Christopher T. Burns, PhD , University of Louisville
The proposed research seeks to use periodic mesoporous organosilicas (PMOs) as supports for palladium mediated insertion polymerization. Developing a new synthetic methodology for the preparation of hydrolytically stable heterocyclic (N^N) organosilica precursors will lead to the formation of ordered mesoporous organosilicas with molecular-scale periodicity of the N^N ligands in the walls of the silica pores. The number of available organosilica precursors is severely limited by the current synthetic methodology. This in turn limits the range of possible PMOs and their applications. Currently, the precursor synthesis requires the incorporation of a tri(alkoxy)silane functionality. This functional group can be synthesized with small organic molecules; but is not possible with large organic molecules and heterocycles due to the methods of purification. The supported N^N ancillary ligands in these new ordered mesoporous organosilicas will be used for the formation of neutral and cationic (N^N)Pd(II) organometallic species. The cationic organometallic functionalized periodic mesoporous organosilicas will be used as catalysts for insertion polymerization of ethylene and ethylene/CO copolymerization. If successful, the synthesis of new hydrid inorganic-organic periodic mesoporous silicas incorporating heterocycles will lead to the development of new supported catalysts not just for polymerization catalysis but organometallic mediated carbon-carbon bond formation reactions as a whole. Specifically, the materials and concepts developed from this research will allow for the exploration of catalyst site-isolation as a means to prevent aggregation of active catalyst species. Ligand design can be simplified by the elimination of steric bulk in the ancillary ligand and focus can be directed solely towards the development of ligands with stronger donor character (i.e. N-heterocyclic carbenes).
Impact on my Career
This ACS-PRF DNI award has significantly impacted the early stages of my faculty appointment in several ways. 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 PRF DNI grant also provided much needed and appreciated funding for chemicals and supplies with which to conduct our research. From a professional standpoint, this grant has allowed me to attend two scientific meetings during the past year, including a Gordon Research Conference on organometallic chemistry and the 2011 ACS Fall National meeting in Denver, CO. My attendance at both of these conferences allowed me the opportunity to discuss and seek input on my group’s research from experts in the field. From an instructional perspective, I have incorporated several concepts that are central to this research project, supported transition metal based catalysis and solid state 29Si, 1H, and 13C NMR, into two graduate level courses I have taught over the past year on “organometallic chemistry” and “spectroscopy for the synthetic chemist”.
Impact on the Students
One Ph.D. student, Suisheng Shang, has been supported by this ACS-PRF DNI grant. Two undergraduate students, Diane Carden and Lindsay Cameron, have actively participated in this project. The students have benefited from conducting research on this project by learning fundamental aspects of synthetic organic chemistry, organometallic chemistry, and materials chemistry. This research project has provided hands on laboratory training for these students in organic and organometallic synthesis, spectroscopic and analytical methods, and catalysis all of which are important to the chemical industry. Mr. Shang has not had the opportunity to present his results at meetings during the past year but will have multiple opportunities during the coming year to attend research meetings and disseminate his research results beginning with 2011 Ohio Inorganic Weekend at the University of Cincinnati at the end of October.
Summary of Results
The first year of this project our research has been focused on the synthesis hydrolytically stable organosilica precursors via a new synthetic methodology that avoids the use of the reactive triethoxysilyl group. We first addressed the synthesis of two hydrolytically stable bis(triallyl)organosilanes, 1,4-bis(triallylsilyl)benzene and 4,4’-bis(triallylsilyl)biphenyl. These were prepared, using our new synthetic methodology, which involved lithiation of 1,4-dibromobenzene or 4,4’-dibromobyphenyl at low temperature with excess n-butyllithium followed by quenching with chlorotriallylsilane. The resulting bis(triallylsilyl)arenes were purified using column chromatography and both were isolated in good yields as analytically pure compounds. The synthesis of PMOs incorporating the benzene and biphenyl 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. Following our success with simple aromatic spacers we turned our attention to the synthesis of the 3,8-bis(triallylsilyl)-1,10-phenanthroline organosilica precursor from 3,8-dibromo-1,10-phenanthroline. After initial difficulties in replicating the synthesis of 3,8-dibromo-1,10-phenanthroline we were able to obtain the white solid in reasonable yields reproducibly. Unfortunately, we have been unable to successfully prepare 3,8-bis(triallylsilyl)-1,10-phenanthroline by low temperature lithiation of 3,8-dibromo-1,10-phenanthroline followed by quenching with chlorotriallylsilane. As we continue to try and successfully synthesize 3,8-bis(triallylsilyl)-1,10-phenanthroline we have begun the investigation of PMOs that incorporate bis(methylene)aryl spacers. The synthesis of bis(R3SiCH2)aryl compounds will require multi-step synthesis but should be a high yielding linear synthesis that is amenable to numerous different aromatic groups. The bis(silyl)- precursors to these new PMOs will again incorporate the triallylsilyl functional group which will allow for purification using column chromatography and avoids the use of hydrolytically unstable tris(alkoxysilane)- precursors. 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 periodic mesoporous organosilicas. Access to a larger library of PMO precursors is crucial as periodic mesoporous materials are certain to find new uses in diverse applications such as adsorbents, optical devices, sensors, mass and ion conductors, biomimetic systems, and catalysis.