Zinc and Magnesium Catalysts for the Hydrosilylation of Carbon Dioxide

The terminal zinc and magnesium hydride compounds, [κ 3 -Tism Pr i Benz ]ZnH and [Tism Pr i Benz ]MgH, which feature the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, react with B(C 6 F 5 ) 3 to afford the ion pairs, {[Tism Pr i Benz ]M}[HB(C 6 F 5 ) 3 ] (M = Zn, Mg), which are rare examples of these metals in trigonal monopyramidal coordination environments. Significantly, in combination with B(C 6 F 5 ) 3, {[Tism Pr i Benz ]M}-[HB(C 6 F 5 ) 3 ] generates catalytic systems for the hydrosilylation of CO 2 by R 3 SiH to afford sequentially the bis(silyl)acetal, H 2 C(OSiR 3 ) 2, and CH 4 (R 3 SiH = PhSiH 3, Et 3 SiH, and Ph 3 SiH). 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 CO 2 involving a magnesium compound. Also of note, the selectivity of the catalytic systems may be controlled by the nature of the silane, with PhSiH 3 favoring CH 4, and Ph 3 SiH favoring the bis(silyl)acetal, H 2 C(OSiPh 3 ) 2 .