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

Abstract

The tetranuclear complex [Cu₄(L)₄(H₂O)₈][ClO₄]₄·2H₂O (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 [Cu₄(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) Å, 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.