PREPARATION, CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF A SYN-ANTI CARBOXYLATE-BRIDGED DINUCLEAR COPPER(II) COMPLEX AND ITS PRECURSOR, A HYDROGEN-BONDED POLYNUCLEAR COPPER(II) COMPLEX

Reaction of the polynuclear zig-zag chain complex [{Cu(L)(H2O)}n] (H2L=5-[(2-carboxyphenyl)azo]-1,3-dimethylbarbituric acid) and pyrazole (Hpz) produces the complex of formula [{Cu(L)(Hpz)(H2O)}.H2O] (1). By heating 1 at 110-degrees-C and further recrystallization in dry acetone the complex [{Cu(L)(Hpz)}2] (2) is isolated. Complex l crystallizes in the triclinic space group P1BAR, with a=8.011(l), b=10.399(2), c=12.815(3) angstrom, alpha=104.15(2), beta=107.43(2), gamma=105.80(2)-degrees and Z=2, and complex 2 crystallizes in the monoclinic space group P2(1)/c, with a=9.539(5), b=20.791(10), c=8.723(4) angstrom, beta=103.88(l)-degrees and Z=2. The structure of 1 consists of neutral [{Cu(L)(Hpz)(H2O)}.H2O] molecules which are interconnected by hydrogen bonds giving rise to a zig-zag chain of copper(II) ions with two different Cu ... Cu distances of 7.862(2) and 7.221(l) angstrom. The structure of 2 consists of centrosymmetrical dinuclear molecules [{Cu(L)(Hpz)12], in which copper(II) atoms of the mononuclear fragments are connected by carboxylate groups displaying syn-anti conformation, with a Cu ... Cu distance of 5.054(2) angstrom. In both complexes, copper atoms exhibit distorted square-pyramidal environments. In the basal plane, four bonds are formed with the doubly deprotonated L2- ligand and the molecule of pyrazole, while the apical position is occupied either by a molecule of water in 1 or an oxygen atom belonging to the carboxylate group of an adjacent and symmetry-related fragment in 2. From magnetic susceptibility measurements, complex 1 is found to exhibit a weak intermolecular antiferromagnetic exchange interaction with J=-0.30 cm-1 whereas a Curie-Weiss law with C=0.40 cm3 mol-1 K and 0=0.45 K is observed for 2. Magnetic experimental data are discussed on the basis of the structural results and compared to those of other related complexes.