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

JOSÉ A. OLABE*

JOURNAL OF THE CHEMICAL SOCIETY, DALTON TRANSACTIONS

RSC

Año: 2008 p. 3633 - 3648

0300-9246

Sodium nitroprusside (SNP, Na2[Fe(CN)5NO].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)5NO]2–, is formed through a reductive nitrosylation reaction of [FeIII(CN)5H2O]2– with NO, or, alternatively, through the coordination of NO2− into [FeII(CN)5H2O]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 [FeIII(CN)5H2O]2–. The more electron rich [Fe(CN)5NO]3– may be prepared from [FeII(CN)5H2O]3– and NO, and is also highly inert toward the dissociation of NO (k = 1.6 ´ 10−5 s−1, 25.0 ºC, pH 10.2). It reacts with O2 leading to SNP, with the intermediacy of a peroxynitrite adduct. The [Fe(CN)5NO]3– ion is labile toward the release of trans-cyanide, forming the [Fe(CN)4NO]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)4NO]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)5HNO]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.