Structure, magnetism and reactivity of a {MnIII(μ-O)2MnIV}3+ core towards oxidation of phenols

Two complexes [(L)(2)Mn-II(OClO3)(CH3CN)](ClO4)center dot CH3CN (1) and [ (L)Mn2(III/IV )(mu-O)(2)](ClO4)(3)center dot 2CH(3)CH center dot Et2O (2), where L is 2-((1H-pyrazol-1-yl)methyl)-1-methyl-1H-benzimidazole, have been synthesized, structurally characterized, UV-Vis and EPR spectral properties, variable-temperature (2-300 K) magnetic susceptibility and redox behavior investigated. Structural analysis reveals that the {Mn-III(mu-O)(2) Mn-IV}(3+) core in 2 is an example of delocalized class III system. Complex 1 has S = 5/2 ground-state. The Me-III and Me-IV centres in 2 are antiferromagnetically coupled (J = -192 cm(-1)) and has S = 1/2 ground-state. Complex 2 exhibits characteristic 16-line EPR spectrum (120 K) centered at g approximate to 2. Reactivity of 2 towards p-X-2-tert-butylphenols (X = H, Me, OMe, Bu-t) has been investigated. The oxidation of phenols generate phenoxyl radical (2,4,6-tri-tert-butylphenol exhibits EPR signal due to radical)/radical-coupled bis-phenol product (in the case of 2,4-di-tert-butylphenol, DTBP). Kinetic experiments have allowed us to evaluate second-order rate constant values for the faster initial step (k(2) x 10(-2) M-1 s(-1)) for p-X-2-tert-butylphenols: 0.6 (X = H), 1.95 (Me), 3.0 (OMe), 1.85 (Bu-t) and for the slower second step (k(2) x 10(-3) M-1 s(-1)) for p-X-2-tert-butylphenols: 0.9 (X = H), 2.5 (Me), 4.0 (OMe). Reaction between {Mn-III(mu-O)(2) Mn-IV}(3+) of 2 and phenols proceeds via hydrogen atom transfer mechanism to produce oxidation products. Initial reaction supposedly generates (Mn-III(O) (OH)Mn-III}(3+) species, which finally ends up as (L)-coordinated Mn-III species (ESI-MS spectrum for the reaction between 2 and DTBP). (C) 2019 Elsevier Ltd. All rights reserved.