A chromium ethylidene complex as a potent catalyst for selective ethylene trimerization

Article abstract of DOI:10.1002/anie.201006953

Going one, twice... Reaction of the mononuclear complex [{η⁵-(tBu)₂C₄H₂N}CrCl ₂(thf)] with AlEt₃ afforded the dinuclear species [{[η⁵-(tBu)₂C₄H₂N]CrEt} ₂(μ-Cl)₂] (see picture; Cr purple, Cl green, C sticks/gray, H white). The complex acts a single-component selective trimerization catalyst; a higher loading of AlEt₃ activator afforded isomerization of 1-hexene to cis, trans 2-hexene. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.201006953

Communications
DOI: 10.1002/anie.201006953
Catalyst Design
A Chromium Ethylidene Complex as a Potent Catalyst for Selective
Ethylene Trimerization**
Sebastiano Licciulli, Khalid Albahily, Valeria Fomitcheva, Ilia Korobkov, Sandro Gambarotta,*
and Robbert Duchateau*
The factors transforming a statistical ethylene oligomeriza-
tion process into selective tri- or tetramerization are being
intensively scrutinized in both industrial and academic
research.[1] Their identification also poses a genuine intellec-
tual challenge, as the design of a selective catalytic system
requires a complete understanding of the mechanistic fea-
tures. The so-called ring-expansion mechanism for selective
trimerization[2,3] is today unchallenged to the point that it is a
paradigm among workers in the field.
Central to the understanding of the mechanism and to
tailoring new catalysts is the question of the metal oxidation
state.[1a] In the case of the chromium systems that are involved
in the majority of these catalysts, the + I state has been
conclusively linked to selective trimerization by both syn-
thetic and spectroscopic work.[4–6] One of the characteristics
of chromium, however, is the possibility of readily intercon-
verting between + I, + II, and + III states as a part of a redox
dynamism.[7] As each one of these oxidation states has its own
distinctive catalytic behavior,[6a,b] stabilizing the desired state
and slowing at the same time the dynamism is central to
obtain selective behavior.
pyrrolide rings in the form of an aluminum pyrrolide complex.
The specific nature of the aluminum residue seems to play a
very important role in the activation of the catalytic center for
this system.[6a,b] However, the importance of its coordination
to the pyrrolide nitrogen atom remains unclear. Is it the
zwitterionic structure introduced by the coordination of
aluminum? Or is it the alkyl transfer that makes the metal
prone to further reduction to the + I state? In an attempt to
address these and other questions that are important for
tailoring the design of new catalysts, we have now probed the
nature of the pyrrolide anion. The anion chosen bears two
bulky tert-butyl groups in the two a positions, which could
possibly prevent coordination of aluminum while maintaining
an enhanced tendency for h5-coordination.[8]
Deprotonation of 2,5-di-tert-butylpyrrole followed by
reaction with [CrCl3(thf)3] afforded [{h5-(tBu)2C4H2N}CrCl2-
(thf)] (1), which was isolated as dark violet-blue crystals.[13]
The pseudo-tetrahedral piano-stool arrangement of 1 was
elucidated by an X-ray crystal structure (Figure 1).[14] When
Mechanistic and synthetic work on the commercially used
pyrrole-based Phillips trimerization system[6a,b] has allowed
the structures of single-component self-activating precatalysts
to be elucidated. All of these species consist of the pyrrolide
ring being either s- or p-bonded to the metal center and of an
aluminate residue, which is invariably s-coordinated to the
pyrrolide nitrogen donor atom. There are two main features
in this particular structural motif. First, the coordination of
the aluminate introduces a zwitterionic type of electronic
structure that in turn provides partial cationic character to the
chromium center. Second, the alkyl group migrates from the
aluminate to chromium during the dissociation of one of the
Figure 1. Structures of 1 (left) and 2 (right). Ellipsoids set at 50%
probability; hydrogen atoms omitted for clarity. Selected bond lengths
[ꢀ] and angles [8]: 1: Cr1–O1 2.045(2), Cr1–N1 2.199(3), Cr1–C4
2.213(3), Cr1–C1 2.249(3), Cr1–C3 2.257(3), Cr1–Cl2 2.2847(10), Cr1–
Cl1 2.2873(10), Cr1–C2 2.286(4), N1–C1 1.377(4), N1–C4 1.387(4),
C1–C2 1.434(5), C2–C3 1.401(5), C3–C4 1.414(5). 2: Cr1–Cl2 2.188(3),
Cr1–C13’ 2.202(15), Cr1–C3 2.226(6), Cr1–C2 2.225(5), Cr1–C1
2.281(5), Cr1–C4 2.289(5), Cr1–N1 2.312(3), Cr1–Cl1 2.3439(19), N1–
C4 1.382(7), N1–C1 1.364(7), C1–C2 1.418(8), C2–C3 1.374(8), C3–C4
1.405(7), C13’–C14’ 1.496(14); Cr1-Cl1-Cr1A 89.39(10), Cl2-Cr1-Cl1
93.76(7), C13’-Cr1-Cl1 99.6(4).
[*] Dr. S. Licciulli, K. Albahily, V. Fomitcheva, 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. I. Korobkov
X-ray Core Facility, University of Ottawa
Ottawa, ON K1N 6N5 (Canada)
Dr. R. Duchateau
activated with AlEt3 (5–15 equivalents), complex 1 is a highly
active and selective trimerization catalyst (Table 1). The
reaction time and also the temperature and quantity of alkyl
aluminum activator affected both yield and purity of 1-hexene
with respect to the side-product 2-hexene.
Department of Chemistry, Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
E-mail: r.duchateau@tue.nl
[**] This work was supported by the Natural Science and Engineering
Council of Canada (NSERC).
In an attempt to isolate the catalytically active species, we
reacted 1 with a stoichiometric amount of AlEt3. The violet-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006953.
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Angew. Chem. Int. Ed. 2011, 50, 2346 –2349

Table 1: Oligomerization studies.[a]
Entry Catalyst AlEt3
[equiv]
magnetic complex, formulated
as [{[h5-(tBu)2C4H2N]Cr}2(m-Cl)2-
(m-CHCH3)] (3), was isolated as
deep-blue crystals from the afore-
mentioned blue solutions.[14] The
crystal structure revealed a dimeric
chromium complex with a face-
sharing bi-tetrahedral arrangement
(Figure 2). The two distorted tetra-
hedra are bridged by two chlorine
atoms and one carbon atom of a C2
T
Alkenes
PE
[g]
1-hexene/
2-hexene
[mol%]
1-hexene
[mol%]
Oligomerization
activity
[8C] [mL]
[gmolÀ1 CrhÀ1
]
1
2
3
4
5
6
7
8
1[b]
1[b]
1[b]
2[c]
2[d]
2[c]
2[c]
2[c]
2[c]
2[c]
2[c]
3[c]
3[d]
4[c]
10
5
5
0
5
10
10
10
10
15
50
0
85 31
85 20
115
120
115 32
85 35
100 39
110 40
150
85 25
85 15
120
115 71
0
0
99
99
84
83
75
98
89
95
93
89
73
91
92
0
700600
452000
97100
55200
1486400
791000
881400
917300
90400
565000
339000
0
4
2
0.2 99
0.3 99
0.6 99
0
0
99
99
0.4 99
À
residue. The C C distance of this
9
4
0
0
0
0
95
99
99
0
particular unit (C13–C14 1.505(9))
10
11
12
13
14
À
is in the range of a C C single bond.
0
Chemical degradation experiments
carried out with a D2O/H2O mix-
ture in C6D6 followed by centrifu-
gation and NMR spectroscopic
characterization of the solution
5
10
1.3 99
0
91
0
3295900
0
0
0
0
[a] Conditions: Methylcyclohexane 100 mL, 30 min reaction time, 40 bar PE=poly(ethylene).
[b] 60 mmol. [c] 30 mmol. [d] 15 mmol.
blue solution turned immediately dark red, affording deep-
red crystals of the dinuclear species [{[h5-(tBu)2C4H2N]CrEt}2-
(m-Cl)2] (2; Scheme 1).[14] The dinuclear nature and the
connectivity of this rare example of a chromium(III) ethyl
derivative were clarified by an X-ray crystal structure
(Figure 1). The structure consists of two edge-sharing tetra-
Figure 2. Structures of 3 (left) and 4 (right). Ellipsoids set at 50%
probability; hydrogen atoms are omitted for clarity. Selected bond
lengths [ꢀ] and angles [8]: 3: Cr1–C13 2.048(13), Cr1–C3 2.239(2), Cr1–
C2 2.238(2), Cr1–C1 2.256(2), Cr1–C4 2.260(2), Cr1–N1 2.2791(18),
Cr1–Cl1 2.359(3), Cr1–Cl1A 2.384(3), Cr1–Cr1A 2.6739(6), N1–C1
1.377(3), N1–C4 1.377(3), C1–C2 1.417(4), C2–C3 1.423(4), C3–C4
1.415(4), C13–C14 1.505(9); Cl1–Cr1-Cl1A 99.58(8), Cr1-Cl1-Cr1A
68.63(8), Cr1-C13-Cr1A 81.7(2). 4: Cr1–N1 2.0121(19), Cr1–O1
2.0774(17), Cr1–Cl1A 2.3909(7), Cr1–Cl1 2.4048(7).
phase helped to partially simplify the spectrum. The
1
2H NMR spectrum acquired with H decoupling showed two
Scheme 1. Reactivity of complexes 1–4.
peaks at d = 0.748 and 0.730 ppm, thus indicating the presence
of the two isotopomers. 13C NMR spectroscopy unequivocally
showed the presence of a mixture of the isotopomers
CD2HCH3, CDH2CH3, and CH3CH3. The 1H NMR spectrum
showed complete absence of olefinic residues. The resonances
of a mixture of partly deuterated ethanes were present at d =
0.778 ppm as a complex signal resulting from both 1H–1H and
1H–2H coupling. Deuterium decoupling could not be properly
carried out owing to dilution and line broadening. However,
the 1H13C HMQC signal at d = 0.778 ppm in the proton
section clearly showed coupling at d = 6.78 ppm of the
13C NMR spectrum. The 13C resonance was not clearly visible
owing to low intensity and very possibly line broadening.
hedra with two identical piano-stool units connected by two
bridging chlorides. The low magnetic moment is to be
expected for the planar {Cr2Cl2} core, with an intermetallic
distance that is shorter than the sum of the Van der Waals
radii. The complex is reasonably thermally robust, but slowly
turned brown upon heating for about 30 minutes in toluene.
During the preparation of 2, it was discovered that if only
0.5 equivalent of AlEt3 was added dropwise to solutions of 1
and the reaction was slowly carried out at room temperature,
a distinctively different blue color developed, thus indicat-
ing the formation of a different species. The color did not
change appreciably upon heating overnight. A new para-
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Communications
These findings compare reasonably well with literature data.[9]
Finally, ESI-MS analysis provided the expected value for the
molecular ion of the proposed formulation of 3.
activity and good selectivity with the usual isomerization to
cis/trans 2-hexene (30:70). The similarities in catalytic behav-
ior between 2 and 3 when activated does not relate to the
preliminary conversion of 2 to 3, but rather to the fact that
both species may possibly generate the same catalytically
active chromium(I) intermediate.
The formation of 3 is puzzling. In principle, the most direct
À
way to form the bridging ethylidene could be a simple C H
s-bond metathesis from the two ethyl groups present in 2.
This does not seem to be the case though, as thermolysis of 2
led to brown decomposition products with no evidence for the
formation of the blue compound 3, which could only be
formed from 1 under the conditions previously discussed
(Scheme 1). Therefore, its formation seems to be a reaction
pathway alternative, and not subsequent to the formation of 2,
that is triggered at higher temperature by a lower Al:Cr ratio.
Attempts to form 3 rationally by treating solutions of
analytically pure 2 with either AlEt3 or Et2AlCl failed. The
reactions afforded instead the dinuclear, square-planar com-
plex [{[s-(tBu)2C4H2N]CrII(thf)}2(m-Cl)2] (4) that was isolated
as blue crystals (Figure 2) after crystallization of the dried
reaction residue from THF.[14] The retention of chlorine in the
structure suggests that formation of 4 also proceeds by a non-
straightforward mechanism. The s-bonding of the pyrrolide
anion in 4 is most likely due to the treatment with THF, which
was required for triggering crystallization. Complex 4 is
catalytically inactive.
Finally, the variable amount of isomerization of 1-hexene
observed during this work (also observed with the commercial
process using the Phillips catalytic system) is simply due to the
alkyl aluminum activator. The degree of isomerization was
found to increase with the temperature and the loading of
AlEt3 and did not depend on chromium. In a blank run with
only AlEt3, we found that at 1108C, pure 1-hexene can be
isomerized (93%) in 1 hour to a thermodynamic mixture of
cis and trans isomers (30:70%) of 2-hexene. No 3-hexene was
ever observed. It is therefore important to strike the right
balance between reaction conditions and activator loading to
maximize 1-hexene production.
In conclusion, we have presented a novel ethylene
trimerization system related to the previously reported
intermediates of the Phillips system.[6a,b] In contrast to those
complexes, however, the bulky p-bonded pyrrolide ligand
does not allow retention of AlR3 at the pyrrole nitrogen atom
without detracting from the trimerization performance. In
this system, the role of organoaluminum seems to be
exclusively confined to that of an alkylating agent. The
serendipitous discovery of the first case of a Schrock-type
chromium ethylidene is quite intriguing. We found it partic-
ularly stimulating regarding the recent mechanistic debate[10]
regarding the possibility of a dinuclear Schrock type of
alkylidene being a catalytically active intermediate in the
oligomerization or even a polymerization process.[11] The
isolation of such a species, which is closely reminiscent of the
Takai olefination intermediate,[12] indicates that chromium
Schrock carbenes may indeed exist. We are currently
examining the reactivity of this unprecedented and catalyti-
cally active chromium ethylidene species.
Complex 2 is indeed a single-component, self-activating
precatalyst that produces highly pure 1-hexene. The low
activity is likely to be ascribed to both the robust dimeric
structure, which is not easily dissociated in solution, and to
partial catalyst poisoning owing to impurities (Figure 3).
Received: November 16, 2010
Revised: December 7, 2010
Published online: February 11, 2011
Figure 3. Plot of the single-component catalytic behavior of 2.
Keywords: carbene ligands · chromium · ethylene trimerization ·
1-hexene · homogeneous catalysis
.
Furthermore, the formation of the chromium(I) catalytically
active species from 2 will require a preliminary ethyl/chlorine
exchange to generate a ligand–CrEt2 function that is ready to
undergo thermal reductive elimination. When treated with a
few equivalents of AlEt3, 2 became an excellent trimerization
catalyst, producing a large amount of 1-hexene with a minor
extent of isomerization. The reaction outcome was very
sensitive to the temperature and catalyst and activator
loadings: Higher loadings of AlEt3 always resulted in an
increase of 2-hexene in combination with higher temper-
atures.
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