Formation and protonation of aminopolyphosphonate complexes of alkaline-earth and divalent transition-metal ions in aqueous solution

Abstract

The formation and protonation of N-methyliminobis(methylenephosphonic acid)(midmp, H₄L) and N,N-dimethylaminomethylphosphonic acid (dmamp, H₂L) complexes with alkaline-earth (M = Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺) and divalent transition metals (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, or Cd²⁺) have been investigated at 25.0 °C and at an ionic strength of 0.1 mol dm^–3(KNO₃) by means of potentiometry and ³¹P NMR spectroscopy. The complex-formation constants, K_ML, for the alkaline-earth metals decrease with increasing ion size. The order of K_ML for the transition-metal complexes agrees with the Irving–Williams series for each ligand. The first protonation constants, K_MHL, of the alkaline earth–midmp and transition metal–dmamp complexes are quite large compared with the first protonation constants of the phosphonate group of the free ligand (the second protonation constants of the ligand K_H2L), and decrease with increasing complex-formation constant. The values of K_MHL for the transition metal–midmp complexes are of the same order of magnitude as the value of K_H2L for the ligand. The second protonation constants K_MH2L of all complexes are smaller than K_H2L and there is no significant difference among the complexes. The ³¹P NMR spectra of the metal–ligand solutions have been measured at 25 °C as a function of pH and the chemical shift of each species was evaluated. The results of the potentiometric and NMR studies suggest that the M–N bond of the ML complex is weakened by a decrease in the number of methylenephosphonate groups of the ligand, i.e. protonation of midmp complexes of alkaline-earth and some transition metals and of all the dmamp complexes occurs at the nitrogen atom of the ligand, rupturing the M–N bond.