Activation and reduction of CO2 by metalloporphyrin-based molecular catalysts

Overview & Synopsis

Transforming CO₂ into valuable reduced forms of carbon is an interesting approach towards the recycling of this greenhouse gas, by introducing non-fossil fuel based C1 building blocks back into the carbon cycle. Tetraphenyl iron porphyrins and derivatives have been shown to be efficient and selective molecular catalysts for CO₂ reduction to CO. The introduction of various functions in the second coordination sphere of porphyrins showed great improvements of both the overpotential and the catalytic rates.Inspired by the distal pocket of enzymatic active centers, an iron porphyrin with a carboxylate strap is investigated. Electrochemical, kinetic and computational chemistry studies show that this catalyst operates at a low overpotential, while maintaining high catalytic rates. It is proposed that the carboxylate function, initially acting as an axial ligand of the metal, plays an important role in the insertion and transformation of CO₂, in synergy with a water molecule trapped in the superstructure.Furthermore, two iron porphyrins were synthetized bearing an imidazolium group at various positions with respect to the metal center. The original goal of this study was to establish a correlation between the distance of the cationic group from the metal center and the catalytic performances of the catalyst, which can guide the design of new catalysts for CO₂ reduction.The electrochemical study of these catalysts revealed that these imidazolium functions can be electroactive. Electron paramagnetic resonance was used to describe their various reduced forms. These studies revealed and describe the potential electroactive behavior of the imidazolium groups on these novel iron porphyrins catalysts.