Employment of methyl 2-pyridyl ketone oxime in manganese non-carboxylate chemistry: MnII2MnIV and MnII 2MnIII6 complexes

Abstract

The employment of the anion of methyl 2-pyridyl ketone oxime (mpko -) as a tridentate chelating/bridging ligand in manganese chemistry is described. The inorganic anion (Br-, ClO4-) used in the reaction affects the identity of the product. The reaction of MnBr2 and one equivalent of mpkoH in the presence of a base affords [Mn3(OMe)2(mpko)4Br2] (3), which is mixed-valence (2MnII, MnIV). The central MnIV atom in each of the two, crystallographically independent, centrosymmetric molecules is coordinated by four oximate oxygen atoms belonging to the η1:η1:η1:μ mpko- ligands, and two η1:μ MeO- groups, while six coordination at each terminal MnII atom is completed by four nitrogen atoms belonging to the 'chelating' part of two mpko- ligands, and one Br- ion. The MnII atoms have trigonal prismatic coordination geometry. The reaction of Mn(ClO4)2· 6H2O, mpkoH and OH- (1:2:1) in MeOH gives [Mn 8O4(OMe)(mpko)9(mpkoH)](ClO4) 4 (4), which is also mixed-valence (2MnII, 6Mn III) and possesses the novel [Mn8(μ3-O) 4(μ-OMe)(μ-OR″)2]11+ core. The latter possesses a U-shaped sequence of four fused {MnIIMn III2(μ3-O)}6+ triangular units, with a MnIII-MnIII edge being shared between the central triangles. Variable-temperature, solid-state dc and ac magnetic susceptibility studies were carried out on complexes 3 and 4. The dc susceptibility data for 3 in the 5.0-300 K range have been fit to a model with two J values, revealing weak ferromagnetic MnII⋯MnIV (J = +3.4 cm -1) and MnII⋯MnII (J′ = +0.3 cm-1) exchange interactions. Fitting of the magnetization vs. H/T data by matrix diagonalization and including only axial anisotropy (ZFS, D) gave ground state spin (S) and D values of S = 13/2, D = +0.17 cm-1 for 3 and S = 3, D = -0.09 cm-1 for 4. The combined work demonstrates the usefulness of mpko- in the preparation of interesting Mn clusters, without requiring the co-presence of carboxylate ligands. © The Royal Society of Chemistry 2009.