Polymerizations of methyl methacrylate and rac-lactide by 4-coordinate cobalt(II) complexes supported by N′-substituted N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine derivatives

Article abstract of DOI:10.1016/j.poly.2017.12.021

The novel complexes [L_nCoCl₂] (L_n = L_A ? L_H), containing the N,N-chelating ligands N,N-bis((1H-pyrazol-1-yl)methyl)aniline (L_A), N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)-3,5-dimethylaniline (L_B), N,N-bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline (L_C), N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline (L_D), N,N-bis((1H-pyrazol-1-yl)methyl)-4-methoxyaniline (L_E), N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-4-methoxyaniline (L_F), N,N-bis((1H-pyrazol-1-yl)methyl)-4-fluoroaniline (L_G) and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-4-fluoroaniline (L_H), have been synthesized and structurally characterized. The molecular structures of these Co(II) complexes showed a distorted tetrahedral geometry. No interaction exists between the N_amine atom and the Co(II) centre in the [L_nCoCl₂] complexes, thus resulting in the formation of an eight membered chelate ring. [L_DCoCl₂] exhibited the highest catalytic activity (4.02 × 10⁴ g PMMA/molCo h) for the polymerization of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) at 60 °C and yielded poly(methylmethacrylate) (PMMA) with high molecular weight (M_w) (10.5 × 10⁵ g/mol). Syndiotactic enriched PMMA was obtained with T_g in range 123–130 °C. The MMA polymerization activity of the complexes in the current study should not be solely considered as a function of the total steric hindrance from the ligands around the metal center. The dimethyl derivatives [L_nCoMe₂] (L_n = L_A ? L_H), generated in situ, effectively polymerized rac-lactide (rac-LA) and yielded polylactide (PLA) with good number-average molecular weights and narrower polydispersity indices (PDIs). The electronic density around the metal center in these dimethyl cobalt initiators seemed to enhance the activity, while the stereoselectivity is negatively affected. Thus, the presence or absence of a methyl group in the pyrazole ring moiety influences not only the activity of the Co(II) complexes in the MMA polymerization, but also the stereoselectivity of the Co(II) initiators in the ROP of rac-LA.

Full text of DOI:10.1016/j.poly.2017.12.021

Accepted Manuscript
Polymerizations of methyl methacrylate and rac-lactide by 4-coordinate co-
balt(II) complexes supported by N'-substituted N,N',N-bis((1H-pyrazol-1-
yl)methyl)amine derivatives
Sujin Shin, Saira Nayab, Hyosun Lee
PII:
S0277-5387(17)30804-5
https://doi.org/10.1016/j.poly.2017.12.021
POLY 12978
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
12 October 2017
9 December 2017
Please cite this article as: S. Shin, S. Nayab, H. Lee, Polymerizations of methyl methacrylate and rac-lactide by 4-
coordinate cobalt(II) complexes supported by N'-substituted N,N',N-bis((1H-pyrazol-1-yl)methyl)amine
derivatives, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.12.021
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Polymerizations of methyl methacrylate and rac-lactide by 4-coordinate
cobalt(II) complexes supported by N'-substituted N,N',N-bis((1H-pyrazol-
1-yl)methyl)amine derivatives
Sujin Shina, Saira Nayabb,* and Hyosun Leea,*
aDepartment of Chemistry and Green-Nano Materials Research Center, Kyungpook National
University, 80 Daehakro, Bukgu, Daegu, 41566, Republic of Korea
*E-mail address: hyosunlee@knu.ac.kr (H. Lee)
bDepartment of Chemistry, Shaheed Benazir Bhutto University, Sheringal Dir (U), Khyber
Pakhtunkhwa, Islamic Republic of Pakistan
----------------------------------
*Corresponding author. Tel: +82-53-950-5337; fax: +82-53-950-6330
E-mail address: hyosunlee@knu.ac.kr (H. Lee)
1

Abstract
The novel complexes [LnCoCl2] (Ln = LA – LH), containing the N,N-chelating
ligands N,N-bis((1H-pyrazol-1-yl)methyl)aniline (LA), N,N-bis((3,5-dimethyl-1H-pyrazol-1-
yl)-3,5-dimethylaniline (LB), N,N-bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline (LC),
N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline (LD), N,N-bis((1H-
pyrazol-1-yl)methyl)-4-methoxyaniline
(LE),
N,N-bis((3,5-dimethyl-1H-pyrazol-1-
yl)methyl)-4-methoxyaniline (LF), N,N-bis((1H-pyrazol-1-yl)methyl)-4-fluoroaniline (LG)
and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-4-fluoroaniline (LH), have been
synthesized and structurally characterized. The molecular structures of these Co(II)
complexes showed a distorted tetrahedral geometry. No interaction exists between the Namine
atom and the Co(II) centre in the [LnCoCl2] complexes, thus resulting in the formation of an
eight membered chelate ring. [LDCoCl2] exhibited the highest catalytic activity (4.02 × 104 g
PMMA/molCo h) for the polymerization of methyl methacrylate (MMA) in the presence of
modified methylaluminoxane (MMAO) at 60 C and yielded poly(methylmethacrylate)
(PMMA) with high molecular weight (Mw) (10.5 × 105 g/mol). Syndiotactic enriched PMMA
was obtained with Tg in range 123-130 °C. The MMA polymerization activity of the
complexes in the current study should not be solely considered as a function of the total steric
hindrance from the ligands around the metal center. The dimethyl derivatives [LnCoMe2] (Ln
= LA – LH), generated in situ, effectively polymerized rac-lactide (rac-LA) and yielded
polylactide (PLA) with good number-average molecular weights and narrower polydispersity
indices (PDIs). The electronic density around the metal center in these dimethyl cobalt
initiators seemed to enhance the activity, while the stereoselectivity is negatively affected.
Thus, the presence or absence of a methyl group in the pyrazole ring moiety influences not
only the activity of the Co(II) complexes in the MMA polymerization, but also the
stereoselectivity of the Co(II) initiators in the ROP of rac-LA.
2

Keywords: 4-coordinate cobalt(II) complex, N,N-bis((1H-pyrazol-1-yl)methyl)aniline,
syndiotactic poly(methylmethacrylate), heterotactic polylactide
1. Introduction
Pyrazoles have been extensively studied as N-donor ligands to metals since the first
review of pyrazole-derived ligands in 1972 [1]. Pyrazole-based ligands are attractive due to
their efficient synthesis and various modifications on the linker unit of two or three pyrazole
moieties [2,3]. Since the pioneering work of Driessen in 1982 [4], N-substituted pyrazolyl
amines based ligands and their transition metal complexes, due to their structural stability,
diverse coordination modes and catalytic efficacy, have found profound applicability as
supramolecules for metal-organic frames (MOF) [5], catalysts for organic transformations
[6], biological agents [7], cancer sensors and as hydrolysis and oxidation agents [8].
More recently, N-substituted pyrazolyl amines ligated to a variety of late transition metals,
such as Ru(II), Pt(II), Pd(II), Rh(I), Co(II) and Zn(II), have revealed various useful catalytic
properties in olefin [9] and methyl methacrylate (MMA) polymerizations [10]. Early
transition metal complexes generally cannot tolerate polar functional groups owing to the
highly electron-deficient nature of the metal center. To overcome this drawback, late
transition metal complexes were developed which proved to be better candidates for
polymerization of a polar monomer in controlled fashion, due to their less oxophilic nature
[11,12].
As a part of our ongoing research towards stereoselective MMA and rac-lactide (rac-LA)
polymerization, we have recently investigated a variety of late transition metal complexes,
including Co(II), Zn(II), Pd(II), Cd(II) and Cu(II) with N,N-bis(1H-pyrazolyl-1-
methyl)aniline and its derivatives, which exhibit diverse coordination modes with high
3

activities and mediocre stereoselectivities [13,14]. These results and the potential merit of N-
substituted pyrazolyl amine based ligands, that show fine tuning of the reactivity of the metal
centre to which they are bound, prompted us to explore the complexing properties of N,N′,N-
bis((1H-pyrazol-1-yl)methyl)amine ligands to the Co(II) centre and its catalytic capabilities.
Further, the influence of substituents on the aniline and pyrazole moieties on the catalytic
activity in both methyl methacrylate and rac-LA polymerization is also discussed.
2. Experimental
2.1. Materials
1H-pyrazole, 3,5-dimethylpyrazole, para-formaldehyde, aniline, 3,5-dimethylaniline,
p-anisidine, 4-fluoroaniline, magnesium sulfate (MgSO4), molecular sieve (0.4 nm),
[CoCl2·6H2O] and methyl methacrylate (MMA) were purchased from Sigma-Aldrich (St.
Louis, MO) and anhydrous solvents, such as CH2Cl2, anhydrous EtOH, acetonitrile, hexane,
diethyl ether (Et2O) and DMF, were purchased from Aldrich and Merck (Darmstadt,
Germany) and used without further purification. Modified methylaluminoxane (MMAO) was
purchased from Tosoh Finechem Corporation (Tokyo, Japan) as 5.90 % aluminum (by weight)
in a toluene solution and used without further purification.
2.2. Physical measurements
Elemental analyses (C, H, N) of the synthesized ligands and their corresponding
metal complexes were performed on an elemental analyzer (EA 1108; Carlo-Erba, Milan,
1
Italy). H (operating at 500 MHz) and 13C (operating at 125 MHz) NMR spectra were
recorded on an Avance Digital 500 NMR spectrometer (Bruker, Billerica, MA); chemical
shifts were recorded in ppm units (relative to SiMe4 as an internal standard. Infrared (IR)
4

spectra were recorded on a Bruker FT/IR-Alpha spectrophotometer (neat solid or neat liquid)
and the data were reported in reciprocal centimeters (cm-1). The molecular weights and
molecular weight distributions of the obtained poly(methyl methacrylate) (PMMA) were
determined using gel permeation chromatography (GPC) (THF, Alliance e2695; Waters
Corp., Milford, MA). The glass transition temperature (Tg) was determined using a thermal
analyzer (Q2000; TA Instruments, New Castle, DE).
2.3. Synthetic procedures
2.3.1. Preparation of the ligands
1H-pyrazolyl-1-methanol and 3,5-dimethyl-1H-pyrazolyl-1-methanol as starting
materials were prepared according to the reported method [15]. The syntheses of the ligands
LA, LB, LC, LE and LG were carried out as reported previously [13a,13d,14c,16c].
2.3.1.1. N,N-bis((3,5-dimethylpyrazol-1-yl)methyl)-3,5-dimethylaniline (LD)
A CH2Cl2 solution (50.0 mL) of 3,5-dimethylaniline (2.00 mL, 16.0 mmol) and
molecular sieves (20.0 g, 0.4 nm) were slowly added to a CH2Cl2 (50.0 mL) solution of 3,5-
dimethylpyrazole-1-methanol (4.05 g, 32.1 mmol). The reaction solution was stirred at
ambient temperature for 120 h and the molecular sieves were filtered off. The reaction
solvent was removed under reduced pressure to give a yellow solid, which was recrystallized
in CH2Cl2 at -78 oC (3.80 g, 70.3 %). Analysis calculated for C20H27N5 (%): C, 71.2; H, 8.06;
1
N, 20.8. Found: C, 71.5; H, 8.14; N, 20.3. H NMR (CDCl3, 500 MHz) δ, ppm: 6.65 (s, 2H,
o-NC6H3(CH3)2-), 6.62 (s, 1H, p-NC6H3(CH3)2-), 5.74 (s, 2H, -N=C(CH3)-CH=C(CH3)-N-),
5.47 (s, 4H, -N-CH2-N-), 2.23 (s, 6H, -NC6H3(CH3)2), 2.21 (s, 6H, -N=C(CH3)-CH=C(CH3)-
N-), 2.06 (s, 6H, -N=C(CH3)-CH=C(CH3)-N-). 13C-NMR (CDCl3, 125 MHz) δ, ppm: 147.62
5

(s, 2C, -N=C(CH3)-CH=C(CH3)-N-), 146.39 (s, 1C, ipso-NC6H3(CH3)2-), 139.47 (s, 2C, -
N=C(CH3)-CH=C(CH3)-N-), 138.50 (s, 2C, m-NC6H3(CH3)2-), 124.35 (d, 1C, J = 155 Hz, p-
NC6H3(CH3)2-), 118.47 (d, 2C, J = 156 Hz, o-NC6H3(CH3)2-), 105.62 (d, 2C, J = 172 Hz, -
N=C(CH3)-CH=C(CH3)-N-), 64.21 (t, 2C, J = 151 Hz, -N-CH2-N-), 21.39 (q, 2C, J = 126 Hz,
-NC6H3(CH3)2-), 13.55 (q, 2C, J = 127 Hz, -N=C(CH3)-CH=C(CH3)-N-), 10.94 (q, 2C, J =
129 Hz, -N=C(CH3)-CH=C(CH3)-N-). IR (solid neat; cm-1): 2996 (w), 2977 (w), 2912 (w),
1602 (m), 1549 (m), 1451 (m), 1336 (m), 1279 (m), 1238 (m), 1196 (s), 1028 (m), 960 (m),
885 (m), 812 (m), 773 (s), 685 (w), 623 (w).
2.3.1.2. N,N-bis((3,5-dimethylpyrazol-1-yl)methyl)-4-methoxylaniline (LF)
An analogous method as described for LD was used to synthesize LF, except utilizing
p-anisidine (2.00 g, 16.2 mmol), molecular sieve (20.0 g, 0.4 nm) and 3,5-dimethylpyrazole-
1-methanol (4.10 g, 32.5 mmol) in CH2Cl2 (50.0 mL). The final ligand was obtained as a
brown solid (4.33 g, 78.6 %). Analysis calculated for C19H25N5O (%): C, 67.2; H, 7.42; N,
20.6. Found: C, 67.1; H, 7.47; N, 20.1. 1H NMR (CDCl3, 500 MHz) δ, ppm: 6.83 (d, 2H, J =
8.9 Hz, p-NC6H4-OCH3), 6.74 (d, 2H, J = 8.8 Hz, o-NC6H4-OCH3), 5.71 (s, 2H, -N=C(CH3)-
CH=C(CH3)-N-), 5.38 (s, 4H, -N-CH2-N-), 3.74 (s, 3H, -NC6H4-OCH3), 2.20 (s, 6H, -
13
N=C(CH3)-CH=C(CH3)-N-), 1.93 (s, 6H, -N=C(CH3)-CH=C(CH3)-N-). C NMR (CDCl3,
125 MHz) δ, ppm: 156.60 (s, 1C, p-NC6H4-OCH3), 147.70 (s, 2C, -N=C(CH3)-CH=C(CH3)-
N-), 139.65 (s, 2C, -N=C(CH3)-CH=C(CH3)-N-), 139.35 (s, 1C, ipso-NC6H4-OCH3), 125.45
(d, 2C, J = 159 Hz, o-NC6H4-OCH3), 114.27 (d, 2C, J = 160 Hz, m-NC6H4-OCH3), 105.47 (d,
2C, J = 173 Hz, -N=C(CH3)-CH=C(CH3)-N-), 65.72 (t, 2C, J = 151 Hz, -N-CH2-N-), 55.33
(q, 1C, J = 143 Hz, o-NC6H4-OCH3), 13.58 (q, 2C, J = 127 Hz, -N=C(CH3)-CH=C(CH3)-N-),
10.78 (q, 2C, J = 128 Hz, -N=C(CH3)-CH=C(CH3)-N-). IR (solid neat; cm-1): 2970 (w), 2918
(w), 2836 (w), 1553 (m), 1511 (s), 1419 (m), 1286 (w), 1242 (s), 1179 (s), 1116 (m), 1030 (s),
6

932 (w), 776 (s), 689 (w), 623 (w), 571 (m).
2.3.1.3. N,N-bis((3,5-dimethylpyrazol-1-yl)methyl)-4-fluoroaniline (LH)
An analogous method as described for LD was used to synthesize LH, except utilizing
4-fluoroaniline (2.00 mL, 21.1 mmol) and 3,5-dimethylpyrazole-1-methanol (5.33 g, 42.2
mmol) in CH2Cl2. The final product was obtained as a white solid by recrystallization from
hexane (5.86 g, 86.8 %). Analysis calculated for C18H22N5F (%): C, 66.0; H, 6.77; N, 21.4.
1
Found: C, 65.8; H, 6.89; N, 21.8. H NMR (DMSO, 500 MHz) δ, ppm: 6.93 (s, 2H, p-
NC6H4-F), 6.91 (d, 2H, J = 2.0 Hz, o-NC6H4-F), 5.74 (s, 2H, -N=C(CH3)-CH=C(CH3)-N-),
5.41 (s, 4H, -N-CH2-N), 2.20 (s, 6H, -N=C(CH3)-CH=C(CH3)-N-), 1.99 (s, 6H, -N=C(CH3)-
13
CH=C(CH3)-N-). C-NMR (DMSO, 125 MHz) δ, ppm: 160.41 158.48 (s, 2C, p-NC6H4-F),
147.89 (s, 2C, -N=C(CH3)-CH=C(CH3)-N-), 142.44 (s, 2C, ipso-NC6H4-F), 139.54 (s, 2C, -
N=C(CH3)-CH=C(CH3)-N-), 124.57 (d, 2C, J = 161 Hz, m-NC6H4-F), 115.73 (d, 1C, J = 163
Hz, o-NC6H4-F), 105.68 (d, 2C, J = 172 Hz, -N=C(CH3)-CH=C(CH3)-N-), 65.17 (t, 2C, J =
151 Hz, -N-CH2-N-), 13.54 (q, 2C, J = 127 Hz, -N=C(CH3)-CH=C(CH3)-N-), 10.82 (q, 2C, J
= 128 Hz, -N=C(CH3)-CH=C(CH3)-N-). IR (solid neat; cm-1): 2975 (w), 2950 (w), 2915 (w),
1550 (m), 1512 (m), 1454 (m), 1360 (m), 1252 (m), 1191 (s), 1021 (m), 951 (m), 813 (s), 773
(s), 713 (w), 683 (w), 621 (w).
2.3.2. Preparation of the Co(II) complexes
The synthesis of complexes [LACoCl2] and [LBCoCl2] was carried out as reported
previously [13b,16d].
2.3.2.1.
[N,N-bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline]cobalt(II)
chloride
([LCCoCl2])
7

A solution of LC (1.00 g, 3.55 mmol) in anhydrous EtOH (15.0 mL) was slowly added
to solution of [CoCl2·6H2O] (0.850 g, 3.55 mmol) in anhydrous EtOH (15.0 mL).
Precipitation of a blue solid occurred while stirring at 25 °C for 24 h. The blue powder was
filtered and washed with cold EtOH (20.0 mL × 3), followed by washing with hexane (20.0
mL × 3) to get the final product (1.31 g, 89.7 %). Analysis calculated for C16H19Cl2N5Co (%):
C, 46.7; H, 4.66; N, 17.0. Found: C, 46.6; H, 4.63; N, 17.2. IR (solid neat; cm-1): 3495 (w),
3102 (w), 1593 (w), 1511 (w), 1459 (w), 1404 (m), 1266 (m), 1148 (m), 1065 (s), 991 (w),
850 (w), 819 (w), 783 (s), 702 (m), 611 (m).
2.3.2.2. [N,N-bis((3,5-dimethylpyrazol-1-yl)methyl)-3,5-dimethylaniline]cobalt(II) chloride
([LDCoCl2])
An analogous method as described for [LCCoCl2] was used, except utilizing LD (1.00
g, 2.96 mmol) and [CoCl2·6H2O] (0.710 g, 2.96 mmol) in anhydrous EtOH, giving a blue
solid as the final product (0.770 g, 55.8 %). Analysis calculated for C20H27Cl2N5Co (%): C,
51.4; H, 5.82, N, 15.0. Found: C, 51.7; H, 5.84; N, 14.7. IR (solid neat; cm-1): 2915 (w), 1592
(w), 1554 (m), 1463 (m), 1420 (w), 1374 (m), 1282 (m), 1227 (w), 1157 (m), 1121 (m), 1046
(m), 957 (m), 850 (m), 797 (s), 726 (w), 694 (s).
2.3.3.3. [N,N-bis((1H-pyrazol-1-yl)methyl)-4-methoxyaniline[cobalt(II) chloride ([LECoCl2])
An analogous method as described for [LCCoCl2] was used, except utilizing LE (1.00
g, 3.53 mmol) and [CoCl2·6H2O] (0.84 g, 3.53 mmol) in anhydrous EtOH, giving a blue
powder as the final product (1.25 g, 85.9 %). Analysis calculated for C15H17Cl2N5OCo (%): C,
43.6; H, 4.15; N, 17.0. Found: C, 43.9; H, 4.15; N, 16.6. IR (solid neat; cm-1): 3111 (w), 1609
(w), 1513 (s), 1463 (m), 1408 (m), 1312 (m), 1283 (m), 1244 (m), 1190 (s), 1166 (m), 1098
(s), 1028 (m), 998 (m), 958 (m), 915 (m), 841 (m), 809 (w), 772 (s), 741 (m), 693 (m), 642
8

(w), 611 (s), 574(s)
2.3.3.4. [N,N-bis((3,5-dimethylpyrazol-1-yl)methyl)-4-methoxyanilin] cobalt(II) chloride
([LFCoCl2])
An analogous method as described for [LCCoCl2] was used, except utilizing LF (1.00
g, 2.95 mmol) and [CoCl2·6H2O] (0.700 g, 2.95 mmol) in anhydrous EtOH, giving a blue
crystalline solid as the final product (1.07 g, 82.5 %). Analysis calculated for
C19H25Cl2N5OCo (%): C, 48.6; H, 5.37; N, 14.9. Found: C, 48.2; H, 5.36; N, 15.0. IR (solid
neat; cm-1): 2995 (w), 2924 (w), 2829 (w), 1606 (w), 1585 (w), 1554 (m), 1510 (m), 1463 (m),
1423 (m), 1376 (m), 1350 (m), 1299 (m), 1243 (s), 1206 (m), 1170 (m), 1118 (m), 1035 (s),
935 (m), 835 (m), 798 (s), 742 (m), 702 (s), 632 (m), 567 (s)
2.3.3.5. [N,N-bis((1H-pyrazol-1-yl)methyl)-4-fluoroaniline]cobalt(II) chloride ([LGCoCl2])
An analogous method as described for [LCCoCl2] was used, except utilizing LG (1.00
g, 3.69 mmol) and [CoCl2·6H2O] (0.880 g, 3.69 mmol) in anhydrous EtOH, giving a blue
powder as the final product (1.31 g, 88.4 %). Analysis calculated for C14H14Cl2N5FCo (%): C,
41.9; H, 3.52; N, 17.5. Found: C, 41.9; H, 3.59; N, 17.6. IR (solid neat; cm-1): 3127 (w), 2358
(w), 1638 (w), 1611 (w), 1512 (m), 1456 (m), 1401 (w), 1316 (m), 1282 (m), 1226 (s), 1160
(m), 1101 (m), 1066 (s), 1014 (w), 991 (m), 923 (w), 903 (w), 831 (s), 761 (s), 717 (m), 644
(m), 606 (s), 564 (m)
2.3.3.6. [N,N-bis((3,5-dimethylpyrazol-1-yl)methyl)-4-fluoroaniline]cobalt(II) chloride
([LHCoCl2])
An analogous method as described for [LCCoCl2] was used, except utilizing LH (1.00
g, 3.05 mmol) and [CoCl2·6H2O] (0.730 g, 3.05 mmol) in anhydrous EtOH, to get the final
9

product (1.30 g, 92.9 %). Analysis calculated for C18H22Cl2N5FCo, CH2Cl2 (%): C, 42.1; H,
4.46; N, 12.9. Found: C, 42.1; H, 4.50; N, 13.4. IR (solid neat; cm-1): 3008 (w), 2865 (w),
1552 (m), 1508 (s), 1458 (m), 1383 (m), 1344 (m), 1304 (m), 1255 (m), 1230 (m), 1210 (m),
1175 (s), 1160 (m), 1120 (s), 1045 (m), 1010 (m), 987 (m), 936 (m), 841 (m), 821 (s), 803 (s),
749 (m), 717 (m), 688 (m), 631 (m).
2.4. Catalytic activities for MMA polymerization
The methyl methacrylate (MMA) was extracted with 10% NaOH, washed with water,
dried over MgSO4 and distilled over CaH2 under reduced pressure before use. In a Schenk
flask, the synthesized complex (15.0 μmol, 5.70 mg for [LACoCl2], 6.60 mg for [LBCoCl2],
6.20 mg for [LCCoCl2], 7.00 mg for [LDCoCl2], 6.20 mg for [LECoCl2], 7.00 mg for
[LFCoCl2], 6.00 mg for [LGCoCl2] and 8.20 mg for [LHCoCl2]) was dissolved in dried
toluene (10.0 mL), followed by the addition of MMAO (5.90 wt% in toluene 3.80 mL, 7.50
mmol and [MMAO]0/[catalyst]0 =500) as a co-catalyst. The solution was mixed by stirring
for 20 min at a temperature of 60 °C. MMA (5.00 mL, 47.1 mmol and [MMA]0/[catalyst]0 =
3100) was slowly added to the above reaction solution and stirred for 2 h at 60 °C to obtain a
viscous solution. Methanol (2.00 mL) was added to terminate the polymerization. The
reaction mixture was poured into a large quantity of MeOH (500 mL) to precipitate PMMA
from the toluene solution and HCl (5.00 mL, 35.0 %) was injected to remove the remaining
co-catalyst (MMAO). White PMMA was obtained by filtration, washed with MeOH (250 mL
× 2) and dried under vacuum at 60 °C for 12 h. The polymers were isolated as white solids
and characterized by GPC in THF using standard polystyrene as the reference. Three polymer
o
trials were conducted to confirm the activities of the synthesized complexes at 60 C. The
temperature of the polymerization was optimized by previous work between 0, 25, 50 and 60
10

oC [17].
2.5. Typical procedure for ROP of rac-lactide
In a general polymerization reaction for rac-LA with the dimethyl cobalt initiators,
the catalyst species were generated as follows. The dichloro cobalt complexes (0.500 mmol)
and dried THF (7.40 mL) were added to a 100 mL Schlenk flask under an argon atmosphere.
To this solution MeLi (1.00 mmol, 0.63 mL of a 1.6 M solution in Et2O) was added dropwise
at -78 °C to generate the in situ dimethyl Co(II) species, [LnCOMe2] (Ln = LA – LH). After
being stirred for 2 h at room temperature, the resulting THF solution of the dimethyl Co(II)
complexes, i.e. [LnCOMe2] (Ln = LA – LH), was used as an initiator for the ROP of rac-LA.
The general procedure for the polymerization reaction was as follows. A Schlenk flask (100
mL) was charged with rac-LA (0.901 g, 6.25 mmol) under an argon atmosphere and 5.00 mL
of dried CH2Cl2 was added. The polymerization was initiated by the slow addition of the
catalyst solution (1.00 mL, 0.0625 mmol) via a syringe under argon at -50 °C. The reaction
mixture was stirred for the allotted time and the polymerisation reactions were quenched
using H2O (1.00 mL). Hexane (2.00 mL) was then added to precipitate the polymer. 1H NMR
(CDCl3, 400 MHz) for the obtained polymer, δ, ppm: 5.13-5.20 (m, 1H), 1.51-1.63 (m, 3H).
2.6. X-ray crystallographic studies
Crystals suitable for an X-ray study of the synthesized Co(II) complexes were obtained by
layering hexane on a CH2Cl2 solution or by Et2O diffusion into a DMF solution. X-ray
quality single crystals of some cobalt complexes were coated with paratone-N oil and the
diffraction data were measured at 100(2) K with synchrotron radiation (λ = 0.610 Å) on an
ADSC Quantum-210 detector at 2D SMC with a silicon (111) double crystal monochromator
11

(DCM) at the Pohang Accelerator Laboratory, Korea. The BL2DSMDC program was used
for data collection (detector distance is 63 mm, omega scan; Δω = 1˚, exposure time is 1 s per
frame) and HKL3000sm (Ver. 703r) was used for cell refinement, reduction and absorption
corrections. The structures were solved by direct methods and refined by full-matrix least-
squares refinement using the SHELXS-2014 and SHELXL-2016 computer programs.
The crystals of the other cobalt complexes were picked up with paratone oil and
mounted on a Bruker SMART CCD diffractometer equipped with a graphite-monochromated
Mo-Kα (λ = 0.71073 Å) radiation source under a nitrogen cold stream (200(2) K). Data
collection and integration were performed with SMART and SAINT-Plus software packages
[18]. Multi-scan absorption corrections based on equivalent reflections were applied by
SADABS [19]. The structures were solved by direct methods and refined using a full-matrix
least-squares method on F2 using SHELXTL [20]. Structural refinement and crystallographic
data for [LnCoCl2] (Ln= LC – LH) are presented in Table 1.
3. Results and discussion
3.1. Synthesis and chemical properties
The ligands were obtained in a single step from the substituted aniline and 1H-
pyrazolyl-1-methanol or 3,5-dimethylpyrazol-1-methanol in CH2Cl2 or MeCN, as described
in the literature [13,14c,16c]. The corresponding [LnCoCl2] (Ln= LA – LH) complexes were
obtained (approximately 82 ~ 93 % yields) by direct ligation of the metal starting materials
with the ligands in a 1:1 ratio in dried EtOH (Scheme 1). The synthesized complexes
[LnCoCl2] (Ln= LA – LH) were characterized using IR spectra and elemental analysis data.
Owing to the paramagnetic nature of the Co(II) complexes, we were unable to study the
synthesized complexes via NMR spectroscopy. However, a comparison of the IR spectra of
12

the ligands with those of the complexes was performed in the C–H stretching region. For
instance, characteristic C–H peaks for the LD, LF and LG free ligands in the IR spectra were
observed at 2996, 2970 and 2975 cm-1, while the C–H stretching bands appeared at 3019,
3115 and 3016 cm-1, respectively in their corresponding Co(II) complexes. All the
synthesized complexes were stable towards oxygen and moisture.
3.2. Description of the molecular structures
Crystals suitable for the X-ray study of the Co(II) complexes were obtained by layering
hexane on CH2Cl2 solutions of the cobalt complexes. The molecular structures of [LnCoCl2]
(Ln = LC - LH), with 30 % probability level, are illustrated in Figs. 2-7, respectively. Selected
bond lengths and angles are described in Table 2.
Among the synthesized Co(II) complexes, [LCCoCl2], [LDCoCl2], [LFCoCl2], and
[LHCoCl2] crystallized in the monoclinic system with Cc, P21/c and P21/n space groups.
[LECoCl2] and [LGCoCl2] crystallized in the triclinic system with the P-1 space group. The
central cobalt atom was bonded to the N'-aromatic group substituted N,N',N-bis((1H-pyrazol-
1-yl)methyl)amine framework via the N atoms of the pyrazole moiety in a bidentate binding
mode, resulting in a distorted tetrahedral geometry. No interaction between the Namine atom
and the cobalt metal center exists in the 4-coordinate [LnCoCl2] (Ln = LC – LH) complexes,
resulting in an eight membered chelate ring [13b,14c], which is in contrast to our previously
reported Co(II) complexes with a similar ligand architecture (N'-non aromatic group
substituted N,N',N-bis((1H-pyrazol-1-yl)methyl)amine framework), where a 5-coordinate
distorted trigonal bipyramid geometry resulted via an Namine atom and cobalt centre
coordinative interaction [14b,14c,21].
The average Co-Npyrazole bond lengths in [LnCoCl2] (Ln = LC - LH) lie in the expected
range, 2.000(6)-2.0421(1) Å, as found in similar reported complexes [11a,b,12c]. It is
13

observed that the Co-Npyrazole bond length in [LDCoCl2], [LFCoCl2] and [LHCoCl2] is about
0.012-0.038 Å longer compared to those in [LCCoCl2], [LECoCl2] and [LGCoCl2]. The
difference in the steric environment provided by two methyl groups of the pyrazole ring
might be responsible for this increase in the bond lengths. Similarly, all the Co-Cl bond
lengths lie in the accepted range, i.e. 2.229(5)-2.251(1) Å, of the reported Co(II) pyrazole
amine complexes [11b].
The Clterminal-Co-Clterminal and Npyrazole-Co-Npyrazole angles of [LnCoCl2] (Ln = LC – LH) are in
the range 108.4(7)-117.1(2) and 107.8(2)-114.7(2)°, respectively. The Npyrazole-Co-Npyrazole
angles in the [LCCoCl2], [LDCoCl2], [LFCoCl2], [LGCoCl2] and [LHCoCl2] complexes show
almost ideal tetrahedral angles (109.0°), although those of the [LECoCl2] complex are
approximately 5° larger (Table 2). A useful index for tetrahedral geometries of 4-coordinate
complexes is the τ4 value (for a perfect Td, τ4 is given 1.00 and 0 for a perfect square plane,
D4h) [22a], which is calculated by R. P. Houser et al. who modified the equation of τ5 (for the
geometry of 5-coordinate complexes) and is introduced by A.W. Addition, J. Reedijk and
coworkers [22b]. In addition, the THCDA/100 index suggests useful analysis of the
tetrahedral and the trigonal pyramidal extremes [22c,d,e]. For instance, a value of 1.00 for the
THCDA/100 index represents a perfect tetrahedral geometry, while a value of 0 is for a perfect
trigonal pyramidal geometry. Further, the FCGP/100 index is applied to the tetrahedral
geometry of main group complexes [22f]. Table 3 shows the 4-coordinate geometry indices
for the Co(II) complexes with N'-substituted N,N',N-bis((1H-pyrazol-1-yl)methyl)amine
derivatives. The τ4 values of all the [LnCoCl2] (Ln = LA - LH) complexes, except the complex
[LGCoCl2], fall in the range 0.922 to 0.945, indicated a distortion of the tetrahedral geometry.
Note, the τ4 values of 5-coordinated trigonal bipyramidal Co(II) complexes with a similar
ligand system are between 0.833 and 0.891. These Co(II) complexes have an interaction
between the nitrogen atom of the amine moiety and the cobalt metal center (the bond lengths
14

are from 2.522(6) to 2.382(2) Å, indicating a solid covalent bond) [14,21]. The calculated
bond distance between the nitrogen atom of the amine moiety and the cobalt metal center in
[LGCoCl2] is 2.653 Å and obviously there is a significant interaction to give a τ4 value of a
trigonal bipyramidal geometry, 0.871. In contrast, the calculated bond distances between the
nitrogen atom of the amine moiety and the cobalt metal center in [LnCoCl2] (Ln = LA – LF
and LH) were from 3.909 to 3.298 Å, indicating that there no interaction between these atoms,
thus giving a distorted tetrahedral geometry around the metal center. Another interesting
feature is that the complexes with methyl substituents at the pyrazolyl moiety, [LnCoCl2]
(Ln= LB, LD, LF and LH), are found to have more distorted tetrahedrality than the complexes
with non-methyl substituents at the pyrazolyl moiety, [LnCoCl2] (Ln= LA, LC, LE and LG), as
judged by the τ4 values.
To find the effect of N'-substitution on the tetrahedrality of the complexes, we have
calculated the tilt angle between the N'-aromatic ring on the amine moiety and the plane of
the N,N-bispyrazole ring. For example, the tilt angle between the N'-aromatic ring on the
amine moiety defined by the plane of yz and the xz plane of the N,N-bis pyrazole ring
(containing the metallacyclic ring residue) in [LDCoCl2] and [LHCoCl2] was perpendicular
(90o). It is found that the complexes with methyl substituents at the pyrazolyl moiety,
[LnCoCl2] (Ln= LB, LD, LF and LH; tilt angles 9, 0, 8 and 0 o, respectively), are found to have
less tilt angles than the complexes with non-methyl substituents at the pyrazolyl moiety,
[LnCoCl2] (Ln= LA, LC, LE and LG;tilt angles 43, 38, 15 and 30 o, respectively). Similarly, it
has been found that the orientation of the phenyl moiety is slightly affected by the
substituents of the pyrazolyl rings, as exhibited by our previously reported complexes where
the orientation of the cyclopentyl unit with respect to the plane of the pyrazole ring is
slightly affected by the substituents of the pyrazole moieties [21]. The orientation of the
plane of the N'-aromatic ring moiety is tilted towards one side with respect to the plane of
15

the methyl substituted pyrazole rings in [LnCoCl2] (Ln = LA - LH) by 0-90o. However, the
observed angles are not significantly affected by the substituents attached to the N'-aromatic
phenyl ring (-H, -CH3, -OCH3, -F). Further, the tilt angle between the N'-aromatic ring on
the amine moiety and the plane of the N,N-bispyrazole ring was not significantly related to
the τ4 values of the complexes.
3.3. Methyl methacrylate (MMA) polymerization
Isotactic PMMA, produced through radical processes commercially, has a Tg value
around 65 °C [23]. Thus, non-radical mediated polymerization of MMA by coordination
complexes is of recent interest to achieve PMMA with a high Tg value and hence improved
properties. As a part of the ongoing research in our group, targeted towards the synthesis of
pyrazolyl based initiators in stereoselective polymerization of MMA, the catalytic capabilities
of the N,N',N-bis((1H-pyrazol-1-yl)methyl)amine based Co(II) complexes were tested for
polymerization in the presence of modified methylaluminoxane (MMAO) at 60 °C. All the
synthesized complexes were found to be efficient initiators, yielding PMMA with glass
transition temperatures (Tg) of 127-135 °C [24-26]. The polymers were isolated as white
polymeric materials and characterized by GPC in THF using standard polystyrene as a
reference. The resulting polymer characteristics are summarized in Table 4.
To see the ligand effect on the metal complexes, a blank polymerization of methyl
methacrylate (MMA) was processed with the starting material [CoCl2]/MMAO or solely with
MMAO under the same experimental conditions. It is evident from the polymerization data
(Table 4) that [CoCl2] exhibited slightly higher activity compared to [LACoCl2] and
[LGCoCl2], yielding PMMA with comparable molecular weights. However, the remaining
Co(II) complexes showed not only better activities, but also exhibited better
16

stereoselectivities and yielded PMMA with high molecular weights compared to the starting
material [CoCl2]. The mediocre activities of our current system can be attributed to their low
solubility in the polymerization media. It is apparent that the complexes with methyl
substituents at the pyrazolyl moiety are found to have more enhanced solubility than the
complexes with the non-methyl substituents at the pyrazolyl moiety in the organic solvents.
Thus, [LnCoCl2] (Ln= LB, LD, LF and LH), which have 3,5-dimethyl substituents at the
pyrazolyl residue, gave better activity for the polymerization of methylmethacrylate than the
corresponding [LnCoCl2] complexes (Ln= LA, LC, LE and LG) having a 1-H-pyrazole moiety
(Table 4).
Similarly, no appreciable amount of PMMA resulted with the dichloro Co(II)
complexes in the absence of MMAO. In the case of using [LnCoCl2] (Ln = LA – LH) for MMA
polymerization, [LDCoCl2] exhibited the highest catalytic activity (4.02 × 104 g/mol Co h)
with high molecular weights (10.5 × 105 g/mol).
In a previous study, the higher activity was attributed to steric hindrance provided by
the methyl substituents attached to the pyrazole moiety [14,16c]. The catalytic activities of
initiators bearing methyl substituents attached to the pyrazole moiety exhibited higher
activities compared to their counterparts without methyl substituents on the pyrazole moiety
(Table 4). Thus, this enhance activity of the Co(II) complexes containing two methyl
substituents on the pyrazole ring might be due to greater electron density and local steric
hindrances around the metal center, as shown previously by the N,N-bis(1H-pyrazolyl-1-
methyl)aniline Co(II) system [11a]. Further, the catalytic activity can also be enhanced with
better solubility of the initiators with methyl substituents on the phenyl as well the pyrazolyl
ring in polymerization. These observations are in contrast to the previous report [21b] where
the bulky substituents at the pyrazole moiety negatively affect the MMA activation, probably
the steric bulk around the metal center depresses the monomer approach to the metal sites.
17

Interestingly, the aniline substituents have no significant influence on the MMA
polymerization in this [LnCoCl2] (Ln = LA – LH) system, in which is difficult to observe the
steric and electronic effects by N'-aromatic substitution (aniline moiety). Thus, to evaluate the
total steric hindrance of the bulky ligands towards metal center, it can be predicted and
quantitatively calculated by comparison through a topographic steric map of the Co(II)
complexes by use of the program “SambVca” [27]. Figure 7 shows a ball and stick model,
space-filling mode and a topographic steric map of [LnCoCl2] (Ln = LA – LH) for presenting
the bulky ligands. Note that steric hindrance of [LnCoCl2] (Ln= LB, LD, LF and LH) which
have 3,5-dimethyl substituents at the pyrazolyl residue is bigger than that of the
corresponding [LnCoCl2] (Ln= LA, LC, LE and LG) complexes, having a 1-H-pyrazole moiety,
by judging the buried volume % of the ligands in Fig. 7. Moreover, the more sterically
hindered [LnCoCl2] (Ln= LB, LD, LF and LH) has more open reaction space for the monomer
MMA to approach the metal center in the directions of north and south in the steric map than
the less sterically hindered [LnCoCl2] (Ln= LB, LD, LF and LH). Thus, it may be reasonable to
interpret that the more sterically hindered [LnCoCl2] (Ln= LB, LD, LF and LH), which has 3,5-
dimethyl substituents at the pyrazolyl residue, gives higher activity for the polymerization of
methylmethacrylate than the corresponding the less sterically hindered [LnCoCl2] (Ln= LA,
LC, LE and LG), having the 1-H-pyrazole moiety. However, only considering the total steric
factor from the topographic steric map of [LnCoCl2] (Ln = LA – LH), no relationship was
found between the total steric factor of ligands and the activity for the MMA polymerization
of the Co(II) complexes. Finally, both the steric and electronic effects of the catalysts are
carefully considered to correlate the polymerization activity and the structure of the
complexes. Further, the solubility of the complexes or active species in the polymerization
solvent also substantially influences the polymerization activity.
The behavior of the Co(II) system under investigation as compared with that of the
18

previously reported N,N-bis{(1-pyrazolyl)methyl}aniline Co(II)/MMAO system yielded
PMMA with high molecular weights and narrow PDIs with improved syndiotactic
enchainment and a higher Tg value of 60 °C [11b]. Similarly, in comparison with our
previously reported 5-coordinated Co(II) complexes bearing N,N-bis((1H-pyrazol-1-
yl)methyl)cyclopentanamine
and
N,N-bis((3,5-dimethyl-1H-pyrazol-1-
yl)methyl)cyclopentanamine [21], the current catalytic system exhibited comparable
activities and stereoselectivities, yielding PMMA with a higher molecular weight (11.5 ×
105 g/mol) and Tg value (Table 4).
1
The tacticity of PMMA was analyzed by H NMR spectroscopy. The tacticity of
PMMA can be identified as syndiotactic (rr, 0.85 ppm), heterotactic (mr, 1.02 ppm) and
isotactic (mm, 1.21 ppm) [28,29]. The syndiotacticity of the resultant PMMA ranged from
0.67 to 0.70, and was similar for all the [LnCoCl2] complexes used, regardless of the aniline
substituents. Although the moderate syndiotacticity was not sufficient to confer a mechanism
of the coordination polymerisation, a steric effect was not clearly seen for [LnCoCl2] (Ln = LA
– LH) during the MMA polymerisation. From this data, it is also evident that the
syndiotacticity was not affected by variation of the substituents on aniline as well as the
pyrazole ring. Moreover, the molecular weights of the resultant PMMAs and the molecular
weight distributions are also not significantly influenced by the substituents on the ligand
framework. It can be concluded that the ligand architecture in the current study has made no
dramatic effect on the activity of resultant PMMA as the activity, in some cases, is essentially
the same both for the synthesized cobalt complexes and the starting material. The MMA
polymerization activity of the complexes in the current study should be considered as a
function of steric bulk in the local part of the ligand around the metal center. Further
modification of the ligand architecture to improve the catalytic performance and the resultant
stereo-control of the MMA polymerization are presently on-going in our laboratory.
19

3.4. rac-LA polymerization
The ring opening polymerization of rac-LA was effectively initiated by the dimethyl
derivatives [LnCoMe2] (Ln = LA – LH), yielding PLAs with high molecular weights and
narrow molecular weight distributions (Mw/Mn = 1.04−1.24) at -50 °C in CH2Cl2. The active
catalytic species were generated in situ, by treating the dichloro cobalt complexes [LnCoCl2]
(Ln = LA – LH) with two equivalents of MeLi in THF. The representative polymerization data
are tabulated in Table 5. Complete conversion of rac-LA to PLA was confirmed by the
1
absence of monomer signals in the H NMR spectra. The number average molecular weights
1
(Mn) were determined based on end-group analysis of the H NMR spectra and by GPC in
1
THF relative to polystyrene standards. The Mn determined from the H NMR spectra of the
obtained PLA and those determined by GPC were almost identical to the Mn (corrected using
the Mark–Howink factor of 0.58) [30] value obtained from the monomer to initiator ratio.
The narrower polydispersities (1.14–1.31) indicate that the polymerisation is well controlled
with a single reaction site provided by these dimethyl cobalt complexes. No activity has been
exhibited by the dichloro complexes [LnCoCl2] (Ln = LA – LH) screened for the ROP of rac-
LA under same experimental conditions.
The polymerization data revealed that the Co(II) complexes effectively polymerized
the ROP of rac-LA (Table 5) with 100% conversion. In a previous study, complexes with
more sterically demanding ligands are more active towards lactide polymerization, which
might be due to the enhanced electron density on the metal center. However, previously
reported systems where sterically hindered groups tend to block the coordination/insertion of
the incoming monomer, there is an adverse effect on the catalytic activity [31,32]. These
result indicated that there is no corelaton of the steric and electronic factors to the activity for
the ROP of rac-LA. Note, these dimethyl Co(II) complexes exhibited higher activities with
20

better polymerization control and comparable stereoselectivities as compared to the dimethyl
Zn(II) complexes reported by our group [33] (Table 5).
1
The H NMR spectra showed peaks for the methane protons of –CH(CH3)(OH) at 
2.90 ppm and the signals for –C(=O)CH3 at the other terminus overlapped with those of the
PLA backbone. A coordination insertion mechanism [34-36] for the ROP of rac-LA can be
anticipated where initially LA is coordinated to the Co(II) center to yield a 5-coordinated
intermediate, followed by the cleavage of the acyl-oxygen bond, opening the monomer
ring. Further, another molecule of rac-LA undergoes ring opening by coordinating to the
Co(II) center in similar manner. The following addition of LA produces hetero-enriched PLA.
The stereo-selectivity of PLA is slightly affected by the structure of the ligand
framework attached to central metal atom during the polymerization process. Microstructural
analysis of the obtained PLA was performed by inspecting the methane proton region of the
1
homo-decoupled H NMR spectra and the Pr values were calculated with the equation Pr =
2I1/(I1 + I2) where I1 = (sis + sii) and I2 = (iis + iii + isi) [37-39]. It is clear from the
polymerization data that all the complexes exhibited a preference for heterotactic
enchainment (Table 5). It has been observed that among the synthesized Co(II) complexes
enhancing the steric hindrance on the pyrazole moiety leads to an increased heterotacticity, i.e.
[LnCoCl2] (Ln= LB, LD, LF and LH) (the highest Pr = 0.85, Table 5, entry 7) compared to
[LnCoCl2] (Ln= LA, LB, LC and LD) (the lowest Pr = 0.78, Table 5, entry 1), indicating that
the larger steric hindrance might probably impede the regularity of monomer insertion
[32,33b].
The polymerization results showed that the catalytic activities of these dimethyl
complexes were not affected by the steric hindrance provided by the ligand framework
around the metal center. The increase in the steric bulk and the electronic effect due to the
methyl substituents on the pyrazole moiety around the metal center have a positive effect,
21

resulting in better stereoselectivities. However, the substituents on aniline moiety do not exert
an influence on the activity and stereoselectivity towards the ROP of rac-LA. More
investigations are ongoing to fully describe the role of the current catalytic system and the
mechanism of the ROP of rac-LA.
4. Conclusion
In summary, we have investigated the synthesis and X-ray crystallographic structures
of [LnCoCl2] (Ln = LA - LH), which were prepared by the reaction of the corresponding metal
starting materials and N,N',N-bis((1H-pyrazol-1-yl)methyl)amine derivatives in high yield
and purity. The molecular structures of the 4-coordinated Co(II) complexes were found to be
distorted tetrahedral, obtained via coordination of the Npyrazole atoms to the metallic center.
[LDCoCl2] exhited highest catalytic activity and yielded high molecular weight syndiotactic
PMMA with slightly broader PDIs. The MMA polymerization activity of the complexes in
the current study should not be considered as a function of the total steric encumbrance in the
ligand around the metal center. However, local steric hindrance and the electronic effect of
the methyl substituents on the pyrazole moiety enhanced the activity of MMA polymerization
compared to the corresponding complex with non-methyl substitution on the pyrazole moiety.
Moreover, the syndiotacticity was not affected much by the substituents of the pyrazole and
aniline moieties. The dimethyl derivatives of the synthesized complexes effectively
polymerized rac-LA and yielded heterotactic PLA. The presence or absence of a methyl
group in the pyrazole influences not only the solubility of the complexes but also does have
some electronic influence, which in turn has an influence on the activity of the Co(II)
complexes in the MMA polymerization and on the stereoselectivity of the Co(II) initiators in
the ROP of rac-LA .
Supplementary materials
22

CCDC 1579005-157010 contains the supplementary crystallographic data for complexes
[LCCoCl2]-[LHCoCl2]. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgments
This research was supported by the National Research Foundation of Korea (NRF), funded
by the Ministry of the Education, Science and Technology (MEST) (Grant No.
2017R1A2B4002100). X-ray crystallography with the PLS-II 2D-SMC beamline was
supported by MSIP and POSTECH.
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27

Scheme 1. Synthetic route to the ligands and their corresponding Co(II) complexes [LnCoCl2] (Ln = LA – LH).
28

Table 1. Crystal data and structural refinement for [LnCoCl2] (Ln = LC – LH).
[LCCoCl2]
C16H19Cl2N5Co, CH2Cl2
496.12
[LDCoCl2]
C20H27Cl2N5Co
467.29
[LECoCl2]
C15H17Cl2CoN5O1
413.17
[LFZnCl2]
C19H25Cl2N5CoO
469.27
[LGCoCl2]
C14H14Cl2N5CoF
401.13
[LHCoCl2]
C18H22Cl2N5CoF, CH2Cl2
542.16
mpirical formula
ormula weight
emperature (K)
200(2)
100(2)
200(2)
100(2)
200(2)
100(2)
Wavelength (Å)
0.71073
0.610
0.71073
0.610
0.71073
0.610
Crystal system
Monoclinic
Cc
Monoclinic
P21/c
Triclinic
P-1
Monoclinic
P21/n
Triclinic
Monoclinic
P21/n
pace group
P-1
Unit cell dimensions
(Å)
13.6758(2)
19.180(2)
8.6605(1)
90
12.891(3)
15.504(3)
13.943(3)
90
8.5385(9)
13.5955(2)
15.8732(2)
84.027(2)
84.864(2)
86.870(2)
1823.4(3), 2
1.505
19.411(4)
12.463(3)
19.504(4)
90
7.1932(7)
10.2051(9)
12.8954(1)
101.366(2)
103.410(2)
108.419(2)
835.31(1), 2
1.595
8.3130(2)
15.881(3)
18.312(4)
90
(Å)
(Å)
(°)
99.607(2)
90
114.04(3)
90
114.76(3)
90
102.64(3)
90
(°)
(°)
Volume (Å3), Z
2239.8(4), 4
1.471
2545.0(1), 4
1.220
4284.5(2), 8
1.455
2358.9(9), 4
1.527
Density (calculated) (Mg/m3)
Absorption coefficient (mm-1)
(000)
1.255
0.588
1.246
0.701
1.361
0.786
1012
972
844
1944
406
1108
Crystal size (mm3)
heta range for data collection (°)
ndex ranges
0.32 × 0.22 × 0.18
1.85 to 28.31
0.100 × 0.050 × 0.030
1.776 to 25.999
0.26 × 0.24 × 0.23
1.29 to 27.02
0.200 × 0.050 × 0.030
1.666 to 27.999
0.38 × 0.17 × 0.12
2.20 to 28.28
0.150 × 0.050 × 0.030
1.472 to 28.000
-12  h  12, -24  k  24,
-28  l  28
32367
-12  h  18, -25  k  23, -18  h  18, -21  k  22, -10  h  10, -17  k  12, -29  h  29, -19  k  19, -9  h  9, -13  k  13,
-11  l  11
-20  l  20
-20  l  20
-30  l  30
-17  l  11
Reflections collected
8180
27619
12485
57036
6093
29