An Aluminum-Based Lewis Superacid and Its Weakly Coordinating Anions Derived from an Organotellurium Ligand

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.