Influence of Ionic Liquid Film Thickness and Flow Rate on Macrocyclization Efficiency and Selectivity in Supported Ionic Liquid-Liquid Phase Catalysis.

Supported ionic-liquid phase (SILP) technology in a biphasic setting with n-heptane as the transport phase was applied to the Ru-alkylidene-N-heterocyclic carbene (NHC) catalyzed macrocyclization of α,ω-dienes to elucidate the effect of ionic liquid (IL)-film thickness, flow rate as well as substrate and product concentration on macrocyclization efficiency, and Z-selectivity. To understand the molecular-level behavior of the substrates and products at the n-heptane/IL interphase, atomistic molecular dynamics simulations were conducted and correlated with experimental observations. The thickness of the IL layer strongly influences the Z/E ratio of the products in that a thin IL layer favors higher Z/E ratios by confining the catalyst between the pore wall and the liquid-liquid interphase whereas a thick IL layer favors formation of the E-product and Ru-hydride catalyzed isomerization reactions.

Feed injection in liquid chromatography: Reducing the effect of large-volume injections from purely organic diluents in reversed-phase liquid chromatography.

In liquid chromatography (LC), discrepancies in liquid properties such as elution strength and viscosity lead to a mismatch between the sample diluent and mobile phase. This mismatch can result in peak deformation, including peak splitting or even breakthrough, particularly when large sample volumes are injected. The formation of a T-junction between sample solution and mobile phase flow stream, a technique previously used in supercritical fluid chromatography, is the key enabler of feed injection in LC.

Mesoionic N-Heterocyclic Olefins as Initiators for the Lewis Pair Polymerization of Epoxides.

Mesoionic N-heterocyclic olefins (mNHOs) have recently emerged as a novel class of highly nucleophilic and super-basic σ-donor compounds. Making use of these properties in synthetic polymer chemistry, it is shown that a combination of a specific mNHO and a Mg-based Lewis acid (magnesium bis(hexamethyldisilazide), Mg(HMDS)) delivers poly(propylene oxide) in quantitative yields from the polymerization of the corresponding epoxide (0.1 mol% mNHO loading).

Stereoselective Ring Expansion Metathesis Polymerization with Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Complexes.

Molybdenum alkylidyne -heterocyclic carbene (NHC) complexes of the type [Mo(C--CHY)(OC(R)(CF)) (L)(NHC)][B(Ar)] (Y = OMe, NO; R = CH, CF; L = none, pivalonitrile, tetrahydrofuran; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-3,4-dihydroimidazol-2-ylidene (IMesH), 1,3-dimesityl-3,4-dichloroimidazol-2-ylidene (IMesCl), 1,3-diisopropylimidazol-2-ylidene (IPr); B(Ar) = tetrakis(3,5-bis(trifluoromethyl)phen-1-yl)borate) were used in the ring expansion metathesis polymerization (REMP) of cyclic olefins. With -cyclooctene (COE) cyclic, low molecular weight oligomers were obtained at low monomer concentrations and the cyclic nature of the polymer was confirmed by MALDI-TOF measurements. High-molecular weight cyclic poly(COE) became available at high monomer concentrations.

The reduction behavior of sulfurized polyacrylonitrile (SPAN) in lithium-sulfur batteries using a carbonate electrolyte: a computational study.

Lithium-sulfur batteries (LSBs) have attracted attention due to their high theoretical energy density. This and various other advantages, such as the availability and non-toxicity of sulfur, raise interest in LSBs against the background of the energy revolution. However, a polysulfide shuttle mechanism can adversely affect the electrochemical performance of the cell.

Neutral and Cationic Molybdenum Imido Alkylidene Cyclic Alkyl Amino Carbene (CAAC) Complexes for Olefin Metathesis.

The first neutral and cationic Mo imido alkylidene cyclic alkyl amino carbene (CAAC) complexes of the general formulae [Mo(N-Ar)(CHCMe Ph)(X) (CAAC)] and [Mo(N-Ar)(CHCMe Ph)(X)(CAAC)][B(Ar ) ] (X=Br, Cl, OTf, OC F ; CAAC=1-(2,6-iPr -C H )-3,3,5,5-tetramethyltetrahydropyrrol-2-ylidene) have been synthesized from molybdenum imido bishalide alkylidene DME precursors. Different combinations of the imido and "X" ligands have been employed to understand synthetic peculiarities. Selected complexes have been characterized by single-crystal X-ray analysis.

Method of Manufacturing Structural, Optically Transparent Glass Fiber-Reinforced Polymers (tGFRP) Using Infusion Techniques with Epoxy Resin Systems and E-Glass Fabrics.

Recently, fiber-reinforced, epoxy-based, optically transparent composites were successfully produced using resin transfer molding (RTM) techniques. Generally, the production of structural, optically transparent composites is challenging since it requires the combination of a very smooth mold surface with a sufficient control of resin flow that leads to no visible voids. Furthermore, it requires a minimum deviation of the refractive indices (RIs) of the matrix polymer and the reinforcement fibers.

Effect of liquid confinement on regioselectivity in the hydrosilylation of alkynes with cationic Rh(I) N-heterocyclic carbene catalysts.

Polymeric mesoporous monoliths were prepared ring-opening metathesis polymerization (ROMP) from norbornene (NBE), 1,4,4a,5,8,8a-hexahydro-1,4,5,8-,-dimethanonaphthalene (DMN-H), tris(norborn-2-enylmethylenoxy)methylsilane and the 1-generation Grubbs catalyst [RuCl(PCy)(CHCH)] in the presence of 2-propanol and toluene and surface grafted with 1-(2-((norborn-5-ene-2-carbonyl)oxy)ethyl)-3-ethyl-1-imidazol-3-ium tetrafluoroborate. Subsequently, a supported ionic-liquid-phase (SILP) system was created by immobilizing the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF] with the cationic catalyst [Rh((1-pyrid-1-yl)-3-mesitylimidazol-2-ylidene)(COD)BF] (Rh-1; COD = 1,4-cyclooctadiene) dissolved therein. The regio- and stereoselectivity of Rh-1 dissolved in the IL and supported on the mesoporous monolith, referred to as Rh@SILPROMP, in the hydrosilylation of 1-alkynes with HSiMePh was studied and compared to that of the homogeneous catalyst Rh-1 under biphasic conditions using methyl -butyl ether (MTBE) as a second organic phase.

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts.

Cationic d group 6 olefin metathesis catalysts have been recently shown to display in most instances superior activity in comparison to their neutral congeners. Furthermore, their catalytic performance is greatly improved upon immobilization on silica. In this context, we have developed the new family of molecular cationic molybdenum oxo alkylidene complexes stabilized by N-heterocyclic carbenes of the general formula [Mo(O)(CHCMe)(IMes)(OR)[X]] (IMes = 1,3-dimesitylimidazol-2-ylidene; R = 1,3-dimesityl-CH, CF; X = B(3,5-(CF)CH) , B(Ar), tetrakis(perfluoro--butoxy)aluminate (PFTA)).

Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts.

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length.

Influence of the Silicon-Carbon Interface on the Structure and Electrochemical Performance of a Phenolic Resin-Derived Si@C Core-Shell Nanocomposite-Based Anode.

Silicon is one of the most promising materials when it comes to lithium-ion battery anodes because of its high theoretical capacity and the low working potential Li/Li. However, the drastic volume change during lithiation and delithiation leads to a rapid failure of the electrode. In order to accommodate the large volume change, Si@C core-shell nanocomposites have been investigated, as they efficiently protect the Si surface from being exposed to the electrolyte and thus limit side reactions and improve the cycling stability through a stable solid electrolyte interface layer.

Olefin Metathesis in Confinement: Towards Covalent Organic Framework Scaffolds for Increased Macrocyclization Selectivity.

Covalent organic frameworks (COFs) offer vast structural and chemical diversity enabling a wide and growing range of applications. While COFs are well-established as heterogeneous catalysts, so far, their high and ordered porosity has scarcely been utilized to its full potential when it comes to spatially confined reactions in COF pores to alter the outcome of reactions. Here, we present a highly porous and crystalline, large-pore COF as catalytic support in α,ω-diene ring-closing metathesis reactions, leading to increased macrocyclization selectivity.

Special Issue on Contemporary Challenges in Catalysis.

This special issue on Contemporary Challenges in Catalysis has been organized by three Collaborative Research Centers funded by the German Research Foundation and covers a wide range of aspects and challenges of catalytic research, as described in the Editorial by M. Buchmeiser.

Transparent Fiber-Reinforced Composites Based on a Thermoset Resin Using Liquid Composite Molding (LCM) Techniques.

In this study, optically transparent glass fiber-reinforced polymers (tGFRPs) were produced using a thermoset matrix and an E-glass fabric. In situ polymerization was combined with liquid composite molding (LCM) techniques both in a resin transfer molding (RTM) mold and a lite-RTM (L-RTM) setup between two glass plates. The RTM specimens were used for mechanical characterization while the L-RTM samples were used for transmittance measurements.

Rh(I)/(III)-N-Heterocyclic Carbene Complexes: Effect of Steric Confinement Upon Immobilization on Regio- and Stereoselectivity in the Hydrosilylation of Alkynes.

Rh(I) NHC and Rh(III) Cp* NHC complexes (Cp*=pentamethylcyclopentadienyl, NHC=N-heterocyclic carbene=pyrid-2-ylimidazol-2-ylidene (Py-Im), thiophen-2-ylimidazol-2-ylidene) are presented. Selected catalysts were selectively immobilized inside the mesopores of SBA-15 with average pore diameters of 5.0 and 6.

On the Origin of -Selectivity in the Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidene Heterocyclic Carbene Complexes.

The understanding and control of stereoselectivity is a central aspect in ring-opening metathesis polymerization (ROMP). Herein, we report detailed quantum chemical studies on the reaction mechanism of -selective ROMP of norborn-2-ene (NBE) with Mo(-2,6-Me-CH)(CHCMe)(IMes)(OTf) (, IMes = 1,3-dimesitylimidazol-2-ylidene) as a first step to stereoselective polymerization. Four different reaction pathways based on an ene or ene approach of NBE to either the - or -isomer of the neutral precatalyst have been studied.

Poly(Ethylene Furanoate) along Its Life-Cycle from a Polycondensation Approach to High-Performance Yarn and Its Recyclate.

We report on the pilot scale synthesis and melt spinning of poly(ethylene furanoate) (PEF), a promising bio-based fiber polymer that can heave mechanical properties in the range of commercial poly(ethylene terephthalate) (PET) fibers. Catalyst optimization and solid state polycondensation (SSP) allowed for intrinsic viscosities of PEF of up to 0.85 dL·g.

Melt-Spinning of an Intrinsically Flame-Retardant Polyacrylonitrile Copolymer.

Poly(acrylonitrile) (PAN) fibers have two essential drawbacks: they are usually processed by solution-spinning, which is inferior to melt spinning in terms of productivity and costs, and they are flammable in air. Here, we report on the synthesis and melt-spinning of an intrinsically flame-retardant PAN-copolymer with phosphorus-containing dimethylphosphonomethyl acrylate (DPA) as primary comonomer. Furthermore, the copolymerization parameters of the aqueous suspension polymerization of acrylonitrile (AN) and DPA were determined applying both the Fineman and Ross and Kelen and Tüdõs methods.

Cationic Group VI Metal Imido Alkylidene N-Heterocyclic Carbene Nitrile Complexes: Bench-Stable, Functional-Group-Tolerant Olefin Metathesis Catalysts.

Despite their excellent selectivities and activities, Mo-and W-based catalysts for olefin metathesis have not gained the same widespread use as Ru-based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air-stable cationic-at-metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio-) esters, (thio-) ethers and alcohols without the need for prior activation, for example, by a Lewis acid.

A Spirocyclic Parabanic Acid Masked N-Heterocyclic Carbene as Thermally Latent Pre-Catalyst for Polyamide 6 Synthesis and Epoxide Curing.

1,3-Dicyclcohexyl-6,9-dimethyl-1,3,6,9-tetraazaspiro[4.4]non-7-ene-2,4-dione, a spirocyclic parabanic acid derivative of N,N-dimethylimidazole, is used as thermally latent, protected N-heterocyclic carbene (NHC) in polymerizing anhydride-cured epoxide resins, and azepan-2-one, respectively. The protected carbene is synthesized from 1,3-dimethylimidazolium-2-carboxylate in the presence of two equivalents of cyclohexyl isocyanate.

Reversible N-Heterocyclic Carbene-Induced α-H Abstraction in Tungsten(VI) Imido Dialkyl Dialkoxide Complexes.

The first reversible N-heterocyclic carbene (NHC) induced α-H abstraction in tungsten(VI) imido-dialkyl dialkoxide complexes is reported. Treatment of W(NAr)(CH Ph) (OtBu) (Ar=2,6-dichlorophenyl, 2,6-dimethylphenyl, 2,6-diisopropylphenyl) with different NHCs leads to the formation of complexes of the type W(NAr)(CHPh)(NHC)(CH Ph)(OtBu) in excellent isolated yields of up to 96 %. The highly unusual release of the tert-butoxide ligand as tBuOH in the course of the reaction was observed.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalysts.

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols.

Olefin Metathesis in Confined Geometries: A Biomimetic Approach toward Selective Macrocyclization.

The synthesis of macrocycles is severely impeded by concomitant oligomer formation. Here, we present a biomimetic approach that utilizes spatial confinement to increase macrocyclization selectivity in the ring-closing metathesis of various dienes at elevated substrate concentration up to 25 mM using an olefin metathesis catalyst selectively immobilized inside ordered mesoporous silicas with defined pore diameters. By this approach, the ratio between macro(mono)cyclization (MMC) product and all undesired oligomerization products (O) resulting from acyclic diene metathesis polymerization was increased from 0.