Hydrogenation of Carbon Dioxide Catalyzed by Ruthenium Trimethylphosphine Complexes: A Mechanistic Investigation Using High-Pressure NMR Spectroscopy

Complex cis-(PMe3)4RuCl(OAc) (1) acts as a catalyst for CO2 hydrogenation into formic acid in the presence of a base and an alcohol cocatalyst. NMR spectroscopy has revealed that 1 exists in solution in equilibrium with fac-(PMe3)3RuCl(η2-OAc) (2), [(PMe3)4Ru(η2-OAc)]Cl (3a), and free PMe3. Complex 2 was found to be a poor CO2 hydrogenation catalyst under the conditions of catalysis used for 1. Complex 3a can be prepared by adding certain alcohols, such as MeOH, EtOH, or C6H5OH, to a solution of 1 in CDCl3. The chloride ion of 3a was exchanged for the noncoordinating anion BPh4 − or B(ArF)4 − (B(ArF)4 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) to produce [(PMe3)4Ru(η2-OAc)]BPh4 (3b) and [(PMe3)4Ru(η2-OAc)]B(ArF)4 (3c). Complexes 3b and 3c were found to be as efficient as 1 in the catalytic hydrogenation of CO2 to formic acid in the presence of an alcohol cocatalyst. In contrast to 1, 3b and 3c continued to show high catalytic activity in the absence of the alcohol cocatalyst. High-pressure NMR spectroscopy was used to investigate the mechanism of CO2 hydrogenation via 3b,c in the presence of base. The observations were inconsistent with the previously reported phosphine-loss mechanism; a new mechanism is proposed involving an unsaturated, cationic ruthenium complex of the form [(PMe3)4RuH]+ (B) as the active catalyst. The role of the base in this system includes not only trapping of the formic acid product but also initiation of the catalysis by aiding the conversion of 3b,c to B. Crystallographically determined structures are reported for complexes 2, 3b, {[(PMe3)3Ru]2(μ-Cl)2(μ-OAc)}BPh4, and {[(PMe3)3Ru]2(μ-Cl)3}Cl.