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)](ClO(4))(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))](ClO(4))(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(-), NO(3)(-), HSO(4)(-), ClO(4)(-), H(2)PO(4)(-), HCO(3)(-), 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 µM, 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···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.