Group 6 carbene complexes derived from lithiated azoles and the crystal structure of a molybdenum thiazolinylidene complex

Fischer-type (alkoxy)azolyl carbene complexes and Öfele–Lappert-type azolylinylidene complexes were synthesised by reaction of 1-phenylpyrazol-3-yllithium, 4-methylthiazol-2-yllithium, benzothiazol-2-yllithium, 1-methylimidazol-2-yllithium with M(CO)₅L (L=CO, THF or Cl⁻; M=Cr, Mo or W) and subsequent alkylation with CF₃SO₃CH₃. The alkylation of Fischer-type carbene complexes containing an azolyl as the organic substituent proceeded via ring opening of tetrahydrofuran. When the alkylation is carried out in THF, the carbocation CH₃O(CH₂)₄⁺ acts as an electrophile. Protonation rather than alkylation of coordinated imidazolyl furnished cyclic imine complexes. Changing the donor atom of a coordinated thiazole from N to C by deprotonation and alkylation afforded a carbene complex.     

Robust partial least squares regression: Part I,algorithmic developments

The work summarized in this paper presents the first part of a three‐paper series on robust partial least squares (RPLS) regression. Motivated by recent research activities in this area, this part provides a detailed algorithmic analysis of associated techniques, showing that existing work (i) may not represent a true robust formulation of partial least squares (PLS), (ii) may lead to convergence problems or (iii) may be insensitive to a certain type of outlier. On the basis of this analysis, Part I introduces a new conceptual RPLS algorithm that overcomes the deficiencies of existing work. The second part of this work details this new RPLS technique, compares its peformance with existing RPLS methods and provides an analysis on the computational efficiency and sensitivity of these algorithms. Whilst the first two parts of this work discuss algorithmic developments of RPLS, the final part concentrates on practical issues of RPLS implementations. This third part is devoted to practitioners of chemistry and chemical engineering covering a wide range of applications involving a calibration experiment, the analysis of recorded data from an industrial debutanizer process and data from a number of Raman spectroscopy experiments. Copyright © 2007 John Wiley & Sons, Ltd.     

Reversible Transformation between a [PdL2]2+ “Figure-of-Eight” Complex and a [Pd2L2]4+ Dimer: Switching On and Off Self-Recognition

Structural changes to metallosupramolecular assemblies resulting in the release or uptake of guests are currently well established, whereas transformations turning on and off specific self-recognition are far less developed. We report a novel ligand (2,6-bis(1-(3-pyridin-4-yl)phenyl-1H-1,2,3-triazol-4-yl)pyridine) possessing a tridentate central metal-binding site flanked by two pendant pyridyl arms. In a 2:1 ratio with Pd^II metal ions, a spiro-type [PdL₂]²⁺ “Figure-of-eight” complex forms with the central tridentate binding pocket unoccupied. The introduction of an additional one equivalent of Pd^II metal ion results in the conversion to a dimeric [Pd₂L₂]⁴⁺ molecule with the tridentate pocket occupied. There is site-specific self-recognition between dimers in solution with strong NOE peaks between adjacent molecules. The self-recognition between dimers can be turned off in two ways: firstly, adding another equivalent of Pd^II metal ion brings about binding to the previously uncoordinated pyridyl arms that are key to the self-recognition event, and; secondly, addition of sufficient ligand to return the stoichiometry to 2:1 regenerates the [PdL₂]²⁺ complex. Hence, the self-recognition event can be turned on or off through simple variation of L:Pd^II stoichiometry.     

Development and validation of a liquid chromatography‐tandem mass spectrometry (LC‐MS/MS) method for the quantification of tigecycline in rat brain tissues

Tigecycline (TIG), a derivative of minocycline, is the first in the novel class of glycylcyclines and is currently indicated for the treatment of complicated skin structure and intra‐abdominal infections. A selective, accurate and reversed‐phase high‐performance liquid chromatography‐tandem mass spectrometry (HPLC‐MS/MS) method was developed for the determination of TIG in rat brain tissues. Sample preparation was based on protein precipitation and solid phase extraction using Supel‐Select HLB (30 mg/1 mL) cartridges. The samples were separated on a YMC Triart C₁₈ column (150 mm x 3.0 mm. 3.0 µm) using gradient elution. Positive electrospray ionization (ESI+) was used for the detection mechanism with the multiple reaction monitoring (MRM) mode. The method was validated over the concentration range of 150–1200 ng/mL for rat brain tissue. The precision and accuracy for all brain analyses were within the acceptable limit. The mean extraction recovery in rat brain was 83.6%. This validated method was successfully applied to a pharmacokinetic study in female Sprague Dawley rats, which were given a dose of 25 mg/kg TIG intraperitoneally at various time‐points. Copyright © 2015 John Wiley & Sons, Ltd.     

Construction and characterization of a small-bore electrospray ionization source

A simple, economical, and efficient electrospray ionization (ESI) source has been constructed in the configuration of a probe that makes use of a standard 13 mm vacuum lock. The principal components have been placed inside a glass tube making use of the electrical insulating properties of the glass while allowing for visual adjustments to be readily made. The ESI source, a variation of an atmospheric pressure ionization interface, is a modified version of designs published by Chait et al. (Rapid Comm. Mass Spectrom. 1990, 4, 81–87) and Knapp et al. (Anal. Chem. 1991, 63, 1658–1660) wherein a heated metal capillary is used for desolvation. The ESI probe has been tested on three different Extrel quadrupole mass spectrometers, with removable ion volumes, using polypeptides and small proteins. No modifications to the standard electron ionization/chemical ionization lens assembly were required to obtain excellent results other than removal of the ion volume. The spectra acquired were in excellent agreement with those previously published.     

Condensation phenomena in capillaries with diffusion of water vapour in air under influence of a constant temperature gradient

A capillary tube filled with water and air is subjected to a temperature varying linearly along its wall. The temperature is highest at the water-filled end. The axis of the capillary may be placed horizontally or vertically. The rate of water transport in the stationary state can be explained by diffusion of water vapour in air. Formation of water droplets on the capillary wall in the air-filled part is observed in a number of cases. The observed position of the boundary between the wet and the dry part of the capillary wall agrees well with the one calculated from diffusion theory. However, the accuracy of the theoretical results is limited because of uncertainties with regard to the true value of the diffusion coefficient of water vapour in air and the unknown influence of diffuso-phoresis.     

Borwein–Preiss vector variational principle

This article extends to the vector setting the results of our previous work Kruger et al. (J Math Anal Appl 435(2):1183–1193, 2016) which refined and slightly strengthened the metric space version of the Borwein–Preiss variational principle due to Li and Shi (J Math Anal Appl 246(1):308–319, 2000. doi: 10.1006/jmaa.2000.6813). We introduce and characterize two seemingly new natural concepts of \(\varepsilon \)-minimality, one of them dependent on the chosen element in the ordering cone and the fixed “gauge-type” function.     

Synthesis and NMR elucidation of novel penta-cycloundecane amine derivatives as potential antituberculosis agents

The synthesis and NMR elucidation of five novel penta-cycloundecane amine derivatives are reported. These compounds are potential antituberculosis agents. The (1)H and (13)C spectra showed major overlapping of methine signals of the cage skeleton making it extremely difficult to elucidate these compounds. The overlapping occurs as a result of the additions made to the carbonyl carbon (C-8/C-11) of the cage. The two-dimensional NMR technique proved to be a useful tool in overcoming this problem. All compounds reported are meso compounds thereby not only simplifying the NMR structure elucidation, but also making it indeed possible.     

Hydrogen‐bonding motifs and thermotropic polymorphism in redetermined halide salts of hexamethylenediamine

The redetermined crystal structures of hexane‐1,6‐diammonium dichloride, C₆H₁₈N₂²⁺·2Cl⁻, (I), hexane‐1,6‐diammonium dibromide, C₆H₁₈N₂²⁺·2Br⁻, (II), and hexane‐1,6‐diammonium diiodide, C₆H₁₈N₂²⁺·2I⁻, (III), are described, focusing on their hydrogen‐bonding motifs. The chloride and bromide salts are isomorphous, with both demonstrating a small deviation from planarity [173.89 (10) and 173.0 (2)°, respectively] in the central C—C—C—C torsion angle of the hydrocarbon backbone. The chloride and bromide salts also show marked similarities in their hydrogen‐bonding interactions, with subtle differences evident in the hydrogen‐bond lengths reported. Bifurcated interactions are exhibited between the N‐donor atoms and the halide acceptors in the chloride and bromide salts. The iodide salt is very different in molecular structure, packing and intermolecular interactions. The hydrocarbon chain of the iodide straddles an inversion centre and the ammonium groups on the diammonium cation of the iodide salt are offset from the planar hydrocarbon backbone by a torsion angle of 69.6 (4)°. All three salts exhibit thermotropic polymorphism, as is evident from differential scanning calorimetry analysis and variable‐temperature powder X‐ray diffraction studies.