On the origin of dinitrogen hydrogenation promoted by [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2)

The origin of the hydrogenation of the dinitrogen ligand in [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2) has been investigated by a combined computational and experimental study. Density functional theory calculations on the zirconocene dinitrogen complex demonstrate significant imido character in the zirconium nitrogen bonds, arising from effective pi-back-bonding from the low-valent zirconium and the side-on bound N2 ligand. The twisted ground-state structure of the N2 complex is a key requirement for nitrogen hydrogenation, as calculations on the model complex [(eta5-C5H5)2Zr]2(mu2,eta2,eta2-N2) reveal reduced overlap as the dihedral angle between the zirconocene wedges approaches 0 degrees . Experimentally, isotopic labeling studies on the microscopic reverse are consistent with a 1,2-addition mechanism for nitrogen hydrogenation.     

α- and β-cyclodextrin complexes with n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates. A molecular mechanics study of 1:1 and 1:2 associations

Molecular mechanics (MM) methods were employed to evaluate stabilization upon formation of inclusion compounds between two different guest molecules and α- and β-cyclodextrins (CDs) for two different stoichiometries 1:1 and 1:2. The two guest molecules studied were n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates with variety of chain lengths. The computed stability for the inclusion compounds between α-CDs and n-alkyl carboxylic acids reproduced experimental data reported in the literature. The transition between 1:1/1:2 complexes occurred at an alkyl chain length of n_C=9. It was previously demonstrated by diffusion coefficients measures that a stable 1:2 stoichiometry inclusion compound could be formed between n-alkyl p-hydroxy benzoates and α-CD for the chain length n_C>4. The computed results reproduced the experimental ones. The combination between OPLS and GB/SA resulted in better agreements with experiments than those obtained with MM2 and MM3.     

Construction of giant dendrimers using a tripodal building block

Giant pentane-soluble organo-silicon dendrimers have been synthesized using a triallylphenol brick according to a new divergent construction that uses a hydrosilylation-nucleophilic substitution sequence up to the ninth generation (G(9)). All the reactions were monitored by (1)H, (13)C, and (29)Si NMR until G(9), indicating that they were clean at the NMR accuracy until this last generation. MALDI TOF mass spectra were recorded for G(1) to G(4) and show the nature and amounts of defects that are intrinsic to the divergent construction. This technique and SEC (recorded up to G(5)) confirm the monodispersity (1.00 to 1.02) from G(1) to G(5). HRTEM and AFM images recorded for the high generations disclose the expected smooth dendrimer size progression and the globular shape. At G(9), the theoretical number of termini (TNT) is 177 407 branches (abbreviation: G(9)-177 047). It is estimated that more than 10(5) terminal branches are actually present in the G(9) dendrimer, far beyond the De Gennes "dense-packing" limit (6000 branches), and it is believed that the branch termini turn inside the dendrimer toward the core. This is corroborated by lower reaction rates and yields for the highest generation numbers presumably due to intradendritic reactions. It is probable that the dendritic construction is limited by the density of branches inside the dendrimer, i.e., far beyond the dense-packing limit.     

The reaction of eremanthin and isoeremanthin with bromine. Unprecedented simultaneous addition of Br2 to two isolated but topographically related double bonds.

The reaction of $\text{1}$ with Br₂ in CHCl₃ proceeds with the simultaneous and stereospecific incorporation of two moles of Br₂ to give $\text{2}$ thus providing strong experimental evidence for the existence of Br₃.     

An unusual product obtained from condensation between ethylenediamine and formaldehyde in basic medium

Reaction between ethylenediamine and formaldehyde normally affords 1,3,6,8-tetraazatricyclo[4.4.1.1^3.8]dodecane (TATD) but in this case also afforded 3,3′-ethane-1,2-diyl-bis-1,3,5-triazabicyclo[3.2.1]octane (ETABOC), this being an unusual product from such condensation.     

Physical and optical properties of the quaternary sulfides SrCu2MS4 and EuCu2MS4 (M=Ge and Sn)

Four quaternary sulfides SrCu₂MS₄ and EuCu₂MS₄ (M=Ge and Sn) were prepared from a thoroughly ground mixture of EuS or SrS, Cu, or Sn, and S in stoichiometric proportions. Electrical conductivity measurements on pressed pellets showed that all the phases are semiconductors. The optical band gaps were assessed at 2.8 eV for SrCu₂GeS₄, 2.1 eV for SrCu₂SnS₄, 2.2 eV for EuCu₂SnS₄, and 1.6 eV for EuCu₂GeS₄. Both Sr-based compounds present a temperature-independent paramagnetism, of about +135×10⁻⁶ and +92×10⁻⁶ emu/mol, for SrCu₂SnS₄ and SrCu₂GeS₄, respectively. In the case of the europium compounds, they follow a Curie-Weiss dependence above 1.8 K (EuCu₂GeS₄) and above 4 K (for EuCu₂SnS₄), with values of the magnetic effective moment μ_eff and the Curie-Weiss temperature Θ, equal to 6.27 μ_B and −2.8 K for EuCu₂GeS₄, and 6.81 μ_B and +0.7 K, for EuCu₂SnS₄. The experimental magnetic moments confirm that the europium ion is in divalent state, similar to Sr in the related compounds.     

Polypropylene-clay nanocomposites: effect of compatibilizing agents on clay dispersion

In this work, polypropylene-clay nanocomposites are obtained and studied by using two different coupling agents, diethyl maleate and maleic anhydride. Two different clays, a commercial montmorillonite (Nanomer I30.TC) and a sodium bentonite purified and modified with octadecylammonium ions have also been used. The relative influence of each factor, matrix and clay modification, can be observed from structural analysis (SAXS, TEM) and mechanical properties. An explanation of the results is proposed according to the microstructure and chemical nature of the systems and the thermodynamic interactions operating during nanocomposite preparation.     

Structure and thermodynamics of alpha-, beta-, and gamma-cyclodextrin dimers. Molecular dynamics studies of the solvent effect and free binding energies

The alpha-, beta-, and gamma-cyclodextrin (CyDs) dimers were studied by molecular dynamics (MD) simulations in water as an explicit solvent. The relative stability of dimers and the involved molecular interactions were determined. Three possible starting orientations were considered for the dimers: head-to-head, head-to-tail, and tail-to-tail. MD simulations were performed over a period of 5 ns to ensure the stability of the system for both the CyD dimers and monomers. The MM-PBSA methodology was used to obtain the free binding energy of the dimers and to determine the most stable arrangement for each solvated CyD. In a vacuum, MD simulations provided the head-to-head orientation as the most stable orientation for the three CyDs, while in aqueous solution the, the head-to-tail orientation was found to be the most stable for the alpha-CyD dimer and the tail-to-tail orientation the most stable for the beta- and gamma-CyD dimers.     

Comparative Study on Triclosan Interactions in Solution and in the Solid State with Natural and Chemically Modified Cyclodextrins

The aim of this study was to investigate the interactions of triclosan (TRI), a poorly water-soluble antimicrobial drug, with natural crystalline cyclodextrins (α-, β- and γ-Cd) and the corresponding hydroxypropylated amorphous derivatives (HPα-, HPβ-, and HPγ-Cd) and evaluate their effectiveness as complexing and solubilizing agents towards the drug. Equimolar solid systems were prepared using different techniques (physical mixing (PM), kneading (KN) and coevaporation (COE)) in order to evaluate the influence of the preparation method on the performance of the end products. Drug–carrier interactions were investigated both in aqueous solution, using phase-solubility analysis, fluorescence and circular dichroism (CD) techniques, and in the solid state, using differential scanning calorimetry (DSC) supported by thermograumetric analysis (TGA), X-ray powder diffractometry (XRPD) and scanning electron microscopy (SEM) analysis. Among the native cyclodextrins, β-Cd seemed to have the most suitable cavity to fit the drug molecule, whereas the α-Cd cavity was too small and the γ-Cd cavity too large to establish stable interactions with the guest. However, due to the B _S -type phase solubility diagram, its solubilizing efficiency was very limited. The presence of the hydroxypropylic substituents improved, in all cases, Cd solubilizing and complexing efficacies towards the drug. This was particularly evident in the case of HPγ-Cd, whose stability constant was about 200-fold higher than that of the native γ-Cd. HPβ-Cd was the most effective carrier for TRI, showing a solubilizing power about 20 times higher than the corresponding native Cd and about 2-fold that of the other hydroxypropyl derivatives. Moreover, a clear influence of the preparation method on the properties of the final products was observed. The COE method with hydroxypropylated cyclodextrins seemed the most suitable technique in achieving the complete drug amorphization and/or inclusion complexation. Received in final form: 24 January 2005