A density-functional investigation of aluminium(III)-citrate complexes

Density-functional calculations have been performed on tridentate, hydrated aluminium-citrate complexes using fully (quadruply) and triply deprotonated citric acid ligands. Water molecules in the inner solvation sphere have been included explicitly in the quantum-chemical calculation, whereas the remaining solvent effects have been computed using the polarized continuum model (PCM). As is to be expected, solvation effects play an important role for the calculation of formation energies of the complexes. Optimized geometries are in good agreement with X-ray data. Reaction enthalpies and Gibbs reaction energies have been computed for the substitution of water molecules of the aluminium-hexaaquo complex by citrate molecules. Formation of the tridentate complexes is strongly favored by entropy effects in analogy to previous findings for bidentate acetate and oxalate complexes. Comparison of the stability of acetate, oxalate and citrate complexes shows a pronounced preference for the latter.