13 Binuclear Organoplatinum Complexes as Models for Catalytic Intermediates M. P. BROWN, J. R. F I S H E R , and S. J. F R A N K L I N
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Donnan Laboratories, University of Liverpool, U.K. R. J. P U D D E P H A T T and M . A. T H O M S O N Department of Chemistry, University of Western Ontario, London, Canada, N6A 5B7
Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenylphosphino) methane ligands, has been prepared and structurally characterized. Included are diplatinum (I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered.
M
ononuclear platinum complexes often have been used as models for catalytic intermediates since systematic studies of syn thesis, reactivity, and mechanism are often convenient and because metallic platinum is a very important catalyst. H o w e v e r , u s i n g b i n u clear or polynuclear platinum complexes as models for proposed inter mediates i n heterogeneous catalysis has not been studied, probably because p l a n n e d routes to such complexes have not been available. T h i s chapter describes our first studies i n this area.
Desirable
Functional
Groups
in Binuclear
Complexes
In an oversimplification, we can identify the following two ways in w h i c h the nature of heterogeneous catalytic reactions may differ from homogeneous catalytic reactions with mononuclear catalysts. 0065-2393/82/0196-0231$05.00/0 © 1982 American Chemical Society In Catalytic Aspects of Metal Phosphine Complexes; Alyea, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
M E T A L PHOSPHINE COMPLEXES
232
Table I.
Bonding Modes of Some Important Functional Groups in Terminal or Bridging Situations
Ligand
Terminal
Ligand
Bridging
Ligand H
Hydride
Pt-H
Pt
/
\ Ο
/ Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0196.ch013
Carbon Monoxide
Pt-OsO
I
Pt=CH
\
Pt
Pt CH
Carbene
2
/ \ Pt Pt CH —CH / \
2
2
CH P tt— - IJI P CH
Alkene
Pt
2
2
Pt
Pt
2
Η CH Pt—III CH
Alkyne
Η \
/ C = C
/
\
Pt
Pt
/ Alkyl
Pt-CH
3
H C
3
Pt
\ Pt
1. L i g a n d s m a y b i n d t o m o r e t h a n o n e m e t a l c e n t e r b y b r i d g i n g a n d the reactivity of the c o o r d i n a t e d l i g a n d w i l l t h e r e f o r e b e d i f f e r e n t . W e set o u t to p r e p a r e c o m p l e x e s w i t h the b r i d g i n g functional groups s h o w n i n T a b l e I a n d to c o m p a r e t h e i r r e a c t i v i t y w i t h those h a v i n g t e r m i n a l ligands. 2. C o o p e r a t i v e effects i n w h i c h , for e x a m p l e , a r e a g e n t is a c t i v a t e d b y c o o r d i n a t i o n to o n e m e t a l c e n t e r a n d t h e n i n d u c e d to r e a c t w i t h a l i g a n d o n a n a d j a c e n t m e t a l c e n t e r or i n a b r i d g i n g p o s i t i o n c a n o c c u r . A s e a r c h for s u c h m o d e l reactions has b e e n i n i t i a t e d . Some Diplatinutn(l)
Complexes
We have u s e d the l i g a n d bis(diphenylphosphino)methane (dppm) to s t a b i l i z e b i n u c l e a r p l a t i n u m c o m p l e x e s . W h e n c h e l a t i n g , t h i s l i g a n d forms a s t r a i n e d f o u r - m e m b e r e d r i n g a n d it therefore tends to b r i d g e b e t w e e n m e t a l a t o m s . O u r first p l a t i n u m ( I ) c o m p l e x w a s p r e p a r e d b y t h e f o l l o w i n g r e a c t i o n ( E q u a t i o n 1), P P = d p p m ( 1 , 2 ) .
In Catalytic Aspects of Metal Phosphine Complexes; Alyea, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
BROWN E T A L .
13.
P
Binuclear
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