Red-Light-Driven Photocatalytic Reduction of CO2 using Os(II)-Re(I) Supramolecular Complexes

The novel supramolecular complexes, which are composed of an [Os(5dmb)(2)(BL)](2+)-type complex (5dmb = 5,5'-dimethyl-2,2'-bipyridine; BL = 1,2-bis(4'-methyl-[2,2'-bipyridin]-4-yl)ethane) as a photosensitizer and cis,trans-[Re(BL)(CO)(2){P(p-X-C6H4)(3)}(2)](+)-type complexes (X = F, Cl) as a catalyst, have been synthesized. They photocatalyzed selective reduction of CO2 to CO under red-light irradiation (lambda > 620 nm). The photocatalytic abilities were affected by the phosphine ligands on the Re unit, and the supramolecule with P(p-Cl-C6H4)(3) ligands exhibited better photocatalysis (Phi(CO) = 0.12, TONCO = 1138, TOFCO = 3.3 min(-1)). The detailed studies clarified the electron balance and material balance; i.e., one molecule of the sacrificial electron donor (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]-imidazole (BIH)) donated two electrons, one molecule of CO2 accepted the two electrons, and another CO2 molecule served as an O2- acceptor to give each molecule of the two-electron oxidized compound of BIH, CO, and HCO3-.