Trialkylborane-Assisted CO2 Reduction by Late Transition Metal Hydrides

Trialkylborane additives promote reduction of CO2 to formate by bis(diphosphine) Ni(II) and Rh(III) hydride complexes. The late transition metal hydrides, which can be formed from dihydrogen, transfer hydride to CO2 to give a formate–borane adduct. The borane must be of appropriate Lewis acidity: weaker acids do not show significant hydride transfer enhancement, while stronger acids abstract hydride without CO2 reduction. The mechanism likely involves a pre-equilibrium hydride transfer followed by formation of a stabilizing formate–borane adduct.