Photochemical conversion of CO2 to fuels or useful chemicals using renewable solar energy is an attractive solution to both the world's need for fuels and the reduction of greenhouse gases. Rhenium(I) and ruthenium (II) diimine complexes have been shown to act as photocatalysts and/or electrocatalysts for CO2 reduction to CO. We have studied these photochemical systems focusing on the identification of intermediates and the bond formation/cleavage reactions between the metal center and CO2. For example, we have produced the one-electron-reduced monomer (i.e. Re(dmb)(CO)3S where dmb = 4,4′-dimethy-2,2′-bipyridine and S = solvent) either by reductive quenching of the excited states of fac- [Re(dmb)(CO)3(CH3CN)]PF6 or by photo-induced homolysis of [Re(dmb)(CO)3]2. We previously found that: (1) the remarkably slow dimerization of Re(dmb)(CO)3S is due to the absence of a vacant coordination site for Re-Re bond formation, and the extra electron is located on the dmb ligand; (2) the reaction of Re(dmb)(CO)3S with CO2 forms a CO2-bridged binuclear species (CO)3(dmb)Re-CO(O)-Re(dmb)(CO)3 as an intermediate in CO formation; and (3) the kinetics and mechanism of reactions are consistent with the interaction of the CO2-bridged binuclear species with CO to form CO and CO_3~(2-). .
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