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 [Ru-2(bpy)(4)(mbpibH(2))] (ClO4)(4) (mbpibH(2) 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-, ClO4-, H2PO4-, HCO3-, N-3(-), and SCN- anions in water (without organic solvent) at physiological conditions via formation of multiple hydrogen bonding interaction with binding constants of K-A(H2O) = 345 +/- 21 and K-E(H2O) = 878 +/- 41, respectively. The detection limits of A and E toward CN- in water are 100 and 5 mu M, respectively. Receptor E has an appropriate pK(a2)* 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 center dot center dot center dot 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.