Evidence of the Lewis-Amphoteric Character of Tris(pentafluoroethyl)silanide, [Si(C2 F5 )3 ]

Abstract

According to a first view on the geometrical and electronic structure of the tris(pentafluoroethyl)silanide, this anion appears as a Lewis base. Quantum chemical calculations on perfluoroalkylated silanides show significantly lower HOMO and LUMO energy levels in comparison to their non-fluorinated counterparts, which implies reduced Lewis basicity and increased Lewis acidity of the [Si(C₂ F₅ )₃ ]^- ion. With these findings and a HOMO-LUMO gap of 4.80 eV similar to N-heterocyclic silylenes (NHSis), perfluoroalkyl silanides are predestined to exhibit Lewis-amphoteric character similar to silylenes. Deprotonation of Si(C₂ F₅ )₃ H with sterically demanding phosphazene bases afforded thermally stable phosphazenium salts of the [Si(C₂ F₅ )₃ ]^- anion, which add to benzaldehyde, benzophenone, CS₂ , and CO₂ in various manners. This behavior also mirrors the reactivity of silylenes towards ketones as well as heterocumulenes and is rationalized by Lewis amphotericity being inherent in these silanides.

Keywords: Lewis amphoteric; activation of small molecules; perfluoroalkyl; silanides; weakly coordinating cations.