Cation-Radical Salt

Another redox reaction leading to arenediazonium salts was described by Morkov-nik et al. (1988). They showed that the perchlorates of the cation-radicals of 4-A,A-dimethylamino- and 4-morpholinoaniline (2.63) react with gaseous nitric oxide in acetone in a closed vessel. The characteristic red coloration of these cation-radical salts (Michaelis and Granick, 1943) disappears within 20 min., and after addition of ether the diazonium perchlorate is obtained in 84% and 92% yields, respectively. This reaction (Scheme 2-39) is important in the context of the mechanism of diazotization by the classical method (see Sec. 3.1). 

The first studies on cation-radical Diels-Alder reactions were undertaken by Bauld in 1981 who showed [33a] the powerful catalytic effect of aminium cation radical salts on certain Diels-Alder cycloadditions. For example, the reaction of 1,3-cyclohexadiene with trans, iraw5-2,4-hexadiene in the presence of Ar3N is complete in 1 h and gives only the endo adduct (Equation 1.14) [33]. 

Cation-radical salts with pentafluorophenyl gold(III) anions such as (TTFPh2)2.s[Au(C6F5)2Cl2] and (TTFPh2)[Au(C6F5)2l2], where TTFPh2 is the donor molecule 4,4 -diphenyltetrathiafulvalene, can be performed by electrocrystallization techniques [83]. 

Cerrada, E., Laguna, M., Bartolome, J., Campo, J., Orera, V. and Jones, P.G. (1998) Cation-radical salts with organometallic gold anions. X-ray structure of TTFPh]2[Au(C6F5)2]. Synthetic Metals, 92(3), 245—251. 

Since the discovery of the first organic conductors based on TTF, [TTF]C1 in 1972 [38] and TTF - TCNQ in 1973 [39], TTF has been the elementary building block of hundreds of conducting salts [40] (1) charge-transfer salts if an electron acceptor such as TCNQ is used, and (2) cation radical salts when an innocent anion is introduced by electrocrystallization [41]. In both cases, a mixed-valence state of the TTF is required to allow for a metallic conductivity (Scheme 5), as the fully oxidized salts of TTF+ cation radicals most often either behave as Mott insulators (weakly interacting spins) or associate into... 

Scheme 7 Examples of ortho-diiodo TTFs and selenium analogues used in cation radical salts...

TTF derivatives fused with tetrahydrofurans have been shown to form semiconducting cation radical salts <2002MCLC203>. 

Crystalline cation-radical salts are generally isolated via three preparative procedures by using a nitrosonium (NO+) salt, antimony pentachloride (SbCIs), or triethyloxonium hexachloroantimonate (Et30+SbCl ) as a mild one-electron oxidant. 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

The strongly oxidizing SbCl5 is an effective oxidant for the preparation of cation-radical salts of hexachloroantimonate (SbCl ) from a variety of organic donors, such as para-substituted triarylamines, fully-substituted hydroquinone ethers, tetraarylethylenes, etc.176 For example, the treatment of the hydroquinone ether EA (2 mmol) with SbCl5 (3 mmol) in anhydrous dichloromethane at — 78°C immediately results in an orange-red solution from which the crystalline cation radical salt readily precipitates in quantitative yield upon the slow addition of anhydrous diethyl ether (or hexane)173 (equation 36). 

Triethyloxonium hexachloroantimonate is a selective and mild one-electron oxidant for the facile preparation and isolation of crystalline cation-radical salts from a variety of aromatic and olefinic donors.177 Thus, in a general procedure, a slurry of Et30 + SbCl (3 mmol) and dimethoxytriptycene, DMT (2 mmol) is stirred in dichloromethane at 0°C. The heterogeneous mixture immediately takes on a bright yellow-green coloration (lmax = 466 nm), and on continued stirring (for about 1 h) it is transformed to yield a dark solution of DMT+. The highly pure DMT+ SbCl salt is isolated in quantitative yield by precipitation with diethyl ether (equation 38). 

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

The synthesis of the bis-l,3-dithiolium radical cation (TTF+) in 1970 [1] enabled dramatic growth in the field of molecular conductors in the decades thereafter. TTF and several of its homologues are depicted in Scheme 1. The field of low dimensional molecular metals was further motivated by the discovery of the TTF-TCNQ charge-transfer complex in 1973 [2, 3]. Seven years later, superconductivity was induced in the cation-radical salt, (TMTSF)2PF6, upon application of 12 kbar pressure [4]. Shortly thereafter, superconductivity below 1.4 K was observed at ambient pressure in the perchlorate analog [5]. 

The electron-donor molecule most studied as a component of cation-radical salts is BEDT-TTF. Over 50 superconducting salts containing this molecule have been reported in the literature. Several distinct packing motifs [6-8] of the BEDT-TTF radical cations have yielded superconducting salts, but the most studied of these are... 

Scheme 1 Electron-donor molecules that have been utilized as components of cation radical salts with organometallic anions...

Among organometallic anions, those of the M(CF3)4 (M = Cu, Ag and Au) family have the been most studied as components of cation radical salts. These anions were originally chosen because of their surprising stability and solubility in organic... 

A needle-like morphology sometimes grows as a minority phase during the electrocrystallization process. This phase is particularly intriguing because it has notably higher Tcs (7.2-11.1 K) that approach the record for cation radical salts. (The subscript H designates the phase with the higher Tc.) To date, this phase has only been identified for six compositions, four of which contain the... 

Fig. 9 Well isolated spin chain of [0-Me2TTF(PPh2)2][W(CO)4]2 + cations in the [o-Me2TTF (PPh2)2][W(CO)4]2 [Mo6019] cation radical salt. Hydrogen atoms, phenyl rings, [Mo6Oi9]-2 anions and CO groups have been omitted for clarity...

Cation Radical Salts of the Unsymmetrical Diimine-Dithiolene Complexes. 41... 

Cation Radical Salts of the Organometallic Au(III) Dithiolene Complexes. .. 44... 

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. 

Matsubayashi et al. revealed donor abilities of the unsymmetrical diimine-dithiolene complexes [11-14]. The unsymmetrical complexes provided cation radical salts with various anions including I3, Br3 and TCNQ by use of chemical oxidation [11-14]. The electrical resistivities of the cation radical salts measured with their compressed pellets at room temperature are summarized in Table 1. The electrical resistivities of the dmit complexes were very high. The cation radical salts of the CgH4Sg-complexes, which have the BEDT-TTF moiety [22, 23], exhibited lower resistivity than those of dmit complexes, except for [(Bu-pia)Pt(CgH4Sg)] salts. However, crystal structures of these salts were not reported, and details of their electrical properties and electronic states were not discussed based on their crystal structures. 

Crystal Structure of the Cation Radical Salt [(bpy)Pt(C8H4S8)][BF4]... 

Crystal structure data are indispensable for the discussion of the conduction mechanism in the cation radical salts based on the unsymmetrical complexes. In 2002,... 

Table 1 Cation radical salts based on the unsymmetrical donors and their...

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