Since the first report of ethylene tetramerisation to 1-octene using Cr-bis(diphenylphosphino)amine (PNP) catalysts, numerous studies have investigate the influence of ligand structure on the outcome of the catalysis. Although these studies have highlighted some key “ligand structure–reaction selectivity” relationships, a systematic study of the effect of steric bulk and basicity of various N-substituted PNP ligands has not been reported to date. The present study characterises in more detail the subtle effects of varying the N-substituent on overall oligomerisation performance and selectivity over a broad range of Cr-PNP based catalyst systems. A clear distinction was made between the basicity and steric bulk effects of each N-substituent, with the latter being the dominant parameter influencing the reaction selectivity. The experimental evidence suggests that for alkyl PNP ligands, branching (i.e., further substitution) of the carbon atom directly attached to the N-atom of the PNP ligand was essential for obtaining highly selective tetramerisation catalysts. The best systems achieved total alpha selectivity as high as 88% with exceptional catalyst activities (up to 3,200,000 g/(gCr h)).
