Atropisomeric Hydrogen Bonding Control for CO₂ Binding and Enhancement of Electrocatalytic Reduction at Iron Porphyrins

Abstract

The manipulation of the second coordination sphere for improving the electrocatalytic CO2 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 CO2 in a similar fashion found in the carbon monoxide dehydrogenase (CODH) enzyme. The αα topological isomer with the two urea groups on the same side of the porphyrin provides a stronger binding affinity to tether the incoming CO2 in comparison to the αβ disposition. However, the electrocatalytic activity of the αβ 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 αα 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 αβ isomer and the enhanced electrocatalytic performance on addition of stronger acid indicate the free access of protons to the bound CO2 on the opposite side of the urea arm.

Shaping the Electrocatalytic Performance of Metal Complexes for CO2 Reduction

“…Overpotential ( η ) refers to the additional potential needed to be applied to the system past the thermodynamic potential of the reaction. The high Faraday efficiency of iron porphyrins towards selective CO 2 ‐to‐CO production and their good stability, [26–34] have left most of the design improvements focused on increasing TOF and lowering overpotential.…”

“…We further interrogated the dissymmetric functional features of these nearby amino acid residues by comparing the αα and αβ atropisomers in catalysts 16 and 17 , respectively. This subtle topological change induced a higher log TOF max of 4.71 and overpotential ( η’ =0.66) for the αβ atropisomer compared to the αα analogue [32] . While the αα atropisomer exhibited higher CO 2 binding aptitude compared to the αβ analogue, the former exhibited high values of kinetic isotope effect (7.60) when using H 2 O/D 2 O as proton source in DMF, implicating that proton transfer is involved in the rate‐determining step and suggesting the eventual presence of a tight hydrogen bonding network.…”

Ligand Engineering in Nickel Phthalocyanine to Boost the Electrocatalytic Reduction of CO2

“…Electrocatalytic conversion of CO 2 into value‐added chemicals using the surplus sustainable energy can be an appealing technology to mitigate environmental and energy issues. [ 1–4 ] Developing efficient catalysts is indispensable for the electrochemical CO 2 reduction reaction (CO 2 RR) to overcome the kinetic barrier of low electron affinity and inherent thermodynamic stability of CO 2 . [ 5–8 ] Metal complexes with abundant redox‐active sites for CO 2 activation or adsorption have attracted much attention.…”

Alternating substrate/ligand-metal coordination enables a low-energy pathway for C-O bond cleavage in the electrocatalytic reduction of carbon dioxide

“…4,5,7,10, Also, several metal complexes with intramolecular Brønsted acidic groups positioned close to the reactive center have been shown to facilitate C-O cleavage. 3,10,29,35,[37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] Examples of catalytic rate enhancements for CO2 electroreduction by Lewis acidic metal ions in molecular catalysts are comparatively rare. 32,[53][54][55] In some of these systems, cyclic intermediates that comprise the catalytic metal center, two molecules of CO2 and an exogenous alkali or alkaline earth metal Lewis acid have been proposed [53][54][55] (Figs.…”