Coordination chemistry of vitamin C. Part III. Interaction of L-ascorbic acid with Al(III), La(III), and Pb(II) ions. Evidence for metal chelate formation in the solid and aqueous solution

Abstract

The interaction of L-ascorbic acid with Al(III), La(III), and Pb(II) has been investigated in aqueous solution at pH = 6–7. The solid salts of the type Al(L-ascorbate) 3, La(L-ascorbate 3·4H 2O and Pb(L-ascorbate) 2·2H 2O were isolated and characterized by 13C NMR and Fourier Transform infrared (FT-IR) spectroscopy. Spectroscopic evidence suggested that in aqueous solution, metal chelation can occur through ascorbate anion O-3 and O-2 atoms, whereas in the solid state metal coordination is through the O-1, O-3 and O-2, O-3 of the acid anions as well as the H 2O molecules. The aggregation of the Pb(II)- and La(III)-ascorbate salts occur via the carbonyl O-1 atom, while association of the Al(III)-ascorbate would be through other anion donor groups. The structural properties of these metal-ascorbate complexes are different in the solid and aqueous solution.