Analysis on the caged structure of polyhedral oligomeric dodecaphenyl silsesquioxane and its condensation mechanism

Polyhedral oligomeric dodecaphenyl silsesquioxane (DPS) and octaphenyl silsesquioxane (OPS) are two kinds of caged inorganic-organic hybrids and high thermal stable compounds. Concerning the caged structure of DPS, there remain some inconformity among researchers’ conclusions. Some researchers hold that the structure is in the shape of a gem containing 4 tetragons and 4 pentagons, while other articles consider the structure to be a cyclic ladder with 6 tetragons and 2 hexagons. In this paper, the correct structure of DPS is found to be gem‑like, deduced by detailed NMR spectra analysis and Euler's formula calculation, and proved by a single crystal diffraction pattern. A simulation calculation by Materials Studio (MS) proves that the gem‑like structure (E_total=41.57 kcal·mol^-1) is more stable than the cyclic ladder-like one (E_total=43.37 kcal·mol^-1). For the DPS and OPS, the distances between adjacent phenyl groups are considered to influence the chemical shifts of H, C and Si atoms in NMR due to shielding effects, deshielding effects and electron densities. The cyclic tetramer with E_total=20.40kcal·mol^-1 is proved to be the stable cyclic oligomeric phenyl silsesquioxanes with the smallest polymerization degree by the simulations. The linear dimer with E_total=14.49 kcal·mol^-1 is proved to be the stable linear oligomeric silsesquioxanes with the smallest polymerization degree. A condensation mechanism of DPS and OPS is proposed based on the simulation results.