Lionel Magna

New titanium complexes of general formula [Ti(OR)₂(OiPr)₂], containing functionalized alkoxido ligands, were developed for the selective catalytic dimerization of ethylene to 1‐butene using a combined High Throughput Screening (HTS) / Design of Experiment (DoE) approach. First, a library of 19 ligands was elaborated and a primary screening spotted the phosphorus‐functionalized alkoxido ligands as most promising. A second, more focused library containing 8 alkoxidophosphane ligands was then developed. A longer linear spacer between the alkoxido and the phosphorus functions, as in [Ti(19)₂(OiPr)₂], was found beneficial for this catalytic reaction. After identification of the best co‐catalyst (AlEt₃) and co‐ligand (OnBu), final optimization of the reaction conditions was performed using a design of experiments (DoE) approach. The complex [Ti(19)₂(OnBu)₂] was shown to selectively dimerize ethylene in 1‐butene (C₄(α)=93 % (99⁺%)) at 30 bar C₂H₄ and 55 °C with AlEt₃ as co‐catalyst, resulting in very high activity and selectivity for a molecular titanium catalyst (13000 g g_Ti⁻¹ h⁻¹, 93 % 1‐butene).

The reactivity of a phenoxy-imine-ether system (FI)TiCl₃/MAO was studied toward selective ethylene trimerization. This system was shown to either trimerize or polymerize ethylene depending on the reaction temperature. Its selectivity switches from a significant production of the trimerization product, 1-hexene (85 wt%, 520–450 kg_1-hexene g_Ti ⁻¹ h⁻¹) between 30 and 40 °C, to a moderate polyethylene formation (70–80 wt%, 60–70 kg_polyethylene g_Ti ⁻¹ h⁻¹) at a higher reaction temperature (T > 60 °C). Polymerization was investigated based on an original “polymer-to-catalyst” strategy aiming at identifying the active species responsible for this side reaction. Using DSC, SEC and high temperature ¹³C NMR analyses, polyethylenes were found to exhibit high molar masses (>10⁵ g mol⁻¹) and a low 1-hexene content (<1 mol%) at any temperature. Kinetic studies support that trimerization and polymerization species are generated from the catalyst precursor at 40 °C but a parallel process may occur at a higher temperature. The increase of dispersity to 4.6 at 80 °C suggests a change from single to multi-site catalysis. The poor comonomer incorporation ability of the active species is reminiscent of a molecular Ziegler–Natta or a bulky post-metallocene catalyst.

A series of tridentate aryloxy‐based ligands were synthesized and characterized for their coordination behaviour towards Ti^IV. Coordination studies revealed that the nature of the central atom (amine vs. ether) and the type of bridging spacer (aromatic vs. aliphatic) are important aryloxy ligand parameters and influence the ligand coordination mode and the formation of stable titanium complexes. This series of titanium complexes were evaluated in ethylene oligomerization and polymerization after activation with methylaluminoxane (MAO) and showed the preferential formation of polyethylene. In some cases, the formation of a small amount of 1‐hexene suggests the existence of several catalytic centres in the reaction mixture.

Multinuclear aluminum cocatalysts have been obtained by the reaction of various phenols, alcohols or diols with trimethylaluminum and were used in situ or as isolated, well-defined species, for the activation of an iron(ii) or an iron(iii) pre-catalyst for the oligomerization of ethylene.

Since the discovery that bis(imino)pyridine ligands are able to confer high activities in ethylene oligomerization and polymerization to their iron complexes, considerable attention has been focused on catalyst design for these reactions and this research constitutes an ever-growing area in molecular catalysis. The tuning of the ligand structures and properties, and thus of catalysts, generally represents the basis for subsequent work contributing to process development and industrialization. Significant effort is therefore devoted to generate structural diversity in order to access the required catalyst stability and selectivity. This feature article outlines nitrogen-containing ligands that have been developed for the iron-catalyzed oligomerization of ethylene since the seminal discovery of the properties of bis(imino)pyridine ligands.