Zinc and Magnesium Catalysts for the Hydrosilylation of Carbon Dioxide

The terminal zinc and magnesium hydride compounds, [κ³-Tism^PriBenz]­ZnH and [Tism^PriBenz]­MgH, which feature the tris­[(1-isopropyl­benz­imidazol-2-yl)­dimethylsilyl]­methyl ligand, react with B­(C₆F₅)₃ to afford the ion pairs, {[Tism^PriBenz]­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^PriBenz]­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.