TTF structure

The second family of superconducting materials is based on cation-radical salts of another donor, bis(ethylenedithio)-TTF (abbreviated BEDT-TTF or ET), which exhibit two-dimensional network mostly due to inter-stack S S interactions [100]. Another important feature of BEDT-TTF salts is their tendency to give polymorphs. For instance, (BEDT-TTF)2I3 salt affords four polymorphs (a, p, 0, and k phases), of which only the first undergoes metal-insulator transition, while the others are superconductors at ambient pressure (see Chapter 10 of this book). It is quite surprising that of all numerous BEDT-TTF structural analogs synthesized to date, only two salts of unsymmetrical derivatives, DMET [101] and MDT-TTF [102], led to superconductors. 

In this section, syntheses of 1,3-dithioles of a general formula 649 are discussed. This includes TTF structures 6, previously described in CHEC(1984) and CHEC-II(1996), and additionally new TTE-like structures containing two 1,3-dithiole heterocycles separated by Jt-conjugated spacer. Substituents and those in the spacer may contain... 

Many different chemical modifications have been carried out on the TTF skeleton to prepare building blocks for macro- and supramolecular systems of increased electrical conductivity. Direct selective functionalization of the TTF structure is usually a problem, because of its D h symmetry with four identical attachment sites. Therefore, versatile TTF building blocks have been developed and many problems concerning the selective functionalization of TTFs have been solved. For instance, the potential building block 673 was prepared in quantitative yield starting from the l,3-dithiole-2-thione 667 via intermediates 668-672 (Scheme 95) <1999S803>. 

As shown in Eq. (42), polymers containing the TTF structure are synthesized by this method. Their charge-transfer complex formation with some acceptors and oxidation by halogens have been examined. 

Finally in this section, salts of compound 88 lacking the fully unsaturated TTF structure have been reported to be organic superconductors <01JA4174> and their magnetic properties have been described <0fCC2538>. 

The structures of the black crystalline benzene solvate C6o-4C6H6, the black charge-transfer complex with bis(ethylenedithio)tetrathiafulvene, [C6o(BEDT-TTF)2], and the black ferrocene adduct [C6o Fe(Cp)2)2] (Fig. 8.7b) ) have also been solved and all feature the packing of Cso clusters. 

The first three-dimensional macrobicyclic TTF derivatives have been obtained <96CC615> and work on TTF-containing crown ethers and thioethers has continued <96LA551, 96JCS(P1)1995> with structures of this type being used to obtain the first TTF-containing... 

Scheme 5 Electronic structures of TTF salts, showing charge localization into uniform chains or dicationic dimers in the TTF+ X salts (left) or the partially filled conduction bands in mixed-valence (TTF X salts (right)...

Fig. 4 Illustration of the head-to-tail dyadic association of ortho-dihalo TTFs in the structure of neutral EDS-TSF-I2...

The (EDT-TTF-I)2Br salt described above [36] and the 1 1 (TTFI4)I salt reported by Gompper [51] were the only structurally characterized salts with simple halide anions until Imakubo recently described an extensive series of Cl" and Br" salts from several ortho-diiodo tetrathiafulvalene, tetraselena-fulvalene and dithiadiselenafulvalene derivatives (Scheme 8) [62], The X-ray crystal structure analysis of the nine salts described there show a variety of halogen bonded motifs, demonstrating the adaptability of the supramolecu-lar interactions to other structural requirements imposed by the nature of the heteroatoms (O, S, Se) in the TTF frame. Indeed, in (EDT-TTF-l2)2X-(H20)2 (X = Cl, Br), a bimolecular motif (Fig. 6) associates two partially oxidized EDT-TTF-I2 molecules with one Br" anion and one water molecule. 

In all these salts, the intermolecular I-Cr(Br ) distances are much shorter than the corresponding Danis distances (Table 2), indicating very strong interactions, most probably attributable to an important electrostatic contribution. Furthermore, these salts are highly conducting and exhibit a variety of electronic structures, from completely two-dimensional to quasi one-dimensional, with original TTF- TTF overlap patterns observed here... 

Moreover, it was shown that the presence of Hal Hal interactions between the partially oxidized molecules also contribute to the electronic delocalization. Indeed, the presence of non-zero atomic coefficients on the halogen atoms in the HOMO of EDT-TTF-Br2 or EDT-TTF-I2 [66], together with the short Hal Hal contacts, leads to a sizeable increase of the band dispersion and stabilizes a rare (V structure through the side-by-side arrangement of the inversion-centred dyads connected by Hal- Hal interactions. Both 13 salts are semiconductors with room temperature conductivities around... 

IScm a large value when compared with other ( / structures such as (X-(BEDT-TTF)2(IC12). 

Only a few recent examples of halogen bonding of halogenated TTFs with halometallate anions have been described and this most probably represents a broad development field in this area. Indeed, these halometallate salts offer a wide variety of complexes and specifically here a large variation of the ratio of anion charge vs number of halide atoms, as already discussed above in the comparison between Br and 13 salts. This point is illustrated by the superb structures of two salts of EDT-TTF-I2 with the polymeric one-dimensional PbV (Fig. 9) and two-dimensional (Pbs/eDi/eh) anions where shorter C -1- -Ianion contacts are observed with the linear Pbl3 chains... 

Table 4 Structural characteristics of the metal dithiolene salts of halogenated TTFs... 

The salt (TTF)2[Ni(tdas)2l (390) was obtained by electrocrystallization with a room temperature conductivity of 0.1 Scm-. This indicates the potential of these complexes as components of conducting materials. Some tdas complexes have been characterized by X-ray crystal structure.1062- 64... 

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