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Bulky cyclopentadienyl chromium complexes as ethene trimerisation catalyst precursors
| Content Provider | Semantic Scholar |
|---|---|
| Author | Blom, Burgert |
| Copyright Year | 2004 |
| Abstract | The process of ethene trimerisation to 1-hexene, has recently attracted considerable interest, in light of the usefulness of 1-hexene industrially. In this project the process of catalytic ethene trimerisation , using bulky cyclopentad ienyl chromium complexes was investigated. Firstly, a series of known and novel bulky cyclopentadiene ligands, of the type, C5Ph4RH (R = H, alkyl, aromatic or substituted aromatic group) were synthesised. Four routes were investigated. (i) Making use of an in situ generated bis-lithium intermediate, resulting in the formation of 1 ,2,3,4,5-pentaphenyl cyclopenta-1 ,3-diene and 1 ,2,3,4-tetraphenylcyclopenta-1 ,3-diene. (ii) The attempted use of a metallation I alkylation route which was unsuccessful. (iii) The use of tetracyclone as starting material , reacted with alkyl likthium reagents, or alkyl Grignard reagents which resulted in the formation of ligands, of the type C5Ph4R, with R in the 5 position of the ring (R = n-butyl , tertbutyl, hexyl) . Unexpected side reactions afforded the isolation of two ligands of the type C5Ph4R, R = 1-propeneyl and ethyl, both with the terminal alkene function , saturated. (iv) The use of a Pd catalysed route, making use of Cp2ZrCI2 as source of cyclopentadiene, resulted in the successful isolation of the cyclopentadiene, C5Ar5H (Ar = C6Me2H3). All new compounds were successfully characterised using NMR, IR, MS and elemental analysis. A stable Fe(ll) model complex, bis-tetraphenylcyclopentadienyl iron, was prepared and cyclic voltametry conducted on it, to investigate the effect of phenyl substituents on a Cp ligand in the reduction I oxidation potential of the iron(ll) centre. This study showed that phenyl rings on a cyclopentadiene ligand have a significant effect on the redox potential of Fe(ll) ; this trend could possibly be expected with other metals, in particular chromium . Possible trimerisation catalyst precursors of the type [(CsArnHs-n)CrCI2h (Ar = Ph or CsMe2H3) , were next synthesised . A route making use of a radical to form target dinuclear Cr complexes was looked at, but was not particularly successful. A convenient route using CrC13(thfh and a sodium or potassium salt of the ligands prepared resulted in the isolation and characterisation of [(CsPhs)CrCI2b. [(CsPh4H)CrCI2b. [(C5ArsH)CrCI2]2 (Ar = CsMe2H3) , all dinuclear pseudooctahedral Cr(lll) complexes. EPR spectroscopy on [(CsPhs)CrCI2h and [(CsPh4H)CrCI2b in weakly co-ordinating dichloromethane, at low temperatures demonstrated the presence of an equilibrium between the dinuclear complex and the monomer. EPR spectra showing signals typical of mononuclear Cr(lll) with S = 312 were observed , supporting this dissociation hypothesis. The X-ray crystal structure of CrCI3(thfh was determined with an R value of 3.3 %. High pressure testing of a catalytic system containing the cyclopentadiene ligands; together with Cr-2-ethyl hexanoate, triethylaluminium and hexachloroethane was investigated. Catalytic testing of one of the model complexes, [(C5Ph5)CrCI2]2, demonstrated not only catalytic activity towards ethene trimerisation, but a link between the activity and selectivity of the model complex, and the in situ system. This result has shed light on the nature and structure of the catalytic precursor. Density Functional Theory (OFT) studies were also conducted on a representative catalytic precursor species, CpCrMe2 , using Materials Studio TM. In the first instance, the role of the alkylating agent (typically triethylaluminium) was neglected, and a series of geometry optimisations and transition state searched conducted on the catalytic process starting from CpCrMe2. The process was investigated in a stepwise fashion until formation of the product, 1-hexene. High energy barriers were identified along the pathway, particularly in the last step of the process, where an energy barrier of 88 kJ .mol" was identified. The addition of an alkylating agent was also investigated, at two key high energy stages. It was found on both occasions that the presence of an alkylating agent significantly reduces the energy barriers, by providing a more stable transition state, and hence a lower energy pathway. In this instance, the oxidation state of the chromium was four, as opposed to three looked at in all our earlier calculations. Finally, the system, CpPhCr was investigated. It was found that the addition of a phenyl ring onto the Cp moiety has a significant effect on sequential ethene binding energies. Hence, the importance of the phenyl rings in the catalytic ethene trimerisation process was highlighted. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://open.uct.ac.za/bitstream/handle/11427/22101/thesis_sci_2004_blom_burgert.pdf?isAllowed=y&sequence=1 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
