The reductive activation of CO2 across a Ti═Ti double bond: synthetic, structural, and mechanistic studies

[Image: see text] The reactivity of the bis(pentalene)dititanium double-sandwich compound Ti(2)Pn(†)(2) (1) (Pn(†) = 1,4-{Si(i)Pr(3)}(2)C(8)H(4)) with CO(2) is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO(2) reaction is performed at −78 °C, the 1:1 adduct 4 is formed, and low-temperature spectroscopic measurements are consistent with a CO(2) molecule bound symmetrically to the two Ti centers in a μ:η(2),η(2) binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO(2) is quantitatively reduced over a period of minutes to give the bis(oxo)-bridged dimer 2 and the dicarbonyl complex 3. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (5) and monocarbonyl (7) double-sandwich complexes, which have been independently synthesized and structurally characterized. 5 is thermally unstable with respect to a μ-O dimer in which the Ti–Ti bond has been cleaved and one pentalene ligand binds in an η(8) fashion to each of the formally Ti(III) centers. The molecular structure of 7 shows a “side-on” bound carbonyl ligand. Bonding of the double-sandwich species Ti(2)Pn(2) (Pn = C(8)H(6)) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO(2) reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono(oxo) di-Ti(III) species to yield di-Ti(II) and di-Ti(IV) products. 1 forms a structurally characterized, thermally stable CS(2) adduct 8 that shows symmetrical binding to the Ti(2) unit and supports the formulation of 4. The reaction of 1 with COS forms a thermally unstable complex 9 that undergoes scission to give mono(μ-S) mono(CO) species 10. Ph(3)PS is an effective sulfur transfer agent for 1, enabling the synthesis of mono(μ-S) complex 11 with a double-sandwich structure and bis(μ-S) dimer 12 in which the Ti–Ti bond has been cleaved

Short-term effects of amelogenin gene splice products A+4 and A-4 implanted in the exposed rat molar pulp

In order to study the short-time effects of two bioactive low-molecular amelogenins A+4 and A-4, half-moon cavities were prepared in the mesial aspect of the first maxillary molars, and after pulp exposure, agarose beads alone (controls) or beads soaked in A+4 or A-4 (experimental) were implanted into the pulp. After 1, 3 or 7 days, the rats were killed and the teeth studied by immunohistochemistry. Cell proliferation was studied by PCNA labeling, positive at 3 days, but decreasing at day 7 for A+4, whilst constantly high between 3 and 7 days for A-4. The differentiation toward the osteo/odontoblast lineage shown by RP59 labeling was more apparent for A-4 compared with A+4. Osteopontin-positive cells were alike at days 3 and 7 for A-4. In contrast, for A+4, the weak labeling detected at day 3 became stronger at day 7. Dentin sialoprotein (DSP), an in vivo odontoblast marker, was not detectable until day 7 where a few cells became DSP positive after A-4 stimulation, but not for A+4. These results suggest that A +/- 4 promote the proliferation of some pulp cells. Some of them further differentiate into osteoblast-like progenitors, the effects being more precocious for A-4 (day 3) compared with A+4 (day 7). The present data suggest that A +/- 4 promote early recruitment of osteogenic progenitors, and evidence functional differences between A+4 and A-4

Déformation décrochante d’un bassin à litage rhéologique : modélisation analogique à l’échelle crustale

International audienceIn sedimentary basins, sediment type and deposit and burial conditions generate successivelayers with various rheologies reacting differently to boundary stresses. We used analoguemodelling to explore how a rheological-layered basin reacts to strike-slip displacements appliedto boundaries. Several experimental set-ups were tested by varying horizontal rheological layering(alternating viscous and cohesive layers) and displacement rates. All experiments generated a complexthree-dimensional deformation pattern localized into a narrow vertical band with thickened, thinnedand folded domains alternating along both strike and depth. This results in a succession of positiveand negative flower structures connected to the basal velocity discontinuity at depth and in a decouplingof deformation from one layer to another. Thinning and thickening variations display an alongstrikewavelength proportional to the layer thickness, an amplitude controlled by rheological layering,and in particular the strength contrast between each layer. The simultaneous genesis of shorteningand stretching structures during only one deformation stage is symptomatic of strike-slip boundaryconditions. These results indicate that caution should be exercised when postulating polyphaseddeformation fromoverprinting of different deformation styles.Dans les bassins sédimentaires, la nature des sédiments, leurs conditions de dépôt et d’enfouissementproduisent un litage mécanique dont les niveaux rhéologiques réagissent différemmentaux contraintes appliquées aux limites. Une modélisation analogique a été déployée pour explorer ladéformation d’un système sédimentaires multicouche subissant des conditions décrochantes à ses limites.Plusieurs conditions expérimentales ont été testées : variation de la stratification rhéologique horizontale (alternance de couches visqueuses et cohésives) et quantité de déplacement. Toutes lesexpériences montrent une déformation complexe localisée dans un couloir vertical étroit, formée parune succession de structures en fleur positives et négatives connectées à la base du modèle. Les niveauxprésentent des variations dépaisseur (zones épaissies, amincies et ployées). Les variations destyle de déformation se produisent à chaque interface qui découple la déformation d’une coucheà l’autre. Les variations d’épaisseur d’un niveau présentent une longueur d’onde proportionnelle àl’épaisseur de la couche dont l’amplitude est contrôlée par la stratification rhéologique, et en particulierle contraste de résistance entre chaque niveau. La co-existence des structures en raccourcissementet en étirement pendant la même phase de déformation sont caractéristiques des conditionsaux limites décrochantes. Ce résultat remet en cause l’interprétation systématique d’une déformationpolyphasée, la présence de structures accommodant de déformation différentes

Rheological study of quaternary ammonium‐containing star‐shaped polylactic acid

International audienceQuaternary ammonium-containing star-shaped polylactic acid (PLA-Q) is analyzed in melt rheology and found to follow a clustering behavior. The viscosity of this polymer is largely higher than its analog, of the same structure, without ammonium ions. This difference is attributed to physical interactions taking place between the ammoniums and hindering the flow of the melt polymer. The frequency sweeps of the storage (G′) and loss (G′′) modulus of the melt PLA-Q are achieved at different temperatures and different shearing times. The iso-temperature and iso-time superposition of the resulting curves were achieved using, respectively, a temperature and a time shift factors, this superposition allowed the determination of the flow activation energy; its value corresponds to an ionic process confirming the ionic interactions. When the two shift factors are used simultaneously, a unique master curve of both G′ and G′′ is obtained; this curve follows the Rouse model indicating the presence of two types of ionic associations within the melt polymer: a large number of multiplets having small size and a small number of clusters of larger size