Tandem Electrocatalysis: Cathodic Epoxidation of Alkenes by a Manganese Salophen Catalyst

Le résumé fourni par la source

Electrocatalytic oxygen-atom transfer (eOAT) driven by the oxygen reduction reaction (ORR) offers a direct route to couple O 2 activation with the synthesis of oxygenated products. Although homogeneous Mn-based complexes have shown promise as ORR and eOAT catalysts separately, the factors that control their eOAT reactivity remain incompletely understood. Here we present a detailed study of a Mn-salophen complex (Mn-Sal), including its proton-coupled reduction and its cathodic reactivity with O 2 and styrene, using electrochemical and theoretical methods. Mn-Sal undergoes proton-coupled reduction to form Mn-Sal-H, with an estimated pKₐ of 13 derived from Pourbaix analysis of pH-dependent cyclic voltammetry. In the presence of O 2 and absence of a proton source, reduction of Mn-Sal generates Mn-Sal-O 2 , whose reduction and oxidation features were detected by cyclic voltammetry and supported by DFT calculations. When both O 2 and protons are present, Mn-Sal catalyzes the two-electron ORR to H 2 O 2 , as confirmed by rotating ring-disk electrode experiments. Kinetic analysis using foot-of-the-wave methods indicates that catalytic turnover depends on proton concentration but is insensitive to dissolved O₂, consistent with rate limitations arising from an early proton-involved chemical step following electron transfer. The electrocatalytic epoxidation of styrene by Mn-Sal was evaluated using multiple proton sources but was found to proceed only in the presence of imidazolium cations, underscoring the critical role of imidazole in redirecting ORR toward productive eOAT catalysis. Faradaic efficiencies exceeding 40% were achieved at-0.75 V vs Fc⁺/Fc. Complementary density functional theory calculations were used to model the catalytic mechanism and elucidated the unique role of the imidazole/imidazolium pair. Together, these results identify the hydrogen-bonded adduct between imidazolium ion and Mn-hydroperoxide as key intermediate for steering Mn-mediated ORR away from peroxide release and toward selective electrosynthetic oxygenation.

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