Metal-silicon bonding energetics in organo-Group 4 and organo—f-element complexes. Implications for bonding and reactivity

Abstract

Metal-silicon bond disruption enthalpies have been measured for a series of U, Zr, and Sm metallocene complexes: Cp 3USi(TMS) 3, Cp 2Zr(Cl)Si(TMS) 3, Cp 2Zr(Me)Si(TMS) 3, Cp 2Zr(TMS)Si(TMS) 3, Cp 2Zr(TMS)O tBu, Cp′ 2SmSiH(TMS) 2 (Cp = η 5-C 5H 5, Cp′ = η 5-Me 5C 5, TMS = trimethylsilyl). Data were obtained by anaerobic batch-titration solution calorimetry in toluene. Derived metal-ligand bond enthalpies D(L n M-R) in kcal mol −1 are: D[Cp 3USi(TMS) 3] = 37(3), D[Cp 2(Cl)ZrSi(TMS) 3]=57(3), D[Cp 2(Me)ZrSi(TMS) 3]=56(5), D[Cp 2(Si(TMS) 3)ZrMe] = 66(5), D[Cp 2(O tBu)Zr-TMS] = 60(5), D[Cp 2(TMS)ZrSi(TMS) 3] = 42(11), D[Cp 2(Si(TMS) 3)Zr-TMS] = 45(7), D[Cp′ 2SmSiH(TMS) 2] = 43(5). These results show that metal-silicon bond disruption enthalpies involving these electron-deficient metals are substantially smaller than those of the corresponding metal hydride and hydrocarbyl bonds. These data in combination with previously measured metal-ligand bond enthalpies allow thermodynamic analyses of a variety of stoichiometric and catalytic transformations involving metal silyl functionalities. The latter include potential pathways for dehydrogenative silane polymerization, dehydrogenative silane-hydrocarbon coupling, olefin hydrosilylation, and dehydrogenative silane-amine coupling. It is not uncommon for there to be multiple pathways which effect the same catalytic transformation and which contain no steps having major enthalpic impediments.