Atropisomeric Hydrogen Bonding Control for CO2Binding and Enhancement of Electrocatalytic Reduction at Iron Porphyrins

The manipulation of the second coordination sphere for improving the electrocatalytic CO(2)reduction has led to breakthroughs with hydrogen bonding, local proton source, or electrostatic effects. We have developed two atropisomers of an iron porphyrin complex with two urea functions acting as multiple hydrogen-bonding tweezers to lock the metal-bound CO(2)in a similar fashion found in the carbon monoxide dehydrogenase (CODH) enzyme. The alpha alpha topological isomer with the two urea groups on the same side of the porphyrin provides a stronger binding affinity to tether the incoming CO(2)in comparison to the alpha beta disposition. However, the electrocatalytic activity of the alpha beta atropisomer outperforms its congener with one of the highest reported turnover frequencies at low overpotential. The strong H/D kinetic isotope effect (KIE) observed for the alpha alpha system indicates the existence of a tight water hydrogen-bonding network for proton delivery which is disrupted by addition of an acid source. The small H/D KIE for the alpha beta isomer and the enhanced electrocatalytic performance on addition of stronger acid indicate the free access of protons to the bound CO(2)on the opposite side of the urea arm.