Oriented Electrostatic Effects on O₂ and CO₂ Reduction by a Polycationic Iron Porphyrin

Next-generation energy technologies require improved methods for rapid and efficient chemical-to-electrical energy transformations. One new approach has been to include atomically positioned, electrostatic motifs in molecular catalysts to stabilize high-energy, charged intermediates. For example, an iron porphyrin bearing four cationic, o-N,N,N-trimethylanilinium groups (o-[N(CH₃)₃]⁺) has recently been used to catalyze the complex, multistep O₂ and CO₂ reduction reactions (ORR and CO₂RR) with fast rates and at low overpotentials. The success of this catalyst is attributed, at least in part, to specific charge-charge interactions between the atomically positioned o-[N(CH₃)₃]⁺ groups and the bound substrate. However, by nature of the mono-ortho substitution pattern, there are four possible atropisomers of this metalloporphyrin and thus four unique electrostatic environments. This work reports that each of the four individual atropisomers catalyzes both the ORR and CO₂RR with fast rates and low overpotentials. The maximum turnover frequencies vary among the atropisomers, by a factor of 60 for the ORR and a factor of 5 for CO₂RR. For the ORR, the αβαβ isomer is the fastest and has the highest overpotential, while for the CO₂RR the αααα isomer is the fastest and has the highest overpotential. The role of charge positioning is complex and can affect more than a single step such as CO₂ binding. These data offer a first-of-a-kind perspective on atomically positioned charge and highlight the significance of high charge density, rather than orientation, on the thermodynamics and kinetics of multistep molecular electrochemical transformations.