Synthesis, structure and catalytic applications of octahedral nickel(II) benzoylhydrazone complex: Suzuki-Miyaura cross-coupling reaction of aryl bromides with arylboronic acid

Article abstract of DOI:10.1016/j.ica.2016.05.015

A simple route for the synthesis of a new octahedral nickel(II) complex (1) containing 4-chloro-benzoic acid pyridin-2-ylmethylene-hydrazide ligand (HL, where H represents the dissociable proton) has been described. The characterization of the complex having the formula [Ni(μ₃-O,N,N-L)₂] has been established by elemental analysis, ESI-MS, spectral methods and single crystal X-ray crystallography. Further, the new complex was found to act as an active homogeneous catalyst for the Suzuki-Miyaura reaction of substituted aryl bromides (mono and dibromides) with various arylboronic acids having different electronic effects under optimized conditions of solvent, base and catalyst loading. The biphenyl and triphenyl derivatives were obtained in moderate to excellent isolated yields.

Full text of DOI:10.1016/j.ica.2016.05.015

Accepted Manuscript
Research paper
Synthesis, structure and catalytic applications of octahedral nickel(II) benzoyl-
hydrazone complex: Suzuki-Miyaura cross-coupling reaction of aryl bromides
with arylboronic acid
Pandimuni Kalpaga Suganthy, Rupesh Narayana Prabhu, Venugopal
Shanmugham Sridevi
PII:
S0020-1693(16)30244-4
http://dx.doi.org/10.1016/j.ica.2016.05.015
ICA 17051
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
7 October 2015
20 April 2016
8 May 2016
Please cite this article as: P.K. Suganthy, R.N. Prabhu, V.S. Sridevi, Synthesis, structure and catalytic applications
of octahedral nickel(II) benzoylhydrazone complex: Suzuki-Miyaura cross-coupling reaction of aryl bromides with
arylboronic acid, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.05.015
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Synthesis, structure and catalytic applications of octahedral nickel(II) benzoylhydrazone
complex: Suzuki-Miyaura cross-coupling reaction of aryl bromides with arylboronic acid
Pandimuni Kalpaga Suganthy,a Rupesh Narayana Prabhu,b Venugopal Shanmugham Sridevia,∗
a Department of Chemistry, Periyar E.V.R. College, Tiruchirappalli 620 023, Tamil Nadu, India
b
Department of Chemistry, Srimad Andavan Arts & Science College, Tiruchirappalli 620 005,
Tamil Nadu, India
Abstract
A simple route for the synthesis of a new octahedral nickel(II) complex (1) containing 4-
chloro-benzoic acid pyridin-2-ylmethylene-hydrazide ligand (HL, where H represents the
dissociable proton) has been described. The characterisation of the complex having the formula
[Ni(µ3-O,N,N-L)2] has been established by elemental analysis, ESI-MS, spectral methods and
single crystal X–ray crystallography. Further, the new complex was found to act as an active
homogeneous catalyst for the Suzuki-Miyaura reaction of substituted aryl bromides (mono and
dibromides) with various arylboronic acids having different electronic effects under optimized
conditions of solvent, base and catalyst loading. The biphenyl and triphenyl derivatives were
obtained in moderate to excellent isolated yields.
∗ Corresponding author. Tel.: +91–431–242–0079; fax: +91–431–2332010;
e-mail: sridevi.evr@gmail.com
1

Keywords: Benzoylhydrazone ligand, Nickel(II) complex, Characterisation, Crystal structure,
Suzuki-Miyaura reaction.
1. Introduction
Schiff bases are considered as ‘privileged ligands’ due to the fact that they are able to
stabilize many different metals in distinct oxidation states, have chelating structure and are
kinetically inert. Schiff bases can control the performance of metals in a diverse range of
applications and can act as effective catalysts in organic synthesis [1-5], dyes and light emitting
materials [6-8], biologically active substances [9,10], etc.. Acid hydrazones are unique and
multifaceted N- and O- donor Schiff bases that possess synthetic flexibility and exhibit amide–
imidol tautomerism. From a structural perspective, acid hydrazones are fascinating on account of
their coordination versatility towards a diverse range of transition metals (Scheme 1). The
coordination behaviour of acid hydrazones depend on metal ion, the pH of the medium, the
nature of the substituent and position of hydrazone group relative to other nucleus. They can
coordinate to a metal ion, via the azomethine–N either in the neutral amide form (Ia) or in the
monoanionic imidolate form (Ib), as bidentate N,O donor ligands forming five–membered
chelate rings with the metal [11-13]. When a third donor site (D) is incorporated into the acid
hydrazone ligands, tricoordination (II) may take place resulting in the formation of 5,5- or 5,6-
membered chelate rings [14,15].
2

R2
R2
D
R1
R1
R1
N
N
N
M
M
M
HN
R
N
R
N
R
O
O
O
II
Ia
Ib
Scheme 1. Coordination modes of acid hydrazone ligands
Biaryl and triaryl moieties are important structural framework found in fine chemicals,
pharmaceuticals, agrochemicals, natural products and electronic materials [16-18]. Pd–catalyzed
Suzuki–Miyaura reaction between an arylhalide and an arylboronic acid has become a mainstay
of modern synthetic organic chemistry in the construction of carbon-carbon bonds for the
preparation of bi- and triaryl compounds [19-21]. This C-C cross-coupling reaction offers many
advantages such as mild reaction conditions, commercial availability of a large number of air and
water stable boronic acid derivatives, easy handling, easy removal of boron residues as well as
the low toxicity of reagents and by-products, thus explaining the increasing interest in academic
and industrial research. However, the high cost of palladium has resulted in the exploration of
less costly transition metals as alternatives to palladium systems for cross–coupling reactions
while still retaining high levels of efficiency and functional group tolerance. Nickel appears to be
the most promising replacement for palladium as catalysts for Suzuki–Miyaura reactions. The
low cost, low toxicity and high reactivity of nickel is attractive, and a range of substrates have
been shown to undergo nickel–catalyzed C–C coupling reactions. Nickel complexes containing
various ligands have been developed as effective catalysts for the Suzuki–Miyaura reaction [22-
25]. In addition, to the best of our knowledge the use of octahedral nickel(II) complex containing
3

a benzoylhydrazone ligand as catalyst for the Suzuki–Miyaura cross–coupling reaction has not
been explored so far.
As a part of our ongoing research on the synthesis, structural characterisation and
catalytic applications of transition metal complexes [26-28], herein we report the synthesis and
characterisation of a new octahedral Ni(II) complex (1) bearing a N,N,O– donor
benzoylhydrazone ligand. The structure of the complex has been probed with the help of single
crystal X-ray diffraction analysis. Further, the catalytic efficiency of the complex has been
investigated for the homogeneous Suzuki–Miyaura cross–coupling reaction of aryl bromides
(mono and dibromides) with substituted phenylboronic acids under optimized reaction
conditions.
2. Experimental Section
2.1. Materials
Ni(OAc)2·4H2O was purchased from Loba Chemie Pvt. Ltd, and used as received.
Pyridine-2-carboxaldehyde, 4-chlorobenzoylhydrazide as well as the substrates for Suzuki-
Miyaura reactions were purchased from Sigma-Aldrich and used without any further
purification. All other reagents used were purchased from commercially available sources. The
solvents were freshly distilled prior to use following the standard procedures [29].
2.2. Physical measurements and instrumentation
Microanalysis (C, H, N) was recorded by an Elementar Vario EL III analyzer. Magnetic
susceptibility measurement was performed with the help of a Sherwood scientific balance. A
Digisun DI-909 conductivity meter was used to measure the solution electrical conductivity.
Mass spectrometric analysis was performed using the ESI technique on a Waters Q-TOF Micro
4

mass spectrometer in solution of DMF. Infrared spectra were recorded in KBr pellets with a
Perkin–Elmer 597 spectrophotometer in the range 4000–400 cm–1. The NMR spectra were
recorded with a Bruker 400 MHz spectrometer. The electronic spectra were recorded in DMSO
solution with a Cary 300 Bio UV–Vis Varian spectrophotometer in the range 800–260 nm using
cuvettes of 1 cm path length.
2.3. Synthesis of the nickel(II) complex (1)
An ethanolic solution (10 mL) of Ni(OAc)2·4H2O (0.10 mmol) was added dropwise to a
stirred ethanolic solution (10 mL) of HL (0.20 mmol). The reaction mixture was refluxed for 1 h
and the progress of the reaction was monitored using TLC. The deep red solution formed was
filtered and cooled to room temperature. Red single crystals of the complex (1) suitable for
single crystal X-ray diffraction studies were obtained on slow evaporation of the reaction
mixture over a period of 10-15 days. Colour: Red; Yield: 130 mg, 75%; Anal. Cald. for
C26H18Cl2N6NiO2: C, 54.02; H, 2.78; N, 14.54%. Found: C, 54.18; H, 2.81; N, 14.47%. ESI-MS
Found m/z: 579.00 (11%), 577.00 (100%), 566.00 (10%), 434.61 (10%), 421.86 (80%), 423.00
(19%), 356.00 (10%), 300.83 (56%), 255.97 (10%). µeff (300 K) = 2.96 BM. Molar conductance,
ΛM (1 × 10−3 M, DMF): 8 S cm2 mole-1. Selected IR frequencies (KBr, cm-1) (Symbols: s, strong;
m, medium; w, weak): 2912(s), 1586(s), 1492(s), 1460(s), 1281(s), 1136(m), 1075(s), 915(w),
710(m), 693(m), 520(w). UV-Vis (DMSO, λmax/nm; ε/dm3 mol-1 cm-1): 411 (sh, 10920), 393
(13320), 303 (5790), 287 (5730).
2.4. X-ray crystallography
Single crystals of complex 1 suitable for X–ray diffraction analysis were obtained by
slow evaporation of the reaction mixture of the complex at room temperature. The data
collection was carried out using Bruker AXS Kappa APEX II single crystal X–ray diffractometer
5

using monochromated Mo Kα radiation (λ = 0.71073 Å). The absorption corrections were
performed by multi-scan method using SADABS software [30]. Corrections were made for
Lorentz and polarization effects. The structures were solved by SIR92 [31] and refined by full–
matrix least squares on F2 using SHELXL 97 [32]. All non–hydrogen atoms were refined
anisotropically and the hydrogen atoms in these structures were located from the difference
Fourier map and constrained to the ideal positions in the refinement procedure. The unit cell
parameters were determined by the method of difference vectors using reflections scanned from
three different zones of the reciprocal lattice. The intensity data were measured using ω and ϕ
scan with a frame width of 0.5°. Frame integration and data reduction were performed using the
Bruker SAINT–Plus (Version 7.06a) software [33]. The relevant data concerning data collection
and details of structure refinement for the complex are summarized in Table 1.
6

Table 1. Crystal data and structure refinement for the complex 1
Empirical formula
Formula weight
Colour
C26H18Cl2N6NiO2
576.07
Red
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
293(2)
0.71073
Monoclinic
P21/n
10.126(2)
13.539(3)
17.517(4)
93.428(3)
2397.2(9)
4
b (Å)
c (Å)
β (°)
Volume (Å3)
Z
Calculated density (g m–3)
1.596
Absorption coefficient (mm–1) 1.071
F(000)
1176
Crystal size (mm)
0.38 x 0.28 x 0.19
1.90 to 26.00
Theta range (°)
Limiting indices
–12 ≤ h ≤ 12, –16 ≤ k ≤ 16, –21 ≤ l ≤ 21
4698/4286
Full–matrix least–squares on F2
Reflections collected/unique
Refinement method
Data/restraints/parameters
Goodness–of–fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
Largest diff. peak and hole
4698/0/334
1.079
R1 = 0.0386, wR2 = 0.0952
R1 = 0.0425, wR2 = 0.0976
0.006 and 0.001 e. Å–3
7

2.5. Typical procedure for the Suzuki-Miyaura reactions
To a mixture of a monobromide (1.0 mmol), arylboronic acid (1.0 mmol) and K2CO3 (2.0
mmol) in toluene (10 mL), 1 (0.5 mol%) were added. The resultant mixture was refluxed for 8 h,
the reaction mixture was then cooled to room temperature, H2O (10 mL) added and the organic
layer was extracted with EtOAc (3 x 20 mL). The solvent was stripped off and the remaining
residue was purified by column chromatography (n-hexane–EtOAc) to yield the pure product.
The products were confirmed by 1H NMR. For the double cross-coupling reaction the procedure
was similar to that mentioned above where dibromide (1.0 mmol), arylboronic acid (2.0 mmol),
1 (1.0 mol%) and K2CO3 (4.0 mmol) in toluene (10 mL) were used.
3. Results and discussion
3.1. Synthesis and some properties
The Schiff base, 4–chloro–benzoic acid pyridin–2–ylmethylene–hydrazide, (HL, where H
stands for the dissociable proton) was obtained in ~85% yield by the condensation of pyridine–
2–carboxaldehyde and 4–chlorobenzhydrazide by following a procedure reported earlier [34].
1
The purity of HL was authenticated by IR, ESI-MS and H NMR measurements. Treatment of
Ni(OAc)2·4H2O with two equivalents of HL in ethanol under reflux resulted in the formation of
the new red octahedral nickel(II) complex (1) of the formula, [Ni(µ3-O,N,N-L)2] (Scheme 2).
The complex is soluble in solvents such as dichloromethane, chloroform, methanol, toluene,
dimethylformamide, dimethyl sulphoxide, acetonitrile, etc., producing intense red solution. It is
non-hygroscopic and air stable in both solid and liquid states at room temperature. The complex
has non-electrolytic nature in solution. The observed elemental analysis data of 1 was consistent
with its composition and it appears from the formulation that the Schiff base coordinate to the
nickel(II) centre as a monobasic tridentate ligand. The mass spectral study for the complex was
8

investigated and selected mass fragmentations are listed in the Experimental Section. The
obtained results definitely confirmed the molecular formula of the complex. The ESI spectrum of
1 show distinct m/z {M+H}+ and {M+2+H}+ peak patterns at 577 and 579 respectively, due to
Ni-58 and Ni-60 isotopes. Magnetic susceptibility studies revealed that the complex is
paramagnetic (µeff = 2.96 BM) at room temperature and is consistent with high spin
configuration of a distorted octahedral environment around the nickel(II) centre [35].
Cl
N
N
.
O
2
Ni(OAc)2 4H2O
N
N
N
N
Ni
HN
N
EtOH, reflux, 1h
O
1
O
N
Cl
Cl
HL
Scheme 2. Synthesis of the nickel(II) complex
3.2. Spectral characterization
Infrared spectra of HL and 1 showed many vibrations of variable intensities in the region
4000-400 cm-1. However, the assignment of the individual band to a specific vibration has not
been attempted. The complex displays a νC=N stretch at 1586 cm-1 which is at a lower frequency
than that of the free HL (1610 cm-1) indicating coordination of azomethine nitrogen to Ni(II) ion.
This is further supported by an increase in the νC−H stretching frequency of the azomethine
moiety in 1 (2912 cm-1) when compared to that of the free HL (2859 cm-1). The free HL
displayed νN–H and νC=O stretching frequencies at 3225 and 1670 cm-1 respectively. These bands
were not observed in 1, suggesting enolisation and subsequent coordination of the imidolate
9

oxygen to Ni(II) ion. This is further supported by the appearance of new stretching frequency
due to νC–O (1281 cm-1) in 1 [36]. The electronic spectra of the free HL and 1 were recorded in
DMSO solution at room temperature. The free HL showed strong absorption at 298 nm (ε =
22400 M-1 cm-1) which was ascribed to the ligand centered n → π* transition. The band showed
a hypochromic shift in 1; the decrease in intensity may be attributed to the coordination of the
hetero-atoms in HL to the metal centre. In addition, 1 displayed absorptions at 393 nm and
shoulder at 411 nm which were assigned to ligand-to-metal-charge-transfer (LMCT) transitions.
The electronic absorption data of 1 in solution suggests an octahedral structure [37], which was
further confirmed with X-ray crystal structure analysis.
Figure 1. Electronic spectra of HL (dashed line) and 1 (solid line) in DMSO.
10

3.3. Single crystal X-ray structure of 1
The molecular structure of the complex 1 has been probed by single crystal X–ray
diffraction to confirm the coordination mode of the ligand and geometry of the complex. The
ORTEP view of complex 1 with ellipsoids of 30% probability along with the atom numbering
scheme is shown in Figure 2 whereas selected bond parameters are listed in Table 2. The
complex crystallizes in a monoclinic system with P21/n space group. The complex consists of
two aroylhydrazone ligands coordinating to the nickel(II) ion in a tridentate fashion to afford a
six–coordinate geometry by employing the following donor atoms of each L- ion: a pyridyl–N,
an imine–N and an enolate–O. The chelate donor atoms adopt a meridional coordination mode;
thus the two imine-N atoms occupy trans positions whereas each of the two pairs of pyridyl–N
atoms and enolate–O atoms takes adjacent positions to give a trans,cis,cis–coordination
arrangement consistent with the rigidity and planarity of the Schiff-base ligand. Nickel is
therefore sitting in a N4O2 coordination environment from the two ligand units and the complex
has a distorted octahedral geometry as is reflected in the bond parameters around metal centre.
Two five membered chelate rings are formed by each ligand. Around the nickel(II) ion, the
chelate bite angles in the five membered rings formed by the pyridine-N and imine-N (78.44°
and 78.37°) are slightly larger than that formed by the amide-O and imine-N (76.10° and 75.86°),
indicating a better chelation in the latter ring. The N–N, N–C and C–O bond distances in the =N–
N=C(O–)– moiety of the coordinated benzoylhydrazone are consistent with the enolate form of
the amide function. The Ni–N(pyridine), Ni–N(imine) and Ni–O bond distances observed are
also in good agreement with reported data on related nickel(II) acid hydrazone complexes [35-
41]. The trans angles, O(1)–Ni–N(1) = 154.37°, N(2A)–Ni–N(2) = 171.08° and O(1A)–Ni–
11

N(1A) = 154.21°, show significant deviation from the expected 180° which could be due to the
rigidity of the coordinated planar ligand.
Figure 2. ORTEP view of 1 at 30% probability.
12

Table 2. Selected bond parameters for the complex 1
Bond lengths (Å)
Bond angles (°)
Ni-N1
2.1127(19)
1.9821(19)
2.0936(17)
1.359(2)
N2-Ni-N1
78.44(7)
76.10(7)
154.37(7)
108.04(17)
125.8(2)
78.37(8)
75.86(7)
154.21(7)
108.41(18)
125.5(2)
90.65(8)
171.08(8)
93.39(7)
Ni-N2
N2-Ni-O1
Ni-O1
O1-Ni-N1
N3-N2
C7-N3-N2
N3-C7
1.340(3)
O1-C7-N3
O1-C7
1.264(3)
N2A-Ni-N1A
N2A-Ni-O1A
O1A-Ni-N1A
C7A-N3A-N2A
O1A-C7A-N3A
N1-Ni-N1A
N2A-Ni-N2
O1-Ni-O1A
Cl1-C11
Ni-N1A
Ni-N2A
Ni-O1A
N3A-N2A
N3A-C7A
O1A-C7A
Cl1A-C11A
1.742(2)
2.136(2)
1.9804(19)
2.1128(17)
1.358(3)
1.343(3)
1.262(3)
1.748(3)
ESD in parenthesis
3.4. Catalytic activity of 1
3.4.1 Optimization of solvent and base
The coupling reaction between 4ˈ–bromoacetophenone with 4–methylphenylboronic acid
to yield 1-(4'-methyl-biphenyl-4-yl)-ethanone using 1 as catalyst was selected as a model
reaction (Table 3). Various solvents were screened in the beginning (entries 1–5). As can be
inferred from the scrutiny of the obtained results, the reaction proceeded relatively well in
toluene under reflux. Among the different bases screened (entries 5–10) in toluene, K2CO3 gave
excellent results. Other bases were less active or inactive under the studied conditions.
Controlled experiments also indicated that no cross–coupling product was observed in the
absence of the catalyst or base. For further optimization reaction, K2CO3 in toluene was used.
13

Table 3. Optimization of reaction conditionsa
Complex 1
Br (HO) B
+
Solvent, base
Reflux 8 h
2
O
O
Entry
Solvent
Base
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
DMF
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
48
EtOH
52
1,4-Dioxane
MeCN
67
60
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
94
98
KOH
42
CH3COOK
Et3N
27
<10
<10
Pyridine
a
Reaction conditions: 4ˈ-bromoacetophenone (1 mmol), 4-methyl-phenylboronic
acid (1 mmol), base (2 mmol), complex 1 (1 mol%), solvent (10 mL), reflux for 8 h.
b Isolated yield after column chromatography based on arylbromide (average of two
runs).
3.4.2 Optimization of catalyst loading
In order to achieve maximum conversion, the above coupling reaction was carried out
under different catalyst loadings (Table 4). Good isolated yields of the coupled product were
obtained when 1.0 or 0.5 mol % of the catalyst (entries 1 and 2) was used. Attempts to further
decrease the catalyst loading to 0.25 or 0.125 mol % (entries 3 and 4) failed, leading to poor
isolated yield. Hence, for further reactions with various substrates, catalyst loading of 0.5 mol %
was used.
14

Table 4. Effect of catalyst loadinga
Complex 1
Br (HO) B
+
Toluene, K2CO3
Reflux 8 h
2
O
O
Entry
Catalyst (mol%)
Yieldb (%)
1
2
3
4
1.0
98
95
70
36
0.5
0.25
0.125
a
Reaction conditions: 4ˈ–bromoacetophenone (1 mmol), 4–methylphenyl–boronic
acid (1 mmol), K2CO3 (2 mmol), toluene (10 mL), reflux for 8 h.
b Isolated yield after column chromatography based on arylbromide (average of two
runs).
3.4.3. Suzuki–Miyaura cross-coupling reaction of monobromides
Using the above mentioned optimized reaction conditions, the scope of the catalytic
system was explored with various substituted phenylboronic acids and phenylbromides (Table
5). It was observed that all the arylbromides reacted smoothly with the various arylboronic acid
derivatives and the desired biaryl compounds (entries 1–20) were formed in moderate to
excellent isolated yields. It is also inferred that for a given boronic acid derivative, aryl bromides
with electron-withdrawing groups (such as 4–COCH3 or 4–CHO) coupled effectively and gave
better isolated yields of the desired product than that of (unsubstituted) bromobenzene, which in
turn gave better yields than the aryl bromides containing electron–donating groups (such as 4–
CH3 or 4–OCH3) [42]. On the other hand, electron–donating groups (such as 4–CH3 or 4–OCH3)
on arylboronic acid gave the best isolated yields of the desired product, whereas electron–
withdrawing group (such as 4–Cl) on the arylboronic acid reduced the yields of the desired
product. The yield with phenyl boronic acid (unsubstituted) is somewhere between the higher
and lower ranges [43].
15

Table 5. Suzuki–Miyaura reaction of aryl bromides with arylboronic acidsa
Complex 1
R1
Br (HO) B
R2
+
R1
R2
Toluene, K2CO3
Reflux 8 h
2
Entry
R1
R2
Yieldb (%)
1
4-COCH3
4-COCH3
4-COCH3
4-COCH3
4-CHO
4-CHO
4-CHO
4-CHO
4-H
4-Cl
88
92
95
97
84
89
93
95
76
83
89
92
69
77
84
88
64
71
78
83
2
4-H
3
4-CH3
4-OCH3
4-Cl
4
5
6
4-H
7
4-CH3
4-OCH3
4-Cl
8
9
10
11
12
13
14
15
16
17
18
19
20
4-H
4-H
4-H
4-CH3
4-OCH3
4-Cl
4-H
4-CH3
4-CH3
4-H
4-CH3
4-CH3
4-OCH3
4-Cl
4-CH3
4-OCH3
4-OCH3
4-OCH3
4-OCH3
4-H
4-CH3
4-OCH3
a
Reaction conditions: Aryl bromide (1.0 mmol), arylboronic acid (1.0 mmol),
complex 1 (0.5 mol%), K2CO3 (2.0 mmol), toluene (10 mL), reflux for 8 h.
b Isolated yield after column chromatography based on arylbromide (average of two
runs).
16

3.4.4. Suzuki–Miyaura cross-coupling reaction of dibromides
Encouraged by these promising results, the protocol was also applied to the double
Suzuki–Miyaura coupling reaction of dibromides with arylboronic acid derivatives to yield
triaryl compounds (Table 6). The trend in the reactivity was similar to that observed as in the
case of monobromo derivatives.
Table 6. Suzuki-Miyaura reaction of dibromobenzenes with arylboronic acidsa
R1
B(OH)2
R1
Complex 1
+
Toluene, K2CO3
Reflux 8 h
Br
Br
R2
R1
R2
R2
Entry
R2
Yieldb (%)
1
2
3
4
5
6
7
8
H
4-Cl
67
74
81
89
60
68
77
82
H
4-H
H
4-CH3
4-OCH3
4-Cl
H
CH3
CH3
CH3
CH3
4-H
4-CH3
4-OCH3
a
Reaction conditions: Dibromide (1.0 mmol), arylboronic acid (2.0 mmol),
complex 1 (1.0 mol%), K2CO3 (2.0 mmol), toluene (10 mL), reflux for 8 h.
b
Isolated yield after column chromatography based on dibromide (average of two
runs).
The ultimate objective in designing the Suzuki–Miyaura coupling catalyst is to form
active species that can catalyze the coupling of the less expensive and much less reactive aryl
chloride substrates. However, no catalytic activity was observed in the coupling of 4ˈ–
17

chloroacetophenone with 4–methoxyphenylboronic acid using complex 1 in toluene/K2CO3
under reflux even after 24 h. This trend is consistent with the increasing strength of carbon–
halide bonds from aryl bromide to aryl chloride. Furthermore, the catalytic efficiency of complex
1 is found to be comparable or even superior to some of the previously reported nickel(II)
catalysts for the Suzuki–Miyaura reaction of arylbromides with arylboronic acids [44-49]. When
compared to nickel, palladium complexes are much more efficient catalysts for this coupling
reaction and excellent conversions have been reported even for the less reactive aryl chlorides at
low catalyst loading [50,51]. Much recent attention in Suzuki–Miyaura cross-coupling has been
focused on the nickel catalysts because nickel is much cheaper and more abundant in earth than
the palladium metal. Therefore, the use of nickel catalysts would be far more cost-effective,
unless a coupling reaction is workable with a very low level of palladium loading, or only with a
very high nickel catalyst loading. On the other hand, the redox state of palladium is typically
Pd(0)/Pd(II), albeit the catalysis chemistry of high valent palladium such as Pd(III) and Pd(IV)
has been increasingly investigated in recent direct C–H functionalization. In contrast, the early
transition metal nickel usually displays Ni(0)/Ni(II) as well as Ni(I)/Ni(III) oxidation states and
is more nucleophilic due to its smaller size. As such, nickel cannot be simply considered to be a
substitute for palladium, it possesses distinctive catalytic properties that palladium does not have.
Indeed, extensive studies have clearly demonstrated that the nickel-based catalysts were more
versatile and powerful catalysts for the C–C, C–N, and C–P bond forming reactions of a diverse
class of electrophiles, which are conventionally challenging in the presence of palladium
catalysts [23,25,52].
18

4. Conclusions
In conclusion, a new octahedral nickel(II) complex bearing monoanionic tridentate
N,N,O- donor benzoylhydrazone ligand was synthesized and characterized by ESI-MS,
analytical, spectral and X–ray diffraction studies. The utility of the complex as an effective
catalyst for the Suzuki–Miyaura cross–coupling reaction has been highlighted by the coupling
reaction of a variety of aryl bromides with various arylboronic acids under optimized reaction
conditions. The biphenyl and triphenyl derivatives were obtained in moderate to excellent
isolated yields.
Appendix A. Supplementary material
Crystallographic data for the structural analysis have been deposited with Cambridge
crystallographic center, CCDC No. 916211. Copies of this may 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 e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data (ESI-MS of HL and 1) associated with this article
can be found in the online version.
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22

Graphical Abstract - Pictogram
Cl
1 (0.5 mol%)
Toluene, K2CO3
N
HO
O
R1
Br
+
B
HO
R2
N
N
R1
R2
N
N
Ni
110 0C, 8 h
O
N
1
Cl
23

Graphical Abstract - Synopsis
The synthesis and characterization of octahedral nickel(II) benzoylhydrazone complex is
described. The complex was used as a potential catalyst for the Suzuki-Miyaura cross-coupling
reaction of different aryl bromides with various arylboronic acids. Under optimized reaction
conditions bi- and triaryls were obtained in moderate to excellent isolated yield.
24

Highlights
1. New Ni(II) benzoylhydrazone complex is synthesised and characterized.
2. The catalytic activity of the complex is explored for the Suzuki-Miyaura reaction.
3. Aryl bromides coupled effectively with boronic acids to form bi- and triaryls.
25