Reactivity of Heterobimetallic Alkoxysilyl and Siloxy Complexes in the Catalytic Dehydrogenative Coupling of Tin Hydrides Pierre Braunstein* and Xavier Morise Laboratoire de Chimie de Coordination, Associe ´ au CNRS (UMR 7513), Universite ´ Louis Pasteur, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France Received April 25, 1997 Heterobimetallic complexes [(OC) 3 (R 3 Si)Fe(µ-dppm)Pd(η 3 -allyl)] 1 (R ) OMe, Me, OSiMe 3 , OSi(H)Me 2 ) and [(OC) 3 Fe{µ-Si(OR) 2 (OR)}(µ-dppm)PdCl] 6 (R ) Me, SiMe 3 ) are effective catalyst in the dehydrogenative coupling of triorganotin hydrides HSnR′ 3 (R′ ) Ph, n Bu). Although the elementary transformations during catalysis appear to take place at palladium, the function of the iron fragment is to provide the palladium center with the appropriate coordination environment through metal-metal bonding and the Si-containing ligand. Indeed, complexes 1 and 6 revealed a higher catalytic activity than mononuclear Pd catalysts. Modifications of the substituents at silicon resulted in considerable variations of the TON (turnover number) and TOF (turnover frequency) values as well as in the lifetime of the catalysts. In the case of siloxy derivatives, TON and TOF values higher than in the case of the alkoxysilyl analogs have been obtained whereas the lifetime of the catalyst is much longer for the latter. Possible mechanisms which rationalize the role of the silicon ligand are discussed. Solvent effects have also been observed. One of the key features of these systems is the retention of the bimetallic nature of the catalyst. TON and TOF higher than 2 × 10 5 and 3 × 10 7 h -1 , respectively, have been obtained in the case of HSn n Bu 3 . The catalytic activity of 1 toward the dehydropolymerization of tin dihydrides has been tested. Introduction During the last few years, we have been interested in studying the properties of metal-silicon bonds in a heterobimetallic environment. 1,2 This led to the dis- covery of a new class of complexes that display unusual Fe-Si-O-M four-membered rings involving the silicon backbone. The labile character of the dative oxygen- metal interaction, shown by VT 1 H NMR studies, may represent an interesting tool for catalytic purposes where the storage of a “masked” coordination site plays a key role. We have already reported that treatment of [(OC) 3 - {(MeO) 3 Si}Fe(µ-dppm)Pd(η 3 -allyl)] 1a (dppm ) Ph 2 - PCH 2 PPh 2 ) with excess HSnPh 3 afforded, via propene elimination, the trinuclear complex [(OC) 3 Fe{Si(OMe) 2 - (OMe)}(µ-dppm)Pd(SnPh 3 )] (2a), which is stabilized by an intramolecular Fe-Si-O-Pd four-membered ring (eq 1). 1b This reaction was also accompanied by evolu- tion of H 2 and formation of Sn 2 Ph 6 (ca. 10%). Interest- ingly when HSnPh 3 was added to a solution of [(OC) 3 {(Me 3 SiO) 2 MeSi}Fe(µ-dppm)Pd(η 3 -allyl)] 1c, for - mation of the Fe-Pd-Sn complex 2c was not evidenced. Instead a vigorous gas evolution and a complete trans- formation of HSnPh 3 into Sn 2 Ph 6 occurred (eq 1). These observations clearly indicate that not only are complexes 1a and 1c catalysts (precursors) for the dehydrogenative tin-tin coupling of HSnPh 3 , but that the nature of the (1) (a) Braunstein, P.; Knorr, M.; Tiripicchio, A.; Tiripicchio Camel- lini, M. Angew. Chem., Int. Ed. Engl. 1989, 28, 1361. (b) Braunstein, P.; Knorr, M.; Piana, H.; Schubert, U. Organometallics 1991, 10, 828. (c) Braunstein, P.; Knorr, M.; Schubert, U.; Lanfranchi, M.; Tiripicchio, A. J. Chem. Soc., Dalton Trans. 1991, 1507. (d) Braunstein, P.; Knorr, M.; Villarroya, E.; DeCian, A.; Fischer, J. Organometallics 1991, 10, 3714. (e) Knorr, M.; Braunstein, P. Bull. Soc. Chem. Fr. 1992, 129, 663. (f) Knorr, M.; Faure, T.; Braunstein, P. J. Organomet. Chem. 1993, 447, C4. (g) Braunstein, P.; Faure, T.; Knorr, M.; Balegroune, F.; Grandjean, D. J. Organomet. Chem. 1993, 462, 271. (h) Braunstein, P.; Knorr, M.; Strampfer, M.; Tiripicchio, A.; Ugozzoli, F. Organome- tallics 1994, 13, 3038. (i) Knorr, M.; Braunstein, P.; Tiripicchio, A.; Ugozzoli, F. Organometallics 1995, 14, 4910. (j) Braunstein, P.; Knorr, M.; Sta ¨ hrfeldt, T. J. Chem. Soc., Dalton Trans. 1994, 1913 (k) Braunstein, P.; Faure, T.; Knorr, M.; Sta ¨ hrfeldt, T.; DeCian, A.; Fischer, J. Gazz. Chim. Ital. 1995, 125, 35. (l) Braunstein, P.; Knorr, M. J. Organomet. Chem. 1995, 500, 21. (2) Braunstein, P.; Morise, X.; Blin, J. J. Chem. Soc., Chem. Commun. 1995, 1455. 540 Organometallics 1998, 17, 540-550 S0276-7333(97)00353-1 CCC: $15.00 © 1998 American Chemical Society Publication on Web 01/24/1998