Study of the interaction between [Cu(bipy)] 2+ and oxalate in dimethyl sulfoxide. Crystal structure of [Cu 2(bipy) 2(H 2O) 2ox]SO 4·[Cu(bipy)ox

Abstract

A study of complex formation between [Cu(bipy)] 2+ and ox 2− (bipy and ox 2− 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 ▪ and ▪ are log β 110 = 11.165(1) and log β 210 = 13.185(5) at 25 °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 [Cu 2(bipy) 2ox] 2+ units, respectively, as shown by X-ray diffraction studies. Well-formed single crystals of [Cu 2(bipy) 2(H 2O) 2ox]SO 4· [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) Å, β = 92.63(3)°, V = 3846(3) Å 3, Z = 4, D x = 1.670 g cm −3, μ(Mo Kα) = 18.38 cm −1, F(000) = 1956 and T = 298 K. A total of 3214 reflections was collected over the range 2⩽θ⩽25°; of these, 1099 (independent and with I⩾2.5σ( 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 [Cu 2(bipy) 2ox] 2+ dinuclear units, neutral axiosymmetric [Cu(bipy)ox] mononuclear entities and SO 4 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 ligands, that have been observed in solution, are found in the same compound in the solid state.