Mechanism of Photocatalytic Reduction of CO2 by Re(bpy)(CO)3Cl from Differences in Carbon Isotope Discrimination

The rhenium complex Re(bpy)(CO)(3)Cl (1, bpy = 2,2'-bipyridine) catalyzes CO2 reduction to CO in mixtures containing triethanolamine (TEOA) as a sacrificial reductant. The mechanism of this reaction under photocatalytic conditions remains to be fully characterized. Here, we report the competitive carbon kinetic isotope effects (C-13 KIEs) on photocatalytic CO2 reduction by 1 and analyze the results of experimental measurements by comparing with computed KIEs via density functional theory (DFT) calculations as a means of formulating a chemical mechanism and illustrating the utility of this approach. The C-13 KIEs, k(C-12)/k(C-13), in acetonitrile (ACN) and dimethylformamide (DMF) were determined to be 1.0718 +/- 0.0036 and 1.0685 +/- 0.0075, respectively. When [Ru(bpy)(3)]Cl-2 is added to the reaction mixture in acetonitrile as a photosensitizer, the reduction of CO2 exhibited a C-13 KIE = 1.0703 +/- 0.0043. These values are consistent with the calculated isotope effect of CO2 binding to the one-electron reduced [Re-I(bpy(center dot-))(CO)(3)] species. The findings reported here provide strong evidence that the reactions in the two different solvents have the same first irreversible step and proceed with similar reactive intermediates upon reduction. Theoretically, we found that the major contribution for the large C-13 isotope effects comes from a dominant zero-point energy (ZPE) term. These results lay the groundwork for combined experimental and theoretical approaches for analysis of competitive isotope effects toward understanding CO2 reduction catalyzed by other complexes.