Mechanistic Studies on the Selective Reduction of CO2 to the Aldehyde Level by a Bis(phosphino)boryl (PBP)-Supported Nickel Complex

This work describes a thorough investigation of the mechanism of a highly selective hydrosilylation of CO2 to the formaldehyde level catalyzed by a bis(phosphino)boryl (PBP)Ni(II) complex in the presence of B(C6F5)(3). CO2 activation by insertion into the Ni-H bond of the catalyst precursor 2 is shown to occur very easily, because of the trans influence exerted by the boryl ligand. During catalysis, the limiting step is B(C6F5)(3) dissociation from the active species (PBP)Ni-OCHO center dot B(C5F6)(3) (4), which controls the amount of free borane that can lead to over -reduction to methane. Free borane activates the silane by formation of [R3Si-H center dot center dot center dot B(C6F5)(3)], which can then transfer the silylium (R3Si+) fragment to the oxygen atoms of the Ni formate and Ni acetal intermediates. The ion pair [(PBP)Ni][HB(C6F5)(3)] (5) is the key species that activates CO2 in the catalytic cycle (and silylformate in a second step) with [HB(C6F5)(3)](-) as the source of hydride. Hydride transfer to [(PBP)Ni-OCO](+) is virtually barrierless, whereas hydride transfer to [(PBP)Ni-OCHOSiR3](+) has the second -highest energy barrier of the process (25.2 kcal mol(-1)). Therefore, the (PBP)Ni framework is instrumental in both reduction steps of the catalysis and controls the selectivity of the reaction by sequestering B(C6F5)(3).