Tetrathiafulvalene cation-radicals

The redox reaction shown in Scheme 7.60 results in the formation of an amide a-radical and tetrathiafulvalene cation-radical. These initially formed a-radical and cation-radical combine to give salts of the S-arylated tetrathiafulvalene (a minor product) and C-alkylated tetrathiafulvalene (the main product). The latter demonstrates an unprecedented carbon-carbon bond formation with the cation-radical of tetrathiafulvalene the structure depicted was confirmed by single crystal x-ray analysis (Begley et al. 1994). 

Keywords Cation-radical salts, Molecular conductors, Organic superconductors, Organometallic anions, Tetrakis(trifluoromethyl)metallates, Tetrathiafulvalene... 

The convergent synthesis of a range of aryl ester dendrimers with peripheral tetrathiafulvalene units was also reported (Devonport et al. 1998). The dendrimers acquire some amount of the cation-radical tetrathiafulvalene tips on reacting with iodine in solutions. Another promising material is polyphenylene dendrimers functionalized with benzophenone units. On being transformed into the potassium ketyl derivatives, the dendrimers bring forth intermolecular biradicals. These technically attractive species contain four-membered cycles that coordinatively combine two potassium cations... 

It needs to be noted that Q + Q = Q + equilibrium is possible due to the rapid consumption of Q reducer. More often, the interaction of two identical ion-radicals can lead to their dimerization, but not disproportionation. For instance, the anion-radicals of tetracyanoethylene undergo dimerization (Lu et al. 2003). According to Miller and Novoa (2007), this dimerization proceeds as four-center carbon-carbon bonding. Rosokha and Kochi (2007) proposed the term n interaction for dimerization of the cation-radicals of tetrathiafulvalene. Because the n dimers are double-charged species, solvent polarity is essential for their formation. The n interaction overcomes the electrostatic repulsion, even in moderately polar enviromnents. 

Calculations indicate that the unpaired electron density in the cation-radical of tetrathiafulvalene resides principally on sulfur, hut with the internal carbon being the site of second highest density. The product of coupling of an a-carbonyl radical to sulfur, an a-carbonyl-sulfonium salt would be destabilized by the adjacent dipoles. The transition state would be expected to mirror this, thus slowing down the C-S coupling and permitting the observed coupling to the carbon of tetrathiafulvalene. 

The incorporation of tetrathiafulvalene (TTF) into dendrimers presents a fascinating prospect for the following reasons (1) oxidation of TTF to the cation radical and dication species occurs sequentially and reversibly at very accessible potentials (for unsubstituted TTF, = +0.34 and eY = +0.78 V, vs. Ag/AgCl) (2) the oxidation potentials can be finely tuned by the attachment of appropriate substi-... 

The copper-porphyrin complex gives cation radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation radical, which belongs to the -rr type (Ehlinger Scheidt 1999). A new bifunctional tetrathiafulvalene-type donor molecule (D-ct-D) with a copper iodine bridge has recently been synthesized. Its cation radical salt, (D-variable valence (Ramos et al. 1997). 

The convergent synthesis of a range of aryl ester dendrimers with peripheral tetrathi-afulvalene units was also reported (Devonport et al. 1998). The dendrimers acquire some amount of the cation radical tetrathiafulvalene tips upon reaction with iodine in solutions. 

In contrast, the cation-radical salt (TTF)[Au(C6F5)C1] (TTF = tetrathiafulvalene) was prepared by combination of tetrathiafulvalene with an organoaurate(I) salt, under a controlled current55 [Eq. (9)]. 

One-dimensional stacking of the molecules may provide a framework for possible band structure formation. In this case, repeat units should preferably have an overall planar geometry. This concept can be extended to two-dimensional (2D) structural arrangements where interactions between the molecules are found along two directions as in cation-radical salts derived from TTF-like molecules such as BEDT-TTF (BEDT-TTF = C10H8S8, bis(ethylenedithio)-tetrathiafulvalene) (13), or even to three-dimensional (3D) systems such as the doped A Ceo fullerides (47). 

Charge-transfer processes occurring across the liquid-liquid interface have also been studied by EPR. The Galvani potential difference between the two immiscible solvents, water and 1,2-dichloroethane (DCE), was controlled electrochemically by means of a bipotentiostat. The water phase contained potassium ferrocyanide, which, in the DCE phase, by electrochemical polarization of the interface, can reduce a compound such as tetracyanoquinodimethane to its radical anion or oxidize a compound such as tetrathiafulvalene to its cation radical. Both radicals were detected by EPR spectroscopy [79]. 

Cyclic voltammetry on the cation 192 was not reversible attempted reoxidation of the reduction product of 192 produced the cation-radical of the tetraselenafulvalene 193. Apparently, the initially formed radicals dimerize rapidly with elimination of the SeEt groups at ambient temperatures. The cation-radical of 193 and various substituted derivatives have been reported,As with the sulfur analogs (see Section III,B,7,b), no comprehensive survey of the solid-state properties of these materials is attempted. In general, it is found that 193 and its derivatives, although rather more difficult to oxidize than tetrathiafulvalene counterparts, nevertheless do form electrically conducting salts and complexes. 

Table 5 Half-Wave Potentials for the Oxidation of Some Tetrathiafulvalenes and Related Compounds to the Corresponding Cation Radicals and Dications ... 

Similarly to tetrathioethylenes and tetrathiafulvalenes, 1,4-dithiin (LV) and its benzoder-ivatives (LVI and LVII) are electron-rich compounds that can be easily oxidized to their corresponding cation radicals and dications. There are several reviews [2, 3, 186, 187] on the chemistry of the radical cations of dibenzo-1,4-dithiin (LVH) and related compounds. 

Interestingly, an unusual mixed-valence state of a stacked tetrathiafulvalene (TTF) dimer was observed within the coordination host 1 at ambient temperature in an aqueous solution [10], When excess TTF was added to an aqueous solution of 1, the colorless solution became dark green in color because of the formation of a 1 D (TTF)2 complex (Fig. 4.3a). Electrochemical studies revealed that an initial one-electron oxidation occurred at -150 mV that led to the mixed-valence dimer, whereas a second one-electron oxidation at -300 mV afforded the cation radical dimer. The mixed-valence state was also indicated by the appearance of a broad absorption band in the near-infrared region ( max = -2000 nm) of the UV-vis spectrum (Fig. 4.3b). The host framework effectively forced the two molecules of TTF into close proximity in the cavity. As a result, the labile mixed-valence dimer was protected from oxygen and solvent molecules. 

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