Frédéric Capet

Ab initio structure determination of the high-temperature phase of anhydrous caffeine by X-ray powder diffraction

The high-temperature phase I of anhydrous caffeine was obtained by heating and annealing the purified commercial form II at 450 K. This phase I can be maintained at low temperature in a metastable state. A powder X-ray diffraction pattern was recorded at 278 K with a laboratory diffractometer equipped with an INEL curved position-sensitive detector CPS120. Phase I is dynamically orientationally disordered (the so-called plastic phase). The Rietveld refinements were achieved with rigid-body constraints. It was assumed that on each site, a molecule can adopt three preferential orientations with equal occupation probability. Under a deep undercooling of phase I, below 250 K, the metastable state enters in a glassy crystal state.

Catalytic Conversion of Alcohols into Carboxylic Acid Salts in Water: Scope, Recycling, and Mechanistic Insights

The catalytic conversion of alcohols into carboxylic acid salts in water was performed in the presence of ruthenium complexes supported by aliphatic PNP pincer ligands preformed or formed in situ. High activity toward a wide substrate scope was achieved with turnover number values of up to 4000. The air-stable catalytic system can be recycled by using toluene as a catalyst-immobilizing phase; the activity is maintained after five consecutive runs. Finally, mechanistic studies allowed some fundamental aspects related to water activation to be unveiled and to the mechanism postulated.

Theoretical and experimental investigations of the thermoelectric properties of Bi2S3

Electronic and transport properties of Bi2S3 with various dopants are studied using density functional theory and experimental characterizations. First, principle calculations of thermoelectric properties are used to evaluate the thermoelectric potential of the orthorhombic Bi2S3 structure. The computational screening of extrinsic defects is used to select the most favorable n-type dopants. Among all the dopants considered, hafnium and chlorine are identified as prospective dopants, whereas, e.g., germanium is found to be unfavorable. This is confirmed by experiment. Seebeck coefficient (S) and electrical conductivity (σ) measurements are performed at room temperature on pellets obtained by spark plasma sintering. An increase of power factors (S2·σ) from around 50 up to 500 μW K−2 m−1 is observed for differently doped compounds. In several series of samples, we observed an optimum of power factor above 500 μW K−2 m−1 at room temperature for a chlorine equivalence of 0.25 mol. % BiCl3. The obtained results...

Asymmetric Intramolecular Hydroamination of Alkenes in Mild and Wet Conditions—Structure and Reactivity of Cationic Binuclear Gold(I) Catalysts

A selected diphosphine binuclear gold(I) chloride complex was combined with a silver salt to catalyze efficiently, for the first time, the asymmetric intramolecular hydroamination of alkenes with high conversions and enantioselectivities, in mild conditions and in presence of water. Both enantiomers of the products could be obtained by controlling the molecular ion‐pairs through the solvent polarity. The gold(I) cationic active species was characterized unambiguously at the solid state by X‐ray analysis and in solution by DOSY 1H NMR experiments. No contribution of silver chloride was observed on the bonding mode of the catalyst.

Inorganic polar blocks into controlled acentric assemblies.

We show here a strategy to predict the crystal structure, formulate, and prepare new noncentrosymmetric (NCS) bismuth-phosphate based compounds. It is based on the cooperative-arrangement of polar building units (BUs) which can be created at particular stoichiometric conditions. The arrangement of such BUs into NCS compounds arise from the shortest-periodicity of repartition of the cationic charges in NCS structures than in the plausible, but never observed centrosymetric polytypes. This work validates the possibilities for the prediction of an extended series of novel compounds, tuning the size of BUs within a variety of controlled edifices. Despite their closed chemical composition, all the predicted terms appeared strikingly stable at precise stoichiometries.

SURFACE TEMPERATURE OF PROTOPLANETARY DISKS PROBED BY ANNEALING EXPERIMENTS REFLECTING SPITZER OBSERVATIONS

Pyroxenes and olivines are the dominant crystalline silicates observed in protoplanetary disks. Recent spectral observations from the Spitzer Space Telescope indicate that the abundance of olivine, generally associated with silica polymorphs, relative to pyroxene is higher in the outer cold part of the disk than in the inner warmer part. The interpretation of these unexpected results requires a comprehensive knowledge of the thermal processing of Mg-rich silicate dust. In this respect, amorphous analogs were thermally annealed to identify microscopic crystallization mechanisms. We show that pyroxenes are not produced in significant proportions below the glass transition temperature of the amorphous precursor. The annealing of amorphous enstatite leads to a mineralogical assemblage dominated by forsterite, with only minute amounts of pyroxenes at temperatures as high as the glass transition temperature of enstatite (1050 K). The decoupling of cation mobility in amorphous silicates, favors the crystallization of the most Mg-enriched silicates. These results are consistent with Spitzer observations of silicate dust and also with the documented mineralogy of presolar silicates, making the low-temperature annealing a likely formation process for these objects. Based on these laboratory experiments and Spitzer observations, it appears that the reported zoned mineralogy may directly records and calibrates the thermal gradient at the scale of protoplanetary disks.

A sharp change in the mineralogy of annealed protoplanetary dust at the glass transition temperature

The crystallinity of silicate dust detected in protoplanetary disks contrasts with the dominantly amorphous nature of dust in the interstellar medium. The amorphous-to-crystal transition is therefore a valuable probe to constrain physical properties of disks such as temperature gradients or the extent of radial mixing. However, it requires a comprehensive knowledge of the behaviour of amorphous Mg silicates during thermal processing. In this respect, amorphous analogues of enstatite composition (MgSiO3) were thermally annealed around the glass transition temperature (Tg ∼ 1040 K). We show that enstatite is not produced below Tg. Instead, the annealing leads to a mineralogical assemblage dominated by forsterite (Mg2SiO4). A sharp change is observed at Tg, and pyroxene becomes the dominant mineral species. The annealing conditions marginally change the mineralogical assemblages produced above Tg. A notable exception is the recovery of protoenstatite (the high-temperature polymorph), which appears to be easily quenched in dust analogues. Detection of this phase would then provide an excellent probe for fast thermal events in disks. Our data suggest that below ∼1000 K, the mineralogy of silicate dust of solar composition should naturally be dominated by olivine while above this temperature pyroxenes should dominate. They also show that silica polymorphs recently detected in cold regions of disks are the natural by-products of the formation of forsterite out of enstatite precursor. The low-temperature crystallization pathway reported, combined with the evaporation/condensation process, captures the essential features of the zoned mineralogy of protoplanetary disks.

Oxidative Transformations of Biosourced Alcohols Catalyzed by Earth-Abundant Transition Metals

The catalytic acceptorless dehydrogenative oxidation of biosourced alcohols into carboxylic acid salts was achieved using earth‐abundant Fe and Mn complexes that feature aliphatic PNP pincer ligands in good to excellent yields. The Fe derivatives were characterized by using 57Fe NMR spectroscopy. Mn pincer catalysts are catalytically more efficient than their Fe counterparts thanks to their robustness under basic conditions. Attempts to generate aldehydes from alcohols were not successful using the Fe and Mn species, but a commercially available Ru analogue achieves this transformation selectively under very mild conditions in the presence of a large excess of acetone as a hydrogen acceptor.

Selective Hydrosilylation of Esters to Aldehydes Catalysed by Iridium(III) Metallacycles through Trapping of Transient Silyl Cations.

The combination of an iridium(III) metallacycle and 1,3,5-trimethoxybenzene catalyses rapidly and selectively the reduction of esters to aldehydes at room temperature with high yields through hydrosilylation followed by hydrolysis. The ester reduction involves the trapping of transient silyl cations by the 1,3,5-trimethoxybenzene co-catalyst, supposedly by formation of an arenium intermediate whose role was addressed by DFT calculations.

Efficient and Selective Hydrosilylation of Secondary and Tertiary Amides Catalyzed by an Iridium(III) Metallacycle: Development and Mechanistic Investigation

Readily accessible cationic IrIII metallacycles catalyze efficiently the chemoselective hydrosilylation of tertiary and secondary amides to amines. The catalyst described herein operates at low loadings using inexpensive 1,1,3,3‐tetramethyldisiloxane and allows fast reactions with high yields, selectivities, and turnover numbers. A transient iminium intermediate has been observed for the first time by using mass spectrometry, and the activation of the catalyst and the silane reagent have been studied by using DFT calculations. These fundamental insights support the present and future improvements of IrIII metallacycles through proper ligand modifications and enable further broad applications of catalysts based on metallacycles.