Abstract
AbstractThe work carried out for this thesis is concerned with the design and synthesis of new ligands based on 1,3,5-trialkyl-1,3,5-triazacyclohexanes (R3TAC) and their corresponding chromium(III) chloride complexes, with the aim of improving the lifetime of these complexes as active selective linear α-olefin trimerisation catalysts. This is attempted by the retardation of catalyst decomposition by a known pathway. Specifically, additional steric protection is added to the backside of the ligand by the substitution of an alkyne with substituents of increasing steric bulk onto a carbon atom of the R3TAC ring, leading to the synthesis 1,3,5-trialkyl-2-ethynyl-1,3,5-triazacyclohexanes (R’CCR3TAC) and their corresponding chromium(III) chloride complexes.
Chapter 1 is a broad literature review of selective olefin oligomerisation via the metallayclic mechanism. Discussed is the justification for the mechanism, the many chromium based catalysts and their ligands, other metal based catalysts and an overview of the use of the co-catalyst methylaluminoxane (MAO).
Chapter 2 describes the preparation of R’CCR3TACs. It was found that R’CCR3TACs are not as stable as R3TACs and under certain conditions can decompose into their corresponding imines, RNC(H)CCR’ and RNCH2. The latter of which can reoligomerise into R3TAC, leading to a complex mixture of identifiable products. Also described is the synthesis of an ethynyl bridged bis-TAC compound, (Et3TAC)2CC. Additionally a literature review of R3TAC chemistry is given.
Chapter 3 details the synthesis and characterization of (R’CCR3TAC)CrCl3 complexes. Despite challenges of the ligand decomposition described in chapter 2, which leads to a mixture of chromium complexes, tuning the solubility of the ligands allowed for clean isolation of some (R’CCR3TAC)CrCl3 complexes. Select complexes were characterized by X-ray crystallography, IR and UV/Vis and were found to be very similar to (R3TAC)CrCl3 complexes. Paramagnetic NMR did however show deviations in their solution state structures. Also described is the synthesis of (2,4,9-triazaadamantane)CrCl3 complexes. Additionally, a literature review of the coordination chemistry of complexes ligated by R3TAC is given.
Chapter 4 investigates the effect of the steric bulk of the alkyne substituent (R’) on the catalysis of selective 1-hexene trimerisation. By following the reaction progress, it is found that increasing the steric bulk of the R’ substituent does have a positive effect on catalysis at low [Al]/[Cr] but this effect is lost at high [Al]/[Cr]. However, the examination of the rate constants of the reaction cannot conclude that this is due to the slowing of active catalyst decomposition. Alternative explanations for these observations are discussed. It was also found that the presence of the alkyne had a noticeable effect on the isomer distribution compared to the distribution given by the catalysts non alkyne substituted (R3TAC)CrCl3 analogues. Additionally, a literature review of selective linear α-olefin trimerization catalysts is given.
Chapter 5 contains the conclusions and future work of this thesis.
Chapter 6 contains the experimental data for this thesis.
Chapter 7 contains the references for this thesis.
Appendix I contains the catalysis reaction progress graphs.
Appendix II contains the NMR spectra.
Appendix III contains the X-ray crystallography data.
| Date of Award | 11 Sept 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Randolf Kohn (Supervisor) & Michael Hill (Supervisor) |