Showing papers in "Chemistry: A European Journal in 2006"
TL;DR: The biological approach to forming crystals is proving to be most surprising as mentioned in this paper, as mollusks build their shells by using a hydrophobic silk gel, very acidic aspartic acid rich proteins and apparently also an amorphous precursor phase from which the crystals form.
Abstract: The biological approach to forming crystals is proving to be most surprising. Mollusks build their shells by using a hydrophobic silk gel, very acidic aspartic acid rich proteins, and apparently also an amorphous precursor phase from which the crystals form. All this takes place in a highly structured chitinous framework. Here we present ideas on how these disparate components work together to produce the highly structured pearly nacreous layer of the mollusk shell.
903 citations
TL;DR: The development of N,N-dialkyl(thio)urea derivatives is charted, from early "proof-of-concept" materials to contemporary active chiral (bifunctional) promoters of highly selective asymmetric transformations.
Abstract: Over the last decade the potential for N,N-dialkyl(thio)urea derivatives to serve as active metal-free organocatalysts for a wide range of synthetically useful reactions susceptible to the influence of general acid catalysis has begun to be realised. This article charts the development of these catalysts (with emphasis on the design principles involved), from early "proof-of-concept" materials to contemporary active chiral (bifunctional) promoters of highly selective asymmetric transformations.
753 citations
TL;DR: Methods for synthetically tailoring the properties of bis-cyclometalated iridium(III) materials are focused on, and the factors governing the nature of their lowest excited state are explored.
Abstract: Phosphorescent iridium(III) complexes are being widely explored for their utility in diverse photophysical applications. The performance of these materials in such roles depends heavily on their excited-state properties, which can be tuned through ligand and substituent effects. This concept article focuses on methods for synthetically tailoring the properties of bis-cyclometalated iridium(III) materials, and explores the factors governing the nature of their lowest excited state.
696 citations
TL;DR: The utilization of complex 4 led to the development of a reliable, easily employed Suzuki-Miyama protocol and Employing various reaction conditions allowed a large array of hindered biaryl and drug-like heteroaromatic compounds to be synthesized without difficulty.
Abstract: The synthesis of NHC-PdCl(2)-3-chloropyridine (NHC=N-heterocyclic carbene) complexes from readily available starting materials in air is described. The 2,6-diisopropylphenyl derivative was found to be highly catalytically active in alkyl-alkyl Suzuki and Negishi cross-coupling reactions. The synthesis, ease-of-use, and activity of this complex are substantial improvements over in situ catalyst generation and all current Pd-NHC complexes. The utilization of complex 4 led to the development of a reliable, easily employed Suzuki-Miyama protocol. Employing various reaction conditions allowed a large array of hindered biaryl and drug-like heteroaromatic compounds to be synthesized without difficulty.
689 citations
TL;DR: It was demonstrated that the active sites in [Cu3(btc)2] are hard Lewis acids by using the ethylene ketal of 2-bromopropiophenone as a test substrate.
Abstract: An optimized procedure was designed for the preparation of the microporous metal-organic framework (MOF) [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate). The crystalline material was characterized by X-ray diffraction, optical microscopy, SEM, X-ray photoelectron spectroscopy, N2 sorption, thermogravimetry, and IR spectroscopy of adsorbed CO. CO adsorbs on a small number of Cu2O impurities, and particularly on the free CuII coordination sites in the framework. [Cu3(btc)2] is a highly selective Lewis acid catalyst for the isomerization of terpene derivatives, such as the rearrangement of alpha-pinene oxide to campholenic aldehyde and the cyclization of citronellal to isopulegol. By using the ethylene ketal of 2-bromopropiophenone as a test substrate, it was demonstrated that the active sites in [Cu3(btc)2] are hard Lewis acids. Catalyst stability, re-usability, and heterogeneity are critically assessed.
626 citations
TL;DR: Preliminary results for the direct electrophilic transfer of the trifluoromethyl moiety onto organic nucleophiles show modest reactivity in polar aprotic solvents under relatively mild conditions.
Abstract: The synthesis of a new family of 10-I-3 hypervalent iodine compounds is described in which the CF3 functionality participates directly in the hypervalent bond. These materials are accessible by nucleophilic ligand substitution at iodine using Me3SiCF3 in the presence of a substoichiometric amount of fluoride. The expected T-shaped geometry at iodine was verified by X-ray crystallographic analyses of three of the products (1-trifluoromethyl-1,2-benziodoxol-3-(1 H)-one and two substituted 1-trifluoromethyl-1,3-dihydro-1,2-benziodoxoles). Preliminary results for the direct electrophilic transfer of the trifluoromethyl moiety onto organic nucleophiles show modest reactivity in polar aprotic solvents under relatively mild conditions. The overall process can be understood as a formal umpolung of the CF3 group.
488 citations
TL;DR: This paper tries to introduce an example of such an interaction, aimed at solving health issues within the world of biomaterials, within the field of drug delivery systems and tissue engineering.
Abstract: Chemistry, materials science and medicine are research areas that converge in the field of drug delivery systems and tissue engineering. This paper tries to introduce an example of such an interaction, aimed at solving health issues within the world of biomaterials. Ordered mesoporous materials can be loaded with different organic molecules that would be released afterwards, in a controlled fashion, inside a living body. These materials can also react with the body fluids giving rise to carbonated nanoapatite particles as the products of such a chemical interaction; these particles, equivalent to biological apatites, enable the regeneration of bone tissue.
485 citations
TL;DR: Detailed structural comparison of these complexes indicates that the increase in metal nuclearity induces the progressive increase in the connectivities of the ultimate nets: that is, the metalnuclearity plays a significant role in tuning the connectivity of a specific network.
Abstract: To investigate the relationship between network connectivity and metal nuclearity, we designed and synthesized a series of three-dimensional (3D) entangled coordination frameworks based on different metal cores, namely [Zn(2)(bdc)(2)(L)(2)]2H(2)O (1), [Zn(bdc)(L)(0.5)] (2), [Zn(oba)(L)(0.5)] (3) and [Cd(3)(bdc)(3)(L)(2)(H(2)O)(2)] (4) by self-assembly of d(10) metal salts with the flexible long-chain ligand 1,4-bis(1,2,4-triazol-1-yl)butane (L), and with the rigid and nonrigid aromatic dicarboxylate ligands 1,4-benzenedicarboxylate (bdc) and 4,4'-oxybis(benzoate) (oba). Compound 1 exhibits a threefold interpenetrated diamondoid array typically based on a tetrahedral second building unit (SBU) at a single Zn center. Compound 2 adopts a threefold interpenetrated alpha-polonium-type network that is built from bimetallic cores as six-connected vertices. The structure of 3 also consists of dinuclear units; it comprises a novel (3,4)-connected threefold interpenetrated net with complex (4610)(46(2)10(3)) topology when single zinc centers act as four-connected nodes (or the alpha-Po topology if dinuclear units are considered as six-connected nodes). Compound 4, derived from a crosslinked fivefold interpenetrated diamond-like substructure, is an unusual example of a self-penetrating coordination framework displaying an unprecedented eight-connected 4(20)6(8) topology with trinuclear cadmium clusters as eight-connected nodes which, to our knowledge, not only defines a new topology for eight-connected coordination networks, but also represents the highest connected topology presently known for self-penetrating systems. Detailed structural comparison of these complexes indicates that the increase in metal nuclearity induces the progressive increase in the connectivities of the ultimate nets: that is, the metal nuclearity plays a significant role in tuning the connectivity of a specific network. The thermal and luminescent properties of these compounds are discussed.
473 citations
TL;DR: A new type of "task specific ionic liquid" was synthesized by the reaction of tetrabutylphosphonium hydroxide with amino acids, including glycine, L-alanine, T(C4)4][AA), and no changes in absorption capacity and kinetics were found after four cycles of absorption/desorption.
Abstract: A new type of "task specific ionic liquid", tetrabutylphosphonium amino acid [P(C 4 ) 4 ][AA], was synthesized by the reaction of tetrabutylphosphonium hydroxide [P(C 4 ) 4 ][OH] with amino acids, including glycine, L-alanine, L-β-alanine, L-serine, and L-lysine. The liquids produced were characterized by NMR, IR spectroscopies, and elemental analysis, and their thermal decomposition temperature, glass transition temperature, electrical conductivity, density, and viscosity were recorded in detail. The [P(C 4 ) 4 ][AA] supported on porous silica gel effected fast and reversible CO 2 absorption when compared with bubbling CO 2 into the bulk of the ionic liquid. No changes in absorption capacity and kinetics were found after four cycles of absorption/ desorption. The CO 2 absorption capacity at equilibrium was 50 mol % of the ionic liquids. In the presence of water (1 wt%), the ionic liquids could absorb equimolar amounts of CO 2 . The CO 2 absorption mechanisms of the ionic liquids with and without water were different.
469 citations
TL;DR: A facile one-pot template-free method for the preparation of amine-functionalized magnetite nanoparticles and hollow nanospheres by using FeCl(3)6 H(2)O as single iron source is developed.
Abstract: To demonstrate their applications in biological and medical fields such as in immunoassays, magnetic separation of cells or proteins, drug or gene delivery, and magnetic resonance imaging, the template-free syntheses of water-soluble and surface functionalized magnetic nanomaterials have become essential and are challenging. Herein, we developed a facile one-pot template-free method for the preparation of amine-functionalized magnetite nanoparticles and hollow nanospheres by using FeCl(3)6 H(2)O as single iron source. These magnetic nanomaterials were characterized by TEM, SEM, XRD, and FTIR technologies. Their magnetic properties were also studied by using a superconducting quantum interference device (SQUID) magnetometer at room temperature. Then the amine-functionalized magnetite nanoparticles were applied to immunoassays and magnetic resonance imaging in live mice.
457 citations
TL;DR: General mechanistic principles for aqueous and nonaqueous sol-gel processes are presented, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents.
Abstract: Nonaqueous-solution routes to metal oxide nanoparticles are a valuable alternative to the known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review we give a detailed overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape, and, in some cases, also determines the assembly behavior. We have a closer look at the following reaction systems in this order: 1) metal halides in alcohols, 2) metal alkoxides, acetates, and acetylacetonates in alcohols, 3) metal alkoxides in ketones, and 4) metal acetylacetonates in benzylamine. All these systems offer some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. In the second part we present general mechanistic principles for aqueous and nonaqueous sol-gel processes, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents. Depending on the system several mechanisms have been postulated: 1) alkyl halide elimination, 2) elimination of organic ethers, 3) ester elimination, 4) C--C bond formation between benzylic alcohols and alkoxides, 5) ketimine and aldol-like condensation reactions, 6) oxidation of metal nanoparticles, and 7) thermal decomposition methods.
TL;DR: The electronic properties of gas-phase 1-butyl-3-methylimidazolium Cl ion pairs, [C(4)C(1)im]Cl, are analysed in order to deepen the understanding of ionic liquids in general.
Abstract: In this paper we analyse the electronic properties of gas-phase 1-butyl-3-methylimidazolium Cl ion pairs, [C(4)C(1)im]Cl, in order to deepen our understanding of ionic liquids in general. Examination of charge densities, natural bond orbitals (NBO), and delocalised molecular orbitals computed at the B3LYP and MP2/6-31(++)G(d,p) levels have enabled us to explain a number of experimental phenomena: the relative acidity of different sites on the imidazolium ring, variations in hydrogen-bond donor and acceptor abilities, the apparent contradiction of the hydrogen-bond-donor parameters for different types of solute, the low probability of finding a Cl(-) anion at the rear of the imidazolium ring and the expansion of the imidazolium ring in the presence of a strong hydrogen-bond acceptor. The unreactive but coordinating environment and large electrochemical window have also been accounted for, as has the strong electron-donating character of the carbon atoms to the rear of the ring in associated imidazolylidenes. The electronic structure of the [C(4)C(1)im](+) cation is best described by a C(4)==C(5) double bond at the rear, and a delocalised three-centre 4 e(-) component across the front (N(1)-C(2)-N(3)) of the imidazolium ring; delocalisation between these regions is also significant. Hydrogen-bond formation is driven by Coulombic stabilisation, which compensates for an associated destabilisation of the electronic part of the system. Interactions are dominated by a large positive charge at C(2) and the build up of pi-electron density above and below the ring, particularly that associated with the double bond between C(4) and C(5). The NBO partial charges have been computed and compared with those used in a number of classical simulations.
TL;DR: In a preliminary study on the gas sensitivity of SnO2 hollow spheres, considerably reduced "recovery times" were noted, exemplifying the distinct properties imparted by the hollow structure.
Abstract: A general method for the synthesis of metal oxide hollow spheres has been developed by using carbonaceous polysaccharide microspheres prepared from saccharide solution as templates. Hollow spheres of a series of metal oxides (SnO2, Al2O3, Ga2O3, CoO, NiO, Mn3O4, Cr2O3, La2O3, Y2O3, Lu2O3, CeO2, TiO2, and ZrO2) have been prepared in this way. The method involves the initial absorption of metal ions from solution into the functional surface layer of carbonaceous saccharide microspheres; these are then densified and cross-linked in a subsequent calcination and oxidation procedure to form metal oxide hollow spheres. Metal salts are used as starting materials, which widens the accessible field of metal oxide hollow spheres. The carbonaceous colloids used as templates have integral and uniform surface functional layers, which makes surface modification unnecessary and ensures homogeneity of the shell. Macroporous films or cheese-like nanostructures of oxides can also be prepared by slightly modified procedures. XRD, TEM, HRTEM, and SAED have been used to characterize the structures. In a preliminary study on the gas sensitivity of SnO2 hollow spheres, considerably reduced “recovery times” were noted, exemplifying the distinct properties imparted by the hollow structure. These hollow or porous nanostructures have the potential for diverse applications, such as in gas sensitivity or catalysis, or as advanced ceramic materials.
TL;DR: An overview of microfluidic devices available for chemical synthesis is presented and the potential of microreactor technology in organic synthesis is evaluated.
Abstract: Will microreactors replace the round-bottomed flask to perform chemical reactions in the near future? Recent developments in the construction of microstructured reaction devices and their wide-ranging applications in many different areas of chemistry suggest that they can have a significant impact on the way chemists conduct their experiments. Miniaturizing reactions offers many advantages for the synthetic organic chemist: high-throughput scanning of reaction conditions, precise control of reaction variables, the use of small quantities of reagents, increased safety parameters, and ready scale-up of synthetic procedures. A wide range of single- and multiphase reactions have now been performed in microfluidic-based devices. Certainly, microreactors cannot be applied to all chemistries yet and microfluidic systems also have disadvantages. Limited reaction-time range, high sensitivity to precipitating products, and new physical, chemical, and analytical challenges have to be overcome. This concept article presents an overview of microfluidic devices available for chemical synthesis and evaluates the potential of microreactor technology in organic synthesis.
TL;DR: This is the first user-friendly Negishi protocol capable of routinely cross-coupling all combinations of alkyl and aryl centers and the first time Pd-NHC (NHC=N-heterocyclic carbene) methodology has surpassed the related phosphine-ligated Negishi processes both in activity and use.
Abstract: We have developed the first user-friendly Negishi protocol capable of routinely cross-coupling all combinations of alkyl and aryl centers. The use of an easily synthesized, air stable, highly active, well-defined precatalyst PEPPSI-IPr (1; PEPPSI=pyridine-enhanced precatalyst preparation, stabilization and initiation; IPr=diisopropylphenylimidazolium derivative) substantially increases the scope, reliability, and ease-of-use of the Negishi reaction. All organohalides and routinely used pseudohalides were excellent coupling partners, with the use of chlorides, bromides, iodides, triflates, tosylates, and mesylates resulting in high yield of the coupled product. Furthermore, all reactions were performed by using general laboratory techniques, with no glove-box necessary as the precatalyst was weighed and stored in air. Utilization of this methodology allowed for the easy synthesis of an assortment of sterically encumbered biaryls and druglike heteroaromatics, demonstrating the value of the PEPPSI-IPr system. Furthermore, this is also the first time Pd-NHC (NHC=N-heterocyclic carbene) methodology has surpassed the related phosphine-ligated Negishi processes both in activity and use.
TL;DR: An electrostatic model is proposed based on two assumptions: 1) the presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the Halogen atom.
Abstract: A study of the halogen···hal- ogen contacts in organic compounds using ab initio calculations and the re- sults of previously reported crystallo- graphic studies show that these interac- tions are controlled by electrostatics. These contacts can be represented by the geometric parameters of the C� X1···X2� C moieties (where q1 = C� X1···X2 and q2 = X1···X2 � C; ri = X1···X2 distance). The distributions of the con- tacts within the sum of van der Waals radii (rvdW) versus qi (q1 = q2) show a maximum at q � 1508 for X = Cl, Br, and I. This maximum is not seen in the distribution of F···F contacts. These re- sults are in good agreement with our ab initio calculations. The theoretical results show that the position of the maximum depends on three factors: 1) The type of halogen atom, 2) the hy- bridization of the ipso carbon atom, and 3) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom. Calculations show that the strength of these contacts decreases in the follow- ing order: I···I > Br···Br > Cl···Cl. Their relative strengths decrease as a func- tion of the hybridization of the ipso carbon atom in the following order: sp 2 > sp > sp 3 . Attaching an electroneg- ative atom to the carbon atom strengthens the halogen···halogen con- tacts. An electrostatic model is pro- posed based on two assumptions: 1) The presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the halogen atom.
TL;DR: The natures of the ligand substituents were shown to have a significant influence on the degree of control of the polymerizations, and in particular on the tacticity of thepoly(lactic acid)s (PLAs).
Abstract: A series of new alkoxy-amino-bis(phenols) (H2L 1-6) has been synthesized by Mannich condensations of substituted phenols, formaldehyde, and amino ethers or diamines. The coordination properties of these dianionic ligands towards yttrium, lanthanum, and neodymium have been studied. The resulting Group 3 metal complexes have been used as initiators for the ring-opening polymerization of rac-lactide to provide poly(lactic acid)s (PLAs). The polymerizations are living, as evidenced by the narrow polydispersities of the isolated polymers, together with the linear natures of number average molecular weight versus conversion plots and monomer-to-catalyst ratios. Complex [Y(L6){N(SiHMe2)2}(THF)] (17) polymerized rac-lactide to heterotactic PLA (Pr = 0.90 at 20 degrees C) and meso-lactide to syndiotactic PLA (Pr = 0.75 at 20 degrees C). The in situ formation of [Y(L6)(OiPr)(THF)] (18) from 17 and 2-propanol resulted in narrower molecular weight distributions (PDI = 1.06). With complex 18, highly heterotactic PLAs with narrow molecular weight distributions were obtained with high activities and productivities at room temperature. The natures of the ligand substituents were shown to have a significant influence on the degree of control of the polymerizations, and in particular on the tacticity of the polymer.
TL;DR: Different strategies are presented, including functionalisation of the cation, anion or both cation and anion in the same ionic liquid, leading to what has been termed dual-functionalised ionic liquids.
Abstract: Some of the recent developments concerning the synthesis, properties and applications of functionalised ionic liquids are highlighted. Various strategies are presented, including functionalisation of the cation, anion or both cation and anion in the same ionic liquid, leading to what has been termed dual-functionalised ionic liquids. Particular attention is given to the application of functionalised ionic liquids as reaction media, to stabilise nanoparticles/modify surfaces and to generate porous materials.
TL;DR: Both chemical and geometrical models need to be considered for X...X interactions in hexahalogenated benzenes, where nonspecificity of the weak interlayer interactions here is demonstrated by the structure of twinned crystals of these compounds.
Abstract: The nature of intermolecular interactions between halogen atoms, X···X (X=C1, Br, I), continues to be of topical interest because these interactions may be used as design elements in crystal engineering. Hexahalogenated benzenes (C 6 Cl 6-n Br n , C 6 Cl 6-n I n , C 6 Br 6-n I n ) crystallise in two main packing modes, which take the monoclinic space group P2 1/n and the triclinic space group P1. The former, which is isostructural to C 6 Cl 6 , is more common. For molecules that lack inversion symmetry, adoption of this monoclinic structure would necessarily lead to crystallographic disorder. In C 6 Cl 6 , the planar molecules form Cl...Cl contacts and also π···π stacking interactions. When crystals of C 6 Cl 6 are compressed mechanically along their needle length, that is, [010], a bending deformation takes place, because of the stronger interactions in the stacking direction. Further compression propagates consecutively in a snakelike motion through the crystal, similar to what has been suggested for the motion of dislocations. The bending of C 6 Cl 6 crystals is related to the weakness of the Cl···Cl interactions compared with the stronger π···π stacking interactions. The triclinic packing is less common and is restricted to molecules that have a symmetrical (1,3,5- and 2,4,6-) halogen substitution pattern. This packing type is characterised by specific, polarisation-induced X···X interactions that result in threefold-symmetrical X 3 synthons, especially when X=I; this leads to a layered pseudohexagonal structure in which successive planar layers are inversion related and stacked so that bumps in one layer fit into the hollows of the next in a space-filling manner. The triclinic crystals shear on application of a mechanical stress only along the plane of deformation. This shearing arises from the sliding of layers against one another. Nonspecificity of the weak interlayer interactions here is demonstrated by the structure of twinned crystals of these compounds. One of the compounds studied (1,3,5-tribromo-2,4,6-triiodobenzene) is dimorphic, adopting both the monoclinic and triclinic structures, and the reasons for polymorphism are suggested. To summarise, both chemical and geometrical models need to be considered for X···X interactions in hexahalogenated benzenes. The X···X interactions in the monoclinic group are nonspecific, whereas in the triclinic group some X···X interactions are anisotropic, chemically specific and crystal-structure directing.
TL;DR: A new and readily available bidentate ligand, namely, pyrrolidine-2-phosphonic acid phenyl monoester (PPAPM), has been developed for the copper-catalyzed formation of C-N, C-O, and P-C bonds, and various N-arylation products were synthesized by using the CuI/ PPAPM catalyst system.
Abstract: A new and readily available bidentate ligand, namely, pyrrolidine-2-phosphonic acid phenyl monoester (PPAPM), has been developed for the copper-catalyzed formation of C-N, C-O, and P-C bonds, and various N-, O-, and P-arylation products were synthesized in good to excellent yields by using the CuI/PPAPM catalyst system. Addition of the PPAPM ligand greatly increases the reactivity of the copper catalyst, and the resulting versatile and efficient catalyst system is of widespread and practical application in cross-coupling reactions.
TL;DR: The concepts article describes enabling techniques (solid-phase assisted synthesis, new reactor design, microwave irradiation and new solvents) in organic chemistry and emphasizes the combination of several of them for creating new synthetic technology platforms.
Abstract: The concepts article describes enabling techniques (solid-phase assisted synthesis, new reactor design, microwave irradiation and new solvents) in organic chemistry and emphasizes the combination of several of them for creating new synthetic technology platforms. Particular focus is put on the combination of immobilized catalysts as well as biocatalysts with continuous flow processes. In this context, the PASSflow continuous flow technique fulfils both chemical as well as chemical engineering requirements. It combines reactor design with optimized, monolithic solid phases as well as reversible immobilization techniques for performing small as well as large scale synthesis with heterogenized catalysts under continuous flow conditions.
TL;DR: Iron(III) is also able to catalyze exo and endo skeletal rearrangements of 1,6-enynes, although the scope of this transformation is more limited.
Abstract: Gold(I) complexes are the most active catalysts for alkoxy- or hydroxycyclization and for skeletal rearrangement reactions of 1,6-enynes. Intramolecular alkoxycyclizations also proceed efficiently in the presence of gold(I) catalysts. The first examples of the skeletal rearrangement of enynes by the endocyclic cyclization pathway are also documented. Iron(III) is also able to catalyze exo and endo skeletal rearrangements of 1,6-enynes, although the scope of this transformation is more limited. The gold(I)-catalyzed endocyclic cyclization proceeds by a mechanism different from those followed in the presence of PdII, HgII, or RhI catalysts.
TL;DR: These new cyclic quaternary ammonium salts show desirable properties, including low melting points, high thermal stabilities, low viscosities, high conductivities, and wide electrochemical windows, and may thus be potential candidates for use as electrolytes in high-energy storage devices.
Abstract: New cyclic quaternary ammonium salts, composed of N-alkyl-(alkyl ether)-N-methylpyrrolidinium, -oxazolidinium, -piperidinium, or -morpholinium cations (alkyl=nC 4 H 9 , alkyl ether=CH 3 OCH 2 , CH 3 OCH 2 CH 2 ) and a perfluoroalkyltrifluoroborate anion ([R F BF 3 ] - , R F =CF 3 , C 2 F 5 , nC 3 F 7 , nC 4 F 9 ), were synthesized and characterized. Most of these salts are liquids at room temperature. The key properties of these salts-phase transitions, thermal stability, density, viscosity, conductivity, and electrochemical windows-were measured and compared to those of their corresponding [BF 4 ] - and [(CF 3 SO 2 ) 2 N] - salts. The structural effect on all the above properties was intensively studied in terms of the identity of the cation and anion, variation of the side chain in the cation (i.e., alkyl versus alkyl ether), and change in the length of the perfluoroalkyl group (R F ) in the [R F BF 3 ] - ion. The reduction of Li + ions and reoxidation of Li metal took place in pure N-butyl-N-methyl-pyrrolidinium pentafluoroethyltrifluor-oborate as the supporting electrolyte. Such comprehensive studies enhance the knowledge necessary to design and optimize ionic liquids for many applications, including electrolytes. Some of these new salts show desirable properties, including low melting points, high thermal stabilities, low viscosities, high conductivities, and wide electrochemical windows, and may thus be potential candidates for use as electrolytes in high-energy storage devices. In addition, many salts are ionic plastic crystals.
TL;DR: For the first time, an internal alkyne was successfully used in this copper-catalyzed cycloaddition reaction and DFT calculations allowed for the proposition of a new mechanistic pathway for disubstituted alkynes.
Abstract: A versatile and highly efficient catalyst for the Huisgen cycloaddition reaction has been developed. Previously isolated or in situ generated azides yielded 1,2,3-triazoles with differently substituted alkynes in the presence of a [(NHC)CuBr] complex (NHC = N-heterocyclic carbene). Extremely high reaction rates and excellent yields were obtained in all cases. This catalytic system fulfils the requirements of "click chemistry" with its mild and convenient conditions, notably in water or solvent free reactions and simple isolation with no purification step. Furthermore, for the first time, an internal alkyne was successfully used in this copper-catalyzed cycloaddition reaction. DFT calculations on this particular system allowed for the proposition of a new mechanistic pathway for disubstituted alkynes.
TL;DR: Doping of the LaF(3) core with selected luminescent Ln(3+) ions allows the particles to display a range of emission lines from the visible to the near-infrared region (lambda=450-1650 nm), indicating that there is minimal nonspecific binding of FITC-avidin to the silica particles.
Abstract: The preparation of nearly monodisperse (40 nm), silica-coated LaF(3):Ln(3+) nanoparticles and their bioconjugation to FITC-avidin (FITC=fluorescein isothiocyanate) is described in this report. Doping of the LaF(3) core with selected luminescent Ln(3+) ions allows the particles to display a range of emission lines from the visible to the near-infrared region (lambda=450-1650 nm). First, the use of Tb(3+) and Eu(3+) ions resulted in green (lambda=541 nm) and red (lambda=591 and 612 nm) emissions, respectively, by energy downconversion processes. Second, the use of Nd(3+) gave emission lines at lambda=870, 1070 and 1350 nm and Er(3+) gave an emission line at lambda=1540 nm by energy downconversion processes. Additionally, the Er(3+) ions gave green and red emissions and Tm(3+) ions gave an emission at lambda=800 nm by upconversion processes when codoped with Yb(3+) (lambda(ex)=980 nm). Bioconjugation of avidin, which has a bound fluorophore (FITC) as the reporter, was carried out by means of surface modification of the silica particles with 3-aminopropyltrimethoxysilane, followed by reaction with the biotin-N-hydroxysuccinimide activated ester to form an amide bond, imparting biological activity to the particles. A 25-fold or better increase in the FITC signal relative to the non-biotinylated silica particles indicated that there is minimal nonspecific binding of FITC-avidin to the silica particles.
TL;DR: The parallels between organic reactions conducted with water as the solvent and reactions conducted under high pressure can be understood in light of theories of aqueous solvation and hydrophobic effects to promote reactions of nonpolar compounds.
Abstract: The parallels between organic reactions conducted with water as the solvent and reactions conducted under high pressure can be understood in light of theories of aqueous solvation and hydrophobic effects. Such parallels provide powerful tools for promoting reactions of nonpolar compounds.
TL;DR: Terminal substitution at the allyl moiety of the palladium complex facilitates its activation at room temperature leading to very active catalytic species enabling the present catalytic transformations to be performed rapidly using very mild reaction conditions.
Abstract: The use of second-generation [(NHC)Pd(R-allyl)Cl] complexes for Suzuki-Miyaura and Buchwald-Hartwig cross-coupling reactions involving heteroaromatic halides at room temperature is reported. The first examples of room temperature Suzuki-Miyaura cross-coupling of deactivated aryl chlorides with alkenyl boronic acids are also disclosed. Terminal substitution at the allyl moiety of the palladium complex facilitates its activation at room temperature leading to very active catalytic species enabling the present catalytic transformations to be performed rapidly using very mild reaction conditions. Catalyst loadings can be as low as 10 ppm for the Buchwald-Hartwig aryl amination and 50 ppm for the Suzuki-Miyaura reaction.
TL;DR: The results confirm the classical view of steric repulsion between ortho-Hydrogen atoms in biphenyl and they falsify the hypothesis of hydrogen-hydrogen bonding.
Abstract: Based on an Atoms-in-Mole- cules (AIM) analysis, Matta et al. (Chem. Eur. J. 2003, 9, 1940) recently claimed evidence for the existence of hydrogen-hydrogen bonding between ortho-hydrogen atoms, pointing to- wards each other from adjacent phenyl groups in planar biphenyl. This AIM result is opposed to the classical view that nonbonded steric repulsion be- tween the ortho-hydrogen atoms is re- sponsible for the higher energy of the planar as compared to the twisted ge- ometry of biphenyl. In the present work, we address the question if hydro- gen-hydrogen bonding in biphenyl exists, as suggested by AIM, or not. To this end, we have analyzed the poten- tial energy surface for internal rotation of biphenyl in terms of two interacting phenyl radicals using density functional theory (DFT) at BP86/TZ2P. A de- tailed analysis of the bonding mecha- nism and a quantitative bond energy decomposition in the framework of Kohn-Sham DFT show that Pauli (or overlap) repulsion, mainly between C orthoH ortho phenyl MOs, prevents bi- phenyl from being planar and forces it to adopt a twisted equilibrium geome- try. Furthermore, a derivative of bi- phenyl in which all four ortho-hydro- gen atoms have been removed does adopt a planar equilibrium geometry. Thus, our results confirm the classical view of steric repulsion between ortho- hydrogen atoms in biphenyl and they falsify the hypothesis of hydrogen-hy- drogen bonding.
TL;DR: It is shown that these beta-D-glucose-stabilized Au nanocrystals function as effective catalyst for the reduction of 4-nitrophenol in the presence of NaBH4 (otherwise unfeasible if only the strong reducing agent NaBh4 is employed), which was reflected by the UV/Vis spectra of the catalytic reaction kinetics.
Abstract: We present a straightforward, economically viable, and "green" approach for the synthesis and stabilization of relatively monodisperse Au nanocrystals with an average diameter of 8.2 nm (standard deviation, SD=2.3 nm) by using nontoxic and renewable biochemical of beta-D-glucose and by simply adjusting the pH environment in aqueous medium. The beta-D-glucose acts both as reducing agent and capping agent for the synthesis and stabilization of Au nanocrystals in the system. The UV/Vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), electron diffraction (ED), and X-ray diffraction (XRD) techniques were employed to systematically characterize Au nanocrystals synthesized. Additionally, it is shown that these beta-D-glucose-stabilized Au nanocrystals function as effective catalyst for the reduction of 4-nitrophenol in the presence of NaBH4 (otherwise unfeasible if only the strong reducing agent NaBH4 is employed), which was reflected by the UV/Vis spectra of the catalytic reaction kinetics.
TL;DR: It is shown that the equilibrium distances of the covalent bonds are not determined by the maximum overlap of the sigma valence orbitals, which nearly always has its largest value at clearly shorter distances than the equilibrium bond length.
Abstract: The chemical bonds in the diatomic molecules Li(2)-F(2) and Na(2)-Cl(2) at different bond lengths have been analyzed by the energy decomposition analysis (EDA) method using DFT calculations at the BP86/TZ2P level. The interatomic interactions are discussed in terms of quasiclassical electrostatic interactions DeltaE(elstat), Pauli repulsion DeltaE(Pauli) and attractive orbital interactions DeltaE(orb). The energy terms are compared with the orbital overlaps at different interatomic distances. The quasiclassical electrostatic interactions between two electrons occupying 1s, 2s, 2p(sigma), and 2p(pi) orbitals have been calculated and the results are analyzed and discussed. It is shown that the equilibrium distances of the covalent bonds are not determined by the maximum overlap of the sigma valence orbitals, which nearly always has its largest value at clearly shorter distances than the equilibrium bond length. The crucial interaction that prevents shorter bonds is not the loss of attractive interactions, but a sharp increase in the Pauli repulsion between electrons in valence orbitals. The attractive interactions of DeltaE(orb) and the repulsive interactions of DeltaE(Pauli) are both determined by the orbital overlap. The net effect of the two terms depends on the occupation of the valence orbitals, but the onset of attractive orbital interactions occurs at longer distances than Pauli repulsion, because overlap of occupied orbitals with vacant orbitals starts earlier than overlap between occupied orbitals. The contribution of DeltaE(elstat) in most nonpolar covalent bonds is strongly attractive. This comes from the deviation of quasiclassical electron-electron repulsion and nuclear-electron attraction from Coulomb's law for point charges. The actual strength of DeltaE(elstat) depends on the size and shape of the occupied valence orbitals. The attractive electrostatic contributions in the diatomic molecules Li(2)-F(2) come from the s and p(sigma) electrons, while the p(pi) electrons do not compensate for nuclear-nuclear repulsion. It is the interplay of the three terms DeltaE(orb), DeltaE(Pauli), and DeltaE(elstat) that determines the bond energies and equilibrium distances of covalently bonded molecules. Molecules like N(2) and O(2), which are usually considered as covalently bonded, would not be bonded without the quasiclassical attraction DeltaE(elstat).