Bis(benzoxazol-2-yl)methanides in main group organometallic syntheses and enquiry-based education
Bis(benzoxazol-2-yl)methanides in main group organometallic syntheses and enquiry-based education
by Nico Graw
Date of Examination:2022-09-29
Date of issue:2023-02-27
Advisor:Prof. Dr. Dietmar Stalke
Referee:Prof. Dr. Dietmar Stalke
Referee:Prof. Dr. Anna Krawczuk
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Abstract
English
The dissertation entitled “Bis(benzoxazol-2-yl)methanides in main group organometallic syntheses and enquiry based education” constitutes three main chapters. Each chapter concerns a different project the author worked on during his PhD studies. The first chapter is dedicated to novel X-ray diffraction instrumentation and its application for single crystal X-ray diffraction using the characteristic X-ray radiation of indium (λ(In Kα)=0.51340 Å). After a short summary of the development of instrumentation for single crystal X-ray diffraction (SC–XRD) since the discovery of X-rays by C. W. Röntgen, the work principle and potential advantages of the novel MetalJet X-ray source from Excillum are discussed. Because the source uses a liquid metal alloy anode made from gallium and indium, there is no risk of melting. Therefore, higher energy loads are tolerable resulting in a more intense and more focussed beam than in conventional electron impact sources. This source had been used for SC–XRD studies before employing characteristic gallium radiation. However, the indium Kα radiation that by design is also emitted from the source, has a shorter wavelength than all other commercially available in-house X-ray sources. This reduces absorption effects during measurements and allows for data collection up to a higher theoretical resolution limit. The aim of this work was therefore to evaluate if indium Kα radiation from a MetalJet source can be used for SC–XRD. The source used was implemented in a Bruker diffractometer with a Photon II detector and Montel optics optimised for the indium Kα wavelength from Incoatec. This setup was carefully aligned, and first test measurements were performed. As expected, the X-ray beam obtained was contaminated with gallium Kα radiation. To quantify and minimise this spectral impurity, experiments using a test crystal and variable amounts of aluminium attenuation were conducted and evaluated by means of crystallographic twin refinements. Afterwards it became apparent that another impurity was present, which could be identified to be tin Kα radiation. Discussions with the manufacturer revealed that tin was also a component of the used alloy. Experiments were performed to also attenuate the tin contamination using palladium foil. However, due to the very small energy difference between characteristic indium and tin radiation, efficient attenuation of the Sn Kα was only possible by also reducing the intensity of the indium radiation significantly as well. In an attempt to circumvent this problem, the influence of the Montel optics was studied but was shown to be minimal. Discussions with the manufacturer finally led to the sourcing of a new, tin-free alloy which could also be used in the MetalJet source. With the whole diffraction setup optimised for this purpose, measurements o a set of test crystals were performed and compared to a well-established in-house diffractometer using a silver X-ray source since this is the closed to indium radiation in terms of wavelength. All datasets were collected up to high resolution and were analysed in terms of data quality (Rint, Rrim, ). Independent atom model refinements were performed for all datasets and the quality (R1, wR2, residual density) of the final structural models was compared. The outcome of this study showed that a In Kα radiation from a MetalJet X-ray source has close to no benefit over established silver X-ray sources for standard SC–XRD experiments. The advantage of this instrumentation becomes only clear if more sophisticated experiments are performed such as diffraction on extremely small crystals, high-pressure crystallography using diamond anvil cells or collection of high-resolution datasets for multipolar refinements. In the second chapter of this dissertation a project committed to improve visuospatial thinking skills in students with a focus on chemical crystallography is presented. As a basis a skill course was developed combining information and hints on how to properly draw chemical structures. This included basic rules of Lewis diagrams but also covered different ways to convey stereochemical information such as for example wedge-and-dash notation or Newman projections. The guide then discusses cyclic systems and their most common conformations, aromatic systems and finally also polyhedra as structural motifs. In a second step, physical models of symmetry elements were designed and built, which can be used during a lecture or seminar, and which allow the objects on which the symmetry operation is to be performed to be assembled freely and stepwise. As a final step a design principle for tangible models of crystallographic space groups was devised. While lattice models for archetype compounds such as sodium chloride or quartz are commonly available, models addressing space group symmetry have not been described before. These models can be visually linked to the conventional 2D notations used by the International Union of Crystallography and as with the models of singular symmetry elements they allow to assemble the chemical content of the unit cell manually and stepwise to emphasise how a solid-state structure is determined by symmetry. The third chapter of this dissertation is dedicated to synthesis. Bis(benzoxazole-2-yl)methanes, which have been shown before to be a good platform for the synthesis of s-, d- and f-block complexes, were used to synthesise tetrylenes that can be understood as heavier analogous of N-heterocyclic carbenes. Germylenes, stannylenes and plumbylenes were prepared by either salt elimination or coupled deprotonation-metalation reactions from the respective group 14 halide or amide precursors. The obtained tetrylenes were characterised and chosen examples were used to study their reactivity. Additionally, the reactivity of bis(benzoxazole-2-yl)methanide in the form of its potassium salt towards antimony trichloride was studied. Surprisingly, this reaction afforded highly symmetric 2,2,4,4,5,5-hexa(4-methylbenzoxazol-2 -yl)-1,3-distibabicyclo[1.1.1]pentane, the structure of which could be confirmed by SC–XRD. While bicyclo[1.1.1] pentanes are known with 1 and 3 positions substituted by group 14 elements, no experimental evidence had been reported for group 15 elements so far. Additionally and in contrast to the known bicyclo[1.1.1]pentanes, the compound reported herein shows a remarkable stability towards oxygen and water and can be handled under ambient atmosphere. Since the compound is virtually insoluble, it was further studied using powder X-ray diffraction and the bonding situation was analysed in detail according to the QTAIM formalism using theoretically derived electron density distributions for the gas as well as the solid-state.
Keywords: X-ray diffraction; Tetrylene; main group; chemistry; indium radiation; crystallography