Structure and thermodynamic properties of aminopoly-phosphonate complexes of the alkaline-earth metal lons

Abstract

The formation equilibria and thermodynamic properties of the alkaline-earth metal (M = Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺) complexes of nitrilotris(methylenephosphonic acid)(H₆ntmp) and ethylenediamine-tetrakis(methylenephosphonic acid)(H₈edtmp) have been studied by means of potentiometry, NMR spectroscopy and calorimetry at 25.0 °C and at an ionic strength of 0.1 mol dm^–3(KNO₃). The complex formation constants and the protonation constants of the complexes were determined from the potentiometric titration data. Thermodynamic parameters of the protonation of the ligand, and the formation and protonation of the metal complexes of ntmp and edtmp were evaluated from the calorimetric titration data. The first protonation on the nitrogen atom of ntmp is significantly exothermic and it is followed by the endothermic protonation on the phosphonate O^–. The protonations of edtmp are exothermic up to the third step. The formation of magnesium complexes of ntmp and edmp is endothermic and entropy driven, whereas that of the complexes of Ca²⁺, Sr²⁺ and Ba²⁺ is weakly exothermic. This behaviour is analogous to that of aminopolycarboxylate complexes. The results of calorimetric measurements of ntmp complexes support the structure of protonated alkaline-earth metal–ntmp complexes predicted from the stability and NMR data, i.e. the first protonation of the metal complexes occurs on the nitrogen atom of the ligand, rupturing the M–N bond. The thermodynamic behaviour of the edtmp complexes is considerably complicated. In the analysis of the pH dependence of the ³¹P NMR signals, the chemical shift of each species of ligand and its metal complexes was evaluated by using the formation constants obtained by potentiometry. The change in the chemical shift of the complexes upon protonation supports the structures of the alkaline-earth metal complexes predicted from the thermodynamic parameters.