The investigation of complexation properties and hard-soft acid-base relationship between thiacrown ethers and metal ions
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Background: There are numerious work on thiacrown ethers that are utilized for binding metal ions. Usually, relative binding ratios have been reported for few selected cations in these works. This study aims to develop a new method to explain cation binding properties by using the hardness parameters for ligands and metal ions, heteroatom rate (S/O) for ligands and extraction percentages. Methods: In this work, complexation constants and selectivity factors of a previously synthesized mixed donor soft bases of thia-oxacrown ethers with metal salts hard Lewis acids (Mg2+, Ca2+, Cr3+), intermediate Lewis acids (Zn2+, Pb2+, Fe3+, Co2+), and soft Lewis acid (Ag+) were determined in a chloroform-dichloromethane by using a liquid-liquid ion pairs metal extraction method. Results: The synthesized compounds were 1,10-dithia-18-crown-6 (B1), 1,7-dithia-15-crown-5 (B2), 1,7-dithia-12-crown-4 (B3), 1,7-dithia-18-crown-6 (B4), 1,10-dithia-21-crown-7 (B5) and 1,4-dithia-12crown-4 (B6). Based on various experiments using the aforementioned extraction method, following results have been observed: first, calcium ion has been the most extractable ion for all ligands in chloroform. At the same time, the most selective ligands for Zn+2, Mg2+, Ca2+, Fe3+, Cr3+, Co2+ and Pb2+ in dichloromethane are B1, B1, B1, B1, B5, B5, B4-B5, respectively. Besides these results, in competitive liquid-liquid ion pairs metal extraction studies, all of the ligands have exhibited a large selectivity for iron and zinc ions. Conclusion: In the light of results, one can conclude that the increase in the heteroatom rate in thiacrown ethers softens the ligands; therefore, the ligands complex with soft metals selectively.