Synthesis, characterization and single crystal X-ray studies of pincer type Ni(II)-Schiff base complexes: Application in synthesis of 2-substituted benzimidazoles

Article abstract of DOI:10.1016/j.jorganchem.2019.03.018

Five new pincer type Ni(II)-Schiff base complexes of the general formula [NiL¹(PPh₃)] 1, [NiL²(PPh₃)] 2, [NiL³(PPh₃)] 3, [NiL⁴(PPh₃)] 4 and [NiL⁴(4-MePy)] 5

Full text of DOI:10.1016/j.jorganchem.2019.03.018

Accepted Manuscript
Synthesis, characterization and single crystal X-ray studies of pincer type Ni(II)-Schiff
base complexes: Application in synthesis of 2-substituted benzimidazoles
Bhumika Agrahari, Samaresh layek, Rakesh Ganguly, Necmi Dege, Devendra D.
Pathak
PII:
S0022-328X(19)30111-1
DOI:
https://doi.org/10.1016/j.jorganchem.2019.03.018
JOM 20746
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 18 February 2019
Revised Date: 19 March 2019
Accepted Date: 22 March 2019
Please cite this article as: B. Agrahari, S. layek, R. Ganguly, N. Dege, D.D. Pathak, Synthesis,
characterization and single crystal X-ray studies of pincer type Ni(II)-Schiff base complexes: Application
in synthesis of 2-substituted benzimidazoles, Journal of Organometallic Chemistry (2019), doi: https://
doi.org/10.1016/j.jorganchem.2019.03.018.
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Synthesis, characterization and single crystal X-ray studies of pincer type
Ni(II)-Schiff base complexes: Application in synthesis of 2-substituted
benzimidazoles
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Bhumika Agraharia, Samaresh layeka, Rakesh Gangulyb, Necmi Degec, Devendra D. Pathaka*
aDepartment of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad-826004,
India
b Division of Chemistry & Biological Chemistry, Nanyang Technological University, Singapore-639798,
Singapore
c Ondokuz Mayıs University, Arts and Sciences Faculty, Department of Physics, 55139 Samsun, Turkey
Email: ddpathak@yahoo.com
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*Phone number: +91 9431126250
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Abstract
Five new pincer type Ni(II)-Schiff base complexes of the general formula [NiL1(PPh3)] 1,
[NiL2(PPh3)] 2, [NiL3(PPh3)] 3, [NiL4(PPh3)] 4 and [NiL4(4-MePy)] 5 [where H2L1 = 2-(2,3-
dihydroxybenzylideneamino)phenol, H2L2 = N-(2,3-dihydroxybenzylidene)benzohydrazide,
H2L3 = 2-(2,3-dihydroxybenzylidene)hydrazinecarbothioamide, H2L4 = 5-(diethylamino)-2-
(2-hydroxybenzylideneamino)phenol, 4-MePy = 4-Methylpyridine] were synthesised by the
reaction of the Ni(OAc)2.4H2O with the corresponding Schiff base ligand in methanol as
coloured crystalline solids in high yields. All the five complexes were fully characterized by
FT-IR, UV-Vis, H NMR, 13C{1H} NMR, 31P{1H} NMR, mass spectrometry and single
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crystal X-ray diffraction studies. The crystal structures of all five new complexes confirmed
the tridentate nature of the pincer type Schiff base ligands (ONO and ONS) and distorted
square planar geometry around the metal centre in all cases. The monodentate ligand
(triphenylphosphine/4-Methylpyridine) occupied the fourth site at nickel. The catalytic
potential of the complexes has been demonstrated in the synthesis of a series of 2-substituted
benzimidazoles at room temperature using low catalyst loading (0.5 mol %), and without the
use of any additives. All organic products were isolated in high yields (85-96 %) and fully
characterized by 1H and 13C{1H} NMR studies.
Keywords: Pincer type, nickel, crystal structure, benzimidazole, catalysis
1. Introduction
Nitrogen containing heterocyclic compounds are essential building blocks of numerous
natural products, [1] pharmaceuticals [2] and organic/polymeric materials [3]. In particular,
benzimidazoles are emerged as an important class of heterocyclic system and become a
significant intermediates in synthetic organic chemistry due to their miscellaneous
applications [4]. Benzimidazoles are medicinally important bioactive heterocyclic scaffolds
and exhibit a broad spectrum of biological and pharmacological properties including anti-

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bacterial, [5] anti-fungal, [6] anti-inflammatory, [7] anti-ulcer, [8] anti-cancer, [9] and anti-
HIV activities [10] (Fig. 1). The presence of imidazole ring is an integral part of several
natural products such as α-amino acid histidine, proteins, histamine, purines and biotin [11].
Besides biological applications, benzimidazoles have also found applications in industry,
chemical UVB filters, pigments, optical brighteners for coatings and thermostable
membranes for fuel cells [12].
A Typical synthesis of benzimidazole entails the treatment of 1,2-phenylenediamine
either with carboxylic acids or their derivatives under strongly acidic conditions [13] or with
aldehydes under oxidative conditions using various oxidative reagents and catalysts such as
I2/KI/K2CO3/H2O, CAN/H2O2, In(OTf)3, Ce(NO3)3.6H2O, Co(OH)2/CoO(II), Nano-Ni(II)/Y
zeolite, Cu (II)-salen, CuFe2O4, CuO nano-particles etc. [14-34]. Although, these approaches
are widely used for the synthesis of benzimidazoles, these are associated with certain
drawbacks, such as formation of by-product, requirement of high reaction temperature,
prolonged reaction time, expensive catalysts, toxic solvents as well as low yields of the
products. In order to overcome these drawbacks, there is a need to develop a new stable,
cheap catalysts capable of catalysing the synthesis of 2-substituted benzimidazoles under
mild conditions.
Transition metal pincer type complexes are reported to have high stability and exhibit
excellent catalytic activities in a number of homogeneous catalytic processes [35]. Tridentate
pincer-type complexes have engendered a lot of interest to stabilizes a large number of metal
complexes [35]. There complexes can be easily fined tuned by simple structural
modifications to achieve the best catalytic activities [36]. In the area of nickel chemistry,
complexes of pincer ligands have attracted considerable attention due to their low cost as
compared to the precious metals, low toxicity, high reactivity and enhanced catalytic and
electrochemical properties [37]. They have been successfully used as catalysts in variety of
organic transformation such as C-C and C-heteroatom bond formation reactions,
hydrosilylation of aldehydes and ketones, hydroamination of nitriles etc. [38]. Consequently,
prompted by these results and as a continuation of our ongoing research on transition metal-
catalysed organic synthesis [39], we report herein the synthesis and crystal structure of five
new pincer type nickel(II)-Schiff-base complexes (1-5) having ONO and ONS donor atom
and their catalytic activity in the synthesis of a series of benzimidazoles with low catalyst
loading (0.5 mol %) at room temperature.
(Fig. 1)

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2. Results and discussion
Four Schiff base ligands H2L1-H2L4 were synthesized by the reported methods [40-
43]. The ligands were obtained as red (H2L1), off-white (H2L2 and H2L3) and yellow (H2L4)
solids, on refluxing ethanolic solution of corresponding aldehyde and amine for 5-8 h. The
reaction of ligand H2L1-H2L4 with Ni(OAc)2.4H2O and PPh3/4-Methylpyridine in 1:1:1 ratio
in methanol at room temperature afforded the complexes [NiL1(PPh3)] 1, [NiL2(PPh3)] 2,
[NiL3(PPh3)] 3, [NiL4(PPh3)] 4, and [NiL4(4-MePy)] 5, respectively as a colored crystalline
solid in high yields (87-91% yields), (Scheme 1). Red block crystals suitable for X-ray
crystallography were obtained by slow evaporation of the solution at room temperature in
DMF. The complexes were air stable, insoluble in water and benzene, and soluble in other
common organic solvents such as CH2Cl2, CHCl3, CH3CN, DMF and DMSO. The complexes
were fully characterized by FT-IR, UV-Vis, 1H NMR, 13C{1H} NMR, P{1H} NMR, mass
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spectrometry and their structures were determined by single crystal X-ray diffraction studies.
(Scheme 1)
2.1. FT-IR spectra of ligands and complexes
The FT-IR spectral data of ligands H2L1-H2L4 and complexes 1-5 are given in Table
1. The FT-IR spectra of ligands H2L1-H2L4 (Fig. S1) showed band at 3369, 3480, 3532 and
3454 cm-1 due to presence of ν OH group and band at 1623, 1622, 1620 and 1622 cm-1, were
attributed to azomethine ν C=N group, respectively [44]. The FT-IR spectrum of the H2L2
displayed a band at 1667 cm-1 due to C=O group and in H2L3, band at 3141 cm-1 and 1227
cm-1 was due to presence of NH and C=S group, respectively. However, the FT-IR spectra of
the complexes 1-5 showed band at 1592, 1604, 1593, 1604 and 1613 cm-1, attributed to ν
C=N respectively. A comparison of the spectra of ligands H2L1-H2L4 with complexes 1-5
indicated that in all complexes the ν C=N has been lowered by 9-31 cm-1, supporting the
coordination of azomethine nitrogen atom to nickel [30]. The absence of C=O stretching
vibrations in complex 2 and N-H and C=S stretching vibrations in complex 3 confirms the
coordination of oxygen in enol form via deprotonation of the -OH group and sulphur in thiol
form via deprotonation of the -SH group to the nickel, respectively [45].
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(Table 1)
2.2. UV-Vis spectra of ligands and complexes
The electronic absorption spectra of the ligands H2L1-H2L4 and complexes 1-5 were
recorded in methanol within 200-800 nm at room temperature (Fig. S2). The electronic
absorption spectra of the ligands H2L1-H2L4 showed band in the region 228-279 nm and 315-

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408 nm due to π-π* transitions of the aromatic rings and n-π* transitions of the azomethine
group, respectively. However the electronic spectra of complexes 1-5 showed three
absorption band in the region 307-336, 363-423, and 414-490 nm (Fig. S2). The peaks in the
regions 307-336 nm were attributed to the π-π* transition. The n-π* transition corresponding
to the azomethine groups observed in the range of 363-423 nm have been attributed to ligand
to metal charge transfer (LMCT) transition (3A2g→3T2g) and the shoulder at 414-490 nm to
forbidden (3A2g→ 3T1g) transition [46].
2.3. 1H, 13C{1H} and 31P{1H} NMR spectra of complexes 1-5
The 1H NMR spectra of complexes 1-5 were recorded in CDCl3 at room temperature
(Fig. S3, S7, S11, S15, S19). The 1H NMR of ligands H2L1-H2L4 as reported [40-43] showed
singlet at
(H2L3),
proton (HC=N-) appeared as singlet at
δ
14.19,
14.19 and 9.58 (H2L4) due to presence of phenolic OH proton. The azomethine
8.85, 8.58, 8.36 and 8.61 in ligands H2L1-H2L4,
δ 9.69 and δ δ δ 9.13 and δ 8.85
8.88 (H2L1), 11.14 and 9.20 (H2L2),
δ
δ
respectively. The phenyl proton of the free ligands were observed in the range of
δ 5.91-7.93
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as a complex multiplets. However, a comparision of the H NMR spectra of complexes 1-5
with the free ligands H2L1-H2L4 exhibited upfield shift of azomethine (HC=N-) proton at
8.49, 8.36, 8.26, 8.18 and 8.49, respectively and appeared as a doublet and the phenyl protons
of the ligand and PPh3/4-Methylpyridine moieties were observed in the range of 7.90-5.15
δ
δ
as a complex multiplets [34]. Splitting of imine signal into a doublet was observed and
attributed to coupling of imine proton with the phosphorus/nitrogen atom of auxiliary ligand
[47]. The 1H NMR spectrum of complexes 1-3 showed only one singlet for OH at
δ 4.75, 4.93
and 4.97, respectively. The absence of other phenolic OH signals in complex 1-3 confirms
the coordination of phenolic oxygen of H2L1-H2L3 to the metal ion. In complexes 4 and 5, the
CH2 and CH3 protons of 4-(diethylamino)-2 hydroxybenzaldehyde moiety appeared as
quartet and triplet at δ 3.20 and 3.30 and δ 1.06 and 1.13, respectively [46, 48].
13C{1H} NMR spectra of the complexes 1-5 were recorded in CDCl3 at room
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temperature (Fig. S4, S8, S12, S16, S20). The C NMR spectra of complexes 1-5 displayed
signal at 165.9, 173.3, 171.1, 164.57 and 164.723, corresponding to azomethine carbon,
respectively. In complexes 2 and 3, a signal at 173.2 and 170.9 was due to presence of N=C-
O and N=C-S, carbon respectively. The signals appeared in the region of δ 113.56-150.57
(complex 1), 149.42-113.47 (complex 2), 152.45-113.24 (complex 3), 151.73-93.53 (complex
4) and 152.13-99.76 (complex 5) were accounted to various aromatic carbons of the ligands
H2L1-H2L4 as well as tripenylphosphine/4-Methylpyridine. In complexes 4-5, the CH2 and

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CH3 carbon of 4-(diethylamino)-2-hydroxybenzaldehyde appeared at 44.82, 44.3 and 13.27,
12.8 respectively and the signal at 21.02 in complex 5 was attributed to methyl carbon of 4-
Methylpyridine [49].
The 31P{1H} NMR spectra of complexes 1-4 were recorded in CDCl3 at room
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temperature (Fig. S5, S9, S13, S17). The P{1H}NMR spectra of complexes 1-4 showed a
singlet at δ 16.29, δ 19.17, δ 23.93 and δ 26.87, respectively. The free PPh3 ligand exhibit a
resonance at δ -6.4. The downfield shifting of the resonance clearly indicates the coordination
of the PPh3 ligand [50].
2.4. Single crystal X-ray Studies
Diffraction quality crystals of the complexes (1-5) were grown over a period of two
weeks by standing a concentrated solution of the complex in DMF at room temperature. A
summary of the crystallographic and refinement data of complexes 1-5 are given in Table 2.
The structures of the complexes, 1-5 have been elucidated by single-crystal X-ray diffraction
studies and the ORTEP diagram of the complexes are shown in Fig. 2. ORTEP structure and
crystallographic and refinement data reveals that complexes crystallizes in triclinic (1 and 4)
and monoclinic (2, 3 and 5) system, and consist of tridentate ligand and PPh3/4-
Methylpyridine. The ligand is coordinated through the ONO/ONS donor atom and fourth
coordination site was occupied by phosphorous/nitrogen atom forming four-coordinated
species with distorted square planar geometry. Distortion is mainly caused by the presence of
the doubly deprotonated tridentate Schiff base ligand, which forms one five-membered and
one six-membered ring with nickel atom. Selected bond distances and bond angle are given
in Table S1. In complexes 1-5, bond angles O1-Ni-N1 are 86.84(6), 83.70(12), 95.08(11),
86.63(9) and 175.86(3), O1-Ni-P1 and N1-Ni-P1 in complexes 1-4 are 91.31(4), 91.39(9),
86.09(8), 86.72(6) and 173.81(5), 174.76(10), 176.13(8), 169.45(6) respectively. In
complexes 3 and 5, the bond angles O1-Ni1-S1, N1-Ni1-S1 and N2-Ni-N1 are 176.89(8),
87.65(8) and 174.57(10), respectively. The Ni(1)-O(1) and Ni(1)-N(1) bond lengths in
complexes 1-5 are 1.836(13), 1.842(2), 1.856(2), 1.838(18), 1.839(2) and 1.888(15),
1.848(3), 1.894(3), 1.874(2), 1.914(2), respectively which are in close agreement with the
previously reported Ni(II) complexes [45]. In complex 5, Ni(1)-S(1) bond length is 2.124(11)
is nearly similar to previously reported Ni(II) complex published by K. Natarajan et al.
having Ni-S bond length 2.127 [46].
(Table 2)
(Fig. 2)

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2.5. Catalytic studies
All the complexes were screened for their catalytic activity in the synthesis of 2-
substituted benzimidazoles (Scheme 2). Initially benzaldehyde and o-phenylenediamine were
chosen as model substrates. Various parameters such as catalyst loading, solvent and time
were studied and optimized. The results are summarised in Table 3. When the reaction was
carried out in absence of catalyst in ethanol at room temperature, 10 % yield of the product
was observed after 12 h (Table 3, entry 1). However, in presence of 0.2 mol % of catalyst 1,
75 % yield of the desired product was obtained in 5 h (Table 3, entry 2). Further, increasing
the catalyst loading from 0.2 mol % to 0.5 mol % and 0.8 mol % resulted in 94 % yield of the
desired product in 2h (Table 3, entries 3-4). Thus 0.5 mol % of catalyst loading was found to
be optimum. The reaction was also performed using complexes 2-5 as catalyst, (Table 3,
entries 3-8). Complex 1 and complex 2 showed superior catalytic activity towards
benzimidazoles synthesis. The lower catalytic activity of complexes 3 (ONS) may be due to
larger atomic (or Van der Waals) radius of sulfur atom which may lower the activity of the
catalyst due to steric effect [51]. Presence of electron donating group at para position of
aldehyde in complex 4 and weaker σ-donor properties of 4-methylpyridine in complex 5 may
be the reason for the lower catalytic activity [52]. In order to found the best solvent, the
reaction was carried out in different solvents such as EtOH, MeOH, CHCl3, CH3CN and
DMF (Table 3). Among all solvents, EtOH gave the best yield of product and was found to
be the best solvent (Table 3, entry 3). Thus, the optimal reaction conditions and best yield
(94%) was achieved in the presence of 0.5 mol% of complex 1 in ethanol solvent at room
temperature within 2 h (Table 3, entry 3).
Subsequently, all reactions were carried out under optimized conditions with a
structurally diverse range of aldehydes to give corresponding benzimidazoles (Table 4). It
can be concluded that the nature of substituent such as electron donating and electron
withdrawing group on aldehyde resulted in good yield. However aldehydes containing
electron-withdrawing groups (Br, Cl, F, NO2) gave products in higher yields than those
containing electron-donating groups (CH3, OCH3, OH) (Table 4, entries 4b-4j). This may be
due to increase in the electrophilicity of the carbonyl carbon of aldehydes by the electron
withdrawing group [53]. The para- and meta- substituted aldehydes resulted in good yield as
compared to substituent at ortho position (Table 4). This may be due to steric effect. The
reaction with heteroaromatic aldehydes i.e. quinoline-2-carbaxaldehyde and thiophene-2-
carbaxaldehyde resulted in good yield of the desired product (Table 4, entry 4k-l). The
isolated products were fully characterized by H and 13C NMR as given in ESI (Fig. S22-
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S33). In order to establish the efficacy of the new catalysts, a comparison was made with
some previously reported catalysts for 2-substituted benzimidazole synthesis in terms of
catalyst loading, temperature, time [16-18,20,32,34] etc. (Table 5). The results indicate that
our catalytic system exhibits better catalytic activity as compared to other reported catalyst.
A plausible mechanism for benzimidazoles synthesis, based on previous reports [19,
54-55] is suggested in Scheme 3. Initially triphenylphosphine ligand dissociate from the
complex to provide a coordination centre [50, 56]. Further, the reaction presumably
proceeds via activation of aldehyde by Ni(II) followed by imine formation and the resulting
imine further reacts with another -NH2 group of 1,2-phenylenediamine resulting in the
formation of dihydroimidazole. Subsequently dihydroimidazole undergoes aromatization
under aerial oxidation to give benzimidazole.
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(Table 3)
(Table 4)
(Table 5)
3. Conclusion
In conclusion, five new pincer type Ni(II)-Schiff-base complexes 1-5, have been synthesized
and characterized by various spectroscopic techniques and structure of the complexes was
confirmed by single crystal X-ray structure determination. The catalytic application of Ni(II)
complexes has been demonstrated in the synthesis of a series of 2-substituted benzimidazoles
from various aldehydes and o-phenylenediamine. Complex 1 and complex 2 showed superior
catalytic activity than complexes 3-5 towards benzimidazoles synthesis as high yields of
product were obtained with these complexes. The reactions proceeded smoothly at a low
catalyst loading at room temperature without use of additives and base.
4. Experiment
4.1. Materials and instrumentations:
All reagents and solvents for the synthesis and analysis were purchased from Merck
and Sigma Aldrich and used as received without further purifications.
FT-IR spectra were recorded on a Perkin Elmer Spectrometer in the range of 400-4000 cm-1
using KBr pellets. Electronic absorption spectral analysis was recorded on a Shimadzu UV-
1800 Spectrophotometer in the wavelength range of 200-800 nm. The H and 13C NMR
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spectra of the isolated products were recorded on a Bruker AvIII HD-400 MHz spectrometer
in DMSO-d6 using TMS as the internal Standard. Melting points were recorded on a Yazawa
micro melting point apparatus.

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4.2. General synthesis of Ni(II) complexes 1-5
The H2L1-H2L4 was synthesized by the reported method [40-43]. A solution of the
corresponding ligand H2L1-H2L4 (0.50 mmol) in 2 mL methanol was added drop-wise to the
methanolic solution of Ni(OAc)2.4H2O (0.50 mmol, 4 mL) with constant stirring at room
temperature. After stirring the solution for 10 minutes, PPh3/4-Methylpyridine (0.50 mmol)
dissolved in 2 mL methanol was added via a syringe to the reaction. After 6-9 h of stirring at
room temperature, the resultant precipitate was filtered, washed with cold ethanol and dried
in vacuum over anhydrous CaCl2. The precipitate was re-crystallized from DMF. Suitable
single crystals for X-ray crystallography were grown over a period of two weeks from a
concentrated solution of the complex in DMF.
[NiL1(PPh3)] 1: Yields: 0.495 g, 91%,. FT-IR (KBr), cm-1: 3443 ν (ΟΗ), 1592 ν (C=N), 3056
ν (CHar), 1474 ν (C=Cring), 517 ν (Ni-O), 443 ν (Ni-N). 1H NMR (CDCl3, 25 0C, 400 MHz):
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δ
8.49 (d, 1H, H-CN),
Ar H), 7.00-6.94 (m, 2H, Ar H), 6.65-6.64 (m, 2H, Ar H), 6.59-6.53 (m, 2H, Ar H),
(s,1H,OH).13C{1H}NMR(CDCl3,250C,100MHz):
165.99, 150.57, 148.58, 134.47, 132.15, 1
30.99, 128.81, 122.90, 119.39, 118.06, 116.02, 114.83, 114.20,113.56, 31P{1H}NMR (CDCl3,
δ
7.90-7.86 (m, 6H, Ar H), 7.72-7.70 (m, 1H, Ar H), 7.55-7.46 (m, 9H,
δ
4.74
δ
25 0C, 160 MHz):
δ
16.29. ESI-MS (ES+; m/z): Calculated for C31H24NNiO3P 548.19, Found
548.11.
[NiL2(PPh3)] 2: Yields: 0.500 g, 87%,. FT-IR (KBr), cm-1: 3455 ν (ΟΗ), 1524 ν (C=N), 3052
1
ν (CHar), 1213 ν (C-O), 1433 ν (C=Cring), 508 ν (Ni-O), 426 ν (Ni-N). H NMR (CDCl3, 25
0C, 400 MHz):
7.55-7.54 (m, 9H, Ar H), 7.49-7.47 (m, 3H, Ar H) 7.25-7.23 (m, 1H, Ar H), 6.91-6.53 (m,
2H, Ar H),
4.92 (s,1H, OH). 13C{1H} NMR (CDCl3, 25 0C, 100
MHz): 173.30, 173.20, 149.43, 148.96, 147.84,134.57, 134.37, 134.26,131.15,139.6,130.25,
128.96, 128.87, 128.15,128.00, 122.15 , 118.24,116.09, 113.07, 31P{1H}NMR (CDCl3, 25
δ 8.35 (d, 1H, H-CN), δ 7.86-7.81 (m, 6H, Ar H), 7.73-7.72 (m, 2H, Ar H),
δ
δ
0C, 160 MHz):
δ
19.17. ESI-MS (ES+; m/z): Calculated for C32H25N2NiO3P 575.22, Found
575.12.
[NiL3(PPh3)] 3: Yields: 0.536 g, 89%. FT-IR (KBr), cm-1: 3432-3350 ν (NH2), 3577 ν (OH),
1593 ν (C=N). 1H NMR (CDCl3, 25 C, 400 MHz):
δ 8.26 (d, 1H, H-CN), δ 8.24 (s, 1H, Ar
0
H),
(m, 1H, Ar H), 6.69-6.67 (m, 1H, Ar H), 6.55-6.49 (m, 1H, Ar H),
(s,1H,NH2).13C{1H}NMR(CDCl3,250C,100MHz):
171.10, 170.99, 152.45, 149.00, 147.16, 1
δ
7.83-7.78 (m, 6H, Ar H), 7.55-7.52 (m, 3H, Ar H), 7.48-7.44 (m, 5H, Ar H), 6.84-6.81
δ
4.97 (s,1H, OH), 4.59
δ
δ

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34.37, 134.26, 131.23,128.91, 128.77,128.67, 128.44,122.33, 116.46, 115.62, 113.00, 31P{1H
} NMR (CDCl3, 25 0C, 160 MHz):
δ
23.93. ESI-MS (ES+; m/z): Calculated for
C26H22N3NiO2PS 529.19, Found 530.08.
[NiL4(PPh3)] 4: Yields: 0.536 g, 88%. FT-IR (KBr), cm-1: 1604 ν (C=N), 2975 ν (CHar),
1468 ν (C=Cring). 1H NMR (CDCl3, 25 0C, 400 MHz):
8.18 (d, 1H, H-CN), 7.88 (m, 10H,
δ
δ
Ar H), 7.58-7.57 (m, 2H, Ar H), 7.03 (m, 3H, Ar H), 6.85-6.84 (m, 1H, Ar H), 6.56-6.50 (m,
3H, Ar H), 6.12-6.10 (m, 1H, Ar H), 5.32-5.28 (m, 2H, Ar H), 5.15 (m, 1H, Ar H), 3.24-3.15
13
(q, 4H, -CH2), ),
δ
1.06-1.04 (t, 6H, -CH3). C{1H}NMR(CDCl3,250C,100MHz):
δ
164.57,
151.73, 149.88, 147.13, 135.13, 134.43, 130.70, 128.61, 126.92, 117.85, 114.25,
0
113.38,104.04, 100.90, 93.53, 44.82, 13.27. 31P{1H} NMR (CDCl3, 25 C, 160 MHz):
δ
26.87. ESI-MS (ES+; m/z): Calculated for C35H33N2NiO2P 603.31, Found 604.17.
[NiL4(4-MePy)] 5: Yields: 0.376 g, 87%. FT-IR (KBr), cm-1: 1613 ν (C=N), 2975 ν (CHar),
0
1486 ν (C=Cring). 1H NMR (CDCl3, 25 C, 400 MHz):
δ 8.49 (d, 1H, H-CN), δ 7.77 (d, 1H,
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
Ar H), δ 7.42-7.39 (m, 1H, Ar H), 7.15-7.13 (m, 3H, Ar H), 6.86-6.83 (m, 1H, Ar H), 6.65-
6.63 (m, 1H, Ar H), 6.47-6.43 (m, 1H, Ar H), 6.17-6.14 (m, 1H, Ar H), 6.03 (m, 1H, Ar H),
3.34-3.29 (q, 4H, -CH2), 1.15-1.12 (t, 6H, -CH3). 13C{1H} NMR(CDCl3, 25OC, 100MHz):
164.72, 163.66, 152.13, 149.95, 133.91, 126.82, 125.10,116.70, 114.33, 133.13, 112.19,
112.9, 103.89, 99.76, 44,37, 21.02, 12.86. ESI-MS (ES+; m/z): Calculated for C23H25N3NiO2
434.17, Found 434.15.
4.3 General procedure for the synthesis of 2-substituted benzimidazoles
A mixture of aldehyde (1.0 mmol) and o-phenylenediamine (1.0 mmol) and complex
1 (0.5 mol %) was stirred at room temperature in ethanol (5mL) for 2-3 h. The progress of the
reaction was monitored by TLC. After completion of the reaction, the solvent was removed
and the product washed with water and extracted with ethyl acetate (3x10 mL). The
combined organic layers were dried over Na2SO4 and the solvent removed under reduced
pressure to give crude product which was purified by column chromatography by using
petroleum ether/ethyl acetate as an eluent. The products were confirmed by 1H and 13C NMR.
4.4 Crystallographic studies
The X-ray diffraction data were collected on a Bruker Kappa diffractometer at 103(2)
K (complexes 1-2), 203(2) K (complex 3) and 296 K (complexes 4-5) equipped with a CCD
detector, employing Cu radiation (complex 1-2) and Mo K α radiation (complex 3-5) (λ =
1.54178 Å (complex 1-2) and 0.71073 Å (complexes 3-5), with the SMART suite of
programs [57]. All data were processed and corrected for Lorentz and polarization effects

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with SAINT and for absorption effects with SADABS [58]. Structural solution and
refinement were carried out with the SHELXTL suite of programs [59]. The structures were
refined (weighted least squares refinement on F2) to convergence. All the non-hydrogen
atoms in all the compounds were refined anisotropically by full-matrix least-squares
refinement. A summary of the crystallographic and refinement data of complexes 1-5 are
given in Table 2.
Acknowledgments
We are thankful to the CRF IIT (ISM), SAIF Panjab University, Chandigarh and IISER
Bhopal for providing help in the analysis of the samples. Bhumika Agrahari and Samaresh
Layek acknowledges the receipt of IIT (ISM) fellowship.
Supplementary data
CCDC 1473738, 1526857, 1585092, 1536826 and 1537251 contain the
supplementary crystallographic data for this paper. This data can be obtained free of charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or
from
the
Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or mail. Supporting information also include experimental details and
characterization data and their spectra.
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List of Scheme’s caption
Scheme 1 Synthesis of pincer type Ni(II) complexes 1-5
Scheme 2 The catalytic activity of Ni(II) complexes in benzimidazoles synthesis
Scheme 3 Plausible mechanism for benzimidazole synthesis
List of Figure’s caption
Fig. 1 Benzimidazoles moiety containing drugs
Fig. 2 ORTEP diagram of [NiL1(PPh3)] 1, [NiL2(PPh3)] 2, [NiL3(PPh3)] 3 (DMF molecule
omitted, [NiL3(PPh3)] 4 and [NiL4(4-MePy)] 5 complexes
List of Table’s caption
Table 1 FT-IR spectral data of ligands H2L1-H2L4 and nickel(II) complexes 1-5
Table 2 Crystal data and refinement parameters of complexes 1-5
Table 3 Optimization of reaction conditions for synthesis of benzimidazolesa
Table 4 The reaction of various aldehydes with o-phenylenediamines under optimized
reaction conditions
Table 5 A comparison study of synthesized Ni(II) complex with the previous reported
catalysts for benzimidazole synthesis

16
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505
506
507
Fig. 1 Benzimidazole moiety containing drugs
508
509
Scheme 1 Synthesis of pincer type Ni(II) complexes 1-5
CHO
H
N
NH2
NH2
0.5 mol % Catalyst
Ethanol, rt, 2-3 h
+
R
R
N
510
511
Scheme 2 The catalytic activity of Ni(II) complexes in benzimidazoles synthesis
[NiL1(PPh3)] 1
[NiL2(PPh3)] 2
[NiL3(PPh3)] 3

17
ACCEPTED MANUSCRIPT
[NiL4(PPh3)] 4
[NiL4(4-MePy)] 5
Fig. 2 ORTEP diagram of [NiL1(PPh3)] 1, [NiL2(PPh3)] 2, [NiL3(PPh3)] 3 (DMF molecule
omitted, [NiL3(PPh3)] 4 and [NiL4(4-MePy)] 5 complexes with the non C-H atom labelling
scheme (Thermal ellipsoids are drawn at the 50% probability level.)
512
513
514
Table 1 FT-IR spectral data of ligands and nickel(II) complexes 1-5
υ OH υ NH υ C=O υ C=N υ C=S
(cm-1) (cm-1) (cm-1)
(cm-1) (cm-1)
Compound
HL1
HL2
3369
3480
3532
3454
3443
3453
3586
-
-
-
1623
1622
1620
1622
1592
1604
1593
1604
1613
-
-
3291
3141
1667
HL3
-
-
-
-
-
-
-
1227
HL4
-
-
-
-
-
-
-
-
-
-
-
-
[NiL1(PPh3)] 1
[NiL2(PPh3)] 2
[NiL3(PPh3)] 3
[NiL4(PPh3)] 4
[NiL4(4-MePy)] 5
-
515
516
517
Table 2 Crystal data and refinement parameters of complexes 1-5
CCDC
1473738
1526857
1585092
1536826
1537251
Chemical
formula
Formula
weight
C31H24NNiO3P C32H25N2NiO3P C29H28N4NiO3PS C35H33N2NiO2P C23H25N3NiO2
548.19 g/mol
575.22 g/mol
602.29 g/mol
603.31 g/mol
434.17 g/mol
Temperature 103(2) K
Wavelength 1.54178 Å
Crystal size 0.100 x 0.140 0.160 x 0.060 x 0.140 x 0.220 x
103(2) K
1.54178 Å
203(2) K
0.71073 Å
296 K
296 K
0.71073 Å
0.37 × 0.31 ×
0.23 mm
red block
0.71073 Å
0.62 × 0.45 ×
0.32 mm
red block
x 0.220 mm
red block
0.040 mm
red block
0.280 mm
red block
Crystal
habit
Crystal
system
triclinic
monoclinic
monoclinic
triclinic
monoclinic
Space group P -1
P 1 21/n 1
P 1 21/n 1
P1
P21/n
Unit cell

18
ACCEPTED MANUSCRIPT
dimensions
a (Å)
β (Å)
γ (Å)
a
76.3139(5)°
88.3431(5)°
76.5697(5)°
8.98080(10) Å 15.7729(3) Å
10.01310(10)
Å
90°
90°
82.472 (4)°
75.563 (4)°
74.087 (4)°
10.1793 (6) Å
15.6331 (8) Å
90°
101.269°
90°
10.5869 Å
7.6769 Å
101.4877(12)° 102.702(16)°
90°
90°
9.960(3) Å
15.038(4) Å
b
15.3705(2) Å
22.1748(3) Å
c
14.4375(2) Å
19.637(6) Å
20.4722 (10) Å 26.0646 Å
Volume
Z
1226.48(3) Å3 5268.31(14) Å3 2869.2(15) Å3
3027.2 (3) Å3
4
2077.5 (3) Å3
4
8
4
2
Density
(calculated)
Absorption
coefficient
F(000)
1.484 g/cm3
1.450 g/cm3
1.394 g/cm3
1.324 Mg m-3
1.388 Mg m−3
2.043 mm-1
1.942 mm-1
0.842 mm-1
0.73 mm-1
0.96 mm-1
568
2384
1252
1264
912
Goodness-
of-fit on F2
∆/σmax
1.064
1.001
1.026
0.938
1.053
0.001
0.001
0.001
0.001
0.001
Final R
indices
R1 = 0.0349,
wR2 = 0.0899
R1 = 0.0609,
wR2 = 0.1402,
R1 = 0.0541,
wR2 = 0.1140,
R1 = 0.0392,
wR2 = 0.0951
R1 = 0.0440,
wR2 = 0.1268
I>2σ(I)
R indices
(all data)
R1 = 0.0366,
wR2 = 0.0932 wR2 = 0.1638
R1=0.1040,
R1 = 0.0978,
wR2 = 0.1324
R1 = 0.0640,
wR2 = 0.1018
R1 = 0.0512,
wR2 = 0.1314
518
519
Table 3 Optimization of reaction conditions for synthesis of benzimidazolesa
520
Entry
Catalyst
-
Catalyst
loading
(mol %)
-
Solvent
Time
Yieldb
(%)
1
2
3
4
5
6
7
8
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
CHCl3
CH3CN
DMF
12
5
2
2
2
2
2
2
3
4
4
4
10
75
94
94
93
82
89
85
75
45
58
84
Complex 1 0.2
Complex 1 0.5
Complex 1 0.8
Complex 2 0.5
Complex 3 0.5
Complex 4 0.5
Complex 5 0.5
Complex 1 0.5
Complex 1 0.5
Complex 1 0.5
Complex 1 0.5
9
10
11
12
521
aReaction conditions: Aldehyde (1 mmol), o-phenylenediamines (1 mmol), RT.
522
523
bYield after column chromatography.

19
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524
525
Table 4 The reaction of various aldehydes with o-phenylenediamines under optimized
reaction conditionsa,b
526
527
aReaction conditions: Aldehyde (1 mmol) and o-phenylenediamines (1 mmol);
528
529
530
531
532
bYield after column chromatography.
Table 5 A comparison study of synthesized Ni(II) complex with the previous reported
catalysts for benzimidazole synthesis
Entry
Catalyst
Catalyst
loading
(mol %)
Temperature
(oC)
Time
(h)
Yields
(%)
Reference
1
2
Cu(II)-salen
CuFe2O4
5
50
50
110
3
24
90
89
[34a]
[34 b]
nanoparticle
Zn(OTf)2
MoO3
Co(OH)2 /
CoO(II)
3
4
5
10
2
10
80
50
rt
8
0.5
4-9
95
93
98
[32]
[34 c]
[18]
6
7
8
9
Ce(NO3)3·6 H2O
In(OTf)3
Ni(NO3)2.6H2O
Pincer Ni(II)
Complexes
30
5
50
0.5
80
rt
80
rt
1.5-6
0.5
0.5
94
95
89
96
[17]
[16]
[20]
2-3
This work
533

20
ACCEPTED MANUSCRIPT
534
535
Scheme 3 Plausible mechanism for benzimidazole synthesis
536
537
538

ACCEPTED MANUSCRIPT
Highlights:
•
•
•
•
Synthesis of five new pincer type Ni(II)-Schiff-base complexes
Characterization by single crystal X-ray, FT-IR, UV-Vis, NMR and mass spectrometry
Efficient catalysts for the synthesis of 2-substituted benzimidazoles
Reaction proceed using low catalyst loading at room temperature