Zinc(II) complexes containing N′-aromatic group substituted N,N′,N-bis((1H-pyrazol-1-yl)methyl)amines: Synthesis, characterization, and polymerizations of methyl methacrylate and rac-lactide

Article abstract of DOI:10.1080/00958972.2018.1437266

A novel series of Zn(II) complexes [L_nZnCl₂] (L_n?=?L_A???L_F) based on N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine bidentate ligands, N,N-bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline [L_A], N

Full text of DOI:10.1080/00958972.2018.1437266

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Zinc(II) complexes containing N′-aromatic
group substituted N,N′,N-bis((1H-pyrazol-1-
yl)methyl)amines: Synthesis, characterization, and
polymerizations of methyl methacrylate and rac-
lactide
Sujin Shin, Hyungwoo Cho, Hyosun Lee, Saira Nayab & Younghak Kim
To cite this article: Sujin Shin, Hyungwoo Cho, Hyosun Lee, Saira Nayab & Younghak Kim
(2018): Zinc(II) complexes containing N′-aromatic group substituted N,N′,N-bis((1H-pyrazol-1-
yl)methyl)amines: Synthesis, characterization, and polymerizations of methyl methacrylate and rac-
lactide, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2018.1437266
To link to this article: https://doi.org/10.1080/00958972.2018.1437266
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Journal of Coordination Chemistry, 2018
https://doi.org/10.1080/00958972.2018.1437266
Zinc(II) complexes containing N′-aromatic group substituted
N,N′,N-bis((1H-pyrazol-1-yl)methyl)amines: Synthesis,
characterization, and polymerizations of methyl methacrylate
and rac-lactide
Sujin Shina, Hyungwoo Choa, Hyosun Leea, Saira Nayabb and Younghak Kimc
adepartment of Chemistry and Green-nano materials research Center, Kyungpook national university,
b
daegu, republic of Korea; department of Chemistry, shaheed Benazir Bhutto university, sheringal dir (u),
c
islamic republic of Pakistan; Pohang accelerator laboratory, Pohang university of science and technology,
Pohang, republic of Korea
ABSTRACT
ARTICLE HISTORY
received 3 october 2017
accepted 14 January 2018
A novel series of Zn(II) complexes [LnZnCl2] (Ln = LA − LF) based on
N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine bidentate ligands, N,N-
bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline [LA], N,N-bis((1H-
pyrazol-1-yl)methyl)-2,6-dimethylaniline [LB], N,N-bis((1H-pyrazol-1-yl)
methyl)-2,6-diethylaniline [LC], N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-
diisopropylaniline [LD], N,N-bis((1H-pyrazol-1-yl)methyl)-4-bromoaniline
[LE] and N,N-bis((1H-pyrazol-1-yl)methyl)benzhydrylamine [LF],
has been synthesized and characterized. X-ray structures of these
Zn(II) complexes showed a distorted tetrahedral geometry. No
interaction exists between the Namine and the Zn(II) center in the
[LnZnCl2] (Ln = LA − LF) complexes, resulting in formation of an eight-
membered chelate ring. [LFZnCl2] exhibited the highest catalytic
activity (3.95 × 104 g PMMA/mol·Zn·h) for the polymerization of methyl
methacrylate (MMA) in the presence of modified methylaluminoxane
(MMAO) at 60 °C and yielded high molecular weight (Mw) (11.0 × 105 g/
mol) of poly(methylmethacrylate) (PMMA). All the complexes resulted
in syndiotactic enriched PMMA with high Tg (125–131 °C). The steric
bulk of ligand architecture plays an influential role in controlling
the catalytic activity and stereoregularity of the resultant PMMA.
Further, alkyl derivatives [LnZnMe2] (Ln = LA − LF) of synthesized Zn(II)
complexes, generated in situ, showed moderate to high activities
toward ring opening polymerization (ROP) of rac-lactide (rac-LA) and
yielded heterotactic polylactide (PLA) with Pr up to 0.95 at −50 °C. The
activity and stereoselectivity toward ROP of rac-LA by these dimethyl
Zn(II) complexes should be considered as a combined effect of steric
hindrance and electronic density around the metal center.
KEYWORDS
Zinc(ii) complexes;
N,N′,N-bis((1h-pyrazol-
1-yl)methyl)amines;
syndiotactic-enriched
poly(methylmethacrylate);
hetero-enriched polylactide;
steric maps
CONTACT hyosun lee
hyosunlee@knu.ac.kr; younghak Kim
iyhkim@postech.ac.kr
supplemental data for this article can be accessed at https://doi.org/10.1080/00958972.2018.1437266.
© 2018 informa uK limited, trading as taylor & francis Group

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S. SHIN ET AL.
1. Introduction
Considerable efforts have been made toward the synthesis of pyrazolyl-based transition
metal complexes due to their efficient synthesis and various modifications on a linker unit
of two or three pyrazole moieties [1–8]. The pyrazolyl group has allowed the construction
of various bi- or polydentate ligands, thus furnishing metal complexes of variable coordina-
tion geometries and nuclearities [9, 10].
In particular, the N,N-bispyrazolyl-based chelating ligands were first reported by Driessen
in 1982 [11]. Since then, owing to their structural stability and catalytic ability, a variety of
pyrazolyl complexes have been employed as cancer sensors and as oxidation and hydrolysis
agents [1, 5, 6, 12]. For instance, Driessen et al. synthesized and characterized bi- and triden-
tate pyrazole ligands such as 1-[2-ethylamino]ethyl]-3,5-dimethylpyrazole (deae), bis-[3,5-di-
methylpyrazolyl]methyl]ethylamine (bdmae) and bis-[(3,5-diemtylpyrazolyl)ethyl]ethylamine
(ddae) [13, 14]. Specifically, N-substituted pyrazolyl amines and their derivatives are versatile
ligands due to their ability to change the nature, number, and position of substituents of
the amine or pyrazole moiety, thus allowing a fine tuning of the reactivity of the metal center
to which they are bound. A variety of transition metal complexes, including Rh(I), Ru(II), Pd(II),
Pt(II), Zn(II), and Co(II), ligated to N-substituted pyrazolyl amines have exhibited diverse
potentially useful chemical properties [15–20], including roles as industrial catalysts in olefin
oligomerization or polymerization [21, 22], as bioinorganic materials in pharmaceutical
preparations [23–26], and as metal ion extractants [27].
As an extension of our continuing interest in the field of stereoselective methyl meth-
acrylate (MMA) and rac-lactide (rac-LA) polymerization, we have recently investigated Co(II),
Zn(II), Pd(II), Cd(II) and Cu(II) complexes with N,N-bis(1H-pyrazolyl-1-methyl)aniline and
N-substituted N-(pyridin-2-ylmethyl)amine and their derivatives, exhibiting diverse coordi-
nation modes with moderate to high activities and stereoselectivities [28–31].
Recently, the coordination chemistry of Zn(II) toward a polymerization of lactide has
received increased attention due to concerns regarding its impact on the environment and
its toxicological effects on health [32, 33]. Choosing the appropriate ligand architecture is
crucial for controlled polymerization and resultant stereo-regularities. The fact that small

JOURNAL OF COORDINATION CHEMISTRY
3
variations in ligand architecture may lead to profound changes in catalytic capabilities and
stereoselectivities of polymerization encouraged us to synthesize new Zn(II) complexes
bearing N′-aromatic-group substituted pyrazolyl amine derivatives. We report in this article
the synthesis, structures and reactivities of a series of mononuclear Zn(II) complexes toward
MMA polymerization as well as their dimethyl derivatives, generated in situ, in ring opening
polymerization (ROP) of rac-LA. Furthermore, the effect of structural modification of ligand
architecture on their catalytic activities and stereoregularities will also be discussed.
2. Experimental
2.1. Materials
1H-pyrazole, para-formaldehyde, 3,5-dimethylaniline, 2,6-dimethylaniline, 2,6-diethylaniline,
2,6-diisopropylaniline, 4-bromoaniline, benzhydrylamine, magnesium sulfate (MgSO4), anhy-
drous [ZnCl2] and methyl methacrylate (MMA) were purchased from Sigma-Aldrich (St. Louis,
MO) and anhydrous solvents, such as C2H5OH, DMF, hexane, and CH2Cl2 were purchased
from Merck (Darmstadt, Germany) and used without further purification. Modified meth-
ylaluminoxane (MMAO) was purchased from Tosoh Finechem Corporation (Tokyo, Japan) as
5.9% aluminum (by weight) in a toluene solution and used without further purification.
1H-pyrazolyl-1-methanol, as starting material, was prepared according to the reported
method [11]. The synthesis of LA − LE was carried out as reported previously [34–38].
2.2. Physical measurements
Elemental analyses (C, H, and N) of the synthesized ligands and their corresponding dichloro
Zn(II) complexes were performed on an elemental analyzer (EA 1108; Carlo-Erba, Milan, Italy).
1H-NMR (operating at 500 MHz) and 13C-NMR (operating at 125 MHz) spectra were recorded
on an Avance Digital 500 NMR spectrometer (Bruker, Billerica, MA); chemical shifts were
recorded in ppm units (δ) relative to SiMe4 as an internal standard. Infrared (IR) spectra were
recorded on a Bruker FT/IR-Alpha (neat) and the data were reported in reciprocal centimeters
(cm−1). The molecular weights and molecular weight distributions of the obtained poly(meth-
ylmethacrylate) (PMMA) were determined using gel permeation chromatography (GPC) (THF,
Alliance e2695; Waters Corp., Milford, MA). 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 ligands and corresponding complexes
2.3.1.1. N,N-bis((1H-pyrazol-1-yl)methyl)benzhydrylamine
(LF).
A
solution
of
benzhydrylamine (2.00 mL, 11.6 mmol) in CH2Cl2 (40.0 mL) was slowly added to a solution
of 1H-1-pyrazolyl-1-methanol (2.28 g, 23.2 mmol) in CH2Cl2 (40.0 mL). The reaction solution
was stirred at ambient temperature for 72 h and dried over MgSO4. The solvent was removed
under reduced pressure and the crude residue was vacuum distilled to give a yellow oil
(2.93 g, 73.7%). Analysis calculated for C21H21N5 (%): C, 73.4; H, 6.16; N, 20.4. Found: C, 73.8;
1
H, 6.26; N, 19.9. H-NMR (DMSO, 500 MHz): δ 7.55 (d, 2H, J = 1.5 Hz, –N=CH–CH=CH–N–),
7.51 (d, 2H, J = 2.3 Hz, –N=CH–CH=CH–N–), 7.39 (d, 4H, J = 7.3 Hz, –N–CH-m-(C6H5)2−), 7.31
(t, 4H, J = 7.6 Hz, –N–CH-o-(C6H5)2−), 7.22 (t, 2H, J = 7.3 Hz, –N–CH-p-(C6H5)2−), 6.29 (dd, 2H,

4
S. SHIN ET AL.
J = 2.0 Hz, J = 2.0 Hz, –N=CH–CH=CH–N–), 5.12 (s, 1H, –N–CH–(C6H5)2−), 4.99 (s, 4H, –N–
CH2–N–). 13C-NMR (DMSO, 125 MHz): δ 141.40 (s, 2C, –N–CH-ipso-(C6H5)2−), 139.25 (d, 2C,
J = 185 Hz, –N=CH–CH=CH–N–), 130.23 (d, 2C, J = 187 Hz, –N=CH–CH=CH–N–), 128.66 (d,
4C, J = 161 Hz, –N–CH-m-(C6H5)2−), 128.04 (d, 4C, J = 159 Hz, –N–CH-o-(C6H5)2−), 127.35 (d,
2C, J = 161 Hz, –N–CH-p-(C6H5)2−), 105.39 (d, 2C, J = 176 Hz, –N=CH–CH=CH–N–), 67.70 (d,
1C, J = 137 Hz, –N–CH–(C6H5)2−), 64.11 (t, 2C, J = 151 Hz, –N–CH2–N–). IR (oily liquid neat;
cm−1): 3029 (w), 1598 (w), 1513 (m), 1492 (w), 1451 (m), 1391 (m), 1283 (m), 1188 (s), 1139
(m), 1083 (s), 1044 (s), 963 (m), 741 (s), 701 (s), 612 (m).
2.3.1.2. N,N-bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylanilinezinc(II)chloride([LAZnCl2]).
A
solution of LA (0.800 g, 2.84 mmol) in anhydrous EtOH (10.0 mL) was slowly added to a
solution of [ZnCl2] (0.390 g, 2.84 mmol) in anhydrous EtOH (10.0 mL). Precipitation of white
material occurred while stirring at 25 °C for 12 h. The white powder was filtered and washed
with cold EtOH (20.0 mL × 3), followed by washing with hexane (20.0 mL × 3) to yield a final
crystalline product (1.00 g, 84.0%). Analysis calculated for C16H19Cl2N5Zn (%): C, 46.0; H, 4.59;
N, 16.8. Found: C, 45.8; H, 4.58; N, 16.9. 1H-NMR (DMSO, 500 MHz): δ 7.81 (d, 2H, J = 2.1 Hz,
–N=CH–CH=CH–N–), 7.51 (d, 2H, J = 1.2 Hz, –N=CH–CH=CH–N–), 6.77 (s, 2H, o-NC6H3(CH3)2−),
6.44 (s, 1H, p–NC6H3(CH3)2−), 6.25 (dd, 2H, J = 2.0 Hz, J = 2.0 Hz, –N=CH–CH=CH–N–), 5.87 (s,
4H, –N–CH2–N–), 2.16 (s, 6H, –NC6H3(CH3)2). 13C-NMR (DMSO, 125 MHz): δ 145.49 (s, 1C, ipso-
NC6H3(CH3)2−), 139.43 (d, 2C, J = 185 Hz, –N=CH–CH=CH–N–), 138.31 (s, 2C, m-NC6H3(CH3)2−),
130.02 (d, 2C, J = 188 Hz, –N=CH–CH=CH–N–), 121.46 (d, 1C, J = 157 Hz, p-NC6H3(CH3)2−),
112.37 (d, 2C, J = 156 Hz, o-NC6H3(CH3)2−), 105.89 (d, 2C, J = 177 Hz, –N=CH–CH=CH–N–),
66.19 (t, 2C, J = 153 Hz, –N–CH2–N–), 21.72 (q, 2C, J = 126 Hz, –NC6H3(CH3)2−). IR (solid neat;
cm−1): 3114 (w), 2917 (w), 1603 (m), 1517 (w), 1480 (w), 1415 (m), 1325 (m), 1248 (m), 1167
(s), 1073 (s), 958 (w), 823 (s), 764 (s), 687 (m), 613 (m), 585 (w).
2.3.1.3. N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-dimethylanilinezinc(II)chloride([LBZnCl2]). Following
aprocedure similar to that described for [LAZnCl2], EtOH solution of LB (0.800 g, 2.84 mmol) and
[ZnCl2] (0.390 g, 2.84 mmol) was used instead. After workup the white crystalline solid was ob-
tained as a final product (1.05 g, 88.5%). Analysis calculated for C16H19Cl2N5Zn (%): C, 46.0; H, 4.59;
N, 16.8. Found: C, 45.8; H, 4.61; N, 16.5. 1H-NMR (DMSO, 500 MHz): δ7.65 (d, 2H, J = 2.0 Hz, –N=CH–
CH=CH–N–), 7.55 (d, 2H, J = 1.4 Hz, –N=CH–CH = CH–N–), 6.97–7.03 (m, 3H, m,o-NC6H3(CH3)2−),
6.27 (dd, 2H, J = 2.1 Hz, J = 2.1 Hz, –N=CH–CH=CH–N–), 5.47 (s, 4H, –N–CH2–N–), 1.67 (s, 6H,
–NC6H3–(CH3)2). 13C-NMR (DMSO, 125 MHz): δ 143.73 (s, 2C, ipso-NC6H3(CH3)2−), 139.85 (d, 2C,
J = 186 Hz, –N=CH–CH=CH–N–), 137.59 (s, 2C, –N=CH–CH=CH–N–), 130.75 (d, 2C, J = 189 Hz, m-
NC6H3(CH3)2−), 128.84 (d, 2C, J = 157 Hz, o-NC6H3(CH3)2−), 126.82 (d, 1C, J = 160 Hz, p-NC6H3(CH3)2−),
106.03 (d, 2C, J = 179 Hz, –N=CH–CH=CH–N–), 68.47 (t, 2C, J = 152 Hz, –N–CH2–N–), 17.59 (q,
2C, J = 127 Hz, –NC6H3(CH3)2−). IR (solid neat; cm−1): 3136 (w), 2928 (w), 1605 (w), 1515 (w), 1469
(w), 1410 (m), 1301 (m), 1257 (m), 1168 (s), 1067 (s), 976 (w), 949 (w), 917 (w), 779 (s), 742 (s), 639
(m), 610 (s), 587 (m).
2.3.1.4. N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-diethylaniline zinc(II) chloride ([LCZnCl2]). Following
a procedure similar to that described for [LAZnCl2], EtOH solution of LC (0.800 g, 2.59 mmol)
and [ZnCl2] (0.350 g, 2.59 mmol) were used instead. After workup the white crystalline
solid was obtained as a final product (0.960 g, 83.3%). Analysis calculated for C18H23Cl2N5Zn

JOURNAL OF COORDINATION CHEMISTRY
1
5
(%): C, 48.5; H, 5.20; N, 15.7. Found: C, 48.0; H, 5.39; N, 15.1. H-NMR (DMSO, 500 MHz): δ
7.66 (d, 2H, J = 2.3 Hz, –N=CH–CH=CH–N–), 7.57 (d, 2H, J = 1.5 Hz, –N=CH–CH=CH–N–),
7.17 (t, 1H, J = 7.6 Hz, p-NC6H3(CH2CH3)2−), 7.06 (d, 2H, J = 7.6 Hz, m-NC6H3(CH2CH3)2−), 6.28
(dd, 2H, J = 2.1 Hz, J = 2.1 Hz, –N=CH–CH=CH–N–), 5.50 (s, 4H, –N–CH2–N–), 1.99 (q, 4H,
J = 7.5 Hz, –NC6H3(CH2CH3)2−), 0.91 (t, 6H, J = 7.6 Hz, –NC6H3(CH2CH3)2−). 13C-NMR (DMSO,
125 MHz): δ 143.10 (s, 1C, ipso-NC6H3(CH2CH3)2−), 142.67 (s, 2C, o-NC6H3(CH2CH3)2−), 139.58
(d, 2C, J = 186 Hz, –N=CH–CH=CH–N–), 130.32 (d, 2C, J = 187 Hz, –N=CH–CH=CH–N–), 127.19
(d, 2C, J = 160 Hz, m-NC6H3(CH2CH3)2−), 126.26 (d, 1C, J = 157 Hz, p-NC6H3(CH2CH3)2−), 105.89
(d, 2C, J = 177 Hz, –N=CH–CH=CH–N–), 69.18 (t, 2C, J = 152 Hz, –N–CH2–N–), 22.65 (t, 2C,
J = 127 Hz, –NC6H3(CH2CH3)2−), 14.77 (q, 2C, J = 126 Hz, –NC6H3(CH2CH3)2−). IR (solid neat;
cm−1): 3120 (w), 2970 (w), 2876 (w), 1516 (w), 1460 (w), 1413 (m), 1303 (m), 1257 (m), 1173
(s), 1066 (s), 977 (w), 912 (w), 819 (w), 770 (s), 741 (m), 614 (m), 587 (w).
2.3.1.5. N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-diisopropylanilinezinc(II)chloride([LDZnCl2]). Following
a procedure similar to that described for [LAZnCl2], EtOH solution of LD (0.800 g, 2.37 mmol)
and [ZnCl2] (0.320 g, 2.37 mmol) were used instead. After workup the white crystalline
solid was obtained as a final product (1.05 g, 93.4%). Analysis calculated for C20H27Cl2N5Zn
1
(%): C, 50.7; H, 5.74; N, 14.8. Found: C, 51.1; H, 6.23; N, 13.9. H-NMR (DMSO, 500 MHz): δ
7.76 (d, J = 1.8 Hz, –N=CH–CH=CH–N–), 7.54 (d, 2H, J = 1.4 Hz, –N=CH–CH=CH–N–), 7.20
(t, 1H, J = 7.6 Hz, p-NC6H3(CH(CH3)2)2−), 7.08 (d, 2H, J = 7.6 Hz, m-NC6H3(CH(CH3)2)2−), 6.27
(dd, 2H, J = 2.0 Hz, J = 2.0 Hz, –N=CH–CH=CH–N–), 5.55 (s, 4H, –N–CH2–N–), 2.64 (m, 2H, –
NC6H3(CH(CH3)2)2−), 0.90 (d, 12H, J = 6.7 Hz, –NC6H3(CH(CH3)2)2−). 13C-NMR (DMSO, 125 MHz):
δ 148.24 (s, 1C, ipso-NC6H3(CH(CH3)2)2−), 141.29 (s, 2C, –N=CH–CH=CH–N–), 139.87 (d, 2C,
J = 184 Hz, o-NC6H3(CH(CH3)2)2−), 130.91 (d, 2C, J = 187 Hz, –N=CH–CH=CH–N–), 127.95 (d,
2C, J = 183 Hz, m-NC6H3(CH(CH3)2)2−), 124.63 (d, 1C, J = 157 Hz, p-NC6H3(CH(CH3)2)2−), 105.91
(d, 2C, J = 176 Hz, –N=CH–CH=CH–N–), 70.72 (t, 2C, J = 151 Hz, –N–CH2–N–), 27.99 (d, 2C,
J = 130 Hz, –NC6H3(CH(CH3)2)2−), 24.79 (q, 4C, J = 126 Hz, –NC6H3(CH(CH3)2)2−). IR (solid neat;
cm−1): 3108 (w), 2970 (w), 2870 (w), 1515 (w), 1408 (m), 1300 (m), 1254 (m), 1174 (s), 1072
(s), 978 (m), 812 (m), 779 (s), 609 (s), 539 (m).
2.3.1.6. N,N-bis((1H-pyrazol-1-yl)methyl)-4-bromoanilinezinc(II)chloride([LEZnCl2]). Following
a procedure similar to that described for [LAZnCl2], EtOH solution of LE (0.800 g, 2.41 mmol)
and [ZnCl2] (0.390 g, 2.41 mmol) were used instead. After workup the white crystalline solid
was obtained as a final product (0.89 g, 79.3%). Analysis calculated for C14H14BrCl2N5Zn (%):
C, 35.9; H, 3.01; N, 15.0. Found: C, 36.2; H, 3.04; N, 14.8. 1H-NMR (DMSO, 500 MHz): δ 7.84 (d,
2H, J = 2.1 Hz, –N=CH–CH=CH–N–), 7.52 (d, 2H, J = 1.4 Hz, –N=CH–CH=CH–N–), 7.34 (d, 2H,
J = 9.2 Hz, m-NC6H4Br-), 7.16 (d, 2H, J = 9.2 Hz, o-NC6H4Br–), 6.27 (dd, 2H, J = 2.0 Hz, J = 2.0 Hz,
–N=CH–CH=CH–N–), 5.91 (s, 4H, –N–CH2–N–). 13C-NMR (DMSO, 125 MHz): δ 144.82 (s, 1C,
ipso-NC6H4Br–), 139.67 (d, 2C, J = 181 Hz, –N=CH–CH=CH–N–), 131.90 (d, 2C, J = 165 Hz,
m-NC6H4Br–), 130.20 (d, 2C, J = 184 Hz, –N=CH–CH = CH–N–), 116.69 (d, 2C, J = 155 Hz, o-
NC6H4Br–), 111.37 (s, 1C, p-NC6H4Br–), 106.13 (d, 2C, J = 186 Hz, –N=CH–CH=CH–N–), 66.10
(t, 2C, J = 152 Hz, –N–CH2–N–). IR (solid neat; cm−1): 3112 (w), 1589 (w), 1494 (m), 1467 (w),
1403 (m), 1319 (m), 1261 (m), 1192 (s), 1160 (s), 1069 (s), 940 (m), 815 (s), 767 (s), 735 (s), 613
(m), 591 (w).

6
S. SHIN ET AL.
2.3.1.7. N,N-bis((1H-pyrazol-1-yl)methyl)benzhydrylamine zinc(II) chloride ([LFZnCl2]). Following
a procedure similar to that described for [LAZnCl2], EtOH solution of LF (0.800 g, 2.33 mmol)
and [ZnCl2] (0.320 g, 2.33 mmol) were used instead. The solution was stirred for 12 h. After
workup the white crystalline solid was obtained as a final product (0.860 g, 76.8%). Analysis
calculated for C21H21Cl2N5Zn (%): C, 52.6; H, 4.41; N, 14.6. Found: C, 52.5; H, 4.41; N, 15.1.
1H-NMR (DMSO, 500 MHz): δ 7.54 (d, 2H, J = 1.4 Hz, –N=CH–CH=CH–N–), 7.51 (d, 2H, J = 2.1 Hz,
–N=CH–CH=CH–N–), 7.38 (d, 4H, J = 7.4 Hz, –N–CH-m-(C6H5)2−), 7.31 (t, 4H, J = 7.6 Hz, –N–
CH-o-(C6H5)2−), 7.23 (t, 2H, J = 7.2 Hz, –N–CH-p-(C6H5)2−), 6.28 (dd, 2H, J = 2.1 Hz, J = 2.1 Hz,
–N=CH–CH=CH–N–), 5.13 (s, 1H, –N–CH–(C6H5)2−), 5.01 (s, 4H, –N–CH2–N–). 13C-NMR (DMSO,
125 MHz): δ 141.24 (s, 2C, –N–CH-ipso-(C6H5)2−), 139.38 (d, 2C, J = 187 Hz, –N=CH–CH=CH–N–),
130.51 (d, 2C, J = 188 Hz, –N=CH–CH=CH–N–), 128.65 (d, 4C, J = 160 Hz, –N–CH-m-(C6H5)2−),
128.07 (d, 4C, J = 158 Hz, –N–CH-o-(C6H5)2−), 127.36 (d, 2C, J = 161 Hz, –N–CH-p-(C6H5)2−),
105.42 (d, 2C, J = 176 Hz, –N=CH–CH=CH–N–), 67.63 (d, 1C, J = 137 Hz, –N–CH–(C6H5)2−), 64.48
(t, 2C, J = 151 Hz, –N–CH2–N–). IR (solid neat; cm−1): 3115 (w), 1514 (w), 1494 (w), 1448 (w),
1404 (m), 1373 (m), 1245 (m), 1168 (m), 1134 (m), 1070 (s), 968 (m), 917 (w), 776 (s), 736 (s),
695 (s), 607 (m), 561 (w).
2.4. Typical procedures for MMA polymerization
The methyl methacrylate (MMA) was extracted with 10% sodium hydroxide, washed with
water, dried over MgSO4, and distilled over CaH2 under reduced pressure before use. In a
Schlenk line, complex (15.0 μmol, 6.30 mg for [LAZnCl2]; 6.30 mg for [LBZnCl2]; 6.70 mg for
[LCZnCl2]; 7.10 mg for [LDZnCl2]; 7.00 mg for [LEZnCl2]; 7.20 mg for [LFZnCl2];) was dissolved
in dried toluene (10.0 mL) followed by the addition of modified 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 stirred
at 60 °C for 20 min. The MMA (5.00 mL, 47.10 mmol, [MMA]0/[catalyst]0 = 3100) was added
to the above reaction mixture and stirred at 60 °C for 2 h to obtain a viscous solution. MeOH
(2.00 mL) was added to terminate the polymerization. The reaction mixture was poured into
a large quantity of MeOH (500 mL), and 35% HCl (5.00 mL) was injected to remove the
remaining co-catalyst.The resulting polymer was filtered and washed with MeOH (250 mL × 2)
to yield PMMA, which was vacuum-dried at 60 °C. The polymers were isolated as white solids
and characterized by GPC in THF using standard polystyrene as the reference.
2.5. Typical procedure for ROP of rac-lactide
In a general polymerization reaction for rac-LA with the dimethyl zinc initiators, the catalyst
species were generated as follows. 0.50 mmol of zinc complexes and dried THF (7.37 mL)
were added to a 100 mL Schlenk flask under argon atmosphere. To this solution was added
MeLi (1.00 mmol, 0.63 mL of 1.6 M solution in diethyl ether) dropwise at −78 °C to generate
in situ dimethyl Zn(II) species. After stirring for 2 h at room temperature, the resulting THF
solution of dimethyl Zn(II) complexes, i.e. [LnZnMe2] (Ln = LA − LF) was used as an initiator
for 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 argon atmos-
phere and 5.0 mL of dried CH2Cl2 was added. The polymerization was initiated by slow addi-
tion of the catalyst solution (1.0 mL, 0.0625 mmol) via a syringe under argon at −50 °C. The
reaction mixture was stirred for the allotted time and the polymerization reactions were

JOURNAL OF COORDINATION CHEMISTRY
7
quenched using H2O (1.0 mL). Then hexane (2.0 mL) was added to precipitate the polymer.
The monomer conversion was measured by 1H-NMR spectroscopy. 1H-NMR (CDCl3, 500 MHz)
for the obtained polymer: δ = 5.13–5.20 (m, 1H), 1.51–1.63 (m, 3H).
2.6. X-ray crystallographic studies
X-ray quality crystals for crystallographic studies of the synthesized Zn(II) complexes were
obtained from layering of ether on methanol solution. A colorless single crystal of each zinc
complex was picked up with paraton-N oil and mounted on a Bruker SMART CCD diffrac-
tometer or PHOTON 100 CMOS equipped with a graphite-monochromated Mo-Kα
(λ = 0.71073 Å) radiation source under nitrogen cold stream (200 K or 223 K). Data collection
and integration were performed with SMART or APEX2 and SAINT-Plus software packages
[39]. Multi-scan absorption corrections based on equivalent reflections were applied by
SADABS [40]. Structures were solved by direct methods and refined using a full-matrix least-
squares on F2 using SHELXTL or SHELXL-2016 [41]. In [LCZnCl2], the C17 and C18 atoms of
the [LC] ligand were refined as disordered over two set of sites (C17a/C17b and C18a/C18b)
with refined occupancies.
Crystallographic and structural data are summarized in Table 1. The void volumes of
[LCZnCl2] and [LDZnCl2], estimated by PLATON, are 1629 and 1699 Å3, respectively. The sol-
vent molecules, presumably methanol or water, were not identified or refined because of
their severe disorder and distribution over multiple sites, so that they could not be reliably
modeled with discrete atoms. Thus, the final refinement was performed with modification
of the structure factors to allow for the contribution of disordered solvent electron density
using the SQUEEZE option of PLATON. The electron densities assigned thus were 299 elec-
trons/unit cell for [LCZnCl2] and 387 electrons/unit cell for [LDZnCl2], respectively.
3. Results and discussion
3.1. Synthesis and chemical properties
Ligands in the current study were obtained by the single step condensation reaction of the
substituted aniline and 1H-pyrazolyl-1-methanol in CH2Cl2 or MeCN. The corresponding
[LnZnCl2] (Ln = LA − LF) complexes were obtained (approximately 76 ~ 94% yields) by straight-
forward reaction of metal starting material [ZnCl2] with ligands in a 1:1 ratio in anhydrous
EtOH (Scheme 1). All the synthesized complexes were stable and stored without any decom-
position. The formation and identity of the synthesized complexes were structurally char-
acterized using 1H-NMR, 13C-NMR, IR, elemental analysis, and X-ray diffraction studies. There
is a slight difference of proton and carbon signals in 1H-NMR and 13C-NMR spectrum of the
Zn(II) complexes relative to their respective free ligand signals. For example, the resonance
for methylene –N–CH2–N– protons appeared at δ 4.99 (s, 4H) in LF, however, in [LFZnCl2] the
signals appeared at δ 5.01 (s, 4H) ppm. Similarly the aromatic protons of the pyrazolyl moiety
resonate at δ 7.55 (d, 2H, J = 1.5 Hz, –N=CH–CH=CH–N–) and δ 7.51 (d, 2H, J = 2.3 Hz, –N=CH–
CH=CH–N–) in LF while in the corresponding Zn(II) complex these signals appeared at δ 7.54
and 7.51 ppm, respectively.
Comparison of the IR spectra of the ligands with those of the synthesized dichloro Zn(II)
complexes was also performed. New absorption bands at 585, 587, 587, 539, 591, and

8
S. SHIN ET AL.

JOURNAL OF COORDINATION CHEMISTRY
9

10
S. SHIN ET AL.
561 cm−1 have been observed for newly formed Zn–N bonds in the synthesized complexes
[LnZnCl2] (Ln = LA − LF). Elemental analyses of the synthesized complexes were consistent
with the proposed structures in Scheme 1 and confirmed the purity of isolated complexes
[LnZnCl2] (Ln = LA − LF).
3.2. Description of X-ray structures
Single crystals for X-ray diffraction studies of [LnZnCl2] (Ln = LA − LF) were obtained by lay-
ering hexane on the CH2Cl2 solution of the synthesized complexes. Selected bond angles (°)
and lengths (Å) for [LnZnCl2] (Ln = LA − LF) are compiled in Table 2. The crystal structures of
[LnZnCl2] (Ln = LA − LF) at 30% probability with the atomic labeling are shown in Figures 1–6.
Among the synthesized complexes, [LAZnCl2], [LBZnCl2], and [LEZnCl2] crystallized in mon-
oclinic system, while [LCZnCl2] (Figures 3a and 3b) and [LDZnCl2] crystallized in the trigonal
system, and [LFZnCl2] crystallized in the triclinic system.
It is evident from the molecular structures of the synthesized Zn(II) complexes that the
central zinc ion of each complex adopts distorted tetrahedral geometry by coordinating
with two pyrazole nitrogen atoms in a chelating manner and two chloro ligands. The average
Zn–Npyrazole bond lengths lie in the range of 2.014(5)–2.058(4) Å for the synthesized Zn(II)
complexes and do not vary much from similar reported complexes [42–46]. Similarly, an
average Zn–Cl (2.226 Å) bond distance is in good agreement with the analogous distance
Figure 1. orteP drawing of [LAZnCl2] with thermal ellipsoids at 30% probability.
note: all hydrogens are omitted for clarity.

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 2. orteP drawing of [LBZnCl2] with thermal ellipsoids at 30% probability.
note: all hydrogens are omitted for clarity.
Scheme 1. synthetic route of ligands and their corresponding Zn(ii) complexes [LnZnCl2] (ln = la − lf).

12
S. SHIN ET AL.

JOURNAL OF COORDINATION CHEMISTRY
13

14
S. SHIN ET AL.
Figure 3a. orteP drawing of [LCZnCl2] with thermal ellipsoids at 30% probability.
note: all hydrogens are omitted for clarity.
found in similar complexes [47–49]. Lack of interaction between Namine and the Zn(II) center
in these four-coordinate complexes resulted in an eight-membered chelate ring which is
consistent with our previously reported complexes [29]. The crystallographic data revealed
that much smaller Npyrazole–Zn–Npyrazole angles have been observed compared to Clterminal
–
Zn–Clterminal bite angles around the central zinc ion in [LnZnCl2] (Ln = LA − LF) (Table 2). This
is a common structural feature for [LZnX2] type of complexes, where L acts as a bidentate
chelating ligand attached to the zinc center and X is a halide, with the ligands forming a
distorted tetrahedral geometry around the zinc center. It has been observed that the Npyrazole
–
Zn–Npyrazole angles lie in an accepted range i.e. 101.9(1)–113.5(1)° [29, 35, 50]. It is evident
that Npyrazole-Zn-Npyrazole angles are slightly affected by the substituents attached to aniline
moiety. For instance, the presence of bulky substituents at ortho position leads to wider
Npyraole-Zn-Npyrazole angles (Table 2). However, the Cl(1)-Zn-Cl(2) angles are not much affected
by the substituents of the phenyl moiety. Similarly, the bond angles Npyrazole–Zn(1)–Cl(1) and
Npyrazole–Zn(1)–Cl(2) lie in the [104.0(5)°–112.2(2)°] and [104.8(7)–112.7(1)] range for [LnZnCl2]
(Ln = LA − LF), respectively. These values are similar to those reported previously for similar
complexes [29, 50]. τ4 values [51] provide useful information regarding geometry of four-co-
ordinate complexes, i.e. for perfect tetrahedron, τ4 is 1.00 and 0 for perfect square planar,
D4h [52]. In addition, THCDA/100 index provides useful analysis of tetrahedral complexes; a
value 1.00 of THCDA/100 index represents perfect tetrahedral geometry, whereas a 0 value

JOURNAL OF COORDINATION CHEMISTRY
15
Figure 3b. orteP drawing of [LCZnCl2] with thermal ellipsoids at 30% probability.
note: all hydrogens are omitted for clarity. the C17 and C18 atoms were refined as disordered over two set of sites (C17a/
C17b and C18a/C18b) with refined occupancies.
Table 4. tilt angles of [LnZnCl2] (ln = l’and la – li).
Complexes
Tilt angles (°)
References
[(N,N-bis((1h-pyrazol-1-yl)methyl)aniline)ZnCl2] = [L′ZnCl2]
52.7
49
1.99
0.41
2.14
50.8
–
8.25
40.7
9.3
[42]
[LAZnCl2]
this work
this work
this work
this work
this work
this work
[42]
[LBZnCl2]
[LCZnCl2]
[LDZnCl2]
[LEZnCl2]
[LFZnCl2]
[(N,N-bis((1h-pyrazol-1-yl)methyl)-p-methoxyaniline)ZnCl2] = [LGZnCl2]
[(N,N-bis((1h-pyrazol-1-yl)methyl)-p-fluoroaniline)ZnCl2] = [LHZnCl2]
[(N,N-bis((3,5-dimethylpyrazol-1-yl)methyl)aniline)ZnCl2] = [LIZnCl2]
[42]
[42]
of THCDA/100 refers to perfect trigonal pyramidal geometry (C3v) [53, 54]. Similarly, for useful
structural information the FCGP/100 index (Table 3) can be applied to the tetrahedral geom-
etry of main group complexes, i.e. 1.00 value defines perfect trigonal pyramidal (C3v) and 0
value reflects perfect tetrahedral geometry [55]. Table 3 shows four-coordinate geometry
indexes for the Zn(II) complexes with N′-aromatic-group substituted N,N′,N-bis((1H-pyrazol-
1-yl)methyl)amine derivatives. τ4 values for all the [LnZnCl2] (Ln = LA − LF) complexes falls

16
S. SHIN ET AL.
Figure 4. orteP drawing of [LDZnCl2] with thermal ellipsoids at 30% probability.
note: all hydrogens are omitted for clarity.
into the range of 0.897–0.950 and indicated the distortion from ideal tetrahedral geometry.
All these values are comparable to those observed in reported complexes with similar ligand
architecture (Table 3) [42, 50].
The tilt angles (Table 4) between the N′-aromatic rings on the amine moiety and the plane
of the N,N-bispyrazole ring were calculated to find the effect of N′-substitution toward tet-
rahedrality of synthesized complexes. It has been observed that the complexes with alkyl
substituents at an ortho position of the aromatic ring of amine moieties have smaller tilt
angles [LnZnCl2] (Ln = LB, LC and LD tilt angles; 2°, 0°, and 2°, respectively) compared to com-
plexes without alkyl moieties at ortho position of aromatic ring of amine moiety [LnZnCl2]
(Ln = LA and LE tilt angles; 51° and 53°, respectively). This is consistent with previously reported
complexes’tilt angles (Table 4); more steric bulk in the vicinity of the metal center results in
smaller tilt angles [42]. 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 complexes.
3.3. Methyl methacrylate (MMA) polymerization
As part of our ongoing investigations toward the stereoselective application of bis-pyrazolyl-
based initiators in MMA polymerization, the catalytic capabilities of N′-substituted

JOURNAL OF COORDINATION CHEMISTRY
17
Figure 5. orteP drawing of [LEZnCl2] with thermal ellipsoids at 30% probability.
note: all hydrogens and water molecule are omitted for clarity.
N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine-based Zn(II) complexes were assessed toward
MMA polymerization in the presence of modified MMAO at 60 °C. All the complexes [LnZnCl2]
(Ln = LA − LF) polymerized MMA effectively yielding PMMA withTg ranging from 125 to 131 °C.
The polymers were isolated as white polymeric material and characterized by GPC in THF
using standard polystyrene as a reference. The results of polymerization are summarized in
Table 5.
The blank polymerization of methyl methacrylate (MMA) was performed using starting
materials, i.e. [ZnCl2]/MMAO or solely MMAO under the same experimental conditions in
order to realize the ligand effect on the metal center. It is clear from polymerization data
that the [ZnCl2]/MMAO system exhibited comparable activity with [LDZnCl2] (Table 5, entry
7) and resulted in PMMA with low molecular weights. However, the rest of the Zn(II) initiators
showed better activities as well as superior stereoselectivities and yielded PMMA with higher
molecular weights compared to starting materials. Similarly, no polymerization activity was
observed with dichloro Zn(II) complexes in the absence of MMAO.
In the case of using [LnZnCl2] (Ln = LA − LF) for MMA polymerization, [LFZnCl2] exhibited
the highest activity (3.95 × 104 g/mol·Zn·h) and yielded PMMA with high molecular weights

18
S. SHIN ET AL.
Figure 6. orteP drawing of [LFZnCl2] with thermal ellipsoids at 30% probability.
note: all hydrogens and water molecule are omitted for clarity.
(110 × 105 g/mol) compared to the rest of its analogs under the same experimental condi-
tions. It has been observed that substituents at the ortho position of the phenyl group of
the amine moiety exhibited significant influence on the activities of these Zn(II) complexes,
whereas the stereoselectivities were not much affected. [LCZnCl2] and [LDZnCl2], with an
ethyl (–CH2CH3) and isopropyl (–CH(CH3)2) groups attached at the ortho position of the phenyl
group of the amine moiety, respectively, exhibited the lowest activities among the synthe-
sized Zn(II) complexes. However, when the substituents –CH2CH3 and –CH(CH3)2 at the ortho
position were changed to –H and –CH3, an increase in activities was observed i.e. for [LAZnCl2]
and [LBZnCl2], respectively. A trivial tendency between the steric bulk and activities was
found in complexes bearing alkyl groups, the ortho position of the phenyl group of the amine
moiety, in the order of –CH(CH3)2 < –CH2CH3 < –CH3 < H (Table 5). It is also evident from our
previous report that the bulky substituents attached to amine or pyrazole moiety negatively
affect MMA activation; probably the steric bulk around the metal center depresses the mon-
omer approach to the metal sites [35]. Clearly, in the current study the lower activity of the
complexes with bulkier ortho amine substituents toward MMA polymerization might be due
to the steric clashes of attached ligand and incoming monomer to a metal center.

JOURNAL OF COORDINATION CHEMISTRY
19
Table 5. mma polymerization by [LnZnCl2] (ln = l′ and la − lf) complexes in the presence of mmao.
e
Yieldb
(%)
Activityc
× 104
Tgd
Tacticity (%)
Mw
Catalysta
(g/molcat·h)
(°C)
mm
9.20
37.2
9.24
10.8
mr
rr
× 105 (g/mol)
PDIf
Entry
1
[ZnCl2]g
11.1
8.97
13.0
15.9
14.7
14.0
12.4
23.1
25.3
0.81
0.34
1.45
1.73
1.40
2.03
2.48
2.29
2.18
1.94
3.60
3.95
2.70
1.13
4.83
129
120
130
128
130
130
128
125
125
132
122
145
24.2
10.9
27.0
22.2
23.1
20.5
17.6
22.2
20.8
22.8
24.1
23.3
66.6
51.9
65.8
69.4
65.4
69.2
66.7
73.5
66.7
70.0
65.6
68.1
1.33
6.78
8.00
10.3
10.6
10.5
10.2
11.2
11.0
8.91
9.20
9.39
1.58
2.09
1.75
2.33
2.48
2.38
2.48
2.47
2.41
1.89
1.73
1.77
2
MMAOh
3
[L′ZnCl2]i
[LAZnCl2]
[LBZnCl2]
[LCZnCl2]
[LDZnCl2]
[LEZnCl2]
[LFZnCl2]
[LGZnCl2]j
[LHZnCl2] j
[LIZnCl2] j
4
5
11.5
10.3
6
7
8.82
8
11.1
12.5
9
10
11
12
7.31
10.3
8.51
a[m(ii) catalyst]0 = 15 μmol, [mma]0/[mmao]0/[m(ii) catalyst]0 = 3100:500:1, temp = 60 °C, time = 2 h.
byield defined (a mass of dried polymer recovered)/(a mass of monomer used).
cactivity is (g Pmma)/(molcat·h).
dTg is glass transition temperature determined using a thermal analyzer.
edetermined using gel permeation chromatography (GPC) eluted with thf at room temperature by filtration with polysty-
rene calibration.
fMn refers to the number average of molecular weights of Pmma.
git is a blank polymerization in which anhydrous [ZnCl2] were also activated by mmao.
hit is a blank polymerization which was done solely by mmao.
iit is polymerization with [L′ZnCl2]; l′ = N,N-bis((1h-pyrazol-1-yl)methyl)aniline activated by mmao [42].
jdata of mma polymerization obtained from reference [42].
In order to evaluate the steric hindrance exerted by ligand architecture toward the metal
center, a quantitative calculation through comparison of a topographic map of Zn(II) com-
plexes using the “SambVca” [56, 57] program has been carried out. Ball and stick models,
space-filling models, and topographic steric maps of [LnZnCl2] (Ln = LA − LF) for presenting
steric bulk of ligands are shown in Figure 7. The steric hindrance of [LFZnCl2] bearing at the
amine moiety is smaller compared to [LnCoCl2] (Ln = LA − LD) complexes, judging from the
buried volume % of ligands in Figure 7. Similarly, the complex [LEZnCl2] with an elec-
tron-withdrawing bromo group at the ortho position of the amine moiety exhibited lesser
steric bulk (buried volume 41.9%). It is evident from the steric map that the bulkier the
substituents at ortho position of phenyl group of amine moiety the less the space provided
for monomer approach to the metal center (as evident from their steric maps that only
monomer approach may occur only from the bottom, as shown from Figure 7, and hence
they showed lower activities. For comparison, the steric maps and models of Zn(II) complexes
[LnZnCl2] (Ln = LG − LH) [42] are presented in Figure 7. Note that the presence of 3,5-dimethyl
substituents at the pyrazole rings in [LGZnCl2] (with 46.4% buried volume) provides greater
steric hindrance than that of the corresponding [LnZnCl2] (Ln = LH and LI) which has a

20
S. SHIN ET AL.
Figure 7a. Ball and stick model, space-filling model, and topographic steric map of [LnZnCl2] (ln = l′ and
la − li) for presenting steric bulk ligands.

JOURNAL OF COORDINATION CHEMISTRY
21
Figure 7b.

22
S. SHIN ET AL.
Figure 7c.
1-H-pyrazole moiety. Moreover, [LGZnCl2], with more sterically hindered 3,5-dimethyl sub-
stituents at the pyrazolyl residue, gave higher activity (4.68 × 104 g PMMA/mol.Zn.h) (Table
5) for MMA polymerization. This might be due to more open reaction space for entering
monomer MMA to the metal center at the top and bottom in the steric map than the less
sterically hindered complexes [LnZnCl2] (Ln = LA − LF, LH and LI) having 1-H-pyrazole. Thus, it
may be a reasonable interpretation that steric hindrance at the proper proximity around the
metal center may lead to better activity toward MMA polymerization.
Compared with our previously reported Zn(II) complex bearing N′-cyclohexyl substituted
N,N-bispyrazolyl [35], the current catalytic system exhibited higher activities (3.95 × 104 g
PMMA/mol·Zn·h) and yielding PMMA with improved molecular weights i.e. (11.0 × 105 g/
mol) and slightly broader PDIs. The PDI range was slightly narrowed when the molecular
weight of PMMA was increased. These results are explained by the narrower PDI range of
the higher molecular weight polymer [58, 59]. Similarly, in comparison with our reported
Zn(II) complexes bearing N,N-bis((1H-pyrazol-1-yl)methyl)-3-methoxypropan-1-amine and
3-methoxy-N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)propan-1-amine [50], the catalytic
behavior of the current system yielded PMMA with comparable syndiotacticity (rr = 66.7)
and high molecular weights and Tg at 60 °C (Table 5).
Syndiotactic-enriched PMMA has been observed in all the synthesized complexes, as
analyzed by 1H-NMR spectroscopy [60, 61]. The syndiotactic enchainment ranged from 0.65
to 0.74 for the current systems and was found to be higher compared to the starting mate-
rials. These mediocre stereoselectivities of Zn(II) complexes are slightly higher when

JOURNAL OF COORDINATION CHEMISTRY
23
compared with reported complexes [35, 42]. Thus, the resultant syndiotacticity of PMMA
was not high enough to confer a mechanism of coordination polymerization for the catalytic
species in the current report. It is presumed that the presence of the benzhydrylamine group
in [LFZnCl2] generates a coordination sphere around the metal center to effectively accom-
modate any steric clashes between incoming monomer, propagating polymer chain and
ligand architecture during MMA polymerization. Thus, it can be concluded that the MMA
polymerization activity of Zn(II) complexes of the current study should be considered as a
function of steric bulk around the metal center. Further modification in ligand architecture
to improve the catalytic performance and the resultant stereo-control of the MMA polym-
erization is presently going on in our laboratory.
3.4. rac-LA polymerization
The synthesis of high molecular weight PLA generally relies on the controlled ROP of LA using
well-defined metal complexes. Dimethyl derivatives of the dichloro zinc complexes were
assessed for activity in the ROP of rac-LA in CH2Cl2. The dimethyl derivatives were generated
in situ by treating the dichloro zinc complexes with two equiv. of MeLi in THF. The polymeriza-
tion was conducted at −50 °C and dimethyl Zn(II) complexes were found to be highly efficient
initiators of ROP of rac-LA. Completion of the reaction was confirmed by the absence of mon-
omer signals in the 1H-NMR spectra. The experimental Mn values (corrected using the Mark–
Houwink factor of 0.58) [62–64] and narrower polydispersity indicate that the polymerization
was well controlled and occurred through a living polymerization mechanism with a single
reaction site provided by these dimethyl zinc complexes. The results of rac-LA polymerization
are presented in Table 6. The importance of Mark–Houwink is given as follows; the molecular
weights (Mw) and number-average molecular weights (Mn) of the obtained PLA samples were
analyzed using GPC, in THF (1 mL/min rate at 25 °C) and were calibrated against polystyrene
standards. The calculated Mn values were also obtained using ([molecular weight of rac-LA] ×
[rac-LA]/[initiator]) × (conversion) formula. Generally, the values were in reasonable agreement;
however, there were always some discrepancies. In order to compensate such discrepancies
attention must be paid to the values obtained by GPC, as the Mark–Houwink parame-
ters, k and a are not known for PLA. Thus using the same instrument to estimate the molecular
weight of PLA, it has been possible to apply a correction factor (0.58 Mn by GPC) [65].
1H-NMR spectra showed peaks for the methane protons of –CH(CH3)(OH) at d 2.90 ppm
and the signals for –C(=O)CH3 at the other terminus overlapped with those of PLA backbones.
Considering the experimental findings for the ROP of rac-LA, under the conditions men-
tioned, we proposed the mechanism as outlined in Scheme 2. A coordination insertion
mechanism [66–68] for the ROP of LA may be predicted, where the pre-coordination of the
LA monomer to the Cu(II) center yields a monomeric 5-coordinated intermediate. The Me
group then acts as a nucleophile and cleavage of acyl-oxygen bond opens the LA ring. The
successive addition of LA produced heterotactic PLA.
It is evident from polymerization data that variation in the steric bulk of alkyl groups at
the ortho position of the aniline moiety has a significant effect on the activity of these Zn(II)
complexes. The tendency between steric bulk and catalytic activity toward rac-LA for com-
plexes bearing alkyl groups, at the ortho position, occurs in the order –CH(CH3)2 > –CH2CH3 > –
CH3 > H (Table 6). However, the introduction of a bromo group at the ortho position of the
amine moiety in [LEZnCl2] significantly decreased the activity, 84% conversion in 12 h (Table
6, entry 7), which might be due to a decrease in electronic density around the metal center.

24
S. SHIN ET AL.
Table 6. Polymerization of rac-lactide with in situ generated [LnZnMe2] (ln = l′ and la − lf) from the
reaction of (ln = l′ and la − lf) [LnZnMe2] and meli.
Catalysta
Conv.b (%)
Mnc (g/mol) × 103 (Calcd.)
Mnd (g/mol) × 103 (GPC)
PDIe
Prf
Entry
1
2
3
4
5
6
7
8
MeLi
85
100
78
90
97
97
84
86
12.25
14.41
11.24
12.97
13.98
13.98
12.11
12.40
3.88
7.43
11.58
16.20
19.03
13.25
7.99
1.14
1.36
1.15
1.22
1.33
1.26
1.08
1.20
0.67
0.84
0.91
0.95
0.91
0.91
0.91
0.88
[L′ZnCl2]g
[LAZnCl2]
[LBZnCl2]
[LCZnCl2]
[LDZnCl2]
[LEZnCl2]
[LFZnCl2]
6.44
aConditions: [initiator] = 0.0625 mmol, [rac-la]/[initiator] = 100, 5.0 ml of solvent (Ch2Cl2), polymerization time = 12 h,
temp = −50 °C.
bmonomer conversion (%) determined by 1h-nmr spectroscopy.
cCalculated from ([molecular weight of rac-la] × [mol concentration of used rac-la]/[mol concentration of initiator]) ×
(conversion).
dexperimental values (corrected using the mark-houwink factor of 0.58) [62–64].
edetermined by gel permeation chromatography (GPC) in thf, relative to polystyrene standard.
1
fProbability of heterotactic enchainment (Pr) were calculated on the basis of homonuclear decoupled h-nmr spectra
according to literature [78–80].
git is polymerization carried out with [l′Znme2] in which l′ = N,N-bis((1h-pyrazol-1-yl)methyl)aniline [42].
Scheme 2. Proposed mechanism for roP of rac-la catalyzed by N,N′,N-bis((1h-pyrazol-1-yl)methyl)amine-
based Zn(ii) complexes.

JOURNAL OF COORDINATION CHEMISTRY
25
These results are in contrast to previously reported systems where sterically hindered groups
tended to block the coordination/insertion of incoming monomer and hence were adverse
to the catalytic activity [69]. Surprisingly, [L′ZnCl2], the least sterically demanding, showed
100% conversion and moderate heterotacticity despite having no substituents on the aniline
moiety compared to the rest of the complexes of the current study. In addition, [LFZnCl2],
with –CHPh2 groups, also showed lower activity and heterotacticity compared to [LnZnCl2]
(Ln = LB − LD). The lower activity of complex [LFZnCl2] might be ascribed to the difference of
electronic effects between alkylamine [LnZnCl2] (Ln = LA − LD) and benzhydrylamine groups.
In contrast, [LAZnCl2] with –Me groups at the meta position of the amine moiety also showed
lower activity, such as 78% conversion with 0.91 heterotactic enchainment. Thus, the activity
of these dimethyl Zn(II) complexes can be considered as a composite result of steric and
electronic effects.
Compared with our previously reported dimethyl Zn(II) complexes these initiators exhibited
significantly lower activities [70–72]. They exhibited higher activities than zinc guanidinate
complexes and ketoiminate complexes [73–75]. However, compared to the highly effective zinc
initiators, based on phenolate diamines/diimines [76, 77], the catalytic efficacy of Zn(II) com-
plexes in the current study are lower, but produce PLA with better stereoselectivity (Table 6).
The stereo-selectivity of PLA is greatly influenced by the structure of the ancillary ligands
attached to the center metal ion during the polymerization process. Microstructural analysis
isi
sis
iii
sii/iis
iis/sii
Figure 8. homonuclear decoupled 1h-nmr spectrum of polylactide produced by dimethyl derivative
of [LBZnCl2] (for the preparation conditions, see entry 4 in table 6); Pr = 2 I1 [I1 + I2] = 2 (7.04 + 1.42)/
[(7.04 + 1.42) + (0.96 + 1.48 + 7.00)] ≈ 0.95.

26
S. SHIN ET AL.
isi
sis
iii
sii/iis
iis/sii
Figure 9. homonuclear decoupled 1h-nmr spectrum of polylactide produced by dimethyl derivative
of [LCZnCl2] (for the preparation conditions, see entry 5 in table 6); Pr = 2 I1/[I1 + I2] = 2 (7.24 + 1.41) /
[(7.4\24 + 1.41) + (1.17 + 1.48 + 7.76)] ≈ 0.91.
of the obtained PLA was performed by inspecting the methane proton region of homodecou-
pled 1H-NMR spectra and Pr values were calculated with the equation Pr = 2I1/(I1 + I2) where
I1 = (sis + sii) and I2 = (iis + iii + isi) [78–80]. It is evident from polymerization data that all the
complexes exhibited a strong preference for heterotactic enchainment. [LBZnMe2] produced
PLA with a highest degree of heterotacticity with Pr = 0.95 compared to the rest of its analogs
under the same experimental conditions (Figure 8; Table 6, entry 3). This high degree of het-
erotacticity obtained with [LBZnMe2] is comparable with those obtained with a hetero-bime-
tallic complex [((R)-{ONNO})2Y·Li] bearing a fluorinated dialkyloxo ligand in ROP of rac-LA [81].
The current system is superior in terms of heterotacticity compared to our previous
reported dimethyl complexes [70–72]. It has been observed that enhancing the steric hin-
drance on amine moiety resulted in decreased heterotacticity for complexes [LCZnMe2] and
[LDZnMe2] (Pr = 0.91, Figure 9; Table 6, entries 5 and 6) compared to complex [LBZnMe2]
(Pr = 0.95, Figure 8; Table 6, entry 4) with less steric bulk around the metal center. Thus, these
results indicated that the larger steric hindrance might probably impede the regularity of
monomer insertion [63]. The stereoselectivity toward ROP of rac-LA could be realized by
variation of the substituents attached to amine moiety. Proper complexity of substituents
in the ligand might provide an electronic environment for better accommodation of any

JOURNAL OF COORDINATION CHEMISTRY
27
potential steric clashes between the propagating polymer chain, ligand architecture and
the alternative insertion of incoming monomers. These results imply that polymerization
proceeded by chain end mechanism. More investigations are underway to fully elaborate
structure-activity relationship to produce PLA with better control and stereoregularities.
4. Conclusion
We investigated the synthesis and X-ray crystallographic structures of [LnZnCl2] (Ln = LA − LF),
which were prepared by reaction of the corresponding metal starting materials with N,N′,N-
bis((1H-pyrazol-1-yl)methyl)amine ligands. The molecular structures of monomeric, four-co-
ordinate Zn(II) complexes were distorted tetrahedral obtained via coordination of Npyrazole
atoms to the metal center. [LFZnCl2] exhited highest catalytic activity and yielded high
molecular weight PMMA with syndiotactic enchainemnt ca. 67%. The MMA polymerization
activity of the complexes in the current study should be considered in reference to the steric
bulk provided by the ligand architecture around the metal center. Dimethyl Zn(II) complexes
[LnZnMe2] (Ln = LA − LF), generated in situ, polymerized rac-LA in a controlled fashion and
yielded highly heterotactic PLA. The activities and stereoregularities toward PLA can be
considered as a composite result of steric and electronic effects.
Supplementary materials
CCDC 1548747-1548748, 1548750-1548751, and 1549025-1549026 contain the supplementary crys-
tallographic data for complexes [LAZnCl2], [LBZnCl2], [LCZnCl2], [LDZnCl2], [LEZnCl2], and [LFZnCl2],
respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK;
Fax: (+44) 1223-336-033; or E-mail: deposit@ccdc.cam.ac.uk.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This research was supported by the National Research Foundation of Korea (NRF), funded by the
Ministry of the Education, Science, and Technology (MEST) [grant number 2017R1A2B4002100]. X-ray
crystallography with PLS-II 2D-SMC beamline was supported by MSIP and POSTECH.
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