Density Functional Theory Studies on the Structures and Water-Exchange Reactions of Aqueous Al(III)–Oxalate Complexes

Abstract

The structures and water-exchange reactions of aqueous aluminum-oxalate complexes are investigated using density functional theory. The present work includes (1) The structures of Al(C2O4)(H2O)4+ and Al(C2O4)2(H2O)2– were optimized at the level of B3LYP/6-311+G(d,p). The geometries obtained suggest that the AlOH2 bond lengths trans to C2O42‑ ligand in Al(C2O4)(H2O)4+ are much longer than the AlOH2 bond lengths cis to C2O42‑. For Al(C2O4)2(H2O)2–, the close energies between cis and trans isomers imply the coexistence in aqueous solution. The 27Al NMR and 13C NMR chemical shifts computed with the consideration of sufficient solvent effect using HF GIAO method and 6-311+G(d,p) basis set are in agreement with the experimental values available, indicating the appropriateness of the applied models; (2) The water-exchange reactions of Al(III)–oxalate complexes were simulated at the same computational level. The results show that water exchange proceeds via dissociative pathway and the activation energy barriers are sensitive to the solvent effect. The energy barriers obtained indicate that the coordinated H2O cis to C2O42‑ in Al(C2O4)(H2O)4+ is more labile than trans H2O. The water-exchange rate constants (kex) of trans- and cis-Al(C2O4)2(H2O)2– were estimated by four methods and their respective characteristics were explored; (3) The significance of the study on the aqueous aluminum-oxalate complexes to environmental chemistry is discussed. The influences of ubiquitous organic ligands in environment on aluminum chemistry behavior can be elucidated by extending this study to a series of Al(III)–organic system