Chemistry and reactivity of dinuclear manganese oxamate complexes: Aerobic catechol oxidation catalyzed by high-valent bis(oxo)-bridged dimanganese(IV) complexes with a homologous series of binucleating 4,5-disubstituted- o-phenylenedioxamate ligands

Abstract

The high-valent bis(oxo)-bridged dimanganese(IV) complexes with the series of binucleating 4,5-X 2- o-phenylenebis(oxamate) ligands (opbaX 2; X = H, Cl, Me) ( 1a– c) have been synthesized and characterized structurally, spectroscopically and magnetically. Complexes 1a– c possess unique Mn 2(μ-O) 2 core structures with two o-phenylenediamidate type additional bridges which lead to exceptionally short Mn–Mn distances (2.63–2.65 Å) and fairly bent Mn–O–Mn angles (94.1°–94.6°). The cyclovoltammograms of 1a– c in acetonitrile (25 °C, 0.1 M Bu 4NPF 6) show an irreversible one-electron oxidation peak at moderately high anodic potentials ( E ap = 0.50–0.85 V versus SCE), while no reductions are observed in the potential range studied (down to −2.0 V versus SCE). These dinuclear manganese oxamate complexes are excellent catalysts for the aerobic oxidation of 3,5-di- tert-butylcatechol to the corresponding o-quinone in acetonitrile at 25 °C. The order of increasing catecholase activity ( k obs) with the electron donor character of the ligand substituents as 1b (X = Cl) < 1a (X = H) < 1c (X = Me) correlates with Hammett σ + values ( ρ = −0.95). A mechanism involving initial activation of the catechol substrate by coordination to the dimetal center and subsequent oxidation to quinone by O 2 is proposed, which is consistent with the observed saturation kinetics.