Alain Michalowicz

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).

[Ti(MgCl)2· xTHF]q: a reagent for the McMurry reaction and a novel inorganic Grignard complex

Abstract The stepwise formation of two distinct bimetallic titanium/magnesium complexes during the reduction of TiCl 3 (or TiCl 4 ) by magnesium in tetrahydrofuran (THF) has been identified. [TiMgCl 2 · x THF] ( 1 ) is produced in a first stage, but reacts further with excess Mg to give [Ti(MgCl) 2 · x THF] ( 2 ). The reaction is reversible in the presence of excess TiCl 3 . X-ray absorption spectroscopy has been used to identify the local environment in 2 . The most striking feature is the existence of the shortest TiMg interatomic distance yet observed. The TiMg bond length, 2.72(1) A, is close to the sum of the Pauling single bond metallic radii (2.69 A) and the complex is termed an inorganic Grignard reagent by analogy with other complexes containing direct transition metalmagnesium bonds. Overall, a dimeric model is compatible with the EXAFS-derived structural parameters and solubility characteristics. The titanium atoms are linked through two bridging MgCl 2 Mg units and their coordination shells are completed by THF ligands. The structure of 2 , and the existence of intermediate 1 , can be used to rationalize the known catalytic activity of the TiCl 3 /Mg/THF system with respect to ether cleavage.

XAS investigations on nanocrystalline Mg2FeH6 used as a negative electrode of Li-ion batteries

In the frame of research on new metallic hydrides as conversion reaction materials for negative electrodes of Li-ion batteries, the MgFe2H6 complex hydride has been investigated in and ex situ using XRD, XAS and magnetic measurements at different states: initially ball milled complex hydride (CH), electrochemically desorbed (ED) and thermally desorbed (TD). Fe–H bonding is clearly evidenced by EXAFS in the CH sample. It is also observed that the electrochemical reaction leads to a nanocrystalline state that needs local probe analyses to be fully investigated and interpreted. From the XAS and magnetic data, the different routes (ED and TD) for dehydrogenation of the complex hydride are compared. For both electrochemically and thermally driven reactions, the hydrogen depletion from Mg2FeH6 hydride leads to decomposition into its constituting elements Mg and Fe. However, different Fe structures are observed: bulk α-Fe and amorphous Fe nanoparticles for TD and ED samples, respectively.

Multiple solutions in data fitting: a trap in EXAFS structural analysis and some ideas to avoid it

A possible source of error on interatomic distance determination in EXAFS multishell data analysis is described on the basis of fitting a simulated signal for a cluster of Rh atoms with an interaction of O atoms; a fit of an experimental signal is also presented. The origin of this type of mistake is briefly discussed.

F‐test in EXAFS fitting of structural models

A. M i c h a l o w i c z * , K Provost * * , S. Larue l le ÷, A. M i m o u n i * a n d G. V la ic x The meaning and the way to estimate these quantities is discussed elsewhere (Lytle, S&C report, 1988 and present XAFS conference, 1998). We want to compare two fits noted AX2~ and AX22 with respectively the degrees of freedom v~ and v2. In order to avoid confusing notation we shall not use A%zv in the following formulas.

EXAFS characterization of oxaliplatin anticancer drug and its degradation in chloride media.

Oxaliplatin is a second-generation platinum-based anticancer drug. Its degradation is studied in solution, in the presence of chloride ions (in neutral or acidic media) in excess. In both cases the degradation product precipitates immediately. The EXAFS spectra of these products show that they are identical. EXAFS modeling and refinement of the first coordination sphere shows that two light atoms are replaced by two chloride ions. The complete refinement of the local structure is possible by studying the multiple-scattering signal. The results show that the main multiple-scattering contribution is due to the binding oxalato group and that the degradation product is [Cl(2)-(diaminocyclohexane)-Pt(II)].

EXAFS identification of the active species in supported niobium sulfide hydrotreatment catalysts

Abstract We present a Nb K-edge EXAFS study of carbon- and alumina-supported niobium sulfide hydrotreatment catalysts. NbS 2 and NbS 3 entities have been evidenced on the carbon support, and NbS 2 and Nb 1− y S ones on the alumina support. EXAFS spectra of carbon-supported species, which are highly disordered, were analyzed through an asymmetric distance distribution model. Considering their poor dispersion, Nb 1− y S species can be thought to be intrinsically highly active. Their activity, and that of NbS 3 entities as well, can possibly be related to cation clustering. Niobium sulfides appear as very sensitive to support effects.

Local order and electronic structure of Pb1−xLaxZr0.40Ti0.60O3 materials and its relation with ferroelectric properties

The knowledge of electronic and local structures is a fundamental step towards understanding the properties of ferroelectric ceramics. X-ray absorption near-edge structure (XANES) of Pb1−xLaxZr0.40Ti0.60O3 ferroelectric samples was measured in order to know how the local order and electronic structure are related to their ferroelectric property, which was tailored by the substitution of lead by lanthanum atoms. The analysis of XANES spectra collected at Ti K- and L-edges XANES showed that the substitution of Pb by La leads to a decrement of local distortion around Ti atoms on the TiO6 octahedron. The analysis of O K-edge XANES spectra showed that the hybridization between O 2p and Pb 6sp states is related to the displacement of Ti atoms in the TiO6 octahedra. Based on these results, it is possible to determine that the degree of ferroelectricity in these samples and the manifestation of relaxor behavior are directly related to the weakening of O 2p and Pb 6sp hybridization.

Short-range structure of Pb1−xBaxZr0.65Ti0.35O3 ceramic compounds probed by XAS and Raman scattering techniques

Ti K-edge x-ray absorption near-edge spectroscopy (XANES) and Raman scattering were used to study the solid solution effects on the structural and vibrational properties of Pb1−xBaxZr0.65Ti0.35O3 with 0.0<x<0.40. Compared with x-ray diffraction techniques, which indicates that the average crystal symmetry changes with the substitution of Pb by Ba ions or with temperature variations for samples with x=0.00, 0.10, and 0.20, local structural probes such as XANES and Raman scattering results demonstrate that at local level, the symmetry changes are much less prominent. Theoretical XANES spectra calculation corroborate with the interpretation of the XANES experimental data.

EXAFS structural study of platinum-based anticancer drugs degradation in presence of sulfur nucleophilic species

Three platinum complexes, cisplatin, carboplatin and oxaliplatin are currently used worldwide. Investigation of their main structural modifications in presence of sulfur nucleophiles is of particular interest because of the implication of thiol and thioether groups in biochemical mechanism of action, resistance mechanism and in vivo or in vitro detoxification. We present the main structural results we have obtained concerning the reaction of these drugs with diverse sulfur nucleophiles (cysteine, glutathione, methionine, thiosulfate and thiocyanate), monitored in solution or as precipitates by EXAFS spectroscopy. The reactivities of the carboxylate and amine ligands of both carboplatin and oxaliplatin are compared, on the basis of first-coordination sphere modeling. Among the new results of this EXAFS study, we present the first observation of oxaliplatin diaminocyclohexane ligand displacement by sulfur nucleophiles.