Roussin s red ester

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

Liaw et al. reported that conversions between the neutral sparteine [Fe(NO)2] complex 133 and the anionic Fe(NO)2 [Fe(NO)2(S2C3Hg)] 137 proceed via the cationic sparteine Fe(NO)2 -complex 135 through oxidation by NO" " and transfer of the [Fe(NO)2] -unit to the chelating ligand S-(CH2)3-S 136 (Scheme 35). The resulting anionic complex 137 is stable in contrast to the cationic complex 135. The cationic complex 135 also acts as a [Fe(NO)2] donor in the presence of the DNIC [(PhS)2Fe(NO)2] 138 to yield Roussin s red ester 139. The bidentate thiol ligand S-(CH2)3-S 136 promotes the stability of the anionic DNIC Fe(NO)2 ... 

Francois-Zacharie Roussin (1827-1894) discovered the first polynuclear cluster compound of nitric oxide as a result of his study of the reactions of iron(II) sulphate, ammonium sulphide and nitrous acid (HNO2), and this black crystalline material was subsequently characterised as a salt of [Fe4S3(NO)7] (Roussin s black salt), 2. It may be converted into a red salt on treatment with alkaline, which reacts with ethyl bromide to give Roussin s red ester, [Fe2(SEt)2(NO)4], 3 [7]. 

DNICs can exist in equilibrium with their dimeric analogs, Roussin s red esters [108]. Several factors including solvent polarity, concentration, and the nature of the thiolate ligand can influence which species predominates in solution [49, 68, 109-112]. In the case of LMW thiolate DNICs, an excess of ligand (usually ca. 20-fold) is required to maintain the mononucleated form. If the concentration of thiol compound is low, the ions are promptly condensed into a binuclear RRE structure via formation of two RS-bridges and release of other two RS-ligands, according to Eq. (1). 

Lu TT, Tsou CC et al (2008) Anionic Roussin s Red Esters (RREs) syn-/anti-[Fe(p-SEt) (N0)2] the critical role of thiolate ligands in regulating the transformation of RREs into dinitrosyl iron complexes and the anionic RREs. Inorg Chem 47 6040-6050... 

Tsai ML, Hsieh CH, Liaw WF (2007) Dinitrosyl iron complexes (DNICs) containing S/N/0 ligation transformation of Roussin s red ester into the neutral Fe(NO)2 DNICs. Inorg Chem 46 5110-5117... 

Lu TT, Huang HW, Liaw WF (2009) Anionic mixed thiolate-sulfide-bridged Roussin s red esters [(NO)2Fe(p-SR)(p-S)Fe(NO)2] (R = Et, Me, Ph) a key intermediate for transformation of dinitrosyl iron complexes (DNICs) to [2Fe-2S] clusters. Inorg Chem 48 9027-9035... 

Tsai ML, Liaw WF (2006) Neutral Fe(NO)2 dinitrosyliron complex (DNIC)[ (SCgH4-o-NHCOPh)(Im)Fe(NO)2] (Im = Imidazole) interconversion among the anionic/neutral Fe(NO)2 DNICs and Roussin s red ester. Inorg Chem 45 6583-6585... 

Chen YJ, Ku WC et al (2008) Nitric oxide physiological responses and delivery mechanisms probed by water-soluble Roussin s red ester and Fe(NO)2) DNIC. J Am Chem Soc 130 10929-10938... 

Salts containing the anions [Fe2S2(NO)4]2 and [Fe4S3(NO)7] are often called, from their colors, Roussin s red and black salts, respectively similarly the organic derivatives [Fe2(SR)2(NO)4] are often called esters of Roussin s red salt. These names are nonsystematic and, in the case of [Fe2(SR)2(NO)4], scarcely accurate nonetheless they are convenient and they carry the sanction of long usage, and will on occasion be used here. 

Wecksler SR, Mikhailovsky A, Korystov D, Buller F. Single- and two-photon properties of a dye-derivatized Roussin s red salt ester (Fe2( i-RS)2(NO)4) with a large TPA cross section. Inorg Chem 2007 46 395-402. 

The most familiar complexes in this class are Roussin s red and black salts140 obtained as a mixture from the reaction of iron(III) sulfide with nitric oxide and an alkali metal sulfide.141 The dimeric and diamagnetic red salt, K2[Fe(NO)2S]2 gives rise to a series of stable esters [Fe(NO)2SR]2 (R = Me, Et, Ph etc.) which are also diamagnetic, dark red and soluble in organic solvents. Some preparative routes142 to the red salt and its esters are shown in Scheme 3. 

Special attention was also paid to understanding the basic properties and reactivity patterns of the dinitrosyl iron complexes and Roussin s red salt esters, and to resolve the mechanism of their biological functions and NO emission (43-47). The quantum yield of NO release from [Fe2(p2-SR)2(NO)4] under visible light irradiation can be considerably enhanced by incorporating into the ester a proper chromophore then single- or two-photon processes were observed (46,48,49). 

Wang R, Camacho-Femandez MA et al (2009) Neutral and reduced Roussin s red salt ester [Fe2 p-RS>2(NO)4] (R = N-Pr, T-Bu, 6-Methyl-2-Pyridyl and 4,6-Dimethyl-2-Pyrimidyl) synthesis. X-ray crystal structures, spectroscopic, electrochemical and density functional theoretical investigations. Dalton Trans 777-786... 

Thomas JT, Robertson JH, Cox EG (1958) The crystal structure of Roussin s red ethyl ester. Acta Crystallogr 11 599-604... 

CftHioClftHgaS, Diethyl sulfide mercury chloride, 29, 632 CftHi oFe2N40i,S2 f Roussin s red ethyl ester, 22, 579 CftHioHgS2, Mercury ethylmercaptide, 5, 142, 151 30B, 374 CftHioNftNiS2 f Bis(thioacethydrazidato)nickel(II), 35B, 788 CftHioNi02S2, Cyclohexakis(bis-(M 1-hydroxyethane-2-thiolato)-nickel-(II)), 35B, 788... 

Figure 3.4 Iron-sulfur nitrosyl compounds (a) Roussin s black salt and (b) Roussin s methyl red ester.

Figure 25.6 The stnicmre of Roussin s salts (a) the ethyl ester [Fe(NO>2SEt]2 of the red salt showing pseudo-tetrahedral coordination of each iron (Fe-Fe = 272 pm), and (b) the anion of the black salt Cs[Fe4(N0)7S3].H20 showing a pyramid of 4 Fe atoms with an S atom above each of its three non-horizontal faces (Fejp —Fhast = 271 pm, Fei 5c Fcbas 357 pm). (The anion may alternatively be viewed as an FesSj ring with the chair" conformation.) Note that even the short Fe-Fc distances are appreciably greater than the Fe-Fe "single-bond distance of 250pm.

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