Cation-Radical Salts of Arenes

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, ... 

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. 

Table 1. Radical cation salts of simple arenes...

Kispert, L. D., J. Joseph et al. (1987). EPR study of arene radical cation salt crystals. Synthetic Metals 20 67-72. 

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]. 

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. 

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. 

In appropriately polar solvents, diazonium salts decompose to nitrogen and aryl cations, and formation of arenes may take place by transfer of hydrogen from the reducing agent or from the solvent. Depending on reaction conditions, an ionic mechanism (equation 87)," or a radical mechanism (equation 88) " may be operating, and experimental evidence is claimed for both. However, the radical mechanism seems to have more support. 

The exciplexes formed between excited arenes (e.g. phenanthrene, naphthalene, 2,3-dimethylnaphthalene) and the acceptor 1,4-dicyanobenzene in ether solutions containing 1-propylamine are quenched by the addition of tetramethylammonium tetrafluoroborate, and products of addition, (129)-(131), are obtained. The authors suggest that under the relatively non-polar conditions the added salt promotes charge separation in the exciplex so that the amine can attack the arene radical cation. 

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. 

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