Mechanistic\nContrasts between Manganese and Rhenium\nBipyridine Electrocatalysts for the Reduction of Carbon Dioxide

Abstract

[Re­(bpy)­(CO)₃]⁻ is a well-established\nhomogeneous electrocatalyst for the reduction of CO₂ to\nCO. Recently, substitution of the more abundant transition metal Mn\nfor Re yielded a similarly active electrocatalyst, [Mn­(bpy)­(CO)₃]⁻. Compared to the Re catalyst, this Mn\ncatalyst operates at a lower applied reduction potential but requires\nthe presence of a weak acid in the solution for catalytic activity.\nIn this study, we employ quantum chemistry combined with continuum\nsolvation and microkinetics to examine the mechanism of CO₂ reduction by each catalyst. We use cyclic voltammetry experiments\nto determine the turnover frequencies of the Mn catalyst with phenol\nas the added weak acid. The computed turnover frequencies for both\ncatalysts agree to within one order of magnitude of the experimental\nones. The different operating potentials for these catalysts indicate\nthat different reduction pathways may be favored during catalysis.\nWe model two different pathways for both catalysts and find that,\nat their respective operating potentials, the Mn catalyst indeed is\npredicted to take a different reaction route than the Re catalyst.\nThe Mn catalyst can access both catalytic pathways, depending on the\napplied potential, while the Re catalyst does not show this flexibility.\nOur microkinetics analysis predicts which intermediates should be\nobservable during catalysis. These intermediates for the two catalyzed\nreactions have qualitatively different electronic configurations,\ndepending on the applied potential. The observable intermediate at\nhigher applied potentials possesses an unpaired electron and therefore\nshould be EPR-active; however, the observable intermediate at lower\napplied potentials, accessible only for the Mn catalyst, is diamagnetic\nand therefore should be EPR-silent. The differences between both catalysts\nare rationalized on the basis of their electronic structure and different\nligand binding affinities.