Catalytic Aspects of Metal Phosphine Complexes - American

16

Downloaded by AUBURN UNIV on November 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0196.ch016

The Preparation, Characterization, and Properties of Highly Soluble Transition-Metal Complexes of Long-Chain Tertiary Phosphines S. F R A N K S and F. R. H A R T L E Y Department of Chemistry and Metallurgy, The Royal Military College of Science, Shrivenham, England J. R. C H I P P E R F I E L D Department of Chemistry, The University of Hull, Hull, England

Two series of phosphines, H ) 39

form

and

II

(P(

I (PR R

R')

2

complexes,

2

trans-[PtL 'HCl],

isolating and purifying influence

2

in

association

olefins and (2)

2

are discussed.

the latter reaction is more complex

complexes

SnCl

for

2

addition

is reported.

in the presence

than suggested

have

on (1) 2

with

2

hosphine

trans-[RhL Cl(CO)]

of the type [PtL Cl ]

the oxidative

to trans-[RhL Cl(CO)]

P

and

char-

= I only. The problems

the complexes

catalysts

which

19

trans-[PdL2-

2

of the two series of phosphines

hydrogenation Cl ]

9

2

4

and L'

n-C19-

n-C H ),

cis-[PtL Cl ],

[PtL ],

2

where L = I and II

-

5

-

21

have been prepared,

acterized, and used to prepare Cl ],

10

R' = C H

3

very soluble

= n-C H

3

been

2

selective or [PdL2-

polyunsaturated of methyl

iodide

The mechanism of

of The

of

triarylphosphines

previously.

used

widely

as

homogeneous

catalysts, a n d i n many o f their reactions the solvent plays a very

important part. A c c o r d i n g l y we d e c i d e d to prepare phosphine c o m plexes w i t h rather different solubility properties to those previously available. In particular two series of tertiary phosphine

complexes

were prepared that w o u l d confer extreme solubility i n hydrocarbon solvents such as alkanes. Part of the rationalization for d o i n g this was the v i e w that if alkanes were ever going to be activated using homo0065-2393/82/0196-0273$05.00/0 © 1982 American Chemical Society

Alyea and Meek; Catalytic Aspects of Metal Phosphine Complexes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

274

M E T A L PHOSPHINE COMPLEXES

g e n e o u s c a t a l y s t s , t h e a c t i v a t i o n w o u l d h a v e to b e

effected i n the

presence o f solvents that w e r e c h e m i c a l l y more inert t h a n the alkanes themselves.

N o s u c h solvents are, o f course, a v a i l a b l e a n d i f h o m o

g e n e o u s c a t a l y s t s a r e to b e u s e d t h e n t h o s e c a t a l y s t s m u s t freely i n the alkanes

dissolve

themselves.

T w o series o f tertiary p h o s p h i n e l i g a n d s that are v e r y s o l u b l e i n h y d r o c a r b o n s o l v e n t s w e r e p r e p a r e d . T h e first s e r i e s , P ( n - C H m

2 m

+i)3,

w h e r e m = 1 0 to 1 9 , w e r e p r e p a r e d b y t r e a t i n g t h e G r i g n a r d r e a g e n t of the n - a l k y l b r o m i d e w i t h p h o s p h o r u s t r i c h l o r i d e i n tetrahydrofuran (THF)

(1 ). T h e y w e r e p u r i f i e d b y r e c r y s t a l l i z a t i o n f r o m a m i x t u r e o f


c h l o r o f o r m a n d e t h a n o l . T h e l o w e r m e m b e r s o f t h e series w e r e w a x y materials, C

1 0

-C

b e i n g soft w a x e s a n d C

1 2

w h i l e the h i g h e r m e m b e r s

1 3

-C

being hard

1 5

waxes,

o f the series w e r e c r y s t a l l i n e . A l l w e r e

fairly l o w melting—the m e l t i n g points increasing steadily from 3 7 ° 4 0 ° C for P ( n - C

1 0

H

2 1

)

3

t o 6 0 ° - 6 2 ° C for P ( n - C

1 9

H

3 9

) . They

were

3

ex

t r e m e l y s o l u b l e i n h y d r o c a r b o n s o l v e n t s s u c h as h e x a n e a n d c h l o r i nated

solvents

such

as

dichloromethane,

chloroform,

rachloride, a n d 1,2-dichloroethane a n d moderately

carbon

tet

soluble in T H F ,

b e n z e n e , a n d o t h e r a r o m a t i c s o l v e n t s , h o t a l c o h o l s s u c h as m e t h a n o l and ethanol, a n d w a r m acetone. T h e y were, however, insoluble i n c o l d m e t h a n o l , e t h a n o l , a n d acetone. O n e x p o s u r e to a i r t h e y w e r e o x i d i z e d r e a d i l y to a c o m p l e x

mixture of products o f the type

(η = 0 , 1, o r 2 ) a n d P ( 0 ) ( O R ) _ R 3

n

n

P(OR) _ R 3

n

w

(η = 0 , 1, 2 , o r 3 ) . H o w e v e r treat

m e n t w i t h a s l i g h t e x c e s s o f h y d r o g e n p e r o x i d e ( 6 % w/v) r e s u l t e d i n a s m o o t h o x i d a t i o n to t h e t r i a l k y l p h o s p h i n e o x i d e s w h i c h w e r e s t a b l e i n air a n d m o r e c r y s t a l l i n e t h a n the c o r r e s p o n d i n g tertiary p h o s p h i n e s . T h e s e p h o s p h i n e oxides h a d very s i m i l a r s o l u b i l i t y characteristics to the p a r e n t t r i a l k y l p h o s p h i n e s . I n contrast to t r i a l k y l p h o s p h i n e oxides below PO(n-C H 8

1 7

) , the l o n g - c h a i n t r i a l k y l p h o s p h i n e oxides w e r e not 3

deliquescent. T h e s e c o n d series o f p h o s p h i n e s p r e p a r e d w e r e a series o f t r i s ( p alkylaryl)phosphines n-C H 9

1 9

with

alkyl

groups

ranging

from

n-C H 3

7

to

. T h e y w e r e p r e p a r e d b y t r e a t i n g the G r i g n a r d reagent o f the

corresponding ρ-alkylbromobenzene w i t h phosphorus t r i c h l o r i d e i n THF devise

(2).

H o w e v e r i n o r d e r to use these routes it was necessary

a preparative

r o u t e for s y n t h e s i z i n g

to

p-alkylbromobenzenes.

M a n y o f t h e m e t h o d s for p r e p a r i n g a l k y l b r o m o b e n z e n e s

yield a mix

ture o f isomers. S i n c e these are l i q u i d s w i t h s i m i l a r b o i l i n g p o i n t s , t h e i r s e p a r a t i o n is d i f f i c u l t . A t h r e e - s t a g e s y n t h e s i s w a s a d o p t e d

eventually

(see R e a c t i o n s 1, 2 , a n d 3 ) w h i c h g a v e t h e p u r e p a r a i s o m e r i n o v e r a l l y i e l d s o f 4 6 t o 6 0 % . T h e c o m p o u n d ρ-propylbromobenzene

also c a n

b e p r e p a r e d i n t h i s w a y b u t i t is p r e p a r e d m o r e e a s i l y b y

reacting

a l l y l b r o m i d e w i t h the m o n o - G r i g n a r d reagent o f ρ

-dibromobenzene

f o l l o w e d b y h y d r o g e n a t i n g the r e s u l t i n g ρ - p r o p e n y l b r o m o b e n z e n e y i e l d the d e s i r e d p r o d u c t .


to

16.

FRANKS E T A L .

Long-Chain

Tertiary

275

Phosphines


H

After c o m p l e t i n g this w o r k o u r attention was d r a w n to the s y n thetic

procedure

of

Manassen

and Dror

(3)

based

b r o m o b e n z e n e w i t h a n acyl chloride i n the presence

on

reacting

of aluminum

t r i c h l o r i d e . T h i s t h e n is f o l l o w e d b y a W o l f f - K i s c h n e r r e d u c t i o n o f t h e a c y l g r o u p to a n a l k y l g r o u p u s i n g a l k a l i n e h y d r a z i n e . A l t h o u g h these a u t h o r s d o n o t g i v e o v e r a l l y i e l d s , t h e a c y l a t i o n s t e p (4) g i v e s o n l y 3 0 % of the desired product together w i t h 2 5 % o f C H C O R . T h u s o u r pro e

5

cedure gives m u c h higher overall yields w i t h only minor quantities of s i d e p r o d u c t s t o b e s e p a r a t e d off i n t h e final v a c u u m d i s t i l l a t i o n . T h e tris(p-alkylaryl)phosphine w i t h a propyl group was a crystal l i n e s o l i d , w h e r e a s t h e η-butyl to n - o c t y l d e r i v a t i v e s w e r e v i s c o u s o i l s . The n-nonyl derivative

was a viscous, waxy solid. T h e solids were

purified b y recrystallization from ethanol a n d the oils b y v a c u u m dis tillation. A l l o f the tris(p-alkylaryl)phosphines w e r e e x t r e m e l y

soluble

i n h y d r o c a r b o n s o l v e n t s s u c h as h e x a n e a n d c h l o r i n a t e d s o l v e n t s s u c h as d i c h l o r o m e t h a n e , c h l o r o f o r m , a n d c a r b o n t e t r a c h l o r i d e , a n d m o d e r a t e l y s o l u b l e i n T H F , d i e t h y l e t h e r , a n d a r o m a t i c s o l v e n t s s u c h as b e n z e n e a n d t o l u e n e . T h e l o w e r m e m b e r s o f t h e series w e r e s o l u b l e i n e t h a n o l b u t t h i s d e c r e a s e d as t h e a l k y l c h a i n l e n g t h i n c r e a s e d . T h e higher m e m b e r s w e r e i n s o l u b l e i n c o l d ethanol, methanol, a n d other p o l a r solvents. P - 3 1 N M R c h e m i c a l shifts w e r e m e a s u r e d i n C D C 1

3

solution relative to a trimethylphosphate external standard a n d w e r e f o u n d t o b e as f o l l o w s : ρ - a l k y l = C H , 2

+ 10.6; C H , 5

n

+10.6; C H 7

1 5

5

, +10.8; C H 8

+ 1 0 . 6 ; C H , + 1 0 . 5 ; C4H9, 3

1 7

7

, +10.7, a n d C H 9

1 9

+10.8

p p m . A l l o f the tris(p-alkylaryl)phosphines w e r e sensitive to oxygen b o t h as s o l i d s o r o i l s a n d i n s o l u t i o n . T h i s c o n t r a s t s w i t h t r i p h e n y l p h o s p h i n e w h i c h is f a i r l y r e s i s t a n t t o o x i d a t i o n b o t h as a s o l i d a n d i n solution. U n l i k e the trialkylphosphines w h i c h y i e l d a complex mixture o f products o n oxidation i n the air, the tris(p-alkylaryl)phosphines give a single p r o d u c t , the p h o s p h i n e o x i d e . T h e sensitivity to a i r o x i d a t i o n


276

M E T A L PHOSPHINE COMPLEXES

i n c r e a s e d as t h e a l k y l c h a i n o n t h e p h e n y l r i n g i n c r e a s e d i n l e n g t h . O x i d a t i o n w i t h h y d r o g e n p e r o x i d e ( 6 % w/v) g a v e a c o n v e n i e n t p r e parative route to the t r i s ( p - a l k y l a r y l ) p h o s p h i n e oxides. F i v e series o f t r a n s i t i o n - m e t a l c o m p l e x e s o f t h e n e w t e r t i a r y p h o s phines have been prepared: cis-[Pt(PR ) Cl ], frans-[Pd(PR ) Cl ], trans- [ P t ( P R ' ) H C l ] , [ P t ( P R ) ] , a n d trans- [ R h ( P R ) C l ( C O ) ] w h e r e R = n - a l k y l or p - a l k y l a r y l a n d R ' = n - a l k y l (5). A l l o f t h e c o m p l e x e s d i s p l a y e d e s s e n t i a l l y t h e s a m e s o l u b i l i t y c h a r a c t e r i s t i c s as t h e p a r e n t p h o s p h i n e s . A c c o r d i n g l y the synthetic routes that w e r e u s e d w e r e chosen because they gave the cleanest reactions w i t h largely volatile side products a n d a m i n i m u m of isomers. 3


3

2

3

2

2

3

4

3

2

2

2

cis-[P/(PR )2C;/ ] 3

2

W h e n [ P t ( C O D ) C l ] w a s t r e a t e d w i t h t w o e q u i v a l e n t s o f t h e ter t i a r y p h o s p h i n e , c y c l o o c t a d i e n e w a s d i s p l a c e d a n d cis- [ P t ( P R ) C l ] was formed. T h e displaced cyclooctadiene was removed easily i n vacuo. T h e lower triarylphosphine complexes were recrystallized from a 6 0 : 4 0 mixture of ethanol and chloroform. The higher triarylphos p h i n e c o m p l e x e s c o u l d b e o b t a i n e d o n l y as e i t h e r w a x e s o r v i s c o u s oils. These were purified chromatographically on a l u m i n a (Brockman a c t i v i t y I, n e u t r a l ) e l u t i n g w i t h a 6 0 : 4 0 m i x t u r e o f c h l o r o f o r m a n d m e t h a n o l . T h e t r i a l k y l p h o s p h i n e c o m p l e x e s w e r e r e c r y s t a l l i z e d from a m i x t u r e o f c h l o r o f o r m a n d e t h a n o l e x c e p t for t h e t r i o c t y l p h o s p h i n e c o m p l e x w h i c h w a s p u r i f i e d c h r o m a t o g r a p h i c a l l y to y i e l d a y e l l o w w a x . T h e m e l t i n g p o i n t s o f t h e t r i a l k y l p h o s p h i n e c o m p l e x e s are i n t e r e s t i n g (see F i g u r e 1) i n t h a t t h e l o n g - c h a i n a l k y l g r o u p s c l e a r l y d i s r u p t t h e p a c k i n g i n t h e c r y s t a l s t r u c t u r e so t h a t t h e m e l t i n g p o i n t i n i t i a l l y d e c r e a s e s s h a r p l y u n t i l t h e t r i - n - o c t y l p h o s p h i n e is r e a c h e d after w h i c h a s l i g h t r i s e o c c u r s , p r e s u m a b l y d u e t o i n c r e a s i n g m o l e c u lar w e i g h t . T h e i n i t i a l s h a r p d r o p i n F i g u r e 1 has b e e n o b s e r v e d p r e v i o u s l y for p l a t i n u m ( I I ) c o m p l e x e s w i t h η i n t h e 2 - 4 r a n g e as w e l l as for p a l l a d i u m ( I I ) c o m p l e x e s w i t h η i n t h e 2 - 5 r a n g e (6, 7 ) . O n h e a t i n g t h e cis isomers d e c o m p o s e b u t t h e y d o not either i s o m e r i z e or g i v e a n y m e t a l l a t i o n . W h e n [ P t ( M e C N ) C l ] is u s e d as t h e s t a r t i n g p l a t i n u m ( I I ) c o m p l e x , a m i x t u r e o f cis- a n d trans- [ P t ( P R ) C l ] is f o r m e d . T h i s m i x t u r e is v i r t u a l l y i m p o s s i b l e t o s e p a r a t e b e c a u s e o f t h e v e r y s i m i l a r p h y s i c a l characteristics o f the two isomers. 2

3

2

2

2

3

trans-[

2

2

2

PdiPR^Ck]

W h e n [ P d ( C O D ) C l ] was treated w i t h t w o e q u i v a l e n t s of the tris(p-alkylaryl) phosphines, P A r , £rans-[Pd(PAr ) Cl ] complexes w e r e f o r m e d . A p a r t f r o m t h e i r t r a n s g e o m e t r y t h e y w e r e v e r y s i m i l a r to t h e i r p l a t i n u m ( I I ) a n a l o g u e s , a l t h o u g h less c r y s t a l l i n e so t h a t p u r i f i c a 2

3

3

2

2


16.

m i

FRANKS E T A L .

Long-Chain

Tertiary

277

Phosphines

(°C) 300


200

100

0 L

Figure

Melting

16

12

8 points of cis-[Pt{P(n-C H ) } Cl ] m

2m+1 3

2

complexes

2

tion d e p e n d e d m o r e o n c o l u m n c h r o m a t o g r a p h y . W i t h the t r i a l k y l p h o s p h i n e s , P R ' , t w o p r o d u c t s w e r e o b t a i n e d . T h e first w a s a y e l l o w m a t e r i a l , trans- [ P d ( P R ' ) 2 C l ] , w h i c h w a s v e r y s i m i l a r to t h e trans[ P d ( P A r ) C l ] complexes. H o w e v e r a second, darker p r o d u c t was also p r e s e n t a n d i n a n i n c r e a s i n g a m o u n t as t h e c h a i n l e n g t h i n c r e a s e d . H - l , C - 1 3 , a n d P-31 N M R analyses a n d I R s p e c t r o s c o p y c o m b i n e d to s u g g e s t t h a t t h i s d a r k e r m a t e r i a l is a m i x t u r e o f c h l o r i d e - b r i d g e d , d i m e r i c S p e c i e s I a n d II. I n spite o f r e p e a t e d attempts it w a s i m p o s s i b l e to s e p a r a t e t h e s e s p e c i e s b y c o l u m n c h r o m a t o g r a p h y . T h e m e t a l l a t e d s p e c i e s , I I , a p p e a r to b e a c o m p l e x m i x t u r e o f p r o d u c t s ; t h e five3

3

3

2

2

R 'P

Cl

3

\

Cl

/

Pd

2

Cl

/

\

\

/

C ll ι

Pd

/ \

PR ' 3

R T

Cl

2

/ \ (CH ) 2

CH CH (CH ) 3

n

/ P

CH—(CH ) CH 2

\

d

/ P

V

d

\ (CH ) 2

b

PIV

2

m

II


a

3

278

M E T A L PHOSPHINE C O M P L E X E S

m e m b e r e d r i n g s p e c i e s (n = b = 2) w o u l d b e e x p e c t e d to p r e d o m i n a t e . It is s u r p r i s i n g , i n v i e w o f t h e w o r k o f S h a w (8, 9) w h i c h h a s s h o w n p l a t i n u m ( I I ) to b e m o r e s u s c e p t i b l e to m e t a l l a t i o n t h a n p a l l a d i u m ( I I ) , t h a t i n t h e p r e s e n t w o r k t h e r e v e r s e is f o u n d . I n s p i t e o f r e p e a t e d a t t e m p t s w e w e r e u n a b l e to t h e r m a l l y i n d u c e m e t a l l a t i o n at platinum(II). trans-[Pt(PR '^HCl] 3

T h e cis-[Pt(PR )2Cl ] c o m p o u n d s , where R' = a l k y l , w e r e con v e r t e d s m o o t h l y to trans- [ P t ( P R ' ) H C l ] b y t r e a t i n g t h e m w i t h h y drazine hydrate i n a mixture of ethanol and chloroform. Chloroform m u s t b e a d d e d since the c o m p l e x e s are i n s o l u b l e i n p u r e e t h a n o l . T h e h y d r i d e c o m p l e x e s are c r e a m - c o l o r e d , l o w - m e l t i n g , c r y s t a l l i n e solids that closely resemble the p h y s i c a l properties o f the parent dichloro complexes. 3

,

2


3

2

[Pt(PR) ]

3 4

W h e n [ P t ( C O D ) ] (10) is t r e a t e d w i t h 4 e q u i v o f t e r t i a r y p h o s p h i n e i n a h e x a n e s o l u t i o n u n d e r n i t r o g e n , [ P t ( P R ) ] is f o r m e d . T h e s e platinum(O) c o m p l e x e s are y e l l o w , air-sensitive, l o w - m e l t i n g , crystal l i n e s o l i d s w i t h s o l u b i l i t y p r o p e r t i e s t h a t a r e v e r y s i m i l a r to t h e p a r e n t p h o s p h i n e s . W h e n d i s s o l v e d i n s o l u t i o n these c o m p l e x e s r e v e r s i b l y d i s s o c i a t e (see E q u a t i o n 4). T h u s t h e P - 3 1 N M R s p e c t r a i n b e n z e n e 2

3

[Pt(PR ) ] = ^ 3

4

[Pt(PR ) ] 3

4

[Pt(PR ) ]

3

3

(4)

2

s o l u t i o n at a m b i e n t t e m p e r a t u r e s s h o w a b r o a d s i n g l e t o n l y w i t h n o platinum-phosphorus coupling. A

spectrophotometric

study of

k i n e t i c s o f p h o s p h i n e d i s s o c i a t i o n at 25°C i n a 4 x 1 0 ~ m o l L " 4

zene solution showed the

following

sure) » P ( —

order:

t h a t t h e rate o f t h e P i n - C ^ H ^

first

(fc^obs)

1

the ben

step decreased

too

large

to

in

mea

O - C H ^ f c ^ o b s ) = 3.5 x 1 0 " s e c " ) > P P h ( M o b s ) 4

1

3

=

1.5 x 1 0 " s e c " ) . T h e rates o f t h e s e c o n d s t e p w e r e m o r e s i m i l a r , d e 3

1

c r e a s i n g i n t h e f o l l o w i n g o r d e r : P ( -

Catalytic Aspects of Metal Phosphine Complexes - American

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