Chemosensing of Chloride Based on a Luminescent Platinum(II) NCN Pincer Complex in Aqueous Media

The new luminescent platinum­(II) pincer complex [Pt­(NCN)­(S)]­TfO (I; NCN = 1,3-bis­(2-N-phenylbenzimidazolyl)­benzene, S = solvent, and TfO^– = triflate anion) was synthesized and studied as a chemosensor for chloride in aqueous media. In 50 vol % aqueous DMF or CH₃CN chloride quenches the fluorescence with association constants of 1.2 × 10³ and 81 M^–1, respectively. On the other hand, in the micellar medium of cetyltrimethylammonium hydrogensulfate at pH 7.0 additions of inorganic anions to I enhance the fluorescence with a pronounced selectivity toward chloride, which shows also much tighter binding to the receptor with association constant 7.9 × 10⁴ M^–1 in comparison to that in mixed organic solvents. On basis of ¹H NMR titration experiments and the crystal structure of the neutral chloro complex of I the binding mode of chloride is proposed involving the coordination of chloride to the Pt­(II) atom with simultaneous formation of intramolecular short C–H···Cl–Pt contacts. The combination of the cyclometalated platinum complex I with a cationic surfactant allows for the detection of chloride in the micromolar concentration range in samples of bottled mineral water.