Synthesis, characterization, theoretical and antimicrobial studies of tridentate hydrazone metal complexes of Zn(II), Cd(II), Cu(II) and Co(III)

Article abstract of DOI:10.1016/j.poly.2018.03.012

The synthesis of four new complexes involving a tridentate hydrazone ligand (H₂L = hydrazinyl-(pyridin-2-yl)salicyladimine) with Zn(II), Cd(II), Cu(II) and Co(III) ions is reported. Complexes [Zn(HL)₂] (1), [Cd(HL)₂] (2) and [Cu₂(HL)₂(OAc)₂] (3) were synthesized by a one-pot reaction. A trivalent cobalt complex, [Co(HL)₂]Cl(CH₃OH) (4), was obtained from the reaction of H₂L with CoCl₂. The hydrazone ligand and the metal complexes were characterized by elemental analyses, IR and single-crystal X-ray diffraction. The location of the HOMO and LUMO frontier orbitals on the molecular structures of 1–4 were employed to explain influence of the central atoms on the electronic properties of the complexes. The antimicrobial activities of H₂L and the metal complexes were investigated on three bacteria strains (Staphylococcus aureus, Bacillus subtilis and Escherichia coli).

Full text of DOI:10.1016/j.poly.2018.03.012

Accepted Manuscript
Synthesis, characterization, theoretical and antimicrobial studies of tridentate
hydrazone metal complexes of Zn(II), Cd(II), Cu(II) and Co(III)
Wei Cao, Yaoming Liu, Ting Zhang, Jinping Jia
PII:
S0277-5387(18)30137-2
https://doi.org/10.1016/j.poly.2018.03.012
POLY 13078
DOI:
Reference:
Polyhedron
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Accepted Date:
29 January 2018
12 March 2018
Please cite this article as: W. Cao, Y. Liu, T. Zhang, J. Jia, Synthesis, characterization, theoretical and antimicrobial
studies of tridentate hydrazone metal complexes of Zn(II), Cd(II), Cu(II) and Co(III), Polyhedron (2018), doi:
https://doi.org/10.1016/j.poly.2018.03.012
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Synthesis, characterization, theoretical and antimicrobial studies of
tridentate hydrazone metal complexes of Zn(II), Cd(II), Cu(II) and
Co(III)
Wei Cao*, Yaoming Liu, Ting Zhang, Jinping Jia
Scientific Instrument Center, Shanxi University, Taiyuan, 030006, P. R. China.
E-mail: caowei@sxu.edu.cn
Abstract
The synthesis of four new complexes involving a tridentate hydrazone ligand (H2L
= hydrazinyl-(pyridin-2-yl)salicyladimine) with Zn(II), Cd(II), Cu(II) and Co(III) ions
is reported. Complexes [Zn(HL)2] (1), [Cd(HL)2] (2) and [Cu2(HL)2(OAc)2] (3) were
synthesized by a one-pot reaction. A trivalent cobalt complex, [Co(HL)2]Cl(CH3OH)
(4), was obtained from the reaction of H2L with CoCl2. The hydrazone ligand and the
metal complexes were characterized by elemental analyses, IR and single-crystal
X-ray diffraction. The location of the HOMO and LUMO frontier orbitals on the
molecular structures of 1-4 were employed to explain influence of the central atoms
on the electronic properties of the complexes. The antimicrobial activities of H2L and
the metal complexes were investigated on three bacteria strains (Staphylococcus
aureus, Bacillus subtilis and Escherichia coli).
Keywords: Schiff base complex; Hydrazone; Synthesis; Crystal structure;
Antimicrobial activity
1. Introduction
Metal coordination compounds have been a subject of particular interest due to
their broad range of applications in science and technology [1-3]. Among the various
complexes, transition metal complexes containing a Schiff base ligand are important,
mostly due to their important applications in various areas such as catalysis [4-6],
chemosensors [7,8], luminescent materials [9,10], energy materials [11,12],

magneto-structural chemistry [13,14] and biological fields [15-19]. Schiff base
complexes have been significantly active over the years because of the wide variety of
possible structures for the ligands, depending on the aldehyde and amine used.
Hydrazones, a member of the Schiff base family with a triatomic >C=N-NH- linkage,
play an important role in the development of coordination chemistry. Reports on the
synthesis, characterization and structural studies of hydrazone ligands show the
importance of the hydrazone complexes in various fields, including analytical and
biological fields [20-24]. Concerning the bioactivity, in many cases complexes
display higher activity than the parent ligands [25-27], suggesting that preparation of
new hydrazine complexes can be an interesting task to satisfy the requirements of an
application.
With the above consideration in mind, we selected the Schiff base
hydrazinyl-(pyridin-2-yl)salicyladimine (H2L) as the target ligand. When a functional
group (–OH) is present close to the azomethine group, the Schiff bases bear an
excellent coordinating ability to act as multidentate ligands. We have synthesized four
new metal compounds with different structures and have characterized these
complexes by a series experiments. In addition, the antibacterial activities of H2L and
the metal compounds were also studied.
2. Experimental
2.1 Materials and physical measurements
Salicylaldehyde and 2-hydrazinopyridine were purchased from J&K Scientific and
used without further purification. The other reagents and solvents were analytical
grade reagents from Beijing Chemical Reagent Factory. Elemental analyses were
conducted using a Vario EL elemental analyzer. Fourier transform infrared (FT-IR)
spectra were measured on a Thermo Scientific Nicolet iS50 spectrometer. X-ray
single crystal structures were determined on a Bruker D8 VENTURE diffractometer.
2.2. Synthesis of H2L
A methanol (2 mL) solution of salicylaldehyde (0.3 mmol, 0.030 mL) and

2-hydrazinopyridine (0.3 mmol, 0.033 g) was sealed in a Teflon-lined stainless steel
autoclave and heated at 80 °C for 1 day, then cooled to room temperature. White
crystals were obtained (yield 82.5%). Anal. Calcd for C12H11N3O: C, 67.59; H, 5.20;
N, 19.70. Found: C, 67.46; H, 5.57; N, 19.59%. IR (cm-1): 2993m, 1600s, 1586s,
1496m, 1446s, 1309s, 1267m, 1156m, 770s, 751m.
2.3. Synthesis of the metal complexes
2.3.1. Synthesis of [Zn(HL)2] (1)
A methanol (3 mL) solution of Zn(OAc)2·2H2O (0.1 mmol, 0.022 g),
2-hydrazinopyridine (0.2 mmol, 0.022 g) and salicylaldehyde (0.2 mmol, 0.020 mL)
was sealed in a Teflon-lined stainless steel autoclave and heated at 80 °C for 3 days,
then cooled to room temperature. Yellow crystals suitable for X-ray crystallography
were obtained (yield 64.7%). Anal. Calcd for C24H20N6O2Zn: C, 58.84; H, 4.12; N,
17.16. Found: C, 58.61; H, 4.15; N, 16.95%. IR (cm-1): 3040w, 3003w, 1622vs,
1597m, 1539s, 1481s, 1468s, 1443m, 1427m, 1333s, 1310m, 1287m, 1265w, 1198m,
1152m, 1099m, 1038m, 754m, 735m.
2.3.2. Synthesis of [Cd(HL)2] (2)
The Cd(II) complex was synthesized similarly to the Zn(II) complex, except that
Zn(OAc)2·2H2O was replaced by Cd(NO3)2·4H2O, to produce yellow needle-like
crystals (yield 35.9%). Anal. Calcd for C24H20CdN6O2: C, 53.69; H, 3.75; N,15.65.
Found: C, 53.52; H, 4.05; N, 15.56%. IR (cm−1): 3166w, 1648vs, 1604s, 1486m,
1411m, 1303s, 1274vs, 1254s, 1212m, 1189s, 1098vs, 1039m, 999m, 860s, 756vs.
2.3.3. Synthesis of [Cu2(HL)2(OAc)2] (3)
The Cu(II) complex was synthesized similarly to the Zn(II) complex, except that
Zn(OAc)2·2H2O was replaced by Cu(OAc)2·H2O, to produce black crystals (yield
2.3%). Anal. Calcd for C28H26Cu2N6O6: C, 50.22; H, 3.91; N, 12.55. Found: C, 49.97;
H, 3.76; N, 12.56%. IR (cm-1): 3035w, 1710s, 1620vs, 1599s, 1485s, 1450m, 1400m,
1326s, 1308s, 1287m, 1265w, 1200m, 1099m, 1038m, 754m, 735m.

2.3.4. Synthesis of [Co(HL)2]Cl(CH3OH) (4)
A methanol (4 mL) solution of CoCl2·2H2O (0.1 mmol, 0.022 g) and H2L (0.2
mmol, 0.042 g) was sealed in a Teflon-lined stainless steel autoclave and heated at
80 °C for 6 hours, then cooled to room temperature. Dark red crystals suitable for
X-ray crystallography were obtained (yield 60.6%). Anal. Calcd for C25H24ClCoN6O3:
C, 54.51; H, 4.39; N, 15.26. Found: C, 54.58; H, 4.09; N, 15.29%. IR (cm-1): 3040m,
1650vs, 1610m, 1484s, 1468s, 1431s, 1396m, 1305m, 1284m, 1269m, 1199m, 1155m,
1120m, 747m.
2.4. X-ray crystallography
Single-crystal X-ray diffraction data of H2L, 1, 2, 3 and 4 were collected on a
Bruker D8 VENTURE diffractometer with graphite monochromated Mo-Kα radiation
(λ = 0.71073 Å) at 293 K. The structures were solved by the direct method [28] and
refined by full matrix least squares based on F2 using the SHELX 2014 program [29].
All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in
calculated positions. The detailed crystallographic data and structure refinement
parameters of the complexes are listed in Table 1 for comparison.
Table 1. Crystal data and structure refinement parameters of compounds H2L, 1, 2, 3 and 4.
Compound
Formula
Fw
H2L
C12H11N3O
213.24
1
2
3
4
C24H20N6O2Zn C24H20CdN6O2 C28H26Cu2N6O6 C25H24ClCoN6O3
489.83
536.86
669.62
Monoclinic,
C2/c
550.88
Monoclinic,
P21/n
Crystal system,
space group
a (Å)
Monoclinic,
P21/n
Orthorhombic,
Pbcn
Orthorhombic,
Pbcn
4.66(7)
20.66(2)
9.11(7)
11.28(9)
90
20.77(4)
8.99(2)
11.49(2)
90
24.85 (4)
8.30(1)
15.19(3)
90
11.47 (6)
18.86(1)
11.55(6)
90
b (Å)
21.10(3)
10.90(2)
90
c (Å)
α (°)

β (°)
γ (°)
V (Å3)
Z,
94.2(2)
90
90
90
120.7(6)
90
98.9 (2)
90
90
90
1069.6(3)
2122.6(3)
4, 1.533
2145.4(7)
4, 1.662
2692.8(8)
4, 1.652
2468.4(2)
4, 1.482
calculated 4, 1.324
density (Mg/m3)
F (000)
448
1008
1080
1368
1136
Reflections
collected/unique
Goodness-of-fit
on F2
6325
/
2435 13630 / 2443 39095 / 2464 20605 / 3085 23620
/
6119
[R(int)=0.0252] [R(int)=0.0470] [R(int)=0.1044] [R(int)=0.0783] [R(int) = 0.0250]
1.022
1.062
1.024
1.106
1.035
Final R indices R1 = 0.0449,
R1 = 0.0395,
wR2 = 0.0980
R1 = 0.0617,
wR2 = 0.1077
R1
=
0.0523, R1
=
0.0489, R1 = 0.0402,
[I>2σ(I)]
R indices
(all data)
wR2 = 0.1017
R1 = 0.0883,
wR2 = 0.1205
wR2 = 0.1113
wR2 = 0.1036
0.0957, R1 = 0.0492,
wR2 = 0.1321 wR2 = 0.1065
wR2 = 0.1000
R1
= 0.0933, R1 =
wR2 = 0.1328
R1 =Σ(|Fo|- |Fc|)/|Fo|; wR2 = {Σ[(w|F2 |- |F2 |)2/Σw(F2 )2]}1/2.
o
c
o
2.5. Calculation Methods
In this work, density functional theory (DFT) calculations using the Gaussian 09
program were carried out at the B3LYP functional level [30]. Additionally, the basis
set LANL2DZ was employed for the Zn, Cd, Cu and Co atoms, and the other atoms
are described with 6-31+G(d) [31]. The CPCM [32,33] was also employed in the
calculations, and the solvent was methanol. Complexes 1, 2 and 3 are closed shell
systems and for complex 4, the spin state is two.
2.6. Antimicrobial activity
A microtitre bioassay was used to determine the antibacterial activity [34]. The
compounds were dissolved in DMSO (20% of the final volume) and sterile water (80%
of the final volume). The samples (200 µL) were two-fold serially diluted with culture
medium in 96-well microplates to give concentrations from 0.001 to 0.660 mg/mL.

Amoxicillin was used as a positive control for each bacterium, with solvent and
bacteria-free wells being included as negative controls. Microplates were covered and
incubated at 37 °C for 24 h. To indicate bacterial growth, 40 µL of 0.2 mg/mL
p-iodonitrotetrazoliumchloride (INT) were added to each well and incubated at 37 °C
for 30 min. Clear wells with INT after incubation indicate inhibition of bacterial
growth. The minimum inhibitory concentration (MIC) was considered to be the
lowest concentration of the test substance exhibiting no visible growth of bacteria in
the microplate well. The results were repeated in triplicate and are expressed in
micromolar concentrations (µM).
3. Results and discussion
The hydrazone ligand H2L was prepared from the reaction of salicylaldehyde with
2-hydrazinopyridine as the literature reported [35,36]. The IR spectrum exhibit a v
(C=N) vibration at 1600 cm-1. The structure was confirmed by single-crystal X-ray
crystallography (Fig.1). Both intramolecular and intermolecular hydrogen bonds
could be observed in H2L (Fig. S1). Compounds 1-3 were obtained using two
equivalents of salicylaldehyde and 2-hydrazinopyridine with one equivalent of the
metal salts. Compound 4 was prepared from the direct reaction of H2L and
CoCl2·2H2O in methanol. All the compounds are air and moisture stable crystals at
room temperature.
Fig. 1. Structure of H2L.
3.1. Description of the crystal structures
3.1.1. X-ray crystal structures of 1 and 2
The iso-structural metal complexes 1 and 2 crystallize in the orthorhombic system
with Pbcn space group, so only the Zn(II) complex (1) will be described. The analysis

of the [Zn(HL)2] coordination compound indicates that the ligand to metal ratio is 2:1.
The molecular structure of 1 is shown in Fig. 2, and selected bond lengths and angles
are summarized in Table 2. In the compound [Zn(HL)2], the Zn(II) atom is six
coordinated by two Schiff base anion ligands. Each of the Schiff base ligands loses
one hydrogen atom and coordinates to the metal ion through the hydrazine N atom
and the phenolic O atom. The coordination environment can be best described as
strongly distorted octahedral, as is often observed for hexacoordinated Zn(II)
complexes. In this complex, the two deprotonated ligands are almost perpendicular to
each other. The Zn(1)–O(1), Zn(1)–N(1) and Zn(1)–N(3) bond lengths are 2.073(2),
2.186(2) and 2.152(2) Å, respectively (Table 2), which are in agreement with the
reported values for such bonds [37]. The crystal packing of complex 1 is stabilized
mainly by N(2)–H(2A)⋯O(1) hydrogen bonds. As a result, a 1D polymeric chain is
formed along the c-axis (Fig. 3, Table 3). Then, with the help of van der Waals
(C(6)–H(6)⋯H(11)–C(11)) interactions, adjacent molecular units are linked together
into an infinite 2-D supramolecular structure (Fig. S2). The structure of complex 2 is
shown in the Supporting Information (Fig. S3-S5).
(a)
(b)
Fig. 2. (a) Crystal structure of complex 1; (b) Intermolecular hydrogen bonds of 1. The relevant
coordination atoms are labelled. All H atoms are omitted for clarity, except those forming
hydrogen bonds.

Fig. 3. The infinite 1-D chain supramolecular structure of 1.
Table 2. Selected bond distances (Å) and angles (deg) for H2L, 1, 2, 3 and 4.
H2L
C(1)-N(1)
1.343(2)
1.368(2)
1.367(2)
1.371(2)
1.391(2)
1.333(2)
1.377(2)
124.37(2)
118.02(2)
119.59(2)
118.67(2)
114.81(1)
122.70(2)
122.47(2)
122.00(1)
C(6)-N(3)
C(6)-C(7)
C(7)-C(12)
C(7)-C(8)
C(8)-O(1)
C(8)-C(9)
1.282(2)
1.448(2)
1.396(2)
1.401(2)
1.357(2)
1.382(2)
C(9)-C(10)
C(10)-C(11)
C(11)-C(12)
N(2)-N(3)
1.371(3)
1.376(3)
1.376(2)
1.359(2)
0.8600
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
N(2)-H(2A)
O(1)-H(1A)
C(5)-N(1)
0.8200
C(5)-N(2)
N(1)-C(1)-C(2)
C(3)-C(2)-C(1)
C(2)-C(3)-C(4)
C(3)-C(4)-C(5)
N(1)-C(5)-N(2)
N(1)-C(5)-C(4)
N(2)-C(5)-C(4)
N(3)-C(6)-C(7)
C(12)-C(7)-C(8)
C(12)-C(7)-C(6)
C(8)-C(7)-C(6)
O(1)-C(8)-C(9)
O(1)-C(8)-C(7)
C(9)-C(8)-C(7)
C(10)-C(9)-C(8)
C(9)-C(10)-C(11)
117.57(2)
119.87(2)
122.56(1)
118.19(2)
121.39(1)
120.42(2)
120.48(2)
120.33(2)
C(12)-C(11)-C(10)
C(11)-C(12)-C(7)
C(5)-N(1)-C(1)
N(3)-N(2)-C(5)
N(3)-N(2)-H(2A)
C(5)-N(2)-H(2A)
C(6)-N(3)-N(2)
C(8)-O(1)-H(1A)
119.55(2)
121.64(2)
116.65(1)
119.70(1)
120.1
120.1
118.25(1)
109.5
1
Zn(1)-N(1)
2.186(2)
157.5(7)
83.8(7)
Zn(1)-O(1)
2.073(2)
93.2(8)
89.3(1)
Zn(1)-N(3)
2.152(2)
74.1(7)
95.9(7)
O(1)-Zn(1)-N(1)
O(1)-Zn(1)-N(3)
O(1)-Zn(1)-N(1)a
O(1)a-Zn(1)-O(1)
N(3)-Zn(1)-N(1)
N(3)-Zn(1)-N(1)a

O(1)-Zn(1)-N(3)a
106.5(7)
N(1)-Zn(1)-N(1)a
92.9(1)
N(3)a-Zn(1)-N(3)
165.8(1)
2
Cd(1)-O(1)
2.227(4)
148.5(1)
94.8(2)
Cd(1)-N(1)
2.332(4)
79.08(1)
92.1(2)
Cd(1)-N(3)
2.343(4)
69.71(1)
166.5(2)
95.2(2)
O(1)-Cd(1)-N(1)
O(1)-Cd(1)-N(1)b
O(1)-Cd(1)-N(3)b
O(1)-Cd(1)-N(3)
O(1)b-Cd(1)-O(1)
N(1)-Cd(1)-N(3)b
N(1)-Cd(1)-N(3)
N(3)b-Cd(1)-N(3)
N(1)-Cd(1)-N(1)b
110.6(1)
100.8(1)
3
Cu(1)-O(1)
1.899(3)
1.995(3)
91.60(1)
89.96(1)
170.48(1)
92.29(1)
Cu(1)-N(3)
1.960(3)
2.393(2)
171.75(1)
80.45(1)
109.96(1)
Cu(1)-O(2)
1.976(2)
Cu(1)-N(1)
Cu(1)-O(2)c
O(1)-Cu(1)-N(3)
O(1)-Cu(1)-O(2)
O(1)-Cu(1)-N(1)
O(1)-Cu(1)-O(2)c
N(3)-Cu(1)-O(2)
N(3)-Cu(1)-N(1)
N(3)-Cu(1)-O(2)c
O(2)-Cu(1)-O(2)c
N(1)-Cu(1)-O(2)c
O(2)-Cu(1)-N(1)
78.07(9)
95.33(1)
97.19(1)
4
Co(1)-O(2)
1.880(1)
1.891(1)
94.79(6)
87.19(7)
90.44(7)
177.96(7)
87.65(7)
Co(1)-N(6)
1.886(2)
1.934(2)
176.67(7)
94.43(7)
87.95(6)
83.18(7)
92.36(7)
Co(1)-N(3)
1.888(2)
1.943(2)
94.72(7)
82.96(7)
94.83(7)
177.06(7)
89.64(7)
Co(1)-O(1)
Co(1)-N(1)
Co(1)-N(4)
O(2)-Co(1)-N(6)
O(2)-Co(1)-N(3)
O(2)-Co(1)-O(1)
O(2)-Co(1)-N(4)
O(2)-Co(1)-N(1)
N(6)-Co(1)-N(3)
N(6)-Co(1)-N(1)
N(6)-Co(1)-O(1)
N(6)-Co(1)-N(4)
N(3)-Co(1)-O(1)
N(3)-Co(1)-N(1)
N(3)-Co(1)-N(4)
O(1)-Co(1)-N(1)
N(1)-Co(1)-N(4)
O(1)-Co(1)-N(4)
b
c
a Symmetry code: (-x+1, y, -z+1/2). Symmetry code: (-x+1, y, -z+3/2). Symmetry code: (-x+1,
-y+1, -z+1).
3.1.2. X-ray crystal structure of 3
Compound 3 crystallizes in the monoclinic space group C2/c and features a dimeric
Cu(II) moiety. X-ray crystal structure analysis reveals that 3 forms a 2:2 ligand to
metal stoichiometry and consists of two Cu(II) ions, two deprotonated ligands (HL)
and two acetate anions. Fig. 4a shows the molecular structure of 3 and the

coordination configuration. The geometries of the Cu(II) atoms can be best described
as slightly distorted square-pyramidal, with a CuN2O3 coordination. In this complex
the O(1), N(1), O(2) and N(3) donors define the equatorial plane, with the Cu(II) atom
lying in this plane, and the O(2A) (Symmetry code: -x+1, -y+1, -z+1) donor occupies
the apical position. Bond lengths in the coordination sphere follow a wide range
(Cu(1)–O(1) = 1.899(3) Å, Cu(1)–N(1) = 1.995(3) Å, Cu(1)–O(2) = 1.976(2) Å,
Cu(1)–O(2A) = 2.393(2) Å, Cu(1)–N(3) = 1.960(3) Å). The oxygen atoms O(2) and
O(2A) from two acetate ions linked the two Cu(II) atoms at distances of 1.976(2) and
2.393(2) Å. All the coordination distances are in the normal ranges, consistent with
those for comparable copper(II) coordination environments [38,39]. In a previous
study, H2L has been used with copper chloride to generate a 1 D coordination polymer.
The introduction of a methoxy substituent on the phenol ring of H2L converts the
original 1 D polymeric complex into a dinuclear complex [36]. In compound 3, as
shown in Figs. 5 and S6, intermolecular hydrogen bonds, N(2)-H(2)⋯O(3) and
C(4)-H(4)⋯O(3) with distances of 2.745 and 3.487 Å respectively, produce a
supramolecular polymeric association within 2D sheets.
(a)
(b)
Fig. 4. (a) Crystal structure of complex 3; (b) Crystal structure of complex 4. The relevant
coordination atoms are labelled. All H atoms are omitted for clarity.

Fig. 5. 1-D chain supramolecular structure of 3 formed via hydrogen bonds (dark red).
3.1.3. X-ray crystal structure of 4
Complex 4 has been prepared via the reaction of H2L with CoCl2·2H2O in methanol,
however, over the course of the coordination reaction the divalent cobalt ion is
oxidized to the Co(III) ion. In the structure, the cobalt centre features the anticipated
six-coordinate geometry with bond distances typical for trivalent cobalt (all distances
less than 2 Å) and similar to related Co(III) complexes previously reported [40,41].
The geometry of the coordination sphere is best considered to be octahedral (Fig. 4b).
In this complex the O(1), N(1), N(3) and N(6) donors define the equatorial plane and
the Co(III) atom lies in this plane. The O(2) and N(4) donors occupy the apical
positions with an O-Co-N angle of 177.96(7)°. The CoN4O2 sphere thus formed is like
the FeN4O2 sphere observed for Fe(III) complexes by Kou and co-workers [42]. An
uncoordinated Cl- anion is present, thereby balancing the charge of the complex. In
the structure of 4, the non-bonded nitrogen atoms at the center of the ligand are
protonated and form intramolecular hydrogen bonds to adjacent chloride atoms,
which assist in the formation of a 1D chain (Fig. 6). In addition, one co-crystallized
solvent methanol molecule was also found in the X-ray structure (Fig. S7).
Fig. 6. 1-D chain supramolecular structure of 4 combined via hydrogen bonds (dark red).

Table 3. Hydrogen bonding distances (Å) and angles (°) for complexes 1, 2, 3 and 4.
D-H...A
d(D-H) d(H...A)
d(D...A)
2.745
2.751
2.745
3.209
3.487
3.270
3.111
3.156
3.192
3.033
3.374
3.458
3.084
∠DHA
135.06
153.68
147.87
127.21
166.16
132.17
174.34
175.31
152.87
111.66
154.02
151.06
110.54
Symmetry code
x, -y, z-1/2
N(2)-H(2A)...O(1)
N(2)-H(13)...O(1)
N(2)-H(2)...O(3)
C(1)-H(1)...O(1)
C(4)-H(4)...O(3)
C(6)-H(6)...O(3)
O(3)-H(3A)...Cl(1)
N(2)-H(26)...Cl(1)
N(5)-H(27)...Cl(1)
C(1)-H(1)... N(6)
C(1)-H(1)...O(3)
C(2)-H(2)...O(2)
0.860
0.903
0.860
0.930
0.930
0.930
0.820
0.818
0.889
0.930
0.930
0.930
2.068
1.912
1.978
2.560
2.577
2.572
2.294
2.340
2.374
2.564
2.514
2.615
2.632
1
2
x, -y+1, z-1/2
-x+1, -y, -z+1
-x+1, -y+1, -z+1
x, -y, z-1/2
3
-x+1, -y, -z+1
x+1/2, -y+1/2, z+1/2
4
x+1/2, -y+1/2, z+1/2
x+1/2, -y+1/2, z+1/2
C(13)-H(13)... N(3) 0.930
3.2. Theoretical studies
To better understand the influence of the Zn(II), Cd(II), Cu(II) and Co(III) ions on
the electronic properties of H2L, density functional theory (DFT) calculations using
the Gaussian 09 program were carried out at the B3LYP/6-31+G(d) level [30].
Selected optimized bond distances of these complexes are given in Table S1. From
Table S1, it can be seen that the calculated bond lengths are in good agreement with
the experimental measurements. Therefore, the theoretical method and basis set that
we employed are suitable for geometry optimizations of the studied systems. The
optimized structures, distributions on the highest occupied molecular orbitals
(HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of complexes 1-4 are
shown in Fig. 7. The HOMOs and the LUMOs of 1 and 2 are promotions of electrons
delocalized predominantly all over the molecules. The HOMO of 3 is similar to that
of 1, but the LUMO was mostly positioned at the center of the molecule. For

compound 4, the α-HOMO, β-HOMO and α-LUMO were delocalized throughout the
entire molecule, while the β-LUMO was mostly positioned at the center of the
molecule. The computed energy levels of the HOMOs and LUMOs for 1-4 are shown
in Table 4; it can be seen from Table 4 that a change of the central atom has a great
influence on the frontier molecular orbital energy level. Therefore, the central atoms
have an obvious influence on the electron distributions of the frontline molecular
orbitals.
Fig. 7. HOMO and LUMO graphical representation of each molecule of the compounds.
Table 4. Computed energy levels (eV) of the HOMOs and LUMOs for complexes 1, 2, 3 and 4.
4
1
2
3
α
β
HOMO
LUMO
ΔEH-L
-5.01
-1.43
3.58
-4.98
-1.35
3.63
-5.74
-2.42
3.32
-5.45
-1.72
3.73
-5.40
-2.85
2.55
3.3. Antimicrobial activity
The antibacterial activities of the hydrazone ligand H2L as well as its metal

complexes were tested against Staphylococcus aureus and Bacillus subtilis as
Gram-positive and Escherichia coli as a Gram-negative microorganisms. The
obtained results were compared with amoxicillin as a standard, with different
concentrations.
As shown in Table 5, the hydrazone ligand and its metal complexes had different
behavior against the same bacteria. H2L was significantly more toxic against Gram-
positive than Gram-negative bacteria, which may be due to the different cell wall
structures of the tested microorganisms [43]. Complexes 2-4 were found to have
higher activity than 1 against the bacteria strains studied under the test conditions. In
the Bacillus subtilis strains, H2L showed an outstanding activity among all the
compounds tested, exhibiting a MIC value of 0.6 µM, even better than the effect of
amoxicillin. For the Staphylococcus aureus strains, although amoxicillin was the most
effective compound, 2 and 3 showed excellent activities, with MIC values of 5 and 7
µM. In addition, with the Escherichia coli strains, 3 and 4 showed outstanding activity,
both with MIC values of 40 µM.
The results obtained revealed that the active complexes show, in general, higher
antimicrobial properties than the ligand. Complex 3 was the most active among the
series of diverse metal complexes. According to chelating theory, chelation could
enhance the lipophilicity of the molecules, favoring permeation of the complexes
through the lipid layer of the cell membrane [44,45]. Besides the chelate effect, the
following factors should also be considered: (i) the nature of the metal centre, i.e.
copper is an element with antimicrobial properties; (ii) the nuclearity of the metals in
a complex, with dinuclear complexes usually being more potent than mononuclear
ones [46,47]. In addition, variation of the antimicrobial activities of metal complexes
against different bacteria depends on the impermeability of the cell or differences in
the ribosomes in the microbial cells [48].
Table 5. Minimal inhibitory concentrations (MICs) of H2L, 1, 2, 3 and 4 against bacterial strains.
Compound
MIC (µM)

Escherichia coli
Staphylococcus aureus
Bacillus subtilis
H2L
400
600
200
40
60
0.6
700
8
700
5
1
2
3
7
4
40
400
0.04
60
12
4
Amoxycilin
200
Conclusions
In this work, the tridentate Schiff base ligand H2L, containing -NH-N=CH- and
-OH groups, was synthesized. Zinc(II), cadmium(II), copper(II) and cobalt(III)
complexes with this ligand were prepared by two methods and characterized by
single-crystal X-ray diffraction. The mononuclear complexes [M(HL)2] (M(II) = Zn,
Cd) and dinuclear copper(II) complex [Cu2(HL)2(OAc)2] were synthesized by a one
pot reaction of salicylaldehyde, 2-hydrazinopyridine and the corresponding metal salts.
The Co(III) Schiff-base complex ([Co(HL)2]Cl(CH3OH)) has been prepared in
CH3OH via the reaction of H2L with CoCl2, in which the Co(II) core was oxidized to
Co(III). Hydrogen bonds play a crucial role in the construction of 1-D chains and 2-D
layers in all the supramolecular metal complexes. The location of the HOMO and
LUMO graphics help us to better understand the electronic properties of the
complexes. H2L and the three metal complexes have antibacterial activities. However,
the metal complexes have more excellent antibacterial properties, which are possibly
attributed to the nature of metal center and the structure of the metal complexes.
These observations have been guiding us for the development of new hydrazide
derivatives and their metal complexes that possess varied biological activities.
.
Acknowledgements
We are thankful for financial support from the Fundamental Research Funds for
Shanxi University (No. 304545009) and the Natural Science Foundation of Shanxi

Province in China (No. 201701D221031). The theoretical studies were supported by
the high- performance computing platform of Shanxi University
Appendix A. Supplementary data
CCDC 1826685, 1583570, 1583571, 1583572 and 1583573 contain the
supplementary crystallographic data for complexes H2L, 1, 2, 3 and 4. 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.
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A series of complexes encompassing hydrazinyl-(pyridin-2-yl)salicyladimine as a
ligand have been synthesized and structurally characterized. X-ray studies revealed
that these complexes display mononuclear or binuclear structures with different
coordination geometries. Antibacterial assays were carried out and DFT calculations
were used to explain the influence of the metal ions on the electronic properties of the
complexes.