| Abstract
| - The syntheses and structural, spectral, and electrochemical characterization of the dioxo-bridged dinuclear Mn(III)complexes [LMn(μ-O)2MnL](ClO4)2, of the tripodal ligands tris(6-methyl-2-pyridylmethyl)amine (L1) and bis(6-methyl-2-pyridylmethyl)(2-(2-pyridyl)ethyl)amine (L2), and the Mn(II) complex of bis(2-(2-pyridyl)ethyl)(6-methyl-2-pyridylmethyl)amine (L3) are described. Addition of aqueous H2O2 to methanol solutions of the Mn(II) complexes of L1 and L2produced green solutions in a fast reaction from which subsequently precipitated brown solids of the dioxo-bridgeddinuclear complexes 1 and 2, respectively, which have the general formula [LMnIII(μ-O)2MnIIIL](ClO4)2. Addition of30% aqueous H2O2 to the methanol solution of the Mn(II) complex of L3 ([MnIIL3(CH3CN)(H2O)](ClO4)2 (3)) showeda very sluggish change gradually precipitating an insoluble black gummy solid, but no dioxo-bridged manganesecomplex is produced. By contrast, the Mn(II) complex of the ligand bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine(L3a) has been reported to react with aqueous H2O2 to form the dioxo-bridged MnIIIMnIV complex. In cyclic voltammetricexperiments in acetonitrile solution, complex 1 shows two reversible peaks at E1/2 = 0.87 and 1.70 V (vs Ag/AgCl)assigned to the MnIII2 ↔ MnIIIMnIV and the MnIIIMnIV ↔ MnIV2 processes, respectively. Complex 2 also shows tworeversible peaks, one at E1/2 = 0.78 V and a second peak at E1/2 = 1.58 V (vs Ag/AgCl) assigned to theMnIII2 ↔ MnIIIMnIV and MnIIIMnIV ↔ MnIV2 redox processes, respectively. These potentials are the highest so farobserved for the dioxo-bridged dinuclear manganese complexes of the type of tripodal ligands used here. The bulkelectrolytic oxidation of complexes 1 and 2, at a controlled anodic potential of 1.98 V (vs Ag/AgCl), produced thegreen MnIV2 complexes that have been spectrally characterized. The Mn(II) complex of L3 shows a quasi reversiblepeak at an anodic potential of Ep,a of 1.96 V (vs Ag/AgCl) assigned to the oxidation Mn(II) to Mn(III) complex. Itis about 0.17 V higher than the Ep,a of the Mn(II) complex of L3a. The higher oxidation potential is attributable tothe steric effect of the methyl substituent at the 6-positon of the pyridyl donor of L3.
- Aqueous H2O2 oxidized Mn(II) complexes of tris(6-methyl-2-pyridylmethyl)amine (L1) and bis(6-methyl-2-pyridylmethyl)(2-(2-pyridyl)ethyl)amine (L2) to the Mn(IV) complexes of the formula [Mn2(μ-O)2L2]4+ as metastable intermediate products in methanol solution, subsequently reduced to the stable [Mn2(μ-O)2L2]2+. The Mn(II) complex of bis(2-(2-pyridyl)ethyl)(6-methyl-2-pyridylmethyl)amine (L3) however failed to produce the dioxo product. The MnIII2 and MnIV2 dioxo forms are interconverted as shown below.
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