Bridging Efficiency within Multinuclear Homogeneous Catalysts in the Photocatalytic Reduction of Carbon Dioxide

A trinuclear complex consisting of one [Ru(dmb)(3)](2+) (dmb=4,4-dimethyl-2,2-bipyridine) (Ru) and two [Re(dmb)(CO)(3)Cl] (Re) building blocks, [Re(CO)(3)Cl(dmb-dmb)Ru(dmb)(dmb-dmb)Re(CO)(3)Cl](PF6)(2) (Re-Ru-Re), is presented. Photophysical properties of Re-Ru-Re and the individual components with different or no covalent linkages are thoroughly investigated and compared. To elucidate the role of the single covalent bonds, photocatalytic reduction of CO2 is performed with the trinuclear complex and a series of model systems featuring systematic absence of linkages between the metal centers. Photoluminescence spectra and quantum yields reveal efficient energy transfer from the excited state of Re to Ru if these fragments are covalently linked. Moreover, intramolecular electron transfer from the one-electron reduced species of Ru to Re occurs if there is covalent bonding, leading to a higher photostability and thus the highest turnover number in photocatalytic CO2 reduction of 199 for the trinuclear complex Re-Ru-Re within the systems under investigation. Optimized experimental conditions reveal the highest turnover number (315) reported to date for Re-I/Ru-II-based homogeneous catalysts in photocatalytic CO2 reduction.