Synthesis and screening of ligands for catalytic olefin oligomerisation reactions

A set of reliable, one-pot reactions have enabled the preparation of a wide variety of novel bidentate (and potentially monodentate) ligands bearing a combination of P, N and C atoms across the backbone. The coordination capabilities of these phosphines have been explored with a variety of transition metal centres including PtII, PdII, RuII, IrIII, AuI and Cr0 confirming the versatility of this group of compounds when acting as bi- and monodentate ligands as well as the ability to bridge two metal centres. Reaction of a substituted hydroxymethylphosphine (R'2PCH2OH) with a primary amine in methanol gave the diphosphine ligands R'2PCH2N(R)CH2PR'2 including a PCNCP backbone. The steric and electronic properties of these ligands were tuned by the simple interchanging of the R/R' substituents attached to the P and N atoms. These compounds have been fully characterised by spectroscopic and analytical methods and reacted with transition metal precursors of the type MCl2(COD) (M = Pd/Pt) to yield complexes incorporating six-membered chelate rings. In addition to the aforementioned symmetric PCNCP ligands, the new non-symmetric ditertiary compounds Ph2PCH2N(R)CH2PAd were prepared using a three-step sequence of condensation reactions. Novel ligands incorporating a PCNP backbone were synthesised via a single condensation reaction between Ph2PCH2OH and a primary amine to give the secondary aminophosphines Ph2PCH2N(H)R. Deprotonation of the secondary amine followed by addition of neat R'2PCl resulted in the formation of the ligands Ph2PCH2N(R)PR'2. This group of compounds was characterised utilising similar techniques to their PCNCP analogues with their coordinative capabilities also assessed. The catalytic properties of a select group of the diphosphines prepared, when coordinated to a CrIII precursor, were analysed at Sasol Technology (Pty) Ltd under their standard conditions for ethylene oligomerisation reactions. Initial results established that ligands were effective as catalysts with the exception of those incorporating phospha-adamantane cages. Ligand 3.8 with a PCNP backbone was found to be comparable, in terms of activity, to Sasol’s current PNP tetramerisation systems.