Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

Synthesis, structure, and transformation of novel osmium azine and ylide complexes

Three novel triosmium complexes with unusual coordination characteristics are reported. Treatment of the hydridotriosmium cluster (mu-H)2Os3(CO)10 with CNNPPh3 in CH2Cl2 gave complexes (mu-H)Os3(CO)(10)(mu2-eta2-C(H)NNPPh3) (1) and (mu-H)Os3(CO)10(mu2-eta1-CHPPh3) (2). Complex 1 represents the first example of the existence of a coordinated phosphinazine ligand. An in-situ 1H NMR study showed that the reaction of (mu-H)2Os3(CO)10 with CNNPPh3 produced complex 1 as the initial product in 100% conversion. The latter is not stable in solution and slowly eliminates nitrogen to form an unusual ylide complex 2 in quantitative yield. The thermolysis of 2 in refluxing toluene afforded (mu-H)3Os3(CO)9(mu3-eta1-CCO2CH2Ph) (3) as a colorless compound. Complexes 1-3 were characterized by spectroscopic methods and single-crystal X-ray diffraction analysis. The interesting feature of structure 3 is the presence of a mu3-alkylidyne ligand where the symmetrically triply bridged CCO2CH2Ph fragment lies perpendicular to and above the triosmium triangle.     

Reversing a rotaxane recognition motif: threading oligoethylene glycol derivatives through a dicationic cyclophane

An already well-established recognition motif-namely one in which the NH2+ centers in the rod sections of the dumbbell components of rotaxanes are encircled by macrocyclic polyether components-has been turned simultaneously outside-in and inside-out, a fact that has been proved beyond any doubt by the stoppering of both ends of a [2]pseudorotaxane to give a stable [2]rotaxane. The [2]pseudorotaxane is formed in nitromethane when a benzylic dibromide, obtained after reacting an excess of 1,4-bis(bromomethyl)benzene with hexaethylene glycol, is added to an equimolar amount of a dicationic cyclophane in which two -CH2OCH2- chains link a pair of dibenzylammonium ions through the para positions on their phenyl rings. When the [2]pseudorotaxane is reacted in nitromethane with triphenylphosphine, a [2]rotaxane and the corresponding free dumbbell compound are isolated in 58 and 31% yields, respectively. The structure of the [2]rotaxane is established by using mass spectrometry (FABMS and ESMS) and NMR (1H and 13C) spectroscopy in nitromethane-d3. The [2]rotaxane exhibits quite dramatic changes in the 1H chemical shifts of the signals for its CH2N+ and CH2O protons compared with those in the free dumbbell compound. The 1H NMR spectrum of the [2]pseudorotaxane shows many similar features. Titration experiments with three of the six different CH2O probes give an average Ka value of 2900 +/- 750 M-1 in nitromethane-d3. The new recognition motif for the template-directed synthesis of rotaxanes can now be exploited at both the molecular and macromolecular levels of structure with numerous potential applications in sight.     

Translational isomerism in a [3]catenane and a [3]rotaxane

[structure: see text] Post-assembly covalent modification using Wittig chemistry of [2]rotaxane ylides, wherein NH(2)(+) centers in the dumbbell-shaped components are recognized by dibenzo[24]crown-8 (DB24C8) rings, has afforded a [3]catenane and a [3]rotaxane with a precise and synthetically prescribed shortage of DB24C8 rings. The nondegenerate pairs of translational isomers present in both of these interlocked molecular compounds provide the fundamental platform on which to construct sensory devices and nanochemomechanical systems.     

Intramolecular 1,3-dipolar cycloaddition of cyclo-1,3-diene-tethered nitrile oxides

Intramolecular 1,3-dipolar cycloaddition of cyclo-1,3-diene- and -1,3,5-triene-tethered nitrile oxides gave tricyclic isoxazolines as a single stereoisomer in most cases. The relative stereochemistry of tricycle-fused isoxazolines resulting from 1,3-dipolar cycloaddition of cyclo-1,3-diene-tethered nitrile oxides is cis-cis, whereas from cyclohepta-1,3,5-triene-tethered nitrile oxides the cis-trans isomer predominates.     

Palladium complexes with ethylene-bridged bis(N-heterocyclic carbene) for C-C coupling reactions

A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)₂ produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs.     

Superstable theories with few countable models

Summary We prove here: Theorem. LetT be a countable complete superstable non -stable theory with fewer than continuum many countable models. Then there is a definable groupG with locally modular regular generics, such thatG is not connected-by-finite and any type inG ^eq orthogonal to the generics has Morley rank. Corollary. LetT be a countable complete superstable theory in which no infinite group is definable. ThenT has either at most countably many, or exactly continuum many countable models, up to isomorphism.Supported by NSF grant DMS 90-06628     

Copper-catalyzed hydrostannation of activated alkynes

[reactions: see text] [(Ph3P)CuH]6 effectively catalyzes the hydrostannation of activated alkynes with exclusive regioselectivity for alpha-stannation. Syn hydrostannation is observed exclusively for alkynoates. Anti or syn hydrostannation adducts are obtained as products for alkynone substrates.