The reaction of monobromodiborane with phosphine

INORG.

NUCL,

CHEM.

LETTERS

Vol. 3,

Pp.

87-92,

1967.

PergQrnon press

Ltd.

Printed

In

Great

THE REACTION OF MONOBROMODIBORANE WITH PHOSPHINE

J. Chemistry

E.

Drake and J.

Department,

Simpson

The U n i v e r s i t y ,

Southampton,

England

(Recelved16Decem~r1966) Monobromodiborane at

-78 °.

(B2HsBr)

With reactants

-45 °

to give

with

a two-fold

equimolar

diborane

Reaction typical

with

in equimolar

of

the

is

monobromodibbrane into

vacuum line.

from -196 ° to -78 ° and held was formed and

negligible

for

the

the

formation

with

evolution

of diborane

solid

changed

to a colorless

further

increases

viscous series

liquid. of

in

adduct.

liquid

volatile

is c o n s i s t e n t

(approximately

for

products with

the

87

A white

- a s would be remained

was a sharp

increase

temperature

(-45 ° ) the

which was stable hydrogen

25 m l )

t o warm up

one hour.

The p r e s s u r e

At t h i s

slowly

In a

were allowed

-45 ° when there

(O.32m.mole).

it

and phosphine

vessel

temperature

The

addition).

was negligible

of

temperature,

No o t h e r

reactions

that

o f a 1:1

up t o a t e m p e r a t u r e

at

nearly

(PH3BH3).

(O.66m.mole)

a reaction

pressure

phosphine

occurs

temperature

(1:1

The r e a c t a n t s

at

gives

horane

which

and phosphine

on a c o n v e n t i o n a l

expected

t o O° a t

of monobromodiborane

were distilled

decomposition

decomposition

of hydrogen.

experiment,

with

m o n o b r o m o b o r a n e !PH3BH2Br) , b u t

phosphine,

stable

a 1:1 a d d u c t

amounts,

evolution

(O.66m.mole)

solid

forms

o f PH3BH2Br a n d p h o s p h i n e

monobromoborane adduct polymerises

readily

and phosphine

excess

amounts

(1)

t o O° .

was evolved

leaving

were formed. observations:

With a

The f o l l o w i n g

B,l'~otn.

88

REACTION OF JIONOBROMOOIBORANE WITH PHOSPHINE

_78 °

B2H5Br + PH3

Vol. 3,Ne.3

H

)

H3P.B.H.BH 3 ( s ) Br

H H3P.B.H.BH 3 (s) Br

_45 °

H3P. BH2Br (1>

> O°

The d e c ~ p o s i t i o n described

for

(2),

which gives

is analogous

in the polymertsation

dissociates

into diborane

to that

(H2BPH2) n and H2 on

b u t o n l y u n d e r more s e v e r e c o n d i t i o n s .

The e x p e r i m e n t was r e p e a t e d

reaction vessel.

1/n(H2PBHBr) n ( 1 ) + H2.

of the monobromoborane adduct

that result

PH3BH3 r e a d i l y

}B2H 6 + H3P.BH2Br (I)

)

the borane adduct

pol~er~sation,

conditions

)

Under the

o f PH3BH2Br , t h e a d d u c t

and p h o s p h i n e .

using a standard

nmr t u b e ( 3 ) a s t h e

The m i x t u r e o f m o n o b r o m o d i b o r a n e ( 7 . 0 m . m o l e ) and

phosphine

( 7 . O m . m o l e ) g a v e t h e e x p e c t e d amount o f d i b o r a n e

(3.5m.mole)

at

T h i s was d i s t i l l e d

The p r o t o n

-45 ° .

o f f and t h e t u b e was s e a l e d .

nmr s p e c t r u m o f t h e c o l o r l e s s A-60 and i s c o n s i s t e n t

with

e x p e c t e d -PH 3 r e s o n a n c e . downfield

from external

p h o s p h o r u s a t o m and i t s

liquid

was r e c o r d e d a t

the structure

PH3BH2Br.

There is a doublet TMS, w h i c h i s attached

centered

the result

- 2 0 ° on a V a r i a n FIG. at

1 shows t h e 5 . 1 ppm

of coupling

between the

h y d r o g e n a t o m s (JPH = 405 c p s ) .

Coupling of the two hydrogen atoms on boron with those on phosphorus splits each component of the phosphine doublet into a 1:2:1 triplet (JHI~" 6 cps). broad

The -BH~ resonance is not resolved but appears as a

'hump' centered at approximately 1.9 ppm downfield of TMS.

However,

the integration shows that the ratio of the intensities of the

-PH 3 and -BH~ resonances is 3:2.

The spectrum must be run below O °

because above this temperature bubbles are formed and the liquid becomes very viscous.

All fine structure in the spectrum is lost but a doublet

due to hydrogen atoms on phosphorus can still be seen, centered at 5.8 ppm downfleld of TMS (JPH m 422 cps).

This is at least consistent with the

formation of polymers of the type (PH2BHBr) n. Reaction of monobromodlborane and phosphine (1:2 addition).

In a

Vol. 3, No. 3

REACTION OF MONOBROMODIBORANE WITH PHOSPHINE

typical experiment monobromodiborane were

distilled

into

the

reaction

89

(0.65m.mole) and p h o s p h l n e

vessel

on t h e

vacuum line,

(1.30~.mole)

The reactants

If J

t 5 . 1 ppm Proton

Nmr, S p e c t r u m

JPH = 4 0 5 c p s

JHH'

FIG.

were for

allowed

the

1:1

value at

t o warm up f r o m

one hour.

indicated

-45 °

A white

by the adduct.

and

of

the

so when solid

adduct.

the

the

(a

total

as

solid

of O.96m.mole).

was hydrogen. observations:

-196 °

to

to the

-63 °

= 6 cps

be expected the

llquidifled

was

series

of

reactants

was

the

formation

of

to

-23 ° .

only is

However,

for

was a sharp

to diborane

reactions

to a negligible

be expected

there

the

temperature

of

raised

decomposed

that

dropped

- as would

At room t e m p e r a t u r e

The f o l l o w i n g

at

for

pressure

- 2 3 ° a n d O°

component

H P.BH'Br 3 2

and an excess

temperature

Between

of

-78 ° and held

was formed

- as would

On w a r m i n g

monobromoborane pressure

solid

pressure

remained

part

1.

the

rise

in

and phosphtne

volatile

consistent

speclee with

the

90

REACTION OF MONOBROMODIBORANE WITH PHOSPHINE

H

-78 °

B2H5Br + PH3

H 3 P . B . H . B H 3 (s) Br

H

-63 °

H3P.B.H.BH3 Br

+ PH 3

(s)

>O °

The experiment (2.75m.mole) reaction

give a clear for a short

repeated

and p h o s p h l n e

time

H 3 P . B H 3 (s) + H 3 P . B H 2 B r

)

~B2H 6 + PH 3

)

I/n(H2PBHBr) n + H 2.

in a sealed (5.5m.mole)

The solld and

liquid.

nmr spectrum was presence

(1)

was

at -23 ° .

)

During

in order

(cf.

from some d e c o m p o s i t i o n

nmr tube using m o n o b r o m o d i b o r a n e in a 1:2 ratio and stopping

llquld

phases

this process

FIG.

is shown

1).

clear

in FIG.

Distortion

to the polymerlc

were vlgorously

the tube had

to get a c o m p l e t e l y

run at -20 ° and

of P H 3 B H 2 B r

liquld.

2.

5. I 2.

Proton

Nmr S p e c t r u m

JPH = 375 c p s

4.4

of the trlplets

up

th'e arises

of the need

ppm

o f H3P. BH; a n d H3P.BH~Br JHH' = 8 c p s

to

The proton

It shows

form as a result

the

shaken

to be w a r m e d

I

FIG.

(I)

0°

H3P.BH3 (s) H3P.BH2Br

Vol. 3, No. 3

to

Vol. 3, No. 3

REACTION

OF MONOBROMODIBORANE WITH PHOSPHINE

warm the sample for a short time.

91

The -PH 3 resonance in PH3BH 3 is seen

as a doublet centered at 4.4 ppm downfield of TMS (JPH ffi 375 cps).

Each

doublet is split into a 1:3:3:1 quartet as expected (JHH' m 8 cps). This is in good agreement with t h e reported spectrum of the pure adduct (4).

A comparison of the intensities of the -PH 3 resonances for the

two adducts indicates that they are formed in approximately equimolar amounts.

A trace of PH3BH 3 is formed in the 1:1 addition as can be seen

from FIG. 1. DiscUssion

R e p l a c e m e n t o f one h y d r o g e n a t o m i n PH3BH3 by b r o m i n e h a s q u i t e striking effect on the properties.

a

The JPH coupling constant changes

from 375 cps to 405 cps indicating an increase in the is' character of the P-H bond.

The low field shift of the -PH 3 resonance to 5.1 ppm

from 4.4 ppm, and the increased

'acceptor pOwer* of'HH2Br' compared with

'BH 3 t (5) may be rationalised by reference to the electronegatlvity of bromine.

Some features of the reactions support this.

Thus the

monobromoborane adduct is formed more readily than the borane adduct under identical conditions,

and its mode of decomposition also suggests

a strengthening of the P-B bond when a bromine atom is attached to boron.

Further,

the adduct PH3BH2Br polymerises to give (PH2BHBr) n + H2,

rather than (PH2BH2) n + HBr, although there is no obvious mechanistic reason why hydrogen evolution should be preferred. stronger P-B(Br) bonds may be a contributory factor.

The formation of The increased

strength of the P-B(Br) bond may also contribute to the non-dissociation of PH3BH2Br in conditions that result in the breaking of the P-B bond in the dissociation of PH3BH 3. References 1. 2.

H. I. SCHLESINGER and A. B. BURG, J. Amer. Chem. Soc. 5_~3, 4321 (1931). E. L. GAMBLE and P. GILMONT, J. Amer. Chem. Soc. 62,717

A. B. BURG and R. I .

WAGNER, J .

Amer. Chem. S o c . 7 5 ,

(1940), and

3872 ( 1 9 5 3 ) .

92

REACTION OF MONOBROMODIBORANE

WITH PHOSPHINE

3.

Varlan Associates, P a l o Alto, Callfornla, U.S.A.

4.

R. W. RUDOLPH, R. W. PARRY and C. F. FARRAN~

5.

A. B. BURG and E. S. KULJIAN~ J. Amer. Chem. Soc. 72j 3103 (1950).

VoI° 3, He, 3

Inor~. Chem. ~, 723 (1966).