Nonclassical Recrystallization

Applications in the fields of materials science and nanotechnology increasingly demand monodisperse nanoparticles in size and shape. Up to now, no general purification procedure exists to thoroughly narrow the size and shape distributions of nanoparticles. Here, we show by analytical ultracentrifugation (AUC) as an absolute and quantitative high-resolution method that multiple recrystallizations of nanocrystals to mesocrystals is a very efficient tool to generate nanocrystals with an excellent and so-far unsurpassed size-distribution (PDI_c=1.0001) and shape. Similar to the crystallization of molecular building blocks, nonclassical recrystallization removes “colloidal” impurities (i.e., nanoparticles, which are different in shape and size from the majority) by assembling them into a mesocrystal. In the case of nanocrystals, this assembly can be size- and shape-selective, since mesocrystals show both long-range packing ordering and preferable crystallographic orientation of nanocrystals. Besides the generation of highly monodisperse nanoparticles, these findings provide highly relevant insights into the crystallization of mesocrystals.     

Gated Channels and Selectivity Tuning of CO2 over N2 Sorption by Post‐Synthetic Modification of a UiO‐66‐Type Metal–Organic Framework

The highly porous and stable metal–organic framework (MOF) UiO‐66 was altered using post‐synthetic modifications (PSMs). Prefunctionalization allowed the introduction of carbon double bonds into the framework through a four‐step synthesis from 2‐bromo‐1,4‐benzenedicarboxylic acid; the organic linker 2‐allyl‐1,4‐benzenedicarboxylic acid was obtained. The corresponding functionalized MOF (UiO‐66‐allyl) served as a platform for further PSMs. From UiO‐66‐allyl, epoxy, dibromide, thioether, diamine, and amino alcohol functionalities were synthesized. The abilities of these compounds to adsorb CO₂ and N₂ were compared, which revealed the structure–selectivity correlations. All synthesized MOFs showed profound thermal stability together with an increased ability for selective CO₂ uptake and molecular gate functionalities at low temperatures.     

Thermal and Sc(OTf)3 catalyzed 1,3-dipolar cycloaddition of open-chain nitrones to α,β-unsaturated lactones: combined experimental and computational studies

The stereoselectivity of 1,3-dipolar cycloaddition reactions of C-phenyl open-chain nitrones and α,β-unsaturated γ- and δ-lactones was investigated under thermal and catalytic conditions. It was found that under thermal conditions, the endo approach of the reactants was preferred leading to the thermodynamic product. In the presence of Sc(OTf)₃ the exo adduct was obtained in high yield and selectivity. The energies of the cycloaddition reactions were investigated by means of molecular orbital calculations at the B3LYP/6-31+G(d,p) and MP3/6-31+G(d,p) theory level. Different reaction channels and reactant approaches, fitting the individual regio- and stereochemical preferences, are discussed. The computational results were compared with the corresponding experimental data and found to be in good agreement.     

Hybridorubrins A–D: Azaphilone Heterodimers from Stromata of Hypoxylon fragiforme and Insights into the Biosynthetic Machinery for Azaphilone Diversification

The diversity of azaphilones in stromatal extracts of the fungus Hypoxylon fragiforme was investigated and linked to their biosynthetic machineries by using bioinformatics. Nineteen azaphilone-type compounds were isolated and characterized by NMR spectroscopy and mass spectrometry, and their absolute stereoconfigurations were assigned by using Mosher ester analysis and electronic circular dichroism spectroscopy. Four unprecedented bis-azaphilones, named hybridorubrins A–D, were elucidated, in addition to new fragirubrins F and G and various known mitorubrin derivatives. Only the hybridorubrins, which are composed of mitorubrin and fragirubrin moieties, exhibited strong inhibition of Staphylococcus aureus biofilm formation. Analysis of the genome of H. fragiforme revealed the presence of two separate biosynthetic gene clusters (BGCs) hfaza1 and hfaza2 responsible for azaphilone formation. While the hfaza1 BGC likely encodes the assembly of the backbone and addition of fatty acid moieties to yield the (R)-configured series of fragirubrins, the hfaza2 BGC contains the necessary genes to synthesise the widely distributed (S)-mitorubrins. This study is the first example of two distant cross-acting fungal BGCs collaborating to produce two families of azaphilones and bis-azaphilones derived therefrom.