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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. [ABSTRACT FROM AUTHOR]

The [Fe(II)L(CN)(2)].H(2)O complex, dicyano[2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene]iron(II) monohydrate, exhibits a thermal induced metal-ligand bond break reversible in the solid state and associated to a spin crossover that corresponds to an unprecedented structurally characterized modification of the coordination metal environment from a hepta-coordinate high spin state to a hexa-coordinate low spin state.

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 [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 comparative photostability of curcumin 1, and two non-phenolic curcuminoids: 1,7-diphenyl-1,6-heptadiene-3,5-dione 2 (unsubstituted curcumin) and dimethylcurcumin 3 in non-degassed dilute solutions (approximately 3-5 x 10(-5) mol l(-1)) has been established by UV-visible absorption spectroscopy; disappearance quantum yields were measured. The similar behavior of the three studied curcuminoids is indicative of only a moderate role of phenol groups in the photodegradation process. Structural analysis of the photodegradation products of compound 2 in more concentrated solution (approximately 3.6 x 10(-3) mol l(-1)) shows formation of benzaldehyde, cinnamaldehyde, 2'-hydroxy-5',6'-benzochalcone 4, flavanone 5 and some other unidentified photoproducts. Flavanone 5 is formed by irradiation of chalcone 4. It represents a unique example of photochemical conversion of a diarylheptanoid molecule into a flavonoid, another very important class of natural products.

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)].