The reaction mechanisms followed by ethylene polymerization and selective oligomerization (tri- and tetramerization) are conceptually very different, being a non-redox chain growth and a redox metallacycle ring expansion pathway respectively. With chromium being the metal of choice, metal oxidation states and the variation of ancillary ligand able to support specific metal oxidation states responsible for selective trimerization, tetramerization or nonselective oligomerization/polymerization were varied. In this research project we have explored a broad range of novel pyridine containing modified PN ligand scaffolds with the aim of probing the role of the pyridine donor substituent in stabilizing lower oxidation states and ultimately affecting the selectivity of the ethylene in the catalytic cycle. In this study, pyridine PNP pincer ligands in conjunction with chromium salts and alkylaluminium activators have been explored. Their catalytic activities toward ethylene oligomerization were rationalized through the isolation of different chromium-aluminate intermediates in different oxidation states during the catalytic reaction. Moreover, we explored other pyridine modification of NNP type ligand. Its anionic ligand shows a rare example of a Cr(II) hydride cluster which shows a high activity as a nonselective ethylene oligomerization catalyst. Finally, a cyclic PNPN type ligand was explored. The ligand enabled the isolation of a mix-valent Cr(I)/Cr(II) species which was found to be inactive toward ethylene oligomerization due to its geometric constraint.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/31024 |
| Date | January 2014 |
| Creators | Alzamly, Ahmed |
| Contributors | Gambarotta, Sandro |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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