Zinc and Magnesium Catalysts for the Hydrosilylation of Carbon Dioxide.

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