Diazonium salts metal complexes

With regard to the mechanism of these Pd°-catalyzed reactions, little is known in addition to what is shown in Scheme 10-62. In our opinion, the much higher yields with diazonium tetrafluoroborates compared with the chlorides and bromides, and the low yields and diazo tar formation in the one-pot method using arylamines and tert-butyl nitrites (Kikukawa et al., 1981 a) indicate a heterolytic mechanism for reactions under optimal conditions. The arylpalladium compound is probably a tetra-fluoroborate salt of the cation Ar-Pd+, which dissociates into Ar+ +Pd° before or after addition to the alkene. An aryldiazenido complex of Pd(PPh3)3 (10.25) was obtained together with its dediazoniation product, the corresponding arylpalladium complex 10.26, in the reaction of Scheme 10-64 by Yamashita et al. (1980). Aryldiazenido complexes with compounds of transition metals other than Pd are discussed in the context of metal complexes with diazo compounds (Zollinger, 1995, Sec. 10.1). 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

Direct dyes have only modest fastness to washing, which may be improved by after-treatments such as metal-complex formation (section 5.5.3) or by diazotisation of the dye on the fibre and further coupling of the diazonium salt with an insoluble coupling component (section 1.6.14). In addition to their use on cotton and viscose, direct dyes are important in the dyeing of leather. The cheapest members of this class are also used in the coloration of paper, since for this purpose fastness properties are largely irrelevant and price is all-important. 

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]. 

On the other hand, Hartman and Biffar (1977) reported that decomposition of arenediazonium tetrafluoroborates in dichloromethane in the presence of copper metal is catalysed by dicyclohexyl- 18-crown-6. Electron-withdrawing substituents in the aryl ring enhance the rate of the reaction. The main function of the crown ether is probably to solubilize the salt. The effect of the complexation on the rate was not investigated in detail. Similar enhanced solubilization of diazonium salts in apolar solvents was reported and used by Martin and Bloch (1971) in pyrolysis experiments aimed at the generation of the dehydrocyclopentadienyl anion. 

Another group of stable diazonium salts are the so-called diazonium metal double salts the zinc double chlorides are particularly important. The term is misleading, as they are not associations between two salts but, in the case of Zn, are formed from two arenediazonium ions with the complex anion ZnCL 2-. This fact became obvious from crystal structure studies38-41. The reason for their increased stability relative to ArN2+Cl- salts is that ZnCLt2- complex ions are less nucleophilic than free chloride ions. 

Because of the small concentration of the 2 1 complex the last term can be ignored. From the extreme rate values in the absence of zinc and with an excess of zinc, 2i and 22 are determined as 2.4 X 104 min.-1 and 1.57 min.-1 respectively. These values can be combined with the trend in the rate constants to give the stability constant of the reactive complex, presumably Zn(OR)(OAc), as 3 X 107. For the simple zinc complex in water the literature values of the stability constant for the 1 1 complex vary from 2.5 X 108 to 6.3 X 108. The diazo coupling reaction of the complex indicates the smaller effect of coordination vis a vis protonation since this reaction is very sensitive to such effects and does not proceed with phenols. Unfortunately the choice of cations for such a reaction is restricted since the cation should not interfere with the analytical methods used to obtain the kinetic data nor should it introduce additional reactions such as occur with transition metal cations which can catalyze the decomposition of the diazonium salt via a redox process. 

Terminally metallizable dyes (30) are obtained by the interaction of a diazonium salt and a coupling component containing a chelating system, for example salicylic acid, catechol, salicyl-aldoxime or 8-hydroxyquinoline, and their coordination chemistry is typical of these compounds. Such dyes were rarely used as preformed metal complexes but were usually applied to cotton and then converted to their copper complexes on the fibre to improve their fastness to wet treatments. A typical example is the blue dyestuff (31). 

Metal-Complex (Formazan) Dyes. The hydrazone from 2-carboxyphenylhydra-zine-4-sulfonic acid and benzaldehyde is suspended in water and then dissolved by adding aqueous sodium hydroxide to obtain pH 6.5 -7.0. This solution is added to the aqueous diazonium salt solution obtained from a typical aqueous diazotiza-tion of 4-(2-sulfooxyethylsulfonyl)-2-aminophenyl-6-sulfonic acid. The mixture is then dripped into an aqueous solution of copper sulfate, while the pH is maintained with soda at 5.5 - 6.5. After complete coupling the pH is adjusted to 1 with concentrated hydrochloric acid. The strongly acidic solution is then neutralized with alkali to pH 5.5. The copper - formazan complex is salted out along with sodium chloride, filtered, washed with dilute aqueous sodium chloride solution, and dried. A dark powder results which gives a dark blue solution in water. It consists of an electrolyte-containing powdered sodium salt of the acid 25 ... 

Following in the wake of the first reaction shown to yield diazenido complexes (210) many workers have investigated the reactions of diazonium salts with transition metal complexes. This category involves two distinct subsections. 

The original work in this area was the insertion of diazonium salts into the Pt—H bond of [Pt(H)Cl(PEt3)2j to yield [Pt(NHNAr)-Cl(PEt3)2]+ (267). Although this strategy occasionally results in the formation of diazenido complexes if the resulting diazene is sufficiently acidic (see Section IV,A,2), it has been extended to the metal hydrides of Ru (175, 218), Os (218), Rh (218), and IR (52, 331) as exemplified in Eq. (23). 

In the photometric determination of copper, a coupling product formed between the diazonium salt from 2-amino-pyridine and resorcinol, or 4-(2-pyridinylazo)-l,3-benzenediol 21, has been used. Here the formed copper complex under acetate buffer exhibits an absorption peak at 520 nm, which is measured photometrically <2003KPU28>. Similarly for photometric determination of iron(ll), a coupling product formed between the diazonium salt of 2-amino-4,6-dihydroxypyrimidine and 8-hydroxyquinoline, or 6-hydroxy-2-(8-hydroxy-7-quinolinyl)azo-4(l//)-pyrimidinone 22, has been used. This reagent forms a blue complex with iron(n) ions with an absorption maximum at 625 nm that does not interfere with the presence of other metals <2003KD95>. 

Morken and Lavastre used the formation of a colored side product to identify catalysts for the allylation of /i-dicarbonyl compounds [8]. The researchers employed 1-naphthyl allyl carbonate 5 as an allyl source and the diazonium salt of fast red as an indicator. Formation of the active 7z>allyl complex furnishes C02 and 1-naphthoxide which deprotonates the 1,3-dicarbonyl compounds which can, in turn, react with the 71-allyl metal complex. 1-Naphthol is the only species in the reaction mixture that can react with the diazonium salt 6 to generate the bright red azo dye fast red. Thus the red color is indicative of successful formation of the active re-allyl complex (Figure 5.4.3). 

Homolytic substitution of pyridinium salts has scarcely been studied. The phenylation of pyridine in acetic acid solution by benzene-diazonium borofluoride has been reported,392 1 as has the phenylation of pyridine-metal complexes.3926... 

Copper complexes (152) of o-hydroxy-o -(p-aminoethylamino)diarylazo compounds have been prepared ° by the reaction of the corresponding o-chloro-o -hydroxydiarylazo compounds and ethylenediamine in the presence of copper(II) ions (c/. Section 58.2.3. l(iii)(c)). Dyestuffs of this type have been evaluated on nylon but are reported to have very poor fastness properties. Copper complexes such as (153) have been preparedby similar methods. Other, related tetradentate diarylazo compounds, e.g. (154), (155) and (156), are obtained by the reaction of a suitable diazonium salt with the appropriate diarylamine. Chromium(III) and cobalt(III) complexes of dyes of this type, in which the coordination sphere of the metal is completed by a colourless bidentate ligand such as ethylenediamine, salicylic acid or 8-hydroxyquinoline, are reported to have dyeing properties on wool similar to those of the comparably charged 2 1 complexes derived from tri-... 

Though the almost unique catalytic effects of cuprous salts, or of metallic copper, in the Sandmeyer reaction have been contested by Hodgson (7), the careful kinetic work of Cowdrey and Davies (8) has established that the anion (CuCh)" does play a special role. These workers, however, suggested that an intramolecular decomposition of complex cuprous salts (At.N2, CuCh) produced the aryl halide. If this concept is correct, then the Sandmeyer reaction should not yield detectable free aryl radicals and should differ from many associated reactions of diazonium salts (9,10) for which the homolytic character is in less doubt. 

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