A highly selective ethylene tetramerization catalyst

Article abstract of DOI:10.1002/anie.201106517

Small change, big difference: The introduction of an additional CH ₂ group into the bridge of ligands with two P/N units leads to a different selectivity of the corresponding chromium-based catalysts. Whereas 1 produces an ethylene trimerization system, 2 provides an unprecedented ethylene tetramerization system that produces 1-octene with high purity and little or no polymer side products (see scheme; DMAO=Me₃Al-depleted methylaluminoxane). Copyright

Full text of DOI:10.1002/anie.201106517

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Angewandte
Communications
DOI: 10.1002/anie.201106517
Ethylene Tetramerization
A Highly Selective Ethylene Tetramerization Catalyst**
Yacoob Shaikh, Khalid Albahily, Matthew Sutcliffe, Valeria Fomitcheva, Sandro Gambarotta,*
Ilia Korobkov, and Robbert Duchateau*
The issue of selectivity has been the most challenging aspect
of ethylene oligomerization since its discovery.[1] In the last
three decades, trimerization systems with high selectivity have
been discovered,[2] but tetramerization catalysts with high
selectivity remain elusive.[3] Central to the future success of
this endeavor is to understand the factors responsible for the
selectivity of the catalytic cycle.[4]
for the selectivity of the trimerization cycle has also been
invoked for the rationalization of the tetramerization.[3b,8]
Following this mechanism, it is conceivable that high selec-
tivity cannot be reached. The selectivity in this mechanism is
determined by the rate of the reduction/elimination step
compared to the rate of further ring expansion. If the seven-
membered ring is capable of expanding readily into the nine-
membered ring, it is hard to imagine why additional
expansion should not occur equally fast.[9] In the end, a
distribution of oligomers is to be expected and 1-octene may
be a dominant product.
Trivalent chromium complexes are the catalyst precursors
that are most commonly used for these transformations. In the
presence of alkyl aluminum activators, these species are
reduced to the divalent state (responsible for nonselective
oligomerization and/or polymerization) or the monovalent
state (responsible for selectivity).[5] To complicate the sce-
nario, there is also the possibility for these monovalent and
divalent species to undergo disproportionative redox pro-
cesses that give inactive zero-valent chromium together with
higher-valent species that are readily available for further
cycles of reduction/reoxidation.[6] Because of the presence of
such a redox dynamism in the catalytic cycle, the ancillary
ligand system determines the selectivity by preferentially
stabilizing one particular oxidation state. For example, when
highly reactive monovalent species are provided with a
sufficiently long lifetime, selective oligomerization is initiated
by the so-called redox reaction/ring-expansion mecha-
nism.[2,5,7]
As mentioned above, selective ethylene tetramerization
remains exceedingly rare. With a maximum of around 77% in
the case of the process developed by SK Energy,[3d] the
selectivity is definitely good, but still far from the levels
obtained with the trimerization systems. In addition, the
unavoidable formation of polymers poses serious reactor-
fouling problems that complicate industrial application. The
same redox reaction/ring expansion mechanism that accounts
Rosenthal and co-workers[10] first emphasized this prob-
lem and postulated an alternative mechanism for the highly
selective formation of 1-octene. According to their hypoth-
esis, a dimetallic system with two low-valent chromium
centers that are not linked with each other may independently
form two five-membered metallacycles. Cooperative dime-
tallic reductive elimination selectively affords 1-octene. It is
by following this fascinating hypothesis that we have recently
for the first time observed the formation of 1-octene that was
uncontaminated by other olefins.[3e] Albeit a step forward, this
particular system also produced large amounts of waxes,
possibly as a result of incomplete reduction of the precatalyst
to the monovalent state.
The combination of N and P donor atoms within an
ancillary ligand system has resulted in selective oligomeriza-
tion systems.[3,11] Recent studies in our research group on
chromium complexes that are stabilized by a simple mono-
anionic [Ph2PN(R)]À ligand system showed its versatility for
assembling polymetallic structures and self-activating species,
which occasionally show selective catalytic behavior.[12,13] To
study how the nuclearity of the catalysts may affect the
catalytic behavior, we have now linked two N/P-units with
both ethylenic and propylenic bridges in order to stimulate
the formation of dimetallic species. Herein, we describe the
first catalytic system that has a good activity and is capable of
producing pure 1-octene (up to 91% purity) with little or no
polymer side products.
The ligands Ph2PN(R)(CH2)nN(R)PPh2 (a-Me: n = 2, R =
Me; b-Me: n = 3, R = Me; b-Et: n = 3, R = Et; b-iPr: n = 3,
R = iPr; Scheme 1) do not react with chromium salts in THF.
Nonetheless, they readily ligate trivalent chromium salts in
toluene as indicated by color change and dissolution upon
mixing. The complexes were isolated as microcrystalline
materials from toluene solutions after centrifugation and
upon layering with hexane. Analytical and ESI-MS data were
in agreement with simple ligand coordination. In the case of
the reaction of [CrCl3(THF)3] with a-Me, the precipitated
complex was isolated, resuspended in toluene, and subse-
quently treated with alkyl aluminum reagents. When Et2AlCl
[*] Y. Shaikh, Dr. K. Albahily, M. Sutcliffe, V. Fomitcheva,
Prof. Dr. S. Gambarotta
Department of Chemistry, University of Ottawa
10 Marie Curie, Ottawa, ON K1N 6N5 (Canada)
E-mail: sgambaro@uottawa.ca
Dr. I. Korobkov
X-Ray Core Facility, University of Ottawa
10 Marie Curie, Ottawa, ON K1N 6N5 (Canada)
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
[**] This work was supported by the Natural Science and Engineering
Council of Canada (NSERC), the University of Ottawa and the
Eindhoven University of Technology.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106517.
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Angewandte
Chemie
conveniently obtained by mixing either [CrCl3(THF)3] or
[CrCl2(THF)2] with the appropriate ligand, activator, and
solvent in the reactor which was pressurized with ethylene.
The reactions were carried out in toluene by using methyl-
aluminoxane (MAO) as co-catalyst and produced oligomers
with Shulz–Flory (S-F) distributions, albeit with high catalytic
activity. Interestingly, the product mixtures were polymer-free
(Table 1). Conversely, when MeCy was used as solvent and
TMA-depleted MAO (DMAO) as co-catalyst, a reasonably
selective oligomerization catalyst was obtained. Although the
catalytic activity is significantly lower than for the MAO-
activated system in toluene, the product distributions were
interesting because they consisted of mixtures of exclusively
1-hexene and 1-octene. Generally, 1-hexene was the dominant
product, except when [CrCl2(THF)2] was used or when a
mixture of DMAO and Et3Al was employed as a co-catalyst.
The moderate predominance of ethylene tetramerization that
we observed with the system a-Me/[CrCl3(THF)3]/DMAO
prompted us to modify the ligand.
Scheme 1.
(DEAC) and Me3Al (TMA) were used as alkyl aluminum
reagents, two crystalline products could be obtained. The
formulations ({[Ph2PN(Me)(CH2)2N(Me)PPh2]Cr(m-Cl)}2[(m-
Cl)2(AlEt2)])+(EtAlCl3)À·(toluene)1.5
(1a-Me)
and
[{[Ph2PN(Me)(CH2)2N(Me)PPh2]Cr}(m-Cl)3]+[AlMe2Cl2]À
(2a-Me) were obtained by X-ray crystal structure determi-
nations (Figure 1a and b, respectively).
The simple introduction of an additional methylene group
into the ethylene bridge that links the two N/P units (b-Me)
had a remarkable impact on catalytic behavior (Table 2).
Even in this case, the microcrystalline adducts with chromium
salts showed the same catalytic behavior as the in situ
generated species. Whereas use of MAO and DMAO in
toluene resulted in an S-F distribution of oligomers (albeit
with good catalytic activity and no polymer formation), the
switch to MeCy as solvent resulted in a highly selective and
reasonably active ethylene tetramerization system. Even
more interesting, the product mixtures were polymer-free
with the only by-product being 1-hexene. Increasing the
temperature to 1008C had a positive effect on the selectivity
toward 1-octene while the activity remained the same. As it
was observed for a-Me, use of [CrCl2(THF)2] helps to increase
the selectivity toward 1-octene to a record high of 91%. The
addition of organoaluminum or organozinc activators to
DMAO considerably improved the outcome of the catalytic
runs in terms of both activity and selectivity. The addition of
Et3Al resulted in a peak activity toward to formation of 1-
octene, while a good selectivity was maintained, although
some polymer was also produced as a function of the amount
of Et3Al. Other additives such as iBu3Al or Et2Zn resulted in
good selectivity for 1-octene, but also in a somewhat lower
catalytic activity and the production of small amounts of
Figure 1. Partial thermal ellipsoid drawings for a) 1a-Me and b) 2a-Me
cations. The [EtAlCl3] and [Me2AlCl2] counteranions have been omitted
for clarity. Thermal ellipsoids are shown at 50% probability.
Both complexes showed similar dinuclear structures, in
which each ligand chelates a single chromium atom with the
two P donors. The Cr–N distances are well beyond bonding
distance, and vary between 3.419 and 3.618 ꢀ. The dimetallic
structures are formed by either two bridging chlorine atoms
and one bridging [(m-Cl)2AlEt2] unit (1a-Me), or by three
bridging chlorine atoms (2a-Me). In both cases, the counter
À
À
anions ([EtAlCl3 ] and [Me2AlCl2 ], respectively) are not
connected with the dimetallic frame but part of the overall
ionic structure. In both complexes the chromium centers are
divalent. Reduction of the metal center is rather expected as it
is often observed when trivalent
chromium precursors are treated
Table 1: Ethylene oligomerization with a-Me/[CrCl3(THF)3] and 1a-Me.[a]
with alkyl aluminum reagents.[6,14]
The catalytic activity of com-
Cat.
Co-cat.
Equiv
LAO
[g]
PE
[g]
Activity
C6
C8
C10–18
[ggCrÀ1 hÀ1
]
[mol%]
plexes 1a-Me and 2a-Me was low
because of their poor solubility in
both methylcyclohexane (MeCy)
and toluene. Test reactions that
were carried out on the precipi-
tated ligand–CrCl3 adducts showed
no difference in catalytic behavior
to the in situ generated species.
Therefore, extensive catalytic test-
ing was carried out with in situ
generated complexes, which were
a-Me[b]
a-Me[c]
a-Me[c,d]
a-Me[c]
a-Me[c]
a-Me[c]
1a-Me[b]
1a-Me[c]
MAO
DMAO
DMAO
DMAO/Me3Al
DMAO/Et3Al
DMAO/iBu3Al
MAO
500
500
500
500/100
500/100
500/100
500
97
6
3
3
3
1
50
–
0
1.1
0
0
0
0
0
–
88917
6910
2750
2750
2750
917
28
70
40
50
37
56
17
–
26
31
60
50
61
44
23
–
46
0
0
0
2
0
60
–
45833
–
DMAO
500
[a] Conditions: ligand (30 mmol), [CrCl3(THF)3] (30 mmol), solvent (100 mL), 808C, ethylene (40 bar),
30 min. [b] Toluene as solvent. [c] MeCy as solvent. [d] [CrCl2(THF)2]. LAO=linear alpha olefin,
PE=polyethylene.
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Angewandte
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Table 2: Ethylene oligomerization with various ligands.[a]
responsible for the unique selectiv-
ity toward ethylene tetramerization.
Although we have no conclusive
data, we speculate that such a
remarkable sensitivity could be
explained in terms of monomer
Cat.
Co-cat.
Equiv
LAO
[g]
PE
[g]
Activity
C6
C8
[mol%]
C10–18
[ggCr hÀ1
]
b-Me[e]
b-Me[e]
b-Me[f]
b-Me[f,h]
b-Me[f,i]
b-Me[b,f]
b-Me[c,f]
b-Me[d,f]
b-Me[f,j]
b-Me[f,k]
b-Me[f,l]
b-Me[f,m]
b-Me[f,n]
b-Me[f]
b-Me[f]
b-Me[f]
b-Me[f,g]
b-Me[f]
b-Me[f]
b-Me[f]
b-Me[f]
b-Et[f]
MAO
DMAO
DMAO
DMAO
DMAO
DMAO
DMAO
DMAO
DMAO
DMAO
DMAO
DMAO
500
500
500
500
500
500
500
500
500
500
500
500
18
22
8
6
3
8
8
2
1
0
0
0
1.0
3.0
0
0
0
16500
11000
7333
6621
6012
7333
7333
1833
555
5480
11494
22186
20346
11000
12814
25424
9424
5096
7186
2326
1834
8488
5718
6012
5956
35
15
19
31
33
29
15
9
35
28
27
59
27
25
18
11
23
24
10
53
12
33
59
73
41
30
26
81
69
67
68
85
91
65
72
67
15
17
75
77
88
77
72
89
36
87
67
41
27
59
35
59
0
0
0
4
0
0
0
0
6
24
56
0
5
1
0
4
versus
dimer
formation
(Scheme 2). In fact, the formation
of dichromium systems (both
mono- and divalent) is indeed
highly dependent on steric interac-
tions within the ligand scaffold.[15]
Thus, if we assume that monomeric
and dimeric structures may coexist
and that each of the two arrange-
ments is responsible for the forma-
tion of either 1-hexene or 1-octene,
it is natural that changes on the
ligand scaffold may lead the cata-
lytic cycle into either direction.
Given that the ligand system is
neutral, the formation of a dimeric
structure might or might not require
reversible dissociation of one
ligand. The marked dependence on
the nature of the activator may be
difficult to rationalize and yet, it
may fit with the same assumption,
given that the aggregation with
alkyl aluminum is also affecting
the sterics of the catalytically
active species (see for example the
0
5
0.7
1.1
2.6
3.0
0
0.7
4.1
0.2
0.4
0.6
1.1
0
11
21
18
12
13
22
10
5
7
1
2
8
DMAO
500
DMAO/Me3Al
DMAO/Et3Al
DMAO/Et3Al
DMAO/Et3Al
DMAO/Et3Al
DMAO/iBu3Al
DMAO/Et2AlCl
DMAO/Et2Zn
DMAO/Et3Al
DMAO
500/100
500/25
500/100
500/100
500/250
500/100
500/100
500/100
500/100
500
1
11
1
0
0
0.9
1.6
0.4
2.5
b-iPr[f]
4
6
3
b-iPr[f]
DMAO/Et3Al
DMAO
500/100
500
0
0
b-PiPr[f]
[a] Conditions: ligand (30 mmol), [CrCl3(THF)3] (30 mmol), solvent (100 mL), 808C, ethylene (40 bar),
30 min. [b] 608C. [c] 1008C. [d] [CrCl2(THF)2]. [e] Toluene as solvent. [f] MeCy as solvent. [g] Ethylene
(20 bar). [h] Ligand (60 mmol), [CrCl3(THF)3] (30 mmol). [i] Ligand (30 mmol), [CrCl3(THF)3] (60 mmol).
[j] Catalyst (15 mmol). [k] Catalyst (22 mmol). [l] Catalyst (45 mmol). [m] Catalyst (60 mmol). [n] Catalyst
(90 mmol).
polymer. The introduction of bulkier groups into the molec-
ular scaffold at either the N (b-Et, b-iPr) or P (b-PiPr) atoms
of the ligands decreased both activity and selectivity. Chang-
ing the Cr/ligand ratio negatively affected the activity and the
selectivity. Variation of the overall catalyst loading dramat-
ically affected the selectivity (Table 2, entries 2 and 9–13).
The overall behavior has a non-straightforward interpretation
and clearly showed the existences of two parallel trends. After
the initial rise of the concentration of octene as a function of
the loading, the selectivity switches toward 1-hexene to then
decrease to an S-F distribution at the highest loadings. We
take this as an indication for the existence of multiple
equilibria in the catalytic mixture.
structure of 1a-Me). However, even in the case that a
dimetallic mechanism would be followed by the reaction, it
would not necessarily imply bimetallic reductive elimination.
The selectivity is caused by monovalent chromium, the
oxidation state of which is the only one known to date that is
capable of performing the initial reductive coupling of the two
ethylene molecules.[5] Unlike Sasolꢁs PNP-based and SK
Energyꢁs PC2P-based catalysts,[3a,b] the current system shows a
strong solvent and co-catalyst dependence. Selective ethylene
tetramerization has only been observed in MeCy. In toluene
S-F distributions of a olefins were invariably obtained.
In conclusion, we have reported the first tetramerization
catalyst with good activity that is capable of producing 1-
octene with high purity and little or no polymer as a by-
product. Both solvent and co-catalyst have a dramatic impact
on the selectivity. Previously,[3e] we have observed a tetrame-
rization system that produces 1-octene in more than 99%
purity along with a large amount of wax. With the results
reported herein, we have made a step forward by indicating
that a potentially useful tetramerization system of high
selectivity can indeed be obtained. Whether the present
catalytic system may or may not be linked to Rosenthal and
co-workersꢁ hypothesis of a dimetallic system is not yet clear.
The remarkable change in catalytic behavior upon such a
minor modification on the ligand framework as going from a-
Me to b-Me is puzzling, to say the least. Regrettably, these
species have so far eluded crystallographic characterization
Given the similarity between a-Me and b-Me, the switch
of selectivity from 1-hexene to 1-octene cannot be understood
on the ground of differences in electronic features. Therefore,
the longer N(CH2)3N chain of b-Me remains the sole factor
Scheme 2.
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Chem. 2010, 122, 9411; Angew. Chem. Int. Ed. 2010, 49, 9225;
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between nuclearity and tetramerization behavior. Nonethe-
less, the fact that we are able to observe a tetramerization
system of such high selectivity (up to 91%) is encouraging and
we are currently attempting to isolate self-activating catalyti-
cally active species as well as improving catalyst performance.
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Keywords: 1-octene · chromium · ethylene · oligomerization ·
tetramerization
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