Shape recognition of alkylammonium ions by 1,3-bridged calix[5]arene crown-6 ethers: endo- vs exo-cavity complexation

A series of tri-O-substituted 1,3-bridged calix[5]arene crown-6 ethers bearing alkyl, arylalkyl, alkoxyalkyl, and alkoxycarbonylmethyl residues at the lower rim and either (t)()Bu or H substituents at the upper rim have been synthesized. (1)H NMR studies have shown that p-tert-butylcalix[5]crowns, irrespective of the size and nature of their lower rim pendant groups, adopt preorganized conelike conformations, whereas p-H-calix[5]crowns with bulky substituents preferentially exist in solution as partial cone conformers (C(1) symmetry). Calix[5]crown derivatives behave as mono- or ditopic receptors for isomeric butylammonium ions, forming endo-cavity (inside the calixarene cup) and/or exo-cavity (at the crown ether moiety) 1:1 complexes according to the shape of the guest. These two binding modes can be clearly distinguished and monitored by (1)H NMR titration experiments.     

Kinetic Buffers

This paper proposes a new type of molecular device that is able to act as an inverse proton sponge to slowly decrease the pH inside a reaction vessel. This makes the automatic monitoring of the concentration of pH‐sensitive systems possible. The device is a composite formed of an alkyl chloride, which kinetically produces acidity, and a buffer that thermodynamically modulates the variation in pH value. Profiles of pH versus time (pH–t plots) have been generated under various experimental conditions by computer simulation, and the device has been tested by carrying out automatic spectrophotometric titrations, without using an autoburette. To underline the wide variety of possible applications, this new system has been used to realize and monitor HCl uptake by a di‐copper(II) bistren complex in a single run, in a completely automatic experiment.     

Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

Three (donor–π–acceptor)⁺ systems with a methyl pyridinium or quinolinium as the electron‐deficient group, a dimethyl amino as the electron‐donor group, and an ethylene or butadiene group as the spacer have been investigated in a joint spectroscopic and TD‐DFT computational study. A negative solvatochromism has been revealed in the absorption spectra, which implies a solution color change, and interpreted by considering the variation in the permanent dipole moment modulus and orientation upon photoexcitation. The fluorescence efficiency decreases upon increasing solvent polarity, in agreement with the excited‐state optimized geometries (planar in low‐polarity media and twisted in high‐polarity media). Femtosecond transient absorption has revealed the occurrence of a fast photoinduced intramolecular charge transfer (ICT) and the molecular factors that determine an efficient ICT. Considering the crucial role of the ICT in tuning the nonlinear optical (NLO) properties, these compounds can be considered promising NLO materials.     

Hybridization of Nafion membranes with an acid functionalised polysiloxane: Effect of morphology on water sorption and proton conductivity

Polysiloxane-modified hybrid Nafion membranes were prepared by casting a mixture of Nafion solution and a precursor of acid functionalised polysiloxane based on tetraethoxysilane and a mercaptan-organoalkoxysilane.Scanning Electron Microscopy (SEM) and Atomic Force Microscopy analysis revealed that the functionalised polysiloxane was dispersed either as finely nanosized inclusions or as coarse domains depending on the rate of the solvent evaporation during the casting procedure. In particular the slower is the rate of solvent evaporation the more interpenetrated and homogenously dispersed at nanosized level is the polysiloxane inside the Nafion membrane.The hybridization process increases the thermal stability of the membranes of about 50 °C relatively to the unmodified Nafion. Small angle X-ray scattering (SAXS) analysis reveals that the hybrid membranes exhibited the typical morphology of Nafion consisting of distinct hydrophilic and hydrophobic domains.Water vapor sorption and proton conductivity were measured varying the temperature (up to 120 °C) and the water activity conditions (from 0.1 to 0.8). The polysiloxane network always increases the water vapor uptake of the membranes and increases significantly the proton conductivity at higher temperature depending on the type of morphology developed by the manufacturing method. In particular hybrid membranes exhibiting nanosized polysiloxane dispersion show a proton conductivity which is up to one-and-half time higher than Nafion recast membrane at high temperature and low water content.     

Crystal structure ofcis-diaqua bis(2,2′-bipyridyl)chromium(III) nitrate, [Cr(bipy)2(H2O)2] (NO3)3

cis-Diaquabis(2,2-bipyridyl)chromium(III) nitrate crystallizes in the mono-clinic system, space groupP2₁/c, witha=16.102(6),b=10.792(5),c=15.076(6) Å,=112.90(5)°, andZ=4. The saltlike structure was determined from X-ray crystallography and refined toR=7.5% for 3342 observed reflections. The Cr atom is octahedrally surrounded by twocis water molecules and by two chelated 2,2-bypiridyl ligands forming two nearly orthogonal planes. Selected bond distances are: Cr-O, 2.00(1) and 1.98(1) Å, Cr-N, 2.04 Å (mean).     

The reaction of di-2-pyridyl ketone with antimony(III) fluoride. Preparation and crystal structure of difluoro (hydroxodi-2-pyridylmethoxo-NN′O)antimony(III)

The synthesis and the crystal and molecular structure of the difluoro(hydroxodi-2-pyridylmethoxo-NNO) antimony(III) compound determined by single crystal X-ray diffraction analysis are reported. Crystals are orthorhombic, space group Pbcn,a=12.732(2),b=12.165(2),c=15.936(2) Å. Refinement based on 1516 observed reflections led to a finalR value of 0.025 (Rw=0.028). In the crystal the di-2-pyridyl ketone behaves asfacial-(NNO)-terdentate in the Sb coordination, the square pyramid geometry around the metal is completed by the coordination of two fluorine atoms.     

New anti-HIV aptamers based on tetra-end-linked DNA G-quadruplexes: effect of the base sequence on anti-HIV activity

This communication reports on the synthesis and biophysical, biological and SAR studies of a small library of new anti-HIV aptamers based on the tetra-end-linked G-quadruplex structure. The new aptamers showed EC(50) values against HIV-1 in the range of 0.04-0.15 μM as well as affinities for the HIV-1 gp120 envelope in the same order of magnitude.     

(E)‐5‐[(4‐Nitro­phenyl­hydrazono)­phenyl­acetyl]‐3‐phenyl­isoxazole

The title compound, C₂₃H₁₆N₄O₄, can be considered as consisting of two connected fragments: a nitro­phenyl­hydrazone moiety, which assumes an E configuration, and an isoxazole moiety. In this latter fragment, the weak π‐electron delocalization shortens the carbonyl–isoxazole O?O distance [2.643 (2) Å] to less than the van der Waals radii sum.     

Heterocyclic azallyl badicals

Methyl-thlyl radical selectively abstracts allylic hydrogens from oxazolyn and thiazolin derivatives, yielding nitrogen centered azallyl radicals.     

X-ray and mössbauer studies of tricyclohexyltin(IV) halides. The crystal structures of (cyclo-C6H11)3SnX (X = F,Br and I)

The structures of tricyclohexyltin fluoride (I), bromide (II) and iodide (III) have been determined by X-ray analysis. Compound I crystallizes in the space group P2₁/m with a = 10.422(6), b = 17.238(9), c = 5.769(3) Å, β = 104.6(1)° and Z = 2. Compounds II and III crystallize in the space group Pcmn with a = 10.427(6), b = 16.914(9), c = 11.366(6) Å, Z = 4; and a = 10.400(6), b = 16.900(10), c = 11.400(4) Å, Z = 4, respectively. All three structures consist of discrete tetrahedral (cyclo-C₆H₁₁)₃SnX units.The temperature dependence of the Mössbauer resonance areas has been examined in order to obtain information about the relationship between chemical structure and lattice dynamics.