Functionalised Polyhedral Oligomeric Silsesquioxane with Encapsulated Fluoride – First Observation of Fluxional Si…F Interactions in POSS

The synthesis of a styryl functionalised POSS incorporating an encapsulated fluoride ion within a (SiO1.5)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 SiF coupling constant (2.5 Hz) is substantially lower than expected for a covalent Si-F bond and arises from a fluxional SiF 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 SiF interactions are observed, and above which an increasing SiF covalent character occurs.