Lithium-Templated Formation of Polyhedral Oligomeric Silsesquioxanes (POSS).

A coordination complex, lithium hepta(-butyl)silsesquioxane trisilanolate (; Li-T), a stable intermediate in silsesquioxane (SQ) syntheses, was successfully isolated in 65% yield and found to be highly soluble in nonpolar solvents such as hexane. The structure of Li-T was confirmed by NMR, IR spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, electrospray ionization mass spectrometry, and computational simulation, providing detailed elucidation of the intermolecular self-association of the SQ cage with a box-shaped LiO polyhedron through strong coordination bonds. After acid treatment, Li-T undergoes lithium-proton cationic exchange, yielding hepta(-butyl)silsesquioxane trisilanol (; H-T) quantitatively. The high yield of H-T seems to be influenced by Li-O bonding in the Li-T complex that affects the selective formation of hepta(-butyl)silsesquioxane trisilanolate and the bulky -butyl groups which may prevent decomposition or SQ cage-rearrangement even at reflux under alkaline conditions. Single-crystal X-ray crystallography confirms the presence of the dumbbell-shaped SQ partial cages through strong intermolecular hydrogen bonds. Interestingly, lowering the polarity of the reaction solution by adding dichloromethane results in formation of the cubic octa(-butyl)silsesquioxane (; T) cage in a good yield (47%), which is isolated by crystallization from the reaction solution.