Fluorometric chemosensors. Interaction of toxic heavy metal ions Pb^II, Cd^II, and Hg^II with novel mixed-donor phenanthroline-containing macrocycles: Spectrofluorometric, conductometric, and crystallographic studies

The macrocycles L¹-L³ incorporating N₂S₃-, N₂S₂O-, and N₂S₂-donor sets, respectively, and containing the 1,10-phenanthroline unit interact in acetonitrile solution with heavy metal ions such as Pb^II, Cd^II, and Hg^II to give 1:1 ML, 1:2 ML₂, and 2:1 M₂L complex species, which specifically modulate the photochemical properties of the ligands. The stoichiometry of the complex species formed during spectrofluorometric titrations and their formation constants in MeCN at 25 °C were determined from fluorescence vs M^II/L molar ratio data. The complexes [Pb(L¹)][ClO₄]₂· 1/2H₂O (1), [Pb(L²)][ClO₄]2·MeNO₂ (1a), [Pb(L³)₂][ClO₄]₂·2MeCN (1b), and [Cd(L³)][NO₃]₂ (2b) were also characterized by X-ray diffraction studies. The conformation adopted by L¹-L³ in these species reveals the aliphatic portion of the rings folded over the plane containing the heteroaromatic moiety with the ligands trying to encapsulate the metal center within their cavity. In 1, 1a, and 2b the metal ion completes the coordination sphere by interacting with counteranion units and solvent molecules. On the contrary, the 1:2 complex 1b shows Pb^II sandwiched between two symmetry-related molecules of L3 reaching an overall [4N + 4S] eight-coordination.