Ethylene tetramerization: structure and reactivity investigation of metal based catalyst precursors

English: Transition metal-catalyzed ethylene oligomerization typically produces a broad range of α-olefins. The selective tetramerisation of ethylene to 1-octene is possible with the use of diphosphinoamine ligands together with chromium(lll) and an aluminoxane activator as catalyst system. The aim of this study was therefore to synthesize and characterise a range of diphosphinoamine (PNP) ligands with varying catalytic activity and selectivity (see Figure 1). The stereo-electronic properties of these ligands were evaluated by varying the alkyl group in a systematic way. The single crystal X-ray crystallograph ic study of the above-mentioned PNP ligands revealed a decreasing trend of the P-N-P bond angle to decrease as the steric bulk of the alkyl moiety increases. Figure in PDF full text. An important part of this investigation was concerned with the synthesis and evaluation of the solid state characteristics of the PNP ligands bonded to Cr(lll). This was extended by complexing other metal cations (eg. Pt(ll) and Pd(ll)) with the various PNP ligands in order to provide further information on the coordination mode of these ligands. The study was also supplemented with theoretical chemistry. The comparison between the optimised structure and the crystal data revealed small differences, resulting in the possibility that predictions can be made in terms of ligand design. The calculated structure indicated that the phenyl ring arrangement is affected by the steric bulk of the nitrogen-coordinated alkyl moiety which could ultimately affect the catalytic selectivity. A steric parameter was defined (Effective Tolman-based N-substituent steric effect (θN·sub)) in which the steric bulk of the nitrogen-coordinated alkyl substituent was quantified. A comparison between θN·sub for various diphosphinoamine ligands (free and metal-coordinated) and the catalytic selectivity (for 1-C6 and 1-C8) revealed a relatively good correlation with the catalytic selectivity increasing as θN·sub for the varying ligands increased. It was also possible to compare the θN·sub* (from theoretically calculated structures) with the catalytic selectivity and a similar trend was observed for the optimized structures than the solid state structures (θN·sub). The kinetics of the substitution of two chlorido ligands of the [PtCl2(PNP-i-Pent)] complex with Br- was investigated. Two consecutive reversible reactions were observed and analysed separately. Three platinum(II) species [(PtCl2(PNP-i-Pent)], [(PtBrCl(PNP-i-Pent)] and [(PtBr2(PNP-i-Pent)], are present in equilibrium after the completion of the two reactions and was characterized with 31P NMR. An associative mechanism was identified for both reactions, with large negative entropy and low positive enthalpy values.