COMPLEX-FORMATION BETWEEN OXALATE AND (2,2'-6',2''-TERPYRIDYL)-COPPER(II) IN DIMETHYL-SULFOXIDE SOLUTION - SYNTHESIS AND CRYSTAL-STRUCTURES OF MONONUCLEAR AND DINUCLEAR COMPLEXES

The crystal and molecular structures of two new complexes [{Cu(terpy)(H2O)}2(ox)][{Cu(terpy)}2(ox)]-[ClO4]4.2H2O 1 and [Cu(terpy)(H2O)(ox)].4H2O 2 (terpy = 2,2':6',2"-terpyridine and ox = oxalate) have been determined by X-ray diffraction. Crystals of 1 are monoclinic, space group P21/c, with Z = 2, a = 13.443(2), b = 23.183(4), c = 12.394(1) angstrom and beta = 116.29(1)-degrees, whereas those of 2 are triclinic, space group P1BAR, with Z = 2, a = 10.192(2), b = 12.319(2), c = 8.397(3) angstrom, alpha = 86.65(3), beta = 96.80(3) and gamma = 106.14(1)-degrees. The structure of 1 contains two different centrosymmetrical copper(II) dinuclear dicationic units, unco-ordinated perchlorate groups and lattice water. In both dinuclear units the terpyridyl group is terminal and the oxalate acts as an asymmetrical bis(chelating) bridge. However, the copper atom is five-co-ordinate in one dinuclear unit and six-co-ordinate in the other. The structure of 2 consists of neutral mononuclear [Cu(terpy)(H2O)(ox)] entities and unco-ordinated water molecules. The copper atom is in a six-co-ordinate, tetragonally elongated, octahedral environment. The stability constants of the oxalato complexes of [Cu(terpy)]2+ [equations (i)-(iii)] [GRAPHICS] were determined by potentiometry in dimethyl sulphoxide solution: log beta-111 = 12.397(4), log beta-210 = 10.621(6) and log beta-110 = 7.394(2) (25-degrees-C, 0.1 mol dm-3 [NBu4][ClO4]). The co-ordination modes of oxalate in the Cu(II)L-ox2- system (L being tri- or bi-dentate N-donor ligands) are discussed in the light of available thermodynamic and structural parameters.