Dioxo-Bridged Dinuclear Manganese(lll) 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(lll) complexes [LMn(nu-O)2MnL](C1O4)2, of the tripodal ligands tris(6-methyl-2-pyridylmethyl)amine (L1) and bis(6-methyl2-pyridylmethyl)(2-(2-pyridyl)ethyl)amine (L^2, and the Mn(ll) 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(ll) complexes of L1 and L^2 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 [LMnlll(mu-O)2Mn111L](C1o4)2. Addition of 30% aqueous H2O2 to the methanol solution of the Mn(ll) complex of L3 ([MnllL3(CH3CN)(H2o)](CIo)4)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(ll) complex of the ligand bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine (L^3a) has been reported to react with aqueous H2O2 to form the dioxo-bridged MnlllMnlv 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/AgGI) assigned to the Mnlll1- MnlllMnlv and the MnlllMnlv - Mnlv(2) processes, respectively. Complex 2 also shows two reversible peaks, one at E1/2 = 0.78 V and a second peak at E1/2 = 1.58V (vs Ag/AgCI) assigned to the Mnlll2 - MnlllMnlv and MnlllMn^lv Mn^lv(2) 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 land 2, at a controlled anodic potential of 1.98 V (vs Ag/AgCI), produced the green Mn^lv(2) complexes that have been spectrally characterized. The Mn(ll) complex of L3 shows a quasi reversible peak at an anodic potential of Ep,a of 1.96 V (vs Ag/AgCI) assigned to the oxidation Mn(ll) to Mn(lll) complex. It is about 0.17 V higher than the ep,a of the Mn(ll) complex of L^.3a The higher oxidation potential is attributable to the steric effect of the methyl substituent at the 6-positon of the pyridyl donor of L^3