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

Abstract

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.

Hydrogen: efficient promoter for PNP/Cr(III)/MAO catalyzed ethylene tetramerization toward 1-octene

“…species [11], hydrogenation of inactive Ti allylic species [12,13] and hydrogenation of unsaturated chain ends that poison active sites. Although progress has been made in understanding the mechanism of PNP/Cr(III)/MAO catalyzed ethylene tetramerization [5,14], most notably the role of a metallacyclic reaction manifold, definitive answers relating to the oxidation state of chromium and specific aspects of ligand control remain elusive. The plausible machenism of how H 2 promotes to reactivate the deactive metal centre is shown in Fig.…”

“…Since its discovery, a rapid increase in the number of publications and patents have been reported for selective ethylene tetramerization, among which much attention has been paid to the electronic and steric factors of the PNP ligands, reaction mechanism, reaction kinetics and new ligands [2][3][4][5].…”

Investigating the Active Species in a [(R‐SN(H)S‐R)CrCl3] Ethene Trimerization System: Mononuclear or Dinuclear?

“…We did not calculate the minimum energy crossing points (MECPs). In multiple publications the MECP has been demonstrated to be lower in energy than the rate‐determining step . Subsequent oxidative coupling of the two ethene molecules ( TSA1 , ΔΔG ≠ =4.4 kcal mol −1 ) yields chromacyclopentane A4 .…”

Ligand Control of the Metal Coordination Sphere: Structures, Reactivity and Catalysis

“…for the fabrication of lubricants (C 10 ), biodegradable detergents (C 10 , C 12 , C 14 ), surfactants and lubricant additives (C 12 -C 20 ) and many other useful chemicals. These features explain the continuing interest in both academia and industry for this research area [14][15][16][17][18][19].…”