Symmetric Functionalization of Polyhedral Phenylsilsesquioxanes as a Route to Nano-Building Blocks.

Abstract

The design and synthesis of nanometer scale structures is of intense current interest. Herein we report on the ability to use symmetric, robust, mutable silsesquioxane ([RSiO3/2]n) nano-building blocks to produce well-defined 3-D structures for electronic or adsorption applications. We are able to show the systematic effects of supermolecular coordination to modulate the density of the molecular packing. This dissertation describes the synthesis of the elusive decaphenylsilsesquioxane, and exploration of the substitutionally specific para iodination of the octa-, deca- and dodeca-(p-iodophenyl)-silsesquioxanes, whose single crystal X-ray diffraction structures are reported. Octa(p-iodophenyl)-silsesquioxane shows supermolecular coordination via Desiraju’s halogen-halogen short-contact synthon, forming an open structure with a solvent accessible cavity comprising 40% of the unit cell. The application of palladium, nickel and copper catalyzed cross-coupling techniques using the carbon-iodine bond is explored in order to divergently synthesize crystalline derivatives. These derivatives include the octa(diphenylacetylene)silsesquioxane and the octa(hexaphenylbenzene)silsesquioxane (56 Aryl), whose single crystal X-ray diffraction structures are reported. We show that 56 Aryl, which contains more carbon atoms than any other discrete molecule in the Cambridge Structural Database, crystallizes into an extremely open structure with a solvent accessible cavity comprising 55% of the volume. The supermolecular ordering driven by the bulky hexaphenylbenzene moieties gives nanometer-scale channels along the ab plane. Substitutional specificity is explored in the bromination of octaphenylsilsesquioxane (OPS), and single-crystal X-ray diffraction structures are reported for the octa-, hexadeca- and tetraicosa-brominated derivatives. Precise synthetic control is demonstrated by the unique catalyst-free bromination of OPS, providing the octa(o-bromophenyl)-silsesquioxane in low yield. An iron tribromide catalyzed hexadeca-bromination gives the crystalline octa(2,5-dibromophenyl)- silsesquioxane in good yield, with a high density of 2.3 g/cc. Finally, the iron tribromidecatalyzed tetraicosa-bromination of OPS is described, producing a low yield of crystals with a solid solution of substitution patterns coordinated by bromine-bromine short contacts. Lastly, Scholl-type dehydrogenative cyclization of the octa(hexaarylbenzene)silsesquioxane systems is explored by the addition of iron trichloride/nitromethane to a refluxing solution of the silsesquioxane in stannic chloride/dichloromethane. The products are analyzed by Proton Nuclear Magnetic Resonance Spectroscopy, Matrix Assisted Laser Desorption and Ionization – Time of Flight Mass Spectroscopy and Diffuse Reflectance Fourier Transform Infra Red Spectroscopy.