Synthesis and ethylene trimerisation capability of new chromium(II) and chromium(III) heteroscorpionate complexes

Article abstract of DOI:10.1039/b926333k

Reaction of (Me₂pz)₂CHSiMe₂N(H)R (R = ⁱPr or Ph) or (Me₂pz)₂CHSiMe ₂NMe₂ with CrCl₃(THF)₃ or CrCl ₂(THF)₂ gave Cr{(Me₂pz)₂CHSiMe ₂NR¹R²}Cl₃ (R¹ = H, R² = ⁱPr (10) or Ph (11); R¹ = R² = Me (15)) or Cr{(Me₂pz)₂CHSiMe₂NR ¹R²}Cl₂(THF) (R¹ = H, R² = ⁱPr (12) or Ph (13); R¹ = R² = Me (16)), respectively. Compounds 10 and 11 were crystallographically characterized and the magnetic behaviour of all the new compounds was evaluated using SQUID magnetometry. Reaction of CrCl₃(THF)₃ with Li{C(Me ₂pz)₃}(THF) gave the zwitterionic complex Cr{C(Me ₂pz)₃}Cl₂(THF) (17) containing an apical carbanion. Reaction of the analogous phenol-based ligand (Me₂pz) ₂CHArOH (ArO = 2-O-3,5-C₆H₂^tBu ₂) with CrCl₃(THF)₃ gave Cr{(Me ₂pz)₂CHArOH}Cl₃ (19) whereas the corresponding reaction with CrCl₂(THF)₂ unexpectedly gave the Cr(iii) phenolate derivative Cr{(Me₂pz)₂CHArO}Cl₂(THF) (20) which could also be prepared from CrCl₃(THF)₃ and the sodiated ligand [Na{(Me₂pz)₂CHArO}(THF)]₂. Reaction of the corresponding ether (Me₂pz)₂CHArOMe with CrCl₃(THF)₃ or CrCl₂(THF)₂ gave Cr{(Me₂pz)₂CHArOMe}Cl₃ (23) and Cr{(Me ₂pz)₂CHArOMe}Cl₂(THF) (24), respectively. The catalytic performance in ethylene oligomerisation/polymerisation of all of the new Cr(ii) and Cr(iii) complexes was evaluated. Most of the complexes showed high activity, but produced a Schultz-Flory distribution of α-olefins. Compound 23 had an exceptionally low α-value of 0.37 and showed a preference for 1-hexene and 1-octene formation. While replacing a secondary amine (10-13) for a tertiary amine (15-16) resulted in loss of catalytic activity, replacing a phenol (19) for an anisole (23) group afforded a more selective and more active catalyst. Changing from MAO to DIBAL-O as cocatalyst induced a switch in selectivity to ethylene polymerisation.