Phosphine-free ruthenium NCN-ligand complexes and their use in catalytic CO2 hydrogenation

Abstract

This work investigates the hydrogenation of carbon dioxide to formate catalysed by the phosphine-free Ru complexes Ru(OtBu)(κ³-NCN)(^tBubpy) and RuH(OtBu)(κ²-NCN)(^tBubpy) (OtBu = tert-butoxide, κ²-NCN = 1,3-di(2-methylpyridyl)-4,5-diphenyl-1H-imidazol-2-ylidene, where one pyridyl moiety is not coordinated to Ru, ^tBubpy = 4,4′-di-tert-butyl-2,2′-dipyridyl). A catalytic cycle is proposed for this reaction, supported by computational studies and the characterization of the hydride and the formate intermediates proposed to be involved. Modest catalytic turnovers are demonstrated at relatively low pressures and temperatures. The proposed rate determining step is heterolytic H₂ splitting to regenerate the Ru–H complex, which has an estimated hydricity of approx. 27 kcal mol⁻¹. The κ²-NCN ligand in the hydride complex undergoes a variety of dynamic processes as detected by EXSY spectroscopy including a pyridyl “roll-over” carbon–hydrogen – ruthenium hydride exchange, possibly occuring via a Perutz–Sabo-Etienne CAM mechanism.