Electrocatalytic Oxygen Atom Transfer by a Manganese Based Complex

Le résumé fourni par la source

The development of environmentally benign oxidation methods is a critical goal in sustainable chemistry. Oxygen atom transfer (OAT) reactions, which utilize molecular oxygen (O₂) as the terminal oxidant, offer a direct and atom-economical route to functionalize organic substrates. However, selective reduction of O₂ remains a challenge due to competing pathways that lead to water formation, unselective oxidants, or catalyst degradation. In this work, we present a mechanistic and electrochemical investigation of a manganese(III) salen complex capable of catalyzing the selective two-electron reduction of O₂ to hydrogen peroxide (H₂O₂), which then serves as an in situ oxidant for downstream OAT processes. Inspired by the redox versatility of metalloenzymes such as manganese catalases and oxygen-evolving complexes in nature, we selected a Mn-salen scaffold for its stability, tunability, and ability to support high-valent oxidation states. A synthetic Mn(III)-salen chloride complex (L₁Mn(III)Cl) was prepared following literature protocols and characterized by 1 H-NMR, 13 C-NMR and mass spectrometry. Electrochemical data under Ar-saturated conditions indicate a reversible Mn(III)/Mn(II) couple, with a diffusion-controlled redox process and no evidence of surface adsorption. Upon saturation with O₂, the CVs show catalytic current enhancement in the presence of a proton donor such as 2-fluorophenol. DFT simulations reveal that the O₂ binds to the Mn center with concurrent dissociation of the axial Cl⁻ ligand, consistent with the formation of Mn–O₂ adducts. The calculated free energy change for this process (ΔG ≈ –1.11 eV) confirms the thermodynamic favorability of O₂ coordination and Cl⁻ release. Structural changes observed in the optimized geometries indicate a redox event involving both the metal and ligand backbone, as bond lengths and angles vary significantly across oxidation states. To probe the catalytic reduction of O₂ and the mechanism of H₂O₂ formation, we employed rotating ring-disk electrode (RRDE) experiments. The disk potential was swept from 0 V to –0.55 V vs Ag/AgCl while the ring was held at 1.2 V, enabling real-time detection of H₂O₂ produced at the disk. These measurements revealed that approximately 80% of the reduced oxygen was selectively converted to H₂O₂, validating the high selectivity of the Mn-salen catalyst under electrochemical conditions. Further validation of the system’s ability to mediate OAT reactions came from tandem electrosynthesis experiments. Under conditions of O₂ saturation, 0.4 M 2-fluorophenol, and a constant applied potential of –0.7 V vs Fc⁺/Fc, the Mn-salen catalyst facilitated the oxidation of triphenylphosphine (PPh₃) to triphenylphosphine oxide (PPh₃O). Product formation was confirmed by ³¹P NMR spectroscopy. This indirect oxidation pathway leverages the electrogenerated H₂O₂ as a reactive intermediate, illustrating the potential of this platform for green, selective oxidation of organic substrates. Kinetic analysis of the catalytic current response using foot-of-the-wave analysis (FOWA) supports an ECEC-type mechanism, where the first chemical step is rate-limiting and depends linearly on the proton donor concentration. A maximum turnover frequency (TOFₘₐₓ) of 0.54 s⁻¹ and a rate constant k₁ = 85.84 s⁻¹ were obtained, reflecting efficient catalyst turnover under mild conditions. Looking ahead, we aim to expand the substrate scope to include benzylic and aliphatic alcohols, styrenes, sulfides, and toluene derivatives, focusing on substrates that are soluble in acetonitrile, stable under electrochemical conditions, and yield selective oxidation products with minimal side reactions. This manganese salen platform offers a versatile, cost-effective, and scalable solution for developing noble-metal-free electrosynthetic OAT chemistry, with implications for green oxidation processes in fine chemicals, materials science, and pharmaceutical synthesis.

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