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 wa s prepared by controlled oxidation of manganous nitrate with n-tetrabu tylammonium permanganate in the presence of benzoic acid (PhCOOH) and 2,2'-bipyridine (bpy). Its structure was determined in a single-crysta l 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 ani ons are located in disordered positions, the analysis of bond lengths and steric considerations led us to assign to the complex an asymmetri c structure OH)Mn-III(mu-O)(mu-PhCOO)(2)Mn-III(bpy)(ONO)(2))]. The pro duct, with a chemical formula C34H29Mn2N5O10, crystallizes in the mono clinic system, space group C2/c, with a = 16.607(4) Angstrom, b = 25.6 19(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 signif icant zero-field splittings in line with the strong Jahn-Teller distor tion of the high spin d(4) manganese(III) ions. In the present work we interpreted the magnetic properties of the complex using a spin Hamil tonian 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 p arameters with enough accuracy, we resorted to variable-temperature va riable-field magnetization measurements over the 2-300 K range at fiel ds of 0.5, 1.0, 2.5, and 5 T. The whole set of data was fit with a sin gle set of parameters through diagonalization of the complete spin Ham iltonian. 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 octahe dral environment. A magnetostructural relationship linking the ferroma gnetic 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 m olecular orbital calculations of the extended Huckel type.