Electrochemical and light-driven CO2 reduction by amine-functionalized rhenium catalysts: A comparison between primary and tertiary amine substitutions

Abstract

Properly positioned second-coordination sphere functional groups can play an integral role in enhancing catalytic activity, altering product selectivity, and/or reducing the energy cost for desired chemical transformations. In this context, we have investigated the effect of second-sphere tertiary amine groups in the fac-Re(bpy)(CO)3Cl (ReBpy) catalyst scaffold (bpy is 2,2′-bipyridine). A family of isomeric Re(bpy) catalysts bearing N,N-dimethylaniline (DMA) substituents at the 6 position of 2,2′-bipyridine has been synthesized and characterized, where the -NMe2 group of each complex is installed at the ortho, meta, or para position of the pendant phenyl ring to systematically vary the distance between the amine and the rhenium metal center. The para-substituted complex (Re-pNMe2) leads the series with the highest turnover frequency (168 s−1) and highest turnover number (80) measured under optimized electrochemical and photochemical conditions, respectively. Spectroelectrochemical experiments were performed to identify reduced intermediates that are involved in the catalytic cycles of these complexes. In addition, the catalytic parameters have been compared with the analogous and previously reported series of Re catalysts bearing –NH2 functionality in the second-coordination sphere. Our results demonstrate that, unlike the aniline-substituted primary amine systems, catalysis with the DMA-substituted tertiary amine complexes is predominantly governed by inductive electronic effects.