Synthesis, characterization, and ethylene polymerization behavior of Cr(III) catalysts based on bis(pyrazolylmethyl)pyridine and its derivatives

Article abstract of DOI:10.1016/j.molcata.2015.04.007

New chromium(III) [Cr(III)] catalysts based on 2,6-bis(pyrazol-1-ylmethyl) pyridine derivatives have been synthesized, characterized, and evaluated for ethylene polymerization. All ligands with substituents on the pyrazole rings were analyzed by single-cr

Full text of DOI:10.1016/j.molcata.2015.04.007

Journal of Molecular Catalysis A: Chemical 404–405 (2015) 204–210
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis, characterization, and ethylene polymerization behavior of
Cr(III) catalysts based on bis(pyrazolylmethyl)pyridine and
its derivatives
Jeong Oh Woo, Sung Kwon Kang, Jong-Eun Park, Kyung-sun Son∗
Department of Chemistry, Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
New chromium(III) [Cr(III)] catalysts based on 2,6-bis(pyrazol-1-ylmethyl) pyridine derivatives have
been synthesized, characterized, and evaluated for ethylene polymerization. All ligands with sub-
stituents on the pyrazole rings were analyzed by single-crystal X-ray diffraction to clearly identify isomer
structures. Additionally, X-ray analyses of the new Cr(III) complex bearing 2,6-bis[(4,5-dimethyl-1H-
pyrazol-1-yl) methyl]pyridine showed tridentate coordination on the mer-octahedral chromium sphere.
Upon activation with dry methyaluminoxane, the precatalysts produce polyethylene (PE) as a major
product, and their catalytic performances were affected by the substituents on the pyrazole units; the
introduction of functional groups on the pyrazole raised PE compositions and the molecular weight of
Received 12 January 2015
Received in revised form 9 April 2015
Accepted 13 April 2015
Available online 14 April 2015
Keywords:
Polymerization
Chromium catalyst
Polyethylene
the PE.
© 2015 Elsevier B.V. All rights reserved.
Organometallic catalyst
1. Introduction
As an extension of this idea, we chose the combination of pyridyl
and pyrazolyl units with various substituents to search for new
The development of homogeneous chromium (Cr)-based cata-
lysts for olefin polymerization has received considerable attention,
spurred by the extensive use of Cr catalysts in commercial ethy-
lene polymerization [1,2]. At the interface of late transition metal
and early metal metallocene catalysts, Cr has been used in various
polymerization studies using a variety of ligands possessing homo-
and heteroatom donor sets [3–7]. Since the discovery of Brookhart-
[8,9] and Gibson-type [10,11] catalysts, new catalyst families of
tridentate nitrogen-based ligands such as 2,6-bis(imino) pyridine
increased interest in the design of further non-metallocene olefin
polymerization catalysts [12–18].
polymerization catalysts and to further investigate ligand struc-
tures in catalytic performance [32]. In this report, we present the
synthesis and characterization of ethylene polymerization catalysts
based on Cr(III) with 2,6-bis(pyrazol-1-ylmethyl) pyridine deriva-
tives [33–35], as well as their catalytic behavior toward ethylene
polymerization. The molecular structures of the new ligands were
determined by single-crystal X-ray diffraction to confirm unam-
biguously the structures of the isomers. To better understand the
molecular structures of the Cr(III) precatalysts, their single crystals
were also obtained and analyzed structurally.
Another interesting, but less explored, type of tridentate
nitrogen-based ligands in olefin polymerization is tris(pyrazolyl)
borate [19] or tris(pyrazolyl) methane [20,21], in which the coor-
dination chemistry has been studied extensively, but not in terms
of olefin polymerization [22–31]. One of the benefits regarding the
pyrazolyl unit is the possibility of modifying the architecture of the
ligands due to the three possible substituting positions on the pyra-
zole ring, permitting tuning of the electronic and steric properties
depending on the substituents used, offering great possibilities in
modifying catalytic behavior.
2. Experimental
2.1. General considerations
All manipulations were carried out under a nitrogen atmosphere
using standard Schlenk techniques or in a purified nitrogen-filled
dry box, unless otherwise stated. Methylaluminoxane (MAO, 10%
in toluene) was purchased from Albemarle Corporation (Baton
Rouge, LA, USA). Trimethyl aluminum (TMA)-free dry methy-
laluminoxane (dMAO) was prepared by removing all volatiles
from MAO in vacuo. Pyrazole, 3-phenyl-1H-pyrazole, 3-tert-butyl-
1H-pyrazole, p-toluenesulfonyl chloride, 2,6-pyridinedimethanol,
CrCl3(THF)3, and Cr(acac)3 were purchased from Sigma–Aldrich (St.
Louis, MO, USA). 3,4-Dimethyl-1H-pyrazole, 3-trifluoromethyl-1H-
∗
Corresponding author. Tel.: +82 0 42 821 6547.
E-mail address: kson@cnu.ac.kr (K.-s. Son).
http://dx.doi.org/10.1016/j.molcata.2015.04.007
1381-1169/© 2015 Elsevier B.V. All rights reserved.

J.O. Woo et al. / Journal of Molecular Catalysis A: Chemical 404–405 (2015) 204–210
205
Fig. 2. Structures of Cr(III) complexes Cr1 and Cr2.
(0.8 g, 20 mmol) in THF/water (7.5/7.5 mL). After 4 h of stirring, the
mixture was poured into 20 mL of water and extracted with methy-
lene chloride (3 × 7.5 mL). The organic phase was washed with
saturated aqueous NaCl solution and distilled water and dried over
Na2SO4; the solvent was removed in vacuo to afford 2,6-pyridine-
dimethylene-ditosylate (0.79 g, 88%) as a white powder. 1H NMR
(300 MHz, CDCl3): ı = 7.80 (d, J = 7.5 Hz, 4H, 2,6-H tosyl), 7.69 (t,
J = 7.5 Hz, 1H, 4-H Py), 7.32 (d, 2H, 3,5-H Py, d, 4H, 3,5-H tosyl), 5.04
(s, 4H, PyCH2tosyl), and 2.44 (s, 6H, tosyl-CH3) Fig. 1.
Fig. 1. Structures of the ligands L1–L6.
pyrazole, and 3-(4-methoxyphenyl)-1H-pyrazole were purchased
from Alfa Aesar (Ward Hill, MA, USA).
All solvents were dried and distilled prior to use according
to standard methods. For instance, toluene was refluxed over
sodium/benzophenone until the purple color appeared and dis-
tilled under nitrogen prior to use. All other chemical reagents were
purchased from commercial sources and used as received unless
stated otherwise.
2.2.2. Preparation of 2,6-bis[(pyrazol-1-yl) methyl]pyridine (L1)
[33,36–38]
At 0 ◦C, a solution of pyrazole (0.22 g, 3.2 mmol) in dry THF (5 mL)
was added dropwise to a suspension of NaH (0.08 g, 3.2 mmol) in
dry THF (5 mL). After 15 min of stirring, a clear solution of NaPz
was obtained. A solution of 2,6-pyridine-dimethylene-ditosylate
(0.73 g, 1.6 mmol) in dry THF (7.5 mL) was added to this solution;
the mixture was stirred overnight and filtered, and the solvent was
removed. The crude product was purified by column chromatogra-
phy on silica gel with ethyl acetate (EA) as eluent to afford 0.30 g
(76%) of pure ligand as a white solid. 1H NMR (300 MHz, acetone-
d6): ı 7.77(d, J = 2.4 Hz, 2H, 5-H Pz), 7.66 (t, J = 7.8 Hz, 1H, 4-H Py),
7.48 (d, J = 1.5 Hz, 2H, 3-H Pz), 6.83 (d, J = 7.8 Hz, 2H, 3,5-H Py), 6.30 (t,
J = 2.1 Hz, 2H, 4-H Pz), and 5.44 (s, 4H, PyCH2Pz). 13C NMR (75 MHz,
acetone-d6): ı 157.9 (2C, 2,6-C Py), 140.1 (2C, 3-C Pz), 138.6 (2C, 5-C
Pz), 131.0 (1C, 4-C Py), 120.9 (2C, 3,5-C Py), 106.4 (2C, 4-C Pz), 57.7
(2C, PyCH2Pz).
1H and 13C NMR spectra of the ligand products were recorded
at 25 ◦C on a Fourier 300 NMR spectrometer (Bruker, Billerica,
MA, USA) with tetramethylsilane (TMS) as an internal reference.
13C NMR spectra of the polyethylene (PE) samples were recorded
on a Bruker Avance III 600 at 130 ◦C in a 50:50 v/v solution of
1,2-dichlorobenzene/1,2-dichlorobenzene-d4 in the presence of
Cr(III) acetylacetonate (1 mM) to reduce the relaxation time of the
aliphatic carbons. The oligomers (liquid products) were analyzed by
gas chromatography–mass spectrometry (GC–MS) on a Clarus 600
with an Elit-5 (30 m × 0.25 mm × 0.25 ␮m column; PerkinElmer,
Waltham, MA, USA) using nonane as an internal standard. Molecu-
lar weights and polydispersity indices (PDIs) of PE were determined
by high-temperature gel permeation chromatography (GPC) on a
PL-GPC 220 instrument with a refractive index (RI) detector (Agi-
lent, Santa Clara, CA, USA), calibrated with polystyrene standards
at 160 ◦C and 1,2,4-trichlorobenzene as the eluent. Melting points
of the polymer were determined by differential scanning calorime-
try (DSC) with a DSC-1 instrument (Mettler–Toledo, Columbus, OH,
USA) in standard DSC run mode. The instrument was initially cal-
ibrated for the melting point of an indium standard. The polymer
sample was first heated to 160 ◦C to remove thermal history, and a
second heating cycle was used for collecting DSC thermogram data
at a ramping rate of 5 ◦C min−1. Elemental analysis was performed
on a Flash EA 1112 automatic elemental analyzer (Thermo Fisher
Scientific, Waltham, MA, USA). X-ray crystal data were obtained
using a SMART APEX II (Bruker) single crystal X-ray diffractome-
ter equipped with a Bruker SMART charge-coupled device (CCD)
area detector. The Fourier-transform infrared (FTIR) spectrum was
recorded with an FTIR 4100 spectrometer (JASCO, Tokyo, Japan).
Mass spectra were recorded on a JEOL JMS-600W (EI) spectrometer
or a Bruker micrOTOF-QII (ESI) spectrometer.
2.2.3. Preparation of 2,6-bis[(4,5-dimethyl-1H-pyrazol-1-yl)
methyl]pyridine (L2)
At 0 ◦C,
a solution of 3,4-dimethyl-1H-pyrazole (1.66 g,
17.3 mmol) in dry THF (30 mL) was added dropwise to a suspen-
sion of NaH (0.41 g, 17.3 mmol) in dry THF (30 mL). After 15 min of
stirring, a solution of 2,6-pyridine-dimethylene-ditosylate (3.87 g,
8.64 mmol) in dry THF (45 mL) was added to this solution; the
mixture was stirred overnight and filtered, and the solvent was
removed. The crude product was purified by column chromatog-
raphy on silica gel with EA as eluent to afford 1.63 g (64%) of pure
ligand as a white solid. Single crystals were obtained by slow dif-
fusion of hexane into a concentrated solution of the ligand in THF
at room temperature. 1H NMR (300 MHz, acetone-d6): ı 7.63 (t,
J = 7.8 Hz, 1H, 4-H Py), 7.19 (s, 2H, 3-H Pz), 6.72 (d, J = 7.8 Hz, 2H,
3,5-H Py), 5.31 (s, 4H, PyCH2Pz), 2.14 (s, 6H, Pz-5-Me), and 1.97 (s,
6H, Pz-4-Me). 13C NMR (75 MHz, acetone-d6): ı 158.3 (2C, 2,6-C
Py), 139.6 (2C, 3-C Pz), 138.6 (2C, 5-C Pz), 136.4 (1C, 4-C Py), 120.6
(2C, 3,5-C Py), 114.0 (2C, 4-C Pz), 55.4 (2C, PyCH2Pz), 9.18 (2C, Pz-
5-Me), and 8.79 (2C, Pz-4-Me). HRMS (ESI) Calcd for C17H21N5Na
([M+Na]+): 318.1695, Found: 318.1709.
2.2. Synthesis of ligands
2.2.4. Preparation of 2,6-bis[(3-(tert-butyl)-1H-pyrazol-1-yl)
2.2.1. Preparation of 2,6-pyridine-dimethylene-ditosylate
methyl]pyridine (L3)
At 0 ◦C,
a
solution of p-toluenesulfonyl chloride (0.76 g,
At 0 ◦C,
a
solution of 3-tert-butyl-1H-pyrazole (0.05 g,
4.0 mmol) in tetrahydrofuran (THF; 7.5 mL) was added to a stirred
solution of 2,6-pyridinedimethanol (0.28 g, 2.0 mmol) and NaOH
0.40 mmol) in dry THF (1 mL) was added dropwise to a sus-
pension of NaH (0.01 g, 0.40 mmol) in dry THF (1 mL). After 15 min

206
J.O. Woo et al. / Journal of Molecular Catalysis A: Chemical 404–405 (2015) 204–210
of stirring,
a
solution of 2,6-pyridine-dimethylene-ditosylate
1.71 mmol) in dry THF (7.5 mL) was added to this solution; the
mixture was stirred overnight and filtered, and the solvent was
removed. The crude product was purified by column chromatog-
raphy on silica gel with hexane:EA = 1:1 as eluent to afford 0.40 g
(51%) of pure ligand as a white solid. Single crystals were obtained
by slow diffusion of hexane into a concentrated solution of the lig-
and in THF at room temperature. 1H NMR (300 MHz, acetone-d6): ı
7.81 (d, J = 2.1 Hz, 2H, 5-H Pz), 7.77 (d, J = 9.0 Hz, 4H, 2,6-H Ph), 7.70
(t, J = 7.8 Hz, 1H, 4-H Py), 6.96 (t, J = 7.2 Hz, 4H, 3,5-H Ph, 2H, 3,5-H
Py), 6.65 (d, J = 2.4 Hz, 2H, 4-H Pz), 5.47 (s, 4H, PyCH2Pz), and 3.81
(s, 6H, PhOCH3). 13C NMR (75 MHz, acetone-d6): ı 160.3 (2C, 4-C
Ph), 158.1 (2C, 2,6-C Py), 152.2 (2C, 3-C Pz), 138.8 (1C, 4-C Py), 132.8
(2C, 5-C Pz), 127.5 (2C, 1-C Ph), 127.4 (4C, 2,6-C Ph), 121.2 (2C, 3,5-C
Py), 114.7 (4C, 3,5-C Ph), 103.1 (2C, 4-C Pz), 57.9 (2C, PyCH2Pz), and
55.6 (2C, PhOCH3). HRMS (EI) Calcd for C27H25N5O2 (M+): 451.2008,
Found: 451.2008.
(0.09 g, 0.20 mmol) in dry THF (1.5 mL) was added to this solution;
the mixture was stirred overnight and filtered, and the solvent
was removed. The crude product was purified by column chro-
matography on silica gel with EA as eluent to afford 0.05 g (72%)
of pure ligand as a white solid. Single crystals were obtained by
slow diffusion of hexane into a concentrated solution of the ligand
in THF at room temperature. 1H NMR (300 MHz, acetone-d6): ı
7.64 (t, J = 7.8Hz, 1H, 4-H Py), 7.61 (d, J = 2.4 Hz, 2H, 5-H Pz), 6.79 (d,
J = 7.8 Hz, 2H, 3,5-H Py), 6.17 (d, J = 2.4 Hz, 2H, 4-H Pz), 5.36 (s, 4H,
PyCH2Pz), and 1.27 (s, 18H, tBu). 13C NMR (75 MHz, acetone-d6): ı
162.6 (2C, 3-C Pz), 158.3 (2C, 2,6-C Py), 138.6 (1C, 4-C Py), 131.4 (2C,
5-C Pz), 120.7 (2C, 3,5-C Py), 102.6 (2C, 4-C Pz), 57.6 (2C, PyCH2Pz),
32.7 (2C, C(CH3)3), and 31.0 (6C, C(CH3)3). HRMS (EI) Calcd for
C
21H29N5 (M+): 351.2423, Found: 351.2417.
2.2.5. Preparation of 2,6-bis[(3-phenyl-1H-pyrazol-1-yl)
methyl]pyridine (L4) [39,40]
2.3. Synthesis of LCrCl3
At 0 ◦C, a solution of 3-phenyl-1H-pyrazole (0.61 g, 5.3 mmol) in
dry THF (10 mL) was added dropwise to a suspension of NaH (0.13 g,
5.34 mmol) in dry THF (10 mL). After 15 min of stirring, a solution
of 2,6-pyridine-dimethylene-ditosylate (1.20 g, 2.67 mmol) in dry
THF (15 mL) was added to this solution; the mixture was stirred
overnight and filtered, and the solvent was removed. The crude
product was purified by column chromatography on silica gel with
hexane:EA = 1:1 as eluent to afford 0.41 g (40%) of pure ligand as a
white oil that solidified with time. Single crystals were obtained by
slow diffusion of hexane into a concentrated solution of the ligand
in THF at room temperature. 1H NMR (300 MHz, acetone-d6): ı 7.84
(m, 2H, 4H, 5-H Pz, 2,6-H Ph), 7.65 (t, J = 7.8 Hz, 1H, 4-H Py), 7.36
(t, J = 7.2 Hz, 4H, 3,5-H Ph), 7.26 (t, J = 7.5 Hz, 2H, 4-H Ph), 6.96 (d,
J = 7.8 Hz, 2H, 3,5-H Py), 6.72 (d, J = 2.4 Hz, 2H, 4-H Pz), and 5.48 (s,
4H, PyCH2Pz). 13C NMR (75 MHz, acetone-d6): ı 157.9 (2C, 2,6-C
Py), 152.2 (2C, 3-C Pz), 138.9 (1C, 4-C Py), 134.8 (2C, 1-C Ph), 132.9
(2C, 5-C Pz), 129.3(4C, 3,5-C Ph), 128.2 (2C, 4-C Ph), 126.1 (4C, 2,6-C
Ph), 121.2 (2C, 3,5-C Py), 103.7 (2C, 4-C Pz), and 57.9 (2C, PyCH2Pz).
Cr complexes were synthesized by the reaction of CrCl3(THF)3
with the corresponding ligands in THF. A typical synthetic proce-
dure was as follows Fig. 2.
2.3.1. Synthesis of trichloro[2,6-bis[(pyrazol-1-yl)
methyl]pyridine]chromium(III) (Cr1)
L1 (0.072 g, 0.3 mmol) in dry THF (4 mL) was added to a solution
of CrCl3(THF)3 (0.112 g, 0.3 mmol) in dry THF (6 mL); the resulting
mixture was stirred at 60 ◦C for 24 h, giving a green suspension.
After the solvent was removed, the product was washed repeatedly
with THF and diethyl ether, and dried in vacuo. The green powder
(0.091 g) was obtained in a yield of 76%. Calc. for C13H13Cl3CrN5: C,
39.27; H, 3.30; N, 17.61. Found: C, 39.21; H, 3.34; N, 17.45%.
2.3.2. Synthesis of
trichloro[2,6-bis((4,5-dimethyl-1H-pyrazol-1-yl) methyl)
pyridine]chromium (III) (Cr2)
The complex Cr2 was prepared as a green powder by a similar
procedure in a yield of 79%. Single crystals were obtained by slow
diffusion of diethyl ether into a concentrated solution of the com-
plex in dimethyl sulfoxide (DMSO) at room temperature. Calc. for
2.2.6. Preparation of
2,6-bis[(3-(trifluoromethyl)-1H-pyrazol-1-yl)
methyl]pyridine (L5)
At 0 ◦C, a solution of 3-trifluoromethyl-1H-pyrazole (0.84 g,
6.20 mmol) in dry THF (10 mL) was added dropwise to a suspen-
sion of NaH (0.15 g, 6.20 mmol) in dry THF (10 mL). After 15 min of
stirring, a solution of 2,6-pyridine-dimethylene-ditosylate (1.39 g,
3.10 mmol) in dry THF (15 mL) was added to this solution; the
mixture was stirred overnight and filtered, and the solvent was
removed. The crude product was purified by column chromatog-
raphy on silica gel with EA as eluent to afford 0.53 g (46%) of
pure ligand as a white solid. Single crystals were obtained by slow
diffusion of hexane into a concentrated solution of the ligand in
THF at room temperature. 1H NMR (300 MHz, acetone-d6): ı 7.97
(d, J = 1.5 Hz, 2H, 5-H Pz), 7.81 (t, J = 7.8Hz, 1H, 4-H Py), 7.15 (d,
J = 7.5 Hz, 2H, 3,5-H Py), 6.69 (d, J = 2.4 Hz, 2H, 4-H Pz), and 5.55 (s,
4H, PyCH2Pz). 13C NMR (75 MHz, acetone-d6): ı 156.7 (2C, 2,6-C Py),
142.8 (2C, 3-C Pz), 139.2 (1C, 4-C Py), 133.6 (2C, 5-C Pz), 122.7 (2C,
CF3), 122.0 (2C, 3,5-C Py), 105.2 (2C, 4-C Pz), and 58.3 (2C, PyCH2Pz).
19F NMR (564 MHz, acetone-d6): ı 62.19 (CF3). HRMS (ESI) Calcd for
C17H21Cl3CrN5: C, 45.00; H, 4.66; N, 15.43. Found: C, 45.02; H, 4.77;
N, 15.47%.
2.4. General procedure for ethylene polymerization
These reactions were carried out in a 125 mL Parr reactor
equipped with a pressure controller. The desired amounts of dMAO
(0.174 g, 3 mmol, 300 eq) and dry toluene (46 mL) were added to
the reactor. After the desired reaction temperature was achieved
(45 ◦C), the reactor was charged with ethylene at a constant pres-
sure (10 bar), and (i) a toluene solution (4 mL) of Cr(acac)3 (3.5 mg,
0.01 mmol) and NNN ligand (0.02 mmol) or (ii) a toluene solution
(4 mL) of Cr complex (0.01 mmol) was injected into the reactor to
start the reaction. After 15 min, the reaction was terminated by
discontinuing the ethylene feed and adding methanol (10 mL) to
the reactor. Then, the reactor was cooled to below 5 ◦C in an ice
bath. After releasing the excess ethylene from the reactor, nonane
(1 mL) was added as an internal standard for the GC–MS analysis
of the liquid phase. A small amount of the reaction solution was
collected and analyzed by GC–MS to determine the distribution of
oligomers obtained. The remainder of the mixture was quenched
with methanol/HCl (10 vol%) to precipitate the solid product, which
was isolated by filtration, dried in a vacuum oven at 60 ◦C, and
finally characterized by GPC and DSC.
C
15H11F6N5Na ([M+Na]+): 398.0816, Found: 398.0812.
2.2.7. Preparation of
2,6-bis[(3-(4-methoxyphenyl)-1H-pyrazol-1-yl)
methyl]pyridine (L6)
At 0 ◦C, a solution of 3-(4-methoxyphenyl)-1H-pyrazole (0.59 g,
3.42 mmol) in dry THF (5 mL) was added dropwise to a suspen-
sion of NaH (0.08 g, 3.42 mmol) in dry THF (5 mL). After 15 min of
stirring, a solution of 2,6-pyridine-dimethylene-ditosylate (0.77 g,

J.O. Woo et al. / Journal of Molecular Catalysis A: Chemical 404–405 (2015) 204–210
207
Fig. 3. Thermal ellipsoid representation (25% probability boundaries) of the molec-
ular structure of L2 [symmetry code: (i) -x+1, y, -z+3/2].
Fig. 4. Thermal ellipsoid representation (25% probability boundaries) of the molec-
ular structure of L3.
3. Results and discussion
3.1. Preparation and characterization of L1–L6 and Cr1–Cr2
Fig. 6. Thermal ellipsoid representation (25% probability boundaries) of the molec-
ular structure of L5.
The new ligands derived from 2,6-bis(pyrazol-1-ylmethyl)
pyridine (L1–L6) were prepared as described previously
(Scheme 1) [33], starting from substituted pyrazole salt with
2,6-pyridinedimethanol. All tridentate ligands were characterized
by 1H and 13C NMR spectroscopy. Additionally, ligands L2–L6
were further characterized by X-ray crystallography to confirm
unambiguously the structures of the isomers. Suitable single
crystals were obtained from slow diffusion of n-hexane into a
concentrated solution of the ligand in THF at room temperature.
Perspective views of the molecules are shown in Figs. 3–7, and
bond distances and angles are included in Tables S1–S10. The
pyrazole groups connected to the methylene spacers were placed
on opposite sides of the pyridine plane, and all bond distances and
bond angles fell within expected ranges. The substituents were
located at the 3-position of the pyrazole rings in L3–L6, whereas
the methyl groups in L2 were found at the 4,5-positions.
isolated complexes were consistent with the expected theoretical
values, assuming the general formula LCrCl3. They are powder-like
and exhibit low solubility in common organic solvents, excluding
DMSO. The solubility of Cr complexes in DMSO allowed us to obtain
single crystals of Cr2, which were analyzed by X-ray crystallogra-
phy to determine structural information. The crystal was obtained
from a slow diffusion of diethyl ether into a solution of DMSO at
room temperature. The compound has a monoclinic crystalline sys-
tem and its molecular structure is shown in Fig. 8. This is the first
crystal structure determined by X-ray crystallography of a Cr(III)
complex bearing 2,6-bis(pyrazol-1-ylmethyl) pyridine. As shown
in Fig. 8, solid-state structure of Cr2 revealed that coordination
sphere of the Cr center adopts an octahedral geometry with a tri-
dentate ligand and three chlorines. In the molecular structure of
Cr2, the donor nitrogen atoms form a mer configuration; the basal
plane consists of the three nitrogen atoms from the pyridine and
pyrazoles and one equatorial chloride, with the remaining chlorides
occupying the apical positions [37]. The structure can be com-
pared to that of 2,6-bis(2-benzimidazolyl) pyridine Cr(III) complex
reported by Zhang et al. [16] Similarities include the fact that the
bond lengths between the Cr and the mutually trans-disposed chlo-
rine atoms, Cr1-Cl2 and Cr1-Cl2i (2.3329(16) Å), are longer than the
bond length of Cr1-Cl3 (2.303(2) Å). However, the Cr-N(pyridine)
To better understand the molecular structures of the Cr(III) pre-
catalysts, we tried to prepare and obtain suitable single crystals.
The Cr precatalysts Cr1 and Cr2 were formed through the treatment
of CrCl3(THF)3 with equimolar amounts of L1 and L2, respectively.
These complexes were isolated in good yield (>76%) as neutral octa-
hedral complexes. Experimental values of elemental analysis of the
Fig. 5. Thermal ellipsoid representation (25% probability boundaries) of the molec-
ular structure of L4.
Fig. 7. Thermal ellipsoid representation (25% probability boundaries) of the molec-
ular structure of L6.

208
J.O. Woo et al. / Journal of Molecular Catalysis A: Chemical 404–405 (2015) 204–210
Fig. 8. Thermal ellipsoid representation (25% probability boundaries) of the molec-
ular structure of Cr2 [symmetry code: (i) -x+1, y, -z+1/2].
Fig. 9. Gel permeation chromatography (GPC) traces for polyethylene samples
obtained from entries 1, 2, 7, and 8 in Tables 1 and 2.
bond distance [Cr1-N10 = 2.132(6) Å] is about 0.079 Å longer than
the Cr-N(pyrazole) bond distances [Cr1-N4 and Cr1-N4i = 2.053(5)
Å], in contrast to some other Cr complexes [14,16,17], probably due
to steric influences. All bond angles in the equatorial plane are very
close to a right angle [N4-Cr1-N10 and N4i-Cr1-N10, 88.85(16)◦;
N4-Cr1-Cl3 and N4i-Cr1-Cl3, 91.15(16)◦] (Tables S11–S12).
The IR spectra of ligands L1 and L2 show that the C = N stretch-
ing frequency of pyridine appears at 1594 cm−1 (Figs. S13, S15).
In complexes Cr1 and Cr2, the C = N stretching vibrations shift
to 1609 cm−1 (Figs. S14, S16), indicating effective coordination
between the metal and the nitrogen atom of the pyridine ring.
the central metal was also observed in a similar Cr(III) complex
bearing 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl) pyridine or 2,6-
bis(indazol-2-ylmethyl) pyridine, which was reported to produce
high molecular-weight PE of Tm 133–135 ◦C [41]. It is also interest-
ing to compare our results to a previous report with a similar Cr(III)
catalyst, such as [2,6-bis(2-benzimidazolyl) pyridyl]chromium
chlorides, which has afforded ethylene oligomers and PE with selec-
tivity sensitive to the cocatalysts [15,16]. However, the relationship
between the polymerization results and the presence of an elec-
tron donating or electron withdrawing group in the pyrazole rings
remains unclear.
3.2. Effects of the ligand structure on in situ ethylene
polymerization
3.3. Ethylene polymerization with Cr1 and Cr2
Ligands L1–L6 and Cr(acac)3 were reacted in situ and tested for
ethylene polymerization; the results are summarized in Table 1.
In all experiments, the in situ complexes were activated with
excess dMAO at 45 ◦C in toluene to generate the active species.
Regardless of the type of substituents, they converted ethylene to
linear polyethylene with moderate activities (85–140 kg molCr−1
The precatalysts Cr1 and Cr2 were also activated with excess
dMAO in the presence of ethylene. As summarized in Table 2, the
complex Cr2 showed activities twice higher than analog Cr1 (213
vs. 109 kg molCr−1 −1). In addition, the substitution of hydrogen
h
on the pyrazole rings resulted in increased PE compositions (%) and
molecular weights of PE, consistent with the trends of substituent
effects shown in Table 1.
h
−1) depending on the ligand environment. Analysis of the data
indicated that the substitution of hydrogen on the pyrazole rings
(entry 1 vs. entries 2–6) greatly affected the product distribution,
leading to increases in PE compositions (%) and in the molec-
ular weights of PE, as determined by GPC measurements and
deduced by the melting points (Tm). This could have been due to
steric hindrance arising around the central Cr by the substituted
bis(pyrazolylmethyl) pyridine ligands, resulting in a reduction of
the rate of ␤-H transfer, giving the higher molecular weight PE.
Note that the presence of substituents at the 3-position of the
pyrazole rings afforded longer PE. For example, entries 3–6 employ-
ing L3–L6 with a substituent at the 3-position generated higher
molecular weight PE than L2 (entry 2), which has a hydrogen at
the 3-position of the pyrazoles. The effect of steric factors around
Further comparisons between the in situ precatalysts (Table 1)
and LCrCl3 complexes (Table 2) highlight other differences between
these Cr systems. GPC analysis of the polymers produced by Cr1
and Cr2 exhibited narrower polydispersities (Mw/Mn = 1.7–2.5)
and lower molecular weights (Mw = 1633–3528 g mol−1
)
compared to the in situ precatalysts (Mw/Mn = 5.0–20.0,
Mw = 10,935–58,874 g mol−1). As shown in Fig. 9, some of the
GPC traces of PE exhibited bimodal behavior, which was also
observed in the catalytic systems of [2,6-bis(2-benzimidazolyl)
pyridyl]chromium chlorides, [16] pyridinebis(imino) chromium
chlorides, [14] and [bis(pyridylmethyl) amine]chromium com-
plexes [13]. Based on our results, we propose the presence of two
Scheme 1. Synthesis of 2,6-Bis(pyrazol-1-ylmethyl) pyridine Derivatives.

J.O. Woo et al. / Journal of Molecular Catalysis A: Chemical 404–405 (2015) 204–210
209
Table 1
Effects of various ligands (L1–L6) on ethylene polymerization/oligomerizationa
Rꢀ
R”
Oligomer distribution (wt%)c
PE
Tm
Mw
Mn
e
e
Activityb
PDIe,f
Entry
R
Ligand
(wt%) (◦C)d
1-C6 (wt%)
1-C8 (wt%)
C10–C40 (wt%)
1
2
3
4
5
6
L1
L2
L3
L4
L5
L6
H
H
H
Me
H
H
H
H
Me
H
H
H
115
87
140
90
106
85
0.10
0.27
0.18
0.13
0.06
0.10
0.83
0.59
2.05
0.84
0.22
0.46
13.61
14.14
8.20
10.86
4.81
85.46 128.9
85.00 128.7
89.57 130.2
88.18 129.2
94.90 129.9
90.31 129.3
10,935
14,611
37,659
18,689
58,874
27,423
2175
2096
2452
2275
2943
2249
5.0
6.9
15.3
8.2
20.0
12.1
t Bu
Ph
CF3
MeO-Ph
H
H
9.13
a
General reaction conditions: toluene (50 mL), 10 ␮mol Cr(acac)3, 20 ␮mol ligand, 300 equivalent dMAO, 10 bar ethylene, 45 ◦C, 15 min.
b
c
d
e
f
In units of kg (mol Cr)−1 h−1
.
The % yield measured by GC-MS.
Determined by DSC.
Determined by high-temperature GPC.
Polydispersity (PDI) = Mw/Mn.
Table 2
Ethylene polymerization/oligomerization behavior of Cr1 and Cr2a.
e
e
Activityb
PDIe,f
Entry
Oligomer distribution (wt%)c
PE
(wt%)
Tm
(◦C)d
Mw
Mn
Complex
1-C6 (wt%)
1-C8 (wt%)
C10–C40 (wt%)
7
8
Cr1
Cr2
109
213
0.46
0.16
1.04
0.95
38.56
14.05
59.93
84.84
122.5
125.8
1633
3528
950
1378
1.7
2.5
a
General reaction conditions: toluene (50 mL), 10 ␮mol Cr complex, 300 equivalent dMAO, 10 bar ethylene, 45 ◦C, 15 min.
b
c
d
e
f
In units of kg (mol Cr)−1 h−1
.
The % yield measured by GC–MS.
Determined by DSC.
Determined by high-temperature GPC.
Polydispersity (PDI) = Mw/Mn.
active species in the in situ polymerization reaction: one species
that is activated from the ligand-binding Cr complex (and also
exists during the polymerization with Cr1 and Cr2), while the
other is associated with the Cr precursor, Cr(acac)3.
The NMR spectra demonstrate that all of the PEs are long, lin-
ear ␣-olefins. The 13C NMR spectrum of the PE obtained in Fig. S33
demonstrates that the PE sample is highly linear with the presence
of end vinyl groups. This is based on the observation of one sig-
nal from the methylene groups (–CH2–) in the 13C NMR spectrum,
which is evidence of the high linearity of the polymer.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molcata.2015
.04.007.
References
[1] K.H. Theopold, Eur. J. Inorg. Chem. (1998) 15–24.
[2] K.M. Smith, Curr. Org. Chem. 10 (2006) 955–963.
[3] R.D. Kohn, M. Haufe, S. Mihan, D. Lilge, Chem. Commun. (2000) 1927–1928.
[4] D.J. Jones, V.C. Gibson, S.M. Green, P.J. Maddox, Chem. Commun. (2002)
1038–1039.
[5] T. Xu, Y. Mu, W. Gao, J. Ni, L. Ye, Y. Tao, J. Am. Chem. Soc. 129 (2007)
2236–2237.
4. Conclusions
[6] F. Junges, M.C.A. Kuhn, A.H.D.P. dos Santos, C.R.K. Rabello, C.M. Thomas, J.-F.
Carpentier, O.L. Casagrande, Organometallics 26 (2007) 4010–4014.
[7] J. Al Thagfi, G.G. Lavoie, Organometallics 31 (2012) 2463–2469.
[8] B.L. Small, M. Brookhart, A.M.A. Bennett, J. Am. Chem. Soc. 120 (1998)
4049–4050.
[9] B.L. Small, M. Brookhart, J. Am. Chem. Soc. 120 (1998) 7143–7144.
[10] G.J.P. Britovsek, M. Bruce, V.C. Gibson, B.S. Kimberley, P.J. Maddox, S.
Mastroianni, S.J. McTavish, C. Redshaw, G.A. Solan, S. Strömberg, A.J.P. White,
D.J. Williams, J. Am. Chem. Soc. 121 (1999) 8728–8740.
[11] V.C. Gibson, C. Redshaw, G.A. Solan, Chem. Rev. 107 (2007) 1745–1776.
[12] M.A. Esteruelas, A.M. López, L. Méndez, M. Oliván, E. On˜ate, Organometallics
22 (2003) 395–406.
[13] M.J. Carney, N.J. Robertson, J.A. Halfen, L.N. Zakharov, A.L. Rheingold,
Organometallics 23 (2004) 6184–6190.
[14] B.L. Small, M.J. Carney, D.M. Holman, C.E. O’Rourke, J.A. Halfen,
Macromolecules 37 (2004) 4375–4386.
[15] A.K. Tomov, J.J. Chirinos, D.J. Jones, R.J. Long, V.C. Gibson, J. Am. Chem. Soc. 127
(2005) 10166–10167.
[16] W. Zhang, W.-H. Sun, S. Zhang, J. Hou, K. Wedeking, S. Schultz, R. Fröhlich, H.
Song, Organometallics 25 (2006) 1961–1969.
In summary, we synthesized and characterized new ligand
derivatives of 2,6-bis(pyrazol-1-ylmethyl) pyridine and the octa-
hedral Cr(III) complexes bearing the tridentate ligands in a mer
configuration, which were shown to be good precatalysts for ethy-
lene polymerization. In the presence of dMAO, these precatalysts
exhibited moderate activities (85–213 kg molCr−1 h−1) in ethylene
polymerization. The introduction of substituents on the pyrazole
rings increased PE compositions (%) and the molecular weights of
the PE. The results presented here demonstrate that the catalytic
performances are largely affected by the ligand substituents, indi-
cating that rational ligand modifications can lead to predictable
product properties. Further studies will focus on increasing our
understanding of the relationship between the activity and the
electronic effects of the ligands, and studying the influence of the
cocatalyst.
[17] L. Xiao, M. Zhang, W.-H. Sun, Polyhedron 29 (2010) 142–147.
[18] D. Gong, X. Jia, B. Wang, X. Zhang, L. Jiang, J. Organomet. Chem. 702 (2012)
10–18.
[19] S. Trofimenko, Chem. Rev. 93 (1993) 943–980.
[20] H.R. Bigmore, S.C. Lawrence, P. Mountford, C.S. Tredget, Dalton Trans. (2005)
635–651.
Acknowledgments
[21] C. Pettinari, R. Pettinari, Coord. Chem. Rev. 249 (2005) 525–543.
[22] D.P. Long, P.A. Bianconi, J. Am. Chem. Soc. 118 (1996) 12453–12454.
[23] S. Murtuza, O.L. Casagrande, R.F. Jordan, Organometallics 21 (2002)
1882–1890.
We gratefully acknowledge financial support from LG
Chem and the National Research Foundation of Korea (NRF-
2012R1A1A1005839).

210
J.O. Woo et al. / Journal of Molecular Catalysis A: Chemical 404–405 (2015) 204–210
[24] K. Michiue, R.F. Jordan, Macromolecules 36 (2003) 9707–9709.
[25] K. Michiue, R.F. Jordan, Organometallics 23 (2004) 460–470.
[26] J.T. Dixon, M.J. Green, F.M. Hess, D.H. Morgan, J. Organomet. Chem. 689 (2004)
3641–3668.
[27] R. Rojas, M. Valderrama, G. Wu, Inorg. Chem. Commun. 7 (2004) 1295–1297.
[28] I. García-Orozco, R. Quijada, K. Vera, M. Valderrama, J. Mol. Catal. A: Chem.
260 (2006) 70–76.
H.B. Ortega, P. Joskowics, Appl. Catal. A 280 (2005) 165–173.
[35] H. Abbo, S.J. Titinchi, Catal. Lett. 139 (2010) 90–96.
[36] R. Gupta, R. Mukherjee, Polyhedron 20 (2001) 2545–2549.
[37] A.K. Sharma, A. De, V. Balamurugan, R. Mukherjee, Inorg. Chim. Acta 372
(2011) 327–332.
[38] K. -s. Son, J.O. Woo, D. Kim, S.K. Kang, Acta Crystallographica Section E 70
(2014) o973.
[29] K. Michiue, R.F. Jordan, J. Mol. Catal. A: Chem. 282 (2008) 107–116.
[30] J. Zhang, A. Li, T.S.A. Hor, Organometallics 28 (2009) 2935–2937.
[31] J. Zhang, A. Li, T.S.A. Hor, Dalton Trans. (2009) 9327–9333.
[32] D. Zabel, A. Schubert, G. Wolmershäuser, R.L. Jones, W.R. Thiel, Eur. J. Inorg.
Chem. (2008) 3648–3654.
[39] K. -s. Son, J.O. Woo, D. Kim, S.K. Kang, Acta Crystallographica Section E 71
(2015) m75–m76.
[40] C.L. Foster, C.A. Kilner, M. Thornton-Pett, M.A. Halcrow, Polyhedron 21 (2002)
1031–1041.
[41] J. Hurtado, D.M.-L. Carey, A. Mun˜oz-Castro, R. Arratia-Pérez, R. Quijada, G. Wu,
[33] D.L. Reger, R.F. Semeniuc, M.D. Smith, Cryst. Growth Des. 5 (2005) 1181–1190.
[34] A.R. Karam, E.L. Catarí, F. López-Linares, G. Agrifoglio, C.L. Albano, A.
Díaz-Barrios, T.E. Lehmann, S.V. Pekerar, L.A. Albornoz, R. Atencio, T. González,
R. Rojas, M. Valderrama, J. Organomet. Chem. 694 (2009) 2636–2641.