Radical ion salts

Single-Stack Donor. Ion-radical salts can also be formed from electron donors such as tetrathiafulvalene (TTE) or TMPD (N,N,N N-tetramethyl- phenylene diamine) with inorganic acceptors such as halogens. The resulting stmcture of compounds such as TTE(A)... 

Eullerene-based donor-acceptor complexes and ion-radical salts with tetrathia-fulvalenes, metalloporphyrins, and cyclic amines as donors 99UK23. 

Several crystal structure determinations of [AuX2l salts with different cations have been carried out for [AuC12],1823,1904,2854,3065-3068 [AuBr2],3065,3069,3070 or [AuF]-.3065 3066 3071-3074 Many other structural determinations have been reported in which [AuX2]- salts act as counterions of conducting or superconducting ion radical salts as bis-ethylenedithiotetrathiafulvalene (ET) and related organic donors. 

In contrast, ferrocene, l,T-dimethylferrocene, decamethylferrocene, and 1,2-diferrocenylethane all react with DDQ forming a dark green 1 1 ion-radical salt. Both (DDQ) and ferrocenium constituents of the salts were characterized by their typical electron spectra (Salman et al. 2004). Sometimes, even a weak charge transfer can transform the ferrocenyl substituent into the full-value hole reservoir acting intramolecularly. Scheme 1.31 gives two such examples. One of them describes the effect of charge-transfer coordination between Sc + as an acceptor and a ferrocene derivative as a donor (Qkamoto et al. 2003). The other example introduces the effect of charge-transfer coordination between pyridine as a donor and a ferrocenium derivative as an acceptor (Hillard et al. 2006). 

The problem of preparation of pure ion-radical salts in solid state is very important technically. This problem is decisive in new application fields snch as organic conductors, semiconductors, and magnets. Especially, Chapter 8 of this book considers methods for the preparation of solid ion-radical salts for these materials. 

Research on spin-charge separation in distonic ion-radicals has been carried out in recent years with an emphasis on theoretical calculations. Experiments were performed to prove their existence and observe their behavior in a mass spectrometer chamber. The next step is likely to emphasize the synthesis of the distonic ion-radical salts, which could be stable under common conditions. Applications of the salts would be possible in creating magnetic, conductible media and other materials possess practically useful properties. The attractive strength of distonic ion-radicals is that they can enter ionic reactions at the charged site and radical reactions at the radical site. Success in this direction can open a new window in terms of organic reactivity. 

Magnetic susceptibility of paramagnetic particles is used to determine the concentration of ion-radicals but yields no structural information. The method often demands solid samples of ion-radical salts. Many ion-radical salts are unstable in the solid state, and this requirement turns out to be a decisive limit. Fortunately, there are special ways to determine magnetic susceptibility of paramagnetic particles in solutions (Selwood 1958). However, instruments for such measurements are rarely used in chemical laboratories. Besides, special devices should be used to conduct investigations at different temperatures. 

Organic ion-radicals exist as salts with counterions. As seen in the preceding chapters, neutral molecules with strong acceptor/donor properties form rather stable ion-radical salts. Under certain conditions (see later), components of an ion-radical salt pack up in a crystal lattice in a special manner. Different ion-radical parts (cations and anions separately) line up one over the other in the form of endlessly long, practically linear one-dimensional (1-D), piles-chains. These piles-chains form a crystal. 

Thus, the ion-radical salt of BTDM-TTF (ET) with TCNQ has a high condnctivity 1.3 X 10 cm at room temperatnre with metallic behavior down to 26 K (Rovira et al. 1994). The strik-... 

There is a rule that donor and acceptor components for a planned ion-radical salt should have more or less symmetrical molecular geometry. However, if substituents are not so bulky, they can be located unsymmetrically without detriment to conductivity (Tatemitsu et al. 1985). Scheme 8.11 also testifies that such a rule should be understood dialectically. 

The direct donor-to-acceptor interaction is the best way to limit chemical impurities. In this case, the reaction mixture contains minimal amounts of substances that are not included in the structure of a given ion-radical salt. The oxidation of donors in the presence of anions or ion exchange usually results in the formation of less pure crystals. 

The correct choice of the method for growing crystals is very important. Slow crystallization of a reaction mixture at a fixed diffusion rate can be achieved by means of a H-shaped tnbe. Separate flasks with the solutions of a donor and an acceptor are joined by the H-shaped overtnmed tnbe. The tube is filled with a solvent. A donor and an acceptor slowly diffuse together, react, and form a crystalline product. As a rule, this process proceeds at room temperature. The method leads to the most perfect crystals of an ion-radical salt. 

In the H-shaped tube, all processes take place in a slow, controlled way. One simple modification consists of separating the two halves of the H-shaped tnbe with a fitted glass disk (medium, fine, or ultra-fine porous) to slowdown the diffusion. Another modification involves diffusion inside the reaction solvent containing a polymer. In this case, diffusion is retarded due to an increase of the solution viscosity (Scott et al. 1974, Berg et al. 1976). Sometimes, the synthesis of ion-radical salts is conducted ultrasoni-cally if the starting materials are insoluble in the desired solvents (see, e.g., Neilands et al. 1997). 

As mentioned earlier, much attention was being given to the formation of ion-radical conductors in the appropriate crystalline form. Meanwhile, Ziolkovskiy et al. (2004) reported data on high conductivity at 77-300 K of the methyl-TCNQ anion-radical salts with A-alkylpyridinium cations that keep their conductivity after crystallization from the melted forms. The melting temperatures of the salts described are rather low and the melting proceeds without salt destruction. This feature opens a possibility to create definite, much essential constructive elements directly from the liquid phase. Importantly, these salts also possess affinity to metals due to the metal-nitrogen coordina-tive ability. The authors notice that such ion-radical salts are promising for use in electronics and microelectronics. 

Electric conductivity of ion-radical salts arises from the mobility of their unpaired electrons. At the same time, each of the unpaired electrons possesses a magnetic moment. This small magnetic moment is associated with the electron quantum-mechanical spin. Spin-originated magnetism as a phenomenon is described in many sources (see, e.g., monographs by Khan 1993, Bauld 1997, Itokh and Kinoshita 2001 and reviews by Miller 2000, Miller and Epstein 1994, 1995, Wudl and Thompson 1992). This section is, naturally, devoted to the organic magnets based on ion-radicals. 

The problem of the ferromagnetism of a solid ion-radical salt has two important points. One is ferromagnetism at the level of a salt as the molecule, which consists of two paramagnetic species. Another is ferromagnetism at the level of a solid sample formed from assemblies of the spin-bearing molecules. These molecules may contain one or more magnetic centers. 

The theoretical design of donor oligomers that gives parallel spins upon electron transfer is reported (Mizonchi et al. 1995). TTF and TSeF were nsed as the donor units in the corresponding ion-radical salts. Reviews by Enoki and Miyazaki (2004) as well as Turner and Day (2005) consider and explained magnetic properties of these systems from physical background. 

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