Structures of Mn(II) complexes of bis(2-pyridylmethyl)amine (bpa) and (2-pyridylmethyl)(6-methyl-2-pyridylmethyl)amine (Mebpa): geometric isomerism in the solid state

Abstract

The crystal structures of [Mn(bpa)2](ClO4)2 (1), [bpa = bis(2-pyridylmethyl)amine], and Mn(6-Mebpa)2(ClO4)2 (2), [6-Mebpa = (6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine] have been determined. In 1, two facial [Mn(bpa)2]2+ isomers are observed in the same unit cell, one with C i (1a) and the other with C2 (1b) symmetries. In 2, only the isomer with C2 symmetry is observed. The structure of [Mn(bpa)2]2+ with only C2 symmetry has been reported previously (Inorg. Chem., 31, 4611 (1992)). The bond length order Mn–Namine > Mn–Npyridyl, observed in the C2 and the C i isomers in the crystals of 1, is the reverse of the order observed in the structure of [Mn(bpa)2](ClO4)2 which contains only the C2 isomer in the unit cell. The structure of 2 in which only the C2 isomer is found, also shows the bond length order Mn–Npyridyl > Mn–Namine. In cyclic voltammetric experiments in acetonitrile solutions, 1 and 2 show irreversible anodic peaks at E p = 1.60 and 1.90 V respectively, (vs. Ag/AgCl), assigned to the oxidation of Mn(II) to Mn(III). The substantially higher oxidation potential of 2 is attributable to a higher rearrangement energy in complex 2 due to the steric effect of the methyl substituent.