Approche structurale du phénomène de transition de spin par diffraction des rayons X sous contraintes (T,P, hv)

Le phénomène de transition de spin correspond au changement d'état de spin d'un ion de transition sous l'action d'une perturbation extérieure (T, P, B, hv). Dans certains complexes du Fe(II), par exemple, l'ion peut basculer de l'état haut spin paramagnétique (S=2) vers l'état bas spin diamagnétique (S=0). Avant d'envisager des applications industrielles, il est nécessaire de bien comprendre et de maîtriser les différences de comportement magnétique de ces composés. C'est dans ce contexte que l'équipe des sciences moléculaires de l'ICMCB étudie depuis quelques années des complexes mononucléaires du Fe(II) aux caractéristiques magnétiques de transition de spin très variées. Dans ce travail de thèse, les paramètres structuraux à l'origine des différences de comportement d'un complexe à l'autre sont identifiés. De plus, des corrélations directes entre propriétés magnétiques et les propriétés structurales des complexes de la série [FeLn(NCS)2] sont mises en évidence. Par ailleurs, pour la première fois, les structures cristallines de ces complexes dans des états métastables atteints par effet de trempe thermique ou par photo-excitation à très basse température ont été déterminées.The spin transition phenomenon corresponds to the spin state modification of a transition metal ion under an external perturbation (T, P, B, hv). Some iron (II) complexes, for example, can change from the paramagnetic spin state (S = 2) to the diamagnetic low spin state (S = 0). Before thinking to potential industrial applications, it is necessary to understand and control the differences of magnetic behaviour of such compounds. To this aim, the molecular science group of the ICMCB is studying, for a few years, mononuclear iron (II) complexes, which present very distinct spin transition magnetic features. In this thesis, the structural parameters which govern the behaviour differences from one complex to the other are identified. Moreover, direct correlations between the magnetic features and the structural properties of the [FeLn(NCS)2] series of complexes are pointed out. In addition, for the first time, the crystal structures of theses complexes in metastable high spin states obtained by thermal trapping or by light irradiation at very low temperature have been determined.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

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

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 [bold 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]

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 Å3 (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

A CuII Chain Compound Showing a Ferromagnetic Coupling through Triple N1,N2-1,2,4-Triazole Bridges

[Cu(hyetrz)3](CF3SO3)2·H2O [hyetrz = 4-(2-hydroxyethyl)-1,2,4-triazole] represents the first structurally characterised ferromagnetically coupled CuII chain compound containing triple N1,N2-1,2,4-triazole bridges. catena-[-Tris{4-(2-hydroxyethyl)-1,2,4-triazole-N1,N2}copper(II)] bis(trifluoromethanesulfonate) hydrate (C14H23F6S2O10CuN9) crystallises in the triclinic space group P , a = 13.54(3), b = 14.37(3), c = 15.61(4) Å, = 95.9(1), = 104.9(1), = 106.5(1)°, V = 2763(11) Å3, Z = 4 (CuII units). The CuII ions are linked by triple N1,N2-1,2,4-triazole bridges yielding an alternating chain with Cu1-Cu2 = 3.8842(4) Å and Cu2-Cu3 = 3.9354(4) Å. Analysis of the magnetic data according to a high-temperature series expansion gives a J value of +1.45(3) cm-1. The nature and the magnitude of the ferromagnetic exchange have been discussed on the basis of the structural features

Peroxovanadium(V) Complexes of L-Lactic Acid as Studied by NMR Spectroscopy

A multinuclear (1H, 13C, 17O, 51V) 1D and 2D NMR study of the complexation of l-lactic acid with vanadium(V) and hydrogen peroxide shows that four peroxo complexes are formed in aqueous solution in the pH range 1-7. Two isomeric 2:2:1 (metal:ligand:peroxo) complexes, together with a 2:2:2 species, are found over the entire pH range. At pH values below 2 an additional 1:1:1 complex is formed. The acid was found in all cases to act as a bidentate ligand through the carboxylic and the adjacent hydroxyl groups. To the best of our knowledge, this is the first report of dinuclear peroxovanadium complexes in which one of the metal centres is an oxovanadium centre (2:2:1 species)

Crystal structure, magnetic properties, and 57Fe Mössbauer spectroscopy of the two-dimensional coordination polymers [M(1,2-bis(1,2,4-triazol-4-yl)ethane)2(NCS)2] (MII = Fe, Co).

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

Thermal trapped iron(II) high spin state investigated by X-ray diffraction

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..

Studies on the photochemistry of 1,7-diphenyl-1,6-heptadiene-3,5-dione, a non-phenolic curcuminoid model

he 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 (3–5 × 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

A reversible metal–ligand bond break associated to a spin-crossover

The [FeIIL(CN)2]·H2O 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