Chemistry and reactivity of mononuclear manganese oxamate complexes: Oxidative carbon-carbon bond cleavage of vic-diols by dioxygen and aldehydes catalyzed by a trans-dipyridine manganese(III) complex with a tetradentate o-phenylenedioxamate ligand

Abstract

[EN] Two new mononuclear octahedral manganese(III) complexes with the tetradentate equatorial ligand o-phenylenebis(oxamate) (opba) and two aquo (1a) or two pyridine (1b) axial ligands have been synthesized and characterized structurally, magnetically, and electrochemically. The cyclovoltammogram of 1a in acetonitrile (25 degrees C, 0.1 M Bu4NPF6) shows an irreversible one-electron oxidation peak at a high anodic potential (E-ap = 1.03 V versus SCE), while that of 1b shows two well-separated one-electron oxidation peaks at moderate to high anodic potentials (E-ap = 0.92 and 1.27 V versus SCE), the first redox-wave being quasireversible in nature. The access to formally high-valent Mn-IV and Mn-V oxidation states for 1b is attributed to the stabilization effect of axial pyridine ligand coordination. Complex 1b has been used as an efficient catalyst for the aerobic oxidative cleavage of aromatic vic-diols with co-oxidation of pivalaldehyde to pivalic acid. The corresponding aldehydes or ketones are obtained in fair to good yields and moderate selectivities in acetonitrile at 40 degrees C. Under the same experimental conditions, complex la shows lower efficiencies and selectivities than 1b. The modulation of catalytic activity by the axial pyridine ligands highlights the role of oxomanganese(V) species as the putative intermediates in these C-C bond cleavage oxidation reactions.