Experimental and Theoretical Studies on the Synthesis, Electronic Structure and Biphasic Extraction Properties of Functionalized Thia-Crown Ethers

Abstract In the present study, three thia-crown ether derivatives (AE1, AE2, AE3) were synthesized using diacyl chlorides and tri(ethylene glycol) dithiol, and characterized by infrared, proton nuclear magnetic resonance, and mass spectrometry. Their metal complexation properties with K+, Na+, Ag+, Ca2+, Mg2+, Zn2+, and Fe2+ were investigated using liquid–liquid ion-pair extraction in chloroform and dichloromethane. Extraction efficiencies, equilibrium constant, and Gibbs free energies were calculated. AE1 and AE2 exhibited the highest extraction efficiencies for K+ and Zn2+ (>95%), whereas Na+ and Ca2+ showed the lowest. The solvent type significantly influenced extraction selectivity, with dichloromethane enhancing K+ selectivity. The results correlated well with the hard–soft acid–base principle, where AE3, having the highest hardness, favored harder metal ions, while AE1 and AE2, being softer, preferred softer or borderline cations. Density Functional Theory calculations supported the experimental findings, providing insights into ligand electronic structures through frontier molecular orbital energy gaps and global hardness values. The study demonstrates that structural differences among ligands and solvent polarity critically affect metal ion selectivity and efficiency.