Nitrosyl reaction

It was also found that N02 accelerated the observed rates of reductive nitrosylation (kso, = 3.1 0.1 M 1 s 1 in 16mM acetate at pH 4.96) (69). Since nitrite is a product of the reductive nitrosylation reaction in aqueous solution, the system is, in principle, autocatalytic. 

Finally, a single example of desulfurization, using a tertiary phosphine, to effect cluster transformation, has been reported (26) for a heterometallic nitrosyl. Reaction of complex 18 f(f/5-C5H4Me)2V2S4Fe-(NO)2] with tributylphosphine provided [(f/5-C5H4Me)2V2S3Fe(NO)2], of unknown constitution, in 80% yield. 

In this section the bonding modes of nitric oxide have been described. We next focus on the reactivity patterns that the modes of nitrosyl coordination impart to the bound nitric oxide molecule, and how they have been observed in nitrosyl reaction chemistry. 

Studies on nitrosylation reactions of metalloporphyrins are emerging and have been reviewed. 21 Reactions involving FeNO 7 and FeNO 6 species (considered as ferroheme and ferriheme, respectively) are crucial to enzymatic functions (activation of guanylyl cyclase, cytochrome oxidase, catalase inhibition). Reversible photodissociations of NO from nitrosyl metalloporphyrins have been studied by ns-pulsed laser techniques, providing values for kt and kA (Equation (12)). Dissociation processes are very slow, particularly for ferroheme complexes however, kA values in the 10-5—101 s 1 range have been measured for several five-coordinated FeNO 7 tetraarylpor-phyrins.103 The spread in the kA values is not well understood yet. 

Complexes of the MNO 6 class are most commonly found and can be prepared by a direct mixing of NO with Fe(III) precursors5,8 in a reductive nitrosylation reaction (Equation 7.1) ... 

NO is a radical species, but unlike most other radicals, it is not particularly reactive. However, it does react very rapidly with iron to give well authenticated iron-nitrosyls. Reaction with iron is responsible for the activation of guanylate cyclase it may also be responsible for the cytotoxicity of NO and it has been incorporated into a number of analytical procedures for the detection and quantitation of NO. 

Fe(II) porph3uins (ca. 10 M s ) and very slow NO dissociation (70). On the other hand, FeNO species show lower association rates (ca. 10 M s ) and significantly higher NO dissociation rates (between 1 and 500s ) (70). A final very important point concerning the FeNO complexes is their reactivity toward nucleophiles, usually OH and nitrite, which results in the reduction of the porphyrin metal center and the oxidation of NO to N02 in a process called reductive nitrosylation. The net outcome of the reductive nitrosylation reaction is the formation of FeNO species, when excess NO reacts with ferric porphyrins (71,72). For a recent review of the above described reactions, see the review by Ford and coworkers (61). 

In siunmary, while iron porphyrins cannot discriminate NO from HNO due to the reductive nitrosylation reaction, both Mn and Co porph5uins tend to differentiate NO from HNO Co(II) and Mn(II) porphyrins react rapidly with NO but not witb HNO, while Co(III) and Mn(III) porphyrins react rapidly with HNO but not with NO. On the other hand, Mn porphyrins tend to show an important shift in the UV-vis spectra (Soret band) when going from Mn(III) to Mn(II)NO porphyrins, while Co and Fe porph5rrins do not (see Scheme 2). 

In spite of the above mechanistic insight into the initial stages of NO coordination to the ferriheme systems, the fact is that the so-called reductive nitrosylation reaction has been observed for the water-soluble ferri-heme model Fe(III)TPPS in aqueous solution (H2TPPS = tetraanionic form of meso-tetrakis(p-sulfonatophenyl)por-phyrin), as well as for Cyt, metMb and metHb reacting with NO at various pHs, even in slightly acidic media, pH range 6-8 (25,50). Figure 4 describes the proposed reaction scheme for the overall reductive nitrosylation process. 

The products of aU the nitrosylation reactions described and analyzed so far for both the heme- or nonheme complexes have been consistently described as diamagnetic low-spin (Fe NO ) species, independently of the high-spin or low-spin nature of the Fe(III) reactants. Strikingly, there is an evident contrast between the values of feoff that measure the lability of NO, between moderately labile heme and the undoubtedly inert nonheme nitrosylated... 

Nitrosylation reactions of iron(ll) aqua and chelate complexes 200... 

In order to prove the vahdity of the above statement, the rate and mechanism of nitrosylation reactions were further examined, for example, for the (TMPS)Fe (L) system in which the number and chemical nature of the axial hgands (L), as well as the spin state of the iron(III) center, were carefully tuned by the selection of appropriate reaction conditions. Thus, the mono-hydroxido species, Fe (TMPS)(OH), was obtained in basic aqueous solution (at pH>pK, of coordinated water in (TMPS)Fe(H20)2) (J2) Fe (TMPS)(CN)(H20) and Fe (TMPS)(CN)2 were formed in situ under conditions of small and large excess of CN, respectively (16) and Fe (TMPS)(OH)(MeIm) was generated in [emim][NTf2] (ionic liquid) containing trace amounts of Melm (17). The nitrosylation reactions of all above Fe (TMPS) derivatives were investigated as a function of temperature and pressure. The mechanistic conclusions drawn from the obtained activation parameters are presented below. 

Since the previous study has postulated that the mechanism of nitroprusside formation from nitric oxide and [Fe (CN)5(Fi20)] involves the reduction of the latter species to [Fe (CN)5(H20)], it was essential to examine the binding properties and reactivity of nitric oxide toward the reduced form of the pentacyanoferrate complex. In this context, the nitrosylation reaction of [Fe (CN)5(H20)] (Equation 4) was investigated kineticaUy and mech-... 

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