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

Reaction of (Me2pz)2CHSiMe2N(H)R (R = iPr or Ph) or (Me2pz)2CHSiMe2NMe2 with CrCl3(THF)3 or CrCl2(THF)2 gave Cr{(Me2pz)2CHSiMe2NR1R2}Cl3 (R1 = H, R2 = iPr (10) or Ph (11); R1 = R2 = Me (15)) or Cr{(Me2pz)2CHSiMe2NR1R2}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(Me2pz)3}(THF) gave the zwitterionic complex Cr{C(Me2pz)3}Cl2(THF) (17) containing an apical carbanion. Reaction of the analogous phenol-based ligand (Me2pz)2CHArOH (ArO = 2-O-3,5-C6H2tBu2) with CrCl3(THF)3 gave Cr{(Me2pz)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{(Me2pz)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 a-olefins. Compound 23 had an exceptionally low a-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.