Chromium Aryl Complexes with N‑Donor Ligands as Catalyst Precursors for Selective Ethylene Trimerization

A series
of 8-amino-2-arylquinoline ligands (1–6) were synthesized and reacted with CH3CrCl2(thf)3. Under these conditions a CH bond of the
2-aryl substituent is metalated, leading to organochromium complexes
with monoanionic tridentate ligands (8–13). The presence of a chromium–carbon σ bond in these
complexes has been established by X-ray analysis. Furthermore, 8-(piperidin-1-yl)­quinoline
(14) was used as neutral bidentate ligand in addition
to an external aryl group, leading to complex 15. Finally,
the tris-aryl complex 18 was synthesized, which features
a rare five-coordinate chromium­(III) metal center. All chromium complexes
were tested as catalysts for the selective trimerization of ethylene
after activation with methylaluminoxane (MAO). Several of the new
catalyst precursors show good behavior for the selective trimerization
of ethylene. Although chlorido ligands in the catalyst precursor will
be substituted by methyl groups during the activation with MAO, there
is a clear difference in the catalytic behavior when the complex contains
a methyl (or aryl) group prior to addition of MAO. The mechanism of
catalyst activation has been studied in more detail with the tris-aryl
complex 18.