The Challenge of Selectivity in Ethylene Oligomerization: Ligand Design and Metal Valence States

Catalytic ethylene oligomerization is a well understood industrially viable process. The large majority of scientific literature and patents concerning this process has been developed with the use of chromium catalysts. Commercial systems producing selective tri/tetramerization, non-selective oligomerization and polymerization are all based on this metal with the exception of a few systems based on other transition metals (Zr, Ti, Ni etc.). This versatility raises interesting questions about chromium's unique behaviour. Essentially, selective or non-selective oligomerization and polymerization processes could be regarded as belonging to the same category of C-C bond forming reactions, though different mechanisms are involved.;The first part of this thesis explores a variety of chromium complexes for ethylene oligomerization purposes. In order to gather further information about the unique behaviour of chromium, we have explored a variety of nitrogen and phosphorus containing ligands. We started with a simple bi-dentate anionic amidophosphine (NP) ligand and assessed the role of the ligand's negative charge and number of donor atoms in determining the type of catalytic behaviour in relation to the metal oxidation state. This ligand proved capable of generating a series of chromium dimeric, tetrameric or polymeric and even heterobimetallic chromium-aluminate complexes in different valence states. This allowed us to isolate a "single component" self activating Cr(II) complex as well as a rare example of mixed valence Cr(I)/Cr(II) species. Additionally, each of these species acted as switchable catalyst depending on the type of co-catalyst and solvent used (Chapter 2).;To systematize out comprehension of the NP ligand, we have replaced the acidic proton in an (NP) anionic ligand with an alky group. An impressive selectivity switch from trimerization to tetramerization (>90%) was observed (Chapter 3). One of the important achievements during the study of this thesis work was the finding of a first ever selective tetramerization catalyst reaching >99% selectivity for 1-octene with Cr-amino-bispyridine ligand systems (Chapter 4). In search of a clue to introduce the selectivity in the catalytic cycle we have uncovered the ability of simple dicyclohexyl amido ligand to stabilize the monovalent state of chromium (Chapter 5). Extending this concept, a variety of bi-dentate and tri-dentate amidinate (NCN) and amidate (NCO) ligands were also assessed for highly active and non-selective polymer free oligomerization behaviours as they have shown enough ability to stabilize the divalent oxidation state of chromium (Chapter 6).;The second part of this thesis explores a few nickel based complexes. Aiming of our study at low valent nickel complexes and their catalytic performances towards ethylene oligomerization, we have chosen different well established ligands in this field. To this end, we were able to isolate a novel nickel dinitrogen complex Ni(0), a nickel hydride complex Ni(II) and even the first example of a nickel alky bis(imino)pyridine complex Ni(I). The "non-innocent" nature of the ligand and its involvement in the organometallic chemistry of the metal centre has been studied. All of these complexes were assessed for their ethylene oligomerization behaviour (Chapter 7). Even in this case, we were looking for highly reactive monovalent species and have isolated a rare example of heterobimetallic Ni(I) species with a dianionic tripyrrolide ligand (Chapter 8). Characteristics C-C bond forming and C-C bond cleavage properties of nickel were revealed by isolating corresponding dimethyl pyrrolid-nickel complexes (Chapter 9). All of these complexes were tested for their unique ethylene dimerization behaviour.