A SIGMA-BOND METATHESIS MECHANISM FOR DEHYDROPOLYMERIZATION OF SILANES TO POLYSILANES BY D(0) METAL-CATALYSTS

A mechanism for the dehydropolymerization of hydrosilanes to polysilanes, as catalyzed by early-transition-metal metallocene derivatives, is proposed. This mechanism is based on two sigma-bond metathesis reactions that pass through four-center transition states: (1) the dehydrometalation of silane, H(SiHR)nH, with a metal hydride to give hydrogen and a silyl derivative, M(SiHR)nH, and (2) coupling of the metal silyl derivative with more hydrosilane, H(SiHR)mH, to produce H(SiHR)n(SiHR)mH and regenerate the active metal hydride catalyst. This proposal is based on a number of observed, stoichiometric sigma-bond metathesis reactions involving zirconocene and hafnocene complexes. These reactions, which involve silicon, hydrogen, and a d0 metal center, are rather facile and apparently reflect the tendency of silicon to expand its coordination sphere under these conditions. One reaction of this type is rapid MH/SiH hydrogen exchange, for example between PhSiH3 and CpCp*MHCl (1, M = Zr, or 2, M = Hf), which is observed via deuterium labeling. Hydrogenolysis of CpCp*M[Si(SiMe3)3]Cl (3, M = Zr, and 4, M = Hf) provides a convenient route to the monomeric hydride complexes 1 and 2, respectively. The first step in the proposed polymerization mechanism, which is the reverse of M-Si bond hydrogenolysis, is observed in stoichiometric reactions of 1 or 2 with PhSiH3 to give hydrogen and the phenylsilyl complexes CpCp*M(SiH2Ph)Cl (5, M = Zr, and 6, M = Hf). The thermolytic decomposition of 6 to 2 results in Si-Si bond formation, with the production of polysilane oligomers. This second-order reaction exhibits a deuterium isotope effect at 75-degrees-C of 2.9 (2) and activation parameters (DELTA-H(double dagger) = 19.5 (2) kcal mol-1 and DELTA-S(double dagger) = -21 (6) eu) that suggest a four-center transition state. The second-order reaction of 6 with PhSiH3 (to give 2 and (SiHPh)n polysilanes) was also studied kinetically and found to exhibit similar kinetic parameters. This sigma-bond metathesis reaction, which corresponds to the second step in the proposed mechanism, is believed to pass through a four-center transition state that results from interaction of the CpCp*(Cl)Hf-SiH2Ph and H-SiH2Ph bonds. Evidence for the role of hydride complexes as true catalysts is obtained by comparing gel permeation chromatograms for polysilanes obtained from both 3 and 1 as catalysts. Observed reactions of CpCp*Hf[Si(SiMe3)3]Me (7) with PhSiH3, to give CpCp*Hf(SiH2Ph)Me (8) and then CpCp*Hf(H)Me (9), model proposed processes for the activation of catalyst precursors. The step-growth character of the reaction is illustrated by the slow dehydrocoupling of PhSiH3 by Cp*2HfH2, which allows observation of early polysilane intermediates (di-, tri-, and tetrasilane). The participation of M(SiHPh)nH complexes as intermediates in dehydrocoupling was investigated. Partly on the basis of the observed reaction of CpCp*Hf[(SiHPh)3H]Cl (11) with PhSiH3 to give 6 and H(SiHPh)3H, it is concluded that a preferred dehydrocoupling pathway involves monosilyl intermediates, MSiH2Ph, and chain growth by one monomer unit per catalytic cycle. Implications of the proposed mechanism are discussed.