Functionalised polyhedral oligomeric silsesquioxane with encapsulated fluoride - first observation of fluxional Si center dot center dot center dot F interactions in POSS

摘要

The synthesis of a styryl functionalised POSS incorporating an encapsulated fluoride ion within a (SiO1.5)(8) cage (T-8-F) is reported. It was characterised by single crystal XRD, MALDI-MS, FTIR, solution (Si-29, F-19, C-13, H-1) and solid state (Si-29, F-19) NMR. In the absence of H-1 decoupling, the Si-29 solution NMR spectrum exhibited a triplet of doublets. In contrast, H-1, F-19 and H-1/F-19 double-decoupling resulted in two, three and one signal, respectively, being consistent with a single Si site whose Si-29 NMR signal is modulated by both the proximal aromatic-ring protons and fluoride. The associated Si center dot center dot center dot F coupling constant (2.5 Hz) is substantially lower than expected for a covalent Si-F bond and arises from a fluxional Si center dot center dot center dot F covalent effect in which the F- interacts equivalently with all eight Si atoms. Additional variable temperature NMR studies demonstrated a threshold at -5 degrees C below which no Si center dot center dot center dot F interactions are observed, and above which an increasing Si center dot center dot center dot F covalent character occurs.