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.

The reduction of CO 2 by electrogenerated Fe(0)TPP can be directed toward formate with a good selectivity (ca. 70%) upon addition of a tertiary amine in the presence of a weak proton donor. Whereas most systems generating formate from CO 2 involve the initial formation of a hydride, we demonstrate herein that the hydride pathway is avoided by preferential binding of CO 2 that may then be protonated by weak acids. The net result is that hydrogen production via the hydride is disfavored, thus leading to high yields of formate. These results demonstrate an alternative route for the activation of CO 2 to formate.

Hydrogen evolution from a weak acid, acetic acid, occurs at extremely high rates for iron tetraphenylporphyrin in the presence tertiary amine as a cocatalyst. Kinetic analysis of H2 evolution for a range of amines with varying coordinative and acid–base properties reveals a dramatic rate enhancement derived from enhanced proton activity of a heteroconjugated adduct between acetic acid and the amine. Additionally, nonbulky tertiary amines (quinuclidine and diazabicyclooctane) result in a further increase in activity resulting from a trans effect for a TPPFe­(II)–H hydride intermediate. These findings reveal the design principle of a cocatalyst for creating more efficient molecular systems for hydrogen evolution electrocatalysis.

Experimental section. NMR, UV−vis, MS, and additional crystal structures (Figures S1−S19) (PDF) Crystallographic data (CIF)