Large polyhedral oligomeric silsesquioxane cages : the isolation of functionalized POSS with an unprecedented Si18O27 core

The synthesis of organo-functionalized polyhedral oligomeric silsesquioxanes (POSS, (R-SiO1.5)n, Tn) is an area of significant activity. To date, T14 is the largest such cage synthesized and isolated as a single isomer. Herein, we report an unprecedented, single-isomer styryl-functionalized T18 POSS. Unambiguously identified among nine possible isomers by multinuclear solution NMR (1H, 13C, and 29Si), MALDI-MS, FTIR, and computational studies, this is the largest single-isomer functionalized Tn compound isolated to date. A ring-strain model was developed to correlate the 29Si resonances with the number of 6-, 5-, and/or 4-Si-atom rings that each non-equivalent Si atom is part of. The model successfully predicts the speciation of non-equivalent Si atoms in other families of Tn compounds, demonstrating its general applicability for assigning 29Si resonances to Si atoms in cage silsesquioxanes and providing a useful tool for predicting Si-atom environments.