Isolation and Characterization of a Class II Mixed-Valence Chromium(I)/(II) Self-Activating Ethylene Trimerization Catalyst

Reduction of the tetranuclear {[(t-Bu)­NPPh2]­Cr­[μ-(t-Bu)­NPPh2]2Cr}2(μ-Cl)2 (1) with either
KC8 or vinyl Grignard afforded the dinuclear, mixed-valence
(Me3P)­Cr­[μ-(t-Bu)­NPPh2]3Cr (2) with the two metals possessing distinctively
different coordination environments. According to the formulation
of 2 as Cr­(I)/Cr­(II) mixed-valence species, 2 acts as a self-activating catalyst, producing under pressure of
ethylene a mixture of 1-butene and 1-hexene. Activation with three
different activators selectively produced three different products,
namely, 1-butene with TEAL, 1-hexene with DMAO/TEAL, and LAO-free
highly linear HMWPE with DMAO. Mixtures of 1-hexene and 1-butene were
also obtained upon activation with vinyl Grignard. In this case it
was possible to isolate, albeit in very low yield, an intriguing butadiene/butadiene-diyl
cluster, {[(η4-butadiene)Cr (μ,η4-butadienediyl)­(μ-NP)­Mg]2(μ-Cl)4Mg­(THF)2}­{[(THF)3Mg]2(μ-Cl)3}2 (3), which is also a highly selective
self-activating trimerization catalyst.