Nickel(II) Complexes with Mono(imino)pyrrole Ligands: Preparation, Structure, and MMA Polymerization Behavior

Article abstract of DOI:10.1134/S1070328420050073

Abstract: A simplified synthetic method was initiated to prepare the corresponding nickel complexes NiL₂ (I–III) with direct condensation of mono(imino)pyrroles (L¹–L³) and nickel dichloride, the structures and methyl meth

Full text of DOI:10.1134/S1070328420050073

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2020, Vol. 46, No. 5, pp. 355–364. © Pleiades Publishing, Ltd., 2020.
Nickel(II) Complexes with Mono(imino)pyrrole Ligands:
Preparation, Structure, and MMA Polymerization Behavior
B. Y. Sua, *, D. D. Pana, T. Y. Yana, A. Zhanga, L. Wanga, L. Q. Dinga, and R. Zhoua
aCollege of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an, Shaanxi, 710065 P.R. China
*e-mail: subiyun@xsyu.edu.cn
Received July 9, 2019; revised October 12, 2019; accepted November 26, 2019
Abstract—A simplified synthetic method was initiated to prepare the corresponding nickel complexes NiL2
(I–III) with direct condensation of mono(imino)pyrroles (L1–L3) and nickel dichloride, the structures and
methyl methacrylate (MMA) catalytic polymerization behavior of this series of mono(imino)pyrrole nickel
complexes were presented. The mono(imine)pyrrole ligands and the corresponding nickel complexes
were determined by 1H NMR, 13C NMR, IR and MS, etc. Complexes I and III were further characterized by
X-ray crystal diffraction (CIF files CCDC nos. 1890965 (I), 1890964 (III)). Both of the structures showed
that the ligand chelated to nickel with 2 : 1 molar ratio. The systematic studies were focused on the relation-
ship between the catalytic behavior of these complexes for MMA polymerization and catalyst structure, reac-
tion time, reaction temperature, and ratio of monomer with catalyst. The optimum reaction conditions of the
molar ratio of monomer to catalyst is of 1200 : 1, the polymerization temperature of 100°C, time of 10 h, the
nickel complex with two bulky substituents on the o-position of phenyl ring linked with imine showed excel-
lent catalytic activities for MMA polymerization (4.791 × 104 g mol–1 h–1), high molecular weight (Mn =
65.873 × 103 g mol–1), and narrow molecular mass distribution (polymer dispersity index = 3.9877), and azo-
diisobutyronitrile acted as co-catalyst during MMA polymerization.
Keywords: mono(imino)pyrrole, nickel complex, MMA polymerization, catalytic activity
DOI: 10.1134/S1070328420050073
Ni catalyst was as high as 1.1 × 107 g mol–1 h–1. In
2003, α-diimine Ni catalyst was firstly applied into the
polymerization of methyl methacrylate (MMA) under
the activation of co-catalyst methylaluminoxane
(MAO) [13]. Authors of [14, 15] first developed a class
Ni or Pd catalysts bearing [N,O] chelating ligands,
called Schiff base catalysts, and showed that ethylene
polymerization can be carried out without the aid of
MAO activation. A chelated salicylaldimine Schiff
base nickel catalyst was synthesized with high activity
of 1.5 × 105 g mol–1 h–1 when nAl/nNi is 100 in [16, 17].
An asymmetric [N,O] neutral Ni complex, used as
catalytic system with the aid of MAO for polymeriza-
tion of MMA, has been reported in [18]. Authors of
[19] designed a series of bis[N,O] ligand Ni complexes
as the main catalyst and used MAO as a promoter to
catalyze the polymerization of MMA in toluene. Hu
Yangjian et al. [20] designed the [N,O] chelated nickel
complex as catalyst [10], using aluminum alkyl as pro-
moter and n-hexane as solvent, the solution polymer-
ization of MMA indicated that the catalytic activity
could reach 1.107 × 105 g mol–1 h–1.
INTRODUCTION
Poly(methyl methacrylate) (PMMA) is a kind of
excellent and cost-effective variety in synthetic trans-
parent materials [1]. It is the most widely used class of
acrylic resin, commonly known as plexiglass, and has
excellent transparency, surface visibility and arc resis-
tance. PMMA is mainly used in the automotive indus-
try, pharmaceutical industry, consumer goods and
electronic products [2]. In 2015, China has become
the world’s largest consumer of PMMA with a of 80 ×
104 t [3–5]. As a polar monomer, the catalyst used for
the polymerization of methyl methacrylate is mainly
metallocene catalyst [6, 7], recently much work has
been devoted to late transition metal catalysts due to
their low affinity for oxygen and strong tolerance to
polar groups, which make them to be used for
homopolymerization of polar monomers and copoly-
merization of olefins and polar monomers [8]. In
those catalysts, Ni and Pd-based olefin polymeriza-
tion catalysts [9] are the most important class of late
transition metal catalyst systems [10]. In 1995,
authors of [11, 12] developed Ni and Pd catalysts con-
taining α-diimine ligands and applied them to the
Herein we reported a series of nickel complexes
polymerization of ethylene. These catalysts showed with asymmetric [N,N] mono(imino)pyrrole ligands
very high catalytic activities, especially the activity of and studied their catalytic performance for PMMA. In
355

356
SU et al.
1
FT-IR (ν, cm–1): 1609.0 ν(C=N). H NMR (δ,
order to increase coordination ability of heteroatom
N, CH3 group was introduced into the side arm of pyr-
role imine, to get novel five-membered heterocyclic
imine compounds mono(imino)pyrroles [21]. During
the synthesis of mono(imino)pyrroles, an efficient
microwave irradiation method was used to accelerate
Schiff base condensation, which offered several
advantages, such as shorter reaction time, improved
yield, and a simplified reaction process as compared
with traditional liquid phase reflux reaction. In addi-
tion, this solvent-free reaction was in keeping with the
green chemistry principle. The novel direct coordina-
tion of mono(imino)pyrrole ligand with nickel dichlo-
ride, without traditional deprotonation process, was
applied successfully in the preparation of nickel com-
plexes [22]. As the mono(imino)pyrrole nickel com-
plex was used in polymerization of MMA with the aid
of co-catalyst azodiisobutyronitrile (AIBN), the
excellent activity was afforded, this was in sharp con-
trast with the polymerization result of ethylene using
the similar nickel catalyst system [23].
ppm): 9.76 (s., pyrrole N–H), 7.20 (t., benzene ring
aromatic H), 7.15 (t., benzene ring aromatic H), 6.99
(t., benzene ring aromatic H), 6.82 (t., pyrrole ring
aromatic H), 6.66 (d., benzene ring aromatic H), 6.65
(d., pyrrole ring aromatic H), 6.23 (t., pyrrole ring
aromatic H), 2.08 (s., –N=C(CH3)–), 2.04 (s., phe-
nyl–CH3). 13C NMR (δ, ppm): 159.66, 157.38,
149.03, 139.77, 133.56, 130.25, 125.36, 124.01, 120.85,
118.39, 116.33, 115.12, 111.76, 79.43, 76.78, 31.98,
19.50.
For C13H14N2
Anal. calcd., %
Found, %
C, 78.75
C, 78.22
H, 7.12
H, 6.95
N, 14.13
N, 14.68
Synthesis of ligand 2-{1-[(3-methylphenyl)imino]-
ethyl}pyrrole (L2) was carried out by the same proce-
dure used for L1. White solid of L2 was obtained with a
yield of 25.7%; mp 128.9–130.0°C; elutes: petroleum
ether–EtOAc (3 : 1, V/V).
EXPERIMENTAL
1
FT-IR (ν, cm–1): 1594.5 ν(C=N). H NMR (δ,
Methods and materials. All experiments concerning
air and moisture sensitive compounds were carried out
under nitrogen protection using standard Schlenk
techniques. Solvents were refluxed over an appropri-
ate drying agent and distilled prior to use. CHN anal-
yses were performed with a HP-MOD 1106 micro
analyzer. Melting points were determined in a X-5
Micro-melting point apparatus. Fourier transform
infrared (FT-IR) spectra were obtained with a Perkin-
ppm): 9.67 (s., pyrrole N–H), 7.23 (t., benzene ring
aromatic H), 7.19 (t., benzene ring aromatic H), 6.90
(t., benzene ring aromatic H), 6.84 (t., pyrrole ring
aromatic H), 6.60 (d., benzene ring aromatic H), 6.59
(t., pyrrole ring aromatic H), 6.24 (t., pyrrole ring aro-
matic H), 2.34 (s., phenyl–CH3), 2.12 (s.,
‒N=C(CH3)–). 13C NMR (δ, ppm): 157.45, 157.39,
150.75, 138.77, 132.59, 128.79, 124.12, 121.05–121.87,
117.39, 112.29, 109.76–109.80, 76.73–77.37, 29.74,
21.51.
1
Elmer FT-IR 2000 spectrometer. H NMR spectra
were recorded with a Bruker DMX-300 spectrometer.
Mass spectral (MS) analyses were performed with a
Kratos AEI MS-50 instrument using the electron
impact (EI) method. Single-crystal XRD was per-
formed using a Bruker Smart 1000 CCD at a tempera-
ture of 296 K. Powder X-ray diffractions (PXRD) were
tested using Shimadzu XRD-6000. Microwave-
assisted reactions were carried out in a Midea PJ 21B-
A800w(21L)domesticoven.2-Acetylpyrrole,2-meth-
ylaniline, 3-methylaniline, and 4-methylaniline were
purchased from Acros Co. and used as received. AIBN
in toluene was from Cologne Chemicals Co. LTD.
Polymerization MMA monomer, produced by
Chengdu Cologne Chemicals Co. LTD, was used
without further purification.
Synthesis of ligand 2-{1-[(2-methylphenyl)imino]-
ethyl}pyrrole (L1). 2-Acetylpyrrole (152.8 mg,
1.400 mmol) and 2-methylaniline (300.0 mg,
2.802 mmol) were added to a 50 mL beaker with a
1 : 2 M ratio, then 0.5 mL of glacial acetic acid was
added and blended well. The mixture was put in a
microwave oven and irradiated at 600 W for 5 min. The
obtained black brown product was purified by chro-
matographic column to afford a mass of white solid
with a yield of 40.9%; mp 123.3–125.7°C; elutes:
petroleum ether–EtOAc (5 : 1, V/V).
For C13H14N2
Anal. calcd., %
Found, %
C, 78.75
C, 78.84
H, 7.12
H, 6.79
N, 14.13
N, 13.99
Synthesis
of
ligand
2-{1-[(4-methylphe-
nyl)imino]ethyl}pyrrole (L3) was carried out by the
same procedure used for L1. White solid of L3 was
obtained with a yield 26.7%; mp 134.3–135.0°C;
elutes: petroleum ether–EtOAc (3 : 1, V/V).
FT-IR (ν, cm–1): 1631.8 ν(C=N). H NMR (δ,
1
ppm): 9.72 (s., pyrrole N–H), 7.14 (t., benzene ring
aromatic H), 6.81 (t., benzene ring aromatic H), 6.71
(t., pyrrole ring aromatic H), 6.64 (d., pyrrole ring
aromatic H), 6.23 (t., pyrrole ring aromatic H), 2.35
(s., phenyl–CH3), 2.12 (s., –N=C(CH3)–). 13C NMR
(δ, ppm): 157.57, 148.24, 132.76, 132.68, 129.53,
121.72, 120.36, 112.08, 109.70, 76.74–77.37, 20.90,
16.41.
For C13H14N2
Anal. calcd., %
Found, %
C, 78.75
C, 78.26
H, 7.12
H, 6.77
N, 14.13
N, 13.89
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NICKEL(II) COMPLEXES WITH MONO(IMINO)PYRROLE LIGANDS
357
Synthesis of bis{2-[(2-methylphenyl)iminoethyl]- Schlenk tube containing a magneton, air was expelled
1H-pyrrol-1-ido-k2N,N′}nickel(II) (I). NiCl2 · 6H2O
(0.45 mmol) was dissolved in 10 mL of MeOH and
added dropwise into 10 mL of MeOH solution of L1
(0.45 mmol) with stirring. The mixture was stirred for
8 h at room temperature and then concentrated to 5–
6 mL. After placing for 2 h without disturbance, the
red brown solid was precipitated from the solution.
The powder was collected and washed with small
amount of distilled water for three times, the red
brown powder was obtained after vacuum drying suit-
able for PXRD analysis. Red brown crystals of com-
plex I suitable for XRD analysis was grown from tri-
chloromethane–acetone–methanol (1 : 1 : 1, V/V/V)
solution at room temperature. The yield was 65.9%.
FT-IR (ν, cm–1): 1598.3 ν(C=N), 1588.6 ν(C=N).
by three vacuum/nitrogen cycles before appropriate
amounts of monomer MMA, initiator AIBN
(12.1 μmol), and toluene (5 mL) were added. All liq-
uids were transferred into the Schlenk tube with dried
syringes under nitrogen. The tube was capped under
N2 atmosphere, and then immediately immersed in an
oil bath previously heated to the desired temperature.
After the reaction mixture was stirring for a certain
period of time, the polymerization was stopped and
the reaction quenched with acidic ethanol solution.
The precipitated polymer was filtered and washed sev-
eral times with ethanol and then dried under vacuum
at room temperature for 24 h. Molecular weight and
molecular weight distribution of the obtained polymer
were tested with the PL-GPC50 polystyrene column
of Agilent, UK, at temperature of 40°C, using THF as
mobile phase. A small amount of the treated polymer
was dissolved in a chromatographic grade of THF to
form a uniform solution, and then injected into the
tester with a syringe with a filter needle, the obtained
gel chromatographic data was compared with the sty-
rene standard sample to get final Mn and Mw/Mn. The
catalytic activity was calculated by the total mass of
PMMA product (g) dividing the molar amount of
metal catalyst (mol) and polymerization time (h).
For C26H26N4Ni
Anal. calcd., % C, 68.90 H, 5.78 N, 12.36 Ni, 12.96
Found, %
C, 68.44 H, 5.29 N, 12.33 Ni, 13.94
Synthesis of bis{2-[(3-methylphenyl)iminoethyl]-1H-
pyrrol-1-ido-k2N,N′}nickel(II) (II) was carried out by
the same procedure used for I. Yellow brown powder
of II was prepared suitable for PXRD analysis.
Regarding the crystal of complex II, different meth-
ods, such as the direct evaporation of the original solu-
tion, the liquid-liquid diffusion, and the gas-liquid
diffusion method were used to grow crystals of com-
plex II but crystals did not come out. Similar method
applied to complexes I and III was also applied but it
didn’t work for complex II. The solvents used were
chloroform, acetone, methanol, ethanol and the dif-
ferent combination ratios, but the results were not sat-
isfactory. The yield was 65.7%. FT-IR (ν, cm–1):
1611.4 ν(C=N), 1610.2 ν(C=N).
X-ray structure determination. Diffraction intensity
data of the single crystals of I and III were collected on
a Bruker SMART 1000 diffractometer with graphite-
monochromated MoKα radiation (λ = 0.71073 Å).
The Smart program package was used to determine
the unit cell parameters. A SADABS absorption cor-
rection was applied [24]. The structures were solved by
direct methods and refined by full-matrix least-
squares techniques with anisotropic thermal parame-
ters for non-hydrogen atoms. Hydrogen atoms were
placed at calculation positions and were included in
the structure calculation without further refinement of
the parameters. All calculations were performed using
the SHELXS-97 program [25, 26]. Crystal data and
structure refinement for complexes I and III are sum-
marized in Table 1.
For C26H26N4Ni
Anal. calcd., % C, 68.90 H, 5.78 N, 12.36 Ni, 12.96
Found, %
C, 68.78 H, 5.65 N, 12.23 Ni, 13.34
Synthesis of bis{2-[(4-methylphenyl)iminoethyl]-
1H-pyrrol-1-ido-k2N,N′}nickel(II) (III) was carried
out by the same procedure used for I. Yellow brown
powder of III was prepared suitable for PXRD analy-
sis. Yellow brown crystals of complex III suitable for
XRD analysis was grown from trichloromethane–ace-
tone–methanol (1 : 1 : 1, V/V/V) solution at room
temperature. The yield was 65.2%. FT-IR (ν, cm–1):
1611.6 ν(C═N), 1609.8 ν(C=N).
Supplementary material for structures has been depos-
ited with the Cambridge Crystallographic Data Centre
(CCDC nos. 1890965 (I), 1890964 (III); deposit@ccdc.
cam.ac.uk or http://www.ccdc.cam.ac.uk).
For C26H26N4Ni
Anal. calcd., % C, 68.90 H, 5.78 N, 12.36 Ni, 12.96
Found, %
C, 68.91 H, 5.70 N, 12.35 Ni, 13.04
RESULTS AND DISCUSSION
The syntheses of ligands L1–L3 and corresponding
tain amount of nickel complex was placed into a Ni(II) complexes I–III are shown in Scheme 1.
General procedure for MMA polymerization. A cer-
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358
SU et al.
NH2
R3
R1
R2
R1
R2
R3
N
N
N
N
.
NiCl2 H2O
R3
R2
Ni
R1
HOAc, M. W.
Methanol
N
H
N
H
R2
R3
N
O
R1
L1−L3
(I)−(III)
L1/I: R1 = CH3, R2 = H, R3 = H
L2/II: R1 = H, R2 = CH3, R3 = H
L3/III: R1 = H, R2 = H, R3 = CH3
Scheme 1.
Initially, 2-acetylpyrrole reacts with a series of ani- membered chelate rings formed by atoms
line derivatives by Schiff base condensation to get Ni(1)/N(1)/C(1)/C(5)N(2) and Ni(2)/N(3)/C(18)/
three novel mono(imino)pyrrole ligands (L1–L3),
herein, a new microwave irradiation method are
employed to accelerate the reactions instead of con-
ventional liquid-phase reaction [27–29] and all the
reactions proceeded under mild microwave conditions
(600 W, 5–8 min), then the combination of column
chromatography and recrystallization methods was
used to obtain pure products.
Complexes L1–L3 were synthesized by reacting
Ni(II) dichloride with a 1 : 1 stoichiometric amount of
ligands L1–L3, respectively, in methanol at room tem-
perature. All complexes were isolated as air-stable sol-
ids in high purity. In the synthesis of imino pyrrole
transition metal complexes, the weak acidities in pyr-
role N–H were always thought to interfere with coor-
dination, so the most commonly used method was to
deprotonate the ligand using butyl lithium to produce
a lithium complex [30], then reacted with a transition
metal to get an imino pyrrole transition metal com-
plex. Accidently, it was found that mono(imino)pyr-
role nickel complex can be synthesized with the direct
chelation of mono(imino)pyrrole ligand and Ni(II)
dichloride under very mild conditions (e.g. methanol
solvent, room temperature) [22]. The process is of
significance in terms of the procedure simplification
and cost reduction.
C(14)/N(4) are essentially coplanar, and the maxi-
mum deviations from their planes are –0.023(2) and
‒0.021(7) Å, respectively, for atoms N(1) and N(4).
While in complex III, the five-membered chelate rings
formed by Ni(1)/N(1)/C(3)/C(4)N(2) and Ni(2)/
N(6)/C(30)/C(29)/N(5) are also essentially coplanar,
the maximum deviations of atoms N(1) and N(5) from
their planes are –0.025(7) and –0.023(9) Å, respec-
tively. In addition, we notice that in complexes I and
III, the two equatorial Ni–Nimine distances (mean
1.942(2) Å in I, mean 1.941(2) Å in III) are substan-
tially longer than the equatorial Ni–Npyrrole bonds
(mean 1.906(2) Å in I, 1.918(2) Å in III), this has been
considered as combination result of the anionic nature
of pyrrolyl nitrogen and the steric bulk of methyl sub-
stituents on phenyl ring. All Ni–N bond lengths in
complexes I and III are obviously shorter than the
normal values for typical Ni–N bonds (2.07 Å) [31],
particularly Ni–Nimine distances. This may indicate a
stronger σ-donor character of imino N atom induced
by the methyl (C(6)) substituent in iminio carbon
(C(5)), which may also cause a higher degree of steric
congestion around Ni cation, as exhibited by I.
From the bond angles data, it could be found that
both in I and III sum of all the angles around nickel
center are ca. 360°, indicating that the central Ni
atoms are in essentially square-planar conformation.
In complex I, the phenyl substituents at imine nitro-
gen atoms are nearly perpendicular to NiN4 square
plane (86.38°, 87.03°) and parallel to each other (0°).
In complex III, the phenyl substituents at imine nitro-
gen atoms are also nearly perpendicular to NiN4
square plane (75.93°, 72.50°) and form a acute angle to
each other (19.62°). The difference about relative
space position of phenyl substituents to NiN4 square
and each other between I and III, maybe because dif-
Complexes I and III were further characterized
using single-crystal XRD analysis. The crystal struc-
tures of I and III are shown in Fig. 1, and selected
bond lengths and angles are listed in Table 2.
As shown by Fig. 1, complexes I and III have the
similar molecular structure, in both of which Ni atom
is coordinated by two inverted N,N′-bidentate
mono(imino)pyrrole ligands using two imino and two
pyrrolide nitrogen atoms to form a four-coordinate
geometry. Ni atom is located in a crystallographic
inversion center with the pyrrolide rings and the imine ferent hindrance effect brought by different methyl
groups trans to each other. In complex I, the five- substituent position on the phenyl ring. It is noticeable
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY
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NICKEL(II) COMPLEXES WITH MONO(IMINO)PYRROLE LIGANDS
Table 1. Crystallographic data and and structure refinement for I and III
359
Value
Parameter
I
III
Empirical formula
C26H26N4Ni
453.22
C26H26N4Ni
453.22
Formula weight
Crystal system
Space group
Triclinic
Monoclinic
P1
C2/c
a, Å
b, Å
c, Å
10.0928(11)
10.4996(11)
11.6897(12)
22.730(6)
9.728(2)
31.025(7)
α, deg
β, deg
γ, deg
78.160(2)
66.297(2)
89.174(2)
1106.9(2)
2; 1.360
0.90
90
98.210(6)
90
6790(3)
4; 1.330
0.88
Volume, Å3
Z; ρcalcd, mg m–3
μ, mm–1
Crystal size, mm
Diffractometer
0.31 × 0.24 × 0.16
Bruker APEX-II CCD
0.35 × 0.25 × 0.14
Bruker APEX-II CCD
Absorption correction
F(000)
Multi-scan (SADABS; Bruker, 2008) Multi-scan (SADABS; Bruker, 2008)
476
2856
Crystal color; habit
θ Range for data collection, deg
Index ranges
Brown-yellow; needle
2.2–26.3
Yellow; needle
2.4–17.5
–27 ≤ h ≤ 28,
–11 ≤ k ≤ 12,
–3 ≤ l ≤ 38
–11 ≤ h ≤ 11,
–8 ≤ k ≤ 12,
–11 ≤ l ≤ 13
Tmin, Tmax
0.770, 0.873
0.625, 0.745
Number of measured/independent/
5613/3901/3102
17921/6764/4328
observed (I > 2σ(I)) reflections
Rint
0.018
0.597
0.146
0.597
(sinθ/λ)max, Å−1
R(F2 > 2σ(F2)), wR(F2); S
Number of parameters
Number of restraints
0.044, 0.12; 1.19
0.176, 0.35; 1.25
287
0
426
0
Δρmax/Δρmin, e Å–3
0.46/–0.28
0.79/–2.76
that the pyrrolyl rings and the substituted phenyl ring and N(6)C(30)C(29) (111.89°) for III, in relation to
those observed in the free organic ligands L1 and L3. In
addition, angles at the pyrrolyl nitrogen CNpyrroleC are
decreased upon coordination, which is compensated
by an increase of the angles at C atoms bound to pyr-
rolyl nitrogen. Besides, the bond lengths within pyr-
role ring appear to be significantly affected, both C–C
and C–N distances appear to increase in varying
degrees as the ligand chelating with nickel. Mean-
while, the bond lengths in NCCN backbone of the
ligand change upon coordination to nickel, the C–
Npyrrole and C–Nimine distances increase, whereas C–C
linking to imine groups lay trans to each other in both
of complexes I and III.
Comparison of structure data of Ni(II) complexes
and their free ligands [32–36] also highlights some
structural differences. The first feature to note is that
ligand bite angles portrayed by NiminoNiNpyrrole are very
acute (at 82.88° and 82.92° in I, 82.82° and 84.52° in
III), this value is obtained decreases in the angles
defined by N(1)C(1)C(5) (115.24°), N(2)C(5)C(1)
(114.70°), N(4)C(14)C(18) (114.81°), and N(3)C(18)-
C(14) (115.24°) for I, N(1)C(3)C(4) (117.16°),
N(2)C(4)C(3) (115.13°), N(5)C(29)C(30) (117.13°) distances decrease. All the imine C=N bonds only
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360
SU et al.
(I)
C(6)
C(2A)
C(4A)
C(1A)
C(13)
C(12)
C(3A)
C(9A)
C(5)
C(11)
C(10)
C(9)
N(1A)
N(2A)
N(2)
C(7)
C(8A)
Ni(1)
C(8)
C(10A)
C(7A)
N(1)
C(11A)
C(12A)
C(4)
C(3)
C(5A)
C(1)
C(13A)
C(2)
C(6A)
(III)
C(5)
C(16)
C(2)
C(12)
C(3)
C(4)
C(10)
C(9)
C(11)
C(12)
C(1)
N(2)
N(1)
C(6)
C(20)
Ni(1)
N(3)
C(7)
C(22)
C(25)
C(8)
C(19)
C(21)
N(4)
C(13)
C(14)
C(24)
C(23)
C(17)
C(16)
C(15)
C(18)
Fig. 1. Molecular structure of I and III.
slightly increase upon coordination (C(5)–N(2) and for comparison, as shown in Fig. 2b. The position and
N(2A)–C(5A) in I, C(4)–N(2) and N(4)–C(17) in
III), indicating that π-back-donation from the nickel
center to the imine fragment is not strong.
height of all peaks in the powder diffraction patterns
for two synthesized complexes II were consistent
totally, so it was preliminarily determined that the syn-
thesized complex II was of good purity.
XRPD are used to characterize the purity of com-
plexes I–III, the results are displayed in Fig. 2. From
Figs. 2a and 2c, it is found that all the peaks in XRPD
patterns are highly consistent with that simulated by
SC-XRD, indicating good purity of the complexes of
I and III. Using the similar procedures, two copies of
A series of MMA polymerization experiments were
carried out to investigate the catalytic performance of
mono(imino)pyrrole nickel complexes I–III. The
influences of polymerization conditions were also
the complex II were prepared and measured by XRPD studied, employing 22 μmol of nickel catalyst, 12 μmol
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NICKEL(II) COMPLEXES WITH MONO(IMINO)PYRROLE LIGANDS
Table 2. Selected bond lengths (Å) and angles (deg) for I and III
361
I
III
Bond
d, Å
Bond
N(3)–C(16)
d, Å
N(1)–C(1)
1.372(4)
1.412(5)
1.499(5)
1.309(4)
1.451(4)
1.907(3)
1.944(3)
1.907(3)
1.944(3)
1.372(4)
1.412(5)
1.499(5)
1.309(4)
1.449(4)
1.421(17)
1.42(2)
1.51(2)
C(1)–C(5A)
C(5A)–C(6A)
N(2A)–C(5A)
N(2A)–C(7A)
Ni(1)–N(1)
Ni(1)–N(2A)
Ni(1)–N(1A)
Ni(1)–N(2)
C(1A)–N(1A)
C(1A)–C(5)
C(5)–C(6)
C(16)–C(17)
C(17)–C(18)
N(4)–C(17)
N(4)–C(19)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(1)
Ni(1)–N(2)
N(1)–C(3)
C(3)–C(4)
C(4)–C(5)
N(2)–C(4)
N(2)–C(6)
1.33(2)
1.419(17)
1.917(13)
1.929(10)
1.930(14)
1.944(11)
1.351(19)
1.39(2)
1.495(18)
1.349(19)
1.404(18)
C(5)–N(2)
N(2)–C(7)
Angle
ω, deg
Angle
ω, deg
N(1)Ni(1)N(2A)
N(1A)Ni(1)N(2)
N(1)Ni(1)N(2)
N(1A)Ni(1)N(2A)
82.88(10)
82.88(10)
97.12(10)
97.12(10)
N(1)Ni(1)N(2)
N(3)Ni(1)N(4)
N(3)Ni(1)N(2)
N(4)Ni(1)N(1)
83.6(5)
84.5(5)
95.4(5)
96.9(5)
of AIBN as co-catalyst and 5 mL of toluene as solvent. ylene polymerization process by the similar nickel cat-
The experimental data are listed in Table 3.
alysts [19]. Catalyst I shows the highest catalytic activ-
ities so it is selected for further condition optimization.
From the polymerization data shown in Table 3, it
can be concluded that MMA cannot be polymerized
without any main catalyst (nickel complex) or co-cat-
alyst (AIBN), as illustrated in run 0. Run 1 shows that
the main nickel catalyst cannot catalyze MMA polym-
erization alone. While the initiator AIBN alone can
promote MMA polymerization to obtain very little
PMMA, as illustrated in run 2. As run 2 and run 3 are
compared, it could be found that the catalytic activity
is enhanced from 0.190 × 104 to 4.791 × 104 g mol–1 h–1
by adding both of nickel catalyst and AIBN, the fact
shows that these two reagents produce good synergis-
tic catalysis effect, which greatly promotes the rate of
MMA polymerization.
Temperature
significantly
influences
the
catalytic activity for MMA polymerization. The runs 3
and 6–9 in Table 3 show that the activities of I are
increased sequentially from 1.321 × 104 to 4.791 × 104 g
mol–1 h–1, as the temperature raise from 70 to 100°C,
while the catalytic activities decrease slowly from
4.791 × 104 to 4.695 × 104 g mol–1 h–1, as the tempera-
ture raise further to 110°C. Although Mn and PDI of
PMMA indicate unregular correlation with polymer-
ization temperature, both of which get the highest
value at 100°C, 65.873 × 103 g mol–1 and 3.9877,
respectively. At lower reaction temperature, molecules
cannot get sufficient energy to form activated catalyst-
monomer complex and active center, so indicate rela-
tively lower insertion rate of monomer and slower
polymerization rate. At the higher temperature, the
monomer has sufficient thermal energy to insert into
the activation center and exacerbate chain propagation
rate. Moreover, higher temperature will make viscosity
of the system decrease, which is conducive to mass
transfer and diffusion of monomers.
By comparing runs 3–5 in Table 3, a trend can be
easily found that the activities (4.791 × 104 to 3.598 ×
104 g mol–1 h–1) and polymer dispersity index (PDI)
(3.9877–2.8948) are decreased sequentially along
with the order of catalysts I, II, III, indicating the
methyl group in different position of benzene ring
(ortho, meta and para) will influence the catalytic per-
formance dramatically. The methyl group near C=N
bond brings higher steric effects for the coordination
catalysis process of MMA, which positively correlates
with the polymerization activity and PDI. This intrin-
The runs 3 and 10–13 in Table 3 show that the
activity of catalyst I increases sequentially from
sic rule is in accordance with the rules observed in eth- 1.205 × 104 to 4.791 × 104 g mol–1 h–1, as the polymer-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 46 No. 5 2020

362
SU et al.
4.791 × 104 g mol–1 h–1, as the molar ratio of monomer
(a)
and nickel catalyst (n(M)/n(C)) raise from 800 : 1 to
1200 : 1, while the activities decrease from 4.791 × 104
to 3.085 × 104 g mol–1 h–1, as n(M)/n(C) increase fur-
ther to 2400 : 1. The molecular weight distribution is in
the range of 2.5211 and 3.9877. The small n(M)/n(C)
means relatively large concentration of catalyst active
centers and low concentration of monomers, the sep-
arate active center easily leads to lower molecular mass
(Mn = 30.983 × 103 g mol–1) while the low concentra-
tion of monomer is inclined to result in a poor catalyst
activity (2.183 × 104 g mol–1 h–1). While too much
monomer doesn’t make sure higher activity, for exam-
ple, as n(M)/n(C) raise to 1400 : 1 to 2400 : 1, the con-
centration of the catalyst active centers is relatively low
as compared with the monomer amount, which means
low conversion rate. At the same time, too much
monomer may raise the viscosity of polymerization
system, which affect the diffusion of monomer to
active center, and cause lower catalytic activity and
molecular mass.
10
20
30
40
50
(b)
10
20
30
40
50
Thus, a series of mono(imino)pyrrole ligands (L1–
L3) and the corresponding Ni(II) complexes (I–III)
have been synthesized. Ligands were synthesized using
microwave irradiation instead of a conventional liq-
uid-phase reaction. Direct coordination strategy was
adopted to generate the nickel(II) complexes. Both
innovations bring several advantages to the prepara-
tion of the mono(imino)pyrrole nickel catalysts, such
as cost and time savings, operation simplification. The
crystal structures of I and III indicate similar molecu-
lar structure, in both of which Ni(II) are coordinated
by two inverted N,N′-bidentate mono(imino)pyrrole
ligands using two imino and two pyrrolide N atoms to
form a four-coordinate geometry. Ni(II) is located in
a crystallographic inversion center with the pyrrolide
rings and the imine groups trans to each other. As the
nickel complexes are applied into MMA polymeriza-
tion catalysis with the aids of co-catalyst AIBN, the
results show that the relative molecular mass of
PMMA is in the magnitude of 104, PDI is between
1.5–4.0, and all catalysts have the medium activities in
the range of 4.791 × 104– 3.598 × 104 g mol–1 h–1. The
ligand structure, monomer ratio, polymerization time
and temperature all influence the catalytic perfor-
mance to some extent. Experimental data show that
the closer –CH3 on aniline ring to the imine group,
the higher activity is likely obtained. As the molar ratio
of monomer to catalyst I is of 1200 : 1, the polymeriza-
tion temperature of 100oC, time of 10 h, MMA polym-
erization provides the relatively higher activity
(4.791 × 104 g mol–1 h–1), larger molecular weight
(Mn = 65.873 × 103 g mol–1), as well as the narrower
(c)
10
20
30
2θ, deg
40
50
Fig. 2. PXRD of I (a), II (b), III (c): experiment is shown
in black, simulations are shown in red.
ization time prolongs from 4 to 10 h, while the cata-
lytic activities decrease from 4.791 × 104 to 3.148 ×
104 g mol–1 h–1 as the time raise further to 12 h. The
molecular mass of I increases sequentially from
55.202 × 103 to 87.945 × 103 g mol–1, as the time raise
from 4 to 8 h, while the molecular mass decrease
slowly from 87.945 × 103 to 65.873 × 103 g mol–1, as
the time prolongs further to 12 h. The molecular
weight distribution is between 2.1467 and 3.9877.
From above results, it could be concluded that at the
optimum reaction conditions (polymerization tem-
perature 100°C and time 10 h) the high activity can
be obtained, while too high temperature or too long
reaction time do not help for the production of active
center.
The runs 3 and 14–17 in Table 3 show that the
activities of I increase sequentially from 2.183 × 104 to molecular mass distribution (PDI = 3.9877).
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NICKEL(II) COMPLEXES WITH MONO(IMINO)PYRROLE LIGANDS
Table 3. MMA polymerization with nickel catalysts/AIBNa
363
Mnd
(103 g mol–1)
PDIe
(Mw/Mn)
n(M) : n(C)b
Activityc
Run
Catalyst
T, h
Temperature, °C
0
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
10
10
4
100
100
100
100
100
100
70
No. catalyst and AIBN
No. AIBN
No catalyst
1200 : 1
I
0.190
4.791
3.753
3.598
1.321
1.355
1.397
4.695
1.205
1.495
2.239
3.148
2.183
4.381
3.619
3.085
20.795
67.824
19.954
34.618
55.210
59.733
54.315
47.441
55.202
65.529
87.945
65.873
30.983
72.599
101.814
15.120
1.3644
3.9877
3.0307
2.8948
1.7541
1.9814
2.4463
2.2305
2.1467
2.5627
2.7784
3.1268
2.5877
2.9168
2.6544
2.5211
I
II
III
I
I
I
I
I
I
I
1200 : 1
1200 : 1
1200 : 1
1200 : 1
1200 : 1
1200 : 1
1200 : 1
1200 : 1
1200 : 1
1200 : 1
800 : 1
1600 : 1
2000 : 1
2400 : 1
80
90
9
110
100
100
100
100
100
100
100
100
10
11
12
13
14
15
16
17
6
8
I
I
I
I
12
10
10
10
10
I
a
b
c
d
e
Reaction conditions: 22 μmol Ni catalyst, 5 mL toluene, 12 μmol AIBN.
n(M) : n(C): molar ratio of MMA monomer to Ni catalyst.
4
–1 –1
The catalytic activity (10 g mol
h ).
M : number average molecular weight of polymer PMMA.
n
PDI = (M /M ).
w
n
FUNDING
This work was supported by the National Natural Sci-
ence Foundation of China (grant no. 51674200); the Sci-
ence and Technology Research Program of Shaanxi Prov-
ince (grant nos. 2018JM2035 and 2019JM-421).
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