Carbon Dioxide Reduction by Iron Hangman Porphyrins

Iron hangman porphyrins with phenol, guanidinium, and sulfonic acid proton donor groups placed above the Fe porphyrin platform reduce CO2 to CO with Faradaic efficiencies >93%. Computations show that the activation of CO2 at the Fe center is enhanced by the hanging group. Intramolecular hydrogen bonding from the phenol and guanidinium groups results in a 2.1–6.6 kcal/mol stabilization of CO2 within the hangman pocket; the hanging sulfonate group is deprotonated, thus resulting in destabilization of the CO2 adduct due to unfavorable electrostatic interactions. Electrochemical studies show that Fe hangman porphyrins exhibit canonical S-curve character; together with computation results, the apparent rate constant for CO2 reduction is found to be governed by CO2 binding within the hangman cleft.