Thermodynamics of protonation and complexation of EDTA derivatives and metal cations in water

Two EDTA derivatives, ethylenedinitrilo-N,N′-diacetic-N,N′-bis(1-phenylethylacetamido) acid (EDTAMBA) and ethylenedinitrilo-N,N′-diacetic-N,N′-bis(2-pyridylacetamido) acid (EDTAPA) have been synthesised and characterised by IR and ¹H NMR spectroscopy. The pK_a values for the dissociation of the tertiary amino groups are lower by about three units relative to EDTA. Enthalpies of protonation of EDTAMBA and EDTAPA in water have been determined by titration calorimetry. Thermodynamic parameters of protonation show that the differences observed between the first protonation constants of these derivatives in water is of entropic origin. For EDTAPA, the enthalpy term is dominant in the differences found between the first and second protonation constants of this ligand. Stability constants (determined by potentiometry) and derived Gibbs energies, enthalpy (derived from calorimetry) and entropy data for these ligands and metal cations in water are reported. These data show that, unlike EDTA, there is hardly any variation in the Gibbs energies of complexation (or stability constants) of EDTAMBA and EDTAPA with metal cations in water, as a result of an almost complete compensation between enthalpy and entropy.