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), T-n) is an area of significant activity. To date, T-14 is the largest such cage synthesized and isolated as a single isomer. Herein, we report an unprecedented, single-isomer styryl-functionalized T-18 POSS. Unambiguously identified among nine possible isomers by multinuclear solution NMR (H-1, C-13, and Si-29), MALDI-MS, FTIR, and computational studies, this is the largest single-isomer functionalized T-n compound isolated to date. A ring-strain model was developed to correlate the Si-29 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 T-n compounds, demonstrating its general applicability for assigning Si-29 resonances to Si atoms in cage silsesquioxanes and providing a useful tool for predicting Si-atom environments.