Ruthenium(II) complexes of thiosemicarbazones: Synthesis, X-ray crystal structures, spectroscopy, electrochemistry, DFT studies and fluoride sensing properties

Six Ru(II) complexes with formula [Ru(terpy)(L)]X (L = tridentate N,N,S- donor thiosemicarbazone ligands, X = HE) have been synthesized and characterized by using standard analytical and spectroscopic techniques. The X-ray crystal structures of the complexes [Ru(terpy)(L-2)]PF6(2) and [Ru(terpy)(L-3)]PF6(3) have been determined. It was found that during complexation the hydroxyimino functionality of the parent ligands is converted to imine, which participates in coordination through the N-atom. The complexes are found to undergo two quasi-reversible oxidations on the positive side of the potential window (0 to +0.8V) and three successive quasi-reversible/irreversible reductions on the negative side of the potential window (0 to -2V). The fluoride sensing properties of all the complexes have been studied in solution using absorption spectra and H-1 NMR studies, as well as by cyclic voltammetric (CV) measurements. From the absorption titration studies the association constants and detection limit values of fluoride by the complexes have been determined. The association constant values were found to be reasonably high (logK(a) > 5) for complexes 3-6, and these values are one order of magnitude greater than those observed for the corresponding free thiosemicarbazones. Selective fluoride sensing by these complexes is also signaled by the development of vivid colors, visible with the naked eye. 2017 Elsevier B.V. All rights reserved.