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

DFT was used to investigate molecular structure and metal affinity of the systems CH3CO2M (1), CH3-O-SO3M (2), CH3-NH-SO3M (3), (CH3-O-PO3M)− (4), CH3-O-PO3M2 (5), CH3-O-(CH3)PO2M (6), and 1,4-DiOMe IdoA-2SM2 (7; 2So 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, ΔGo, 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-2SM2 (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.