Polyhedral Oligomeric Silsesquioxane D-(RSiO).

While smaller polyhedral oligomeric silsesquioxanes TR (POSS) are readily accessible or even commercially available, unambiguously authenticated larger systems (n>12) have barely been reported. Synthesis and isolation procedures are lengthy, and yields are often very low. Herein, we present the surprisingly straightforward and high-yielding access to the phenyl-substituted derivative of a so far only postulated second D-symmetric T isomer and with that the largest crystallographically characterized POSS cage with organic substituents. Treatment of the commercially available incompletely condensed TPh(OH) silsesquioxane with catalytic amounts of trifluoromethanesulfonic acid results in high yields of the TPh framework, which is isolated in crystalline form by a simple work-up. D-TPh was analyzed by single crystal X-ray diffraction, multinuclear NMR spectroscopy and thermal analysis. The relative energies of all four theoretically possible TPh isomers were determined by optimization of the corresponding structure using DFT methods.