Zinc (II), palladium (II) and cadmium (II) complexes containing 4-methoxy-N-(pyridin-2-ylmethylene) aniline derivatives: Synthesis, characterization and methyl methacrylate polymerization

Article abstract of DOI:10.1002/aoc.4797

A series of Zn (II), Pd (II) and Cd (II) complexes, [(L)_nMX₂]_m (L = L-a–L-c; M = Zn, Pd; X = Cl; M = Cd; X = Br; n, m = 1 or 2), containing 4-methoxy-N-(pyridin-2-ylmethylene) aniline (L-a), 4-methoxy-N-(pyridin-2-ylmethyl) aniline (L-b) and 4-methoxy-N-methyl-N-(pyridin-2-ylmethyl) aniline (L-c) have been synthesized and characterized. The X-ray crystal structures of Pd (II) complexes [L₁PdCl₂] (L = L-b and L-c) revealed distorted square planar geometries obtained via coordinative interaction of the nitrogen atoms of pyridine and amine moieties and two chloro ligands. The geometry around Zn (II) center in [(L-a)ZnCl₂] and [(L-c)ZnCl₂] can be best described as distorted tetrahedral, whereas [(L-b)₂ZnCl₂] and [(L-b)₂CdBr₂] achieved 6-coordinated octahedral geometries around Zn and Cd centers through 2-equivalent ligands, respectively. In addition, a dimeric [(L-c)Cd(μ-Br)Br]₂ complex exhibited typical 5-coordinated trigonal bipyramidal geometry around Cd center. The polymerization of methyl methacrylate in the presence of modified methylaluminoxane was evaluated by all the synthesized complexes at 60°C. Among these complexes, [(L-b)PdCl₂] showed the highest catalytic activity [3.80 × 10⁴ g poly (methyl methacrylate) (PMMA)/mol Pd hr^?1], yielding high molecular weight (9.12 × 10⁵ g mol^?1) PMMA. Syndio-enriched PMMA (characterized using ¹H-NMR spectroscopy) of about 0.68 was obtained with T_g in the range 120–128°C. Unlike imine and amine moieties, the introduction of N-methyl moiety has an adverse effect on the catalytic activity, but the syndiotacticity remained unaffected.

Full text of DOI:10.1002/aoc.4797

Received: 10 October 2018
DOI: 10.1002/aoc.4797
Revised: 12 December 2018
Accepted: 21 December 2018
F U L L P A P E R
Zinc (II), palladium (II) and cadmium (II) complexes
containing 4‐methoxy‐N‐(pyridin‐2‐ylmethylene) aniline
derivatives: Synthesis, characterization and methyl
methacrylate polymerization
Suhyun Park1 | Jungkyu K. Lee1 | Hyosun Lee1 | Saira Nayab2 | Jong Won Shin3
1 Department of Chemistry and Green‐
Nano Materials Research Centre,
A series of Zn (II), Pd (II) and Cd (II) complexes, [(L)nMX2]m (L = L‐a–L‐c; M
Kyungpook National University, 80
= Zn, Pd; X = Cl; M = Cd; X = Br; n, m = 1 or 2), containing 4‐methoxy‐N‐
Daehakro, Buk‐gu, Daegu‐city 41566,
Republic of Korea
(pyridin‐2‐ylmethylene) aniline (L‐a), 4‐methoxy‐N‐(pyridin‐2‐ylmethyl) ani-
line (L‐b) and 4‐methoxy‐N‐methyl‐N‐(pyridin‐2‐ylmethyl) aniline (L‐c) have
been synthesized and characterized. The X‐ray crystal structures of Pd (II) com-
plexes [L1PdCl2] (L = L‐b and L‐c) revealed distorted square planar geometries
obtained via coordinative interaction of the nitrogen atoms of pyridine and
amine moieties and two chloro ligands. The geometry around Zn (II) center
in [(L‐a)ZnCl2] and [(L‐c)ZnCl2] can be best described as distorted tetrahe-
dral, whereas [(L‐b)2ZnCl2] and [(L‐b)2CdBr2] achieved 6‐coordinated octa-
hedral geometries around Zn and Cd centers through 2‐equivalent ligands,
respectively. In addition, a dimeric [(L‐c)Cd(μ‐Br)Br]2 complex exhibited
typical 5‐coordinated trigonal bipyramidal geometry around Cd center. The
polymerization of methyl methacrylate in the presence of modified
methylaluminoxane was evaluated by all the synthesized complexes at 60°C.
Among these complexes, [(L‐b)PdCl2] showed the highest catalytic activity
[3.80 × 104 g poly (methyl methacrylate) (PMMA)/mol Pd hr−1], yielding high
2 Department of Chemistry, Shaheed
Benazir Bhutto University, Sheringal Dir
(U), Khyber Pakhtunkhwa, Islamic
Republic of Pakistan
3 Daegu‐Gyeongbuk Branch, Korea
Institute of Science and Technology
Information (KISTI), 90 Yutongdanji‐ro,
Buk‐gu, Daegu 41515, Republic of Korea
Correspondence
Hyosun Lee, Department of Chemistry
and Green‐Nano Materials Research
Centre, Kyungpook National University,
80 Daehakro, Buk‐gu, Daegu‐city, 41566,
Republic of Korea.
Email: hyosunlee@knu.ac.kr
Jong Won Shin, Daegu‐Gyeongbuk
Branch, Korea Institute of Science and
Technology Information (KISTI), 90
Yutongdanji‐ro, Buk‐gu, Daegu 41515,
Republic of Korea.
molecular weight (9.12 × 105 g mol−1
) PMMA. Syndio‐enriched PMMA
1
(characterized using H‐NMR spectroscopy) of about 0.68 was obtained with
Tg in the range 120–128°C. Unlike imine and amine moieties, the introduction
of N‐methyl moiety has an adverse effect on the catalytic activity, but the
syndiotacticity remained unaffected.
Email: jwshin@kisti.re.kr
Funding information
Ministry of Education, Science, and Tech-
nology (MEST), Grant/Award Number:
2017R1A2B4002100
KEYWORDS
4‐methoxy‐N‐methyl‐N‐(pyridin‐2‐ylmethyl) aniline, 4‐methoxy‐N‐(pyridin‐2‐ylmethyl) aniline, 4‐
methoxy‐N‐(pyridin‐2‐ylmethylene) aniline, methyl methacrylate polymerization, molecular
structures
1 | INTRODUCTION
and N‐methyl‐1‐(pyridin‐2‐yl) methanamine[8,9] moieties
have been broadly researched owing to their diverse coor-
Transition metal complexes containing ligands with
pyridinylmethanimine,[1–4] pyridinylmethanamine[5–7]
dination geometries ranging from bidentate[10–12] to
polydentate coordination modes.[13–15] In addition, the
Appl Organometal Chem. 2019;e4797.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4797

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PARK ET AL.
steric and electronic properties can be easily
modified by appropriate variation on N‐donors of
pyridinylmethanimine moiety, and resulted in stable
complexes with a variety of transition or inner transition
metals.[16,17] These metal complexes have found a wide
range of applications in various fields, such as synthesis
and spectroscopic studies,[18] as catalysts for transfer
hydrogenation of ketones,[19] oxidation,[20] supramolecu-
lar metal–organic frameworks,[21] olefin polymeriza-
tion,[22] ε‐caprolactone[23] and methyl methacrylate
(MMA) polymerization.[24,25]
(Et2O) and dichloromethane (CH2Cl2) were purchased
from Merck, and used without further purification. Mod-
ified methylaluminoxane (MMAO) was purchased from
Tosoh Fine Chemicals as 5.90% weight aluminum of a
toluene solution and used without further purification.
Ligands L‐a,[45] L‐b[46] and L‐c[47] have been prepared
by literature methods. Complexes [(L‐a)PdCl2],[48] [(L‐
a)ZnCl2],[49] ([(L‐a)Cd(μ–Br)Br]2
and [(L‐b)
[50]
PdCl2][46] have been synthesized as described previously.
Elemental analyses (C, H, N) of the prepared complexes
were carried out on an elemental analyzer (EA 1108;
1
On the other hand, the metal‐mediated poly (methyl
methacrylate) (PMMA) polymerization is of growing
interest both in industry and academia due to its signifi-
cant optical properties.[26–30] The optical properties of
PMMA are far improved with high Tg, which in turn
depend on tacticity of PMMA, i.e. Tg of isotactic PMMA
is about 65°C, whereas syndiotactic PMMA[31] exhibits
Tg about 140°C. That is why many attempts aiming to
increase the syndiotacticity of PMMA were planned.
Recently, we have studied Zn (II), Cd (II), Co (II), Pt
(II) and Pd (II) complexes with N,N‐di(2‐picolyl) amine, 2‐
iminomethylpyridine, N‐methyl‐N‐((pyridin‐2‐yl)methyl)
cyclohexanamine, N‐(pyridin‐2‐ylmethyl) aniline and N,
N‐di(2‐picolyl) cycloalkylamine ligands in MMA poly-
merization with promising results,[25,32–38] as a slight dif-
ference in ligand architecture is able to influence the
stereoregular outcomes during polymerization reactions.
Thus, suitable ligand architecture around the metal cen-
ter is vital for achieving a high degree of polymerization
as these complexes/initiators regulate various steps
involved in polymerization.[39–43] Our current findings
demonstrate the synthesis, X‐ray structures of Zn (II),
Pd (II) and Cd (II) complexes ligated to 4‐methoxy‐N‐
(pyridin‐2‐ylmethylene) aniline derivatives and their cat-
alytic efficiency for MMA polymerization. Moreover, the
electronic and steric effect from slight variation of N atom
substituents on the catalytic efficacy of stereoregulated
MMA polymerization is especially discussed.
Carlo‐Erba, Milan, Italy). H‐NMR (500 MHz) and 13C‐
NMR (125 MHz) spectra were recorded on a Bruker
Avance Digital 500 NMR spectrometer; chemical shifts
were reported in ppm units (δ) relative to SiMe4 as the
internal standard. Infrared (IR) spectra were recorded
on Bruker FT/IR‐Alpha (neat) and the data are reported
in reciprocal centimeters (cm−1). The molecular weights
(Mn) and molecular weight distribution [polydispersity
indexes (PDIs)] of the obtained PMMA were carried out
using gel permeation chromatography (GPC; in THF,
Alliance e2695; Waters, Milford, MA, USA). Glass transi-
tion temperature (Tg) was determined using a thermal
analyzer (DSC 4000; PerkinElmer).
2.2 | Preparation of Pd (II), Zn (II) and Cd
(II) complexes
2.2.1 | 4‐methoxy‐N‐(pyridin‐2‐ylmethyl)
anilinezinc (II) chloride, [(L‐b)2ZnCl2]
[L‐b] (0.401 g, 1.87 mmol) solution in EtOH (10.0 mL)
was treated with ZnCl2 (0.127 g, 0.935 mmol) solution
in EtOH (10.0 mL) at ambient temperature. The resul-
tant precipitate was filtered and washed with cold EtOH
and Et2O to give white crystalline solid as final product
(0.422 g, 79.9%). X‐ray crystallographies were obtained
from Et2O (5.00 mL) diffusion into acetone solution
(3.00 mL). X‐ray quality crystals for [(L‐b)2ZnCl2] were
obtained from slow diffusion of Et2O (10.0 mL) into
DMF solution (5.00 mL) of [(L‐b)2ZnCl2]. Analysis cal-
culated for C26H28Cl2N4O2Zn (%): C, 55.3; H, 5.00; N,
9.92. Found: C, 55.3; H, 5.02; N, 9.81. 1H‐NMR
(DMSO‐d6, 500 MHz, δ, ppm): 8.53 (ddd, 1H, J = 4.88
Hz, –N=C–CH=CH–CH=CH–), 7.75 (td, 1H, J = 7.63
Hz, –N=C–CH=CH–CH=CH–), 7.38 (d, 1H, J = 7.93
Hz, –N=C–CH=CH–CH=CH–), 7.26 (ddd, 1H, J =
7.32 Hz, –N=C–CH=CH–CH=CH–), 6.69 (d, 2H, J =
9.00 Hz, –CH3OC6H4–), 6.55 (d, 2H, J = 8.85 Hz, –CH3-
OC6H4–), 5.95 (t, 1H, J = 6.10 Hz, –NH–CH2–NC5H4–),
4.31 (d, 2H, J = 6.10 Hz, –NH–CH2–NC5H4–), 3.61 (s,
3H, –CH3OC6H4–). 13C‐NMR (DMSO‐d6, 125 MHz, δ,
2 | EXPERIMENTAL
2.1 | Physical measurements and
instrumentations
PdCl2 was purchased from Kojima Chemicals, and [Pd
(MeCN)2Cl2] was prepared according to the literature
method.[44] [ZnCl2], [CdBr2·4H2O], NaBH4, CH3I, 2‐
pyridinecarboxaldehyde, 4‐methoxyaniline, magnesium
sulfate (MgSO4) and MMA were purchased from Aldrich.
Anhydrous solvents such as acetonitrile (CH3CN), etha-
nol (EtOH), dimethylformamide (DMF), diethyl ether

PARK ET AL.
3 of 15
ppm): 159.60 (s, 1C, ipso‐N=C–CH=CH–CH=CH–),
151.45 (s, 1C, ipso‐CH3OC6H4–), 148.67 (d, 1C, J =
178.93 Hz, –N=C–CH=CH–CH=CH–), 142.19 (s, 1C,
ipso‐CH3OC6H4–), 137.26 (d, 1C, J = 163.49 Hz, –N=C–
CH=CH–CH=CH–), 122.28 (d, 1C, J = 164.40 Hz, –
N=C–CH=CH–CH=CH–), 121.55 (d, 1C, J = 163.49
Hz, –N=C–CH=CH–CH=CH–), 114.54 (d, 2C, J =
158.95 Hz, m‐CH3OC6H4–), 114.25 (d, 2C, J = 158.04
Hz, o‐CH3OC6H4–), 55.29 (q, 1C, J = 143.51 Hz,
–CH3OC6H4–), 49.58 (t, 1C, J = 135.33 Hz, –CH2–
NC5H4–). IR (solid neat; cm−1): 3240 (w), 2994 (w),
2913 (w), 2838 (w), 1603 (m), 1570 (w), 1508 (s), 1440
(m), 1307 (w), 1241 (s), 1215 (m), 1183 (m), 1072 (m),
1008 (s), 904 (m), 853 (s), 816 (m), 761 (s), 730 (m),
642 (w).
2.2.3 | 4‐Methoxy‐N‐methyl‐N‐(pyridin‐2‐
ylmethyl) anilinepalladium (II) chloride
([(L‐c)PdCl2])
[(L‐c)PdCl2] was prepared according to a similar proce-
dure as described for [(L‐b)2ZnCl2], except utilizing [L‐
c] (0.400 g, 1.75 mmol) and [Pd (MeCN)2Cl2] (0.454 g,
1.75 mmol) in acetonitrile (50.0 mL) to give an orange
solid (0.603 g, 84.9%). X‐ray quality crystals for [(L‐c)
PdCl2] were obtained from slow diffusion of Et2O (10.0
mL) into DMF solution (5.00 mL) of [(L‐c)PdCl2]. Anal-
ysis calculated for C14H16Cl2N2OPd (%): C, 41.5; H, 3.98;
N, 6.91. Found: C, 41.9; H, 3.99; N, 6.91. 1H‐NMR
(DMSO‐d6, 500 MHz, δ, ppm): 8.65 (dd, 1H, J = 5.80 Hz,
–N=C–CH=CH–CH=CH–), 8.11 (td, 1H, J = 7.63 Hz, –
N=C–CH=CH–CH=CH–), 7.85–7.80 (m, 3H, –N=C–
CH=CH–CH=CH–, –CH3OC6H4–), 7.49 (t, 1H, J = 6.26
Hz, –N=C–CH=CH–CH=CH–), 6.91 (d, 2H, J = 9.16
Hz, –CH3OC6H4–), 5.21 (d, 1H, J = 16.02 Hz, –NCH3–
CH2–NC5H4–), 4.89 (d, 1H, J = 16.17 Hz, –NCH3–CH2–
NC5H4–), 3.71 (s, 3H, –CH3OC6H4–), 3.07 (s, 3H, –
NCH3–CH2–NC5H4–). 13C‐NMR (DMSO‐d6, 125 MHz, δ,
ppm): 160.07 (s, 1C, ipso‐N=C–CH=CH–CH=CH–),
157.56 (s, 1C, ipso‐CH3OC6H4–), 149.27 (d, 1C, J =
188.01 Hz, –N=C–CH=CH–CH=CH–), 142.72 (s, 1C,
ipso‐CH3OC6H4–), 140.65 (d, 1C, J = 168.03 Hz, –N=C–
CH=CH–CH=CH–), 124.18 (d, 1C, J = 171.66 Hz, –
N=C–CH=CH–CH=CH–), 123.44 (d, 2C, J = 160.76 Hz,
o‐CH3OC6H4–), 122.78 (d, 1C, J = 171.66 Hz, –N=C–
CH=CH–CH=CH–), 114.38 (d, 2C, J = 162.58 Hz, m‐
CH3OC6H4–), 67.45 (t, 1C, J = 141.69 Hz, –NCH3–CH2–
NC5H4–), 55.36 (q, 1C, J = 144.41 Hz, –CH3OC6H4–),
54.54 (q, 1C, J = 141.69 Hz, –NCH3–CH2–NC5H4–). IR
(solid neat; cm−1): 3108 (w), 3069 (w), 3054 (w), 3034
(w), 3001 (w), 2940 (w), 2830 (w), 1608 (m), 1509 (m),
1250 (s), 1185 (m), 1034 (s), 995 (w), 947 (m), 829 (w),
806 (s), 774 (m), 736 (s), 693 (m), 575 (w).
2.2.2 | 4‐Methoxy‐N‐(pyridin‐2‐ylmethyl)
anilinecadmium (II) bromide ([(L‐
b)2CdBr2])
[(L‐b)2CdBr2] was prepared according to a similar proce-
dure as described for [(L‐b)2ZnCl2], except utilizing [L‐
b] (0.401 g, 1.87 mmol) and CdBr2·4H2O (0.322 g, 0.935
mmol) to give a white solid (0.605 g, 92.3%). X‐ray quality
crystals for [(L‐b)2CdBr2] were obtained from slow diffu-
sion of Et2O (10.0 mL) into DMF solution (5.00 mL) of
[(L‐b)2CdBr2]. Analysis calculated for C26H28Br2CdN4O2
(%): C, 44.6; H, 4.03; N, 8.00. Found: C, 45.0; H, 4.01; N,
1
7.98. H‐NMR (DMSO‐d6, 500 MHz, δ, ppm): 8.53 (ddd,
1H, J = 4.88 Hz, –N=C–CH=CH–CH=CH–), 7.76 (td,
1H, J = 7.63 Hz, –N=C–CH=CH–CH=CH–), 7.39 (d,
1H, J = 7.78 Hz, –N=C–CH=CH–CH=CH–), 7.27 (ddd,
1H, J = 7.32 Hz, –N=C–CH=CH–CH=CH–), 6.69 (d,
2H, J = 9.00 Hz, –CH3OC6H4–), 6.57 (d, 2H, J = 9.00 Hz,
–CH3OC6H4–), 5.93 (t, 1H, J = 5.65 Hz, –NH–CH2–
NC5H4–), 4.30 (d, 2H, J = 5.80 Hz, –NH–CH2–NC5H4–),
3.61 (s, 3H, –CH3OC6H4–). 13C‐NMR (DMSO‐d6, 125
MHz, δ, ppm): 159.15 (s, 1C, ipso‐N=C–CH=CH–
CH=CH–), 151.76 (s, 1C, ipso‐CH3OC6H4–), 148.70 (d,
1C, J = 178.93 Hz, –N=C–CH=CH–CH=CH–), 142.04
(s, 1C, ipso‐CH3OC6H4–), 137.36 (d, 1C, J = 163.49 Hz, –
N=C–CH=CH–CH=CH–), 122.38 (d, 1C, J = 165.30
Hz, –N=C–CH=CH–CH=CH–), 121.81 (d, 1C, J =
163.49 Hz, –N=C–CH=CH–CH=CH–), 114.85 (d, 2C, J
= 158.04 Hz, m‐CH3OC6H4–), 114.41 (d, 2C, J = 158.04
Hz, o‐CH3OC6H4–), 55.27 (q, 1C, J = 142.60 Hz,
–CH3OC6H4–), 49.76 (t, 1C, J = 136.24 Hz, –CH2–
NC5H4–). IR (solid neat; cm−1): 3235 (w), 2989 (w), 2960
(w), 2907 (w), 2836 (w), 1600 (m), 1507 (s), 1462 (s),
1438 (m), 1240 (s), 1181 (m), 1149 (m), 1118 (w), 1071
(w), 1033 (w), 1000 (m), 964 (w), 895 (m), 845 (s), 763 (s).
2.2.4 | 4‐Methoxy‐N‐methyl‐N‐(pyridin‐2‐
ylmethyl) anilinezinc (II) chloride ([(L‐c)
ZnCl2])
[(L‐c)ZnCl2] was prepared according to a similar proce-
dure as described for [(L‐b)2ZnCl2], except utilizing [L‐
c] (0.400 g, 1.75 mmol) and [ZnCl2] (0.239 g, 1.75 mmol)
to give a beige solid (0.429 g, 67.2%). X‐ray quality crystals
for [(L‐c)ZnCl2] were obtained from layering of hexane
(3.00 mL) on the chloroform solution (2.00 mL) of [(L‐
c)ZnCl2]. Analysis calculated for C14H16Cl2N2OZn (%):
C, 46.1; H, 4.42; N, 7.68. Found: C, 46.5; H, 4.43; N,
1
7.59. H‐NMR (DMSO‐d6, 500 MHz, δ, ppm): 8.52 (ddd,
1H, J = 4.73 Hz, –N=C–CH=CH–CH=CH–), 7.70 (td,

4 of 15
PARK ET AL.
1H, J = 7.78 Hz, –N=C–CH=CH–CH=CH–), 7.23 (ddd,
1H, J = 7.48 Hz, –N=C–CH=CH–CH=CH–), 7.15 (d,
1H, J = 7.78 Hz, –N=C–CH=CH–CH=CH–), 6.76 (d,
2H, J = 9.16 Hz, –CH3OC6H4–), 6.66 (d, 2H, J = 9.16 Hz,
–CH3OC6H4–), 4.53 (s, 2H, –NCH3–CH2–NC5H4–), 3.63
(s, 3H, –CH3OC6H4–), 3.00 (s, 3H, –NCH3–CH2–
NC5H4–). 13C‐NMR (DMSO‐d6, 125 MHz, δ, ppm):
159.28 (s, 1C, ipso‐N=C–CH=CH–CH=CH–), 151.16
(s, 1C, ipso‐CH3OC6H4–), 149.17 (d, 1C, J = 178.02 Hz, –
N=C–CH=CH–CH=CH–), 143.60 (s, 1C, ipso‐
CH3OC6H4–), 137.10 (d, 1C, J = 162.58 Hz, –N=C–
CH=CH–CH=CH–), 122.23 (d, 1C, J = 165.30 Hz, –
N=C–CH=CH–CH=CH–), 121.39 (d, 1C, J = 163.49 Hz,
–N=C–CH=CH–CH=CH–), 114.62 (d, 2C, J = 158.95
Hz, o‐CH3OC6H4–), 113.81 (d, 2C, J = 158.04 Hz, m‐
CH3OC6H4–), 58.46 (t, 1C, J = 136.24 Hz, –NCH3–CH2–
NC5H4–), 55.32 (q, 1C, J = 143.51 Hz, –CH3OC6H4–),
39.74 (q, 1C, J = 135.33 Hz, –NCH3–CH2–NC5H4–). IR
(solid neat; cm−1): 3077 (w), 3023 (w), 2988 (w), 2950
(w), 2927 (w), 2906 (w), 2833 (w), 1606 (m), 1572 (w),
1512 (s), 1442 (m), 1302 (m), 1253 (m), 1028 (s), 918
(m), 837 (s), 766 (m), 752 (m), 650 (w), 569 (m).
2C, J = 158.04 Hz, m‐CH3OC6H4–), 58.46 (t, 1C, J =
135.33 Hz, –NCH3–CH2–NC5H4–), 55.27 (q, 1C, J =
143.51 Hz, –CH3OC6H4–), 40.36 (q, 1C, J = 135.33 Hz, –
NCH3–CH2–NC5H4–). IR (solid neat; cm−1): 3073 (w),
3032 (w), 3015 (w), 3003 (w), 2976 (w), 2950 (w), 2927
(w), 2898 (w), 2827 (w), 1605 (w), 1572 (w), 1512 (s),
1447 (m), 1250 (m), 794 (s), 779 (m), 765 (m), 746 (w),
716 (m), 647 (w).
2.3 | Catalytic activity for methyl
methacrylate polymerization
In a Schlenk flask, the complex (15.0 μmol, 5.84 mg for
[(L‐a)PdCl2], 5.23 mg for [(L‐a)ZnCl2], 14.5 mg for
[(L‐a)Cd(μ‐Br)Br]2, 5.87 mg for [(L‐b)PdCl2], 8.47 mg
for [(L‐b)2ZnCl2], 10.5 mg for [(L‐b)2CdBr2], 6.08 mg
for [(L‐c)PdCl2], 5.47 mg for [(L‐c)ZnCl2] and 15.0 mg
for [(L‐c)Cd(μ‐Br)Br]2) 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/[M (II)
catalyst]0 = 500) as a co‐catalyst. The solution was stirred
for 20 min at 60°C. MMA (5.00 mL, 47.1 mmol and
[MMA]0/[M (II) catalyst]0 = 3100) was added to the
above reaction mixture and stirred for 2 hr to obtain a vis-
cous solution. MeOH (2.00 mL) was added to terminate
polymerization. The reaction mixture was poured into a
large quantity of MeOH (500 mL), and 35.0% HCl (5.00
mL) was injected to remove the remaining co‐catalyst
(MMAO). The product was obtained by filtration and
washed with MeOH (250 mL × 2) to yield PMMA, and
dried under vacuum at 60°C for 12 hr.
2.2.5 | 4‐Methoxy‐N‐methyl‐N‐(pyridin‐2‐
ylmethyl) anilinecadmium (II) bromide
([(L‐c)cd(μ‐Br)Br]2)
[(L‐c)Cd(μ‐Br)Br]2 was prepared according to a similar
procedure as described for [(L‐b)2ZnCl2], except utilizing
[L‐c] (0.400 g, 1.75 mmol) and [CdBr2·4H2O] (0.602 g,
1.75 mmol) in anhydrous EtOH to give a beige solid
(1.17 g, 66.8%). X‐ray quality crystals for [(L‐c)Cd(μ‐Br)
Br]2 were obtained from slow diffusion of Et2O (5.00
mL) into acetone solution (3.00 mL) of [(L‐c)Cd(μ‐Br)
Br]2. Analysis calculated for C28H32Br4Cd2N4O2 (%): C,
33.6; H, 3.22; N, 5.60. Found: C, 34.0; H, 3.29; N, 5.60.
1H‐NMR (DMSO‐d6, 500 MHz, δ, ppm): 8.53 (ddd, 1H,
J = 4.88 Hz, –N=C–CH=CH–CH=CH–), 7.72 (td, 1H, J
= 7.63 Hz, –N=C–CH=CH–CH=CH–), 7.25 (ddd, 1H, J
= 7.32 Hz, –N=C–CH=CH–CH=CH–), 7.18 (d, 1H, J =
7.78 Hz, –N=C–CH=CH–CH=CH–), 6.76 (d, 2H, J =
9.31 Hz, –CH3OC6H4–), 6.70 (d, 2H, J = 9.31 Hz, –CH3-
OC6H4–), 4.52 (s, 2H, –NCH3–CH2–NC5H4–), 3.63 (s,
3H, –CH3OC6H4–), 2.98 (s, 3H, –NCH3–CH2–NC5H4–).
13C‐NMR (DMSO‐d6, 125 MHz, δ, ppm): 158.83 (s, 1C,
ipso‐N=C–CH=CH–CH=CH–), 151.46 (s, 1C, ipso‐
CH3OC6H4–), 149.15 (d, 1C, J = 178.02 Hz, –N=C–
CH=CH–CH=CH–), 143.55 (s, 1C, ipso‐CH3OC6H4–),
137.10 (d, 1C, J = 163.49 Hz, –N=C–CH=CH–CH=CH–),
122.25 (d, 1C, J = 165.30 Hz, –N=C–CH=CH–CH=CH–),
121.54 (d, 1C, J = 163.49 Hz, –N=C–CH=CH–CH=CH–),
114.50 (d, 2C, J = 158.95 Hz, o‐CH3OC6H4–), 114.46 (d,
2.4 | X‐ray crystallographic studies
The X‐ray quality single crystals were coated with
paratone‐N oil and the diffraction data measured at
100(2) K with synchrotron radiation (λ = 0.700 Å) for
[(L‐a)ZnCl2], [(L‐b)2ZnCl2], [(L‐c)PdCl2], [(L‐c)
ZnCl2] and [(L‐c)Cd(μ‐Br)Br]2 on an ADSC
Quantum‐210 detector at 2D SMC with a silicon (111)
double‐crystal monochromator at the Pohang Accelera-
tor Laboratory, South Korea. The PAL BL2D‐SMDC pro-
gram[51] 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)[52] was used
for cell refinement, reduction and absorption correction.
An orange cubic‐shaped crystal of [(L‐b)PdCl2] and a
colorless cubic‐shaped crystal of [(L‐b)2CdBr2] were
picked up with paraton‐N oil and mounted on a Bruker
SMART CCD diffractometer equipped with a graphite‐
monochromated Mo‐Kα (λ = 0.71073 Å) radiation source
under nitrogen cold stream [200(2) K] at the Western

PARK ET AL.
5 of 15
Seoul Center of Korea Basic Science Institute. Data col-
lection and integration were performed with SMART
(Bruker, 2000) and SAINT‐Plus (Bruker, 2001) software
packages.[53] Semi‐empirical absorption corrections
based on equivalent reflections were applied by
SADABS.[54] All measured structures were solved by
dual‐space algorithms related with intrinsic phasing
using SHELXT‐2018/2,[55] and refined by full‐matrix
least‐squares refinement using the SHELXL‐2018/3[56]
computer program. The positions of all non‐hydrogen
atoms were refined with anisotropic displacement
factors. All hydrogen atoms were placed using a riding
model, and their positions were constrained relative
to their parent atoms using the appropriate HFIX
command in SHELXL‐2018/3 computer program.
Crystallographic refinements and structural data are
summarized in Table 1.
3.2 | Description of molecular structures
The ORTEP drawings of synthesized complexes are
shown in Figure 1 ([(L‐a)ZnCl2]), Figure 2 ([(L‐
b)2ZnCl2]), Figure 3 ([(L‐b)PdCl2]), Figure 4 ([(L‐
b)2CdBr2]), Figure 5 ([(L‐c)ZnCl2]), Figure 6 ([(L‐c)
PdCl2]) and Figure 7 ([(L‐c)Cd(μ‐Br)Br]2). The selected
bond lengths and angles are listed in Table 2. [(L‐a)
ZnCl2] crystallized in the triclinic system with the P1
space group. The synthesized complexes, [(L‐c)MCl2]
(M = Zn and Pd) and [(L‐b)2MX2] (M = Zn; X = Cl; M
= Cd; X = Br), crystallized in the monoclinic system with
the P21/c space groups, while [(L‐b)PdCl2] and [(L‐c)
Cd(μ‐Br)Br]2 crystallized in the orthorhombic system
with Pbca space groups.
Pd (II) atom adopted slightly distorted square planar
geometry in [(L)PdCl2] (L = L‐b and L‐c) by coordinat-
ing with two N atoms of bidentate ligand and two
chloro ligands (Figures 3 and 6).[14,25,46,49] The bond
lengths of Pd‐Npyridine in [(L)PdCl2] (L = L‐b and L‐c)
were in the range of 2.024(3)–2.026(4) Å, while that of
Pd‐Namine ranged from 2.053(4) to 2.094(3) Å, similar to
the Pd‐N bond length of square planar Pd (II) com-
plexes.[12,25,46] The Pd‐Npyridine bond length in [(L‐b)
PdCl2] differs by approximately 0.002 Å compared with
[(L‐c)PdCl2]. On the other hand, the length of Pd‐N-
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and chemical properties
Ligands (Ln = L‐a–L‐c) in the current study were synthe-
sized as reported previously.[25,45–47] Complexes, [(L)
nMX2]m (L = L‐a–L‐c; M = Zn, Pd; X = Cl; M = Cd; X
= Br; n, m = 1 or 2; 67–98% yields), were obtained by
direct ligation of metal starting materials with ligands
in 1:1 ratio in CH3CN or EtOH at ambient temperature
(Scheme 1). The structures of these complexes were
length was longer by about 0.04 Å in [(L‐c)PdCl2].
amine
The Pd‐Clterminal bond lengths ranged from 2.2876(1)
to 2.3166(1) Å. The N(2)‐C(6) distances of 1.499(6) Å
([(L‐b)PdCl2]) and 1.498(3) Å ([(L‐c)PdCl2]) were in
the range of accepted carbon‐nitrogen single bonds.
The C(5)–C(6) bond distances of the complexes ranged
from 1.489(3) to 1.495(7) Å. The Me subsistent at the
amine moiety slightly affected the bond lengths in [(L‐
c)PdCl2].
The Npyridine‐Pd(1)‐Clterminal and Namine‐Pd(1)‐
Clterminal angles in [LPdCl2] (L = L‐b and L‐c) were
nearly linear in the range of 170.52(1)–176.67(5)°. The
average Npyridine‐Pd(1)‐Namine bond angles of five‐
membered rings ranged from 81.51(8)° to 82.55(2)°, and
were slightly affected by substituents of amine moieties.
The Clterminal‐Pd(1)‐Clterminal angles in [(L)PdCl2] (L =
L‐b and L‐c) agree well with the traditional angles for
square‐planar Pd (II)[14,25,46,49] and Pt (II)[46] complexes.
The geometry of [(L)ZnCl2] (L = L‐a and L‐c) can be
best described as distorted tetrahedral around the Zn cen-
ter (Figures S1 and 5). The Zn‐Npyridine lengths lie in the
2.0551(1)–2.1844(2) Å range. The Zn‐Nimine and Zn‐N-
characterized
by
1H‐NMR,
13C‐NMR,
Fourier
transform‐infrared (FT‐IR) and X‐ray diffraction. 1H‐
NMR spectra were consistent with ligands and Zn (II),
Pd (II) and Cd (II) complexes formulation. 1H‐NMR
peaks of the metal complexes were shifted to low field
by approximately δ 0.01–4.68 compared with ligands
due to the resonance effects of the N and C atoms of
the donating moieties of ligand. Characteristic C–H
peaks in the IR spectra were observed at 3108, 3077
and 3073 cm−1 for [(L‐c)ZnCl2], [(L‐c)PdCl2] and [(L‐
c)Cd(μ‐Br)Br]2, respectively. Moreover, the N–H
stretching band appeared at 3240 and 3235 cm−1 for
[(L‐b)2ZnCl2] and [(L‐b)2CdBr2], respectively. A con-
siderable difference in IR stretching band for >C=N‐
group is observed due to complexation and appeared at
(1600–1603 cm−1) range. Elemental analysis of the syn-
thesized complexes was consistent with the proposed
structures in Scheme 1 and confirmed the purity of the
isolated complexes, [(L)nMX2]m (L = L‐a–L‐c; M = Zn,
Pd; X = Cl; M = Cd; X = Br; n, m = 1 or 2). All the syn-
thesized complexes were stable in the air and could be
stored for months at room temperature.
lengths in [(L)ZnCl2] (Ln = L‐a and L‐c) were
amine
found to be 2.088(2) and 2.1107(1) Å, respectively, similar
to the Zn‐N bond lengths of tetrahedral Zn (II) com-
plexes.[10,49,57] The Zn‐Clterminal bond lengths ranged from
2.1940(5) to 2.2186(1) Å. The N(2)‐C(6) distance of

6 of 15
PARK ET AL.

PARK ET AL.
7 of 15
SCHEME 1 Synthesis of ligands [L] (L = L‐a–L‐c) and complexes [(L)nMX2]m (L = L‐a–L‐c; M = Zn, Pd; X = Cl; M = Cd; X = Br; n, m
= 1 or 2)
FIGURE 1 ORTEP drawing of [(L‐a)ZnCl2] with thermal
ellipsoids at 70% probability. All hydrogen atoms are omitted for
FIGURE 2 ORTEP drawing of [(L‐b)2ZnCl2] with thermal
clarity
ellipsoids at 70% probability. All hydrogen atoms are omitted for
clarity
1.286(3) Å in [(L‐a)ZnCl2] and 1.4827(2) Å in [(L‐c)
ZnCl2] were in the range of accepted carbon‐nitrogen
single or double bonds, respectively. The C(5)–C(6) bond
distances of the complexes ranged from 1.470(3) to
1.510(2) Å, reflecting the lack of delocalized π‐electrons
in the pyridine ring. The average Npyridine‐Zn(1)‐Namine
and Clterminal‐Zn(1)‐Clterminal bond angles ranged from
80.92(8)° to 82.19(5)° and 114.06(3)° to 119.03(2)°,

8 of 15
PARK ET AL.
FIGURE 3 ORTEP drawing of [(L‐b)PdCl2] with thermal
ellipsoids at 30% probability. All hydrogen atoms are omitted for
clarity
FIGURE 6 ORTEP drawing of [(L‐c)PdCl2] with thermal
ellipsoids at 70% probability. All hydrogen atoms are omitted for
clarity
FIGURE 4 ORTEP drawing of [(L‐b)2CdBr2] with thermal
ellipsoids at 60% probability. All hydrogen atoms are omitted for
clarity
FIGURE 7 ORTEP drawing of [(L‐c)Cd(μ‐Br)Br]2 with thermal
ellipsoids at 30% probability. All hydrogen atoms are omitted for
clarity
respectively, which agreed well with the angles reported
in related Zn (II)[49] systems.
The molecular structures of six‐coordinated [(L‐
b)2MX2] (M = Zn; X = Cl; M = Cd; X = Br) complexes
revealed an octahedral geometry around Zn and Cd cen-
ters through 2‐equivalents ligands ([L‐b]), included two
N atoms of pyridine moieties, two N atoms of 4‐
methoxyaniline moieties and two halogen ligands,
resulting in the formation of two five‐membered rings
(Figures 2 and 4).[11,15,58] The bond lengths of M‐Npyridine
and M‐Namine in [(L‐b)2MX2] (M = Zn; X = Cl; M = Cd;
X = Br) were similar to the M‐N bond length of octahe-
dral M (II) complexes.
.
The M‐Npyridine length differs by approximately 0.2 Å,
ranging in size from [(L‐b)2ZnCl2] < [(L‐b)2CdBr2].
Moreover, the length of Cd‐Namine was longer than Zn‐
FIGURE
5
ORTEP drawing of [(L‐c)ZnCl2 CHCl3] with
thermal ellipsoids at 30% probability. All hydrogen atoms are
omitted for clarity

PARK ET AL.
9 of 15

10 of 15
PARK ET AL.
Namine by about 0.2 Å [Zn‐Namine = 2.2090(2) Å, Cd‐N-
amine = 2.398(8) Å]. The average equatorial bond lengths,
Zn‐(L‐b)eq and Cd‐(L‐b)eq, are 2.197(1) and 2.388(1) Å,
respectively. The axial bond lengths related with
halogen ligands in [(L‐b)2ZnCl2] and [(L‐b)2CdBr2]
are 2.4737(7) and 2.2.7109(1) Å, respectively. The [(L‐
b)2CdBr2] bond lengths are much longer than the [(L‐
b)2ZnCl2] bond lengths, which can be attributed to size
effects of the coordinated metal ions. The average bond
angles N(1)‐M(1)‐N(2) ranged from 70.2(3)° to 74.77(5)°,
and the N(1)‐M(1)‐N(1)#1, N(2)‐M(1)‐N(2)#1 and X(1)‐
M(1)‐X(1)#1 were linear, corresponding to similar angles
in literature.
The dimeric Cd (II) complex, [(L‐c)Cd(μ‐Br)Br]2,
exhibited a distorted trigonal bipyramidal geometry
around Cd center, coordinating via two N atoms of 4‐
methoxy‐N‐methyl‐N‐(pyridin‐2‐ylmethyl) aniline ligand,
two bridged bromides and one terminal bromide. Cd–N-
length in [(L‐c)Cd(μ‐Br)Br]2 was 2.281(2) Å,
pyridine
while that of Cd–Namine was 2.494(3) Å,[50,59–61] and the
#1
Cd–Brbridged
,
Cd–Brterminal and Cd–Brbridged
bond
lengths were 2.6082(5) Å, 2.5318(4) Å and 2.8420(5) Å,
respectively. The base of the pyramid has been con-
#1
structed from bidentate imine ligand and Brbridged
ligand, whereas the axial positions are occupied by termi-
nal and Brbridged ligands. It has been observed that axial
bromo ligand length is longer compared with equatorial
bromo ligand. The Npyridine–Cd(1)–Brbridged, Namine
Cd(1)–Brterminal, Npyridine–Cd(1)–Brterminal and Brterminal
–
–
Cd(1)–Brbridged angles for [(L‐c)Cd(μ‐Br)Br]2 were
128.16(6)°, 96.87(5)°, 111.06(6)° and 120.224(2)°, respec-
tively. Npyridine‐Cd(1)‐Namine angle was 72.89(8)°, whereas
#1
Namine–Cd(1)–Brbridged
angle in [(L‐c)Cd(μ‐Br)Br]2
was 160.96(5)°.
The new geometric parameters for the 4‐coordinate
compounds (τ4) as improved simple metrics for
quantitatively evaluating the geometry are presented in
Table 3.[62] We have observed the degree of distortion
based on the bond angles around the Pd and Zn centers
using these structure indexes (τ4). The τ4 value of [(L)
PdCl2] (L = L‐a and L‐c) and [(L‐a)PtCl2] showed a
square planar geometry distorted by 0.00695–0.108°,
whereas [(L)ZnCl2] (L = L‐a and L‐c) showed a tetrahe-
dral geometry distorted by 0.866–0.885°.
Similarly, from comparison of Nimine–Cd–Npyridine
angles for [(L‐a)Cd(μ‐Br)Br]2 (72.3°)[50] and [(L‐c)
Cd(μ‐Br)Br]2 [72.89(8)°] along with 5‐coordinate
structure indices (τ5) values it has been concluded that
[(L‐a)Cd(μ‐Br)Br]2 showed more distorted trigonal
bipyramidal geometry compared with [(L‐c)Cd(μ‐Br)
Br]2. The τ5 value of [(L‐a)Cd(μ‐Br)Br]2 (0.600) and
[(L‐c)Cd(μ‐Br)Br]2 (0.546) depending on the bond
angles around the metal center showed trigonal

PARK ET AL.
11 of 15
TABLE 3 Four‐coordinate geometry indices (τ4) for [(L)MCl2] (L = L‐a–L‐c; M = Zn, Pd) and representative examples from the literature
Complexes
Geometry
τ4
THCDA/100
FCGP/100
Reference
Square planar (D4h
[(L‐a)PdCl2]a,b
[(L‐a)ZnCl2]c
[(L‐b)PdCl2]c
[(L‐c)PdCl2]
)
Square planar
Square planar
Tetrahedral
0.00
−1.43
−1.33
0.395
−1.26
−1.34
0.432
1.00
−0.400
−0.249
0.349
[62]
0.00695
0.866
0.108
0.0626
0.885
1.00
[48]
[49]
Square planar
Square planar
Tetrahedral
−0.200
−0.265
0.452
[46]
This work
This work
[62]
[(L‐c)ZnCl2]
Tetrahedral (Td)
Tetrahedral
0.00
aAssume that the geometry of [(L‐a)PdCl2] is 4‐coordinated.
bFour‐coordinate geometry indices of [(L‐a)PdCl2] are calculated using CIF file of [(L‐a)PtCl2].[63]
cAlthough complex of [(L‐a)ZnCl2] and [(L‐b)PdCl2] is reported in the literature[46,49] with no X‐ray crystallographic structure so CIF file of it is not available.
In this work, we investigated the X‐ray crystallographic structures.
TABLE 4 Five‐coordinate geometry indices (τ5) for [(L)Cd(μ‐Br)Br]2 (L = L‐a and L‐c) and representative examples from the literature
Complexes
Geometry
τ5
Reference
[64,65]
trigonal bipyramidal
[(L‐a)Cd(μ‐Br)Br]2
[(L‐c)Cd(μ‐Br)Br]2
square‐pyramidal
trigonal bipyramidal
trigonal bipyramidal
trigonal bipyramidal
square‐pyramidal
1.00
0.600
0.546
0.00
[50]
This work
[64,65]
bipyramidal geometry distorted by 0.546–0.600°,
while an ideal trigonal bipyramidal arrangement has τ5
value 14;= 1 (Table 4).[64,65]
Further, the plane angles between 4‐methoxyaniline
moiety and the plane of metal and pyridine were 6.010°
for [(L‐a)ZnCl2], 59.21° for [(L‐b)2ZnCl2], 77.51° for
[(L‐b)PdCl2], 59.81° for [(L‐b)2CdCl2], 34.60° for [(L‐
c)ZnCl2], 80.16° for [(L‐c)PdCl2] and 70.68° for [(L‐c)
Cd(μ‐Br)Br]2, respectively.
number of structural units and molecules, are summa-
rized in Table 5.
The catalytic activity of all the complexes showed a
general tendency of L‐a > L‐c > L‐b at 60°C, with the
exception of [(L‐b)PdCl2]. The imine‐bearing complexes
[(L‐a)MX2] (M = Zn; X = Cl; M = Cd; X = Br) exhibited
catalytic activities twice as high as the corresponding
metal precursors/salts. However, the catalytic activity of
[(L‐b)PdCl2] was much greater than [(L‐a)PdCl2], pre-
sumably the electron‐rich cloud around the Pd metal in
[(L‐b)PdCl2] by amine ligand provided increased activity
compared with the electronic effect of imine ligand.
It can be seen that [(L‐b)2MX2] (M = Zn; X = Cl; M
= Cd; X = Br) exhibited lower activities, which might be
due to the fact that two equivalent amine and two halo-
gen ligands around the metal center generate a coordina-
tion sphere that could not effectively accommodate any
steric clashes between the incoming monomer, propagat-
ing polymer chain and ligand architecture during MMA
polymerization. In addition, the catalyst's solubility in
reaction media has significantly affected the catalytic
activities. [(L‐c)Cd(μ‐Br)Br]2 owing to its poor solubility
exhibited lower activity (Table 5, entry 17).
3.3 | Methyl methacrylate polymerization
As
the
a
part of our ongoing investigations towards
stereoselective MMA catalyst based on
pyridinylmethanimine moiety, the catalytic capabilities
of 4‐methoxy‐N‐(pyridin‐2‐ylmethylene) aniline‐based
Zn (II), Pd (II) and Cd (II) complexes were investigated
for MMA polymerization in the presence of MMAO in
toluene at 60°C. All the synthesized complexes polymer-
ized MMA, yielding PMMA with Tg ranging from 121 to
128°C.[66] The polymers were isolated as white solids
and characterized by GPC in THF using standard polysty-
rene as the reference. The polymerization results, includ-
ing tacticity based on the isotactic (mm), heterotactic (mr)
and syndiotactic (rr),[29,30,67,68] and PDI, which represent
the average degree of polymerization in terms of the
To evaluate the total steric hindrance by ligands
toward the metal center, it can be predicted and quantita-
tively calculated by comparison through a topographic
steric map of [(L)nMX2]m (L = L‐a–L‐c; M = Zn, Pd; X

12 of 15
PARK ET AL.
TABLE 5 Polymerization of MMA by [(L)nMX2]m (L = L‐a–L‐c; M = Zn, Pd; X = Cl; M = Cd; X = Br; n, m = 1 or 2) complexes in the
presence of MMAO
d
Tg
Tacticity
e
Yieldb
(g)
Activityc
(g mol−1 Cat hr−1) × 104
Mw
Mw/
Mn
f
Entry
Catalystsa
(°C)
%mm
%mr
%rr
(g mol−1) × 105
1
[PdCl2]g
[ZnCl2]g
[CdBr2·4H2O]g
MMAOh
[(L‐a)PdCl2]i
[(L‐a)ZnCl2]i
[(L‐a)Cd(μ‐Br)Br]2
[(L‐b)PdCl2]i
[(L‐b)2ZnCl2]
[(L‐b)2CdBr2]
[(L‐c)PdCl2]
[(L‐c)ZnCl2]
[(L‐c)Cd(μ‐Br)Br]2
12.6
11.1
10.2
6.83
22.9
19.9
21.6
24.3
14.1
12.9
19.8
14.3
17.5
1.97
1.73
1.58
1.07
3.58
3.10
3.37
3.80
2.20
2.02
3.08
2.23
2.73
129
129
131
120
128
124
121
123
126
126
126
121
125
10.2
9.20
7.60
37.2
10.3
7.43
7.04
7.43
6.94
6.99
7.53
7.69
7.59
23.5
24.2
22.8
10.9
20.7
25.0
22.5
25.0
23.6
23.1
24.0
23.1
24.1
67.7
66.6
69.6
51.9
69.0
67.6
70.4
67.6
69.4
69.9
68.5
69.9
69.0
9.27
1.33
0.66
6.78
10.8
9.14
9.78
9.12
9.52
9.55
9.62
10.2
9.37
1.58
1.58
2.90
2.09
2.96
2.62
2.49
2.46
2.40
2.54
2.54
2.52
2.59
2
3
4
5
6
i
7
8
9
10
11
12
13
a[M (II) catalyst]0 = 15 μmol, and [MMA]0/[MMAO]0/[M (II) catalyst]0 = 3100:500:1, polymerization temp. = 60°C and time = 2 hr.
bYield defined as a mass of dried polymer recovered/mass of monomer used.
cActivity is calculated (g of PMMA)/(mol−1 M hr−1).
dTg is glass transition temperature, which is determined by a thermal analyzer.
eDetermined by GPC eluted with THF at room temperature by filtration with polystyrene calibration.
fMn refers to the number average of molecular weights of PMMA.
gIt is a blank polymerization in which PdCl2, ZnCl2 and CdBr2·4H2O were also activated by MMAO.
hIt is a blank polymerization that was done solely by MMAO.
iComplexes of [(L‐a)PdCl2], [(L‐a)ZnCl2], [(L‐a)Cd(μ‐Br)Br]2 and [(L‐b)PdCl2] are reported in the literature.[46,48–50]
= Cl; M = Cd; X = Br; n, m = 1 or 2) complexes by use of
program ‘SambVca’ (see Supplementary materials).[69]
Figure S1 in the Supplementary material shows the ball
and stick model, space‐filling model and topographic ste-
ric map of [(L)nMX2]m (L = L‐a–L‐c; M = Zn, Pd; X =
Cl; M = Cd; X = Br; n, m = 1 or 2) for presenting steric
bulk of ligands around the metal center. Only considering
total steric factor by topographic steric maps, it is evident
that less sterically hindered complexes exhibited better
catalytic performance. By comparing Pd complexes, [(L)
PdCl2] (L = L‐a–L‐c), it is evident that [(L‐b)PdCl2]
(buried volume % is 37.7, activity 3.80 × 104 g mol−1 Pd
hr−1) has higher activity compared with more sterically
demanding [(L‐c)PdCl2] (buried volume % is 42.4,
activity 2.73 × 104 g mol−1 Cd hr−1); [(L‐a)ZnCl2] (bur-
ied volume % 36.4, activity 3.10 × 104 g mol−1 Zn hr−1
)
> [(L‐c)ZnCl2] (buried volume % 42.3, activity 2.23 ×
104 g mol−1 Zn hr−1). However, the opposite trend has
been observed in [(L‐b)MX2] (M = Zn; X = Cl; M = Cd;
X = Br) complexes. Despite apparently greater steric
crowding, the buried volume was found to be lower com-
pared with [(L‐c)MX2] (M = Zn; X = Cl; M = Cd; X =
Br) complexes. However, the polymerization activities
were 2.20 × 104 g mol−1 Zn hr−1 and 2.02 × 104 g mol−1
Cd hr−1, respectively. Thus, both steric and electronic
effects of catalysts are needed to correlate the polymeriza-
tion activity and structure of complexes. In addition, the
solubility of complexes or active species in polymerization
solvent also influenced the polymerization activity.
activity 3.08 × 104 g mol−1 Pd hr−1
)
in 4‐methoxy‐N‐
(pyridin‐2‐ylmethylene) aniline derivatives bearing Pd
(II) system. Similarly, the order of activity for [(L)
nMX2]m (L = L‐a–L‐c; M = Zn, Pd; X = Cl; M = Cd; X
= Br; n, m = 1 or 2) was found to be [(L‐a)Cd(μ‐Br)
Br]2 (buried volume % 31.4, activity 3.37 × 104 g mol−1
Cd hr−1) > [(L‐c)Cd(μ‐Br)Br]2 (buried volume % 34.1,
The influence of metal center variation is also studied.
It is evident that using [(L)nMX2]m (L = L‐a–L‐c; M =
Zn, Pd; X = Cl; M = Cd; X = Br; n, m = 1 or 2) as a poly-
merization catalyst, [(L‐b)PdCl2] exhibited the highest
catalytic activity (3.80 × 104 g PMMA mol−1 Pd hr−1).
The current results are consistent with our previously

PARK ET AL.
13 of 15
reported Zn (II), Pd (II) and Cd (II) initiators bearing N‐
substituted 2‐iminoalkylpyridines for MMA polymeriza-
tion, where the iminopyridine effectively creates
electron‐rich clouds around the palladium metal but does
not affect Cd (II) and Zn (II) complexes.[32,33]
ligand architecture and the resultant stereoselectivity of
the MMA polymerization are ongoing.
4 | CONCLUSIONS
Previously studied Pd (II) complexes with N‐(pyridin‐
2‐ylmethyl) aniline derivatives[37] resulted in comparable
activities and yielded PMMA with low molecular weights
compared with [(L‐b)PdCl2]. Our current system is supe-
rior in terms of activity and resulted in syndio‐enriched
high molecular weight PMMA compared with N‐methyl‐-
N‐(pyridin‐2‐ylmethyl) aniline‐based Pd (II)[25] complexes
in our earlier report. Similarly, compared with previous
Zn (II) complexes with N‐(pyridin‐2‐ylmethylene) aniline
and N‐phenyl‐(pyridin‐2‐yl)methanimine,[38,70] current
Zn (II) systems effectively catalyzed MMA polymeriza-
tion. The Cd (II) complexes, [(L‐c)Cd(μ‐Br)Br]2 and
[(L‐c)Cd(μ‐Br)Br]2, exhibited lower activities compared
with our previously reported Cd (II) complexes bearing
N,N‐di(2‐picolyl) amine ligand,[71] but resulted in high
molecular weight PMMA with improved Tg (Table 5).
Moreover, the PDIs of the resultant PMMA ranged from
2.40 to 2.96, which is in agreement with the fact that
the PDI range narrowed with increasing molecular
weights.[63,72,73] Judging from the low activity of [(L‐c)
MX2] (M = Zn; X = Cl; M = Cd; X = Br) similar to blank
polymerization, the MMA polymerization activity of
metal complexes in the current study should be consid-
ered as a function of the electron density about the metal
center and steric factor of the substituents on the N atom
of N‐methylamine.
In summary, we have synthesized and structurally
characterized a series of new Zn (II), Pd (II) and Cd (II)
complexes supported by 4‐methoxy‐N‐(pyridin‐2‐
ylmethylene) aniline derivatives. The molecular struc-
tures of 4‐coordinate Pd (II) and Zn (II) complexes [(L)
MCl2] (L = L‐a and L‐c; M = Zn and Pd) were distorted
square planar and distorted tetrahedral, respectively,
whereas the 6‐coordinated Zn (II) and Cd (II) complexes
[(L‐b)2MX2] (M = Zn; X = Cl; M = Cd; X = Br) exhib-
ited distorted octahedral geometries. However, the
dimeric [(L‐c)Cd(μ‐Br)Br]2 complex adopted distorted
trigonal bipyramidal geometry. The catalytic activity of
[(L‐b)PdCl2] toward the polymerization of MMA in the
presence of MMAO resulted in higher activity of 3.80 ×
104 g PMMA mol−1 Pd hr−1 compared with the rest
of its analogs under identical conditions. Moderate
syndiotacticity has resulted with all complexes with Tg
in range of 121–128°C. The MMA polymerization activity
in the current study should be considered as a function of
the electron density about the metal center, and steric
factor of imine, amine and N‐amine moieties of ligand
framework. The molecular weights of resultant PMMA
were significantly affected by the steric bulk around the
metal centers, whereas no clear trends could be found
according to the steric bulk of the substituents for the
PDIs and syndiotacticity.
The syndiotacticity of PMMA was found to be 0.68–
0.70, similar to all [(L)nMX2]m (L = L‐a–L‐c; M = Zn,
Pd; X = Cl; M = Cd; X = Br; n, m = 1 or 2) initiators
regardless of the ligand architecture and metal center
variation. However, the syndiotacticity was higher com-
pared with co‐catalyst, MMAO. Although the resultant
syndiotactic enchainment was not satisfactorily high to
predict a coordination polymerization mechanism, it
was found to be similar for all of the synthesized com-
plexes. The molecular weights were much influenced by
the steric encumbrance provided by amine and N‐
methylamine moieties, while having little dependence
on the PDIs of resultant PMMA. The molecular weights
of the resultant PMMA should be considered as a
function of steric bulk around the metal center. In addi-
tion, experiments using 2,6,6,6‐tetramethylpiperidinyloxy
(TEMPO), a 5‐equivalent radical inhibitor during the
MMA polymerization, did not significantly reduce the
yield of PMMA and showed a reduction in yield up to
2–5%, which supports the fact that the mechanism of
polymerization is not a radical one. Further investiga-
tions to improve the catalytic performance in terms of
ACKNOWLEDGEMENT
This research was supported by the National Research
Foundation of Korea (NRF), funded by the Ministry of
Education, Science, and Technology (MEST) (Grant no.
2017R1A2B4002100).
ORCID
Hyosun Lee https://orcid.org/0000-0002-0107-3100
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