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

The aqueous decomposition of the iridium coordinated nitrosothiols (RSNOs) trans-K[IrCl(4)(CH(3)CN) NOS-Ph] (1), and K(2)[IrCl(5)(NOECyS)] (2, ECyS = cysteine ethyl ester), was studied by MS analysis of the gaseous products, ESI-MS, NMR, and UV-Vis spectroscopy. Bent NO (NO , nitroxyl anion), sulfenic acids and nitrite were observed as coordinated products in solution, while nitrous oxide (N(2)O) and nitrogen were detected in the gas phase. The formation of coordinated NO and N(2)O, a nitroxyl dimerization product, allows us to propose the formation of free nitroxyl (HNO) as an intermediate. Complex 1 decomposes 300 times slower than free PhSNO does. In both cases (1 and 2) kinetic results show a first order decomposition behavior and a very negative Delta S(not equal), 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 H(2)O on to the sulfur atom of the coordinated RSNO, producing an NO complex and free sulfenic acid, followed by two competing reactions: a ligand exchange reaction of this NO with the sulfenic acid or, to a minor extent, coordination of N(2)O to produce an NO / N(2)O complex which finally renders free N(2) and coordinated NO(2) . Some of the produced NO is likely to be released from the metal center producing nitroxyl by protonation and finally N(2)O by dimerization and loss of H(2)O. In conclusion, the decomposition of these coordinated RSNOs occurs through a different mechanism than for the decomposition of free RSNOs. It involves the formation of sulfenic acids and coordinated NO , which is released from the complexes and protonated at the reaction pH producing nitroxyl (HNO), and ultimately N(2)O. (C) 2010 Elsevier B.V. All rights reserved.36618590CONICETUBA [UBACYT X065]ANPCyT [PICT 2006-2396]UBA [UBACYT X065]ANPCyT [PICT 2006-2396