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.