Switchable Chromium(II) Ethylene Oligomerization/Polymerization Catalyst

The tri- and divalent chromium derivatives
of [N-Me,tripyrr] ([N-Me,tripyrr] = 2,5-bis(diphenyl(pyrrol-2-yl)methyl)-1-methylpyrrole) [N-Me,tripyrr]CrCl
(1), and [N-Me,tripyrr]Cr(L)2·(THF)0.5 (2) [L = THF (2a),
pyridine (2b)] were obtained in good yield from the
reaction of the dipotassium salt of the ligand with CrCl3(THF)3 and CrCl2(THF)2, respectively. Reaction of the
dilithium salt of the ligand with CrCl2(THF)2 led to the
divalent analogue [N-Me,tripyrr]Cr(μ-Cl)Li(THF)3 (3),
wherein a Li−Cl unit was retained by the metal center.
The compounds were tested for ethylene oligomerization
activity upon activation with two different alumoxanes.
In the case of MAO, the Cr(II) catalyst precursors 2b and
3 showed a marked increase in activity with respect to
the trivalent 1. The product distributions from compounds 1 and 2b were nearly identical, indicating that
the Cr(II) oxidation state provides better catalyst precursors. Use of the i-BuAlO activator instead of MAO
afforded exclusively polymerization with high activity
and produced linear, low molecular weight polyethylene.