STUDY OF THE INTERACTION BETWEEN [CU(BIPY)]2+ AND OXALATE IN DIMETHYL-SULFOXIDE - CRYSTAL-STRUCTURE OF [CU2(BIPY)2(H2O)2OX]SO4.[CU(BIPY)OX]

A study of complex formation between [Cu(bipy)]2+ and ox2- (bipy and ox2- being 2,2'-bipyridyl and the dianion of oxalic acid), has been carried out by potentiometry in dimethyl sulfoxide solution. The constants of the equilibria [Cu(bipy)]2+ + ox2- reversible-beta-110 [Cu(bipy)ox] and 2[Cu(bipy)]2+ + ox2- reversible-beta-210 [Cu2(bipy)2ox]2+ are log beta-110 = 11.165(1) and log beta-210 = 13.185(5) at 25-degrees-C and 0.1 mol dm-3 tetra-n-butylammonium perchlorate. The high values of these constants are consistent with the symmetrical bidentate and bis-bidentate modes of oxalate in [Cu(bipy)ox] and [Cu2(bipy)2ox]2+ units, respectively, as shown by X-ray diffraction studies. Well-formed single crystals of [Cu2(bipy)2(H2O)2ox]SO4. [Cu(bipy)ox] were grown from aqueous solutions and characterized by X-ray diffraction. The system is monoclinic, space group C2/c, with a = 22.706(5), b = 10.485(3), c = 16.172(4) angstrom, beta = 92.63(3)-degrees, V = 3846(3) angstrom3, Z = 4, D(x) = 1.670 g cm-3, mu-(Mo K-alpha) = 18.38 cm-1, F(000) = 1956 and T = 298 K. A total of 3214 reflections was collected over the range 2 less-than-or-equal-to theta less-than-or-equal-to 25-degrees; of these, 1099 (independent and with I greater-than-or-equal-to 2.5 sigma-(I) were used in the structural analysis. The final R and R(w) residuals were 0.067 and 0.070, respectively. The structure is made up of cationic centrosymmetric [Cu2(bipy)2ox]2+ dinuclear units, neutral axi-symmetric [Cu(bipy)ox] mononuclear entities and So4(2-) as a counterion. Each copper atom of the dinuclear unit shows a square-pyramidal environment with the two nitrogen atoms of 2,2'-bipyridyl and two oxygen atoms of oxalato bridge building the basal plane and an oxygen atom of a water molecule filling the apical position. The copper atom of the mononuclear unit is bound to two oxygen atoms of oxalate and two nitrogen atoms of 2,2'-bipyridyl forming a four-fold surrounding slightly deviating from planarity. In these complexes, copper coordination is 4 + 1 + 1 (dinuclear unit) or 4 + 2 (mononuclear unit) because of the weak binding of the sulfate group in a bis-monodentate fashion linking alternately dinuclear and mononuclear entities. This is a rare case where the two copper(II) complexes bound to the same ligand, that have been observed in solution, are found in the same compound in the solid state.