SIGMA-BOND METATHESIS REACTIONS FOR D(0) METAL SILICON BONDS THAT PRODUCE ZIRCONOCENE AND HAFNOCENE HYDROSILYL COMPLEXES

Reactions of zirconocene and hafnocene silyl derivatives CpCp'M(SiR3)Cl (CP = eta-5-C5H5; Cp' = Cp or Cp*, Cp* = eta-5-C5Me5; M = Zr or Hf; R = Me or SiMe3) with hydrosilanes have been investigated. The observed products depend on the nature of the starting materials, since in some cases the initial sigma-bond metathesis products react further via dehydrocoupling processes. For example, Cp2Zr(SiMe3)Cl reacts with PhSiH3 to give the direct products Me3SiH and Cp2Zr(SiH2Ph)Cl, which then combines rapidly with PhSiH3 to produce [CP2ZrHCl]n, PhH2Si-SiH2Ph, and PhH2Si-SiHPh-SiH2Ph. Thus, hydrosilyl complexes obtained from such reactions can be contaminated by significant quantities of the corresponding hydride. The sigma-bond metathesis reactions of CpCp*Hf[Si(SiMe3)3]Cl (2) with hydrosilanes RR'SiH2 cleanly give the isolable metal silyl derivatives CpCp*Hf(SiHRR')Cl (SiHRR' = SiH2Ph (8), SiH2(p-Tol) (9), SiH2(p-MeOC6H4) (10), SiH2(p-FC6H4) (11), SiH2Mes (12, Mes = 2,4,6-Me3C6H2), SiH2CH2Ph (13), SiH2Cy (14), SiHPh2 (15), SiHMePh (16, as a 7:5 mixture of diastereomers), and SiHPhSiH2Ph (17, as a 1:1 mixture of diastereomers)). In general, primary or secondary silanes will undergo such reactions, provided that a less sterically hindered silyl ligand is introduced at hafnium. Tertiary silanes such as Me3SiH and Et3SiH do not react under comparable conditions. Reactions of 2 with bis(silyl) compounds have provided the bimetallic complex 1,4-CpCp*(Cl)HfSiH2C6H4SiH2Hf(Cl)CpCp* (19) and the thiophene derivative 2,5-CpCp*(Cl)HfSiH2(C4H2S)SiH2Hf(Cl)CpCp* (20). The thermal (dark) reaction of 2 with PhSiH3 obeys a second-order rate law, rate = k[2][PhSiH3], with DELTA-H(double dagger) = 16.4 (7) kcal mol-1, DELTA-S(double dagger) = -27 (2) eu, and k(H)/k(D) (70-degrees-C) = 2.5 (1). These parameters suggest that the above sigma-bond metathesis reactions involve four-center transition states similar to those that have been proposed for "hydrocarbon activation" reactions of d0 metal complexes. Visible light accelerates the reactions of 2 with hydrosilanes, via excitation of a low-intensity transition at 405 nm (epsilon = 445 dm3 mol-1 cm-1), which appears to have considerable silyl-to-metal charge-transfer character. The quantum yield for the reaction of 2 with 1 or 20 equiv of PhSiH3 is 1.0 +/- 0.1. Use of radical traps failed to provide evidence for intermediate radicals in this photochemistry. Possible mechanisms for the observed photochemical conversions are discussed. Reactions of 2 with alkoxyhydrosilanes HSi(OMe)2R' provide routes to the new complexes CpCp*Hf(SiH2Me)Cl (23, R' = Me) and CpCp*Hf(SiH3)Cl (24, R' = OMe). These reactions appear to proceed via initial, metal-catalyzed redistribution of the alkoxyhydrosilanes (e.g., HSiMe(OMe)2 is redistributed to MeSi(OMe)3 and MeSiH3), followed by trapping of the new hydrosilane by 2. The Hf-Si distance observed for 2 (2.881 (4) and 2.888 (4) angstrom for the two independent molecules) is much longer than that observed for 8 (2.729 (3) angstrom), apparently because of greater ligand-ligand repulsion in 2.