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 <b>2</b> is shown to occur very easily, because of the <i>trans</i> influence exerted by the boryl ligand. During catalysis, the limiting step is B­(C₆F₅)₃ dissociation from the active species (PBP)­Ni–OCHO·B­(C₅F₆)₃ (<b>4</b>), 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···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] (<b>5</b>) 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