Selective Ni-Catalyzed Hydroboration of CO2 to the Formaldehyde Level Enabled by New PSiP Ligation

The synthesis and characterization of group 10 metal pincer complexes supported by a new bis­(indolylphosphino)­silyl ligand are described, including the synthesis of Ni, Pd, and Pt hydride species. Solution NMR and single-crystal X-ray data revealed that a significant amount of structural variability is possible for such hydride complexes, particularly in the case of Ni, where terminal Ni-H as well as complexes involving η2-SiH coordination are both accessible and may even coexist, in ratios dependent on factors such as the nature of additional coligands, including N2 from the reaction atmosphere, as well as solvent and temperature. Nickel and palladium hydride complexes of this new ligand were found to exhibit divergent selectivity in the catalytic hydroboration of CO2 with pinacolborane (HBPin). While the Pd catalyst exhibited moderate activity for CO2 hydroboration to the formate level, the analogous Ni species exhibited unprecedented selectivity (97%) for hydroboration of CO2 to the formaldehyde level to provide the bis­(boryl)­acetal PinBOCH2OBPin in high yield, under mild conditions. The HBPin-derived bis­(boryl)­acetal can be successfully isolated and utilized as a source of methylene for the formation of C–N and C–P bonds.