Diazonium salts, structure

X-ray structures 291 f., see also Arenediazonium ions, Dediazoniations, Diazonium salt structures... 

Diazonium metal double salts 42 f., see also Diazonium salts, structure Diazonium salts, isolation 24 f. 

For a review of diazonium salt structures, see Sortiso, S. in Patai The Chemistry of Diazonium and Diazo Groups, pt. 1, Ref. 557, p. 95. 

Owing to their particular interest two individual reactions will now be discussed separately. The reaction of methoxycarbonylhydrazine and 3-bromo-2,4-pentanedione affords, in addition to the expected pyrazole (608), a pyrazolium salt (609), the structure of which was established by X-ray crystallography (74TL1987). Aryldiazonium salts have been used instead of arylhydrazines in the synthesis of pyrazolines (610) and pyrazoles (611) (82JOC81). These compounds are formed by free radical decomposition of diazonium salts by titanium(n) chloride in the presence of a,/3-ethylenic ketones. 

Chemical evidence for the structure of imidazol-4-ones has been summarized,although rather different conclusions would now be drawm from this evidence. For example, in the light of present knowledge, the ease with which imidazol-4-ones react with diazonium salts suggests that an appreciable amount of the 4-hydroxyimidazole exists... 

Diazonium salt (Section 24.8) A compound with the general structure RN2+ X-. 

As explained in the preceding section, we will discuss the structure of aromatic diazonium salts on the basis of evidence from X-ray investigations. We will supplement those results with data obtained by other physical methods, in particular NMR and IR spectroscopy. Earlier experience with the more stable arenediazonium salts enabled those scientists who first obtained alkanediazonium ions in solution to characterize them by NMR spectroscopy (see Zollinger, 1995, Sec. 2.1). 

This summary of X-ray investigations of aromatic diazonium salts will concentrate on those results that are essential to understanding the structure and reactivity of these compounds. Most important are, first of all ... 

The packing arrangement of cation and anion in diazonium salts has important implications not only for the structure of diazonium ions, as discussed above, but also for the solid-state chemistry of these compounds, in particular with regard to halogeno-de-diazoniations such as the Schiemann reaction. TWo of the papers of Gougoutas (1978, with Johnson, and 1979) contain, in addition to the X-ray analyses, experimental results on bromo- and iodo-de-diazoniation, which can be interpreted on the basis of the structural information (see Secs. 10.4-10.6). 

Ultraviolet spectra of diazonium salt solutions were recorded for the first time by Hantzsch and Lifschitz as early as 1912. However, electron spectra did not provide significant information on the structure of diazonium ions, either at that time or later. For example, Anderson and coworkers (Anderson and Steedly, 1954 Anderson and Manning, 1955), compared spectra of 4-amino-benzenediazonium salts with those of diphenylquinomethane (4.18). Their conclusion that the structures of these compounds are analogous is basically correct, but the arguments given by Anderson can easily be refuted, as shown by Sorriso (1978, p. 102). 

The arenediazocyanides have been known since 1879. They played an important role in the Hantzsch-Bamberger debate on the (Z)/( ,)-isomerism of diazo compounds (see Sec. 7.1). When an aqueous solution of a diazonium salt is added to a solution of sodium or potassium cyanide, both in relatively high concentration, at a temperature below 0°C, a yellow to red (Z)-arenediazocyanide starts to crystallize. Hantzsch and Schulze (1895 a) found that these compounds rearrange into the (ii)-isomers, which have a bathochromically shifted visible absorption (see Sec. 7.1). Under strongly alkaline conditions a 1 2 adduct is formed, to which Stephenson and Waters (1939) assigned the structure 6.36. It was never corroborated, however, by modern instrumental analysis. 

The first diazonium-salt-crown-ether adduct was isolated and identified as a 1 1 complex by Haymore et al. (1975). Unfortunately Haymore never published the X-ray structural analysis of benzenediazonium hexafluorophosphate with 18-crown-6 which he performed in 1980. ORTEP drawings with measured bond angles and lengths from Haymore s investigation can be found in a review chapter by Bartsch (1983, p. 893). A few data from Haymore s work (e.g., R = 0.064) were also mentioned by Cram and Doxsee (1986, footnote 7). Groth (1981) published the results of his X-ray investigation of 4-methoxybenzenediazonium tetrafluoroborate and 21-crown-7 (R = 0.042) and Xu et al. (1986) those of 4-methoxybenzenediazonium tetrafluoroborate and dibenzo-24-crown-8 (R = 0.086). 

However, an evaluation of the observed (overall) rate constants as a function of the water concentration (5 to 25 % in acetonitrile) does not yield constant values for ki and k2/k i. This result can be tentatively explained as due to changes in the water structure. Arnett et al. (1977) have found that bulk water has an H-bond acceptor capacity towards pyridinium ions about twice that of monomeric water and twice as strong an H-bond donor property towards pyridines. In the present case this should lead to an increase in the N — H stretching frequency in the o-complex (H-acceptor effect) and possibly to increased stabilization of the incipient triazene compound (H-donor effect). Water reduces the ion pairing of the diazonium salt and therefore increases its reactivity (Penton and Zollinger, 1971 Hashida et al., 1974 Juri and Bartsch, 1980), resulting in an increase in the rate of formation of the o-complex (ik ). 

The diazonium salt s stability depends on the structure of the aromatic group and above all, of the anion salt. 

Thus, in the first case, if the amine is o-aminobenzoic acid, an explosive diazonium salt with a zwitterion structure is obtained ... 

The hydroxyphenylbenzotriazole structure was constructed by a coupling of the diazonium salt of o-nitroaniline with 4-ethyl-phenol, followed by reduction of the nitro-azobenzene to the benzotriazole with zinc powder and NaOH. After blocking of the phenol by acetylation, bromination and dehydrobromination were performed as described earlier, and treatment with aqueous NaOH... 

Whilst azo compounds prepared from diazonium salts and phenolic or keto-enol coupling components are often depicted in the hydroxyazo form (4.11), an alternative tautomeric structure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer (4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms, exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, but without hydrogen bonding, can be drawn for p-hydroxyazo compounds. 

Superior passive stabilised diazo compounds are afforded by the diazoamino compounds (triazenes) that arise by reaction of diazonium salts with a variety of secondary amines [114]. Typically, sarcosine (CH3NHCH2COOH), which gives products based on structure 4.114, as well as N-methyltaurine (CH3NHCH2CH2SO3H) and N methylaniline-4 Sulphonic acid,... 

The coupling of Naphtol AS or its phenyl-substituted derivatives with diazonium salts from variously substituted anilines in aqueous alkaline solution (section 4-11) gave incomplete reactions and impure products in some instances, probably because these coupling components have inadequate solubility in aqueous media. Pure dyes in ca. 90% yields were obtained by reaction in dimethylformamide in the presence of sodium acetate. Metallisation of these o,o -dihydroxyazo ligands with sodium chromium salicylate or a cobalt(II) salt gave metal-complex dyes in 80-100% yields [22]. Specific structural isomers of these complexes were identified by i.r., n.m.r., Raman and UV/visible spectroscopy [23]. 

Table 1. Newly synthesized diazonium salts and chemical structure of Methylone used in this experiment...

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