Raman Spectroscopy as a Method to Investigate Catalytic Intermediates: CO2 Reducing [Re(Cl)(bpy-R)(CO)3] Catalyst

Complexes of the type [Re(Cl)(bpy-R)(CO)3] (1, bpy = bipyridine, R = (t)Bu, H, CF3) show high catalytic activity for electrochemical CO2 reduction. Application of Raman spectroscopy to these complexes as well as to the doubly reduced species [Re(bpy-R)(CO)3](-) (3), which are the postulated active species, and the monoreduced complex [Re(Cl)(bpy-CF3)(CO)3](-) (2) and comparison with state-of-the-art quantum chemical calculations allows accurate investigation of electronic structures as well as geometries. For doubly reduced complexes, calculations point out a formal closed-shell singlet state only compatible with a formal {Re(I)(bpy-R)(2-)} moiety. In contrast, based on molecular orbital analysis and the change of the actual charge distribution during the overall two-electron reduction, the system is better described as {Re(0)(bpy-R(•))(-)}. Interestingly, the Raman spectra of the monoreduced and doubly reduced complexes with the CF3-substituted bpy ligand are virtually identical, which points to the same overall electronic structure of the bpy species in both complexes. Additional Raman experiments and calculations of [Re(COOH)(bpy)(CO)3] (4) and [Re(bpy)(CO)4]OTf (5), which are proposed to be intermediates of the catalytic cycle for CO2 reduction, confirm the presence of neutral bpy showing that the reducing equivalent stored at the bidentate ligand is involved in the activation of CO2. As such, Raman spectroscopy combined with quantum chemical calculations is an ideal tool to investigate catalysts with redox active ligands, since the spectra give local information about the electronic and geometric structure of the molecule.