CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF MNCU(OBBZ)(H2O)3.DMF (OBBZ=N,N'-OXAMIDOBIS(BENZOATO) AND DMF=DIMETHYLFORMAMIDE), A PRECURSOR OF THE MOLECULAR-BASED MAGNET MNCU(OBBZ).H2O WITH T(C)=14-K

After 3 years of efforts, we succeeded in obtaining single crystals of MnCu(obbz)(H2O)3.DMF (1) (obbz = N,N'-oxamidobis(benzoato) and DMF = dimethylformamide). 1 is a precursor of the molecular-based magnet MnCu-(obbz)-H2O(2) exhibiting a spontaneous magnetization below T(c) = 14 K. 1 crystallizes in the orthorhombic system, space group Pbca. The lattice parameters are a = 10.3006 (6) angstrom, b = 22.375 (2) angstrom, c = 19.639 (1) angstrom, and Z = 8 (MnCu units). The structure consists of alternating bimetallic chains running along the c axis, the Mn(II) and Cu(II) ions being alternately bridged by oxamido and carboxylato groups. The conformation around the carboxylato bridge is of the syn-anti type. The chains stack on top of one another along the a axis, and are separated by noncoordinated DMF molecules along the b axis. The magnetic susceptibility of 1, investigated down to 2 K, is characteristic of one-dimensional ferrimagnetic behavior with a minimum in the chi(M)T versus T curve at 35 K (chi(M) is the molar magnetic susceptibility and T is the temperature). Two theoretical models have been used to interpret these data. Model 1 consists of treating S(Mn) = 5/2 as a classical spin and S(Cu) = 1/2 as a quantum spin, with two interaction parameters J1 and J2. This model might be of limited applicability in the case of 1, due to the low value of the ratio J2/J1. Model 2 describes the system as consisting of chains of oxamido-bridged MnIICuII units weakly coupled in a ferromagnetic fashion through carboxylato bridges. Knowing the crystal structure of 1, it has been possible to propose a scheme describing the irreversible transformation from 1 to the molecular-based magnet 2. In 2 each Mn(II) ion belonging to a chain would bind to two free carboxylato oxygen atoms belonging to the adjacent chains just above and below along the a axis, affording a two-dimensional network. The interplane interactions would be favored by the removal of the DMF molecules.