Complexes of Al(III) with D-gluconic acid

The complexation of Al(III) withD-gluconic acid was studied in solution by means of pH-potentiometry, ESI mass spectrometry and one- and two-dimensional NMR spectroscopy. Six complexes were found to form in solution from pH 2 to 10: [AlL]2+, [AlLH-1]+, [AlLH-2], [AlLH-3]-, [AlL2H11] and [AlL2H-2]-. NMR spectroscopy indicated very complicated chemical exchange processes between the free ligand and gluconic acid molecules bound in the metal complexes, with different coordination modes resulting in changes both of the chemical shift and of the line shape of the signals. A solid complex [AlL2H-1].2(H2O) was isolated as a microcrystalline powder and characterized. The structures of the complexes are discussed on the basis of the spectroscopic results and MM force field calculations.