Employment of methyl 2-pyridyl ketone oxime in manganese non-carboxylate chemistry: Mn2IIMnIV and Mn2IIMn6II complexes

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 [Mn-3(OMe)(2)(mpko)(4)Br-2] (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 eta(1):eta(1):eta(1):mu mpko(-) ligands, and two eta(1):mu 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)center dot 6H(2)O, 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 [Mn-8(mu(3)-O)(4)(mu-OMe)(mu-OR '')(2)](11+) core. The latter possesses a U-shaped sequence of four fused {(MnMn2III)-Mn-II(mu(3)-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 center dot center dot center dot Mn-IV (J = +3.4 cm(-1)) and Mn-II center dot center dot center dot 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.