General, Modular Access toward Immobilized Chiral Phosphoric Acid Catalysts and Their Application in Flow Chemistry.

Chiral phosphoric acids (CPAs) are among the most frequently used organocatalysts, with an ever-increasing number of applications. However, these catalysts are only obtained in a multistep synthesis and are poorly recyclable, which significantly deteriorates their environmental and economic performance. We herein report a conceptually different, general strategy for the direct immobilization of CPAs on a broad scope of solid supports including silica, polystyrene, and aluminum oxide. Solid-state catalysts were obtained in high yields and thoroughly characterized with elemental analysis by inductively coupled plasma-optical emission spectrometry (ICP-OES), nitrogen sorption measurements, thermogravimetric analysis, scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDX) images, and solid-state NMR spectroscopy. Further, the immobilized catalysts were applied to a variety of synthetically valuable, highly stereoselective transformations under batch and flow conditions including transfer hydrogenations, a Friedländer condensation/transfer hydrogenation sequence, and Mannich reactions under cryogenic flow conditions. Generally, high yields and stereoselectivities were observed along with robust catalyst stability and reusability. After being used for 10 runs under batch conditions, no loss of selectivity or catalytic activity was observed. Under continuous-flow conditions, the heterogeneous system was in operation for 19 h and the high enantioselectivity remained unchanged throughout the entire process. We expect our approach to extend the applicability of CPAs to a higher level, with a focus on flow chemistry and a more environmentally friendly and resource-efficient use of these powerful catalysts.

On the Localization of Persistent Currents Due to Trapped Magnetic Flux at the Stacking Faults of Graphite at Room Temperature.

Granular superconductivity at high temperatures in graphite can emerge at certain two-dimensional (2D) stacking faults (SFs) between regions with twisted (around the c-axis) or untwisted crystalline regions with Bernal (ABA…) and/or rhombohedral (ABCABCA…) stacking order. One way to observe experimentally such 2D superconductivity is to measure the frozen magnetic flux produced by a permanent current loop that remains after removing an external magnetic field applied normal to the SFs. Magnetic force microscopy was used to localize and characterize such a permanent current path found in one natural graphite sample out of ∼50 measured graphite samples of different origins. The position of the current path drifts with time and roughly follows a logarithmic time dependence similar to the one for flux creep in type II superconductors. We demonstrate that a ≃10 nm deep scratch on the sample surface at the position of the current path causes a change in its location. A further scratch was enough to irreversibly destroy the remanent state of the sample at room temperature. Our studies clarify some of the reasons for the difficulties of finding a trapped flux in a remanent state at room temperature in graphite samples with SFs.

Nanometers-Thick Ferromagnetic Surface Produced by Laser Cutting of Diamond.

In this work, we demonstrate that cutting diamond crystals with a laser (532 nm wavelength, 0.5 mJ energy, 200 ns pulse duration at 15 kHz) produced a ≲20 nm thick surface layer with magnetic order at room temperature. We measured the magnetic moment of five natural and six CVD diamond crystals of different sizes, nitrogen contents and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300 K was observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals were much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface.

Adsorption of Olefins at Cupric Ions in Metal-Organic Framework HKUST-1 Explored by Hyperfine Spectroscopy.

Metal-organic frameworks (MOFs) represent a promising platform for gas storage and separation. In this work, adsorption of olefins in Zn-doped HKUST-1 metal-organic framework is explored with hyperfine spectroscopy. By means of electron-nuclear double resonance and hyperfine sublevel correlation spectroscopy, we detect the interaction between the electron spins of the Cu2+ sites of the MOF and the 1H nuclear spins of adsorbed C2H4 and C4H8. Further analysis of the measured spectra allows us to precisely locate the protons in the vicinity of the Cu2+ ions and thereby establish ensemble-averaged structural models of the olefin molecules adsorbed at the open metal sites of HKUST-1.

Structure and Dynamics of Asymmetric Poly(styrene-b-1,4-isoprene) Diblock Copolymer under 1D and 2D Nanoconfinement.

The impact of 1- and 2-dimensional (2D) confinement on the structure and dynamics of poly(styrene-b-1,4-isoprene) P(S-b-I) diblock copolymer is investigated by a combination of Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Grazing-Incidence Small-Angle X-ray Scattering (GISAXS), and Broadband Dielectric Spectroscopy (BDS). 1D confinement is achieved by spin coating the P(S-b-I) to form nanometric thin films on silicon substrates, while in the 2D confinement, the copolymer is infiltrated into cylindrical anodized aluminum oxide (AAO) nanopores. After dissolving the AAO matrix having mean pore diameter of 150 nm, the SEM images of the exposed P(S-b-I) show straight nanorods. For the thin films, GISAXS and AFM reveal hexagonally packed cylinders of PS in a PI matrix. Three dielectrically active relaxation modes assigned to the two segmental modes of the styrene and isoprene blocks and the normal mode of the latter are studied selectively by BDS. The dynamic glass transition, related to the segmental modes of the styrene and isoprene blocks, is independent of the dimensionality and the finite sizes (down to 18 nm) of confinement, but the normal mode is influenced by both factors with 2D geometrical constraints exerting greater impact. This reflects the considerable difference in the length scales on which the two kinds of fluctuations take place.

Intrinsically microporous polyesters from betulin - toward renewable materials for gas separation made from birch bark.

Betulin, an abundant triterpene, can be extracted from birch bark and can be used as a renewable monomer in the synthesis of microporous polyesters. Cross-linked networks and hyperbranched polymers are accessible by an A(2) + B(3) reaction, with betulin being the A(2) monomer and B(3) being a trifunctional acid chloride. Reaction of betulin with a diacid dichloride results in linear, soluble polyesters. The present communication proves that the polyreaction follows the classic schemes of polycondensation reactions. The resulting polymers are analyzed with regard to their micro-porosity by gas sorption, NMR spectroscopy, and X-ray scattering methods. The polymers feature intrinsic microporosity, having ultrasmall pores, which makes them candidates for gas separation membranes, e.g., for the separation of CO(2) from N(2) .

Micropore analysis of polymer networks by gas sorption and 129Xe NMR spectroscopy: toward a better understanding of intrinsic microporosity.

The microporosity of two microporous polymer networks is investigated in detail. Both networks are based on a central spirobifluorene motif but have different linker groups, namely, imide and thiophene units. The microporosity of the networks is based on the "polymers of intrinsic microporosity (PIM)" design strategy. Nitrogen, argon, and carbon dioxide were used as sorbates in order to analyze the microporosity in greater detail. The gas sorption data was analyzed with respect to important parameters such as specific surface area, pore volume, and pore size (distribution). It is shown that the results can be strongly model dependent and swelling effects have to be regarded. (129)Xe NMR was used as an independent technique for the estimation of the average pore size of the polymer networks. The results indicate that both networks are mainly ultramicroporous (pore sizes < 0.8 nm) in the dry state, which was not expected based on the molecular design. Phase separation and network defects might influence the overall network morphology strongly. Finally, the observed swelling indicates that this "soft" microporous matter might have a different micropore size in the solvent swollen/filled state that in the dry state.

P--P bond cleavage of tetraphenyltetraphosphane-1,4-diide facilitated by nickel(0).

One equivalent of [Na2(thf)5-(P4Ph4)] (1) reacts with one equivalent of [Ni(cod)2] (cod=1,5-cyclooctadiene) to give the unexpected ionic compound [Na(Et2O)3][Na3(Et2O)2Ni3(micro-P2Ph2)2-(P2Ph2)3] (2), whereas the reaction of [Ni(cod)2] with the less reactive [K2(pmdeta)2(P4Ph4)] (3) leads to the formation of [K(pmdeta)]2[Ni(P4Ph4)-(P2Ph2)] (4) (PMDETA=NMe(CH2CH2NMe2)2), in which K--Ni interactions are observed. The calculations for 4 confirm the structural parameters obtained by X-ray diffraction studies. A shared electron number (SEN) analysis was applied to investigate the K...Ni interactions. These studies indicate a SEN value of a typical three-center, two-electron bond for K1-Ni-K2 indicating a covalent contribution in the interaction between nickel and potassium.

Element-organic frameworks with high permanent porosity.

Microporous hydrophobic polysilanes with high specific surface areas (700-1100 m2 g(-1)) for applications in gas adsorption are obtained using an organolithiation route.

Dielectric spectroscopy of nano BaTiO3 confined in MCM-41 mesoporous molecular sieve materials.

Dielectric properties of barium titanate (BaTiO3) particles, synthesized directly in the pores of MCM-41 materials, have been investigated in the frequency range from 20 Hz to 1 MHz for temperature intervals from 100 K to 500 K. The dielectric spectra of BaTiO3 confined in these molecular sieves were compared with the results obtained from the investigation of pure MCM-41 materials. Obtained results confirmed successful incorporation of BaTiO3 into porous matrix, but no phase transition from paraelectric to ferroelectric phase was observed due to the particle size being smaller than the critical size. Also, the overall dielectric response of investigated materials is strongly influenced by adsorbed water molecules.

The (P4HMes4)- anion: lability, fluxionality, and structural ambiguity (Mes = 2,4,6-Me3C6H2).

The synthesis and structural characterization of the tetramesityltetraphosphanide anion, (P4HMes4)- (1), is described. It is shown that 1 partially decomposes in solution and displays an unsymmetrical structure in which, depending on conditions, the proton may or may not fluctuate between the terminal P atoms of the P4 chain.

Characterization of the metal-organic framework compound Cu3(benzene 1,3,5-tricarboxylate)2 by means of 129Xe nuclear magnetic and electron paramagnetic resonance spectroscopy.

129Xe NMR measurements of adsorbed xenon are shown for the first time to be a suitable tool to characterize the porosity and the properties of the metal-organic framework Cu3(BTC)2(H2O)3 (BTC = benzene 1,3,5-tricarboxylate). The NMR experiments are performed at room temperature and over a wide range of xenon pressure and on two different synthesized Cu3(BTC)2 samples. 129Xe NMR results reveal that in dependence on the kind of the synthesis pathway either one or two signals are observed which can be attributed to two kinds of fast exchange of xenon atoms in two pores with different pore sizes. Coadsorption experiments of xenon and ethylene demonstrate that the xenon atoms prefer to fill the greater pores of the material because the smaller pores are occupied with residual molecules from the synthesis procedure and additionally adsorbed ethylene. Besides the NMR experiments a series of electron paramagnetic resonance (EPR) measurements are performed to estimate the state of copper having a strong influence on the chemical shift of the adsorbed xenon. The EPR experiments demonstrate that spin exchange between the interconnected copper dimers is taking place across the BTC linker molecules in the Cu3(BTC)2 framework.

Direct synthesis and catalytic evaluation of AlSBA-1.

The direct synthesis of the new mesoporous molecular sieve AlSBA-1 containing exclusively tetrahedrally coordinated aluminium and the catalytic activity of the novel material in the isomerization of n-decane are reported.

Adsorption and desorption behavior of NO on H-ZSM-5, Na-ZSM-5, and Na-A as studied by EPR.

Nitric monoxide probe molecules are used to characterize the Lewis acid properties of sodium cations and aluminum defect centers in various zeolite materials. The adsorption-desorption behavior of NO probe molecules is studied at different temperatures for Na-A, Na-ZSM-5, H-ZSM-5, and silicalite. Adsorbed NO molecules form paramagnetic adsorption complexes with Lewis acid sites which can be examined by EPR transitions ((Delta)m(S)+/-1) at g approximately 2.0. Otherwise the desorption of NO into the gas phase can be monitored by the typical nine-line EPR spectrum ((Delta)m(J)+/-1) of the (2)Pi(3/2) state at g approximately 0.7776. This gas-phase signal is used to study the overall adsorption-desorption properties of the zeolites in the temperature range 150 K less than or approximately T less than or approximately 300 K. At lower temperatures the probe molecules are adsorbed at the Lewis acid sites inside the nanoporous materials and produce an intensive spectrum at T less than or approximately 110 K. But at intermediate temperatures 110 K less than or approximately T less than or approximately 150 K the NO molecules are adsorbed only for a few hundred picoseconds because the lifetime of the adsorption complexes is limited by the beginning desorption processes. The decreasing lifetime of the adsorption complex with rising temperature results in an increasing homogeneous line broadening of their EPR signals. An analysis of the line-broadening effects provides an opportunity for determining the specific desorption energies E(A)(H-ZSM-5)=(20.2+/-7.3) kJ/mol, E(A)(Na-ZSM-5)=(4.1+/-1.5) kJ/mol, and E(A)(Na-A)=(7.1+/-2.1) kJ/mol for NO probe molecules at sodium cations and aluminum defect centers just below the desorption temperature.