Direct bonds between metal atoms: Zn, Cd, and Hg compounds with metal-metal bonds

The synthesis and characterization of [Zn(2)(eta(5)-C(5)Me(5))(2)], a stable molecular compound with a Zn-Zn bond and the first example of a dimetallocene structure, has opened a new chapter in the organometallic chemistry of zinc and in metallocene chemistry. The existence of two directly bonded zinc atoms demonstrates that the [Zn-Zn](2+) unit, the lightest Group 12 homologue of the well-known [Hg-Hg](2+) ion, can be stabilized by appropriate ligands. Activity in this area has increased enormously in the few years since the determination of the structure of this molecule. Numerous theoretical studies have been devoted to the investigation of the electronic, structural, and spectroscopic properties of this and related compounds, and new metal-metal coordination and organometallic compounds of zinc, cadmium, and mercury have been synthesized and structurally characterized. This Minireview gives an overview of activity in this field during the past three to four years.     

Rapid determination of sterols in vegetable oils by CEC using methacrylate ester‐based monolithic columns

A method for the determination of sterols in vegetable oils by CEC with UV–Vis detection, using methacrylate ester‐based monolithic columns, has been developed. To prepare the columns, polymerization mixtures containing monomers of different hydrophobicities were tried. The influence of composition of polymerization mixture was optimized in terms of porogenic solvent, monomers/porogens and monomer/crosslinker ratios. The composition of the mobile phase was also studied. The optimum monolith was obtained with lauryl methacrylate monomer at 60:40% (wt:wt) lauryl methacrylate/ethylene dimethacrylate ratio and 60 wt% porogens with 20 wt% of 1,4‐butanediol (12 wt% 1,4‐butanediol in the polymerization mixture). Excellent resolution between sterols was achieved in less than 7 min with an 85:10:5 v/v/v ACN–2‐propanol–water buffer containing 5 mM Tris at pH 8.0. The limits of detection were lower than 0.04 mM, and inter‐day and column‐to‐column reproducibilities at 0.75 mM were better than 6.2%. The method was applied to the determination of sterols in vegetable oils with different botanical origins and to detect olive oil adulteration with sunflower and soybean oils.     

Preparation and evaluation of butyl acrylate-based monolithic columns for CEC using ammonium peroxodisulfate as a chemical initiator

Acrylate-ester-based monoliths for CEC using peroxodisulfate as a chemical initiator were prepared. The influence of two ternary porogenic solvents on the physical and chromatographic properties of butyl acrylate monolithic stationary phases was investigated. The composition and the ratio of porogenic solvent were adjusted to obtain highly permeable rigid monoliths with adequate column efficiency. Among the prepared butyl acrylate monoliths, those polymerized from a ternary porogenic solvent of acetonitrile/ethanol/water exhibited the most promising performance with a minimum plate height for naphthalene of 10.5 microm and a bed permeability of 7.3 x 10(-14) m(2). A comparison in terms of efficiency and permeability with thermal and UV initiation using alpha,alpha'-AIBN was also performed. The resulting monolithic stationary phases were evaluated in terms of reproducibility, giving RSD values below 5.1% in the electrochromatographic properties studied.     

Solid-supported cross metathesis and the role of the homodimerization of the non-immobilized olefin

We have prepared immobilized olefins as models for the cross metathesis using different olefin partners in the presence of second generation Grubbs and Hoveyda-Grubbs precatalysts. We have demonstrated that solid-phase cross metathesis is strongly dependent on the degree of homodimerization of the non-immobilized olefin and the reactivity of such a homodimer. As in the homogeneous phase, the Hoveyda-Grubbs precatalyst was better for immobilized alpha,beta-unsaturated carbonyl compounds.     

Glycolipids from sponges. 20. J-Coupling analysis for stereochemical assignments in furanosides: structure elucidation of vesparioside B, a glycosphingolipid from the marine sponge Spheciospongia vesparia

Reinvestigation of the glycosphingolipid composition of the marine sponge Spheciospongia vesparia revealed the presence of vesparioside B ( 2a), a new furanose-rich hexaglycosylated glycosphingolipid that is the most complex glycosphingolipid isolated from a marine sponge to date. The structure of the new compound was elucidated using extensive 2D NMR studies and chemical degradation. Particularly useful for structure elucidation of vesparioside B was a quantum mechanical computational study, showing that in furanosides a vicinal coupling constant <2.0 Hz (for H-1/H-2 or H-3/H-4) or <3.5 Hz (for H-2/H-3) is a proof of the trans orientation of the relevant protons. This general rule, combined with ROE data, allowed us to elucidate the relative stereochemistry (including anomeric configuration) of the three furanose five-membered rings.     

On the Chemical and Structural Rearrange of Hg-organyltellurolate Clusters: Synthesis, X-ray Characterization, TGA and Raman Evaluation of [(PhTe)6(Ph3P)2Hg5Cl4]?·?2THF and [(PhTe)8Hg6Py2Cl4]?·?Py (Py = pyridine)

(PhTe)₂Hg reacts with HgCl₂ and triphenylphosphine to give the clusters [(PhTe)₆(Ph₃P)₂Hg₅Cl₄] · 2THF (1) and [(PhTe)₈Hg₆Py₂Cl₄] · Py (Py = pyridine) (2). In 1 each tellurium atom connects two Hg atoms, with Hg–Te distances around 2.75 Å. Phosphorus atoms of the triphenylphosphine groups complete the tetrahedral coordination of the Hg1 atoms. The second set of Hg atoms (Hg2) is also coordinated to chlorine atoms. The three-dimensional assembling of 1 encloses circular channels with medium diameter of 12.5 Å. The cluster 2 results from substitution of the oxidized triphenylphosphine ligands of 1 by one molecule of Hg(PhTe)₂ in the course of the synthetic procedures. The alternated Te–Hg bonds of 2 close a 12-membered ring with two inverted 4-membered rings in the middle. Thermogravimetrical evaluations and Raman scattering lines of 1 and 2 are also discussed.

Is ammonia a better solvent than water for contact ion pairs?

The existence of a charge-transfer-to-solvent process when a KI contact ion pair (CIP) dissolved in supercritical water (SCW) is excited by UV light was confirmed by use of electronic structure calculations applied to molecular dynamics trajectories. We observed similar behavior with fluid density as that found for the KI-CIP in supercritical ammonia (SCA); nevertheless, there are some distinct features in the two supercritical solvents. First, the effect of the solvent field due to the molecules lying beyond the first solvation shell is very different in SCW compared with that observed in SCA; in SCW it actually has a destabilizing effect over the ground and excited states. Second, our results for the thermodynamic behavior of the CIP indicate that SCA is better solvent than SCW for this species. The differences found can be attributed to the solvent molecules surrounding the CIP and bridging the two ions; they shield more efficiently the ion pair from long-range solvent effects in SCA. The different behavior is partially attributed to a stronger solvent-solvent interaction in SCW than in SCA.     

Redox and acid-base coupling in ultrathin polyelectrolyte films

A single layer of poly(allylamine) with a covalently attached osmium pyridine-bipyridine complex adsorbed onto a Au surface modified by mercaptopropanesulfonate has been studied theoretically with a molecular approach and experimentally by cyclic voltammetry. These investigations have been carried out at different pHs and ionic strengths of the electrolyte solution in contact with the redox polyelectrolyte modified electrode. The theory predicts strong coupling between the acid-base and redox equilibria, particularly for low ionic strength, pH close to the pKa, and high concentration of redox sites. The coupling leads to a decrease in the peak potential at pH values above the apparent pKa of the weak polyelectrolyte, in good agreement with the experimental pH dependence at 4 mM NaNO3. Theoretical calculations suggest that the inflection point in the peak position versus pH curves can be used to estimate the apparent pKa of the amino groups in the polymer. Comparison of the apparent pKa for PAH-Os in the film with that of poly(allylamine) reported in the literature shows that the underlying charged thiol strongly influences charge regulation in the film. A systematic study of the film thickness and the degree of protonation in sulfonate and amino groups for solutions of different pH and ionic strength shows the coupling between the different interactions. It is found that the variation of the film properties has a non-monotonic dependence on bulk pH and salt concentration. For example, the film thickness shows a maximum with electrolyte ionic strength, whose origin is attributed to the balance between electrostatic amino-amino repulsions and amino-sulfonate attractions.