Mechanism of the photoreduction of carbon dioxide catalyzed by the benchmarking rhenium dimethylbipyridine complexes; operando measurements by XAFS and FT-IR

The photoreduction of CO2 catalyzed by [fac-Re(dmbpy)(CO)3Cl] was precisely investigated through operando measurements of X-ray absorption fine structure spectroscopy (XAFS) and laser-flash FT-IR transient spectroscopy. In the early stage of the photoreaction, Re0-species assigned as penta-coordinated complex, [Re0(dmbpy)(CO)3], appeared and a delayed rise of ReII-species, [ReII(dmbpy)(CO)3CO2] (3) / [ReII(dmbpy)(CO)3COOH]+ (3’), were observed in XAFS measurements. The valence state of Re-center in the catalytic cycle changed as ReI → Re0 → ReII to be a clear evidence of an oxidative addition of CO2 to the Re0 center. In the laser-flash induced FT-IR measurements, a rise and decay of 3 followed by a delayed rise and decay of 3’ was observed. The second electron transfer from the carbon radical of TEA to 3’ induced the formation of [ReI(dmbpy)(CO)4]+ through [ReI(dmbpy)COOH]. The rate determining step of the catalytic cycle was revealed to be the protonation process of forming 3’ from 3.