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.

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 Ci (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 Ci 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 Ep = 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. [ABSTRACT FROM AUTHOR]