15-Hydroxybenzomonothia-15-crown-5 with the sulfur atom linked with the benzene ring and the derived sulfoxide: synthesis, structure, and complexation with the metal cations

Abstract

Novel 15-hydroxybenzomonothia-15-crown-5 containing the sulfur atom linked with the benzene ring and its S-oxide were synthesized. The stability constants for the complexes of the obtained benzocrown ethers and a reference 15-hydroxybenzo-15-crown-5 with Na, Ca, AgI, Cd, HgII, and PbII perchlorates were determined by 1H NMR titration. In MeCN-d3, the benzothiacrown ether demonstrates a high selectivity towards the thio- and oxothiophilic Hg2+ (logK 1 = 7.1) and Pb2+ ions (logK 1 = 7.4). In MeCN-d3-D2O mixtures, the stabilities of the most of complexes decrease sharply due to competitive hydration of the metal cations except for the “soft” Ag+ and Hg2+ ions having low affinity for the “hard” oxygen atoms and, on the contrary, very high affinity for the “soft” SII atoms. This results in the change in selectivity of complexation: at the water content in solution of 20%, the benzothiacrown ether binds preferably the Hg2+ (logK 1 = 5.0) and Ag+ ions (logK 1 = 2.7). In MeCN-d3, the benzothiacrown-derived sulfoxide is a weak and non-selective complexing agent towards all the metal cations under study; the reference 15-hydroxybenzo-15-crown-5 forms more stable complexes with the oxophilic sodium, calcium, and lead(ii) cations. The conformational features of the benzocrown ethers and their metal complexes established by NMR spectroscopy and X-ray diffraction are discussed. The found characteristics of the complexing ability of benzomonothia-15-crown-5 where the sulfur atom is in conjugation with the benzene ring reveal that the macrocyclic ligands with such a structure are promising as high-selective and efficient complexing agents for the “soft” mercury(ii) and silver(i) cations in acetonitrile-water mixtures.