Oriented Electrostatic Effects on O_2 and CO_2 Reduction by a Polycationic Iron Porphyrin

摘要

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(CH_3)_3]~+) has recently been used to catalyze the complex, multistep O_2 and CO_2 reduction reactions (ORR and CO_2RR) 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(CH_3)_3]~+ groups and the bound substrate. However, by nature of the mono-ortho substitution pattern, there are four possible atro-pisomers of this metalloporphyrin and thus four unique electro- static environments. This work reports that each of the four individual atropisomers catalyzes both the ORR and CO_2RR 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 CO_2RR. For the ORR, the αβαβ isomer is the fastest and has the highest overpotential, while for the CO_2RR the αβαβ 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 CO_2 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.