Isomeric olefin tetracarbonyl complexes of tungsten(I): an infrared spectroelectrochemical study at low temperatures

In situ electrolysis within an optically transparent thin-layer electrochemical (OTTLE) cell was applied at 293-243 K in combination with FTIR spectroscopy to monitor spectral changes in the carbonyl stretching region accompanying oxidation of four tetracarbonyl olefin complexes of tungsten(0), viz., trans-[W(CO)(4)(eta(2)-ethene)(2)], trans-[W(CO)(4)(eta(2)-norbornene)(2)], [W(CO)(4)(eta(4)-cycloocta-1,5-diene)], and [W(CO)(4)(eta(4)-norbornadiene)]. In all cases, the one-electron-oxidized radical cations (17-electron complexes) have been identified by their characteristic nu(CO) patterns. For the bidentate diene ligands, the cis stereochemistry is essentially fixed in both the 18- and 17-electron complexes. The radical cation of the trans-bis(ethene) complex was observed only at 243 K, while at room temperature it isomerized rapidly to the corresponding cis-isomer. The thermal stability of the three studied radical cations in the cis configuration correlates with the relative strength of the W-CO bonds in the positions trans to the olefin ligand, which are more affected by the oxidation than the axial W-CO bonds. For the bulky norbornene ligands, their trans configuration in the bis(norbornene) complex remains preserved after the oxidation in the whole temperature range studied. The limited thermal stability of the radical cations of the trans-bis(alkene) complexes is ascribed to dissociation of the alkene ligands. The spectroelectrochemical results are in very good agreement with data obtained earlier by DFT (B3LYP) calculations

Sequential Photosubstitution of Carbon Monoxide by (E)-Cyclooctene in Hexacarbonyltungsten: Structural Aspects, Multistep Photokinetics, and Quantum Yields

The photochemical conversion of W(CO)6 (1) into a trans-W(CO)4(η2-olefin)2 complex has been investigated using (E)-cyclooctene (eco) as a model olefin possessing extraordinary coordination properties. trans-W(CO)4(η2-eco)2 (4) is generated as an equimolar mixture of two diastereoisomers (4a, S symmetry; 4b, D2 symmetry) which can be separated by fractional crystallization. The entire reaction sequence involves the intermediate formation of W(CO)(η2-eco) (2) and cis-W(CO)(η2-eco)2 (3:  two diastereoisomers, 3a and 3b, with apparent C and C symmetry, respectively). Complexes 2 and 3, although difficult to isolate from the photochemical reaction mixture, are conveniently accessible via alternative thermal ligand exchange routes. The molecular structures of 2 and 4a in the crystal were determined by X-ray diffraction techniques. The olefin double bonds, with trans-orthogonal arrangement in 4a, are eclipsed to a OC−W−CO axis in either case. The course of the conversion of 1 into the olefin-substituted products was monitored by quantitative IR spectroscopy. Photokinetic equations developed for this study describe the concentrations of all four components as implicit functions of the amount of light absorbed by the system, of the quantum yields of the individual photoprocesses, and of the UV−vis absorbance coefficients of the compounds involved. Based on these functional relationships, the individual quantum yields at λexc = 365 nm (Φ12 = 0.73, Φ23 = 0.34, Φ24 = 0.16, Φ34 = 0.15) were evaluated from a series of experimental data sets by an iterative procedure which involves variation of the quantum yield input data until the best fit of the computed to the measured concentrations is achieved. Low-temperature matrix isolation techniques were employed to characterize the W(CO)(η2-eco) fragment (5) as a key intermediate in the photolysis of W(CO)5(η2-eco) (2)

Récolte de la cyanobactérie Arthrospira platensis par techniques à membranes, interactions Procédé/Biomasse

Le travail présenté concerne la récolte de la cyanobactérie Arthrospira platensis par des techniques à membranes dans le compartiment IVa de la boucle biorégénérative MELiSSA, développée par l'ESA en vue de vols spatiaux habités de longue durée. Une attention particulière est portée aux interactions Procédé/Biomasse, ainsi qu'aux contraintes spécifiques liées à la récolte en environnement spatial. Une première campagne expérimentale de sélection a orienté le choix vers une membrane inorganique tubulaire d'UF (ATZ 50 kD). Paramètres opératoires, modes de filtration et de nettoyage ont été investigués aux échelles " laboratoire " puis " pilote " pour les deux options envisageables. Dans le cas " basse concentration ", le circuit d'ultrafiltration permet de concentrer une biomasse diluée de 50 à 1000 mg/ afin de la recycler en photobioréacteur (PBR). La deuxième option consiste à concentrer directement la suspension en sortie du PBR. L'étude a montré la pertinence de l'UF pour la récolte cellulaire dans de bonnes conditions (taux de récupération et qualité du concentrat) jusqu'à un facteur de réduction volumique 20. Le mécanisme de colmatage est lié aux conditions d'écoulement, mais aussi aux caractéristiques et à l'activité de la suspension elle même. Si la lyse des cellules est préjudiciable aux performances, les exopolysacclharides excrétés par A. platensis jouent un rôle déterminant dans le colmatage qui n'est pas uniquement particulaire. Le taux d'EPS peut être minimisé d'une part en contrôlant les conditions de culture et d'autre part en réduisant le stress mécanique, fonction des paramètres d'UF et des caractéristiques des éléments constitutifs du circuit.The present work is devoted to the harvesting of a cyanobacterium Arthrospira platensis by membranes techniques in compartment IVa of the bioregenerative loop MELiSSA set up by ESA for long time spatial missions. We focus on Process/Biomass interactions, and on specific spatial constraints. First a comparative study has been realized and consequently an ultrafiltration inorganic tubular membrane has been chosen (ATZ 50 kD). Operating parameters, cleaning process and filtration modes have been investigated both at laboratory and pilot scales in order to perform the two following options : Low concentration , where un-harvested cells are recycled in the photobioreactor (from 50 to 1000 mg/l); and High concentration , for which biomass is concentrated at the photobioreactor outlet. The study has underlined the validity of UF as a way of harvesting (biomass recovery and integrity). The fouling phenomenon is linked both to hydrodynamics and biomass, especially exopolysaccharides excretion and is not only a particular contribution. EPS rate could be minimized by culture conditions and by the mechanical shear stress reduction degradation, a function of UF parameters and characteristics of constitutive set-up elements.NANTES-BU Sciences (441092104) / SudocNANTES-Ecole Centrale (441092306) / SudocSudocFranceF