Selective Recognition of Cyanide Anion via Formation of Multipoint NH and Phenyl CH Hydrogen Bonding with Acyclic Ruthenium Bipyridine Imidazole Receptors in Water.

Five imidazole-based anion receptors A–E are designed for cyanide anion recognition via hydrogen bonding interaction in water. Only receptors A [Ru(bpy)2(mpipH)](ClO4)2 (bpy is bipyridine and mpipH is 2-(4-methylphenyl)-imidazo[4,5-f]-1,10-phenanthroline) and E [Ru2(bpy)4(mbpibH2)](ClO4)4 (mbpibH2 is 1,3-bis([1,10]-phenanthroline-[5,6-d]imidazol-2-yl)-benzene) selectively recognize CN– from OAc–, F–, Cl–, Br–, I–,NO3–, HSO4–, CIO4–, H2PO4–, HCO3–, N3–, and SCN– anions in water (without organic solvent) at physiological conditions via formation of multiple hydrogen bonding interaction with binding constants of KA(H2O) = 345 ± 21 and KE(H2O) = 878 ± 41, respectively. The detection limits of A and E toward CN– in water are 100 and 5 μM, respectively. Receptor E has an appropriate pKa2* value (8.75) of N–H proton and a C-shape cavity structure with three-point hydrogen bonding, consisting of two NH and one cooperative phenyl CH hydrogen bonds. Appropriate acidity of N–H proton and multipoint hydrogen bonding are both important in enhancing the selectivity and sensitivity toward CN– in water. The phenyl CH···CN– hydrogen bonding interaction is observed by the HMBC NMR technique for the first time, which provides an efficient approach to directly probe the binding site of the receptor toward CN–. Moreover, CN– induced emission lifetime change of the receptor has been exploited in water for the first time. The energy-optimized structure of E–CN adduct is also proposed on the basis of theoretical calculations. [ABSTRACT FROM AUTHOR]