Novel Bis ( phospholyl )amine Ligands in Selective Ethylene Oligomerisation

1. Tom Stennett - University of Bristol, UK 10th European Workshop on Phosphorus Chemistry 19.3.2013 Novel Bis(phospholyl)amine Ligands in Selective Ethylene Oligomerisation

2. Linear α-Olefins • Linear hydrocarbons with a terminal double bond. • Important bulk chemicals used as comonomers in polyethylene production (C4-C8), detergent precursors (C10-C14) and lubricants (C16-C18). • Synthesis by oligomerisation of ethylene using a transition metal catalyst

3. Ethylene Oligomerisation • Traditional ethylene oligomerisation catalysts operate via a Cossee-Arlman mechanism of linear chain growth: • This always produces a distribution of oligomers • Workers at Union Carbide Corporation in 1967 found that polyethylene produced by a Cr catalyst contained C4 side chains – showing the presence of 1-hexene – but no 1-butene or higher oligomers

4. Selective Ethylene Oligomerisation • An alternative mechanism involving metallacycles was postulated to explain the selectivity to 1-hexene: J. R. Briggs, J. Chem. Soc. Chem. Commun., 1989, 674-5

5. PNP Ligands • Diphosphazane (PNP) ligands are probably the most important ligand class in selective ethylene oligomerisation • Minor ligand modifications can give good selectivity to 1-octene Wass et al., Chem. Commun., 2002, 858. Bollmann et al., J. Am. Chem. Soc., 2004, 126, 14712.

6. Phosphole PNPs • Unlike pyrrole, phosphole has negligible aromaticity due to poor orbital overlap, resulting in a reactive phosphorus atom: • Phosphole ligands exist, but are much less studied than phosphines Hydroformylation (with Rh) (Réau 1989) Ethylene dimerisation (Réau 2004)

7. Phosphole PNPs • Phosphole PNPs have so far been overlooked – can we make them? • PNP coupling analogous to that of traditional arylphosphine PNPs: • 31P NMR – sharp singlets at 72 ppm (R = Me) and 67 ppm (R = Et).

8. Phosphole PNPs • Phosphole PNPs have so far been overlooked – can we make them? Yields 24-57% Stennett, Wass et al., submitted.

9. Bulky Phosphole PNPs • Using isopropylamine, synthesis gets stuck at halfway: • However, changing the solvent to PhCl affords the symmetrical products

10. NMR spectra of bulky ligands • Isopropyl ligands display broadened NMR spectra due to restricted P-N rotation: • Ligand 6 exists as two species at room temperature 9 8 6

11. Variable Temperature NMR • Dibenzophosphole group has a lower rotational barrier than tetraethylphosphole group: 6 90 ˚C 7 60 ˚C 20 ˚C -20 ˚C

12. Variable Temperature NMR • Minor isomer of ligand 8 at low T has equivalent phosphines. • Ligand 9 only has one isomer at low T. 9 8 90 ˚C 60 ˚C 20 ˚C -20 ˚C

13. Identity of Rotamers • X-ray crystallography and DFT studies were carried out to find out the nature of the isomers. 9 8 • Rotation of the P-N bond showed a second low-energy minimum with the phosphole groups parallel:

14. Oligomerisation Results

15. Oligomerisation Results

16. Oligomerisation Results

17. Oligomerisation Results

18. Oligomerisation Results

19. Cr(CO)4 complexes • Chromium carbonyl complexes are a useful way of probing the steric and electronic properties of new ligands

20. Rationalising the Selectivity • Steric clash occurs between ethyl arms and N-substituent:

21. Conclusions • PNP ligands based on 2,3,4,5-tetraethylphosphole form active and highly tunable catalysts for selective ethylene oligomerisation • Dibenzophosphole-based PNPs give poor catalytic results • Proximal and remote steric effects have a large influence on the catalyst activity and selectivity

22. Acknowledgements

23. Thanks for your attention!