Extremely Active Ethylene Tetramerization Catalyst Avoiding the Use of Methylaluminoxane: [iPrN{P(C6H4-p-SiR3)2}2CrCl2]+[B(C6F5)4]?

Article abstract of DOI:10.1002/cctc.201900898

Sasol's original ethylene tetramerization catalyst requires the use of expensive MMAO, a low working temperature (～60 °C), and generates polyethylene (PE) as a side product. In this study, we developed an upgraded catalytic system that successfully avoids the need for MMAO. [(PNP)CrCl₂]⁺[B(C₆F₅)₄]^?-type species was obtained from the reaction of CrCl₃(THF)₃, [PhN(H)Me₂]⁺[B(C₆F₅)₄]^?, and iPrN[P(C₆H₄-p-Si(nBu)₃)₂]₂ (2) as well as from simply reacting 2 with [CrCl₂(NCCH₃)₄]⁺[B(C₆F₅)₄]^?. The bulky (nBu)₃Si-substituents play the crucial role of preventing the formation of the inactive [(PNP)₂CrCl₂]⁺[B(C₆F₅)₄]^?. The prepared [2-CrCl₂]⁺[B(C₆F₅)₄]^? combined with iBu₃Al was extremely active ('4000 kg/g-Cr/h), performed well at a high temperature of up to 90 °C, and generated a negligible amount of PE (0.03 wt%). Screening the performance with a series of iPrN[P(C₆H₄-p-SiR₃)₂]₂ further supported that bulky R₃Si-substituents are crucial not only to achieve extremely high activities but also to minimize the generation of PE. Structure of a [(PNP)CrCl₂]⁺[B(C₆F₅)₄]^? species was elucidated by X-ray crystallography.

Full text of DOI:10.1002/cctc.201900898

Accepted Article
Title: Extremely Active Ethylene Tetramerization Catalyst
Avoiding the Use of Methylaluminoxane: [iPrN{P(C6H4-p-
SiR3)2}2CrCl2]+[B(C6F5)4]–
Authors: Hee Soo Park, Tae Hee Kim, Jun Won Baek, Hyun Ju Lee,
Tae Jin Kim, Ji Yeon Ryu, Junseong Lee, and Bun Yeoul Lee
This manuscript has been accepted after peer review and appears as an
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201900898
Link to VoR: http://dx.doi.org/10.1002/cctc.201900898
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10.1002/cctc.201900898
ChemCatChem
FULL PAPER
Extremely Active Ethylene Tetramerization Catalyst Avoiding the
Use of Methylaluminoxane: [iPrN{P(C6H4-p-
SiR3)2}2CrCl2]+[B(C6F5)4]–
Hee Soo Park,[a] Tae Hee Kim,[a] Jun Won Baek,[a] Hyun Ju Lee,[a] Tae Jin Kim,[a] Ji Yeon Ryu,[b]
Junseong Lee,[b] and Bun Yeoul Lee*[a]
Abstract: Sasol's original ethylene tetramerization catalyst requires
the use of expensive MMAO, a low working temperature (~60 °C),
and generates polyethylene (PE) as a side product. In this study, we
developed an upgraded catalytic system that successfully avoids the
need for MMAO. [(PNP)CrCl2]+[B(C6F5)4]–-type species was obtained
from the reaction of CrCl3(THF)3, [PhN(H)Me2]+[B(C6F5)4]–, and
iPrN[P(C6H4-p-Si(nBu)3)2]2 (2) as well as from simply reacting 2 with
[CrCl2(NCCH3)4]+[B(C6F5)4]–. The bulky (nBu)3Si-substituents play the
crucial role of preventing the formation of the inactive
trimerization catalyst in the early 1990s, which was
commercialized in the early 2000s.[11-13] Another type of Cr-
based catalytic system that generates 1-octene as the main
product was also discovered at Sasol in the early 2000s.[14]
While the Phillips trimerization catalyst is constructed with (2-
ethylhexanoate)3Cr(III), 2,5-dimethylpyrrole, and inexpensive
Et3Al·ClAlEt2 working at a high temperature above 90 °C, the
original Sasol tetramerization catalyst is composed of Cr(acac)3,
iPrN(PPh2)2 (abbreviated as PNP ligand), and expensive
modified-methylaluminoxane (MMAO), which works best at a
relatively low temperature of 60 °C. In large scale commercial
operations with the original Sasol system, the use of expensive
MMAO in excess (Al/Cr, ~500) is an economic disadvantage.[15]
MMAO is inevitably expensive because it is prepared using
Me3Al. Whereas Et3Al and iBu3Al are produced atom-
economically using inexpensive raw materials such as Al, H2,
and ethylene (or isobutene), Me3Al is produced using Al, CH3Cl,
and Na metal with concomitant generation of byproduct NaCl.
Even though the reported activity of the typical Sasol system
calculated based on Cr was extremely high (1200 kg/g-Cr), the
productivity calculated based on the feed amount of MMAO was
unsatisfactory (~2.0 kg/g-MMAO).[16, 17] Consequently, attempts
have been made to replace expensive MMAO with inexpensive
iBu3Al or Et3Al in combination with a stoichiometric amount of
[(PNP)2CrCl2]+[B(C6F5)4]–.
The
prepared
[2-CrCl2]+[B(C6F5)4]–
combined with iBu3Al was extremely active (> 4000 kg/g-Cr/h),
performed well at a high temperature of up to 90 °C, and generated
a negligible amount of PE (0.03 wt%). Screening the performance
with a series of iPrN[P(C6H4-p-SiR3)2]2 further supported that bulky
R3Si-substituents are crucial not only to achieve extremely high
activities but also to minimize the generation of PE. Structure of a
[(PNP)CrCl2]+[B(C6F5)4]– species was elucidated by X-ray
crystallography.
Introduction
Ethylene oligomerization catalysts typically generate 1-alkenes
with an even number of carbon atoms such as 1-hexene, 1-
octene, 1-decene, etc., which are also called whole linear a-
olefins (LAOs).[1-5] 1-Hexene and 1-octene are used as
comonomers in the ethylene polymerization processes, and their
demand has increased with the increase in the production
capacity of linear low-density polyethylene (LLDPE) and
polyolefin elastomer (POE). Whereas nickel- and zirconium-
based catalysts are used in the Shell higher-olefin process and
discrete non-coordinating anions (e.g., [Ph3C]+[B(C6F5)4]– or
18-23]
[PhN(H)Me2]+[B(C6F5)4]–).[15,
However, most of these
attempts, especially with commonly used [Ph3C]+[B(C6F5)4]– and
[PhN(H)Me2]+[B(C6F5)4]–, have been unsuccessful.[24, 25] The low
working temperature (~60 °C) is also an issue. A large amount
of heat is generated in the oligomerization process that cannot
be removed with ambient air or water when the working
temperature is not sufficiently high. In this regard, a catalytic
system that can work at a high temperature (e.g., 100 °C) has
been pursued, especially in the industry.[26-28] The typical Sasol
system generates a small amount of PE as a side product and
the amount increases with an increase in the reaction
temperature. Generation of PE, though its amount is small, also
poses a critical problem that is often referred to as 'fouling' of the
operation of the continuous commercial process. In this study,
we developed an extremely active catalytic system that
eliminates the issues associated with the original Sasol system.
Catalyst development for ethylene oligomerization is an ongoing
research topic in both academia and industry.[29-45]
Sabic/Linde Alpha-SABLIN process, respectively, to produce a
7]
wide range of LAOs (Flory–Schulz distribution),[6,
Cr-based
catalysts that can selectively generate 1-hexene and/or 1-octene
have also been discovered.[8-10] Phillips discovered the ethylene
[a]
[b]
Hee Soo Park, Tae Hee Kim, Jun Won Baek, Hyun Ju Lee, Tae Jin
Kim, and Prof. Bun Yeoul Lee
Laboratory of Polymer Synthesis
Department of Molecular Science and Technology, Ajou University,
206 Worldcup-ro, Yeongtong-gu, Suwon 16499, Korea
E-mail: bunyeoul@ajou.ac.kr
Ji Yeon Ryu, Prof. Junseong Lee
Department of Chemistry, Chonnam National University, 77
Yongbong-ro, Buk-gu, Gwangju 500-757, South Korea
Results and Discussion
Supporting information for this article is available on the WWW
under http://doi.org/10.1002/cctc.XXXXX.
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Preparation
of
[(PNP)CrCl2]+[B(C6F5)4]–
using
[H(OEt2)2]+[B(C6F5)4]– with [PhN(H)Me2]+[B(C6F5)4]–, which is
commonly used for olefin polymerization. Gratifyingly, the
catalyst precursor prepared with [PhN(H)Me2]+[B(C6F5)4]– and 2
according to the procedure shown in Scheme 1 was also as
highly active as the one prepared with [H(OEt2)2]+[B(C6F5)4]–.
The temperature rose from 30 °C to 95 °C in 15 min because of
a large exotherm and the activity was calculated to be 5600
kg/g-Cr/h (entry 2 in Table 1). [H(OEt2)2]+[B(C6F5)4]– immediately
reacted with Et2Al(acac) to generate the ethane bubble. In
contrast, an ethane bubble was not detected when
[PhN(H)Me2]+[B(C6F5)4]– was added to Et2Al(acac) in acetonitrile.
Consequently, we attempted to prepare the catalyst precursor
without the use of Et2Al(acac). After stirring a solution containing
CrCl3(THF)3 and [PhN(H)Me2]+[B(C6F5)4]– (2.0 eq) overnight at
room temperature, the solvent was changed from acetonitrile to
CH2Cl2 because 2 was insoluble in acetonitrile. Upon addition of
2 in CH2Cl2, the color immediately changed from reddish brown
to dark blue. The prepared Cr species was also as highly active
as the one prepared with the use of Et2Al(acac) (4500 kg/g-Cr/h;
entry 3). Finally, we tried to prepare the catalyst precursor with
1.0 equivalent of [PhN(H)Me2]+[B(C6F5)4]– instead of 2.0
equivalents. Gratifyingly, the catalyst precursor prepared using
1.0 equiv [PhN(H)Me2]+[B(C6F5)4]– exhibited almost the same
[PhN(H)Me2]+[B(C6F5)4]–. Recently, we reported a MAO-free
catalytic system composed of (acac)AlEt2, [H(OEt2)2]+[B(C6F5)4]–
(2.0 eq), CrCl3(THF)3, and PNP ligand that exhibited high activity
(up to 4000 kg/g-Cr/h) when combined with common and
inexpensive alkylaluminum iBu3Al (Scheme 1).[46] We presumed
that the chloride-bridged adduct (i.e., [(PNP)Cr(Cl)(THF)(m2-
Cl)2Al(acac)]2+[B(C6F5)4]– ) might be formed between the cationic
2
Cr species [(PNP)CrCl2]+[B(C6F5)4]– and the cationic Al species
[ClAl(acac)]+[B(C6F5)4]–. The role of aluminum part might be
blocking the formation of bis(PNP)-CrIII species (i.e.,
[(PNP)2CrCl2]+[B(C6F5)4]–). The bis(PNP)-CrIII species, which
was known to be inactive, was mainly isolated in attempts to
prepare
the
mono(PNP)-CrIII
complex
(i.e.,
[(PNP)CrCl2]+[B(C6F5)4]–).[15, 23] The aluminum part may detach
from the Cr-center by the action of iBu3Al, transforming
[(PNP)Cr(Cl)(THF)(m2-Cl)2Al(acac)]2+[B(C6F5)4]–
to
2
[(PNP)Cr(iBu)2]+[B(C6F5)4]– or [(PNP)Cr(iBu)2]2+[B(C6F5)4]– , from
2
which the catalytic cycle involving cationic CrI/III or dicationic
CrII/IV species may operate.[47-49] The thus-prepared complex with
[CH3(CH2)16]2C(H)N[P(C6H5)2]2 (1) exhibited high activity (1120
kg/g-Cr/h). Eventually, we found that the PNP ligand substituted
with R3Si-groups at the para-positions of the phenyl moieties
(e.g., iPrN[P(C6H4-p-Si(nBu)3)2]2 (2)) showed much enhanced
catalytic performance. The catalyst precursor prepared with 2
exhibited the activity of 4000 kg/g-Cr/h at 75 °C with generation
of a small amount of PE (0.15%). When the oligomerization
process was performed with the catalyst precursor derived from
1 at 80 °C, a significant amount of PE was generated (10%).
The advantages gained by the introduction of the R3Si-
substituents were attributed to the restricted rotation around P-N
bonds in 2. Owing to the restricted rotation, two fairly sharp
signals were observed in the 31P NMR spectrum of 2 (Scheme 1).
activity
as
the
one
prepared
with
2.0
equiv
[PhN(H)Me2]+[B(C6F5)4]– (4700 kg/g-Cr/h; entry 4).
[CrCl2(NCCH3)4]+
2
Cl-
[PhN(H)Me2]+[B(C6F5)4]-
+
CrCl3(THF)3
[PhN(H)Me2]+
[B(C6F5)4]-
CH3CN
CH2Cl2
SiBu3
Bu3Si
SiBu3 Bu3Si
Bu3Si
SiBu3
[B(C6F5)4]-
[B(C6F5)4]-
Cl
Cl
P
P
P
P
P
+
[PhN(H)Me2]+Cl-
Insoluble
Cr+
Cr+
Cl
N
N
N
Cl
P
in methylcyclohexane
5.3-6.1Å
Bu3Si
Bu3Si
SiBu3
SiBu3
SiBu3 Bu3Si
Soluble in methylcyclohexane
Robust due to restricted rotation with -SiBu3
Not generated
due to steric repulsion between -SiBu3 groups
Scheme 2. Preparation of catalyst precursor using [PhN(H)Me2]+[B(C6F5)4]–.
The final products in the reaction of [PhN(H)Me2]+[B(C6F5)4]– (1.0
eq), CrCl3(THF)3, and
2
were mostly soluble in
methylcyclohexane. The methylcyclohexane-soluble portion (92
wt%) was taken by filtration for the use in the oligomerization
studies. The amount of the methylcyclohexane-insoluble portion
was small (8 wt%), which was freely soluble in CH2Cl2. Analysis
of the 1H NMR spectrum indicated that the insoluble solid
contained the N,N-dimethylaniline unit (Figure S1) and
gravimetric analysis after treatment with AgNO3 indicated the
presence of Cl- anion. The weights of methylcyclohexane-
soluble portion, methylcyclohexane-insoluble fraction, and AgCl
precipitated in the gravimetric analysis were indicative of the
Scheme 1. Preparation of the catalyst precursors discussed in a previous
work.[46]
Even though [H(OEt2)2]+[B(C6F5)4]– used in Scheme 1 can be
simply prepared by the treatment of Li+[B(C6F5)4]- with HCl in
diethyl ether, it is not commonly used in olefin polymerization.[50]
We found that [H(OEt2)2]+[B(C6F5)4]– decomposes in diethyl ether
during the course of recrystallization, leading to a rather low
yield during preparation (65%) as well as low purity. Therefore,
we attempted to prepare the catalyst precursor by replacing
quantitative
generation
of
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Table 1. Screening of the catalyst precursors prepared by varying the component and composition (A– = [B(C6F5)4]–).[a]
Catalyst (+ CrCl3(THF)3 +
iPrN[P(C6H4-p-
Temp
(°C);[b]
time (min)
Activity (kg/g-
Cr/h)
Entry
1-C8 (wt%)
1-C6 (wt%)
cy-C6 (wt%)
>C10 (wt%)
PE (wt%)[c]
Si(nBu)3)2]2)
(acac)AlEt2 +
1[d]
30-75; 30
30-95; 15
[H(OEt2)2]+A– (2.0 eq)
4000
5600
54.6
52.4
28.3
29.5
3.7
3.4
13.3
14.5
0.15
0.13
(acac)AlEt2 +
2
[PhN(H)Me2]+A– (2.0 eq)
3
4
[PhN(H)Me2]+A– (2.0 eq)
[PhN(H)Me2]+A– (1.0 eq)
30-76; 30
20-96; 25
4500
4700
53.2
45.4
26.7
32.4
3.4
3.0
16.5
19.0
0.11
0.01
[a] Conditions: Cr-complex (0.20 mmol), methylcyclohexane (20 mL), ethylene (45 bar), iBu3Al (80 mmol). [b] Temperature was not controlled. [c] Insoluble portion
isolated at 80 °C. [d] Data from reference 43.
[2-CrCl2]+[B(C6F5)4]–
[PhN(H)Me2]+Cl– (Scheme 2).
Identifying the reaction conditions for obtaining [2-
CrCl2]+[B(C6F5)4]– from the reaction of [PhN(H)Me2]+[B(C6F5)4]–
(1.0 eq), CrCl3(THF)3, and 2 was rather tricky. Treatment of
species
along
with
byproduct
with
methylcyclohexane
to
afford
the
desired
[2-
CrCl2]+[B(C6F5)4]–. [PhN(H)Me2]+Cl– remained as an insoluble
solid.
When the isolated [2-CrCl2]+[B(C6F5)4]– was dissolved in C6D6,
no signal was observed in the 31P NMR spectrum, which is
indicative of persistent and complete coordination of the PNP
ligand to the paramagnetic CrIII center (Figure 1a). In the 1H
NMR spectrum, there were no signals in the region of 7–8 ppm
corresponding to the -C6H4- moieties in proximity with the
paramagnetic CrIII center. In contrast, signals corresponding to
the -Si(nBu)3 moieties, which were farther from the CrIII center,
were observed at 0–2 ppm (Figure 1b). In the 19F NMR spectrum,
ortho-, para-, and meta-F in [B(C6F5)4]– were observed at –
131.72, –166.20, and –162.45 ppm (Figure 1c), which was
indicative of an outer-sphere ion pair. In the EPR spectrum, two
signals were observed at the g values of 4.72 and 2.02 (Figure
1d). These signal features resembled that of CrIII(acac)3, for
which two signals were also observed at the g values of 3.75
and 1.98.[51]
acetonitrile
to
a
mixture
was
of
CrCl3(THF)3
necessary.
and
When
[PhN(H)Me2]+[B(C6F5)4]–
[PhN(H)Me2]+[B(C6F5)4]– was mixed with CrCl3(THF)3 in CH2Cl2,
some solids gradually precipitated, from which the extremely
active species was not generated. In contrast, the dissolution of
both [PhN(H)Me2]+[B(C6F5)4]– and CrCl3(THF)3 in CH3CN and
subsequent thorough removal of volatiles (i.e., CH3CN and THF)
after stirring overnight afforded CH2Cl2-soluble species. The
NMR studies of paramagnetic Cr(III) species yielded limited
information, but it was possible to monitor the status of the
ligand. When a ligand was coordinated to the paramagnetic
Cr(III)-center, its signal was either not observed or severely
abnormal. When it was free from the paramagnetic Cr(III)-center
through decoordination, the ligand signal appeared in the typical
range. When the species isolated via thorough evacuation from
the reaction pot of [PhN(H)Me2]+[B(C6F5)4]– and CrCl3(THF)3 in
Ar
Ar
Ar
Ar
Me
Me
Me
Me
P
P
rotation
Ar
Ar
C
N
C
N
1
CH3CN was dissolved in CD3CN for H NMR study, the signals
P
P
H
H
corresponding to [PhN(H)Me2]+ and CH3CN were observed in
approximately 1:6 mole ratio (Figure S2). When it was dissolved
in CD2Cl2, the signals corresponding to the two species were
also observed, but the ratio was, in contrast, approximately 1:2
(Figure S3). It was confirmed in another 1H NMR experiment that
THF in CrCl3(THF)3 was completely detached from the Cr-center
when dissolved in CH3CN and that the detached THF was
completely removed during the course of removal of acetonitrile.
From these NMR studies, we concluded that CrCl3(THF)3 was
transformed to a dissociated species [Cr(NCCH3)6]3+[Cl–]3 in
highly polar CH3CN (dielectric constant, 37.5), which was then
transformed to [CrCl2(NCCH3)4]+Cl– when dissolved in CH2Cl2.
When PNP ligand 2 was added to the CH2Cl2 solution containing
[CrCl2(NCCH3)4]+, Cl–, [PhN(H)Me2]+, and [B(C6F5)4]–, the PNP
ligand coordinated to the Cr-center to generate [2-CrCl2]+
species, which was extracted out, combined with [B(C6F5)4]–,
Ar
Ar
the most stable conformer
demetallation
metallation
Ar
P
Ar
Ar
Ar
N
Me
Me
Me
Me
P
Cr+Cl2
Ar
Cr+Cl2
C
C
N
P
P
H
H
Ar
Ar
Ar
Ar
Ar
Ar
N
Ar
Me
Me
Me
Me
C
P
P
Cl
Cr+
C
N
P
Cl
P
H
H
Ar
Ar
Ar
Ar
inactive species
Scheme 3. Fluxionality in PNP ligand and [(PNP)CrCl2]+ species.
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i) nBuLi
ii) (Et2N)PCl2
i) nBuLi
ii) R3SiCl
iii) PCl3
Br
Br
R3Si
Br
SiR3
R3Si
SiR3
-SiR3
R3Si
3 -Si(iPr)3
P
iPrNH2
4 -SiEt3
N
5 -SiMe3
P
P
6 -Si(iPr)Et2
7 -Si(iPr)Me2
8 -Si(1-octyl)Me2
Cl
R3Si
SiR3
Scheme 4. Synthesis of PNP ligands bearing various R3Si-substituents.
SiR3
SiR3
R3Si
R3Si
Figure 1. 31P, 1H, and 19F NMR and EPR spectra of [2-CrCl2]+[B(C6F5)4]–.
[B(C6F5)4]-
[CrCl2(NCCH3)4]+[B(C6F5)4]-
P
P
P
P
Cl
Cr+
N
N
Cl
The bulky -Si(nBu)3 substituents played a crucial role. When the
typical PNP ligand bearing no substituents (e.g., 1) was
employed in the preparation scheme, the portion that can be
extracted with methylcyclohexane was small (~20 wt%), which,
moreover, was inactive. We guessed that the bulky -Si(nBu)3
substituents hindered the formation of the inactive bis(PNP)-CrIII
species (Scheme 2). In the structure of the bis(PNP)-CrIII
complex [iPrN(PPh2)2CrCl2]+[B(C6F5)4]–, the nearest distance
between the two para-carbons in the two PNP ligands was 5.28
Å (Scheme 2).[23] The space between the two para-carbons was
too small to accommodate the two bulky -Si(nBu)3 groups,
preventing the formation of bis(PNP)-CrIII species. In previous
R3Si
R3Si
SiR3
SiR3
Scheme
5.
Preparation
of
[(PNP)CrCl2]+[B(C6F5)4]–
using
[CrCl2(NCCH3)4]+[B(C6F5)4]–.
attempts
to
prepare
[(PNP)CrCl2]+[B(C6F5)4]–
and
[(PNP)CrCl2]+[Al(OC(CF3)3)4]–, the inactive [(PNP)2CrCl2]+
species were mainly isolated. We further observed that the
mono(PNP)-CrIII species [iPrN(PPh2)2CrCl2L2]+[B(C6F5)4]− (L =
CH3CN or THF) was easily transformed to the bis(PNP)-CrIII
species [{iPrN(PPh2)2}2CrCl2]+[B(C6F5)4]–. Fluxionality of the PNP
ligand through the rotation around N-P bonds and weak bonding
between the first-row transition metal and phosphine ligand may
cause the facile transformation of the active mono(PNP)-CrIII
species to the inactive bis(PNP)-CrIII species (Scheme 3). By
introduction of bulky -Si(nBu)3 substituents, the rotation around
the N-P bonds was restricted to render a stable and discrete
molecule [2-CrCl2]+[B(C6F5)4]–. It was stable as a solution in
methylcyclohexane.
In
contrast,
[2-
CrAlCl3(acac)(THF)]+[B(C6F5)4]– prepared with (acac)AlEt2 and
2.0 equiv [H(OEt2)2]+[B(C6F5)4]– was not fully soluble in
methylcyclohexane and, moreover, it was unstable even as a
solid, the activity being reduced by ~half after storage for several
days.
Figure 2. Thermal ellipsoid plot of [6-Cr(NCCH3)Cl(m2-Cl)]+[B(C6F5)4]–.
Selected bond distances (Å) and angles (°): Cr-Cl(1), 2.378(3); Cr-Cl(2),
2.256(3); Cr-P(1), 2.462(3); Cr-P(2), 2.434(3); Cr-N(2), 2.079(11); Cl(1)-Cr-
Cl(2), 92.02(12); Cl(2)-Cr-P(1), 93.10(12); Cl(1)-Cr-P(1), 165.30(12). Inserted
is the imaginary bis(PNP)-CrIII species constructed with the "6-Cr" fragment.
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Table 2. Results of ethylene tetramerization performed using the catalyst precursors obtained by combining 1–8 with [CrCl2(NCCH3)4]+[B(C6F5)4]–.[a]
Temp
Activity
(kg/g-Cr/h)
Entry
1[d]
PNP (substituent)
iPrN(PPh2)2
(°C);[b] time
(min)
1-C8 (wt%)
68.0
1-C6 (wt%)
10.5
cy-C6 (wt%)
4.5
>C10 (wt%)
14.9
PE (wt%)[c]
1.3
45; 30
26
2[d]
3
1 ((C16H33)2CHN(PPh2)2)
2 (-Si(nBu)3)
2 (-Si(nBu)3)
2 (-Si(nBu)3)
2 (-Si(nBu)3)
3 (-Si(iPr)3)
45; 30
84
6310
70.3
45.1
49.5
52.8
67.0
55.8
58.1
14.3
34.5
32.4
25.7
15.9
27.2
25.6
4.7
3.0
3.3
3.6
3.4
3.2
3.3
9.4
1.0
24-98; 15
24-90; 15
28-75; 30
26-60; 30
25-84; 30
25-85; 30
25-35; 30
22-89; 30
25-79;30
24-82; 30
17.2
14.6
17.8
13.4
13.7
12.7
0.03
0.04
0.03
0.03
0.05
0.18
4
5400
5
4420
6
3640
7
3020
8
4 (-SiEt3)
2760
9
5 (-SiMe3)
negligible
3260
10
11
12
6 (-Si(iPr)Et2)
7 (-Si(iPr)Me2)
8 (-Si(Oct)Me2)
53.8
63.0
55.5
28.6
21.8
25.3
3.1
3.8
3.5
14.3
11.2
15.5
0.02
0.01
0.06
2430
2890
[a] Conditions: Cr-complex (0.20 mmol), methylcyclohexane (20 mL), ethylene (45 bar), iBu3Al (80 mmol). [b] Temperature was controlled for entries 1, 2, and 4–6;
uncontrolled for entries 3 and 7–12. [c] Insoluble portion isolated at 80 °C. [d] Data from reference 23.
Preparation of the PNP ligand bearing various -SiR3
substituents. PNP ligands bearing various -SiR3 substituents at
the para-position of the phenyl moieties were prepared with an
aim to either improve the performance further or elucidate the
structure by growing single crystals. A synthetic scheme is
shown in Scheme 4.[52] 1,4-Dibromobenzene was treated with
1.0 equiv of nBuLi to generate 1-bromo-4-lithiobenzene, which
was then reacted with R3SiCl (or iPr3SiOTf) to obtain 1-bromo-4-
(R3Si)benzene. Treatment of 1-bromo-4-(R3Si)benzene with
nBuLi and subsequently with 0.50 equiv (Et2N)PCl2 afforded [4-
Preparation
of
[(PNP)CrCl2]+[B(C6F5)4]–
using
[CrCl2(NCCH3)4]+[B(C6F5)4]–. When the prepared PNP ligands
3–8 were reacted in CH2Cl2 containing [CrCl2(NCCH3)4]+, Cl–,
[PhN(H)Me2]+, and [B(C6F5)4]– ions, as shown in Scheme 2, the
desired complexes [(PNP)CrCl2]+[B(C6F5)4]– were not extracted
out with methylcyclohexane probably because of their low
solubility. Previously, we reported another route to prepare the
[(PNP)CrCl2]+[B(C6F5)4]– species; the PNP ligand was reacted
with [CrCl2(NCCH3)4]+[B(C6F5)4]– that was prepared by the
treatment of CrCl3(THF)3 with [Ag(NCCH3)4]+[B(C6F5)4]–.[23] When
2 was reacted with [CrCl2(NCCH3)4]+[B(C6F5)4]– in CH2Cl2, no
solids remained in the methylcyclohexane-extraction procedure.
(R3Si)C6H4]2P(NEt2),
which
was
transformed
to
[4-
(R3Si)C6H4]2PCl by treatment with PCl3. By reacting [4-
(R3Si)C6H4]2PCl with 0.50 equiv iPrNH2, the desired PNP ligands
1
The 31P, H, 19F NMR and ESR spectra were almost the same
iPrN[P(C6H4-p-SiR3)2]2 (R3Si-
=
(iPr)3Si-, Et3Si-, Me3Si-,
as those of the species generated with [PhN(H)Me2]+[B(C6F5)4]–
Et2(iPr)Si-, Me2(iPr)Si-, and Me2(1-octyl)Si-) were obtained in a
facile manner. Two signals were observed in their 31P NMR
spectra because of the restricted rotation around P-N bonds
(Figure S5), which indicated that [2-CrCl2]+[B(C6F5)4]– was also
successfully generated in the reaction of
[CrCl2(NCCH3)4]+[B(C6F5)4]– (Scheme 5).
2
with
(Scheme
1
and Figure S4). The restricted rotation was
Eventually, we revealed the structure of the complex generated
responsible for the compound existing as a conformer for which
from the reaction of 6 with [CrCl2(NCCH3)4]+[B(C6F5)4]–. It was
the two phosphorus atoms were inequivalent (Schemes 1 and 3). soluble in hot hexane and cooling the solution deposited pale
The bulkier the R3Si-substituent, the sharper the signals; i.e., the
signals were sharp in the 31P NMR spectra of 2, 3, and 6,
whereas those in the spectrum of 5 were relatively very broad
(Figure S4).
green colored single crystals. X-ray crystallographic studies
confirmed the formation of the desired species [6-
CrCl2]+[B(C6F5)4]–, but the Cr atom persistently adopted an
octahedral coordination,
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10.1002/cctc.201900898
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forming chloride-bridged dinuclear species with additional
substituents (2400–3300 kg/g-Cr/h vs. 6300 kg/g-Cr/h). The Cr-
CH3CN donor, i.e., [6-CrCl(m2-Cl)(NCCH3)]+[B(C6F5)4]– (Figure 2). complex prepared with
5
bearing the smallest Me3Si-
The sum of the bond angles around the nitrogen atom (N1) in
the PNP ligand was perfectly 360°, which is indicative of the
delocalization of the nitrogen lone pair through the phosphorous
atoms by adopting the sp2-hybridization. Metrical parameters
around the Cr center (e.g., Cr-P and Cr-Cl distances) did not
severely differ from those observed for [iPrN(PPh2)2-
CrCl2(THF)2]+[B(C6F5)4]–.[23] The Cr atom was situated on a plane
formed by three carbon atoms that were selected from the three
-Si(iPr)Et2 substituents and protruded the most from the PNP
ligand toward Cr (i.e., C(22), C(37), and C(51)); the distance
from the plane to the Cr atom was just 0.46 Å. This observation
allowed us to infer that the formation of the inactive bis(PNP)CrIII
species (i.e., 62-CrCl2]+[B(C6F5)4]–) was difficult because of steric
repulsion (Figure 2). We guess that the Cr complex prepared
with 2 bearing the bulkiest -Si(nBu)3 substituent might be
substituents and accordingly exhibiting broad 31P NMR signals
showed negligible activity, which might be attributed to the
formation of the inactive bis(PNP)CrIII species. The Me3Si-
substituent may be too small to prevent the formation of the
inactive [52-CrCl2]+[B(C6F5)4]– complex. In the case of 4 bearing
relatively small-sized Et3Si-substituents, the amount of the
generated PE was relatively higher compared to those of the
other cases (0.18 wt% vs. 0.01–0.06 wt%), even though the
activity was comparable (2760 kg/g-Cr/h vs. 2430–3260 kg/g-
Cr/h). The rotation barrier around P-N bonds may not be
sufficiently high in the case of 4, which renders the demetallation
process relatively facile, leading to the generation of a relatively
high amount of PE (Scheme 3).
present as
a
monomeric form, consequently exhibiting
Conclusions
contrastingly higher solubility in methylcyclohexane and higher
activity than other complexes including the dimeric [6-CrCl(m2-
Cl)(NCCH3)]+[B(C6F5)4]–.
Dissolving
commonly
used
CrCl3(THF)3
and
[PhN(H)Me2]+[B(C6F5)4]– in CH3CN generated a mixed salt
composed of [CrCl2(NCCH3)4]+, Cl–, [PhN(H)Me2]+, and
[B(C6F5)4]–. Treatment of a PNP ligand bearing bulky (nBu)3Si-
substituents, i.e., iPrN[P(C6H4-p-Si(nBu)3)2]2 (2), to the mixed
salt, afforded [2-CrCl2]+[B(C6F5)4]–, which was extracted using
methylcyclohexane, leaving the byproduct [PhN(H)Me2]+Cl– as
an insoluble solid. The isolated [2-CrCl2]+[B(C6F5)4]–, when
combined with iBu3Al, was extremely active (> 4000 kg/g-Cr/h),
worked well at a high temperature of up to 90 °C, and generated
minimal amount of PE (0.03 wt%). The activity attained in this
work is comparable with the highest activity reported for MAO-
based system.[8] The activity attained is also more than 4 times
higher than the commercially used Chevron Phillips trimerization
catalyst (~1000 Kg/g-Cr/h).[13] The extremely active [2-
CrCl2]+[B(C6F5)4]– was also prepared by simply reacting 2 and
[CrCl2(NCCH3)4]+[B(C6F5)4]–. A series of PNP ligands bearing
various R3Si-substituents (i.e., iPrN[P(C6H4-p-SiR3)2]2, where
R3Si- = (nBu)3Si-, iPr3Si-, Et3Si-, Me3Si-, Et2(iPr)Si-, Me2(iPr)Si-,
and Me2(1-octyl)Si-) were prepared. Performance screening
after reacting the prepared iPrN[P(C6H4-p-SiR3)2]2 with
[CrCl2(NCCH3)4]+[B(C6F5)4]– further supported that the bulky
R3Si-substituents are crucial not only to achieve extremely high
activity but also to minimize the generation of the problematic
side product PE. X-ray crystallographic studies revealed that the
Oligomerization studies with [(PNP)CrCl2]+[B(C6F5)4]–. The
reaction product of [CrCl2(NCCH3)4]+[B(C6F5)4]– with a typical
PNP ligand with no substituent (e.g., iPrN(PPh2)2 or 1) exhibited
a very low activity (26 or 84 kg/g-Cr/h; entry 1 and 2 in Table
2).[23] In contrast, [2-CrCl2]+[B(C6F5)4]–, which was prepared by
reacting [CrCl2(NCCH3)4]+[B(C6F5)4]– with 2, showed extremely
high activity (6300 kg/g-Cr/h; entry 3 in Table 2). The activity
was somewhat higher than the species generated with
[PhN(H)Me2]+[B(C6F5)4]– (6300 kg/g-Cr/h vs. 4700 kg/g-Cr/h).
We speculated that [2-CrCl2]+[B(C6F5)4]– was generated with
somewhat higher purity in the reaction of
2
and
[CrCl2(NCCH3)4]+[B(C6F5)4]–. The catalyst survived at a high
temperature of up to 98 °C with generation of a minimal amount
of PE (0.03 wt%). In contrast, the reactor was fully filled with PE
when the oligomerization was performed with the Cr-complex
prepared with
1 bearing no -SiR3 substituents (i.e., [1-
CrAlCl3(acac)(THF)]+[B(C6F5)4]–).[46] By lowering the reaction
temperature, the 1-octene/1-hexene ratio increased accordingly,
reaching 67/16 at a temperature of 60 °C (entries 3–6). When
methylcyclohexane solvent was replaced with toluene, the
activity was significantly reduced to 1200 kg/g-Cr/h. These 1-
octene/1-hexene selectivity dependency on the temperature and
the solvent effect have been generally observed in the Sasol-
type catalytic system. The catalyst survived for a long time over
1 h with negligible deactivation, which was inferred from
monitoring the ethylene consumption by the reaction time with
mass flow controller; the ethylene consumption was almost
linearly increased by the reaction time up to 75 min (Figure S6).
When [2-CrCl2]+[B(C6F5)4]– was combined with Et2AlCl instead of
iBu3Al, no activity was observed, and replacement of iBu3Al with
MMAO resulted in generation of huge amount of PE (4.8 g).
The reaction products of [CrCl2(NCCH3)4]+[B(C6F5)4]– with the
other PNP ligands bearing iPr3Si-, Et3Si-, Et2(iPr)Si-, Me2(iPr)Si-,
and Me2(1-octyl)Si-substituents (i.e., 3–4 and 6–8) were also
highly active, though their activities did not complete with that of
the species generated with 2 bearing the bulkiest (nBu)3Si-
prepared [(PNP)-CrCl2]+[B(C6F5)4]– species existed as
a
dinuclear CrIII complex sharing the chloride ligand and
coordinated by an additional CH3CN donor.
Experimental Section
General Remarks. All manipulations were performed in an inert
atmosphere using standard glove box and Schlenk techniques. CH2Cl2
and acetonitrile were stirred over CaH2 and transferred to the reservoir
under vacuum. Methylcyclohexane (anhydrous grade) used for the
oligomerization reactions was purchased from Aldrich and purified over a
Na/K alloy. Ethylene was purified by contact with molecular sieves and
copper for more than 12 h under 48 bar pressure. 1H NMR (600 MHz),
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10.1002/cctc.201900898
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FULL PAPER
13C NMR (150 MHz), and 31P NMR (243 MHz) spectra were recorded
using a JEOL ECZ 600 spectrometer. Continuous wave X-band EPR
spectra were collected at 5 K on a Bruker EMX plus 6/1 spectrometer
with the experimental parameters of 1 mW microwave power, 10 G
modulation amplitude, and 5 scans at KBSI Western Seoul Center Korea.
Gas chromatography-flame ionization detection (GC-FID) analysis was
performed using a YL instrument 6500GC system equipped with an HP-
31P (243 MHz, C6D6): δ 43.40, 54.62 ppm. HRMS (FAB): m/z calcd (M+
C39H59NP2Si4) 716.1820, found 716.3276.
iPrN[P(C6H4-p-Si(iPr)Et2)2]2 (6). Compound 6 was prepared following
the same procedure and experimental conditions as those employed for
2, using ClP[C6H4-p-Si(iPr)Et2] (1.58 g, 3.30 mmol). A white glassy solid
was obtained (1.02 g, 68%). 1H NMR (600 MHz, C6D6): 8.05-7.24 (br,
8H), 7.44 (d, J = 7.2 Hz, 8H), 3.85 (m, 1H, NCH), 1.25 (d, J = 6.6 Hz, 6H,
NCHCH3), 1.06-0.95 (br, 52H), 0.80 (q, J = 7.2 Hz, 24H, SiCH2) ppm.
13C{1H} NMR (150 MHz, C6D6): δ 2.28, 7.89, 12.12, 18.19, 24.53 (t, JP-C
PONA
(50
m
×
0.200
mm
×
0.50
mm)
column.
[CrCl2(NCCH3)4]+[B(C6F5)4]– and CrCl3(THF)3 were prepared according to
the literature method.[23, 53] Compound ClP[C6H4-p-SiR3]2 was prepared
according to the scheme reported in literature (see SI).[52]
=5.7 Hz), 52.70 (t, JP-C =10.1 Hz), 132.22-133.09 (br), 134.70 (d, JP-C
=
5.7 Hz), 137.07, 141.1-141.5 (br) ppm. 31P (243 MHz, C6D6): δ 43.18,
54.62 ppm. HRMS (FAB): m/z calcd (M+ C55H91NP2Si4) 940.6073, found
940.5778.
iPrN[P(C6H4-p-Si(nBu)3)2]2 (2). A solution of iPrNH2 (0.174 g, 2.94
mmol) in CH2Cl2 (10 mL) was added dropwise to a solution of [p-
(nBu)3Si-C6H4]2PCl (3.64 g, 5.89 mmol) and Et3N (2.98 g, 29.4 mmol) in
CH2Cl2 (30 mL). After the resulting solution was stirred overnight at room
temperature, the volatiles were removed using a vacuum line. The
residue was treated with hexane (40 mL) and the insoluble byproduct
(Et3NH)+Cl- was removed by Celite-aided filtration. The filtrate was
passed through a short pad of silica gel which had been treated
beforehand with hexane/Et3N (v/v, 50:1). Removal of solvent afforded a
colorless oil (2.72 g, 70%). The isolated compound was pure by the
analysis of NMR spectra and used for the next step without further
iPrN[P(C6H4-p-Si(iPr)Me2)2]2 (7). Compound 7 was prepared following
the same procedure and experimental conditions as those employed for
2, using ClP[C6H4-p-Si(iPr)Me2]2 (0.56 g, 1.3 mmol). A white glassy solid
was obtained (0.25 g, 45%). 1H NMR (600 MHz, C6D6): 8.09-7.20 (br,
8H), 7.44 (d, J = 6.0 Hz, 8H), 3.90 (m, 1H, NCH), 1.29 (d, J = 6.0 Hz, 6H,
NCHCH3), 0.97 (d, J = 6.0 Hz, 24H, CH3), 0.89 (m, 4H, SiCH), 0.19 (d, J
= 4.8 Hz, 24H, SiCH3) ppm. 13C{1H} NMR (150 MHz, C6D6): δ –5.2, 14.09,
17.84, 24.64 (t, JP-C =5.7 Hz), 52.53 (t, JP-C =10.1 Hz), 132.41-132.89 (br),
134.07 (d, JP-C = 5.9 Hz), 139.17, 141.2-141.6 (br) ppm. 31P (243 MHz,
C6D6): δ 43.43, 54.23 ppm. HRMS (FAB): m/z calcd (M+ C47H75NP2Si4)
828.3946, found 828.4533.
1
purification. H NMR (600 MHz, C6D6): δ 8.10-7.27 (br, 8H), 7.50 (d, J =
7.2 Hz, 8H), 3.86 (m, 1H, NCH), 1.42-1.33 (br, 48H), 1.24 (d, J = 7.2 Hz,
6H, NCHCH3,), 0.92 (t, J = 6.6 Hz, 36H, CH3), 0.87-0.82 (br, 24H, SiCH2)
ppm. 13C{1H} NMR (150 MHz, C6D6): δ 12.58 (nBu), 14.05 (nBu), 24.51 (t,
JP-C = 7.2 Hz), 26.54 (nBu), 27.22 (nBu), 52.54 (t, JP-C =10.1 Hz), 132.4-
133.0 (br), 134.2 (d, JP-C = 5.7 Hz), 138.83, 141.1-141.5 (br) ppm. 31P
(243 MHz, C6D6): δ 42.17, 54.96 ppm. HRMS (FAB): m/z calcd (M+
C75H131NP2Si4) 1221.1389, found 1220.8917.
iPrN[P(C6H4-p-Si(1-octyl)Me2)2]2 (8) Compound
8
was prepared
following the same procedure and experimental conditions as those
employed for 2, using ClP[C6H4-p-Si(1-octyl)Me2]2 (1.82 g, 3.24 mmol). A
colorless viscous oil was obtained (1.22 g, 68%). 1H NMR (600 MHz,
C6D6): 8.14-7.22 (br, 8H), 7.48 (d, J = 6.6 Hz, 8H), 3.92 (m, 1H, NCH),
1.41-1.19 (br, 48H), 1.31 (d, J = 6.6 Hz, 6H, NCHCH3), 0.92 (t, J = 7.2 Hz,
12H, CH3), 0.78-0.72 (br, 8H, SiCH2), 0.25 (d, J = 1.8 Hz, 24H, SiCH3)
ppm. 13C{1H} NMR (150 MHz, C6D6): δ –2.82, 14.41, 16.07, 23.14, 24.38,
24.69, 29.8 (d, JP-C = 5.7 Hz), 32.39, 34.10, 52.53 (t, JP-C =10.1 Hz),
132.8 (d, JP-C = 21.45 Hz), 133.73 (d, JP-C = 5.7 Hz), 140.32, 141.2-141.7
(br) ppm. 31P (243 MHz, C6D6): δ 42.90, 54.96 ppm. HRMS (FAB): m/z
calcd (M+ C67H115NP2Si4) 1108.9262, found 1108.7655.
iPrN[P(C6H4-p-Si(iPr)3)2]2 (3). Compound 3 was prepared following the
same procedure and experimental conditions as those employed for 2,
using ClP[C6H4-p-Si(iPr)3]2 (1.00 g, 1.88 mmol). An analytically pure
compound was obtained through recrystallization in toluene at –30 °C
(0.160 g, 19%). 1H NMR (600 MHz, C6D6): δ 8.12-7.26 (br, 8H), 7.45 (d, J
= 6.6 Hz, 8H), 3.83 (m, 1H, NCH), 1.32 (m, 12H, SiCH), 1.23 (d, J = 6.6
Hz, 6H, NCHCH3), 1.10 (d, J = 7.8 Hz, 72H, CH3) ppm. 13C{1H } NMR
(150 MHz, C6D6): δ 11.10, 18.83, 24.41(t, JP-C = 5.7 Hz), 52.85 (t, JP-C
=10.1 Hz), 132.1-132.9 (br), 135.33 (d, JP-C = 5.7 Hz), 135.49-135.65 (br)
ppm. 31P (243 MHz, C6D6): δ 43.63, 54.34 ppm. HRMS (FAB): m/z calcd
(M+ C63H107NP2Si4) 1052.8199, found 1052.7030.
Preparation of [2-CrCl2]+[B(C6F5)4]– using [PhN(H)Me2]+[B(C6F5)4]–
and CrCl3(THF)3 (Scheme 2). [PhN(H)Me2]+[B(C6F5)4]– (0.182 g, 0.227
mmol) in acetonitrile (2.9 mL) was added to a solution of CrCl3(THF)3
(0.0851 g, 0.227 mmol) in acetonitrile (2.9 mL). After stirring overnight at
25 °C, the solvent was removed using the vacuum line. The obtained
green residue was dissolved in CH2Cl2 (2.0 mL) and a solution of 2
(0.277 g, 0.227 mmol) in CH2Cl2 (4.6 mL) was added dropwise. Upon
addition, the color of the solution changed immediately from green to
bluish-green. After stirring at 25 °C for 2 h, the solvent was removed
using a vacuum line. The obtained bluish-green residue was dissolved in
methylcyclohexane and the insoluble portion was removed by Celite-
aided filtration. Removal of solvent afforded a bluish-green viscous oil
(0.457 g, 99% based on the formula of [2-CrCl2]+[B(C6F5)4]–). The
insoluble portion (white solid contaminated with a small amount of green
solid) collected on top of Celite was extracted using CH2Cl2. In the 1H
NMR spectrum of the residue obtained after the removal of the solvent,
the signals corresponding to N,N-dimethylaniline unit could be observed,
although the signals were very broad (Figure S1). The weight of the
isolated solid (41 mg) agreed roughly with the quantitative generation of
the byproduct [PhN(H)Me2]+Cl– (36 mg). When the isolated solid was
dissolved in CH3CN (1.0 mL) and then treated with AgNO3 (78 mg, 0.46
mmol) in CH3CN (1.0 mL), a white solid precipitated (38 mg), which
agreed roughly with the value expected for the quantitative amount of
AgCl (33 mg).
iPrN[P(C6H4-p-SiEt3)2]2 (4). Compound 4 was prepared and purified
following the same procedure and experimental conditions as those
employed for 2, using ClP[C6H4-p-SiEt3]2 (1.00 g, 2.23 mmol). A colorless
viscous oil was obtained (0.428 g, 50%). 1H NMR (600 MHz, C6D6): 8.16-
7.20 (br, 8H), 7.44 (d, J = 6.6 Hz, 8H), 3.87 (m, 1H, NCH), 1.27 (d, J =
6.0 Hz, 6H, NCHCH3), 0.98 (t, J = 7.8 Hz, 36H, CH3), 0.74 (q, J = 7.2 Hz,
24H, SiCH2) ppm. 13C{1H} NMR (150 MHz, C6D6): δ 3.70, 7.72, 24.58 (t,
JP-C = 5.7 Hz), 52.61 (t, JP-C =11.6 Hz), 133.1-132.5 (br), 134.32, 137.97,
141.02-141.58 (br) ppm. 31P (243 MHz, C6D6): δ 43.01, 54.90 ppm.
HRMS (FAB): m/z calcd (M+ C51H83NP2Si4) 884.5009, found 884.5153.
iPrN[P(C6H4-p-SiMe3)2]2 (5). Compound 5 was prepared by the same
procedure and experimental conditions as those employed for 2, using
ClP[C6H4-p-SiMe3]2 (1.54 g, 4.23 mmol). A colorless viscous oil was
obtained (0.564 g, 37%). 1H NMR (600 MHz, C6D6): 7.93-7.32 (br, 8H),
7.43 (d, J = 7.2 Hz, 8H), 3.94 (m, 1H, NCH), 1.33 (d, J = 6.6 Hz, 6H,
NCHCH3), 0.20 (s, J = 7.2 Hz, 36H, SiCH3) ppm. 13C{1H} NMR (150 MHz,
C6D6): δ –1.10, 24.76 (t, JP-C =5.7 Hz), 52.52 (t, JP-C =10.1 Hz), 132.80 (d,
JP-C = 20.1 Hz), 133.50 (d, JP-C = 4.2 Hz), 140.98, 141.3-161.6 (br) ppm.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201900898
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FULL PAPER
Preparation of [2-CrCl2]+[B(C6F5)4]– using [CrCl2(NCCH3)4]+[B(C6F5)4]–
(Scheme 5). A solution of 2 (0.10 g, 0.082 mmol) in CH2Cl2 (1.8 mL) was
added dropwise to a solution of [CrCl2(NCCH3)4]+[B(C6F5)4]– (0.079 g,
0.082 mmol) in CH2Cl2 (0.5 mL). After stirring at room temperature for 2.5
h, the solvent was removed using a vacuum line to obtain a green solid
(0.16 mg, 99% based on the formula of [2-CrCl2]+[B(C6F5)4]–).
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(CCDC# 1913318): C166H196B2Cl4Cr2F40N4OP4Si8,
M
=
3639.28,
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Acknowledgements
[26] M. S. Mokhadinyana, M. C. Maumela, M. M. Mogorosi, M. J. Overett,
J.-A. van den Berg, W. J. Van Rensburg, K. Blann, WO 2014/181250A1
(2014).
This research was supported by C1 Gas Refinery Program
through the National Research Foundation of Korea (NRF)
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Mansell, Organometallics 2018, 37, 1062-1073.
funded
by
the
Ministry
of
Science
and
ICT
(2019M3D3A1A01069100).
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Britovsek, Catal. Sci. Tech. 2018, 8, 1314-1321.
Keywords: chromium • ethylene tetramerization • 1-octene
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ChemCatChem. 2017, 9, 2467-2472.
production • oligomerization • phosphane ligands
[32] A. M. Lifschitz, N. A. Hirscher, H. B. Lee, J. A. Buss, T. Agapie,
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Entry for the Table of Contents
FULL PAPER
R3Si-substituents: Extremely active
catalyst that avoids the need for
expensive MAO was prepared for
ethylene tetramerization. It was
extremely active (> 4000 kg/g-Cr/h),
performed well at a high temperature
of up to 90 °C with generation of a
negligible amount of PE. The bulky
(nBu)3Si-substituents play the crucial
roles.
Hee Soo Park, Tae Hee Kim, Jun Won
Baek, Hyun Ju Lee, Tae Jin Kim, Ji
Yeon Ryu, Prof. Junseong Lee, Prof.
Bun Yeoul Lee
Page No. – Page No.
Extremely Active Ethylene
Tetramerization Catalyst Avoiding the
Use of Methylaluminoxane:
[iPrN{P(C6H4-p-
SiR3)2}2CrCl2]+[B(C6F5)4]–
This article is protected by copyright. All rights reserved.