Synthesis, crystal structures, catalytic application and antibacterial activities of Cu(II) and Zn(II) complexes bearing salicylaldehyde-imine ligands

Article abstract of DOI:10.1016/j.ica.2020.119639

Three novel copper(II) and zinc(II) complexes based on salicylaldehyde-imine ligands, namely [Cu(HL1)(CH₃CH₂OH)]·(CH₃COO) (1), [Zn₃(HL1)₂(CH₃COO)₄] (2), and [Zn₂(L2)]·

Full text of DOI:10.1016/j.ica.2020.119639

InorganicaChimicaActa508(2020)119639
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis, crystal structures, catalytic application and antibacterial activities
of Cu(II) and Zn(II) complexes bearing salicylaldehyde-imine ligands
Chao Liu⁎, Ming-Xing Chen, Ming Li
School of Chemistry and Chemical Engineering, Suzhou University, Suzhou 234000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cu(II) complex
Zn(II) complex
Crystal structure
Catalytic application
Antibacterial activity
Three novel copper(II) and zinc(II) complexes based on salicylaldehyde-imine ligands, namely [Cu(HL1)
(CH3CH2OH)]·(CH3COO) (1), [Zn3(HL1)2(CH3COO)4] (2), and [Zn2(L2)]·(CH3COO) (3) [H2L1 = N,N′-bis(sali-
cylidene)diethylenetriamine and H3L2 = 2,2′-(1E,1′E)-(2,2′-(2-(2-hydroxyphenyl)imidazolidine-1,3-diyl)bis
(ethane-2,1-diyl))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)diphenol] have been synthesized. The
complexes were characterized by IR, melting point, and elemental analyses, and the crystal structures were
further determined by single crystal X-ray diffraction. These three complexes demonstrated excellent catalytic
activities in the decomposition reaction of H2O2, and the decomposition percent of the H2O2 ranged from 89% to
98% under the optimized reaction conditions. Additionally, the compounds were screened as antibacterial
agents against three gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa)
and three gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus and Corynebacterium xerosis). The results
showed that complex 2 possessed stronger antibacterial activities than the two ligands and other two complexes.
1. Introduction
coordination abilities, salicylaldehyde-imines are very effective che-
lating agents which can coordinate to transition metal ions, forming
Nowadays, transition metal complexes have gained considerable
attention on basis of their potential applications in catalytic fields
[1–4]. In consideration of the concepts of green chemistry and the de-
mands of chemical production, it has an extremely important practical
significance to develop economic, efficient and low toxic transition
metal complexes as catalyst [5,6]. Copper and zinc complexes display
encouraging characteristics mentioned above and can catalyze various
organic synthesis reactions [7,8]. As a typical sterilization disinfectant
and bleaching agent, hydrogen peroxide was widely used in various
aspects of industrial production and daily life, which caused serious
water pollution yet [9]. Copper and zinc complexes as well as inorganic
metal salt and metal oxide, possess application potentials in terms of
catalyzing the decomposition reaction of hydrogen peroxide [10–13].
In addition, they also play important roles in the development of broad-
spectrum biological agents including antibacterial, anticancer, anti-
tumor, etc. [14–16].
various mononuclear, binuclear, and multinuclear complexes with
fascinating structures and interesting properties [17–24]. These find-
ings will contribute to further researching of transition metal complexes
bearing salicylaldehyde-imine ligands as catalyst or bioactivity agent. It
is known that some complexes bearing salicylaldehyde-imine ligands
show excellent catalytic activities, and others exhibit good biological
activities [25,26]. However, as far as we know, the synthesis and ap-
plication of the complexes with both catalytic activities and biological
activities have rarely been reported.
Therefore, three novel copper(II) and zinc(II) complexes bearing
salicylaldehyde-imine ligands (H2L1 and H3L2) have been synthesized
by the condensation reactions of salicylaldehyde with diethylene-
triamine and triethylenetetramine. The complexes have been char-
acterized by various detection techniques and their structures have
been determined by single crystal X-ray diffraction. [Cu(HL1)
(CH3CH2OH)]·(CH3COO) exhibits a mononuclear structure composed of
one acetate anion and one [Cu(HL)(CH3CH2OH)]+ cation,
[Zn3(HL1)2(CH3COO)4] exhibits a trinuclear structure composed of two
ZnN2O3 coordination units and one ZnO6 coordination unit, and
[Zn2(L2)]·(CH3COO) is a binuclear complex composed of two ZnN2O3
coordination units. Stimulated by the prior studies, the catalytic
It is well known that the construction of functional complexes is
mostly attributed to functional attributes and structural characteristics
of the ligands [2,3,15,16]. Consequently, considerable research atten-
tion has been focused on the design and synthesis of the suitable ligand.
Owing to their stable structures, various coordination modes and strong
⁎ Corresponding author.
E-mail address: kykrliu@163.com (C. Liu).
https://doi.org/10.1016/j.ica.2020.119639
Received 4 March 2020; Received in revised form 29 March 2020; Accepted 29 March 2020
Availableonline31March2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

C. Liu, et al.
InorganicaChimicaActa508(2020)119639
activities of the complexes in the decomposition of H2O2 have been
investigated. In addition, in order to further investigate their biological
activities, the antibacterial activities of the complexes against gram-
negative bacteria and gram-positive bacteria were tested.
766(s), 706(m), 640(w), 608(w), 520(w).
2.2.4. Synthesis of [Zn3(HL1)2(CH3COO)4] (2)
The complex was prepared as a white crystal in 83% yield by the
treatment of Zn(OAc)2·2H2O (1.22 g, 5.57 mmol) with H2L1 (1.00 mL,
3.71 mmol) in 40 mL of ethanol using the similar procedure for the
preparation of complex 1. m.p. 172.5–174.0 °C. Anal. Calcd for
2. Experimental
2.1. Materials and methods
C44H50N6O12Zn3 (%): C, 50.28; H, 4.79; N, 8.00. Found: C, 50.24; H,
5.01; N, 7.95. IR (KBr, cm−1): 3431(m), 3224(s), 2924(s), 2853(m),
1646(s), 1575(s), 1471(s), 1443(s), 1416(m), 1307(s), 1187(m),
1154(w), 1127(m), 1039(m), 1006(m), 897(w), 826(w),750(s),
662(m), 602(w), 531(w).
Analytical grade diethylenetriamine, triethylenetetramine and sali-
cylaldehyde were acquired from Sigma–Aldrich and used as received.
All other reagents and solvents were purchased from commercially
available sources without any further purification. The six bacteria in
the antibacterial test were presented by the school of pharmacy of
Anhui medical university. FT-IR spectra were measured by using KBr
2.2.5. Synthesis of [Zn2(L2)]·(CH3COO) (3)
The complex was prepared as a white crystal in 79% yield by the
treatment of Zn(OAc)2·2H2O (0.96 g, 4.36 mmol) with H3L2 (1.00 g,
2.18 mmol) in 40 mL of ethanol using the similar procedure for the
preparation of complex 1. m.p. 197.0–199.5 °C. Anal. Calcd for
pellet technique with
a FTS-40 spectrophotometer in the region
400–4000 cm−1 1H NMR and 13C NMR spectra for analyses of com-
.
pounds were recorded on a Bruker AM-400 NMR spectrometer in CDCl3
solution. Elemental analyses for C, H and N were carried out on a Vario
EL III elemental analyzer.
C
29H30N4O5Zn2 (%): C, 53.97; H, 4.69; N, 8.68. Found: C, 53.73; H,
4.95; N, 8.70. IR (KBr, cm−1): 3471(s), 3325(s) 2968(m), 1635(s),
1553(s), 1449(s), 1411(s), 1339(m), 1312(m), 1258(m), 1252(m),
1191(m), 1148(s), 1022(m), 919(m), 853(w), 750(m),711(w), 679(m),
609(w).
2.2. Synthesis
2.2.1. Synthesis of H2L1
To a solution of diethylenetriamine (2.58 mL, 23.95 mmol) in water
(40 mL) was added salicylaldehyde (5.00 mL, 47.90 mmol) slowly by a
syringe. A biphasic reaction system was formed due to the slight solu-
bility of salicylaldehyde in water. The reaction mixture was stirred at
room temperature for 1 h and the color of the solution gradually
changed from colorless to deep yellow. After completing the reaction,
the organic layer was separated using a separatory funnel and the
aqueous layer was extracted with dichloromethane (2 × 15 mL). Then,
the organic fractions were combined and washed with water
(2 × 10 mL) and n-hexane (2 × 10 mL), respectively. Removal of the
residual solvents gave a yellow oily product H2L1. Yield: 92%. Anal.
Calcd for C18H21N3O2 (%): C, 69.43; H, 6.80; N, 13.49. Found: C, 69.41;
H, 7.08; N, 13.28. 1H NMR (400 MHz, CDCl3): δ = 13.37 (br, 2H, OH),
8.35 (s, 2H, N = CH), 7.30–6.83 (m, 8H, Ar-H), 3.71 (t, 4H, C=NCH2),
3.00 (s, 4H, CH2NHCH2). 13C NMR (100 MHz, CDCl3): δ = 166.1,
161.1, 132.3, 131.3, 118.7, 118.6, 117.0, 59.5, 49.7.
2.3. Crystal structure determination
Single crystals of complexes 1, 2 and 3 were carefully selected and
glued to thin glass fibers. The diffraction data were collected on a
Bruker APEX-II CCD diffractometer for complex 2 and a Bruker P4
diffractometer for complexes 1 and 3 at 296(2) K equipped with Mo Kα
radiation (λ = 0.71073 Å). After data reduction and cell refinement
were performed with the SAINT program, a multi-scan empirical ab-
sorption correction was applied to the data by using the SADABS pro-
gram [27]. The crystal structures were solved by direct methods and
refined by full-matrix least-squares on F2 using SHELXL-97 [28] for
complex 2 and SHELXL-2018/3 [29] for complexes 1 and 3. All non-
hydrogen atoms were refined by anisotropic thermal parameters. The
hydrogen atoms were set in calculated positions and the isotropic
thermal parameters were fixed during the structure refinement. The
crystallographic data and structure refinement parameters of complexes
1, 2 and 3 are summarized in Table 1, the selected bond lengths and
angles are listed in Table S1, and the hydrogen bonding parameters are
summarized in Table S2. CCDC: 1585125, complex 1; 1585124, com-
plex 2; 1585126, complex 3.
2.2.2. Synthesis of H3L2
The compound was prepared as a yellow solid in 88% yield by the
treatment of triethylenetetramine (2.38 mL, 15.97 mmol) with salicy-
laldehyde (5.00 mL, 47.90 mmol) in 40 mL of water using the similar
procedure for the preparation of H2L1. Anal. Calcd for C27H30N4O3 (%):
C, 70.72; H, 6.59; N, 12.22. Found: C, 70.89; H, 6.64; N, 11.96. 1H NMR
(400 MHz, CDCl3): δ = 13.18 (br, 2H, OH), 10.61 (br, 1H, OH), 8.25 (s,
2H, N = CH), 7.30–6.78 (m, 12H, Ar-H), 3.84 (s, 1H, NCHN), 3.59 (t,
4H, C=NCH2), 2.97 (t, 4H, C=NCH2CH2), 2.67 (t, 4H, NCH2CH2N).
13C NMR (100 MHz, CDCl3): δ = 166.0, 161.1, 158.2, 132.2, 131.4,
130.9, 130.2, 120.8, 118.7, 118.5, 117.0, 89.6, 58.5, 52.8, 51.2.
2.4. General procedure for decomposition of hydrogen peroxide [30]
To a 50 mL round-bottom flask was added the synthesized com-
pound (1.0 mmol) and DMF (5 mL), and then an aqueous solution of
H2O2 (20 mL, 15 wt%) was carefully injected into the solution by a
syringe. The reaction was carried out at room temperature for 24 h.
After completing the reaction, the residual concentration of H2O2 was
titrated with a KMnO4 standard solution and the decomposition percent
of H2O2 was further calculated.
2.2.3. Synthesis of [Cu(HL1)(CH3CH2OH)]·(CH3COO) (1)
To a solution of Cu(OAc)2·H2O (0.74 g, 3.71 mmol) in ethanol
(30 mL) was added an ethanol solution of H2L1 (1.00 mL, 3.71 mmol) at
room temperature. The reaction mixture was stirred under reflux for
20 h and then filtered off after being cooled to room temperature. The
filtrate was evaporated to dryness under reduced pressure to provide a
dark blue residue. The residue was dissolved in a co-solvent of ethanol
and dichloromethane (v: v = 1: 1) and blue single crystals of 1 were
obtained at room temperature after 48 h. Yield: 71%, m.p.
115.0–116.5 °C. Anal. Calcd for C22H28N3O5Cu (%): C, 55.28; H, 5.90;
N, 8.79. Found: C, 55.47; H, 6.15; N, 8.65. IR (KBr, cm−1): 3443(vs),
3142(m), 2928(s), 2573(m), 2388(m), 1635(s), 1597(s), 1553(vs),
1449(s), 1307(m), 1247(m), 1148(m), 1050(m), 919(m), 864(w),
2.5. Biological activity assay
All the synthesized ligands and complexes have been screened for
their antibacterial activities in vitro. Three gram-negative bacteria in-
cluding Escherichia coli (ATCC 25922), Klebsiella pneumoniae (ATCC
70063) and Pseudomonas aeruginosa (ATCC 27853) and three gram-
positive bacteria including Bacillus subtilis (ATCC 6633), Staphylococcus
aureus (ATCC 25923) and Corynebacterium xerosis (ATCC 373) were
used as the test microorganisms.
The primary screening was carried out by the disc diffusion method
[31]. Dimethylsulphoxide (DMSO) with no inhibiting effects on the
2

C. Liu, et al.
InorganicaChimicaActa508(2020)119639
Table 1
Crystallographic data and structure refinement parameters for complexes 1, 2 and 3.
Complex
1
2
3
Formula
C22H29N3O5Cu
C44H50N6O12Zn3
C29H30N4O5Zn2
Fw
479.03
1051.07
645.31
Temperature
296(2)
296(2)
296(2)
Crystal system
Monoclinic
Tetragonal
Triclinic
Space group
a (Å)
Cc
I41/a
24.454(6)
Pī
16.6146(16)
10.144(2)
b (Å)
7.5227(7)
24.454(6)
11.501(2)
c (Å)
18.9472(17)
16.024(4)
16.118(3)
α
90.00
101.911(3)
90.00
2317.2(4)
90.00
90.00
90.00
9582(4)
91.97(3)
93.83(3)
110.58(3)
1753.1(7)
β
γ
Volume (Å3)
Z
4
8
2
Dcalcd/(g·cm−3
)
1.370
0.979
1004
1.457
1.555
4336
1.223
1.405
664
μ (mm−1
F (0 0 0)
)
Crystal size (mm)
Reflections collected
Unique reflections (Rint
Gof
0.22 × 0.25 × 0.28
7772
3203 (0.025)
1.055
0.22 × 0.25 × 0.27
26,223
4221 (0.048)
1.034
0.11 × 0.12 × 0.13
16,272
8577(0.025)
)
1.040
Final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0320, wR2 = 0.0813
R1 = 0.0351, wR2 = 0.0833
0.44 and −0.31
R1 = 0.0318, wR2 = 0.0729
R1 = 0.0554, wR2 = 0.0843
0.43 and −0.18
R1 = 0.0374, wR2 = 0.1006
R1 = 0.0515, wR2 = 0.1063
0.37 and −0.53
Largest diff. peak and hole (e Å−3
)
aR = Σ(||F0| − Fc||)/Σ|F0|, bwR = [Σw(|F0|2 − |Fc|2)2/Σw(F02)]1/2
.
microorganisms was used as solvent to dissolve the compounds. The
standard drugs, ciprofloxacin (for gram-positive) and gentamicin (for
gram-negative), were used as positive references. The discs (diameter
6 mm) impregnated separately with the compounds at the concentra-
tion of 10 mg/mL were placed on the inoculated agar plates and all the
plates were incubated at 37 °C for 24 h. By measuring the inhibition
zone diameters, the antibacterial activities were evaluated and re-
corded. The secondary screening was carried out by measuring the
minimum inhibitory concentration (MIC). The compounds were dis-
solved in DMSO to prepare the solutions with the initial concentration
of 256 μg/mL, whose final concentration ranged from 256 to 0.5 μg/mL
with two-fold serial dilutions. A standard suspension of the micro-
organism was diluted in broth media to obtain a final concentration of
1 × 106 colony-forming units(CFU)/mL. The sterile micro plates were
incubated for 24 h at 37 °C for all bacteria strains. MIC was the lowest
antibacterial concentration and chosen as the evaluation criterion. All
the assays were repeated in triplicate.
authenticated by elemental analyses and NMR spectroscopy. Compared
with the similar compounds synthesized in organic solvents [32,33],
these ligands could be prepared more effectively by using water as
solvent at room temperature.
Complexes 1 and 2 have been synthesized by the reactions of H2L1
with Cu(OAc)2·H2O (1:1 M ratio) and Zn(OAc)2·2H2O (2:3 M ratio)
under refluxing, respectively (Fig. 1 and Fig. 2). Moreover, we also tried
to synthesize the other two Cu(II) and Zn(II) complexes by the reactions
of H3L2 with Cu(OAc)2·H2O and Zn(OAc)2·2H2O, respectively. But un-
fortunately, only complex 3 has been synthesized as a white crystal in
79% yield (Fig. 3), the copper complex hasn’t been obtained under the
same conditions [1–45]. By summarizing the preparation of the Cu(II)
and Zn(II) complexes with similar ligands [34], we would try to syn-
thesize the copper complex using different methods in the following
study. The complexes were stable in air and were fully characterized by
IR, melting point, elemental analyses, and single crystal X-ray diffrac-
tion.
3. Results and discussion
3.2. Structure description
3.1. Synthesis of the ligands and the complexes
3.2.1. Structure of complex 1
The results of the single crystal X-ray diffraction analysis reveal that
the asymmetric unit of complex 1 contains one acetate anion and one
The ligands H2L1 and H3L2 have been synthesized by the con-
densation of salicylaldehyde with diethylenetriamine (2:1 M ratio) and
triethylenetetramine (3:1 M ratio) in water (Scheme 1). H2L1 and H3L2
were obtained as a yellow oil in 92% yield and a yellow solid in 88%
yield, respectively. The purity and structure of the ligands were
N
N
N
N
OH
N
OH
OH
H2N
N
H
NH2
N
H
N
H2N
NH2
H
NH
N
CHO
OH
H2O
H2O
OH
OH
H2L1
H3L2
Fig. 1. Crystal structure of complex 1 with 30% probability ellipsoids (H atoms
Scheme 1. Synthesis of the ligands.
are omitted for clarity).
3

C. Liu, et al.
InorganicaChimicaActa508(2020)119639
O4 and C16—H16…O2(iv)). Under the weak interactions, the crystal
structure is further stabilized. The crystal packing diagram of complex 1
is shown in Fig. S1.
3.2.2. Structure of complex 2
Complex 2 is a bridged trinuclear Zn(II) molecule, which crystallizes
in tetragonal crystal system with I41/a space group. As shown in Fig. 2,
the asymmetric unit consists of only half of the molecule, which con-
tains three crystallographically independent Zn(II) cations, two depro-
tonated N2O Schiff base ligands and four acetates. Zn2 ion has the same
coordination environment with Zn2(i) ion, where Zn2 ion is penta-co-
ordinated by one oxygen atom (O2(i)) and two nitrogen atoms (N1 and
N2) from one tridentate chelating H2L1 ligand, one bridging oxygen
atom (O5) from one monodentate acetate, and one oxygen atom (O8)
from one bidentate bridging acetate, exhibiting a slightly distorted
trigonal bipyramidal ZnN2O3 coordination geometry. The three atoms
(O5, O8 and N1) define the basal plane of the trigonal bipyramidal
structure, while N2 and O2(i) atoms are located in the axial position
with the N2—Zn2—O2(i) angle of 167.84(9)°. The Zn2—N bond dis-
tances are in the ranges of 2.022(3) and 2.184(3) Å and the Zn2—O
bond distances are in the ranges of 1.992(2) and 2.144(2) Å, which are
similar to those in other related Zn(II) complexes [38,39]. The sum of
the bond angles O5—Zn2—O8 (104.58(8)°), O5—Zn2—N1
(140.16(10)°) and O8—Zn2—N1 (113.87(10)°) around the metal center
is 358.61°, which further confirms that the four atoms are nearly co-
planar. Zn1 ion is six-coordinated by two oxygen atoms (O6 and O6(i))
from two bidentate bridging acetates, two bridging oxygen atoms (O5
and O5(i)) from two monodentate acetates, and two bridging hydroxyl
oxygen atoms (O2 and O2(i)) from two H2L1 ligands. A slightly dis-
torted octahedral ZnO6 coordination structure is observed in the Zn1
ion, with two hydroxyl oxygen atoms (O6 and O6(i)) in the axial po-
sition and four oxygen atoms (O2, O5, O2(i) and O5(i)) from four
acetates in the equatorial plane. The Zn1—O bond lengths fall in the
ranges of 2.054(2) and 2.2168(19)Å, while the bond angles around Zn1
ion fall in the ranges of 79.72(7) ° and 100.28(7)°. Zn1 ion affords a
bridged trinuclear Zn3 building subunit together with the Zn2 ion and
its symmetric Zn2(i) ion. The adjacent Zn(II) ions (Zn1, Zn2 and Zn2(i))
are bridged by two acetates in a μ2-η1:η1-bridging coordination mode,
two acetates in a μ2-η2-bridging mode, and two H2L1 ligands in a μ2-
η1:η1:η2-chelating/bridging mode, with a nonbonding Zn1…Zn2 dis-
tance of 3.0571(6) A˚. As shown in Fig. S2, one intramolecular hy-
drogen bond (O1—H1…N4) and three types of nonclassical hydrogen
bonds (C9—H9A…O4, C11—H11B…O4(ii) and C17—H17…O5) are
also observed in complex 2. By the hydrogen bond interactions, the
complex molecules are extended into an infinite 3D supramolecular
framework structure.
Fig. 2. Crystal structure of complex 2 with 30% probability ellipsoids (H atoms
are omitted for clarity). Symmetry code: (i) 2 − x, −y, −z.
Fig. 3. Crystal structure of complex 3 with 30% probability ellipsoids (H atoms
are omitted for clarity).
mononuclear [Cu(HL)(CH3CH2OH)]+ cation, which crystallizes in
monoclinic crystal system with Cc space group. The crystal structure of
complex 1 is shown in Fig. 1. The Cu(II) ion is coordinated by an
ethanol molecule in an η1 mode and a deprotonated N3O Schiff base
ligand in an η4 mode, resulting in a slightly distorted square-pyramidal
CuN3O2 coordination geometry. The τ value is applicable to the penta-
coordinated structure as an important index of the degree of trigonality
[35], which helps us distinguish the square pyramidal geometry. The
central Cu1 ion, with trigonality index τ = 0.206 which deviates from 0
(ideal square pyramid) and 1 (ideal trigonal bipyramid), is situated in a
distorted square pyramidal environment. The basal plane consists of
one hydroxyl oxygen atom (O4), two imino nitrogen atoms (N1 and N3)
and one secondary amine nitrogen atom (N2) from the ligand. The axial
position is occupied by one oxygen atom (O5) from the ethanol mole-
cule. The Cu1—N1, Cu1—N2, and Cu—N3 bond distances are 1.990(5),
2.010(5) and 1.936(5)Å, respectively, which are in agreement with
those of the reported structures [36,37]. The Cu1—O5 bond (2.323(3)
Å) is significantly longer than the Cu1—O4 bond (1.906(4) Å), which
indicates that the strength of the Cu1—O5 bond is weaker than that of
the Cu1—O4 bond. The bond angles N1—Cu1—N2 (84.28(19)°),
N2—Cu1—N3 (85.1(2)°), O4—Cu1—N3 (93.8(2)°) and O4—Cu1—N1
(94.59(18)°) are added up to 357.77° unequal to 360°. The bond angles
O5—Cu1—N1 (99.03(18)°), O5—Cu1—N2 (91.78(17)°), O5—Cu1—N3
(97.65(18)°) and O4—Cu1—O5 (95.91(16)°) are all different to the
ideal value of 90°. All those indicate that the central Cu1 ion adopts a
distorted square pyramidal geometry. Furthermore, there is one acetate
anion in the asymmetric unit, which acts as the counter anion to bal-
ance the valence charge of the [Cu(HL)(CH3CH2OH)]+ cation. There
exist three types of intermolecular hydrogen bonds (N2—H2A…O1(i),
O3—H3A…O1(ii) and O5—H5A…O2(i)) and four types of nonclassical
hydrogen bonds (C9—H9A…O2(iii), C10—H10B…O2(iii), C12—H12…
3.2.3. Structure of complex 3
Complex 3 is a bridged binuclear Zn(II) molecule, which crystallizes
in triclinic crystal system with Pī space group. As shown in Fig. 3, the
asymmetric unit of complex 3 contains two crystallographically in-
dependent Zn(II) cations, one deprotonated N4O3 Schiff base ligand and
one acetate. Zn1 ion has the same coordination environment with Zn2
ion, where Zn1 is penta-coordinated by two oxygen atoms (O1 and O5)
and two nitrogen atoms (N1 and N2) from one seven-dentate chelating
H3L2 ligand, and one oxygen atom (O2) from one bidentate bridging
acetate, exhibiting a slightly distorted trigonal bipyramidal ZnN2O3
coordination geometry. The basal plane is defined by one hydroxyl
oxygen atom (O5), one imino nitrogen atoms (N1) and one oxygen
atom (O2), while the axial position is occupied by one secondary amine
nitrogen atom (N2) and one hydroxyl oxygen atom (O1) with the
O1—Zn1—N2 angle of 162.63(8)°. The Zn—N bond distances range
from 2.014(2) to 2.406(2) A˚ and the Zn—O bond distances are in the
ranges of 1.9795(19) and 1.9945(19) Å. The bond angles O5—Zn1—N1
(139.15(8)°),
O2—Zn1—O5
(109.38(8)°)
and
O2—Zn1—N1
(109.73(9)°) around the metal center are added up to 358.26°,
4

C. Liu, et al.
InorganicaChimicaActa508(2020)119639
Table 2
showed that the catalytic reaction could be performed efficiently with a
catalyst loading ranging between 1.0 and 2.0 mmol (Table 2, entries
8–10, 13–15). It is interesting to find that when the loading of com-
plexes 2 and 3 was 1.0 mmol equal to that of complex 1, the decom-
position percents of 93% and 89% have been obtained, respectively
(Table 2, entries 8 and 13). Further increase in the catalyst loading did
not improve the catalytic activities of complexes 2 and 3 appreciably. In
consideration of the decomposition percent and the cost of catalyst, a
catalyst loading of 1.0 mmol is the most suitable for the decomposition
reaction. Additionally, the two ligands were used to catalyze the de-
composition of H2O2. But unfortunately, the residual concentration of
H2O2 remained virtually unchanged, which indicated that the ligands
showed poor catalytic activities (Table 2, entries 16–17). The decom-
position reaction could also be catalyzed by some acetates such as Cu
(OAc)2·H2O and Zn(OAc)2·2H2O [30,42]. The results of the literatures
showed that the decomposition percents only reached 60% and 74%
with equal amount of Cu(OAc)2·H2O and Zn(OAc)2·2H2O as catalyst
(Table 2, entries 18–19). Compared to Cu(OAc)2·H2O and Zn
(OAc)2·2H2O, our complexes showed better catalytic activities in the
decomposition of H2O2, which has a certain practical significance for
the development of new efficient catalysts.
Optimization of decomposition conditions of hydrogen peroxide.
Entry
Catalyst
Catalyst loading (mmol)
Decomposition percent (%)
1
2
3
4
5
6
7
8
Complex 1
0.1
0.5
1.0
1.5
2.0
0.1
0.5
1.0
1.5
2.0
0.1
0.5
1.0
1.5
2.0
1.0
1.0
1.0
1.0
17
59
97
97
98
25
47
93
93
93
21
52
89
91
92
0
Complex 2
Complex 3
9
10
11
12
13
14
15
16
17
18
19
H2L1
H3L2
Cu(OAc)2·H2O
Zn(OAc)2·2H2O
0
60
74
Taking into account of the above finding and the metal content in
catalyst, complex 1 was chosen as the catalyst for the decomposition
reaction of H2O2.
approximately equal to 360°, indicating that Zn1 ion is nearly coplanar
with the three coordination atoms. Interestingly, it is observed that
there exist two kinds of different coordination modes for the hydroxyl
oxygen atoms from the ligand. One hydroxyl oxygen atom (O1) is only
coordinated to Zn1 ion in an η1 mode, while the other hydroxyl oxygen
atom (O5) is coordinated to both Zn1 and Zn2 ions in a μ2-η1:η1-brid-
ging mode. Zn2 ion affords a binuclear Zn2 building subunit together
with the Zn1 ion. The adjacent Zn(II) ions (Zn1 and Zn2) are bridged by
one bidentate acetate and one hydroxyl oxygen atoms (O3) from the
ligand, with a nonbonding Zn1…Zn2 distance of 3.2293(11) A˚. Unlike
complexes 1 and 2, there exist no hydrogen bonds in complex 3. The
crystal packing diagram of complex 3 is shown in Fig. S3.
3.4. Antibacterial activity
Copper and zinc complexes have a wide range of biological activ-
ities, especially in antibacterial activities, so in order to investigate the
biological activities of the complexes, their antibacterial activities
against gram-negative bacteria and gram-positive bacteria were
screened. For the primary screening, the inhibition zone diameters were
tested and the results were presented in Table 3. The two Schiff base
ligands exhibited very weak activities against the bacteria and the
solvent DMSO had no inhibiting effects. Compared to the two ligands,
complexes 1, 2 and 3 had larger bacterial inhibition zones. Complex 1
exhibited stronger antibacterial activities against S. aureus with an in-
hibition zone diameter of 14 mm than other five bacteria with inhibi-
tion zone diameters between 11 mm and 13 mm. In addition, complex 2
exhibited excellent inhibitory activities against K. pneumoniae with an
inhibition zone diameter of 17 mm, while complex 3 exhibited stronger
activities against S. aureus and C. xerosis with an inhibition zone dia-
meter of 14 mm, respectively.
After above primary screening, the MIC values of the compounds
were also measured, and two standard drugs (ciprofloxacin and gen-
tamicin) were used as the positive controls (listed in Table 4). Com-
plexes 1, 2 and 3 showed antibacterial activities against the six bac-
teria, with the MIC values between 16 and 128 µg/mL, while the MIC
values of the ligands were all beyond 128 µg/mL, confirming their poor
activities. The antibacterial activities of complexes might be related to
the chelation and the biological function of the metal ion, which could
be explained by Tweedy’s chelation theory [43]. Chelation reduces the
3.3. Catalytic activity
Hydrogen peroxide, a typical inorganic reductant and oxidant,
could be decomposed with some suitable transition metal compounds
as catalyst [40,41]. In order to investigate the catalytic activities of the
synthesized compounds, the catalytic decomposition of H2O2 was ex-
amined at room temperature.
As shown in Table 2, there is a close relationship between the de-
composition percent of H2O2 and the catalyst loading. In the presence
of complex 1 as catalyst, the decomposition reaction of H2O2 could be
carried out well. When the catalyst loading was below 1.0 mmol, the
decomposition percent increased significantly with the increasing of the
catalyst loading (Table 2, entries 1–3). Increasing the catalyst loading to
1.0 mmol, the decomposition percent reached up to 97%. When the
catalyst loading was beyond 1.0 mmol (Table 2, entries 4–5), the de-
composition percent remained almost constant. In the meantime, the
catalytic activities of complexes 2 and 3 were also surveyed. The results
Table 3
Inhibition zone diameter of the ligands, complexes 1, 2 and 3 and the standard drugs.
Compound
Inhibition zone diameter (mm)
B. subtilis
S. aureus
C. xerosis
E. coli
K. pneumoniae
P. aeruginosa
DMSO
H2L1
−
8
−
7
−
8
−
7
−
8
−
8
H3L2
8
9
9
7
8
10
12
15
12
−
27
Complex 1
Complex 2
Complex 3
Ciprofloxacin
Gentamicin
13
14
13
23
−
14
16
14
28
−
13
15
14
27
−
12
15
12
−
25
11
17
13
−
25
5

C. Liu, et al.
InorganicaChimicaActa508(2020)119639
Table 4
MIC of the ligands, complexes 1, 2 and 3 and the standard drugs.
Compound
MIC (μg/mL)
B. subtilis
S. aureus
C. xerosis
E. coli
K. pneumoniae
P. aeruginosa
H2L1
H3L2
256
256
64
> 256
128
32
16
32
256
256
64
32
64
> 256
> 256
128
64
128
−
> 256
256
128
16
64
−
256
256
128
64
128
−
Complex 1
Complex 2
Complex 3
Ciprofloxacin
Gentamicin
64
128
1.0
−
0.5
−
1.0
−
0.5
2.0
1.0
polarity of the metal ion because the positive charge of the ion is par-
tially shared by the donor groups of the ligand, and then the abilities of
complexes to cross cell membranes are enhanced. The MIC value of
complex 1 against the gram-positive bacteria S. aureus was 32 μg/mL
and those against two other gram-positive bacteria were both 64 μg/
mL. Interestingly, it could be observed that complex 1 showed anti-
bacterial activities against the three gram-negative bacteria with the
same MIC value of 128 µg/mL. The MIC values of complex 2 were ob-
served between 16 and 64 µg/mL, while those of complex 3 containing
the same Zn(II) ion were between 32 and 128 µg/mL. In comparison
with complex 2, the MIC values of complex 3 increased two-fold to
four-fold against the same bacteria, respectively. The strong anti-
bacterial activities of complex 2 may be mainly due to two factors. On
the one hand, complex 2 exhibits a trinuclear Zn3 molecule structure,
while complex 3 is a binuclear Zn2 molecule. On the other hand, two
H2L1 ligands exist in complex 2, while only one H3L2 ligand exists in
complex 3. According to the relevant literatures as well as the mem-
brane theory of cell permeability [44,45], the special structure of
complex 2 is more beneficial for its penetration through the cell
membranes. Compared with ciprofloxacin and gentamicin, complex 2
showed slightly weaker antibacterial activities against the tested bac-
teria.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (no. 21977001) and Innovative Research Team of
Anhui Provincial Education Department (2016SCXPTTD).
Appendix A. Supplementary data
CCDC 1585125, 1585124 and 1585126 contain the supplementary
crystallographic data for complexes 1, 2 and 3. These data can be ob-
tained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html or from Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1E2, UK; fax:(+44) 1223-336-033; or E-mail: deposit@
ccdc.cam.ac.uk. Supplementary data to this article can be found online
at https://doi.org/10.1016/j.ica.2020.119639.
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solvothermal conditions. Complex 1 exhibits a mononuclear structure,
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