Switchable chromium(II) ethylene oligomerization/polymerization catalyst

Article abstract of DOI:10.1021/om050699n

The tri- and divalent chromium derivatives of [N-Me,tripyrr] ([N-Me,tripyrr] = 2,5-bis(diphenyl(pyrrol-2-yl)methyl)-1 -methylpyrrole) [N-Me, tripyrr]CrCl (1), and [N-Me,tripyrr]Cr(L)₂·(THF)_0.5 (2) [L = THF (2a), pyridine (2b)] were obtained in good yield from the reaction of the dipotassium salt of the ligand with CrCl₃-(THF)₃ and CrCl₂(THF)₂, respectively. Reaction of the dilithium salt of the ligand with CrCl₂(THF)₂ led to the divalent analogue [N-Me,tripyrr]Cr(μ-Cl)Li(THF)₃ (3), wherein a Li-Cl unit was retained by the metal center. The compounds were tested for ethylene oligomerization activity upon activation with two different alumoxanes. In the case of MAO, the Cr(II) catalyst precursors 2b and 3 showed a marked increase in activity with respect to the trivalent 1. The product distributions from compounds 1 and 2b were nearly identical, indicating that the Cr(II) oxidation state provides better catalyst precursors. Use of the i-BuAlO activator instead of MAO afforded exclusively polymerization with high activity and produced linear, low molecular weight polyethylene.

Full text of DOI:10.1021/om050699n

5214
Organometallics 2005, 24, 5214-5216
Switchable Chromium(II) Ethylene Oligomerization/
Polymerization Catalyst
Patrick Crewdson,† Sandro Gambarotta,*,† Marie-Charlotte Djoman,†
Ilia Korobkov,† and Robbert Duchateau*,‡
Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada,
and Department of Chemistry, Eindhoven University of Technology, P. O. Box 513,
5600 MB, The Netherlands
Received August 12, 2005
Summary: The tri- and divalent chromium derivatives
of [N-Me,tripyrr] ([N-Me,tripyrr] ) 2,5-bis(diphenyl-
(pyrrol-2-yl)methyl)-1-methylpyrrole) [N-Me,tripyrr]CrCl
(1), and [N-Me,tripyrr]Cr(L)2‚(THF)0.5 (2) [L ) THF (2a),
pyridine (2b)] were obtained in good yield from the
reaction of the dipotassium salt of the ligand with CrCl3-
(THF)3 and CrCl2(THF)2, respectively. Reaction of the
dilithium salt of the ligand with CrCl2(THF)2 led to the
divalent analogue [N-Me,tripyrr]Cr(µ-Cl)Li(THF)3 (3),
wherein a Li-Cl unit was retained by the metal center.
The compounds were tested for ethylene oligomerization
activity upon activation with two different alumoxanes.
In the case of MAO, the Cr(II) catalyst precursors 2b and
3 showed a marked increase in activity with respect to
the trivalent 1. The product distributions from com-
pounds 1 and 2b were nearly identical, indicating that
the Cr(II) oxidation state provides better catalyst precur-
sors. Use of the i-BuAlO activator instead of MAO
afforded exclusively polymerization with high activity
and produced linear, low molecular weight polyethylene.
cycle. While the reductive coupling of ethylene is a two-
electron process, the oxidation state of chromium in the
catalytically active intermediate remains uncertain5
despite the fact that all the known chromium catalyst
precursors are based on the trivalent state as a rule with
very few exceptions.2a,4a,7
The ability of the [PNP]CrCl3 [PNP ) Ar2PN(R)PAr2;
R ) Cy, Me; Ar ) Ph, Ar] catalyst to exceed the Flory-
Schultz distribution,8 effectively producing the highly
desirable 1-octene, has recently marked a milestone in
this field.4 A subsequent investigation aimed at isolating
intermediates of this remarkable system9 has pointed
out that the catalyst precursor (1) is more active if it is
(3) (a) Briggs, J. R. (Union Carbide) US-A 4,668,838. (b) Araki, Y.;
Nakamura, H.; Nanba, Y.; Okano, T. (Mitsubishi Chemicals) US-A
5,856,612, 1997. (c) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.; Grimm,
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Wasserscheid, P.; Kuhlmann, S. J. Am. Chem. Soc. 2004, 126, 14712.
(g) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Morgan, D. H.;
Dixon, J. T.; Bollmann, A.; Maumela, H.; Hess, F.; Englert, U. J. Am.
Chem. Soc. 2003, 125, 5272. (h) McGuinness, D. S.; Wasserscheid, P.;
Morgan, D. H.; Dixon, J. T. Organometallics 2005, 24, 552.
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Wass, D. F. Chem. Commun. 2002, 858. (b) Blann, K.; Bollmann, A.;
Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H.; Morgan, D. H.;
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E.; Maumela, H.; Morgan, D. H.; Neveling, A.; Otto, S. Chem. Commun.
2005, 622.
Since the original discovery by Manyik of the ability
of a mixture of CrCl3, pyrrole, and MAO to transform
ethylene into 1-hexene with high yield and good selec-
tivity,1 this fascinating process has received a steady
increase in attention.2 Today, encouraging progress has
been made in the direction of making the reaction more
selective.3,4 There is a rather general consensus that the
oligomerization proceeds with a preliminary reductive
coupling of ethylene and formation of a chromocyclo-
pentane intermediate,5,6 followed by further insertions
before reductive elimination completes the catalytic
(5) (a) de Bruin, T. J. M.; Magna, L.; Raybaud, P.; Toulhoat, H.
Organometallics 2003, 24, 3404. (b) Tobisch, S.; Ziegler, T. Organo-
metallics 2005, 26, 256, and references therein. (c) Blok, A. N. J.;
Budzelaar, P. H. M.; Gal, A. W. Organometallics 2003, 22, 2564. (d)
van Rensburg, W. J.; Grove, C.; Steynberg, J. P.; Stark, K. B.; Huyser,
J. J.; Steynberg, P. J. Organometallics 2004, 23, 1207. (e) Tobisch, S.;
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J. T.; Nair, J. J.; Hunter, R. Adv. Synth. Catal. 2003, 939. (h) Yang,
Y.; Kim, H.; Lee, J.; Paik, H.; Jang, H. G. Appl. Catal. A 2000, 193,
29. (i) Bercaw, J. E. The 15th International Symposium on Olefin
Metathesis and Related Chemistry (ISOM XV) July 28-Aug 1, 2003,
Kyoto, Japan. (j) Theopold, K. H. Eur. J. Inorg. Chem. 1998, 15. (k)
Meijboom, N.; Schaverien, C. J.; Orpen, A. G. Organometallics 1990,
9, 774. (l) Fang, Y.; Liu, Y.; Ke, Y.; Cuo, C.; Zhu, N.; Mi, X.; Ma, Z.;
Hu, Y. Appl. Catal. A Gen. 2002, 235, 33. (m) Ko¨hn, R. D.; Haufe, M.;
Mihan, S.; Lilge, D. Chem. Commun. 2000, 1927. (n) Agapie, T.;
Schofer, S. J.; Labinger, J.; Bercaw, J. E. J. Am. Chem. Soc. 2004,
126, 1304.
* To whom correspondence should be addressed. E-mail: sgambaro@
science.uottawa.ca.
† University of Ottawa.
‡ Eindhoven University of Technology.
(1) Manyik, R. M.; Walker, W. E.; Wilson, T. P. (Union Carbide
Corporation) US 3300458, 1967.
(2) (a) Reagan, W. K. (Phillips Petroleum Company) EP 0417477,
1991. (b) Conroy, B. K.; Pettijohn, T. M.; Benham, E. A. (Phillips
Petroleum Company) EP 0608447, 1994. (c) Freeman, J. W.; Buster,
J. L.; Knudsen, R. D. (Phillips Petroleum Company) US 5,856,257,
1999. (d) Reagan, W. K.; Freeman, J. W.; Conroy, B. K.; Pettijohn, T.
M.; Benham, E. A. (Phillips Petroleum Company) US 5,451,645, 1995.
(e) Sugimura, K.; Nitabara, T. S.; Fujita, T. (Mitsui Chemicals
Incorporated) JP 10,324,710, 1998. (f) Ban, K.; Hayashi, T.; Suzuki,
Y. (Mitsui Chemicals Incorporated) JP 11,060,627, 1999. (g) Salo, V.
B.; Guan, Z. Organometallics 2003, 22, 5033. (h) Wilke, G. Angew.
Chem., Int. Ed. Engl. 1988, 27, 185. (i) Andes, C.; Harkins, S. B.;
Murtuza, S.; Oyler, K.; Sen, A. J. Am. Chem. Soc. 2001, 123, 7423. (j)
Romano, A. M.; Proto, A.; Santi, R.; Sommazzi, A.; Grande, M.; Masi,
F. PCT Int. Appl. WO 0168572, 2001. (k) McGuinness, D. S.; Gibson,
V. C.; Wass, D. F.; Steed, J. W. J. Am. Chem. Soc. 2003, 125, 12716.
(l) Ruther, T. H.; Braussaud, N.; Cavell, K. J. Organometallics 2001,
20, 1247.
(6) See for example: Dixon, J. T.; Green, M.; Hess, F. M.; Morgan,
D. H. J. Organomet. Chem. 2004, 689, 3641.
(7) See for example: (a) Oguri, M.; Mimura, H.; Aoyama, H.; Okada,
H.; Koie, Y. (Tosoh Corporation) JP 11,092,407, 1999. (b) Murakita,
S.; Yamamoto, T.; Okada, H.; Osamu, Y. JP 2,001,149,788, 2001.
(8) Flory, P. J. J. Am. Chem. Soc. 1936, 58, 1877.
(9) Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau,
R. Submitted.
10.1021/om050699n CCC: $30.25 © 2005 American Chemical Society
Publication on Web 09/23/2005

Communications
Organometallics, Vol. 24, No. 22, 2005 5215
Scheme 1
Scheme 2
based on divalent chromium; (2) may possess a cationic
nature; and (3) may retain a Cl-AlMe3 in the coordina-
tion sphere of the metal center. These findings, which
gave some insight into this catalytic system, prompted
us to try to probe their generality. Given the well-known
ability of the pyrrolide anion to promote very high
catalytic activity,7 we have selected the dianion of
2,5-bis(diphenyl(1H-pyrrol-2-yl)methyl)-1-methylpyr-
role [N-Me,tripyrr] as a ligand. The rationale for this
selection was twofold. The presence of two anionic
pyrrolide rings and consequent chelating effect were
regarded as beneficial for preventing ligand dissociation.
Second, the presence of a third pyrrole ring in the ligand
framework, with the N atom alkylated and held by the
chelating geometry at a close proximity to the metal
center in a sort of “forced π-bonding”, could provide
electron density on demand to the metal center via
slippage of the π-bonded ring. This idea was suggested
by the fact that ligand hemilability may be a winning
card for good activity and selectivity, as in the case of a
Ti-based catalytic system.10
afforded the corresponding tri- and divalent complexes
[N-Me,tripyrr]CrCl (1) (Scheme 1) and [N-Me,tripyrr]-
Cr(THF)2 (2a) (Scheme 2).11 Complex 2a could not be
separated from the KCl byproduct and recrystallized
due to its very poor solubility. Nevertheless, its in situ
treatment with pyridine conveniently transformed this
species into the corresponding bis-pyridine adduct
[N-Me,tripyrr]Cr(pyridine)2‚(THF)0.5 (2b), which was
isolated in crystalline, analytically pure form and high
yield (94%).11 The crystal structures of both 1 and 2b11
showed the expected arrangement. The only substantial
difference arises from the π-oriented alkylated pyrrole
ring, which shows a regular π-bonding in the first case
but gives instead a nonbonding contact in the second
[Cr-centroid ) 2.00 and 2.70, respectively]. This is
probably the result of the diminished Lewis acidity of
the divalent chromium atom, in turn confirming that
the ligand may provide π-coordination on demand.
When the reaction of CrCl2(THF)2 was carried out
with the dilithium salt of the ligand, another divalent
complex, [N-Me, tripyrr]Cr(µ-Cl)Li(THF)3 (3), was ob-
tained in crystalline form and moderate yield.11 The
crystal structure (Scheme 2) of this species10 differs from
1 in two main points. First, the chlorine atom is bridging
a Li(THF)3 unit, and second, there is only one bonding
The reaction of the dipotassium salt of the tripyr-
role ligand with either CrCl3(THF)3 or CrCl2(THF)2
(10) (a) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem.,
Int. Ed. 2001, 40, 2516. (b) Deckers, P. J. W.; Hessen, B.; Teuben, J.
H. Organometallics 2002, 21, 5122.
(11) See Supporting Information.

5216 Organometallics, Vol. 24, No. 22, 2005
Communications
Table 1. Catalytic Activity Results with 2000 equiv of MAO, 40 bar, 50 °C, 30 µmol of Catalyst, 150 mL
Total Toluene Volumea
product distribution 1-olefins (mol %)
olefin activity
(g/g Cr h)
catalyst
olig. (g)
PE (g)
C4
C6
C8
C10
C12
C14
1
3813
23 296
20 708
n/a
8.25
54.0
48.0
0
1.75
2.30
16.2
0.42
14.74
12.35
0
40.09
36.77
13.87
0
22.50
22.68
32.74
0
11.61
13.16
23.29
0
5.98
7.77
15.41
0
3.36
2.70
9.18
0
2b
3
blank
0
a Blank reaction was carried out with CrCl2(THF)2.
contact with the former π-oriented ring with the N atom
[Cr-N(1) ) 2.239(13) Å], which in turn assumes a
rather obvious sp3 character.
lated pyrrole ring, this has a direct impact on the metal
Lewis acidity. On the other hand, the coordination of a
chlorine atom attached to other metallic residues such
as Me3Al has indeed been indicated by both experi-
mental5h and theoretical work5d as an important point
for both high catalytic activity and preferential forma-
tion of 1-octene. Therefore, it is tempting to suggest that
this structural motif may be somehow preserved during
the activation of 3 by MAO.
Finally, activation of 2b with 400 equiv of IBAO
(tetraisobutyldialuminumoxane) afforded an even more
unexpected change of reactivity. First, no oligomeriza-
tion products could be detected in the reaction mixture.
Second, the reaction produced only highly linear, low
molecular weight polyethylene (MW ) 11 000, PD ) 2.3)
with a rather respectable activity (884 kg PE/mmol/h)
when compared to other existing chromium polymeri-
zation catalysts.13 This unprecedented switch from
oligomerization toward polymerization, as an exclusive
function of the nature of the Al activator, is a fact with
no straightforward explanation. Its understanding will
provide a further step toward clarifying the factors that
control the reactivity and performance of divalent
chromium catalysts.
The catalytic activity of these species in terms of
ethylene oligomerization (Table 1) was very high while
in combination with a large amount of MAO. The
product distribution (Table 1) clearly indicates that
complexes 1 and 2b are related given the very similar
distribution of oligomers and the small amount of PE
formed. In the presence of a reducing medium, such as
provided by the MAO activator, it is hard to imagine a
reaction pathway where the divalent 2b is oxidized to
a higher oxidation state. Vice versa, it is quite reason-
able to expect that the trivalent 1 may instead be
reduced to the divalent state.12 Therefore, we propose
that the trivalent 1 is simply a precursor to a divalent
species. Accordingly, the activity of the divalent 2b was
found to be 6 times greater than the trivalent 1. This is
remarkable while considering that 2b has two molecules
of pyridine coordinated to the metal center. On the other
hand, complex 2b displays reversible color change from
purple to emerald green in warm toluene, most likely
as a result of pyridine dissociation equilibrium. Inter-
esting questions arise about the role of MAO in the
activation process of 2b. Obviously, further reduction
to the monovalent state cannot be ruled out at this
stage.
The divalent 3 displays a somewhat different type of
selectivity in the sense that 1-octene is the major
component of the reaction mixture. Also no significant
amount of C4 was detected in this case despite identical
workup and the quantity of PE was substantially larger.
It should be reiterated that the retention of LiCl through
the bridging chloride, instead of the presence of the
two pyridines, is the only difference between the two
divalent compounds 3 and 2b. As indicated by the
different interaction of the metal center with the alky-
Acknowledgment. This work was supported by
the Natural Science and Engineering Council of
Canada (NSERC) and by the Eindhoven University of
Technology.
Supporting Information Available: Complete crystal-
lographic data (CIF) for the complexes and full details on
preparation, characterization, and oligomerization experi-
ments. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM050699N
(13) For highly active Cr catalysts see: (a) Esteruelas, M. A.; Lopez,
A. M.; Mendez, L.; Olivan, M.; Onate, E. Organometallics 2003, 22,
395-406. (b) Enders, M.; Fernandez, P.; Ludwig, G.; Pritzkow, H
Organometallics 2001, 20, 5005.
(12) See for example: Sugiyama, H.; Aharonian, G.; Gambarotta,
S.; Yap, G. P. A.; Budzelaar, P. H. J. Am. Chem. Soc. 2002, 124, 12268.