Synthesis, crystal structure, fluorescence, electrochemical property, and SOD-like activity of an unexpected nickel(II) complex with a quinazoline-type ligand

Article abstract of DOI:10.1080/00958972.2013.857016

An unexpected mononuclear Ni(II) complex, [Ni(L²) ₂]·CH₃OH (HL² = 1-(2-{[(E)-3,5-dichloro- 2-hydroxybenzylidene]amino}phenyl)ethanone oxime), has been synthesized via complexation of Ni(II) acetate tetrahydrate with HL¹. HL¹ and its corresponding Ni(II) complex were characterized by IR, ¹H-NMR spectra, HRMS, as well as by elemental analysis, UV-vis, and emission spectroscopy. The crystal structure of the complex has been determined by single-crystal X-ray diffraction. Each complex links two other molecules into an infinite 1-D chain via intermolecular hydrogen bonds. Moreover, the electrochemical properties of the nickel complex were studied by cyclic voltammetry. Superoxide dismutase-like activities of HL¹ and Ni(II) complex were also investigated.

Full text of DOI:10.1080/00958972.2013.857016

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Synthesis, crystal structure,
fluorescence, electrochemical
property, and SOD-like activity of an
unexpected nickel(II) complex with a
quinazoline-type ligand
Lan-Qin Chaia, Gang Liua, Yu-Li Zhanga, Jiao-Jiao Huanga & Jun-
Feng Tongb
a School of Chemical and Biological Engineering, Lanzhou Jiaotong
University, Lanzhou, P.R. China
b Key Laboratory of Opto-Electronic Technology and Intelligent
Control, Lanzhou Jiaotong University, Ministry of Education,
Lanzhou, P.R. China
Accepted author version posted online: 21 Oct 2013.Published
online: 26 Nov 2013.
To cite this article: Lan-Qin Chai, Gang Liu, Yu-Li Zhang, Jiao-Jiao Huang & Jun-Feng Tong (2013)
Synthesis, crystal structure, fluorescence, electrochemical property, and SOD-like activity of an
unexpected nickel(II) complex with a quinazoline-type ligand, Journal of Coordination Chemistry,
66:22, 3926-3938, DOI: 10.1080/00958972.2013.857016
To link to this article: http://dx.doi.org/10.1080/00958972.2013.857016
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 22, 3926–3938, http://dx.doi.org/10.1080/00958972.2013.857016
Synthesis, crystal structure, fluorescence, electrochemical
property, and SOD-like activity of an unexpected nickel(II)
complex with a quinazoline-type ligand
LAN-QIN CHAI*†, GANG LIU†, YU-LI ZHANG†, JIAO-JIAO HUANG† and
JUN-FENG TONG‡
†School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, P.R. China
‡Key Laboratory of Opto-Electronic Technology and Intelligent Control, Lanzhou Jiaotong
University, Ministry of Education, Lanzhou, P.R. China
(Received 17 May 2013; accepted 8 October 2013)
An unexpected mononuclear Ni(II) complex, [Ni(L2)2]·CH3OH (HL2 = 1-(2-{[(E)-3,5-dichloro-2-
hydroxybenzylidene]amino}phenyl)ethanone oxime), has been synthesized via complexation of Ni
(II) acetate tetrahydrate with HL1. HL1 and its corresponding Ni(II) complex were characterized by
1
IR, H-NMR spectra, HRMS, as well as by elemental analysis, UV–vis, and emission spectroscopy.
The crystal structure of the complex has been determined by single-crystal X-ray diffraction. Each
complex links two other molecules into an infinite 1-D chain via intermolecular hydrogen bonds.
Moreover, the electrochemical properties of the nickel complex were studied by cyclic voltammetry.
Superoxide dismutase-like activities of HL1 and Ni(II) complex were also investigated.
Keywords: Quinazoline-type ligand; Crystal structure; Fluorescence behavior; Cyclic voltammetry;
Superoxide dismutase activity
1. Introduction
Oxime-type compounds are versatile ligands in coordination chemistry. A number of
complexes with oxime-type ligands have been reported with interesting structures and
potential applications [1]. For example, Dilber et al. [2] report a new macrobicyclic
vic-dioxime and macrobicyclic groups might have selective ion binding properties and
could be used for remediation of heavy metal pollution or recovery of precious metals from
heavy metal wastes. The quinazoline ring system along with many alkaloids is widely
recognized in inorganic syntheses and medicinal applications [3], for example, in HIV
reverse transcriptase inhibitors [4]. Some of them or their metal complexes are used as
biological models in understanding the structure of biomolecules and biological processes
[5–7]. Such compounds have become the center of creating new drugs, some with highly
active compounds have been commercialized such as fungicide fuquinconazole, anti-cancer
drug, antihypertensive pyrazosin, etc. [8–10]. Some transition metal complexes with
quinazoline analogs have been investigated in coordination chemistry and biological
*Corresponding author. Email: dongwk@mail.lzjtu.cn
© 2013 Taylor & Francis

Nickel(II) with quinazoline-type ligand
3927
chemistry, although much remains to be understood [11, 12]. Nickel(II) complexes with
oxime-type ligands have been widely investigated in coordination chemistry and biological
chemistry [2, 13–15], but cleavage of two C–N bonds of Schiff bases have not been
observed in reaction with metal salts.
In this paper, we report the synthesis, structural characterization, and fluorescence of a
mononuclear Ni(II) complex, [Ni(L2)2]·CH3OH, which is an unexpected mononuclear Ni
(II) complex possessing a Schiff base-type Ni–N4O2 instead of an anticipated quinazoline-
type Ni–N4O2 complex. Fluorescence of the Ni(II) complex in DMF solution is also investi-
gated; electrochemistry and superoxide dismutase (SOD) activity are also described.
2. Experimental
2.1. Materials and physical measurements
All chemicals were of analytical reagent grade from Tianjin Chemical Reagent Factory and
used without purification. C, H, and N analyzes were obtained using a GmbH VarioEL
V3.00 automatic elemental analysis instrument. Elemental analysis for Ni was determined
by an IRIS ER/S·WP-1 ICP atomic emission spectrometer. IR spectra were recorded on a
Vertex70 and Nicolet Instrument Corporation NEXUS 670 FT-IR spectrophotometer, with
samples prepared as KBr (500–4000 cm−1) and CsI (100–500 cm−1) pellets. 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 voltam-
metry measurements were performed using a 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/AgCl reference electrode was used in DMF
1
containing 0.05 M L−1 tetrabutylammonium perchlorate at scan rates of 100 mV s−1. H
NMR spectra were determined on a Mercury plus (400 or 300 MHz) instrument using
CDCl3 as solvent and TMS as internal standard. MS (ESI) were measured on a Bruker
Esquire 6000 mass spectrometer. Single-crystal X-ray structure determination was carried
out on a Bruker Smart 1000 CCD area detector diffractometer. Melting points were
obtained by use of a microscopic melting point apparatus made by Beijing Taike Instrument
Limited Company and are uncorrected.
2.2. Preparation of HL1 and its complex
2.2.1. Synthesis and structural characterization of HL1. The synthetic route of quinazo-
line-type ligand (HL1) is shown in scheme 1.
(E)-1-(2-Aminophenyl)ethanone oxime was synthesized as white powder according to
an analogous method reported earlier [16, 17]. Yield 94.7%; m.p. 106–107 °C. Anal. Calcd
for C8H10N2O (Mw 150.18) (%): C, 63.98; H, 6.71; N, 18.65. Found (%): C, 64.24; H,
6.58; N, 18.83. IR (KBr) ν cm−1: 3389 (N–OH), 3248 (N–H), 3133 (Ar–H), 1602 (C=N).
1H NMR (300 MHz, CDCl3): 11.30 (s, 1H, NOH), 7.63–7.25 (m, 4H, ArH), 5.82 (s, 2H,
NH2), 2.56 (s, 3H, CH3).
2-(3,5-Dichloro-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline 3-oxide: To an
ethanol solution (4 mL) of 3,5-dichloro-2-hydroxybenzaldehyde (0.1909 g, 1.0 mM) was
added an ethanol solution (4 mL) of (E)-1-(2-aminophenyl) ethanone oxime (0.1502 g,

3928
L.-Q. Chai et al.
Scheme 1. Synthetic route of HL1.
1.0 mM). After stirring at 55–60 °C for 12 h, the mixture was filtered, precipitates were col-
lected on a suction filter to afford HL1 (0.2315 g, 71.3%) as pale yellow powder [16]. m.p.
1
217–218 °C. H NMR (400 MHz, CDCl3) δ 13.59 (br, 1H, OH), 7.39 (dd, J = 7.6 Hz,
J = 4.0 Hz, 1H, CHarom), 7.33 (d, J = 7.6 Hz, 1H, CHarom), 7.31 (d, J = 2.4 Hz, 1H,
CHarom), 7.11 (d, J = 2.4 Hz, 1H, CHarom), 7.00 (d, J = 4 Hz, 1H, CHarom), 6.99 (dd,
J = 8.0 Hz, J = 4.0 Hz, 1H, CHarom), 6.40 (br, 1H, NH), 4.95 (br, 1H, CH), 2.48 (s, 3H,
CH3). Anal. Calcd for C15H12Cl2N2O2 (Mw 323.17) (m/z = 323.2) (%): C, 55.75; H, 3.74;
N, 8.67. Found (%): C, 55.93; H, 3.97; N, 8.86.
2.2.2. Preparation of the Ni(II) complex. A solution of Ni(II) acetate tetrahydrate
(0.0013 g, 0.005 mM) in methanol (2 mL) was added dropwise to a solution of HL1
(0.0065 g, 0.01 mM) in dichloromethane (4 mL). The color of the mixing solution turned
brown immediately from light yellow. The solvent was partially evaporated for one week at
room temperature and several brown needle-like single crystals suitable for X-ray crystallo-
graphic analysis were obtained. Anal. Calcd for C31H26Cl4N4NiO5 [Ni(L2)2]·CH3OH (Mw
735.07) (%): C, 50.65; H, 3.57; N, 7.62; Ni, 7.98. Found (%): C, 50.78; H, 3.63; N, 7.67;
Ni, 7.93.
2.3. X-ray structure determination of [Ni(L2)2]·CH3OH
The X-ray diffraction measurement for the Ni(II) complex was performed on a Bruker
Smart 1000 CCD diffractometer with graphite-monochromated Mo-Kα radiation
(λ = 0.71073 Å) at 298(2) K. Data reduction and cell refinement were performed using
SAINT [18]. The structure was solved by direct methods (SHELXS-97) and subsequent
difference-Fourier map revealed positions of the remaining atoms, and all non-hydrogen
atoms were refined anisotropically using full-matrix least-squares on F2 with SHELXL-97
[19, 20]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The
hydrogens were generated geometrically. Crystallographic data and structural refinements
for the Ni(II) complex are listed in table 1.

Nickel(II) with quinazoline-type ligand
3929
Table 1. Crystallographic data and data collection parameters for [Ni(L2)2]·CH3OH.
Empirical formula
Formula weight
C31H26Cl4N4NiO5
735.07
Temperature (K)
298(2)
Wavelength (Å)
0.71073
Crystal system
Space group
Monoclinic
P21/c
Unit cell dimensions (Å, °)
a
22.436(2)
b
7.7150(5)
c
19.0950(18)
109.798(2)
3109.9(5), 4
1.570
1.016
1504
β
Volume (Å3), Z
Calculated density (Mg/m3)
Absorption coefficient (mm–1
F (000)
)
Crystal size (mm3)
0.10 × 0.08 × 0.06
2.43–26.20
θ Range for data collection (°)
Index ranges
−26 ≤ h ≤ 26
−8 ≤ k ≤ 9
−22 ≤ l ≤ 12
15323
Reflections collected
Independent reflections
2723 [R(int) = 0.1086]
99.8
5489/0/406
Completeness to θ = 25.02 (%)
Data/restraints/parameters
Goodness-of-fit on F2
1.015
Final R indices [I >2σ(I)]
R indices (all data)
R1 = 0.0478, wR2 = 0.1012
R1 = 0.0932, wR2 = 0.1217
0.408 and –0.470
Largest difference peak and hole (e Å−3
)
w = 1/[σ2(Fo2) + (0.0158P)2], where P = (Fo2 + 2Fc2)/3.
2.4. Scavenger measurements of superoxide radicals
Superoxide radicals were measured in the test system using NBT/VitB2/MET [21]. HL1 and
the Ni(II) complex were dissolved in DMF (C = 4.0 μM), each solution was added to a
solution containing MET (0.01 M L−1), NBT (4.6 × 10−5 M L−1), VitB2 (3.3 × 10−6 M L−1),
and phosphate buffer (0.067 M L−1). After incubating the mixture at 30 °C for 10 min and
illuminating with a fluorescent lamp for 3 min, the absorbance (Ai) of these samples were
measured at 560 nm. The results are shown in figure 8. The suppression ratio was calculated
using the following equation:
Suppression ratioð%Þ ¼ ½ðA0 ꢀ AiÞ=ðA0Þꢁ ꢂ 100%
where Ai is the absorbance in the presence of HL1 and Ni(II) complex, and A0 is the absor-
bance in the absence of HL1 and Ni(II) complex.
The antioxidant activity was described as the 50% inhibitory concentration (IC50). IC50
values were calculated from regression lines, where x was the concentration of HL1 and Ni
(II) complex in μM and y was the percent inhibition of HL1 and Ni(II) complex.
3. Results and discussion
3.1. Crystal structure of [Ni(L2)2]·CH3OH
The anticipated quinazoline-type complex [Ni(L1)2] was not formed, but an unexpected
Schiff base-type complex [Ni(L2)2] was obtained, which was formed in the course of

3930
L.-Q. Chai et al.
complexation of HL1 by Ni(II) acetate tetrahydrate. The ligand HL1 changed to be a new
C=N–O ligand HL2 after forming the Ni(II) complex. In the process of reaction between
Ni(II) and HL1, unexpected cleavage of C–N bonds in HL1 took place forming HL2, which
coordinated to Ni(II) ion forming a mononuclear Ni(II) complex with a six-membered
ring Schiff base-type complex instead of an anticipated quinazoline-type Ni–N4O2 complex
[22, 23]. The result suggests that in the metal complex the Schiff base configuration is more
stable than that of quinazoline configuration (scheme 2).
X-ray structural studies reveal that the metal complex has 2 : 1 ligand to metal stoichiom-
etry, crystallizes in the monoclinic system with P21/c space group and consists of one Ni
(II), two (L2)−, and one non-coordinated methanol. Ni(II) is bonded to oxygen and nitrogen
of the two (L2)− molecules in a cis arrangement in which Ni(II) is six-coordinated. The
molecular structures of HL1 and the Ni(II) complex are illustrated in figure 1, and relevant
bond lengths and angles are summarized in table 2.
As shown in figure 2, the coordination environment around Ni(II) is octahedral with
some distortion, by one imine nitrogen (N1), one Schiff base nitrogen (N4), and two depro-
tonated phenolic oxygens (O2 and O4) defining the cis-N2O2 basal plane, plus one imine
nitrogen (N3) and one Schiff base nitrogen (N2) occupying apical positions from two
deprotonated (L2)− units [24, 25]. The coordination environment around Ni(II) is best
regarded as a slightly distorted octahedral geometry with the distance of the apical N2 and
N3 to the N2O2 basal plane being 1.991(4) and 2.042(4) Å. The Ni is in the N2O2 basal
plane. The bond lengths of Ni–O are longer than Ni–N. These structural features, from ste-
ric hindrance of the L1− fragments, account for the distortion of the ligand in the Ni(II)
complex. The dihedral angle between two six-membered rings of methylene as vertex is
81.14(3)°. After formation of the Ni(II) complex, the dihedral angle turns to 69.30(3)°,
while the dihedral angles between every two metal chelating rings adjacent to each other
(the plane defined by Ni1O2C11C10C9N2, the adjacent plane defined by
Ni1N1C2C3C4N2Ni1 and Ni1O4C26C25C24N2, the other adjacent plane defined by
Ni1N3C17C18C19N4Ni1) for the Ni(II) complex are 51.63(3)° and 50.85(3)°, respectively.
Scheme 2. Complexation of HL1 with Ni(II) acetate.

Nickel(II) with quinazoline-type ligand
3931
Figure 1. Crystal structures of HL1 (a) and the Ni(II) complex (b) with atom numbering. Displacement ellipsoids
for non-H atoms are drawn at the 30% probability level.
Molecules of complex are connected by intramolecular C–H⋯O and O–H⋯O hydrogen
bonds (figure 3), intermolecular hydrogen bonds, and C–X⋯π (Ph) interactions (table 3),
which play a role in stabilizing the structure of the crystal. Two pairs of intermolecular
O3–H3⋯O2 and C1–H1A⋯O1 hydrogen bonds form two five-membered rings. Three

3932
L.-Q. Chai et al.
Table 2. Selected bond distances (Å) and angles (°) for the Ni(II) complex.
Bond
Ni1–O2
Ni1–O4
Bond
Dist.
1.999(5)
2.043(5)
Angles
Bond
Ni1–N2
Ni1–N3
Bond
Dist.
Bond
Ni1–N4
Ni1–N1
Bond
Dist.
2.009(5)
2.047(5)
Angles
2.042(6)
2.130(6)
Angles
O2–Ni1–N2
O2–Ni1–O4
O2–Ni1–N3
O4–Ni1–N3
N4–Ni1–N1
C2–N1–Ni1
C4–N2–Ni1
C24–N4–Ni1
C26–O4–Ni1
89.8(2)
87.7(2)
86.8(2)
95.0(2)
92.1(2)
123.9(6)
116.6(5)
123.2(5)
116.0(4)
O2–Ni1–N4
N2–Ni1–O4
N2–Ni1–N3
O2–Ni1–N1
O4–Ni1–N1
O1–N1–Ni1
C17–N3–Ni1
C19–N4–Ni1
166.6(2)
91.1(2)
172.9(3)
97.4(2)
169.5(2)
120.9(4)
131.0(6)
117.1(5)
N2–Ni1–N4
N4–Ni1–O4
N4–Ni1–N3
N2–Ni1–N1
N3–Ni1–N1
C9–N2–Ni1
O3–N3–Ni1
C11–O2–Ni1
101.3(2)
84.5(2)
83.0(2)
79.8(2)
94.4(2)
123.5(5)
115.0(4)
123.1(4)
Figure 2. Coordination configuration of Ni(II) complex.
intermolecular O1–H1⋯O5, O5–H5⋯O2 and C7–H7⋯O4 hydrogen bonds between aro-
matic carbon, phenolic oxygen, and non-coordinated methanol form a seven-membered
ring. A pair of C27–Cl3⋯Cg(6) hydrogen bonds exist between aromatic rings [26].
Intermolecular C–X⋯π (Ph) hydrogen bonds play an important role in stabilizing the crystal
structure. With the help of hydrogen bonds, adjacent molecular units are linked by interac-
tions to give an infinite 1-D chain supramolecular structure along the b-axis, as shown in
figure 4.
3.2. IR spectra
The main FT-IR absorptions of HL1 and the Ni(II) complex from 400 to 4000 cm−1 are
given in table 4. The free ligand exhibits a characteristic C=N stretch at 1600 cm−1, while
C=N of the Ni(II) complex is observed at 1614 cm−1. The C=N stretch is shifted to higher
frequency by ca. 14 cm−1 upon complexation, indicating a decrease in C=N bond order due
to the coordination of Ni(II) with oxime nitrogen [27]. The Ar–O stretch is a strong band at
1263–1213 cm−1 as reported for similar ligands [28–30], occurring at 1258 cm−1 for HL1
and at 1230 cm−1 for the Ni(II) complex. The Ar–O stretching frequency shifts to lower
frequency, indicating that Ni–O bond was formed between Ni(II) and oxygen of the
phenol [31].

Nickel(II) with quinazoline-type ligand
3933
Figure 3. Hydrogen bonds of the Ni(II) complex.
Table 3. Hydrogen-bonding distances (Å) and angles (°) for the Ni(II) complex.
D–H⋯A
d(D–H)
d(H⋯A)
d(D⋯A)
<D–H⋯A
O3–H3⋯O2
C1–H1A⋯O1
O1–H1⋯O5
O5–H5⋯O2
C7–H7⋯O4i
C27–Cl3⋯Cg(6)ii
0.82
0.96
0.82
0.82
93
2.06
2.26
1.84
1.89
2.57
3.76
2.709(7)
2.5975
2.654(8)
2.601(7)
3.493(9)
4.616(8)
136
100
174
145
170
109
1.73
Symmetry code: (i) x, ½ − y, −1/2 + z; (ii) x, y, z. Cg(6) is the centroid for benzene ring C10–C15.
Figure 4. View of the 1-D chain motif of the Ni(II) complex along the b-axis.
Table 4. Selected FT-IR bands for the ligand and its Ni(II) complex (cm−1).
Compound
ν(O–H)
ν(N–H)
ν(C=N)
νN→O
νAr–O
ν(Ni–N)
ν(Ni–O)
HL1
Complex
3225
3434
3067
–
1600
1614
1278
–
1258
1230
–
526
–
423
The IR spectrum of HL1 shows unexpected strong absorption due to ν(N→O) at 1278 cm−1,
which disappears in the complex, indicating the C–N bond in the Ni(II) complex has broken
and oxime N participated in coordination. The N–H in HL1 at 3067 cm−1 disappears in the

3934
L.-Q. Chai et al.
complex, indicating the nitrogen coordinated to Ni(II). The O–H stretch of free HL1 appears at
3225 cm−1, while the infrared spectrum of the Ni(II) complex shows the expected strong
absorption due to ν(O–H) at 3434 cm−1, which is evidence of O–H from oxime.
The far-infrared spectrum of the Ni(II) complex is obtained from 100–500 cm−1 to
identify frequencies due to Ni–O and Ni–N. The FT-IR spectrum of the complex shows
ν(Ni–N) and ν(Ni–O) at 526 and 423 cm−1, respectively.
3.3. UV–vis absorption spectra
UV–vis absorption spectra of HL1 and the Ni(II) complex were determined in
5 × 10−5 M L−1 DMF solution. Absorptions of the Ni(II) complex were different from
those of HL1 as shown in figure 5. Compared with the Ni(II) complex, an important
feature of the absorption spectrum of HL1 was that three absorptions were observed at
241, 305, and 364 nm, respectively. The latter absorptions were absent in the spectrum of
the Ni(II) complex. The absorption at 254 nm in the Ni(II) complex could be due to the
d–d transition of Ni(II). The Ni(II) complex with 3d8 electronic configuration could be
3
assigned to A2g→3T1g transition of Ni(II) in an octahedral environment [32]. These
observations were in line with common spectral features of d-block metal complexes [33].
The intense band near 439 nm was assigned for ligand-to-metal charge transfer transitions
[32].
3.4. Fluorescence spectra
Fluorescent properties of HL1 and its corresponding Ni(II) complex were investigated at
room temperature (figure 6). The ligand exhibits two intense emissions at 427 and 472 nm
upon excitation at 364 nm, which should be assigned to the intraligand π–π* transition [34,
35]. The Ni(II) complex shows an intense broad photoluminescence with maximum
emission at ca. 484 nm upon excitation at 437 nm, which is red-shifted to that of HL1. This
Figure 5. UV–vis absorption spectra: HL1 (solid line) and the Ni(II) complex (dashed line) in DMF
(5 × 10−5 M L−1).

Nickel(II) with quinazoline-type ligand
3935
Figure 6. Emission spectra of HL1 (solid line) and corresponding Ni(II) complex (dashed line) in dilute DMF at
room temperature (c = 5 × 10−5 M L−1).
red shift might be related to the auxochrome group (–Cl), which makes the conjugated sys-
tem larger. The compounds emit strong fluorescence in the blue light area and may be
potential functional fluorescent material which can emit blue light.
3.5. Electrochemical studies
The electrochemical property of the Ni(II) complex has been studied in DMF containing
0.05 M L−1 tetrabutylammonium perchlorate over the potential range from –2 to 0.7 V (vs.
Ag/AgCl) using CV techniques, with scanning rate of 100 mV s−1. As it observed in figure
7, the Ni(II) complex exhibits two reversible redox waves, which are assigned to
consecutive Ni(II)/Ni(I) and Ni(III)/Ni(II) redox processes [36]. The first pair of oxidation–
reduction peaks correspond to the oxidation–reduction couples Ni(III)/Ni(II), Epa1 = 0.197 V,
Epc1 = –0.896 V, with the average formal potential [E1/2 = (Epa1 + Epc1)/2] is –0.347 V.
The peak-to-peak separation between the anodic and cathodic processes (ΔEp1) is 1.093 V
and the proportion of the peak current (ipc1/ipa1) is 1.523. The second pair of oxidation–
reduction peaks correspond to Ni(II)/Ni(I), Epa2 = –0.724 V, Epc2 = –1.560 V, with the
average formal potential [E1/2 = (Epa2 + Epc2)/2] of 1.142 V, the peak-to-peak separation
between the anodic and cathodic processes (ΔEp2) of 0.836 V and the proportion of the
peak current (ipc2/ipa2) of 0.771. These features are indicative of a quasi-reversible elec-
trode process [37].
3.6. SOD-like activity
The SOD-like activities of HL1 and Ni(II) complex were investigated (figure 8). The aver-
age suppression ratio against Oꢃ2ꢀ increases with increasing concentration in the range of the
tested concentration. The observed IC50 values of the compounds were compared with ear-
lier reported values for nickel(II) complexes [38, 39]. The catalytic activity of NiSOD [40],

3936
L.-Q. Chai et al.
Figure 7. Cyclic voltammogram of Ni(II) complex in DMF solution. Scan rate at 100 mV s−1
.
Figure 8. Scavenging effects of HL1 (solid line) and corresponding Ni(II) complex (dashed line) on Oꢃ2ꢀ
.
however, is on the same high level as that of Cu–ZnSOD at about 109 (M L−1)−1s−1 per
metal center. The IC50 data of the SOD activity assay along with kinetic catalytic constants
of compounds [41, 42] are presented in table 5. The results indicate that the prepared
compound is more efficient antioxidant than vitamin C, which is the standard superoxide
dismutase [43–45]. The brown color of the octahedral Ni(II) complex exhibits more active
scavenging effects against Oꢃ2ꢀ than HL1 and other nickel complexes under the same
conditions. The scavenging effect of the Ni(II) complex may be ascribed to the chelating
function of ligand with metal ion to achieve significant selectivity of radical scavenging
activity [46].

Nickel(II) with quinazoline-type ligand
Table 5. IC50 values and kinetic constant of compounds.
3937
Compound
IC50 (μM)
kMcCF ((M L−1 −1 −1
)
s
) × 104
Refs.
Vc
852
51.60
8.26
31
34
38
95
35
55
60
–
–
[43]
This work
This work
[38]
[38]
[38]
[38]
[39]
[39]
[39]
HL1
[Ni(L2)2]·CH3OH
[Ni(L)2]
33.08
3.06
1.76
2.5
[Ni(L)(HL)](ClO4)(H2O)
Ni(HL)(bipy)(H2O)(NO3)(ClO4)(H2O)
Ni(HL)(dien)(ClO4)(H2O)
[Ni(L1)2]·2H2O
1.0
2.71
1.73
1.58
[Ni(L2)2](ClO4)2
[Ni(L3)(bipy)](ClO4)2
kMcCF were calculated by K = kNBT × [NBT]/IC50, kNBT (pH 7.8) = 5.94 × 104 (M L−1 −1
)
s−1 [42].
4. Conclusion
We have reported the synthesis and structural characterization of an unexpected
mononuclear Ni(II) complex based on analytical and spectral data. Ni(II) caused cleavage
of C–N bonds in HL1, giving a new C=N–O ligand HL2, which coordinates to Ni(II)
forming Ni–N4O2 complex with a Schiff base-type instead of an anticipated quinazoline
complex. Moreover, the quasi-reversible one-electron Ni(III)/Ni(II) and Ni(II)/Ni(I) redox
waves were determined by cyclic voltammetry. In addition, the Ni(II) complex exhibits
more active scavenging effects against Oꢃ2ꢀ than HL1 and other nickel complexes under the
same conditions. Analytic, spectroscopic, electrochemical, and crystal structure determina-
tions show that nickel(II) has octahedral geometry.
Supplementary material
Further details of the crystal structure investigation(s) may be obtained from the Cambridge
Crystallographic Data Center, Postal Address: CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (Telephone: (44) 01223 762910; Facsimile: (44) 01223 336033; E-mail:
deposit@ccdc.cam.ac.uk on quoting the depository number CCDC 886495).
Funding
We are thankful for the financial support by the Natural Science Foundation of Gansu Province
[No: 1107RJZA165]; the Young Scholars Science Foundation of Lanzhou Jiaotong University
[No: 2011038]; and the Innovation and Technology Fund of Lanzhou Jiaotong University
[No: DXS-2013035].
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