Polyhedral oligomeric silsesquioxane-conjugated bis(diphenylphosphino)amine ligand for chromium(III) catalyzed ethylene trimerization and tetramerization

Article abstract of DOI:10.1016/j.apcata.2018.04.030

Polyhedral oligomeric silsesquioxanes (POSSs) were attached to conventional bis(diphenylphosphino)amine (PNP) ligand as solubility-enhancing materials for catalytic ethylene trimerization and tetramerization. Differently functionalized arylphosphine ligands of the type (Ph)₂PN(POSS)P(Ph)(Ar^R) (R = functional groups) were systematically developed, and their corresponding chromium(III) complexes were formed. The developed precatalysts exhibited excellent tolerance in solvents, including even low-carbon-number hydrocarbons such as n-pentane, n-hexane, or cyclohexane. In particular, the ortho-fluorophenyl-substituted complex showed higher stability even at higher temperatures above 120 °C. The ortho-OCF₃-phenyl-substituted complex showed outstanding catalytic activity, which reached 2287 kg/g Cr/h at 30 bar.

Full text of DOI:10.1016/j.apcata.2018.04.030

AppliedCatalysisA,General560(2018)21–27
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Polyhedral oligomeric silsesquioxane-conjugated bis(diphenylphosphino)
amine ligand for chromium(III) catalyzed ethylene trimerization and
tetramerization
Hoseong Leea,b, Soon Hyeok Honga,⁎
a
Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-gu, Seoul, 08826, Republic of Korea
Department of Chemical R&D Center, SK Innovation, Yuseong-gu, Daejeon, 34124, Republic of Korea
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Polyhedral oligomeric silsesquioxanes (POSSs) were attached to conventional bis(diphenylphosphino)amine
(PNP) ligand as solubility-enhancing materials for catalytic ethylene trimerization and tetramerization.
Differently functionalized arylphosphine ligands of the type (Ph)2PN(POSS)P(Ph)(ArR) (R = functional groups)
were systematically developed, and their corresponding chromium(III) complexes were formed. The developed
precatalysts exhibited excellent tolerance in solvents, including even low-carbon-number hydrocarbons such as
n-pentane, n-hexane, or cyclohexane. In particular, the ortho-fluorophenyl-substituted complex showed higher
stability even at higher temperatures above 120 °C. The ortho-OCF3–phenyl-substituted complex showed out-
standing catalytic activity, which reached 2287 kg/g Cr/h at 30 bar.
Ethylene trimerization
Ethylene tetramerization
Chromium
Ethylene oligomerization
Polyhedral oligomeric silsesquioxane
Homogeneous catalysis
1. Introduction
hydrocarbon solvents, however, has a potential advantage since they
dissolve ethylene well [20–22]. The solubility of ethylene in some hy-
Selective oligomerization of ethylene for linear α-olefins (LAOs),
such as 1-hexene and 1-octene, is a highly challenging subject in cat-
alysis and has been researched extensively [1]. LAOs, which are valu-
able co-monomers in the production of linear low-density polyethylene
(LLDPE), are produced industrially via a generally less-selective oligo-
merization of ethylene. The conventional oligomerization process pro-
duces not only 1-hexene and 1-octene, but also higher-molecular-
weight oligomers according to the Schulz-Flory or Poisson distribution
[2–4]. To address the selectivity issue, various catalytic systems that
utilize chromium complexes have been developed [5–12]. Generally, an
active chromium catalyst can be generated in situ using a ligand and Cr
(acetylacetonate)3 or Cr(ethylhexanoate)3 or CrCl3(THF)3 [13–18]. The
generated precatalyst, however, has poor solubility in many non-polar
solvents, which often causes lower catalytic activities, so that several
attempts have been reported to overcome the low solubility. For in-
stance, K. Blann et al. demonstrated significant improvement on cata-
lytic activity by increasing carbon number and solubility with results of
methyl-(26 kg/g Cr/h), n-pentyl-(43 kg/g Cr/h), and n-decyl-(50 kg/g
Cr/h)PNP precatalysts [19]. Meanwhile, hydrocarbon solvents with
fewer carbon atoms have not been evaluated adequately, and most
selective oligomerization reactions have conventionally been carried
out in toluene or methylcyclohexane (MCH). The use of light
drocarbon solvents has been calculated by Aspen Plus (Table S1). It is
higher in light hydrocarbon solvents at lower temperatures, and also in
a linear aliphatic solvent than in a cyclic one with the same number of
carbon atoms. Furthermore, the processing temperature for the oligo-
merization is still restrictive, although numerous modifications and
developments have been made to improve the catalytic systems. Be-
cause of the intrinsic activity of conventional catalytic systems, op-
eration at 45–60 °C is encouraged; otherwise, at elevated temperatures,
the performance drops significantly, resulting in a drastic decrease in
the productivity [23]. The advantage of operating at higher tempera-
tures is that viscous byproduct polymers can be dissolved, preventing
post-process plugging. In order to enable operation at high tempera-
tures, more innovative catalytic systems are required.
Polyhedral oligomeric silsesquioxanes (POSSs) have been high-
lighted as advanced materials with excellent solubility and thermal
stability [24–29]. Guenthner et al. showed that POSSs have excellent
solubility in non-polar solvents such as n-hexane by measuring the
Hansen solubility parameters [30]. Fina et al. reported that POSSs have
high thermal stability; for instance, isobutyl-substituted POSS has a
pyrolysis temperature above 260 °C [31]. Owing to such advantageous
properties, POSSs have been widely used as scaffolds for organic/in-
organic hybrid catalysts [32], microfluidic formats [33], monolithic
⁎
Corresponding author.
E-mail address: soonhong@snu.ac.kr (S.H. Hong).
https://doi.org/10.1016/j.apcata.2018.04.030
Received 9 December 2017; Received in revised form 22 April 2018; Accepted 23 April 2018
Availableonline24April2018
0926-860X/©2018ElsevierB.V.Allrightsreserved.

H. Lee, S.H. Hong
AppliedCatalysisA,General560(2018)21–27
structures [34–36], tailored internal chemistries of porous materials
[37], conjugates to nanoparticles [38–40] as well as stiffeners for
transparent electronic devices with inherent thermal stability [41].
Duchateau et al. reported the use of titanium- and zirconium-tethered
POSSs as catalysts for ethylene polymerization [42]. However, the use
of chromium-tethered POSSs for ethylene oligomerization has not been
investigated thus far. In this study, versatile chromium complexes based
on POSS-conjugated ligands were developed as catalysts with excellent
solubility and thermal stability for selective ethylene oligomerization.
to yield 4.32 g (70.2%) of the colorless solid.; m/z (APPI) [M + H]+
calcd for C31H72NO12Si8+: 874.3203; found: 874.3178. v (CHCl3)/
cm−1
: 2953s, 2926 w, 2902 w, 2868 w, 1463s, 1398 w, 1388 w,
1366 w, 1332 w, 1229s, 1096s, 907 s, 852 s, 732 s. δH(CDCl3): 2.65 (t,
2 H, -CH2N), 1.84 (m, 7 H, CH), 1.51 (m, 2 H, -CH2-), 1.14 (b, 2 H, NH2),
0.94 (m, 42 H, CH3), 0.58 (m, 16 H, Si-CH2). δC(CDCl3): 44.7, 27.1,
25.7, 23.9, 22.5, 9.2.
2.3.2. Preparation of (Ph)2PN(POSS)P(Ph)2 (L1)
POSS-NH2 (0.6 g, 0.68 mmol) and triethylamine (1.0 mL,
7.20 mmol) were dissolved in dichloromethane (10 mL), and then
chlorodiphenylphosphine (0.317 g, 1.44 mmol) was added. The solu-
tion was stirred at ambient temperature for 1 h. The volatile solvent was
evaporated and the product was washed with methanol (2 × 3 mL).
Further purification was performed by recrystallization in n-hexane/
methanol, recovered, and dried to yield 0.71 g (83%) of (Ph)2PN(POSS)
2. Experimental
2.1. General conditions
All reactions were performed under an inert atmosphere using
standard Schlenk techniques. All solvents and gases were dried and
degassed using standard procedures. Chemicals were purchased from
Sigma Aldrich or Strem and used without further purification unless
otherwise stated. mMAO-3A was obtained from Akzo Nobel
Corporation as a 7% w/w solution in heptane. The IR spectra were
P(Ph)2 as
a white solid.; m/z (APPI) [M +
H]+ calcd for
C
55H90NO12P2Si8+: 1242.4087; found: 1242.4033. v (CHCl3)/cm−1
:
2952s, 2915 w, 2868 w, 2848 w, 1464s, 1434s, 1401 w, 1382 w,
1365 w, 1331 w, 1229s, 1096s, 866 s, 837 s, 741 s, 725 s. δH(CDCl3):
7.35 (b, 8 H, aromatics), 7.26 (b, 12 H, aromatics), 3.17 (m, 2 H,
-CH2N), 1.80 (m, 7 H, -CH-), 1.24 (b, 2 H, -CH2-), 0.92 (m, 42 H, CH3),
0.56 (m, 14 H, Si-CH2), 0.15 (t, 6 H, Si-CH2). δC(CDCl3): 139.8, 132.7,
128.6, 127.9, 55.5, 45.7, 25.6, 23.8, 22.5, 9.2. δP(C6D6): 61.8.
δSi(C6D6): -67.9.
recorded with
a Nicolet 6700 FT-IR Spectrometer from Thermo
Scientific. 1H, 19F, 29Si and 31P nuclear magnetic resonance (NMR)
spectra were recorded on a Bruker AVANCE III HD 500 MHz spectro-
meter in CDCl3 or C6D6. Chemical shifts are reported in ppm with re-
ference to tetramethylsilane. 13C NMR spectra were recorded on a
Bruker AVANCE III HD 600 MHz spectrometer in CDCl3. Chemical shifts
are reported in ppm with reference to internal chloroform. Bruker
Daltonics (Billerica, MA, USA) APPI 7T FT-ICR MS was used for (+)
mode atmospheric pressure photoionization analysis. Quantitative
chromatographic analysis of the oligomerization products was per-
formed using an Agilent 7890 A GC-FID with an HP-PONA column
(50 m × 0.20 mm). The reaction solvent was used as an internal
standard.
2.3.3. Preparation of (Ph)2PN(n-Bu)P(Ph)2 (L2)
(Ph)2PN(n-Bu)P(Ph)2 was prepared according to a modified litera-
ture method [12]. N-Butylamine (0.1 g, 1.37 mmol) and triethylamine
(1.72 mL, 8.4 mmol) were dissolved in dichloromethane (5 mL), and
then chlorodiphenylphosphine (0.618 g, 2.8 mmol) was added. The
solution was stirred at ambient temperature for 1 h. The volatile solvent
was evaporated and the product was washed with methanol
(2 × 3 mL), recovered, and dried to yield 0.5 g (83%) of (Ph)2PN(n-Bu)
+
2.2. Ethylene oligomerization
P(Ph)2 as a white solid.; m/z (APPI) [M + H]+ calcd for C28H30NP2
:
442.1848; found: 442.1843. δH(CDCl3): 7.39 (b, 8 H, aromatics), 7.29
(b, 12 H, aromatics), 3.23 (t, 2 H, -CH2N), 1.07 (b, 2 H, -CH2-), 0.92 (m,
2 H, -CH2-), 0.60 (t, 3 H, CH3). δC(CDCl3): 139.7, 132.7, 128.6, 128.0,
52.8, 33.4, 19.9, 13.6. δP(C6D6): 62.2.
All runs were carried out in a 50-mL stainless steel Parr autoclave
with a magnetic stirrer. In a glovebox, a glass vial was charged with the
ligand (0.5 μmol) and CrCl3(THF)3 (0.5 μmol) followed by 1 mL of di-
chloromethane, and then the solution was stirred for 10 min. After this,
the solvent was removed under reduced pressure and the resultant solid
was suspended in 20 mL of the reaction solvent. The solution was
placed in the autoclave and mMAO (0.14 mL, 0.25 mmol, 500 equiva-
lents) was added; then, the solution was pressurized with ethylene and
stirred at 600 rpm. Ethylene was fed on demand to keep the reactor
pressure constant, and the uptake was monitored using a flow meter.
After 15 min, the autoclave was cooled to 0 °C and depressurized slowly
to atmospheric pressure. The product was quenched by adding 2-
ethylhexanol (1 mL). The crude products were filtered and analyzed
using GC-FID. The polymeric products were recovered by filtration and
dried overnight in an oven at 100 °C.
2.3.4. Preparation of N,N-diethylaminochlorophenylphosphine
N,N-Diethylaminochlorophenylphosphine was prepared according
to a literature method [47]. Pyridine (13.05 g, 165 mmol) was added
dropwise to
a
solution of dichlorophenylphosphine (14.77 g,
82.5 mmol) in n-hexane (80 mL) at −78 °C, and this was followed by
the dropwise addition of diethylamine (12.07 g, 165 mmol). The reac-
tion mixture was warmed to room temperature, stirred for 3 h, and then
filtered to remove the precipitated diethylammonium chloride salt.
Removal of the solvent under reduced pressure gave a pale yellow oil
(17.75 g, 99.75%), which was distilled to yield 13.53 g (76.05%) of
pure N,N-diethylaminochlorophenylphosphine as colorless oil. b.p.
110 °C/150 mbar.
2.3. Ligand preparation
2.3.5. Typical procedure for ClPPh(ArR) for L3–L8, L10, L11, L13, and
L14
2.3.1. Preparation of POSS-NH2
3-Aminopropyl-substituted heptaisobutyl-POSS (POSS-NH2) was
synthesized as described in the literature [43–46]. Isobutyltrisilanol-
POSS (5.57 g, 7.0 mmol) was dissolved in THF (47 mL) and then (3-
aminopropyl)trimethoxysilane (1.64 g, 9.1 mmol) was added. The re-
action mixture was vigorously stirred for 24 h at 25 °C and then the
solvent was removed in vacuo. The crude product which was further
dissolved with 30 mL of n-hexane was filtered out to remove some of
insoluble residue. The obtained clear solution was added to the same
amount of acetonitrile to precipitate the desired product, filtered, wa-
shed with acetonitrile (2 × 20 mL), and dried over in vacuo. The pro-
duct was further purified by recrystallization in n-hexane/acetonitrile
Magnesium turnings (30 mmol) were activated in anhydrous THF
(40 mL), and then functionalized aryl bromide (20 mmol) was added
dropwise. The reaction mixture was stirred overnight at 45 °C. After it
had cooled to room temperature, the reaction mixture was separated
from excess magnesium via decantation to obtain a Grignard reagent
(0.5 M). A portion of the reagent (8 mL, 4 mmol) was added to a dilute
solution of N,N-diethylaminochlorophenylphosphine (3.2 mmol), and
the reaction mixture was refluxed for 3 h. After it had cooled to room
temperature, the volatile solvent was removed in vacuo, and then the
crude mixture was slurried in n-hexane (20 mL). The slurry was filtered
using an activated alumina pad, and the filter cake was washed with n-
22

H. Lee, S.H. Hong
AppliedCatalysisA,General560(2018)21–27
Scheme 1. Synthesis of a POSS-conjugated PNP ligand.
hexane (2 × 4 mL). Afterwards, the volatile solvent was removed in
vacuo to afford pure (Et)2NPPh(ArR) in 71.8%–99.3% yield as colorless
oil or white solid. Yields and characterization data are summarized in
Table S2. The obtained (Et)2NPPh(ArR) was dissolved in Et2O (10 mL),
and then a 2 M HCl/Et2O solution (2.1 equiv.) was added; then, the
mixture was stirred for 1 h at room temperature. White ammonium salt
precipitated and was removed using an activated alumina pad to afford
pure ClPPh(ArR) in 99.9% yield as colorless oil or a white solid. Yields
and characterization data are summarized in Table S3.
2.3.8. Typical procedure for (Ph)2PN(POSS)P(Ph)(ArR) for L3–L15
Prepared POSS-PNH (0.2 g, 0.19 mmol) and triethylamine (0.05 mL,
0.38 mmol) were dissolved in dichloromethane (2 mL), and then pre-
pared ClPPh(ArR) (0.23 mmol) was added. The solution was stirred at
ambient temperature for 1 h. The volatile solvent was evaporated and
the product was washed with methanol (2 × 2 mL), recovered, and
dried to afford (Ph)2PN(POSS)P(Ph)(ArR) in 72%–95% yield as a white
solid. Yields and characterization data are summarized in Table S4.
3. Results and discussion
2.3.6. Typical procedure for ClPPh(ArR) for L9, L12, and L15
3.1. Synthesis of POSS-conjugated catalysts and solvent screening
Functionalized aryl bromide (2 mmol) was dissolved in Et2O (8 mL),
and then a 1.6 M n-BuLi solution (1.25 mL, 2 mmol) was added drop-
wise at −78 °C. After 1 h of stirring, N,N-diethylamino-
chlorophenylphosphine (1.8 mmol) in Et2O (2 mL) was added dropwise.
The mixture was allowed to warm to ambient temperature and stirred
for another 3 h. The volatile solvent was removed in vacuo and the
crude mixture was slurried in n-hexane (10 mL). The slurry was filtered
using an activated alumina pad, and the filter cake was washed with n-
hexane (2 × 2 mL). The volatile solvent was removed in vacuo to afford
pure (Et)2NPPh(ArR) in 75.8%–85.3% yield as colorless oil. Yields and
characterization data are summarized in Table S2. The ClPPh(ArR)
products were obtained in the same manner described above in 99.9%
yield. Yields and characterization data are summarized in Table S3.
An amino-POSS scaffold (POSS-NH2), which can be synthesized by
tethering an aminosilane to an incompletely condensed POSS triol, was
chosen for conjugation to bis(diphenylphosphino)amine (PNP)-ligand-
based chromium complexes. A POSS-PNP ligand (L1) was obtained by
adding chlorodiphenylphosphine to POSS-NH2 (Scheme 1). Absence of
–OH moiety and formation of P-N-P bond were confirmed by FT-IR
analysis (Figure S1). A conventional ligand system, Ph2PN(n-Bu)PPh2
(L2), was also synthesized in a similar way for control experiments. The
corresponding Cr complexes were prepared via the reaction between
the ligands and CrCl3(THF)3 in-situ.
Prior to ligand screening, a pretest was carried out to investigate an
appropriate ratio of ligand/Cr for L1. As expected, when the same
equivalent of ligand/Cr, the best activity and α-olefins selectivity were
observed. In case of a higher equivalence of chromium, binuclear
chromium species with bridging diphosphine ligands would be formed
which would lower the activity [48]. With a higher equivalence of li-
gand, on the other hand, a stable four ligand coordinated chromium
complex could be formed, which would retard the activation of the
catalyst. The yield of each product was determined using gas chroma-
tography with a flame ionization detector (GC-FID) (Figure S2 for ca-
libration curves). The results are summarized in Table 1 (Figure S3).
Further screening was examined to check the activity and the solubility.
L1 and L2 complexes were dissolved in various reaction solvents. The
solubility of the L1 complex in various hydrocarbon solvents was ex-
cellent. Ligand L2, however, did not dissolve in most solvents. Subse-
quently, ethylene oligomerization was carried out and the results are
summarized in Table 2 (Figure S4).
2.3.7. Preparation of POSS-PNH
Synthesized POSS-NH2 (1.98 g, 2.26 mmol) and 1 equiv. of chlor-
odiphenylphosphine (0.50 g, 2.26 mmol) were dissolved in di-
chloromethane (30 mL). The solution was cooled to 0 °C, and then
triethylamine (1.43 mL, 10.3 mmol) was added dropwise. The mixture
was warmed to room temperature and stirred for 1 h. The volatile sol-
vent was evaporated and the solid product was washed with acetoni-
trile several times (5 × 3 mL). Further purification was performed by
recrystallization in n-hexane/acetonitrile, recovered, and dried to yield
2.22 g (92%) of POSS-PNH as a white solid.; m/z (APPI) [M + H]+
calcd for C43H81NO12PSi8+: 1058.3645; found: 1058.3612. v (CHCl3)/
cm−1
: 2952s, 2923 w, 2887 w, 2867 w, 1464s, 1439 w, 1401 w,
1382 w, 1366 w, 1332 w, 1229s, 1096s, 836 s, 739 s, 700 s. δH(C6D6):
7.55 (m, 4 H, aromatics), 7.23 (m, 4 H, aromatics), 7.18 (m, 2 H, aro-
matics), 2.98 (m, 2 H, -CH2N), 2.16 (m, 7 H, -CH-), 1.75 (b, 2 H, -CH2-),
1.16 (m, 42 H, CH3), 0.92 (t, 14 H, Si-CH2), 0.79 (t, 2 H, Si-CH2).
δC(CDCl3): 141.8, 131.3, 128.9, 128.1, 25.7, 23.8, 22.5, 9.3. δP(C6D6):
41.8. δSi(C6D6): -67.3.
L1 showed excellent activities in most solvents tested, and the best
activity was observed in n-pentane. No clear difference was observed
between C6 and C7 solvents, and cyclic hydrocarbon solvents such as
cyclohexane and MCH led to slightly better activities compared to
Table 1
Ethylene oligomerization results according to ligand/Cr ratio.a
Ligand/Cr molar ratio
Productivity
(kg/g Cr/h)
C6
(%)
1-C6/C6
(%)
C8
(%)
1-C8/C8
(%)
C10-14
(%)
Polymer
(%)
Selectivityb
0.5
1.0
1.5
434
1049
995
18.6
23.3
21.3
30.2
42.2
33.9
55.6
63.0
62.0
93.2
95.5
94.7
12.0
10.3
11.7
13.8
3.4
5.0
3.0
2.7
2.9
a
Standard conditions: 50 mL autoclave, 0.5 μmol of CrCl3(THF)3, 500 equiv. of mMAO-3A, 15 min, 20 mL of MCH, 30 bar of ethylene, 45 °C.
C8/C6 selectivity.
b
23

H. Lee, S.H. Hong
AppliedCatalysisA,General560(2018)21–27
Table 2
Ethylene oligomerization results of solvent screening.a
Ligand
L1
Solventb
Productivity
(kg/g Cr/h)
C6
(%)
1-C6/C6
(%)
C8
(%)
1-C8/C8
(%)
C10-14
(%)
Polymer
(%)
Selectivityc
Pen
1250
889
1021
869
1049
125
161
214
228
446
21.2
20.6
21.7
20.8
23.3
20.2
19.2
11.5
11.5
13.1
39.5
36.9
37.2
37.3
42.2
45.9
51.3
30.5
33.5
34.2
64.6
64.5
64.7
63.9
63.0
69.8
69.3
71.5
73.6
70.8
96.9
96.0
95.4
95.6
95.5
97.2
96.1
95.8
96.1
96.5
11.1
10.5
9.9
11.2
10.3
8.8
3.2
4.4
3.7
4.1
3.4
1.2
1.6
1.2
0.4
0.2
3.1
3.1
3.0
3.1
2.7
3.5
3.6
6.2
6.4
5.4
nHx
cHx
Hep
MCH
Pen
nHx
cHx
Hep
MCH
L2
9.9
15.8
14.5
15.9
a
Standard conditions: 50 mL autoclave, 0.5 μmol of ligand, 0.5 μmol of CrCl3(THF)3, 500 equiv. of mMAO-3A, 15 min, 20 mL of solvent, 30 bar of ethylene, 45 °C.
Abbreviations for solvents: Pen; n-pentane, nHx; n-hexane, cHx; cyclohexane, Hep; n-heptane, MCH; methylcyclohexane.
C8/C6 selectivity.
b
c
linear hydrocarbon solvents such as n-hexane and n-heptane. In the case
Unlike the experiments in Table 2, these reactions were performed
at 60 °C to enable comparison with previously reported results for re-
lated PNP systems. The activity was slightly lower at 60 °C than at 45 °C
for L1, which seems to be because of reduced ethylene solubility
(1049 kg/g Cr/h at 45 °C, 997 kg/g Cr/h at 60 °C). The productivity of
L2 was only approximately 60% of that of L1, despite an increase of
temperature to 60 °C (Figure S5).
Various substituents (Me, OMe, F, CF3, OCF3) were introduced at
the ortho-, meta-, and para-positions. ortho-Substituted L3, L6, and L12
were noticeably superior to the others in terms of C6 fraction selectivity.
This result is in agreement with previously reported experimental re-
sults and can be attributed to steric effects [52,53]. A sterically bulky
substituent disturbs the formation of a metallacycle, and consequently,
a metallacycloheptane intermediate is in a more advantageous geo-
metry than a metallacyclononane intermediate. The inductive effect
was observed as well for the methyl-substituted phosphoamino ligands.
The use of L5, which has an electron donating methyl group at the para-
position, resulted in higher oligomers productivity and less polymer
productivity compared to the use of L4.
In the case of the methoxyl-substituted ligands (L6–L8), the L6-
based catalytic system remarkably produced C6 and C10–14 products
with high activity and a high selectivity of 99.9% for 1-hexene. The
formation of C10–14 products with a trace amount of 1-decene is at-
tributed to co-trimerization of ethylene with 1-hexene. Therefore, most
of the products are branched olefins of C10–14 compounds rather than α-
olefins [55]. L6 led to the production of only a trace amount of C8
products without any polymer generation. This can be explained by the
pendant effect of the hemilabile ether moiety, which may coordinate to
the metal center to form a typical trimerization catalyst (Fig. 1a) [54].
It is noteworthy that the use of the L6-based catalytic system resulted in
higher productivity than that reported in the previous literature under
the same pressure (1977 kg/g Cr/h at 30 bar) [53] with excellent se-
lectivity for C6 products and with no polymerization. L8, which has a
methoxyl substituent at the para-position, showed slightly higher ac-
tivity compared to L7.
Fluoro-substituted ligands L9–L11 exhibited unique characteristics.
The activity with the ortho-F ligand was twice as high as those with the
other fluoro-substituted ligands, and the C8/C6 selectivity was main-
tained at approximately 2.3. As previously reported, steric hindrance
limits metallacycle formation, and thus C6 production dominates over
C8 production when there is an ortho substituent; however, the se-
lectivity with L9 did not follow this trend. This result can be explained
as follows. First, the relatively low steric hindrance from a fluoro sub-
stituent may not interfere with metallacycle formation. In addition, a
bulky POSS structure can limit the movement and rotation of the aryl
group, and a repulsive force from the lipophobicity of the fluorine atom
may affect the ligand orientation during the formation of the lipid
metallacycle [56]. In conclusion, lowering the flexibility and crowding
of L2, no increased activity was observed in low-carbon-number sol-
vents, seemingly owing to its low solubility. After the reaction in n-
pentane an insoluble greenish material which was likely derived from
residual chromium(III) complex was observed inside the reactor. A high
1-C8 fraction of the total C8 products (> 95%) and a moderate 1-C6
fraction of the total C6 products (< 52%) were obtained with both
precatalysts. It is known that alpha branching on the N atom is im-
portant in determining the proportion of the 1-C6 fraction of C6 pro-
ducts; with tert-, sec-, and prim-alkyl alpha branching, the proportions of
1-C6 in the C6 products were approximately 97%, 85%, and 45%, re-
spectively [19]. Since a methylene group is at the alpha position to the
N atom in the case of L1, it is consistent with the above tendency. In
other words, improving the selectivity by adjusting the alpha branching
group is possible. It is well reported that the major C6 byproducts are
methylcyclopentane and methylene cyclopentane [49]. It is also known
that the bulkiness of substituents on the N atom increases the C6 frac-
tion and decreases the C8 fraction [19,50,51]. Similarly, the reason that
the C8/C6 selectivity of L1 (2.7–3.1) is somewhat lower than that of L2
(3.5 to 6.4) may be the bulkiness of the POSS structure, even though it
is far away from the PNP moiety. To summarize the first screening,
POSS-containing ligand L1 showed consistent activities in whole sol-
vents, whereas ordinary ligand L2 was behind. It is noteworthy that a
fine powdery polymer was produced rather than a viscous polymer
when a solvent with six or fewer carbons was used. The powdery
polymer can be separated easily using a filter, thereby making it
probably suitable for a continuous process.
3.2. Synthesis of functionalized POSS-conjugated catalysts and their
catalytic activities
Control of activity and selectivity by adjusting the functional groups
on the aryl substituent in the PNP ligand system has already been re-
ported by Sasol Technology [52,53] and British Petroleum [54].
However, many of the studies only focused on alkyl and methoxyl
functional groups. It has also been reported that the catalytic activity
and selectivity depend slightly on the number of substituents on the
four aryl groups in a diphosphinoamine ligand. The C6 fraction, for
instance, can be increased gradually by reducing the number of sub-
stituents. To examine electronic and steric effects simultaneously, we
conducted further studies with POSS-conjugated ligands with various
functional groups on the aryl substituents of the diphosphinoamine, as
shown in Scheme 2. The POSS-PNH precursor was reacted with ClPPh
(ArR) containing various functional groups. The ClPPh(ArR) compounds
were synthesized using Grignard reagents or via lithiation of the re-
levant aryl compounds with N,N-diethylaminochlorophenylphosphine.
Using the synthesized ligands, ethylene oligomerization was carried out
(Table 3).
24

H. Lee, S.H. Hong
AppliedCatalysisA,General560(2018)21–27
Scheme 2. Synthesis of POSS-conjugated PNP ligands. [a] ClPPh2/triethylamine (TEA), dichloromethane (DCM), 0 °C → rt, 1 h. [b] ClPPh(ArR)/TEA, DCM, rt, 1 h.
[c] Diethylamine/pyridine, n-hexane, −78 °C to rt, 3 h. [d] 1) Grignard reagent/tetrahydrofuran (THF), reflux, 3 h, or lithiation/Et2O, −78 °C, 3 h, 2) HCl/Et2O, 1 h.
at the active catalytic site aids in the formation of an intact metalla-
cycle. Notably, the 1-C6/C6 selectivity (70.4%) with L9 increased de-
spite the lack of bulky N-alkyl groups, which was also probably because
of the prevention of the obstruction of metallacycle formation. Neither
the meta- (L10) nor the para-substituted ligand (L11) was beneficial to
productivity.
L12–L14, which have CF3 substituents, showed lower activities than
the others. C6 production with ortho-CF3-substituted L12 was higher
than C8 production. L15 showed much higher activity than the other
ligands. It is also noteworthy that the properties of L15 were quite
different from those of L6 despite structural similarities. For instance,
L15 led to a common distribution of C6, C8, and C10–14, whereas L6
produced mainly C6 and C10–14 products. The higher activity and se-
lectivity with L15 can be attributed to the interference of the co-
ordination of the pendant ether to the metal center by the strongly
electron-withdrawing CF3 group (Fig. 1b). As a result, the co-trimer-
ization to produce > C10–14 did not occur. In terms of energy efficiency,
the methoxyl group in anisole tends to be coplanar with the phenyl
group, whereas the OeCF3 bond in trifluoromethoxybenzene prefers to
be perpendicular to the phenyl group [57]. This tendency can be ex-
plained through the stereoelectronics of the OCF3 group. First, there is a
steric effect caused by the longer CeF bond length (1.41 Å) compared
to that of a CeH bond (1.09 Å) [57]. In addition, the CeF bond is ad-
vantageous since its energy is minimized when oriented antiperiplanar
25

H. Lee, S.H. Hong
AppliedCatalysisA,General560(2018)21–27
Table 3
Ethylene oligomerization results with functionalized POSS-PNP.a
Ligand
Productivity
(kg/g Cr/h)
C6
(%)
1-C6/C6
(%)
C8
(%)
1-C8/C8
(%)
C10-14
(%)
Polymer
(%)
Selectivityb
L1
L2
L3
L4
L5
L6
L7
L8
997
575
1254
606
892
1977
579
717
1745
799
809
730
437
562
2275
24.8
23.0
41.6
23.9
29.6
60.0
29.1
32.2
27.7
25.7
25.6
54.4
27.2
27.8
37.2
47.4
51.7
65.5
46.9
59.3
99.9
57.2
65.1
70.4
44.9
44.6
65.9
51.8
54.2
67.8
64.4
68.6
50.5
65.7
60.1
1.5
60.4
57.4
63.6
63.3
63.9
38.8
60.7
62.5
56.8
95.3
97.0
97.9
96.3
96.5
93.2
96.3
96.6
98.1
95.6
95.8
97.1
95.7
96.3
96.7
8.8
7.6
7.6
9.1
9.7
38.5
9.9
9.7
8.3
10.2
9.7
6.5
11.0
8.8
5.6
2.0
0.8
0.3
1.3
0.6
0.0
0.6
0.7
0.4
0.8
0.8
0.3
1.1
0.9
0.4
2.6
3.0
1.2
2.7
2.0
0.0
2.1
1.8
2.3
2.5
2.5
0.7
2.2
2.2
1.5
L9
L10
L11
L12
L13
L14
L15
a
Standard conditions: 50 mL autoclave, 0.5 μmol of ligand, 0.5 μmol of CrCl3(THF)3, 500 equiv. of mMAO-3A, 15 min, 20 mL of MCH, 30 bar of ethylene, 60 °C.
C8/C6 selectivity.
b
with the nonbonding orbital of oxygen. In other words, the nonbonding
orbital of oxygen and σ* orbital of the CeF bond are hyperconjugated,
lowering the electron density of oxygen and the coordination bond
strength. These hypotheses could be clarified through single crystal x-
ray diffraction and structure analysis, but it was unsuccessful to form a
single crystal due to the good solubility of the complexes.
3.3. Thermal stability
Considering the thermal stability of the catalysts, the oligomeriza-
Fig. 1. Proposed possible interactions in active catalysts: [a] coordination of
pendant ether to metal center, [b] interference of coordination of pendant ether
by electron withdrawing character of trifluoromethoxyl group.
tion reactions with the POSS-containing catalytic systems were con-
ducted at higher temperatures (Table 4). Like with L2, the activity of L1
decreased significantly at above 80 °C (Figure S6). It was observed that
Table 4
Ethylene oligomerization results with different reaction temperatures.a
Ligand
L1
Temp.
(°C)
Productivity
(kg/g Cr/h)
C6
(%)
1-C6/C6
(%)
C8
(%)
1-C8/C8
(%)
C10-14
(%)
Polymer
(%)
Selectivityb
45
60
80
100
120
140
45
60
80
100
120
140
45c
45
1049
997
865
632
513
316
446
575
396
23.3
24.8
26.1
30.7
34.2
40.7
13.1
23.0
24.5
29.0
35.5
36.8
22.7
25.1
27.7
34.4
42.7
46.7
47.3
32.6
35.5
37.2
46.9
51.2
56.2
62.3
42.2
47.4
54.6
63.9
71.0
80.0
34.2
51.7
57.1
67.6
77.5
82.0
58.8
62.8
70.4
80.3
87.5
89.1
89.3
54.8
58.5
67.8
81.7
84.8
88.9
92.4
63.0
64.4
63.9
60.0
57.6
48.6
70.8
68.6
66.8
61.8
56.3
48.3
65.9
64.2
63.6
54.9
43.8
42.4
44.9
59.2
56.8
56.8
46.1
42.5
37.0
32.3
95.5
95.3
96.5
96.9
96.4
96.2
96.5
97.0
97.4
97.2
96.5
96.0
98.2
98.0
98.1
98.3
98.5
98.8
98.7
97.9
97.9
96.7
97.9
97.8
97.9
97.6
10.3
8.8
8.2
7.5
6.5
9.0
15.9
7.6
7.7
7.7
3.4
2.0
1.8
1.8
1.7
1.7
0.2
0.8
1.0
1.5
2.2
6.7
0.3
0.4
0.4
0.4
0.1
0.1
0.1
0.2
0.2
0.4
0.5
0.1
0.2
0.4
2.7
2.6
2.4
2.0
1.7
1.2
5.4
3.0
2.7
2.1
1.6
1.3
2.9
2.6
2.3
1.6
1.0
0.9
0.9
1.8
1.6
1.5
1.0
0.8
0.7
0.5
L2
L9
239
218
128
6.0
8.2
1841
1793
1745
1638
1619
1617
1464
2275
2287
2275
2030
1595
1371
636
11.1
10.3
8.3
10.3
13.4
10.8
7.7
8.0
7.5
5.6
6.5
60
80
100
120
140
45c
45
60
80
100
120
140
L15
6.2
6.6
5.0
a
Standard conditions: 50 mL autoclave, 0.5 μmol of ligand, 0.5 μmol of CrCl3(THF)3, 500 equiv. of mMAO-3A, 15 min, 20 mL of MCH, 30 bar of ethylene.
b
c
C8/C6 selectivity.
n-Pentane as solvent.
26

H. Lee, S.H. Hong
AppliedCatalysisA,General560(2018)21–27
the amount of 1-C6 in the C6 products increased as the temperature
increased, and the C8/C6 selectivity decreased gradually (L1: 2.7 at
45 °C, 1.2 at 140 °C). These results were similar to those of L2. To our
surprise, L9 exhibited excellent activity even at a higher temperature
(1464 kg/g Cr/h at 140 °C). The ortho-F substituent may stabilize the
active catalytic intermediate. It is industrially important for the cata-
lytic activity to remain high at high temperatures, since this allows the
dissolution of byproduct polymers during the process. As previously
mentioned, fouling can be prevented and continuous operation can be
enabled. Although the activity with L15 was twice that of L1 at every
temperature, it decreased sharply at high temperatures like with the
other catalysts. However, it still reached up to 2287 kg/g Cr/h at 45 °C.
It is surprising to see activity comparable to a previously reported value
under the same pressure (30 bar) [58]. Both of ligands, L9 and L15, also
showed good activities in n-pentane at 45℃.
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