Showing papers in "Chemistry: A European Journal in 2004"
TL;DR: Analysis of the hydration process by solid-state NMR has clearly indicated that the trapped water molecules interact with the carboxylate groups through hydrogen bonds, but do not affect the hydroxyl species bridging the aluminum atoms.
Abstract: Aluminum 1,4-benzenedicarboxylate Al(OH)[O2CC6H4CO2]⋅ [HO2CC6H4CO2H]0.70 or MIL-53 as (Al) has been hydrothermally synthesized by heating a mixture of aluminum nitrate, 1,4-benzenedicarboxylic acid, and water, for three days at 220 °C. Its 3 D framework is built up of infinite trans chains of corner-sharing AlO4(OH)2 octahedra. The chains are interconnected by the 1,4-benzenedicarboxylate groups, creating 1 D rhombic-shaped tunnels. Disordered 1,4-benzenedicarboxylic acid molecules are trapped inside these tunnels. Their evacuation upon heating, between 275 and 420 °C, leads to a nanoporous open-framework (MIL-53 ht (Al) or Al(OH)[O2CC6H4CO2]) with empty pores of diameter 8.5 A. This solid exhibits a Langmuir surface area of 1590(1) m2 g−1 together with a remarkable thermal stability, since it starts to decompose only at 500 °C. At room temperature, the solid reversibly absorbs water in its tunnels, causing a very large breathing effect and shrinkage of the pores. Analysis of the hydration process by solid-state NMR (1H, 13C, 27Al) has clearly indicated that the trapped water molecules interact with the carboxylate groups through hydrogen bonds, but do not affect the hydroxyl species bridging the aluminum atoms. The hydrogen bonds between water and the oxygen atoms of the framework are responsible for the contraction of the rhombic channels. The structures of the three forms have been determined by means of powder X-ray diffraction analysis. Crystal data for MIL-53 as (Al) are as follows: orthorhombic system, Pnma (no. 62), a = 17.129(2), b = 6.628(1), c = 12.182(1) A; for MIL-53 ht (Al), orthorhombic system, Imma (no. 74), a = 6.608(1), b = 16.675(3), c = 12.813(2) A; for MIL-53 lt (Al), monoclinic system, Cc (no. 9), a = 19.513(2), b = 7.612(1), c = 6.576(1) A, β = 104.24(1)°.
1,737 citations
TL;DR: Currently available approaches to molecular-scale logic gates are summarized and compared and integration of simple logic functions to produce more complex ones is discussed in terms of recent developments.
Abstract: Currently available approaches to molecular-scale logic gates are summarized and compared. These include: chemically-controlled fluorescent and transmittance-based switches concerned with small molecules, DNA oligonucleotides with fluorescence readout, oligonucleotide reactions with DNA-based catalysts, chemically-gated photochromics, reversibly denaturable proteins, molecular machines with optical and electronic signals, two-photon fluorophores and multichromophoric transient optical switches. The photochemical principles of electron and energy transfer are involved in several of these approaches. More complex molecular logic systems with reconfigurability and superposability provide contrasts with current semiconductor electronics. Integration of simple logic functions to produce more complex ones is also discussed in terms of recent developments.
563 citations
TL;DR: The new method reported here is the most successful to date with regard to Ullmann-type arylation of some of these nucleophiles, providing the arylated products under particularly mild conditions.
Abstract: Mild, efficient, copper-catalyzed N-arylation procedures for nitrogen heterocycles, amides, carbamates, and C-arylation procedures for malonic acid derivatives have been developed that afford high yields of arylated products with excellent selectivity. The N-arylation of imidazole with aryl bromides or iodides was found to be greatly accelerated by inexpensive, air-stable catalyst systems, combining catalytic copper salts or oxides with a set of structurally simple chelating ligands. The reaction was shown to be compatible with a broad range of aryl halides, encompassing sterically hindered, electron-poor, and electron-rich ones, providing the arylated products under particularly mild conditions (50-82 degrees C). The lower limit in ligand and catalyst loading and the scope of Ullmann-type condensations catalyzed by complexes bearing those ligands with respect to the nucleophile class have also been investigated. Chelating Schiff base Chxn-Py-Al (1c) generates a remarkably general copper catalyst for N-arylation of pyrrole, indole, 1,2,4-triazole, amides, and carbamates; and C-arylation of diethyl malonate, ethyl cyanoacetate, and malononitrile with aryl iodides under mild conditions (50-82 degrees C). The new method reported here is the most successful to date with regard to Ullmann-type arylation of some of these nucleophiles.
542 citations
TL;DR: A template-free chemical synthesis is described that produces uniform polyaniline nanofibers with diameters below 100 nm, and the interfacial polymerization can be readily scaled to make gram quantities.
Abstract: The conjugated polymer polyaniline is a promising material for sensors, since its conductivity is highly sensitive to chemical vapors. Nanofibers of polyaniline are found to have superior performance relative to conventional materials due to their much greater exposed surface area. A template-free chemical synthesis is described that produces uniform polyaniline nanofibers with diameters below 100 nm. The interfacial polymerization can be readily scaled to make gram quantities. Resistive-type sensors made from undoped or doped polyaniline nanofibers outperform conventional polyaniline on exposure to acid or base vapors, respectively. The nanofibers show essentially no thickness dependence to their sensitivity.
531 citations
TL;DR: The scope of this novel method for the preparation of polycyclic arenes is illustrated by thetotal synthesis of a series of polyoxygenated phenanthrenes that are close relatives of the anticancer agent combretastatin A-4, as well as by the total synthesis of the aporphine alkaloid O-methyl-dehydroisopiline and its naturally occurring symmetrical dimer.
Abstract: Readily available biphenyl derivatives containing an alkyne unit at one of their ortho-positions are converted into substituted phenanthrenes on exposure to catalytic amounts of either PtCl2, AuCl, AuCl3, GaCl3 or InCl3 in toluene. This 6-endo-dig cyclization likely proceeds through initial pi-complexation of the alkyne unit followed by interception of the resulting eta2-metal species by the adjacent arene ring. The reaction is inherently modular, allowing for substantial structural variations and for the incorporation of substituents at any site of the phenanthrene product. Moreover, it is readily extended to the heterocyclic series as exemplified by the preparation of benzoindoles, benzocarbazoles, naphthothiophenes, as well as bridgehead nitrogen heterocycles such as pyrrolo[1,2-a]quinolines. Depending on the chosen catalyst, biaryls bearing halo-alkyne units can either be converted into the corresponding 10-halo-phenanthrenes or into the isomeric 9-halo-phenanthrenes; in the latter case, the concomitant 1,2-halide shift is best explained by assuming a metal vinylidene species as the reactive intermediate. The scope of this novel method for the preparation of polycyclic arenes is illustrated by the total synthesis of a series of polyoxygenated phenanthrenes that are close relatives of the anticancer agent combretastatin A-4, as well as by the total synthesis of the aporphine alkaloid O-methyl-dehydroisopiline and its naturally occurring symmetrical dimer.
521 citations
TL;DR: A unique feature of synthetic helical polymers for the detection and amplification of chirality is briefly described in this article, which enable the detection of a tiny imbalance in biologically important chiral molecules through a noncovalent bonding interaction with high cooperativity.
Abstract: A unique feature of synthetic helical polymers for the detection and amplification of chirality is briefly described in this article. In sharp contrast to host-guest and supramolecular systems that use small synthetic receptor molecules, chirality can be significantly amplified in a helical polymer, such as poly(phenylacetylene)s with functional pendants, which enable the detection of a tiny imbalance in biologically important chiral molecules through a noncovalent bonding interaction with high cooperativity. The rational design of polymeric receptors can be possible by using chromophoric helical polymers combined with functional groups as the pendants, which target particular chiral guest molecules for developing a highly efficient chirality-sensing system. The chirality sensing of other small molecular and supramolecular systems is also briefly described for comparison.
499 citations
TL;DR: Two protocols to overcome the insoluble nature of carbon nanotubes by affixing large amounts of addends to the nanotube sidewalls are developed, both of which involve reactions with aryl diazonium species.
Abstract: The use of carbon nanotubes in materials applications has been slowed due to nanotube insolubility and their incompatibility with polymers. We recently developed two protocols to overcome the insoluble nature of carbon nanotubes by affixing large amounts of addends to the nanotube sidewalls. Both processes involve reactions with aryl diazonium species. First, solvent-free functionalization techniques remove the need for any solvent during the functionalization step. This delivers functionalized carbon nanotubes with increased solubility in organic solvents and processibility in polymeric blends. Additionally, the solvent-free functionalization process can be done on large scales, thereby paving the way for use in bulk applications such as in structural materials development. The second methodology involves the functionalization of carbon nanotubes that are first dispersed as individual tubes in surfactants within aqueous media. The functionalization then ensues to afford heavily functionalized nanotubes that do not re-rope. They remain as individuals in organic solvents giving enormous increases in solubility. This protocol yields the highest degree of functionalization we have obtained thus far-up to one in nine carbon atoms on the nanotube has an organic addend. The proper characterization and solubility determinations on nanotubes are critical; therefore, this topic is discussed in detail.
411 citations
TL;DR: The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy and all observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules.
Abstract: The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), pore filling by pore condensation (axial-pore-filling mode) occurs at a low relative pressure, corresponding roughly to a single adsorbed monolayer. For SBA-15, owing to its larger pore diameter (8 nm), a gradual increase in the thickness of the adsorbed layer (radial-pore-filling mode) prevails until pore condensation takes place at a higher level of pore filling.
336 citations
TL;DR: This concept article tries to integrate the views of two experts to delineate the specific principles, approaches, and the novel opportunities for chemistry that arise from the rational control of matter and functionality on that scale.
Abstract: Manipulation and control of chemical structures on the mesoscale has recently developed to a very promising and also aesthetically appealing area of chemistry. This concept article tries to integrate the views of two experts to delineate the specific principles, approaches, and the novel opportunities for chemistry that arise from the rational control of matter and functionality on that scale.
318 citations
TL;DR: A generalizable strategy for designing nonfluorescent aptamers that can be turned into fluorescence-signaling reporters that is coupled to the generation of a fluorescent signal through the departure of QDNA, permitting the real-time monitoring of the aptamer-target recognition.
Abstract: The development of aptamer technology considerably broadens the utility of nucleic acids as molecular recognition elements, because it allows the creation of DNA or RNA molecules for binding a wide variety of analytes (targets) with high affinity and specificity. Several recent studies have focused on developing rational design strategies for transducing aptamer-target recognition events into easily detectable signals, so that aptamers can be widely exploited for detection directed applications. We have devised a generalizable strategy for designing nonfluorescent aptamers that can be turned into fluorescence-signaling reporters. The resultant signaling probes are denoted "structure-switching signaling aptamers" as they report target binding by switching structures from DNA/DNA duplex to DNA/target complex. The duplex is formed between a fluorophore-labeled DNA aptamer and an antisense DNA oligonucleotide modified with a quencher (denoted QDNA). In the absence of the target, the aptamer hybridizes with QDNA, bringing the fluorophore into close proximity of the quencher for efficient fluorescence quenching. When this system is exposed to the target, the aptamer switches its binding partner from QDNA to the target. This structure-switching event is coupled to the generation of a fluorescent signal through the departure of QDNA, permitting the real-time monitoring of the aptamer-target recognition. In this article, we discuss the conceptual framework of the structure-switching approach, the essential features of structure-switching signaling aptamers as well as remaining challenges and possible solutions associated with this new methodology.
296 citations
TL;DR: Experimental results imply that reduction of O(2) occurs by different pathways in the two photocatalytic systems.
Abstract: This article examines the photoxidation of a dye (rhodamine-B, RhB) by visible-light irradiation in the presence of a polyoxometalate (12-tungstosilicic acid, H(4)SiW(12)O(40)), and compares it with the analogous process in the presence of TiO(2). The photoreaction processes were examined by UV-visible spectroscopy, fluorescence spectroscopy, high-performance liquid chromatography (HPLC), liquid chromatography/mass spectral techniques (LC-MS), and total organic carbon (TOC) assays in order to identify the intermediates produced. Formation of oxygen species, such as H(2)O(2) and O(2)*-, was also investigated to clarify the details of the reaction pathway. With the use of SiW(12)O(40)(4-) ions as the photocatalyst, the photoreaction leads mainly to N-dealkylation of the chromophore skeleton. In contrast, cleavage of the whole conjugated chromophore structure predominates in the presence of TiO(2). Strong O(2)*-/HO(2)*- ESR signals were detected in the TiO(2) dispersions, whereas only weak ESR signals for the O(2)*- radical ion were seen in the SiW(12)O(40)(4-) solutions during the irradiation period. Experimental results imply that reduction of O(2) occurs by different pathways in the two photocatalytic systems.
TL;DR: It is shown that chromium can be electrodeposited efficiently to yield a crack-free deposit and this approach could circumvent the use of chromic acid for chromium electroplating, which would be a major environmental benefit.
Abstract: A dark green, viscous liquid can be formed by mixing choline chloride with chromium(III) chloride hexahydrate and the physical properties are characteristic of an ionic liquid. The eutectic composition is found to be 1:2 choline chloride/chromium chloride. The viscosity and conductivity are measured as a function of temperature and composition and explained in terms of the ion size and liquid void volume. The electrochemical response of the ionic liquid is also characterised and it is shown that chromium can be electrodeposited efficiently to yield a crack-free deposit. This approach could circumvent the use of chromic acid for chromium electroplating, which would be a major environmental benefit. This method of using hydrated metal salts to form ionic liquids is shown to be valid for a variety of other salt mixtures with choline chloride.
TL;DR: Lewis acids have been classified according to their propensity to hydrolyse in organic solvents and Brønsted acid-catalysed conjugate addition reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asymmetric Lewis acid catalysis are discussed.
Abstract: The mechanism of Lewis acid catalysed hetero-Michael addition reactions of weakly basic nucleophiles to alpha,beta-unsaturated ketones was investigated. Protons, rather than metal ions, were identified as the active catalysts. Other mechanisms have been ruled out by analyses of side products and of stoichiometric enone-catalyst mixtures and by the use of radical inhibitors. No evidence for the involvement of pi-olefin-metal complexes or for carbonyl-metal-ion interactions was obtained. The reactions did not proceed in the presence of the non-coordinating base 2,6-di-tert-butylpyridine. An excellent correlation of catalytic activities with cation hydrolysis constants was obtained. Different reactivities of mono- and dicarbonyl substrates have been rationalised. A (1)H NMR probe for the assessment of proton generation was established and Lewis acids have been classified according to their propensity to hydrolyse in organic solvents. Bronsted acid-catalysed conjugate addition reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asymmetric Lewis acid catalysis are discussed.
TL;DR: Shape-persistent macrocycles with an interior in the nanometer regime allow the attachment of (functional) side groups at defined positions at the ring to display a new design principle for discotic liquid crystals.
Abstract: Shape-persistent macrocycles with an interior in the nanometer regime allow the attachment of (functional) side groups at defined positions at the ring. These side groups can have either a fixed orientation relative to the molecular backbone or they can adapt their orientation according to an external stimulus. The properties and applications of the compounds depend strongly on the orientation of these side groups. Macrocycles with intraannular or adaptable long alkyl groups display a new design principle for discotic liquid crystals. Macrocycles with extraannular (oligo)alkyl groups can be used for surface patterning in the nanometer regime and rings with extraannular oligostyryl groups are able to aggregate to supramolecular hollow polymer brushes.
TL;DR: Dendritic PG monolayers are as protein resistant as poly(ethylene glycol) (PEG) SAMs and are significantly better than dextran-coated surfaces, which are currently used as the background for SPR spectroscopy.
Abstract: Highly protein-resistant, self-assembled monolayers (SAMs) of dendritic polyglycerols (PGs) on gold can easily be obtained by simple chemical modification of these readily available polymers with a surface-active disulfide linker group. Several disulfide-functionalized PGs were synthesized by N,N'-dicyclohexylcarbodiimide-mediated ester coupling of thioctic acid. Monolayers of the disulfide-functionalized PG derivatives spontaneously form on a semitransparent gold surface and effectively prevent the adsorption of proteins, as demonstrated by surface plasmon resonance (SPR) kinetic measurements. A structure-activity relationship relating the polymer architecture to its ability to effectuate protein resistance has been derived from results of different surface characterization techniques (SPR, attenuated total reflectance infrared (ATR-IR), and contact-angle measurements). Dendritic PGs combine the characteristic structural features of several highly protein-resistant surfaces: a highly flexible aliphatic polyether, hydrophilic surface groups, and a highly branched architecture. PG monolayers are as protein resistant as poly(ethylene glycol) (PEG) SAMs and are significantly better than dextran-coated surfaces, which are currently used as the background for SPR spectroscopy. Due to the higher thermal and oxidative stability of the bulk PG as compared to the PEG and the easy accessibility of these materials, dendritic polyglycerols are novel and promising candidates as surface coatings for biomedical applications.
TL;DR: The catalyst demonstrates high performance, is easily recycled, and approximately 100 % selectivity to sulfones).
Abstract: A [(C(18)H(37))(2)N(+)(CH(3))(2)](3)[PW(12)O(40)] catalyst, assembled in an emulsion in diesel, can selectively oxidize the sulfur-containing molecules present in diesel into their corresponding sulfones by using H(2)O(2) as the oxidant under mild conditions. The sulfones can be readily separated from the diesel using an extractant, and the sulfur level of the desulfurized diesel can be lowered from about 500 ppm to 0.1 ppm without changing the properties of the diesel. The catalyst demonstrates high performance (>/=96 % efficiency of H(2)O(2), is easily recycled, and approximately 100 % selectivity to sulfones). Metastable emulsion droplets (water in oil) act like a homogeneous catalyst and are formed when the catalyst (as the surfactant) and H(2)O(2) (30 %) are mixed in the diesel. However, the catalyst can be separated from the diesel after demulsification.
TL;DR: Of the different ligands prepared, the new dialkyl-2-(N-arylindolyl)phosphines (cataCXium P) perform excellently in the palladium-catalyzed amination of aryl and heteroaryl chlorides.
Abstract: The synthesis and application of monodentate N-substituted heteroarylphosphines is described. In general, the ligands are conveniently prepared by selective metallation at the 2-position of the respective N-substituted heterocycle (pyrrole, indole) by using n-butyllithium/tetramethylethylenediamine (TMEDA) followed by quenching with dialkyl- or diarylchlorophosphines. Of the different ligands prepared, the new dialkyl-2-(N-arylindolyl)phosphines (cataCXium P) perform excellently in the palladium-catalyzed amination of aryl and heteroaryl chlorides. Coupling of both activated and deactivated chloroarenes proceeds under mild conditions (room temperature to 60 degrees C). By using optimized conditions remarkable catalyst productivity (total turnover number, TON, up to 8000) and activity (turnover frequency, TOF=14000 h(-1) at 75% conversion) are observed.
TL;DR: The photoisomerization dynamics of azobenzene is simulated, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach and it is shown that in all cases the torsion around the N==N double bond is the preferred mechanism.
Abstract: We have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n-->pi* and pi-->pi* excitations and both cis-->trans and trans-->cis conversions have been considered. We show that in all cases the torsion around the N==N double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time-resolved spectroscopic experiments are interpreted in the light of the simulated dynamics.
TL;DR: Mechanistic results support the idea that thiolate is the reacting species and that the reaction indeed proceeds through an uncomplicated S(N)2 transition state, and it was found that attack by thiol is much faster than previously assumed, if mediated through water chains.
Abstract: The mechanism for thiol/disulfide exchange has been studied with high-level theoretical calculations. Free energies, transition structures, charge densities, and solvent effects along the reaction pathway have been determined for the first time. Mechanistic results agree with experimental data, and support the idea that the thiolate is the reacting species and that the reaction indeed proceeds through an uncomplicated S(N)2 transition state. The transition structures have the charge density evenly concentrated in the attacking and leaving sulfur atoms. The charge densities allow us to rationalize the solvent effects. As transition structures have the charge density more widely distributed than reactants, hydrophobic environments catalyze the reaction. The effect can be so dramatic that disulfide exchange inside the active site of ribonucleotide reductase is estimated to be catalyzed 10(3) times faster than the reaction in water. It was also found that attack by thiol is much faster than previously assumed, if mediated through water chains. Although the present results, as well as experimental data, still suggest that thiolate is the main reaction species, water-mediated thiol attack is almost kinetically competitive, and can eventually become competitive under specific experimental conditions.
TL;DR: A zinc(II)-selective emission ratiometric probe suitable for biological applications is developed with a cation-induced inhibition of excited-state intramolecular proton transfer (ESIPT) with a series of 2-(2'-benzenesulfonamidophenyl)benzimidazole derivatives.
Abstract: To develop a zinc(II)-selective emission ratiometric probe suitable for biological applications, we explored the cation-induced inhibition of excited-state intramolecular proton transfer (ESIPT) with a series of 2-(2'-benzenesulfonamidophenyl)benzimidazole derivatives. In the absence of Zn(II) at neutral pH, the fluorophores undergo ESIPT to yield a highly Stokes' shifted emission from the proton-transfer tautomer. Coordination of Zn(II) inhibits the ESIPT process and yields a significant hypsochromic shift of the fluorescence emission maximum. Whereas the paramagnetic metal cations Cu(II), Fe(II), Ni(II), Co(II), and Mn(II) result in fluorescence quenching, the emission response is not altered by millimolar concentrations of Ca(II) or Mg(II), rendering the sensors selective for Zn(II) among all biologically important metal cations. Due to the modular architecture of the fluorophore, the Zn(II) binding affinity can be readily tuned by implementing simple structural modifications. The synthesized probes are suitable to gauge free Zn(II) concentrations in the micromolar to picomolar range under physiological conditions.
TL;DR: Ionic hydrogenation catalysts based on the inexpensive metals molybdenum and tungsten have been developed based on mechanistic understanding of the individual steps required in the catalytic reaction.
Abstract: Catalytic ionic hydrogenation of ketones occurs by proton transfer to a ketone from a cationic metal dihydride, followed by hydride transfer from a neutral metal hydride. This contrasts with traditional catalysts for ketone hydrogenation that require binding of the ketone to the metal and subsequent insertion of the ketone into a M-H bond. Ionic hydrogenation catalysts based on the inexpensive metals molybdenum and tungsten have been developed based on mechanistic understanding of the individual steps required in the catalytic reaction.
TL;DR: It is shown that molecules that contain a pi system possess a long-range magnetic response, while the induced magnetic field is short-range for molecules without pi systems and the magnetic response is directly connected to nucleus-independent chemical shifts (NICS).
Abstract: The response of a molecule to an applied external magnetic field can be evaluated by a graphical representation of the induced magnetic field. We have applied this technique to four representative, cyclic organic molecules, that is, to aromatic (C 6 H 6 , D 6 h ), anti-aromatic (C 4 H 4 , D 2 h ) and non-aromatic (C 4 H 8 , D 4 h , and C 6 H 1 2 , D 3 d ) molecules. The results show that molecules that contain a π system possess a long-range magnetic response, while the induced magnetic field is short-range for molecules without π systems. The induced magnetic field of aromatic molecules shields the external field. In contrast, the anti-aromatic molecules increase the applied field inside the ring. Aromatic, anti-aromatic, and non-aromatic molecules can be characterized by the appearance of the magnetic response. We also show that the magnetic response is directly connected to nucleus-independent chemical shifts (NICS).
TL;DR: The solid-state structures of two of the carboxylic acid functionalized imidazolium salts have been determined by single-crystal X-ray diffraction analysis.
Abstract: Imidazolium chlorides with one or two carboxylic acid substituent groups, 1-methyl-3-alkylcarboxylic acid imidazolium chloride, [Me{(CH2)(n)COOH}im]Cl (n=l, 3), and 1,3-dialkylcarboxylic acid imidazolium chloride, [{(CH2)(n)COOH}(2)im]Cl (n=1, 3), have been synthesized via their corresponding acid esters. Deprotonation of the carboxylic acid functionalized imidazolium chlorides with triethylamine affords the corresponding zwitterions [Me{(CH2)(n)COO}im] (n=1, 3) and [{(CH2),COOH}{(CH2)(n)COO}im] (n= 1, 3). Subsequent reaction of the zwitterions with strong acids gives the new imidazolium salts [Me{(CH2)(n)COOH }im]X (n=1, 3; X=BF4, CF3SO3) and [{(CH2)(n)COOH)(2)im]X (n=1, 3; X=BF4, CF3SO3), which exhibit melting points as low as -61 degreesC. The solidstate structures of two of the carboxylic acid functionalized imidazolium salts have been determined by single-crystal X-ray diffraction analysis. Extensive hydrogen bonding is present between the chloride and the imidazolium, with eight (ClH)-H-... interactions below 3Angstrom. The pK(a) values of all the salts, determined by potentiometric titration, lie between 1.33 and 4.59 at 25 degreesC.
TL;DR: A detailed study on the photophysical and complexing properties of calixarenes bearing two and four dansyl derivatives (Calix-DANS2 and Calix- DANS4) in a CH3CN/H2O mixture (60:40 v/v) is reported.
Abstract: A detailed study on the photophysical and complexing properties of calixarenes bearing two and four dansyl derivatives (Calix-DANS2 and Calix-DANS4) in a CH 3 CN/H 2 O mixture (60:40 v/v) is reported. Calix-DANS2 shows a high selectivity towards Hg 2 + over interfering cations (Na + , K + , Ca 2 + , Cu 2 + , Zn 2 + , Cd 2 + and Pb 2 + ) and a sensitivity in the 10 - 7 mol L - 1 concentration range. The complexation of mercury ion induces a strong fluorescence quenching due to a well-defined electron transfer process from the fluorophore to the metal center. Calix-DANS4 exhibits an extremely high affinity for Pb 2 + with a high selectivity over various competing ions. The unprecedented detection limit (4 μg L - 1 ) is fully compatible with the level defined by the World Health Organisation. The affinity of Calix-DANS4 for Pb 2 + can be rationalized by the activation of the inert pair of electrons on Pb 2 + . The number of fluorophores involved in the complexation can be determined from a careful time-resolved fluorescence characterization.
TL;DR: The aim was to prepare both diene and dienophiles simultaneously, under very mild and environmentally friendly conditions, thus giving the constituents for a stereocontrolled Diels-Alder reaction, which in turn yields compounds 4 to 8.
Abstract: Here we report on our studies on combinations of amino acids and copper(I) for catalyzing multicomponent reactions (MCRs). We aimed to prepare both diene and dienophiles simultaneously, under very mild and environmentally friendly conditions, thus giving the constituents for a stereocontrolled Diels–Alder reaction, which in turn yields compounds 4 to 8. A diversity-oriented synthesis of polysubstituted spirotriones 4 to 6 were assembled from simple substrates like 1-(triphenylphosphanylidene)-propan-2-one, two aldehydes, and cyclic-1,3-diketones through Wittig/Knoevenagel/Diels–Alder and aldol/Knoevenagel/Diels–Alder reaction sequences in one pot under stereospecific organocatalysis. Chemical diversity libraries of polysubstituted spirotrione-1,2,3-traizoles 8 were assembled from simple substrates by means of Wittig/Knoevenagel/Diels–Alder/Huisgen cycloaddition reaction sequences in one pot under stereospecific organo/CuI catalysis. Functionalized dispirolactones such as 6 are biologically active antioxidants and radical scavengers, and spirotrione-1,2,3-traizoles 8 have found wide applications in chemistry, biology, and materials science. Experimentally simple and environmentally friendly, organocatalytic, asymmetric four-component Diels–Alder (AFCDA) reactions of 1-(triphenylphosphanylidene)- propan-2-one, two different aldehydes, and cyclic-1,3-diketones produced diastereospecific and highly enantioselective substituted spirotriones 4 by means of a Wittig/Knoevenagel/Diels–Alder reaction sequence in one pot. Additionally we have developed an organocatalytic, asymmetric three-component Michael (ATCM) reaction of 1-(triphenylphosphanylidene)-propan-2-one, aldehyde, and cyclic-1,3-diketones that produced Michael adducts 15, 16 through a Wittig/Michael reaction sequence in a highly enantioselective one-pot process.
TL;DR: The key features of these new salts are their low melting points or extremely low glass transition, and considerably low viscosities, which are compared with the reported [BF4](-)-based ones.
Abstract: A series of twenty two hydrophobic ionic liquids, 1-alkyl(alkyl ether)-3-methylimidazolium ([C(m)mim]+ or [C(m)O(n)mim]+; where Cm is 1-alkyl, Cm = nCmH(2m+1), m = 1-4 and 6; C(m)O(n) is 1-alkyl ether, C2O1 = CH3OCH2, C3O1 = CH3OCH2CH2, and C5O2 = CH3(OCH2CH2)2) perfluoroalkyltrifluoroborate ([RFBF3]-, RF = CF3, C2F5, nC3F7, nC4F9), have been prepared and characterized. Some of the important physicochemical properties of these salts including melting point, glass transition, viscosity, density, ionic conductivity, thermal and electrochemical stability, have been determined and were compared with those of the reported [BF4](-)-based ones. The influence of the structure variation in the imidazolium cation and the perfluoroalkyltrifluoroborate ([RFBF3]-) anion on the above physicochemical properties was discussed. The key features of these new salts are their low melting points (-42 to 35 degrees C) or extremely low glass transition (between -87 and -117 degrees C) without melting, and considerably low viscosities (26-77 cP at 25 degrees C).
TL;DR: This work discusses current approaches to utilize active transport and actuation on a molecular scale, and gives an outlook to the future of hybrid "bio/nano"-devices.
Abstract: Biomolecular motors, in particular motor proteins, are ideally suited to introduce chemically powered movement of selected components into devices engineered at the micro- and nanoscale level. The design of such hybrid "bio/nano"-devices requires suitable synthetic environments, and the identification of unique applications. We discuss current approaches to utilize active transport and actuation on a molecular scale, and we give an outlook to the future.
TL;DR: Electrospray ionization mass spectrometry (ESI-MS) is found to gently and efficiently transfer small to large as well as singly to multiply charged [X+]n[A-]m supramolecules of imidazolium ion (X+) ionic liquids to the gas phase, and to reveal "magic numbers" for their most favored assemblies.
Abstract: Electrospray ionization mass spectrometry (ESI-MS) is found to gently and efficiently transfer small to large as well as singly to multiply charged [X + ] n [A - ] m supramolecules of imidazolium ion (X + ) ionic liquids to the gas phase, and to reveal "magic numbers" for their most favored assemblies. Tandem mass spectrometric experiments (ESI-MS/MS) were then used to dissociate, via low-energy collision activation, mixed and loosely bonded [A-----X-----A'] - and [X-----A-----X'] + gaseous supramolecules, as well as their higher homologues, and to estimate and order via Cooks' kinetic method (CKM) and B3LYP/6-311G(d,p) calculations the intrinsic solvent-free magnitude of hydrogen bonds. For the five anions studied, the relative order of intrinsic hydrogen-bond strengths to the 1-n-butyl-3-methylimidazolium ion [X 1 ] + is: CF 3 CO 2 - (zero) > BF 4 - (-3.1) > PF 6 - (-10.0) > InCl 4 - (-16.4) and BPh 4 - (-17.6 kcal mol - 1 ). The relative hydrogen-bond strength for InCl 4 - was measured via CKM whereas those for the other anions were calculated and used as CKM references. A good correlation coefficient (R=0.998) between fragment ion ratios and calculated hydrogen-bond strengths and an effective temperature (T e f f ) of 430 K demonstrate the CKM reliability for measuring hydrogen-bond strengths in gaseous ionic liquid supramolecules. Using CKM and T e f f of 430 K, the intrinsic hydrogen-bond strengths of BF 4 - for the three cations investigated is: 1-n-butyl-3-methyl-imidazolium ion (0) > 1,3-di-[(R)-3-methyl-2-butyl]-imidazolium ion (-2.4) > 1,3-di-[(R)-α-methylbenzyl]-imidazolium ion (-3.0 kcal mol - 1 ). As evidenced by "magic" numbers, greater stabilities are found for the [(X 1 ) 2 (BF 4 ) 3 ] - and [(X 1 ) 5 A 4 ] + supramolecules (A ≠ InCl 4 - ).
TL;DR: The controlled decomposition of an Ru(0) organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF(6), which represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles, is controlled.
Abstract: The controlled decomposition of an Ru(0) organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF(6)), tetrafluoroborate (BMI.BF(4)) or trifluoromethane sulfonate (BMI.CF(3)SO(3)) ionic liquids with H(2) represents a simple and efficient method for the generation of Ru(0) nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of approximately 57 nm that contain dispersed Ru(0) nanoparticles with diameters of 2.6+/-0.4 nm. These nanoparticles dispersed in the ionic liquids are efficient multiphase catalysts for the hydrogenation of alkenes and benzene under mild reaction conditions (4 atm, 75 degrees C). The ternary diagram (benzene/cyclohexene/BMI.PF(6)) indicated a maximum of 1 % cyclohexene concentration in BMI.PF(6), which is attained with 4 % benzene in the ionic phase. This solubility difference in the ionic liquid can be used for the extraction of cyclohexene during benzene hydrogenation by Ru catalysts suspended in BMI.PF(6). Selectivities of up to 39 % in cyclohexene can be attained at very low benzene conversion. Although the maximum yield of 2 % in cyclohexene is too low for technical applications, it represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles.
TL;DR: By integrating porosity with electrical or optical properties, microporous chalcogenides may have unique applications and is reviewed and concepts in the synthesis and crystal structure of tetrahedral clusters and their frameworks are discussed.
Abstract: By integrating porosity with electrical or optical properties, microporous chalcogenides may have unique applications. Here we review recent advances and discuss concepts in the synthesis and crystal structure of tetrahedral clusters and their frameworks. These chalcogenides can be viewed as trivalent metal chalcogenides doped with tetra-, di-, or monovalent metal cations. Low-valent cations help to increase the cluster size, while high-valent cations have the opposite effect.