Ligand Assisted Cleavage of H2 Across a Ru-N Bond within a Four-membered Metallacycle and the Catalytic Hydrogenation of CO2 to Formate

The catalytic hydrogenation of CO2 to formate was achieved using the previously reported dichloro(eta(6)-p-cymene){diphenyl(3-methyl-2-indolyl)phosphine}ruthenium (1) as a catalyst under mild conditions. In this complex, the phosphorus-based ligand adopts a kappa(1)-P coordination mode. The catalytic activity was achieved in the presence of DBU as a base providing a TONmax value of 3,800. In order to explore potential transformations occurring within the catalytic reactions, a series of stoichiometric tests were performed. Complex 1 was reacted with DBU to form chloro(eta(6)-p-cymene){diphenyl(3-methyl-2-indolide)phosphine}ruthenium (2). Structural characterization of complex 2 confirmed a kappa(2)-P,N coordination mode for the ligand resulting in a four membered metallacycle. Reaction of 2 with H-2 led to the formation of hydrochloro(eta(6)-p-cymene){diphenyl(3-methyl-indolyl)phosphine}ruthenium (3), albeit not with a clean conversion. This is the product resulting from the formal addition of hydrogen across the Ru-N bond of the metallacycle. Complex 3 was also synthesized via an alternative route involving the reaction of complex 1 with Me2NH center dot BH3 as a means of converting Ru-Cl to Ru-H