An experimental investigation of phosphino compounds of boron hydrides

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AN EXPERIMENTAL INVESTIGATION OP PHOSPHINO COMPOUNDS OF BORON HYDRIDES

A Thesis Presented to the Faculty of the Department of Chemistry University of Southern California

In Partial Fulfillment of the Requirements for the Degree Master of Science

by Ross Irving Wagner August 1950'

UMI Number: EP41592

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T h is thesis, w ritten by . R 0 8 3 . I M I N G . M 5 M 8 .....

under the guidance of h%.6~. F a c u lty Com m ittee, and app ro ved by a l l its members, has been presented to and accepted by the C o u n cil on G raduate Study and Research in p a r t ia l f u l f i l l ­ ment o f the requirements f o r the degree of

MASTER OF...SCIENCE.................

Date.

Faculty Committee

hair man

....

J

ACKNOWLEDGMENT To Dr. Anton B. Burg for his thoughtful advice, helpful suggestions on theoretical interpretation, and constructive criticism of both the experimental techniques and the preparation of this manuscript • The generous support of this work by the Office of Naval Research is gratefully acknowledged#

TABLE OP CONTENTS CHAPTER I. II.

PAGE

INTRODUCTION TO THE PROBLEM.....................1 PREPARATION AND PURIFICATION OF D I M E T H YL P H O S P H I N E........................... 6

III.

PREPARATION AND PURIFICATION OF DIMETHYLPHOSPHINOBORINE.................... 13 Dimethylphosphine B o r i n © ............

14

The Phosphinolysis Reaction

...............

15

The Phosphinolysis Products

. . . . . .

17

Chemical Properties of the Trimer

. . . .

19

Treatment of the Trimer with Diborane . . .

21

Chemical Properties of the Tetramer

21

. . .

Purification of the Trimer and Tetramer

22

Combustion Analysis of the Trimer and Tetramer...................................23 Vapor Tensions of the Trimer and Tetramer IV.

•

24

ATTEMPTED PREPARATION OF SODIUM DIMETHYLPHOSPHINIDE.......................... 28

V. VI.

DISCUSSION S U M M A R Y ..............

............. 33 38

BIBLIOGRAPHY.........................................39

LIST OP TABLES TABLE I.

PAGE Vapor Tensions of Dimethylphosphine Borine .

II.

....................................16

Vapor Tensions of Dimethylphosphinoborine T r i m e r .......................................26

III.

Vapor Tensions of Dimethylphosphinoborine Tetramer

................................. 27

LIST OP FIGURES FIGURE

PAGE

1.

Apparatus for Preparation of # Dimethylphosphine.............................. 8

2.

Magnetically Operated Separatory Funnel

. . .

12

CHAPTER I INTRODUCTION TO THE PROBLEM DIborane reacts with hydrides of the group V elements in the sense of a Lewis acid to form addition compounds containing two moles of the base per mole of diborane.

With ammonia diborane forms a salt-like product,

the chemical behavior of which suggests an ammonium salt of a monobasic acid2 according to the equation BgHg

*

2

NH3

--- 1-

HH^HgBHHgBHg]-

Substitution of trimethylamine for ammonia gives a volatile product having a molecular weight corresponding to the following formulation, B2H6 * 2 (CHg)3N

2 (CH3 )3N:BH3

The hydride of the second of the group V elements, phosphine, reacts with diborane to give a product somewhat similar to the product obtained with ammonia and diborane.^

^ A. Stock, K. Priederici, and 0, Priess, Ber., 46, 1353 (1913). 2

H. I. Schlesinger and A. B. Burg, J. Am. Chem. Soc., 60, 290 (1938). A. B. Burg and H. I. Schlesinger, J. Am. Chem. Soc., 59, 785 (1937). 4 E. L. Gamble and P. Gilmont, J. Am. Chem. Soc., 62, 717 (1940).

This product has properties corresponding to a phosphonium salt of a monobasic acid*

For example, only half of the

phosphine associated with the diborane is liberated upon treatment with liquid ammonia, in agreement with the equations B2H6 * 2 PH3 —

PH4[H3BPH2BH3]

PH4 [H3BPH2BH3 ]_

«• NH3

f- MH^HgBPHgBHg]

«■ PH3.

Treatment of any of the methyl substituted phosphines with diborane has not been reported. Thus it appears that in some cases, at least, bond­ ing of the group V element to two boron atoms can occur* With the possible exception of the phosphine borine case no compounds containing a B-P-B skeleton are known, whereas types of compounds containing B-N-B bonding have been 5

reported in wider variety.

Burg and Randolph

prepared

dime thy laminodiborane and methylaminodiborane which are derivatives of the previously known compound BgH^N, aminodiborane,

■

thus completing the series

H,C.

H,a

H2Bw BH2 H

H2 < / BH2 H

3X

H

X

H

E

X

H2
— CHgPHg 4- NafCl7

(2)

Then the methylphosphine was put through the same process to yield the desired product in accordance with the equa­ tions CHgPHg «■ Na — »— Na^PHCHg «■ § Hg Na’-PHCHg «• CHgCX — s—

(CHg)gPH

* Na*Cl~

(3) (4)

The apparatus in which these reactions were carried out is shown in Figure 1.

The lower portion of the

------ I-----W. C. Fernelius, Inorganic Syntheses, Vol. II, pp. 141-144, McGraw Hill Book Gompany, I§46• 5 A. Stock, Hydrides of Boron and Silicon, Gornell University Press, 1§33. R. T. Sanderson, Vacuum Manipulation of Volatile Compounds, Wiley and Sons, 194$ •

8

FIGURE APPARATUS

FOR

I

PREPARATION

OF

DIMETHYLPHOSPHINE

9 reaction vessel (A) containing the liquid ammonia was main­ tained at -78° with a dry ice-ether bath during the course of the reactions.

Capsules of sodium were added from (B) and

phosphine or methyl chloride as required was added from (C). The trap (D) was used both to prevent loss of unused react­ ants or solvent, and to store the reaction mixture when (A) required cleaning* In a representative run, a 250 ml* sample of anhydrous liquid ammonia was condensed in (A) under an atmosphere of dry nitrogen.

Sodium capsules containing approximately

0*05 g. each of metal (prepared by drawing molten sodium into 4 mm. glass tubing, cooling, and cutting into 10 mm. lengths) were added from (B) several at a time.

A weighed

supply of phosphine was placed in (C) at -78°; the result­ ing pressure of 1*5 atm. permitted easy delivery to (A). After each addition of sodium, phosphine was introduced until the blue color of the sodium solution was replaced by the light yellow of the sodium phosphinide solution*

Alter­

nate addition of sodium and phosphine was continued until the supply of the latter was exhausted, whereupon the phos­ phine which had passed through the sodium solution without reaction was recycled from the trap (D)•

At this point

3.65 g. (0.159 mole) of sodium had reacted according to equation (1) above.

This amount corresponds to 86% of that

required by the 6.29 g. (0.185 mole) of phosphine used.

10 Any further addition of sodium to obtain a more complete reaction with the dissolved phosphine was not justified because of the slow rate of reaction at these low concen­ trations and the low temperature* The tube (C) was replaced by another containing a sample of methyl chloride equivalent to the sodium used and maintained at -10° (1.5 atm*).

The methyl chloride

condensed as it was passed into the liquid ammonia solution and it was necessary to keep the inlet tube free of precip­ itated sodium chloride by intermittent flushing with nitro­ gen.

The yellow color of the sodium salt faded out and

sodium chloride precipitated; then there appeared a second liquid phase more dense than ammonia— presumably monomethylphosphine, formed according to equation (2). The volatile mixture was distilled away from the sodium chloride into (D); the reaction vessel (A) was re­ moved and cleaned. it.

The mixture was then redistilled into

Sodium was added in portions of 0.10-0.25 g. and

sufficient time was allowed after each addition for the sodium solution to change into the chrome-yellow sodium monomethylphosphinide solution according to equation (3)• At first, a 0.10 g. portion of sodium was consumed in 10 minutes but the reaction became progressively slower as it neared completion until 0.10 g. of sodium required three hours to react at -78°.

At this point 79$ (0.146 mole) of

11 the theoretical amount of sodium had been used, relative to the original phosphine. Methyl chloride in amount equivalent to the sodium usediin this reaction was introduced and reacted to give again a second phase more dense than ammonia (equation (4)}.

The two-phase mixture was distilled from the sodium

chloride into the trap (D) and introduced into the vacuum system. A mechanical separation of the phases was accom­ plished at -78° in the magnetically operated separatory funnel shown in Figure 2.

Fractional distillation of the

ammonia-rich phase through a helix-packed, vacuum-jacketed column indicated that it contained a small percentage of the desired product, the recovery of which was not justified from the standpoint of time required.

Fractional condensa­

tion of the phosphine-rich phase yielded primarily dimethyl­ phosphine after the removal of some ammonia and monomethyl phosphine.

The dimethylphosphine was further purified by

distillation through a fractionating column with a vapor take-off head at -110° and the boiler at -80° to yield a product having a vapor pressure of 342 mm. at 0°.

Since

some of the product was not isolated and purified, the yield was not calculated but it is estimated to be at least 25-30$ based on phosphine.

12

FIGURE MAGNETICALLY

OPERATED

2 S E P A R A T OR Y

F UNN E L

CHAPTER III PREPARATION AND PURIFICATION OF DIMETHYLPHOSPHINOBORINE The current literature contains no mention of dimethylphosphinoborine or any other alkyl-substituted phosphinoboron hydride.

Consequently, the method of prepa­

ration parallels as closely as possible the method used in the preparation of dimethylaminoborine,1 the nitrogen ana­ logue of the desired product.

Due to differences in physical

and chemical properties of the nitrogen and phosphorus ana­ logues encountered in the synthetic sequences, the methods differ for-reasons of either convenience or necessity. The first step in such a synthesis is the formation of-an addition compound of the base, in this case dimethyl­ phosphine, with diborane in agreement with the equation (CH3)2PH 4- BgHg —

2 (CH3)2HP:BH3 .

(1)

The addition compound, dimethylphosphine borine, upon heat­ ing, yields the theoretical amount of hydrogen in accord­ ance with the equation (CH3 )2HP:BH3 —

(CH3)gPBHg ♦ Hg.

Thus, a material balance) indicates that the residue remain­ ing after the removal of the hydrogen is some form of

Burg and Randolph, loc. cit.

14 dimethylphosphinoborine * This residue consists primarily of two difficultly volatile solid materials, the molecular weights and analy­ ses of which indicate the trimer and tetramer of dimethyl­ phosphinoborine,

In addition, there is a very small amount

of a liquid of similar volatility as well as a small quantity of less volatile solid material, presumably higher polymers.

The yield of trimer plus tetramer is nearly

quantitative; hence a monomer, dimer, pentamer, or higher polymers cannot have formed In more than trace quantities. Dime thylpho sphi ne Borine. In a preliminary experio ment, 15.0 cc. of diborane and 30.0 cc. of dimethylphos­ phine were condensed into a high temperature tensimeter^ held at -196°.

The mercury in the manometer arms was

allowed to rise, and the temperature in the tubulature of the tensimeter was raised -to -80°, whereupon a vigorous reaction occurred.

That a 2:1 addition compound of di­

methylphosphine and diborane formed in accordance with equation (1), was indicated by the decrease in pressure to zero.

The melting point of the product (dimethylphosphine

borine) was observed as -22.6° (SOg vapor tension thermomg All volumes refer to cc. of gas reduced to standard conditions of 760 mm. pressure and 273.2°A. 3 A. B. Burg and H. I. Schlesinger, J. Am.* Chem. Soc., 59, 785 (1937).

15 eter read 419 mm.).

4

The tenslmeter was immersed in an

oil bath and the vapor tension was measured as a function of temperature.

The data given in Table I determine the

equation

lo*l