Zinc and Magnesium Catalysts for the Hydrosilylation of Carbon Dioxide

The terminal zinc and magnesium hydride compounds, [κ³-Tism^Pr]ZnH and [Tism^Pr]MgH, which feature the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, react with B(C₆F₅)₃ to afford the ion pairs, {[Tism^Pr]M}[HB(C₆F₅)₃] (M = Zn, Mg), which are rare examples of these metals in trigonal monopyramidal coordination environments. Significantly, in combination with B(C₆F₅)₃, {[Tism^Pr]M}-[HB(C₆F₅)₃] generates catalytic systems for the hydrosilylation of CO₂ by R₃SiH to afford sequentially the bis(silyl)acetal, H₂C(OSiR₃)₂, and CH₄ (R₃SiH = PhSiH₃, Et₃SiH, and Ph₃SiH). In contrast to many other catalysts for these transformations, both the zinc and magnesium catalytic systems are active at room temperature, and the latter provides the first example of catalytic hydrosilylation of CO₂ involving a magnesium compound. Also of note, the selectivity of the catalytic systems may be controlled by the nature of the silane, with PhSiH₃ favoring CH₄, and Ph₃SiH favoring the bis(silyl)acetal, H₂C(OSiPh₃)₂.