Oliver Presly

A Permanent Mesoporous Organic Cage with an Exceptionally High Surface Area

Recently, porous organic cage crystals have become a real alternative to extended framework materials with high specific surface areas in the desolvated state. Although major progress in this area has been made, the resulting porous compounds are restricted to the microporous regime, owing to the relatively small molecular sizes of the cages, or the collapse of larger structures upon desolvation. Herein, we present the synthesis of a shape-persistent cage compound by the reversible formation of 24 boronic ester units of 12 triptycene tetraol molecules and 8 triboronic acid molecules. The cage compound bears a cavity of a minimum inner diameter of 2.6 nm and a maximum inner diameter of 3.1 nm, as determined by single-crystal X-ray analysis. The porous molecular crystals could be activated for gas sorption by removing enclathrated solvent molecules, resulting in a mesoporous material with a very high specific surface area of 3758 m(2)  g(-1) and a pore diameter of 2.3 nm, as measured by nitrogen gas sorption.

[Au14(PPh3)8(NO3)4]: An Example of a New Class of Au(NO3)-Ligated Superatom Complexes†

Ultrasmall gold nanoparticles (usNPs) have been of considerable interest because of their unique properties, which arise from size quantization effects and thus make them attractive for applications in different areas of nanoscience and nanotechnology. 4] Among usNPs of defined composition, and especially for gold clusters, an outstanding so-called “magic” stability has been observed for certain compositions. This stability has been explained through their specific geometric or electronic structure. In the full-shell cluster model, an usNP is considered as a cut-out of the bulk fcc structure. 6] Accordingly, a central metal atom is surrounded by shells of closely packed metal atoms, each shell having (10n+2) atoms, n indicating the number of shells. Clusters possessing fully occupied shells consist of 13, 55, 147, ... atoms and are supposed to show high stability for geometric reasons, such as the well-studied Schmid cluster [Au55(PPh3)12Cl6]. [7] In another model, which goes back to the counting rules introduced by Mingos, metal clusters are considered as superatom complexes formulated as [LS·ANXM] z with electron-withdrawing ligands X or weak Lewis base ligands L attached to the core with metal atoms A and an overall core charge z. 10] Accordingly, for Au the 6s electrons are counted and corrected by the number of electrons that are located at electron-withdrawing ligands and corrected by the charge of the cluster. The remaining 6s electrons are placed within the binding delocalized orbitals of the cluster, which is analogous to valence electrons of the atomic theory. Exceptional stability is associated with a total number of: n* 1⁄4 2, 6, 12, 20, 30, . . . ð1Þ

Applications of leverage analysis in structure refinement

Leverages measure the influence that observations (intensity data and restraints) have on the fit obtained in crystal structure refinement. Further analysis enables the influence that observations have on specific parameters to be measured. The results of leverage analyses are discussed in the context of the amino acid alanine and an incomplete high-pressure data set of the complex bis(salicylaldoximato)copper(II). Leverage analysis can reveal situations where weak data are influential and allows an assessment of the influence of restraints. Analysis of the high-pressure refinement of the copper complex shows that the influence of the highest-leverage intensity observations increases when completeness is reduced, but low leverages stay low. The influence of restraints, notably those applying the Hirshfeld rigid-bond criterion, also increases dramatically. In alanine the precision of the Flack parameter is determined by medium-resolution data with moderate intensities. The results of a leverage analysis can be incorporated into a weighting scheme designed to optimize the precision of a selected parameter. This was applied to absolute structure refinement of light-atom crystal structures. The standard uncertainty of the Flack parameter could be reduced to around 0.1 even for a hydrocarbon.

Handling difficult samples: data-processing improvements

A research project will often define at its inception a material or family of closely related materials for study. Sometimes obtaining crystal structure(s) is deemed to be highly useful and of high importance, yet unforeseen problems in obtaining high quality diffraction data can lead to difficulties in realising project goals. This may be for a number of reasons such as sample sensitivities, a propensity for twinning or simply weak diffraction.