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Published on: November 12, 2016
An Aluminum-Based Lewis Superacid and Its Weakly Coordinating Anions Derived from an Organotellurium Ligand
Daniel Wegener1, Niklas Limberg1, Moritz Bubenik1
1Fachbereich Biologie, Chemie, Pharmazie, Institut für Chemie und Biochemie - Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34/36, Berlin 14195, Germany.
Abstract:
Lewis acids and their related weakly coordinating anions (WCAs) are central species in chemistry, and tuning their properties toward different purposes is still a challenging field of research. In this work, the properties of the Lewis acid Al-(OTeR)3 have been investigated using the advantages of the OTeF3(C6F5)2 ligand (OTeR). Its Lewis acidity was evaluated by means of fluoride ion affinity (FIA) calculations, indicating that it is a Lewis superacid. Complementary analysis using the Gutmann-Beckett method by the synthesis of the Al-(OTeR)3·OPEt3 adduct rendered similar results, yet for the heavier Ga species this adduct formation was not possible, as only GaEt-(OTeR)2·OPEt3 was obtained. The reported new Lewis acid was further stabilized as acid-base adducts with tetrahydrofuran and dimethyl carbonate. The isolation of the free Lewis acid proved challenging due to fluoride abstraction from its own ligand, as shown by quantum-chemical calculations. Derived from the Lewis superacid Al-(OTeR)3 two weakly coordinating anions, the fluoride adduct [FAl-(OTeR)3]- and the even less coordinating mixed anion [(F5TeO)-Al-(OTeR)3]-, were synthesized. Among them, the synthetically useful silver salt Ag-[(F5TeO)-Al-(OTeR)3] stands out, which could be used to generate a strong Brønsted acid by reaction with HCl as well as the [Ph3C]+ cation via reaction with Ph3CCl. Electrostatic potential surface analysis confirmed the more efficient delocalization of the negative charge and enhanced shielding of oxygen atoms in [(F5TeO)-Al-(OTeR)3]- compared to [FAl-(OTeR)3]- and [Al-(OTeF5)4]-, and therefore its potential as a promising new WCA.
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