Coordinated nitroxyl anion is produced and released as nitrous oxide by the decomposition of iridium-coordinated nitrosothiols

KAZHDAN, D.; PERISSINOTTI, L.L.; WATANABE, B.; EBERLIN, M.N.; MILAGRE, H.; B.G. VAZ; ESTRIN, D.A.; DOCTOROVICH, F.

INORGANICA CHIMICA ACTA

ELSEVIER SCIENCE SA

Año: 2011 vol. 366 p. 85 - 90

0020-1693

The aqueous decomposition of the iridium coordinated nitrosothiols (RSNOs) trans-K[IrCl4(CH3CN)NOS-Ph] (1), and K2[IrCl5(NOECyS)] (2, ECyS = cysteine ethyl ester), was studied by MS analysis of the gaseousproducts, ESI-MS, NMR, and UV–Vis spectroscopy. Bent NO (NOÀ, nitroxyl anion), sulfenic acids andnitrite were observed as coordinated products in solution, while nitrous oxide (N2O) and nitrogen weredetected in the gas phase. The formation of coordinated NOÀ and N2O, a nitroxyl dimerization product,allows us to propose the formation of free nitroxyl (HNO) as an intermediate. Complex 1 decomposes300 times slower than free PhSNO does. In both cases (1 and 2) kinetic results show a first order decom-position behavior and a very negative DS– , which strongly indicates an associative rate-determining step.A proposed decomposition mechanism, supported by the experimental data and DFT calculations,involves, as the first step, nucleophilic attack of H2O on to the sulfur atom of the coordinated RSNO, pro-ducing an NOÀ complex and free sulfenic acid, followed by two competing reactions: a ligand exchangereaction of this NOÀ with the sulfenic acid or, to a minor extent, coordination of N2O to produce an NOÀ/N2O complex which finally renders free N2 and coordinated NO2À. Some of the produced NOÀ is likely tobe released from the metal center producing nitroxyl by protonation and finally N2O by dimerization andloss of H2O. In conclusion, the decomposition of these coordinated RSNOs occurs through a differentmechanism than for the decomposition of free RSNOs. It involves the formation of sulfenic acids and coor-dinated NOÀ, which is released from the complexes and protonated at the reaction pH producing nitroxyl(HNO), and ultimately NO.