Polymerization of Group 14 Hydrides by Dehydrogenative Coupling

Chapter 7

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 22, 2016 | http://pubs.acs.org Publication Date: January 7, 1988 | doi: 10.1021/bk-1988-0360.ch007

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


90

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

2


7.

HARROD

91

Polymerization of Group 14 Hydrides

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.


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.


or the on the for is give two


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 .


2

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

2



7. HARROD

93


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



94

Cp' ZrMe 2

Ph Ph H4Si-Si4t:H H £ )

2


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


(4)

7.

HARROD

95


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 ·


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


96


Table I .

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

Oligophenylsilylene


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 .


7. HARROD

97

Polymerization of Group H Hydrides

Cp TiMe 2

+

2

RSiH

3

.Me Cp Ti 2

+

CH.

+

MeRSiH

SiH R


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 .



98


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


7. HARROD


99


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.



100 2. 3. 4. 5. 6.


7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

Burkhard, C. A. J. Am. Chem. Soc. 1949, 71, 963. Yajima, S; Shishodo, T.; Kayano, H. Nature 1976, 264, 237. Miller, R. D.; Hofer, D.; McKean, D. R.; Wilson, C. G.; West, R.; Trefonas, P. T. III ACS Symposium Series No. 266; American Chemical Society, Washington, DC, 1984, pp. 293-310. Stolka, M.; Yuh, H.-J.; McGrane, K.; Pai, D. M. J. Polymer Sci., Part A, Polymer Chemistry 1986, 25, 823. Trefonas, P.; Damewood, J. R.; West, R. Organometallics 1985, 4, 1318. Harrah, L. Α.; Ziegler, J. M. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 209. Gobbi, G. C.; Fleming, W. W.; Sooriyakamuran, R.; Miller, R. D. J. Am. Chem. Soc. 1986, 108, 5624. For an excellent review of the current status of organopolysilane chemistry, see West, R., J. Organometal. Chem. 1986, 300, 327. Helmer, B; West, R. J. Organometal. Chem. 1982, 236, 21. Trefonas, P.; Djurovich, P. I.; Zhang, X.-H.; West, R.; Miller, R. D.; Hofer, D. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 819. Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Organomet. Chem. 1985, 279, C11. Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Am. Chem. Soc. 1986, 108, 4059. Aitken, C. T.; Harrod, J. F.; Samuel, E, Can. J. Chem. 1986, 64, 1677. Finkbeiner, H. L.; Hay, A. S.; White, D. M. Polymerization by oxidative coupling, in Polymerization Processes, Wiley-Interscience, New York, 1977, p. 537. Malek, A.; Harrod, J. F., unpublished results. Aitken, C.; Harrod, J. F., unpublished results. Barry, J.-P. ; Harrod, J. F., unpublished results. The reactivity of organoactinides with C-H bonds has been extensively studied by Marks et al.; see e.g., Bruno, J. W.; Smith, G. M.; Marks, T. J.; Fair, C. K.; Schultz, A. J.; Williams, J. M. J. Am. Chem. Soc. 1986, 108, 40. Hennge, E.; Lunzer, F. Monatheft. Chem. 1976, 107, 371. Harrod, J. F. ; Yun, S. S. Organometallics, in press. Aitken, C. T.; Harrod, J. F.; Gill, U. Can. J. Chem., in press. Speier, J. L. Adv. Organomet. Chem. 1979, 17, 407. Harrod, J. F.; Chalk, A. J. In Organic Synthesis via Metal Carbonyls, Wender, I., Pino, P. Eds.; Wiley: New York, 1977; Vol. 2, p. 673. Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1982, 104, 107. Stein, H. J.; Peercy, P. S. Appl. Phys. Lett. 1979, 34(9), 604. DeMeo, Ε. Α.; Taylor, R. W. Science 1984, 224, 245. Harrod, J. F.; Malek, Α.; Rochon, F.; Melanson, R. Organo metallics, in press. See e.g., Sommer, L. H.; Lyons, J. E. J. Am. Chem. Soc. 1967, 89, 1521. Blum, Y; Laine, R. M. Organometallics 1986, 5, 2081.

RECEIVED September 1, 1987


Polymerization of Group 14 Hydrides by Dehydrogenative Coupling

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