Fluorine-free Lewis acids: Extreme Lewis acidities based on bulky and strongly electron-withdrawing carborane substituents

• Applicant: Professor Dr. Norbert W. Mitzel
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 557181584

Lewis acids (LA) are fundamentally important building blocks of chemistry. They are widely used in synthesis, catalysis, molecular recognition and supramolecular chemistry. In the recent decade, more and more strong molecular Lewis acids have been developed, the so-called Lewis superacids (LSA), which are by definition more acidic than antimony pentafluoride. Most LSAs derive their high acidity from the strong electron-withdrawing effect of highly fluorinated substituents on atoms that are in principle capable of forming Lewis acidity. However, per- and polyfluoroalkyl substances (PFAS) are persistent and partly toxic compounds for which a far-reaching ban is currently being discussed due to these problematic properties. The aim of this project is to utilize the strong electron-withdrawing properties of ortho-carboranyl (oCb) ligands to generate new fluorine-free LA (or even LSA) or new strong Lewis acid functions. To this end, ortho-carborane-substituted compounds of elements capable of acting as LA functions will be prepared, specifically: Al, Ga, In, Si, Sn and Sb (specific examples include Ga(oCb)3, (oCb)3SiH or (oCb)2SbCl). Comparative studies of these compounds will be used to quantify the electron-withdrawing properties of ortho-carboranyl ligands. The comparison will be based on: Lewis acidity measurements and calculations, spectroscopic properties, structural parameters and electronic properties from quantum mechanical calculations. These findings will be used to determine the elements to which bound ortho-carboranyl ligands are particularly well suited to realize their special properties in electron withdrawal and steric requirements for advantageous use in the chemistry of LA or Lewis acidic functions. One aim is to determine how many of the bulky ortho-carboranyl ligands can be bound to specific atoms in order to maximize their effect. A combination of ortho-carboranyl ligands with other functions (halogens, hydrides) on the respective atoms should result in new starting substances for further functionalization. Halogen functions then allow further nucleophilic substitution, while hydride functions are suitable for hydrometallation reactions. The findings will be used to provide new strong Lewis acids/functions, whose properties will be demonstrated in Lewis acid catalysis and new poly-Lewis acids. The findings of this project should result in concrete proposals for the developmnt of a fluorine-free Lewis (super) acid chemistry for a focused follow-up project.

• DFG Programme: Research Grants