Addition and Redox Reactivity of Hydrogen Sulfides (H2S/HS-) with nitroprusside: New chemistry of nitrososulfide ligands

SANDRA L. QUIROGA, ALEJANDRA E. ALMARAZ, VALENTÍN T. AMOREBIETA, LAURA L. PERISSINOTTI, JOSÉ A. OLABE

Chemistry A European Journal

Wiley-VCH

Lugar: Weinheim; Año: 2011 vol. 17 p. 4145 - 4156

0947-6539

Abstract: The nitroprusside ion [Fe(CN)5NO]2? (NP) reacts with excess HS?in the pH range 8.5-12.5, in anaerobic medium (?Gmelin? reaction). The progress of the addition process of HS? into the bound NO+-ligand was monitored by stopped-flow UV-vis/EPR, FTIR, mass spectrometry, and chemical analysis. Theoretical calculations were employed for the characterization of the initial adducts and reaction intermediates. The shapes of the absorbance-time curves at 535-575 nm depend on the pH and concentration ratio of the reactants, R = [HS?] / [NP]. The initial adduct [Fe(CN)5N(O)SH]3? (AH, λmax ≈ 570 nm) forms in the course of a reversible process, with kad = 190 ± 20 M?1 s?1, k?ad = 0.3 ± 0.05 s?1. Deprotonation of AH (pKa = 10.5 ± 0.1, at 25.0 ºC, I = 1 M), leads to [Fe(CN)5N(O)S]4? (A, λmax = 535 nm, ε = 6000 ± 300 M?1 cm?1). [Fe(CN)5NO?]3? and HS?22? radicals form through the spontaneous, intramolecular decomposition of AH and A. The minor formation of the [Fe(CN)5NO]3? ion equilibrates with [Fe(CN)4NO]2? through cyanide labilization, and generates the ?g = 2.03? iron-dinitrosyls, [Fe(NO)2(SH)2]?, which are labile toward NO-release. Alternative nucleophilic attack of HS? on AH and A generates reactive intermediates, [Fe(CN)5N(OH)(SH)2]3? and [Fe(CN)5N(OH)(S)(SH)]4?, respectively, which decompose through multielectronic nitrosyl reductions, leading to NH3 and hydrogen disulfide, HS2?. N2O is also produced at pHs ≥ 11. Biological relevance relates to the role of NO, NO? and other bound-intermediates, eventually able to be released to the medium and rapidly trapped by substrates. Structure and reactivity comparisons of the new nitrososulfide ligands with free and bound nitrosothiolates are provided.