Wurster salts

Wurster process Wurster salts Wurtz-Fittigcoupling Wurtz-Fittig reaction Wurtzite... 

LDPE or HDPE extracts has been determined colorimet-rically at 430 nm by oxidation with H202 in the presence of H2S04 [66]. p-Phenylenediamine derivatives such as Flexzone 3C, used as antiozonants in rubber products, have been determined colorimetrically after oxidation to the corresponding Wurster salts [67]. A wide range of amine AOs in polyolefins has been determined by the p-nitroaniline spectrophotometric procedure [68]. Monoethanolamine (MEA) in a slip agent in PE film has been determined as a salicylaldehyde derivative by spectrophotometric quantification at 385 nm [69]. Table 5.6 contains additional examples of the use of 1JV/VIS spectrophotometry for the determination of additives in polymers. 

Both hole and charge are not necessary to be localized together on one atom and they can be delocalized over the whole molecule. In fact, the delocalization of the unpaired electron in conjugated system can lead to stable radical cations such as the Wurster salt. This compound is isolable and the chemical structure including its resonance forms are shown in Scheme 1. Aryl amine moieties are thought to be a main core structure in HTMs because amine atom is relatively easy to lose one electron and the resulting radical cation can be stabilized by resonance effect of adjacent aryl substituent. It is worth to note that the Wurster salt mentioned above is stabilized by two factors. One is a resonance effect by aryl substituents and the other is stabilized by counter ion, perchlorate. 

Scheme 1. The possible resonance forms of Wurster salt...

Crystalline cation-radical salts have been known for many years, the earliest examples—Wurster salts—having been prepared almost 100 years ago. Interest in the salts has quickened in the last decade or so because it is possible to examine their spectroscopic properties both in the solid state and in solution. 

Wurster salts (Sidgwick, 1966) are usually obtained by oxidizing diaminobenzenes (p-phenylenediamines) with bromine in methanol-acetic acid solution. Ibis leads to the Wurster bromide (Wurster and Sendtner, 1879), but if oxidation is carried out in the presence of a large amount of perchlorate ion the Wurster perchlorate is formed. This is the way in which Wurster s Blue [ 1 ] perchlorate (the most widely studied cation radical salt of all) and Wurster s Red [2] perchlorate are made (Michaelis and Granick, 1943). The chlorides and iodides are also known (Oohashi and Sakata, 1973 Sakata and Nagakura, 1970). 

There are some pitfalls in the preparation of Wurster salts by this method. The cation radical of a p-phenylcnediamine may readily undergo further oxidation to the dication (or the corresponding diimine). This, in turn, may be reactive in aqueous solution (as with p-phenylenediamine itself) and become hydrolysed. Further, steric inhibition of resonance may prevent stabilization of the cation radical (e.g. [5]). Thus, while the salts of cation radicals [1], [2], and [3] are readily isolable, those of (4] and [5] are not (Michaelis et al., 1939). 

Formation of dicationic dimers (90) has been longer known than formation of the monocationic type (89), no doubt because of the availability of and interest in the very stable Wurster Salts. Hausser and Murrell (1957) proposed that the long wave-length absorption band (near 800 nm) of Wurster s Blue perchlorate in ethanol at —90° was caused by two associated, cation radicals lying in parallel planes. Since that time a considerable number of workers have explored the dimerization of Wurster and analogous cation radicals, (e.g. Kawamori et al., 1966 Kimura et al., 1968). Not only does Wurster s Blue cation radical (i.e. TMPD +) associate with itself, but it also forms a spin paired dimer with p-phenylenediamine cation radical (PD +). In fact, Takimoto et al. (1968) conclude from absorption spectroscopy that solutions of TMPD + and PD + in ethanol-ether at —195° contain (PD"+)2 and (PD"+-TMPD +)2 but very little of (TMPD +)2. Dimerization of unlike cation radicals is known in other systems too. Perylene" and naphthacene4-each forms an (M"+)2 dimer in sulfuric acid at reduced temperatures (Kimura et al., 1971). Mixtures of the two cation radicals in sulfuric acid leads to a mixed dimer too, (Perylene "+, naphthacene +), the heat of formation of which (—7-7 kcal mole-1) incidentally, lies between that of the perylene"+ (—8-8) and naphthaccne + (— 5 6) dimers (Yamazakiand Kimura, 1972). 

Most oxy or halo free radicals readily oxidize phenjienediamines to radical cations called Wurster salts (12), which are stable in water-ethanol solutions at pH 3. On the other hand, carbon and sulfur radicals generally do not produce Wurster salts. The intense color of Wurster salts can be used as a quick spot test for variously substituted phenylenediamines when oxidized with bromine in carbon tetrachloride solution. For example, Al,AT-diaIkyl substitution gives red, N,N,N N gives blue, and N-alkyl-TST-aryl gives light blue or green. 

The best known of these is the ozonation of tertiary amines to amine oxides (II) (i). Henbest and Stratford (11) and Shulman (17) have shown that competing with this is an ozone attack on the alpha position of an alkyl side chain to produce various decomposition products of III. Henbest (11) showed that amine oxide formation is favored in chloroform and methanol, while side chain oxidation is predominant in hydrocarbon solvents. Also of considerable interest are the reported conversions, during ozonation, of phenylenediamines to Wursters salts (VII) (8, 14), of liquid ammonia to ammonium ozonate (VA) at a low temperature 18), and of amines to amine hydrochlorides (VB) in chlorinated hydrocarbon solvents 17, 19), Finally, an early report states that azobenzene and quinone are obtained upon ozonation of aniline (15). 

The methyl groups on the ring in octamethyl-p-phenylene interfere with this interaction by affecting the molecular geometry. Thus there is no resonance stabilization as in the Wurster salt cation and it is not as stable. 

Mechanisms may also be written involving Wurster salts (XXV) to provide the di-, tri-, and polynuclear indo dyes formed in these reactions. Lee and Adams [28] have generated the Wurster salt of p-phenylenediamine by electrochemical oxidation in buffered media. Above pH 6, the radical stability decreases rapidly, indicating the low stability of these species under hair-dyeing conditions therefore, the diiminium ion is probably the active intermediate in actual hair dyeing. 

In situ quantitation has been performed by absorption ultraviolet or visible light measurements of the color developed with Wursters salts. 

Wurster s salts Stable radical cations formed... 

Radicals in which the odd electron is on a nitrogen next to an aromatic ring are stabilized by resonance analogous to that of tri-phenylmethyl. In the case of Wurster s salts, the nitrogen analogs of semiquinones, there are two equivalent resonance structures in the acid form, but in the less stable basic form one of the structures requires separation of charge. Evidence for the unpaired electron has been obtained by measurement of the paramagnetism.144... 

The typical transformation of all p-phenylenediamine derivatives by oxidising agents in acid solution consists in a change into a salt of the quinonediimine series. The dye just observed, so called Wurster s red , was long regarded as a simple quinonimonium salt ... 

From Wurster s salts and the Weitz system i/it is well known that the intermediate oxidation level of the radical cations ( violenes ) is characterized by an e ecially long wavelength absorption. This general behaviour may be demonstrated with 32, n = 1 In Fig. 7 the similar absorption curves of RED and OX can be seen from... 

Several other organic systems have been stndied as potential electrochromes because of their redox behaviour. These include carbazoles, methoxybiphenyls, fluorenones, benzoquinones, naphthaquinones and anthraqninones, tetracyanoquinodimethane, tetrathiafnlvalene and pyrazolines. ° Of particnlar interest are the 1,4-phenylenedi-amines, which form highly colonred species on oxidation. These, known as Wurster s salts, exemplified by Wnrster s Bine (1.97), are anodically colouring and this type of material has found nse in composite electrochromic systems for car rearview mirrors (see 1.5.4.1). 

Some easily formed cation radical salts, particularly those of the Wurster s blue type, may be prepared and isolated in protic media.36 Such cation radical salts may then be dried and slurried in aprotic solvents (their solubility is usually very low in such media) and will produce emission when caused to react with etheral solutions of aromatic anion radicals and other energetic reductants.15,24... 

More recently, charge-transfer emission was anticipated when solutions of hydrocarbon anion radical salts in dimethoxyethane were mixed with Wurster s blue perchlorate.15 Emission was seen in every instance however, with eight anion radicals derived from 3 to 5 ring-fused aromatic hydrocarbons, the emission was derived from the hydrocarbon rather than the complex. Preliminary studies with smaller hydrocarbons, biphenyl and naphthalene, did show emission in the region (18 kK) where charge transfer was expected. The question as to what pairs of ion radicals will be emissive under what conditions has only begun to be considered. Much opportunity for further experimentation exists in this area. 

Wurster in 1879 had already prepared crystalline salts containing radical cation 23 (equation 12). Subsequently, radical cations of many different structural types have been found, especially by E. Weitz and S. Hunig, and recently these include a cyclophane structure 24 containing two radical cations (Figure 3). Leonor Michaelis made extensive studies of oxidations in biological systems, " and reported in 1931 the formation of the radical cation species 25, which he designated as a semiquinone. Michaelis also studied the oxidation of quinones, and demonstrated the formation of semiquinone radical anions such as 26 (equation 13). Dimroth established quantitative linear free energy correlations of the effects of oxidants on the rates of formation of these species. ... 

Many radical ion salts such as Wurster s Blue perchlorate130 and charge transfer salts of tetracyanoquinodimethane131 are situated in their crystal lattices such that the unpaired electrons are coupled, and low-lying triplet exciton states are observable by ESR at low temperatures. 

A particularly stable cation-radical of the semiquinone type is formed by mild oxidation of N,N,N, N -tctramethyl-l, 4-bcnzencdiamine. The cation, which is isolable as a brilliant-blue perchlorate salt, 2, is called Wurster s Blue ... 

Exercise 26-17 Write resonance structures that account for the stability of the cation of Wurster s salts, such as Wurster s Blue, 2. Explain why N,N,N, N -2,3,5,6-octamethyl-1,4-benzenediamine does not form a similarly stable cation radical. 

The starting point is the Wurster s Blue radical cation, discovered in 1879 and thoroughly investigated since then The twofold N pyramidal IVWW. /V -tetraalkyl-p-phenylenediamine precursors are completely flattened on two-electron oxidation as proven by crystal structure analyses of the resulting redox salts (Scheme 8a). 

The semiquinones, among which are Wurster s red and quinhydrone, form a remarkable class of intensely coloured compounds. Here there is no question of a molecular compound of one molecule of 0j -dimethyl-/>-phenylenediamine (I) and one molecule of this compound, oxidized by two atoms of bromine to the dimethylquinone diimonium salt (II). Wurster s red is indeed a true monomeric semiquinone resonating between two electron configurations, hence the colour. 

The history of organic radical ions is intertwined with the history of quinhy-drones , molecular aggregates between substrates that are readily oxidized and compounds that are readily reduced. In the absence of modem analytical methods, particularly magnetic resonance techniques, it was often difficult to ascertain whether one was dealing with a homogeneous radical ion salt, such as Wurster s Blue, or with a quinhydrone, such as the prototypical complex formed between benzoquinone and benzohydroquinone. Indeed, in several cases radical ions were mistaken for molecular complexes [54,55]. Furthermore, there are instances where a free radical ion and a molecular complex have a similar appearance, at least subjectively, so that it is not clear which of the two species was observed originally. 

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