Add time:08/09/2019         Source:sciencedirect.com

Two complexes [(L)2MnII(OClO3)(CH3CN)](ClO4)·CH3CN (1) and [(L)4MnIII/IV2(μ-O)2](ClO4)3·2CH3CN·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 {MnIII(μ-O)2MnIV}3+ core in 2 is an example of delocalized class III system. Complex 1 has S = 5/2 ground-state. The MnIII and MnIV 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 ≈ 2. Reactivity of 2 towards p-X-2-tert-butylphenols (X = H, Me, OMe, tBu) 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 (k2 × 10−2 M−1 s−1) for p-X-2-tert-butylphenols: 0.6 (X = H), 1.95 (Me), 3.0 (OMe), 1.85 (tBu) and for the slower second step (k2 × 10−3 M−1 s−1) for p-X-2-tert-butylphenols: 0.9 (X = H), 2.5 (Me), 4.0 (OMe). Reaction between {MnIII(μ-O)2MnIV}3+ of 2 and phenols proceeds via hydrogen atom transfer mechanism to produce oxidation products. Initial reaction supposedly generates {MnIII(O)(OH)MnIII}3+ species, which finally ends up as (L)-coordinated MnII species (ESI-MS spectrum for the reaction between 2 and DTBP).

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