Ligands for selective metal ion extraction: A molecular modeling approach

We present a density functional theory (DFT) based study on interaction of alkali metal cations (Li+ and Na+) with macrocyclic crown ethers of different ring sizes. The minimum energy structures, binding energies, and binding enthalpies of crown ether–cation complexes have been determined with a correlated hybrid density functional, namely Becke’s three-parameter functional, B3LYP using a split valence basis function, 6-311++G(d, p). Geometry optimizations for all the crown ether–cation complexes were carried out with several initial guess structures based on semi-empirical PM3 optimized results. For both metal ions, the calculated values of binding energy and binding enthalpy increase with the increase in size of the crown ether ring, i.e. with the increase in the number of donor oxygen atoms in crown ether. The calculated values of gas phase binding energy for lithium ions are always higher than those for sodium ions in the case of all macrocyclic crown ethers studied at present. The calculated values of binding enthalpy are in good agreement with the reported experimental data.

We have reported fully relaxed geometries, binding energy and binding enthalpies for M+– ether complexes (M+ = Li+ and Na+) based on the density functional theory at 6-311++G (d,p) level using B3LYP exchange functional in the gas phase for crown ether of different ring sizes. It is noticed that the Li+ ion sits at the center but above the oxygen plane of 9C3 ether. However, it fits nicely inside the cavity of 12C4 ether. Sodium ion sits above the oxygen plane of 15C5 ether but fits quite nicely in the cavity of 18C6 ether. For a given metal ion the binding energy increases with the increase in the number of donor oxygen. The calculated values of binding enthalpy (298K) for Li+ and Na+ ions with 12C4 ether are in good agreement with the experimental results. The density functional theory can be suitably applied instead of computationally costly MP2 method for prediction of structures, binding energies and binding enthalpies of metal ion–crown ether complexes. It is noted that binding enthalpy for Li+– crown ether complexes is higher than that for the Na+ complexes. A systematic detailed study on theoretical work considering solvent effect is in progress with the aim to design a suitable macrocyclic ligand for better extraction of alkali metal ions. Results of such studies will be communicated for a journal publication in near future.