Mechanism of photocatalytic reduction of CO₂ by Re(bpy)(CO)₃Cl from differences in carbon isotope discrimination

The rhenium complex Re(bpy)(CO)₃Cl (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 (¹³C 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 ¹³C KIEs, k(¹²C)/k(¹³C), in acetonitrile (ACN) and dimethylformamide (DMF) were determined to be 1.0718 ± 0.0036 and 1.0685 ± 0.0075, respectively. When [Ru(bpy)₃]Cl₂ is added to the reaction mixture in acetonitrile as a photosensitizer, the reduction of CO₂ exhibited a ¹³C 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•-)(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 ¹³C 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.