Recognition and mechanistic investigation of anion sensing by ruthenium(II) arene complexes and bio-imaging application.

In this work, four new ruthenium complexes [Ru(η--cymene)(L1)Cl] 1, [Ru(η--cymene)(L2)Cl] 2, [Ru(η--cymene)(L3)Cl] 3 and [Ru(η--cymene)(L4)Cl] 4 [HL1 = (2-cyanophenyl)glycine; HL2 = (5-chloro-2-cyanophenyl)glycine; HL3 = (2-cyano-3-fluorophenyl)glycine; HL4 = (4-cyanophenyl)glycine] were synthesized and well characterized by several spectroscopic and analytical techniques. Complexes 1 and 3 were found to be fluorescent in most of the solvents; however, 2 and 4 were found to be fluorescent mostly in EtOAc, DMF and ethanol. Amongst these four complexes, 3 has shown selective sensing against CO and SO anions by quenching of fluorescence. The LOD values are found to be in the sub-micromolar range. Investigations of the sensing mechanism performed by computation and NMR studies indicate a possible adduct formation between the NH group of the ligand and the anion(s) through hydrogen bond formation, which ultimately might lead to proton transfer to the bi-negative anion. The quantum yield of the complex 3 was found to decrease on addition of CO and SO anions from 0.46 to 0.13 and 0.12, respectively. The Job's plot indicates the binding between the probe and anion in a 1 : 1 ratio for both CO and SO anions. Along with that, all the complexes were found to be biocompatible when tested against several cell lines showing very high IC values. It can also be observed that 1 is capable of penetrating within the cells and can act as a cell imaging agent showing fluorescence, and thus can be used for bio-imaging purposes.