Angewandte
Chemie
DOI: 10.1002/anie.201003465
Catalyst Design
Towards Selective Ethylene Tetramerization**
Sebastiano Licciulli, Indira Thapa, Khalid Albahily, Ilia Korobkov, Sandro Gambarotta,*
Robbert Duchateau,* Reynald Chevalier, and Katrin Schuhen
The metallacycle mechanism formulated to account for the
excellent selectivity of catalytic ethylene-trimerization pro-
cesses is today well-established.[1–3] Further ring expansion
through the insertion of a fourth ethylene unit is believed to
afford 1-octene.[4] However, selectivity becomes a major
challenge in this event. If a fourth molecule of ethylene can be
inserted readily into a seven-membered ring, it is clearly very
difficult to prevent further expansion of the nine-membered
ring (and thus the formation of heavier oligomers). In fact, in
the entire patent and academic literature, only two homoge-
neous catalytic systems, discovered by researchers at Sasol[4a]
and SK Energy,[4b] have been described that are capable of
producing a substantial excess of 1-octene (about 70%) over
other a-olefins. The search for a catalytic system capable of
producing 1-octene as the only product is still being pursued
actively. One may even question whether selective tetrame-
rization to produce solely 1-octene may ever be possible,
unless an alternate mechanistic pathway is followed.
C3H6Si(OEt)3; 1g: R = C4H8OEt) were synthesized
readily. The monomethyl derivative [{(2-
CrIII
C5H4N)2NCH2SiMe3}CrMeCl2(thf)] (2c) was prepared by
treating 1c with a stoichiometric amount of MeLi in THF or
by the direct treatment of [CrMeCl2(thf)3] with the ligand.
The structures were all very similar (Figure 1).
In our search for highly selective ethylene-tetramerization
catalysts, we selected the 2,2’-dipyridylamine ligand with an
alkylated central nitrogen atom. Alkylation of the central
N atom to prevent anionization was deemed necessary to
maintain the possibility of cationizing the monovalent metal
center, as necessary for catalytic activity.[1p] Alkylation of the
central nitrogen atom was also expected to diminish the
established tendency of the ligand to form multiply bonded
dimers or higher aggregates,[5] because of sterically induced
deformation of the ligand backbone.[6]
Figure 1. Thermal-ellipsoid (50% probability) plots of 1b, 1g, and
2c.[11]
When activated with methylaluminoxane (MAO) at 508C,
all trivalent complexes underwent a vigorously exothermic
reaction after an induction period of several minutes to form
large amounts of a heavy a-olefin. In all cases, the 13C NMR
spectrum showed the presence of a vinylic residue and a
complete lack of branching: features indicative of linear a-
olefins. Under isoparabolic conditions, with temperatures
rising to a maximum of 1108C, we observed that, aside from
polyethylene (PE) wax, a sizeable amount of highly pure 1-
octene was formed, as shown by NMR spectroscopy and GC–
MS (Figure 2). The thermal behavior of the reaction was
remarkable. After an induction period of about 4 min, the
temperature increased very rapidly to reach about 1108C.
When the reaction temperature was maintained constant at
808C with the aid of a cooling coil in the interior of the
reactor, the waxy a-olefin appeared to be the sole product of
the reaction (only traces of 1-octene were detected; Table 1,
entry 7). A lower catalyst loading and variable pressure did
not affect the activity significantly.[7] Attempts to diminish the
amount of PE wax by carrying out the catalytic reaction in the
presence of hydrogen gas did not affect the outcome. MAO
appears to be the only usable activator, since no catalytic
activity was observed with other common alkyl aluminum
compounds, including trimethylaluminum (TMA), triiso-
A range of substituted (2-C5H4N)2NR derivatives and the
corresponding trivalent chromium complexes [{(2-
C5H4N)2NR}CrCl3(thf)] (1a: R = Me; 1b: R = CH2CMe3;
1c: R = CH2SiMe3; 1d: R = C16H33; 1e: R = benzyl; 1 f: R =
[*] Dr. S. Licciulli, I. Thapa, K. Albahily, Dr. I. Korobkov,
Prof. Dr. S. Gambarotta
Department of Chemistry, University of Ottawa
10 Marie Curie, Ottawa, ON K1N 6N5 (Canada)
Fax: (+1)613-562-5170
E-mail: sgambaro@uottawa.ca
Dr. R. Duchateau
Department of Chemistry, Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
E-mail: r.duchateau@tue.nl
Dr. R. Chevalier, Dr. K. Schuhen
Lyondellbasell Industries, Polyolefine GmbH
Industriepark Hoechst, 65926 Frankfurt (Germany)
[**] This research was supported by the Natural Science and Engineer-
ing Council of Canada (NSERC), by the Eindhoven University of
Technology, and by LyondellBasell.
butylaluminum,
tetraisobutyldialuminoxane,
triethyl-
aluminum, and diethylaluminum chloride.[8] Interestingly,
even the use of TMA-depleted MAO gave no reaction.
Conversely, when a small amount of TMA (10%) was added
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2010, 49, 9225 –9228
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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