Enhancement of Oxidative Asynchronicity in Hydrogen Atom Abstraction by a Mononuclear Mn(III)-OMe Complex via Protonation and Sodium Ion Binding

This paper reports the preparation of three mononuclear Mn(III) complexes supported by a pentadentate, dianionic N₄O donor ligand, L²⁻ (H₂L = 2-((2-((2-hydroxybenzyl)(methyl)amino)ethyl)(methyl)amino)-N-(quinolin-8-yl)acetamide): [MnIII(L)(OMe)] (1OMe), {Na[MnIII(L)(OMe)]}¹⁺ (Na+1OMe), and [MnIII(L)(MeOH)]¹⁺ (1MeOH+). The complexes were characterized by analytical and spectroscopic techniques, including magnetic measurements, X-ray crystallography for Na+1OMe and 1MeOH+, and ²³Na NMR spectroscopy for Na+1OMe. Complexes 1OMe and Na+1OMe contain a terminal MnIII-OMe unit, while Na+1OMe forms a Na⁺ Lewis acid adduct through interaction with methoxide and phenoxide oxygen atoms. Complex 1MeOH+ features a terminal methanol ligand, serving as the protonated analog of [MnIII(L)(OMe)]. These complexes abstract hydrogen atoms from 4-R-2,6-di-tert-butylphenols (R = OMe, Me, H, Br), with rates increasing in the order 1OMe < Na+1OMe < 1MeOH+, consistent with anodically shifted MnIII/MnII redox potentials. Hammett and Bell-Evans-Polanyi analyses, large negative ΔS‡ values, and kinetic isotope effects support an apparent concerted proton-electron transfer (CPET) pathway. However, nonzero slopes (-0.262 to -0.286) in Marcus-type plots indicate an oxidatively asynchronous CPET pathway. Density functional theory calculations, including electrostatic potential mapping, asynchronicity factor (η) evaluation, and free-energy analysis of electron- and proton-transfer steps, further support the ET-dominated concerted pathway. The study reveals that binding of H⁺ or Na⁺ to the Mn(III)-OMe complex increases the oxidative asynchronicity factor, facilitating phenolic O-H bond activation during hydrogen-atom transfer.