Structure, magnetism and reactivity of a {MnIII(μ-O)2MnIV}3+ core towards oxidation of phenols. (1st November 2019)

• Record Type: Journal Article
• Authors: Kumar, Akhilesh; Sengupta, Arunava; Demeshko, Serhiy; Mukherjee, Rabindranath
• Is Part Of: Polyhedron. Volume 172(2019)
• Page Start: 226
• Page End: 235
• Publication Date: 2019-11-01
• Subjects: MnII and MnIII/IV2(μ-O)2 -- Crystal structures -- Magnetism -- Kinetics of phenol oxidation -- Hydrogen atom transfer; Chemistry, Inorganic -- Periodicals; Chimie inorganique -- Périodiques; Organometaalverbindingen; Anorganische chemie; 546.05
• DOI: 10.1016/j.poly.2019.06.032
• Languages: English
• ISSNs: 0277-5387

Abstract

Graphical abstract: Systematic kinetic studies have been made for the reaction between bis( μ -oxo)dimanganese(III, IV) and p -X-2- tert -butylphenols (X = H, Me, OMe, t Bu); the oxidation of phenols generate phenoxyl radical/radical-coupled product through hydrogen-atom transfer reaction. Abstract: Two complexes [(L)2 Mn II (OClO3 )(CH3 CN)](ClO4 )·CH3 CN (1 ) and [(L)4 Mn III/IV 2 ( μ -O)2 ](ClO4 )3 ·2CH3 CN·Et2 O (2 ), where L is 2-((1 H -pyrazol-1-yl)methyl)-1-methyl-1 H -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 ( μ -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 Mn III and Mn 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 ≈ 2. Reactivity of 2 towards p -X-2- tert -butylphenols (X = H, Me, OMe, t Bu) 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 × 10 −2 M −1 s −1 ) for p -X-2- tert -butylphenols: 0.6 (X = H), 1.95 (Me), 3.0 (OMe), 1.85 ( t Bu) and for the slower second step ( k 2 × 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 ( μ -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 II species (ESI-MS spectrum for the reaction between 2 and DTBP).