Naphthalene, Cation-Radical Salts

As it was first shown on naphthalene radical cation salts of various aromatic hydrocarbons can be prepared by anodic oxydation and subsequent crystallization in solvents like methylenechloride, chlorobenzene, 1,1,2-trichloroethane or THF if an electrolyte like NR4X with suitable anions X is present (R n-hutyl X BF4, C104, PF5 , AsF6% SbF6, ...) l... 

A few examples in which aromatic cation radicals have been isolated as crystalline salts, actually consist of mixed valence units. For example, crystal structure analysis showed that the naphthalene radical cation (NAP) + forms a mixed valence dimer (NAPy4 in which the two components are arranged face to face in n-stacks with an interplanar separation significantly closer than van der Waals contacts. Such an intermolecular organization arises from the sponta-... 

The stability of radical-cation salts varies from rapid decomposition in air (e.g., naphthalene salts) to stability for several months under ambient conditions (e.g., perylene and decacyclene salts). The stability correlates with the oxidation potential As a rule of thumb, salts of extendend aromatic systems are more stable than those of smaller ones, and salts of the ideal 2 1 composition are more stable than those of other compositions. 

Another view has recently been proposed by Wegner.Naphthalene and other simple aromarics can be oxidize electrochemi-cally to form monomelic radial cat n salts (Ar. X ) which have conductivities of 10 to 10 s/cm. The crystal structures of these reveal that the aromatic moieties form stacks, along which the charges and the electrons are presumably delocalized. The structure is formally analogous to that deduced for oxidized (doped) polyacetylene in which the polyene chains are arranged in stacks. This leads to the idea that intermolecular delocalization is the important feature which leads to high conductivity. Other data are consistent with this rationale. Biphenyl and terphenyl radical cation salts have crystal structures very similar to that of oxidized (doped) poly(p-phenylene lO). In the older literature oligoanilines (26) are reported upon iodine treatment to yield conductivities up to 1 s/cm the aniline moieties are stacked in these materials as well. Poly(N-vinyl-carbazole) (27) forms radical cation structures by oxidation with... 

The direct reaction between a radical cation derived from an aromatic hydrocarbon (naphthalene) and SCN has been studied by mixing the salt (CioH8)2PF6 with a solution of BU4NSCN in CH2CI2 at —78 C [158]. Two major products were isolated, 1-naphthylthiocyanate (kinetic control, 28%) and 2-naphthyl-wo-thiocyanate (thermodynamic control, 8%). The mechanistic details leading to this product distribution are not clear. 

A photochemical study- of the electron-transfer reactions of a sulfonium salt, 4-cyanobenzylmethylphenylsulfonium tetrafluoroborate, has been reported to give phenyl methyl sulfide quantitatively. 9,10-Dimethylanthracene and naphthalene have both been used in the sensitised reactions of triphenyl-sulfonium hexafluoroantimonate and the reactions encountered involve the singlet states and produce the radical-cation of the sensitiser and phenyl radicals. The phenyl radicals are formed by bond fission within the neutral triphenylmethyl radical. The reactions of some aromatic sulfonium compounds have been patented for use in resin compositions. ... 

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. 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

This scheme explains the loss of substrate selectivity in nitration with nitronium salts and the high position selectivity which is determined by the spin density distribution in the radical cation (see above) and by the relative stability of the formed a-complexes. In favour of this scheme naphthalene is electrochemically oxidized in CH3CN with NO2 at a lower oxidation potential than that of NOj but sufficient to fonn the radical cation of naphthalene 1- and 2-nitronaphthalenes are formed in the same ratio (9 1) as from nitration by HNO3 and H SO in CH3CN. Later, however, the formation of nitronaphthalenes was shown to be mainly, if not completely, due to the nitration of naphthalene by catalyzed by the acid... 

The authors proposed the following picture of the silylene anion-radical formation. Treatment of the starting material by the naphthalene anion-radical salt with lithium or sodium (the metals are denoted here as M) results in two-electron reduction of >Si=Si< bond with the formation of >SiM—MSi< intermediate. The existence of this intermediate was experimentally proven. The crown ether removes the alkali cation, leaving behind the >Si - Si< counterpart. This sharply increases electrostatic repulsion within the silicon-silicon bond and generates the driving force for its dissociation. In a control experiment, with the alkali cation inserted into the crown ether, >Si — Si< species does dissociate into two [>Si ] particles. 

A similar situation is observed in the case of the cation-radical derived from dimethylhexa-phenyl diphosphafnlveninm dication as a resnlt of redaction of the latter by the sodinm salt of the naphthalene anion-radical (Biaso et al. 2006). According to ESR, x-ray experimental data, and results of DFT calculations, the diphosphafnlveninm cation-radical detains the excess electron density within the exocyclic double bond (see Scheme 3.41). Stabilization is gained by conjngation with the two pentavalent phosphorus atoms. Biaso et al. (2006) rationalize the electronic strnctnre throngh two valence-isomeric fluidic forms of the distonic type. 

Dektar and Hacker have studied the sensitized photolysis of sulfonium salts extensively [70,83]. Like diphenyliodonium cation, triphenylsulfonium is reduced by anthracene singlet to triphenylsulfur radical which cleaves incage to yield phenyl radical and diphenylsulfide [91]. Naphthalene sensitized photolysis of triarylsulfonium salts yielded some of the same photoproducts observed in direct photolysis, namely arylated diarylsulfides [94a] ... 

The spectrophotometric technique determines K whenever the fraction of free ions is very low. The concentration of the free ions may be reduced to an insignificant level by the addition of some readily dissociated salt sharing a common cation with the investigated radical anion. On the other hand, the potentiometric technique yields K, and its value can be used to calculate K if the necessary dissociation constants are known. These constants may be derived from conductometric data (5). For an anthracene and pyrene pair incorporating Na+ as the counterion and tetrahydrofuran (THF) as the solvent, the ratio of equation 8 is only 1.6, which is equivalent to 10 millivolts (mV). However, for an anthracene and naphthalene pair, the ratio is 30.3, which is equivalent to 90 mV. 

Eberson and Radner (19-23) have explored some of the scope of the reaction of N02 with ArH +. They prepared the solid hexafluorophosphates of the naphthalene cation radical and some methylnaphthalene cation radicals, for example, C10H8,+PF6, and carried out reactions of the solid salts with N02 in dichloromethane at temperatures near-25 °C (19-23). Reaction was found to occur according to equation 12. The results of their reactions are summarized in Table I (19). This table also lists the results of nitrating the parent hydrocarbons by the conventional route (N02 + ) and with N204. The results prompted Eberson and Radner to dismiss the possibility that conventional nitration of these compounds was preceded by electron transfer (equations 11 and 12), because the proportions of isomers from the reaction of ArH + with N02 were quite different from those from the reactions of ArH with N02+ and with N204. Eberson and Radner set out to determine whether or not the conventional nitration of naphthalene and methylnaph-thalenes involved the electron-transfer step. In so doing they became, to our knowledge, the first to show that a neutral radical (N02) will react with an aromatic cation radical. 

Fig. 1. The crystal structure of neutral naphthalene (A) and of the cation-radical salt (naphthalene)2 5 (B).

The room temperature conductivity of polycrystalline samples of the naphthalene cation radical salts lies between 0.1 and 1 (Q cm) . Single crystals measured along the stack direction show a conductivity of several 100 (Q cm) depending on the quality of the crystals and to some extent on the nature of the counterions, solvent inclusion etc. The naphthalin cation radical salts can be stored for sometime at room temperature, if moisture is excluded. The corresponding radical ion complexes of perylene, pyrene, fluoranthene, and other arenes of higher number of fused rings exhibit a much greater stability and can be handled under normal laboratory conditions. 

Yamashita, T, Tsurusako, T, Nakamura, N., Yasuda, M., and Shima, K., Electron transfer photosensitized oxygenation of stilbene and naphthalene derivatives in the presence of acetate ion controlling the reaction of the cation radicals by weak-nucleophilic salts. Bull. Chem. Soc. Jpn., 66, 857, 1993. 

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