Zwitterionic late transition metal alkene polymerisation catalysts containing aminofulvene-aldiminate (AFA) ligands
Rahman, Mohammed Mahmudur
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Over recent years significant progress has been made in the design and development of late transition metal cationic catalysts for olefin polymerisation. Never-the-less, the activation of catalyst precursors and generation of active species still remains a challenge. In this respect, zwitterionic catalysts could offer a range of advantages over the traditional two component catalytic systems. For example, stable zwitterions are well-defined, single component catalysts which do not require Lewis acid co-catalysts for activation. Therefore, this eliminates the possibility of anions coordinating to the active site and could provide highly active catalysts. Moreover, this could reduce the production costs. In this thesis the 6-aminofulvene-2-aldiminate (AFA) ligand system has been employed to develop zwitterionic, charge-neutral complexes, analogues of Brookhart-type cationic alkene polymerisation catalyst containing 1,2-diimine ligand. Chapter 1 of the thesis provides a comprehensive literature review of the late transition metal (Group 10) α-diimine catalytic systems and the zwitterionic early and late transition metal alkene polymerisation catalysts. Chapter 2 describes the synthesis and characterisation of some novel zwitterionic complexes [(Ph2AFA)Pd(Me)(DMAP)], [(Ph2AFA)(N,N-dimethylbenzylamine-2-C,N)- Pd(II)] and [(Ph2AFA)Ni(η 3-C3H5)] and their possible application as catalyst precursors in alkene polymerisation. In principle, upon activation these complexes should exhibit higher catalytic activity. The ideal catalyst precursor for a highly active palladium based system would be a halide-bridged dimer of the form [(Ph2AFA)Pd(μ-X)]2. Chapter 2 describes several efforts towards the synthesis of such complexes using a range of R2AFA ligands. Even with the introduction of bulky N-substituents such as cyclohexyl or tert-butyl, the halidebridged dimers could not be synthesised. Instead, the reaction between the deprotonated ligand and [PdCl2(NCPh)2] provides bis-chelated complexes [(R2AFA)2Pd]. In order to introduce more steric bulk into the AFAH ligand which might lead to a halide-bridged dimer, two more ligands N,N’-bis(2,6-diisopropyl)phenyl-6-aminofulvene-2-aldimine and N,N’-di-(2,4,6-trimethyl)phenyl-6-aminofulvene-2-aldimine have been synthesised and characterised. It has been found that the presence of the 2,6-diisopropylphenyl substituents in N,N'-bis(2,6-diisopropyl)phenyl-6-aminofulvene-2-aldimine not only prevents the coordination of two ligands to the same metal, but precludes complexation all together. Chapter 2 also describes several efforts to develop a hemi-labile complex for alkene polymerisation. Chapter 3 describes the synthesis of metalloligands of aminofulvene-aldimine (AFA) and corresponding bimetallic complexes. The AFA ligand affords transition metal complexes via both η 5- as well as κ 2-coordination modes. A new synthetic methodology has been developed to synthesise metalloligands [Cp*RuII(Ph2AFA)H][BF4], [Cp*RhIII(Cy2AFA)H][BF4]2 and [Cp*RhIII(Cy2AFA)]- [BF4]. The basicity of the monocationic Rh metalloligand is found to be significantly lower than that of its Ru analogues. This is significant as it opens a potentially easy synthetic route to bimetallic complexes. The bimetallic complex [Cp*RhIII(Cy2AFAPdCl2)][BF4] has been developed for alkene polymerisation in an attempt to investigate the charge effect in alkene polymerisation catalysis. Upon activation this monocationic Rh/Pd bimetallic complex would provide a dicationic active species which would in principle be a more highly active catalyst than the Brookhart mono cationic diimine catalysts. Chapter 4 describes all the experimental procedure and polymerisation tests in this thesis.