New magnesium-chromium-aluminum spinels with the compositions Mg0.967Al1.985Cr0.048O4 and Mg0.75Al1.3125Cr0.9375O4 were synthesized by thermal transformation of organochromiumoxane magnesiumoxane alumoxanes, in which a chromium atom was incorporated into the molecular structure of the oligomer. The crystallographic parameters of these spinels were calculated for the first time. The crystal lattice constant of the magnesium-chromium-aluminum spinels depends not only on the ratio of the components but also on the sizes of the crystallites. Thermal conductivity of the synthesized spinels was studied and, depending on the chromium content, temperature, and crystal lattice constant, was found to vary in the range from 3.6 to 16.1 W m−1 K−1.
The mechanical response of a cantilever made of a magnetoactive elastomer (MAE) of magnetically soft type positioned on a solid plane and subjected to a uniform magnetic field is studied. Under a field normal to the plane, the cantilever may lose its straight configuration either by lifting its free end or by forming an arc. Under cyclic variation of the field, both deformation modes exhibit a substantial magnetomechanical hysteresis when evolving from/to straight configuration under the field strength sweep. Theoretical analysis shows that these deformation scenarios are similar to that of the first-order transition, that is, a jump-like change of the characteristic parameter followed by hysteresis. However, unlike the customary case, hereby the initial state remains stable under arbitrary strong fields, the onset of instability requires a perturbation of finite strength. The effect is essentially different from the known behaviour of MAE cantilevers under an in-plane field where the transition to the curved configuration is of the second-order (reversible) kind. If to consider a horizontal plane-supported MAE cantilever as an element of a magnetic cilia assembly, it behaves in the way opposite to that of conventional systems where in the initial state the MAE cilia stand normally to the surface.
Efficient approaches have been developed for the synthesis of heteromultifunctional cone calix[4]arenes containing four amino groups at the wide rim and one, two or four propargyl or 2-azidoethyl groups at the narrow rim of the macrocycle, which can be used for expanding functionalization of the calixarene core in the well-known amine acylation (or similar reactions) and CuAAC 'click' reactions. Two different strategies were implemented to obtain propargylated and 2-azidoethylated p-aminocalixarenes. In the case of propargylated calixarenes, sterically crowding silyl protection was introduced into the alkyne groups of p-tert-butyl-calix[4]arene (multiple) propargyl ethers, and the resulting compounds were ipso-nitrated followed by reduction of the nitro groups. To prepare 2-azidoethylated macrocycles, the ipso-nitration/reduction sequence was applied top-tert-butylcalix[4]arenes containing 2-tosyloxyethyl groups at the narrow rims followed by replacement of the tosyloxy groups with azide ones. In all cases, p-aminocalix[4]arenes were obtained as the readily cleavable tert-butoxycarbonyl (Boc) derivatives, which was crucial for certain transformation and purification steps. To confirm the functionalization capabilities of the five obtained multifunctional calixarenes, they were reacted with excess benzyl azide or phenylacetylene, taken as representatives, under copper(I) catalysis, resulting in the narrow-rim triazolated macrocycles. By removing the Boc protecting groups and involving the free amino groups in reactions with p-tolyl isocyanate, a series of narrow-rim triazolated tetraureacalix[4]arenes was obtained. Examination of the 1H NMR spectra of the tetraureas in CDCl3 showed that in most cases triazole heterocycles do not intervene the formation of homo-and heterodimeric capsules by these compounds. Thus, considering the synthetic value of CuAAC and amine transformations, p-aminocalix[4]arenes enriched with alkyne or azide functionalities can be readily used as multifunctional platforms to obtain even higher functionalized macrocycles. As an example, they can be used for the preparation of sophisticated supramolecular assemblies with homo-or heterodimeric calixarene cores and virtually any functional units attached to them via triazole groups.
Co-condensation of organoalumoxanes with Mg(acac)2 and Cr(acac)3 in a solution of EtOH was studied. The interaction of the components at a temperature of ∼70 °C and molar ratios Al/Mg = 2–150 and Al/Cr = 40–200 proceeds with the formation of soluble ceramic-forming chromium-containing magnesiumoxane alumoxanes, that is, precursors of chromium-modified aluminum magnesium ceramics. The introduction of a larger quantity of Cr(acac)3 (Al/Cr < 40) leads to the formation of crystals in the reaction mixture, which, according to scanning electron microscopy, have a probable composition Cr0.86Al0.11Mg0.03(acac)3. The thermal transformation of organochromiumoxane magnesiumoxane alumoxanes into ceramic phases at temperatures of 700, 900, and 1500 °C was studied. The obtained chromium-containing magnesiumoxane alumoxanes are precursors of components (matrix binders, fibers, coatings, powders, etc.) of chromium-modified ceramic composites with spinel and mixed spinel-corundum compositions.
The dynamics of restricted internal rotation of benzene rings in N-benzylideneaniline derivatives and their protonated forms have been studied by NMR spectroscopy and quantum chemical calculations. 15N–1H and 15N–13C spin–spin coupling constants have been determined for a series of [15N]-enriched N-benzylideneanilines substituted at the ortho position of the benzene ring linked to the CH=N carbon atom. The conformation of their molecules has been found to strongly depend on the substituent nature. The presence of a methyl group weakly affects the molecular conformation in going from the neutral to protonated form. In contrast, the protonated forms of compounds with fluoro, hydroxy, and methoxy substituents are additionally stabilized (by 6–7 kcal/mol) due to hydrogen bonding between the heteroatom and proton on the azomethine nitrogen atom, which leads to rotation of the substituted benzene ring by 180°. The change of conformation upon protonation of such molecules can be utilized in the design of new pH-dependent molecular switches. The energy of the pH-induced molecular switching of N-(2-fluorobenzylidene)[15N]aniline is 7 kcal/mol, which is one of the highest values for pH-dependent molecular switches.
