The coordination chemistry of nitrosyl in cyanoferrates. An exhibit of bioinorganic relevant reactions

Abstract

Sodium nitroprusside (SNP, Na₂[Fe(CN)₅(NO)]·2H₂O) is a widely used NO-donor hypotensive agent, containing the formally described nitrosonium (NO⁺) ligand, which may be redox-interconverted to the corresponding one-electron (NO) and two-electron (NO⁻/HNO) reduced bound species. Thus, the chemistry of the three nitrosyl ligands may be explored with adequate, biologically relevant substrates. The nitrosonium complex, [Fe(CN)₅(NO)]²⁻, is formed through a reductive nitrosylation reaction of [Fe^III(CN)₅(H₂O)]²⁻ with NO, or, alternatively, through the coordination of NO₂⁻ to [Fe^II(CN)₅(H₂O)]³⁻ and further proton-assisted dehydration. It is extremely inert toward NO⁺-dissociation, and behaves as an electrophile toward different bases: OH⁻, amines, thiolates, etc. Also, SNP releases NO upon UV-vis photo-activation, with formation of [Fe^III(CN)₅(H₂O)]²⁻. The more electron rich [Fe(CN)₅(NO)]³⁻ may be prepared from [Fe^II(CN)₅(H₂O)]³⁻ and NO, and is also highly inert toward the dissociation of NO (k = 1.6 × 10⁻⁵ s⁻¹, 25.0 °C, pH 10.2). It reacts with O₂ leading to SNP, with the intermediacy of a peroxynitrite adduct. The [Fe(CN)₅(NO)]³⁻ ion is labile toward the release of trans-cyanide, forming the [Fe(CN)₄(NO)]²⁻ ion. Both complexes exist in a pH-dependent equilibrium, and decompose thermally in the hours time scale, releasing cyanides and NO. The latter may further bind to [Fe(CN)₄(NO)]²⁻ with formation of a singlet dinitrosyl species, [Fe(CN)₄(NO)₂]²⁻, which in turn is unstable toward disproportionation into SNP and N₂O, and toward the parallel formation of a tetrahedral paramagnetic dinitrosyl compound, [Fe(CN)₂(NO)₂]. Emerging studies with the putative nitroxyl complex, [Fe(CN)₅(HNO)]³⁻, should allow for a complete picture of the three nitrosyl ligands in the same pentacyano fragment. The present Perspective, based on an adequate characterization of structural and spectroscopic properties, will focus on the kinetic and mechanistic description of the above mentioned reactions, which display a versatile scenario, fundamentally related to the biologically relevant processes associated with NO reactivity.