Ruthenium(II/III) complexes of redox non-innocent bis (thiosemicarbazone) ligands: Synthesis, X-ray crystal structures, electrochemical, DNA binding and DFT studies

Six tetradentate bis-thiosemicarbazone ligands formed by condensation of pyruvaldehyde with 4-(aryl) thiosmicarbazides and their Ru(II) complexes are reported. The complexes are of formula [Ru (PPh3)(2)(L)], where L is the dianionic form of the bis-thiosemicarbazone ligands. X-ray crystal structures of two of the complexes suggest that the thiosmicarbazone ligands act in a tetradentate fashion occupying the basal plane of the Ru(II) coordination octahedron and each of the two thiosemicarbazone moiety forms five membered chelate ring involving deprotonated thiolato sulfur and the imine nitrogen atoms; the two triphenylphosphine ligands are trans to each other. The complexes undergo Ru(II)/Ru(III) oxidation at a relatively low potential of 0.05-0.12 V versus Ag/AgCl. One of the Ru(II) complex has been chemically oxidized to the corresponding Ru(III) complex [Ru(PPh3)(2)(L)]PF6 using ferrocenium hexafluorophosphate as an oxidizing agent. E.P.R. spectra of the Ru(III) complex and DFT calculations on the Ru(III) as well as two of the Ru(II) complexes suggest that there is extensive mixing between the ligand and metal valence orbitals. The DNA-binding properties of the Ru(II) complexes were studied by both IJV-Vis spectroscopy and fluorescence quenching of ethidium bromide-DNA complex. The complexes showed strong DNA -binding ability with K-app of the order of 10(6) M-1. (C) 2017 Elsevier Ltd. All rights reserved.