The structure and physical properties of tetramethyl-tetrathiofulvalène (TMTTF) salts are reviewed. New results are given about (TMTTF)2 PF6, (TMTTF)2 ClO4 and the selenium analog (TMTSF)2 ClO4. An experimental relationship is shown between the electrical and magnetic properties and the closest contacts S-S or Se-Se of neighbouring stacks.

In order to evaluate the potentiality of organic donor molecules for molecular organic ferromagnets, with in principle C3 molecular symmetry, as in the hexakis (alkylthio) benzene, we have prepared single crystals of the charge transfer complex of tris [ethylene dithio] benzene (tEDTB) with TCNQF4, which have been subsequently oxidized with bromine and arsenic pentafluoride gas. The stoichiometry of this complex is 2:1 and it crystallizes in the triclinic system, space group P1 with the following cell parameters: a=11.515, b=11.044, c=10.460 A, α=111.50°, β=122.50°, γ=93.82°. The crystal structure consists of columns of alternating donor dyads and acceptors monads (DDADDA). The degree of charge transfer is estimated to be 0.2-0.3 from the methods based on bond lengths. While (tEDTB)2- TCNQF4 does not give a significant ESR spectrum, after treatment with the bromine or AsF5 gas, a single intense line, centered around the free electron g value, appears without any evidence of either a ground or an excited triplet state.

The nature and the mechanism of the magnetic hysteresis for the thermal spin crossover exhibited by an iron ( II) compound is investigated by means of variable-temperature powder and single-crystal x-ray diffraction. The unit cell temperature dependence clearly evidences the amplitude of the strong structural rearrangement that accompanies the spin crossover-corresponding to a variation of 8.6% for one of the unit cell parameters-as well as the structural hysteresis width. In this regard, the present x-ray study reveals significant differences in the spin crossover features according to the nature of the sample-powder or single crystal - that should be taken into account in the analysis of physical properties. Concerning the interplay between structural and magnetic transitions, quenching effects show that the structural transition and the spin crossover are indissociable. Furthermore, investigations of the mechanism itself of the thermal spin crossover confirm the presence of spin-like domains in the conversion region, either in the cooling or in the warming loops. The non-dependence with temperature of these domains inside the hysteresis loop demonstrates the stability of the microscopic and macroscopic structures in the corresponding thermodynamic conditions. This result is of interest in the context of the potential use of hysteresis loops to obtain high-temperature photo-conversion.

New coordination polymers of the formula [M(btre)(2)(NCS)(2)] (btre = 1,2-bis(1,2,4-triazol-4-yl)ethane; M(II) = Fe, Co) have been synthesized, and their crystal structures have been determined at 293 K by X-ray analysis. The Fe(II) compound (C(7)H(8)FeN(7)S(2)) crystallizes in the monoclinic space group P2(1)/n, a = 12.439(5) A, b = 8.941(2) A, c = 9.321(3) A, beta = 90.88(2) degrees , V = 1036.6(6) A(3), Z = 2, 3791 reflections [I > 3sigma(I)], R(F) = 0.036, wR2 = 0.123. The Co(II) compound is isostructural to the Fe(II) compound. The crystal structure consists of a 2D sheet in which the metal ions are linked by bis monodentate (N1, N1') 1,2,4-triazole ligands. The structure is stabilized by pi-bond interactions between two adjacent sheets and by S...S interactions. Temperature-dependent SQUID, (57)Fe Mössbauer, and X-ray diffraction measurements indicate that [Fe(btre)(2)(NCS)(2)] retains a HS ground state upon cooling from 293 K down to 8 K. The surprising absence of spin-crossover behavior for this Fe(II)-1,2,4-triazole polymeric coordination compound that has been confirmed by pressure experiments up to approximately 12 kbar and by light irradiation experiments at 10 K is discussed on the basis of its structural features. Insight into the origin of the cooperative effects of the spin transition in [Fe(btr)(2)(NCS)(2)].H(2)O (btr = 4,4'-bis-1,2,4-triazole) is also given thanks to a re-evaluation of its distortion parameters in the high- and low-spin states.

The interplay between the solid-state spin-crossover features and the structural properties is analyzed for the [Mn-III(pyrol)(3)tren] complex on the basis of high-pressure and low-temperature single-crystal x-ray-diffraction experiments. In particular, the low-temperature (30 K, 10(5) Pa) low spin crystal structure is compared to the low-temperature (60 K, 10(5) Pa) high spin and to the high-pressure (293 K, 1.00 GPa) high spin crystal structures. The low-temperature structural properties show the structural modifications due to the spin crossover in a Mn(III) complex. Comparison of these structural modifications to those described for mononuclear Fe(II) spin-crossover compounds emphasizes significant differences, such as in bond length variation and polyhedron distortion, for example. Elsewhere, analysis of the high-pressure data shows that the internal stress on the metal ion is not the cause of the occurrence of the thermal spin crossover, contrary to a general belief.

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.

The interplay between the spin crossover and the structural properties of the complexes in the solid state is still under investigation. In particular the following questions may be asked. What are the structural modifications of the metal coordination sphere at the spin crossover? How are the dimensions and the symmetry of the crystallographic unit cell affected by the spin crossover? Conversely, how may structural properties influence the spin crossover behavior? Do intramolecular parameters account for the features of the spin crossover? What are the relevant characteristics of the crystal packing for the cooperativity? Do the above questions have general answers that can be used for all the spin crossover compounds? This contribution tries to give answers to these questions. The discussion is based on a large structural data set provided in the literature for the six-coordinated iron(II) mononuclear complexes of general formula [FeLn(NCS)(2)]. The effects of temperature, light and pressure on the X-ray diffraction crystal structures are reviewed. The structural modifications due to the spin crossover are first estimated, these include the expansion and the distortion of the FeN6 octahedron, the isotropic and the anisotropic changes of the unit cell. The influence of the structural properties on the features of the spin crossover is then discussed. For example, intramolecular properties such as Fe-N bond lengths are in general not relevant to account for the spin crossover features. In contrast, hydrogen bonds play a paramount role in the propagation of the spin conversion throughout the crystal lattice.

Some recent improvements made to the high-pressure single-crystal x-ray diffraction (XRD) experiments in use in our laboratory are reviewed. In particular a set-up to perform high-pressure (0–3.0 GPa)–low-temperature (300–9 K) investigations as well as a protocol for high-pressure XRD data collection using a CCD detector are shown. Examples of studies are also presented.

[Cu(hyetrz)(3)](CF3SO3)(2)(H2O)-H-. thyetrz = 4-(2'-hydroxyethyl)-1,2,4-triazole] represents the first structurally characterised ferromagnetically coupled Cull chain compound containing triple N-1,N-2-1,2,4-triazole bridges. catena-[mu-Tris(4-(2'-hydroxyethyl)-1,2,4-triazole-N-1,N-2}copper(II)] bis(trifluorome-thanesulfonate) hydrate (C14H23F6S2O10CuN9) crystallises in the triclinic space group P1, a = 13,54(3), b = 14.37(3), c = 15.61(4) Angstrom alpha = 95,9(1), beta = 104.9(1) gamma = 106.5(1)degrees V = 2763(11) Angstrom(3), Z = 4 (Cu-II units), The Cu-II ions are linked by triple N-1,N-2-1,2,4-triazole bridges yielding an alternating chain with Cu1-Cu2 = 3,8842(4) A and Cu2-Cu3 = 3.9354(4) Angstrom. Analysis of the magnetic data according to a [high-temperature series expansion gives a J value of +1 45(3) cm. The nature and the magnitude of the ferromagnetic exchange have been discussed on the basis of the structural features. (C) Wiley-VCH Verlag GmbH & Co, KGaA, 69451 Weinheim, Germany, 2003.

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)].

The possibilities to trap by flash cooling the high spin (HS) state of iron(II) in the [Fe(PM–BiA)2(NCS)2] complex have been investigated by X-ray diffraction. This study reveals that trapping the HS state is possible under some conditions depending on the final temperature. If the latter is lower than the T(LIESST) temperature, the HS→LS (low spin) relaxation is slow enough to determine the trapped HS crystal structure by X-ray diffraction. The crystal structure of this complex in the 30 K trapped HS state shows differences from either the room temperature (HS) or the 30 K (LS) crystal structures, as for example differences in the strength of the S⋯H–C hydrogen bond like intermolecular interaction or the π–π interactions, known to play a crucial role in this compound for the propagation of the change in spin at the spin crossover (SCO), i.e. the cooperativity. The differences in intermolecular interactions are directly linked to the differences between the crystallographic unit cell modifications induced by pure thermal effects and those induced by the SCO.

Our research interests include the direct observation of unstable intermediates in a crystal by X-ray crystallography.Our target compounds are open-shell species having more than one spin.Open-shell species such as radical, carbene, and nitrene have been extensively studied spectroscopically in inert matrices or in a gas phase, for the understanding of their role as intermediates in various chemical reactions and also for the investigation of their potential use for magnetic materials.To design and develop functional materials, it is essential to obtain structural knowledge.However, few diffraction studies of open-shell species have been reported except stable radicals and relatively stable singlet species, because of the difficulty of their isolation and their high lability.Our basic technique used is a well-known cryo-trapping method to freeze reactive species.A key to retaining crystallinity during photoreaction is to design suitable reaction cavity.This talk will review the in situ study of photo-induced unstable reactive intermediates, radical, carbene, and nitrene, by X-ray analysis and also introduce some applications of an X-ray vacuum camera developed by Toriumi (Himeji Inst.Tech.) in combination with synchrotron radiation at BL02B1 in SPring-8.

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 Angstrom(3) (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.

Exploiting the prior knowledge of standard protein geometry in structure refinement is common practice.The prior geometrical data is a wealthy source of phase information.However, the application of these data requires assignment of the protein structure.This is typically achieved by model building in an experimentally phased map.We have developed an N-particle formalism that allows a rigorous treatment of stereo-chemical information without the need of prior assignments.In effect, the optimization works on loose atoms and the topology is developed in the optimization process.Our method, called Conditional Dynamics, enhances the radius of convergence in refinement and may, in principle, be applied to random starting models for ab initio phasing.Calculations using a simplified test case, consisting of a polyalanine four helical bundle, showed i. a large radius of convergence even when the resolution is limited to 3.5 Å, and ii.successful ab initio optimization against 2.0 Å resolution data.Recently, we have developed a conditional dynamics-force field containing stereo-chemical restraints for all prevalent configurations in protein structures.We are now testing our method against real protein-diffraction data with respect to map improvement and ab initio phasing, i.e. modeling starting from random models.

[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).

The crystal structure of (TMTSF)2ClO4 has been determined at (7 K, 1 bar) and at (7 K, 5 kbar) with a high accuracy. For the latter, low temperature and pressure were applied simultaneously using a X-ray diffraction instrumentation designed in our laboratory, these results are the first for molecular compounds. The effects of lowering the temperature are not the same as those produced by increasing the pressure. At (7 K, 1 bar) the anion ordering which occurs in this compound, and which is characterised by the appearance of b * /2 superlattice reflections, is well observed. This anion ordering leads to the presence of two independent stacks of TMTSF cations which is the only case found in the Bechgaard salts family. The comparison of the low temperature crystal structures under atmospheric pressure and at 5 kbar shows that the centres of mass are nearly the same, independent of the pressure: the interchain interactions do not depend on the doubling of the unit cell. Under pressure, the ordering (0, 1/2, 0) does not occur at any temperature. These structural data are confirmed by the quantum chemical calculations which show that the difference in the site energy of the two independent cations is 100 meV.

The crystal structures in both irradiated and nonirradiated states of a photoinduced molecular switch based on the spin-crossover phenomenon are presented. From the structural point of view, the light-induced metastable high-spin state of the spin-crossover complex [Fe(phen)2(NCS)2] (phen = 1,10-phenanthroline) shows significant differences with the low-spin state but also with the thermally induced high-spin state.