Structure Determination of Catecholato-Silanes in Different Phases and by Different Methods

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We propose to study the structure of a series of catecholatosilanes in the gaseous, solid and solution state by a variety of methods including gas electron diffraction, single crystal and powder X-ray diffraction, CP/MAS-NMR as well as DOSY-NMR spectroscopy and ESI mass spectrometry. The finding of extreme Lewis acidities for perhalogenated bis(catecholato)silanes has very recently drawn the attention to this class of compounds. The fluoride ion affinity of the perchloro-derivative even exceeds that of antimony pentafluoride and thus it qualifies as a Lewis superacid. Simple bis(catecholato)silanes and tris(catecholato)silicates, in contrast, are known for almost 100 years. However, there is a distinct paucity in structural information, because many of these compounds solidify glassy and can adopt various forms of aggregation. There were even speculations about planar tetra-coordinate silicon configurations in the past. In order to further enhance the Lewis acidity of silicon based Lewis acids, and to improve their catalytic efficiencies, insights on the structure of the isolated monomeric units (obtained in the proposed study via GED), the possible occurrence of higher aggregates (DOSY-NMR in solution, ESI-MS) and their interaction in the solid state (single crystal or powder XRD and CP/MAS-NMR) are needed. The planned experiments will provide sufficient and redundant information that allows an unequivocal statement about the structure of the class of bis(catecholato)silanes in different environments. The study of a range of compounds with a variety of substituents differing in steric bulk and electron-withdrawing character will give insights into the bonding situation which determine the acidic properties and aggregation preferences. The new data will thus clarify a longstanding debate and provide a basis for future developments. The preparative work and spectroscopic investigations will be undertaken at the University of Heidelberg, gas phase structure determination will be undertaken within the core facility GED@BI (Centre of Gas Electron Diffraction and Small Molecule Structures) at Bielefeld University.