Synthesis, structural features, and methyl methacrylate polymerisation of binuclear zinc(II) complexes with tetradentate pyrazolyl ligands

Article abstract of DOI:10.1016/j.molstruc.2014.01.038

The reaction of ZnCl₂ with ancillary ligands, including 1,4-bis-(N,N-di-(1H-pyrazolyl-1-methyl)amine)benzene (L₁) and 4,4′-bis-(N,N-di(1H-pyrazolyl-1-methyl)phenyl)methane (L₂), in ethanol yields Zn(II) chloride complexes, i.e., 1,4-bis-(N,N-di-(1H-pyrazolyl-1- methyl)amine)benzene(dichloro)Zn(II) [L₁Zn₂Cl₄] and 4,4′-bis-(N,N-di-(1H-pyrazolyl-1-methyl)phenyl)methane(dichloro) Zn(II) [L₂Zn₂Cl₄]. The X-ray crystal structures of Zn(II) complexes revealed that they are binuclear, and each zinc atom has a distorted tetrahedral geometry which involves a nitrogen atom from two pyrazole groups and two chloro ligands. The catalytic activity of [L₁Zn ₂Cl₄] and [L₂Zn₂Cl₄] for the polymerisation of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) increased by twofold compared to the corresponding monomeric Zn(II) complex, N,N-bis(1H-pyrazolyl-1-methyl)aniline(dichloro)Zn(II) [LZnCl₂], at 60 C.

Full text of DOI:10.1016/j.molstruc.2014.01.038

Journal of Molecular Structure 1063 (2014) 70–76
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, structural features, and methyl methacrylate polymerisation
of binuclear zinc(II) complexes with tetradentate pyrazolyl ligands
Sunghoon Kim a, Dongil Kim a, Ha-Jin Lee b,c, Hyosun Lee a,
⇑
a Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu-city 702-701, Republic of Korea
b Jeonju Center, Korea Basic Science Institute (KBSI), 634-18 Keumam-dong, Dukjin-gu, Jeonju-city 561-180, Republic of Korea
c Department of Chemistry, Chonbuk National University, Dukjin-gu, Jeonju-city 561-756, Republic of Korea
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
The bispyrazole-containing binuclear and tetrahedral zinc (II) complexes, [L1Zn2Cl4] and [L2Zn2Cl4] have
been structurally characterised. The catalytic activity of Zn(II) complexes toward the polymerisation of
MMA in the presence of MMAO increased by two-fold compared to the corresponding monomeric Zn(II)
complex, [LZnCl2] at 60 °C.
ꢀ
The bispyrazole-containing binuclear
and tetrahedral Zn(II) complexes are
synthesized.
The Zn(II) complexes were active
toward polymerization of MMA.
The activity increased by two-fold
compared to the monomeric Zn(II)
complex.
ꢀ
ꢀ
C9
N5
C6
C3
N3
N2
C5
C4
C2
C7
C8
Zn1
N4
C1
N1
Cl1
Cl2
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of ZnCl2 with ancillary ligands, including 1,4-bis-(N,N-di-(1H-pyrazolyl-1-methyl)amine)
benzene (L1) and 4,40-bis-(N,N-di(1H-pyrazolyl-1-methyl)phenyl)methane (L2), in ethanol yields
Zn(II) chloride complexes, i.e., 1,4-bis-(N,N-di-(1H-pyrazolyl-1-methyl)amine)benzene(dichloro)Zn(II)
[L1Zn2Cl4] and 4,40-bis-(N,N-di-(1H-pyrazolyl-1-methyl)phenyl)methane(dichloro)Zn(II) [L2Zn2Cl4]. The
X-ray crystal structures of Zn(II) complexes revealed that they are binuclear, and each zinc atom has a
distorted tetrahedral geometry which involves a nitrogen atom from two pyrazole groups and two chloro
ligands. The catalytic activity of [L1Zn2Cl4] and [L2Zn2Cl4] for the polymerisation of methyl methacrylate
(MMA) in the presence of modified methylaluminoxane (MMAO) increased by twofold compared to the
corresponding monomeric Zn(II) complex, N,N-bis(1H-pyrazolyl-1-methyl)aniline(dichloro)Zn(II)
[LZnCl2], at 60 °C.
Received 27 December 2013
Received in revised form 15 January 2014
Accepted 15 January 2014
Available online 23 January 2014
Keywords:
1,4-Bis-(N,N-di-(1H-pyrazolyl-1-
methyl)amine)benzene
4,40-Bis-(N,N-di(1H-pyrazolyl-1-
methyl)phenyl)methane
Binuclear zinc(II) complex
MMA polymerization
Ó 2014 Elsevier B.V. All rights reserved.
⇑
Corresponding author. Tel.: +82 1053971576; fax: +82 539506330.
E-mail address: hyosunlee@knu.ac.kr (H. Lee).
http://dx.doi.org/10.1016/j.molstruc.2014.01.038
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

S. Kim et al. / Journal of Molecular Structure 1063 (2014) 70–76
71
Corporation as 6.9% weight aluminum of a toluene solution and used
without further purification. Elemental analyses (C, H, N) of the pre-
pared complexes were carried out on an elemental analyzer (EA
1108; Carlo-Erba, Milan, Italy). 1H NMR (400 MHz) and 13C NMR
(75.46 MHz operating) were recorded on a Bruker Advance Digital
400 NMR spectrometer and chemical shifts were recorded in ppm
units using SiMe4 as an internal standard. Electronic absorption
spectra were obtained on an Ocean Optics USB4000 spectrophotom-
eter (Ocean Optics, Dunedin, FL). Absorption spectra were obtained
on Jasco V-530 UV/VIS spectrophotometer. The molecular weight
and molecular weight distribution of the obtained PMMA were
carried out using gel permeation chromatography (GPC) (CHCl3,
Alliance e2695; Waters Corp., Milford, MA). Glass transition temper-
ature (Tg) was determined using a thermal analyzer (Q2000; TA
Instruments, New Castle, DE).
1. Introduction
Bis(1H-pyrazol-1-ly)methylaniline and its various derivatives
form a variety of complexes due to the possible binding modes
of N,N-bidentate, N,N0,N-tridentate, and N,N0,N0,N-tetradentate
with proper linker units of amine derivatives depending on the
transition metals. These derivatives were first introduced by Dries-
sen in 1982 [1]. Driessen et al. synthesised and characterised N-
bidentate and N-tetradentate pyrazole ligands, which contain a
proper amine linker unit, such as 1-(2-ethylaminoethyl]-3,5-
dimethylpyrazole (deae), bis-(3,5-dimethylpyrazolylmethyl)ethyl-
amine (bdmae), or bis-(3,5-diemtylpyrazolylethyl)ethylamine
(ddae), with several transition metals [2–9]. For example, metal
complexes such as monomeric/dimeric Rh(I) [10–12], Co(II)
[6,13–16], dimeric Fe(II) [17,18], metals of group 6 (Cr, Mo, W)
[19,20], metals of group 10 (Ni, Pd, Pt) [21–25], Cu(II) [26–31],
Cd(II) [32,33], and Zn(II) [3,34–37] complexed with N-substituted
pyrazole ligands have unique structural properties and catalytic
activities. Although a large variety of transition metal complexes
with N-substituted pyrazole derivatives have been characterised,
further studies are required on the design and synthesis of transi-
tion metals ligated with bis N-substituted pyrazole derivatives to
manipulate the amine linker unit. In addition, the zinc-containing
complexes have drawn a huge interest for the photo-induced opti-
cal and nonlinear optics applications [38,39]. Along with this fact,
we are interested in the zinc complexes as catalyst for producing
the polymethyl methacrylate (PMMA). PMMA is very universal
polymers as optical usage. Usually, the higher the glass transition
temperature (Tg) represents the higher optical quality and syndio-
tacticity content of PMMA. The glass transition temperature (Tg) of
isotactic PMMA, which is produced by radical process in commer-
cial is around 65 °C. Thus, the research on non-radical mediated
polymerization of MMA have been attracted and some transition
metal complexes used successfully [37,40–47]. Previously, we
reported tetrahedral Co(II) and Zn(II) complexes with N,N-
bis(1H-pyrazolyl-1-methyl)aniline and its derivatives, as well as
the binding mode of N,N-bidentate for methyl methacrylate
(MMA) polymerisation [37,45]. In addition, the Cd(II) complex with
ligand N,N-bis(3,5-dimethyl-1H-pyrazolyl-1-methyl)aniline has
revealed the binding mode of N,N0,N-tridentate by binding the ani-
line nitrogen to cadmium [48]. Although the molecular structure of
the ligand N,N,N0,N0-tetra-[(3,5-di-substituted-1-pyrazolyl)methyl]
-para-phenylenediamine was reported by Daoudi et al. in 2003
[49,50], the synthesis, crystal structure, and catalytic activity of
this transition metal complex with the bis-N,N-bidentate
ligand has not been investigated [51]. Thus, we report the
synthesis, X-ray crystal structure, and MMA polymerisation of
binuclear Zn(II) complexes with the tetradentate ligands 1,4-bis-
(N,N-di(1H-pyrazolyl-1-methyl)amine)benzene and 4,40-bis-(N,N-
di(1H-pyrazolyl-1-methyl)phenyl)methane, which contain four
pyrazoles as N-donor atoms. We expect to observe the ‘‘effect of
double metal existence in one molecule’’ on MMA polymerisation
[42,46] compared to the previously reported corresponding mono-
meric Zn(II) complex.
2.2. Synthesis of ligands and complexes
2.2.1. Preparation of organic compounds
The 1H-pyrazolyl-1-methanol as starting material were pre-
pared in processes described in literature [1]. The CH2Cl2 solution
(100 mL) of pyrazole (20.4 g, 0.30 mol) was added a CH2Cl2 solu-
tion (100 mL) of para-formaldehyde (9.00 g, 0.30 mol). The solu-
tion was reflux for 5 days and the filtrate solvent was removed
under reduced pressure to give white powder (28.5 g, 98.0%). 1H
NMR (CDCl3, 400 MHz) for 1H-pyrazolyl-1-methanol: d 7.71 (s,
1H), 7.59 (d, 1H, J = 2.24 Hz), 7.56 (d, 1H, J = 1.48 Hz), 6.29 (t, 1H,
J = 1.8 Hz), 5.51 (s, 2H).
2.2.2. 1,4-Bis-(N,N-di(1H-pyrazolyl-1-methyl)amine)benzene (L1)
L1 was prepared by a similar procedure as described in the lit-
erature [6,49,52–54]. The CH2Cl2 solution (10.0 mL) of para-phen-
ylenediamine (1.08 g, 0.010 mol) was added a CH2Cl2 solution
(50.0 mL) of 1H-1-pyrazolyl-1-methanol (3.92 g, 0.040 mol). The
reaction solution was dried over the MgSO4 after stirring the reac-
tion mixture at room temperature for 3 days. The white solid prod-
uct was obtained (4.07 g, 95.0%). Analysis calculated for C22H24N10
:
C, 61.67%; H, 5.65%; N, 32.69%. Found: C, 61.69%; H, 5.65%; N,
31.81%. 1H NMR (CDCl3, 400 MHz): d 7.56 (d, 4H, J = 1.6 Hz,
AN@CHACH@CHANA), 7.42 (d, 4H, J = 2.0 Hz, AN@CHACH
@CHANA), 7.02 (s, 4H, o-,m-NC6H5NA), 6.25 (dd, 4H, J = 1.6 Hz,
J = 2.0 Hz, AN@CHACH@CHANA), 5.62 (s, 8H, ACH2A). 13C NMR
(CDCl3, 400 MHz): d 140.67 (2C, ipso-NC6H5NA), 139.89 (4C,
AN@CHACH@CHANA), 129.03 (4C, AN@CHACH@CHANA),
118.48 (4C, o-,m-NC6H5NA), 106.17 (4C, AN@CHACH@CHANA),
66.82 (4C, ACH2A). IR (solid neat; cmꢁ1): 3102(w), 2361(s),
1697(s), 1522(w), 1470(s), 1370(s), 1265(s), 1206(s), 1156(s),
1089(s), 1046(s), 947(s), 880(s), 750(s), 650(s), 610(s).
2.2.3. 4,40-Bis-(N,N-di(1H-pyrazolyl-1-methyl))phenyl)methane (L2)
L2 was prepared by analogous method as described for L1 ex-
cept utilizing 4,40-diaminophenylmethane. The white solid product
was obtained (4.77 g, 92%). Analysis calculated for C29H30N10: C,
66.49%; H, 5.73%; N, 26.25%. Found: C, 67.16%; H, 5.73%; N,
26.25%. 1H NMR (CDCl3, 400 MHz): d 7.56 (d, 4H, J = 1.6 Hz,
AN@CHACH@CHANA), 7.43 (d, 4H, J = 2.0 Hz, AN@CHA
CH@CHANA), 7.02 (d, 4H, J = 8.8 Hz, m-NC6H4ACH2AC6H4NA),
7.00 (d, 4H, J = 8.8 Hz, o-NC6H4ACH2AC6H4NA), 6.25 (dd, 4H,
J = 1.6 Hz, J = 2.0 Hz, AN@CHACH@CHANA), 5.69 (s, 8H, ACH2A),
3.80 (s, 2H, ANC6H4ACH2AC6H4NA). 13C NMR (CDCl3, 400 MHz):
2. Experimental
2.1. Materials and instrumentation
ZnCl2, pyrazole, para-formaldehyde, para-phenylenediamine,
4,40-diaminophenylmethane were purchased from Aldrich and
anhydrous solvents such as ethanol, dimethylformamide (DMF),
diethyl ether, acetone, dichloromethane were purchased from
Merck and used without further purification. Modified methylalu-
minoxane (MMAO) was purchased from Tosoh Finechem
d
144.44 (2C, -ipsoANC6H4ACH2AC6H4NA), 140.35 (4C,
AN@CHACH@CHANA), 134.52 (4C, m-NC6H4ACH2AC6H4NA),
130.32 (4C, AN@CHACH@CHANA), 129.26 (2C, p-NC6H4ACH2AC6
H4NA), 116.37 (4C, o-NC6H4ACH2AC6H4NA), 106.65 (4C,
AN@CHACH@CHANA), 66.90 (4C, ACH2A), 40.42 (1C, ANC6H4
ACH2AC6H4NA). IR (solid neat; cmꢁ1): 3110(w), 2969(w),

72
S. Kim et al. / Journal of Molecular Structure 1063 (2014) 70–76
2903(w), 1611(w), 1512(s), 1461(s), 1342(w), 1260(s), 1217(s),
1172(s), 1082(s), 1049(s), 956(s), 903(s), 844(s), 791(s), 745(s),
648(s), 604(s).
AC6H4NA), 6.24 (dd, 4H, J = 1.6 Hz, J = 2.0 Hz, AN@CHACH
@CHANA), 5.69 (s, 8H, ACH2A), 3.65(s, 2H, ANC6H4ACH2AC6H4
NA). 13C NMR (DMSO-d6, 400 MHz): d 143.54 (2C, -ipso-NC6H4
ACH2AC6H4NA), 139.41 (4C, AN@CHACH@CHANA), 133.02 (4C,
m-NC6H4ACH2AC6H4NA), 129.97 (4C, AN@CHACH@CHANA),
129.41 (2C, p-NC6H4ACH2AC6H4NA), 114.45 (4C, o-NC6H4ACH2
AC6H4NA), 105.91 (4C, AN@CHACH@CHANA), 66.30 (4C, ACH2
A), 39.84 (1C, ANC6H4ACH2AC6H4NA). IR (solid neat; cmꢁ1):
3110(w), 2968(w), 2888(w), 1746(w), 1696(s), 1618(s), 1518(w),
1469(s), 1411(s), 1317(s), 1257(s), 1170(s), 1072(s), 953(s),
764(s), 647(s), 612(s).
2.3. Synthesis of Zn(II) complexes
2.3.1. 1,4-Bis-(N,N-di(1H-pyrazolyl-1-methyl)aminebenzene
(dichloro)zinc(II)) ([L1Zn2Cl4])
A solution of L1 (0.214 g, 0.50 mmol) in dried ethanol (10.0 mL)
was added to a solution of ZnCl2 (0.136 g, 1.00 mmol) in dried eth-
anol (10.0 mL) at room temperature. Precipitation of white mate-
rial occurred while stirring at room temperature for 12 h. The
white powder was filtered and washed with ethanol (50.0 mL), fol-
lowed by washing with diethyl ether (50.0 mL) (0.29 g, 83%). The
X-ray crystals of [L1Zn2Cl4] were obtained within five days from
diethyl ether (10.0 mL) diffusion into a DMF solution (10.0 mL) of
[L1Zn2Cl4] (0.10 g). Analysis calculated for C22H24Cl4N10Zn2: C,
37.69%; H, 3.45%; N, 19.98%. Found: C, 37.69%; H, 3.54%; N,
20.01%. 1H NMR (DMSO-d6, 400 MHz): d 7.80 (d, 4H, J = 2.0 Hz,
AN@CHACH@CHANA), 7.49 (d, 4H, J = 1.2 Hz, AN@CHACH
@CHANA), 7.00 (s, 4H, o-,m-NC6H5NA), 6.24 (dd, 4H, J = 2.0 Hz,
J = 1.6 Hz, AN@CHACH@CHANA), 5.81 (s, 8H, ACH2A). 13C NMR
2.4. Representative polymerization procedure of MMA
Methyl methacrylate (MMA) was extracted with 10% sodium
hydroxide, washed with water, dried over magnesium sulfate,
and distilled over calcium hydride under reduced pressure before
use. To a 100-mL Schlenk flask containing the complex (11 mg,
15 lmol for [L1Zn2Cl4] and 12 mg, 15 lmol for L2Zn2Cl2) in toluene
(1 mL) was added MMAO (modified methylaluminoxane, 6.9 wt%
in toluene, 3.25 mL, [MMAO]0/[Zn(II) catalyst]0 = 500) under a dry
argon atmosphere. After the mixture had been stirred at room tem-
perature for 10 min, it was transferred into MMA (5.0 mL,
47.0 mmol, [MMA]0/[Zn(II) catalyst]0 = 3100). Then, the reaction
flask was immersed in an oil bath at 60 °C and stirred for 2 h.
The resulting polymer was precipitated in methanol (400 mL)
and HCl (3 mL) was added with stirring for 10 min. The polymer
was filtered and washed with methanol (400 mL ꢂ 3) to give
PMMA, which was vacuum-dried at 60 °C.
(DMSO-d6, 400 MHz)
d 139.36 (4C, AN@CHACH@CHANA),
138.87 (2C, ipso-NC6H5A), 129.95 (4C, AN@CHACH@CHANA),
115.84 (4C, o-,m-NC6H5NA), 105.87 (4C, AN@CHACH@CHANA),
66.68 (4C, ACH2A). IR (solid neat; cmꢁ1): 3123(w), 2364(s),
1694(s), 1648(w), 1522(s), 1469(s), 1409(s), 1316(s), 1251(s),
1194(s), 1163(s), 1069(s), 992(s), 949(s), 867(s), 784(s), 736(s),
643(s), 611(s), 566(s).
2.3.2. 4,40-Bis-(N,N-di(1H-pyrazolyl-1-methyl))phenylmethane
(dichloro)zinc(II)) ([L2Zn2Cl4])
2.5. X-ray crystallographic analysis
The [L2Zn2Cl4] was prepared according to the similar procedure
described for [L1Zn2Cl4]. The white powder was filtered and
washed with ethanol (50.0 mL), followed by washing with diethyl
ether (50.0 mL) (0.34 g, 87%). The X-ray crystals of L2Zn2Cl4 were
obtained within five days from diethyl ether (10.0 mL) diffusion
into an acetone solution (10.0 mL) of [L2Zn2Cl4] (0.10 g). Analysis
calculated for C29H30Cl4N10Zn2: C, 44.02%; H, 3.82%; N, 17.70%.
Found: C, 43.92%; H, 3.76%; N, 17.75%. 1H NMR (DMSO-d6,
400 MHz): d 8.14 (d, 4H, J = 1.6 Hz, AN@CHACH@CHANA), 7.49
(d, 4H, J = 1.6 Hz, AN@CHACH@CHANA), 7.03 (d, 4H, J = 8.8 Hz,
m-NC6H4ACH2AC6H4NA), 6.97 (d, 4H, J = 8.8 Hz, o-NC6H4ACH2
A colorless cubic-shaped crystal was picked up with paraton oil
and mounted on a Bruker SMART CCD diffractometer equipped
with a graphite-monochromated Mo K
a (k = 0.71073 Å) radiation
source and a nitrogen cold stream (ꢁ100 °C). Data collection and
integration were performed with SMART (Bruker, 2000) and
SAINT-Plus (Bruker, 2001) [55]. Semiempirical absorption correc-
tions based on equivalent reflections were applied by SADABS
[56]. The structure was solved by direct methods and refined by
full-matrix least-squares on F2 using SHELXTL [57]. All the non-
hydrogen atoms were refined anisotropically, and hydrogen atoms
were added to their geometrically ideal positions.
NH2
OH
N
N
N
N
N
N
N
N
N
N
N
Cl
Cl
N
N
Cl
Cl
ZnCl2
CH2Cl2
+
Zn
N
N
N
Zn
N
N
N
reflux, 3days
EtOH, rt
N
N
NH2
N
[L1Zn2Cl4]
L1
NH2
N
OH
N
N
N
N
N
N
N
N
N
N
+
N
CH2Cl2
reflux, 3days
ZnCl2
EtOH, rt
Cl
N
N
N
N
Zn
N
N
N
Cl
Zn
Cl
N
N
Cl
N
NH2
L2
[L2Zn2Cl4]
Scheme 1. Synthetic scheme for the preparation of [L1Zn2Cl4] and [L2Zn2Cl4].

S. Kim et al. / Journal of Molecular Structure 1063 (2014) 70–76
73
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
The electronic absorption spectra of ligands and Zn complexes
in dimethylformamide (DMF) appeared similar and mainly con-
sisted of intra-ligand characteristics (Fig. 1). For example, ligands
LZnCl2
L1Zn2Cl4
L2Zn2Cl4
276
283
271
L
and LZnCl2 showed the same two absorption bands at
305
280
314
k1 = 265 nm and k2 = 279 nm with molar absorption coefficients
of 1650 cmꢁ1 Mꢁ1 and 2250 cmꢁ1 Mꢁ1, respectively, which were
attributed to p–
pꢃ interactions of the aniline and pyrazolyl moiety.
265
The absorption spectra of ligands L1 and [L1Zn2Cl4] showed two
bands at approximately k1 = 274 nm and k2 = 283 nm with molar
absorption coefficients of 2670 cmꢁ1 Mꢁ1 and 2740 cmꢁ1 Mꢁ1
,
respectively. The absorption spectra of ligands L2 and L2Zn2Cl4
showed three bands, due to the presence of diphenylmethylene
units between bis-didentate pyrazole, near k1 = 265 nm,
k2 = 305 nm, and k3 = 318 nm with molar absorption coefficients
of 2647 cmꢁ1 Mꢁ1, 2150 cmꢁ1 Mꢁ1, and 2070 cmꢁ1 Mꢁ1, respec-
tively. The absorption band of ligands and complexes showed very
slight red shifts, increasing from [LZnCl2] to [L1Zn2Cl4] and then
[L2Zn2Cl4]. These Zn(II) complexes are substantially disturbed by
the electron-vibration broadening and particularly anharmonic
ones.
220 240 260 280 300 320 340 360 380 400 420
Wavelength (nm)
Fig. 1. UV absorption spectra of Zn(II) complexes.
A single crystal suitable for X-ray analysis was obtained
from diethyl ether diffusion in DMF for [L1Zn2Cl4] and acetone for
[L2Zn2Cl4] solution. Crystal data and structure refinement of
[L1Zn2Cl4] and [L2Zn2Cl4] are listed in Table 1. The structures of
the Zn(II) complexes and the selected bond distances and bond an-
gles of Zn(II) complexes are shown in Figs. 2 and 3, respectively. Both
[L1Zn2Cl4] and [L2Zn2Cl4] crystallised in the triclinic P1 space group,
and the unit cell included a DMF for [L1Zn2Cl4] and an acetone for
[L2Zn2Cl4] solvent. The coordination geometry around the Zn(II)
atom bound by the two nitrogen atoms from pyrazole and two chlo-
rine atoms showed slightly distorted tetrahedral coordination. The
N atom of aniline did not take part in the coordinative bond to the
zinc atom, thus achieving the N,N-bidentate binding mode, as deter-
minedbased on the bond lengths between the N of aniline and the Zn
metal.
3. Results and discussion
The zinc(II) complexes
1,4-bis-N,N-di(1H-pyrazolyl-1-
methyl)aminebenzene(dichloro)zinc(II) [L1Zn2Cl4] and 4,40-bis-
N,N-di(1H-pyrazolyl-1-methyl))phenylmethane(dichloro)zinc(II)
[L2Zn2Cl4] were synthesised using the method illustrated in
Scheme 1.
Ligands were obtained at yields of 95% (L1) and 92% (L2) from
the reaction of 1H-pyrazolyl-1-methanol with p-phenylenedi-
amine and 4,40-diaminophenylmethane in methylene chloride,
respectively. The Zn(II) chloride complexes (83% for [L1Zn2Cl4]
and 87% for [L2Zn2Cl4]) were obtained from the reaction between
the corresponding ligands and anhydrous zinc(II) chloride in anhy-
drous ethanol. The results of 1H NMR, 13C NMR, and elemental
analyses were consistent with ligand and Zn(II) complex formula-
tion. 1H NMR and 13C NMR peaks of the Zn(II) complexes were
shifted slightly down-field compared to the ligands due to reso-
nance effects of the N and C atoms of the pyrazole group.
The average bond lengths of ZnANpyrazole and ZnACl were 2.039
and 2.2115 Å for [L1Zn2Cl4] and 2.0205 and 2.228 Å for [L2Zn2Cl4],
respectively. The bond lengths were slightly affected by the
Table 1
Crystal data and structural refinement of [L1Zn2Cl4] and [L2Zn2Cl4].
[L1Zn2Cl4]
[L2Zn2Cl4]
Empirical formula
Formula weight
Temperature
Crystal system
Space group
C
22H24Cl4N10Zn2
C29H30Cl4N10Zn2
849.25
200(2) K
Triclinic
P1
774.15
200(2) K
Triclinic
P1
Unit cell dimensions
a = 8.8611(7) Å,
a
= 72.191(2)°
a = 11.9336(13) Å,
a = 102.497(3)°
b = 12.7048(9) Å, b = 79.461(2)°
b = 12.1475(14) Å, b = 110.424(4)°
c = 15.6390(13) Å,
1646.2(2) Å3
2
c
= 89.157(2)°
c = 15.664(3) Å,
1927.4(4) Å3
2
c = 105.428(2)°
Volume
Z
Calculated density
Absorption coefficient
F(000)
F(0000)
1.562 Mg mꢁ3
1.821 mmꢁ1
788
1.463 Mg mꢁ3
1.562 mmꢁ1
868
Crystal size
Theta range for data collection
Index range
Reflections collected
Independent reflections
Data completeness
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
0.29 ꢂ 0.18 ꢂ 0.11 mm
1.39° to 28.29°
0.34 ꢂ 0.18 ꢂ 0.17 mm
1.48° to 28.34°
ꢁ11 6 h 6 11, ꢁ16 6 k 6 16, ꢁ17 6 l 6 20
ꢁ15 6 h 6 15, ꢁ8 6 k 6 16, ꢁ20 6 l 6 17
12353
14391
8069[Rint = 0.0377]
0.987
9471[Rint = 0.0663]
0.985
Full-matrix least-squares on F2
8069/0/390
Full-matrix least-squares on F2
9471/6/444
1.130
0.933
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
r
(I)]
R1 = 0.0652, wR2 = 0.1407
R1 = 0.1439, wR2 = 0.2281
1.511 and ꢁ2.387 e Åꢁ3
R1 = 0.0802, wR2 = 0.1943
R1 = 0.1808, wR2 = 0.2892
1.131 and ꢁ1.294 e Åꢁ3

74
S. Kim et al. / Journal of Molecular Structure 1063 (2014) 70–76
AZnANpyrazole angles for the complexes were 119.62(11)° and
107.0(3)° for [L1Zn2Cl4] and 117.27(10)° and 109.5(2)° for [L2Zn2
Cl4], respectively. The angle of C(12)AC(15)AC(16) in [L2Zn2Cl4]
was 112.0(7)°, which was greater than the tetrahedral angle
(109.5°) due to slight steric repulsion by the phenyl rings. The Npyr-
azoleAZnANpyrazole angle showed nearly ideal tetrahedral angles,
but those of the ClAZnACl angle were approximately 10° larger.
The geometry at each zinc centre was best described as a distorted
tetrahedral with two equivalent half-molecules showing overall C2
symmetry. The two zinc atoms for complex [L1Zn2Cl4] were lo-
cated across the phenyl plane due to the lack of steric hindrance.
The bond angles of NAZnACl were in the range of
101.93°ꢄ111.5°, indicative of
a slightly distorted tetrahedral
configuration.
[L1Zn2Cl4] and [L2Zn2Cl4] were activated by modified methyl-
aluminoxane (MMAO) to polymerise MMA, yielding poly(methyl
methacrylate) (PMMA) with a Tg ranging from 120 to 130 °C
[40,41,43]. The polymers were isolated as white solids and charac-
terised by GPC in tetrahydrofuran (THF) using standard polysty-
rene as a reference. The triad microstructure of the PMMA was
analysed using 1H NMR spectroscopy. The results of polymerisa-
tion, including tacticity (isotactic (mm), heterotactic (mr), or syn-
diotactic (rr)), as well as the polydispersity index (PDI) as the
average degree of polymerisation in terms of the number of struc-
tural units and molecules, are summarised in Table 2.
For comparison, catalytic activity of the corresponding mono-
nuclear complex, LZnCl2 [N,N-bis(1H-pyrazolyl-1-methyl)aniline
(dichloro)zinc(II)] for MMA polymerisation, blank polymerisation
of MMA with MMAO itself, and ZnCl2/MMAO were cited [37].
The tacticity of PMMA was identified around syndiotactic
(0.85 ppm), heterotactic (1.02 ppm), and isotactic (1.21 ppm) by
1H NMR [58]. [L1Zn2Cl4] exhibited a wider PDI and similar molec-
ular weights as PMMA, but twofold higher activity compared to
mononuclear [LZnCl2] [59]. In addition, syndiotacticity designated
by [L1Zn2Cl4] was much higher than any other comparable bis-
pyrazoyl-containing Zn complexes. However, [L2Zn2Cl4] showed
Fig. 2. ORTEP drawing of [L1Zn2Cl4] with thermal ellipsoids at 50% probability. All
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°):
Zn(1)AN(1) 2.036(6), Zn(1)AN(4) 2.042(7), Zn(1)ACl(2) 2.222(2), Zn(1)ACl(1)
2.201(3), N(1)AC(1) 1.330(9), N(1)AN(2) 1.345(8), N(2)AC(3) 1.355(10),
C(1)AC(2) 1.385(12), C(2)AC(3) 1.359(12), N(1)AZn(1)AN(4) 107.0(3),
Cl(1)AZn(1)ACl(2) 119.62(11), N(1)AZn(1)ACl(1) 110.3(2), N(1)AZn(1)ACl(2)
101.93(18),
N(4)AZn(1)ACl(1)
111.5(2),
N(4)AZn(1)ACl(2)
105.4(2),
C(4)AN(5)AC(5) 118.5(6).
environment around the zinc metal. These values were in good
agreement with the distances for related zinc complexes with pyr-
azole ligands, specifically [LZnCl2]. The ClAZnACl and Npyrazole
Fig. 3. ORTEP drawing of [L1Zn2Cl4] with thermal ellipsoids at 50% probability. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°):
Zn(1)AN(1) 2.009(6), Zn(1)AN(4) 2.032(6), Zn(1)ACl(2) 2.210(2), Zn(1)ACl(1) 2.246(2), N(1)AC(1) 1.352(9), N(1)AN(2) 1.349(8), N(2)AC(3) 1.329(9), C(1)AC(2) 1.401(11),
C(2)AC(3) 1.372(11), N(1)AZn(1)AN(4) 109.5(2), Cl(1)AZn(1)ACl(2) 117.27(10), N(1)AZn(1)ACl(1) 109.49(19), N(1)AZn(1)ACl(2) 106.84(18), N(4)AZn(1)ACl(1) 107.92(19),
N(4)AZn(1)ACl(2) 105.64(19), C(12)AC(15)AC(16) 112.0(7).

S. Kim et al. / Journal of Molecular Structure 1063 (2014) 70–76
75
Table 2
Polymerisation of MMA by [L1Zn2Cl4] and [L2Zn2Cl4] in the presence of MMAO.
d
Entry
Catalysta
Temp. (time)
Yieldb
(g)
Activityc
Tg
Tacticity
%mm
Mw
Mw/Mn
(°C)
(g/mol Cat h) ꢂ 104
(°C)
%mr
%rr
(g molꢁ1) ꢂ 105
1
2
3
4
5
Zn2Cl2e,[37]
MMAOf,[37]
[LZnCl2]g,[37]
[L1Zn2Cl4]
60 (2 h)
60 (2 h)
60 (2 h)
60 (2 h)
60 (2 h)
0.52
0.42
0.61
1.24
0.96
1.73
1.40
2.03
4.13
3.20
124.24
119.61
130.48
128.76
129.89
9.2
37.2
9.2
8.8
9.2
24.2
10.9
26.0
24.1
30.1
66.6
51.9
64.8
67.1
60.7
4.96
6.78
8.00
7.26
4.41
15.3
2.09
1.75
2.34
6.36
[L2Zn2Cl4]
a
b
c
d
e
f
[Zn(II) catalyst]0 = 15 lmol, and [MMA]0/[MMAO]0/[Zn(II) catalyst]0 = 3100:500:1.
Yield defined as the mass of dried polymer recovered/mass of monomer used.
Activity is g of PMMA/(mol Zn h).
Determined by GPC eluted with THF at room temperature by filtration with polystyrene calibration.
Blank polymerisation in which ZnCl2 was activated by MMAO.
Blank polymerisation performed by MMAO alone.
[LZnCl2] refers to the N,N-bis(1H-pyrazolyl-1-methyl)aniline(dichloro)zinc(II) complex.
g
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The bispyrazole-containing binuclear zinc(II) complexes
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characterised. The coordination geometry around the Zn(II) atom
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Acknowledgment
This research was supported by Kyungpook National University
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Appendix A. Supplementary material
CCDC 973734 and CCDC 973735 contain the supplementary
crystallographic data for [L1Zn2Cl4] and [L2Zn2Cl4], respectively.
These data can be obtained free of charge via http://www.ccdc.ca-
m.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk. Sup-
plementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.molstruc.2014.01.
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