Dioxo-Bridged Dinuclear Manganese(III) and -(IV) Complexes of Pyridyl Donor Tripod Ligands: Combined Effects of Steric Substitution and Chelate Ring Size Variations on Structural, Spectroscopic, and Electrochemical Properties

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 L2
produced green solutions in a fast reaction from which subsequently precipitated brown solids of the dioxo-bridged
dinuclear complexes 1 and 2, respectively, which have the general formula [LMnIII(μ-O)2MnIIIL](ClO4)2. Addition of
30% aqueous H2O2 to the methanol solution of the Mn(II) complex of L3 ([MnIIL3(CH3CN)(H2O)](ClO4)2 (3)) showed
a very sluggish change gradually precipitating an insoluble black gummy solid, but no dioxo-bridged manganese
complex 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 voltammetric
experiments 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 two
reversible peaks, one at E1/2 = 0.78 V and a second peak at E1/2 = 1.58 V (vs Ag/AgCl) assigned to the
MnIII2 ↔ MnIIIMnIV and MnIIIMnIV ↔ MnIV2 redox processes, respectively. These potentials are the highest so far
observed for the dioxo-bridged dinuclear manganese complexes of the type of tripodal ligands used here. The bulk
electrolytic oxidation of complexes 1 and 2, at a controlled anodic potential of 1.98 V (vs Ag/AgCl), produced the
green MnIV2 complexes that have been spectrally characterized. The Mn(II) complex of L3 shows a quasi reversible
peak at an anodic potential of Ep,a of 1.96 V (vs Ag/AgCl) assigned to the oxidation Mn(II) to Mn(III) complex. It
is about 0.17 V higher than the Ep,a of the Mn(II) complex of L3a. The higher oxidation potential is attributable to
the steric effect of the methyl substituent at the 6-positon of the pyridyl donor of L3.