Nitrosation with metal nitrosyl complexes

Nitrosonium (NO+) is a strong oxidant and the reduction potential to NO has been measured in non-aqueous media (1.67 V vs. SCE in CH3CN), and estimated for water (Eq. (3)) (12,15). NO+ is subject to rapid hydrolysis to nitrite (2H+ + N02 ), and therefore if formed in biological media would be short-lived. However, other less water-sensitive chemical species can act as NO+ donors in reactions leading to the nitrosation of various substrates. For example, the reactions of certain metal nitrosyl complexes with nucleophiles such as R SH can lead to the transfer of NO+ as illustrated in Eq. (4). Such reactions will be discussed in greater detail below. 

Until recently most of the mechanistic studies on nitrosation have been concerned with N-nitrosation reactions of amines, including the diazotisation reactions of primary amines. Now, work has been extended to include both O- and S-nitrosation, so that comparisons can be made. Mechanistic studies have also been extended in recent years to include reactions of nitrogen oxides, nitrosamines, alkyl nitrites, thionitrites and transition metal nitrosyl complexes. Many of these reactions have been used preparatively for a long time, but little has been known about their detailed reaction mechanisms. 

In the presence of metal catalysts nitrosations using nitric oxide proceed rapidly and it is clear that a very powerful nitrosating species is produced. Rate measurements on the reaction of diethylamine with nitric oxide in the presence of Cu(II) salts indicated that a copper-nitrosyl complex was that species (Brackman and Smit, 1965). Many metal-nitrosyl complexes are now known... 

The nitrosation mechanism of aliphatic amines by metal nitrosyl complexes was studied with various secondary amines and one primary amine (ethylamine) using pentacyanonitrosylferrate (sodium nitroprusside). Very little mechanistic information is available for reactions with other metal nitrosyl complexes (see also Sects. 2.3, 4.3, and 7.2). 

The different biological effects of NO and HNO are a function of distinct molecular targets for these redox siblings. For example, NO preferentially reacts with reduced metals to directly form a nitrosyl complex (Eq. 13). The identical product is formed by reductive nitrosylation of HNO toward oxidized metals (Eq. 12). Exceptions of course exist (e.g., the reverse of Eq. 19). Perhaps more importantly, HNO reacts with thiols (Eq. 21) and amines (Eq. 25) directly while NO must interact with these species indirectly, following oxidation to a nitrosating or oxidizing agent (36). 

Reductive nitrosylation, on the other hand, can refer to the addition of NO to a metal center Mox with formal reduction of the metal center to yield Mred(NO +), but in the context of ligand reactions reductive nitrosylation refers to the net reactions of NO with metal-bound NO and the ensuing events. Reductive nitrosation of coordinated amines to form nitrosamines occurs through the conjugate base of the amine, and this process has been reported for reactions of NO with [Ni(tacn)2]3 +, 198 with methyl-amine coordinated to a macrocyclic Ni(III) complex,199 with triglycyl complexes of Fe(III), Ni(III), and Cu(III),200 and with Cu(II) macrocyclic complexes.201 Reductive nitrosation of [Ru(NH3)6]3+ produces [Ru(NH3)5N2]2 + with base-catalyzed kinetics the coordinated N2 is produced by hydrolysis after the nitrosation step.170... 

Besides this iron-nitrosyl complex, nitrosyl complexes of other transition metals can be used for nitrosation. As discussed by Bottomley et al. (1973, see also review by Bottomley, 1978), these complexes are not only sources of nitrosyl ions (NO" ) as two-electron acceptors, but also of nitroxide (NO ) as one-electron donor. Bottomley found that they are nitrosating reagents only if their NO stretching frequency is greater than 1886 cm The ruthenium nitrosyls are particularly interesting with respect to their reaction with aliphatic and aromatic primary amines. We discuss them in the context of metal dinitrogen complexes (Sect. 3.3). 

A number of studies have been reported on the reduction of nitrate by metal ions. In dilute HNO3 solution, the ruthenium(ii) species [(py)(bipy)2Ru(OH2)]2+ is oxidized by NO3- to the corresponding Ru complex. The metal nitrosyl cation [(py)(bipy)2Ru(NO)] + is also formed as a product, resulting from the nitrosation reaction of (initial product) HNO2 with ruthenium(ii) aquo-species. This side reaction is eliminated when the HNOj scavenger sulphamate ion (N112803") is present. The reaction mechanism involves an initial substitution of the nitrate to yield the species [(py)(bipy)2Ru(0 -NOa)], which in an intramolecular redox step produces HNOa and a ruthenium(iv) ion,... 

Nitric oxide rapidly reacts with transition metals, which have stable oxidation states differing by one electron (see Chapters 2 and 3). Nitric oxide is unusual in that it reacts with both the ferric (Fe " ) and ferrous forms (Fe " ) of iron. TTie unpaired electron of nitric oxide is partially transferred to the metal forming a principally ionic bond. Complexes of ferric iron with nitric oxide are called nitrosyl compounds and will nitrosate (add an NO group) many compounds, while reducing the iron to the ferrous state (Wade and Castro, 1990). 

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