Functionalised polyhedral oligomeric silsesquioxane with encapsulated fluoride – first observation of fluxional Si⋯F interactions in POSS

Abstract

The synthesis of a styryl functionalised POSS incorporating an encapsulated fluoride ion within a (SiO_1.5)₈ cage (T₈-F) is reported. It was characterised by single crystal XRD, MALDI-MS, FTIR, solution (²⁹Si, ¹⁹F, ¹³C, ¹H) and solid state (²⁹Si, ¹⁹F) NMR. In the absence of ¹H decoupling, the ²⁹Si solution NMR spectrum exhibited a triplet of doublets. In contrast, ¹H, ¹⁹F and ¹H/¹⁹F double-decoupling resulted in two, three and one signal, respectively, being consistent with a single Si site whose ²⁹Si NMR signal is modulated by both the proximal aromatic-ring protons and fluoride. The associated Si⋯F coupling constant (2.5 Hz) is substantially lower than expected for a covalent Si–F bond and arises from a fluxional Si⋯F covalent effect in which the F⁻ interacts equivalently with all eight Si atoms. Additional variable temperature NMR studies demonstrated a threshold at −5 °C below which no Si⋯F interactions are observed, and above which an increasing Si⋯F covalent character occurs.