Daoud Naoufal

Magnetic Anisotropy in Pentacoordinate Ni(II) and Co(II) Complexes: Unraveling Electronic and Geometrical Contributions.

The magnetic properties of the pentacoordinate [MII (Me4 cyclam)N3 ]+ (Me4 cyclam=tetramethylcyclam; N3 =azido; M=Ni, Co) complexes were investigated. Magnetization and EPR studies indicate that they have an easy plane of magnetization with axial anisotropy parameters D close to 22 and greater than 30 cm-1 for the Ni and Co complexes, respectively. Ab initio calculations reproduced the experimental values of the zero-field splitting parameters and allowed the orientation of the anisotropy tensor axes with respect to the molecular frame to be determined. For M=Ni, the principal anisotropy axis lies along the Ni-Nazido direction perpendicular to the Ni(Me4 cyclam) mean plane, whereas for M=Co it lies in the Co(Me4 cyclam) mean plane and thus perpendicular to the Co-Nazido direction. These orientations match one of the possible solutions experimentally provided by single-crystal cantilever torque magnetometry. To rationalize the geometry and its impact on the orientation of the anisotropy tensor axis, calculations were carried out on model complexes [NiII (NCH)5 ]2+ and [CoII (NCH)5 ]2+ by varying the geometry between square pyramidal and trigonal bipyramidal. The geometry of the complexes was found to be the result of a compromise between the electronic configuration of the metal ion and the structure-orienting effect of the Me4 cyclam macrocycle. Moreover, the orientation of the anisotropy axes is mainly dependent on the geometry of the complexes.

Multi-functional hybrid materials for proton conductivity

In this paper, a new multifunctional hybrid organic–inorganic silica containing poly(ethylene glycol) [PEG] and sulfonic acid [SO3H] groups was synthesized by a sol–gel process consisting of co-hydrolysis and co-condensation of a mixture of bis(triethoxysilylpropyl urethane)–PEG-[400, 600 and 1000] in the presence of various amounts (0, 10 and 50% molar) of a monosilylated precursor affording SH units i.e. 3-mercaptopropyltrimethoxysilane (MPTMS). These materials were characterized by 13C, 29Si solid state NMR, IR, TGA, XPS and elemental analyses which confirm that the siloxane units in these materials remain integrally intact after chemical modification and the organic units thermally stable up to 150 °C. Evaluation of proton conductivity recorded at ambient temperature on hydrated samples indicates that by using the sol–gel process, it is possible to achieve mechanically and chemically stable hybrid materials exhibiting proton conductivity performances comparable to that of Nafion.

Conformational Effects through Hydrogen Bonding in a Constrained γ-Peptide Template: From Intraresidue Seven-Membered Rings to a Gel-Forming Sheet Structure

A series of three short oligomers (di-, tri-, and tetramers) of cis-2-(aminomethyl)cyclobutane carboxylic acid, a γ-amino acid featuring a cyclobutane ring constraint, were prepared, and their conformational behavior was examined spectroscopically and by molecular modeling. In dilute solutions, these peptides showed a number of low-energy conformers, including ribbonlike structures pleated around a rarely observed series of intramolecular seven-membered hydrogen bonds. In more concentrated solutions, these interactions defer to an organized supramolecular assembly, leading to thermoreversible organogel formation notably for the tripeptide, which produced fibrillar xerogels. In the solid state, the dipeptide adopted a fully extended conformation featuring a one-dimensional network of intermolecularly H-bonded molecules stacked in an antiparallel sheet alignment. This work provides unique insight into the interplay between inter- and intramolecular H-bonded conformer topologies for the same peptide template.

Two Compartmentalized Inner Receptors for the Tetramethylammonium Guest within a Keplerate-Type Capsule

The host-guest interactions between the spherical porous Keplerate anion, [Mo132O372(CH3CO2)30(H2O)72](42-) (abbreviated {Mo132}) and the tetramethylammonium cation have been investigated extensively by one- and two-dimensional (EXSY, ROESY, and DOSY) and variable-temperature NMR. Evidence of two inner receptor sites specific for a NMe4(+) guest appears consistent with a quite striking compartmentalization phenomenon. ROESY NMR analyses showed that both sites exhibit a close spatial proximity with the hanging inner acetate groups, while a quantitative EXSY study revealed that these two sites are differentiated by their exchange rates. These NMR data support the hypothesis that these two inner sites could be delimited by the hanging inner acetate groups forming triangular (S1) or pentagonal (S2) hydrophobic pockets on the inner side of the capsule wall. Furthermore, the stability constants associated with the trapping process of the NMe4(+) guest on both the S1 and S2 sites have been determined, showing that the stability constant of the S1 sites decreases significantly as the concentration of the capsule increases gradually, while that of the S2 sites remains nearly unaffected. Such an observation has been interpreted as a result of the plugging process of the {Mo9O9} pores by the counterions NH4(+), which causes unfavorable electrostatic interactions for the NMe4(+) coordination on the proximal S1 site. Finally, the thermodynamic parameters of the NMe4(+) transfer from the solvated situation to the interior of the capsule were estimated from variable-temperature NMR experiments that provide the split of the global process into two successive events corresponding to the plugging and transfer across the inorganic shell.

Multifunctional Silica Nanoparticles Modified via Silylated-Decaborate Precursors

A new class of multifunctional silica nanoparticles carrying boron clusters (10-vertex closo-decaborate) and incorporating luminescent centers (fluorescein) has been developed as potential probes/carriers for potential application in boron neutron capture therapy (BNCT). These silica nanoparticles were charged in situ with silylated-fluorescein fluorophores via the Stober method and their surface was further functionalized with decaborate-triethoxysilane precursors. The resulting decaborate dye-doped silica nanoparticles were characterized by TEM, solid state NMR, DLS, nitrogen sorption, elemental analysis, and fluorescence spectroscopy.

β-Cyclodextrin-Mediated Enantioselective Photochemical Electrocyclization of 1,3-Dihydro-2H-azepin-2-one

The photochemical electrocyclization reaction of the title compound in the presence of β-cyclodextrin was examined in different conditions. No enantioselectivity was observed in solution, but solid-state reactions of a 1:1 complex as a suspension or a thin film, followed by reduction, provided (1R,5R)-2-azabicyclo[3.2.0]heptan-3-one in isolated yields up to 79% and with ee values up to 45%.

Study of the controlled temperature reaction between closo-decahydrodecaborate and alcohols in H2SO4 medium

The reactions of the ammonium salt of closo-decahydrodecaborate (NH4)2B10H10 with alcohols were studied in concentrated sulfuric acid medium. Alcohols ROH (R = CH3 ,C 2H5 ,C 3H7, iso-C3H7 ,C 4H9, 2-C4H9, ter-C4H9, C5H11) played the role of solvent and ligand, simultaneously. These Electrophile Induced Nucleophilic Substitution Reactions were conducted at two different temperatures (25 ◦ C or reflux) in atmospheric air and produced (2-B10H9OR) 2− , (2,7(8)-B10H8(OR)2) 2- and/or (B10H7(OR)3) 2- . The rate of the reaction and the nature of the product formed depended on the temperature, on the alcohol, and on the reaction duration. The reactions at 25 ◦ C, if proceeded, produced mainly the monoalkoxy derivative, (2-B10H9OR) 2- . For the longer chain alcohols (4 carbons and above) the increase in the reaction duration and the temperature resulted in higher substitution, thus producing the dialkoxy (2,7(8)-B10H8(OR)2) 2- and trialkoxy (B10H7(OR)3) 2- derivatives. Meanwhile only the monoalkoxy (2-B10H9OR) 2- plus the dialkyloxonium (2-B10H9OR2) − derivatives were produced with the shorter alcohols (1 to 3 carbons) at refluxing temperature. All these derivatives were isolated in their pure forms on DEAE-cellulose column with adequate yields, and characterized by 11 B, 13 C, 1 H NMR, negative ESI and TLC.

Synthesis, characterization and mechanism of formation of 6-substituted nido-B10H13 decaboranes by the opening reaction of closo-decahydrodecaborate [B10H10]2− cage

The synthesis of new 6-LB10H13 (L = ligand) derivatives of decaborane B10H14 with different ligands was achieved by the opening cage reaction of closo-decahydrodecaborate (NH4)2B10H10 in the presence of a strong acid (HNO3 or H2SO4) in organic solvents (hexane or benzene derivatives). This reaction was performed by varying different factors: the acid strength, the solvent polarity, the equivalent amounts of (NH4)2B10H10 with respect to the acid used (1/4 or 1/6) and the reaction time (30 minutes to 24 hours). Nitric acid forced hexane solvent to play the role of ligand whereas sulfuric acid did not. The reaction of (NH4)2B10H10 with HNO3 in hexane produced 6-(C6H13)B10H13 and [B9H13NO3]−. The reaction of (NH4)2B10H10 with H2SO4 in aromatic solvent L (benzene, toluene, o-xylene and m-xylene) produced 6-LB10H13 and 6-(HO3SO)B10H13 as major products with B6H10 and [B9H14]− as minor products. All these derivatives were characterized by 11B NMR, 1H NMR, 13C NMR and negative ESI. This study elucidated some facts and ideas that were not clear concerning the mechanism of opening reaction of closo-[B10H10]2−.