The iron complexes of the tetra-cationic porphyrin ligand tetra(o-N,N,N-trimethylanilinium)porphyrin are among the best known molecular electrocatalysts for the reductions of O2 and CO2 (the ORR and CO2RR), in terms of their turnover frequencies and overpotentials. Due to hindered rotation, there are four atropisomers of this ligand, with the cationic groups positioned above and below the porphyrin plane in different ways: αβαβ, ααββ, αααβ (illustrated) and αααα. For the first time, these four isomers have been purified, isolated and fully characterized, thus giving four catalysts with unique electrostatic environments. While prior studies using this system for the ORR and CO2RR stated that catalysis was by the pure αβαβ isomer, we find that these actually used a mixture of all four isomers. Comparing these molecules with neutral iron(tetraphenylporphyrin), the four positive charges have strong effects on the iron(III/II) reduction potentials, the binding of anionic ligands, and the electrocatalysis. In contrast, the set of isomers with different orientations of the four cationic charges about the iron active site have very similar properties. The implications of this work for molecular electrocatalysis and for the use of (oriented?) electrostatic effects will be discussed.

Polycationic iron electrocatalysts for O2 and CO2 reduction: Electrostatic effects of the charges and their positioning

ACS Spring Meeting 2021

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