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

摘要

The syntheses and structural, spectral, and electrochemical characterization of the dioxo-bridged dinuclear Mn(III) complexes [LMn(mu-O)(2)MnL](CIO4)(2), of the tripodal ligands tris(6-methyl-2-pyridylmethyl)amine (L-1) and bis(6-methyl2-pyridylmethyl)(2-(2-pyridyl)ethyl)amine (L-2), and the Mn(II) complex of bis(2-(2-pyridyl)ethyl)(6-methyl-2-pyridylmethyl)amine (L-3) are described. Addition of aqueous H2O2 to methanol solutions of the Mn(II) complexes of L-1 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 [LMnIII(mu-O)(2)(MnL)-L-III](CIO4)(2). Addition of 30% aqueous H2O2 to the methanol solution of the Mn(II) complex of L-3([(MnL3)-L-II(CH3CN)(H2O)](CIO4)(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 (L-3a) has been reported to react with aqueous H2O2 to form the dioxo-bridged (MnMnIV)-Mn-III complex. In cyclic voltammetric experiments in acetonitrile solution, complex I shows two reversible peaks at E-1/2 = 0.87 and 1.70 V (vs Ag/AgCI) assigned to the Mn-2(III) <----> (MnMnIV)-Mn-III and the (MnMnIV)-Mn-III <----> Mn-2(IV) processes, respectively. Complex 2 also shows two reversible peaks, one at E-1/2 = 0.78 V and a second peak at E-1/2 = 1.58 V (vs Ag/AgCI) assigned to the Mn-2(III) <----> (MnMnIV)-Mn-III and Mn11IMnIV <----> Mn-2(IV) 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/AgCI), produced the green Mn-2(IV) complexes that have been spectrally characterized. The Mn(II) complex of L-3 shows a quasi reversible peak at an anodic potential of E-p,E-a of 1.96 V (vs Ag/AgCI) 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 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. [References: 43]