D. Chasseau

Determining the charge distribution in BEDT-TTF salts

The intramolecular bond lengths of the donor BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene, also ET) are sensitive to the charge carried by the molecule. By considering a large number of ET salts, we have developed a method for determining experimentally the charges of the ET molecules. The standard deviation in charge is only 0.1 for high quality structures.

Photo‐induced spin‐transition: the role of the iron(II) environment distortion

The [FeLn(NCS)2] iron(II) spin-crossover complexes cover a wide range of magnetic behaviour. Owing to the large number of known structural and magnetic data, this series is perfectly adapted to the investigation of the structure-magnetic properties relationship. In this paper we propose a new structural parameter, denoted Theta, which is used to correlate the features of the spin-crossover phenomena with the distortion of the iron environment. In particular, this parameter has shed light on the role of such distortion on the limiting temperature of photo-inscription, known as T(LIESST). A strong dependence of T(LIESST) on Theta is clearly demonstrated. The stronger the distortion the higher the T(LIESST) value. This structure-property dependence represents, for instance, a powerful tool to estimate the highest potential T(LIESST) value for a series of complexes. This limit in the [FeLn(NCS)2] series is estimated to be around 120 K, which probably prevents their use in any industrial application.

Co(II) molecular complexes as a reference for the spin crossover in Fe(II) analogues

The crystal structures of a series of cobalt(II) molecular complexes, [Co(PM-L)2(NCS)2] [PM = N-2-pyridylmethylene, L = 4-(aminobiphenyl) or 4-(phenylethynyl)aniline], are investigated and compared to the analogous iron(II) complexes, [Fe(PM-L)2(NCS)2], already known in the literature. At room temperature, the Co(II) complexes prove to be isostructural with the iron(II) complexes. An interesting point is that the iron complexes, unlike the cobalt complexes, undergo a spin crossover at low temperature. Hence, a comparison of the temperature dependence of the structural properties of the Co(II) and the Fe(II) complexes underlines some structural features of the spin crossover. Comparative deformation of the lattices and thermal expansion tensors are first discussed. Then, new parameters to estimate the distortion and the contraction at the spin crossover of the FeN6 coordination sphere are presented, thereby allowing the estimation of the reduction of the volume of the octahedron to around 3 A3 (25%). As well, comparative discussions on the intermolecular contact modifications with temperature are proposed. In the above considerations the cobalt series is therefore used as a reference to distinguish between the effects of the spin crossover and the purely thermal effects.

Synthesis, structure and preliminary results on electrical and magnetic properties of (Perylene)2 [Pt(mnt)2]

Abstract In this paper we report the preparation, crystal structure and some physical properties of the conducting molecular (Perylene)2 [Pt(S4C4(CN)4)]. The crystal structure consists of segregated stacks of perylene and Pt S4C4 (CN)4 units, with a spacing of 3.3 A between the carbon atoms in the perylenes. The temperature dependence of the electrical conductivity is metallic with a maximum at approximately 15 K. At this temperature a sharp transition to an insulating state occurs. Magnetic suceptibility measurements confirm the existence of a transition in this temperature range.

Towards direct correlations between spin-crossover and structural features in iron(II) complexes.

The [Fe(PM-BiA)(2)(NCS)(2)] complex, where PM is N-2-pyridylmethylene and BiA is 4-aminobiphenyl, crystallizes in two polymorphs. The two phases, denoted (I) and (II), undergo a spin-crossover when the sample is cooled and present distinct spin-transition features as (I) shows a very abrupt spin transition, while (II) exhibits a gradual transition. The two forms of the complex are used to investigate the correlations that exist between the spin-transition features and structural features. This article presents the crystal structures of polymorph (II) at room temperature (high spin) and at 120 K (low spin), including a comparison with those of polymorph (I). This study reveals that the packing, in a first approximation, is similar in both forms. In order to look at the crystal structures in more detail, a new angular parameter, denoted theta(NCS), as well as a particular type of intermolecular hydrogen-bond interaction, which involves the S atoms of the NCS ligands, are investigated. Interestingly, this angle and this intermolecular interaction can be directly connected to the cooperativity of the spin transition. Such a result is extended to all the SCO iron(II) complexes belonging to the same family of the general formula [Fe(PM-L)(2)(NCS)(2)].

Structural approach of the features of the spin crossover transition in iron(II) compounds

We have determined the crystal structures, both in high and low spin state, of four Fe(PM-L) 2 (NCS) 2 complexes, where PM is N-2′-pyridylmethylene and the aromatic subunit L is 4-aminoterphenyl (TeA), 4-(phenylazo)aniline (AzA), 4-aminobiphenyl (BiA) or 4-(phenylethynyl)aniline (PEA). As previously reported, these compounds undergo a spin crossover at low temperature with different features of transition: very smooth and incomplete for Fe(PM-TeA) 2 (NCS) 2 , smooth with almost no hysteresis for Fe(PM-AzA) 2 (NCS) 2 , unusually abrupt for Fe(PM-BiA) 2 (NCS) 2 and abrupt with a very large hysteresis (37 K) for Fe(PM-PEA) 2 (NCS) 2 . In Fe(PM-BiA) 2 (NCS) 2 , Fe(PM-TeA) 2 (NCS) 2 and Fe(PM-AzA) 2 (NCS) 2 the spin conversion is not associated with a large structural phase transition and the space group is the same above and below the temperature of transition: orthorhombic Pccn for the two first and monoclinic P2 1 /c for the third. On the other hand, Fe(PM-PEA) 2 (NCS) 2 undergoes a change in the crystal symmetry from P2 1 /c (high spin) to Pccn (low spin) which corresponds to a strong re-organisation of the iron atom network. The evolution as a function of temperature of the FeN 6 core as well as of the intramolecular characteristics are almost identical in all four compounds. To a first approximation, the crystal packing is similar in all of the structures except that the P2 1 /c structures develop an asymmetrical molecular environment. Nevertheless, a close examination of the intermolecular interactions, classified as intra- and inter-sheet, show some differences. The intrasheet and the intersheet interactions are stronger in Fe(PM-BiA) 2 (NCS) 2 and Fe(PM-PEA) 2 (NCS) 2 than either in Fe(PM-TeA) 2 (NCS) 2 where no ‘second’ neighbour intrasheet contacts are created, or in Fe(PM-AzA) 2 (NCS) 2 where the intersheet interactions are weak. Thus, the abruptness of the transition is attributed to the combination of close intrasheet and intersheet contacts. The hysteresis effect in Fe(PM-PEA) 2 (NCS) 2 is connected to the phase transition which could occur due to an irregular iron atom network associated with very short carbon-carbon intermolecular contacts at high temperature, not found in Fe(PM-AzA) 2 (NCS) 2 which shows the same irregular iron atom network.

A spin transition molecular material with a wide bistability domain

[Fe(hyptrz)3](4-chloro-3-nitrophenylsulfonate)22 H2O (1; hyptrz=4-(3-hydroxypropyl)-1,2,4-triazole) has been synthesized and its physical properties have been investigated by several physical techniques including magnetic susceptibility measurements, calorimetry, and Mössbauer, optical, and EXAFS spectroscopy. Compound 1 exhibits a spin transition below room temperature, together with a very wide thermal hysteresis of about 50 K. This represents the widest hysteresis loop ever observed for an FeII-1,2,4-triazole spin transition material. The cooperativity is discussed on the basis of temperature-dependent EXAFS studies and of the structural features of a CuII analogue. The EXAFS structural model of (1) in both spin states is compared to that obtained for a related material whose spin transition occurs above room temperature. EXAFS spectroscopy suggests that 1,2,4-triazole chain compounds retain a linear character whatever the spin state of the iron(II).

Crystal structures and non-linear optical properties of borane derivatives

[4′-(Dimethylamino)biphenyl-4-yl]dimesitylborane (BNB) and 4-[4′-(Dimethylamino)pnenylazo]phenyldimesityl-borane (BNA) have been investigated by X-ray diffraction; crystallographic data were used to calculate dipole moments in the ground state and the hyperpolarizability coefficient βo was measured by the EFISH technique for each derivative. The results show that the trivalent boron group is an efficient electron acceptor compared to other known acceptors such as the nitro group. Organoboron derivatives are promising candidates for nonlinear optical materials.

A novel conducting charge-transfer salt: (BEDT-TTF)3Cl2·2H2O

The bis(ethylenedithio) tetrathiafulvalene salt (BEDT-TTF)3Cl2·2H2O (1) has been prepared and characterised by single crystal X-ray diffraction, four probe electrical conductivity, and e.s.r. measurements, showing a metallic 2D network of BEDT-TTF molecules, separated by complex anions consisting of units of 4Cl– and 4 H2O molecules (σ= 0.6 S cm–1 at 300 K) with a metal–insulator transition at 100 K; activated behaviour of the spin susceptibility in the metallic regime is tentatively ascribed to Peierls fluctuations, lowering N(IµF) with temperature.

A metal-insulator phase transition close to room temperature: (BEDTTTF)2SbF6 and (BEDTTTF)2AsF6

Abstract We report the synthesis and characterization of two new salts of BEDTTTF: (BEDTTTF)2SbF6 and AsF6. Near to room temperature these two salts show a strong anomaly of conductivity, also visible on the EPR results. The crystal structure of the high temperature phase is described and a short discussion of the nature of the metal-insulator phase transition is given.