Post-synthetic modification of hangman porphyrins synthesized on the gram scale

Influence of the proton relay spacer on hydrogen electrocatalysis by cobalt hangman porphyrins

A cobalt hangman porphyrin system with a phenyl spacer between the porphyrin ring and an internal carboxylic acid group as well as its non-hangman analogue were synthesized and utilized for the study of the proton-coupled electron transfer (PCET) kinetics attendant to electrocatalytic hydrogen evolution. Cyclic voltammetry (CV) together with simulations show that a short distance between the proton relay and the redox active cobalt center as well as the increased proton donating strength results in superior catalytic activity. The mechanism of hydrogen generation is at the nexus of proton transfer–electron transfer (PTET) and concerted proton–electron transfer (CPET), as opposed to an ETPT mechanism that is characteristic of hangman systems with longer proton relay networks.

Double Hangman Iron Porphyrin and the Effect of Electrostatic Nonbonding Interactions on Carbon Dioxide Reduction

Hangman porphyrins influence the reaction rates of small molecule activation by positioning a functional group in the secondary coordination sphere of the metal center. Electrocatalysis by hangman porphyrins has examined only one face modification of the macrocycle with a hanging group, thus allowing for circumvention of secondary sphere effects by reaction of the small molecule on the opposite face of the hangman cleft. We now report the synthesis and characterization of a double hangman Fe porphyrin in which both faces of the macrocycle are modified with a hanging group. With this double hangman architecture, we are able to unequivocally examine the role of electrostatic interactions on the carbon dioxide reduction reaction (CO₂RR) and show that CO₂RR rates are significantly attenuated, consistent with the initial reduction of CO₂ to generate the anion, whose binding is diminished within the negatively charged carboxylic groups of the hangman cleft. The results demonstrate the pronounced role that nonbonding electrostatic interactions may play in CO₂RR and highlight the need to manage deleterious electrostatic interactions during catalytic turnover.

Proton-coupled electron transfer chemistry of hangman macrocycles: Hydrogen and oxygen evolution reactions

The splitting of water into its constituent elements is an important solar fuels conversion reaction for the storage of renewable energy. For each of the half reactions of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), multiple protons and electrons must be coupled to avoid high-energy intermediates. To understand the mechanistic details of the PCET chemistry that underpins HER and OER, we have designed hangman porphyrin and corrole catalysts. In these hangman constructs, a pendant acid/base functionality within the secondary coordination sphere is "hung" above the macrocyclic redox platform on which substrate binds. The two critical thermodynamic properties of a PCET event, the redox potential and pKa may be tuned with the macrocycle and hanging group, respectively. This review outlines the synthesis of these catalysts, as well as the examination of the PCET kinetics of hydrogen and oxygen evolution by the hangman catalysts. The insights provided by these systems provide a guide for the design of future HER and OER catalysts that use a secondary coordination sphere to manage PCET.