Double Hangman Iron Porphyrin and the Effect of Electrostatic Nonbonding Interactions on Carbon Dioxide Reduction

Hangman porphyrins influence the reaction rates of small molecule activation by positioning a functional group in the secondary coordination sphere of the metal center. Electrocatalysis by hangman porphyrins has examined only one face modification of the macrocycle with a hanging group, thus allowing for circumvention of secondary sphere effects by reaction of the small molecule on the opposite face of the hangman cleft. We now report the synthesis and characterization of a double hangman Fe porphyrin in which both faces of the macrocycle are modified with a hanging group. With this double hangman architecture, we are able to unequivocally examine the role of electrostatic interactions on the carbon dioxide reduction reaction (CO2RR) and show that CO2RR rates are significantly attenuated, consistent with the initial reduction of CO2 to generate the anion, whose binding is diminished within the negatively charged carboxylic groups of the hangman cleft. The results demonstrate the pronounced role that nonbonding electrostatic interactions may play in CO2RR and highlight the need to manage deleterious electrostatic interactions during catalytic turnover.