Addition and Redox Reactivity of Hydrogen Sulfides (H 2 S/HS − ) with Nitroprusside: New Chemistry of Nitrososulfide Ligands

The nitroprusside ion [Fe(CN) 5 NO] 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 and FTIR spectroscopy, 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) 5 N(O)SH] 3− ( AH , λ max ≈570 nm) forms in the course of a reversible process, with k ad =190±20  M −1  s −1 , k ‐ad =0.3±0.05 s −1 . Deprotonation of AH (p K a =10.5±0.1, at 25.0 °C, I =1  M ), leads to [Fe(CN) 5 N(O)S] 4− ( A , λ max =535 nm, ε =6000±300  M −1  cm −1 ). [Fe(CN) 5 NO] . 3− and HS 2 . 2− radicals form through the spontaneous decomposition of AH and A . The minor formation of the [Fe(CN) 5 NO] 3− ion equilibrates with [Fe(CN) 4 NO] 2− through cyanide labilization, and generates the “ g =2.03” iron–dinitrosyl, [Fe(NO) 2 (SH) 2 ] − , which is labile toward NO release. Alternative nucleophilic attack of HS − on AH and A generates the reactive intermediates [Fe(CN) 5 N(OH)(SH) 2 ] 3− and [Fe(CN) 5 N(OH)(S)(SH)] 4− , respectively, which decompose through multielectronic nitrosyl reductions, leading to NH 3 and hydrogen disulfide, HS 2 − . N 2 O is also produced at pH≥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.