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Subnanometric monodispersed titanium oxide particles have been dispersed on the surface of a large mesopore SBA-15 silica. TiSBA-15 materials were obtained by reaction between a hexanuclear titanium oxo cluster and a calcined SBA-15 silica in solution. Solids with various Ti loading have been prepared and characterized by chemical analysis, X-ray diffraction, nitrogen adsorption, and UV–visible spectroscopy. Titanium oxide particles containing 6 Ti octahedra show a UV–visible absorption band at ca. 235 nm, close to those usually observed on TiMCM-41 or TiHMS materials obtained by direct synthesis. The catalytic performance of TiSBA compounds has been evaluated in a series of oxidation reactions with H 2 O 2 or TBHP as oxygen donors and compared with that of TiHMS materials obtained by direct synthesis. Data indicate that TiO 2 nanoparticles are active and selective in epoxidation of cyclohexene and in oxidation of aniline and aromatic molecules. Activities and selectivities suggest that isolated tetrahedrally coordinated Ti species are not necessarily the only active species for these reactions. Moreover, results raise the problem of the validity of such oxidation reactions as a test for the determination of the coordination state of Ti species.

β-Ketoesterates derivatives M(mpa)x (M=Y, Ba, Cu; mpaH=methylpivaloylacetate; x=2 or 3) were prepared by ligand exchange reactions starting from methoxides (M=Ba, Cu) or from tristrimethylsilylamide (M=Y) derivatives. Decomplexation of the oligomeric and non-volatile Ba and Y species was achieved by various Lewis bases. Adducts Y(mpa)3L [L=tmeda (N,N,N′,N′-tetramethylethane-1,2-diamine), bipy (2,2′-bipyridine)] and Ba(mpa)2L [L=diglyme, 18-crown-6 ether; pmdta=N,N,N′,N′,N″-pentamethylethylenetriamine] and [Ba(mpa)2(tmedta)2] were obtained. All compounds were synthesised in high yield and characterised by elemental analyses, 1H NMR, FT-IR and TGA. The structures of [Ba(mpa)2(tmeda)2] and [Y(mpa)3(bipy)] were established by single crystal X-ray diffraction. The metals are eight-coordinated with a slightly distorted square antiprismatic surrounding. The yttrium complexes sublime as adducts. The barium complexes, by contrast, are dissociated in the vapour phase. Better solubility properties in polar and non-polar with respect to the 2,2,6,6-tetramethylheptane-3,5-dionates analogues were observed especially for the copper species. These compounds are thus potential metal-oxide precursors for liquid injection MOCVD using solvents such as hexane or diglyme.

Conventional MOCVD techniques require molecules displaying volatility and constant vapor pressure. Metal oxide precursors, i.e., b-diketonates, or classical or functionalized metal alkoxides are mostly solids. The various approaches used to tailor volatility are discussed with barium derivatives as an example. The relationships between sublimation temperature and molecular weight suggest that volatility can be optimized on the basis of molecular weight. Aerosol-assisted CVD (AACVD) can use a larger range of precursors since volatility is no longer crucial. The solvent is an undesired ballast in a CVD process. High solubility of the precursors in the selected solvent is thus desirable. ‘Stability’ here includes the absence of precipitation which would change the stoichiometry of the feed solution for multicomponent oxides. Precipitation is often promoted by hydrolysis; stability toward moisture is thus desirable. The use of mixtures of precursors based on different ligands (b-diketonates, bketoesterates, alkoxides) can lead to ligand exchange reactions giving homometallic species, sometimes of low solubility, or mixed-metal species by self-assembly, thus improving solubility and stability toward moisture. These aspects are illustrated in compositions related to high-Tc superconductors. Novel copper, yttrium, cerum(IV), barium-copper, yttriumcopper and praseodynium-copper species are reported. # 1998 John Wiley & Sons, Ltd.

The title compound was obtained by segregation of the mixed-metal CeTi species formed in THF by reacting Ti(OEt)4 and the cerium chloride ethanol adduct resulting from dehydration of CeCl3·7H2O by ethylorthoformate. The overall structure is based on a bidimensional meshlike polymer. Each CeCl3(THF)(H2O) unit is connected to three others by double halide bridges which make infinite chains spreading out along the a and b axis. These layers are stacked along c. Each cerium atom is surrounded by six chlorine atoms [2.819(1) to 2.927(1) A] and two oxygens of the THF and water ligands [2.530(3) and 2.495(3) A]. The overall sterochemistry of the 8-coordinate metal is that of an antiprism. Intermolecular H-bonds are observed between the water and chlorine ligands, OH⋯Cl 3.11–3.28 A within and between the layers.

Alcoholysis of alkoxides or oxoalkoxides of trivalent metals such as yttrium or bismuth with 2-methoxyethanol at room temperature gives highly soluble cyclic or linear oligomers or polymers [M(0C 2 H 4 0Me) 3 ] m M = Y, m = 10; M = Bi, m = ∞, which have been structurally characterized. Their common feature is the preponderance of the bridging μ 2 ,n 2 or μ,n 1 coordination modes over the chelating mode. The 2-methoxyethoxide group generally favors high coordination numbers, and thus even for divalent metals, this is achieved by μ 3 ,n 2 or μ 3 ,n 1 coordination modes as in Cd 9 (0C 2 H 4 0Me) 18 ,20HC 2 H 4 0Me. The lability of the alkoxyethoxide ligand allows further tailoring of the properties of the precursors, especially the reduction of the size of the metal aggregates by modifiers such as acetylacetone, added either as a free ligand or via metal acetylacetonates.

Various routes to lead(II) alkoxides using PbCl2, Pb(OAC)2 and Pb[N(SiMe3)2]2 as starting materials have been explored. The formation of homoleptic alkoxides is limited to [Pb(OBut)2]m, while condensation to the oxoalkoxides Pb4O(OR)6 or Pb6O4(OR)4(R = Et, Pri) seems favoured. The various products have been characterized by microanalysis, IR and NMR spectroscopy. [Pb(OBut)2]m and Pb4O(OR)6 are soluble precursors of lead oxide, while the tert-butoxide derivates are also volatile. Their reactivity (alcoholysis reactions, formation of complexes) has been examined and 207Pb NMR was used to follow their condensation and/or complexation.

Characterization of Zr{sub 2}(O{sup i}Pr){sub 8}({sup i}PrOH){sub 2} by {sup 1}H and {sup 13}C NMR spectroscopy, ir spectroscopy, and single-crystal x-ray diffraction ({minus}100{degree}C) is reported. The unit cell contains four half-dimers in the asymmetric unit, all of which differ only in the rotational conformation about Zr-O and O-C bonds. In each dimer, the edge-shared bioctahedron has two {mu}-O{sup i}Pr groups. On opposite sides of this Zr{sub 2}({mu}-OR){sub 2} plane, each dimer forms two hydrogen bonds, one each between a coordinated alcohol and a terminal alkoxide. The NMR spectra at 25{degree}C are so simple as to be structurally uninformative, a result of rapid fluxionality which includes, as one component, proton migration among all O{sup i}Pr units. At {minus}80{degree}C in toluene, the NMR spectra are now too complex to be accounted for by a single edge-shared bioctahedral structure. The hafnium analogue is isomorphous with the zirconium compound. Although Ce{sub 2}(O{sup i}Pr){sub 8}({sup i}PrOH){sub 2} is not isomorphous, it exhibits an analogous hydrogen-bonded structure in which the O{hor ellipsis}O distance is as short as it is in the Zr analogue, in spite of a metal-metal separation which is longer by 0.28 {angstrom}. The generality of hydrogen bonding between M-OR and M-O(H)R groups whenmore » they are aligned parallel in a metal cluster is reviewed. 44 refs., 2 figs., 4 tabs.« less

Mg has been widely used as p-doping species despite its intrinsic difficulties. It is nowadays well established that during the growth process of Mg doped GaN, atomic H is generated from the decomposition of NH3 and Mg-H complexes are formed in the layer. This has been for instance shown by the occurrence of LO mode in IR absorption, and by the observation of the Mg-H local vibration modes. This H passivation limits the electrical activity of Mg, therefore an activation process is required to get full activation of the Mg atoms. In the present study, bismethylcyclopentadienyl magnesium [(MeCp)2Mg] was used as precursor. However, this precursor reacts in the gas phase with NH3 to produce tiny solid particles as evidenced by a very bright diffuse emission visible along the laser beam used for reflectometry measurements. This simplest obvious product would be [(MeCp)Mg(NH2)]m(m≥2). To limit this drawback, Ga and Mg precursor lines have been separated. With proper in situ heat treatment, doping densities up to 1.5×1018 cm−3 have been obtained. PL spectra of lightly Mg doped samples (1016 cm−3) are dominated by shallow donor-acceptor pairs whereas for higher doping densities ( 1018 cm−3), the luminescence is dominated by a broad band in the 2.7-2.9 eV range. GaN LEDs were fabricated from Si doped (n-type) and Mg-doped (p-type) GaN, these LEDs emit in the blue-UV range.

Alcoholysis of alkoxides or oxoalkoxides of trivalent metals such as yttrium or bismuth with 2-methoxyethanol at room temperature gives highly soluble cyclic or linear oligomers or polymers [M(0C 2 H 4 0Me) 3 ] m M = Y, m = 10; M = Bi, m = ∞, which have been structurally characterized. Their common feature is the preponderance of the bridging μ 2 ,n 2 or μ,n 1 coordination modes over the chelating mode. The 2-methoxyethoxide group generally favors high coordination numbers, and thus even for divalent metals, this is achieved by μ 3 ,n 2 or μ 3 ,n 1 coordination modes as in Cd 9 (0C 2 H 4 0Me) 18 ,20HC 2 H 4 0Me. The lability of the alkoxyethoxide ligand allows further tailoring of the properties of the precursors, especially the reduction of the size of the metal aggregates by modifiers such as acetylacetone, added either as a free ligand or via metal acetylacetonates.

The niobium(III) molecular derivatives Nb 2 Cl 4 (acac) 2 (acacH) 2 (PhPMe 2 ) 2 (acacH = acetylacetone), Nb 2 Cl 5 (dpm)(PhPMe 2 ) 2 and Nb 2 Cl 3 (dpm) 3 (dpmH = dipivaloylmethane) were isolated in high yields by reacting the corresponding β-diketone with Nb 2 Cl 6 (PhPMe 2 ) 4 or Nb 2 Cl 6 (SMe 2 ) 3 . These products were characterized by elemental analysis, mass spectra, infra-red and 1 H and 31 P NMR spectroscopy. Unlike dipivaloylmethanato derivatives, extensive dissociation reactions were observed for Nb 2 Cl 4 (acac) 2 (acacH) 2 (PhPMe 2 ) 2 in solution, which therefore contains mainly Nb 2 Cl 4 (acac) 2 (PhPMe 2 ) 2 species.

The reduction of niobium(V) chloride by magnesium in CH 2 Cl 2 and in the presence of various ethers and sulfides has been investigated. New niobium(III) species - molecular: Nb 2 Cl 6 (p-dioxane) 2 , [Nbcl 2 (OC 6 H 4 CHO)(THF) 2 ] 2 and ‘Nb 2 Cl 6 (THF 4 ’; and cluster: Nb 3 Cl 8 (SMe 2 ) 2 (Et 2 ) - have been isolated in good yields. The access to a previously reported niobiun(III) molecular compound, Nb 2 Cl 6 (SMe 2 ) 3 , has been improved. All these compounds were, characterized by elemental analysis, infrared and proton NMR spectroscopy. Dimeric species, with formally a double bond between the metallic centers, account for the diamagnetism of all the molecular compounds.

The coordination ability of about 40 potential monodentate and bidentate ligands with niobium and tantalum pentamethoxides has been investigated and is discussed.

Reduction of tantalum pentahalides Ta 2 X 10 (X = Cl,Br) by magnesium turnings in CH 2 Cl 2 was investigated in the presence of various phosphanes, and found to be highly dependent on the ligand. In the case of phenyldimethylphosphane it led to tantalum(III) derivatives, Ta 2 X 6 (PhPMe 2 ) 4 (X = Cl, Br) and [TaBr 3 (PhPMe 2 )] n in high yields. These compounds are diamagnetic in the solid and in solution. The chloride is very stable, both in the solid and in solution, while spontaneous reduction of the metal was observed in solution for the bromo derivatives, yielding ionic species. Reaction between Me 3 SnCp and Ta 2 Cl 6 (PhPMe 2 ) 4 led to Cp 2 Ta 2 Cl 4 (PhPMe 2 ) 2 , the first dimeric cyclopentadienyltantalum(III) derivative reported, while the addition of molecular hydrogen under normal pressure gave a stable hydride Ta 2 Cl 6 (PhPMe 2 ) 4 H 2 . All products were characterized by elemental analysis, IR, 1 H and 31 P spectroscopy, molecular weight data and mass spectrometry.

The preparation and properties are described of trans-[(Ph3P)2(CO)M(RNSNR)] [ClO4] (M  RhI, IrI; R  Me, Et, i-Pr, t-Bu) and of cis- or trans-[L2Pt(RNSNR)X] [ClO4] (X  Cl−, L  Et2S, PhMe2As, PhMe2P, R  Me, t-Bu; X  CH3, L  PhMe2P, R  Me). 1H and 13C NMR data show the existence of various isomers in solution which may interconvert via intra- and inter-molecular exchange processes. A general reaction scheme for the intramolecular exchange processes is discussed.

The niobium(V) oxyyalkoxide derivatives [NbO(OCH2CF3)3 · MeCN]2, NbO(OCH2CF3)2(OCHCF2) (CF3CH2OH)0.5, [NbO(OEt)3]2, [NbOCl2(OCH3)· CH3OH]2 and NbO(OCH3)(OC6H4CHO)2 have been obtained by direct alkoxylation of niobium oxychloride, and characterised. The molecular constitution of the solutions of [NbO(OEt)3]2 and [NbO(OCH2CF3)3· MeCN]2 in non-polar solvents was tentatively interpreted as a dynamic equilibrium between various μ-alkoxo and μ-oxo isomers, the latter being favoured by dilution. In polar media, dimeric unit are retained for [NbO(OEt)3]2, while the triflouroethoxide derivative is split into monomeric species.

Molecular tantalum(III) adducts Ta 2 X 6 (γ-pic) 4 (X = Ci or Br) have been obtained in high yields by reducing Ta 2 X 10 by magnesium in the presence of the ligand. Preliminary results of their reactivity are given. Ta(IV) β-diketonates or carboxylates, Ta 2 CI 4 (dpm) 4 and Ta 2 CI 4 ( t BuCO) 2 4 ), 2 t BuCo 2 H or oxo derivatives were obtained by treating Ta 2 CI 6 - (γ-pic) 4 with the weak protonic acids: dipivaloylmethane (dpmH), pivalic acid and acetic respectively. Reactions between Ta 2 Cl 6 (γ-pic) 4 and LiO t Bu led to the formation of the tantalum(III) alkoxide Ta 2 CI 2 (O t Bu) 4 . All new derivatives, which are diamagnetic, were characterized by elemental analysis, IR and 1 H NMR spectroscopy.

The niobium(V) and tantalum(V) oxyalkoxide derivatives NbO(OMe)2(OC6H4CHO), NbO(OMe)2[HB(pz)3] (pz = C3H3N2), NbOCl(OCH2CCl3)2· 2DMSO and TaOCl2(OMe)·2DMSO have been obtained from the pentaalkoxides or from the alkoxychlorides and characterized by microanalysis, molecular weight measurements, IR and NMR spectroscopy. The structure in solution of these oxo derivatives — all monomeric — which was mainly investigated through low-temperature 1H NMR studies, is discussed.

The ability of various crown-ethers L (dibenzo-18-crown-6, 18-crown-6 and 15-crown-5) to coordinate and reduce niobium and tantalum pentachlorides has been explored. Compounds of different stoichiometries, such as (NbCl5)2·L (L = DBC, 18-CRW-6, 15-CRW-5), NbCl5·L (L = DBC, 18-CRW-6), (NbCl4)2(15-CRW-5), (NbCl4)3·L2 (L = 18-CRW-6, 15-CRW-5) and TaCl5)2(DBC); TaCl5(15-CRW-5), (TaCl4)2(18-CRW-6), were isolated and characterized by elemental analysis, magnetic susceptibility measurements, infra-red and nmr spectroscopy. The proton nmr data were tentatively interpreted to mean that disproportionation reactions, leading to mono and polynuclear species having various ligand to metal ratios, occur in solution.

Heterometallic [ZnM 2 (OR) 3 ] m species (M = Al, Ga; R = C 2 H 4 OMe, C 2 H 4 NMe 2 ) were prepared and characterized by elemental analysis, FT-IR and multinuclear NMR ('H, 13 C, 27 Al). Their Hydrolysis was investigated. The powders were analysed by FT-IR, TGA/TDA and XRD. [ZnGa 2 (OC 2 H 4 OMe) 8 ] 2 allowed to obtain ZnGa 2 O 4 at 300°C. Particles size in solution and aging phenomena were analysed. Stable colloidal suspensions (150 - 200nm) for elaboration of thin films by spin-coating techniques were obtained. These films transparent and were characterized by XRD, XPS, TMAFM, SEM, EDAX and UV-V spectroscopy.

A serie of M(mpa) x derivatives (M = Y. Ba, Cu; mpaH = methylpivaloylacetate; x = 2 or 3) and their adducts M(mpa) x L y [L= TMEDA (N,N,N',N'-tetramethylethane-1,2-diamine), 2,2'-bipyridine, diglyme and 18-crown-6 ether; y = 1, 2] were synthesised and fully characterised. X-ray structure determinations of Ba(mpa) 2 (TMEDA) 2 and Y(mpa) 3 (bipy) are described. Better solubility properties than the M(thd) x analogues (thdH = 2,2,6,6-tetramethylheptane-3,5-dione), in non polar and polar solvents such as hexane and diglyme, have proved promising metal-oxide Liquid Injection MOCVD precursors. Their thermal behaviour was investigated by thermogravimetric analysis.