Visible-Light-Driven CO Reduction with Homobimetallic Complexes. Cooperativity between Metals and Activation of Different Pathways.

Visible-light-driven reduction of CO to both CO and formate (HCOO) was achieved in acetonitrile solutions using a homobimetallic Cu bisquaterpyridine complex. In the presence of a weak acid (water) as coreactant, the reaction rate was enhanced, and a total of ca. 766 TON (turnover number) was reached for the CO reduction, with 60% selectivity for formate and 28% selectivity for CO, using Ru(phen) as a sensitizer and amines as sacrificial electron donors. Mechanistic studies revealed that with the help of cooperativity between two Cu centers, a bridging hydride is generated in the presence of a proton source (water) and further reacts with CO to give HCOO. A second product, CO, was also produced in a parallel competitive pathway upon direct coordination of CO to the reduced complex. Mechanistic studies further allowed comparison of the observed reactivity to the monometallic Cu quaterpyridine complex, which only produced CO, and to the related homobimetallic Co bisquaterpyridine complex, that has been previously shown to generate formate following a mechanism not involving the formation of an intermediate hydride species.