Novel square-planar cyclic tetranuclear copper(II) complex containing oximate bridges.: Synthesis, crystal structure and magnetic properties of tetrakis[diaqua(μ-1,3-dimethylviolurato)-copper(II)] tetraperchlorate dihydrate

The tetranuclear complex [Cu-4(L)(4)(H2O)(8)][ClO4](4). 2H(2)O (HL = 1,3-dimethylvioluric acid, i.e. 1,3-dimethyl-5-(hydroxyimino)-2,4,6(1H,3H,5H)-pyrimidinetrione) has been prepared and its crystal structure determined by X-ray diffraction methods. The structure consists of a tetranuclear cation [Cu-4(L)(4)(H2O)(8)](4+), four non-co-ordinated perchlorate anions and two water molecules located on a 4-fold rotation-inversion axis. Within each cation the four copper(II) ions, linked through four bis(bidentate) L- ligands, are arranged at the vertices of a strict square-planar parallelogram with edge and diagonal Cu ... Cu distances of 4.764(3) and 6.738(4) Angstrom, respectively. Each oxime group acts as a bridge between adjacent copper ions with an anti conformation and each copper is co-ordinated by two L- ligands. The copper atoms exhibit a CuO5N elongated distorted octahedral co-ordination environment, with three oxygen atoms and one nitrogen atom of the L- ligands at equatorial positions and the water oxygens at axial positions. From variable-temperature magnetic susceptibility measurements the compound was found to exhibit a very strong antiferromagnetic exchange interaction through the oximate bridges with J = -349 cm(-1) [-J(S-Cu. S-CuA + S-CuA. S-CuC + S-CuC. S-CuB + S-CuB. S-Cu)], leading to complete spin coupling at 70 K.