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

Benzothiazole 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 Pb 2+ 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 Pb 2+ . The complexation of 2a , b with Ag + , Cd 2+ , Pb 2+ , and Hg 2+ in water–acetonitrile mixtures with different contents of water ( P W = 0–75%, v/v) was studied by 1 H NMR, UV–Vis spectroscopy, and polarography. In anhydrous acetonitrile, the stability constants of 1:1 complexes change in the sequence Cd 2+ < Pb 2+ ≤ Ag + << Hg 2+ in the case of 2a and in the sequence Cd 2+ < Ag + < Pb 2+ << Hg 2+ in the case of 2b . As P W increases, the thermodynamic stability of Ag + complexes increases. The opposite effect is observed for the complexes with Cd 2+ , Pb 2+ , and Hg 2+ . When P W ∼ 50%, the stability constants of complexes with Cd 2+ and Pb 2+ become too small to be measured. The selectivity of ligands 2a , b toward Hg 2+ versus Ag + is very high at any P W values (selectivity coefficients > 10 4 ). The complexation of 2a , b with Hg 2+ at P W ≤ 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.