The electronic properties of low-dimensional molecular metals and superconductors [1] are, to a large extent, determined by the nature of the energy states near the Fermi level. These energy states usually derive from large collections of a single p-type orbital because the building units are conjugated molecules with large highest occupied molecular orbital±low-est unoccupied molecular orbital (HOMO±LUMO) gaps.[2]The way in which these orbitals overlap in the solid state and spread into energy bands depends on the structure which itself results from a complex balance of weak intermolecular interactions within a narrow energy range of less than 1 eV (96 kJ mol ±1 ).[3] Thus, low-dimensional patterns are constructed out of weak intermolecular interactions between planar units which make for small intermolecular interaction energies [4] and anisotropic transfer integrals. Remarkably then, within this class of molecular solids, the chemistry of weak intermolecular interactions blends with a rich supramolecular physics of low-lying excitations where electron correlations become more relevant with reducing dimensionality, to reach paramount importance in a strict one-dimensional regime. [5] Here, at ambient pressure, below a temperature T r , strongly interacting electrons are no longer free to move along, the metallic character is lost, and the system becomes localized; this is typically exemplified by the observation of an activated electrical conductivity. An understanding of what localizes electrons in such one-dimensional systems is relevant to many research areas including crystal engineering, supramolecular chemistry, materials science, and physics. Despite its importance, the mechanism of electron localization in one-dimension is difficult to study. For example, an essential component of the physics of the electron gas is the electron count within the underlying one-dimensional mixedvalence system. In radical ions salts it is fixed by the stoichiometry of the compound which rules the net degree of charge transferred into or out of the extended chain motif. Among the model systems presently available, a prominent one is the prototypical, triclinic (P1 Å ) mixed valence (TMTSF) 2 X and (TMTTF) 2 X (X = BF 4 ± , ReO 4 ± , PF 6 ± , AsF 6 ± , etc.; TMTSF and TMTTF stand for tetramethyltetraselenafulvane and tetramethyltetrathiafulvane, respectively) series (hereafter the (TM) 2 X series), [6] for which one hole is shared by the two p-donor HOMOs in the asymetric unit. For a uniform stacking, this would give a band quarter-filled with holes (or threequarter filled with electrons). The stacks, however, are not uniform but slightly dimerized instead which opens a gap at the center of the band and makes the systems effectively halffilled. [7] Thus, both the stoichiometry and the formation of weak dimers in the structure of the (TM) 2 X series manifest themselves into two vectors at one-quarter and one-half, respectively, along the reciprocal of the stacking axis. The very fact that both vectors are commensurate with a...
The ethylenedithiotetrathiafulvalene (EDT-TTF) directly functionalized with a primary amido group, which is both a hydrogen bond donor and acceptor group, is prepared from the corresponding ester. The electron-donating ability of EDT-TTF-CONH 2 (1), which is comparable to that of bisethylenedithiotetrathiofulvalene (BEDT-TTF) despite the presence of the electron-withdrawing amidic group, allows the successful electrocrystallization of air-stable cation radical salts. Two completely different salts are obtained with the isosteric AsF 6 À and ReO 4 À ions; the former has 6:1 stoichiometry, and the latter has 2:1 stoichiometry. Compound (1) 6 (AsF 6 ) crystallizes in the P3 Å space group, and the three crystallographically independent donor molecules are linked to each other through a combination of NÀH´´´O and CÀH´´´O hydrogen bonds. This strong trimeric motif organizes around the AsF 6 À ion located on the 3 Å axis, exemplifying the templating effect of the octahedral anion on the whole structure. The presence of a uniform spin chain, as identified in the crystal structure, is confirmed by the Bonner ± Fischer behavior of the magnetic susceptibility. In the 2:1 ReO 4 À salt, two crystallographically independent organic slabs are interconnected through NÀH´´´O(Re) hydrogen bonds, demonstrating the overlooked hydrogen-bond acceptor capability of this anion. The salt exhibits metallic behavior with a weak localization below 200 K. Both structures reveal the occurrence of a strong CÀH´´´O hydrogen bond involving the aromatic CH group of the EDT-TTF core, which is activated by the neighboring amidic moiety. Together with the NH´´´O hydrogen bond, it gives rise to a cyclic motif noted R 2 1 (7) in Etters graph set analysis.
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