Rate studies of silylation with silylamides

The rates of silylation of p-nitrophenol with N,O-bis(trialkylsilyl)acetamides in dioxane have been measured, the reaction shown to be slowed down by replacing methyl by ethyl in the trialkylsilyl group. The rates of methanolysis of some N,O-bis(aryldimethylsilyl)acetamides (I) and N-aryldimethylsilylacetamides (II) have been measured. The reactions of compounds II were found to be acid catalyzed and accelerated by electron-withdrawing substituents in the benzene ring. At 30°C the methanolysis was shown to be entropy controlled. Compounds of series I were found to be aproximately 1000 times more reactive than those of series II. Introducing a methyl at nitrogen in the monosilylamides produced a similar rate enhancing effect as introduction of a second silyl group. Promotion of (p-d)π coordination of silicon to oxygen or nitrogen in the ground state of the silylamide molecule is suggested as the factor responsible for this effect.     

Silica-supported lanthanide silylamides for methyl methacrylate polymerisation: controlled grafting induces controlled reactivity

Lanthanide silylamides were grafted onto non-porous silica dehydroxylated at various temperatures, and the surface species nature and relative distribution were correlated with MMA polymerisation activity and selectivity.     

Thiomalates of alkaline earth metals

Solid state 1:1 complexes of divalent Mg, Ca, Sr and Ba with thiomalic acid(tma) have been isolated and characterised by elemental analysis, IR spectra and thermal studies. It is shown that tma coordinates to the metal ions through carboxylic oxygen atoms. Thermal studies of these complexes show that desulphurisation preceeds decarbonylation reaction leading to the formation of metal carbonates in all the cases except Mg where MgO is the end product. Thermal stability of the anhydrous thiomaltes follows the order Mg ∼ Ca > Sr > Ba. Structures have been proposed based on the information obtained from these studies.     

New boryl radicals derived from N-heteroaryl boranes: generation and reactivity

Recently, boryl radicals have been the subject of revived interest. These structures were generated by hydrogen-abstraction reactions from the corresponding boranes (i.e., from amine or phosphine boranes). However, the classical issue remains their high B--H bond-dissociation energy (BDE), thereby preventing a very efficient hydrogen-abstraction process. In the present paper, new N-heteroaryl boranes that exhibiting low B--H BDE are presented; excellent hydrogen-transfer properties have been found. Both the generation and the reactivity of the associated boryl radicals have been investigated through their direct observation in laser flash photolysis. The boryl radical interactions with double bonds, oxygen, oxidizing agent, and alkyl halides have been studied. Some selected applications of N-heteroaryl boryl radicals as new polymerization-initiating structures are proposed to evidence their high intrinsic reactivity.     

Study of the alkaline earth metals with 8-hydroxyquinolinate derivatives

Metal complexes of calcium with 5,7-dibromo, 7-iodo and 5-chloro-7-iodo-8-hydroxyquinolate were precipitated in aqueous ammonia and acetone medium, except for the solid state compound with 5,7-dichloro-8-hydroxyquinoline which hasn"t been obtained under these conditions. The complexes obtained through the mentioned precipitation are Ca[(C₉H₄ONBr₂)₂]₃·H₂O, Ca[(C₉H₅ONI)₂]·2H₂O and Ca[(C₉H₄ONICl)₂]·2.5H₂O. Their intermediate from the thermal decomposition found through TG/DTA curves in air indicated the presence of different kinds of calcium carbonates related to the reversibility and crystalline structure, depending on the original compounds. The initial compounds and the intermediate from the thermal decomposition were also characterized through IR spectra and X-ray diffraction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Growth of carbon nanotubes with alkaline earth carbonate as support

Multiwalled carbon nanotubes (MWCNTs) were grown by chemical vapor deposition by applying C(2)H(2) fluxed over Fe(1)(-x)Co(x) catalyst supported by alkaline earth carbonate. Detailed investigations of the chemical process occurring prior and during the growth allowed us a significant improvement of the nanotube production rate and quality. We observed a strong influence of the catalyst stoichiometry on the carbon deposition rate and the nanotube characteristics. We also found evidence for the active role of the support in the growth process, which is explained by the decomposition of the carbonate at the growth temperature. Using the optimized parameters obtained from our study performed in a fixed bed furnace, we could improve the production rate to about 500 g/day of purified MWCNTs in our large-scale rotary tube furnace.     

Luminescence of vanadium and rare earth ions in alkaline earth sulfates

The luminescence of samples MeSO₄V, RE (Me = Mg, Ca, Ba) depends strongly on the nature of the Me ions. The amount of association of the V⁵⁺ and RE³⁺ ions can be estimated from the measured quantum efficiencies.     

Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides

The reaction of some organometallic complexes with the surfaces of inorganic oxides leads to the formation of surface organometallic complexes, chemically bound to the surface yet retaining many features of their molecular structure. These surface organometallic complexes can therefore be considered to belong to both the molecular and solid states. In cases where such complexes have been structurally characterised, their reactivity can be interpreted with molecular concepts. In this review article, the stoichiometric and catalytic reactivity of some relatively well-defined surface organometallic fragments is surveyed. Many elementary steps which have precedent in molecular organometallic chemistry and homogeneous catalysis have now been demonstrated with surface organometallic fragments, including reversible ligand binding, oxidative addition, reductive elimination, protonation, heterolytic metal—carbon bond cleavage, electrophilic CH bond activation and insertion into metal—carbon bonds. In some cases, the supported organometallic complexes are highly effective low temperature catalysts, a phenomenon which is not always observed with molecular analogues nor with conventionally prepared heterogeneous catalysts. Applications of surface organometallic chemistry to catalytic alkane hydrogenolysis, olefin isomerisation and hydrogenation, the Fischer—Tropsch synthesis and the water—gas shift reaction are discussed. Proposed mechanisms for several representative catalytic cycles are presented.     

Ionic conductivity of some alkaline earth halides

The ionic conductivity above and below the melting temperature has been measured for the fluorides, chlorides, and bromides of calcium, strontium, and barium and for magnesium chloride. The observed behavior is of three types: I (MgCl₂, CaCl₂, CaBr₂, BaBr₂), there is an increase in conductivity of several orders of magnitude on melting; II (BaCl₂, SrBr₂), there is a solid-solid transition accompanied by a large increase in conductivity with little subsequent change on melting; and III (CaF₂, SrF₂, BaF₂, SrCl₂), the conductivity of the solid is continuous and changes only slightly on melting.     

Vibrational spectra of four alkaline earth cyclo-hexaphosphates

FT-IR and FT-Raman spectra of four alkaline earth (Mg, Ca, Sr and Ba) cyclo-hexaphosphates have been recorded and analysed. FT-Raman spectra of the deuterated analogues of these compounds are used to clear the ambiguity regarding the OH vibrations in the low frequency region. The spectra reveal that the P6O18(6) anion ring in all compounds have independent PO4 tetrahedra present in it. The P-O(P)and P-O(M')(M'-Mg, Ca, Sr and Ba) bonds become stronger as the cation size decreases. The P6O18(6-) anion ring is distorted in the Mg and Ca compounds. In the Sr and Ba compounds, free and hydrogen bonded water molecules of varying strength are present, whereas in Mg and Ca compounds no free water molecules exist. The POP bridge angle of the Mg, Ca and Sr compounds are estimated using the correlations between the POP bridge stretching frequencies and the bridge angle value.     

Thermogravimetric investigation of the alkaline earth diliturates

Thermolysis curves for calcium, strontium and barium diliturates were obtained. Calcium dilituratc forms an octahydrate from aqueous solution while strontium, dilituratc forms a. heptahydrate and barium diliturate forms a tetrahydrate.Methods for the thermogravimetrie determination of strontium and barium were developed and found to be quite accurate; calcium can only be determined semi-qnartitatively by precipitation as the diliturate salt.     

The thermal decomposition of alkaline earth formates

The thermal decomposition of alkaline earth formates has been studied. Experiments with single crystals, compacts and powders of strontium formate demonstrate that the method of sample presentation can affect the rate of decomposition.For anhydrous calcuim, strontium and barium formates, isothermal kinetic runs show that the decompositions follow the Erofeev law, In(l-α) = ktⁿ; the measured activation energies were 199.4, 228 and 270 kJ/mol respectively.     

Basicities of alumina-supported alkaline earth metal oxides

A series of alumina-supported alkaline earth metal oxide catalysts were prepared by incipient-wetness impregnation. These catalysts were characterized by nitrogen-sorption to determine their surface areas and pore size distributions. The basicities of these catalysts were characterized by temperature-programmed desorption of carbon dioxide. The TPD results demonstrate that all of the catalysts have one-peak profiles. The basicity increases with increasing atomic number of the alkaline earth metal. The alumina-supported alkaline earth oxides exhibit the same basic properties as bulk metal oxides. However, the presence of alumina can increase the mechanical strength of the catalyst, since the alkaline earth oxides have a weak mechanical strength. The basic properties of the catalysts are strongly influenced by the calcination temperature.     

Spectroscopic characterization of alkaline earth uranyl carbonates

A series of alkaline uranyl carbonates, M[UO₂(CO₃)₃]·nH₂O (M=Mg₂, Ca₂, Sr₂, Ba₂, Na₂Ca, and CaMg) was synthesized and characterized by inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS) after nitric acid digestion, X-ray powder diffraction (XRD), and thermal analysis (TGA/DTA). The molecular structure of these compounds was characterized by extended X-ray absorption fine-structure (EXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). Crystalline Ba₂[UO₂(CO₃)₃]·6H₂O was obtained for the first time. The EXAFS analysis showed that this compound consists of (UO₂)(CO₃)₃ clusters similar to the other alkaline earth uranyl carbonates. The average U-Ba distance is 3.90±0.02 Å.Fluorescence wavelengths and life times were measured using time-resolved laser-induced fluorescence spectroscopy (TRLFS). The U-O bond distances determined by EXAFS, TRLFS, XPS, and Raman spectroscopy agree within the experimental uncertainties. The spectroscopic signatures observed could be useful for identifying uranyl carbonate species adsorbed on mineral surfaces.     

Studies on interaction of polyvinyl alcohol with alkaline earth metal valerates

Summary The interaction of polyvinylalcohol (PVA) with valerates of (Sr, Ca, Mg) has been studied by conductivity measurements. The specific conductivity-soap concentration plots showed two transition points in the curves which denote the starting and completion of the interaction. The influence of salt and temperature has been examined. The salt concentration shifts the first transition point to slightly higher concentration where as the second diminishes. Temperature was found not to effect the transition points in polymersoap solutions and even the CMC of soaps were uneffected with temperature.

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Zusammenfassung Die Wechselwirkung zwischen Polyvinylalkohol und Si-, Ca- and Mg-Valeraten wurde konduktometrisch untersucht.Die Abhängigkeit des spezifischen Leitvermögens von der Konzentration zeigt drei Bereiche unterschiedlicher Neigung. Die beiden Übergangspunkte weisen auf den Beginn und die Beendigung einer besonderen Wechselwirkung hin. Erhöhung der Salzkonzentration verschiebt den 1. Übergangspunkt zu etwas höheren PVA-Konzentrationen, den 2. dagegen zu etwas niedrigeren.

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With 5 figures and 1 table