Stability constants and multinuclear NMR measurements of phosphonic acid derivatives with aluminum in aqueous solutions

Abstract

We are reporting the stability constants of the different complexes between phosphonoacetic acid (PAA), phosphonoformic acid (PFA), aminomethylphosphonic acid (AMPA), aminoethylphosphonic acid (AEPA) and methylenediphosphonic acid (MDP) with the aluminum metal ion in aqueous solutions. (In this study the term aluminum is reserved for the 3+ ion.) The affinity of the phosphonic acid derivatives to chelate aluminum has been tested by potentiometric titrations with I = 0.10 M KNO 3 at 25 ± 0.1 °C. The proposed aluminum–ligand complex structures have been confirmed by 31P NMR, 13C NMR, and 27Al NMR experiments. Both PAA and PFA formed simple one to one complexes. The respective values for PAA are log β 111 = 13.57, log β 110 = 10.58, and log β 11−1 = 5.84. The respective values for PFA are log β 112 = 15.24, log β 111 = 13.11, and log β 110 = 6.88. In contrast to PAA and PFA, the major species formed with AMPA and AEPA consist of a series of dimeric complexes. The 31P NMR spectra of these complexes indicate that the amine groups do not co-ordinate to aluminum and remain protonated. In addition to these dimeric complexes, a monoprotonated monomer of Al–AMPA also has been detected. The 27Al NMR experiments indicated that the aluminum is hexacoordinated in all the complexes in this study and the hydroxide ion did not remove aluminium from its complexes. Among the ligands studied, PAA and PFA were able to solubilize aluminum at physiological pH. A comparison between the acidities and the chelating properties of the ligands used is presented.