Radical-cation salts of the arenes

In Sect. 9.5, we treated the radical-anion salts of DCNQl. There, we especially emphasized the often strong effects of minor variations in the organic anions and/or the inorganic cations on the physical properties of the CT salts. In this Sect. 9.6, we treat the radical-cation salts of the arenes. First, we want to present some different experimental methods for studying the physical properties of CT salts using as an example the (Fa)2PF6 crystal. 

The Peierls instability and the high degree of one-dimensionality are observable in a whole series of different experiments. These include the dc conductivity in low applied fields (see Sect 9.6.1 and Fig. 1.13), the diffuse reflections of the Ikp superlattice in X-ray scattering (Sect 9.6.2), the reflection spectra from the FIR up to the UV spectral ranges (Sect 9.6.3), the magnetic susceptibihty (Sect 9.6.4), the conduction electron spin resonance, and nuclear resonances (Sect 9.6.5), as well as the nonlinear electrical conductivity at high apphed electric fields or at high frequencies (Sect. 9.6.6). Most of these methods are also employed for the study of the other radical-ion salts, e.g. TTF-TCNQ or the DCNQl salts. They will therefore be treated as examples in this Sect 9.6. 

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Table I. Conditions of the Electrocrystallization and Composition of Radical-Cation Salts of Arenes...

As knowm for typical one-dimensional organic metals most radical cation salts of arenes are arranged in columnar structures with the crystallization of planar aienes in segregated stacks and crystallographically uniform spacing within the stack elements which are present in a partial oxydized form. Therefore anisotropies of at least factor 100 have been reported together with specific electrical conductivities within the stack direction of nearly 1000 S/cm, ... 

Mariano and coworkers have exploited this PET bond cleavage chemistry in intramolecular as well as intermolecular cyclization reactions [38]. The combined iminium-benzylsilane functionalities undergo intramolecular PET to provide an intramolecular amine radical/benzyl silane cation-radical pair by exciting either the iminium salt or the arene chromophore [38]. Cleavage of the benzyl C—Si bond presumably takes place with assistance of moderately weak n ldeophiles such as methanol. The diradical couples intramolecularly to provide an indolizidine derivative [38] in 90% yield. 

Polycyclic arenes, e.g. perylene, have been widely studied in the preparation of molecular conductors, some of the radical cations show semiconducting or metallic behavior [418]. Introduction of one or more sulfur atoms at the periphery of such systems, i.e. thia arene derivatives, generally imparts greater stability to the radical-cation salts, coupled with increase conductivity [419]. For compound 116, X-ray structure studied have been reported on the pure donor and some radical-ion salts [420]. 

The exciplexes formed between arenes and good acceptors such as dicyanobenzene can dissociate to radical ion pairs if the solvent used is sufficiently polar. The radical cation of the arene is then susceptible to attack by nucleophiles and this can lead to products of addition or substitution of the arene. A preliminary account of how salts such as tetrabutylammonium tetrafluoroborate can serve to mediate the charge separation in less polar solvents was reported last year. A second paper from the same group has now been published which describes the formation of the amino substituted dihydrophenanthrene (86) during the irradiation of a solution of phenanthrene, dicyanobenzene and propylamine in relatively non-polar solvents such as THF in the presence of tetrabutylammonium tetrafluoroborate. In the absence of the salt no product is formed. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

The mechanism by which a nucleophile displaces the diazonium group depends on the nucleophile. While some displacements involve phenyl cations, others involve radicals. Nucleophiles, e.g. CN , Cl and Br , replace the diazonium group if the appropriate cuprous salt is added to the solution containing the arene diazonium salt. The reaction of an arene diazonium salt with cuprous salt is known as a Sandmeyer reaction. 

Nuclear597 or side-chain588,598 acetoxylation of arenes can be performed with good yields by persulfate and copper(II) salts in acetic acid (equations 268 and 269). As previously shown for cyclohexene (equation 263), persulfate oxidizes the aromatic ring to a radical cation which loses a proton to give a carbon radical, which is further oxidized by copper(II) acetate to the final acetoxylated product. 

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