Catherine Bied

Nanostructuring organo-silicas: combination of intermolecular interactions and molecular recognition properties to generate self-assembled hybrids with phenylene or adenine⋯thymine bridging units

Owing to hydrophobic interactions, silyl linkers containing long alkylene chains allowed the synthesis of self-organised hybrids. Lamellar organo-silicas with phenylene or a hydrogen-bonded adenine⋯thymine complex as the bridging units are reported.

Sol–gel immobilized and reusable copper-catalyst for arylation of phenols from aryl bromides

A simple β-diamide ligand was immobilized by the sol–gel process on hybrid silica for Cu-mediated O-arylation reactions. Combined with 5% of CuI, the latter can easily be recovered and reused to generate diarylethers under smooth conditions from cheap aryl bromides in an eco-friendly solvent (MIBK). Besides, negligible metal leaching occurred after reaction in solution from the supported catalyst.

Organic–inorganic hybrid silica materials containing imidazolium and dihydroimidazolium salts as recyclable organocatalysts for Knoevenagel condensations

Organic–inorganic hybrid silica materials containing imidazolium and dihydroimidazolium salts prepared from monosilylated and disilylated monomers by sol–gel methodologies are active and reusable organocatalysts for the Knoevenagel condensation of aromatic aldehydes with malononitrile and ethyl cyanoacetate under solvent-free conditions. Our immobilized systems present higher activities than related homogeneous bis-imidazolium salts, showing the cooperative effect of the matrix surface and the additional advantage of easy recycling. The best performances were obtained with the material derived from polycondensation of a disilylated dihydroimidazolium salt in the absence of tetraethoxysilane (TEOS).

Structuring of bridged silsesquioxanes via cooperative weak interactions: H-bonding of urea groups and hydrophobic interactions of long alkylene chains

Lamellar-bridged silsesquioxanes were obtained by the acid-catalysed hydrolytic condensation of a series of bissilylated organobridged molecular precursors. It was found that exploiting cooperative effects between the molecular interactions created by long hydrophobic hydrocarbon chains and the H-bonding between urea groups in the precursors favoured the nanostructuring of the corresponding hybrid silicas.

Structural ordering of self-assembled alkylene-bridged silsesquioxanes probed by X-ray diffraction experiments

Powder X-ray diffraction experiments have been performed to analyse the structural order in alkylene bridged silsesquioxanes. These organic–inorganic hybrids were obtained by the hydrolysis–condensation of a series of six α,ω-bis[triethoxysilyl(propyl)ureido]alkylenes associating hydrophobic alkylene units with various chain length and H-bonding urea groups. The sol–gel hydrolysis in ethanol with a stoichiometric amount of water and using fluoride anion as catalyst only afforded amorphous bridged silsesquioxanes. Conversely, the hydrolysis with a large excess of water under acidic catalysis produced materials as layered sheets. A lamellar periodicity on the molecular scale level was observed for longer alkylene chain (Cn C8–C12). Under similar reaction conditions, the hydrolysis of the short alkylene chain precursor (C6) led to an amorphous solid. The medium to long range arrangement of the organic fragment in the solids is attributable to a combination of the hydrogen bonding interactions between the urea groups and of the van der Waals interactions between the long alkylene chains. In excess water, the hydrophobic nature of the long alkylene chains prevails and favours the formation of the intermolecular H-bonds between the urea groups leading to ordered assemblies of the organic substructures in the silsesquioxane network.

In situ X-ray measurements to probe a new solid-state polycondensation mechanism for the design of supramolecular organo-bridged silsesquioxanes.

A long-range ordered organic/inorganic material is synthesized from a bis-silane, (EtO)(3)Si-(CH(2))(3)-NHCONH-C(6)H(4)-NHCONH-(CH(2))(3)-Si(OEt)(3). This crosslinked sol-gel solid exhibits a supramolecular organization via intermolecular hydrogen bonding interactions between urea groups (-NHCONH-) and covalent siloxane bonding, triple bond Si-O-Si triple bond. Time-resolved in situ X-ray measurements (coupling small- and wide-angle X-ray scattering techniques) are performed to follow the different steps involved in the synthetic process. A new mechanism based on the crystallization of the hydrolyzed species followed by their polycondensation in solid state is proposed.

Chiral amino-urea derivatives of (1R,2R)-1,2-diaminocyclohexane as ligands in the ruthenium catalysed asymmetric reduction of aromatic ketones by hydride transfer

Abstract Several new chiral urea and thiourea ligands have been prepared by reaction of (1R,2R)-1,2-diaminocyclohexane with various organic isocyanates and isothiocyanates. These were used as ligands in the ruthenium catalysed enantioselective reduction of aromatic ketones by isopropanol. The reduction proceeded at room temperature using 2 mol% of ruthenium catalyst to give good yields of the (R)-alcohol with enantiomeric excesses of up to 83%. By contrast, the use of bis-urea ligands gave much lower enantioselectivities. Amino-thiourea ligands led to the (S)-alcohol with low enantiomeric excess.

Preparation and Characterization of New Templated Hybrid Materials Containing a Chiral Diamine Ligand

New hybrid silica-based materials containing a chiral diamine ligand have been prepared by using primary amines as templates. Amorphous materials with high porosities and high surface areas have been characterized. They have been used to immobilize chiral rhodium complexes which catalyse the enantioselective reduction of acetophenone by hydride transfer, with low enantiomeric excess.

Chirality Transcription of Molecular Precursors to Hybrid Silicas

Silylated racemic, chiral (R,R)- and (S,S)-diureidocyclohexane derivatives have been synthesized in quantitative yields by the reaction of respectively trans racemic, (R,R)- or (S,S)-1,2-diaminocyclohexanes with 3-isocyanatopropyltriethoxysilane. The enantiomeric compounds have the ability to auto-associate through inter-molecular hydrogen bonding owing to the presence of the urea groups. Consequently they formed organic gels in non-polar solvents such as cyclohexane and mesitylene at very low concentration (ca < 10 mg per ml). Such gelating properties were not observed in the case of the racemic mixture. Under acidic aqueous media the controlled hydrolysis-condensation of the (R,R)- and (S,S)-enantiomers led to the hybrid silicates with fibrous structures. Interestingly right- or left-handed helices were respectively observed by Scanning Electronic Microscopy (SEM) images. Conversely, a featureless structure was obtained from the racemic mixture under the same conditions. A significantly reduced wavelength distance (Δ ν = 60 cm−1) between the C=O stretching mode and the NH bending mode was observed by FTIR spectroscopy, respectively at 1636 and 1576 cm−1. These results confirm the presence of the H-bonded urea groups in the solid materials. The solid state 13C NMR spectra exhibit in all cases a signal at 160 ppm attributed to the C=O, and a series of chemical shifts at 55, 43, 34, 25 and 10 ppm corresponding to the different sp3 carbon atoms. The preservation of the C–Si bonds was confirmed by the solid state 29Si CP-MAS spectra in which only T2 (−58 ppm) and T3 (−67 ppm) structures, respectively assigned to C—Si(OH)(OSi)2 and C—Si(Si)3, were observed. The formation of the right- and left-handed helical morphologies demonstrates the transcription of chirality from the enantiomers to the hybrid solids.

Photoluminescence Changes Induced by Self-Organisation in Bridged Silsesquioxanes

Two organo-bridged silsequioxanes derived from the same molecular precursor were synthesized with completely different local structure, namely long-range ordering. The photoluminescence features (emission, excitation, and time-resolved modes) were studied in the temperature range 13-300 K and compared with those of the molecular precursor. The effects of the self-assembling of the nanobuild blocks on these emission properties were discussed in terms of the magnitude of the hydrogen bonds between adjacent organic groups.