Complex formation between oxalate and (2,2′:6′,2″-terpyridyl)copper(II) in dimethyl sulphoxide solution. Synthesis and crystal structures of mono- and di-nuclear complexes

Abstract

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)Å and β= 116.29(1)°, whereas those of 2 are triclinic, space group P, with Z= 2, a= 10.192(2), b= 12.319(2), c= 8.397(3)Å, α= 86.65(3), β= 96.80(3) and γ= 106.14(1)°. 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, teragonally elongated, octahedral environment. The stability constants of the oxalato complexes of [Cu(terpy)]²⁺[equations (i)–(iii)][Cu(terpy)]²⁺+ ox^2–+ H⁺[graphic omitted][Cu(terpy)(Hox)]⁺(i) 2[Cu(terpy)]²⁺+ ox^2–[graphic omitted] [{Cu(terpy)}₂(ox)]²⁺(ii)[Cu(terpy)]²⁺+ ox^2–[graphic omitted][Cu(terpy)(ox)](iii) were determined by potentiometry in dimethyl sulphoxide solution: log β₁₁₁= 12.397(4), log β₂₁₀= 10.621(6) and log β₁₁₀= 7.394(2)(25 °C, 0.1 mol dm^–3[NBu₄][ClO₄]). The co-ordination modes of oxalate in the Cu^IIL–ox^2– system (L being tri- or bi-dentate N-donor ligands) are discussed in the light of available thermodynamic and structural parameters.