New dithiacrown–ether butadienyl dyes: synthesis, structure, and complex formation with heavy metal cations

Abstract

AbstractBenzothiazole type butadienyl dyes containing a dithia‐15‐crown‐5 (2a) or dithia‐18‐crown‐6 ether (2b) moieties were synthesized. The structures of dyes 2a,b and their complexes with Ag+ and Pb2+ were studied by an X‐ray crystallography. It was found that the conformations of dithiacrown–ether moieties of dyes 2a,b are unfavorable for complex formation and change significantly upon binding of Ag+ or Pb2+. The complexation of 2a,b with Ag+, Cd2+, Pb2+, and Hg2+ in water–acetonitrile mixtures with different contents of water (PW = 0–75%, v/v) was studied by 1H NMR, UV–Vis spectroscopy, and polarography. In anhydrous acetonitrile, the stability constants of 1:1 complexes change in the sequence Cd2+ < Pb2+ ≤ Ag+ << Hg2+ in the case of 2a and in the sequence Cd2+ < Ag+ < Pb2+ << Hg2+ in the case of 2b. As PW increases, the thermodynamic stability of Ag+ complexes increases. The opposite effect is observed for the complexes with Cd2+, Pb2+, and Hg2+. When PW ∼ 50%, the stability constants of complexes with Cd2+ and Pb2+ become too small to be measured. The selectivity of ligands 2a,b toward Hg2+ versus Ag+ is very high at any PW values (selectivity coefficients > 104). The complexation of 2a,b with Hg2+ at PW ≤ 50% is accompanied by a substantial hypsochromic effect. This allows dithiacrown‐containing butadienyl dyes to be used as selective optical molecular sensors for heavy metal ions, in particular, in aqueous solutions. Copyright © 2009 John Wiley & Sons, Ltd.