Unexpected Solvent Effect in Electrocatalytic CO₂ to CO Conversion Revealed Using Asymmetric Metalloporphyrins

Abstract

Rapid and efficient electrochemical CO reduction is an ongoing challenge for the production of sustainable fuels and chemicals. In this work, electrochemical CO reduction is investigated using metalloporphyrin catalysts (metal = Mn, Fe, Co, Ni, Cu) that feature one hydroxyphenyl group, and three other phenyl groups, in the porphyrin heterocycle (5-(2-hydroxyphenyl)-10,15,20-triphenylporphyrin, TPOH). These complexes, which are minimal versions of related complexes bearing up to eight proton relays, were designed to allow more straightforward determination of the role of the 2-hydroxylphenyl functional group. The iron-substituted version of TPOH supports robust reduction of CO in acetonitrile solvent, where carbon monoxide is the only detected product. Addition of weak Brønsted acids (1 M water or 8 mM phenol) gives rise to almost 100-fold enhancement in turnover frequency. Surprisingly, the iron analogue is a poor catalyst when the solvent is changed to dimethylformamide. These results lead to the proposal of a model where the hydroxyphenyl group behaves as a local proton source, a hydrogen bond donor to CO-bound intermediates, and a hydrogen bonding partner to Brønsted acids. The observations from this model suggest improvements for existing electrocatalytic CO reduction systems.

Transition metal-based catalysts for the electrochemical CO2reduction: from atoms and molecules to nanostructured materials

“…121 The choice for a suitable reaction medium is another crucial aspect to consider for catalyst optimization. 122,123 An asymmetric tetraphenylporphyrin iron catalyst 8 containing only a single proton relay revealed to be a poor CO 2 RR catalyst in DMF, whereas a strong enhancement in the catalytic response was observed in CH 3 CN. 122 In an attempt to mimic the key stabilizing role of amino acid residues by H-bonding interaction in enzymatic systems, a number of varying H-bond donor groups have been incorporated in the structure of iron porphyrin complexes.…”

“…122,123 An asymmetric tetraphenylporphyrin iron catalyst 8 containing only a single proton relay revealed to be a poor CO 2 RR catalyst in DMF, whereas a strong enhancement in the catalytic response was observed in CH 3 CN. 122 In an attempt to mimic the key stabilizing role of amino acid residues by H-bonding interaction in enzymatic systems, a number of varying H-bond donor groups have been incorporated in the structure of iron porphyrin complexes. 99,[124][125][126][127] The effect of different hanging proton donors (phenol (9), guanidine (10) and sulfonate (11) has been explored in a series of iron hangman porphyrins.…”

Electrocatalytic CO2 Reduction to Formate with Molecular Fe(III) Complexes Containing Pendent Proton Relays

“…We propose that the zwitterionic state of 3 in the interactions with the sacrificial proton donor will be much stronger than for the neutral methoxy groups in complex 2, for purely electrostatic reasons, which would be expected to inhibit the catalytic response. 45,46 Consistent with this interpretation, the experimental results with a sterically hindered proton source demonstrated catalytic increases in current only with 2. This suggests that for complex 2, the outer-sphere pendent Lewis basic site is operating as a more accessible site for protonation, and more efficiently shuttling the proton to the metal center.…”

Proton Relays in Molecular Catalysis of Electrochemical Reactions: Origin and Limitations of the Boosting Effect

“…It is also worth noting that, for this purpose, the role of proton relays is mostly thought as involving stabilization by means of H‐bonding, following the Sabatier principle, rather by their direct role as proton donors . Several examples of pre‐positioning proton relays in the catalysis molecule are also available in molecular catalysis of electrochemical CO 2 ‐to‐CO conversion, with the purpose of improving catalytic efficiency (increase of turnover frequency and/or decrease of overpotential) . Here too, stabilization of intermediate Sabatier adduct between CO 2 and the active form of the catalyst is frequently invoked to explain good catalytic efficiencies.…”