bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electrophosphorescent Devices Based on Cationic Complexes: Control of Switch‐on Voltage and Efficiency Through Modification of Charge Injection and Charge Transport
Authors: Plummer E. A. , van Dijken A. , Hofstraat J. W. , De Cola L. , Brunner K. 
Published: February 2005

AbstractThis paper reports an analysis of the properties of polymer light‐emitting devices (PLEDs) doped with iridium complexes. Devices based on charged and neutral complexes doped into poly(vinylcarbazole) (PVK) are presented, and the role of the ions and the charge‐transport properties of the complexes are discussed. In devices with the charged complexes, the concentration of the complex is found to have a profound effect on both the switch‐on voltage and the efficiency. At higher doping concentrations the efficiency is increased and the switch‐on voltage decreased. The increase in efficiency and decrease in switch‐on voltage at higher dopant concentration are found to be due to an alternative charge transport path via the iridium dopant [Ir(bpy)]+ (bis(2‐phenylpyridine‐C2,N′)(2,2′‐bipyridine)iridium hexafluorophosphate). However, at lower concentrations the complex becomes an electron trap and the efficiency is reduced. The devices are found to be significantly less efficient than those with neutral complexes. This difference is attributed to the ionic content and the charge trapping properties of the charged complexes. The low efficiency of the charged‐complex‐based devices could be overcome by utilizing a hole‐blocking layer; devices with efficiencies as high as 23 cd A–1 were obtained.
Titania and Mixed Titania/Aluminum, Gallium, or Indium Oxide Spheres: Sol–Gel/Template Synthesis and Photocatalytic Properties
Authors: Deshpande A. S. , Shchukin D. G. , Ustinovich E. , Antonietti M. , Caruso R. A. 
Published: February 2005

AbstractPorous polymer beads have been used as templates in which sol–gel chemistry was conducted for the formation of porous titanium dioxide and titania/aluminum, gallium, or indium oxide spheres. The addition of 5, 10, and 15 wt.‐% of the second metal oxide to titania was studied, resulting in little variation in the final porous‐sphere diameter, but in a decreased titania nanocrystal size and an increased specific surface area of the material. The crystallinity of the samples was observed after heating at 550, 750, and 950 °C as anatase to rutile phase transitions became apparent and peaks from the added metal oxide were observed with the increase in temperature. Photocatalytic decomposition of 2‐chlorophenol was monitored in the presence of the titania and titania/metal‐oxide spheres showing that a 5 wt.‐% addition of the second metal oxide gave best photocatalytic results for all the metal oxides studied. At a pH of 6 the pure titania spheres were less photocatalytically active than the Degussa P25 titania, however the mixed titania/5 wt.‐% metal‐oxide samples were more active than the standard in the order In (least active), Ga, then Al (most active). Variation of the solution pH (between pH 2 and 10) had little influence on the photocatalytic activity of the titania/5 wt.‐% aluminum oxide material, more effect on the titanium/5 wt. % gallium oxide, and the most pronounced effect on the titanium/5 wt.‐% indium oxide, with increased activity at higher pH values. The adsorption of pyridine to the titania samples containing the second metal oxide indicated the presence of Lewis‐acid sites.
Biomimetic Pattern Transfer
Authors: Wu L.‐Q. , Ghodssi R. , Elabd Y. A. , Payne G. F. 
Published: February 2005

AbstractBiological systems routinely use phenols to construct complex materials with diverse functions. Typically, these phenolic materials are generated using oxidative enzymes to initiate a cascade of uncatalyzed reactions. We mimic these processes to micropattern films of the aminopolysaccharide chitosan. Specifically, we microfabricate silicon wafers to have gold patterns, cast a chitosan film onto the patterned wafers, and commence pattern transfer by polarizing the underlying gold surfaces to electrochemically initiate the phenol reaction cascade. The electrochemically initiated reactions lead to modification of the chitosan film's chemistry, structure, and fluorescence. Further, electrochemically initiated modification of the chitosan film is localized to the interfacial region between the film and the anode, with resolution in the lateral direction of at least 20 μm. These results demonstrate that electrochemical pattern transfer provides a promising new method for micropatterning flexible films.
Enhanced Electro‐Optic Behavior for Shaped Polymer Cholesteric Liquid‐Crystal Flakes Made Using Soft Lithography
Authors: Trajkovska‐Petkoska A. , Varshneya R. , Kosc T. Z. , Marshall K. L. , Jacobs S. D. 
Published: February 2005

AbstractPolymer cholesteric liquid‐crystal (PCLC) flakes were investigated for their electro‐optical behavior under an applied alternating‐current field. Shaped flakes, fabricated using soft lithography and suspended in dielectric‐fluid‐filled cells, reoriented more uniformly than randomly shaped flakes made by fracturing of PCLC films. Extensive characterization found shaped flakes to be smooth and uniform in size, shape, and thickness. Reorientation in applied fields as low as tens of mVrms μm–1 was fastest for flakes with lateral aspect ratios greater than 1:1, confirming theoretical predictions based on Maxwell–Wagner polarization. Brilliant reflective colors and inherent polarization make shaped PCLC flakes of interest for particle displays.
Cover Picture: Enhanced Electro‐Optic Behavior for Shaped Polymer Cholesteric Liquid‐Crystal Flakes Made Using Soft Lithography (Adv. Funct. Mater. 2/2005)
Authors: Trajkovska‐Petkoska A. , Varshneya R. , Kosc T. Z. , Marshall K. L. , Jacobs S. D. 
Published: February 2005

AbstractThe cover shows a variety of shaped flakes fabricated from polymer cholesteric liquid‐crystal material using soft lithography. In work reported by Jacobs and co‐workers on p. 217, the micrometer‐sized flakes exhibit brilliant circularly polarized selective reflection colors without polarizers or color filters when placed in a fluid‐filled electro‐optic cell. With the application of a low‐magnitude alternating current field, the flakes reorient in hundreds of milliseconds and the colors disappear.Polymer cholesteric liquid‐crystal (PCLC) flakes were investigated for their electro‐optical behavior under an applied alternating‐current field. Shaped flakes, fabricated using soft lithography and suspended in dielectric‐fluid‐filled cells, reoriented more uniformly than randomly shaped flakes made by fracturing of PCLC films. Extensive characterization found shaped flakes to be smooth and uniform in size, shape, and thickness. Reorientation in applied fields as low as tens of mVrms μm–1 was fastest for flakes with lateral aspect ratios greater than 1:1, confirming theoretical predictions based on Maxwell–Wagner polarization. Brilliant reflective colors and inherent polarization make shaped PCLC flakes of interest for particle displays.
On the Stereochemical Course of Self‐Replication in Secondary Cycle Sharpless Aminohydroxylation
Authors: Muñiz Kilian 
Published: February 2005

AbstractAn investigation on a potential asymmetric self‐replication process within the secondary cycle of Sharpless asymmetric aminohydroxylation is discussed. The reaction of two model olefins is investigated within this process and two different models for an asymmetric process are discussed. Analysis of the results on the basis of these models and a mathematical description for development of the enantiomeric excesses as a function of olefin conversion lead to the conclusion that for such processes there is no possibility for any inhibitory effect and that asymmetric self‐replication in secondary cycle aminohydroxylation reactions is not a feasible process.
Mononuclear Ruthenium Catalysts for the Direct Propargylation of Heterocycles with Propargyl Alcohols
Authors: Bustelo Emilio , Dixneuf Pierre H. 
Published: February 2005

AbstractA new family of mononuclear ruthenium catalysts for the catalytic direct propargylation by propargylic alcohols of heterocycles is presented. The catalyst activity can be improved by tuning the ligand's influence and electronic properties at the metal centre. Whereas [(p‐cymene)RuCl(PR3)][OTf] (PR3=PCy3, PPh3) complexes catalyse the propargylation of furan or 2‐methylfuran by the alkynol HC≡CCH(OH)Ph in moderate yield, mononuclear [(p‐cymene)RuCl(CO)(PR3)][OTf] complexes are more active to achieve the same reaction. Low temperature NMR experiments performed on the stoichiometric reaction of the parent [(p‐cymene)RuCl(PR3)][B(ArF)4] and the alkynol show the unexpected in situ formation, via allenylidene and hydroxycarbene intermediates, of the carbonyl complexes [(p‐cymene)RuCl(CO)(PR3)][B(ArF)4] [ArF=3,5‐(CF3)2C6H3] that appear to be the best catalyst precursors.
A Practical and Chemoselective Reduction of Nitroarenes to Anilines Using Activated Iron
Authors: Liu Yugang , Lu Yansong , Prashad Mahavir , Repič Oljan , Blacklock Thomas J. 
Published: February 2005

AbstractReduction of nitroarenes to anilines using activated iron generated by Fe/HCl or Zn/FeSO4 is described. A variety of functional groups such as alkyne, ketone, enone, nitrile, lactone, and aromatic halide are well tolerated under these conditions.
Silica Gel Confined Ionic Liquid+Metal Complexes for Oxygen‐Free Carbonylation of Amines and Nitrobenzene to Ureas
Authors: Shi Feng , Zhang Qinghua , Gu Yanlong , Deng Youquan 
Published: February 2005

AbstractA new kind of silica gel confined ionic liquid containing a metal complex as heterogenized catalysts was prepared for the carbonylation of amines and nitrobenzene without molecular oxygen to afford the corresponding ureas with greatly enhanced catalytic activity (TOF exceeded 11000 mol⋅mol−1⋅h−1), with a much lower amount of ionic liquids being needed, with easy catalyst separation and possible reusability, and avoidance of the using of explosive CO+O2gas mixtures. Such an enhancement in catalytic activity may be derived from the effect of a high concentration of ionic liquid containing a metal complex due to the confinement into the nano‐pores or cavities of the silica gel matrix.
Metallodendritic Catalysis for Redox and CarbonCarbon Bond Formation Reactions: A Step towards Green Chemistry.
Authors: Astruc Didier , Heuzé Karine , Gatard Sylvain , Méry Denise , Nlate Sylvain , Plault Lauriane 
Published: February 2005

AbstractMetallodendritic catalysts combine the advantages of homogeneous and heterogeneous catalysts: they are soluble and perfectly well defined on the molecular level, and yet they can be recovered by precipitation, ultra‐filtration or ultra‐centrifugation (as biomolecules) and recycled several times. In this article, we summarize our recent work in this field with examples operating under ambient conditions in metathesis, Pd‐catalyzed Sonogashira coupling, redox catalysis of nitrate and nitrite cathodic reduction to ammonia and various oxidation reactions by H2O2 catalyzed by polyoxometallates. The dendritic effects on the catalytic efficiencies are scrutinized, i.e., the comparison of the metallodentritic catalysts with their monomeric models and among the dendrimer generations. It is concluded that metallostars or low‐generation metallodendrimers usually are optimized catalysts in terms of efficiency and recovery/re‐use.
Trimethylphosphine‐bis(benzyl)nickel: Synthesis and Characterization
Authors: Wasilke Julia‐Christina , Ziller Joseph W. , Bazan Guillermo C. 
Published: February 2005

AbstractThe reaction of NiCl2(PMe3)2 with 2 equivs. of benzylmagnesium chloride in THF at −120 °C affords Ni(η1‐CH2C6H5)(η3‐CH2C6H5)PMe3. The formation of this thermally unstable compound is accompanied by bisbenzyl and Ni(0) side products. Single crystal X‐ray analysis confirms the presence of the η1‐benzyl and η3‐benzyl ligands. Reaction of Ni(η1‐CH2C6H5)(η3‐CH2C6H5)PMe3 with 2‐(diphenylphosphino)benzoic acid and activation with B(C6F5)3 gave the expected product [2‐(diphenylphosphino)benzoate tris(pentafluorophenyl)borate‐κ2P,O](η3‐benzyl)nickel.
Iridium‐Catalyzed Enantioselective Hydrogenation of Terminal Alkenes
Authors: McIntyre Steven , Hörmann Esther , Menges Frederik , Smidt Sebastian P. , Pfaltz Andreas 
Published: February 2005

AbstractIridium complexes derived from chiral P,N ligands are efficient catalysts for the enantioselective hydrogenation of 2‐aryl‐substituted terminal alkenes. Using 0.1–1 mol % of catalyst at room temperature and ambient hydrogen pressure, high enantioselectivities (88–94% ee), full conversions after short reaction times and essentially quantitative yields were obtained for a range of differently substituted 2‐arylalkenes. Among six iridium complexes that were tested, the most selective catalyst was a complex with a phosphinite‐oxazoline ligand derived from threonine (Ir‐ThrePHOX). In contrast to the hydrogenation of trisubstituted alkenes, a strong pressure effect was observed for this class of substrates. Lowering the hydrogen pressure from 50 to 1 bar resulted in a strong increase of the ee values.
Favored CH Activation in Comparison to Cyclopropyl CC Activation using a Vaska‐Analogous Complex
Authors: Ruhland Klaus , Herdtweck Eberhardt 
Published: February 2005

AbstractThrough ligand exchange reaction of Vaska's complex with a chelating diphosphinite ligand containing a cyclopropyl group, CH activation rather than CC activation of the strained cyclopropyl ring is induced. The activation takes place as a second step preceded by the ligand exchange reaction. The fixation of the CH bond near to the metal through the geometry of the diphosphinite ligand is essential for the activation process.
Oxygen‐Transfer Reactions of Molybdenum‐ and Tungstendioxo Complexes Containing η2‐Pyrazolate Ligands
Authors: Most Kerstin , Hoßbach Jens , Vidović Denis , Magull Jörg , Mösch‐Zanetti Nadia C. 
Published: February 2005

AbstractDioxomolybdenum and ‐tungsten compounds containing sterically demanding pyrazolate ligands have been synthesised by treatment of dioxometal halides with the potassium salts of 3,5‐di‐ tert‐butylpyrazole (t‐Bu2pzH) and 3,5‐di‐tert‐butyl‐ 4‐bromopyrazole (t‐Bu2‐4‐BrpzH). The products [MoO2Cl(η2‐t‐Bu2pz)] (1), [MoO2(η2‐t‐Bu2pz)2] (2), [MoO2(η2‐t‐Bu2‐4‐Brpz)2] (3), [WO2(η2‐t‐Bu2pz)2] (4) and [WO2(η2‐t‐Bu2‐4‐Brpz)2] (5) were characterised by spectroscopic techniques. The X‐ray structure of complex 3 reveals a distorted trigonal prismatic geometry with two η2‐co‐ordinated pyrazolate ligands. These high‐valent compounds participate in oxygen‐transfer reactions and catalyse the oxidation of PPh3 with dimethyl sulphoxide. UV/VIS measurements of the oxo‐transfer reactions and the kinetics of the catalytic process are described. By the reaction of 2 with three equivalents of PEt3 or treatment of [MoOCl2(PMe3)2] with two equivalents of t‐Bu2pzK mononuclear mono‐oxo compounds of the type [MoO(t‐Bu2pz)2(PR3)2] (R=Et 6, R=Me 7) were obtained and characterised by X‐ray diffraction analyses. This points to biologically relevant mononuclear Mo(IV) intermediates in the catalytic process with this type of complex.
Evaluation of (η5‐C5Me5)Hf(R)2[N(Et)C(Me)N(t‐Bu)] (R=Me and i‐Bu) for the Stereospecific Living and Degenerative Transfer Living Ziegler–Natta Polymerization of α‐Olefins
Authors: Kissounko Denis A. , Zhang Yonghui , Harney Matthew B. , Sita Lawrence R. 
Published: February 2005

AbstractThe dialkyl(monocyclopentadienyl)hafnium acetamidinates, Cp*Hf(R)2[N(Et)C(Me)N(t‐Bu)] [Cp*=η5‐C5Me5, R=Me (3a) and i‐Bu (3b)], were prepared through either ‘one–pot’ carbodiimide insertion or two‐step salt elimination protocols starting with commercially available Cp*HfCl3. Protonolysis of 3a and 3b with [PhNHMe2][B(C6F5)4] in chlorobenzene produced the cationic complexes, {[Cp*Hf(R)[N(Et)C(Me)N(t‐Bu)]}[B(C6F5)4] [R=Me (4a) and i‐Bu (4b)], which were characterized in solution. Compounds 4a and 4b were evaluated as initiators for the stereospecific living Ziegler–Natta polymerization of 1‐hexene. These results reveal that, while an extremely high level of stereoselectivity can be achieved to produce isotactic poly(1‐hexene) in a living fashion, the rate constant for polymerization, kp, using either 4a or 4b, is ~60 times less than that of the analogous zirconium initiators. Finally, upon substoichiometric activation of 3a with [PhNHMe2][B(C6F5)4] in a 2 : 1 ratio, degenerative transfer living Ziegler–Natta polymerization of 1‐hexene can be accomplished to produce atactic poly(1‐hexene).
Transition Metal Imido Compounds as Ziegler–Natta Olefin Polymerisation Catalysts
Authors: Bolton Paul D. , Mountford Philip 
Published: February 2005

AbstractTransition metal imido compounds having the general formula [(L)nM(NR)] [where (L)n is a supporting ligand or ligand set, and R typically is alkyl or aryl] have been known for nearly 50 years, and during the last two decades have been the focus of considerable attention. Relatively recently their potential application in the Ziegler–Natta polymerisation of olefins has been realised. In this contribution we review the Ziegler–Natta polymerisation of olefins by transition metal imido compounds. A general introduction to homogeneous Ziegler–Natta olefin polymerisation and key aspects of transition metal imido chemistry is given, followed by a short description of a related imido‐based polymerisation process, namely ring‐opening olefin metathesis polymerisation catalysed by imido‐supported molybdenum and tungsten alkylidenes. A summary of the “design principles” that have been employed in the development of certain imido‐based Ziegler–Natta catalysts is presented, followed by a comprehensive survey of the literature in the title area.
Cationic Alkyl Complexes of the Rare‐Earth Metals: Synthesis, Structure, and Reactivity
Authors: Arndt Stefan , Okuda Jun 
Published: February 2005

AbstractCationic alkyl complexes of the rare‐earth metals [LnRm(L)n](3–m)+ (R=alkyl; m=1, 2; L=Lewis base) were virtually unknown species until recently. Because of their increased Lewis acidity/electrophilicity they should have considerable potential as homogeneous catalysts in olefin polymerization and in organic transformations. They can be generated by treating the neutral rare‐earth metal precursors containing at least two alkyl groups R with suitable Lewis or Brønsted acids in the presence of weakly coordinating anions. Not only monocationic but also dicationic alkyl derivatives have been shown to be accessible. In the context of modeling homogeneous ethylene polymerization using a mixture consisting of LnR3/AlR3/[NMe2HPh][B(C6F5)4], such dications were discovered. Some thermally robust examples of mono‐ and dicationic alkyl complexes have been structurally characterized as solvent‐separated ion pairs. Neutral and monoanionic macrocycles such as crown ethers or aza‐crown ethers as well as amidinato, β‐diketiminato, and substituted cyclopentadienyl ligands are suitable ancillary ligands to stabilize the cationic alkyl fragments.
Ligand Architecture on Stereocontrol of Half‐Metallocene Benzylidene Complexes of Tantalum
Authors: Mashima Kazushi 
Published: February 2005

AbstractThis brief review discloses our studies on half‐metallocene benzylidene complexes of tantalum bearing 1,3‐butadiene, 2,3‐dimethyl‐1,3‐butadiene, o‐xylylene, 1,4‐diaryl‐1,4‐diaza‐1,3‐butadiene, or 1‐aryl‐4‐phenyl‐1‐aza‐1,3‐butadiene as an auxiliary ligand. These auxiliary ligands can control the stereochemistry of the benzylidene moiety and induce intriguing reactivity. Furthermore, the butadiene and o‐xylylene complexes catalyze ring opening metathesis polymerization of norbornene to respectively give polymers with cis‐ and trans‐CC bonds. Such selectivity can be attributed to control of the stereochemistry of the propagating alkylidene chain end by the auxiliary ligand: the butadiene complex favors the anti‐rotamer while the o‐xylylene complex favors the syn‐one.
Exploring Routes to Tantalum(V) Alkylidene Complexes Supported by Amine Tris(phenolate) Ligands
Authors: Groysman Stanislav , Goldberg Israel , Kol Moshe , Genizi Elisheva , Goldschmidt Zeev 
Published: February 2005

AbstractTwo basic organometallic species, tris(neopentyl)mono(neopenylidene)tantalum and pentabenzyltantalum, were evaluated as starting materials on the route to alkyl and alkylidene complexes with amine tris(phenolate) ligands. The reaction of the former with the ligand precursor proceeded via the addition of the OH functionality to the TaC double bond at the first step, forming a sterically congested tetraneopentyl complex, that was characterized by X‐ray crystallography. In contrast, the reaction of the less bulky pentabenzyltantalum with a variety of the ligand precursors led directly and in a high yield to the dibenzyl Ta(V) complexes. These species were surprisingly stable toward thermolysis, decomposing only at 120–130 °C after prolonged periods of time. The unexpected product of this decomposition was found to be a dinuclear μ‐benzylidene Ta(V) complex.
Effect of Cyclopentadienyl Fragment in Copolymerization of Ethylene with Cyclic Olefins Catalyzed by Non‐Bridged (Aryloxo)(cyclopentadienyl)titanium(IV) Complexes
Authors: Wang Wei , Tanaka Tatsuo , Tsubota Miki , Fujiki Michiya , Yamanaka Shozo , Nomura Kotohiro 
Published: February 2005

AbstractThe effect of the cyclopentadienyl fragment in the copolymerization of ethylene with norbornene (NBE) by various non‐bridged (aryloxo)(cyclopentadienyl)titanium(IV) complexes of the type, Cp′TiCl2(OAr) [Cp′=indenyl (1), C5Me5 (Cp*, 2), t‐BuC5H4 (3), 1,2,4‐Me3C5H2 (4); OAr=O‐2,6‐i‐Pr2C6H3], MAO catalyst systems has been explored. The catalytic activity and the NBE incorporation were highly dependent upon substituent on Cp′ employed, and 1 exhibited both a remarkable catalytic activity and an efficient NBE incorporation, affording high molecular weight polymers with unimodal molecular weight distributions. NBE repeat units were observed in the resultant poly(ethylene‐co‐NBE)s prepared by 1, 3, and 4, and the degree of NBE dyads as well as alternating sequences was strongly influenced by the Cp′ employed. These complexes (1,3,4) also exhibited high catalytic activities for ethylene/cyclopentene (CPE) copolymerization, but the CPE incorporation by 1 was less efficient than the NBE incorporation under the same conditions. The ab initio density functional (DF) molecular orbital (MO) methods were utilized to explore the effect of substituents on Cp′ toward the catalytic activity, the NBE incorporation and the monomer sequence distribution. Good correlations between the ΔEcoordEt (coordination energy of ethylene) after NBE insertion and the activity, and between ΔEcoordNBE (coordination energy of NBE) after ethylene (or NBE insertion) and NBE content were observed, strongly suggesting that the capability producing NBE repeat units was dependent upon ΔEcoordNBE after NBE insertion.
Enantioselective Alkynylation of Aromatic Ketones Catalyzed by New Chiral Oxazolidine Ligands
Authors: Kang Yong‐Feng , Liu Lei , Wang Rui , Zhou Yi‐Feng , Yan Wen‐Jin 
Published: February 2005

AbstractNew chiral oxazolidine ligands have been derived conveniently from natural amino acids in good yields. Their use in the enantioselective addition of phenylacetylene to aromatic ketones has been tested with the best ee being up to 88%. We provide a simple, practical and inexpensive method to generate chiral tertiary propargylic alcohols.