Elucidating the Mechanism of Electrocatalytic Halide Activation by Low-Coordinate Manganese and Iron Complexes through Voltammetric Analysis and Density Functional Theory

Abstract Transition metal catalysis has undergone considerable development over the past few decades, with a growing shift toward earth-abundant first-row transition metals contributing to the advancement of more sustainable synthetic methodologies. Among these, Mn(pybox)Cl2 and Fe(pyrox)OTf2 have been proposed as electrocatalysts for reductive activation of organic halides and the generation of carbon-centered radicals for subsequent transformations. Here, we investigate the mechanisms of organic halide activation by these catalysts through a combination of cyclic voltammetry, electrochemical simulations, and density functional theory (DFT) calculations, using benzyl bromide as a representative substrate. Cyclic voltammetry reveals distinct electrochemical behavior for each of the complexes, suggesting differences in both catalyst turnover and radical rebound pathways. Comparison of experimental and simulated voltammograms suggests that Mn(pybox)Cl2 and Fe(pyrox)OTf2 are more likely to undergo benzyl radical rebound to MnI and FeII centers, respectively. DFT calculations, however, indicate that while radical rebound to MnI is thermodynamically favorable, rebound to either FeI or FeII centers is unfavorable. Mechanistic investigations of halide activation reveal that Mn(pybox)Cl2 preferentially activates benzyl bromide through an outer-sphere electron transfer pathway, whereas Fe(pyrox)OTf2 favors an inner-sphere SN2-like mechanism, which effectively associates the benzyl radical with the iron center until dissociation occurs. Overall, this study reveals distinct mechanisms of electrochemical halide activation catalyzed by Mn(pybox)Cl2 and Fe(pyrox)OTf2 and provides mechanistic insights that can guide the future design of first-row transition-metal electrocatalysts for synthetic applications.