Nitrosonium complexes compounds

Among complexes of organic compounds with N-centered electrophiles, a special position is occupied by the nitrosonium complexes. Interest in these complexes has not diminished over the years as they are intermediates in several important organic reactions, such as nitrosation of aromatic compounds (35),... 

Nitrosonium complexes 20a-d of l-R-2-methylacenaphthylenes 21a-d (Scheme 15) can be considered as complexes with two-electron ligands, as unlike complexes of other polycyclic aromatic compounds (8, 52), nitrosonium... 

To date the structure and reactivity of numerous complexes derived from aromatic compounds and nitrosonium cation have been studied (5, 56-63). However, relatively few studies are available on the nitrosonium complexes of cyclophanes (5, 57, 59, 61, 62), cf ref. (63). The interaction of [2.2]paracyclophane with nitrosonium tetrachloroaluminate was studied by H and 13C NMR spectroscopy using deuterium isotope perturbation technique (64). It was found that the resulting nitrosonium complexes containing one (25) or two NO groups (26) are involved in fast interconversion (on the NMR time scale) (Scheme 17). 

Here again measurements are confined to the nitrosyl pentacyanides. There is a large literature on the ESR spectrum of K3[Cr(NO)(CN)5] 48, 68, 71 and references therein) and some disagreement as to the interpretation, but the formulation of the complex as a nitrosonium coordination compound is upheld by the results, and this is also the conclusion reached from ESR studies on the isoelectronic complex K2[Mn(NO)(CN)5] 68). In the case of [Fe(NO)(CN)s], a species not yet isolated from solution, the delocalization of the unpaired electron onto the ligand appears to be such that the complex may almost be regarded as an example of nitric oxide donating an electron pair only to iron(II) 45). A study of C hyperfine interaction in the ion has recently been made 15). 

Formation of Nitrosonium Complexes from Organic Compounds and... 

The most important physiological nitrogen substrate of peroxidases is undoubtedly nitric oxide. In 1996, Ishiropoulos et al. [252] suggested that nitric oxide is able to interact with HRP Compounds I and II. Glover et al. [253] measured the rate constants for the reactions of NO with HRP Compounds I and II (Table 22.2) and proposed that these reactions may occur in in vivo inflammatory processes. The interaction of NO with peroxidases may proceed by two ways through the NO one-electron oxidation or the formation of peroxidase NO complexes. One-electron oxidation of nitric oxide will yield nitrosonium cation NO+ [253,254], which is extremely unstable and rapidly hydrolyzed to nitrite. On the other hand, in the presence of high concentrations of nitric oxide and the competitor ligand Cl, the formation of peroxidase NO complexes becomes more favorable. It has been shown [255]... 

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

Nitrous acid catalysis also takes place in the nitration of such compounds (naphthalene) that are unable to undergo nitrosation on the given conditions or whose nitrosation proceeds slower than nitration. As accepted, the nitrosonium ion is formed from HNOj in acid media. The nitrosonium ion oxidizes an aromatic substrate into a cation-radical and transforms into nitric oxide. The latter reduces nitronium cation to nitrogen dioxide that gives a a-complex with the aromatic cation-radical ... 

A number of hexahalogenotellurates(IV) have been investigated. A few rather unstable hexafluoro complexes are known, for example the nitrosonium compounds of composition (NO)2XF6 have been obtained40 from the reaction of nitrosulfuryl fluoride, NOS02F, with selenium or tellurium tetrachloride. The selenium compound loses nitrosyl fluoride at room temperature to leave the pentafluoroselenate(lV), whereas the tellurium analogue remains... 

A common reaction of nitrosonium ion with low-valent transition metal compounds results in 1-electron oxidation of the complex, with the evolution of NO(g). Thus, nitrosyl complexes were not isolated in the following reactions (30) ... 

The reaction of phenols with nitrous acid gives the ortho- and para-nitroso products, which are formed through a neutral dienone intermediate, the proton loss from the latter being the rate-limiting step" " . It has been shown that the nitrous acid can act as a catalyst for the formation of the nitro derivatives. Thus the conventional preparation of nitro compounds by the oxidation of nitroso compounds may be replaced by methods using an electron-transfer pathway in certain cases. In the latter method, the phenoxide reacts with nitrosonium ion to give the phenoxy radical and nitric oxide radical. The nitric oxide radical is in equilibrium with the nitronium radical by reaction with nitronium ion. The reaction of the phenoxy radical with the nitroninm radical resnlts in the formation of the ortho- and para-mixo prodncts" . Leis and coworkers carried ont kinetic stndies on the reaction of phenolate ions with alkyl nitrites and fonnd that the initially formed product is the 0-nitrite ester, which evolves by a complex mechanism to give the ortho-and the para-nitro products". ... 

Many methods have been used to prepare NO complexes. The reactions of transition metal complexes with gaseous NO or solutions of the dissolved gas have been used to form NO complexes. One example of such a reaction to form, a homoleptic nitrosyl complex is shown in Equation 4.30. Alternatively, the reactions of metal complexes with nitrosonium salts form metal-nitrosyl complexes. (Note Alcohol solvents should be used with caution, because they convert NO " to alkyl nitrites. In addition, tetrahydrofuran should not be used as solvent because it can be polymerized by NO". ) Two examples of such processes are shown in Equations 4.31 and 4.32. A range of organic nitroso compounds release "NO" under appropriate conditions and have been used to introduce NO into transition metal complexes. The generation of a metal-nitrosyl complex from F3CNO is shown in Equation 4.33. ° Inorganic nitrites have also been used to prepare NO complexes, ° as shown in Equation 4.34. ... 

Another study by Kochi et al. examined the chemistry of nitrosation [52]. Several [NO, arene]" Jt-complexes were characterized by UV-visible, NMR, and IR spectroscopy. X-ray quality crystals were also obtained at -78°C with mesitylene, hexamethylbenzene, and other arenas. The highly colored nitroso jt-complexes were prepared directly from nitrosonium salts (i.e., NO+ SbCl ) and the aromatic compound. Structural studies revealed the distance between the ring and the electrophile is roughly 1A less than the sum of the van der Waals radii and the N—O bond length is significantly lengthened as a result of strong donor-acceptor interaction. 

The compound Tr-CsHsNiNO is regarded as formed by a three-electron donation from the nitric oxide ligand. Most NO complexes are best considered as being formed by an initial one-electron transfer to the metal prior to donation from the nitrosonium ion (NO ). The nitric oxide ligand, a so-called odd molecule, is then effectively a three-electron donor. Such a bonding mode is supported by considerable infrared, ESR, and other structural data. It should be noted, however, that recent interpretation of ESR data in terms of MOT indicates that the nitric oxide ligand also can be described by the formal structures NO- and NO , in some complexes. 

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