Isolation of single-component trimerization and polymerization chromium catalysts: The role of the metal oxidation state

Article abstract of DOI:10.1002/anie.200803434

(Chemical Equation Presented) One more trimer: A series of chromium complexes of different oxidation states are employed as catalysts for the polymerization/oligomerization of ethylene. The results suggest a link between chromium oxidation state and catalytic performance, with Cr^III leading to oligomerization, Cr^II to polymerization, and Cr ^I to selective trimerization.

Full text of DOI:10.1002/anie.200803434

Angewandte
Chemie
DOI: 10.1002/anie.200803434
Catalyst Design
Isolation of Single-Component Trimerization and Polymerization
Chromium Catalysts: The Role of the Metal Oxidation State**
Amir Jabri, Chris B. Mason, Yan Sim, Sandro Gambarotta,* Tara J. Burchell, and
Robbert Duchateau*
Chromium catalysts account for over 90% of the patent and
scientific literature of ethylene oligomerization.[1] This metal
also provides many potent polymerization catalysts.[2] There-
fore, its complexes are ideal systems for studying the factors
that determine catalyst activity and selectivity for polymer-
ization or selective oligomerization of ethylene, respectively.
One of the main unsolved issues in all the chromium systems
is the metal oxidation states responsible for either ethylene
polymerization or selective oligomerization. Given the com-
pletely different nature of the two mechanisms (nonredox[3]
versus redox[4]), the metal oxidation state is expected to play a
critical role in determining the type of process and its
selectivity. Whereas trivalent chromium complexes are gen-
erally used as catalyst precursors for both polymerization and
oligomerization, it is well-established that trivalent organo-
chromium species have the tendency to spontaneously reduce
towards the + 2 or even lower oxidation states.[5] Accordingly,
chromium(II) complexes can also be catalyst precursors for
both polymerization and oligomerization processes and
regularly display a catalytic activity higher than their trivalent
counterparts.[6] Conversely, further reduction towards the
Isolating active intermediates of the catalytic cycle is
central to the clarification of different aspects of the complex
mechanistic puzzle and, most of all, addressing the problem of
selectivity. For this purpose, we have recently analyzed two
established selective oligomerization systems,[5] demonstrat-
ing that the CrIII precursor is readily reduced to the + 2
oxidation state. By isolating reactive intermediates, we also
realized that the CrII complex may be reoxidized to CrIII by
disproportionation,[5d] suggesting that CrII produces polyeth-
ylene or a Schulz-Flory distribution of ethylene oligomers by
a nonredox Cossee-Arlman mechanism, whereas CrIII might
form a redox couple with CrI that is responsible for selective
ethylene oligomerization.
To gain greater insight into the chromium oxidation state,
we have focused on the highly active and selective Phillips
trimerization catalyst.[1a] This remarkable system consists of a
mixture of pyrrole, a chromium salt, and a mixture of Et3Al
and Et2AlCl. The ratio AlX3/Cr [X = Et, Cl] was unusually
low in this system (< 10:1), suggesting that a single-compo-
nent, catalytically active species could be isolated and
characterized and, in turn, elucidating the metal oxidation
state responsible for such an outstanding selective catalytic
activity. Herein we report our findings.
As expected, isolation of pyrrole-based single-component
catalysts proved problematic, with the preferred 1,2-dime-
thylpyrrole ligand, as recommended by the original Phillips
patent,[1a] yielding only oily or microcrystalline products. By
using 2,3,4,5-tetrahydro-1H-carbazole (also recommended in
the Phillips patent) and Cr(2-ethylhexanoate)3, [CrCl3(thf)3],
or [CrCl2(thf)2] in the presence of AlEt3 and AlEt2Cl, we
isolated characterizable products. By using [CrCl3(thf)3] or
[CrCl2(thf)2] and following the method, outlined in the patent,
of treating the chromium precursor with a mixture of the
ligand and AlEt3, the same square-planar CrII species, [Cr{h2-
AlEt2Cl(2,3,4,5-C12H4N)}2] (1) was obtained as a blue para-
magnetic crystalline material (meff = 4.78 mB, Scheme 1). The
use of AlEt2Cl in combination with AlEt3, as required by the
patent method,[1a] did not substantially modify the outcome of
the reaction in this case. Conversely, when halogen-free
chromium(III) octanoate (also suggested in the original
patent)[1] was used, the presence of AlEt2Cl was crucial to
the successful isolation of a characterizable material. The
reaction afforded the new paramagnetic CrI species [Cr{h6,h6-
(C12H12NAlEt2)2(m-Cl)}] (2) in crystalline form. Complex 2
can also be obtained from the one-electron reduction of 1
monovalent state is also a realistic alternative scenario.[7]
A
CrI intermediate might be sufficiently reducing to start the
redox ring-expansion mechanism, leading to oligomerization.
Furthermore, the tendency of CrI to form p complexes with
ethylene and even with arenes is documented.[8] Seminal work
by Jolly and co-workers has further demonstrated the
tendency of a deliberately prepared seven-membered metal-
lacycle to reductively eliminate towards CrI and 1-hexene.[7b]
Although not a catalyst system for trimerization, this system
lends substantial credibility to the ring-expansion mechanism
powered by the CrI/CrIII redox couple, and encourages further
efforts to isolate and prepare CrI derivatives.
[*] A. Jabri, C. B. Mason, Y. Sim, Prof. Dr. S. Gambarotta,
Dr. T. J. Burchell
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 (The Netherlands)
E-mail: r.duchateau@tue.nl
[**] This work was supported by the Natural Science and Engineering
Council of Canada (NSERC) and by the Dutch Polymer Institute.
W. K. Gerritsen (University Eindhoven) is gratefully acknowledged
for the GPC analysis.
with potassium metal. The magnetic moment of 2 (meff
=
1.87 mB) was as expected for the low-spin configuration of d5
CrI in a pseudooctahedral ligand field with a formal electron
count of 17.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803434.
Angew. Chem. Int. Ed. 2008, 47, 9717 –9721
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9717

Communications
Scheme 1. Synthesis of complexes 1 and 2.
Figure 1. Partial molecular structures of 1 and 2 with thermal ellipsoids
set at 50% probability. Hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢀ] and angles: 1: Cr1-N1 2.1477(17), Cr1-Cl1
2.3589(9), Al1-N1 1.9510(15), Al1-Cl1 2.3139(10), N1-Cr1-Cl1 84.21(5),
Al1-Cl1-Cr1 83.23(4), N1-Al1-Cl1 89.97(6), Cl1A-Cr1-Cl1 180.0, N1A-
Cr1-Cl1 95.79(5); 2: Al1-N1 1.887(4), Al1-Cl1 2.326(2), Al2-Cl1
2.337(2), Al2-N2 1.884(4), N1-Al1-Cl1 103.56(14), Al1-Cl1-Al2
117.55(8), N2-Al2-Cl1 101.26(14), Cr1-centroid 1.64(5), centroid-Cr1-
centroid 176.2(5).
The formation under identical conditions of 1 and 2,
containing CrII and CrI, respectively, depending on the
counteranion of the chromium starting salt, clearly reiterates
the ease of chromium reduction in these systems. The further
reduction to the monovalent state, as obtained by using a
nonchlorinated starting material, is not completely surprising.
It can be readily explained by assuming more extensive
alkylation of the octanoate, compared to the chloride, in the
early stage of the reaction. Interestingly, the structure of 2
contains one chlorine atom, acquired from AlEt2Cl.
In all cases, the connectivity has been elucidated by single-
crystal X-ray structure determinations (Figure 1). Complexes
1 and 2 display either the characteristic square-planar or
sandwich coordination geometry, as expected for high-spin d4
CrII and low-spin d5 CrI ions, respectively. In complex 1, the
coordination environment is defined by the N atoms of the
carbazolyl ligands, which, in turn, bridge an AlEt2Cl residue.
The chlorine atoms bridge between the aluminum atoms and
chromium atom, thus occupying the two remaining coordi-
nation sites of the square-planar chromium center. Complex 2
comprises a chromium(I) center sandwiched between the
phenyl moieties of the two carbazolyl ligands, in a structure
closely reminiscent of chromium(I) bis(arene) complexes.[9]
Each nitrogen atom of the carbazolyl anions is s bonded to an
Et2Al residue. The two aluminum atoms are connected
through one residual chlorine atom, which is too distant
from chromium to form a bonding interaction.
simple exposure to ethylene gas (Table 1). An interesting
comparison can be made between this species and the
recently reported [V(h5-{2,5-Me2C4H2N(AlClMe2)})2], which
also acts as a single-component, single-site polymerization
catalyst.[10] The unique ability of this species to be catalytically
À
active without a preformed V C bond was explained by
assuming ring slippage and switching of the pyrrolide bonding
mode from p to s, prior to dissociation and alkyl transfer from
Al to V. From this point of view, the s-ligated complex 1 may
be regarded as being one stage further toward self-trans-
formation into the catalytically active species, only requiring
alkyl transfer and dissociation of the neutral (L)AlR2 residue.
The square-planar coordination geometry, as dictated by the
d4 electronic configuration of CrII, is clearly the factor
responsible for this difference to the vanadium catalyst.
The monovalent state of chromium is known to be
stabilized by aromatic compunds (for example, toluene used
as a solvent), with which it forms catalytically inactive h6-
arene compounds.[8] Accordingly, monovalent 2 produces
only insignificant traces of polymer while in solution in
aromatic solvents (Table 1). However, in methylcyclohexane,
Complex 1 acts as a single-component ethylene polymer-
ization catalyst, initiating a fast polymerization reaction upon
Table 1: Catalytic performances of complexes 1–3
Catalyst
Solvent
Cocatalyst
Activity
Polymer
[g]
Total
olig. [mL]
1-hexene
[%]
C8
[%]
C10 (isomers)
[%]
[gmmolÀ1 hÀ1
]
(1,2-dimethylpyrro- methylcyclohexane AlEt3 (7 equiv) 1170
0.2
11.5
1.5
92
4
4
4
le)[a]
AlEt2Cl
(7 equiv)
(2,3,4,5-tetrahydro- methylcyclohexane AlEt3 (7 equiv) 150
Traces
93
3
1H-carbazole)[a]
AlEt2Cl
(7 equiv)
none
none
none
1
2
2
3
methylcyclohexane
methylcyclohexane
toluene
320
220
20
3.2
Traces
0.2
none
2.2
traces
2.7
–
93
<1
30
–
4
–
–
3
–
methylcyclohexane
none
270
traces
17
10
[a] Benchmark chromium catalysts with the stated ligands.[1a] All runs carried out under 600 psi at 1158C for 30 min. with 20 mmol of catalyst in a total
volume of 100 mL.
9718
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Chemie
2 is an unprecedented single-component trimerization cata-
lyst, producing 1-hexene with only trace amounts of higher
oligomers. As in the Phillips catalytic system, 2 is inactive in
toluene, even at elevated temperatures.[1a] The poisoning
behavior of toluene in the Phillips catalytic systems can now
be understood in terms of the nature of the catalytically active
species (Scheme 2). Assuming that complex 2 dissociates one
ligand to vacate the necessary coordination sites, in a similar
manner to [V(h5-{2,5-Me2C4H2N(AlClMe2)})2], it is conceiv-
Figure 2. Molecular structure of 3 with thermal ellipsoids set at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond
lengths [ꢀ] and angles [8]:Cr1-Cr1A 2.2992(15), Cr1-centroid 1.94(6),
Cr1-C5 2.063(5), Cr1-C9 2.060(5), Cr1-C5A 2.075(5), Cr1-C9A 2.088(4),
Al1-N1 2.015(4), C5-C9A 1.343(6), Si1-C5 1.875(5), C9-Cr1-C5
100.67(18), C9A-C5-Si1 140.1(4).
nonselective, single-component oligomerization catalyst
(Table 1). As a trivalent chromium complex,[11] this species
was not expected to be a single-component selective catalyst.
In fact, the lack of selectivity seems to rule out that alkyne
reductive elimination might occur by simple dissociation. In
this event, it would have been reasonable to expect the same
level of selectivity reported in the Phillips patent. Thus, dimer
versus monomer dissociation probably generates empty
coordination sites and in turn initiates the catalytic cycle.
Unsurprisingly, the activity was only moderate, albeit signifi-
cant, and no evidence for incorporation of alkyne in the
oligomers was detected in the GC of catalytic mixtures. No
evidence for alkyne dissociation could be found in the NMR
spectra of the reaction mixtures. Again, unless one assumes
that alkyl transfer from aluminum to chromium occurs, there
Scheme 2. Catalytic activity of complexes 1 and 2. P.E.=polyethylene.
able that toluene could simply form an inert {CrI(p-arene)}
structure.[8] The same ligand dissociation in methylcyclohex-
ane would allow ethylene to coordinate, without the unfav-
orable competition of other p systems. In contrast to complex
1, dissociation of a LAlR2 fragment from 2 does not imply
alkyl transfer, which is, furthermore, unnecessary for trime-
rization by a redox ring-expansion mechanism.
À
is no initiating Cr R function in this system.
In the original Phillips method, 1,2-dimethylpyrrole is the
preferred ligand, and gives by far the best activity. This ligand
does not incorporate a condensed aromatic ring and, there-
fore, formation of a complex analogue to 2 is not possible,
raising the question whether a monovalent species might still
be generated with standard pyrrolide anions. In an attempt to
address this issue we have carried out a preparation similar to
that for 1, using simple pyrrole. The reaction afforded an
insoluble material which, however, could still be crystallized if
a stoichiometric amount of bis(trimethylsilyl)acetylene was
added as a trapping agent. The complex, formulated as
[{Cr(h5-C4H4NAlEt2Cl)}2{m2,h2-C2(SiMe3)2}2] (3) is nearly dia-
magnetic, with the same small residual paramagnetism often
detected for dinuclear chromium compounds with a short
Cr···Cr contact (in this case, 2.3 ꢀ). The structure (Figure 2)
consists of a symmetry-generated dimer, with each chromium
center p bonded to a pyrrolyl(AlEt3) anion. The bridging
interaction between the two Cr-containing residues is pro-
vided by the two alkynes which are m2,h2-side-on bonded to
both metal centers. Complex 3 is the first example of a well-
characterized Cr(alkyne) complex not to contain phosphine
or carbonyl ligands. Its formation is the result of the oxidative
addition of a transient monovalent Cr species to the alkyne,
affording what appears to be a trivalent species, given the
In summary, this study, on a system closely related to the
commercial Phillips ethylene trimerization system, has eluci-
dated a link between the metal oxidation state and the type of
catalytic behavior: CrIII led to nonselective oligomerization,
CrII to polymerization and CrI to selective trimerization.
Although these findings give a reasonably realistic picture of
the complexity of an important catalytic system, they
certainly cannot be generalized at such an early stage. For
example, the three complexes also differ by chlorine content,
which, in principle, could be another factor affecting the
catalytic behavior. Nonetheless, an attractive working
hypothesis for the design of novel trimerization catalysts
emerged with zwitterionic ligands facially bound to CrI.
Experimental Section
Polymerization and oligomerization results: Samples were tested in a
200 mL high-pressure Bꢁchi reactor containing a heating/cooling
jacket. A preweighed amount of catalyst was dissolved in toluene
(10 mL) under N2 and injected into the preheated reactor already
charged with co-catalyst and toluene (total volume 100 mL). Sol-
utions were heated using a thermostatic bath and charged with
ethylene, maintaining the pressure throughout the run. The oligo/
polymerizations were quenched by venting the reactor and addition
of EtOH and HCl. The resulting polymer was isolated by filtration,
sonicated with an acidified ethanol solution, rinsed, and thoroughly
dried prior to mass determination. Oligomers were analyzed using
À
elongation of the coordinated alkyne C C bond, which is
clearly indicative of a two-electron reduction. Complex 3 is a
Angew. Chem. Int. Ed. 2008, 47, 9717 –9721
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Communications
GC and 1H NMR spectroscopy. Gel-permeation chromatography
Crystallographic data: Data for X-ray crystal structure determi-
nation were collected with a Bruker diffractometer equipped with a
1 K Smart CCD area detector and solved by direct methods.
1: C32H44Al2Cl2CrN2, Mr = 633.55, monoclinic, P21/n, T=
200(2) K, a = 10.053(5) ꢀ, b = 9.809(5) ꢀ c = 8.263(2) ꢀ, b =
92.743(9)8, V= 1606.9(13) ꢀ3, Z = 2, 4471 unique (Rint = 0.0391),
GOF = 1.068, final R1[I>2s(I)] = 0.0376, wR2 = 0.0914. 2:
C32H44Al2ClCrN2, Mr = 598.10, monoclinic, P2(1), T= 203(2) K, a =
10.543(7) ꢀ, b = 28.920(19) c = 10.762(7) ꢀ, b = 110.679(9)8, V=
3070(3) ꢀ3, Z = 4, 10848 unique reflections [Rint = 0.0546], GOF =
0.997, final R1[I>2s(I)] = 0.0552, wR2 = 0.1262. 3: C36H74Al2Cr2N2Si4,
Mr = 805.29, monoclinic, P21/n, T= 203(2) K, a = 11.4912(16), b =
12.3763(18), c = 16.618(2), b = 99.366(2), V= 2331.8 ꢀ3, Z = 2, 1804
unique reflections (Rint = 0.0329), GOF = 1.0151, final R1[I>2s(I)] =
0.0613 and wR2 = 0.1356. CCDC 676890 (1), 676891 (2), and 676892
(3) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif
(GPC) analysis of the polyethylene was referenced to polystyrene
(Mw = 65500, PDI = 1.02) standards. All reactions were carried out
under a dry nitrogen atmosphere. Solvents were dried using an
aluminum oxide solvent-purification system. Samples for magnetic
susceptibility analysis were preweighed inside a drybox equipped with
an analytical balance, and measured on a Johnson–Matthey magnetic
susceptibility balance. [CrCl2(thf)2] and [CrCl3(thf)3] were prepared
according to standard procedures. Cr(2-ethylhexanoate) was used as
received (Strem). Pyrrole and 2,5-dimethylpyrrole (Aldrich) were
purified by distillation and recrystallization, respectively, before use.
2,3-Dimethylindole and 2,3,4,5-tetrahydrocarbazole (Aldrich) were
used as received. The alkylaluminum reagents AlEt2Cl (Aldrich),
AlEt3 (Aldrich), and MAO (Chemtura) were used as received. Mass
spectra were recorded on a Micromass Quattro-LC Electrospray-
Triple Quadrupole Mass Spectrometer. Experiments were carried out
in the negative mode.
Synthesis of [Cr{h2-AlEt2Cl(2,3,4,5-C12H4N)}2] (1): Method A: A
purple suspension of [CrCl3(thf)3] (374 mg, 1 mmol) in pentane
(5 mL) was added a colorless solution of 2,3,4,5-tetrahydrocarbazole
(340 mg, 2 mmol) in pentane (5 mL) containing AlEt3 (684 mg,
6 mmol). The resulting deep-green solution was centrifuged and
allowed to stand for 3 days at À358C, affording 1 (259 mg, 0.41 mmol,
41%) as blue crystals.
Method B: A blue suspension of [CrCl2(thf)2] (267 mg, 1 mmol)
in pentane (10 mL) was added to a colorless solution of 2,3,4,5-
tetrahydrocarbazole (340 mg, 2 mmol) in pentane (10 mL) containing
AlEt3 (684 mg, 6 mmol), resulting in a blue solution. The solution was
stirred for a period of 1 h, centrifuged, and stored at À358C for 3 days
affording blue crystals of 1 (530 mg, 0.84 mmol, 84%). Elemental
analysis (%) calcd for C32H44Al2Cl2CrN2: C 60.60, H 7.00, N 4.42;
found: C 60.00, H 6.98, N 4.38.
Received: July 16, 2008
Revised: August 14, 2008
Published online: November 5, 2008
Keywords: alkenes · chromium · homogeneous catalysis ·
.
oligomerization · polymerization
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Synthesis of [Cr{h6,h6-(C12H12NAlEt2)2(m-Cl)}] (2): Method A: A
green solution of Cr(2-ethylhexanoate)3 (481 mg, 1 mmol) in toluene
(10 mL) was added to a solution of AlEt3 (570 mg, 6 mmol) and
2,3,4,5-tetrahydrocarbazole (340 mg, 2 mmol) in toluene (10 mL).
The mixture was stirred for 10 min and afforded a brown solution.
Addition of AlEt2Cl (170 mg, 1 mmol), filtration, and layering with
hexanes for one week at À358C afforded 2 as brown crystals (97 mg,
0.16 mmol, 16%).
Method B: The addition of finely divided potassium (40 mg,
1 mmol) to a blue solution of 1 (632 mg, 1 mmol) in toluene (10 mL)
and stirring for 24 h gradually turned the color of the mixture to dark
brown. The solution was filtered to remove solids, layered with
hexanes, and stored at À358C for one week, affording 2 as dark brown
crystals (540 mg, 0.90 mmol, 90%). Elemental analysis calcd for
C32H44Al2ClCrN2: C 64.26, H 7.42, N 4.68; found: C 64.23, H 7.41,
N 4.67.
Synthesis of [{Cr(h5-C4H4NAlEt2Cl)}2{m2,h2-C2(SiMe3)2}2](3): A
purple suspension of [CrCl3(thf)3] (374 mg, 1 mmol) and pyrrole
(134 mg, 2 mmol) in hexane (5 mL) was combined with a colorless
solution of AlEt3 (798 mg, 7 mmol) and C2(SiMe3)2 (170 mg, 1 mmol)
in hexanes (5 mL), resulting in a dark green solution. The solution was
stirred for 1 h, centrifuged, and stored at À358C for 3 days to give 1 as
dark green crystals (140 mg, 0.17 mmol, 34%). Further product was
isolated by concentrating the solution to half the volume and storing
for an additional week at À358C (total yield 44%). 1H NMR
(500 MHz, [D8]toluene): d = 3.34 (m, 4H; Pyr), 1.43 (br m 12H;
SiMe3), 1.20 (br m 12H; SiMe3), 1.01 (m, 4H; Pyr), 0.86 (br m, 12H;
SiMe3), 0.12 (br s, AlCH2CH3), À0.12 ppm (br s, 18H; AlCH2CH3);
13C{1H} NMR (125.7 MHz, [D8]toluene): d = 70.8 (s, Pyr), 32.1 (s,
SiMe3), 25.0 (s, Pyr), 23.1 (s, AlCH2CH3), 14.4 (s, SiMe3), 10.4 (s,
SiMe3), 9.5 (s, C2(SiMe3)2), À5.8 ppm (AlCH2CH3). ESI MS (toluene/
NEt3 0.2%, rel. int.): m/z 804.1 (75) [{MÀH}À], 732.1 (100)
[{MÀSiMe3}À]. Elemental analysis calcd for C36H74Al2Cr2N2Si4:
C 53.69, H 9.26, N 3.48; found: C 53.67, H 9.21, N 3.47.
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