Selective ethylene oligomerization with in-situ-generated chromium catalysts supported by trifluoromethyl-containing ligands

Article abstract of DOI:10.1002/pola.28915

A series of pyrrole-containing diarylphosphine and diarylphosphine oxide ligands were prepared. The catalytic activity of the corresponding in-situ-generated chromium catalysts was investigated during selective ethylene oligomerization reactions. Variatio

Full text of DOI:10.1002/pola.28915

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Selective Ethylene Oligomerization with In-Situ-Generated Chromium
Catalysts Supported by Trifluoromethyl-Containing Ligands
Eung Man Choi, Jong-Eun Park, Gyeong Su Park, Bonggeun Shong,* Sung Kwon Kang, and
Kyung-sun Son
1Department of Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea
Correspondence to: K.-s. Son (E-mail: kson@cnu.ac.kr)
Received 1 August 2017; accepted 8 November 2017; published online 26 November 2017
DOI: 10.1002/pola.28915
ABSTRACT: A series of pyrrole-containing diarylphosphine and
diarylphosphine oxide ligands were prepared. The catalytic
activity of the corresponding in-situ-generated chromium cata-
lysts was investigated during selective ethylene oligomeriza-
tion reactions. Variations in the ligand system were introduced
by modifying the diarylphosphine and pyrrole moieties that
affect the steric and electronic properties. Minor changes in the
ligand structure and the composition of activators significantly
changed the catalytic activity, selectivity toward linear alpha-
olefins (LAO) versus polyethylene (PE), and the distribution of
oligomeric products. The presence of trifluoromethyl groups
on the diphenyl rings in ligand 3 promoted oxidation to form
the corresponding phosphine oxide structure, 3o, which dra-
matically enhanced the catalytic activity of ethylene trimeriza-
tion. The in-situ-generated chromium complex based on 3o
activated by DMAO (dry methylaluminoxane)/TIBA (triisobuty-
laluminum) was used to achieve activity of about 1250 g
(mmol of Cr)21 h21 with 98.5 mol % 1-hexene, along with a
C
V
negligible amount of PE side product. 2017 Wiley Periodicals,
Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 444–450
KEYWORDS:
catalysts;
1-hexene;
a-olefins;
oligomers;
polyethylene
that the ortho-methoxy-substituted aryl group on bis(diaryl-
phosphino)amine played an important role in the ethylene
trimerization and is essential for catalytic activity.14,15 Sasol
Technology also reported that related Cr-diphosphinoamine
systems containing alkyl or methoxy substituents in the
ortho position of the aryl groups favor trimerization over tet-
ramerization, supporting that ortho-substitution at the diary-
lphosphines is key to switching between tetramerization and
trimerization.16 As far as we know, modification on the dia-
rylphosphines of other P,N-ligand scaffolds has not yet been
investigated.
INTRODUCTION The selective oligomerization of ethylene to
1-hexene and 1-octene is one of the most exciting advance-
ments in olefin catalysis in the past few decades.1–3 Tradi-
tionally, the oligomerization of ethylene by alkyl aluminum
or transition metal catalysts follows the Cossee–Arlman
mechanism, yielding a Schulz–Flory distribution of oligomers
that must be separated by distillation. However, trimerization
to 1-hexene or tetramerization to 1-octene provides a selec-
tive route to these valuable reagents,4 which are comono-
mers for linear low-density polyethylene (LLDPE)
production. For this reason, many selective ethylene trimeri-
zation and tetramerization catalysts have been developed
based on chromium and other transition metals.1,2,5
Moreover, phosphine oxides are also important but have
received much less attention. They are often considered a sign
of incomplete exclusion of oxygen when the corresponding
phosphines are the target of synthesis. As far as we know, ami-
nophosphine oxide has not yet been investigated as a ligand
for chromium-catalyzed selective ethylene oligomerization.
The combination of nitrogen (N) and phosphorus (P) donor
atoms within an ancillary ligand system for selective oligo-
merization has been expanded since the discovery of chro-
mium bis(diphenylphosphino)amine.1,2,5–7 Although a variety
of amine groups on the aminophosphine ligands were exten-
sively studied for selectivity control,7–10 previous studies on
the modification of diarylphosphine parts are quite lim-
ited.11–13 One such study, by Wass and coworkers , claimed
Recent studies by Yang et al. on chromium complexes that
are stabilized by a simple N-pyrrolyldiphenylphosphine ligand
system showed its versatility for the selective formation of
*Present address: Bonggeun Shong, Department of Chemical Engineering, Hongik University, Seoul 04066, Republic of Korea.
Additional Supporting Information may be found in the online version of this article.
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1-hexene and 1-octene.17 We modified the diarylphosphine
moiety by introducing methyl or trifluoromethyl groups to
study how the substituted diarylphosphine affects catalytic
behavior during selective ethylene oligomerization. Herein, we
report a catalytic system that has a high activity and selectivity
for 1-hexene (up to 99% purity) with only minor by-products.
The influence of ligands and activators was systematically
studied under various reaction conditions.
under vacuum to give a crude product, which was purified
as described below.
1: Diphenyl(1H-Pyrrol-1-yl)Phosphine
Following the general synthesis, the crude product as a yel-
low oil was purified by column chromatography, with ethyl
acetate:hexane (1:9) used as the eluent. The solvent was
removed under vacuum to give the product as a white solid
(0.266 g, 0.99 mmol, 21.7%). 1H NMR (300 MHz, CDCl3, d):
6.32 (dd, J 5 3.8, 1.9 Hz, 2H), 6.82 (td, J 5 3.8, 1.9 Hz, 2H),
7.23–7.31 (m, 4H), 7.38 (d, J 5 3.4 Hz, 6H); 13C NMR (150
MHz, CDCl3, d): 111.62, 125.64, 128.76, 129.86, 132.10,
137.14; 31P NMR (242 MHz, CDCl3, d): 47.79; purity 5 98%
on the basis of 31P NMR analysis. HRMS (ESI, m/z): [M 1 H]1
calcd for C16H15NP, 252.0942; found, 252.0930.
EXPERIMENTAL
General Considerations
All oligomerizations were carried out under an N2 atmo-
sphere using a glove box or by using standard Schlenk tech-
niques, except column chromatography at ambient
conditions. All solvents were dried and distilled prior to use
according to standard methods. Methylaluminoxane (MAO;
10 wt % in toluene) was purchased from Albemarle Corp.
(Baton Rouge, LA, USA). TIBA, pyrrole, 2,4-dimethylpyrrole,
2,5-dimethylpyrrole (DMP), triethylamine, tetrahydrofuran
(THF), and nonane were purchased from Sigma-Aldrich (St.
Louis, MO, USA). Chlorodiphenylphosphine, chlorodi(p-tolyl)-
2: 1-(Di-p-Tolylphosphino)Pyrrole
Following the general synthesis, pyrrole (0.31 mL, 4.4
mmol), Et3N (0.62 mL, 4.4 mmol), and chlorodi(p-tolyl)phos-
phine (0.22 g, 1 mmol) were used. The crude product as a
white oil was purified by column chromatography on silica
gel, with ethyl acetate:hexane (1:9) as the eluent. The solvent
was removed under vacuum to give the product as a white
solid (0.09 g, 0.32 mmol, 33.7%). Single crystals were
obtained by slow diffusion of hexane into a concentrated
solution of the product in THF at room temperature. 1H
NMR (300 MHz, CDCl3, d): 2.36 (s, 6H), 6.29 (dd, J 5 3.6, 1.7
Hz, 2H), 6.81 (dt, J 5 3.8, 2.0 Hz, 2H), 7.14–7.20 (m, 8H); 13C
NMR (150 MHz, CDCl3, d) 21.50, 111.36, 125.50, 129.52,
132.15, 133.93, 139.98; 31P NMR (242 MHz, CDCl3, d):
47.78; purity 5 94% on the basis of 31P NMR analysis. HRMS
(ESI, m/z): [M 1 H]1 calcd for C18H19NP, 280.12551; found,
280.1263.
phosphine,
bis(3,5-di(trifluoromethyl)phenyl)chlorophos-
phine, and methylcyclohexane were purchased from Alfa-
Aesar (Haverhill, MA, USA). Cr(acac)3 and CrCl3(THF)3 were
used as purchased from Strem Chemicals (Newburyport, MA,
USA). Hexane, ethyl acetate, methanol, and hydrogen perox-
ide (34.5 wt % aqueous solution) were purchased from Sam-
chun Pure Chemical Co. (Korea).
1
The H NMR spectra of the products were recorded at 25 8C
on a Fourier 300 NMR spectrometer (Bruker, Billerica, MA,
USA) with tetramethylsilane (TMS) as an internal reference.
The 13C NMR spectra and 31P NMR spectra of the products
were recorded at 25 8C with Avance III 600 equipment
(Bruker) and using TMS as an internal reference. The
oligomers (liquid products) were analyzed by gas chroma-
tography with a flame ionization detector (GC-FID) using
Agilent 6890 (Agilent Technologies, Santa Clara, CA, USA)
equipment with an Agilent J&W GC capillary column, with
nonane used as the internal standard. X-ray crystal data
were obtained using a SMART APEX II (Bruker) single-
crystal X-ray diffractometer equipped with a Bruker SMART
charge-coupled device (CCD) area detector. Mass spectra
3: Bis(3,5-Bis(Trifluoromethyl)Phenyl)(1H-Pyrrol-1-
yl)Phosphine
Following the general procedure, pyrrole (4.4 mmol), Et3N
(0.63 mL, 4.5 mmol), and bis(3,5-(trifluoromethyl)phenyl)-
chlorophosphine (0.49 g, 1 mmol) were used. The crude
product (3) was recrystallized in hexane to yield an ivory
solid, which was isolated and characterized immediately. 1H
NMR (300 MHz, CDCl3, d): d 5 6.45 (dd, J 5 1.8 Hz, 2H), 6.81
(dd, J 5 3.6, 1.8 Hz, 2H), 7.66 (d, J 5 6.6 Hz, 4H), 7.97 (s,
1
2H); 13C NMR (75 MHz, CDCl3, d): 114.12, 123.0 (q, JC–F
5 273 Hz), 124.58, 125.33, 131.82, 132.81, 139.36; 31P NMR
(242 MHz, CDCl3, d): 42.55; purity 5 93% on the basis of 31P
NMR analysis.
were recorded with
spectrometer.
a
Bruker micrOTOF-QII (ESI)
General Procedure for Ligand Synthesis
Pyrrole (1.4 mL, 20 mmol), Et3N (2.8 mL, 20 mmol), and
THF (6 mL) were charged to a Schlenk flask under an inert
3o: Bis(3,5-Bis(Trifluoromethyl)Phenyl)(1H-Pyrrol-1-
yl)Phosphine Oxide
The product 3 was purified by column chromatography on
silica gel, with ethyl acetate:hexane (1:9) as the eluent. On
recrystallization from hexane, a white solid was obtained,
which was isolated and dried under vacuum (0.17g, 0.32
mmol, 31.5%). Single crystals were obtained by slow diffu-
sion of hexane into a concentrated solution of the product in
THF at room temperature. 1H NMR (300 MHz, CDCl3, d):
6.53 (dd, J 5 4.3, 2.6 Hz, 2H), 6.84 (dd, J 5 4.3, 3.0 Hz, 2H),
8.11 (d, J 5 12.8 Hz, 4H), 8.18 (s, 2H); 13C NMR (150 MHz,
atmosphere.
Subsequently,
chlorodiphenylphosphine
(0.85 mL, 4.6 mmol) was added dropwise at 0 8C. The reac-
tion mixture was stirred for 10 min at 0 8C, and for 10 min
at room temperature, and then heated to reflux for a further
15 h. The colorless precipitate that formed was removed by
filtration and washed with THF. The combined filtrates were
evaporated to dryness under vacuum. The resulting oil was
re-dissolved in hexane and filtered. The solvent was removed
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isolated by filtration and dried in a vacuum oven at 60 8C.
The oligomers were analyzed by GC-FID for oligomer
composition.
Computational Details
The DFT calculations were performed using the Orca pro-
gram (ver. 3.0.3).18 Geometry optimization was carried out
with B97-D3 functional,19,20 which showed good perfor-
mance for modeling organometallic complexes,21 with def2-
TZVP(-f) used as the basis set.22 Single point energies at the
optimized geometries were obtained with double-hybrid
PWPB95-D3 functional23 and def2-TZVP as the basis set. Sol-
vation by methylcyclohexane was accounted for using the
conductor-like screening model (COSMO)24 in all calcula-
tions. Transition state geometries were initially estimated
and then confirmed after optimization to give an imaginary
vibrational mode along the reaction coordinate.
FIGURE 1 Structures of the ligands examined in this study.
1
CDCl3, d): 115.48, 122.56 (q, JC–F 5 273 Hz), 123.10, 127.79,
132.13, 132.24 (1JC–P 5 129 Hz), 133.28 (qd, 2JC–F 5 34 Hz
and 3JC–P 5 14 Hz); 31P NMR (242 MHz, CDCl3, d): 18.04;
purity 5 98% on the basis of 31P NMR analysis. HRMS (ESI,
m/z): [M 1 Na]1 calcd for C20H10F12NOPNa, 562.0206;
found, 562.0207.
RESULTS AND DISCUSSION
The ligands used in this study (Fig. 1) were prepared by
reaction of pyrrole with triethylamine and then chlorodiaryl-
phosphine to form each ligand via salt elimination followed
by column chromatography (Scheme S1).17,25 Among them, 1
and 2 had an aminophosphine structure with no oxygen (O)
bound to the phosphine atom even after purification by col-
umn chromatography under ambient conditions; their struc-
tures were confirmed by spectroscopic data and X-ray single-
crystal analysis of 2 (Fig. 2). In contrast, the same procedure
using chlorodiarylphosphine bearing trifluoromethyl groups
on the phenyl rings yielded phosphine oxide, 3o, confirmed
by X-ray single-crystal analysis (Fig. 3). It is well known that
related phosphine compounds are readily oxidized upon
exposure to air, resulting in phosphine oxides.26–28 To obtain
3, all synthetic and purification processes were carried out
under an N2 atmosphere. Although attempts to isolate a sin-
gle crystal of 4 were unsuccessful, the spectroscopic analysis
of 4 supported a phosphine structure (see Supporting Infor-
mation for details).
4: Bis(3,5-Bis(Trifluoromethyl)Phenyl)(3,5-Dimethyl-1H-
Pyrrol-2-yl)Phosphine
Following the general procedure, 2,4-dimethylpyrrole
(0.2 mL, 2 mmol), Et3N (0.3 mL, 2.2 mmol), bis(3,5-(trifluor-
omethyl)phenyl)chlorophosphine (0.49 g,
1 mmol), and
dichloromethane were used. The product as a red oil was
purified by column chromatography on silica gel, with ethyl
acetate as the eluent. The solvent was removed under vac-
uum to give the product as a red oil (0.28 g, 0.49 mmol,
49.0%). 1H NMR (300 MHz, CDCl3, d): 2.15 (s, 3H), 2.24 (s,
3H), 5.95 (s, 1H), 7.42 (s, NAH), 7.69 (d, J 5 6.4 Hz, 4H),
7.86 (s, 2H); 13C NMR (150 MHz, CDCl3, d): 12.47, 13.39,
111.51, 112.57, 122.91, 123.20 (q, 1JC–F 5 273 Hz), 132.13,
132.37, 134.22, 135.30, 140.42; 31P NMR (242 MHz, CDCl3,
d): 231.98. Purity 91% on the basis of 31P NMR analysis.
Procedure for Ethylene Oligomerization
These reactions were carried out in a 125 mL Parr reactor
equipped with
a pressure controller. The reactor was
To elucidate the effects of ligand modification on activity and
selectivity, ethylene oligomerization reactions were carried
charged with methylcyclohexane (47 mL) and the desired
amount of Al co-catalysts. After the solution was stirred for
15 min at the set temperature using a thermostat bath, it
was saturated with N2 gas. The catalyst solution of Cr(acac)3
(0.01 mmol) and the ligand (0.02 mmol) was methylcyclo-
hexane (3 mL). Time zero for the reaction was considered
the point at which the catalyst solution was injected into the
reactor with ethylene gas. After 30 min, the reaction was ter-
minated by discontinuing the ethylene feed and cooling the
reactor to below 0 8C in an ice bath. After releasing the
excess ethylene from the reactor, nonane (1 mL) was added
as an internal standard for GC-FID analysis of the liquid
phase. The mixture was quenched with methanol (5 mL). A
small amount of reaction solution was collected and ana-
lyzed by GC-FID to determine the distribution of oligomers
obtained. The remainder of the mixture was quenched with
a mixture of methanol (450 mL) and diluted hydrochloric
acid (5 mL) to precipitate the solid product, which was
FIGURE 2 Thermal ellipsoid representation (30% probability
boundaries) of the X-ray crystal structure of 2. [Color figure
can be viewed at wileyonlinelibrary.com]
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various compositions.29–31 Interestingly, when the above
experiments were carried out on activation of a dry methyla-
luminoxane/triisobutylaluminum (DMAO/TIBA) mixture, a
considerable change in the catalytic behavior was observed
(entries 4–6); the in-situ activation of 1, 2, or 3o with a
DMAO (500 eq)/TIBA (50 eq) mixture in the presence of
Cr(acac)3 resulted in a large amount of the liquid product,
linear alpha-olefins (LAO) with 1-hexene as a major product,
whereas the PE formation was highly reduced. In particular,
the reaction of 3o/Cr(acac)3 under this condition produced
96.3 mol % of 1-hexene out of 0.96 g LAO, along with only a
trace amount of PE (0.06 g, 6.2 wt %). This promising cata-
lytic performance of 3o/Cr(acac)3 prompted an investigation
into this catalytic system under various reaction conditions,
with the aim of further improving the activity and selectivity
for 1-hexene formation.
FIGURE 3 Thermal ellipsoid representation (30% probability
boundaries) of the X-ray crystal structure of 3o. [Color figure
can be viewed at wileyonlinelibrary.com]
When the influence of the reaction conditions on the cata-
lytic behavior was systematically investigated with 3o/Cr(a-
cac)3, both the selectivity and catalytic activity were
dramatically influenced by the choice of the activator and
solvent, as summarized in Table 2. First, under typical ethyl-
ene oligomerization conditions, on activation with a toluene
solution of MAO (10 wt %), ethylene oligomerization with
out by using Cr(III) pre-catalysts generated in situ by mixing
the ancillary ligands with chromium(III) acetylacetonate
(Cr(acac)3) and alkyl aluminum co-catalyst. Attempts to
obtain further insights into the connectivity in the chromium
complexes by isolating single crystals suitable for X-ray dif-
fraction analysis were unsuccessful. The pertinent results
regarding the catalytic behavior and properties of the poly-
mers are summarized in Tables 1 and 2.
3o/Cr(acac)3 resulted in
a nonselective distribution of
oligomers including 66 mol % of C10–C40 (entry 7). Various
examples are reported where the presence of toluene poi-
sons the selective ethylene oligomerization catalyst and
results in a nonselective distribution of ethylene oligom-
ers.32–34 The switch in catalytic behavior may be due to
interference of toluene with the catalytically active chromium
species via coordination, although direct evidence has not
been reported.35 Thus, to avoid using toluene, DMAO was
chosen as an activator instead of MAO in toluene. Whereas
the use of MAO in toluene resulted in a statistical distribu-
tion of oligomers, the switch to the toluene-free system using
DMAO and methylcyclohexane as a solvent changed the
selectivity from higher oligomers to 1-hexene (67 mol %),
although the major product was a significant amount of PE
(entry 8). This shift in selectivity indicates that, for the cur-
rent system, a small amount of toluene plays a crucial role
in determining the selectivity for 1-hexene versus higher
oligomers.
First, we compared the catalytic behaviors of 1/Cr(acac)3, 2/
Cr(acac)3, and 3o/Cr(acac)3 upon activation of dry methyla-
luminoxane (DMAO) under 10 bar of ethylene (entries 1–3
in Table 1). During this initial screening, the ethylene oligo-
merization selectivity toward 1-hexene of the catalyst sys-
tems containing 2 and 3o were much lower than that of 1,
affording a large amount of polyethylene (PE) as the major
product. The amount of PE was too high to be part of the
statistical product distribution, suggesting that a polymeriza-
tion catalyst co-exists in the catalytic system. Previous
reports that highlighted the profound influence of this type
of activator on the ethylene oligomerization selectivity
prompted us to use different aluminum co-catalysts of
TABLE 1 Ethylene Oligomerization with 1–3o/Cr(acac)3
Al/Cr
PE
Activitya
1-Cb6
1-Cb8
C10-Cb40
Entry
Ligand
Activator
(equiv)
PE (g)
LAO (g)
(wt %)
(g/mmol Cr/h)
(mol %)
(mol %)
(mol %)
1
2
3
4
5
6
1
DMAO
500
0.059
0.563
0.489
0.069
0.054
0.063
0.229
0.065
0.050
0.337
0.321
0.957
20.5
89.6
90.7
17.0
14.4
6.2
57.6
90.1
66.4
48.7
88.3
94.7
96.3
3.9
6.0
2
DMAO
500
125.7
107.8
81.3
10.0
14.0
3.2
23.6
37.3
1.0
3o
1
DMAO
500
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
500/50
500/50
500/50
2
74.9
2.4
1.5
3o
203.9
0.8
0.5
a
General conditions: Cr(acac)3 0.01 mmol, ligand 0.02 mmol, ethylene 10
In units of [g of total products (PE 1 oligomers)]Á(mmol Cr)21Áh21
.
b
bar, methylcyclohexane 50 mL, 80 8C, 30 min.
The % yield as measured by GC-FID.
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TABLE 2 Effects of Activator Compositions and Reaction Conditions on Selective Ethylene Oligomerization
Al/Cr
PE
Activitya
1-Cb6
1-Cb8
C10-Cb40
Entry
Ligand
Activator
(equiv)
PE (g)
LAO (g)
(wt %)
(g/mmol Cr/h)
(mol %)
(mol %)
(mol %)
7
3o
3o
3o
3o
3o
1
MAO
500
0.348
0.863
0.103
0.097
0.036
0.075
0.068
0.241
0.169
0.105
0.111
0.060
0.145
0.031
1.536
0.234
2.122
5.252
6.204
0.963
0.918
1.226
0.169
0.964
3.965
1.503
5.299
4.223
18.5
78.7
4.6
376.8
219.4
444.9
1069.8
1248.0
207.6
197.2
293.5
77.5
17.3
67.1
98.7
98.9
98.5
85.1
93.1
96.8
85.0
97.8
98.4
98.5
96.4
97.5
16.9
11.2
0.5
0.3
0.2
9.1
3.6
0.9
4.9
0.4
0.5
0.6
0.7
0.5
65.7
21.8
0.8
0.8
1.2
2.0
1.3
2.1
6.8
0.6
1.2
0.9
2.8
2.0
8
DMAO
500
9
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
DMAO/TIBA
500/100
500/250
250/250
250/250
250/250
250/250
250/250
250/250
250/250
250/250
250/250
250/250
10
11
12
13
14
15
16d
17e
18f
19g
20
1.8
0.6c
7.2
2
6.9
3
16.4
43.6
9.8
4
3o
3o
3o
3o
DMP
213.8
815.1
312.6
1088.7
850.8
2.7
3.8
2.7
0.7
d
e
f
General conditions: Cr(acac)3 0.01 mmol, ligand 0.02 mmol, ethylene 20
CrCl3(THF)3 0.01 mmol.
60 8C.
bar, methylcyclohexane 50 mL, 80 8C, 30 min.
a
In units of [kg of total products (PE 1 oligomers)]Á(mol Cr)21Áh21
.
Ligand 0.01 mmol.
Methylcyclohexane 100 mL.
b
c
g
The % yield as measured by GC-FID.
M
w 5 1625 g/mol, polydispersity (Mw/Mn) 5 31.4.
Next, we carried out ethylene oligomerization under various
DMAO/TIBA ratios. Both the selectivity and catalytic activity
were significantly affected by the composition of the aluminum
co-catalysts. As aforementioned, the activation of 3o/Cr(acac)3
with DMAO as the only activator generated 67 mol % 1-hexene
in 0.23 g LAO, along with 0.86 g of PE (entry 8). However,
when TIBA was added, a very positive effect on the catalytic
activity was observed; 3o/Cr(acac)3 activated by 500 eq/100
eq of DMAO/TIBA (entry 9) showed a 10-fold increase in the
formation of LAO (0.23 g to 2.12 g) and a significant drop in
PE (0.86 g to 0.10 g). The selectivity for 1-hexene was also
improved by up to 98.7 mol %. An increase in the amount of
TIBA up to 250 eq resulted in even higher LAO yields (5.25 g)
with 98.9 mol % 1-hexene selectivity (entry 10), implying that
the formation of an active species responsible for selective eth-
ylene trimerization was promoted by TIBA. The best activity of
the catalyst was achieved when the activator composition of
DMAO/TIBA was 250 eq/250 eq, reaching an activity of about
1250 g (mmol of Cr)21 h21 with 98.5 mol % of 1-hexene in the
6.2 g (99.4 wt %) liquid fraction (entry 11). The polymer for-
mation was considerably reduced (0.6 wt %) under this condi-
tion. It is important to avoid PE formation, because the
unwanted solid byproduct poses serious reactor-fouling prob-
lems that complicate industrial applications.
same conditions, ligand 4 promoted ethylene trimerization.
However, activity was much lower, and a significant amount
of PE formed (44%; entry 15), implying that the presence of
CF3 groups is not likely the only parameter for improving
catalytic activity for 1-hexene.
To identify the origin of the high selectivity achieved with
3o, we prepared 3 under air-free conditions successfully and
carried out ethylene oligomerization (entry 14). Interestingly,
the results were very similar to those of 1 and 2 in terms of
activity and selectivity, indicating that phosphine oxide is
critical for improving the activity of the ligand series (Fig. 4).
In addition, we carried out ethylene oligomerization using
2,5-dimethylpyrrole (DMP) (from the well-known Philips
system) under our reaction conditions to compare the
Then, under the reaction conditions used for entry 11, ethyl-
ene oligomerization was carried out with the catalyst system
containing 1 or 2 to compare the performance between the
ligands; the activity achieved with 1 or 2 was considerably
lower than that with 3o (entries 11–13), suggesting that the
highest activity achieved with 3o was due to its phosphine
oxide structure or the presence of CF3 groups. Under the
FIGURE 4 Product distribution depending on the ligands
(entries 11–14). [Color figure can be viewed at wileyonlineli-
brary.com]
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performance with that of our ligands (entry 20).36 Although
DMP showed excellent selectivity and activity [851 g (mmol
of Cr)21 h21 with 97.5 mol % 1-hexene in the 4.2 g liquid
fraction], the performance of 3o is comparable or superior
to that of DMP, supporting the importance of the
[(CF3)2C6H3]2P(O) moiety attached to pyrrole.
ethylene oligomerization capabilities. We found that the
ligand structure, as well as the type and composition of the
activators, had a profound influence on both the selectivity
and activity, whereas the reaction temperature, ligand/Cr
ratio and solvent volume had only modest effects on the
activity. By carefully adjusting the reaction conditions, 3o/
Cr(acac)3 activated by TIBA and DMAO resulted in a highly
active PE-free selective ethylene trimerization system,
achieving an activity of 1248 g (mmol Cr)21 h21 with 98.5%
of 1-hexene, with only a negligible amount of PE (0.6 wt %).
High-temperature gel permeation chromatography and 13C
NMR analyses of the PE produced with 3o/Cr(acac)3 (entry
11) revealed that the polymer is highly branched and poly-
disperse (PDI 5 31.4, Mn 5 1625 g/mol) (Figs. S23 and S25).
The formation of the branched products can be attributed to
the incorporation of in-situ-generated 1-hexene into the poly-
mer chain. This branching is also reflected in the low melting
temperature of the polymer (Tm 5 123 8C; Fig. S24).
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from LG Chem,
Chungnam National University, and the National Research
Foundation of Korea (NRF-2015R1D1A1A01057228).
From the results described so far, we found that both the
selectivity for 1-hexene and the catalytic activities were dra-
matically influenced by the choice of the activators and the
presence of toluene. The influence of other reaction condi-
tions, including Cr precursor (entry 16), reaction tempera-
ture (entry 17), ligand/Cr ratio (entry 18), and the solvent
volume (entry 19), were subsequently investigated and com-
pared to the best result obtained for 3o/Cr(acac)3 (entry
11). Despite the similar selectivity toward 1-hexene, all var-
iations including the use of chromium(III) chloride tetrahy-
drofuran complex (CrCl3(THF)3) instead of Cr(acac)3, lower
reaction temperature (60 8C), and a lower ligand to Cr ratio
were unsuccessful, leading to a reduction in the amount of
LAO produced. Similar selectivities for 1-hexene indicate gen-
eration of the same active species regardless of the type of
Cr precursor and temperature. Changing the ligand/Cr ratio
from 2 to 1 also resulted in the decline of activity, indicating
a possible change in the coordination environment for the
chromium. Doubling the quantity of the reaction solvent did
not enhance the performance of the catalyst and had only
minor effects on activity and selectivity.
REFERENCES AND NOTES
1 D. S. McGuinness, Chem. Rev. 2011, 111, 2321 and references
therein.
2 T. Agapie, Coord. Chem. Rev. 2011, 255, 861 and references
therein.
3 J. T. Dixon, M. J. Green, F. M. Hess, D. H. Morgan, J. Orga-
nomet. Chem. 2004, 689, 3641 and references therein.
4 D. F. Wass, Dalton Trans. 2007, 816 and references therein.
5 K. A. Alferov, G. P. Belov, Y. Meng, Appl. Catal. A: Gen. 2017,
542, 71 and references therein.
6 E. H. Kim, H. M. Lee, M. S. Jeong, J. Y. Ryu, J. Lee, B. Y. Lee,
ACS Omega 2017, 2, 765.
7 C. Fliedel, A. Ghisolfi, P. Braunstein, Chem. Rev. 2016, 116,
9237 and references therein.
8 P. R. Elowe, C. McCann, P. G. Pringle, S. K. Spitzmesser, J. E.
Bercaw, Organometallics 2006, 25, 5255.
9 S. Kuhlmann, K. Blann, A. Bollmann, J. T. Dixon, E. Killian,
ꢀ
M. C. Maumela, H. Maumela, D. H. Morgan, M. Pretorius, N.
Taccardi, P. Wasserscheid, J. Catal. 2007, 245, 279.
To explore the possible reasons for the differences in the
activities of the ligands, we carried out density functional
theory (DFT) calculations on one of the main active inter-
mediates in the hypothetical metallacycle mechanism for eth-
ylene trimerization with 2/Cr and 3o/Cr catalysts (Fig. S26).
A metallacycle mechanism involving the Cr(I)/Cr(III) redox
cycle was assumed,37–40 including coordination of two ethyl-
ene molecules to the catalysts with sextet Cr(I), spin-
crossing into a quartet, and CAC coupling in the quartet
spin state to generate a chromacyclopentane with an oxida-
tion state of Cr(III).41 The 3o/Cr exhibited a lower activation
barrier for CAC coupling (18.0 kcal/mol above the sextet
state; 22.7 kcal/mol for 2/Cr) and less endothermicity for
metallacycle formation (6.3 kcal/mol above the sextet state;
12.0 kcal/mol for 2/Cr); both of these factors are consistent
with a higher activity of 3o/Cr.
10 S. Sa, S. M. Lee, S. Y. Kim, J. Mol. Catal. A: Chem. 2013,
378, 17.
11 M. J. Overett, K. Blann, A. Bollmann, J. T. Dixon, F. Hess, E.
Killian, H. Maumela, D. H. Morgan, A. Neveling, S. Otto, Chem.
Commun. 2005, 622.
12 S. J. Schofer, M. W. Day, L. M. Henling, J. A. Labinger, J. E.
Bercaw, Organometallics 2006, 25, 2743.
13 S.-K. Kim, T.-J. Kim, J.-H. Chung, T.-K. Hahn, S.-S. Chae, H.-
S. Lee, M. Cheong, S. O. Kang, Organometallics 2010, 29, 5805.
14 A. Carter, S. A. Cohen, N. A. Cooley, A. Murphy, J. Scutt, D.
F. Wass, Chem. Commun. 2002, 858.
15 T. E. Stennett, T. W. Hey, L. T. Ball, S. R. Flynn, J. E.
Radcliffe, C. L. McMullin, R. L. Wingad, D. F. Wass, Chem-
CatChem 2013, 5, 2946.
16 K. Blann, A. Bollmann, J. T. Dixon, F. M. Hess, E. Killian, H.
Maumela, D. H. Morgan, A. Neveling, S. Otto, M. J. Overett,
Chem. Commun. 2005, 620.
17 Y. Yang, Z. Liu, B. Liu, R. Duchateau, ACS Catal. 2013, 3,
2353.
CONCLUSIONS
A series of new ligands 1–4 and the corresponding in-situ-
18 F. Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2,
formed chromium catalysts were evaluated for their selective
73.
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2018, 56, 444–450
449

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
19 S. Grimme, J. Comput. Chem. 2006, 27, 1787.
32 Y. Shaikh, K. Albahily, M. Sutcliffe, V. Fomitcheva, S.
Gambarotta, I. Korobkov, R. Duchateau, Angew. Chem. Int. Ed.
2012, 51, 1366.
20 S. Grimme, S. Ehrlich, L. Goerigk, J. Comput. Chem. 2011,
32, 1456.
33 I. Vidyaratne, G. B. Nikiforov, S. I. Gorelsky, S. Gambarotta,
R. Duchateau, I. Korobkov, Angew. Chem. Int. Ed. 2009, 48,
6552.
21 R. K. Raju, A. A. Bengali, E. N. Brothers, Dalton Trans. 2016,
45, 13766.
22 F. Weigend, R. Ahlrichs, Pccp 2005, 7, 3297.
34 Y. Shaikh, J. Gurnham, K. Albahily, S. Gambarotta, I.
Korobkov, Organometallics 2012, 31, 7427.
23 L. Goerigk, S. Grimme, J. Chem. Theory Comput. 2011, 7, 291.
24 A. Klamt, G. Schuurmann, J. Chem. Soc., Perkin Trans.
1993, 2, 799.
35 K. Blann, A. Bollmann, H. de Bod, J. T. Dixon, E. Killian, P.
Nongodlwana, M. C. Maumela, H. Maumela, A. E. McConnell,
25 K. G. Moloy, J. L. Petersen, J. Am. Chem. Soc. 1995, 117, 7696.
ꢀ
D. H. Morgan, M. J. Overett, M. Pretorius, S. Kuhlmann, P.
26 C. R. Hilliard, N. Bhuvanesh, J. A. Gladysz, J. Blumel, Dalton
Trans. 2012, 41, 1742.
Wasserscheid, J. Catal. 2007, 249, 244.
36 J. W. Freeman, J. L. Buster, R. D. Knudsen, U.S. Patent
5,856,257 (Phillips Petroleum Company), 1999.
27 G. S. Nyamato, M. G. Alam, S. O. Ojwach, M. P. Akerman,
J. Organomet. Chem. 2015, 783, 64.
37 Y. Yang, Z. Liu, L. Zhong, P. Qiu, Q. Dong, R. Cheng, J.
Vanderbilt, B. Liu, Organometallics 2011, 30, 5297.
28 S. Teo, Z. Weng, T. S. A. Hor, Organometallics 2008, 27,
4188.
38 A. Jabri, C. B. Mason, Y. Sim, S. Gambarotta, T. J. Burchell,
R. Duchateau, Angew. Chem. Int. Ed. 2008, 47, 9717.
29 S. Vadake Kulangara, D. Haveman, B. Vidjayacoumar, I.
Korobkov, S. Gambarotta, R. Duchateau, Organometallics 2015,
34, 1203.
39 P. H. M. Budzelaar, Can. J. Chem. 2009, 87, 832.
40 D. H. Lee, E. H. Kim, J. Y. Jeon, S. H. Park, M. S. Jeong, Y.
Y. Kang, J. Lee, B. Y. Lee, J. Organomet. Chem. 2016, 803, 13.
30 T. Jiang, X. Liu, Y. Ning, H. Chen, M. Luo, L. Wang, Z.
Huang, Catal. Commun. 2007, 8, 1145.
41 G. J. P. Britovsek, D. S. McGuinness, Chem.: A Eur. J. 2016,
22, 16891.
31 D. S. McGuinness, A. J. Rucklidge, R. P. Tooze, A. M. Z.
Slawin, Organometallics 2007, 26, 2561.
450
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2018, 56, 444–450