A study of the ionic conduction of mica surface by admittance spectroscopy

The ionic conduction on the surface of humid mica has been analyzed by admittance spectroscopy as a function of relative humidity for different surface treatments. Measurements at low frequency indicate that water adsorption proceeds first in the form of a strongly adsorbed uniform thin layer, then with the formation of highly inhomogeneous thick aggregates.     

Click-modified hexahomotrioxacalix[3]arenes as fluorometric and colorimetric dual-modal chemosensors for 2,4,6-trinitrophenol

A new type of chemosensor-based approach to the detection of 2,4,6-trinitrophenol (TNP) is described in this paper. Two hexahomotrioxacalix[3]arene-based chemosensors 1 and 2 were synthesized through click chemistry, which exhibited high binding affinity and selectivity toward TNP as evidenced by UV–vis and fluorescence spectroscopy studies. ¹H NMR titration analysis verified that CH?O hydrogen bonding is demonstrated as the mode of interaction, which possibly facilitates effective charge-transfer.     

Latest Advances Towards Ras Inhibition: A Medicinal Chemistry Perspective

Owing to their high occurrence rate across many human cancers and their lack of druggability so far, mutant forms of the signaling protein Ras are currently among the most attractive (and elusive) oncology targets. This strong appeal explains the sustained effort in the field, and the ensuing progress has rekindled optimism regarding the discovery of Ras inhibitors. In this Minireview, we discuss the most recent advances towards irreversible inhibitors, and highlight approaches to inhibitors of Ras–effector interactions that have been overshadowed by the current focus on direct Ras inhibition. At the same time, we provide a critical assessment from a medicinal chemistry perspective.     

Ueber die Bestimmung des Gerbstoffs in Gerbstoffmaterialien

Ohne Zusammenfassung     

Sub-solidus Relations in the System KF—PbF2 to high Pressures

The phase β-K_0.25Pb_0.75F_1.75 previously found in the KF-PbF₂ system appears to be metastable at low temperatures relative to a mixture of orthorhombic PbF₂ and a new phase suspected to be KPbF₃ II. KPbF₃ II transforms to KPbF₃ I at 298.5°C at atmospheric pressure. The KPbF₃ II/I transition line rises with pressure, but the substance appears to reversibly disproportionate above ～360°C, 5 kbar, possibly to a mixture of PbF₂ and K₄PbF₆. Instead of β-K_0.25Pb_0.75F_1.75, a mixture with this composition yielded, in addition to weak heat events due to the KPbF₃ II/I transition, strong heat events at 254.5°C and atmospheric pressure (thermal hysteresis ～13°C) which were ascribed to the PbF₂ orthorhombic/cubic transition. This transition rises with pressure to 673°C at 37.8 kbar.     

Vanadium complexes possessing N2O2S2-based ligands: highly active procatalysts for the homopolymerization of ethylene and copolymerization of ethylene/1-hexene

The family of ligands containing an N2O2S2 core, namely, 1,2-di(3-Me-5-t-Bu-salicylaldimino-o-phenylthio)ethane (H2L1), 1,3-di(3-Me-5-t-Bu-salicylaldimino-o-phenylthio)propane (H2L2), 1,4-di(3-Me-5-t-Bu-salicylaldimino-o-phenylthio)butane (H2L3), and 1,2-di(3-Me-5-t-Bu-salicylaldamino-o-phenylthio)ethane (H2L4), have been prepared and complexed with a variety of vanadium chlorides and alkoxides to afford complexes of the form [V(X)L1] (X = O (1), Np-tol (2), Cl (3)), [V(O)(L2,3)] (L2 (4), L3 (5)), and [V(L4)] (6). Crystal structure determinations of H2L1 and H2L4 show the molecule to be centrosymmetric about the bridging ethane moiety, with structural determination of 1 and 3 revealing isostructural monomeric complexes in which the ligand chelates in such a way as to afford pseudo-octahedral coordination at the vanadium center. Prolonged reaction of H2L1 with [V(Np-tol)(OEt)3] led, via oxidative cleavage of the C-S bond, to the bimetallic complex [V2L1(3-Me,5-t-Bu-salicylaldimino-o-phenylthiolate)2] [VL'] (7), as characterized by single-crystal X-ray crystallography. 7 was also isolated from the reaction of H2L4 and [VO(On-Pr)3]. The ability of 1-7 to catalyze the homopolymerization of ethylene and the copolymerization of ethylene/1-hexene in the presence of dimethylaluminum chloride (DMAC) has been assessed: screening reveals that for ethylene homopolymerization 1-7 are all highly active (>1000 g/mmol.h.bar), with the highest activity (ca. 11 000 g/mmol.h.bar) observed using catalyst 3; the use of trimethyl aluminum (TMA) or methylaluminoxane (MAO) as the cocatalyst led only to poorly active systems producing negligible polymer. Analysis of the polyethylene produced showed high molecular weight linear polymers with narrow polydispersities. For ethylene/1-hexene copolymerization, activities as high as 1,190 g/mmol.h.bar were achieved (4); analysis of the copolymer indicated an incorporation of 1-hexene in the range of 5-13%.     

Borate binding to siderophores: structure and stability

Well-known as specific iron chelating agents produced by bacteria, it is shown that some, but not all, siderophore classes have an unexpected binding affinity for boron. The relevant criterium is the availability of a vicinal dianionic oxygen containing binding group (i.e., citrate or catecholate). The resulting boron complexes have been characterized by ESI-MS, multinuclear NMR, and DFT calculations. Detailed boron binding constants have been measured for vibrioferrin, rhizoferrin, and petrobactin. The observed affinity of certain siderophores for borate, a common chemical species in the marine but not the terrestrial environment, allows for small, but potentially significant, concentrations of B-siderophores to exist at oceanic pH. We hypothesize that these concentrations could be sufficient for them to function as cell signaling molecules or as mediators of biological boron uptake. In addition, binding of the tetrahedral boron to these siderophores results in a conformation that is different from either the free siderophore or its iron complex and would thus allow a distinction to be made between its iron uptake and any putative cell signaling roles.     

Cycloaddition Reactions of the Metal Phosphorus Double Bonded Complexes CP(CO)2M=PR2 (M = MO,W; R = Alkyl,Aryl)

Abstract

The reaction of the metal phosphorus double bonded species Cp(CO)₂M=PR₂ (M = Mo, W; R = aryl, alkyl) (1)¹⁾ with diverse diazoalkanes, alkenes, alkines and dienes results in the facile formation of the [2+1]-, [2+2]1- and [2+4]-cycloaddition products 2a-c.