Heteroscorpionate bis-pyrazolyl ligands in Cr(III)-catalyzed ethylene oligomerization

Article abstract of DOI:10.1016/j.jorganchem.2016.04.033

We have investigated Cr(III) catalysts supported by the bis-pyrazoyl ligands containing a range of appendices from heteroaryl groups, such as quinolinyl, furanyl, and thiopheneyl, to hydroxy-containing groups such as carboxyl and hydroxyl. These catalysts, upon activation with MAO, are active for ethylene oligomerization. The Cr precatalyst supported by the bis-pyrazolyl ligand with thiopheneyl achieved a activity of 29.8 kg/(g Cr/h) with a total selectivity of 58.5% toward 1-hexene and 1-octene. The complexation reaction of carboxyl-containing ligand with Cr(THF)₃Cl₃ gave an acetoxychromium(III) chloride through the HCl elimination.

Full text of DOI:10.1016/j.jorganchem.2016.04.033

Journal of Organometallic Chemistry 817 (2016) 21e25
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Heteroscorpionate bis-pyrazolyl ligands in Cr(III)-catalyzed ethylene
oligomerization
a
a
b
b, *
Mingfang Zheng , Hongfei Wu , Xuejun Zhang , Jun Zhang
a
Yanshan Branch, Beijing Research Institute of Chemical Industry, SINOPEC, Beijing 102500, China
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and
b
Technology, 130 Mei Long Road, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 February 2016
Received in revised form
We have investigated Cr(III) catalysts supported by the bis-pyrazoyl ligands containing a range of
appendices from heteroaryl groups, such as quinolinyl, furanyl, and thiopheneyl, to hydroxy-containing
groups such as carboxyl and hydroxyl. These catalysts, upon activation with MAO, are active for ethylene
oligomerization. The Cr precatalyst supported by the bis-pyrazolyl ligand with thiopheneyl achieved a
activity of 29.8 kg/(g Cr/h) with a total selectivity of 58.5% toward 1-hexene and 1-octene. The
17 April 2016
Accepted 28 April 2016
Available online 7 May 2016
complexation reaction of carboxyl-containing ligand with Cr(THF)
3
Cl
3
gave an acetoxychromium(III)
chloride through the HCl elimination.
Keywords:
©
2016 Elsevier B.V. All rights reserved.
Ethylene oligomerization
Bis-pyrazoyl ligands
Chromium complex
Ethylene trimerization
Ethylene tetramerization
1
. Introduction
to be efficient to support Cr catalysts in ethylene trimerization
when using alkylaluminium as cocatalysts, probably due to the
presence of a reactive protic nitrogen, which could react with the
alkylaluminium activator [8]. The concept was also extended to the
use of hydroxy moieties with other phosphine ligands [9]. In 2008,
one of us, Hor, and Braunstein et al. have developed Cr(III) catalytic
systems supported by either heteroscorpionate pyrazolyl ligands of
Over the last two decades, to meet high growth in the end-user
applications industry for 1-hexene and 1-octene, the selective
ethylene tri- and tetramerization have been paid a great deal of
attention from both the academic and industrial communities [1].
Since this was first observed with Cr catalysts in 1977 [2], only a few
highly active and selective catalysts for ethylene trimerization have
been established [3], including the Phillips pyrrolide [3a,3b], the BP
diphosphinoamine [3c], and the Sasol mixed heteroatomic systems
0
the type A (X ¼ O, S, NR ) with [NNO], [NNS], or [NNN] donor sets or
those of the type B bearing a third donor group altered between
pyrazolyl, pyridyl and imidazolyl (Fig. 1), exhibiting excellent
selectivity (up to 98 wt%) for ethylene trimerization [10]. As part of
our long-standing interest in the design and synthesis of new li-
gands for selective ethylene oligomerization catalysis, we have
recently developed asymmetric carbon-bridged diphosphine li-
gands [11] and a family of asymmetric PNPO ligands bearing a
phenol moiety [12] for supporting the Cr catalysts. Herein, we
present further the structural adaptation of the bis-pyrazoyl motif
through the introduction of a range of appendices, from heteroaryl
groups, such as quinolinyl, furanyl, and thiopheneyl, to hydroxy-
containing groups such as carboxyl and hydroxyl (C and D, Fig. 1).
The bis-pyrazoyl ligands of type C are first reported. The effect of
the ligands on ethylene trimerization is demonstrated. Isolation of
an acetoxychromium chloride ligated by the bis-pyrazoyl motif
through the HCl elimination is also presented.
[3d]. Other diphosphine ligands [4] and P,N-ligands [5] were also
been developed to support the Cr-based ethylene trimerization
catalysts. In 2004, the researchers from Sasol first reported a Cr-
based ethylene tetramerization catalyst by modification of the BP
diphosphinoamine (PNP) ligand, giving an 1-octene selectivity up
to 70% [6]. Stimulated by the results, an ever increasing number of
publications and patents have been reported for the selective
ethylene tri- and tetramerization, among which a variety of new
ligands have been developed for this purpose [7].
The novel PNPNHP ligands bearing a protic nitrogen linked to
the nitrogen backbone of the typical PNP ligands have been found
*
Corresponding author.
E-mail address: zhangj@ecust.edu.cn (J. Zhang).
http://dx.doi.org/10.1016/j.jorganchem.2016.04.033
022-328X/© 2016 Elsevier B.V. All rights reserved.
0

22
M. Zheng et al. / Journal of Organometallic Chemistry 817 (2016) 21e25
Fig. 2. Selected bis-pyrazolyl ligands 4~7.
carboxyl-containing 4 or the hydroxy-containing 5 and 6 didn’t
show reactivity toward ethylene oligomerization, although all of
them have a reactive hydroxyl group. Compared with the carboxyl-
containing ligand 4, the catalyst supported by the ester-containing
ligand 7 showed lower activity and similar selectivity (Table 1,
entries 4 and 7).
Fig. 1. Selected bis-pyrazolyl ligands for selective ethylene oligomerization.
Ligand 3 having a thiopheneyl group achieved the highest ac-
tivity with a considerable total selectivity toward 1-hexene and 1-
octene, and were then selected for further investigation under
different reaction condition to improve the catalyst performance.
Increasing the Al/Cr molar ratio from 600 to 800 enhanced the
activity and produced slight more total selectivity toward 1-hexene
and 1-octene, with a similar amount of polymer formed (Table 1,
2
. Results and discussion
Avoiding the use of dangerous phosgene, we have synthesized
the new ligands 1~3 in 39e75% yields, by a modified method [13],
from a condensation reaction of the corresponding aldehydes with
0
ꢀ
ꢀ
(
Pz )
2
S¼O (from thionyl chloride) (Scheme 1). The hydroxy-
entries 3 and 8). Increasing temperature from 60 C to 80 C led to a
slight increase in activity and a decrease in selectivity toward either
1-hexene or 1-octene (Table 1, entries 3 and 9). Same trend was also
containing ligands 4~6 and were known compounds and pre-
pared by the literature methods [14e16]. For comparison, the
ligand 7 having a third ester donor was also synthesized according
to the known procedure [17] (Fig. 2).
The ligands 1~7 were examined for ethylene oligomerization in
conjunction with chromium source upon activation with MAO, and
the results are summarized in Table 1.
ꢀ
observed when keeping the reaction temperature at 80 C and
increasing the Al/Cr molar ratio from 600 to 800 (Table 1, entries 3
and 10).
In order to establish the coordination mode of the hetero-
scorpionate ligands, the complexation reaction of carboxyl-
containing 4 with Cr(THF) Cl has been exploitation. The reaction
ꢀ
At 35 bar ethylene and 60 C and in the presence of 600 M excess
3
3
of MAO, all of the Cr precatalysts supported by the dipyrazolyl li-
gands (1~7) were reactive in ethylene oligomerization reaction.
Among the ligands 1~3 bearing a third heteroaryl groups, 3 having
a thiopheneyl group achieved better result, exhibiting the highest
activity with a 55.4% total selectivity toward 1-hexene and 1-octene
3 3
of 4 with Cr(THF) Cl and followed treatment with DMSO led to a Cr
complex 8 (Scheme 2). Single crystals, suitable for X-ray diffraction
study, were obtained by slow evaporation of a DMSO solution of 8,
and the structure is illustrated in Fig. 3, which revealed a HCl
elimination occurred during the complexation reaction. Complex 8
displays a distorted-octahedral geometry, with the Cr atom facially
capped by the anionic tridentate heteroscorpionate ligand and with
coordination of all three donor atoms.
In the previous work of one of our group, treatment of a related
tris(pyrazolyl)methane Cr(III) complex 9 with AlMe3 resulted ul-
timately in reduction to a Cr(II) complex 10 and cation formation
(Scheme 3). The Cr(II) complex, 10, was likely formed through the
methyl-chromium(III) intermediate 11, a methylation product ob-
tained from the reaction of 9 with MAO.
The selective formation of 1-hexene and 1-octene using the Cr
catalysts supported by the bis-pyrazolyl ligands 1~7 could be
explained by the commonly accepted metallacycle mechanism [1a]
(Scheme 4). Based on the previous observation of the formation of
related Cr intermediates 10 and 11, we speculate that upon the
activation of MAO, the Cr catalyst supported by the bis-pyrazolyl
ligand underwent methylation, followed by reduction, and gener-
ating a Cr active intermediate I (Scheme 4). Active Cr species I co-
ordinates with two ethylene molecules, and undergoes oxidative
coupling of them to produce a metallacyclopentane complex III.
Insertion of further ethylene into the metallacyclopentane III pro-
duces larger ring metallacycles. At any point, decomposition of the
metallacycle occurs via the formation of a chromium alkenyl hy-
dride species, which undergoes reductive elimination to product
(Table 1, entries 1e3). Furanyl-containing 2 achieved a highest total
selectivity of up to 68% toward 1-hexene and 1-octene, but gave a
low activity (Table 1, entry 2). It is notable that the Cr catalysts
supported by the ligands 1~3 show much lower selectivities toward
ethylene oligomerization than those bipyrazolyl Cr catalysts
bearing a pyrazolyl, pyridyl, or imidazolyl group, which implies
subtle differences in the ligand structure dramatically influences
the catalytic behavior.
Compared with the hydroxy-containing ligand 5, the carboxyl-
containing ligand 4 exhibited higher activity and similar selec-
tivity toward 1-hexene and 1-octene (Table 1, entries 4 and 5).
Introduction of two phenyl groups in the hydroxy-containing 5
resulted in a dramatic decrease in activity (Table 1, entry 6).
Disappointedly, upon activation with triethylaluminium either the
the corresponding linear a-olefins and the active catalytic species I
again. The selectivity of the transformation is controlled by relative
stability of the different sized metallacycles, which is dramatically
influenced by the supportive ligand.
Scheme 1. Synthesis of bis-pyrazolyl ligands 1~3.

M. Zheng et al. / Journal of Organometallic Chemistry 817 (2016) 21e25
23
Table 1
Ethylene oligomerization with dipyrazolyl ligands 1~7 in conjunction with chromium source and MAO.a
Entry (Lig.)
Activity (kg/g Cr/h)
Oligomer distribution 1-olefins (wt%)
(wt%)b (wt%)b
PE (wt%)c
C
6
C
8
C
10 (wt%)b
C
12 (wt%)b
1
2
3
4
5
6
7
8
9
1
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(3)
(3)
15.7
6.6
19.0
18.1
12.0
3.7
14.1
29.8
19.8
22.1
33.8
39.0
30.1
33.8
30.2
40.5
30.7
33.8
21.4
18.3
26.7
29.0
25.3
24.6
28.3
29.0
26.7
24.7
22.7
19.5
17.3
17.4
17.7
16.2
19.0
15.6
18.3
16.3
18.9
16.8
10.9
9.0
11.4
10.5
11.7
8.2
11.2
10.6
14.5
13.0
22.5
40.7
21.1
19.7
31.0
40.1
39.5
19.7
25.1
23.1
d
e
d,e
0 (3)
a
b
c
ꢀ
Conditions: 10
wt % of liquid products (oligomers).
m
mol Cr(THF)
3
Cl
3
, 1.0 equiv ligand, 600 equiv. MAO, 35 bar of ethylene, 25 mL toluene, 60 C, 30 min.
wt % of total product (oligomers þ polymer).
d
e
8
8
00 equiv. MAO.
ꢀ
0
C.
Scheme 2. Synthesis of bis-pyrazolyl Cr complex 8.
Scheme 3. Reaction of the related tris(pyrazolyl)methane Cr(III) complex 9 with AlMe
3
or MAO. See Ref. 10b.
Fig. 3. Molecular structure of 8 with 30% probability. H atoms have been omitted for
clarity. Selected bond distances (Å) and angles (deg): Cr(1)eO(3) 1.976(3); Cr(1)eO(1)
1.988(3); Cr(1)eN(1) 2.076(3); Cr(1)eN(3) 2.078(3); Cr(1)eCl(1) 2.3041(12); Cr(1)
eCl(2) 2.3132(11); O(3)-Cr(1)-N(1) 85.36(12); O(1)-Cr(1)-N(1) 85.12(13); O(3)-Cr(1)-
N(3) 171.07(12); O(1)-Cr(1)-N(3) 85.03(12); N(1)-Cr(1)-N(3) 87.64(13); O(1)-Cr(1)-
Cl(1) 90.11(9); N(1)-Cr(1)-Cl(1) 174.48(10); N(3)-Cr(1)-Cl(1) 94.73(10).
Scheme 4. Proposed mechanism for the selective ethylene oligomerization via a
metallacycle mechanism.
3
. Conclusion
4. Experimental
In summary, Cr(III) catalysts supported by the bis-pyrazoyl li-
gands having a third quinolinyl, furanyl, thiopheneyl, carboxyl,
hydroxyl, or ester group, which, upon activation with MAO, are
active for ethylene oligomerization. The catalyst with thiopheneyl-
containing ligand 3 achieved a activity of 29.8 kg/(g Cr/h) with a
total selectivity of 58.5% toward 1-hexene and 1-octene. The
4
.1. General information
Unless otherwise stated, all reactions and manipulations were
performed using standard Schlenk techniques. All solvents were
purified by distillation using standard methods. Commercially
available reagents were used without further purification. MMAO-
complexation reaction of carboxyl-containing 4 with Cr(THF)
3 3
Cl
gave an acetoxychromium complex through the HCl elimination.
3
A (modified methylaluminoxane) (7 wt % in heptane solution) was
purchased from Akzo-Nobel. NMR spectra were recorded by using a

24
M. Zheng et al. / Journal of Organometallic Chemistry 817 (2016) 21e25
Bruker 400 MHz spectrometer. Chemical shifts are reported in ppm
from tetramethylsilane with the solvent resonance as the internal
crystallized in DMSO to give Cr complex 8 (0.131 g, 65%). Anal. Calcd
for C14 CrN S (%): C, 37.51; H, 4.72; N, 12.50. Found: C,
37.90; H, 4.35; N, 13.01.
H21Cl
2
4 3
O
1
13
standard ( H NMR CDCl
3
: 7.26 ppm; C NMR CDCl
3
: 77.0 ppm).
Mass spectra were recorded on the HP-5989 instrument by EI
methods. Quantitative gas chromatographic analysis of the prod-
ucts of oligomerization was performed on an Agilent 6890 Series
4.6. Oligomerization of ethylene
ꢀ
ꢀ
GC instrument with a J&W DB-1 column working at 36 C for
A 120 mL stainless steel reactor was dried at 120 C for 3 h under
vacuum, and then cooled down to the desired reaction tempera-
ture. The precatalysts and co-catalysts (MAO) were combined in a
Schlenk vessel in the ratios indicated in Table 1. The resultant
mixture was stirred for 1 min and immediately transferred to the
reactor. Then the reactor was immediately pressurized. After the
specified reaction time, the reaction was stopped by shutting in the
ꢀ
ꢁ1
ꢀ
n
1
0 min and then heating at 10 C min until 250 C. Nonane was
used as an internal standard. Ligands 4~7 are known compounds,
and were prepared according to modified literature methods,
respectively [14e17].
4.2. 8-(Bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl)quinolone (1)
ꢀ
ethylene feed, cooling the system at 0 C, depressurizing, and
In a representative procedure, 3,5-dimethylpyrazole (2.8 g,
0 mmol) was added at 0 C to a stirred suspension of sodium
quenched by addition of 30 mL of 10% aq. HCl. A small sample of the
upper-layer solution was filtered through a layer of Celite and
analysed by GC using nonane as the internal standard. The indi-
vidual oligomerization products were identified by GC-MS. The
remainder of the upper-layer solution was filtered to isolate the
solid polymeric products. The solid products were suspended in
10% aq. HCl and stirred for 24 h, dried under reduced pressure and
weighed.
ꢀ
3
hydride (1.2 g, 30 mmol) in tetrahydrofuran (30 mL). Stirring was
continued for 30 min sulfurous dichloride (1 mL, 14.9 mmol) was
added in one portion via syringe, and the resulting mixture was
allowed to warm to room temperature. After treatment with 8-
quinolinecarboxaldehyde (2.35 g, 14.9 mmol) and pyridine (1.2 g,
1
5 mmol), the reaction mixture was kept at the reflux temperature
for 12 h. Water was added and the aqueous phase extracted into
dichloromethane. The combined organic extracts were dried over
magnesium sulphate and filtered, and the filtrate was evaporated to
dryness under a vacuum. The crude product was purified by column
Acknowledgments
Financial support from Shanghai Pujiang Talent Program
chromatography (silica gel; hexane/EtOAc1:1) to afford 1 (3.5 g,
(11PJ1402500) and the National Natural Science Foundation of
1
7
(
1%). H NMR (400 MHz, CDCl
3
):
d
¼ 8.83 (s, 1H), 8.72 (s, 1H), 8.13
China for financial support (21171056) is greatly acknowledged.
d, J ¼ 8 Hz,1H), 7.81 (d, J ¼ 8 Hz,1H), 7.62 (d, J ¼ 6.8 Hz,1H), 7.53 (d,
J ¼ 7.2 Hz, 1H), 7.35e7.38 (m, 1H), 5.84 (s, 2H), 2.23 (s, 6H), 2.18 (s,
Supplementary data
13
6
1
(
H); CNMR (CDCl
3
, 100 MHz):
d
¼ 149.9, 148.0, 145.2, 139.9, 136.0,
34.4, 129.4, 128.6, 127.9, 126.2, 121.1, 106.1, 68.2, 13.9, 11.2. HRMS
EI): m/z [M] calcd for C20
CCDC 1017399 contained the supplementary crystallographic
data for 8. This data could be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
þ
þ
5
: 331.1797, found: 331.1800.
H
21
N
0
4
.3. 1,1 -(furan-2-ylmethylene)bis(3,5-dimethyl-1H-pyrazole) (2)
Ligand 2 has been prepared similarly to 1 with a yield of 75%,
starting from 3,5-dimethylpyrazole (2.8 g, 30 mmol), sodium
hydride(1.2 g, 30 mmol), sulfurous dichloride (1 mL, 14.9 mmol), 2-
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