Synthesis of new bis(amidine)-cobalt catalysts and their application to styrene polymerization

Article abstract of DOI:10.1021/om401183d

Two new bis(amidine) cobalt(II) complexes were rationally designed and efficiently synthesized. The first synthesized cobalt complex was based on a cyclic bis(amidine) ligand with chiral vicinal diphenyl groups. The structure was verified by single-crystal X-ray crystallography. As expected from the ligand structure, the Co-N bonds in this complex were significantly shorter than those found in the corresponding 1,2-diimine cobalt complex. This indicates that the bis(amidine) ligand has better electron-donating capability than the 1,2-diimine ligand. Upon test polymerization of styrene, the complex exhibited a moderate activity of 5.88 × 10⁵ g PS/(mol Co h). On the basis of this encouraging result, a new 1,2-diaminobenzene-derived bis(amidine) ligand was efficiently synthesized and used to make the corresponding cobalt(II) complex. When subjected to styrene polymerization, the resulting complex showed unusually high polymerization activity [164 × 10⁵ g PS/(mol Co h)] and high conversion (>99%). The resulting polymer was identified as atactic polystyrene by ¹³C NMR spectroscopy analysis. The significantly enhanced styrene polymerization activity of the 1,2-diaminobenzene-originated bis(amidine) cobalt(II) complex is attributed to the improved electron-donating capability of the ligand.

Full text of DOI:10.1021/om401183d

Article
pubs.acs.org/Organometallics
Synthesis of New Bis(amidine)−Cobalt Catalysts and Their
Application to Styrene Polymerization
Wonseok Cho,† Hyemin Cho,† Chun Sun Lee,‡ Bun Yeoul Lee,‡ Bongjin Moon,*,† and Jahyo Kang†,§
†Department of Chemistry, Sogang University, Seoul 121-742, South Korea
‡Department of Molecular Science and Technology, Ajou University, Suwon 443-749, South Korea
S
* Supporting Information
ABSTRACT: Two new bis(amidine) cobalt(II) complexes
were rationally designed and efficiently synthesized. The first
synthesized cobalt complex was based on a cyclic bis(amidine)
ligand with chiral vicinal diphenyl groups. The structure was
verified by single-crystal X-ray crystallography. As expected
from the ligand structure, the Co−N bonds in this complex
were significantly shorter than those found in the correspond-
ing 1,2-diimine cobalt complex. This indicates that the
bis(amidine) ligand has better electron-donating capability
than the 1,2-diimine ligand. Upon test polymerization of
styrene, the complex exhibited a moderate activity of 5.88 ×
105 g PS/(mol Co h). On the basis of this encouraging result, a new 1,2-diaminobenzene-derived bis(amidine) ligand was
efficiently synthesized and used to make the corresponding cobalt(II) complex. When subjected to styrene polymerization, the
resulting complex showed unusually high polymerization activity [164 × 105 g PS/(mol Co h)] and high conversion (>99%).
The resulting polymer was identified as atactic polystyrene by 13C NMR spectroscopy analysis. The significantly enhanced
styrene polymerization activity of the 1,2-diaminobenzene-originated bis(amidine) cobalt(II) complex is attributed to the
improved electron-donating capability of the ligand.
INTRODUCTION
■
Since Ziegler−Natta-type catalysts were developed, the field of
transition metal catalyzed olefin polymerization has continued
to grow worldwide in both industry and academics.1 Early on,
the heterogeneous catalytic system evolved into homogeneous
single-site catalytic systems focused on early transition metal
metallocene complexes.2a,b The research paradigm then shifted
to nonmetallocene late transition metal complexes after
Brookhart and co-workers reported the stunning catalytic
Figure 1. Design of a new styrene polymerization catalyst based on a
activity of new palladium(II)- and nickel(II)-based catalysts
bis(amidine) ligand.
with α-diimine ligands in 1995.3 The beauty of their catalyst
design lies in the easy synthesis and structural variation of
bidentate α-diimine ligands.4 In general, the electron density of
advantage of enhancing structural variation because various
substituents can easily be introduced on the exo-amino groups.
Despite their structural versatility, bis(amidine) ligand-based
nitrogen atoms in α-diimine ligands can be tuned by changing
substituents on the N-aryl rings. However, variation of the N-
aryl groups in Brookhart-type catalysts may have little effect on
the donacity of nitrogen chelating sites. This is because bulky
N-aryl groups are orthogonal to α-diimine groups, so there is
minimal delocalization of the lone-pair electrons of nitrogens to
the N-aryl groups.
transition metal olefin polymerization catalysts have rarely been
studied.5
In this paper, we report the synthesis of two new
bis(amidine) cobalt(II) complexes (6, 12) and their unusually
high activities in styrene polymerization.
We propose a new transition metal complex design based on
bis(amidine) ligands in which the electron-donating ability of
the ligand is enhanced compared to α-diimine-derived catalysts.
We also explored the catalytic activity of the new complexes in
RESULTS AND DISCUSSION
Synthesis of Chiral Bis(amidine) Cobalt(II) Complex
(6). Chiral bis(amidine) ligand 5 was initially designed and
■
olefin polymerization. As shown in Figure 1, bis(amidine) has
two exo-amino groups that could enhance the electron density
Received: December 10, 2013
of nitrogen when coordinated to metal. Bis(amidine) has the
© XXXX American Chemical Society
A
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synthesized to develop a catalyst for asymmetric Kumada-type
coupling reactions.6 The synthesis of chiral cobalt complex 6 is
described in Scheme 1. Commercially available 2-isopropyl
Scheme 1. Synthesis of Chiral Bis(amidine) Cobalt(II)
Complex 6
Figure 2. ORTEP drawing of 6 showing the atomic numbering
scheme. Hydrogen atoms are omitted for clarity.
2.2322(12) and 2.2386(12) Å, respectively, and the Co−N
bond distances are 2.013(3) and 2.025(3) Å. It is noteworthy
that the C1−N1−C12 angle (123.5(3)°) is significantly larger
than the corresponding C−N−C angle (115°) in α-diimine
cobalt complex A.9 This is mainly because the two 2-
isopropylphenyl rings are pushed away due to the two N-
methyl groups (C21 and C22). This implies that the two 2-
isopropylphenyl groups on the amidine nitrogens in 6 may
cause more shielding of the cobalt complex on the front side
than the two 2,6-diisopropylphenyl rings in A. In addition, the
two 2-isopropylphenyl rings on N1 and N2 are located
perpendicular to the plane of Co1−N1−C1−C2−N2, while the
two isopropyl groups are antiperiplanar to each other.
Styrene Polymerization with Catalyst 6. Since complex
6 was envisioned as a good catalyst for styrene polymerization,
test polymerization was conducted using 6 as a catalyst and
methylaluminoxane (MAO) as cocatalyst (Scheme 2). The
reaction conditions were set following typical literature
conditions.10a−d This catalyst exhibited a catalyst activity of
5.88 × 105 g PS/(mol Co h), which is a reasonably high activity
for the first trial.
Most reported styrene polymerization catalysts have been
based on nickel complexes, while cobalt complexes are rarely
investigated. For example, Wang and colleagues reported a
pyrazolylimine nickel catalyst with an activity up to 8.45 × 105 g
PS/(mol Ni h).10a The Li group also reported styrene
polymerization catalysts based on a β-diketiminate nickel(II)
complex10b showing activity up to 8.24 × 105 g PS/(mol Ni h).
The Gomes group studied α-diimine nickel(II) complexes for
styrene polymerization that exhibited activity up to 2.79 × 105 g
PS/(mol Ni h).10c,d We also prepared a bis(amidine) nickel
dichloride complex with ligand 5 (see Supporting Information),
which is analogous to cobalt catalyst 6, and tested its activity in
styrene polymerization. However, this nickel-based catalyst did
not show any styrene polymerization activity under the same
conditions.
aniline (1) was reacted with a half an equivalent of oxalyl
chloride in the presence of triethylamine to provide oxalamide
2 in 92% yield.7 Treatment of oxalamide 2 with PCl5 yielded
oxalimidoyl dichloride 3. Oxalimidoyl dichloride 3 was then
reacted with (1R,2R)-N1,N2-dimethyl-1,2-diphenylethane-1,2-
diamine (4) to provide the desired chiral bis(amidine) 5.
(1R,2R)-N1,N2-Dimethyl-1,2-diphenylethane-1,2-diamine (4)
was prepared following the literature procedure.8 Finally,
refluxing the mixture of chiral bis(amidine) 5 and CoCl2 in
CH2Cl2 provided the desired cobalt(II) complex 6 as sky blue
crystals [mp: decomposed at 315 °C].
X-ray Structure of 6. Single crystals of 6 suitable for X-ray
crystallography were obtained from CH2Cl2 solution by slow
evaporation. The molecular structure determined by single-
crystal X-ray diffraction is shown in Figure 2 as an ORTEP
drawing. Selected bond lengths and angles are listed in Table 1
along with analogous structural data extracted from Rosa’s 1,2-
diimine cobalt catalyst A9 to verify the improved donating effect
of the bis(amidine) ligand compared to that of the 1,2-diimine
ligand. The Co1−N distances in 6 (2.025(3) and 2.013(3) Å)
are significantly shorter than those in A (2.084(6) and 2.067(6)
Å). Conversely, N1−C1 and N2−C2 bond lengths in 6
(1.301(5) and 1.298(5) Å) are longer than those of A
(1.232(8) and 1.283(8) Å). These key structural features imply
that the bis(amidine) ligand gives more electron density to the
cobalt ion and is a better electron-donating ligand than 1,2-
diimine, as was envisioned. The cobalt bound to two amidine
nitrogens adopts tetrahedral geometry with two chlorides. The
Cl1−Co1−Cl2 angle is 116.00(5)°, while the N1−Co1−N2
bite angle is 81.12(13)°. The bond lengths of Co−Cl are
On the basis of these preliminary results, our attention
quickly shifted to the catalytic polymerization activity of cobalt
complex 6. The new bis(amidine) cobalt complex 12, an achiral
version of 6, was prepared as the polymerization catalyst, as
shown in Scheme 3.
B
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Table 1. Comparison of Selected Bond Lengths and Angles
of Rosa’s 1,2-Diimine Cobalt Complex A9 and Chiral
Bis(amidine) Cobalt(II) Complex 6
Scheme 2. Styrene Polymerization with Chiral Bis(amidine)
Cobalt(II) Complex 6
Scheme 3. Synthesis of Achiral Bis(amidine) Cobalt(II)
Complex 12
A
6
Bond Lengths (Å)
Co1−Cl1
Co1−Cl2
Co1−N1
Co1−N2
N1−C1
N2−C2
N3−C1
N4−C2
2.192(2)
2.227(2)
2.084(6)
2.067(6)
1.232(8)
1.283(8)
2.2386(12)
2.2322(12)
2.025(3)
2.013(3)
1.301(5)
1.298(5)
1.342(5)
1.339(5)
sired achiral bis(amidine) ligand 11 was prepared by
methylation of compound 10 with iodomethane and sodium
hydride in DMSO. Finally, refluxing a mixture of bis(amidine)
11 and CoCl2 in EtOH yielded cobalt(II) complex 12 as a
yellowish-green powder [mp: decomposed at 168 °C]. The
desired cobalt complex 12 was obtained in 45% overall yield via
five steps. This is significantly improved compared to the
synthesis of a chiral cobalt complex with a more expensive
chiral diamine (16% overall yield, four steps).
Bond Angles (deg)
Cl1−Co1−Cl2
N1−Co1−N2
C1−N1−C12
C2−N2−C3
C1−N1−C5
C2−N2−C14
115.15(9)
81.5(2)
116.00(5)
81.12(13)
123.5(3)
126.6(3)
115.8(6)
119.1(6)
Styrene Polymerization with Catalyst 12. Styrene
polymerization was attempted with the newly synthesized
achiral cobalt complex 12 under various conditions. Reactions
were initially carried out and monitored in an NMR tube. The
reactions were very fast, with some completed in 10 min and
most completed in less than 60 min (see Supporting
Information). To screen catalytic activity, the reactions were
performed in 3 g of styrene, and the catalyst concentration was
fixed at 16 μM. Toluene was chosen as a polymerization
solvent, and the results are summarized in Table 2.
As a control experiment, polymerization was attempted
without catalyst, but this attempt did not produce any polymer
(entry 1). The use of cocatalyst (MAO) was critical because
polymerization did not proceed without MAO even in the
presence of catalyst 12 (entry 2). Since the presence of both
Synthesis of Achiral Bis(amidine) Cobalt(II) Complex
(12). Since catalyst chirality is not essential for polymerization,
the chiral 1,2-diphenyl-1,2-diaminoethylene moiety in 6 was
replaced with achiral o-phenylenediamine. The bis(amidine)
structure was maintained by adopting a quinoxaline fragment,
and the synthetic scheme was devised so that synthesis was
economical and efficient. Commercially available o-phenyl-
enediamine (7) was reacted with a half an equivalent of oxalic
acid in the presence of aqueous HCl to afford oxalamide 8 in
98% yield. Treatment of oxalamide 8 with SOCl2 yielded 2,3-
dichloroquinoxaline 9.11 2,3-Dichloroquinoxaline 9 was then
reacted with 2-isopropylaniline (1) to afford the desired N2,N3-
bis(2-isopropylphenyl)quinoxaline-2,3-diamine (10). The de-
C
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a
Table 2. Styrene Polymerization with Catalyst 12 or A
b
b
c
entry
catalyst
temp (°C)
Al/Co [mol ratio]
yield (%)
Mw (kg/mol)
Mw/Mn
activity
1
no catalyst
rt
1000
0
0
0
0
2
12
12
12
12
12
12
12
12
A
rt
0
3
60
50
40
rt
1000
1000
1000
1000
800
95
13.0
12.0
13.5
15.6
13.5
16.7
3.61
3.07
5.55
3.07
3.59
2.08
159
154
164
164
159
4
92
5
98
6
98
7
rt
96
8
rt
600
7
11.7
9
rt
400
trace
trace
trace
10
60
1000
1000
11
A
rt
a
b
c
Typical polymerization conditions: styrene 3.0 g; MAO (10 wt % in toluene); 8 mL of toluene. Determined by GPC. In units of (105 g of PS)/
[(mol of Co) × h].
cobalt catalyst and cocatalyst MAO is essential for polymer-
ization, the molar ratio of aluminum in MAO and cobalt
catalyst 12 ([Al]/[Co]) was set at 1000. Initially, the reaction
was run at 60 °C (entry 3). Surprisingly, the polymerization
was too fast even at such a low catalyst loading and was
completed in less than 1 h. The resulting polystyrene (PS) was
isolated in 95% yield after precipitation in methanol. GPC
analysis of the resulting polystyrene showed that the weight
average molecular weight (Mw) was 13 000 g/mol and
polydispersity (Mw/Mn) was 3.61. Catalyst activity was
calculated to be 159 in units of 105 g of PS per 1 mol of
catalyst per 1 h. This activity is remarkably high for a
polystyrene polymerization catalyst.10a−d To optimize the
reaction temperature, polymerizations were performed at
lower temperatures (40 °C and rt; entries 5, 6). Surprisingly,
the reactions did not deteriorate even at room temperature, and
they provided polystyrene with almost the same molecular
weight, polydispersity, and catalyst activity.
Since polymerization was fast enough even at room
temperature, the reaction temperature was fixed at room
temperature for the remaining experiments to optimize the
amount of cocatalyst MAO (entries 7−9). Polymerization was
not significantly affected until the amount of MAO was reduced
to 800 [Al]/[Co] molar ratio. When the ratio was reduced to
400, the reaction was significantly affected, and only trace
amounts of polystyrene were obtained (entry 8). These
experiments showed that MAO cocatalyst is required with at
least 800 molar equiv of cobalt catalyst 12 to achieve
polymerization at room temperature.
izations. Each polymerization mechanism can lead to different
stereoregularities. In some cases, the polystyrene obtained by
transition metal catalyzed polymerization exhibits stereo-
selectivity showing enhanced tacticity control. For example,
titanium catalysts reported by Natta and co-workers yielded
isotactic polystyrene.12 Other titanium catalysts have led to the
synthesis of syndiotactic polystyrene.13a−e However, there are
also many examples in which metal complexes exhibit poor or
nonstereospecific polymerization. One example is the anilidoi-
mino nickel/MAO catalytic system.14 Although this catalyst was
efficient for styrene polymerization and a coordination
mechanism was proposed for the polymerization mechanism,
the resulting polymer was atactic. The tacticities of the
polystyrenes that were obtained with catalysts 6 and 12 were
1
also investigated by H NMR and 13C NMR spectroscopy.
Analysis of the spectra indicated that the polystyrenes obtained
by these catalysts were atactic polymers. Free radical polymer-
ization mechanisms were ruled out because a reaction that
occurred in the presence of radical inhibitor TEMPO (N-oxy-
2,2,6,6-tetramethylpiperidine) also provided polystyrene. In
addition, an ethylene polymerization attempt with catalyst 12
under comparable conditions (800 equiv of MAO, 50 bar
ethylene in toluene, 60 °C, 50 min) provided high-density
polyethylene (Tm = 138.3 °C) with a TON of 7720 h−1 (see
Supporting Information). We believe that this additional
experiment supports that polystyrene was formed via a
coordination polymerization mechanism rather than via a
radical mechanism. A more detailed and systematic study of
ethylene polymerization with catalyst 12 will be reported in due
course.
As a comparison, Rosa’s catalyst A was synthesized following
the literature procedure.9 Its catalytic activity was tested for
styrene polymerization under the same reaction conditions
(entries 10, 11). While catalyst A was reported to have some
catalytic activity for ethylene polymerization, it showed very
poor activity for styrene polymerization. Styrene polymer-
ization did not proceed with Rosa’s catalyst A even at 60 °C.
Styrene is one of only a few monomers that can be
polymerized through all known polymerization mechanisms,
including radical, anionic, cationic, and coordinated polymer-
CONCLUSIONS
■
Two new bis(amidine) ligand-based cobalt(II) complexes, 6
and 12, were efficiently synthesized. The structure of 6 was
verified by X-ray crystallography. The two complexes exhibited
excellent activity for styrene polymerization up to 164 × 105 g
PS/(mol Co h) in the presence of cocatalyst MAO.
Polymerization reactions proceeded very fast even at room
temperature with high conversion (>99%), which is beneficial
D
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yellow solution was cooled to rt, and the solvent evaporated to give a
brown solid. Crystallization from isopropyl alcohol gave the diamine
hydrochloride salt as a white crystalline solid. The salt was dissolved in
saturated potassium carbonate solution and extracted into CH2Cl2.
The organics were dried over potassium carbonate, filtered, and
concentrated in vacuo to give the desired product 4 a white solid (1.60
for energy savings. Since the corresponding cobalt complex
with a 1,2-diimine ligand did not exhibit styrene polymerization
activity, the high catalytic activity of these catalysts was
attributed to the enhanced electron donacity of the bis-
(amidine) ligand. A future study will be undertaken to develop
bis(amidine) ligand-based catalysts and screen catalyst activity
by varying the core metals and structures of ligands.
1
g, 52%). H NMR (500 MHz, CDCl3): δ 7.16−7.09 (m, 6H), 7.05−
7.02 (m, 4H), 3.57 (s, 2H), 2.47 (br s, 2H), 2.26 (s, 6H).
Synthesis of (N,N′Z,N,N′Z)-N,N′-((5R,6R)-1,4-Dimethyl-5,6-
diphenylpiperazine-2,3-diylidene)bis(2-isopropylaniline) (5).
Dimethyldiphenylethylenediamine (0.70 g, 2.91 mmol) and sodium
bicarbonate (1.47 g, 17.5 mmol) were dissolved in THF (23.1 mL).
The mixture was heated at 60 °C for 30 min, and N,N′-bis(2-
isopropylphenyl)oxalodiimidoyl dichloride (1.16 g, 3.20 mmol) in
THF (23.1 mL) was added dropwise with stirring at 60 °C. The
mixture was stirred at this temperature for a further 24 h. After cooling
to rt it was filtered, and the solvent was evaporated to give a yellow
solid. The resulting residue was purified by flash column
chromatography on silica gel to give 0.50 g of the desired chiral
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out with dry,
freshly distilled solvent under anhydrous conditions. Toluene was
dried over sodium/potassium alloy for 48 h and freshly distilled under
nitrogen before use. Styrene was purchased from Sigma-Aldrich, dried
over calcium hydride, and then freshly distilled under vacuum before
use. Methylaluminoxane (10 wt % Al) was generously provided by
Prof. Bun Yeoul Lee’s group at Ajou University. All reagents were
purchased at the highest commercial quality and used without further
purification, unless otherwise stated. All experiments involving
moisture- and/or air-sensitive compounds were performed in oven-
and/or flame-dried glassware with rubber septa under positive pressure
of argon using glovebox. Reactions were monitored by thin-layer
chromatography (TLC) carried out on a 0.25 mm Merck silica gel
plate (60F254) using UV light and p-anisaldehyde solution,
phosphomolybdic acid solution (PMA), and/or ninhydrin solution,
as a visualizing agent. Flash chromatography was performed using
Merck 230−400 mesh silica gel. Melting points were determined using
an electrothermal capillary melting point apparatus and are
uncorrected. Infrared spectra were obtained on a Nicolet Avatar FT-
IR spectrometer. NMR spectra were obtained either on Varian Inova-
500 (500 MHz for 1H, 125 MHz for 13C) or Varian 400 (400 MHz for
1
bis(amidine) ligand 5 (34%). Mp: 95−98 °C. H NMR (400 MHz,
CDCl3): δ 7.43−6.64 (br m, 18H), 4.52 (br s, 2H), 3.40−2.60 (br,
5H), 2.20−1.70 (br, 3H), 1.22−0.68 (br, 12H). 13C NMR (100 MHz,
CDCl3): δ 146.09, 142.05, 139.55, 137.70, 129.24, 128.28, 126.54,
124.95, 121.74, 118.99, 68.14, 37.51, 28.19, 26.43, 22.78. Anal. Calcd
for C36H40N4: C, 81.78; H, 7.63; N, 10.6. Found: C, 81.65; H, 7.64; N,
10.57.
Synthesis of Chiral Bis(amidine) Cobalt(II) Complex (6).
Isopropyl-bis(amidine) ligand (0.63 g, 1.20 mmol) and anhydrous
cobalt chloride (0.15 g, 0.93 mmol) were dissolved in CH2Cl2 (49.8
mL). The mixture was stirred at rt for 12 h, filtered, and concentrated
in vacuo to give a sky blue solid. A solid was obtained either by
recrystallization from CH2Cl2 or precipitation in CH2Cl2/ether to give
0.43 g of the desired complex 6 (70%). Mp: 315−318 °C dec. Anal.
Calcd for C36H40Cl2CoN4: C, 65.66; H, 6.12; N, 8.51. Found: C,
65.76; H, 6.07; N, 8.32.
Synthesis of 2,3-Dihydroxyquinoxaline (8). A solution of oxalic
acid (13.6 g, 108 mmol) in 4 N aqueous HCl (25.0 mL) was added to
a solution of 1,2-diaminobenzene (10.6 g, 97.7 mmol) in 4 N HCl
(75.0 mL), and the resulting solution was heated at reflux for 2 h. The
reaction mixture was cooled to ambient temperature, and the resulting
precipitate was isolated by filtration, washed with water, and dried,
giving 15.5 g (98%) of 8 as an off-white powder. 1H NMR (400 MHz,
DMSO-d6): δ 11.92 (s, 2H), 7.15−7.05 (m, 4H). 13C NMR (100
MHz, DMSO-d6): δ 155.17, 125.55, 122.93, 115.10
Synthesis of 2,3-Dichloroquinoxaline (9). N,N-Dimethylforma-
mide (0.01 mL, 0.19 mmol) was added dropwise to slurry of 2,3-
dihydroxyquinoxaline (3.00 g, 18.5 mmol) and thionyl chloride (3.50
mL, 48.1 mmol) in 1,2-dichloroethane (31.0 mL). The resulting
reaction mixture was heated to reflux for 2 h, then concentrated to
dryness. The residue was dissolved in 1,2-dichloroethane (35.0 mL)
and concentrated to dryness. The resulting solid was recrystallized
from CH3CN/H2O, giving 3.50 g (96%) of 9 as an off-white needles.
1H NMR (400 MHz, CDCl3): δ 8.12−8.08 (m, 2H), 7.99−7.94 (m,
1H, 100 MHz for 13C) NMR spectrometers. H NMR spectra were
1
referenced to tetramethylsilane (δ 0.00 ppm) as an internal standard
and are reported as follows: s = singlet, d = doublet, t = triplet, q =
quartet, m = multiplet. 13C NMR spectra were referenced to the
residual CDCl3 peak (δ 77.16 ppm). Elemental analyses were
performed by the Organic Chemistry Research Center (OCRC) at
Sogang University using a Carlo Erba EA 1180 elemental analyzer.
Synthesis of N1,N2-Bis(2-isopropylphenyl)oxalamide (2). 2-
Isopropylaniline (1, 5.24 mL, 37.0 mmol) and triethylamine (5.16 mL,
37.0 mmol) were dissolved in CH2Cl2 (18.0 mL), and the solution was
cooled to 0 °C. A solution of oxalyl chloride (1.42 mL, 16.8 mmol) in
CH2Cl2 was added dropwise to the mixture with stirring, and ice bath
cooling was applied over 1 h. The mixture was then allowed to stir at rt
for 56 h. The mixture was transferred to a separatory funnel and then
washed with water (∼50.0 mL). The mixture was extracted with
CH2Cl2 three times. The combined organic layers were evaporated to
give a white solid. A solid was recrystallized from isopropyl alcohol to
1
give 5.00 g of the desired oxalamide 2 (92%). H NMR (400 MHz,
CDCl3): δ 9.52 (s, 2H), 8.05 (d, 2H, J = 4 Hz), 7.28−7.23 (m, 4H),
3.15 (m, 2H), 1.32 (d, 12H, J = 4 Hz).
Synthesis of (1Z,2Z)-N′1,N′2-Bis(2-isopropylphenyl)-
oxalimidoyl Dichloride (3). N1,N2-Bis(2-isopropylphenyl)oxalamide
(2.00 g, 5.67 mmol) and phosphorus pentachloride (3.31 g, 12.5
mmol) were dissolved in toluene (8.42 mL). The mixture was heated
at reflux over 30 min, then maintained at reflux for another 5 h to
afford a clear yellow solution. The mixture was allowed to cool to rt
and filtered, and the solvent evaporated to give a yellow solid. The
resulting residue was purified by flash column chromatography on
silica gel to give 1.50 g of the desired oxalimidoyl dichloride 3 (75%).
1H NMR (400 MHz, CDCl3): δ 7.39−7.37 (m, 2H), 7.28−7.25 (m,
2H). 13C NMR (100 MHz, CDCl3) δ 144.57, 139.97, 131.71, 127.84.
Synthesis of N2,N3-Bis(2-isopropylphenyl)quinoxaline-2,3-
diamine (10). Under a nitrogen atmosphere, 2,3-dichloroquinoxaline
(1.00 g, 5.00 mmol) and 2-isopropylaniline (3.50 mL, 25.0 mmol) in a
sealed tube were heated at 150 °C for 2 h. The resulting residue was
purified by flash column chromatography on silica gel, and then the
desired product was obtained in 1.63 g (82%) yield as a yellow
powder. Mp: 113−115 °C. 1H NMR (400 MHz, CDCl3): δ 9.10 (br s,
1H), 8.60 (br s, 1H), 7.65 (br s, 1H), 7.33−6.76 (br m, 10H), 6.29 (br
s, 1H), 3.07 (br s, 2H), 1.10 (br d, 12H, J = 40 Hz). 13C NMR (100
MHz, CDCl3): δ 146.46, 144.33, 141.50, 137.67, 137.21, 136.06,
132.54, 127.10, 126.88, 126.33, 126.15, 125.30, 124.48, 123.57, 122.95,
121.21, 120.34, 113.35, 28.42, 28.13, 23.02, 22.74.
2H), 7.00−6.97 (m, 2H), 3.10 (m, 2H), 1.25 (d, 12H, J = 4 Hz). 13C
NMR (100 MHz, CDCl3): δ 143.53, 139.90, 137.89, 127.08, 126.76,
126.22, 126.09, 125.92, 125.79, 118.93, 118.67, 29.14, 28.98, 23.07,
22.90, 22.75, 22.59.
Synthesis of (1R,2R)-N1,N2-Dimethyl-1,2-diphenylethane-
1,2-diamine (4). (4R,5R)-1,3-Dimethyl-2,4,5-triphenyl-1,3,2-diaza-
phospholidine 2-oxide (4.70 g, 13.0 mmol)8 was dissolved in methanol
(27.0 mL), and 0.8 M HCl/methanol (27.0 mL) was added to the
solution. The resulting mixture was heated at reflux for 1 day. The pale
Synthesis of (N,N′Z,N,N′Z)-N,N′-(1,4-Dimethylquinoxaline-
2,3(1H,4H)-diylidene)bis(2-isopropylaniline) (11). To a solution
of N2,N3-bis(2-isopropylphenyl)quinoxaline-2,3-diamine (2.40 g, 6.00
mmol) in DMSO (50.0 mL) was added NaH (0.70 g, 30.0 mmol), and
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dx.doi.org/10.1021/om401183d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the reaction mixture was stirred at rt for 10 min. Methyl iodide (1.90
mL, 30.0 mmol) was added, and the reaction mixture was stirred at rt
for 1 h. The reaction was quenched by careful addition of MeOH and
diluted with an ice and water mixture. The product was then extracted
with methylene chloride, and the combined organic layers were
washed with brine, dried over MgSO4, filtered, and concentrated in
vacuo. The crude residue was purified by flash column chromatog-
raphy on silica gel to give the desired product in 1.70 g (67%) yield as
a dark yellow powder. Mp: 139−141 °C. 1H NMR (400 MHz,
CDCl3): δ 7.06−6.89 (br m, 10H), 6.42 (br s, 2H), 3.31 (br, 8H), 0.97
(br s, 12H). 13C NMR (100 MHz, CDCl3): δ 145.84, 140.03, 136.73,
130.73, 129.51, 125.54, 125.24, 122.89, 121.72, 118.93, 115.15, 112.72,
105.92, 32.96, 27.75, 23.05.
(4) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169−1203. (b) MacKenzie, P. B.; Moody, L. S.; Ponasik, J. A.;
Farthing, A. K. U.S. Patent 0077809, 2004.
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(6) Lou, S.; Fu, G. C. J. Am. Chem. Soc. 2010, 132, 1264−1266.
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Soc. 2007, 28, 1127−1134.
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158.
́
(9) Rosa, V.; Carabineiro, S. A.; Aviles, T.; Gomes, P. T.; Welter, R.;
Campos, J. M.; Ribeiro, M. R. J. Organomet. Chem. 2008, 693, 769−
775.
Synthesis of Bis(amidine) Cobalt(II) Complex (12).
(N,N′Z,N,N′Z)-N,N′-(1,4-Dimethylquinoxaline-2,3-(1H,4H)-
diylidene)bis(2-isopropylaniline) (0.20 g, 0.47 mmol) and anhydrous
cobalt chloride (0.06 g, 0.46 mmol) were dissolved in EtOH (10.0
mL). The reaction mixture was heated at reflux for 12 h, filtered, and
concentrated in vacuo to give a green solid. The resulting solid was
recrystallized from CH2Cl2/hexanes, giving 0.20 g (88%) of dark green
needles. Mp: 168−175 °C. Anal. Calcd for C28H32Cl2CoN4: C, 60.66;
H, 5.82; N, 10.11. Found: C, 60.52; H, 5.92; N, 9.97.
Polymerization of Styrene. The polymerizations of styrene were
conducted in 25 mL Schlenk tubes that were flamed under vacuum
and backfilled with argon. The desired amounts of solvent (toluene),
styrene, and catalyst suspension in the solvent (16 μM) were injected
in this order via a syringe. The polymerization was then initiated by
addition of MAO in toluene via a syringe. After the desired reaction
time, the reaction mixture was dripped into the mixture of
concentrated hydrochloric acid and methanol [5:95 (v/v)] to
precipitate polystyrene. The white precipitates were isolated by
filtration, washed with methanol, and dried under vacuum at 60 °C to
give polystyrene.
(10) (a) Wang, Y. Y.; Li, B. X.; Zhu, F. M.; Gao, H. Y.; Wu, Q. J.
Appl. Polym. Sci. 2012, 125, 121−125. (b) Li, Y.; Gao, M.; Wu, Q.
Appl. Organomet. Chem. 2008, 22, 659−663. (c) Li, L.; Gomes, C. S.
B.; Gomes, P. T.; Duarte, M. T.; Fan, Z. Dalton Trans. 2011, 40,
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Diogo, H. P.; Ascenso, J. R. Eur. Polym. J. 2011, 47, 1636−1645.
(11) Romer, D. R. J. Heterocycl. Chem. 2009, 46, 317−319.
(12) Mani, R.; Burns, C. M. Macromolecules 1991, 24, 5476−5477.
(13) (a) Ishihara, N.; Seimiya, T.; Kuramoto, M.; Uoi, M.
Macromolecules 1986, 19, 2464−2465. (b) Zambelli, A.; Longo, P.;
Pellecchia, C.; Grassi, A. Macromolecules 1987, 20, 2035−2037.
(c) Ishihara, N.; Kuramoto, M.; Uoi, M. Macromolecules 1988, 21,
3356−3360. (d) Po, R.; Cardi, N. Prog. Polym. Sci. 1996, 21, 47−88.
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914.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data of 6 including CIF files, 1H and 13
C
1
NMR spectra of 5, 10, and 11, H NMR monitoring data of
styrene polymerization with catalyst 12, 13C NMR of the
polystyrene obtained by catalyst 12, synthetic procedure of
chiral bis(amidine) Ni(II) complex, and ethylene polymer-
ization procedure with catalyst 12. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: bjmoon@sogang.ac.kr.
Notes
The authors declare no competing financial interest.
§Deceased on Nov 9, 2012.
ACKNOWLEDGMENTS
■
This work was supported by the Sogang University Research
Grant of 2010 (201010032.01) and by the National Research
Foundation of Korea (NRF) grant funded by the Korean
government (MOE) (NRF-2012R1A1A2009532).
REFERENCES
■
(1) Handbook of Transition Metal Polymerization Catalysts; Hoff, R.,
Mathers, R. T., Eds.; John Wiley & Sons, 2010.
(2) (a) Chen, E. Y.-X. Chem. Rev. 2009, 109, 5157−5214. (b) Hlatky,
G. G. Chem. Rev. 2000, 100, 1347−1376.
(3) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414−6415.
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dx.doi.org/10.1021/om401183d | Organometallics XXXX, XXX, XXX−XXX