationally implanted into the coordination nanospace of MOF materials. MOFs with widely varied fluorometric sensing properties have been developed using the design principles of crystal engineering and structure–property correlations, resulting in a large and rapidly growing body of literature. The MOFs assembled from metal ions and organic

bridging ligands are promising as modified electrode materials as compared to other coordination compounds, owing to their high surface area, tunable structures, porosity and condensation. Scientists have developed all kinds of MOF sensors in various fields of application [88-91]. MOFs have been widely used as chemical sensors because of the precision with which they can be designed to create favorable interactions of the pore interior with diverse analytes. The various MOF sensors reported thus far operate through luminescence, solvatochromic/vapochromic, interferometry, localized surface Plasmon resonance (LSPR), with colloidal crystals or conductivity and electromechanical detection. Most studies were carried out on luminescence-based MOF sensors for the detection of hazardous materials and high explosives. The characteristics of a good sensor are generally summarized as the “4S”: sensitivity, selectivity, stability and speed of response and recovery times. In most luminescent MOF sensors, response is based on quenching, or enhancement of emission intensity upon guest adsorption. However, only selectivity can be achieved by this type of signal transduction, and not sensitivity. The resulting sensor response can be observed by introduction of any analyte to any compound as a sensor which is usually insufficient for accurate and sensitive detection of a specific analyte. In these cases the introduction of an additional sensing/detection parameter such as a shift in emission frequency (wavelength) is expected to greatly enhance signal transduction from one dimension (1D) to two dimensions (2D) and remove false-positive responses, but has not yet been realized. According to the U.S. Environmental Protection Agency, picric acid is widely recognized as an environmental contaminant and as harmful to humans and wildlife. Morsali and co-workers report a ratiometric luminescent sensor, based on a metal-organic framework being able to create sensitivity for detection of picric acid (PA). It is widely used as a staining agent and reagent in laboratory procedures as well as in the manufacture of rocket fuels, fireworks, matches among others. Apart from its acidic properties, picric acid is unstable and readily reacts with other materials to create explosive compounds. Its explosive power is somewhat superior when compared to that of TNT. In contrast to most luminescent MOF sensors that rely on quenching or enhancement of emission intensity upon guest adsorption (1D), we follow a different strategy to significantly improve accuracy and selective detection of a specific analyte. Their 2D signal transduction approach is achieved by introducing an appropriate guest to the interior of the MOF with a different photoinduced emission wavelength than the host framework. Thereby a ratiometric fluorescent sensor with 2D signal transduction was created in which the response now depends on the ratio of emission intensities at two different wavelengths, e.g. the host and the guest. This type of signal transduction eliminates environmental interference such as drift of light source or detector and concentration changes of competing analytes. This strategy is based on creating a sensor with convertibility of diverse responses from different analytes in various concentrations to unit signal and producing a special signal to specific concentrations of a target analyte. In contrast to most reported MOF sensors, and in order to obtain an improved sensor, the effective parameters (4S) were considered and investigated. Interestingly for the first time in this kind of sensors differentiation between response and sensitivity has been observed and thus it shows particular sensitivity to a certain analyte with a specific concentration. TMU-5, [Zn(oba) (4-

bpdh)0.5]n.1.5DMF (H2oba = 4,4-oxybisbenzoic acid and 4-bpdh = 2,5-bis(4-pyridyl)-3,4diaza-2,4-hexadiene, DMF = N,N-dimethylformamide), is a Zn paddlewheel based metalorganic framework in which narrow, interconnected and azine decorated pores generate favourable interactions with acidic analytes [92, 93]. They detail how dye sensitized MOF TMU-5 can act as a ratiometric fluorescent sensor for selective detection of low concentrations of picric acid (PA) in presence of nitroaromatic compounds (4-nitroaniline (NA), nitrobenzene (NB), 4-nitrophenol (NP)) and representatives of volatile organic compounds (VOCs) (acetone, toluene, methanol and ethanol). The TMU-5S sensor has great selectivity to PA which arises from existence of basic azine groups (fluorophores) in the narrow pore walls. In addition, 2D signal transduction can eliminate environmental interference and enables selective response to a specific concentration of PA. The selectivity of MOFs to specific analytes can be explained by donor - acceptor electrontransfer mechanisms. Generally, this behavior arises from specific interactions between the guest and MOF fluorophores (depending on size and type of interaction) as well as the position of molecular orbital and band structure of MOF and analyte [94]. In other effort has been aimed at multifunctional MOFs, flexible D-camphor acid (D-H2cam), rigid 4,4’-bis(1-imidazolyl) biphenyl (bimb) and transition-metal centers (Mn2+/Co2+/Cd2+) were selected to construct new structures with potential applications in luminescence sensing and liquid-phase separation based on the following considerations: (1) the binding between D-H2cam and metal centers, not only adds flexibility and diversity in the structures, but also alters the electronic structures and surface functionalities of the MOFs, which may direct their specific recognition for guest substrates through host–guest interactions; (2) rigid ligand bimb with aromatic π rings may effectively favor intraligand interactions and promote luminescent character. Herein, we present three MOFs [M2(D-cam)2(bimb)2]n.3.5nH2O (M = Mn for 1, Co for 2) and [Cd8(D-cam)8(bimb)4]n (3), which exhibit structural diversity. The charming aspect of these frameworks is that, compound 3 is the very first MOF-based sensor for quantitative detecting three different types of analytes (metal ions, aromatic molecules and pesticides). And also, both compounds 2 and 3 show rapid uptake and ready regeneration for the methyl orange (MO), as well as can selectively MO over methylene blue (MB) with high MO/MB separation ratio. The solid-state photoluminescence (PL) spectra of assynthesized 3 and free ligand bimb were investigated upon excitation at 300 nm under ambient temperature. Compound 3 exhibits a strong band at around 370 nm, and an obvious blue-shift compared with the organic linker (λem = 410 nm), which may arise from the ligand-centered emission. The luminescence lifetime of compound 3 is as follows: τ1 = 1.5032 ns. Interestingly, compound 3 in distinct solvent emulsions displays strong guestdependent luminescence properties, which shows the most significant quenching effect toward acetone, implying that the compound can be considered as candidates for selective detecting of acetone (Figure 8.5a). It is noticeable that acetone exhibits a wide absorption range from 225 to 325 nm whereas the excitation spectrum of compound 3 centered at about 300 nm, which is apparently overlapped by the absorption band of acetone. Therefore, the significant PL quenching effect could be ascribed to the energy transfers between the organic

ligand and solvent molecule upon excitation [95, 96]. Furthermore, a gradual depression of the PL intensity was observed upon the addition of acetone to the DMF emulsion of 3, and the PL intensity of 3 centered at 370 nm versus the volume ratio of acetone could well be fitted to first-order exponential decay function, demonstrating that PL quenching of 3 by acetone is diffusion-controlled.

Figure 8.5 The PL intensities of compound 3 introduced to various pure solvents (a) and different metal ions (b) in DMF. (c) The PL intensities of 3 as a function of Cu2+ at different concentrations in DMF, the inset shows the emission quenching linearity relationship of 3 below 3×10-4 M (I = -160.55284 c +802.64287, R = 0.99376) excited at 300 nm. Meanwhile, the potential applications of 3 in sensing metal ions were also examined. The 5 mg as-synthesized 3 was dispersed in 5 mL DMF individually containing 1.0×10-2 mol.L-1 M(NO3)2 (M= Co2+, Ni2+, Cu2+, Zn2+ and Cd2+) for the sensing studies. As shown in Figure 8.5b, the PL intensities of M-incorporated 3 emulsions are heavily metal ions dependent. Apparently, the introduction of Zn2+ has a negligible effect on the PL intensity, while addition of cations Cd2+, Co2+, Ni2+ and Cu2+ can cause PL quenching in various degrees. Particularly, the PL intensity of 3 is dramatically quenched by about 80% upon addition of

Cu2+, underlining the bright potential of compound 3 for the sensing of Cu2+, which may be interpreted that the d–d transition of Cu2+ wasted the energy of the system [97, 98]. To explore sensitivity toward Cu2+ in detail, a batch of emulsions of 3 dispersed in DMF solutions containing different amounts of Cu2+ were prepared to monitor the emissive response. Clearly, as shown in Figure 8.5c, the PL intensity decreases gradually with the increase of the Cu2+ concentration and the inset illustrates the emission quenching of 3 is linear to Cu(NO3)2 concentration below 3.0×10-4 mol.L-1 at 370 nm. The detection limit of 5.805×10-6 mol.L-1 toward Cu2+ in DMF suspension for compound 3 is calculated from the equation: LOD = kSb/m, where k = 3, Sb is the standard deviation of the blank sample and m is the slope of the calibration graph in the linear range. The investigation of luminescent sensors of Cu2+ with high selectivity and sensitivity is remarkably attractive, due to its potential significance in living biological systems. Rapid and efficient detection of explosives and explosive-like substances is very important to homeland security and environmental safety, therefore, the detecting of nitro-aromatic molecules by MOFs is also an important topic [99]. The intensity of the PL in 3 are largely dependent on the aromatics, exhibiting significant quenching behavior in the case of nitrobenzene, which may be related to the electron transfer from the electron-donating MOF to the highly electron-deficient nitrobenzene molecule upon excitation (Figure 8.6a). To examine sensitivity towards nitrobenzene in more detail, a batch of suspensions of 3 with gradually increasing nitrobenzene contents was prepared to monitor the emissive response. Results show the emission quenching of 3 is linear to nitrobenzene content below 3.27×10-4 mol.L-1 at 300 nm. The detection limit toward nitrobenzene in DMF suspension for compound 3 is 6.972×10-6 mol.L-1.

Figure 8.6 The PL intensities of 3 toward selective aromatic molecules with concentration of 6.5×10-4 M (a) and relevant pesticides with concentration of 1×10-3M (b) in DMF. Owing to the high toxicity, rapid determination and reliable quantification of a trace level of pesticides for pest control have aroused increasing attention [100]. Therefore, inspired by the work to utilize 3 for the sensing nitrobenzene, the potential of complex 3 for the detecting of nitro-containing pesticides was carried out. The effects of 10-3 mol.L-1 relevant pesticides in

DMF on the fluorescence response of 3 were recorded. Strikingly, the results indicate that among the different pesticides tested, compound 3 exhibits the most effective detection of parathion-methyl: the PL intensity decreased to 60% at 1×10-4 mol.L-1, and complete quenching was received at 1×10-3 mol.L-1. The encouraging result reveals 3 could be a promising luminescent probe for nitro-containing pesticide. Furthermore, Figure 8.6b shows the emission quenching of 3 are in linear correlation with parathion-methyl concentration below 1×10-4 mol.L-1 at 370 nm and the detection limit reaches 3.576×10-6 mol.L-1. Compound 3 is the very first MOF-based sensor for quantitative detecting three different types of analytes (metal ions, aromatic molecules and pesticides) simultaneously through fluorescence quenching, although limited MOFs for detecting one or two of them were investigated.

8.6 Conclusion There are several advantages in mixed linkers based MOFs compared to the MOFs synthesized from a single linker. First and foremost, it is conceivable to tune the pore size/surface area of the MOFs in a greater extent by opting the linkers of different sizes and lengths. MOFs with two linkers consent much more design ability. The mixed linkers MOFs are found to be highly flexible which would be realized by variable metal geometry, versatile linker flexibility and weak metal-ligand binding. These flexible MOFs divulge many interesting properties like gated and stepwise adsorption, selective adsorption and molecular switching applications. In particular such materials showed applications in sensing and separation of gases.

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Glossary Bridging Ligand: In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides or to ligands that are specifically designed to link two metals. In naming a complex wherein a single atom bridges two metals, the bridging ligand is preceded by the Greek character ‘mu’, µ, with a subscript number denoting the number of metals bound to the bridging ligand. µ2 is often denoted simply as µ. When describing coordination complexes care should be taken not to confuse µ with η (eta), which relates to haptocity. Ligands that are not bridging are called terminal ligands. Chelating Agent: A chelating agent is a substance whose molecules can form several bonds to a single metal ion. In other words, a chelating agent is a multidentate ligand. An example of a simple chelating agent is ethylenediamine. Chemical Stability: when used in the technical sense in chemistry, means thermodynamic stability of a chemical system. Thermodynamic stability occurs when a system is in its lowest energy state, or chemical equilibrium with its environment. This may be a dynamic equilibrium, where individual atoms or molecules change form, but their overall number in a particular form is conserved. This type of chemical thermodynamic equilibrium will persist indefinitely unless the system is changed. Chemical systems might include changes in the phase of matter or a set of chemical reactions. Coordinate Covalent Bond: A coordinate covalent bond, also known as a dative bond or coordinate bond is a kind of 2-center, 2-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal ions to ligands involves this kind of interaction. Coordination Number: the number of atoms or ions immediately surrounding a central atom in a complex or crystal. Crystal Engineering: Crystal engineering is the design and synthesis of molecular solid state structures with desired properties, based on an understanding and use of intermolecular interactions. The two main strategies currently in use for crystal engineering are based on hydrogen bonding and coordination bonding. These may be understood with key concepts such as the supramolecular synthon and the secondary building unit. Drug Delivery:

refers to approaches, formulations, technologies, and systems for transporting a pharmaceutical compound in the body as needed to safely achieve its desired therapeutic effect. It may involve scientific site-targeting within the body, or it might involve facilitating systemic pharmacokinetics; in any case, it is typically concerned with both quantity and duration of drug presence. Drug delivery is often approached via a drug’s chemical formulation, but it may also involve medical devices or drug-device combination products. Drug delivery is a concept heavily integrated with dosage form and route of administration, the latter sometimes even being considered part of the definition. Fluorescence Anisotropy: or Fluorescence Polarization is the phenomenon where the light emitted by a fluorophore has unequal intensities along different axes of polarization. Interpenetration: An Interpenetrating polymer network (IPN) is a polymer comprising two or more networks which are at least partially interlaced on a polymer scale but not covalently bonded to each other. The network cannot be separated unless chemical bonds are broken. Without breaking chemical bonds; they are polymer blends. Luminescence: Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. Magnetic Susceptibility: (χM), is the degree of magnetization of a material in response to an applied magnetic field. If magnetic susceptibility is positive then the material can be paramagnetic, ferromagnetic, ferrimagnetic, or antiferromagnetic. Metal–Organic Frameworks: (MOFs) are compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. They are a subclass of coordination polymers, with the special feature that they are often porous. Metal-to-Ligand Charge Transfer (MLCT) and Ligand-to-Metal Charge Transfer (LMCT): In inorganic chemistry, most charge-transfer complexes involve electron transfer between metal atoms and ligands. The charge-transfer bands of transition metal complexes result from shift of charge density between molecular orbitals (MO) that are predominantly metal in character and those that are predominantly ligand in character. If the transfer occurs from the MO with ligand-like character to the metal-like one, the complex is called a ligand-to-metal charge-transfer (LMCT) complex. If the electronic charge shifts from the MO with metal-like character to the ligand-like one, the complex is called a metal-to-ligand charge-transfer (MLCT) complex. Thus, a MLCT results in oxidation of the metal center, whereas a LMCT results in the reduction of the metal center. Resonance Raman spectroscopy is also a powerful technique to assign and characterize charge-

transfer bands in these complexes. Multivariate Metal-Organic Frameworks (MTV-MOFs): contain multiple linker types within a single structure. Arrangements of linkers containing different functional groups confer structural diversity and surface heterogeneity and result in a combinatorial explosion in the number of possible structures. Nanostructure: is a structure of intermediate size between microscopic and molecular structures. Nanostructural detail is microstructure at nanoscale. Non-Linear Optical: (NLO), is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization P responds nonlinearly to the electric field E of the light. This nonlinearity is typically only observed at very high light intensities such as provided by pulsed lasers. Nonlinear Optics (NLO): is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization density P responds nonlinearly to the electric field E of the light. The non-linearity is typically observed only at very high light intensities (values of atomic electric fields, typically 108 V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds. Pillared Metal-Organic Frameworks: Pillar-layered MOFs are among the most studied research area related to inorganic polymers. Investigating their properties and application as a multi donor 3-D porous frameworks is of interest. Modifying pillar moieties as a third building blocks of pillarlayered MOFs, together with metal nodes and oxygen donor linkers can enhance controlling structure assembly and led to specific properties into obtained structures. The structure of pillar can be easily modified which cause better designing of desired structural topology and pore environment. Photo Excitation: is the photoelectrochemical process of electron excitation by photon absorption, when the energy of the photon is too low to cause photoionization. The absorption of the photon takes place in accordance with Planck’s quantum theory. Post-Combustion Capture: refers to the removal of CO2 from power station flue gas prior to its compression, transportation and storage in suitable geological formations, as part of carbon capture and storage. A number of different techniques are applicable, almost all of which are adaptations of acid gas removal processes used in the chemical and petrochemical industries. Many of these techniques existed before World War II and, consequently, post combustion capture is the most developed of the various carbon-capture methodologies. Pre-Combustion Capture:

A pre-combustion system involves first converting solid, liquid or gaseous fuel into a mixture of hydrogen and carbon dioxide using one of a number of processes such as ‘gasification’ or ‘reforming’. Reforming of gas is well-established and already used at scale at refineries and chemical plants around the world. Gasification is widely practiced around the world and is similar in some respects to that used for many years to make town gas. The hydrogen produced by these processes may be used, not only to fuel our electricity production, but also in the future to power our cars and heat our homes with near zero emissions. SBUs: Secondary Building Blocks, Subunits of a MOF SCM: Single chain magnets, Single-chain magnets are molecular spin chains displaying slow relaxation of the magnetisation on a macroscopic time scale. Sonochemistry: In chemistry, the study of sonochemistry is concerned with understanding the effect of ultrasound in forming acoustic cavitation in liquids, resulting in the initiation or enhancement of the chemical activity in the solution. Therefore, the chemical effects of ultrasound do not come from a direct interaction of the ultrasonic sound wave with the molecules in the solution. The simplest explanation for this is that sound waves propagating through a liquid at ultrasonic frequencies do so with a wavelength that is significantly longer than that of the bond length between atoms in the molecule. Therefore, the sound wave cannot affect that vibrational energy of the bond, and can therefore not directly increase the internal energy of a molecule. Instead, sonochemistry arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid. The collapse of these bubbles is an almost adiabatic process, thereby resulting in the massive build-up of energy inside the bubble, resulting in extremely high temperatures and pressures in a microscopic region of the sonicated liquid. Syngas: or Synthesis Gas, is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. The name comes from its use as intermediates in creating synthetic natural gas (SNG) and for producing ammonia or methanol. Syngas is usually a product of gasification and the main application is electricity generation. Syngas is combustible and often used as a fuel of internal combustion engines. It has less than half the energy density of natural gas. Topology: topology provides a convenient way of describing and predicting the molecular structure within the constraints of three-dimensional (3-D) space. Given the determinants of chemical bonding and the chemical properties of the atoms, topology provides a model for explaining how the atoms ethereal wave functions must fit together. Molecular topology is a part of mathematical chemistry dealing with the algebraic description of chemical compounds so allowing a unique and easy characterization of them.

π-π Stacking Interactions: In chemistry, pi stacking (also called π-π stacking) refers to attractive, noncovalent interactions between aromatic rings, since they contain pi bonds.

Subject Index A Adsorption 5, 10, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 27, 28, 29, 36, 38, 40, 46, 54, 59, 60, 61, 64, 65, 66, 63, 131, 132, 133, 134, 135, 141, 142, 144, 147, 157, 161, 163, 165, 244, 256, 259, 266, 276, 298

B Building blocks 2, 4, 9, 45, 46, 52, 62, 66, 77, 129, 244, 245, 319, 345

C Catalysis 2, 7, 8, 30, 134, 256, 262, 276, 289, 290

G Gas storage 2, 7, 8, 11, 13, 40, 66, 67, 276, 277, 291

M Magnetism 30, 31, 276 Metal Organic Framework 1, 2, 4, 5, 6, 10, 12, 13, 16, 17, 18, 19, 20, 21, 22, 24, 26, 27, 28, 32, 34, 39, 40, 45, 49, 51, 54, 61, 66, 69, 262, 277, 298, 299, 317, 322, 323, 324 Motifs 61, 139, 168, 177, 282

N Node 45, 51, 132, 140, 177, 251, 261, 289, 290

P Pillar 29, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 60, 61, 63, 64, 65, 66, 67, 68, 69, 70, 71, 77, 78, 129, 130, 131, 132, 133, 134, 136, 137, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 157, 158, 160, 162, 163, 164, 165, 166, 177, 244, 245, 246, 249, 255, 260, 261, 263, 264

Porosity 1, 2, 4, 7, 10, 11, 30, 31, 35, 38, 40, 48, 60, 61, 67, 132, 140, 161, 162, 165, 244, 255, 259, 261, 275, 276, 282, 283, 287, 299, 305, 313, 322

S Self-assembly 52, 60, 61, 134, 247 Sensor 13, 31, 34, 35, 36, 40, 149, 165, 288, 313, 322, 323, 324, 327, 329 Separation 2, 7, 11, 19, 20, 25, 26, 27, 28, 29, 30, 59, 61, 67, 70, 71, 130, 165, 247, 276, 277, 279, 280, 288, 298, 300, 321, 325 Surface area 1, 4, 5, 10, 14, 15, 16, 17, 19, 21, 26, 35, 47, 61, 65, 134, 162, 165, 166, 258, 260, 263, 269, 275, 276, 285, 288, 322, 329

T Topology 2, 4, 5, 6, 8, 11, 31, 45, 49, 50, 52, 55, 77, 129, 136, 137, 139, 144, 244, 250, 278, 294, 318

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