Christopher T. Burns, PhD, University of Louisville
During the second year of this project we have devoted our efforts to the synthesis of hydrolytically stable organosilica precursors and their corresponding PMOs that incorporate bis(methylene)aryl spacers. We first addressed the synthesis of two hydrolytically stable bis(triallyl)organosilanes, 1,4-bis(triallylsilylmethylene)benzene and 4,4’-bis(triallylsilylmethylene)biphenyl. These were prepared by lithiation of 1,4-dibromomethylenebenzene or 4,4’-dibromomethylenebiphenyl at low temperature with excess n-butyllithium followed by quenching with chlorotriallylsilane. A second route that was uncovered involved the reaction of the dibromomethylarene with trichlorosilane in the presence of triethylamine. After isolation, the bis(trichlorosilylmethylene)arene was reacted with an excess of allylmagensium bromide. The resulting bis(triallylsilylmethylene)arenes were purified using column chromatography and both were isolated in good yields as analytically pure compounds. The synthesis of PMOs incorporating the dimethylbenzene and dimethylbiphenyl units was conducted under basic conditions using the ionic tetraalkylammonium halide, octadecyltrimethylammonium chloride (C18TMACl), as the surfactant template. After the formation of the PMO was complete, the surfactant was removed from the pores of the silica by refluxing the PMO in weakly acidic ethanol. We next investigated the preparation of a hydrolytically stable bis(triallyl)organosilane that incorporated a pyridine molecule. We prepared 4,4’-bis(bromomethyl)-1,1’-bipyridine by radical bromination of 5,5’-dimethyl-2,2’-bipyridine using N-bromosuccinimide and AIBN. The 4,4’-bis(bromomethyl)-1,1’-bipyridine was converted to 4,4’-bis(trichlorosilylmethyl)-1,1’-bipyridine using trichlorosilane in the presence of triethylamine. After isolation, the 4,4’-bis(trichlorosilylmethyl)-1,1’-bipyridine was reacted with an excess of allylmagensium bromide to form the desired 4,4’-bis(triallylsilylmethyl)-1,1’-bipyridine. The 4,4’-bis(triallylsilylmethyl)-1,1’-bipyridine was purified using column chromatography and isolated in good yield as analytically pure compound. The synthesis of a PMO incorporating the 4,4’-bis(triallylsilylmethyl)-1,1’-bipyridine was conducted under basic conditions using the ionic tetraalkylammonium halide, octadecyltrimethylammonium chloride (C18TMACl), as the surfactant template. Unfortunately, we have been unable to successfully prepare an ordered PMO that incorporates our 4,4’-bis(silylmethyl)-1,1’-bipyridine unit. We are currently investigating other reaction conditions that could be used to synthesize an ordered material. We are also investigating the use of these bissilane compounds to make thin films that could be used in catalysis and energy applications. An alternate route for the synthesis of bis(allylsilyl) monomers that avoids alkoxysilane precursors is an attractive goal because the high cost of the alkoxysilyl compounds could limit the commercial viability of the resultant solids and thin films. Access to a larger library of PMO precursors is crucial as periodic mesoporous materials are certain to find new uses in many diverse applications.
We have also developed an efficient synthetic methodology for preparation of 2-phosphinimine-5-methylbenzenesulfonate ligands via the Staudinger reaction between a phosphine and n-propyl 2-azido-5-methylbenzenesulfonate followed by sulfonate ester deprotection. This new route directly accesses ortho-substituted-arenesulfonates that incorporate a phosphinimine, a strong s donor, creating a nonsymmetric strong/weak donor ligand which we hav recently reported (Tetrahedron Lett, 2012, 53, 4832-4835). We are investigating the synthesis of neutral Pd(II) alkyl complexes incorporating our 2-phosphinimine-5-methylbenzenesulfonate ligands as catalysts for olefin polymerization.
This ACS-PRF DNI award has significantly impacted the early stages of the career of the PI. From an instructional perspective, the PI has incorporated several concepts that are central to this research project, transition metal based catalysis and 31P, 11B, 19F NMR into senior undergraduate synthesis and spectroscopy course I have taught over the past two years. It has provided funds to support one Ph.D. student in my research laboratory who can focus solely on his research without the outside distractions of teaching responsibilities. The graduate student is currently in his final year and is finishing up his work and preparing several manuscripts for publication. An undergraduate student who worked on this project is currently a graduate student in a chemistry PhD program.