Potentiometric and Multinuclear Magnetic Resonance Study of the Solution Equilibria Between Aluminium(III) Ion and L-Aspartic Acid

Solution equilibria between aluminium(III) ion and L-aspartic acid were studied by potentiometric, 27Al, 13C, and 1H NMR measurements. Glass electrode equilibrium potentiometric studies were performed on solutions with ligand to metal concentration ratios 1:1, 3:1, and 5:1 with the total metal concentration ranging from 0.5 to 5.0 mmol/dm3 in 0.1 mol/dm3 LiCl ionic medium, at 298 K. The pH of the solutions was varied from ca. 2.0 to 5.0. The non-linear least squares treatment of the data performed with the aid of the Hyperquad program, indicated the formation of the following complexes with the respective stability constants log βp,q,r given in parenthesis (p, q, r are stoichiometric indices for metal, ligand, and proton, respectively): Al(HAsp)2+ (log β1,1,1 = 11.90 ± 0.02); Al(Asp)+ (log β1,1,0 = 7.90 ± 0.03); Al(OH)Asp0 (log β1,1,−1 = 3.32 ± 0.04); Al(OH)2Asp− (log β1,1−2 = −1.74 ± 0.08), and Al2(OH) Asp3+ (log β2,1,−1 = 6.30 ± 0.04). 27Al NMR spectra of Al3+ + aspartic acid solutions (pH 3.85) indicate that sharp symmetric resonance at δ∼10 ppm can be assigned to (1, 1, 0) complex. This resonance increases in intensity and slightly broadens upon further increasing the pH. In Al(Asp)+ complex the aspartate is bound tridentately to aluminum. The 1H and 13C NMR spectra of aluminium + aspartic acid solutions at pH 2.5 and 3.0 indicate that β-methylene group undergoes the most pronounced changes upon coordination of aluminum as well as α-carboxylate group in 13C NMR spectrum. Thus, in Al(HAsp)2+ which is the main complex in this pH interval the aspartic acid acts as a bidentate ligand with –COO− and –NH2 donors closing a five-membered ring.