Selective ethylene tri-/tetramerization by in situ-formed chromium catalysts stabilized by N,P-based ancillary ligand systems

Article abstract of DOI:10.1021/cs4004968

A series of N,P-based ancillary ligands have been synthesized, and the corresponding catalysts, formed in situ by mixing one of the N,P-ligands, Cr(acac)₃ and MAO, have been tested for ethylene oligomerization. Under standard ethylene oligomeri

Full text of DOI:10.1021/cs4004968

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Article
Selective ethylene tri-/tetramerization by in situ formed chromium
catalysts stabilized by N, P-based ancillary ligand systems
Yun Yang, Zhen Liu, Boping Liu, and Robbert Duchateau
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs4004968 • Publication Date (Web): 03 Sep 2013
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Selective ethylene triꢀ/tetramerization by in situ
formed chromium catalysts stabilized by N, Pꢀbased
ancillary ligand systems
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Yun Yang,‡, † Zhen Liu, ‡ Boping Liu,*, ‡ Robbert Duchateau*, †
‡State Key Laboratory of Chemical Engineering, East China University of Science and
Technology, Meilong Road 130, Shanghai 200237, China
†Department of Chemical Engineering and Chemistry, Eindhoven University of Technology,
P.O. Box 513, 5600 MB, The Netherlands
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ABSTRACT: A series of N, Pꢀbased ancillary ligands have been synthesized and the
corresponding catalysts, formed in situ by mixing one of the N, Pꢀligands, Cr(acac)3 and MAO,
have been tested for ethylene oligomerization. Under standard ethylene oligomerization
conditions (30 bar of ethylene, 60 °C, methylcyclohexane as solvent), all the in situ formed
complexes show catalytic activity producing oligomers together with varying amounts of
polyethylene. Of all these combinations only the catalyst formed by mixing Nꢀ
pyrrolyldiphenylphosphine with Cr(acac)3 and MAO is capable of selectively oligomerizing
ethylene, producing a mixture of 1ꢀhexene and 1ꢀoctene in varying ratios alongside a small
amount of PE. Further investigations on this catalyst system reveal that the presence of a small
volumeꢀ% of toluene favors the production of 1ꢀoctene. However, in pure toluene as solvent the
selectivity towards 1ꢀhexene/1ꢀoctene is lost and a statistic mixture of αꢀolefins is produced.
Moreover, the choice of the coꢀcatalyst is found to dramatically influence the composition of the
liquid products. By careful adjustment of the reaction conditions (temperature, ethylene pressure,
catalyst loading and ligand/Cr ratio), the 1ꢀhexene/1ꢀoctene molar ratio can be tuned from 0.3 to
20 and a selectivity of 1ꢀoctene formation can be achieved up to 74 %.
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KEYWORDS: selective ethylene oligomerization, ethylene tetramerization, chromium catalyst,
ligand design, solvent effect, DFT calculation
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Introduction
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As important comonomers for the production of linear low density polyethylene, 1ꢀhexene and
1ꢀoctene are two highly desired linear αꢀolefins. To collect these light fractions from a broad
distribution of oligomers, typically obtained by the conventional nonꢀselective oligomerization
processes, requires an economically unfavorable separation step. Selective production of 1ꢀ
hexene and 1ꢀoctene directly by selective ethylene triꢀ or tetramerization is therefore highly
desired and has stimulated both academic and industrial research.
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Among all the transition metalꢀbased catalysts, chromium catalysts have proven to be the most
promising candidates for selective ethylene oligomerization.1ꢀ3 Typical examples are the
ChevronꢀPhillips trimerization catalyst,4 the first and sole trimerization system to be successfully
commercialized, and the few existing tetramerization systems with 1ꢀoctene selectivities in the
range of 70 %.5ꢀ6 On the other hand, the variety of oxidation states and spin states known for
chromium severely complicate mechanistic studies of these catalysts.7ꢀ23 As a consequence,
despite the successful developments of numerous chromiumꢀbased selective ethylene
trimerization and tetramerization systems, the nature of the active species is still not fully
understood although there are strong arguments for a Cr(I)/Cr(III) redox couple.8ꢀ10,15,17ꢀ18,24ꢀ35
Besides the choice of the metal center, the desired catalytic selectivity can be tuned by
designing ancillary ligands with the desired structural and electronic properties to assist the
generation and stabilization of active species for selective ethylene oligomerization. Since the
discovery of the chromium bis(diphenylphosphino)amine systems, which are adaptable to
facilitate either ethylene trimerization36 or tetramerization,5 the family of N, Pꢀbased ligands
eligible for selective ethylene oligomerization has gradually been expanded.37ꢀ46 Recent studies
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on chromium catalysts stabilized by monoanionic N, Pꢀbased ligands41 showed the capability of
producing pure 1ꢀhexene (> 99 %) along with small quantities of polyethylene (PE). This system
could be tuned towards ethylene tetramerization38 (producing 91 % of 1ꢀoctene with 1ꢀhexene as
the only oligomeric byꢀproduct) by assembling two N, Pꢀbased units with a propylenic bridge
(Scheme 1).
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Herein we describe the development of a new series of N, Pꢀbased ligands by replacing the
dialkyl amine groups in the ligands mentioned above with nitrogenꢀcontaining heterocycles. The
catalysts, formed in situ by mixing different combinations of ancillary ligands, chromium
precursors and coꢀcatalysts, were systematically tested under varied reaction conditions.
Scheme 1. Chromium aminodiphenylphosphine systems for ethylene trimerization and
tetramerization.
Results and discussion
All the N, Pꢀbased ligands used in this study (Figure 1) were synthesized in a twoꢀstep process.
The nitrogenꢀcontained heterocyclic compounds were first deprotonated by triethylamine or nꢀ
butyllithium. Then chlorodiphenylphosphine or dichlorophenylphosphine was added dropwise to
form the title compounds via salt elimination.
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Figure 1. Illustration of the series of N, Pꢀbased ancillary ligands used in this study.
A major advantage of chromiumꢀbased selective ethylene oligomerization catalysts stabilized
by neutral ancillary ligands is that they can effectively be generated in situ by mixing the
ancillary ligand with an appropriate chromium precursor and the activator (i.e. MAO).47 This
simple catalyst preparation process significantly speeds up preliminary screening studies. In this
study the N, P ancillary ligands (Figure 1) were mixed with Cr(acac)3 in dry methylcyclohexane
and stirred for at least four hours at room temperature before each run. Under typical ethylene
oligomerization conditions (Table 1), upon activation with MAO (10 wt% in toluene) all the in
situ formed Cr species showed catalytic activity. An instant temperature jump after the injection
of catalyst was observed in all the cases, and after 10 to 15 minutes the temperature started to
decrease gradually. The observed activities are moderate, which is most likely to be the result of
the poor solubility of the catalysts in methylcyclohexane, and/or due to the insufficient
separation of Cr cations and counterions in the highly apolar medium.48 Alternatively, the
catalysts themseleves might simply be not that reactive towards ethylene coordination and
insertion. All catalysts, except the one obtained using ligand 7, produced a statistic mixture of αꢀ
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olefins and varying amounts of PE. Under the conditions applied (methylcyclohexane as solvent,
60 °C, 30 bar of ethylene), the catalyst based on ligand 7 produced a mixture of 1ꢀhexene (56 %)
and 1ꢀoctene (40 %) and only small amounts of higher olefins (4 %) along with a minor amount
of PE (8 wt%). For most runs, the amount of PE obtained is too high to be part of the statistic
product distribution, which suggests that besides the oligomerization catalyst also an independent
polymerization catalyst exists in the system.
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Table 1. Catalytic tests using the mixture of Cr(acac)3 and N, Pꢀbased ligands activated by
MAO.
=
=
Ligand
PE
PE
(wt%)
60
LAO
(g)
Activity
C6
C8
(g)
(g/mmol Cr/h)
(mol%)
0.51
0.79
0.52
0.44
0.72
1.00
0.45
0.44
1.82
0.34
4.83
0.71
0.74
0.64
0.93
4.69
0.85
ꢀ
57
375
82
24.3
42.0
15.3
35.1
37.9
38.2
39.5
38.6
ꢀ
24.6
29.3
32.7
24.0
28.2
29.7
56.4
24.0
ꢀ
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3
4
5
6
7
8
-
14
42
37
79
53
91
52
129
361
86
8
34
100
122
Conditions: 30 ꢁmol of Cr(acac)3 and ligand, 500 equivalents of MAO, methylcyclohexane as
solvent (100 mL total volume), 60 °C, 30 bar of ethylene, 30 minutes.
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Given the closelyꢀrelated structure of the diphenylphosphine ligands 1, 3, 5 and 7, it is
confusing that exclusively the chromium complex stabilized by ligand 7 gave selectivities
towards 1ꢀhexene and 1ꢀoctene. Attempts to obtain insights into the connectivity of the
chromium complexes by isolating single crystals suitable for Xꢀray diffraction analysis,
unfortunately turned out to be unsuccessful. All the crystals grown from the solutions of the
chromium complexes were identified to be the initial reactant chromium precursors (i.e.
CrCl3(THF)3), which is a indication of the weak binding strength between the ligands and the
chromium center in the neutral, nonꢀactivated species. The easy loss of the ancillary ligand also
raises the possibility that the PE formed in the present system might partially arise from a ligandꢀ
free chromium species. This assumption is supported by the fact that upon activation with MAO,
Cr(acac)3 itself enables ethylene polymerization albeit with low activity (Table 1).
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Despite the uncertain features that endow Cr/7 the ability to promote ethylene triꢀ and
tetramerization, its promising catalytic performances encouraged us to further investigate the
Cr/7 system under various reaction conditions.
First the effect of different chromium precursors on the catalytic behavior was investigated
(Table 2). Despite the varying activities, systems using CrCl3(THF)3, Cr(acac)3 or Cr(EH)3 (acac
= acetylacetonate, EH = 2ꢀethylꢀhexanoate) as chromium sources showed similar selectivities
towards 1ꢀhexene and 1ꢀoctene, which indicates that the same active species for selective
ethylene triꢀ and tetramerization is formed regardless the chromium precursor used. In the case
of CrCl2(THF)2, more C16 and C20 products were detected. Probably, these two higher
oligomers are the secondary products of coꢀtrimerization or coꢀtetramerization of ethylene, 1ꢀ
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hexene or 1ꢀoctene. The formation of C17 products from a coꢀoligomerization of ethylene and 1ꢀ
pentene has proved that CrCl2(THF)2/7 is capable of incoporating αꢀolefins, although it is
unclear that these C17 products arise from the combination of 1ꢀpentene with which αꢀolefins
(possible combinations: C5+C8+C2+C2, C5+C5+C5+C2, C6+C6+C5, etc.) Chlorineꢀfree
precursors are favored since they afford higher activity and produce less PE. The enhanced
activity probably resulted from a better solubility of Cr(acac)3 and Cr(EH)3 in
methylcyclohexane.
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Table 2. Catalytic tests using 7 combined with different chromium precursors.
=
=
Cr precursor
PE
PE
(wt%)
73
LAO
(g)
Activity
(g/mmol Cr/h)
193
C6
C8
(g)
(mol%)
58.0
CrCl3(THF)3
CrCl2(THF)2
Cr(acac)3
0.70
0.38
0.29
0.66
0.26
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2.40
3.22
40.4
23.8
37.0
37.2
72
106
51.4
11
538
58.3
Cr(EH)3
17
775
56.6
Conditions: 10 ꢁmol of Cr(acac)3 and 7, 500 equivalents of MAO, methylcyclohexane as solvent
(100 mL total volume), 60 °C, 30 bar of ethylene, 30 minutes.
There are various occasions known where the presence of toluene poisons the selective
ethylene oligomerization catalyst or converts it into a nonꢀselective ethylene oligomerization
catalyst.8ꢀ9,26,37ꢀ38,41 Although direct proof is not available in the literature,8ꢀ9,26,49ꢀ50 it is likely that
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Cr(I)ꢀarene complexes are formed. Hence, in an attempt to further improve the catalytic activity
as well as the overall selectivity for 1ꢀhexene and 1ꢀoctene formation, dried MAO (DMAO) was
used as coꢀcatalyst instead of a toluene solution of MAO. Indeed, the tolueneꢀfree system
showed a doubling in activity, however, the selectivity simultaneously shifted from 1ꢀoctene to
1ꢀhexene (Table 3, entry 2). The addition of extra trimethylaluminum (TMA) (10 %) to the
DMAO did not restore the 1ꢀoctene selectivity.35 In contrast, it led to a further increase of 1ꢀ
hexene selectivity as well as activity (Table 3, entry 3). Interestingly, addition of 3.5 mL (33
mmol, the same volume as the MAO toluene solution) of toluene resulted in the original catalytic
behavior obtained with MAO as coꢀcatalyst (Table 3, entries 1 and 4). This finding indicates that
for the current system a small amount of toluene plays a crucial role with respect to the
selectivity for 1ꢀoctene versus 1ꢀhexene. Using chlorobenzene or hexamethylbenzene instead of
toluene demonstrated that using a less coordinating or a more polar arene has a beneficial effect
on the catalytic activity, but also leads to a slight shift in selectivity towards 1ꢀhexene (Table 3,
entries 5, 7). There seems to be no straightforward interpretation for how the arenes affect 1ꢀ
hexene and 1ꢀoctene selectivity. One possible explanation might be that arenes feature a
hemilabile coordinative behavior during ethylene coordination and insertion, which thereby
influences the kinetics of insertion versus βꢀH transfer of the chromacycloheptane intermediate.
In this regard, toluene should be considered as a reagent to the chromium catalyst rather than a
solvent simply increasing polarity. Considering the shift in selectivity requires a contribution
from considerably large equivalents of toluene (ca. 3,300 equiv.), a relatively weak interaction
between toluene and the chromium center is expected.
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Table 3. The effect of different arenes on the catalytic behavior of Cr(acac)3/7.
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=
=
Entry
Coꢀcatalyst
(equiv.)
Arene
PE
(g)
LAO
(g)
Activity
(g/mmol Cr/h)
538
C6
C8
#
1
2
(mmol)
(mol%)
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MAO (500)
DMAO (500)
toluene (33) 0.29
2.40
4.42
37.0
67.4
58.3
30.9
ꢀ
ꢀ
1.45
1.29
1174
DMAO (500) /
TMA (50)
3
6.84
1626
79.8
18.8
4
5
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7
DMAO (500)
DMAO (500)
DMAO (500)
DMAO (500)
toluene (33) 0.82
1.94
7.85
2.96
0.65
553
1853
710
40.2
71.0
49.9
58.3
55.3
28.4
49.2
38.6
PhCl (33)
toluene (5)
C6Me6 (5)
1.42
0.59
0.17
165
Conditions: 10 ꢁmol of Cr(acac)3 and 7, methylcyclohexane as solvent (100 mL total volume),
60 °C, 30 bar of ethylene, 30 minutes.
A detailed investigation was further carried out to elucidate the influence of toluene on the
selectivity for 1ꢀhexene and 1ꢀoctene by carefully varying the amount of toluene added to the
tolueneꢀfree system (Figure 2). It was found that introduction of 1ꢀ5 volumeꢀ% of toluene
gradually shifts the dominant liquid fraction from 1ꢀhexene to 1ꢀoctene, up to a maximum of
59 % of 1ꢀoctene (for 5 volumeꢀ% of toluene). Moreover, the addition of toluene had a strong
influence on the catalytic activity as well. Just 2 volumeꢀ% of toluene proved to be enough to
result in a threeꢀfold drop of the original catalytic activity. Above 10 volumeꢀ% of toluene, nonꢀ
selective ethylene oligomerization became more and more prominent with increasing toluene
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concentration. Using pure toluene as solvent resulted in a statistic distribution of oligomers with
a minor enrichment in 1ꢀhexene and 1ꢀoctene, indicating the coꢀexistence of active species
enabling selective and nonꢀselective ethylene oligomerizations in the system. The activity of the
nonꢀselective catalyst was found to be comparable to that of the selective catalyst in the absence
of toluene.
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Figure 2. Influence of toluene concentration on activity and selectivity for 1ꢀhexene and 1ꢀ
octene. Dotted lines have no physical meanings. Conditions: 10 ꢁmol of Cr(acac)3 and 7, 500
equivalents of DMAO, 60 °C, 30 bar of ethylene, 30 minutes, methylcyclohexane and/or toluene
as solvent (100 mL total volume).
The influence of other reaction conditions on the catalytic behavior was subsequently
investigated with Cr(acac)3/7 (Table 4). Besides the striking effect of toluene, the selectivity for
1ꢀhexene and 1ꢀoctene as well as the catalytic activity were dramatically influenced by the
choice of the coꢀcatalyst. A recent study on the Sasol CrꢀPNP system shed light on the role of the
coꢀcatalyst during catalysis, suggesting that the selectivity can be affected remarkably by the
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interaction strength between the chromium center and the aluminum species.48 Using modified
MAO (MMAO, 7 % Al in heptane solution) instead of MAO shifted the system to ethylene
trimerization with 91 % 1ꢀhexene selectivity (Table 4, entry 2). The combination of DMAO and
triisobutylaluminum (TIBA) also promoted selective ethylene oligomerization, albeit with lower
1ꢀhexene selectivity (80 %) and a significant amount of PE being formed (30 %; Table 4, entry
3). On the other hand, introduction of diethylaluminum chloride (DEAC) in combination with
DMAO turned the system inactive (Table 4, entry 4). This observation is in contrast with the
positive effect of DEAC on the ChevronꢀPhillips system,47 in which the hemilabile interaction
between chromium and the introduced chlorine from DEAC benefits the catalytic activity.18 In
the present study, addition of either chlorineꢀcontained chromium precursors or chlorineꢀ
contained coꢀcatalysts negetively affected the catalytic performance.
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Besides the abovementioned strong influences, catalyst loading, ligand/Cr ratio, temperature
and ethylene pressure had a minor effect but with clear trends on both catalytic activity and
selectivity (Figure 3). Variation of catalyst loading from 5 ꢁmol to 30 ꢁmol (i.e. 0.05 mM and
0.3 mM) resulted in a gradual decline of 1ꢀoctene selectivity as well as catalytic activity (Table 4,
entries 1, 8, 9, 10). The impact of catalyst loading might result from a shift in equilibria between
different, for example mononuclear and binuclear,7 active species. A dramatic effect of catalyst
loading on selectivity has recently been reported for a chromium/PN ligandꢀbased ethylene
tetramerization system, in which increasing the catalyst loading from 15 ꢁmol to 60 ꢁmol (i.e.
0.15 mM and 0.6 mM) led to varied 1ꢀhexene/1ꢀoctene ratios, and the highest 1ꢀoctene
selectivity was achieved when 30 ꢁmol of catalysts were employed.38 Raising the ligand/Cr ratio
from 1 to 3 resulted in a switch in selectivity from 1ꢀoctene towards 1ꢀhexene, while the highest
activity was achieved when 2 equivalents of ligands were employed (Table 4, entries 1, 11, 12).
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The influence of the ligand/Cr ratio on the catalytic behavior might be attributed to a change in
the coordination environment of chromium. Introduction of more ancillary ligands could give
rise to a competitive coordination to the chromium center of a second ligand 7 on the expenses of
toluene, thereby influencing the subsequent formation of active species responsible for selective
oligomerization. As was also found for the CrꢀPNP system,51 lowering the reaction temperature
was found to be beneficial for the 1ꢀoctene formation with a compromise in catalytic activity
(Table 4, entries 1, 5). Finally, higher ethylene pressure was found to favor both 1ꢀoctene
selectivity and catalytic activity (Table 4, entries 1, 6, 7). The positive effect of low temperature
and high ethylene pressure on 1ꢀoctene formation can be attributed to the enhanced ethylene
solubility in methylcyclohexane solution under these conditions.52 This is in line with the
reaction order in ethylene concentration for ethylene tetramerization being higher than that for
ethylene trimerization, as was found to be the case for other selective ethylene
trimerization/tetramerization systems.5,53ꢀ57 By careful adjustment of the reaction conditions, an
activity of ca. 2,000 g/mmol Cr/h could be reached with 57 % of 1ꢀoctene and 42 % of 1ꢀhexene
of the liquid fraction. Changing the reaction conditions for an optimum 1ꢀoctene selectivity (>
70 %) resulted in an aprox. 50 % reduction of the catalytic activity (Table 4, entries 7, 14).
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Table 4. Catalytic tests using Cr(acac)3/7 as catalyst precursor under different conditions.a
3
4
5
6
7
8
9
=
=
Entry
Coꢀcatalyst
(equiv.)
PE
(g)
PE
(wt%)
11
LAO
(g)
Activity
(g/mmol Cr/h)
538
C6
C8
#
1
(mol%)
37.0 58.3
90.8 4.6
10
11
12
13
14
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23
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31
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53
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59
60
MAO (500)
MMAO (500)
0.29
0.43
2.40
5.71
2b
7
555
DMAO(500) /
TIBA(100)
3b
4
3.59
ꢀ
30
ꢀ
3.65
ꢀ
347
ꢀ
79.9 12.5
DMAO(500) /
DEAC(100)
ꢀ
ꢀ
5c
6d
MAO (500)
MAO (500)
MAO (500)
MAO (500)
MAO (500)
MAO (500)
MAO (500)
MAO (500)
MAO (500)
MAO (500)
MAO (500)
0.57
0.33
0.61
0.23
0.56
0.45
0.49
0.97
0.46
0.63
0.01
28
23
9
1.48
1.10
5.97
2.06
3.68
4.69
5.38
3.63
10.31
3.61
ꢀ
409
286
1316
917
423
361
1174
921
2154
848
ꢀ
29.2 66.1
36.4 60.9
28.1 70.5
36.3 60.5
39.8 57.2
39.5 56.4
45.3 52.9
47.8 47.2
41.5 57.1
22.8 73.7
7e
8f
10
13
8
9g
10b
11h
12i
8
21
4
13e, h
14c, e
blank
15
100
ꢀ
ꢀ
Conditions: a10 ꢁmol of Cr(acac)3 and 7, methylcyclohexane as solvent (100 mL total volume),
60 °C, 30 bar of ethylene, 30 minutes. b30 ꢁmol of Cr(acac)3 and 7. c40 °C. d20 bar of ethylene.
e50 bar of ethylene. 5 ꢁmol of Cr(acac)3 and 7. g20 ꢁmol of Cr(acac)3 and 7. h10 ꢁmol of
f
Cr(acac)3, 20 ꢁmol of 7. i10 ꢁmol of Cr(acac)3, 30 ꢁmol of 7.
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Figure 3. Influence of reaction conditions on activity and selectivity for 1ꢀhexene and 1ꢀoctene.
Dotted lines have no physical meanings. Conditions: 10 ꢁmol of Cr(acac)3 and 7, 500
equivalents of MAO, 60 °C, 30 bar of ethylene, 30 minutes, methylcyclohexane as solvent (100
mL total volume). Adjustments of reaction conditions are shown in the figure.
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Inspired by the works of Gambarotta and coꢀworkers (Scheme 1),38,41 endeavors were also
made to achieve higher 1ꢀoctene selectivity by introducing ligands with double units of 7 linked
by a carbon bridge (Figure 4). Upon activation with MAO in toluene, Cr(acac)3/9 produced a
statistic distribution of oligomers with a minor enrichment in 1ꢀhexene, while in
methylcyclohexane the system shifted to ethylene polymerization. The fairly low catalytic
activities shown in both solvents might have arised from the poor solubility of the catalysts.
Therefore ligand 10, assumed to have an improved solubility in methylcyclohexane, was
synthesized. Cr(acac)3/10 indeed shows an improved productivity, however, the catalyst system
still lacked distinct selectivity towards 1ꢀhexene or 1ꢀoctene.
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Figure 4. Illustration of N, Pꢀbased ligands with two units of 7 linked by a carbon bridge.
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N1
N2
P2
P1
Cr
P1 Cr P2
P1 Cr P2
9
C1
C1
C1
C2
C2
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C2
Cr-PNNP
N1
P1
N2
P2
Cr
P2
C2
P1
C1
Cr
C1
C2
Cr-L10
N2
P2
N1
P2
P2
P1
C1
P1
Cr
P1
Cr
C2
C2
C1
Cr
C2
C1
Cr-L10*
Scheme 2. Comparison of Cr/10 complex (below) with a related chromacyclopentane containing
the PNNP ligand published by Gambarotta.38 Unit of CrꢀP distance is Å.
Considering the fact that 10 has a similar backbone as the PNNP ligand developed by
Gambarotta,38 the quite different catalytic behavior from their corresponding chromium
complexes was unexpected. To explore the possible reason for this distinguishable difference
between the two systems, density functional theory (DFT) calculations were conducted on the
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corresponding chromacyclopentane species, which are important active intermediates in the
hypothetical metallacycle mechanism31ꢀ32,34,58 for selective ethylene oligomerization and are
often found to be involved in the rateꢀdetermining step in the reaction cycle.15,18ꢀ19,23 Interestingly,
the complexes clearly exhibit different coordination geometries in the optimized
chromacyclopentane structures. The complex containing the PNNP ligand synthesized by
Gambarotta (CrꢀPNNP) features an octahedral geometry, with a vacant site feasible for the
coordination of the third ethylene. Conversely, the corresponding chromocyclopentane stabilized
by 10 (CrꢀL10) shows a distorted tetrahedral geometry. Besides, one of the two phosphines is
barely coordinating and 10 can better be regarded as a monodentate ligand (CrꢀL10*, ꢂGCrꢀL10*,
298 K ꢀ ꢂGCrꢀL10, 298 K = ꢀ4.9 kcal/mol). The vacant site created by the leaving phosphine is then
available for the third ethylene to coordinate. We speculate that loss of the bidentate character of
ligand 10 might be the reason that the Cr/10 system lost its selectivity towards 1ꢀhexene or 1ꢀ
octene (Scheme 2). Nevertheless, the possibility should not be preculded that active species
differing from CrꢀL10 is generated from Cr/10 system, which also enables nonꢀselctive ethylene
oligomerization.
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Conclusions
Of a series of diphenylphosphineꢀsubstituted Nꢀhetrocyclic ligands, Nꢀpyrrolyldiphenylꢀ
phosphine proved to afford a selective ethylene oligomerization catalyst when in situ mixed with
Cr(acac)3 and aluminumꢀbased coꢀcatalysts in methylcyclohexane, producing 1ꢀhexene and 1ꢀ
octene with an overall selectivity up to 99 % together with a small amout of PE. In this system,
the 1ꢀhexene/1ꢀoctene ratio can be tuned from 0.3 to 20 by the choice and amount of solvents
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and coꢀcatalysts, as well as by varying the reaction conditions such as temperature, pressure and
ligand/Cr ratio. The most striking feature is the positive effect of the presence of small volume
percentages of toluene on the 1ꢀoctene selectivity for the methylcyclohexaneꢀbased
oligomerizations. When increasing the amount of toluene (> 10 volumeꢀ%) nonꢀselective
ethylene oligomerization becomes prominent and in pure toluene more or less a statistic
distribution of oligomers is obtained. By careful adjustment of the reaction conditions, 1ꢀhexene
and 1ꢀoctene selectivites of up to 91 % and 74 %, respectively, with fairly good activities could
be achieved. Coupling two units of ligand 7 proved to be unfavorable for improving the 1ꢀhexene
or the 1ꢀoctene selectivity, which is probably due to the monodentate rather than bidentate
bonding mode of the ligand as revealed by DFT calculations.
10
11
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Experimental Section
General Procedures: All manipulations for air and/or moisture sensitive materials were carried
out under inert atmosphere in a glovebox or using Schlenk techniques. Dry solvents were
obtained by passing them through a column purification system. Starting materials for ligand
synthesis were purchased from SigmaꢀAldrich and used as received. MAO (10 wt% toluene
solution) was purchased from Aldrich and used as received. DMAO was prepared by pumping
off all the volatile compounds of MAO at 40 °C for 6 hours. All NMR spectra were recorded on
Varian Mercury 400 or 500 MHz spectrometers at 25 °C.
Ligand synthesis:
Nꢀcarbazolyldiphenylphosphine (1) was prepared following the method from the literature.59
31P NMR (400 MHz, CDCl3) δ: 31.68 (s). Purity 97 % on the basis of 31P NMR.
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Phenylbis(1ꢀcarbazolyl)phosphine (2) was prepared following the method from the literature.59
31P NMR (400 MHz, CDCl3) δ: 51.78 (s). Purity 94 % on the basis of 31P NMR.
3
4
5
6
7
8
9
Nꢀindolyldiphenylphosphine (3): nꢀBuLi (4.0 mL, 10 mmol, 2.5 M in hexane) was added
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dropwise to a stirred THF solution (20 mL) of indole (1.18 g, 10 mmol) at ꢀ78 °C. The reaction
mixture was allowed to warm to room temperature and stirred overnight. Petroleum ether (15 mL)
was added to precipitate a white powder, which was isolated by filtration, washed with
petroleum ether twice and then redissolved in THF (40 mL). Ph2PCl (1.8 mL, 10 mmol) was
added dropwise at 0 °C and the reaction mixture was stirred overnight. The solvent was
evaporated under reduced pressure and the resulting oil was dissolved in toluene. The suspension
was filtered and the solvent was removed in vacuo. The product was crystallized from petroleum
ether as colorless crystals (2.4 g, 8.0 mmol, 80 %). 1H NMR (400 MHz, CDCl3) δ: 7.79 (d, 1H),
7.62 (d, 1H), 7.43ꢀ7.29 (m, 10H), 7.20 (d, 1H), 7.17 (d, 1H), 6.98 (t, 1H), 6.64 (d, 1H). 31P NMR
13
(400 MHz, CDCl3) δ: 35.20 (s). C NMR (500 MHz, CDCl3) δ: 141.3, 136.3, 134.3, 132.1,
31
130.5, 130.2, 129.7, 128.8, 128.2, 122.2, 120.8, 112.3, 106.5. Purity 99 % on the basis of P
NMR.
Phenylbis(1ꢀindolyl)phosphine (4): As for 3 using indole (2.35 g, 20 mmol), nꢀBuLi (8.0 mL,
20 mmol, 2.5 M in hexane) and PhPCl2 (1.4 mL, 10 mmol). Recrystallization from petroleum
1
ether gave colorless crystals (2.1 g, 6.2 mmol, 62 %). H NMR (400 MHz, CDCl3) δ: 7.91 (d,
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2H), 7.60 (d, 2H), 7.48ꢀ7.38 (m, 4H), 7.30ꢀ7.25 (m, 2H), 7.21ꢀ7.15 (m, 5H), 7.66 (d, 2H). P
NMR (400 MHz, CDCl3) δ: 48.62 (s). 13C NMR (500 MHz, CDCl3) δ: 141.1, 134.9, 131.0, 130.4,
130.2, 129.0, 128.9, 122.9, 121.5, 112.1, 108.0. Purity 99 % on the basis of 31P NMR.
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Nꢀpyrazolyldiphenylphosphine (5): A Schlenk flask was charged with pyrazole (3.40 g, 50
mmol), triethylamine (7.4 mL, 52.5 mmol), and THF (85 mL). Subsequently, Ph2PCl (9.0 mL,
50 mmol) was added dropwise at 0 °C and the solution was stirred overnight at room
temperature. The colorless precipitate which formed was removed by filtration and was washed
with THF. The solvent was removed in vacuo to give the product as an oil (7.6 g, 30 mmol,
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60 %). H NMR (400 MHz, CDCl3) δ: 7.84 (d, 1H), 7.68 (t, 1H), 7.43ꢀ7.35 (m, 10H), 6.38 (t,
31
13
1H). P NMR (400 MHz, CDCl3) δ: 54.24 (s). C NMR (500 MHz, CDCl3) δ: 144.5, 136.9,
136.3, 132.6, 130.1, 128.6, 107.2. Purity 96 % on the basis of 31P NMR.
Phenylbis(1ꢀpyrazolyl)phosphine (6): A Schlenk flask was charged with pyrazole (4.36 g, 60
mmol), triethylamine (9.0 mL, 64.6 mmol), and THF (100 mL). This was followed by dropwise
addition of Ph2PCl (4.0 mL, 30 mmol) at 0 °C. The solution was stirred overnight at room
temperature. The colorless precipitate which formed was removed by filtration and was washed
with THF. The combined filtrates were dried in vacuo. The resulting oil was redissolved in
petroleum ether and filtered. The solvent was removed in vacuo giving the resulting product as
1
an oil (6.0 g, 25 mmol, 83 %). H NMR (400 MHz, CDCl3) δ: 8.10ꢀ7.76 (m, 4H), 7.71ꢀ7.32 (m,
31
13
5H), 6.41 (m, 2H). P NMR (400 MHz, CDCl3) δ: 73.03 (s). C NMR (500 MHz, CDCl3) δ:
145.1, 136.9, 135.1, 130.7, 128.8, 108.3. Purity 96 % on the basis of 31P NMR.
Nꢀpyrrolyldiphenylphosphine (7) was prepared following the method from the literature.60 31
NMR (400 MHz, CDCl3) δ: 47.73 (s). Purity 94 % on the basis of 31P NMR.
P
Phenylbis(1ꢀpyrrolyl)phosphine (8) was prepared following the method from the literature.60
31P NMR (400 MHz, tolueneꢀd8) δ: 70.03 (s). Purity 99 % on the basis of 31P NMR.
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Diphenylbis(pyrrolyldiphenylphosphino)methane (9): A solution of benzophenone (27.0 g,
150 mmol) and methanesulfonic acid (0.5 mL) in 99 % ethanol was stirred and heated at 60ꢀ70
°C until complete dissolution. Neat pyrrole (10 mL, 140 mmol) was added over a period of 30
min and the mixture was stirred at 60ꢀ70 °C for 4 h, upon which the color of the solution
changed from dark red to dark brown. The mixture was diluted with 99 % ethanol (100 mL) and
allowed to crystallize at 50 °C. A red solid was obtained, which was filtered and washed with
portions of warm ethanol (100 mL, 50 °C) until complete discoloration followed by drying in
vacuo (5.8 g, 19.5 mmol, 13 %). The solid diphenyldipyrrolylmethane was then dissolved in
THF (65 mL). nꢀBuLi (15.6 mL, 39 mmol, 2.5 M in hexane) was added over a period of 20 min
at ꢀ78 °C. After stirring overnight at room temperature, chlorodiphenylphosphine (7.2 mL, 39
mmol) was then added dropwise at 0 °C. After stirring overnight at room temperature, the
solvent was pumped off in vacuo. The resulting oil was redissolved in toluene and insoluble
material was removed by filtration. The clear solution was dried in vacuo and the oily residue
was dissolved in petroleum ether. Beige powder was collected after filtration and drying in vacuo
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1
(5.6 g, 8.4 mmol, 43 %). H NMR (400 MHz, CDCl3) δ: 7.31ꢀ6.99 (m, 30H), 6.71 (q, 1H), 6.49
31
(q, 1H), 6.17 (q, 1H), 6.13 (q, 1H), 6.04 (q, 1H), 5.83 (q, 1H). P NMR (400 MHz, CDCl3) δ:
36.73 (s). 13C NMR (500 MHz, CDCl3) δ: 145.8, 137.8, 132.4, 129.9, 129.0, 128.0, 127.4, 126.3,
116.8, 114.4, 109.6, 107.9. Purity 95 % on the basis of 31P NMR.
Diethylbis(pyrrolyldiphenylphosphino)methane (10): 37 % aqueous HCl (0.5 mL) was added
to a solution of 3ꢀpentanone (15 mL, 0.14 mol) in boiling water (100 mL), which was followed
by the dropwise addition of neat pyrrole (5.0 mL, 0.7 mol). After refluxing for 45 min the
suspension was left to cool to 40–50 °C and then the upper layer was collected and allowed to
cool to room temperature upon which crystals precipitated. The green crystals were separated by
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filtration and redissolved in an ethanol/water mixture. The solution was left overnight at room
temperature and colorless crystals formed. The white crystals were filtered, washed with
ethanol/water and dried overnight in a vacuum oven (40 °C) (4.1 g, 20 mmol, 14.5 %). The dried
diethyldipyrrolylmethane (2.0 g, 10 mmol) and triethylamine (3.5 mL, 25 mmol) were dissolved
in THF (30 mL). PPh2Cl (3.5 mL, 20 mmol) was then added dropwise at 0 °C. After reflux
overnight at 70 °C, the formed white solid was removed by filtration and the filtrate was dried in
vacuo yielding brown oil. The resulting oil was redissolved in petroleum ether and stirred
overnight. Beige powder was collected after filtration and drying in vacuo (2.3 g, 4.0 mmol, 40
%). 1H NMR (400 MHz, CDCl3) δ: 7.43ꢀ7.19 (m, 22H), 6.27 (q, 2H), 6.06 (m, 2H), 1.84 (q, 4H),
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13
0.66 (t, 6H). P NMR (400 MHz, CDCl3) δ: ꢀ27.00 (s). C NMR (500 MHz, CDCl3) δ: 140.9,
31
137.7, 132.7, 128.5, 128.4, 123.1, 118.8, 107.9, 44.1, 30.0, 8.4. Purity 83 % on the basis of P
NMR.
Ethylene Oligomerization: All the ethylene oligomerization tests were performed in a 200 mL
Büchi autoclave. The autoclave was heated in an oven at 150 °C overnight before each run. After
evacuation and rinsing with argon three times, the solvent (volume of solvent = total volume
(100 mL) ꢀ volume of catalyst solution ꢀ volume of coꢀcatalyst solution) was charged into the
preheated autoclave. Then the coꢀcatalyst was injected and the solvent was saturated with
ethylene by pressurizing to 30 bar. After 15 minutes of stirring, the autoclave was temporarily
vented to allow the injection of the catalyst solution. The autoclave was repressurized to the
desired pressure and the pressure was maintained throughout the run. The temperature of the
autoclave was controlled by a thermostatted bath. After 30 minutes, the reaction was quenched
by cooling to 0 °C, depressurizing and injection of a mixture of ethanol and diluted hydrochloric
acid. The polymer was separated by filtration and dried overnight at 60 °C under reduced
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pressure before mass determination. The oligomers were analyzed by GCꢀFID for oligomer
composition and by 1H NMR spectroscopy for activity. Since the activity was measured for 0.5 h
and there was a catalyst decay after 15 min indicated by the temperature drop, it should be noted
that the real productivity per hour might be lower than the calculated data.
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Computational details: All the DFT calculations were performed using the Gaussian 09
program package.61 Geometry optimizations were carried out without any symmetry constraints,
using the unrestricted PBE density functional and Ahlrichs tripleꢀζ basis set TZVP. The nature of
stationary points for all the resulting geometries was verified by analytical frequency calculations
(no imaginary frequencies). Singleꢀpoint energies were obtained at the same level of theory. All
energies reported referred to Gibbs free energy corrections to the total electronic energies at
298.15 K.
ASSOCIATED CONTENT
Supporting Information.
Cartesian coordinates of all optimized chromacyclopentane structures in this work. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Eꢀmail: r.duchateau@tue.nl (Robbert Duchateau), boping@ecust.edu.cn (Boping Liu)
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ACKNOWLEDGMENT
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We’d like to thank the Chinese Scholarship Council, the Natural Science Foundation of China
(No. 21174037) and the Eindhoven University of Technology for financial support.
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