Charge transfer salts

Typical charge-transfer salts form as stacks of planar D and A molecules, though the ratio of D A need not be 1 1, as the interaction can be spread over more than two molecules. The amount of charge transfer (5) per unit in the solid may be less than unity, with partial charges residing on the... 

Single-Stack Acceptor. Simple charge-transfer salts formed from the planar acceptor TCNQ have a stacked arrangement with the TCNQ units facing each other (intermolecular distances of ca 0.3 nm (- 3). Complex salts of TCNQ such as TEA(TCNQ)2 consist of stacks of parallel TCNQ molecules, with cation sites between the stacks (17). The interatomic distance between TCNQ units is not always uniform in these salts, and formation of TCNQ dimers (as in TEA(TCNQ)2) and trimers (as in Cs2(TCNQ)Q can lead to complex crystal stmctures for the chainlike salts. 

Charge-Transfer Salts. Most charge-transfer salts can be prepared by direct mixing of donors and acceptors in solution. Semiconducting salts of TCNQ have been prepared with a variety of both organic and inorganic counterions. Simple salts of the type TCNQ can be obtained by direct reaction of a metal such as copper or silver with TCNQ in solution. Solutions of metal iodides can be used in place of the metals, and precipitation of the TCNQ salt occur direcdy (24). 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

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... 

The charge transfer salt (BDNT)2[Ni(mnt)2] (BDNT = 4,9-bis(l,3-benzodithiol-2-ylidene)-4,9-dihydronaphtho[2,3-c]l,2,5-thiadiazole) also shows ferromagnetic interaction with /=3.4K, which was concluded to arise from the [Ni(mnt)2]- component on the basis of EPR evidence. In the absence of structural data, however, further understanding of this behavior could not be obtained. The anion was shown to be monoanionic, hence the valence of BDNT is +0.5.1051... 

Despite considerable efforts, the formulation in equation (42) remains incomplete owing to the high reactivity of organocuprates as well as their oligomeric nature. Accordingly, we select organoborates as stable electron donors to study alkyl additions to various pyridinium acceptors (by thermal and photoinduced electron transfer) via charge-transfer salts as follows. 

Interestingly, the methyl-transfer reaction between BMeT and 3-cyano-TV-methylpyridinium (with the highest reduction potential, see Table 7) occurs instantaneously and thus precludes the isolation of the charge-transfer salt. 

Table 7 Absorption spectra of [Py+, BR4 ] charge-transfer salts".

X-ray crystallographic analysis of the crystalline [bicumene, NO+] charge-transfer salt confirms that the charge-transfer color arises from a close approach of NO+ to the centroid of the phenyl moiety (see Fig. 10) with a non-bonded contact to an aromatic carbon of 2.63 A.194 The orange solution of bicumene bleaches slowly over a long period in a thermal reaction at room temperature (in the dark) or rapidly via irradiation of the CT band at low temperature. In both cases, l,l,3-trimethyl-3-phenylindane is obtained as the principal organic product (equation 63). 

Fig. 11 Transient spectra obtained upon the application of a 200-fs laser pulse to a solution of [Ph2C(0H)C02, MV2+] charge-transfer salt showing the simultaneous formation of benzophenone ketyl radical (dashed line) and the reduced methyl viologen (dotted line). The inset is the authentic spectrum of ketyl radical. Reproduced with permission from Ref. 92a.

In recent years, the amount of research time devoted to materials chemistry has risen almost exponentially and sulfur-based radicals, such as the charge-transfer salts based upon TTF (tetrathiafulvalene), have played an important role in these developments. These TTF derivatives will not be discussed here but are dealt with elsewhere in this book. Instead we focus on recent developments in the area of group 15/16 free radicals. Up until the latter end of the last century, these radicals posed fundamental questions regarding the structure and bonding in main group chemistry. Now, in many cases, their thermodynamic and kinetic stability allows them to be used in the construction of molecular magnets and conductors. In this overview we will focus on the synthesis and characterisation of these radicals with a particular emphasis on their physical properties. 

The l,l -diferrocenyl-VT electron donor molecule is structurally similar to diferrocenyltetrathiafulvalene but with the TTF moiety replaced by bis(vinylene-dithio)tetrathiafulvalene (VT) [76]. It has currently not been possible to separate the cis- and trans-isomers. The 1 1 polyiodide complex of l,l -diferrocenyl-VT was obtained through reaction with iodine. EPR and Mossbauer spectra indicate that in this charge transfer salt the VT moiety is oxidized while the ferrocene... 

Chemical research of molecular metals was activated by the discovery of the metallic charge transfer salt TTF-TCNQ in 1973 [40]. Two basic molecular architectures have been studied intensively. One is based on organic molecules with the... 

This review concerns the materials made from the combination as charge transfer salts of the above-mentioned metallocenium cations with transition metal bisdithiolene anions, expanding and updating a previous review [34]. Most of the materials studied so far are decamethylmetallocenium based salts, but other compounds based on different metallocenium derivatives have also been reported and will be also referred. 

The film has noticeable planar conductivity, which depends on the number of monolayers as shown in Figure 7.2. The conductivity ofthe film is detectable for two monolayers, but the value is small for very thin films (two to six monolayers). From six monolayers the conductivity begins to increase linearly with the number of monolayers, a feature that is also found in LB films made of charge-transfer salts, and is perhaps a function of the imperfection in continuity of the first monolayers on the metal electrode-quartz substratum boundary. This imperfection came about during the deposition process as a result of different hydrophilic properties of metal and quartz surfaces. 

The charge-transfer salt, II2 pz(/V-Me2)8 TCNQ (100b) consists of integrated stacks with alternating donor cation II2 pz(/V-Mc2)8 + and acceptor anion TCNQ-with complete charge transfer from the pz to TCNQ (Fig. 29) (39). 

Carbocations as electron acceptors in aromatic EDA complexes 192 Bis(arene)iron(II) complexes with arene and ferrocene donors 198 Carbonylmetallate anions as electron donors in charge-transfer salts 204 Aromatic EDA complexes with osmium tetroxide 219... 

CARBONYLMETALLATE ANIONS AS ELECTRON DONORS TN CHARGE-TRANSFER SALTS... 

Isolation and the spectral characterization of charge-transfer salts of carbonylmetallates... 

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