Estudo eletroquímico e fotoquímico da liberação de óxido nítrico nos íons complexos do tipo trans-[Ru(NH3)4LNO]3+, onde L=tionicotinamida e isotionicotinamida

Abstract

Aiming to develop novel nitric oxide (NO) donors to biological systems, it was investigated thionicotinamide and isothionicotinamide as new ligand modulators of metallonitrosyl complexes. Firstly, it was prepared and characterized novel precursor metal complexes: trans-[Ru(NH3)4(tio)SO4]+ and trans-[Ru(NH3)4(isotio)SO4]+. Afterwards, they were used to obtain the nitrosyl compounds trans-[Ru(NO)(NH3)4(tio)]3+ (I) and trans-[Ru(NO)(NH3)4(isotio)]3+ (II). These complexes were characterized by spectroscopic and electrochemical techniques and performed photochemical studies. Infrared spectra of the nitrosyl complexes (I) and (II) exhibited intense vibration bands at 1930 cm-1 and 1924 cm-1, respectively, which are assigned to nitric oxide coordinated to ruthenium as NO+. Chemical shifts and hydrogen splitting in the 1H-NMR spectra of nitrosyl complexes are consistent with the spectrum of thionicotinamide and isothionicotinamide ligands. Additionally, it suggested those ligands are coordinated to ruthenium through their pyridinic nitrogen. Cyclic voltammograms of the ion complexes trans-[Ru(NH3)4TioSO4]+ and trans-[Ru(NH3)4IsotioSO4]+ showed the appearance of an additional signal after successive sweep cycles. This additional electrochemical process was assigned to the formation of the aquo complexes, which is in agreement with an electrochemical-chemical mechanism as reported in the literature eletrochemical-chemical-electrochemical (ECE) mechanism reported in literature. The nitrosyl complexes trans-[Ru(NO)(NH3)4(tio)]3+ and trans-[Ru(NO)(NH3)4(isotio)]3+ exhibited an electrochemical processes at -236 and -286 mV vs Ag/AgCl, respectively. These processes have been characterized as the first NO+ reduction to NO0 form, as a result of addition of an electron in the * orbital. Additional electrochemical processes centered at the RuIII/II were only observed upon formation of the aquo species, which was obtained after successive multieletronic reductions at cathodic potentials below –900 mV. The interconversion studies of nitrosyl to nitrite in the nitrosyl complex ions were investigated using spectrophotometric method. It was showed for complexes (I) and (II) that half of the species containing NO+ had suffered hydroxyl nucleophilic attack generating NO2- only at pH 9.9 and 9.8, respectively. Preliminary photochemical studies of complexes I and II were carried out and final products characterized by electrochemical techniques. Irradiated samples of the nitrosyl complexes trans-[Ru(NO) (NH3)4(tio)]3+ and trans-[Ru(NO)(NH3)4(isotio)]3+ showed the formation of the RuIII/II couple, which matches the electrochemical potential for the aquo complexes. The quantum yield for these complexes were calculated indicating the release of nitric oxide in solution