The electrocrystallization from solutions of cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF) in the presence of different anions X = ClO4(-), PF6(-), and I3(-), affords a new type of 2D molecular metals with composition (CNB-EDT-TTF)4X based on an unprecedented bilayer structure of the donors induced by effective head to head interdonor interactions through the nitrile groups, which is responsible for 2D metallic systems with unusual properties such as the higher band filling, larger effective mass of carriers, and almost degenerated double Fermi surfaces.

Electrocrystallization from solutions of the dissymmetrical ET derivative cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF) in the presence of triiodide I affords two different polymorphs (β″ and κ) with the composition (CNB-EDT-TTF)I, both with a bilayer structure of the donors. These polymorphs differ in the packing patterns (β″- and κ-type) of the donor molecules in each layer, in both cases with bifurcated C-N···H interactions effectively coupling head-to-head donor molecules between layer pairs. Two β″ polymorphs can be obtained with different degrees of anionic ordering. In one disordered phase, β″, with a smaller unit cell, the triiodide anions are disordered over two possible positions in a channel between the donor bilayers, while in the ordered phase, β″, the triiodide anions occupy only one of those positions in this channel, leading to the doubling of the unit cell in the layer plane. These results for β″ phases contrast with the κ polymorph previously reported, for which weaker disorder of the triiodide anions, over two possible orientations with 94 and 6% occupation factors, was observed. While the β″ polymorphs remains metallic down to 1.5 K with a ρ/ρ resistivity ratio of 250, the κ polymorph presents a much smaller resistivity ratio in the range of 4-10 and superconductivity with an onset temperature of 3.5 K.

The electrocrystallization from dichloromethane or tetrahydrofuran solutions of the dissymmetrical bis(ethylenedithio)tetrathiafulvalene (ET) derivative cyanobenzene−ethylenedithio−tetrathiafulvalene (CNB-EDT-TTF) in the presence of perchlorate anion ClO 4− affords two different polymorphs of salts with 4:1 stoichiometry, a previously described triclinic phase β″ T -(CNB-EDT-TTF) 4 ClO 4 (1a), and a new monoclinic phase β″ M -(CNB-EDT-TTF) 4 ClO 4 , (1b), as well as a compound with 1:1 stoichiometry, (CNB-EDT-TTF)ClO 4 (2), depending on the solvent and crystallization conditions. The special conditions necessary for the growth of the unusual 4:1 salts, requiring low current densities to oxidize the donors and subsequent slow diffusion controlled association processes, are discussed. Both 4:1 polymorphs present a bilayer structure of the donors induced by head-to-head C−N•••H pairing interactions associated with a combination of R 2 2 (10) and R 2 4 (10) synthons between donors in nearby layers. These polymorphs share the same β″-type donor packing pattern in the layers, but the anions which are disordered over two possible orientations in the triclinic phase appear ordered in the monoclinic one, with a doubling of the interlayer cell axis, due to an alternation of the orientation of the molecules in the bilayers. The 1:1 salt 2 presents a crystal structure with dimerized donors stacks. The donor molecules in nearby stacks are connected by a helical network of C−N•••H pairing interactions. A new polymorph of the neutral donor β-CNB-EDT-TTF (3) is also described.

SummaryA dissymmetric TTF-type electron donor, cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF), was obtained in high yield, by a cross-coupling reaction with triethyl phosphite between 2-thioxobenzo[d][1,3]dithiole-5-carbonitrile and 5,6-dihydro-[1,3]dithiolo[4,5-b][1,4]dithiin-2-one. This new donor was characterized namely by single crystal X-ray diffraction, cyclic voltammetry, NMR, UV-visible and IR spectroscopy.

Abstract:The preparation and characterization of new salts based on the dissymmetrical TTF derivative CNB-EDT-TTF (cyanobenzene-ethylenedithio-tetrathiafulvalene) and BF 4 − anions, are reported. Depending on the electrocrystallization conditions salts with different stoichiometries, (CNB-EDT-TTF)BF 4 and β"-(CNB-EDT-TTF) 4 BF 4 , can be obtained. The 1:1 salt is an electrical insulator isostructural to the ClO 4 analogue previously described. The 4:1 salt is a new member of the family of 2D metals of this donor with different small anions X, (CNB-EDT-TTF) 4 X, characterized by a bilayer arrangement of the donors and it was obtained in a monoclinic polymorph with a β"-type donor packing pattern. The small anions in this compound are severely disordered between the donor bilayers, which present slightly larger lattice parameters than the isostructural ClO 4 analogue. Both electrical conductivity and thermoelectric power measurements in single crystals denote metallic properties as predicted by electronic band structure calculations. As a consequence of the anion disorder the metallic regime of the electrical conductivity denotes electronic localization effects with a progressive increase of resistivity below~25 K. Because of the larger lattice parameters the intermolecular interactions and electronic bandwidth are decreased compared to other (CNB-EDT-TTF) 4 X salts. The large and positive thermoelectric power S of this compound (~110 µV/K in the range 100-330 K) and its electrical conductivity σ = 20 S/cm at room temperature lead to a power factor S 2 σ = 24 µW/K 2 m, quite large among molecular conductors, placing these compounds as potential candidates for thermoelectric materials.