Single- and Double-Coordination Mechanism in Ethylene Tri- and Tetramerization with Cr/PNP Catalysts

The mechanism of ethylene trimerization
and tetramerization with
a chromium–diphosphinoamine (Cr–PNP) catalyst system
has been studied with combined experimental and theoretical methods.
Of the total product output, 1-octene, cyclopentanes, n-alkanes, and higher (C10+) olefins are formed with a
fractional (∼1.4) order response to ethylene concentration,
whereas 1-hexene formation is approximately first-order in ethylene.
Theoretical studies suggest a mechanism involving a cationic monometallic
catalyst in Cr­(I) and Cr­(III) formal oxidation states. A key feature
of the developed model is the occurrence of a double-coordination
mechanism in which a bis­(ethylene) chromacyclopentane intermediate
is responsible for 1-octene formation as well as the other coproducts
that have a greater than first-order response to ethylene. In contrast,
1-hexene is formed primarily from a mono­(ethylene) chromacyclopentane
intermediate. The selectivity of catalysis is governed by the competition
between single- and double-coordination pathways. The mechanistic
model developed displays excellent correlation with experimental observations
and is able to fully explain the formation of all products generated
with this catalyst.