DFT study on the selectivity of complexation of metal cations with a dioxadithia crown ether ligand

The interactions of a dioxadithia crown ether ligand with Li(+), Na(+), K(+), Mg(2+), Ca(2+), and Zn(2+) cations were investigated using density functional theory (DFT) modeling. The modeling was undertaken to gain insight into the mechanism of the selective complexation of the mono- and dications observed with this ligand experimentally. Two types of conformationally different complexes were located with both mono- and dications. In the first conformer, the cation is bonded to the ether oxygens; in the second conformer, the cation is bonded to the alkoxy and suger oxygens. In general, the complexes formed with dications were found to be more stable than those with monocations, with the stability decreasing with the period number within a given periodic table group of elements. The highest stability was observed for the complexes formed with zinc. The complex formed with lithium was the most stable among those involving monovalent cations. The system interaction energy was decomposed into electrostatic (ES), polarization (P), charge-transfer (CT), exchange (EX), and geometry-deformation (DEF) contributions using the self-consistent charge and configuration method for subsystems (SCCCMS). The stabilizing energy components (ES, P, and CT) exhibit the same trend as the total interaction energy, whereas the destabilizing contributions (EX and DEF) exhibit the opposite trend. It was found that the main contributions responsible for stabilization of the dicationic systems are the P and ES energies; in the monocationic systems, the CT stabilization is equally important. The gas-phase preferences changed when the solvent effect was included. The dioxadithia crown ether ligand preserved its selectivity toward Zn(2+), but the selectivity sequence toward monovalent cations was reversed.