Role of the π-System in Fe-Porphyrins for CO2 Reduction Catalysis

Abstract

Iron porphyrins are extensively utilized as molecular catalysts in the electrocatalytic CO2 reduction reaction (CO2RR). While current research primarily investigates the impact of peripheral substitution and second coordination sphere effects, the role of the π-system in governing Fe-porphyrins’ redox and catalytic properties in CO2RR has received limited systematic investigation. To that end, we have prepared and thoroughly characterized, both spectroscopically and electrochemically, a series of Fe-porphyrins with varying numbers of π-orbitals involved in conjugation. We have observed a correlation between thermodynamics and kinetics with the number of atomic orbitals involved in their π-system. Notably, bicycloporphyrins with the lowest number of atomic orbitals involved in the π-system, [FeCl(TbcTPP)] and [FeCl(TbcP)], exhibit lower rate constants and TOFmax values than the [FeCl(TPP)] reference catalyst. Interestingly, benzoporphyrins [FeCl(TBP)] and [FeCl(TBTPP)], with more atomic orbitals involved in the π-system, display two different catalytic cycles, depending on the applied potential.