Radical-anion Salts

Compared with the conducting anion radical salts of metal complexes, the number of molecular conductors based on cationic metal complexes is still limited. Donor type complexes M(dddt)2 (M = Ni, Pd, Pt Fig. 1) are the most studied system. The M(dddt)2 molecule is a metal complex analogue of the organic donor BEDTTTF. Formally, the central C=C bond of BEDT-TTF is substituted by a metal ion. The HOMO and LUMO of the M(dddt)2 molecule are very similar in orbital character to those of the M(dmit)2 molecule. In addition, the HOMO of the M(dddt)2 molecule is also very similar to that of BEDT-TTF. More than ten cation radical salts of M(dddt)2 with a cation (monovalent) anion ratio of 2 1 or 3 2 are reported [7]. A few of them exhibit metallic behavior down to low temperatures. The HOMO-LUMO band inversion can also occur in the donor system depending on the degree of dimerization. In contrast to the acceptor system, however, the HOMO-LUMO band inversion in the donor system leads a LUMO band with the one-dimensional character to the conduction band. 

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. 

One case of n—5 —n delocalization was demonstrated by Stevenson et al. (2006). The potassinm anion-radical salt of l-(9-methyl-9H-fluoren-9-yl)-4-methyl benzyl is characterized by the delocalization of an nnpaired electron within the fluorenyl moiety only. Its ESR spectrnm completely coincides with the spectrnm of the potassium anion-radical salt of the 9,9-dimethyl fluorene anion-radical in THE However, the cesium anion-radical salt of the fluorenyl methylbenzyl derivative produces the ESR spectrum corresponding to the placement of this cation between the fluorenyl and methylbenzyl moiety. The conditions of n—s—n delocalization appear An unpaired electron spends its time within both fluorenyl and methylbenzyl fragments. The situation is explained in Scheme 3.54. 

The dispersity or homogeneity of the reductant in a reaction system sometimes plays a decisive role. It is also important for synthetic practice. Crandall and Mualla (1986) compared reduction of 7-methylocta-5,6-diene-2-one [H3C-C(CH3)=C=CH-CH2-CH2-C(0)-CH3] in THF by the action of naphthalene-sodium, on the one hand and, by sonically activated sodium on the other. In both the cases, one-electron transfer yields the anion-radical salt of the allenic ketone with sodium. However, only in the case of sonicated sodium is this salt stabilized, eventually giving H3C-C(CH3)=C=CH-CH2-CH2-C(0H)-CH3 along with cyclic products (l-methyl-2-isopropylidene cyclopentanol and l-methyl-2-isopropylcyclopent-2-enol). If naphthalene-sodium is used, only the cyclic alcohols are obtained as mentioned earlier. 

It was found that the intercalation of Cgo fullerene by an alkali metal in stoichiometric ratio (1 1) gives rise to the formation of anion-radical salts, namely, KC50, RbCgg, and CsCgo (Bommeli et al. 1995, Btouet et al. 1996). On slow cooling of the intercalation products, [2 + 2] cycloaddition of the fullerene species that is neighboring a crystal lattice occurs. Linear chain fullerenic polymers are formed. These polymers are stable in air, insoluble in THF, and possess metallic conductivity. They depolymerize only on heating above 320°C. 

Interestingly, if the C50 fullerene doped by alkali metals is rapidly cooled down to the liquid nitrogen temperature, polymerization does not occur. Only monomeric anion-radical salts are obtained. Warming up these monomers to 80-160 K results in dimerization polymerization does not take place. The dimer (KCgo)2 is dielectric (Pekker et al. 1995). It has been shown that the tris(anion)-radical Cgo can polymerize too. Particularly, Na2CsCgo forms a polymer that maintains superconducting properties (Mizuki et al. 1994). 

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. 

A betainic structure is very effective in decreasing 17eff according to the LeBlanc s proposal for the TCNQ anion radical salt (eq. 1), where a is the molecular polarizability of the cation and r is the distance between TCNQ anion radical and a cation [67] ... 

CT solids of fullerene Cgo with a number of different inorganic cations have shown metallic or superconducting properties (for superconductivity, see Sect. 5.2.3). Among the fullerene metals, the best known families are MCso anion radical salts... 

LeBlanc OH Jr (1965) On the electrical conductivities of tetracyanoquinodimethan anion-radical salts. J Chem Phys 42 4307 308... 

More recently, charge-transfer emission was anticipated when solutions of hydrocarbon anion radical salts in dimethoxyethane were mixed with Wurster s blue perchlorate.15 Emission was seen in every instance however, with eight anion radicals derived from 3 to 5 ring-fused aromatic hydrocarbons, the emission was derived from the hydrocarbon rather than the complex. Preliminary studies with smaller hydrocarbons, biphenyl and naphthalene, did show emission in the region (18 kK) where charge transfer was expected. The question as to what pairs of ion radicals will be emissive under what conditions has only begun to be considered. Much opportunity for further experimentation exists in this area. 

Benzene and toluene anion radical salts are intriguing examples. Benzene and toluene anion radicals prepared as Cs+ salts were shown to dimerize readily in THF at 70°C. 3,3 -Bis(cyclohexa-l,4-diene) and the corresponding toluene l,l -dimethyl analog... 

The equilibrium constant for the reaction of the electron transfer from the anion radical salts of aromatic compounds (with the usual isotope content) to neutral molecules of the same compounds containing heavier isotopes is less than unity (entries 1-10 in Table 2-1). This means that for heavier compounds (enriched with neutrons), the electron affinity is smaller. This difference is conserved at different temperatures and reaction mediums (including those favorable to the destruction of ionic pairs—in HMPA and in THF containing 18-crown-6). 

No. Starting aromatic compound Starting anion radical salt °C Solvent Keq Ref.a... 

Any (cation-anion) radical salt has two unpaired electrons One belongs to the cation radical and the other to the anion radical. If the spins of these two electrons have an antiparallel orientation, the corresponding magnetic moments are compensating. If both spins are in parallel orientation, their magnetic moments are added together. 

SPECTRAL AND ELECTROPHYSICAL PROPERTIES OF ANION-RADICAL SALTS OF TCNQ AND METHYL-TCNQ WITH N-ALKYLPIRAZINIUM CATIONS... 

Key words 7,7,8,8-tetracyanoquinodimethane (TCNQ), methyl-TCNQ, anion-radical salts, IR absorption spectra, electron-phonon interaction, electric conductivity. 

Tablet. Properties of anion-radical salts of TCNQ andMTCNQ vmin beginning of the continuos absorption in IR spectrum A - the values of conductivity activation energy obtained from the data of resistive measurements aRT - specific conductivity at room temperature.

Patent 1696428 USSR, subm. 4706057 prior, from June 15, 1989, regist. August 8. 1991 // Anion-radical salt of 2-methil-7,7,8,8-tetracyanoquinodimethane as an addend to organic conducting materials // V.A.Starodub, Ye.M.Gluzman, K.V.Krikunov et al. 

It has been observed that tbe conductivity was actually higher than the conductivity reported for pure diimide anion radical salts, because dendrimeric structures may produce 7t-s lac king formation in three dimensions, a structural feature differing from the usual one-dimensional structures of small molecular stacks. 

The formation of nonstoichiometric cation-radical and anion-radical salts can be treated in an analogous manner to the formation of CT complexes. For instance, chemical or electrochemical oxidation of some of the donor molecules in solution leads to the formation of some oxidized molecules, namely cation radicals, which are actually strong acceptors. The latter could easily interact with neutral donor molecules to produce partially positively charged stacks in which the charges are compensated by the counter anions. Such interactions in cation-radical or anion-radical salts could afford nonstoichiometric salts with a formal partial degree of charge transfer, which is determined by the ratio m n in DmX , where X is a counteranion. Representative examples are listed in Table 12. 

Tens of conductive LB films have been developed so far, including metallic and superconductive LB films. These LB films are classified into the categories anion radical salt, charge-transfer complex, cation radical salt, conducting polymer, and transition metal complex in this section. The LB films, with metallic temperature dependences of conductivity, and the fullerene LB films, which exhibit a superconducting transition, are discussed separately. 

The first conductive LB film, consisting of C PyfTCNQ) doped with iodine (Fig. 3) belongs to the class of anion-radical salts [20]. The insulating C PyCTCNQ) LB film was rendered conductive by iodine doping, and the bulk conductivity was about 0.01 S/cm. The nominal charge of TCNQ of... 

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