Polymerization of Group 14 Hydrides by Dehydrogenative Coupling

Polymerization of Group H Hydrides. 97. Cp2 TiMe2. +. RSiH3 .Me. Cp2 Ti. SiH2 R. +. CH. CP2 Ti. +. MeRSiH0. RSiH0. C p 2 Ti^. SiH2 R. Cp2 Ti. RSiH0. C...
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Chapter 7

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Polymerization of Group 14 Hydrides by Dehydrogenative Coupling John F. Harrod Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada The dehydrocoupling of organosilanes and germanes under the c a t a l y t i c influence of titanocene and zirconocene derivatives is reviewed. Primary organosilanes generally give RSiH -terminated o l i g o organosilylenes containing 10 to 20 s i l i c o n atoms, depending on the catalyst. Polymer s t r u c t u r a l assignments are based on a combination of ir, nmr and ms spectroscopies and on molecular weight studies using gpc and vpo. Primary germanes give three-dimensional gels with the titanium-based catalysts but secondary germanes give short chain linear oligomers. It is proposed that the polymerization proceeds by a r e p e t i t i v e insertion of s i l y l e n e moieties i n t o a m e t a l - s i l i c o n bond. 2

The p r o d u c t i o n o f e x t r e m e l y l o n g c h a i n s o f c a r b o n atoms is g r e a t l y facilitated by t h e common e x i s t e n c e o f compounds with metastable multiple carbon-carbon bonds. I n a d d i t i o n t o the h i g h l y f a v o r a b l e thermodynamics of polymerization o f C=C and CEC b o n d s , the initiation modes, which t y p i c a l l y produce a p r o p a g a t i n g species with one a c t i v e e n d , g e n e r a l l y p r e c l u d e t h e f o r m a t i o n o f r i n g s by intramolecular c o u p l i n g o f the two c h a i n e n d s . The thermodynamic stability o f polymer r e l a t i v e t o o l e f i n is f a v o r a b l e f o r those cases where one o f the c a r b o n s c a r r i e s o n l y hydrogen atoms. If both c a r b o n atoms c a r r y s u b s t i t u e n t s , the c e i l i n g temperature, above which the polymer is u n s t a b l e relative t o monomer, is generally t o o low f o r t h e polymer t o be u s e f u l . The near absence of s i l i c o n a n a l o g s o f the s u b s t i t u t e d o l e f i n s and a c e t y l e n e s has precluded a p a r a l l e l evolution of p o l y s i l y l e n e chemistry. Indeed, the s u c c e s s f u l s t r a t e g y f o r p r o d u c i n g d o u b l e bonds between elements o f the t h i r d and lower p e r i o d s has been t o so encumber t h e atoms on either side o f the d o u b l e bond w i t h b u l k y s u b s t i t u e n t s that the m o l e c u l e has no i n c l i n a t i o n t o p o l y m e r i z e The l a r g e r s i z e o f silicon compared t o c a r b o n makes a much h i g h e r s t e r i c encumbrance n e c e s s a r y t o s t a b i l i z e t h e monomer. The same c o n s i d e r a t i o n s apply even more so t o the h e a v i e r congeners o f group 14.

0097-6156/88/0360-0089$06.00/0 © 1988 American Chemical Society

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

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P o l y ( d i m e t h y l s i l y l e n e ) was f i r s t r e p o r t e d by Burkhard in 1949 (2^, b u t the l a c k o f s o l u b i l i t y and g e n e r a l i n t r a c t a b i l i t y o f t h i s material discouraged further studies at that time. The d e m o n s t r a t i o n by Yajima and H a y a s h i t h a t p o l y ( d i m e t h y l s i l y l e n e ) can be p y r o l y z e d to s i l i c o n c a r b i d e , and the subsequent development o f s i l i c o n carbide fibres v i a this route, lead to a dramatic r i s e in i n t e r e s t in p o l y ( o r g a n o s i l y l e n e s ) and t h e i r chemistry (3^). More recently t h e y have a t t r a c t e d i n c r e a s i n g a t t e n t i o n s i n c e potential uses in the a r e a s o f m i c r o l i t h o g r a p h y (4) and r e p r o g r a p h y (5^) have been identified. Certain p o l y ( o r g a n o s i l y l e n e s ) have also been shown to have unusual thermochromic behavior (6,7 ) and t e m p e r a t u r e - d e p e n d e n t t r a n s i t i o n s in c h a i n c o n f o r m a t i o n ( £ ) . These developments have been made p o s s i b l e by the a p p l i c a t i o n o f m o d i f i e d Wurtz-Fittig-type coupling to the production of linear p o l y s i l y l e n e s , as shown in E q u a t i o n 1 ·

η RR'SiCl

2

+ 2n M

(M is a group 1 m e t a l o r

>

R» Cl-fSi-^Cl R

+

2n MCI

(1)

alloy)

The contributions o f West and c o - w o r k e r s in t h i s area have been noteworthy (9_). The W i s c o n s i n group showed that the solubility of p o l y ( d i m e t h y l s i l y l e n e ) can be g r e a t l y enhanced by inclusion of a second s u b s t i t u e n t t h r o u g h c o p o l y m e r i z a t i o n and t h a t E q u a t i o n 1 has c o n s i d e r a b l e g e n e r a l i t y (J_0 ) . I t is now clear that p o l y ( o r g a n o s i l y l e n e s ) with very h i g h molecular weights (ca. 10 ) can be s y n t h e s i s e d by t h i s type o f r e a c t i o n . The p o l y m e r s thus o b t a i n e d have s u f f i c e d t o a l l o w the development o f some new technologies and to p o i n t the way t o o t h e r s . A number o f a s p e c t s of the W u r t z - F i t t i g - t y p e c o u p l i n g d e t r a c t from i t s attractiveness as a c o m m e r c i a l l y v i a b l e r o u t e to p o l y s i l y l e n e s . Among the most serious d i f f i c u l t i e s a r e the p o o r c o n t r o l o f m o l e c u l a r w e i g h t and polydispersity, p r o d u c t i o n o f low m o l e c u l a r weight eyelies, the hazards associated with handling hot, molten a l k a l i metals, the limited tolerance of functional groups on the silicon to the r e a c t i o n c o n d i t i o n s and r e l a t i v e l y h i g h c o s t . We have r e c e n t l y r e p o r t e d an a l t e r n a t i v e r o u t e t o polysilyl­ enes, involving the c a t a l y t i c e l i m i n a t i o n o f H between two S i - H moieties (11,12,13). The r e a c t i o n is homogeneously c a t a l y s e d by t i t a n o c e n e and z i r c o n o c e n e d e r i v a t i v e s and in p r i n c i p l e s h o u l d be easier to u n d e r s t a n d and c o n t r o l than the heterogeneous WurtzFittig reaction. I t is thus c l e a r t h a t t h e s e polymers p r o v i d e at least an i m p o r t a n t complement to those made by the Wurtz-Fittigt y p e c o u p l i n g and t h a t improvements in the performance o f c a t a l y s t s could lead to a v i a b l e commercial route for the large scale production of polysilanes. In t h i s paper a d e s c r i p t i o n o f the progress we have made t o date in u n d e r s t a n d i n g the reaction mechanism and c h a r a c t e r i z i n g the polymers w i l l be d e s c r i b e d . The scope o f the c a t a l y s i s and some o f i t s present l i m i t a t i o n s will also be d i s c u s s e d . Finally, some o f the major q u e s t i o n s that remain to be answered b e f o r e t h i s c h e m i s t r y c a n be successfully applied to the general synthesis of polysilylenes will be addressed. 6

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Dehydrogenative Coupling The f o r m a t i o n o f an X - X bond by t h e e x t r u s i o n o f H from two X - H containing m o l e c u l e s is a r e a c t i o n of considerable generality. Until recently, such reactions have n o t been systematically exploited f o r the formation o f polymers, although c l o s e l y r e l a t e d r e a c t i o n s in which the thermodynamics o f the p r o c e s s a r e enhanced by s c a v e n g i n g t h e hydrogen w i t h oxygen ( o x i d a t i v e coupling) have been e x p l o i t e d to c o m m e r c i a l advantage (14). Some o f the reasons for the l a c k o f p r o g r e s s in t h i s a r e a a r e t h e r e l a t i v e l y low r e a c t i v i t y o f many o f the more i n t e r e s t i n g t y p e s o f monomer, a l a c k o f thermodynamic d a t a which a l l o w p r e d i c t i o n o f whether a reaction is f e a s i b l e o r n o t , and a l a c k o f s p e c i f i c i t y in c a s e s when there are s e v e r a l X - H bonds in the same m o l e c u l e . In the c o u r s e o f s t u d y i n g the r e a c t i o n s o f S i - Η compounds w i t h dialkyltitanocenes, with a view t o the s y n t h e s i s of new h y d r i d o s i l y l t i t a n o c e n e c o m p l e x e s , we a d v e n t i t i o u s l y d i s c o v e r e d t h a t phenylsilane undergoes facile, quantitative dehydrogenative coupling to a l i n e a r p o l y ( p h e n y l s i l y l e n e ) under the catalytic influence of dimethyltitanocene. The ease w i t h which t h i s r e a c t i o n proceeds i n i t i a l l y i n d u c e d us t o u n d e r e s t i m a t e the s i g n i f i c a n c e o f the o b s e r v a t i o n . Further s t u d i e s q u i c k l y r e v e a l e d t h a t the r a p i d dehydrogen­ a t i v e c o u p l i n g o f p r i m a r y o r g a n o s i l a n e s t o o l i g o m e r s and t h e s l o w e r coupling o f secondary s i l a n e s t o d i m e r s c a n be e f f e c t e d under ambient c o n d i t i o n s w i t h compounds o f t h e type C p M R (M = T i , R = alkyl; M = Z r , R = a l k y l or H)(11,12,13). None o f the other m e t a l l o c e n e s , m e t a l l o c e n e a l k y l s , o r m e t a l l o c e n e h y d r i d e s o f groups 4, 5 o r 6 have shown any measurable a c t i v i t y f o r p o l y m e r i z a t i o n under ambient c o n d i t i o n s , a l t h o u g h vanadocene c a t a l y z e s the slow, s t e p w i s e o l i g o m e r i z a t i o n o f p h e n y l s i l a n e in r e f l u x i n g t o l u e n e ( 1 5 ) · Excessive m e t h y l a t i o n o f the Cp l i g a n d s d e a c t i v a t e s t h e c a t a l y s t s . For example, t h e mixed c y c l o p e n t a d i e n y l - p e n t a m e t h y l c y c l o p e n t a d i e n y l (CpCp ) complexes o f T i and Z r a r e a c t i v e , b u t t h e b i s Cp complexes are not (16,17). However, t h e C p M R complexes, where M = Th o r U a r e c a t a l y t i c a l l y active; the former for the dimerization of primary silanes and the l a t t e r for their o l i g o m e r i z a t i o n (V7 ) . A p r o b l e m w i t h t h e s e compounds is t h a t they are t o o r e a c t i v e and have a tendency t o r e a c t w i t h the C - H bonds o f substituents on the s i l i c o n (JM3). Bis (indenyl) dimethyl titanium does n o t c a t a l y z e the p o l y m e r i z a t i o n r e a c t i o n , b u t does e f f e c t a slow s t e p w i s e o l i g o m e r i z a t i o n o f p h e n y l s i l a n e t o dimer and t r i m e r a t room t e m p e r a t u r e . The b i s ( i n d e n y l ) z i r c o n i u m a n a l o g is a c t i v e as a polymerization catalyst, g i v i n g e s s e n t i a l l y the same p r o d u c t as the b i s ( c y c l o - p e n t a d i e n y l ) complex.

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2

2

2

2

2

Replacement o f one o f the Cp groups in the t i t a n o c e n e zirconocene-based catalysts by an alkyl group d e s t r o y s catalytic activity. It is thus c l e a r t h a t the c o n s t r a i n t s c a t a l y t i c a c t i v i t y are extremely severe. A r e m a r k a b l e f e a t u r e o f the p o l y m e r i z a t i o n r e a c t i o n s is absence o f any e v i d e n c e in nmr s p e c t r a o f r e a c t i o n m i x t u r e s intermediate low m o l e c u l a r weight o l i g o m e r s . This behavior quite distinct from those systems mentioned above, which dimers and t r i m e r s and i t is b e l i e v e d t h a t t h e s e represent mechanistically d i s t i n c t processes.

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

or the on the for is give two

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92

Normally, no s m a l l c y c l o p o l y s i l a n e s a r e o b s e r v e d in these reactions* Two e x c e p t i o n s we have noted a r e the v e r y slow r e a c t i o n of p h e n y l s i l a n e under the i n f l u e n c e o f C p T i M e and the reaction of b e n z y l s i l a n e under the i n f l u e n c e o f d i m e t h y l t i t a n o c e n e a t very long reaction times. From b o t h o f t h e s e r e a c t i o n s we i s o l a t e a s i n g l e isomer o f the c y c l o h e x a s i l a n e , in c a . 10 p e r c e n t y i e l d in the c a s e o f the p h e n y l s i l a n e and c a . 60 p e r c e n t y i e l d in the case of the benzylsilane. These i s o m e r s a r e b e l i e v e d t o be the alltrans isomers. The p h e n y l d e r i v a t i v e is identical to that p r e v i o u s l y r e p o r t e d by Hengge (J_9), the b e n z y l d e r i v a t i v e is a new compound. In the case of the benzylsilane reaction, the cyclohexasilane is p r o d u c e d from p o l y m e r , following essentially complete c o n v e r s i o n o f the monomer t o l i n e a r p o l y s i l a n e o f a b o u t 10 silicon u n i t s average l e n g t h . T h i s o b s e r v a t i o n is p r o b a b l y v e r y i m p o r t a n t in u n d e r s t a n d i n g the f a c t o r s t h a t l i m i t c h a i n l e n g t h .

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2

The Polymers The p o l y m e r i z a t i o n o f p r i m a r y o r g a n o s i l a n e s p r o c e e d s a c c o r d i n g to E q u a t i o n 2. The r a t e o f p o l y m e r i z a t i o n is s t r o n g l y dependent on the

η RSiH

3

R Ψ H-f-Si-^H

+

(n-1 ) H

(2)

2

Η

steric demands o f R. The r e l a t i v e r a t e s f o r a number o f d i f f e r e n t silane reactions, c a t a l y z e d by d i m e t h y l t i t a n o c e n e and measured under c o - h y d r o g e n a t i o n c o n d i t i o n s w i t h c y c l o h e x e n e as described below, is (20); P h e n y l (13) > p - T o l y l (10) B e n z y l (1) > n - h e x y l (1)

> Phenylmethyl (4.6) > Cyclohexyl (0.5).

> PhSiD

3

(3.6)

>

C y c l o h e x y l - and p h e n y l m e t h y l s i l a n e s do n o t p o l y m e r i z e , but give d i m e r . W i t h t i t a n i u m - b a s e d c a t a l y s t s the v a l u e o f η is about 10 and does n o t v a r y v e r y much w i t h R o r e x p e r i m e n t a l conditions; with z i r c o n i u m based catalysts, η c a n be as h i g h as 20. P o l y ( p h e n y l - and p - t o l y s i l y l e n e s ) p r o d u c e d w i t h t h e s e c a t a l y s t s a r e brittle g l a s s e s and p o l y ( b e n z y l - and n - h e x y l s i l y l e n e s ) a r e v i s c o u s oils. A l l o f the polymers a r e a t a c t i c and h i g h l y s o l u b l e in most organic solvents. They have been shown by a variety of spectroscopic methods t o be l i n e a r and S i H R t e r m i n a t e d (21_). The SiH R t e r m i n i can be d e t e c t e d by the use o f S i - n m r u s i n g a DEPT pulse sequence. They a r e a l s o e v i d e n t from the p r e s e n c e of a strong S i H i n f r a r e d b e n d i n g mode a t about 910 cm · The l i n e a r nature of the polymers and t h e i r d e g r e e s o f p o l y m e r i z a t i o n have been d e t e r m i n e d u s i n g mass s p e c t r o s c o p y and p a r t i c u l a r l y by comparing the mass spectrum o f a linear poly(phenylsilylene), produced by d e h y d r o g e n a t i v e c o u p l i n g w i t h t h a t r e p o r t e d by Hengge et a l . (J_9) f o r h e x a p h e n y l - c y c l o h e x a s i l a n e . The d a t a a r e shown in T a b l e I and a t t e n t i o n is drawn t o the f a c t t h a t the c y c l i c p o l y m e r , which does not s e p a r a t e i n t o fragments w i t h a s i n g l e Si-Si bond cleavage, gives heavy i o n s in h i g h abundance w h i l e the linear polymer, which is fragmented by a s i n g l e S i - S i bond cleavage, gives heavy fragments in only very low abundance. The 2

2 9

2

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Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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hexabenzylcyclohexasilane d e s c r i b e d above behaves in v e r y similar fashion. The v e r y s h a r p , c o n v e n t i o n a l nmr s p e c t r a o f t h e s e c y c l i c compounds a l s o c o n f i r m the absence o f s m a l l r i n g compounds in the normal polymer p r o d u c t s , where they would be e a s i l y d e t e c t a b l e . The degrees o f p o l y m e r i z a t i o n d e t e r m i n e d by mass s p e c t r o s c o p y are c o r r o b o r a t e d by vapor p r e s s u r e osmometry and g e l permeation chromatography s t u d i e s (21 ) . The t e r t i a r y hydrogens of the polymer backbone show no detectable f u r t h e r r e a c t i v i t y under p o l y m e r i z a t i o n c o n d i t i o n s but t h e y can be h y d r o s i l a t e d u s i n g the c l a s s i c a l S p e i e r - t y p e (22,23), or our new z i r c o n i u m - b a s e d c a t a l y s t s (see below) ( V7), to give polymers with fully, or p a r t i a l l y substituted backbones. The presence of S i - Η at every silicon atom and the attendant opportunity for further f u n c t i o n a l i z a t i o n is one of the most interesting features of t h e s e new p o l y m e r s . The p o t e n t i a l for u t i l i z a t i o n o f S i - Η groups on p o l y ( s i l y l e n e s ) f o r c r o s s - l i n k i n g and other modes o f f u n c t i o n a l i z a t i o n has a l r e a d y been r e c o g n i z e d by West (8). Synthesis of S i - Η f u n c t i o n a l i z e d polymers by WurtzFittig-type c o u p l i n g has been a c h i e v e d by the c o p o l y m e r i z a t i o n of dialkyldichlorosilanes with methyldichlorosilane. G r e a t c a r e must be e x e r c i s e d t o m a i n t a i n n e u t r a l c o n d i t i o n s d u r i n g the work-up in o r d e r t o a v o i d r e a c t i o n o f the S i - Η f u n c t i o n s ( £ ) . Cohydrogenation of

Olefins

Thermodynamics o n l y s l i g h t l y favor E q u a t i o n 1 and the copious evolution o f H can have u n d e s i r a b l e e f f e c t s on the c o u r s e o f the reaction. The i n c l u s i o n of an i n t e r n a l o l e f i n in the reaction mixture completely suppresses the evolution of hydrogen and increases the rate of polymerization. With titanocene-based catalysts the olefin undergoes h y d r o g é n a t i o n but the polymerization, except f o r a rate i n c r e a s e , p r o c e e d s in the same way as in the absence o f o l e f i n (20_). C o h y d r o g e n a t i o n is u s e f u l f o r the s y n t h e s i s o f polymers from gaseous s i l a n e s , in p a r t i c u l a r S i H and MeSiH , since it allows the progress of the reaction to be monitored by gas uptake. Another i m p o r t a n t advantage of cohydrogenation is t h a t i t can make E q u a t i o n 1 much more thermodynamically viable because o f the h i g h h e a t o f hydrogénation of olefins. In this respect the reaction resembles olefin h y d r o g e n a t i o n - d r i v e n C - H bond a c t i v a t i o n s s t u d i e d by C r a b t r e e (24)· T i t a n o c e n e c a t a l y s t s do n o t c a t a l y z e the h y d r o s i l a t i o n o f most internal olefins, a l t h o u g h t h e y can a t t a c h a c t i v e o l e f i n s s u c h as styrene, or norbornene to the growing polymer c h a i n ends. The zirconocene-based catalysts, on the o t h e r hand, c a n be powerful hydrosilation catalysts and the r e m a r k a b l e copolymer synthesis shown in E q u a t i o n 3 c a n be e a s i l y a c h i e v e d under m i l d conditions 2

4

3

(17). With cyclohexene, polymerization o c c u r s more r a p i d l y than hydrosilation. After p o l y m e r i z a t i o n has proceeded t o completion, t h e r e is a slow h y d r o s i l a t i o n t o i n t r o d u c e c y c l o h e x y l groups onto the polymer c h a i n , to a maximum e x t e n t o f about 50 p e r c e n t o f t h e S i - Η groups. W i t h more r e a c t i v e o l e f i n s , s u c h as s t y r e n e , hydro­ silation occurs more r a p i d l y than p o l y m e r i z a t i o n and the polymerization r e a c t i o n is s u p p r e s s e d . As in the polymerization reaction, the r e a c t i v i t y o f p r i m a r y s i l a n e s is much g r e a t e r than

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

94

Cp' ZrMe 2

Ph Ph H4Si-Si4t:H H £ )

2

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2n PhSiH-

+

(2n-1) H

9

(3)

that of secondary s i l a n e s and the p r o d u c t o f h y d r o s i l a t i o n of styrene by p h e n y l s i l a n e is almost entirely phenyl(1-phenylethyDsilane. This s u g g e s t s t h a t the i n c o r p o r a t i o n o f cyclohexyl groups i n t o the polymer does n o t o c c u r by s i m p l e h y d r o s i l a t i o n . We currently favor a mechanism which i n v o l v e s cleavage of the preformed polymer c h a i n by the c a t a l y s t t o g i v e an intermediate which is capable of y i e l d i n g a l k y l s i l a n e , b u t does not require direct b r e a k i n g o f the t e r t i a r y S i - Η bond o f the polymer c h a i n . C o u p l i n g o f Germanes D i m e t h y l t i t a n o c e n e is e x t r e m e l y a c t i v e f o r the c o u p l i n g o f germanes (15). Even s e c o n d a r y germanes c a n be c o u p l e d r a p i d l y t o o l i g o m e r s , but t h e r e seems to be a s e v e r e c o n s t r a i n t on the c h a i n length, as in the case of primary s i l a n e p o l y m e r i z a t i o n . Polymerization of diphenylgermane can be c a r r i e d out under two d i f f e r e n t regimes using dimethyltitanocene as catalyst. Addition of freshly recrystallized d i m e t h y l t i t a n o c e n e to an e x c e s s o f diphenylgermane r e s u l t s in s t e a d y e v o l u t i o n o f hydrogen a t c a . 60°C., b u t the c o l o u r of the solution remains yellow until a l l of the germane is consumed. During this period, the formation of tetraphenyldigermane and s m a l l amounts o f h i g h e r o l i g o m e r s can be o b s e r v e d by nmr s p e c t r o s c o p y . When almost a l l o f the monomer is consumed t h e r e is a d r a m a t i c change in c o l o u r t o d a r k p u r p l e accompanied by a surge of gas. The r e s u l t i n g p u r p l e s o l u t i o n is much more active for the f u r t h e r dehydrogenative c o u p l i n g of diphenylgermane and produces p r i m a r i l y octaphenyltetragermane. The d a r k p u r p l e p r o d u c t can be p r o d u c e d d i r e c t l y by r e a c t i o n o f dimethyltitanocene and diphenylgermane in a molar r a t i o o f about 1:2. The s t r u c t u r e of this d a r k c o l o u r e d compound w i l l be d i s c u s s e d f u r t h e r below. P r i m a r y germanes undergo c o u p l i n g to i n s o l u b l e g e l s under the influence of dimethyltitanocene, presumably because the backbone hydrogens show s u f f i c i e n t activity to lead to crosslinking. Perhaps the most i n t e r e s t i n g a s p e c t o f t h i s r e a c t i o n is t h a t it points t o the p o s s i b i l i t y t h a t s i m i l a r r e a c t i o n s o f s i l a n e s may be a c h i e v a b l e w i t h more a c t i v e c a t a l y s t s . G e l s produced from c o u p l i n g of primary s i l a n e s c o u l d have i n t e r e s t i n g electronic properties s i n c e t h e y a r e the homologs o f s i l i c o n monohydride, a m a t e r i a l o f c o n s i d e r a b l e c u r r e n t i n t e r e s t t o the e l e c t r o n i c s i n d u s t r y (25^). In the p r e s e n c e o f vanadocene, phenylgermane undergoes f a c i l e s t e p w i s e c o n v e r s i o n to o l i g o m e r s a t about 6 0 ° C . Results obtained w i t h germanes a l s o p r o v i d e models for the kinds of r e a c t i o n s t h a t may be o c c u r r i n g in the silane polymer­ ization reaction as well. F o r example, we have succeeded in c a r r y i n g out the r e a c t i o n shown in E q u a t i o n 4 (26 ) . The analogous reactions with t r i p h e n y l s i l a n e , or t r i p h e n y l s t a n n a n e , were not Cp Ti(CH ) 2

3

2

+ Ph GeH 3

Cp Ti(CH )(GePh ) 2

3

3

+ CH

4

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

(4)

7.

HARROD

95

Polymerization of Group 14 Hydrides

successfully demonstrated, b u t the f a c t t h a t i t o c c u r s w i t h the germane adds credence t o the h y p o t h e s i s t h a t an analogous r e a c t i o n is the first step in the reaction of silanes with dime t h y 1 t i t a n o c e n e ·

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The Mechanism o f P o l y m e r i z a t i o n The compounds J,, 2 and £ have a l l been i s o l a t e d from r e a c t i o n s of phenylsilane with either dimethyltitanocene (12) or dimethylzirconocene (J_3). A l l o f the e v i d e n c e p o i n t s t o the fact that these compounds a r e p r o b a b l y r e s t i n g s p e c i e s and a r e not involved in the c a t a l y t i c c y c l e . They do n e v e r t h e l e s s g i v e some i n d i c a t i o n of the complex s e r i e s o f r e a c t i o n s t h a t t r a n s f o r m the d i m e t h y l m e t a l l o c e n e to a c t i v e c a t a l y s t . Using the titanocene-catalyzed co-hydrogenation of cyclohexene, we have s t u d i e d the k i n e t i c s o f the p o l y m e r i z a t i o n o f a number o f p r i m a r y s i l a n e s (2CJ). The r a t e law was found to be: Rate

=

1

2

k[catalyst] / [RSiH ] 3

1 / 2

[C H 6

1 2



(5)

We a t t r i b u t e the form o f t h i s r a t e law t o be due to the pseudoe q u i l i b r i u m 6. We r e f e r t o 6 as a p s e u d o - e q u i l i b r i u m , because i t is in fact a steady state r a t h e r than a t r u e e q u i l i b r i u m . If R

o

Cp Ti 2

^TiCp

N

2

+

RSlH ^=a 3

2 Cp Ti(H)(SiH R) 2

2

(6)

H 1

4

compound 4 is a p a r t i c i p a n t in the c a t a l y t i c c y c l e , a n y t h i n g which changes the throughput r a t e of the cycle will alter the c o n c e n t r a t i o n o f 4. Compounds 1 c a n be o b s e r v e d in s o l u t i o n by nmr and the R = Ph compound has been s t r u c t u r a l l y c h a r a c t e r i z e d by X ray a n a l y s i s (12). A p s e u d o - e q u i l i b r i u m s u c h as E q u a t i o n 6 leads n a t u r a l l y t o the r a t e law o f E q u a t i o n 5 i f i t is assumed t h a t the e q u i l i b r i u m l i e s to the l e f t and the r a t e o f r e a c t i o n is c o n t r o l l e d by the unimolecular transformation of the titanium(IV)silylhydride, 4. The most p l a u s i b l e first step for the decomposition of 4 is an α - h y d r i d e e l i m i n a t i o n from the SiH R group, f o l l o w e d , o r accompanied, by l o s s o f H from the complex t o give a C p T i = S i H R complex. I t is then assumed that propagation o c c u r s by some k i n d o f r e p e t i t i v e i n s e r t i o n o f the s i l y l e n e i n t o a Ti-Si bond. A p o s s i b l e mechanism o f t h i s k i n d is shown in Scheme 1· Two e q u a l l y p l a u s i b l e r o u t e s f o r the p r o p a g a t i o n s t e p a r e shown on the lower c e n t e r and lower r i g h t o f Scheme 1· This type of mechanism is a t t r a c t i v e s i n c e i t e x p l a i n s why s e c o n d a r y silanes w i l l only dimerize. A number o f f e a t u r e s o f the s i l a n e and germane p o l y m e r i z a t i o n reactions show unequivocally that there are a t least two indépendant mechanisms o p e r a t i n g . T h i s dichotomy is most e v i d e n t in s i t u a t i o n s where the p r o d u c t s o f r e a c t i o n c a n be compared f o r the i n d u c t i o n p e r i o d t h a t p r e c e d e s the f o r m a t i o n o f compound 1, and the p r o d u c t s t h a t are p r o d u c e d a f t e r the f o r m a t i o n o f 1 ( 1 2 , 1 5 , 2 0 ) . 2

2

2

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

96

INORGANIC AND ORGANOMETALLIC POLYMERS

Table I .

Mass Spectra of a Linear Oligo(phenylsilylene) and of Hexaphenylcyclohexasilane

Oligophenylsilylene

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Ion

Mass

Hexaphenylcylcohexasilane

(abundance)

Ion

Mass

(abundance)

636

(30)

530

(20)

Ph Si H

7

532

(0.45)

P h

Ph Si H

3

499

(0.35)

Ph Si H

Ph Si H

4

452

(1.13)

Ph.Si.H,

452

(50)

422

(2.95)

Ph,Si.H

421

(40)

346

(7.18)

Ph Si H

376

(16)

3

316

(12.4)

Ph Si,H

344

(75)

2

287

(27.0)

Ph Si

315

(20)

259

(24.0)

Ph Si

259

(60)

240

(25.0)

Ph Si 2

3

238

(12)

Ph Si H

211

(42)

Ph si

2

210

(16)

Ph SiH

183

(47)

Ph SiH

183

(80)

PhSi

105

(100)

PhSi

105

(100)

5

5

5

4

4

5

Ph Si H 4

4

Ph Si H 3

4

2

3

Ph Si H 3

Ph si 3

Ph Si 3

Ph Si H 2

2

2

1.

3

2

2

S i

6

H

6 6

5

4

4 3

Q

3 3

5

5 4 4 5

4

3

3

2

2

5

5

From réf. 1 .

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

7. HARROD

97

Polymerization of Group H Hydrides

Cp TiMe 2

+

2

RSiH

3

.Me Cp Ti 2

+

CH.

+

MeRSiH

SiH R

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2

CP Ti 2

RSiH

0

0

Cp Ti^ 2

SiH R 2

Cp,Ti=Si RSiH

RSiH,

Cp Ti 2

0

RS1H ^SiH R Cp Ti SiH R

0

2

Cp Ti 2

^TiCp

V*

/

2

2

Cp Ti 2

s

SiHRSiH R 2

2

Si-H R

^SiHRSiH R 2

Cp Ti=Si

Cp Ti

λ

2

2

SiH R 2

-RSiH

+ RSiH

n

0

J-HH,

R ii-H

ft \

Cp Ti=SiRSiH R 2

2

Cp

Tl

2 ^SxRSiH R RH Si 2

k

2

C p

2 ^S 1HRS iHRS i H R T

£

-HJ Cp Ti=Si 2

SiHRSiH R 2

Scheme 1.

Mechanism of t i t a n o c e n e c a t a l y z e d s i l a n e p o l y m e r i z a t i o n .

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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INORGANIC AND ORGANOMETALLIC POLYMERS

In the two regimes the p r o d u c t s a r e q u i t e d i f f e r e n t . For example, in the titanium catalyzed oligomerization of diphenylgermane described above, one c a t a l y t i c regime seems t o i n v o l v e no gross reduction of the titanium(IV) and g i v e s rise mainly to tetraphenyldigermane, the other i n v o l v e s the reduction of the titanium to give the d i p h e n y l g e r m y l analogue o f 1,, which is a p o w e r f u l c a t a l y s t f o r the t e t r a m e r i z a t i o n o f d i p h e n y l g e r m a n e . The c h a r a c t e r i z a t i o n o f the p u r p l e t i t a n i u m i n t e r m e d i a t e as an analogue o f 1 is based on the o b s e r v a t i o n o f an e s r spectrum o f the triplet state and on the s i m i l a r i t y o f i t s H-nmr spectrum to t h a t of 1 (11). A similar dichotomy was o b s e r v e d in the titanium catalyzed polymerization o f p r i m a r y s i l a n e s c o u p l e d t o the h y d r o g é n a t i o n of norbornene (2(0). A t low c a t a l y s t concentration ( c a . 0.004M), essentially complete conversion o f norbornene to an e q u i m o l a r mixture of norbornane and b i s - p h e n y l s i l y l - ( a n d / o r 1,2-diphenyldisilyl)norbornane was observed. Under t h e s e conditions no evidence for reduction of t i t a n i u m was obtained. At higher catalyst concentrations (> 0.02M) rapid reduction of the dimethyltitanocene to J, and 2 o c c u r s and the catalytic reaction produces mainly p o l y s i l a n e (DP c a . 10) and norbornane in c a . 80 per cent y i e l d s , and s i l y l a t e d norbornanes in about 20 p e r cent yield. Our present preferred hypothesis is that the reactions o c c u r r i n g in the regime where g r o s s r e d u c t i o n o f the t i t a n i u m does not occur are metal catalyzed radical chain reactions. Oligomerization under t h e s e c o n d i t i o n s p r o c e e d s by the stepwise coupling of s i l y l or germyl r a d i c a l s . Following reduction of the titanium, we b e l i e v e t h a t p o l y m e r i z a t i o n o c c u r s by some s o r t of rapid a d d i t i o n mechanism in which the intermediates are not observable because they are s h o r t l i v e d , o r because they are spectroscopically s i l e n t ( e . g . , c a n n o t be seen in the nmr because they are paramagnetic). A l i k e l y c a n d i d a t e f o r the mechanism in this c a s e is the r a p i d r e p e t i t i v e i n s e r t i o n o f s i l y l e n e moieties, produced by α - h y d r i d e e l i m i n a t i o n from 4, i n t o a m e t a l - s i l y l b o n d , as shown in Scheme 1 · At the present time we do n o t know what the termination reaction is. G i v e n the f a c t t h a t the polymers i s o l a t e d from co­ hydrogenation reactions do n o t have d i f f e r e n t molecular weight properties from t h o s e p r o d u c e d by s i m p l e polymerization, hydrogenolysis can be e x c l u d e d as a l i k e l y t e r m i n a t i o n r e a c t i o n . This leaves spontaneous homolysis of a metal-Si bond, followed by hydrogen a b s t r a c t i o n to n e u t r a l i z e the r e s u l t i n g s i l y l r a d i c a l , or reductive elimination of a s i l y l with a hydride, o r o f two silyl ligands. There is n o t h i n g in the p r e s e n t l y a v a i l a b l e information that a l l o w s us t o d i s c r i m i n a t e between t h e s e a l t e r n a t i v e s and t h e y must be c o n s i d e r e d as e q u a l l y p l a u s i b l e . I t is c l e a r that any r e a c t i o n t h a t l e a d s t o polymer c h a i n t e r m i n a t i o n w i t h - S i H R groups will stop f u r t h e r c h a i n growth since neither titanium, nor z i r c o n i u m based c a t a l y s t s a r e a c t i v e f o r s e c o n d a r y s i l a n e s . The f o r m a t i o n o f c y c l o h e x a s i l a n e s and the z i r c o n i u m catalyzed hydrosilation of poly(phenylsilylene), r e f e r r e d to above, both suggest that s l o w c l e a v a g e o f polymer c h a i n s may o c c u r in the presence of the catalysts. Such c l e a v a g e may a l s o play an n

2

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7. HARROD

Polymerization of Group 14 Hydrides

99

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important r o l e in l i m i t i n g c h a i n l e n g t h s and i t w i l l be the focus of f u r t h e r study. L i k e many homogeneously c a t a l y z e d r e a c t i o n s , the o v e r a l l c y c l e ( o r c y c l e s ) in t h e s e p o l y m e r i z a t i o n r e a c t i o n s p r o b a b l y c o n t a i n s too many s t e p s to be e a s i l y a n a l y z e d by any s i n g l e approach. Both kinetics and model compound s t u d i e s have thrown l i g h t on some o f the steps. However, as indicated above, many o f the model compounds isolated from the r e a c t i o n s of primary silanes with metallocene a l k y l s and h y d r i d e s a r e too u n r e a c t i v e t o e x p l a i n the polymerization results. Conclusions The catalyzed dehydrogenative coupling of s i l i c o n and germanium hydrides has been achieved with high r e a c t i o n rates and high conversions. T h i s r e p r e s e n t s a major new r o u t e to the s y n t h e s i s o f S i - S i and o f Ge-Ge b o n d s . The low d e g r e e s o f p o l y m e r i z a t i o n o f the products of primary organosilane coupling represent a serious limitation on t h e i r use in a p p l i c a t i o n s where m e c h a n i c a l strength is a pre-requisite. F u r t h e r s t u d i e s on the mechanism(s) of the reaction are being pursued, p a r t i c u l a r l y w i t h a view to under­ standing the n a t u r e o f the c h a i n t e r m i n a t i o n p r o c e s s ( e s ) . It is possible that such knowledge w i l l a l l o w some c o n t r o l over the factors l i m i t i n g chain length. The selective reactions of t e r m i n a l S i - Η groups with appropriate c o u p l e r s is a p r o m i s i n g method f o r the conversion of our oligomers to h i g h m o l e c u l a r weight m a t e r i a l s and we are presently studying such reactions. I t is l i k e l y t h a t runs of twenty silicon atoms already exhibit many o f the desirable features manifest by h i g h e r molecular weight m a t e r i a l s and the coupling together o f t h e s e c h a i n s can g i v e them the dimensional stability and mechanical properties necessary for certain applications· The p r i n c i p l e of forming novel polymeric materials by dehydrogenative coupling is of considerable generality. The catalytic reactions o f S i - Η and OH a r e w e l l know (27)· S i m i l a r reactions with the heavier chalcogens might lead to some interesting new m a t e r i a l s . Of even g r e a t e r interest is the c a t a l y t i c f o r m a t i o n o f h i g h m o l e c u l a r w e i g h t p o l y ( s i l a z a n e s ) by the elimination of H between silanes and p r i m a r y a m i n e s . This r e a c t i o n has a l r e a d y been s u c c e s s f u l l y c a r r i e d o u t w i t h the a i d of t r a n s i t i o n m e t a l complex c a t a l y s t s ( 2 8 ) . 2

Acknowledgments The N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l of Canada, the Fonds FCAR du Q u é b e c , the Dow C o r n i n g C o r p o r a t i o n , and Esso Canada a r e a l l thanked f o r t h e i r f i n a n c i a l s u p p o r t o f this work. My c o l l a b o r a t o r s , without whose e x p e r i m e n t a l s k i l l s none o f the work d e s c r i b e d above would have been done, a r e thanked and their c o n t r i b u t i o n s a r e r e c o g n i z e d by c i t a t i o n in the r e f e r e n c e s .

Literature Cited 1.

Fink, J. M.; Michalczyk, M. J.; Haller, K.; West, R. J. Chem. Soc., Chem. Communs. 1983, 1010.

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

100 2. 3. 4. 5. 6.

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7. 8. 9. 10.

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Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.