Structural and magnetization studies of a new (mu-oxo)bis(mu-carboxylato)dimanganese(III) complex with a terminal hydroxo ligand

The dinuclear Mn-III complex [Mn2O(PhCOO)(2)(bpy)(2)(OH)(NO3)]. H2O was prepared by controlled oxidation of manganous nitrate with n-tetrabutylammonium permanganate in the presence of benzoic acid (PhCOOH) and 2,2'-bipyridine (bpy). Its structure was determined in a single-crystal X-ray diffraction experiment, and consists of a triply-bridged [Mn-2(mu-O)(mu-PhCOO)(2)](2+) dinuclear core. Each manganese(III) ion bears a chelating bpy and a terminal X anion (X = OH- or NO3-) completing a distorded octahedral coordination geometry. Although the terminal anions are located in disordered positions, the analysis of bond lengths and steric considerations led us to assign to the complex an asymmetric structure [(bpy)(OH)Mn-III(mu-O)(mu-PhCOO)(2)Mn-III(bpy)(ONO)(2))]. The product, with a chemical formula C34H29Mn2N5O10, crystallizes in the monoclinic system, space group C2/c, with a = 16.607(4) Angstrom, b = 25.619(6) Angstrom, c = 9.796(3) Angstrom, beta = 100.15(3)degrees, and Z = 4. Magnetic studies performed on a series of related compounds have revealed a moderate ferro- or antiferromagnetic interaction and significant zero-field splittings in line with the strong Jahn-Teller distortion of the high spin d(4) manganese(III) ions. In the present work we interpreted the magnetic properties of the complex using a spin Hamiltonian which includes the Heisenberg exchange, axial and rhombic ZFS, and an anisotropic Lande factor, under the assumption of a pseudo-C-2 symmetry of the dinuclear core. In order to determine the anisotropy parameters with enough accuracy, we resorted to variable-temperature variable-field magnetization measurements over the 2-300 K range at fields of 0.5, 1.0, 2.5, and 5 T. The whole set of data was fit with a single set of parameters through diagonalization of the complete spin Hamiltonian. The best fit values J = +1.0(4) cm(-1), D = +4.5(5) cm(-1), E = 0, g(x) = g(y) = 1.96, and g(z) = 2.00 showed that a ferromagnetic interaction occurs between manganese(III) ions in a compressed octahedral environment. A magnetostructural relationship linking the ferromagnetic behavior of the dinuclear complex to the compression of the Mn(III) coordination sphere (and conversely of the antiferromagnetism to the elongation) is then proposed. It is substantiated by theoretical molecular orbital calculations of the extended Huckel type.