A mononuclear MnIII/‘bis-tris’ complex and its conversion to a mixed-valence MnII/III5cluster

Abstract

The use of the anion of 2,2-bis(hydroxymethyl)-2,2,2-nitrilotriethanol (bis-tris or LH5) as a chelate in Mn cluster chemistry is reported, and two products [Mn(N3)(LH3)] ( 1) and [Mn5(LH2)3(LH5)(MeOH)2.5]Cl4 ( 2) are described. The reaction of Mn(ClO4)2 x 6H2O and NaN3 with LH5 and NEt3 in a 1:1:1:2 molar ratio in DMF gave complex 1. The reaction of 1, NEt3 and MnCl2 x 4H2O in a 2:2:1 ratio in MeOH gave the pentanulear complex 2. Complex 1 contains a distorted-octahedral MnIII atom exhibiting a Jahn-Teller axial elongation. Complex 2 has a mixed-valence MnII2MnIII3 trigonal bipyramidal Mn5 topology, with the apical positions of the [Mn5(micro-OR)7](6+) core occupied by the MnII atoms. Variable-temperature, solid-state dc and ac magnetization studies are reported for 1 and 2 in the 1.8-300 K range. The data for 1 are as expected for a high-spin d4 MnIII complex with S = 2; at low temperature, the effects of zero-field splitting (ZFS) and intermolecular antiferromagnetic interactions via OHO hydrogen bonds become evident. Fitting of magnetization vs. field (H) and temperature (T) data by matrix diagonalization gave S = 2, D = -3.25 cm-1 and |E| = 0.32 cm-1, with g fixed at 2.00, where D and E are the axial and rhombic ZFS parameters, respectively. The analogous fit for 2 gave S = 3 and D = -0.68 cm-1. The combined results demonstrate the usefulness of bis-tris as a new poly-alkoxide chelate for the synthesis of new Mn complexes.