Elucidation of Cooperativity in CO₂ Reduction Using a Xanthene-Bridged Bimetallic Rhenium(I) Complex

Abstract

Mechanistic studies on dinuclear complexes that can activate CO2 are rare. Based on the investigations done for the mononuclear compound (bpy)Re(CO)3Cl (bpy-Re, with bpy = 2,2′-bipyridine), many reports favor a mononuclear catalytic cycle, while the possibility of a dinuclear catalytic species is discussed in the literature in only a few cases. Here, we report the synthesis and characterization of a homobimetallic rhenium(I) compound, in which two (bipyridine)Re(CO)3Cl fragments are brought into close vicinity by attaching them to a xanthene backbone. First, photocatalytic investigations show a significant increase of the catalytic performance compared to the mononuclear parent compound. Second, spectroelectrochemical experiments demonstrate the remarkable fast formation of an intermediate with a Re–Re bond that forms upon reduction of the starting compound, but which is not able to activate CO2. Third, spectroscopic investigations under (photo)catalytic conditions were performed to shed light on the crucial intermediates emerging in the reaction cycle. The assignment of these intermediates is assisted by extensive density functional theory calculations. As a result, the enhanced photocatalytic activity is reasoned by inhibition of deactivation channels and a cooperative reaction mechanism, in which one metal center functions as a photosensitizer to assist the second, catalytically active, metal.