Spectroscopic characterization of intermediates in CO₂ reduction with rhenium photocatalysts

We are investigating fundamental processes for the efficient capture and chemical conversion of solar energy using transition metal complexes. Mechanistic and kinetic knowledge of photo-induced multi-electron transfer reactions and bond forming (and/or breaking) reactions is crucial to understanding photochemical activation of CO₂ and to the design of more efficient photoconversion systems. We have investigated the excited state properties and the reactivities of the photochemically produced species of fac-Re(dmb)(CO)₃X (where dmb = 4,4′-dimethyl-2,2′- bipyridine, X = halide or solvent) by transient UV-vis, transient FTIR, and NMR spectroscopy. The reduced monomer, Re(dmb)(CO)₃S (S = solvent) dimerizes (k_d = 40 M^-1 S^-1) sreversibly in THF. The reaction of Re(dmb)(CO)₃S with CO₂ is very slow (k_CO2 < 0.1 M^-1 s^-1) and liberates CO with a 25-50% yield based on [Re]. A CO₂ bridged dimer, (CO) ₃(dmb)Re-CO(O)-Re(dmb)(CO)₃, is identified as a key intermediate in the CO formation. [Re(dmb)(CO)₃]₂(OCO ₂), Re(dmb)(CO)₃(OC(O)OH) and [Re(dmb)(CO) ₃S]⁺ were detected as oxidation products by means of NMR, IR, and FAB MS.