Tetracyanobenzene complexes

Order-disorder phase transitions are especially common in crystalline 7T-donor acceptor complexes between planar polycyclic aromatic hydrocarbons and other organic compounds. The disordered phase can sometimes be characterized in terms of either a static- or a dynamic-disorder model, as shown in Figure 13.2. The dynamic-disorder model consists of disordered components in motion within the confines of a broad well in the potential energy curve, whereas the static disorder model requires that the disordered components be localized in two or more sites in the asymmetric unit, within one or another of the wells of a multiwell-potential energy curve. When the possible sites for static disorder are not resolved by the effective resolution of the data set, it is difficult to choose between these models. This turned out to be true for an anthracene-tetracyanobenzene complex studied at several temperatures above and below the transition temperature Tj, of 206 It was found ° ° ... 

Copper(ii) 8 - hydroxyquinolinate - bis(l,2,4,5 - tetracyanobenzene) complex (photographic data)... 

In this kind of simple system, the complexities of encounter and dissociation processes, which complicate the data analysis, are avoided. The ps excitation of the CT absorption band of 1,2,4,5-tetracyanobenzene complexes gave an absorption spectrum identical to that of the 1,2,4,5-tetracyanobenzene anion, independent of the delay time. The species was formed immediately after excitation and its decay up to a few ns consisted of the fast and plateau components as shown in Fig.6. The latter corresponds to free ion radicals, while, the former obeys first order decay kinetics. It is our... 

Fig. 11 Fragment of the crystal structures of tetracyanobenzene/bromide complex showing different modes of anion coordination to the acceptor moieties (coordinates from [24])...

Benzpinacols (or their trimethylsilyl ethers) are effective electron donors and readily form vividly colored charge-transfer complexes with common electron acceptors such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), methyl viologen (MV2+), and nitrosonium (NO+) cation.191-194 For example, the exposure of a silylated benzpinacol to chloranil... 

As a final example the spectra of molecular charge-transfer complexes are considered next. Electron acceptors such as pyromellitic dianhydride, chloranil and tetracyanobenzene... 

True CT complexes are formed between unsaturated electron acceptors, among which the derivatives with cyano and nitro groups predominate. The most common strong electron acceptors are 1,2,4,5-tetracyanobenzene, 7,7,8,8-tetracyanoquinodimethane (1), tetracyano-p-benzoquinone (2), tetracyanoethylene (TCNE) (3) and many electron-donor... 

Previously, Ohashi and his co-workers reported the photosubstitution of 1,2,4,5-tetracyanobenzene (TCNB) with toluene via the excitation of the charge-transfer complex between TCNB and toluene [409], The formation of substitution product is explained by the proton transfer from the radical cation of toluene to the radical anion of TCNB followed by the radical coupling and the dehydrocyanation. This type of photosubstitution has been well investigated and a variety of examples are reported. Arnold reported the photoreaction of p-dicyanobenzene (p-DCB) with 2,3-dimethyl-2-butene in the presence of phenanthrene in acetonitrile to give l-(4-cyanophenyl)-2,3-dimethyl-2-butene and 3-(4-cyanophenyl)-2,3-dimethyl-l-butene [410,411], The addition of methanol into this reaction system affords a methanol-incorporated product. This photoreaction was named the photo-NO-CAS reaction (photochemical nucleophile-olefin combination, aromatic substitution) by Arnold. However, a large number of nucleophile-incorporated photoreactions have been reported as three-component addition reactions via photoinduced electron transfer [19,40,113,114,201,410-425], Some examples are shown in Scheme 120. 

Bodige, S. G., Rogers, R. D. and Blackstock, S. C. (1997). Supramolecular networks via pyridine A-oxide C—H O hydrogen bonding in the crystal structures of 2,2 -dithiobis(pyridine A-oxide) and its complexes with 1,2,4,5-tetracyanobenzene and pyromellitic dianhydride. J. Chem. Soc., Chem. Commun., 1669. 

When an alkene molecule loses an electron, a cation radical is formed. The very reactive cation radical (CH3)2C—CHJ is generated from 2-methyl-propene in light in the presence of TiCl4. It can be detected by ESR in the frozen parent compound at 123 K [172], We assume that at higher temperatures these formations are dimerized to dications. The existence of a donor-acceptor complex is a necessary condition for the mechanism generating cation radicals (see Chap. 3, Sect. 5). a-Methylstyrene is cationically polymerized when illuminated in the presence of tetracyanobenzene in methylene chloride. From the two compounds, of which a-methylstyrene is the donor (D) and tetracyanobenzene the acceptor (A), the donor-acceptor complex is generated in the singlet and triplet states it dissociates to solvated ion radicals [173]... 

Various a-nitro ketones, widely used as synthetic intermediates, have been prepared by reaction of silyl enolates with tetranitromethane in the dark at room or low temperature or under photochemical conditions. The highly coloured solutions are due to intermolecular 1 1 electron donor-acceptor complexes formed between the enolate and tetranitromethane. The formation of similar vividly coloured complexes with electron acceptors such as chloranil, tetracyanobenzene and tetracyanoquinonedimethane readily establishes silyl enolates as electron donors. The formation of radical cations as reactive intermediates has been confirmed. 

Charge-transfer complexes of neutral molecules in zeolites have also been examined. Transient experiments with 1,2,4,5-tetracyanobenzene (TCNB) as acceptor and arene donors have been reported. For naphthalene, transient absorption bands centered at 470 and 680 nm due to TCNB and naphthalene radical were observed [138]. The decay was found to be biphasic and was 10 times slower in dehydrated zeolite Y than in the hydrated sample, indicating a strong interaction with the framework. 

Stezowski, J. Phase transition effects a crystallographic characterization of the temperature dependency of the crystal structure of the 1 1 charge transfer complex between anthracene and tetracyanobenzene in the temperature range 297 to 119 K. J. Chem. Phys. 73, 538-547 (1980). 

Boyens, J. C. A., and Levendis, D. C. Static disorder in crystals of the anthracene-tetracyanobenzene charge transfer complex. J. Chem. Phys. 80, 2681-2688 (1984). 

Photochemical generation of the radical cations derived from A -vinylcar-bazole/acceptor charge-transfer complexes and subsequent polymerization is well known. Perhaps somewhat more interesting are the cationic photopolymerizations of styrene and a-methylstyrene. With these monomers of relatively weak electron donor character photolysis of the charge-transfer complexes formed with tetracyanobenzene and pyromellitic dianhydride produces monomer radical cation species from both singlet and triplet states, and the photophysics of the primary processes have been elucidated in some detail. ... 

Full details have been published of the substitution reactions which result from irradiation of the charge-transfer complexes of 1,2,4,5-tetracyanobenzene and toluene81 and of 7,7,8,8-tetracyanoquinodimethane and toluene.82 The former reaction yields 1-benzyl-2,4,5-tricyanobenzene (44) via, it is suggested, the intermediate 1,4-acyclic adduct (45), and this reaction is quenched by trifluoro-acetic acid. In contrast, the formation of aa-dicyano-4-dicyanomethylbibenzyl... 

Finally, electron transfer within an excited charge-transfer complex may lead to products of reaction without the appearance of the ion radicals presumed to have been formed. Thus, irradiation of tetracyanobenzene in mesitylene at the system s charge-transfer band led to the formation of 2,4,5-tricyanodiphenylmethane in a reaction involving as the initial step the transfer of a proton from the cation radical of mesitylene to the tetracyanobenzene anion radical (Yoshino et al., 1971). Direct detection of the cation radical (and the anion radical) was not made. 

Tetracyanobenzene 441 forms charge-transfer complexes with electron donors easily. References dealing with complex formation and properties are given in Table 6. Weak exciplexes were formed with benzene, toluene and xylene. Formation and feedback dissociation of these exciplexes has been found to be strongly dependent on the solvent polarity and the donor ionization potential . 

TABLE 6. Charge-transfer complexes with tetracyanobenzene... 

Weak donor-acceptor complexes, nonpolar in the ground state Anthracene-Tetracyanobenzene (TCNB) 1.6... 

In this chapter, we will not discuss the weak CT complexes further. In these complexes, at least in the ground state, the charge transfer S is not large. They in general crystallise in mixed, alternating stacks. We have already mentioned the example systems anthracene-tetracyanobenzene or A-TCNB (cf Fig. 1.6) and anthracene-pyromellitic acid-dianydride or A-PMDA (cf Fig. 6.14). 

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