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 O-2 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 alpha beta alpha beta isomer is the fastest and has the highest overpotential, while for the CO2RR the alpha alpha alpha alpha 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.

Automated summary

The study demonstrates that a metalloporphyrin catalyst with four cationic groups can stabilize charged intermediates and catalyze ORR and CO2RR reactions with fast rates and low overpotentials. Each of the four atropisomers of the metalloporphyrin show differences in maximum turnover frequencies and overpotentials for the two reactions, highlighting the complexity of charge positioning in affecting the thermodynamics and kinetics of multistep molecular electrochemical transformations.