Highly selective chromium(III) ethylene trimerization catalysts with [NON] and [NSN] heteroscorpionate ligands

Article abstract of DOI:10.1021/om8005239

Cr(III) complexes with [NON] and [NSN] heteroscorpionate ligands derived from bis(pyrazol-l-yl)methane have been prepared, which are active for ethylene trimerization to 1-hexene with high selectivity. Structural studies have established the coordination of the ether or thioether pendant group, which leads to a bicyclic metallaheterocycle flanked by pyrazoyl rings.

Full text of DOI:10.1021/om8005239

Organometallics 2008, 27, 4277–4279
4277
Highly Selective Chromium(III) Ethylene Trimerization Catalysts
with [NON] and [NSN] Heteroscorpionate Ligands
Jun Zhang,† Pierre Braunstein,*,‡ and T. S. Andy Hor*,†
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Kent Ridge, Singapore,
and Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), UniVersite´ Louis
Pasteur, 4 rue Blaise Pascal, F-67070 Strasbourg Ce´dex, France
ReceiVed June 6, 2008
followed by ethylene insertion yields a metallacycloheptane
entity.7 Considerable debate, however, remains concerning the
catalytic initiation, the metal oxidation state, the nature of the
chain growth and termination steps, and, perhaps most impor-
tantly, the interplay of these factors in determining oligomer
selection.8 Although it is clear that the selectivity is sensitive
to the ligand environment of the metal, details of such
dependence remain largely speculative. It is therefore important
to explore a variety of ligand designs and examine the catalytic
efficiency and selectivity of the corresponding metal complexes.
Our interest in heterofunctional, especially hemilabile, ligands
is stimulated by their catalytic relevance.9 Accordingly, the
recent use of neutral heteroscorpionate ligands derived from
bis(pyrazol-1-yl)methane has attracted our attention because they
are adpatable to a range of metals, can be sterically and
electronically tuned, and constitute multidentate hybrid ligands.10
We report herein a novel Cr(III) catalytic system with hitherto
unknown heteroscorpionate pyrazolyl ligands of the type
Pz2CHCH2XR (Pz ) pyrazol-l-yl, X ) O, S, R ) alkyl, aryl)
(Scheme 1). Notwithstanding the large number of known
heteroscorpionates, very few contain an ether or thioether
pendant functional group, and such examples include anisolyl
Summary: Cr(III) complexes with [NON] and [NSN] het-
eroscorpionate ligands deriVed from bis(pyrazol-1-yl)methane
haVe been prepared, which are actiVe for ethylene trimerization
to 1-hexene with high selectiVity. Structural studies haVe
established the coordination of the ether or thioether pendant
group, which leads to a bicyclic metallaheterocycle flanked by
pyrazoyl rings.
The oligomerization of ethylene typically gives a broad
distribution of R-olefins which requires fractional distillation
of the products to give relatively low yields of the desired
fractions.1 In view of the importance of 1-hexene in the
production of linear low-density polyethylene (LLDPE), there
is a critical need to develop a methodology for selective
trimerization of ethylene to 1-hexene.2 Although this was first
achieved with Cr-based catalysts in 1977,3 there are very few
highly active and selective catalysts for this conversion and they
include the Phillips pyrrolide,4 the BP diphosphine,5 and the
Sasol mixed heteroatomic6 systems.
The mechanism of this selective catalytic oligomerization is
generally considered to involve metallacyclic intermediates.
Oxidative addition of two ethylene molecules to the metal
* To whom correspondence should be addressed. Fax: (+65) 6873 1324.
E-mail: andyhor@nus.edu.sg.
(7) Briggs, J. R. J. Chem. Soc., Chem. Commun. 1989, 11, 674.
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I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238. (b) Temple, C.; Jabri,
A.; Crewdson, P.; Gambarotta, A.; Korobkov, I.; Duchateau, R. Angew.
Chem., Int. Ed. 2006, 45, 7050. (c) Temple, C.; Gambarotta, S.; Korobkov,
I.; Duchateau, R. Organometallics 2007, 26, 4598. (d) McGuinness, D. S.;
Brown, D. B.; Tooze, R. P.; Hess, F. M.; Dixon, J. T.; Slawin, A. M. J.
Organometallics 2006, 25, 3605.
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Z. Q.; Teo, S.; Liu, Z. P.; Hor, T. S. A. Organometallics 2007, 26, 2950.
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2004, 689, 18. (h) Weng, Z. Q.; Teo, S.; Koh, L. L.; Hor, T. S. A.
Organometallics 2004, 23, 3603. (i) Yen, S. K.; Koh, L. L.; Hahn, F. E.;
Huynh, H. V.; Hor, T. S. A. Organometallics 2006, 25, 5105. (j) Teo, S.;
Weng, Z. Q.; Hor, T. S. A. Organometallics 2006, 25, 1199. (k) Speiser,
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† National University of Singapore.
‡ Universite´ Louis Pasteur.
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G. V. Z. Phys. Chem., Abt. B 1939, 43, 25. (c) Flory, P. J. J. Am. Chem.
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L. L.; Hor, T. S. A. Chem. Commun. 2006, 1319. (g) Late Transition Metal
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10.1021/om8005239 CCC: $40.75
2008 American Chemical Society
Publication on Web 08/05/2008

4278 Organometallics, Vol. 27, No. 17, 2008
Scheme 1. Synthesis of the Cr(III) Precatalysts 1-8
Communications
Table 1. Ethylene Trimerization with Complexes 1-8a
The exceptional performance of 4 prompted a more thorough
investigation of other ether derivatives. At 80 °C under 25 bar
ethylene and in the presence of 200 equiv of MAO, all of them
(notably 4, 6, and 7) give almost exclusively hexenes with
remarkable selectivity for 1-hexene (entries 4-9). The highest
activity of 16 200 g/((g of Cr) h) was found in 6, which has a
long hexyl chain protruding from the pendant group. It also
gives a high selectivity of 98.9% (entry 6). This could be
attributed to a better solubility in toluene compared to that of
4, which has a shorter ethyl chain. A similar trend has been
observed for the Sasol mixed heteroatomic SNS Cr systems, in
which the n-decyl-substituted precatalyst [CrCl3{HN(CH2CH2-
Sndecyl)2}] is significantly more active than those with shorter
alkyl chains.6b Introduction of bulkier substituents on the
chalcogen (compare entries 5 and 4 and compare entries 2 and
1) lowers the catalyst activity. For 6, the activity is higher when
the reaction is performed under higher dilution (compare entries
8 and 6). Replacement of the alkyl by an aryl group has no
significant influence in either the activity or the selectivty
(compare entries 9 and 4, 6). In the ether-based system,
replacement of the methyl group on the pyrazole rings by
hydrogen decreases significantly the selectivity toward hexenes
and favors the formation of polyethylene (PE) (compare entries
10 and 4-6). With a longer reaction time of 1 h, the activity of
6 decreases by a factor of 2 (compare entries 7 and 6), which
suggests a shorter catalyst lifetime. The activities are in general
amt (wt %)
entry
activity
(complex)
PE
<0.1 96.3 1.7 0.5
<0.1 94.7 5.2
<0.1 95.1 4.7 0.1
<0.1 97.3 0.7 1.3
<0.1 96.1 2.0 1.3
<0.1 98.9 0.6 0.3
<0.1 97.1 0.4 1.1
<0.1 98.1 0.5 0.7
<0.1 96.8 0.3 0.5
51.7 41.3 0.8 1.6
C6
C8 C10 gC12 1-C6 (g/((g of Cr) h))
1 (1)
2 (2)
3 (3)
4 (4)
5 (5)
6 (6)
7 (6)b
8 (6)c
9 (7)
10 (8)
1.4
0
99.2
96.1
97.2
99.3
97.9
99.6
99.3
99.5
99.0
99.5
2800
1050
0
0
3900
0.6
0.5
0.1
1.3
0.6
2.3
4.6
5400
2700
16200
9100
21000
11300
10200
a Conditions: 2.0 µmol of complex, 200 equiv of MAO, 25 bar of
ethylene, 6 mL of toluene, 80 °C, 20 min. The results shown are
representative of at least duplicated experiments. b 60 min. c 80 mL of
toluene.
(MeOC6H4),11 benzyl ether,10a and 2-thienyl groups.12 At
variance with such related ligand designs,13 we placed a
functionalized pendant arm on the spacer link between the
pyrazolyl moieties and avoided the presence of a coordinatively
more inhibitive heteroatom directly on the link. The combination
in a ligand framework of an ether or thioether function with
tunable pyrazole rings could also allow easy stereoelectronic
modifications, which is highly desirable in the study of ligand
effect on selectivity. Upon activation with excess methylalu-
minoxane (MAO), our catalysts generally exhibit considerable
activity and selectivity for ethylene trimerization.
The hybrid pyrazolyl ligands were prepared by heating a
mixture of the appropriate acetal with 2 equiv of pyrazole in
the presence of p-toluenesulfonic acid using a modified literature
procedure.14 Treatment of the appropriate pyrazolyl ligands with
[CrCl3(THF)3] in THF afforded the corresponding Cr(III)
complexes in good yields (85-91%) (Scheme 1).
The catalytic performances of the thioether complexes 1-3
toward ethylene trimerization are summarized in Table 1. With
MAO, these precatalysts are highly selective toward trimeriza-
tion (total C6 selectivities >94.7%) to 1-hexene (>96.1%) with
activities of 1050-3900 g/((g of Cr) h) (entries 1-3). The ether
analogue 4 is even more active and selective (entry 4). Whereas
the beneficial role of a soft donor6b such as S (as in thioether)
or P (in phosphine) has been documented, e.g. in the Sasol
mixed-donor systems, the effect of a harder donor (such as N
or O) is less established.
Figure 1. Molecular structure of 1 (30% probability ellipsoids, H
atoms omitted for clarity). Selected bond distances (Å) and angles
(deg): Cr-N(1) ) 2.102(2), Cr-N(3) ) 2.096(2), Cr-S )
2.4808(7), Cr-Cl(1) ) 2.2784(7), Cr-Cl(3) ) 2.3175(7), Cr-Cl(2)
) 2.3183(8); N(3)-Cr-N(1) ) 84.10(7), N(3)-Cr-Cl(1) )
91.87(6), N(1)-Cr-Cl(3) ) 174.38(6), Cl(1)-Cr-Cl(3) ) 93.36(3),
N(3)-Cr-Cl(2) ) 169.15(5), Cl(1)-Cr-Cl(2) ) 97.71(3),
N(1)-Cr-S ) 89.01(6), Cl(1)-Cr-S ) 176.07(3).
(11) Carrion, M. C.; D´ıaz, A.; Guerrero, A.; Jalo´n, F. A.; Manzano,
B. R.; Rodr´ıguez, A.; Paul, R. L.; Jeffery, J. C. J. Organomet. Chem. 2002,
650, 210.
(12) Canty, A. J.; Honeyman, R. T. J. Organomet. Chem. 1990, 387,
247.
(13) Junges, F.; Kuhn, M. C. A.; dos Santos, A. H. D. P.; Rabello,
C. R. K.; Thomas, C. M.; Carpentier, J.-F.; Casagrande, O. L. Organome-
tallics 2007, 26, 4010.
(14) Trofimenko, S. J. Am. Chem. Soc. 1970, 92, 5118.

Communications
Organometallics, Vol. 27, No. 17, 2008 4279
Å).15 This is consistent with the weak basicity of the thioether
function. The Cr-O bond in 7 (2.131(3) Å) is also longer than
those in related complexes, e.g. [CrCl3{bis[2-(3,5-dimethylpyra-
zolyl)ethyl)]amine}] (Cr-O ) 2.076(1) Å), but shorter than that
in [CrCl3{iamyl-N(2-MeO-4-tBuC6H3)2P}2] (Cr-O ) 2.156(2)
Å).16The Cr-Cl bonds trans to the thioether in 1 (2.2784(7) Å)
and to the ether in 7 (2.269(1) Å) are significantly shorter than
the other Cr-Cl bonds (2.3175(7), 2.3183(8) Å and 2.293(1),
2.296(1) Å, respectively). This, together with the potential
hemilability of the hybrid ligand, could facilitate the entry of
ethylene in the trimerization process.
The heteroscorpionate pyrazolyl ligands reported herein can
be readily prepared from inexpensive materials. They are stable
toward oxidation and hydrolysis. The one-step complexation
at room temperature, giving high yields (85-91%), represents
an additional advantage. Their excellent selectivity is generally
comparable with that of the most efficient ethylene trimerization
catalytic systems reported.4-6 While the activity appears to be
sensitive to the nature of the third donor (O or S), the selectivity
difference is less pronounced. The presence of a hard donor
atom (e.g., MeO ether group in the BP system5 and NH amine
in the Sasol system6) is an important element in these types of
hybrid ligands to support olefin oligomerization. The use of two
pyrazolyl groups in place of the common and generally softer
phosphines and thioethers has given the current system some
notable advantages. The fused and hybrid metallaheterocyclic
ring structures adds stability in an inherently labile or hemilabile
system. These hybrid tridentate ligands thus offer a mix of donor
characters that could help the metal respond to changes
throughout the catalytic cycle. These features could offer an
explanation to the notable activity and high selectivity observed.
Ongoing experiments are directed at the development of other
hybrid ring systems, especially for the use of heterocycles to
support olefin oligomerization.
Figure 2. Molecular structure of 7 · MeCN (30% probability
ellipsoids, MeCN and H atoms omitted for clarity). Selected bond
distances (Å) and angles (deg): Cr-N(1) ) 2.076(3), Cr-N(3) )
2.089(3), Cr-O ) 2.131(3), Cr-Cl(1) ) 2.269(1), Cr-Cl(3) )
2.293(1), Cr-Cl(2) ) 2.296(1); N(1)-Cr-N(3) ) 85.2(1), N(3)-
Cr-Cl(1) ) 91.05(9), Cl(1)-Cr-Cl(3) ) 94.68(5), N(1)-Cr-Cl(3)
) 173.35(9), Cl(1)-Cr-Cl(2) ) 96.63(5), N(3)-Cr-Cl(2) )
170.79(9), O-Cr-Cl(1) ) 172.49(8), N(1)-Cr-O ) 85.0(1).
significant (1050-21 000 g/((g of Cr) h)) under 25 bar of
ethylene pressure but are lower than those of the Cr-PNP
ethylene trimerization system developed by McGuinness and
Wasserscheid et al. (312-37 400 g/((g of Cr) h)), which was
run under a higher (40 bar) ethylene pressure.6a The Cr(III)
pyrazolyl ethylene oligomerization system developed by Car-
pentier and Casagrande et al. also achieved higher activities in
the range of 1834-70 664 g/((g of Cr) h) under 20 bar of gas
pressure.13
In order to establish the coordination mode of the heteroscor-
pionate ligands, complexes 1 and 7 were selected for structural
analysis by single-crystal X-ray diffraction. As there is no
example of a structurally characterized thioether or ether Cr
complex with a bis(pyrazol-1-yl)methane heteroscorpionate
ligand, it was important to establish the denticity and ligation
function of these generally perceived weak donors. Both
complexes display a distorted-octahedral geometry, with the Cr
atom facially capped by the tridentate heteroscorpionate ligand
and with coordination of all three donor atoms (Figures 1 and
2). Coordination of the ether or thioether function enables the
formation of a metallabicyclic structure with two connected six-
membered rings, flanked by two pyrazoyl rings. This offers an
explanation for the high stability of the complexes. The Cr-N bond
lengths span a relatively narrow range (2.096(2), 2.102(2) Å for 1
and 2.076(3), 2.089(3) Å for 7). The Cr-S bond in 1 (2.4808(7)
Å) is slightly longer than those in similar Cr(III) precatalysts such
as [CrCl3{HN(CH2CH2SEt}2] (Cr-Sav ) 2.453 Å), [CrCl3{2,6-
bis(cyclohexylthiomethyl)pyridine}] (Cr-Sav ) 2.439 Å), and
[CrCl3L] (L ) mixed PNS or SNS ligand, Cr-S ) 2.403-2.456
Acknowledgment. We are grateful to the Agency for
Science, Technology & Research (Singapore), the National
University of Singapore (NUS) (Grant No. R143-000-277-
305), the CNRS and the Ministe`re de la Recherche (Paris),
and the French Embassy in Singapore for financial support.
Technical support from the CMMAC of the Chemistry
Department of the NUS is appreciated.
Supporting Information Available: Text and figure giving
details of the syntheses and characterization data for all new
compounds and CIF files giving X-ray crystallographic data for 1
and 7. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM8005239
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