DESIGN OF A MOLECULAR-BASED FERROMAGNET THROUGH POLYMERIZATION REACTION IN THE SOLID-STATE OF MN(II)CU(II) MOLECULAR UNITS - CRYSTAL-STRUCTURE OF MNCU(OBZE)(H2O)4.2H2O (OBZE = OXAMIDO-N-BENZOATO-N'-ETHANOATO)

The compound MnCu(obze)(H2O)4.2H2O has been obtained by slow diffusion of equimolar aqueous solutions of Na2[Cu(obze)].2H2O and Mn(II) perchlorate. The abbreviation obze stands for oxamido-N-benzoato-N'-ethanoato. MnCu(obze)(H2O)4.2H2O crystallizes in the monoclinic system, space group P2(1/n) with a = 17.998 (1) angstrom, b = 6.9949 (6) angstrom, c = 13.784 (1) angstrom, beta = 97.40 (1)degrees, V = 1720.8 (4) angstrom3, and Z = 4 (MnCu units) at 294 K. The structure consists of oxamido-bridged Mn(II)Cu(II) discrete units and noncoordinated water molecules. Cu(II) is in square-planar surroundings and Mn(II) in octahedral surroundings with four water molecules in the coordination sphere. MnCu(obze)(H2O)4.2H2O magnetically behaves as antiferromagnetically coupled Mn(II)Cu(II) isolated pairs with a quintet-septet energy gap 3J = -100.8 cm-1 and may be viewed as an assembly of molecules with a large positive spin density on the manganese side and a small negative spin density on the copper side. These molecules polymerize through very mild experimental conditions to afford a compound of formula MnCu(obze)(H2O)2. The electronic spectrum of this polymeric compound strongly suggests that the Mn(II) ion retains an octahedral environment and therefore is bound to the oxygen atoms of the carboxylato groups, which provides an exchange pathway between the negative spin density of a Mn(II)Cu(II) unit and the positive spin density of the adjacent unit. Temperature dependence of the magnetic susceptibility and temperature and field dependences of the magnetization for MnCu(obze)(H2O)2 have been investigated. This compound shows a three-dimensional magnetic transition at T(c) = 4.6 K. Below T(c), the molecular spins S = 2 are ferromagnetically aligned. The strategy used to design this molecular-based ferromagnet is reminiscent of an idea put forward by McConnell as early as 1963.