Insight into the structures and reactivities of aqueous Al(III)-carboxylate complexes from cluster-based ab initio computational studies – Implications for the ligand-promoted mineral dissolution mechanism

Abstract

The stable geometries of a series of mononuclear bidentate chelating Al(III)-carboxylate complexes in aqueous solution are optimized and their transition-states for water-exchange reactions are modeled with the density functional theory – quantum chemical cluster model (DFT-CM) method. The studied carboxylates include oxalate, malonate, succinate, phthalate, salicylate and benzoate. Thermodynamic stability constants Kaq and water-exchange reaction rate constants kex of the tested Al(III)-carboxylate complexes are estimated from calculated Gibbs free energy changes. The estimated kex values suggest that the coordination of dicarboxylate and salicylate ligands to Al3+ leads to the labilization of inner-shell coordinated waters and that the cis waters of chelate rings are more labile than trans waters. From estimated log Kaq and log kex values of aqueous mononuclear Al(III)-carboxylate complexes, linear correlations between log Kaq, log kex values and experimental apparent rate constants kL of carboxylate ligand-promoted δ-Al2O3 mineral dissolution are established. Based on the calculation results, cis labilizing effects of the carboxylate ligands, strong “log kL ∼ log Kaq” correlations, and weak “log kL ∼ log kex” correlations are discussed. This work offers further insight into intrinsic relationships between mineral surface and aqueous metal complexes and furthers understanding of mineral dissolution kinetics at the molecular level.