Comparison of monomode and multimode microwave equipment in Suzuki-Miyaura reactions—en route to high throughput parallel synthesis under microwave conditions

The microwave heated Suzuki-Miyaura cross-coupling reaction of boronic acids, boronic esters and organotrifluoroborates served as a model reaction in a singlemode equipment. The reaction conditions were optimized with respect to temperature and reaction time and were transferred to multimode equipment which is well suited for multiparallel synthesis in a larger scale. The source of the Pd species chosen included immobilized Pd complexes and Pd particles. In fact the increased time to reach the required reaction time in multimode chambers suitable for 48 parallel reactions has to be taken into account. The nature of the boronic acid has no impact on the efficiency of the catalytic process. However, heterogenized Pd species perform less well in multimode chambers with larger vial volumes, which we ascribe to diffusion phenomena.     

Pyridine ligand rotation in self-assembled trigonal prisms. Evidence for intracage solvent vapor bubbles

The rate of interconversion of the two inequivalent edges of the pyridine rings in the trigonal prism 3c, self-assembled from 3 equiv of the star connector, tetrakis[4-(4-pyridylethynyl)phenyl]cyclobutadienecyclopentadienylcobalt, and 6 equiv of a platinum linker, cis-(Me3P)2Pt(2+) 2 TfO(-), was determined by DNMR in nitromethane. It exhibits a highly unusual bilinear Eyring plot. In the low temperature regime, the activation enthalpy DeltaH(double dagger) is approximately 12 kcal/mol and an activation entropy DeltaS(double dagger) ranges from approximately -15 to approximately 0 cal/mol x K as a function of the nature and concentration of the anions present. The reaction is attributed to hindered rotation of the pyridine rings about the Pt-N bond, facilitated by a tight pairing with a counterion. Above a counterion-dependent limiting temperature, DeltaH(double dagger) and DeltaS(double dagger) change abruptly to approximately 35 kcal/mol and approximately 60 cal/mol x K, respectively. The changes largely compensate, such that the reactions have comparable rates in the two regimes, both amenable to DNMR measurement, but their mechanisms clearly differ. Several kinetic models for the involvement of ion pairing equilibria fit the observed data nearly equally well, and they all contain a reaction step with high DeltaH(double dagger) and DeltaS(double dagger) values in the high-temperature regime. Its mechanism is proposed to involve a counterion-assisted reversible dissociation of one or two adjacent Pt-N bonds, followed by nearly free rotation of the terminal pyridine ring or rings and subsequent bond reclosure, which is similar to the last presumed step in the initial prism assembly. An interpretation of the very high DeltaS(double dagger) value is suggested by molecular dynamics calculations: at equilibrium, there is a bubble of gaseous nitromethane solvent inside the prism, and it collapses when the prism opens as the transition state is reached. A simple calculation of the entropy of cavitation provides quantitative support for this tentative proposal. The presence of such voids might be generally important for the formation and properties of self-assembled cages.     

Reductive degradation of nido-1-CB8H12 into smaller-cage carborane systems via new monocarbaboranes [arachno-5-CB8H13](-) and closo-2-CB6H8

Treatment of the nido-1-CB8H12 (1) carborane with NaBH4 in THF at ambient temperature led to the isolation of the stable [arachno-5-CB8H13]- (2(-)), which was isolated as Na+[5-CB8H13]-.1.5 THF and PPh4 +[5-CB8H13]- in almost quantitative yield. Compound 2(-) underwent a boron-degradation reaction with concentrated hydrochloric acid to afford the arachno-4-CB7H13 (3) carborane in 70 % yield, whereas reaction between 2(-) and excess phenyl acetylene in refluxing THF gave the [closo-2-CB6H7]- (4-) in 66 % yield. Protonation of the Cs+4(-) salt with concentrated H2SO4 or CF3COOH in CH2Cl2 afforded a new, highly volatile 2-CB6H8 (4) carborane in 95 % yield, the deprotonation of which with Et3N in CH2Cl2 leads quantitatively to Et3NH+[2-CB6H7](-) (Et3NH+4(-)). Both compounds 4- and 4 can be deboronated through treatment with concentrated hydrochloric acid in CH2Cl2 to yield the carbahexaborane nido-2-CB5H9 (5) in 60 % yield. New compounds 2-, 3, and 4 were structurally characterised by the ab initio/GIAO/MP2/NMR method. The method gave superior results to those carried out using GIAO-HF when relating the calculated 11B NMR chemical shifts to experimental data.     

Cytocompatibility of surface ground PE doped with calcium salt of 6-carboxycellulose

We studied the physical–chemical properties of polyethylene (PE) films doped with calcium salt of 6-carboxycellulose (Ca-oxy) in concentrations 0–20 wt.% and their interactions with cells. The interaction between the PE/Ca-oxy composite film and 3T3 cells has not been described so far. Modification by surface grinding to enhance material cytocompatibility was studied. The surface wettability of doped films was tested by contact angle measurements and the surface morphology by SEM microscopy and profilometry. Adhesion and proliferation of tissue cells (3T3 mouse fibroblasts) were studied in vitro on pristine and ground doped PE films. The ground samples have a higher contact angle values and the number of proliferated cells is greater on the ground samples than on pristine ones. The presence of Ca-oxy in PE film increases the adhesion of 3T3 cells on the doped substrate and on ground samples adhere more cells than on pristine ones.

Ln(III) methyl and methylidene complexes stabilized by a bulky hydrotris(pyrazolyl)borate ligand

The reaction of Ln(AlMe(4))(3) with bulky hydrotris(pyrazolyl)borate (Tp(t)(Bu,Me))H proceeds via a sequence of methane elimination and C-H bond activation, affording unprecedented rare-earth metal ligand moieties including Ln(Me)[(micro-Me)AlMe(3)] and X-ray structurally characterized "Tebbe-like" Ln[(micro-CH(2))(2)AlMe(2)].     

N-K electron energy-loss near-edge structures for TiN/VN layers: an ab initio and experimental study

We study N-K-edge electron energy-loss near-edge structures for well-defined TiN/VN bilayers grown on a MgO(100) substrate by both calculations and experiments. The structural relaxations and the electronic structure of TiN/VN multilayers are calculated using the Vienna Ab Initio Simulation Package computer code, which uses density functional theory to describe the electronic interaction. The effects of the core hole created in the excitation process are included in the calculations. For VN, off-stoichiometric effects due to nitrogen vacancies are modelled. The partial density of states (PDOS) for the N-K edge of atoms in the vicinity of the TiN/MgO interface revealed that two new peaks appear between 7 and 9 eV instead of a broad shoulder typical for the bulk. For the VN/TiN interface, the PDOS is modified only slightly, owing to similar bonding on both sides of the interface, and is thus very similar to the respective bulk spectra. An experimental spectrum taken at the VN/TiN interface is, however, well described by an average of the simulated spectra for VN and TiN bulk (interface). Such a finding is characteristic of an intermixed interface.      

Koksma–Hlawka type inequalities of fractional order

The Koksma–Hlawka inequality states that the error of numerical integration by a quasi-Monte Carlo rule is bounded above by the variation of the function times the star-discrepancy. In practical applications though functions often do not have bounded variation. Hence here we relax the smoothness assumptions required in the Koksma–Hlawka inequality. We introduce Banach spaces of functions whose fractional derivative of order is in . We show that if α is an integer and p = 2 then one obtains the usual Sobolev space. Using these fractional Banach spaces we generalize the Koksma–Hlawka inequality to functions whose partial fractional derivatives are in . Hence we can also obtain an upper bound on the integration error even for certain functions which do not have bounded variation but satisfy weaker smoothness conditions.      

The construction of good extensible rank-1 lattices

It has been shown by Hickernell and Niederreiter that there exist generating vectors for integration lattices which yield small integration errors for for all integers . This paper provides algorithms for the construction of generating vectors which are finitely extensible for for all integers . The proofs which show that our algorithms yield good extensible rank-1 lattices are based on a sieve principle. Particularly fast algorithms are obtained by using the fast component-by-component construction of Nuyens and Cools. Analogous results are presented for generating vectors with small weighted star discrepancy.

     

SERS-activating effect of chlorides on borate-stabilized silver nanoparticles: formation of new reduced adsorption sites and induced nanoparticle fusion

Changes in morphology, surface reactivity and surface-enhancement of Raman scattering induced by modification of borate-stabilized Ag nanoparticles by adsorbed chlorides have been explored using TEM, EDX analysis and SERS spectra of probing adsorbate 2,2'-bipyridine (bpy) excited at 514.5 nm and evaluated by factor analysis. At fractional coverages of the parent Ag nanoparticles by adsorbed chlorides <0.6, the Ag colloid/Cl(-)/bpy systems were found to be constituted by fractal aggregates of Ag nanoparticles fairly uniform in size (10 +/- 2 nm) and SERS spectra of Ag(+)-bpy surface species were detected. The latter result was interpreted in terms of the presence of oxidized Ag(+) and/or Ag(n)(+) adsorption sites, which have been encountered also in systems with the chemically untreated Ag nanoparticles. At chloride coverages >0.6, a fusion of fractal aggregates into the compact aggregates of touching and/or interpenetrating Ag nanoparticles has been observed and found to be accompanied by the formation of another surface species, Ag-bpy, as well as by the increase of the overall SERS enhancement of bpy by factor of 40. The same Ag-bpy surface species has been detected under the strongly reducing conditions of reduction of silver nitrate by sodium borohydride in the presence of bpy. The formation of Ag-bpy is thus interpreted in terms of the stabilization of reduced Ag(0) adsorption sites by adsorbed bpy. The formation of reduced adsorption sites on Ag nanoparticle surfaces at chloride coverages >0.6 is discussed in terms of local changes in the work function of Ag. Finally, the SERS spectral detection of Ag-bpy species is proposed as a tool for probing the presence of reduced Ag(0) adsorption sites in systems with chemically modified Ag nanoparticles.     

Structure and function of oxide nanostructures: catalytic consequences of size and composition

Redox and acid-base properties of dispersed oxide nanostructures change markedly as their local structure and electronic properties vary with domain size. These changes give rise to catalytic behavior, site structures, and reaction chemistries often unavailable on bulk crystalline oxides. Turnover rates for redox and acid catalysis vary as oxide domains evolve from isolated monomers to two-dimensional oligomers, and ultimately into clusters with bulk-like properties. These reactivity changes reflect the ability of oxide domains to accept or redistribute electron density in kinetically-relevant reduction steps, in the formation of temporary acid sites via reductive processes, and in the stabilization of cationic transition states. Reduction steps are favored by low-lying empty orbitals prevalent in larger clusters, which also favor electron delocalization, stable anions, and strong Br?nsted acidity. Isomerization of xylenes and alkanes, elimination reactions of alkanols, and oxidation of alkanes to alkenes on V, Mo, Nb, and W oxide domains are used here to demonstrate the remarkable catalytic diversity made available by changes in domain size. The reactive and disordered nature of small catalytic domains introduces significant challenges in their synthesis and their structural and mechanistic characterization, which require in situ probes and detailed kinetic analysis. The local structure and electronic properties of these materials must be probed during catalysis and their catalytic function be related to specific kinetically-relevant steps. Structural uniformity can be imposed on oxide clusters by the use of polyoxometalate clusters with thermodynamically stable and well-defined size and connectivity. These clusters provide the compositional diversity and the structural fidelity required to develop composition-function relations from synergistic use of experiments and theory. In these clusters, the valence and electronegativity of the central atom affects the acid strength of the polyoxometalate clusters and the rate constants for acid catalyzed elementary steps via the specific stabilization of cationic transition states in isomerization and elimination reactions.