Derivatives of the pentafluoroorthotellurate group (OTeF5) bearing fluorinated and non-fluorinated aryl substituents have been developed as ligand systems for the synthesis of strong Lewis acids and weakly coordinating anions. The acid cis –PhTeF4OH was obtained on a gram scale and subsequently converted into Ag[cis –PhTeF4O], which served as an efficient cis –PhTeF4O transfer reagent, for instance, in the preparation of [PPh4][cis –PhTeF4O]. More importantly, the synthesis of trans –(C6F5)2TeF3OH (abbreviated as HOTeR) was achieved by fluorination of (C6F5)2Te with the TCICA/KF system to give trans –(C6F5)2TeF4, followed by selective hydrolysis in a MeCN/H2O mixture. Quantum-chemical calculations revealed that HOTeR possesses both a higher acidity and an enhanced resistance towards fluoride abstraction compared to cis –PhTeF4OH, and was therefore selected as building block for further syntheses. The boron-based Lewis acid B(OTeR)3 was synthesized by reacting HOTeR with either BCl3 or BCl3 ·SMe2. This compound exhibits exceptional thermal stability up to 300 °C and represents one of the most sterically encumbered boron centers reported to date, as evidenced by a buried volume analysis. Theoretical and experimental investigations showed that B(OTeR)3 possesses a Lewis acidity comparable to that of the well-known B(C6F5)3. The affinity of B(OTeR)3 towards pyridine was determined by isothermal titration calorimetry (ITC), which resulted to be slightly lower compared to B(OTeF5)3 and B(C6F5)3. The ligand-transfer reactivity of B(OTeR)3 was demonstrated by its reaction with fluoride precursors, affording the Au(III) complex [PPh4][(CF3)3Au(OTeR)] and a hypervalent iodine(III) species. Lastly, investigations on the aluminum analogue Al(OTeR)3 revealed that it is a Lewis superacid according to fluoride ion affinity (FIA) calculations. Complementary analysis using the Gutmann–Beckett method via formation of the Al(OTeR)3·OPEt3 adduct rendered similar results, yet for the heavier Ga analogue only GaEt(OTeR)2 · OPEt3 was obtained. The isolation of free Al(OTeR)3 proved challenging due to its intrinsic reactivity towards fluoride abstraction from its own ligands, as shown by quantum-chemical calculations. As a consequence, the Lewis superacid was stabilized as acid-base adducts with tetrahydrofuran and dimethyl carbonate, which remained synthetically useful. Derived from the Al(OTeR)3 moitey, 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 versatile silver salt Ag[(F5TeO)Al(OTeR)3] stands out, which could be used to generate a strong Brønsted acid as well as the [Ph3C]+ cation. Electrostatic potential surface analysis shows improved charge delocalization and oxygen shielding in [(F5TeO)Al(OTeR)3]– compared to [FAl(OTeR)3]– and [Al(OTeF5)4]– .
