Employment of methyl 2-pyridyl ketone oxime in manganese non-carboxylate chemistry: MnII2MnIV and MnII2MnIII6 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 (2Mn^II, Mn^IV). The central Mn^IV 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 Mn^II atom is completed by four nitrogen atoms belonging to the ‘chelating’ part of two mpko⁻ ligands, and one Br⁻ ion. The Mn^II atoms have trigonal prismatic coordination geometry. The reaction of Mn(ClO₄)₂·6H₂O, mpkoH and OH⁻ (1:2:1) in MeOH gives [Mn₈O₄(OMe)(mpko)₉(mpkoH)](ClO₄)₄ (4), which is also mixed-valence (2Mn^II, 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 {Mn^IIMn^III₂(μ₃-O)}⁶⁺ triangular units, with a Mn^III-Mn^III 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 Mn^II⋯Mn^IV (J = +3.4 cm⁻¹) and Mn^II⋯Mn^II (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.