Wurster’s 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... 

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). 

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. 

Examination of postulated reactions 8) of p-phenylenediamine derivatives with ozone reveals that the antiozonant depletion is characterized by the intermediate formation of intensely colored products, or Wurster s salts (5), which are considered... 

Wurster s salts. Generated by one-electron oxidation of the diamine. Indefinitely stable. 

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 ... 

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. 

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 ... 

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. 

V,V,V,V-Tetramethyl-l,4-phenylenediamine dihydrochloride (Wurster s Reagent) [637-01-4] M 237.2, m 222-224°. Crystallise the salt from isopropyl or n-butyl alcohols, saturated with HCl. Treat... 

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). 

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). 

The aminoarylation of 9-methylacridinium salts with aromatic amines provides a well-established example of a SET from arylamines to the acridinium ion, as evidenced by the formation of diacridanyl and arylamine radical cation species (Scheme 62). Also, treatment of W-methylacridinium ion with W,W -tetramethyl-para-phenylenediamine, as the model compound, gave the characteristic color of the Wurster s Blue radical cation [11, 136, 137]. 

Another, lower-energy type of Peierls transition is possible for magnetic systems (Wurster s Blue Perchlorate, or alkali TCNQ salts). Then one talks about "spin-Peierls" transitions, whose X-ray signature is a 2kp SDW (spin density wave) which, again, is dynamic and diffuse above the ordering temperature, and static (or "pinned") and sharpened below the transition temperature [72]. 

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. ... 

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