Phosphorus Compounds for the Formation of Unprecedented Anionic Zirconium Complexes

Using the nucleophilic ability of tertiary phosphines we have been able to prepare a large variety of unprecedented stable phosphonium anionic zirconocene(IV) complexes.     

Periodic Behavior of Lanthanide Coordination within Reverse Micelles

Trends in lanthanide(III) (Ln^III) coordination were investigated within nanoconfined solvation environments. Ln^III ions were incorporated into the cores of reverse micelles (RMs) formed with malonamide amphiphiles in n‐heptane by contact with aqueous phases containing nitrate and Ln^III; both insert into pre‐organized RM units built up of DMDOHEMA (N,N′‐dimethyl‐N,N′‐dioctylhexylethoxymalonamide) that are either relatively large and hydrated or small and dry, depending on whether the organic phase is acidic or neutral, respectively. Structural aspects of the Ln^III complex formation and the RM morphology were obtained by use of XAS (X‐ray absorption spectroscopy) and SAXS (small‐angle X‐ray scattering). The Ln^III coordination environments were determined through use of L₃‐edge XANES (X‐ray absorption near edge structure) and EXAFS (extended X‐ray absorption fine structure), which provide metrical insights into the chemistry across the period. Hydration numbers for the Eu species were measured using TRLIFS (time‐resolved laser‐induced fluorescence spectroscopy). The picture that emerges from a system‐wide perspective of the Ln? O interatomic distances and number of coordinating oxygen atoms for the extracted complexes of Ln^III in the first half of the series (i.e., Nd, Eu) is that they are different from those in the second half of the series (i.e., Tb, Yb): the number of coordinating oxygen atoms decrease from 9 O for early lanthanides to 8 O for the late ones—a trend that is consistent with the effect of the lanthanide contraction. The environment within the RM, altered by either the presence or absence of acid, also had a pronounced influence on the nitrate coordination mode; for example, the larger, more hydrated, acidic RM core favors monodentate coordination, whereas the small, dry, neutral core favors bidentate coordination to Ln^III. These findings show that the coordination chemistry of lanthanides within nanoconfined environments is neither equivalent to the solid nor bulk solution behaviors. Herein we address atomic‐ and mesoscale phenomena in the under‐explored field of lanthanide coordination and periodic behavior within RMs, providing a consilience of fundamental insights into the chemistry of growing importance in technologies as diverse as nanosynthesis and separations science.     

Implementation of a finite‐volume method for the determination of effective parameters in fissured porous media

Many studies have proposed one‐equation models to represent transport processes in heterogeneous porous media. This approach is based on the assumption that dependent variables such as pressure, temperature, or concentration can be expressed in terms of a single large‐scale averaged quantity in regions having very different chemical and/or mechanical properties. However, one can also develop large‐scale averaged equations that apply to the distinct regions that make up a heterogeneous porous medium. This approach leads to region‐averaged equations that contain traditional convective and dispersive terms, in addition to exchange terms that account for the transfer between the different media. In our approach, the fissures represent one region, and the porous media blocks represent the second region. The analysis leads to upscaled equations having a domain of validity that is clearly identified in terms of time and length‐scale constraints. Closure problems are developed that lead to the prediction of the effective coefficients that appear in the region averaged equations, and the main purpose of this article is to provide solutions to those closure problems. The method of solution makes use of an unstructured grid and a joint element method in order to take care of the special characteristics of the fissured network. This new numerical method uses the theory developed by Quintard and Whitaker and is applied on considerably more complex geometries than previously published results. It has been tested for several special cases such as stratified systems and “sugarbox” media, and we have compared our calculations with other computational methods. © 2000 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 16: 237–263, 2000     

A simple model for polysoaps' thread-like aggregates

The equilibrium polymerization, a model for one-dimensional reversible aggregates is used under conditions of theta and bad solvent to describe thread-like aggregates of polysoaps. In two dimensions, the aggregate size distribution decreases always more slowly than an exponential distribution and the dependence of the mean aggregate size L on the density φ and end-cap energy E of the polysoap cylindrical micelle is of the form L[φexp(E/KT)]^δ with δ<1/2. On the other hand, in three dimensions in the bad solvent regime, the dependence of L on φ becomes exponential explaining the high φ dependence of the viscosity in experimental results.     

Relèvement galoisien des revêtements¶de courbes nodales

Let R be a complete discrete valuation ring of mixed characteristics, with algebraically closed residue field k. We study the existence problem of equivariant liftings to R of Galois covers of nodal curves over k. Using formal geometry, we show that this problem is actually a local one. We apply this local-to-global principle to obtain new results concerning the existence of such liftings. Received: 10 February 2000 / Revised version: 13 September 2000     

Travelling breathers in Klein–Gordon chains

We study the existence of travelling breathers in Klein–Gordon chains, which describe nonlinear oscillators linearly coupled in a local anharmonic potential. In this work, we consider a case when the period of the breather and the inverse of the velocity are commensurable. In a neighborhood of critical values of velocity and coupling, we show by a center manifold reduction that the infinite-dimensional problem can be locally reduced to a eight-dimensional reversible differential equation. To cite this article: Y. Sire, G. James, C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

Dendrimers with a copper(I) bis(phenanthroline) core: synthesis, electronic properties, and kinetics

The copper(I) bis(chelate) complex Cu(L(0))(2) has been prepared from 2,9-diphenethyl-1,10-phenanthroline and Cu(CH(3)CN)(4)BF(4). Derivative Cu(L(0))(2) has been characterized by NMR, UV-vis spectroscopy, and X-ray crystallography. Interestingly, owing to the presence of the ethylene linker, the interligand pi-pi stacking interactions between the phenyl rings and the phenanthroline subunits in Cu(L(0))(2) do not induce significant distortions of the pseudotetrahedral symmetry around the Cu(I) center in the solid state or in solution. Following the synthesis of Cu(L(0))(2), dendrimers Cu(L(1)(-)(4))(2) with a Cu(I) bis(2,9-diphenethyl-1,10-phenanthroline) core surrounded by Fréchet type dendritic branches have been prepared and the kinetics of their cyanide-assisted demetalation studied. Importantly, the surrounding dendritic wedges have no significant influence on the coordination geometry of the Cu(I) center, as deduced from their absorption spectra. Therefore, the variations of the rate constants only reflect changes resulting from the presence of the dendritic branches. The kinetics of the cyanide-mediated demetalation reaction indeed revealed that cyanide diffusion through the dendritic shell is slightly influenced by the size of the branches. Significant effects were observed in the kinetics when going from the third to the fourth generation and have been ascribed to changes in the lipophilicity around the metallic core as a result of dendritic encapsulation.     

Multinuclear magnetic resonance and molecular modeling investigations as unambiguous methods for the determination of silacycle 3D structures

An unexpected six-membered silacycle was obtained using ring-closing metathesis of diallylsilane 3. The ring size of the compound formed was derived using a 2D 13C-13C inadequate experiment. All proton and carbon-13 resonances were assigned from classical 2D NMR experiments and the proton coupling pattern was clarified. Molecular modeling calculation was then applied using NMR data as constraints, giving the 3D structure of the synthesized compound.     

Decreasing the ring size of a cyclic nonribosomal peptide antibiotic by in-frame module deletion in the biosynthetic genes

Many natural products of therapeutical and biotechnological importance are nonribosomally synthesized peptides. Structural hallmarks of this class of compounds are the occurrence of unusual amino acids, mostly cyclic peptide backbones, and numerous further modifications such as acylation, heterocyclic ring formation, and glycosylation. Because of their structural complexity, chemical synthesis is usually an unattractive route to these molecules. In contrast, genetic engineering of the biosynthesis genes emerges as a potentially powerful approach to the combinatorial biosynthesis of useful analogues of the lead compounds. Nonribosomal peptide synthetases (NRPSs) carry out a sequential multistep assembly and modification of the peptides in a thiotemplate process described by the multiple carrier model. The modular architecture of NRPSs suggests straightforward methods for the reprogramming of these enzymes by exchange of catalytic subunits. However, many of the reported engineering attempts faced low product yields or even inactive hybrid enzymes. Using a new approach to obtain hybrid NRPSs, we show here that the deletion of an entire module in an NRPS assembly line caused the secretion of the predicted peptide antibiotic variant with a decreased ring size. Furthermore, a module exchange resulted in a significantly higher product yield than that observed in previous studies.