Patrick Cassoux

BETS as a source of molecular magnetic superconductors (BETS = bis(ethylenedithio)tetraselenafulvalene)

BETS-based salts (BETS = bis(ethylenedithio)tetraselenafulvalene) incorporating Fe3+ ions are most interesting candidate systems for the observation of interplay of conductivity and magnetism. Indeed, in λ-(BETS)2FeCl4, conduction electrons in the BETS layers do interact with spins localised on the (FeCl4)− anions. Systematic studies on (BETS)2FexGa1−xCl 4−yBry mixed-composition compounds, in which the electronic properties can be finely tuned by the chemical composition, have provided a number of interesting results, including the characterisation of a molecular antiferromagnetic metal, a conductor with an antiferromagnetic metal state at ambient pressure, and an antiferromagnetic superconductor. Unprecedented superconductor–insulator transitions and superconductor–metal transitions have been also observed.

Molecular (super)conductors derived from bis-dithiolate metal complexes

Abstract The synthesis, electrochemical behavior, electrocrystallization and solution diffusion preparation of the M(dmit)2 bis-dithiolate metal complexes (dmit2−=2-thioxo-1,3-dithiole-4,5-dithiolato) and derived non integral oxidation state compounds is briefly recalled. The properties of the eight superconductors known in this series, namely, [TTF][Ni(dmit)2]2, [Me4N]0.5[Ni(dmit)2], α-[TTF][Pd(dmit)2]2, α′-[TTF][Pd(dmit)2]2, β-[Me4N]0.5[Pd(dmit)2], [Me2Et2N]0.5[Pd(dmit)2], α-[EDT-TTF][Ni(dmit)2]2, and β′-[Me2Et2P]0.5[Pd(dmit)2], (TTF=tetrathiafulvalene, EDT-TTF=ethylenedithio-tetrathiafulvalene) are described, including crystal structures, conductivities, IR/Raman studies, temperature/pressure phase diagrams, observation of charge density waves, and band structure calculations. The enlargement of this series of conducting compounds, not only by modification of the inorganic counter-cation or the organic cation radical, but also by modification of the dmit ligand itself, is reviewed, but superconductivity seems to be restricted to the dmit-based systems. The studies on these conducting bis-dithiolate metal compounds have been extended to other properties and utilization, such as, for example, conducting Langmuir-Blodgett films, unconventional magnetic properties, and nonlinear optics (NLO). Recent tentative research works on the interplay of conducting, magnetic and NLO properties in such compounds are reported.

Pressure-temperature phase diagram of α′-TTF [Pd(Dmit)2]2

Abstract Temperature dependence of resistivity (up to 24 Kbar) and of E.S.R. (at 1 bar) measurements suggest that centered monoclinic TTF[Pd(Dmit) 2 ] 2 could be an anisotropic and weak antiferromagnetic fluctuating metal. At low pressure, gradual charge and spin localizations at different temperatures are the signature of a Mott-Hubbard insulator. Above 20 Kbar, “high” superconducting onset temperatures (6.5 K) are observed.

Infrared and Raman properties of [M(dmit)2] (M=Ni, Pd) based compounds

Abstract The Raman spectra of [M(dmit)2]−n (dmit = 1,3-dithiole-2-thione-4,5-dithiolato, M = Ni, Pd; 0 ≤ n ≤ 2) based complexes with different cations were studied. It was found that the anion Ag modes frequencies correlate linearly with its charge. In the polarized reflectance spectra of the 1:1, [NBu4][Ni(dmit)2] salt the intra- and intermolecular electronic transitions as well as the vibronic bands were assigned. Electron-molecular vibration coupling constants have been evaluated using the dimer model.

New Tetrathiafulvalene-π-Spacer-Acceptor Derivatives: Synthesis, Crystal Structure, Optical and Electrochemical Properties

4-[2-tetrathiafulvalenyl-ethenyl]pyridine (1) has been prepared from a Wittig reaction between formyltetrathiafulvalene and 4-picolyltriphenylphosphonium chloride hydrochloride. Conversion of the pyridine moiety of 1 by reaction with methyl iodide leads to 4-[2-tetrathiafulvalenyl-ethenyl]-1-methylpyridinium iodide (2a). Neutralization of 1 with a large excess of L-tartaric acid affords 4-[2-tetrathiafulvalenyl-ethenyl]-1-methylpyridinium hydrogen tartrate (3). These TTF-π-spacer-acceptor compounds have been characterized by elemental analysis, and IR and 1H NMR spectroscopy. The crystal structure of 2a has been determined by X-ray diffraction. The cation is essentially planar. Examination of the bond lengths in 2a, UV/Vis spectra and CV data, and calculations indicate that an intramolecular charge transfer occurs in the studied compounds, although it is rather limited, and larger in 2a and 3 than in 1.

Magnetic anisotropy of organic conductor λ-(BETS)2FeCl4

Abstract λ-(BETS) 2 FeCl 4 , where BETS is bis(ethylenedithio)tetraselenafulalene, is an organic molecular metal with magnetic anions. It undergoes a metal-insulator transition at about T M−I ∼ 8 K . This insulating state can be suppressed and a metallic or highly-conducting state can be restored by applying external magnetic field higher than 10 T. The nature of this phase transition and the ground state, whether a spin-Peierls or antiferromagnet, has not been clarified. In this paper, we report the anisotropy of magnetic properties of λ-(BETS) 2 FeCl 4 studied using SQUID magnetometer. The temperature dependence of the static magnetic susceptibility below T M−I was found to show a strong field dependence when the field is applied parallel to the crystalline long axis (H//c). The nonlinear behavior in magnetization (M-H), reminiscent of a spin-flop transition of antiferromagnet, is observed at magnetic fields around 1 T, well below the field necessary for restoration of highly-conducting state. This strong field dependence is absent for other magnetic field directions, suggesting that the ground state of λ-(BETS) 2 FeCl 4 is antiferromagnet rather than spin-Peierls.

A theoretical investigation of the oligomerization of ethylene by titanium compounds: the TiMeCl3 · C2H4 system as a model for a quantitative evaluation of the cossee mechanism

Abstract A quantitative evaluation of the Cossee mechanism is performed on the TiMeCl 3 · C 2 H 4 system. Differential electron density contours maps are found to be more reliable than HOMO electron density maps for the description of ethylene insertion into the TiC bond. Cossees “φ RM ” assumption appears to be valid in this case while the “π-back bonding” formulation seems improbable. The existence of a “four centre intermediate” is related to the magnitude of the activation barrier to insertion.

A CNDO/2 approach to electron delocalization in N4S4 and N4S4F4

Abstract The electronic structure of N 4 S 4 and N 4 S 4 F 4 molecules is investigated within the framework of the CNDO/2 approximation. A pure alternated system is obtained for the fluorinated compound with respect to the Wiberg bond populations. On the other hand, the N 4 S 4 molecule appears to be composed of three highly delocalized “islands”, as defined by Dewar in the case of cyclophosphazenes. The “tub form” of N 4 S 4 is due to a strong spatial interaction between non-bonded sulphur atoms and, for this reason, the N 4 S 4 molecule may be called the “inorganic cyclooctatetraene”.

Synthesis, Electrical Conductivity, and Crystal Structure of (AsPh4)0.25 [Ni(dmit)2]

Abstract The synthesis, conductivity behavior and crystal structure of the ion radical salt (AsPh4)0.25 [Ni(dmit)2] are described. This compound crystallizes in the monoclinic system, space group Pn or P2/n, a = 13.507(7), b = 6.610(1), c = 44.536(7) A, β = 96.08(2)[ddot]. The structure consists of thick layers of stacked Ni(dmit)2 entities parallel to (001) and separated by AsPh4 + cations. Short S… S interstack distances are observed such that two-dimensional networks of closely spaced Ni(dmit)2 are formed in the (001) plane of this structure. The two-dimensional character of the structure is reflected in the low anisotropy in the conductivities measured in the (001) plane (σ = 1 : 1 : 10−3. The room-temperature four-probe conductivities measured along the needle axis (b axis) is 10-15 ω−1 cm−1. Temperature-dependent measurements show a thermally activated conductivity with very low activation energies (ca. 0-0.01 eV).

Effect of pressure on the superconducting transition temperature of tetrathiofulvalene bis-[bis-(4, 5-dimercapto-1, 3-dithiol-2-thione) nickel (II)] (TTF[Ni(DMIT)2]2)

Abstract The effect of pressure on the superconducting transition temperature of TTF[Ni(dmit) 2 ] 2 is determined. This is the first organic superconductor in which the transition temperature increases with increasing pressure.