Calix[4]arene-based bis[2]catenanes: synthesis and chiral resolution

The exclusive formation of hydrogen-bonded dimers between tetraaryl and tetratosylurea calix[4]arenes has been used to prepare a series of ten "bisloop" tetraurea calix[4]arenes 3, in which adjacent phenylurea groups are covalently linked through alpha,omega-dioxyalkane chains. This dimerization with tetratosylurea 2 as template preorganizes the alkenyl residues of tetra(m-alkenyloxyphenyl) ureas 1 and enables their selective connection in high yield (up to 95 %) by olefin metathesis followed by hydrogenation. The "bisloop" calixarenes 3 also exclusively form heterodimers with 1. Thus, in a separated metathesis/hydrogenation sequence, a series of 14 cyclic bis[2]catenanes 4, in which two calix[4]arenes are connected through their wide rims by two pairs of interlocked rings (total size 29 to 41 atoms), were prepared in yields of up to 97 %. Optical resolution of these chiral bis[2]catenanes was studied by HPLC on chiral stationary phases. The single-crystal X-ray structure of one example (4(P,10)) confirmed the interlocking rings and revealed that the hydrogen-bonded dimeric capsule of the calix[4]arene can be "completely" opened.     

1H NMR spectroscopic studies of the conformational isomers of pyrrolidinofullerenes

Mixed bis-adduct derivatives of C60 containing a pyrrolidine and a malonate methano group were synthesized. Three regioisomers, the e', the trans-2, and the trans-3, were isolated and characterized. In-depth NMR studies of these methano-pyrrolidinofullerenes showed that the nitrogen inversion on the pyrrolidine moiety is not a fast event in the 1H NMR time scale as previously regarded. Solvent effects, variable temperature experiments, and protonation of the pyrrolidine nitrogen are addressed.     

Evolution of growth modes for polyelectrolyte bundles

Multivalent ions induce attractions between polyelectrolytes, but lead to finite-sized bundles rather than macroscopic phase separation. The kinetics of aggregation and bundle formation of actin is tracked using two different fluorescently labeled populations of F-actin. It is found that the growth mode of these bundles evolves with time and salt concentration, varying from an initial lateral growth to a longitudinal one at later stages. The results suggest that kinetics play a role in bundle growth, but not in the lateral size of bundles, which is constant for linear and branched topologies.     

Directing aryl-I versus Aryl-Br bond activation by nickel via a ring walking process

Activation of a strong aryl-Br bond of a halogenated vinylarene by nickel(0) is demonstrated in the presence of aryl-I containing substrates. eta2-Coordination of Ni(PEt3)2 to the C=C moiety of halogenated vinylarenes is kinetically preferable and is followed by an intramolecular aryl-halide bond activation process. This "ring-walking" process is quantitative and proceeds under mild reaction conditions in solution. Mechanistic studies indicate that the metal insertion into the aryl-halide bond is not the rate-determining step. The reaction obeys first-order kinetics in the eta2-coordination complexes with almost identical activation parameters for Br and I derivatives. The ring-walking process is kinetically accessible as shown by density functional theory (DFT) calculations at the PBE0/SDB-cc-pVDZ//PBE0/SDD level of theory.     

Fluxional behavior of platinum(0) complexes: intra vs intermolecular reaction pathways

The fluxional behavior of two analogous platinum complexes has been studied in solution by NMR spectroscopy to elucidate the reaction mechanism and to determine the activation parameters. This includes variable temperature NMR spectroscopy, 2D (1)H- (1)H exchange spectroscopy, and spin saturation transfer measurements. A platinum moiety, Pt(PEt 3) 2, translocates between two carbon-carbon double bonds of two vinylpyridine moieties bridged by an arene (i.e., phenyl, anthracene) at elevated temperatures. Magnetization transfer NMR experiments in the presence of free ligands unambiguously revealed an intramolecular pathway for the "phenyl" system. An intermolecular pathway is proposed for the "anthracene" complex.     

Molecular assembly of a 3D-ordered multilayer

Combining strong metal-ligand coordination and pi-pi interactions affords a 3D-ordered molecular-based multilayer. The organization of the assembly is apparent from the optical properties and X-ray reflectivity.     

Synthesis of α-CF3- and α-CCl3 substituted nitrogen heterocycles by aza Diels-Alder and cyclocondensation reactions

Convenient synthetic approaches for α-CF₃- and α-CCl₃ substituted pyridines, hydrogenated pyridines, azanorbornenes, 4H-1,2,4-thiadiazine- and 2H-1,2,6-thiadiazine-1,1-dioxides were developed on the basis of cycloaddition and cyclocondensation reactions of trihaloethaneimines and imidoyl chlorides.     

A Study in Red: The Overlooked Role of Azo-Moieties in Polymeric Carbon Nitride Photocatalysts with Strongly Extended Optical Absorption

The unique optical and photoredox properties of heptazine-based polymeric carbon nitride (PCN) materials make them promising semiconductors for driving various productive photocatalytic conversions. However, their typical absorption onset at ca. 430–450 nm is still far from optimum for efficient sunlight harvesting. Despite many reports of successful attempts to extend the light absorption range of PCNs, the determination of the structural features responsible for the red shift of the light absorption edge beyond 450 nm has often been obstructed by the highly disordered structure of PCNs and/or low content of the moieties responsible for changes in optical and electronic properties. In this work, we implement a high-temperature (900 °C) treatment procedure for turning the conventional melamine-derived yellow PCN into a red carbon nitride. This approach preserves the typical PCN structure but incorporates a new functionality that promotes visible light absorption. A detailed characterization of the prepared material reveals that partial heptazine fragmentation accompanied by de-ammonification leads to the formation of azo-groups in the red PCN, a chromophore moiety whose role in shifting the optical absorption edge of PCNs has been overlooked so far. These azo moieties can be activated under visible-light (470 nm) for H₂ evolution even without any additional co-catalyst, but are also responsible for enhanced charge-trapping and radiative recombination, as shown by spectroscopic studies.     

Antiviral activity of 1,2,4-triazole derivatives (microreview)

Chemistry of Heterocyclic Compounds - The microreview summarizes data published since 2015 on the antiviral properties and synthesis of compounds containing the 1,2,4-triazole ring.     

Mesoporous silicas with covalently immobilized β-cyclodextrin moieties: synthesis,structure, and sorption properties

Mesoporous silicas with chemically attached macrocyclic moieties were successfully prepared by sol-gel condensation of tetraethyl orthosilicate and β-cyclodextrin-silane in the presence of a structure-directing agent. Introduction of β-cyclodextrin groups into the silica framework was confirmed by the results of IR spectral, thermogravimetric, and quantitative chemical analysis of surface compounds. The porous structure of the obtained materials was characterized by nitrogen adsorption-desorption measurements, powder X-ray diffraction, transmission electron microscopy, and dynamic light scattering. It was found that the composition of the reaction mixture used in β-cyclodextrin-silane synthesis significantly affects the structural parameters of the resulting silicas. The increase in (3-aminopropyl)triethoxysilane as well as the coupling agent content in relation to β-cyclodextrin leads ultimately to the lowering or complete loss of hexagonal arrangement of pore channels in the synthesized materials. Formation of hexagonally ordered mesoporous structure was observed at molar composition of the mixture 0.049 TEOS:0.001 β-CD-silane:0.007 CTMAB:0.27 NH₄OH:7.2 H₂O and equimolar ratio of components in β-CD-silane synthesis. The sorption of alizarin yellow on starting silica and synthesized materials with chemically attached β-cyclodextrin moieties was studied in phosphate buffer solutions with pH 7.0. Experimental results of the dye equilibrium sorption were analyzed using Langmuir, Freundlich, and Redlich-Peterson isotherm models. It was proved that the Redlich-Peterson isotherm model is the most appropriate for fitting the equilibrium sorption of alizarin yellow on parent silica with hexagonally arranged mesoporous structure as well as on modified one with chemically immobilized β-cyclodextrin groups.

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