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.