Homogeneous Cr- and Co-mediated Selective Olefin Oligomerisation Catalysis

Abstract

This thesis investigates use of Cr/phosphanyl methanimine ligand (PCN, Ph(PR2)C=NAr, R = alkyl group) combinations in selective ethylene tri-/tetra-merisation catalysis. Additionally, the use of the previously-reported linear α-olefin (LAO) dimerisation catalyst precursor Cp*Co(C2H4)P(OMe)3 (4.5) is further explored in 1- butene and 1-hexene dimerisation, in attempts to improve its reported low TON and low selectivity.
In Chapter 2 a description of the synthesis of variously-substituted PCN ligands 2.1– 2.17 is given. A study of the coordination chemistry of a selection of these PCN ligands to Cr0 revealed that both mono-(κ1-P) and bi-dentate (κ2-P,N) [Cr(CO)4/5(PCN)] complexes 2.18–2.19 and 2.22-2.25 were obtained, with the ratio of [Cr(CO)5(κ1-P- PCN)]:[Cr(CO)4(κ2-P,N-PCN)] complexes present in the crude reaction mixtures being influenced by the steric bulk of the nitrogen substituent of the PCN ligand and reaction conditions. Cationic CrI-PCN complexes [Cr(CO)4/5(κ1-P or κ2-P,N-PCN)] 2.26–2.30 and 2.32 were obtained via one-electron oxidation reactions of the corresponding Cr0-PCN complexes with Ag[Al4]. The synthesis of CrIII-PCN complexes [CrCl3(THF)(κ2-P,N-PCN)] 2.34-2.37 was attempted, but proved successful only in two cases, complexes 2.36 and 2.37.
Chapter 3 details the ethylene oligomerisation catalysis behaviour of the CrI and CrIII- PCN complexes. In all cases, CrIII/PCN-based ethylene tri-/tetra-merisation systems with high activities and selectivities were obtained, with the 1-octene:1-hexene ratios of these systems being influenced by the electronic properties of the phosphorus donor moiety and imine carbon substituents of the PCN ligands. The CrI-PCN complexes yielded low activity ethylene polymerisation systems; their poor performance is attributed to poisoning from CO liberated during reactions. It was found that using in situ versus preformed CrIII-PCN complexes in ethylene oligomerisation catalysis improved the activities and selectivities towards 1-hexene and 1-octene production.
In Chapter 4, the effect of varying different solvents, temperatures, and activation methods in 1-butene/1-hexene dimerisation catalysis mediated by complex 4.5/Brookhart’s acid was assessed. In addition, complexes 4.15 and 4.16 (modified versions of complex 4.5) were synthesised and found to perform worse in 1-butene/1- hexene dimerisation catalysis than complex 4.5 (in terms of activity and selectivity towards desired LAO dimer).
Chapters 5 and 6 describe all experimental data and supporting information.