Thermodynamic Features of Benzene-1,2-Diphosphonic Acid Complexes with Several Metal Ions

Among his many contributions to the advancement of f-element chemistry and separation science, Professor Gregory R. Choppin's research group completed numerous investigations featuring the application of distribution techniques to the determination of metal complexation equilibrium quotients. Most of these studies focused on the chemistry of lanthanide and actinide complexes. In keeping with that tradition, this report discusses the complex formation equilibrium constants for complexes of trivalent europium (Eu³⁺) with benzene-1,2-diphosphonic acid (H₄BzDP) determined using solvent extraction distribution experiments in 0.2 M (Na,H)ClO₄ media in the temperature range of 5 - 45°C. Protonation constants for H_nBzDP^4-n and stoichiometry and stability of BzDP^4- complexes with Zn²⁺, Ni²⁺, and Cu²⁺ have also been determined using potentiometric titration (at I = 0.1 M) and ³¹P NMR spectroscopy. Heats of protonation of H_nBzDP^n-4 species have been determined by titration calorimetry. From the temperature dependence of the complex Eu³⁺-H_nBzDP^n-4 equilibrium constant, a composite enthalpy (ΔH = -15.1 (±1.0) kJ mol^-1) of complexation has been computed. Comparing these thermodynamic parameters with literature reports on other diphosphonic acids and structurally similar carboxylic acids indicates that exothermic heats of complexation are unique to the Eu-BzDP system. Comparisons with thermodynamic data from the literature indicate that the fixed geometry imposed by the benzene ring enhances complex stability.