Synthesis and characterization of Ni(II) complexes bearing of 2-(1H–benzimidazol-2-yl)-phenol derivatives as highly active catalysts for ethylene oligomerization

Article abstract of DOI:10.1002/aoc.4015

Novel Ni(II) complexes of 2-(1H–benzimidazol-2-yl)-phenol derivatives (HL_x: x = 1–5; C1–C5) have been synthesized and characterized. In the mononuclear complexes, the ligands were coordinated as bidentate, via one imine nitrogen and the phenola

Full text of DOI:10.1002/aoc.4015

Received: 10 March 2017
DOI: 10.1002/aoc.4015
Revised: 6 July 2017
Accepted: 6 July 2017
F U L L P A P E R
Synthesis and characterization of Ni(II) complexes bearing
of 2‐(1H–benzimidazol‐2‐yl)‐phenol derivatives as highly
active catalysts for ethylene oligomerization
Marzieh Haghverdi1 | Azadeh Tadjarodi1
| Naeimeh Bahri‐Laleh2 |
Mehdi Nekoomanesh‐Haghighi2
1 Research Laboratory of Inorganic
Novel Ni(II) complexes of 2‐(1H–benzimidazol‐2‐yl)‐phenol derivatives (HLx:
x = 1–5; C1–C5) have been synthesized and characterized. In the mononuclear
complexes, the ligands were coordinated as bidentate, via one imine nitrogen
and the phenolate oxygen atoms. The structures of the compounds were con-
Materials Synthesis, Department of
Chemistry, Iran University of Science and
Technology, Tehran, Iran
2 Polymerization Engineering Department,
Iran Polymer and Petrochemical Institute
(IPPI), Tehran, Iran
firmed on the basis of FT‐IR, UV–Vis, H‐, 13C–NMR, inductively coupled
1
plasma and elemental analyses (C, H and N). The purity of these compounds
was ascertained by melting point (m.p.) and thin‐layer chromatography. The
geometry optimization and vibrational frequency calculations of the compounds
were performed using Gaussian 09 program with B3LYP/TZVP level of theory.
All Ni(II) complexes were activated with diethylaluminum chloride (Et2AlCl), so
that C2 showed the highest activity [6600 kg mol−1 (Ni) h−1], where the ligand
contains a chlorine substituent. Oligomers obtained from the complexes consist
mainly of dimer and trimer, and also exhibit high selectivity for linear 1‐butene
and 1‐hexene. Both the steric and electronic effects of coordinative ligands affect
the catalytic activity and the properties of the catalytic products.
Correspondence
Azadeh Tadjarodi, Research Laboratory of
Inorganic Materials Synthesis,
Department of Chemistry, Iran University
of Science and Technology, Tehran, Iran.
Email: tajarodi@iust.ac.ir
Funding information
Iran University of Science and Technol-
ogy, Iran Polymer and Petrochemical
Institute and Petrochemical Research &
Technology Company
KEYWORDS
benzimidazolylphenols, density functional theory calculations, ethylene oligomerization, nickel (II)
complexes
1 | INTRODUCTION
Favored application of late transition‐metal complexes
as catalysts for ethylene oligomerization was first
The oligomerization of ethylene is the major industrial pro-
cess to produce linear α‐olefins. These linear α‐olefins are
extensively used in the preparation of detergents, plasti-
cizers and synthetic lubricants. They are also known as co‐
monomers to produce linear low‐density polyethylene.[1]
Research on late transition‐metal complexes as cata-
lysts for ethylene oligomerization and polymerization
has attracted considerable attention in both academic
and industrial circles due to their high activity and selec-
tivity, as well as their lower oxophilicity and tolerance of
functional groups.[2]
achieved industrially through employing nickel com-
plexes in the shell higher olefin process.[3] To extend the
efficient catalytic systems, nickel complexes bearing
bidentate ligands such as N^N, N^O and P^N have been
extensively studied.[4] Benzimidazolyl‐phenol is a type of
N^O ligand. The complexes with these ligands are impor-
tant, because of high thermal stability, better catalytic per-
formance, superior optical properties,[5] as well as strong
chelating agents. Benzimidazolyl‐phenol and its deriva-
tives can also coordinate as monodentate or multidentate
via one imine nitrogen atom and a phenolate oxygen atom
Appl Organometal Chem. 2017;e4015.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4015

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HAGHVERDI ET AL.
to form M–N and M–O bonds. Besides, it can also act as a
donor and (or) acceptor in hydrogen bond interactions.[6]
The synthesis of privileged chemicals through an economi-
cal and environmentally friendly method is always desir-
able. The reaction of substituted o‐phenylenediamine with
2‐hydroxy aromatic aldehydes is the common method to
produce benzimidazolyl‐phenol derivatives.[7]
In order to improve catalytic activities of metal com-
plexes and finely tune their products, The design of cata-
lysts for ethylene oligomerization and polymerization is
important. The useful way to modify ligands is by placing
different substituents on the frame of ligands.[8] In the
case of benzimidazolyl‐phenol ligands, it is very difficult
to further modify the imidazole ring of benzimidazole,
therefore the functionalization of benzimidazole will be
fused with the benzene ring.
oligomers was calculated by referencing to the mass of the
solvent on the basis of the prerequisite that the mass of each
fraction was approximately proportional to its integrated
areas in the GC trace. Selectivity for linear 1‐alkenes was
defined as: amount of linear 1‐alkenes of all fractions/total
amount of oligomer products (as a percentage). Melting
points were determined using an electro thermal melting
point apparatus. The completion of reactions was moni-
tored by thin‐layer chromatography (TLC).
2.2 | Synthesis of the ligands
2.2.1 | 2‐(1H–Benzimidazol‐2‐yl)‐phenol
(HL1)
2‐Hydroxybenzaldehyde (1.83 g, 15.0 mmol) and sodium
metabisulfide (1.60 g, 8.5 mmol) were dissolved in 20 ml
of ethanol and 20 ml water, respectively. The reaction
mixture was stirred vigorously and more ethanol was
added. The mixture was kept in a refrigerator for several
hours. The mixture was filtered and sodium hydroxyl (2‐
hydroxyphenyl) methansulfonate salt obtained from the
crude extract. This crude salt and 1,2‐phenylenediamine
(0.43 g, 4.0 mmol) in 10 ml of DMF were refluxed for 4 h
at 130 °C in an oil bath. The completion of reaction was
monitored by TLC. Then, the mixture was poured in an
ice bath. After that the reaction was quenched by adding
water, and 2‐(1H–benzimidazole‐2‐yl) phenol filtered and
recrystallized from ethanol.[9] This method was selected
because of simplicity, high yields and saving time. This
intermediate compound (namely, bisulfite compound of
the aldehyde) was utilized to catalyze the benzimidazole
synthesis, as 1,2‐phenylenediamines and aldehydes do
not produce a cyclization reaction by themselves under
mild conditions and possibly might lead to Schiff bases.
2‐(1H–Benzimidazole‐2‐yl) phenol was used for the
synthesis of the complexes. Brown creamy powder, yield:
71.43%. m.p.: 239–240 °C.1H–NMR (CDCl3, 300 MHz): δ
6.96 (t, 1H, J = 7.35 Hz), 7.14 (d, 1H, J = 8.30 Hz), 7.32
(m, 2H), 7.58 (d, 2H, J = 12.72 Hz), 7.72 (s, 1H), 9.44 (s,
1H, N‐H), 13.07 (s, 1H, O‐H). 13C–NMR (CDCl3, 75
MHz): 112.08, 118.16, 119.02, 123.34, 124.37, 132.04,
151.35, 158.98. IR (KBr disc, cm−1): 3325, 3245, 3053,
2300, 1629, 1591, 1490, 1460, 1417, 1394, 1276, 1130,
838,675. Anal. calcd for C13H10N2O: C, 74.27, H, 4.79; N,
13.33. Found: C, 75.02; H, 4.32; N, 12.22%.
Here we report the synthesis and characterization of
various 1H–benzimidazol‐2‐yl‐phenol derivatives and their
Ni(II) complexes. The optimized geometry and vibrational
frequency calculations of the compounds were derived from
theoretical calculations in B3LYP/TZVP level using
Gaussian 09 program. The experimental results were in
agreement with the theoretical calculations. Also, we
reported the correlation among complex structures,
catalytic activities and product properties. These complexes
have been evaluated in the oligomerization of ethylene
upon treatment with diethylaluminum chloride (Et2AlCl).
2 | EXPERIMENTAL
2.1 | Materials and measurements
All manipulations of air‐ and moisture‐sensitive com-
pounds were performed under a nitrogen atmosphere
using standard Schlenk techniques. Toluene was refluxed
over sodium‐benzophenone and distilled under nitrogen.
All reagents (including Et2AlCl) were purchased from
Aldrich, Merck or Acros Chemicals. Elemental analysis
was carried out on a Perkin Elmer model 2400 series 2
automatic carbon, hydrogen, nitrogen analyzer. The
determination of metal percentage was performed using
inductively coupled plasma (ICP; 3410ARL model). IR
spectra were recorded on Shimadzu FT‐IR 8400 spectrom-
eter by using KBr disc in the range of 400–4000 cm−1. UV
spectra were obtained using a Shimadzu, UV‐1700 spec-
trophotometer in dimethyl sulfoxide (DMSO) solution.
NMR spectra (300 MHz for 1H and 75 MHz for 13C) were
recorded on a Bruker AVANCE‐300 instrument in
DMSO‐d6 using tetramethylsilane as internal standard.
Gas chromatography (GC) analysis was performed using
a Varian CP‐3800 gas chromatograph equipped with a
flame ionization detector and a 100 m (0.25 mm i.d.,
0.5 μm film thickness) CP‐Sil PONA CB. The yield of
2.2.2 | 2‐(5‐Chloro‐1H‐benzimidazol‐2‐yl)‐
phenol (HL2)
HL2 was prepared by using the same procedure as the syn-
thesis of HL1, except that 4‐chloro‐o‐phenylenediamine
was used instead of 1,2‐phenylenediamine. Dark brown

HAGHVERDI ET AL.
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1
powder, yield: 60%. m.p.: 281–282 °C. H–NMR (CDCl3,
300 MHz): δ 7.01 (m, 2H), 7.26 (d, 1H, J = 8.37 Hz),
7.37 (t, 1H, J = 7.59 Hz), 7.64 (s, 1H), 7.74 (s, 1H), 8.04
(d, 1H, J = 7.7 Hz), 12.76 (s, 1H, N‐H), 13.26 (s, 1H, O‐
H). 13C–NMR (CDCl3, 75 MHz): 112.52, 117.23, 117.50,
117.65, 119.27, 123.07, 126.59, 132.05, 152.85, 157.87. IR
(KBr disc, cm−1): 3328, 3014, 2310, 1633, 1583, 1488,
1465, 1421, 1382, 1282, 1134, 1058, 844, 705. Anal. calcd
for C13H9N2OCl: C, 63.81, H, 3.68; N, 11.45. Found: C,
62.11; H, 3.38; N, 10.98%.
7.33 (t, 1H, J = 7.56 Hz), 7.40 (s, 2H), 8.00 (d, 1H, J =
7.69 Hz), 13.11 (s, 2H, N‐H, O‐H). 13C–NMR (CDCl3, 75
MHz): 19.96, 112.79, 117.08, 118.96, 125.88, 131.31,
131.48, 150.83, 157.92. IR (KBr disc, cm−1): 3257, 3124,
3049, 2970, 2858, 2241, 1637, 1589, 1488, 1461, 1417,
1384, 1257, 1130, 852, 675. Anal. calcd for C15H14N2O:
C, 75.63, H, 5.88; N, 11.76. Found: C, 74.29; H, 5.82; N,
11.42%.
The structure of the synthesized ligands is given in
Scheme 1. Except for HL4, these ligands had been synthe-
sized before.[9]
2.2.3 | 2‐(5‐methyl‐1H‐benzimidazol‐2‐yl)‐
phenol (HL3)
2.3 | Synthesis of the complexes C1–C5
2.3.1 | K2[Ni (HL1)2Cl2]2H2O (C1)
HL3 was prepared by using the same procedure as the syn-
thesis of HL1, except that 4‐methyl‐o‐phenylenediamine
was used instead of 1,2‐phenylenediamine. Cream brown-
ish powder, yield: 50%. m.p.: 255–256 °C. 1H–NMR
(CDCl3, 300 MHz): δ 2.42 (s, 3H, CH3), 7.01 (m, 2H),
7.08 (d, 1H, J = 8.16 Hz), 7.35 (t, 1H, J = 7.26 Hz),
7.43 (s, 1H), 7.53 (d, 1H, J = 8.13 Hz), 8.03 (d, 1H,
J = 7.71 Hz), 13.14 (s, 2H, N‐H, O‐H). 13C–NMR (CDCl3,
75 MHz): 21.29, 112.69, 117.15, 119.05, 124.29, 126.05,
131.54, 132.26, 151.40, 157.98. IR (KBr disc, cm−1): 3236,
3082, 3028, 2914, 2858, 2212, 1735, 1637, 1596, 1488,
1456, 1419, 1390, 1261, 1130, 846, 680. Anal. calcd for
C14H12N2O: C, 74.98, H, 5.35; N, 12.49. Found: C, 73.98;
H, 4.79; N, 12.15%.
A solution of HL1 (0.106 g, 0.5 mmol) and KOH (0.028 g,
0.5 mmol) in methanol was added to a solution of
.
NiCl2 6H2O (0.059 g, 0.25 mmol) in methanol. The color
change took place immediately. The resulting solution
was stirred for 24 h at room temperature. The completion
of the reaction was monitored by TLC. The precipitation
appeared in the resulting solution after a few days (by
slow evaporation of the solvent). The resulting precipita-
tion was separated by filtration, washed with diethyl ether
and dried in vacuum. Olive powder, yield: 50%. IR (KBr
disc, cm−1): 3419 (N‐H), 3058 (C‐H), 1623 (C = N), 1527
(C = C), 1255(C‐O), 486 (Ni‐O), 418 (Ni‐N). Anal. calcd
for C26H22N4O4K2Cl2Ni: C, 47.15, H, 3.32, N, 8.46, Ni,
8.86. Found: C, 48.00, H, 2.97, N, 7.60, Ni, 8.81%.
2.2.4 | 2‐(5,6‐Dichloro‐1H‐benzimidazol‐2‐
yl)‐phenol (HL4)
2.3.2 | K2[Ni (HL2)2Cl2]2H2O (C2)
HL4 was prepared by using the same procedure as the syn-
thesis of HL1, except that 4,5‐dichlorobenzene‐1,2‐
diamine was used instead of 1,2‐phenylenediamine. Pink
C2 was prepared in the same way as C1. Olive powder,
yield: 70.87%. IR (KBr disc, cm−1): 3433 (N‐H), 3176 (C‐
H), 1623 (C = N), 1560 (C = C), 1253(C‐O), 480 (Ni‐O),
418 (Ni‐N). Anal. calcd for C26H20N4O4K2Cl4Ni: C,
42.69, H, 2.73, N, 7.66, Ni, 8.03. Found: C, 44.62, H, 2.18,
N, 6.00, Ni, 7.70%.
1
powder, yield: 50%. m.p.: 327–328 °C. H–NMR (CDCl3,
300 MHz): δ 7.01 (m, 2H), 7.38 (t, 1H, J = 7.48 Hz),
7.87 (s, 2H), 8.05 (d, 1H, J = 7.66 Hz), 12.50 (s, 2H, N‐
H, O‐H). 13C–NMR (CDCl3, 75 MHz): 112.54, 116.08,
117.21, 119.36, 124.99, 127.00, 132.30, 133.53, 153.77,
157.71. IR (KBr disc, cm−1): 3332, 3097, 2320, 1637,
1598, 1487, 1444, 1419, 1371, 1298, 1132, 1097, 864, 740.
Anal. calcd for C13H8N2OCl2: C, 55.94, H, 2.89; N, 10.04.
Found: C, 55.68; H, 2.94; N, 9.61%.
2.2.5 | 2‐(5,6‐dimethyl‐1H‐benzimidazol‐2‐
yl)‐phenol (HL5)
HL5 was prepared by using the same procedure as the syn-
thesis of HL1, except that 4,5‐dimethylbenzene‐1,2‐
diamine was used instead of 1,2‐phenylenediamine.
1
Cream powder, yield: 50%. m.p.: 297–298 °C. H–NMR
(CDCl3, 300 MHz): δ 2.32 (s, 6H, CH3), 6.98 (m, 2H),
SCHEME 1 Structure of the synthesized ligands

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HAGHVERDI ET AL.
C1 (−3801), C2 (−4720), C3 (−3879), C4 (−5639) and C5
(−3037). The energy values are in a.u. and indicate that
the complexes have more stability compared with
ligands.[21]
2.3.3 | K2[Ni(HL3)2Cl2] (C3)
C3 was prepared in the same way as C1. Olive powder,
yield: 52.76%. IR (KBr disc, cm−1): 3417 (N‐H), 3180 (C‐
H), 1628 (C = N), 1562 (C = C), 1261(C‐O), 487 (Ni‐O),
439 (Ni‐N). Anal. calcd for C28H22N4O2K2Cl2Ni: C,
51.40, H, 3.36, N, 8.56, Ni, 8.97. Found: C, 49.64, H, 2.97,
N, 8.30, Ni, 9.28%.
2.5 | Procedure for ethylene
oligomerization
Ethylene oligomerization was performed in a stainless‐
steel autoclave (1‐l capacity) equipped with a gas ballast
through a solenoid clave for continuous feeding of ethyl-
ene at constant pressure. First, 100 ml of toluene was
transferred to the fully dried reactor under a nitrogen
atmosphere. The required amount of Et2AlCl was then
injected into the reactor via a syringe. Next, toluene solu-
tion of catalytic precursor was added to the reactor under
a nitrogen atmosphere. Over the reaction temperature,
the reactor was sealed and pressurized to high ethylene
pressure, and the ethylene pressure was maintained dur-
ing feeding of ethylene. After stirring the reaction mixture
for 30 min, the reaction was stopped by cooling the reac-
tor before releasing the excess pressure and a small
amount of the reaction solution was collected, which
was then analyzed by GC to determine the composition
and mass distribution of the oligomers obtained. The
residual reaction solution was quenched with 5% hydro-
chloride acid ethanol to collect the polyethylene obtained;
although polyethylene was not formed.
2.3.4 | K2[Ni (HL4)2Cl2]3H2O (C4)
C4 was prepared in the same way as C1. Olive powder,
yield: 46%. IR (KBr disc, cm−1): 3419 (N‐H), 3060 (C‐H),
1625 (C = N), 1558 (C = C), 1255(C‐O), 451 (Ni‐O), 439
(Ni‐N). Anal. calcd for C26H18N4O4K2Cl6Ni: C, 38.15, H,
2.44, N, 6.84, Ni, 7.18. Found: C, 38.45, H, 1.70, N, 5.00,
Ni, 8.76%.
2.3.5 | [Ni(HL5)2] (C5)
C5 was prepared in the same way as C1. Mustard powder,
yield: 40%. IR (KBr disc, cm−1): 3404 (N‐H), 3055 (C‐H),
1635 (C = N), 1550 (C = C), 1247(C‐O), 476 (Ni‐O), 430
(Ni‐N). Anal. calcd for C30H26N4O2Ni: C, 67.58, H, 4.88,
N, 10.51, Ni, 11.01. Found: C, 66.92, H, 4.69, N, 10.19,
Ni, 12.73%.
2.4 | Computational details
Full geometry optimizations were carried out using the
density functional theory (DFT) method at the B3LYP,
the hybrid GGA functional theory of Becke‐Lee, Parr and
Yang,[10,11] level for HL1–5 ligands and C1–C5 complexes,
because, as reported before, better agreement with the
experimental results can be obtained with B3LYP and it
is a commonly used functional in the molecular simulation
of transition‐metal catalytic systems.[12–16] In all cases, the
electronic configuration of the molecular systems was
described with the triple‐ζ basis set augmented with one‐
polarization functions of Ahlrichs and co‐workers (TZVP
keyword in Gaussian).[17] All calculations were performed
by the Gaussian 09 program.[18] For all the computed com-
plexes, except C5, the triplet was the electronic ground
state and thus open shell calculations were performed. In
the case of C5, the singlet was the ground state and closed
shell calculations were employed.
The vibrational frequency calculations were also per-
formed to ensure that the optimized geometries represent
the local minima and that there are only positive Eigen
values.[19,20] The optimized geometry of compounds
furnished the total energy. The energy values obtained
by using the DFT calculations are: HL1 (−686), HL2
(−1146), HL3 (−726), HL4 (−1606), HL5 (−765), whereas
3 | RESULTS AND DISCUSSION
The analytical data and physical properties of the ligands
and complexes were summarized in Table 1. The C1 and
C2 complexes had the general composition of
K2[Ni(HL1,2)2Cl2]2H2O. Also, the general formulae for
the C3 and C4 complexes were K2[Ni(HL3)2Cl2] and
K2[Ni (HL4)2Cl2]3H2O, respectively, and for C5 was
[Ni(HL5)2]. Six‐coordinated octahedral structures have
been assigned for the C1–C4 complexes, while the C5
complex has square‐planar geometry. These results were
confirmed by IR spectra, UV–Vis, ICP and elemental
analysis (C, H and N). The melting points of the ligands
HL2–HL5 suggest that chloro and methyl substitutions
on the benzimidazole moiety increase melting points with
respect to HL1 ligand.[22] Many attempts have been made
in obtaining single crystals of all complexes, but suitable
crystals have not been found in order to determine the
crystal structure of these compounds. Therefore, we
decided to use DFT calculations in order to determine
the molecular structures of complexes. The optimized
structures, vibrational frequencies, stability of the ligands
and complexes, as well as the various bond lengths and

HAGHVERDI ET AL.
5 of 12
TABLE 1 The analytical data and physical properties of the ligands and complexes
Elemental analysis: found
(Calcd) %
Ni
Yield
%
Compound
C
N
H
m.p. °C
Color
HL1 C13H10N2O
75.02 (74.27)
4.32 (4.79)
12.22 (13.33)
__
__
__
__
__
71.43
60
239–240
Brown creamy
Dark brown
Cream brownish
Pink
HL2 C13H9N2OCl
HL3 C14H12N2O
HL4 C13H8N2OCl2
HL5 C15H14N2O
62.11 (63.81)
73.98 (74.98)
55.68 (55.94)
74.29 (75.63)
48.00 (47.15)
3.38 (3.68)
4.79 (5.35)
2.94 (2.89)
5.82 (5.88)
2.97 (3.32)
10.98 (11.45)
12.15 (12.49)
9.61 (10.04)
11.42 (11.76)
7.60 (8.46)
281–282
255–256
327–328
297–298
__
50
50
50
Cream
K2[Ni (HL1)2Cl2]2H2O
C26H22N4O4K2Cl2Ni
8.81 (8.86)
50
Olive
K2[Ni (HL2)2Cl2]2H2O
C26H20N4O4K2Cl4Ni
44.62 (42.69)
49.64 (51.4)
38.45 (38.15)
66.92 (67.58)
2.18 (2.73)
2.97 (3.36)
1.70 (2.44)
4.69 (4.88)
6.00 (7.66)
8.30 (8.56)
5.00 (6.84)
10.19 (10.51)
7.70 (8.03)
9.28 (8.97)
8.76 (7.18)
12.73 (11.01)
70.87
52.76
46
__
__
__
__
Olive
K2[Ni(HL3)2Cl2]
C28H22N4O2K2Cl2Ni
Olive
K2[Ni (HL4)2Cl2]3H2O
C26H18N4O4K2Cl6Ni
Olive
[Ni(HL5)2] C30H26N4O2Ni
40
Mustard
angles generated from the optimized structure are
discussed. All calculations were performed by the Gauss-
ian 09 program.
given in Table 2. The optimized structure of C4 is shown
in Figure 2.
3.2 | FT‐IR spectra
3.1 | Geometrical parameters
To understand the vibrational properties and structural
characteristics of the ligands and complexes, the DFT cal-
culations with B3LYP/TZVP level have been used, and
the observed bands are assigned based on the results of
normal.
The optimized geometry of HL4 is shown in Figure 1 as a
representative structure. The various bond lengths and
angles generated from the structures of the optimized
ligand (HL4) and the complex (C4) using Gaussian 09 soft-
ware were obtained. They show good resemblance to the
X‐ray structural investigations of similar ligands and com-
plexes reported,[3,8,9,23] such as the computed bond
lengths for HL4 ligand, O(1)‐C(6), O(1)‐H(26), N(3)‐C(7),
N(3)‐C(9), N(8)‐C(7) and N(8)‐C(2) are 1.341, 0.991,
1.378, 1.381, 1.323 and 1.379 Å, respectively. The com-
puted bond angles, C(6)‐O(1)‐H(26), C(7)‐N(3)‐C(9),
C(7)‐N(8)‐C(2) and N(8)‐C(7)‐N(3) in the HL4 ligand are
108.754, 107.703, 106.468 and 111.237°, respectively. Bond
lengths and angles of complexes C1–C5 were compared
with similar compounds in the literature,[3,8,9,24,25] as
The prominent bands in the IR spectra of the ligands
and their complexes are presented in Table 3. The mea-
sured and calculated FT‐IR spectra of the ligand HL4
and its complex, C4, are given in Figures 3a and b, and
4a and b, respectively. In HL1 spectrum, ν(O–H) and
ν(N–H) vibration frequencies appear separately at 3325
and 3245 cm−1, respectively, because of weak intramolec-
ular hydrogen bonding.[26] In HL2–HL5 spectra, the char-
acteristic v(O–H) and v(N–H) vibration frequencies of the
ligands exhibit only a single strong band at ~3300 cm−1
,
probably due to double intramolecular hydrogen bonding
between the phenoxyl hydrogen atom and one of the
C = N nitrogen atoms. The corresponding peak in the the-
oretical spectrum was calculated at ~3253 cm−1. The lack
of this band in the FT‐IR spectra of the complexes indi-
cates coordination through the oxygen atom of the pheno-
lic group to metal. The N–H stretching and bending
vibrations of the ligands at ~3200 and ~1490 cm−1 remain
either unperturbed or undergo a slight shift in the com-
plexes (~3433 and 1481 cm−1), respectively. The C = C
frequencies of phenyl groups in the ligands appeared at
~1591 cm−1 in the IR spectra, and these frequencies are
expected to shift at lower frequency upon complex
FIGURE 1 Optimized structure of HL4 ligand at B3LYP/TZVP
level of theory

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HAGHVERDI ET AL.
TABLE 2 Selected bond lengths (Å) and angles (o) for complexes C1–C5 and similar compounds
C1
C2
C3
C4
C5
Ref.[3]
Ref.[8]
Ref.[9]
Ref.[24]
Ref.[25]
Bond lengths
Ni(49)‐O(1)
2.101
2.099
2.105
2.100
1.868
—
—
2.030
2.031
2.056
2.056
—
—
—
Ni(49)‐ O(8)
2.101
2.128
2.128
2.536
2.530
2.099
2.122
2.122
2.525
2.520
2.105
2.122
2.122
2.545
2.528
2.099
2.119
2.119
2.519
2.510
1.868
1.927
1.927
—
—
—
—
Ni(49)‐N(9)
2.057
—
2.097
—
2.036
2.037
—
2.089
—
Ni(49)‐N(34)
Ni(49)‐Cl(51)
—
2.388
2.384
2.292
2.523
2.264
2.312
Ni(49)‐Cl(52)
—
—
—
Bond angles
O(1)‐Ni(49)‐Cl(52)
N(34)‐Ni(49)‐O(1)
N(34)‐Ni(49)‐Cl(51)
O(8)‐Ni(49)‐Cl(51)
N(9)‐Ni(49)‐O(8)
N(9)‐Ni(49)‐Cl(52)
N(9)‐Ni(49)‐N(34)
Cl(51)‐Ni(49)‐Cl(52)
O(1)‐Ni(49)‐O(8)
88.87
84.98
89.31
91.12
84.99
90.68
178.62
179.99
177.75
88.94
85.39
89.08
91.05
85.42
90.92
178.16
179.99
177.88
88.68
84.91
89.07
91.32
84.92
90.91
178.16
179.99
177.36
89.02
85.37
88.97
90.96
85.37
91.03
177.93
179.98
178.05
—
—
—
—
—
—
—
—
—
—
—
—
—
—
90.57
—
—
—
88.27
—
—
95.23
—
—
94.99
—
—
—
—
90.57
—
—
—
—
—
89.75
—
89.15
—
—
101.95
—
176.25
—
—
174.46
—
—
—
—
174.33
—
169.85
—
for the ligands are expected to appear at 1637 cm−1. These
frequencies are expected to shift at lower frequency upon
complex formation (1625 cm−1).[27] These frequency
changes may support the argument that coordination
occurs via imine nitrogen atom. The corresponding peaks
in the ligands and complexes are also estimated at ~1635
and 1633 cm−1 in the predicted theoretical spectra. In the
IR spectra of the C1–C5 complexes, the coordination
through imine nitrogen atom can also be confirmed by
the appearance of a weak band located at the low wave
numbers (~418 cm−1) that may be assigned to v(Ni‐
N).[28,29] In the high‐frequency region, the bands in the
range of 3014–3097 cm−1 are attributed to the stretching
vibrations of aromatic C‐H for all compounds, and the cor-
responding peaks in the theoretical spectrum are calcu-
lated in the range of 3170–3190 cm−1. In HL3 and HL5,
the bands at 2970–2858 cm−1 are related to the stretching
vibrations of aliphatic C‐H.[30] In HL2 and HL4, the bands
at 1058 and 1097 cm−1 correspond to the stretching vibra-
tions of C‐Cl, respectively. The sharp or medium bands in
the ligands and their complexes in the range of 675–864
cm−1 are due to the out‐of‐plane deformation bands for
the aromatic C–H.[26] In the spectra of free ligands, there
is a band at about 1298 cm−1, which is assigned to v(C–O)
of the phenolic group. The phenolic C–O band of the unco-
ordinated ligand shifts to lower frequencies during com-
plex formation. The phenolic C–O band at about ~1261
cm−1 in spectra of the presented complexes suggests that
the phenolic oxygen atoms coordinated to the metal.[31]
FIGURE 2 Optimized structure of C4 complex at B3LYP/TZVP
level of theory
formation (~1562 cm−1). The corresponding peaks in pre-
dicted theoretical spectra in the ligands and complexes are
also calculated at ~1560 and ~1552 cm−1, respectively.
Similarly, the (C = N) asymmetric stretching vibrations

HAGHVERDI ET AL.
7 of 12
TABLE 3 FT‐IR spectra data of the compounds under study
Compound
HL1
Wavenumber(cm−1), KBr pellets
3325, 3245, 3053, 2300, 1629, 1591, 1490, 1460, 1417, 1394, 1276, 1130, 838, 675
3419, 3058, 2925, 2854, 1623, 1600, 1527, 1481, 1438, 1309, 1255, 1137, 856, 742, 486, 418
3328, 3014, 2310, 1633, 1583, 1488, 1465, 1421, 1382, 1282, 1134, 1058, 844, 705
3433, 3176, 3060, 2858, 1623, 1560, 1481, 1452, 1305, 1253, 1137, 929, 705, 511, 480, 418
C1
HL2
C2
HL3
C3
3236, 3082, 3028, 2914, 2858, 2212, 1735, 1637, 1596, 1488, 1456, 1419, 1390, 1261, 1130, 846, 680
3417, 3180, 3058, 2858, 1628, 1604, 1562, 1481, 1448, 1307, 1261, 1137, 865, 752, 487, 439
3332, 3097, 2320, 1637, 1598, 1487, 1444, 1419, 1371, 1298, 1132, 1097, 864, 740
3419, 3060, 2921, 2852, 1625, 1558, 1481, 1450, 1301, 1255, 1135, 966, 752, 451, 439
3257, 3124, 3049, 2970, 2858, 2241, 1637, 1589, 1488, 1461, 1417, 1384, 1257, 1130, 852, 675
3404, 3055, 2972, 2860, 1635, 1550, 1475, 1442, 1317, 1247, 1134, 840, 746, 476, 430
HL4
C4
HL5
C5
Overall, in the C1–C5 complexes, deprotonation and sub-
sequent involvement of the phenoxyl group in metal coor-
dination could also be supported by the appearance of a
new band at a lower frequency (487 cm−1) region assign-
able to v(Ni‐O). These results are in agreement with the
theoretical calculations. The experimental and calculated
IR spectra of ligands and their complexes agree extremely
well with respect to their peak frequencies, band intensi-
ties and the shapes of the bands.[32]
3.3 | Electronic spectra
The UV–Vis spectra data of HL1–5 ligands and C1, C2, C3,
C4 and C5 complexes are given in Table 4. The UV–Vis
spectra in DMSO solvent were measured from 200 to
FIGURE 3 The comparison of (a) experimental FT‐IR, (b)
theoretical IR, calculated at B3LYP/TZVPlevel, spectra for ligand HL4
800 nm. The observed bands at lower wavelength (210–
300 nm) correspond to π → π* transitions of the aro-
matic rings. The bands in the range of 310–360 nm are
due to n → π* transitions.[31] The electronic spectra of
the C1, C2, C4 and C5 complexes are of little help in the
present case, as the d → d transitions are masked by the
strong charge‐transfer bands.[33] In the electronic spectra
TABLE 4 UV–vis spectra data of the compounds (DMSO as
solvent)
Compound
HL1
UV (nm)
Vis.
250, 290
320
HL2
HL3
HL4
HL5
C1
220, 240, 270, 290
310, 330
320, 340
340, 360
340
210, 240, 300
300
210, 240, 290
__
__
__
__
__
350
C2
390
C3
360, 390
390
FIGURE 4 The comparison of (a) experimental FT‐IR, (b)
theoretical IR, calculated at B3LYP/TZVP level, spectra for
complex C4
C4
C5
370

8 of 12
HAGHVERDI ET AL.
of the C1, C2, C4 and C5 complexes, there are broad
bands at 350, 390 and 370 nm, respectively, because of
the combination of chloride → metal or oxygen →
metal, nitrogen → metal charge transfers (L → Ni
charge transfer). The electronic spectrum of the C3 com-
plex shows the two weak bands. The bands in the visible
region (360 and 390 nm) may be assigned to 3A2g (F) →
3
3T1g (P) and A2g (F) → 3T1g (F) transitions, respec-
tively. The 3A2g (F)
→
3T2g (F) transition is not
observed in the spectrum of the present complex; it is sel-
dom observed as it is inherently weak due to an orbital
selection rule.[34] These are characteristic for high spin
octahedral geometry for the C3 complex.
3.4 | NMR spectra of ligands
1H–NMR spectra data of the ligands and the assignments
of the peaks were given in Table 5. The 1H–NMR spectra
of HL3, HL4 and HL5 ligands exhibit only a singlet broad
peak for the OH and amine NH protons (13.14, 12.5 and
13.11 ppm for HL3, HL4 and HL5, respectively). This
broadening results from strong intramolecular hydrogen
bonding between the amine nitrogen atom with double
bond and phenolic hydrogen atoms. The HL1 and HL2
ligands give two singlet peaks for OH and NH protons
(13.07 and 9.44 ppm for HL1, 13.26 and 12.76 ppm for
HL2, respectively).[35] In HL1–5 ligands, protons of the
phenolic ring moiety appeared at the same position but
with different chemical shifts. This is due to the substitu-
tions on the ring that have affected the phenolic ring
moiety. For example, proton chemical shifts of HL2 and
HL4 (containing electron‐withdrawing groups of Cl),
appear at higher ppm than HL1 (non‐substituted), HL3
and HL5 (containing electron‐donating groups of CH3).
In the 1H–NMR spectra of HL3 (included a methyl
group) and HL5 (included two methyl groups), there
are singlets at 2.42 and 2.32 ppm, respectively. The posi-
tion of protons and the assignments of the peaks in the
benzimidazole moiety in HL1–5 ligands is different
(Table 5). Also, 1H–NMR spectra of the ligands are
shown in Figure S1.
In the 13C–NMR spectra of the HL1–5 ligands, the sig-
nals at about 157.71–158.98 ppm and 150.83–153.77 ppm
are attributed to carbon atoms bonded to the OH oxygen
atom and carbon atom of imidazole group, respec-
tively.[26] The other signals are related to the benzimid-
azole and phenolic ring carbon atoms (Table 6).
.
3.5 | Theoretical calculation and stability
According to the DFT calculations, HL4, HL2 and their
Ni(II) complexes, C4 and C2, have lower energy values

HAGHVERDI ET AL.
9 of 12
TABLE 6 13C–NMR spectral data of the HL1–5 (δC, ppm; in DMSO‐d6)
Ligands
HL1
Chemical shifts (δC, ppm)
112.08, 118.16, 119.02, 123.34, 124.37, 132.04, 151.35, 158.98
HL2
HL3
HL4
HL5
112.52, 117.23, 117.50, 117.65, 119.27, 123.07, 126.59, 132.05, 152.85, 157.87
21.29, 112.69, 117.15, 119.05, 124.29, 126.05, 131.54, 132.26, 151.40, 157.98
112.54, 116.08, 117.21, 119.36, 124.99, 127.00, 132.30, 133.53, 153.77, 157.71
19.96, 112.79, 117.08, 118.96, 125.88, 131.31, 131.48, 150.83, 157.92
of −1606, −1146, −5639, −4720 a.u, respectively. The
total energy values of HL1, HL3, HL5 and their complexes
C1, C3 and C5 are: −686, −726, −765, −3801, −3879 and
−3037 a.u., respectively. The DFT calculations prove that
the chloro derivatives have higher stability than the
methyl derivatives, comparing them with each other. It
is observed that HL4, having two chloro substituents, is
more stable (total energy: −1606 a.u.) than HL2, having
one chloro substituent (−1146 a.u.). The DFT calculations
show that the stability of the HL1 ligand is the lowest
among the ligands. Likewise, C5 has the highest energy
among the complexes and consequently the lowest stabil-
ity, −3037 a.u, because of the absence of chloro atoms
attached to Ni (Figure 5). According to the geometry opti-
mization of C1–C4 structures with DFT calculations, Ni
was located on an octahedral center with the C = N nitro-
gen and hydroxy oxygen atoms of benzimidazole moiety
and phenol ring, respectively, associated with two chlo-
ride ions. But for the C5 complex, there is a square‐planar
geometry with Ni ion in the center that coordinated to the
C = N nitrogen and hydroxy oxygen atoms (molecular
symmetry: D2h for C1–C5 complexes).
3.6 | Ethylene oligomerization
The catalytic activities of all pre‐catalysts for ethylene
oligomerization have been carried out in the presence of
Et2AlCl as co‐catalyst. In all cases, these catalysts generate
butenes and hexenes as the main oligomeric products,
and the distribution of oligomers does not follow
Schulz–Flory rules. No odd carbon number oligomers
were detected in the GC analysis.[36]
In the oligomerization mechanism of this class of cat-
alysts, at first, active species are formed through interac-
tion of nickel complex with co‐catalyst (such as Et2AlCl
and MAO) under an ethylene atmosphere. The catalyti-
cally active species is probably a square‐planar or octahe-
dral hybridized nickel atom interacting with a hydride
(or alkyl), electronegative groups (x), the ligand donor
atoms and ethylene molecule. Then, by using successive
insertion of ethylene, an intermediate species alkyl‐
nickel during the growth step of chains is formed. After
that, by ß‐hydrogen elimination, hydride species and
oligomer chains are released (Figure 6).[37] Overall, the
nickel hydride species can be generated by a variety of
FIGURE 5 Optimized structure of C5 complex at B3LYP/TZVP
FIGURE 6 Reaction mechanism for ethylene oligomerization by
level of theory
nickel (II) catalyst

10 of 12
HAGHVERDI ET AL.
standard organometallic reactions, including ß‐hydride
transfer from an intermediate nickel alkyl and oxidative
addition of a Lewis acid to a zero‐valent nickel species.
The labile cationic nickel species containing vacant sites
should be stabilized by the coordination of an ethylene
molecule. The reaction mechanism for ethylene oligo-
merization with nickel (II) catalysts was given in
Figure 6.
different influences on the catalytic performances. When
the nickel complex contains halide as the electron‐
withdrawing substituent on the aryl ring (C2), the
reactivity is much higher than with alkyl substituent. The
electron‐withdrawing group on the benzimidazol rings
makes the central metal more positive and leads to a
decrease of the electron density in the benzimidazol rings,
so the ethylene can easily coordinate to nickel species,
which finally led to higher catalytic activity.[3] This com-
plex showed high selectivity for 1‐butene. In general, elec-
tron‐withdrawing substituents enhance catalyst activities,
as do less bulky substituents, but when the number of Cl
atoms in the ligand (C4) increases, catalytic activity
decreases. This phenomenon is caused by the low solubil-
ity of complex. The complexes with ligands that have
methyl substituents (C3) enhance their solubility and sta-
bility of active species due to less exposure to impurity
reactants,[38] and for these catalysts only dimerization
was achieved, with a high selectivity for 1‐butene.
Whereas, the C5 complex contains two methyl groups on
the benzimidazol rings that might push more electrons
to the nickel atom, reduce the net charge on the nickel
center and, therefore, result in lower catalytic activity. In
general, increasing steric hindrance leads to a decrease in
the catalytic activities resulting in a lower positive charge
on nickel of the active catalytic species, which, subse-
quently, weakens the metal–ethylene interaction during
3.6.1 | Ethylene oligomerization by nickel
complexes C1–C5
The nickel (II) complexes, C1–C5, were systematically
investigated for the oligomerization of ethylene by using
AlEt2Cl as co‐catalyst. In the C1 complex, dimerization,
trimerization and tetramerization of ethylene are the
major reactions, and the selectivity of 1‐octeneproductin
total is high according to GC analysis, while C4 and C5
complexes showed both dimer and trimer products with
good selectivity modest to high toward 1‐butene and 1‐
hexene. Also, C2 and C3 displayed good selectivity of 1‐
buten with a dimer as the predominate product. The results
of ethylene oligomerization were collected in Table 7.
According to Table 7 (entries 1–5), the oligomeriza-
tion activities decreased in the order of C2 > C3 > C4 >
C5 > C1. From this comparison, we can see that C1
and C2 complexes showed the lowest and highest
catalytic activities, respectively. The R substituents had
the ethylene oligomerization.[36] Also,
a
possible
TABLE 7 Results of ethylene oligomerization with C1–C5a and its comparison with similar complexesd–h
Oligomer distributionc(%)
C6 C8 α‐C4 α‐C6
86.60 12.63 0.77 89.04
100
92.31
100
Temperature
Entry Precatalyst (°C)
Time Pressure Yield
Al/Ni (min) (atm) (g)
1.84
Activityb C4
368
α‐C8
1
C1
30
30
30
30
30
20
30
20
15
30
200
200
200
200
200
1000
700
200
1500
400
30
30
30
30
30
30
10
30
30
30
20
20
20
20
20
20
10
1
57.14 100
2
C2
33.00 6600
11.42 2284
9.20 1840
8.59 1718
100
100
—
—
—
—
—
—
—
—
—
73.3
—
—
—
—
—
—
—
—
—
—
—
3
C3
—
4
C4
66.67 33.33
100
5
C5
97.74
93.8
99.8
92.4
8.0
2.26
6.2
83.89 100
6d
7e
8f
9g
10h
Ref.[3]
Ref.[8]
Ref.[9]
Ref.[24]
Ref.[25]
—
—
—
—
—
2240
5200
301
—
—
—
—
—
—
0.2
—
—
—
—
7.6
1
235
18.7
10.3
10
1740
89.7
aGeneral conditions: 10 μmol of precatalyst, 100 ml toluene as solvent, co‐catalyst: Et2AlCl.
bIn units of kg mol−1 h−1
.
cDetermined by GC. ΣC denotes the total amounts of oligomers.
dGeneral conditions: 5 μmol of precatalyst, toluene (100 ml), co‐catalyst: Et2AlCl.
eGeneral conditions: 5 μmol of precatalyst, toluene (100 ml), co‐catalyst: Et2AlCl.
fGeneral conditions: 5 μmol of precatalyst, toluene (30 ml), co‐catalyst: Et2AlCl.
gGeneral conditions: 5 μmol of precatalyst, toluene (100 ml), co‐catalyst: MAO.
hGeneral conditions: 5 μmol of precatalyst, toluene (100 ml), co‐catalyst: Et2AlCl.

HAGHVERDI ET AL.
11 of 12
explanation is that in late transition‐metal catalytic sys-
tems, both bulkier and more electron‐rich ligands weaken
the interactions between the metal center and the incom-
ing monomer, and thereby retard the rate of ethylene
insertion. The observed activity trends appear to be a
superposition of electronic and steric factors.[39]
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How to cite this article: Haghverdi M, Tadjarodi
A, Bahri‐Laleh N, Nekoomanesh‐Haghighi M.
Synthesis and characterization of Ni(II) complexes
bearing of 2‐(1H–benzimidazol‐2‐yl)‐phenol
derivatives as highly active catalysts for ethylene
oligomerization. Appl Organometal Chem. 2017:
e4015. https://doi.org/10.1002/aoc.4015
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