New Insight into the Role of the Metal Oxidation State in Controlling the Selectivity of the Cr-(SNS) Ethylene Trimerization Catalyst

The tri- and divalent complexes of the 2,6-bis(RSCH2)pyridine [R = Ph, Cy] ligands have been prepared.
Upon activation with MAO, both species are catalysts for ethylene oligomerization of moderate activity.
However, while the trivalent catalysts produced only 1-hexene, the divalent species gave a statistical
distribution of oligomers. This clear difference in catalytic behavior indicates that the two oxidation
states are not interconnected during the catalytic cycle as it happens instead with other oligomerization
catalytic systems. The trivalent precursor is not reduced and the divalent is not oxidized. Treatment of
the trivalent catalyst precursors with either MAO or other R3Al species afforded intractable materials.
Instead, similar reactions with the divalent complexes gave new cationic species, which have been
characterized by X-ray analysis. These complexes have preserved the divalent state of chromium during
the reaction and still produce, upon further activation, a statistical distribution of oligomers. This reiterates
the non-interconvertibility of the di- and trivalent oxidation states and the different degree of selectivity
for which they are responsible.