Two mono- and dinuclear Ni(II) complexes constructed from quinazoline-type ligands: Synthesis, X-ray structures, spectroscopic, electrochemical, thermal, and antimicrobial studies

Article abstract of DOI:10.1016/j.poly.2017.04.010

Two mono- and dinuclear Ni(II) complexes, [Ni(L³)₂]·2H₂O (1) (HL³?=?1-(2-{[(E)-3-methoxy-2-hydroxybenzylidene]amino}phenyl)ethanone oxime) and [Ni₂(L⁴)₂(CH₃COO)_2<

Full text of DOI:10.1016/j.poly.2017.04.010

Accepted Manuscript
Two mono- and dinuclear Ni(II) complexes constructed from quinazoline-type
ligands: Synthesis, X-ray structures, spectroscopic, electrochemical, thermal,
and antimicrobial studies
Lan-Qin Chai, Kong-Yan Zhang, Li-Jian Tang, Jian-Yu Zhang, Hong-Song
Zhang
PII:
S0277-5387(17)30279-6
DOI:
http://dx.doi.org/10.1016/j.poly.2017.04.010
POLY 12588
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
28 November 2016
7 April 2017
Please cite this article as: L-Q. Chai, K-Y. Zhang, L-J. Tang, J-Y. Zhang, H-S. Zhang, Two mono- and dinuclear
Ni(II) complexes constructed from quinazoline-type ligands: Synthesis, X-ray structures, spectroscopic,
electrochemical, thermal, and antimicrobial studies, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.
2017.04.010
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Two mono- and dinuclear Ni(II) complexes constructed from
quinazoline-type ligands: Synthesis, X-ray structures, spectroscopic,
electrochemical, thermal, and antimicrobial studies
Lan-Qin Chai a,*, Kong-Yan Zhang a, Li-Jian Tanga, Jian-Yu Zhang a, and Hong-Song Zhang b*
a School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
b Lanzhou University Second Hospital, Lanzhou, Gansu 730030, China
ABSTRACT
Two mono- and dinuclear Ni(II) complexes, [Ni(L3)2]·2H2O (1) (HL3
=
1-(2-{[(E)-3-methoxy-2-hydroxybenzylidene]amino}phenyl)ethanone
oxime)
and
[Ni2(L4)2(CH3COO)2] (2) (HL4 = 1-(2-{[(E)-2-hydroxybenzylidene]amino}phenyl)ethanone
oxime), have been synthesized via complexation of Ni(II) acetate tetrahydrate with HL1 and
HL2 (HL1 = 2-(2-hydroxy-3-methoxyphenyl)-4-methyl-1, 2-dihydroquinazoline 3-oxide, HL2
= 2-(2-hydroxyphenyl)-4-methyl-1, 2-dihydroquinazoline 3-oxide) originally. HL1, HL2, and
two Ni(II) complexes were characterized by IR, UV-Vis, and fluorescence spectroscopy, as
well as by elemental analysis, respectively. The spectroscopic data of two Ni(II) complexes
were compared with the ligands HL1 and HL2. The differently geometric features of the
obtained complexes 1 and 2 are observed by X-ray crystallography. In the crystal structure,
complexes 1 and 2 form an infinite 1-D chain and 2 into 2-D supramolecular frameworks. The
electrochemical properties of 1 and 2 were also investigated by cyclic voltammetry. Moreover,
thermal property of 1 was also studied. In addition, the antimicrobial activity of HL1, HL2,
and complexes 1 and 2 were also investigated.
Keywords: Mono- and dinuclear Ni(II) complexes; Crystal structures; Spectroscopic studies;
Cyclic voltammetry; Antimicrobial activity
*
Corresponding author.
E-mail address: chailanqin@163.com (L.-Q. Chai), jijiuyish@163.com (H.-S. Zhang).
1

1. Introduction
A large variety of organic chelating ligands have been extensively used to synthesize a
great number of the transition metal complexes. Complexes of Schiff bases have attracted
considerable attention due to their important applications in various areas such as catalysis [1],
luminescent probes [2,3], agriculture [4,5], analytical chemistry, dye and polymer industry
[6–8], magneto-structural chemistry [9], agrochemical, and biological fields [10,11]. The
quinazoline ring system along with many alkaloids is a widely recognized moiety in synthesis
and medicinal application [12], such as it was found in HIV reverse transcriptase inhibitors
[13]. Transition metal complexes with quinazoline and analogous ligands are used as
biological models in understanding the structure of biomolecules and biological processes
[14–16]. Meanwhile, acetate ions often play a principal role in coordination chemistry, which
can adopt various binding modes such as terminal monodentate, chelating to one metal center.
Although these complexes were prepared with numerous Schiff base ligands, including their
applications for DNA cleavage and as antimicrobial agents [17–19], reports on Ni(II)
complexes containing a Schiff base ligand derived from a quinazoline-type ligand are scant.
We have recently studied the coordinating ability of transition metal complexes based on
some quinazoline-type ligands with d-block metal such as cobalt, zinc, and copper [20–22],
herein in continuation to our investigations on two mono- and dinuclear Ni(II) complexes,
[Ni(L3)2]·2H2O
(1)
(HL3
=
1-(2-{[(E)-3-methoxy-2-hydroxybenzylidene]amino}phenyl)ethanone
oxime)
and
[Ni2(L4)2(CH3COO)2] (2) (HL4 = 1-(2-{[(E)-2-hydroxybenzylidene]amino}phenyl)ethanone
oxime) are presented with other quinazoline-type ligands HL1 and HL2 (HL1
2-(2-hydroxy-3-methoxyphenyl)-4-methyl-1,2-dihydroquinazoline 3-oxide,
HL2
=
=
2-(2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline 3-oxide) originally (Scheme 1). The
differently geometric features of the obtained complexes 1 and 2 are observed by X-ray
crystallography. HL1, HL2, and two Ni(II) complexes were characterized by IR, UV-Vis, and
fluorescence spectroscopy, as well as by elemental analysis, respectively. The spectroscopic
data of two Ni(II) complexes were compared with the ligands HL1 and HL2. Moreover, the
electrochemical properties of two Ni(II) complexes were investigated by cyclic voltammetry.
In addtion, thermal and antibacterial activities are also studied.
2

2. Experimental
2.1 Materials and Physical Measurements
Salicylaldehyde and 3-methoxysalicylaldehyde were purchased from Heowns Biochem
Technologies LLC Tianjin and used without further purification. The other reagents and
solvents were analytical grade reagents from Tianjin Chemical Reagent Factory. C, H and N
analyses were carried out with a GmbH VariuoEL V3.00 automatic elemental analyzer.
Elemental analysis for Ni was detected by an IRIS ER/S·WP–1 ICP atomic emission
spectrometer. IR spectra (KBr pellets) were recorded on a VERTEX 70 FT–IR
spectrophotometer. UV–Vis absorption spectra were recorded on a Shimadzu UV–2550
spectrometer. The fluorescence spectrum was taken on a 970 CRT spectrofluorometer
(Spectro, Shanghai). Cyclic voltammetry measurements were performed using Chi660 (The
United States CHI) voltammetric analyzer, a three electrode arrangement made up of a glassy
carbon working electrode, a platinum wire auxiliary electrode and a Ag/AgNO3 reference
electrode was used in DMF containing 0.05 mol L–1 tetrabutylammonium perchlorate at scan
rates of 100 mV s–1. Thermal analysis was carried out under nitrogen at a heating rate of
1
10 ℃min–1 using a Perkin TG/DTA analyzer. H and 13C NMR spectra were measured on
Mercury Plus (400 MHz) spectrometers. Chemical shifts of 1H NMR were expressed in parts
13
per million relative to the singlet (δ = 7.26) for CDCl3. Chemical shifts of C NMR nucle
were expressed in parts per million relative to the central line of the triplet (δ = 77.0) for
CDCl3 and TMS as internal standard. MS (ESI) were measured on Bruker Esquire 6000 mass
spectrometer. X-ray single crystal structures were determined on an Agilent SuperNova Dual
area detector diffractometer.
2.2 Synthesis and structural characterization of HL1 and HL2
2-(2-hydroxy-3-methoxyphenyl)-4-methyl-1, 2-dihydroquinazoline 3-oxide (HL1) was
synthesized by modification of the reported method [20–23]. To an ethanol solution (5 mL) of
3-methoxysalicylaldehyde (152.1 mg, 1.0 mmol) was added an ethanol solution (4 mL) of
(E)-1-(2-aminophenyl) ethanone oxime (150.2 mg, 1.0 mmol). After stirring at 55–60°C for
12 h, the mixture was filtered and precipitates were collected on a suction filter to give yellow
crystalline solid (215.4 mg). Yield: 76.2 %. m.p. 196–197°C. The identidy and purity of the
1
1
product was confirmed by H and 13C NMR spectroscopic analysis. H NMR (400 MHz,
CDCl3): 11.81 (s, 1H, OH), 7.31 – 7.21 (m, 2H, CHatom), 7.94 – 6.84 (m, 2H, CHatom), 6.76 –
6.74 (m, 2H, CHatom), 6.65 – 6.64 (d, 1H, CHatom), 6.61 (t, 1H, CHatom), 6.35 (d, J = 2.4 Hz,
1H, CH), 5.42 (d, J = 7.9 Hz, 1H, NH), 3.83 (s, 3H, OCH3), 2.40 (s, 3H, CH3). 13C NMR (100
3

MHz, CDCl3): δ = 149.3, 145.5, 142.5, 139.7, 131.9, 125.7, 125.4, 119.7, 118.9, 118.1, 115.6,
114.4, 112.6, 74.7, 56.0, 12.4 ppm. Anal. Calcd for C16H16N2O3 (Mw 284.12 ) (%): C, 67.59;
H, 5.67; N, 9.85. Found (%): C 67.56; H 5.72; N 9.89.
2-(2-hydroxyphenyl)-4-methyl-1, 2-dihydroquinazoline 3-oxide (HL2) was synthesized
using similar procedure as for HL1 except for using salicylaldehyde (122.1 mg, 1.0 mmol)
1
instead of 3-methoxysalicylaldehyde. Yield, 74.6 %. m.p. 212–213℃. H NMR (400MHz,
CDCl3): 11.04 (br, 1H, OH), 7.36 – 7.32 (m, 1H, CHatom), 7.27 – 7.16 (m, 3H, CHatom), 7.00 –
3
6.90 (m, 3H, CHatom), 6.72 – 6.67 (m, 1H, CHatom), 6.39 (d, 1H, J HH = 3.2Hz, NH), 4.97 (d,
1H, 3J HH = 2.4Hz, CH), 2.43 (s, 3H, CH3). ESI-MS: calcd for [M+H]+ 255.11, found 255.20.
Anal. Calcd for C15H14N2O2 (Mw 254.28) (%): C 70.85; H 5.55; N 11.02. Found (%): C 70.90;
H 5.59; N 11.07.
2.3 Synthesis of the Ni(II) complexes 1 and 2
To a stirred solution of HL1 (5.71 mg, 0.02 mmol) in ethyl acetate (4 mL) was added
dropwise to a solution of Ni(II) acetate tetrahydrate (2.51 mg, 0.01 mmol) in methanol (2 mL)
at room temperature. The color of the mixing solution turned to light green immediately. The
solvent was partially evaporated for two weeks at ambient temperature and several needle-like
single crystals suitable for X-ray diffraction studies were obtained. Complex 2 was prepared
using similar procedure as for 1 except for using HL2 (5.24 mg, 0.02 mmol) instead of HL1.
1, clear brown crystals, Yield: 73.2 %. Anal. Calcd. for C32H34N4NiO8 [Ni(L3)2]·2H2O
(Mw 661.34) (%): C, 58.12; H, 5.18; N, 8.47; Ni, 8.88. Found (%): C, 58.16; H, 5.22; N, 8.51;
Ni, 8.93.
2, clear green crystals, Yield: 73.6 %. Anal. Calcd. for C34H32N4Ni2O8
[Ni2(L4)2(CH3COO)2] (Mw 742.05) (%): C, 55.03; H, 4.35; N, 7.55; Ni, 15.82. Found (%): C,
55.07; H, 4.40; N, 7.59; Ni, 15.87.
2.4 X-ray Crystallography
X-ray diffraction measurement for complexes 1 and 2 were collected on an Agilent
SuperNova X-ray diffractometer equipped with graphite monochromated Mo-Kα radiation (λ
= 0.71073 Å) at 294(10) K. Data reduction and cell parameters refinement were performed
using OLEX2 [24]. The structures were solved by using Fourier difference techniques and
refined by full-matrix least square methods on F2 data [25, 26]. The non-hydrogen atoms were
generated anisotropically. All hydrogen atoms were assigned geometrically and positioned
4

isotropically. The detailed crystallographic data and structure refinement parameters of
complexes 1 and 2 are listed in Table 1 for comparison.
Table 1. X-ray Crystallographic data and structure refinement parameters for two complexes.
[Ni(L3)2]·2H2O
C32H34N4NiO8
661.34
[Ni2(L4)2(CH3COO)2]
C34H32N4Ni2O8
742.05
Complex
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
293.64(10)
0.71073
293.75(10)
1.54184
monoclinic
I2/c
orthorhombic
Pbca
Unit cell dimensions (Å, º)
a
16.2557(8)
11.4038(4)
16.7725(9)
102.757(5)
3032.5(3), 4
1.449
15.065(2)
13.0058(12)
16.4554(13)
90
b
c
β
Volume (Å3), Z
Calculated density (Mg /m 3)
Absorption coefficient (mm– 1)
F(000)
3224.1(6), 4
1.529
0.699
1.950
1384
1536
Crystal size (mm3)
θ Range for data collection (°)
Index ranges
0.35 × 0.33 × 0.31
3.526 – 25.991
–18 ≤ h ≤ 20;
–14 ≤ k ≤ 8;
–13 ≤ l ≤ 20
2450
0.35 × 0.32 × 0.31
6.091 – 63.822
–14 ≤ h ≤ 18;
–15 ≤ k ≤ 7;
–15 ≤ l ≤ 19
1344
Reflections collected
Independent reflections
Completeness to θ = 26.32 /66.97(%)
Data/restraints/parameters
Goodness-of-fit on F2
2992 [R(int) = 0.0389]
3002 [R(int) = 0.0591]
99.8
99.7
5838 / 0 / 213
7736 / 0 / 223
1.050
1.020
Final R indices [I >2σ(I)]
R indices (all data)
Largest difference peak and hole (e Å-3) 0.30 and –0.31
R1 = 0.0445, wR2 = 0.1027a R1 = 0.0406, wR2 = 0.1195b
R1 = 0.0577, wR2 = 0.1126 R1 = 0.0878, wR2 = 0.1312
0.54 and –0.64
2
2
aw = 1 / [σ2 (Fo ) + (0.0414P)2 + 2.9724P] , bw = 1 / [σ2 (Fo ) + (0.1337P)2], Where P = (Fo2 + 2Fc2) / 3.
3. Results and discussion
As shown in Scheme 1, two mono- and dinuclear Ni(II) complexes, [Ni(L3)2]·2H2O (1)
and [Ni2(L4)2(CH3COO)2] (2), formed unexpectedly in the reaction of HL1 and HL2 with
Ni(II) acetate tetrahydrate. In the process of complexation, HL1 and HL2 show a C–N bond,
which are converted into a C=N–OH group in the ligands HL3 and HL4 after the reaction. The
crystal structures of the complexes confirm the change. Dinuclear Ni(II) complex was
synthesized according to the following reaction: 2HL4
+
2Ni(CH3COO)2
→
5

Ni2(L4)2(CH3COO)2 + 2H+ + 2CH3COO–. The result suggested that in the metal complex the
Schiff base configuration is more stable than that of quinazoline configuration [27].
Complexes 1 and 2 display very good stability in air at room temperature. They are soluble in
polaraprotic solvents such as DMF, DMSO and MeCN, slightly soluble in water, methanol,
ethanol, ethyl acetate and chloroform, and insoluble in n-hexane, diethyl ether and petroleum
ether.
Scheme 1. Synthetic route of the Ni(II) complexes 1 and 2.
3.1 Description of the cystal structures
3.1.1 X-ray cystal structure of 1
The analyses of [Ni(L3)2]·2H2O coordination compound indicate that the ligand to metal
ratio is 2:1. X-ray single-crystal analysis reveals that 1 crystallizes in the monoclinic system
with I2/c space group. The crystal structure and coordination configuration of 1 were
illustrated in Fig. 1. Selected bond distances and angles are presented in Table 2. Complex 1
is mononuclear structure with two non-coordinated water molecules. The coordination
geometry around Ni(II) can be described as distorted octahedral geometry, which is evident
from the cisoid and transoid angles around Ni(II) ions, being in the range 80.04–101.07º and
165.56–170.79º, respectively [28]. The hexa-coordination environment is maintained by two
imino nitrogens (N1, N1), two Schiff base nitrogens (N2, N2), two deprotonated phenolic
oxygens (O2, O2), with the plane defined by (N2, N2, O2, O2) as the cis-N2O2 basal plane
[29]. Due to the distance of Ni(II) to the basal plane is 0 Å, N2, N2, O2, O2 and Ni(II) can be
considered as coplanar (plane equation: 0.9387(3) x – 0.3449 (7) z = – 8.1705 (53))
symmetry.
6

Fig. 1. Intramolecular and intermolecular hydrogen bonds (left) and coordination configuration (right) of 1
(Hydrogen atoms are omitted for clarity except those forming hydrogen bonds). Displacement ellipsoids for
non-H atoms are drawn at 30 % probability level.
Table 2. Selected bond distances (Å) and angles (º) in 1.
Bond
Lengths
2.0253(17) Ni(1)–N(1)
Angles Bond
Bond
Lengths
2.150(2)
Angles
Bond
Lengths
2.052(2)
Angles
Ni(1)–O(2)
Bond
Ni(1)–N(2)
Bond
O(2)–Ni(1)–O(2) 85.31(10)
O(2)–Ni(1)–N(2) 88.08(8)
N(2)–Ni(1)–N(1) 80.04(8)
O(2)–Ni(1)–N(1) 88.26(8)
O(2)–Ni(1)–N(2) 165.56(8) N(1)–Ni(1)–N(1) 170.79(11)
N(2)–Ni(1)–N(1) 94.07(8) N(2)–Ni(1)–N(2) 101.07(12)
O(2)–Ni(1)–N(1) 95.54(8)
There are one pair of intramolecular O1−H1···O2 and four pairs of intermolecular
O4−H4A···O2, O4−H4A···O3, O4−H4B···O2 and O1−H1···O4 hydrogen bonds between 1
and two non-coordinated water molecules (Fig. 1, Table 3). Two five and six-membered rings
can be constructed among four pairs of intermolecular hydrogen bonds. As shown in Fig. 2,
the distances of π···π stacking interactions between the neighboring benzene ring centroid for
Cg5−Cg5 and Cg6−Cg6 ring is 3.8497 (18) Å and 3.8336 (17) Å, respectively (Table 4),
which play a principal role in stabilizing the crystal structure of [Ni(L3)2·2H2O]. With the
help of π···π stacking interactions, adjacent moleculars can be linked together to display an
infinite 1-D chain supramolecular structure along the c-axis.
Table 3. Hydrogen bonding distances (Å) and angles (º) in 1.
D–H···A
d(D–H) d(H···A) d(D···A) ∠DHA Symmetry code
0.82(3) 2.53(3) 2.990(3) 117(3)
0.82(3) 1.94(3) 2.727(3) 161(3)
1–x, y, 1/2–z
x, y, z
O1−H1···O2
O1−H1···O4
O4−H4A···O2
O4−H4A···O3
O4−H4B···O2
0.85
0.85
0.85
2.56
2.12
1.90
2.950(3)
2.927(4)
2.728(3)
109
157
164
1–x, y, 1/2–z
1–x, y, 1/2–z
x, y, z
7

Table 4. π···π stacking interactions (Å, º) in the crystalline state of 1.
e
f
αb
βc
γd
π-π
Cg5···Cg5g 3.8497(18) 9.1 28.68 28.68 3.377 3.377
Cg6···Cg6h 3.8336(17)
28.29 28.29 3.376 3.376
Cg···Cga
CgIPerp CgJPerp Symmetry code
1–x, y, 1/2–z
0
1–x, 1–y, 1–z
a Cg···Cg is the distance between ring centroids. b α = dihedral angle between planes I and J.
e
c Where β is the angle Cg(I)···Cg(J). d γ = angle between CgI···CgJ and CgJPerp
.
CgIPerp is the
f
perpendicular distance of Cg(I) on ring J. CgJPerp is the perpendicular distance of Cg(J) on ring I.
g Cg5 is the centroids for benzene ring C3–C8, h Cg6 is the centroids for benzene ring C10–C15.
Fig. 2. π···π stacking interactions and an infinite 1-D chain supramolecular structure of 1 along the
c-axis.
3.2.2 X-ray crystal structures of 2
X-ray crystal structure analysis reveals that 2 forms a 2:2 ligand to metal stoichiometry,
and crystallizes in the orthorhombic system with Pbca space group and consists of two Ni(II)
ions, two (L4)− molecules, and two acetate anions participating coordination. Fig. 3 shows the
molecular structure of [Ni2(L4)2(CH3COO)2] and coordination configuration. Table 5 contains
selected bond parameters. The overall coordination geometry around Ni(II) is distorted
octahedral. The coordination environment around Ni(II) comprises one Schiff base nitrogen
(N1), one imine nitrogen (N2), two deprotonated phenolic oxygens (O1, O1) and two oxygens
(O3, O4) from acetate anion. Thus two deprotonated phenolic oxygens (O1, O1) serve as a
8

bridge to link two Ni (II) centers. The sets of four donor atoms O1, O1, O4 and N1 describe
the basal plane of Ni1 and the apical positions are occupied by N2 and O3. Ni1 atom is
displaced 0.200 Å above the basal plane, which means Ni1 and four donor atoms are
approximately coplanar. The Ni···Ni distance in the unit is 3.086 Å. The Ni–NSchiff base bond
length is 2.005(7) Å and the Ni–Nimine bond length is 2.081(7) Å. The Ni–O (phenolate) bond
length of 2.001(5) Å and 2.055(5) Å are in accordance with the reported values [30, 31].
Fig. 3. Intramolecular hydrogen bonds and coordination configuration of 2 (hydrogen atoms are omitted for
clarity except those forming hydrogen bonds). Displacement ellipsoids for non-H atoms are drawn at 30 %
probability level.
Table 5. Selected bond distances (Å) and angles (º) in 2.
Bond
Lengths Bond
Lengths Bond
Lengths
2.112(5)
2.081(7)
Angles
Ni(1)–O(1)
Ni(1)–O(4)
Bond
2.001(5) Ni(1)–O(1)
2.141(6) Ni(1)–N(1)
2.055(5) Ni(1)–O(3)
2.005(7) Ni(1)–N(2)
Angles
Bond
Angles
Bond
O(1)–Ni(1)–O(1) 80.9(3) O(1)–Ni(1)–O(3) 91.1(2) O(1)–Ni(1)–O(3) 87.9(2)
O(1)–Ni(1)–O(4) 92.2(2) O(1)–Ni(1)–O(4) 152.4(2) O(1)–Ni(1)–N(1) 91.1(2)
O(1)–Ni(1)–N(2) 94.5(3) O(1)–Ni(1)–N(2) 101.8(3) O(3)–Ni(1)–O(4) 61.8(2)
N(1)–Ni(1)–O(1) 170.0(3) N(1)–Ni(1)–O(3) 98.4(2) N(1)–Ni(1)–O(4) 97.7(3)
N(1)–Ni(1)–N(2) 81.1(3) N(2)–Ni(1)–O(3) 167.1(3) N(2)–Ni(1)–O(4) 105.4(3)
Ni(1)–O(1)–Ni(1) 99.1(3) C(1)–O(1)–Ni(1)
C(16)–O(3)–Ni(1) 88.9(5) C(16)–O(4)–Ni(1) 87.7(5) C(7)–N(1)–Ni(1)
C(8)–N(1)–Ni(1)
122.4(4) C(1)–O(1)–Ni(1)
131.4(5)
123.8(5)
116.6(6) O(2)–N(2)–Ni(1) 122.7(6) C(14)–N(2)–Ni(1) 126.6(5)
In the crystal structure of 2, two pairs of intramolecular O2−H2···O3 and
C15−H15C···O2 hydrogen bonds form two five-membered rings (Fig. 3, Table 6). There is
only one pair of intermolecular C15−H15A···O3 hydrogen bonds. With the help of
intermolecular hydrogen bonds interactions, adjacent molecular units are linked together into
9

an infinite 2-D supramolecular structure along the bc-plane (Fig. 4).
Table 6. Hydrogen bonding distances (Å) and angles (º) for 2.
D–H···A
d(D–H) d(H···A) d(D···A) ∠DHA
Symmetry code
– x, 1–y, –z
x, y, z
0.8(2)
0.96
2.1(2) 2.691(8)
132(–1)
100
O2−H2···O3
C15−H15C···O2
C15−H15A···O3
2.35
2.58
2.687(12)
3.369(11)
0.96
139
x, 1/2–y, 1/2+z
Fig. 4. View of the 2-D supramolecular structure of 2 along the bc-plane.
3.3 FT-IR spectra
The main FT-IR spectra for HL1, HL2, complexes 1 and 2 in the 400–4000 cm−1 region
are shown as Supporting information (Fig. S1). The characteristic C=N stretches of HL1 and
HL2 appers at 1613 and 1615 cm−1, while C=N of 1 and 2 are observed at 1609 and 1622
cm−1 upon complexation, indicating the coordination of metal ions with oxime nitrogens [32,
33]. The Ar–O stretch is a strong band, occurring at 1230, 1236 cm−1 for HL1, HL2 and at
1242, 1297 cm−1 for the complexes 1 and 2. The Ar–O stretching frequency shifts higher,
indicating that Ni–O bonds are formed between Ni (II) and oxygen of the phenol [34]. HL1
and HL2 show unexpected strong absorption due to ν(N→O) at 1260 and 1316 cm−1, which
disappear in the complexes, indicating the C–N bond has broken and oxime N participated in
10

coordination. The N–H in the ligand at 2932 and 2956 cm−1 disappears in the complex,
indicating nitrogen coordinated to Ni (II). The missing –NH– vibration in the spectra of the
complexes is a consequence of the formation of the –N=C– bond. The band at 3263, 3367
cm–1 is due to the ν(O–H) stretching vibration of HL1, HL2, while at 3508, 3182 cm−1 for
complexes 1 and 2, which is the evidence of O–H group from oxime [35]. The far-infrared
spectra of 1 and 2 are obtained from 100 to 500 cm−1 to identify frequencies due to Ni–O and
Ni–N bonds. The FT-IR spectra show ν(Ni–N) and ν(Ni–O) at 557, 422 cm−1 and 514, 458
cm−1, respectively. These results conform to the structural data obtained by X-ray
crystallography.
3.4 UV–vis absorption spectra
UV–Vis aborption spectra of HL1, HL2 and two Ni(II) complexes were determined in
5×10–5 mol L–1 CH2Cl2 solution. Absorptions of complexes 1 and 2 were obviously different
from those of the ligands as shown in Fig. 5, the spectra of the ligands exhibit bands at 235
and 263 or 261 nm (ε = 1,9992, 1,2418 or 1,2116 mol–1 L cm–1), attributed to the π → π*
transition of benzene rings [36, 37]. Similarly, π → π* transitions are observed at 240 nm (ε =
1,9942 mol–1 L cm–1) in the complex 1 and 242 nm (ε = 1,9966 mol–1 L cm–1) in the complex
2. Both the ligands show the same bands at 298 nm (ε = 6,744 mol–1 L cm–1) were assigned to
intra-ligand π → π* transitions of the C=N bonds. Absorptions near 363 nm (ε = 3,429 mol–1
L cm–1) attributed to the n → π* transition associated with azomethine red shift on
complexation, indicating coordination of azomethine nitrogen [38]. The new absorption peaks
observed at 414 nm (ε = 2,158 mol–1 L cm–1) for 1 and at 391 nm (ε = 3,906 mol–1 L cm–1) for
2 were assigned for ligand-to-metal charge transfer (LMCT) transitions and showed
bathochromic shift compared to the ligands [39]. These observations were in line with
common spectral features of d-block metal due to the Ni(II) complex in an octahedral
environment with 3d8 electronic configurationtion.
11

(a)
(b)
Fig. 5. UV–vis absorption spectra: HL1 and 1 (a) HL2 and 2 (b) in CH2Cl2 (5 × 10–5 mol L–1).
3.5 Fluorescence spectra
Fluorescent properties of HL1, HL2 and two Ni(II) complexes were investigated at room
temperature (298 K) (Fig. 6). Both of the ligands and Ni (II) complexes showed broad
emission bands indicating charge transfer nature of the transitions. HL1 and HL2 exhibit
intense emission peaks at 380 and 405 nm upon excitation at 350 nm, which displayed
intraligand π → π* transition due to the presence of a lone pair of electrons of the donor
atoms in the ligand (N, O-donor) [40, 41]. Complexes 1 and 2 show intense broad
photoluminescence with maximum emission at ca. 432, 490 nm, and 421, 498 nm,
respectively upon excitation at 350 nm. In the complexes, the π → π* intramolecular electron
transitions exhibited significant red shift compared to the ligands and this indicates
coordination of nitrogen and oxygen atoms to nickel ion [42, 38]. Furthermore, the maximum
wavelength of the fluorescence emission and the fluorescence excitation spectra showed that
complexes 1 and 2 had larger Stokes shift [32].
12

(a)
(b)
Fig. 6. Emission spectra: HL1 and 1 (a) HL2 and 2 (b) in CH2Cl2 at room temperature (5 × 10–5 mol L–1).
3.6 Electrochemistry studies
The electrochemical property of complexes 1 and 2 have been investigated in DMF
containing 0.05 mol L–1 tetrabutylammonium perchlorate over the potential range from –3.2
to 1.0 V (vs. Ag/AgNO3) using cyclic voltammetry techniques with the scanning rate of 100
mV s–1 [43, 44]. All solutions were nitrogen-purged prior to experiment. As it observed in Fig.
7, complex 1 exhibits one reversible redox wave, which are assigned to consecutive
Ni(II)/Ni(I) redox processes [45, 46]. The oxidation peak was observed at Epa1 = 0.6884 V
with corresponding reduction peak at Epc1 = –2.2072 V and the average formal potential [E1/2
= (Epa1 + Epc1)/2] is –0.7594 V. The peak-to-peak separation between the anodic and
cathodic (△Ep1) is 2.896 V and the proportion of the peak current (ipa1/ipc1) is 0.928. The
cyclovoltammogram of complex 2 displays a oxidation peak at Epa2 = –0.0704 V with
corresponding reduction peak at Epc2 = –1.3426 V corresponding to the Ni(II)/Ni(I) couple.
The average formal potential [E1/2 = (Epa2 + Epc2)/2] is –0.7065 V. The peak separation
13

(△Ep2) is 1.272 V and the proportion of the peak current (ipa2/ipc2) is 1.209 V. The separation
in peak potentials higher than △Ep = 0.059/n V assigned to the quasi-reversibility for the
Ni(II)/Ni(I) redox process of complex 2, which is in good agreement with previously reported
analogues [47, 48]. Conceptually the reduction of electron density on the central metal ions by
coordinated moieties reduces the metal ion at less negative potential and vice versa [49].
Hence the Ni(II) ion in complex 1 reduces at relatively highest negative potential (Epc1 =
−2.2072 V) due to the presence of the electron donating 3-methoxy group in the ligand of the
complex 1 as compared to complex 2 (Epc2 = −1.3426V) with two coordinated acetate group
[49, 50].
Fig. 7. Cyclic voltammogram of complexes 1 and 2 in DMF solution. Scan rate 100 mV s–1.
3.7 Thermal studies
Thermogravimetric (TG) analysis data of 1 was collected in the temperature range from
30 to 800°C with the heating rates of 10°C min–1 under a static nitrogen atmosphere. The
TGA curve of 1 was presented as Supporting information (Fig. S2). Thermal decomposition
process of 1 took place in three steps. The first weight loss in the temperature range
114.8–219.8 ℃ was attributed to loss of two water molecules with mass of 6.67 % (calculated
5.47 %). The second step of thermal decomposition from 219.8 to 286.1 ℃ corresponds to
loss of Schiff base ligand with mass loss of 17.40 % (calculated 16.37 %). The last stage of
decomposition at 286.1–786.6 ℃ was assigned to the removal of partial pyrolysis of the rest
of the organic component leaving nickel oxide as the final residue with mass loss of 32.08 %
(calculated 28.71 %) [51,52].
3.8 Antimicobial activity
The antimicrobial activity of HL1, HL2 and two Ni (II) complexes were tested against
14

Escherichia coli as Gram-negative bacteria and Staphylococcus aureus as Gram-positive
bacteria by the disc diffusion method. The process culture of bacteria used Sabouraud
dextrose agar. The agar was poured into petri dishes and microorganisms were introduced on
the surface of agar plates individually. The plates were placed on a leveled surface and
incubated for 24 h at 37°C, soon afterwards the diameters of the inhibition zones were
measured. The blank sterile discs measuring 5 mm in diameter were dissolved in dimethyl
sulfoxide (DMSO) and soaked in a known concentration of the test compounds, such as the
concentrations were 0.3, 0.625, 1.25, 2.5 and 5.0 mg mL–1. The obtained results were
compared with Amoxycilin as reference standards with different concentrations [53].
The diameter of inhibition zones of HL1, HL2 and two Ni (II) complexes are given in
Table 7, complexes 1 and 2 demonstrated enhanced more antimicrobial activities than
quinazoline-type ligands under the same conditions and the ligands have a weak biological
activity [54]. As observed in Table 7, this increase in the antibacterial of the metal complex
with increase in concentration can be explained on the basis of the chelation theory. Chelation
reduces the polarity of the metal atom mainly due to partial share of positive charge of metal
atom with donor groups and possible delocalization of π-electron within the whole chelate
ring. Further, it enhances the lipophilic character of the central atom. These observations are
similar to earlier reports of biological activities of similar Schiff base complexes of nickel (II)
[9,55,56].
Table 7. Antimicrobial activities of HL1, HL2 and two Ni(II) complexes
Inhibition Zone concentration (mm)
E.coli
S.aureus
Compoud
HL1
0.3
6
0.625 1.25
2.5
8
5.0
9
0.3
7
0.625
7
1.25
8
2.5
9
5.0
10
11
14
15
20
6
6
8
7
HL2
6
8
10
11
13
17
8
9
9
10
12
13
18
Complex 1
Complex 2
Amoxycilin
8
8
9
10
12
16
9
9
10
13
17
9
10
13
10
15
10
14
11
15
12
4. Conclusion
Two
mono- and dinuclear Ni(II) complexes, [Ni(L3)2]·2H2O (1) and
[Ni2(L4)2(CH3COO)2] (2), have been synthesized via complexation of Ni(II) acetate
15

tetrahydrate with HL1 and HL2 originally. HL1, HL2, and two Ni(II) complexes were
characterized by IR, UV-Vis, and fluorescence spectroscopy, as well as by elemental analysis,
respectively. The differently geometric features of the obtained complexes 1 and 2 are
observed by X-ray crystallography. 1 has a 2:1 ligand-to-metal ratio whereas 2 froms a 2:2
ligand to metal stoichiometry. In the crystal structure, complexes 1 and 2 form an infinite 1-D
chain and 2 into 2-D supramolecular frameworks. HL1, HL2 and Ni(II) complexes display
intraligand π → π* transition fluorescence. Moreover, electrochemical properties of
complexes 1 and 2 were measured by cyclic voltammetry. In addition, the activity data
showed the metal complexes to be more antibacterial than the quinazoline-type ligands.
Appendix A. Supplementary data
CCDC 1519513 and 1519512 contain the supplementary crystallographic data for
complexes
1
and 2. These 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 e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
We are thankful for the financial support by the Fundamental Research Funds for the
Gansu Province Universities (No. 214152) and the Natural Science Foundation of Gansu
Province (No. 1508RJZA082).
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19

Graphical Abstract
Two mono- and dinuclear Ni(II) complexes have been synthesized via complexation of Ni(II)
acetate tetrahydrate with HL1 and HL2. The spectroscopic data of two Ni(II) complexes were
compared with the ligands HL1 and HL2. The differently geometric features of the obtained
complexes 1 and 2 are observed by X-ray crystallography. In the crystal structure, complexes
1 and 2 form an infinite 1-D chain and 2 into 2-D supramolecular frameworks. The
electrochemical properties of 1 and 2 were also investigated by cyclic voltammetry. Moreover,
thermal property of 1 was also studied. In addition, the antimicrobial activity of HL1, HL2,
and complexes 1 and 2 were also investigated.
Graphical Abstract
20