Nitrosyl complexes electron-transfer reactions

The nitrosylation of [Fe (CN)5(N02)] led to a particularly interesting result a notoriously fast conversion to nitroprusside was observed in the stopped-flow time scale. As E° for the [Fe" (CN)5(N02)] couple is also 0.4 V (83), we can still anticipate similar rates for the encounter-complex formation and the electron transfer reaction steps (analogs of 4-5). However, fe Kio2- cannot be high enough to account for the fast conversion to final products (its rate constant should be comparable to fe py, s ). Instead, the final step might involve a fast proton-assisted N02 /N0 interconversion (cf Section 2.2.1), which would yield the product without rupture of the initial Fe —N02 bond ... 

Porphyrin-nitrosyl complexes with six other metal ions are also known, and all but one of which has been electrochemically investigated. These are Ru [69, 73, 94-96], Os [5], Rh[97], Cr [98], Mo [99] and Mn [100]. Some nitrosyl metalloporphyrins can be reversibly reduced or oxidized by one or two electrons without loss of the NO ligand and this generally occurs when the electrode reactions involve the 7T-conjugated macrocycle in the case of a metal-centered reduction or oxidation, however, the electron-transfer reactions will most often be accompanied by a loss of the NO ligand, resulting in an irreversible oxidation as shown in Fig. 7 for the case of (TPP)Cr(NO) and (TPP)Mn(NO) in CH2CI2. 

Two major pathways have been shown to exist in nitrite reduction [274]. In the first pathway, nitrite is reduced to NO, while in the second there is a direct conversion of nitrite to NH3 or NH4" ". Two classes of nitrite reductase (NIR), namely the cytochromes cd [274], and the copper nitrite reductase [274], have been identified for the first pathway and two classes of enzyme, namely the siroheme nitrite reductase and cytochrome c nitrite reductase, have been proposed to follow the second pathway. The mechanism of these four enzymes has been recently reviewed [274], and only a brief summary of the electron-transfer reactions of cytochrome cd nitrite reductase will be given here. The initial step in the conversion of NO2 to NO involves a binding of the nitrite ion to the metal of the reduced heme. This first step is followed by the uptake of two protons and the loss of one water molecule to yield an electrophilic ferrous pe +-NO+ species, also formulated as a pe +-NO" complex. The dissociation of NO from this species produces the ferric heme d, which is in turn reduced back to its original state by heme c. Why the eri2yme does not reduce the nitrosyl species, Fe -]s[0 or Fe -NO to its Fe -NO form, prior to dissociation of NO in the heme, has been discussed in the literature [274], and may... 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

The electrochemical transformation of a molybdenum nitrosyl complex [Mo(NO)(dttd)J [dttd = 1,2-bis(2-mercaptophenylthio)ethane] (30) is rather interesting (119). Ethylene is released from the backbone of the sulfur ligand upon electrochemical reduction. The resulting nitrosyl bis(dithiolene) complex reacts with O2 to give free nitrite and a Mo-oxo complex. Multielectron reduction of 30 in the presence of protons releases ethylene and the NO bond is cleaved, forming ammonia and a Mo-oxo complex (Scheme 15). The proposed reaction mechanism involves successive proton-coupled electron-transfer steps reminiscent of schemes proposed for Mo enzymes (120). 

The features of initiation of free radical reactions in polymers by dimers of nitrogen dioxide are considered. The conversion of planar dimers into nitrosyl nitrate in the presence of amide groups of macromolecules has been revealed. Nitrosyl nitrate initiates radical reactions in oxidative primary process of electron transfer with formation of intermediate radical cations and nitric oxide. As a result of subsequent reactions, nitrogen-containing radicals are produced. The dimer conversion has been exhibited by estimation of the oxyaminoxyl radical yield in characteristic reaction of p-benzoquinone with nitrogen dioxide on addition of aromatic polyamide and polyvinylpyrrolidone to reacting system. The isomerisation of planar dimers is efficient in their complexes with amide groups, as confirmed by ab initio calculations. 

The reaction was first-order in the concentration of each reactant and there was no evidence for a reverse reaction step. Entropies of activation for the three reactions were in the range of -110 to -130 J mol" K". The volumes of activation were -13.6 0.3 and -18.0 0.5 cm mol for the substitution of the NH3 and Cl groups, respectively (determination of this parameter for the substitution of H2O was not possible). The authors presented several possible mechanisms for consideration to explain the rapid reactions and the magnitudes of the activation parameters. It was eventually concluded that the most compatible mechanism consistent with the results and product species characterisation was a unique combination of associative ligand binding and concerted electron transfer to yield the stable ruthenium(II) nitrosyl complex. 

It should be emphasized that one of the reasons why most of the MnSOD mimics, in particular pentaazamacrocyclic complexes, have not been tested for the reaction with NO is because of the prevailing opinion that all of them do not have sufficiently high redox potential to reduce NO via outer-sphere electron transfer (redox potential of these MnSOD mimetics is >0.8 V, NHE) 47a). However, these complexes are generally prone to react with different monodentate ligands, and coordination of NO is quite feasible. Once NO coordinates, its redox potential shifts toward significantly more positive values, enabling an inner-sphere electron transfer resulting in the Mn (III)NO nitrosyl species. 

The reactivity of metal nitrosyl complexes (51) with thiols is of particular concern in the mobilization of NO to make it accessible for the vasodilation process. Very recently, it has been reported (52) that the S-atom of cysteine reacts to bind the N-atom of the nitrosyl complex of Ru-edta to form a 1 1 intermediate species. Stopped-flow kinetic studies revealed the formation of a transient species, whose rate of formation was found to be first order with respect both [Ru (pac)(NO)] and RSH. The values of rate constants ( 1) were formd to be in the range (0.2-5) x 10 M s at 25°C. Considering the spectral features and kinetic behavior of various [Ru (pac)(SR )] and [Ru (pac)NO] species as described in the preceding sections, and analysis for the products of the above reaction (N2O), the following mechanism (Scheme 15) for the redox reactions involving electron transfer fi om thiols to coordinated NO, that results in the formation of disrdfide (RSSR) and N2O, has been proposed for the reaction of [Ru (pac)(NO)] with thiols (RSH). 

Remarkably, mononitrosyl iron(—II) complexes displayed great potential in the activation of diazo compormds and carbene-transfer reactions [102]. Generally, the activation of diazo compound can be realized by electrophilic transition metal complexes. However, according to the concept of Umpoirmg [103], the electron-rich, nucleophilic iron(—II) compound 31 is expected to react with diazo compounds of electron-poor carbenes, such as ethyl diazoacetate (Scheme 42). At first the iron center would add the C=N bond of the diazo compound followed by release of N2 and formation of the electrophilic iron carbene moiety. The nitrosyl group in such transformations is assumed to support as an ancillary ligand the N2 release by pulling electron density to the iron center. 

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