Potentiometric and Thermodynamic Studies of Binary and Ternary Transition Metal(II) Complexes of Imidazole-4-acetic Acid and Some Bio-relevant Ligands

Proton–ligand association constants of imidazole-4-acetic acid (IMA) were determined potentiometrically in aqueous solution at different temperatures in the range 15–35 °C. The stepwise stability constants of IMA with some selected bivalent transition metal ions were also determined in 0.1 mol·dm −3 NaNO 3 . The stability of the complexes follows the trend Cu 2+ > Ni 2+ > Co 2+ > Mn 2+ , which is in agreement with the Irving–Williams order of the metal ions. The thermodynamic parameters for Cu(II)–IMA complex formation were derived and discussed. The ternary complexes Cu(IMA)L (IMA = imidazole-4-acetic acid, HL = amino acid, amides or DNA constituents) have been investigated. Ternary complexes of amino acids or amides are formed by a simultaneous mechanism. Amino acids form the complex Cu(IMA)L, whereas amides form two complex species Cu(IMA)L and Cu(IMA)(LH −1 ). The DNA constituents form both 1:1 and 1:2 complexes. The stabilities of ternary complexes are quantitatively compared with their corresponding binary complexes. The concentration distribution of the complexes in solution was evaluated as a function of pH.