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 CO 2 to CO with Faradaic efficiencies >93%. Computations show that the activation of CO 2 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 CO 2 within the hangman pocket; the hanging sulfonate group is deprotonated, thus resulting in destabilization of the CO 2 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 CO 2 reduction is found to be governed by CO 2 binding within the hangman cleft.