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

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 TOF_max 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.