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 finely modulated to tune CO₂ activation. Both charged and hydrogen-bonding substituents 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 led 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 deconvolution of through-bond and through-space effects in these novel XB-capable iron porphyrins and it was found that σ-hole modulation mitigates unfavourable through-space repulsion at the reduced Fe(0) state.