Direct and indirect CO₂hydrogenation catalyzed by Ir(III), Rh(III), Ru(II), and Os(II) half-sandwich complexes to generate formates and N,N-diethylformamide

As a contribution to the valorization of carbon dioxide, new N,O-coordinated Ir(III), Rh(III), Ru(II), and Os(II) half-sandwich complexes have been designed, synthesized, and applied as homogeneous pre-catalysts in the experiments for direct and indirect CO2 hydrogenation under basic conditions. The complexes contain a N,O-donor Schiff base ligand derived from N-(β-naphthyl)-2-oxy-1-naphthaldimine that assisted by partitioning the catalysts to the organic layer in biphasic mixtures, enabling catalyst separation. In the presence of DBU, the Os(II) half-sandwich complex N,O-Os(II) (C2) gave a TON of 889 for [DBUH][HCO2] formation under a partial pressure of 60 bar CO2/H2 at 120 °C over 48 h, assisted by heterogeneous side reactions. Complexes N,O-Rh(III) (C3) and N,O-Ir(III) (C4) exhibited great efficiency in the homogeneous hydrogenation of carbon dioxide to afford formate and N,N-diethylformamide in the presence of DBU and diethylamine, respectively. Group 9 half-sandwich complexes eminently qualify as useful in the direct and indirect hydrogenation of CO2 to afford reaction intermediates that can be used to make carbon-neutral fuel.