Investigating Halogen-Bond-Mediated Through-Space Activation of CO₂ for Electrocatalytic Reduction by Iron Porphyrins

Developing novel molecular catalysts for electrochemical CO2 reduction can play an important role for the development of highly selective transformations towards valuable compounds such as CO, HCOOH and CH3OH. Iron porphyrins have shown promising activity in CO2-to-CO electroreduction, as the porphyrin macrocycle structure can be modulated finely to tune CO2 activation. Both charged and hydrogen-bonding substitutents on the porphyrin macrocycle were previously shown to enhance catalytic reduction. In the current study, we probe if halogen bonding (XB) could be leveraged as a new mode of CO2 activation in iron porphyrin electrochemistry. A range of ortho-substituted iodo- and bromo iron porphyrins were synthesized and their electrochemical behaviour analyzed in detail. While all iodo-compounds and one bromo-compound lead to C-halogen bond cleavage at the potentials necessary for CO2 reduction, all other bromo-complexes were shown to be highly selective for CO2-to-CO electroreduction. Cyclic voltammetry and DFT calculations, combined with a detailed mechanistic analysis enabled to deconvolute through-bond and through-space effects in these novel XB-capable iron porphyrins and finds that σ-hole modulation mitigates unfavourable through-space repulsion at the reduced Fe(0) state.