Mechanisms for CO Production from CO2 Using Reduced Rhenium Tricarbonyl Catalysts

The chemical conversion of CO2 has been studied by numerous experimental groups. Particularly the use of rhenium tricarbonyl-based molecular catalysts has attracted interest owing to their ability to absorb light, store redox equivalents, and convert CO2 into higher-energy products. The mechanism by which these catalysts mediate reduction, particularly to CO and HCOO-, is poorly understood, and studies aimed at elucidating the reaction pathway have likely been hindered by the large number of species present in solution. Herein the mechanism for carbon monoxide production using rhenium tricarbonyl catalysts has been investigated using density functional theory. The investigation presented proceeds from the experimental work of Meyer's group (J. Chem. Soc., Chem. Commun. 1985, 1414-1416) in DMSO and Fujita's group (J. Am. Chem. Soc. 2003, 125, 11976-11987) in dry DMF. The latter work with a simplified reaction mixture, one that removes the photo-induced reduction step with a sacrificial donor, is used for validation of the proposed mechanism, which involves formation of a rhenium carboxylate dimer, [Re(dmb)(CO)(3)](2)(OCO), where dmb = 4,4'-dimethyl-2,2'-bipyridine. CO2 insertion into this species, and subsequent rearrangement, is proposed to yield CO and the carbonate-bridged [Re(dmb)(CO)(3)](2)(OCO2). Structures and energies for the proposed reaction path are presented and compared to previously published experimental observations.