Polydentate Lewis Acid Acceptor Bowls and Chalices

• Applicant: Professor Dr. Norbert W. Mitzel
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 424957011

Supramolecular chemistry deals with the association of molecules to larger structures. These associations are not connected by covalent bonds, but by weaker, reversible interactions. Lewis acid-base interactions have already been studied in great detail, but mostly on complex scaffolds with several base functions that complex relatively simple acid particles (mostly cations). Selectivity is achieved by the spatial arrangement of the base functions. The inverse situation of poly-Lewis acids has only been very poorly investigated so far. The reasons for this lie in the high sensitivity of these compounds to air and water, the larger number of bonds at the Lewis acid centres and the larger changes in the receptor structure during the formation of Lewis acid-base complexes.However, the concept of poly-Lewis acids with defined spatial structure of the acid functions allows the recognition and complexation of anions and Lewis bases as well as the construction of supramolecular architectures. Properties and reactivity of the base fraction can thus be specifically influenced. The aim of this project is therefore to develop a series of new Lewis acid groups and to position them in a defined way on rigid molecular frameworks alongside known functions. In this way, we want to present systems for the optimal binding of Lewis-base substrates. New acid groups are those with strong electron withdrawing substituents (perchloro-catecholate, perfluoroalkyl and perfluoroaryl, possibly combined) at silicon or boron functions. Perfluoroarylboryl, dialkylaluminium and -gallium groups have already been tested. In the run-up to the project, these groups are to be classified systematically and quantitatively on the basis of simple model compounds with regard to their Lewis acid properties. As rigid donor-free and easy to functionalize frameworks, we want to use tribenzotriquinacene derivatives on the one hand, which allow three or six substituents to be attached on both sides in a bowl-like or chalice-like manner, which can then be functionalized with Lewis acids. The second subproject concerns the development of chalice-shaped tridentate Lewis acids by hydrometallation of planar precursors. We recently observed a reaction in which the hydrosilylation of hexadehydrotribenzo[12]annule, a planar ring-shaped molecule with three alkyne functions, leads to selective C3-symmetric triple addition with folding of all benzene rings to one side. The functionalization of the backbone and the spatial orientation of the new Lewis acid centers are therefore concerted. We want to test the chelating Lewis-acid systems with regard to their complexing properties and the selective recognition of multiple Lewis-based substrates. We will also try to build up larger cage-like associates with suitable bases.

• DFG Programme: Research Grants