Thermodynamic, crystallographic and solvent extraction properties of cobalt(II), nickel(II) and copper(II) complexes of ethylenediamine-N,N,N′,N′-tetraacetanilide

Complexes of ethylenediamine-N,N,N′,N′-tetraacetanilide (edtan, C₃₄H₃₆N₆O₁₄) with cobalt(II), nickel(II) and copper(II) in the solid state and in solution are reported for the first time. Thermodynamic data (stability constant, and derived Gibbs energy, enthalpy and entropy changes) for the 1:1 complexation of edtan with the metal ions at 298.15 K in water-saturated butan-1-ol gave the selectivity sequence log₁₀K_S; Ni²⁺, 4.56±0.02; Cu²⁺, 4.41±0.01; Co²⁺, 4.18±0.04 as found from microcalorimetric titration studies. The entropies suggested that the structure of the 1:1 complex with copper(II) contains fewer chelate rings than those for nickel(II) and cobalt(II) (Δ_cS⁰: Cu-21.4, Co 5.7, Ni 3.9 J mol^-1 K^-1). Solid complexes of the metal ions with edtan and perchlorate as the counter anion were prepared. For each, a complex with a 1:1 metal: edtan stoichiometry with non-coordinated perchlorate was isolated. The X-ray structure of [Cu(edtan)(H₂O)][ClO₄]₂·1.5H ₂O (1) revealed a six-coordinate Cu centre with edtan acting as a pentadentate ligand (2N, 3O) with the coordination sphere completed by an oxygen atom from water. In striking contrast to the Cu complex, the Co centre in [Co(edtan)(H₂O)][ClO₄]₂·H ₂O·0.5C₂H₅OH (2) is seven-coordinate with hexadentate edtan (2N, 4O) and one coordinated water molecule. There is thus an excellent confirmation of the results obtained from the microcalorimetric study in that edtan forms four chelate rings to Cu but five to Co in the solid state. The ability of the ligand to extract metal ions from water to the water-saturated butan-1-ol phase was assessed from distribution data as a function of the aqueous phase hydrogen ion concentration and of the ligand concentration in the organic phase. The data showed that Cu²⁺ is selectively extracted over a wide range of aqueous phase hydrogen ion concentrations.