IR and theoretical studies of monocarbonyl Ni complexes formed by adsorption of CO at low pressure on silica-supported Ni(II) ions

This work reports on the reactivity of coordination vacancies of Ni(II) ions grafted onto the tridentate silica support (Ni(II)(3c) ions) with respect to CO used as a probe molecule. The adsorption of CO at 77 K in the 0.3 to 3.5 Pa CO pressure range is studied by FTIR on two samples differing in the dispersion of nickel. Quantum chemical calculations by the DFT method are performed to investigate, using a cluster approach, the binding of Ni to silica and, after CO adsorption, the geometry of the resulting carbonyl Ni complexes. Silica is modeled by using clusters composed of three types of monodentate ligands, SiO(-), SiOSi and/or SiOH, found on the surface of silica. This work is devoted to the monocarbonyl complexes. Whatever the sample, only one type of monocarbonyl is formed from Ni(II)(3c) ions. It is shown that the charge of the silica cluster is the major parameter influencing the CO IR frequency whereas the nature and the size of the silica cluster do not affect the CO bond length, confirming that local electrostatic interactions predominate. Only the 1- charged silica cluster Si(5)O(3)(-), composed of SiO(-), 2SiOSi fragments, respectively, reproduces the Ni[bond]O distances derived from EXAFS for the Ni(II)(3c) grafted site and gives CO frequencies in good agreement with the experimental values. It is shown that CO is stabilized by a magnetic transition from the (3)Ni(2+) triplet to the (1)Ni(2+) singlet state occurring upon adsorption.     

Investigation of the luminescent properties of terbium-anthranilate complexes and application to the determination of anthranilic acid derivatives in aqueous solutions

The luminescent properties of terbium complexes with furosemide (FR), flufenamic (FF) acid, tolfenamic (TF) acid and mefenamic (MF) acid have been investigated in aqueous solutions. For all four compounds, complexation occurs when the carboxylic acid of the aminobenzoic group is dissociated and is greatly favoured in the presence of trioctylphosphine oxide as co-ligand and Triton X-100 as surfactant. Under optimum conditions, luminescence of the lanthanide ion is efficiently sensitised and the lifetime of the resonance level of terbium in the complex is ranging between 1 and 1.9 ms, against 0.4 ms for the aqua ion. The sensitivity of the method for the determination of anthranilic acid derivatives is improved by one to two orders of magnitude with respect to that achieved using native fluorescence or terbium-sensitised luminescence in methanol. The limits of detection are 2×10⁻¹⁰, 5×10⁻¹⁰ and 2×10⁻⁹ mol l⁻¹ for flufenamic acid, furosemide and tolfenamic acid, and mefenamic acid, respectively, with within-run RSD values of less than 1%. The method has been applied to the determination of flufenamic acid in spiked calf sera with and without sample pretreatment. Depending on the method and the analyte concentration, the recovery was ranging between 83 and 113% and the lowest concentration attainable in serum samples was close to 1×10⁻⁷ mol l⁻¹.     

Enantioselective 1,4-Addition of Aliphatic Grignard Reagents to Enones Catalyzed by Readily Available Copper(I) thiolates derived from TADDOL. Preliminary communication

Two simple thiols derived from the parent TADDOL, α,α,α′,α′ tetraphenyl-2,2-dimethyl-1,3-clioxolan-4,5-dimethanol, are used to prepare Cu¹ complexes C and D to catalyze (0.05 equiv.) 1,4-additions of Grignard reagents RMgCl to cyclic enones with enantioselectivities which are comparable to or better than previously reported (enantiomer ratios up to 92:8).     

Hydrogen cycling of niobium and vanadium catalyzed nanostructured magnesium

The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage purposes, is enhanced significantly by the addition of catalysts such as Nb and V and by using nanostructured powders. In situ neutron diffraction on MgNb(0.05) and MgV(0.05) powders give a detailed insight on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteriding), a MgH(1< x < 2) phase is observed, which does not occur in bulk MgH(2) and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst is present as NbH(1). Second, a hitherto unknown Mg-Nb perovskite phase could be identified that has to result from mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2-20 nm. Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH(2) layers.     

Using the diphosphanyl radical as a potential spin label: effect of motion on the EPR spectrum of an R1(R2)P--PR1 radical

The EPR spectrum of the novel radical Mes*(CH3)P--PMes* (Mes*=2,4,6-(tBu)3C6H2) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the 31P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic 31P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P--P bond.     

Mechanochemical assembly of hydrogen bonded organic-organometallic solid compounds

Solvent-free reactions with molecular systems have been exploited to prepare hybrid organic-organometallic solids: grinding of the complex [Fe(eta 5-C5H4COOH)2] with solid bases B generates quantitatively the corresponding hydrogen bonded salts [Fe(eta 5-C5H4COOH)(eta 5-C5H4COO)][HB] (B = 1,4-diazabicyclo[2.2.2]octane, 1,4-phenylenediamine); gas-solid reactions are also possible with volatile bases.     

Une méthode simple de calcul des structures électroniques (σ + π) des borazines et des boroxines substitués de symétrie D3h: une estimation théorique de l'aromaticité comparée de ces molécules

Résumé Une utilisation conjointe des méthodes L.C.A.O.-C.U.V. et d'itération tournante permet d'atteindre des structures électroniques ( + ) cohérentes avec l'U.V.(et donc le gradient de population électronique de liaison qui conditionne l'aromaticité) de huit borazines et boroxines, substitués ou non, de symètrie D _3h . L'échelle d'aromaticité obtenue permet de rendre compte des propriétés magnétiques (RMN, RQ, effet Faraday, diamagnétisme) de ce type de molécules. On parvient en particulier aux deux conclusions essentielles suivantes: les borazines sont nettement plus aromatiques que les boroxines isologues et la greffe de groupements méthyles ou d'atomes de chlore, qu'elle se fasse sur le bore ou sur l'azote, confère toujours au dérivé substitué une aromaticité au moins égale à celle du borazine lui-même.

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A simple method to calculate the - and -electron structure of borazines and of substituted boroxines of symmetry D_3h: A theoretical estimation of the relative aromaticity of those molecules

( + ) electronic structures calculations are performed for eight D _3h borazines and boroxines, using in concert previously published U.V.-consistent H.M.O.-L.C.A.O. and bond by bond iteration methods. An estimation of bond electronic populations gradients is so obtained, allowing to set up a new aromaticity scale for such molecules. The main results are the following: Borazines are much more aromatic than isologous boroxines and B-or N-graft of alkyl groups or chlorine atoms never decreases (and generally increases) aromaticity.

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Zusammenfassung Es wird für acht Borazol- und Boroxolverbindungen mit D _3h -Symmetrie unter Verwendung einer am UV-Spektrum geeichten iterativen HMO-LCAO-Theorie die - und -Elektronenstruktur bestimmt. Hiermit kann eine Abschätzung des Gradienten der Bindungselektronendichten gewonnen werden, aus der eine Einteilung der Moleküle nach ihrem aromatischen Charakter hervorgeht. Es ergibt sich, daß die Borazole aromatischer sind als die isoelektronischen Boroxole und daß eine Substitution der H-Atome am Bor oder Stickstoff durch Chlor oder Methylgruppen die Aromatizität fast immer erhöht, jedenfalls aber nie erniedrigt.

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Recherche effectuée dans le cadre de la R.C.P. n du C.N.R.S.     

Characterization of low-molecular weight peptides in champagne wine by liquid chromatography/tandem mass spectrometry

This paper reports the use of an on-line LC–ESI–MS/MS method for the identification and quantification of di- and tripeptides in champagne wine without laborious sample pretreatment. The identification of these compounds, in their underivatised form, is based on identical retention times and ESI–MS spectra to those of reference standards. The presence of nine dipeptides (Arg–Ile, Ile–Arg, Ile–Val, Lys–Phe, Lys–Tyr, Phe–Lys, Tyr–Gln, Tyr–Lys, Val–Ile) and the absence of two tripeptides (Phe–Arg–Arg and Lys–Met–Asn) have been evidenced in the matrix. Calibration curves for each analyte were established using Phe–Arg as internal standard. The calibration curves were linear in the concentration range 0.1–10 mg L⁻¹ with a determination coefficient, r², better than 0.992. The accuracy for the calibration standard was estimated at between 92 and 102%. This method allows high recovery and satisfies the necessary requirements with respect to accuracy, repeatability and sensitivity. The first application of this analytical method to the measurement of di- and tri-peptides in different vintages of champagne wine is reported. Compositional changes in the peptides occurred depending on the vintage.