Al(III) Complexes of Saccharic Acid and Mucic Acid: a Solution and a Solid State Study

The AlIII-binding abilities of two aldaric acids, D-saccharic acid and mucic acid (the neutral form is denoted as H2L), were studied in solution by means of pH potentiometric, 1H and 13C NMR, and ESI-MS techniques. The most probable conformations and isomeric binding modes of the complexes formed in solution were determined by density functional theory (DFT) calculations. A solid D-saccharic acid complex K2[{Al(LH−2)(H2O)}2]⋅H2O was isolated and crystallographically characterised. The two alcoholic hydroxy groups α to the terminal COO− groups were found to take part in the coordination, but in different ways. One of them coordinates in a bridging mode. Detailed ESI-MS and NMR studies proved that the complex retains its structure in solution. However, depending on the ligand and the pH, such complexes may exist in two isomeric forms. DFT calculations on the ion [{Al(LH−2)(H2O)}2]2− revealed that several orbitals participate in stabilizing the dimeric arrangement.