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 (SiO 1.5 ) 8 cage (T 8 -F) is reported. It was characterised by single crystal XRD, MALDI-MS, FTIR, solution ( 29 Si, 19 F, 13 C, 1 H) and solid state ( 29 Si, 19 F) NMR. In the absence of 1 H decoupling, the 29 Si solution NMR spectrum exhibited a triplet of doublets. In contrast, 1 H, 19 F and 1 H/ 19 F double-decoupling resulted in two, three and one signal, respectively, being consistent with a single Si site whose 29 Si 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.