Oktaedrische versus trigonalprismatische Umlagerungen von MoF6\- und WF6-Derivaten

Title and Table of contents Introduction Theoretical Background, Results, and Discussion Experimental Section Conclusions and Outlook Summary Zusammenfassung References Appendix I and IIDFT calculations performed on MoF6 and WF6 revealed that the octahedral- trigonal prismatic-octahedral rearrangement (Oh-D3h-Oh) has a relatively low energy barrier (6-10 kcal mol^(-1)). Experimental evidence using dynamic 19F NMR spectroscopy on complexes of the type F5M-OR where M = Mo or W and R = -CH2CF3, -C6F5, and -C(CF3)3 confirmed these predictions. At room temperature or below these complexes present in the 19F NMR spectroscopy an AB4-type spectrum typically for octahedral structures. At higher temperatures the non- equivalent metal-bonded fluorine atoms undergo exchange. Experimental evidence supports the intramolecular exchange for such species. The 19F NMR spectra at different temperatures for all studied complexes were simulated successfully using the program gNMR. Two intramolecular exchange mechanisms can be used for the simulations, namely a 3:3 process (Bailar twist) and the 2:4 mechanism. Both mechanisms are experimentally undistinguishable, from theoretical calculations the preferred mechanism is the Bailar twist. Theoretical calculations on species of the type F5M-C6X5 (M = Mo or W, X = H or F) predicted in most of the cases a trigonal prismatic geometry as the ground state. Attempts to synthesize such complexes, as well as species of the type F5M-SCF3 (M = Mo or W) were undertaken. From theoretical calculations on the latter even a lower energy barrier for the Oh-D3h-Oh rearrangement than for the corresponding alkoxy and phenoxy derivatives is predicted. Attempts to synthesize such complexes were unsuccessful.DFT-Rechnungen von MoF6 und WF6 zeigen, dass die Strukturumwandlung Oktaeder- Trigonales Prisma-Oktaeder (Oh-D3h-Oh) eine niedrige Energiebarriere aufweisst (6-10 kcal mol^(-1)). Experimentelle Untersuchungen mit Hilfe der dynamischen 19F-NMR-Spektroskopie an Komplexen des Typs F5M-OR mit M = Mo oder W und R = -CH2CF3, -C6F5, und -C(CF3)3 bestätigen diese Vorhersagen. Diese Komplexe zeigen unterhalb Raumtemperatur 19F-NMR-Spektren von AB4-Typ, die typisch für oktaedrische Geometrie sind. Oberhalb Raumtemperatur tauschen die an das Metall gebundenen nicht äquivalenten Fluoratome aus. Experimentelle Hinweise unterstützen einen intramolekularen Austausch in diesen Molekülen. Die 19F- NMR-Spektren bei verschiedenen Temperaturen wurden mit dem gNMR-Programm simuliert. Zwei intramolekulare Austauschmechanismen, ein 3:3-Prozess (Bailar Twist) und ein 2:4-Prozess, wurden der Simulation zugrunde gelegt. Beide Mechanismen sind experimentell nicht unterscheidbar, seitens des theoretischen Berechnungen ist der Bailar Twist-Prozess der bevorzugte Mechanismus. DFT- Rechnungen der Moleküle des Typs F5M-C6X5 (M = Mo oder W, X = H oder F) sagen für die meisten Komplexe als Grundsustand eine trigonal-prismatische Struktur voraus. Es wurden Versuche unternommen, solche Komplexe und Verbindungen des Typs F5M-SCF3 (M = Mo oder W) zu synthetisieren. Die Berechnungen der F5M- SCF3-Komplexe ergibt eine niedrigere Energibarriere für die Oh-D3h-Oh- Umwandlung als für die entsprechenden Alkoxo- und Phenoxokomplexe. Leider gelang es nicht, diese Verbindungen darzustellen

Electron crystallography and dedicated electron-diffraction instrumentation

Electron diffraction (known also as ED, 3D ED or microED) is gaining momentum in science and industry. The application of electron diffraction in performing nano-crystallography on crystals smaller than 1 µm is a disruptive technology that is opening up fascinating new perspectives for a wide variety of compounds required in the fields of chemical, pharmaceutical and advanced materials research. Electron diffraction enables the characterization of solid compounds complementary to neutron, powder X-ray and single-crystal X-ray diffraction, as it has the unique capability to measure nanometre-sized crystals. The recent introduction of dedicated instrumentation to perform ED experiments is a key aspect of the continued growth and success of this technology. In addition to the ultra-high-speed hybrid-pixel detectors enabling ED data collection in continuous rotation mode, a high-precision goniometer and horizontal layout have been determined as essential features of an electron diffractometer, both of which are embodied in the Eldico ED-1. Four examples of data collected on an Eldico ED-1 are showcased to demonstrate the potential and advantages of a dedicated electron diffractometer, covering selected applications and challenges of electron diffraction: (i) multiple reciprocal lattices, (ii) absolute structure of a chiral compound, and (iii) R-values achieved by kinematic refinement comparable to X-ray data