Structure and stability of Li(I) and Na(I) – Carboxylate, sulfate and phosphate complexes

Abstract

DFT was used to investigate molecular structure and metal affinity of the systems CH 3CO 2M ( 1), CH 3–O–SO 3M ( 2), CH 3–NH–SO 3M ( 3), (CH 3–O–PO 3M) − ( 4), CH 3–O–PO 3M 2 ( 5), CH 3–O–(CH 3)PO 2M ( 6), and 1,4-DiOMe IdoA-2SM 2 ( 7; 2S o conformation) (M = Li + and Na +), respectively. Interaction enthalpies, entropies and Gibbs energies of the metal-coordinated systems were determined on the B3LYP/6-311+G(d,p) level. The computed Gibbs energies, Δ G o , of the isolated systems 1– 7 are negative and span a rather broad energy interval (from −500 to −1500 kJ mol −1). The lithium and sodium binding enthalpies and Gibbs energies of a series of phosphate, carboxylate, N-, and O-sulfate anions indicate that multidentate chelation plays an important role in the binding. In particular, the glycosaminoglycan structural unit of heparin 1,4-DiOMe IdoA-2SM 2 (M = Li + and Na +) with coordinating groups in the hexopyranose ring exhibits enhanced metal ion binding energies. Computations that include the effect of solvation showed that in water the relative stability of Li +⋯Ligand and Na +⋯Ligand ionic bonds is rapidly diminished. The computed interaction Gibbs energy in water is small, slightly negative and/or positive, i.e. destabilizing. Thus in water, both contact ion pairs and solvent-separated ion pairs may coexist.