Synthesis, characterization, and MMA polymerization activity of tetrahedral Co (II) complex bearing N, N-bis(1-pyrazolyl)methyl ligand based on aniline moiety

Article abstract of DOI:10.1016/j.inoche.2010.10.019

A new series of air-stable Co(II) complexes, [bpmaL1]CoCl₂ (1a), [bpmaL2]CoCl₂ (2a), [bpmaL3]CoCl₂ (3a), and [bpmaL4]CoCl₂ (4a), where, for example, [bpmaL1 = N,N-bis{(1-pyrazolyl)methyl}aniline], has been synth

Full text of DOI:10.1016/j.inoche.2010.10.019

Inorganic Chemistry Communications 14 (2011) 189–193
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization, and MMA polymerization activity of tetrahedral Co (II)
complex bearing N, N-bis(1-pyrazolyl)methyl ligand based on aniline moiety
Minkyu Yang a, Won Jin Park a, Keun Byoung Yoon b, Jong Hwa Jeong a, Hyosun Lee a,
⁎
a
Department of Chemistry, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu-city, 702-701, Republic of Korea
b
Department of Polymer Science and Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu-city, 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of air-stable Co(II) complexes, [bpmaL1]CoCl2 (1a), [bpmaL2]CoCl2 (2a), [bpmaL3]CoCl2 (3a),
and [bpmaL4]CoCl2 (4a), where, for example, [bpmaL1=N,N-bis{(1-pyrazolyl)methyl}aniline], has been
synthesized. X-ray crystallography of 2a indicated that it has a dipper-type structure in which the aniline
moiety plays the role of the handle with a tetrahedral cobalt located at the edge with an eight-membered
chelate ring formation. However, there is no indication that the metal is coordinated to the nitrogen of the
aniline group. Moreover, complexes showed the highest catalytic activity in the polymerization of methyl
methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) with an activity of
1.14×106 g PMMA/mol-Co·h at 60 °C and syndiotacticity of PMMA, characterized by 13 C NMR spectroscopy,
which value was ca. 0.57.
Received 14 September 2010
Accepted 21 October 2010
Available online 28 October 2010
Keywords:
N,N-bis(1-H-Pyrazole-1-yl)
Tetrahedral Co (II) complex
Methyl methacrylate
PMMA
© 2010 Elsevier B.V. All rights reserved.
Late transition metal complexes with N,N-chelate ligands [1] have
attracted considerable attention due to their catalytic actions since
the pioneering studies of Keim and Fink [2] and Brookhart et al.
reported the discovery of α-diimine complexes of Ni (II) and Pd (II)
catalysts for the polymerization or oligomerization of α-olefins [3–5].
Subsequently, several structural variations of diimines have also been
reported allowing adjustment of the steric and electronic properties of
the N,N-chelate ligands. These include β-diimines and analogs the Ni(II)
and Pd(II) complexes of which usually exhibit lower catalytic activities
than the α-diimine complexes [6]. Specifically, the highly active 2,6-bis
(imino)pyridines initially discovered independently by the groups of
Brookhart and Gibson [7–11] are well known cobalt and iron systems
[12–16]. A further modification involves appending a donor side arm to
construct tridentate chelating ligands, such as [N,O], [N,N,N], and [N,N,O]
ligands [17,18].
range of pyrazoles enables optimization of the electrophilic properties
of transition metal complexes of pyrazoles and their derivatives for
use as catalysts for olefin polymerization. Thus, many pyrazoles and
their transition metal complexes in different oxidation states have
been isolated and structurally characterized, because of their diverse
potential applications, including roles as industrial catalysts, such as
in olefin oligomerization or polymerization [39,40], as bioinorganic
materials in pharmaceutical preparations [41–45], and as metal ion
extractants [46].
These interesting features prompted us to explore the present
system involving the N,N-bispyrazolylmethyl ligand, based on an
aniline moiety [47,48] and their Co complexes to investigate the steric
and electronic influence of substituents on the aniline ring in
formation of complex and catalytic activity in both α-olefin and
methyl methacrylate polymerization.
Moreover, despite many reports on nitrogen-based metal com-
plexes [19,20], little is known about pyrazole-based ligand late
transition metal complexes as catalysts for olefin polymerization
[21–24] and methyl methacrylate polymerization,[25–27] in compar-
ison with early transition metal complexes [20,28–31]. In particular,
pyrazoles, which are among the fundamental heterocycles, have been
extensively used as N-donor ligands to metals since the first review of
pyrazole-derived ligands in 1972 [32–34]. Pyrazoles and their
derivatives are attractive ligands because their steric and electronic
properties can be fine-tuned by the appropriate choice of substitu-
tions on the 2-N, 3-C, 4-C, and 5-C atoms of pyrazole [35–38]. The
Ligands [bpmaL1 - bpmaL4], were prepared in facile forms with
high yields by dehydration of the alcoholic moiety of 1H-pyrazole-1-yl
methanol,{R. Hüttel, 1952 #348} which readily undergoes sublima-
tion, with aniline or substituted anilines, such as 2,6-diispropylaniline
and 2,4,6-trimethylaniline. Solid ligands [bpmaL2–bpmaL4] and
liquid ligand [bpmaL1] were very hydroscopic, with decomposition
to the starting materials at certain time periods [49]. Synthesis of
bidentate [N,N]-cobalt (II) dichloride complexes, 1a–4a was straight-
forward by reaction of the cobalt metal halide, i.e., CoCl2·6H2O with
the corresponding ligands [bpmaL1–bpmaL4], to give them in high
yield and high purity (Scheme 1) [50]. The bulkiness of the aniline
moiety had no effect on the synthesis of ligands or complexes. Ligands
and their Co complexes were characterized using various spectro-
scopic techniques, such as NMR with 1H-1H COSY and 13C-1H HMQC,
IR, elemental analysis, and UV/Vis spectroscopy. All Co(II) complexes
⁎
Corresponding author.
E-mail address: hyosunlee@knu.ac.kr (H. Lee).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.10.019

190
M. Yang et al. / Inorganic Chemistry Communications 14 (2011) 189–193
R1
R1
R1
HO
R2
R2
1,2-C2H4Cl2/80oC
or MeCN/rt
.
CoCl2 6H2O
R2
R2
N
R3
N
N
+
N
EtOH/rt
R3
R3
R3
R3
N
N
R2
R2
N
N
N
N
N
N
R3
M
NH2
R3
Cl
R3
R3
R3
Cl
R1, R2, R3 = H [bpmaL1]
R1, R2 = Me, R3 = H [bpmaL2]
R1 = H, R2 = CHMe2, R3 = H [bpmaL3]
[bpmaL1]CoCl2 (1a)
[bpmaL2]CoCl2 (2a)
[bpmaL3]CoCl2 (3a)
[bpmaL4]CoCl2 (4a)
R1 = H, R2 = CHMe2, R3 = Me [bpmaL4]
Scheme 1. Synthesis of [bpmaL]CoCl2.
were deeply colored, ranging from dark blue to sky blue, were stable
to both heat and moisture, and were paramagnetic, preventing useful
NMR analysis. The complex 2a was further characterized by X-ray
crystallography. A single crystal of 2a was obtained by slow evaporation
of the compound in toluene at room temperature. X-ray crystallography
of complex 2a indicated that it has a dipper-type structure in which the
aniline moiety played the role of the handle with the tetrahedral cobalt
located at the edge [51]. The ORTEP representation of complex 2a with
the atom labeling for non-hydrogen atoms with some selected bond
angles and bond lengths is shown in Fig. 1. The metal center is bonded to
the ligand through the nitrogen atoms of the pyrazolyl moiety in a
bidentate manner, forming an eight-membered chelate ring. The
geometry around the [CoN2Cl2] core has a slightly distorted tetrahedral
configuration. Geometric distortion from ideal angles can be explained
by the need to accommodate the bulky eight-membered chelate ring.
Such eight-membered chelate rings are rare, primarily because of the
destabilization caused by non-bonding interactions within the eight-
membered ring due to unfavorable torsion angles imposed by the ring
size. The flexibility within the pyrazolyl backbone, the presence of
suitably positioned sterically demanding substituents, and heteroatoms
probably contribute to the stability of the eight-membered chelate ring.
The bond length between Co–N(1) and Co–N(5) are mutually equal and
comparable to similar [N,N-bis{(3,5-dimethylpyrazol-1-ylmethyl}ami-
nobenzene]Co(II) complexes [51]. Although the distance of N(3)–Co(1)
was 3.482 (4) in complex 2a was shorter by ca. 0.4 Å than that in the
comparable [N,N-bis{(3,5-dimethylpyrazol-1-ylmethyl}aminobenzene]
Co(II) complex, there was no indication of bond formation between the
cobalt ion and the nitrogen of the aniline moiety [52,53]. For
comparison, the bond distance between the cobalt ion and
the nitrogen of the pyridine was 2.051(3)Å in the pentacoordinate-
distorted square pyramidal bis(imino)pyridine-Co(II) complex [54,55].
The bite angle of N(5)-Co(1)-N(1) is 107.69(14)° and is slightly smaller
than the ideal tetrahedral angle. The Co–Cl bond lengths are similar and
comparable to the other tetrahedral cobalt (II) complexes with a
chromophore group [CoCl2N2]. The ligand–metal–ligand angle differed
from the ideal value of 109.5°, varying between 116.14(6)° [Cl(1)-Co-Cl
(2)] and 110.50(12)° [N(1)-Co-Cl(2)], showing a slightly distorted
tetrahedral configuration.
C16
C12
C13
C11
C14
C10
The electronic absorption spectra of ligands and Co complexes in
toluene appear to be mainly of an intraligand character (Fig. 2). For
example, ligand [bpmaL1] showed two absorption bands at
λ1 =267 nm and λ2 = 284 nm with molar absorption coefficients of
3604 cm−1 M−1 and 5174 cm−1 M−1, respectively, which were
attributed to π–π* interactions of the aniline and pyrazolyl moiety.
The absorption spectra of cobalt complex 1a showed 3 bands at
almost λ1 = 262 nm, λ2 =285 nm, and λ3 = 608 nm with molar
absorption coefficients of 10155 cm−1 M−1, 15074 cm−1 M−1, and
263 cm−1 M−1, respectively. The absorption band of the pyrazole
moiety showed a slight blue shift, due to coordination of the pyrazole
to the cobalt metal, while the absorption band of the aniline moiety
was very similar to that of the ligand, indicating that the aniline
moiety did not affect coordination to the cobalt ion. Moreover, it was
concluded from the spectra that the substituents on the aniline
moiety (i.e. methyl and isopropyl groups) had no effect on λmax or the
intensity of the absorption band. The spectra of the complexes
showed distinct d–d transitions, which may be assigned to the
transition from 4A2 to 4 T1 (p) [56].
C17
C15
C9
C6
C5
C4
C3
N4
N3
C7
N2
N1
C2
N5
Co1
C8
C1
Cl2
Cl1
Fig. 1. Molecular structure of 2a. Selected bond lengths (Å) and angles (o) are Co(1)-N
(5) 2.011 (4), N(4)-C(6) 1.334 (5), Co(1)-Cl(2) 2.228 (1), N(5)-C(8) 1.337 (6), Co(1)-N
(3) 3.482 (4), N(4)-N(5) 1.348 (5), N(2)-C(4) 1.454 (6), C(1)-C(2) 1.377 (8), N(3)-C(5)
1.424 (6), C(2)-C(3) 1.356 (9), N(3)-C(9) 1.438 (5), C(6)-C(7) 1.337 (7), N(5)-Co(1)-N
(1) 107.69 (14), N(5)-N(4)-C(5) 121.34 (4), N(1)-Co(1)-Cl(2) 110.50 (12), Cl(2)-Co
(1)-Cl(1) 118.4 (4), N(1)-Co(1)-Cl(1) 105.29 (12), C(5)-N(3)-C(9) 118.69(5), N(5)-Co
(1)-N(3) 61.54 (12), C(1)-N(1)-Co(1) 105.0 (5), N(4)-N(5)-Co(1) 126.73 (3), C(3)-C
(2)-C(1) 120.0(3), Cl(2)-Co(1)-N(3) 89.25 (7), N(2)-C(3)-C(2) 109.5 (5), N(2)-N(1)-
Co(1) 126.63 (3), N(3)-C(4)-N(2) 114.5 (4).
The chemical properties paralleled those of Brookhart and
Gibson's N-arylbis(amino)pyridyl Co and Fe complexes, which are
known to be pentacoordinated complexes. However, tetrahedral Co
complexes 1a–4a were very inert to ethylene polymerization or

M. Yang et al. / Inorganic Chemistry Communications 14 (2011) 189–193
191
0.35
0.12
0.10
0.08
0.06
0.04
0.02
0.00
267, 285
286
[bpmaL1]
[bpmaL2]
[bpmaL3]
[bpmaL4]
[bpmaL1]CoCl2
[bpmaL2]CoCl2
[bpmaL3]CoCl2
[bpmaL4]CoCl2
0.30
0.25
287
0.20
285
267, 280
0.15
260, 281
267, 284
285
0.10
0.05
0.00
611
609
607
608
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. UV absorption spectra of ligands [bpmaL] and complexes [bpmaL]CoCl2.
ethylene and 1-octene copolymerization even at an ethylene pressure
of 30 bar in toluene media at various temperatures, unlike bis(imino)
pyridine-Co(II) complex, which shows high catalytic activity for
ethylene polymerization and α-olefin oligomerization. To confirm the
catalytic inertness for olefin oligomerization, dimerization of 1-octene
in toluene was also attempted at 30 °C and 80 °C with no noticeable
result. As the tetrahedral benzo[b]thiophen-2-yl-substituted (imino)
pyridine-Co(II) complexes [57] were reported to have catalytic activity
for oligomerization of ethylene to linear α-olefins, it would not be
appropriate to attribute the lack of catalytic activity of precursor
complexes 1a–4a to their structural properties. Additionally, nocatalytic
activity for olefin polymerization was demonstrated indirectly, by the
failure of bis-hydrocarbyl substituted Co derivative synthesis. Although
alkyl derivatives of the complex cobalt (II) amidodiphosphine halide are
known to be stable [58], the synthesis of dihydrocarbyl derivatives of the
complex 1a–4a by changing the polarity of reagents and solvents at
various temperatures was unsuccessful. The reactions were either
unreactive in less polar environments or decomposed in more polar
surroundings. However, all complexes could be activated withMMAO to
polymerize methyl methacrylate (MMA) to give PMMA with Tg from
110 °C to 120 °C [59–62]. The results of polymerization, including the
influence of temperature and tacticity, are summarized in Table 1 [63].
To confirm the catalytic activity of MMA polymerization, blank
polymerization of MMA was performed with starting Co complex,
CoCl2·6H2O, and MMAO themselves at the given temperature. The
activity of 4a (1.14×106 g/mol-Co·h) was highest, while those of 2a
(8.00×105 g/mol-Co·h) and 3a (6.73×105 g/mol-Co·h) were only
moderate and that of 1a (6.17×104 g/mol-Co·h) was the lowest at
60 °C. These observations indicated that the bulkier the substituents on
the aniline moiety – especially the pyrazole ring – making a crowded
environment around the Co metal center, the higher the catalytic
activity for MMA polymerization. In contrast, high molecular weight
(1.13×106 g/mol) and narrow polydispersity index (PDI = 1.75) were
achieved with the catalyst 1a at 60 °C. Moreover, with increasing
polymerization temperature, the catalytic activity was markedly
increased for the precatalyst complex 4a. Although syndiotacticity of
PMMA, which was calculated by 13 C NMR of PMMA in CDCl3 at 50 °C,
increased as the polymerization temperature increased when using
catalyst 4a, any syndiotacticity correlation with the structural relation-
ship of complexes was obscure [64]. The highest syndiotacticity of 57%
was achieved with catalyst 4a at Tp = 50 °C.
Further detailed analyses of MMA polymerization, including analysis
of tacticity, the catalyst structure relationship, and mechanistic aspects
of active species are currently underway in our laboratory.
Acknowledgments
Thanks are due to Dr. Yeon Kyu Lee from Instrumental Analysis
Center (IAC) facility at Yeungnam University for technical support.
This research was supported by Basic Science Research Programs
Table 1
Polymerization of MMA by [bpmaL]CoCl2 in the presence of MMAO.
d
Entry
Catalysta
Temp.
(°C)
Yieldb
(g)
Activityc
Tg
Tacticity
%mm
Mw
Mw/Mn
(g/mol-Co.h)×105
(°C)
%mr
8.46
10.92
3.81
39.06
32.47
12.16
2.84
52.47
43.11
27.08
%rr
(g/mol )×105
1
2
3
4
5
6
7
8
9
CoCl2·6H2Oe
60
60
0
25
40
50
60
60
60
60
0.80
0.42
0.21
0.21
0.67
1.20
3.43
2.02
2.40
1.85
0.266
0.140
0.07
116.54
119.61
118.35
118.13
115.59
117.78
109.91
118.80
117.68
121.41
4.72
37.20
93.64
60.86
64.63
39.85
41.35
45.93
47.62
55.94
86.82
51.88
2.55
0.08
2.90
47.99
55.81
1.59
9.27
16.98
10.9
6.78
0.64
1.94
0.90
13.4
9.67
0.40
10.6
11.3
1.82
2.09
2.41
2.35
1.47
1.62
2.31
3.11
2.32
1.75
MMAOf
[bpmaL4]CoCl2
[bpmaL4]CoCl2
[bpmaL4]CoCl2
[bpmaL4]CoCl2
[bpmaL4]CoCl2
[bpmaL3]CoCl2
[bpmaL2]CoCl2
[bpmaL1]CoCl2
0.07
0.223
0.400
1.143
0.673
0.800
0.617
10
a
[Co (II) catalyst]0 =15 μmol, and [MMA]0/[MMAO]0/[Co (II) catalyst]0 =3100:500:1.
Yield defined a mass of dried polymer recovered/mass of monomer used.
Activity is g of PMMA/(mol-Co.h).
Determined by GPC eluted with THF at room temperature by filtration with polystyrene calibration.
It is a blank polymerization in which CoCl2·6H2O was also activated by MMAO.
It is a blank polymerization which was done solely by MMAO.
b
c
d
e
f

192
M. Yang et al. / Inorganic Chemistry Communications 14 (2011) 189–193
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through the National Research Foundation of Korea (NRF), funded by
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2009-0075742).
Appendix A. Supplementary material
[48] C. Dowling, V.J. Murphy, G. Parkin, Inorg. Chem. 35 (1996) 2415.
[49] A.R. Katritzky, S. Rachwal, J. Wu, Can. J. Chem. 68 (1990) 446.
[50] Representative synthesis of N,N-Bis{(1-H-Pyrazole-1-yl)methyl}-2,4,6-trimethy-
laniline, [bpmaL2]: 2,4,6-trimethylaniline (3.12 mL, 22.2 mmol) was added slowly
to (1-H-Pyrazole-1-yl)methanol (4.35 g, 44.3 mmol) solution in 1,2-dichloroeth-
ane (150 mL). The reaction solution was dried over MgSO4 after stirring the
reaction mixture at 80 °C for 36 hours. The filtrate solvent was removed under
reduced pressure to give pale yellow residue, which was recrystalized in hexane as
a yellow solid (5.90 g, 90.1%). m.p=71-72 °C. 1 H NMR (CDCl3, 400 MHz): δ 7.56
([-(N2C3H3)2-], d, 2 H, J=0.15 Hz), 7.32 ([-(N2C3H3)2-], d, 2 H, J=0.15 Hz), 6.82
([C6H2(CH3)3 N-], s, 2 H), 6.25 ([-(N2C3H3)2-], t, 2 H, J=0.38 Hz), 5.40 ([-N
The supplementary crystallographic data for 2a can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associat-
ed with this article can be found, in the online version, at doi:10.1016/
j.inoche.2010.10.019.
References
(CH2)2-], s, 4 H), 2.23 ([C6H2(CH3)3 N-], s, 3 H), 1.75 ([C6H2(CH3)3 N-], s, 6 H). 13
C
NMR (CDCl3, 400 MHz) : δ 140.86 ([C6H2(CH3)3 N-], s, 1 C), 139.86 ([-(N2C3H3)2-],
d, 2 C, J=185 Hz), 136.82 ([C6H2(CH3)3 N-], s, 2 C), 136.27 ([C6H2(CH3)3 N-], s,
1 C), 129.54 ([C6H2(CH3)3 N-], d, 2 C, J=155 Hz), 129.21 ([-(N2C3H3)2-], d, 2 C,
J=186 Hz), 105.95 ([-(N2C3H3)2-], d, 2 C, J=176 Hz), 68.25 ([-N(CH2)2], t, 2 C,
J=150 Hz), 20.78 ([C6H2(CH3)3 N-], q, 1 C, J=126 Hz), 17.66 ([C6H2(CH3)3 N-], q,
2 C, J=127 Hz). UV-vis (Toluene)/nm 267 (ε/cm-1 M-1 4704), 285 (ε/cm-1 M-1
12327). Analysis calculated for C17H21N5: C, 69.12; H, 7.17; N, 23.71%. Found: C,
68.83; H, 7.44; N, 23.89%. IR (solid neat/cm-1): 1603 (w), 1509 (m), 1485 (m), 1444
(w), 1420 (w), 1395 (m), 1360 (w), 1349 (w), 1323 (w), 1286 (m), 1260 (m), 1202
(m), 1187 (m), 1152 (s), 1099 (w), 1085 (s), 1042 (s), 966 (m), 954 (m), 943 (w),
917 (w), 891 (w), 867 (w), 851 (w), 790 (m), 753 (s), 732 (s), 701 (m), 652 (m),
616 (s), 589 (s). Representative synthesis of 2a: A solution of [bpmaL2] (2.00 g,
6.77 mmol) in dried ethanol (50 mL) was added to a solution of CoCl2·6H2O
(1.61 g, 6.77 mmol) in dried ethanol (50 mL) at room temperature. A dark blue
solid was precipitated after stirring at room temperature for 24 h. The solid was
filtered and washed with fresh cold ethanol (30 mL × 3), followed by washing with
hexane (30 mL × 2), to give a sky blue-colored crystalline solid (2.65 g, 92.0%). m.
p=207-208 °C. UV-vis (Toluene)/nm 262 (ε/cm-1 M-1 12573), 286 (ε/cm-1 M-1
34372), 609 (ε/cm-1 M-1 1111). Analysis calculated for C17H21Cl2CoN5: C, 48.02; H,
4.98; N, 16.47%. Found: C, 47.45; H, 4.95; N, 16.32%. IR (solid neat/cm-1): 1453 (w),
1405 (m), 1321 (w), 1302 (m), 1250 (m), 1206 (w), 1190 (m),1169 (s), 1158 (m),
1100 (w), 1083 (w), 1064 (s), 991 (w), 975 (m), 941 (w), 921 (w), 907 (w), 878 (w),
847 (m), 815 (w), 765 (s), 735 (s), 687 (w), 675 (w), 644 (m), 608 (s), 588 (s), 555 (s).
[51] Crystallographic data for 2a: C17H21Cl2N5Co, M=235.64, T=293(2) K,
Wavelength 0.71073 Å, Crystal size 0.40 × 0.35 × 0.20 mm, Triclinic, Pī,
a=12.4569(8)Å, b=13.2915(8) Å, c=15.1502(14) Å, U=2295.6(3) Å3, Z=8,
Dc=1.364 Mg/m3, μ (λ=0.71073 Å)=0.996 mm-1, F(000)=976, Reflections
collected 9229, Independent reflections 8526 [R(int)=0.0111], Reflections
observed (N2σ) 4959, Data Completeness 0.993, Refinement method, Full-matrix
least-squares on F@2, Data / restraints / parameters, 8536 / 0 / 258, Goodness-of-
fit on F@2, 1.049, Final R indices [IN2σ (I)], R1=0.0491, wR2=0.1572, R indices
(all data), R1=0.1040, wR2=0.1681. Theta range for data collection 1.64° to
25.55° Index ranges -15≤h≤14, -16≤k≤0, -18≤l≤18, Largest diff. peak and
hole 0.595 and -0.358 e. Å-3. An X-ray-quality single crystal was mounted in a
thin-walled glass capillary on an Enraf-Noius CAD-4 diffractometer with Mo-Kα
radiation (λ = 0.71073 Å). Unit cell parameters were determined by least-
squares analysis of 25 reflections (10°bθ b 13°). Intensity data were collected
with a θ range of 1.83° to 25.46° in ω/2θ scan mode. Three standard reflections
were monitored every 1 h during data collection. The data were corrected for
Lorentz polarization effects and decay. Empirical absorption corrections with
χ-scans were applied to the data. The structure was solved using the Patterson
method and refined by the full-matrix least-squares techniques on F using
SHELX-97 and SHELX-97 program packages. The positions of all non-hydrogen
atoms were refined geometrically using a riding model correction with fixed
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[64] Representative polymerization procedure of MMA: Methyl methacrylate (MMA)
was extracted with 10% sodium hydroxide, washed with water, dried over
magnesium sulfate, and distilled over calcium hydride under reduced pressure
atmosphere. After the mixture had been stirred at room temperature for 10 min, it
was transferred into MMA (5.0 mL, 47.0 mmol, [MMA]0/[Co (II) catalyst]0 =
3100). Then, the reaction flask was immersed in an oil bath at 60 °C and stirred for
2 h. The resulting polymer was precipitated in methanol (400 mL) and HCl (3 mL)
was added with stirring for 10 min. The polymer was filtered and washed with
methanol (400 mL × 3) to give PMMA, which was vacuum-dried at 60 °C.
Additionally, polymerization was examined at different temperatures (0 °C, 25 °C,
40 °C, 50 °C) with 4a.
before use. To
a 100-mL Schlenk flask containing [bpmaL]CoCl2 (5.8 mg,
15.0 μmol) in toluene (1 mL) was added MMAO (modified methylaluminoxane,
6.9 wt% in toluene, 3.25 mL, [MMAO]0/[Catalyst]0 = 500) under a dry argon