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), NaMnIII (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.
Molla et al. (Fri,) studied this question.