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

Abstract

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 spectr...