Synthesis, crystal structure and antibacterial activity of a heterometallic tetranuclear Cd2Tb2 complex derived from 5-chloro-2-(3-ethoxy-2-hydroxybenzylideneamino)phenol

Article abstract of DOI:10.1080/24701556.2021.1897616

A new heterometallic tetranuclear complex [Cd₂Tb₂L₄(CH₃COO)₂(DMF)₂] was prepared from the Schiff base 5-chloro-2-(3-ethoxy-2-hydroxybenzylideneamino)phenol (H₂L). The complex was

Full text of DOI:10.1080/24701556.2021.1897616

Inorganic and Nano-Metal Chemistry
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Synthesis, crystal structure and antibacterial
activity of a heterometallic tetranuclear Cd2Tb2
complex derived from 5-chloro-2-(3-ethoxy-2-
hydroxybenzylideneamino)phenol
Shao-Song Qian
To cite this article: Shao-Song Qian (2021): Synthesis, crystal structure and antibacterial
activity of a heterometallic tetranuclear Cd2Tb2 complex derived from 5-chloro-2-(3-
ethoxy-2-hydroxybenzylideneamino)phenol, Inorganic and Nano-Metal Chemistry, DOI:
10.1080/24701556.2021.1897616
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INORGANIC AND NANO-METAL CHEMISTRY
https://doi.org/10.1080/24701556.2021.1897616
Synthesis, crystal structure and antibacterial activity of a heterometallic
tetranuclear Cd2Tb2 complex derived from 5-chloro-2-(3-ethoxy-2-
hydroxybenzylideneamino)phenol
Shao-Song Qian
School of Life Sciences, Shandong University of Technology, Zibo, P.R. China
ABSTRACT
ARTICLE HISTORY
Received 25 August 2020
Accepted 1 December 2020
A new heterometallic tetranuclear complex [Cd2Tb2L4(CH3COO)2(DMF)2] was prepared from the
Schiff base 5-chloro-2-(3-ethoxy-2-hydroxybenzylideneamino)phenol (H2L). The complex was char-
acterized by IR and UV–vis spectra, as well as single crystal X-ray determination. The Cd atoms are
in octahedral coordination with O5N donor set, and the Tb atoms are in square-antiprismatic
coordination with O7N donor set. The Cd and Tb atoms are bridged by phenolate oxygen and
acetate groups. The Schiff base and the complex showed good antibacterial activities against the
bacteria Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhi.
KEYWORDS
Schiff base; Cd-Tb complex;
heteronuclear complex;
crystal structure;
antibacterial activity
Introduction
NMR spectra were recorded on a Bruker DRX-500 NMR
spectrometer; chemical shifts are referenced to residual solv-
ent peaks (with respect to TMS ¼ d 0 ppm). FTIR spectra
were recorded on a Bruker AXS TENSOR-27 instrument
with KBr pellets. UV–vis spectra were recorded on a
Lambda 35 spectrophotometer.
Schiff base ligands are a favorite choice for the construction
of metal complexes with versatile structures and widely
applications.[1–4] Polynuclear complexes of transition metal
atoms have attracted much attention due to their interesting
catalytic, magnetic, and biological activities.[5–8] Some cad-
mium complexes and terbium complexes are reported to
have various biological activities like antibacterial.[9–12]
Generally, multi-dentate ligands with suitable configuration
are used to construct metal complexes with versatile struc-
tures. The tetra-dentate Schiff bases with the basal skeleton
2-(hydroxybenzylideneamino)phenol (HBP) have received
particular interest in constructing polynuclear com-
plexes.[13–17] Although coordination of HBP and its deriva-
Synthesis of 5-chloro-2-(3-ethoxy-2-
hydroxybenzylideneamino)phenol (H2L)
A solution of 2-amino-4-chlorophenol (0.72 g, 5.0 mmol) in
MeOH (20 mL) was added to a stirred solution of and 3-
ethoxysalicylaldehyde (0.83 g, 5.0 mmol) in MeOH (20 mL)
and the resulting solution was stirred at room temperature
tives to nickel(II), manganese(II), zinc(II), and copper(II) for 1 h. The solvent was evaporated to give orange precipi-
has been reported, this type of ligands remains little investi-
gated. The reported heterometallic polynuclear complexes of
such Schiff bases are focused on their magnetic properties.
Considering the good biological activities of Schiff base
complexes, herein, we report a new heterometallic tetranu-
clear complex, [Cd2Tb2L4(CH3COO)2(DMF)2], where L is
the dianionic form of 5-chloro-2-(3-ethoxy-2-hydroxybenzy-
lideneamino)phenol (H2L; Scheme 1). The preliminary study
on the antibacterial activities was also reported.
tate, which was recrystallized from ethanol to form crystal-
line product.
Yield: 1.1 g (76%). Elemental analysis: Calcd for
C15H14ClNO3: C, 61.76; H, 4.84; N, 4.80. Found: C, 61.57;
H, 4.93; N, 4.72. 1H NMR (500 MHz, DMSO-d6) d/ppm:
13.70 (s, 1H, OH), 10.01 (s, 1H, OH), 8.98 (s, 1H, CH ¼ N),
7.49 (s, 1H, ArH), 7.20–7.15 (m, 2H, ArH), 7.10 (d, 1H,
ArH), 6.97 (d, 1H, ArH), 6.86 (t, 1H, ArH), 4.07 (q, 2H,
CH2), 1.35 (t, 3H, CH3). 13C NMR (126 MHz, DMSO-d6)
d/ppm: 162.96, 151.68, 150.11, 147.17, 135.84, 127.30,
124.13, 123.08, 119.33, 119.10, 118.18, 117.76, 116.99, 64.09,
14.77. IR (KBr, cmꢀ1): 3436 br, 1634s, 1598s, 1505s, 1431 w,
1353 m, 1290 w, 1245 m, 1209s, 1172 w, 1079 w, 1006 m,
Experimental
Materials and measurements
902 m 812 w, 745 m. UV–vis kmax/nm (1.02 ꢁ 10ꢀ5 mol Lꢀ1
,
All starting materials were of analytical reagent grade and
MeOH; e, L molꢀ1 cmꢀ1): 263 (22700), 298 (10120),
were used as received without further purification. All reac-
1
tions were carried out under aerobic conditions. H and 13C 345 (8750).
CONTACT Shao-Song Qian
sdutqss@163.com
School of Life Sciences, Shandong University of Technology, Zibo 255049, P.R. China.
ß 2021 Taylor & Francis Group, LLC

2
S.-S. QIAN
Table 2. Selected bond distances (Å) and angles (ꢂ) for the complex.
Tb1-O1
2.206(6)
Tb1-O4
Tb1-O2
Tb1-O9
Tb1-O6
Cd1-N2
2.332(6)
Tb1-O8
Tb1-O5A
Tb1-N1
Cd1-O2
Cd1-O4
Cd1-O7
2.356(7)
2.415(6)
2.533(8)
2.215(5)
2.229(6)
2.268(7)
149.5(2)
83.1(2)
71.8(2)
84.3(2)
146.6(3)
75.0(3)
2.391(6)
2.475(8)
2.565(6)
2.227(7)
2.264(6)
2.563(5)
113.8(3)
135.9(2)
73.4(2)
94.84(19)
74.30(19)
88.2(3)
140.8(2)
72.7(2)
78.9(3)
80.5(2)
87.7(2)
127.0(2)
79.2(2)
152.9(2)
77.1(2)
116.8(2)
148.6(2)
106.0(2)
111.2(3)
86.2(2)
Cd1-O5
Cd1-O2A
O1-Tb1-O4
O4-Tb1-O8
O4-Tb1-O2
O1-Tb1-O5A
O8-Tb1-O5A
O1-Tb1-O9
O8-Tb1-O9
O5-Tb1-O9A
O4-Tb1-N1
O2-Tb1-N1
O9-Tb1-N1
O4-Tb1-O6
O2-Tb1-O6
O9-Tb1-O6
O2-Cd1-N2
N2-Cd1-O4
N2-Cd1-O5
O2-Cd1-O7
O4-Cd1-O7
O2-Cd1-O2A
O4-Cd1-O2A
O7-Cd1-O2A
O1-Tb1-O8
O1-Tb1-O2
O8-Tb1-O2
O4-Tb1-O5A
O2-Tb1-O5A
O4-Tb1-O9
O2-Tb1-O9
O1-Tb1-N1
O8-Tb1-N1
O5-Tb1-N1A
O1-Tb1-O6
O8-Tb1-O6
O5-Tb1-O6A
N1-Tb1-O6
O2-Cd1-O4
O2-Cd1-O5
O4-Cd1-O5
N2-Cd1-O7
O5-Cd1-O7
N2-Cd1-O2A
O5-Cd1-O2A
70.9(3)
142.5(2)
137.4(2)
66.1(2)
120.8(3)
62.36(19)
124.02(18)
69.2(3)
158.2(2)
83.9(2)
75.3(2)
87.1(2)
96.9(3)
80.6(2)
81.86(19)
Scheme 1. H2L.
Table 1. Crystallographic and refinement data for the complex.
Formula
C70H68Cd2Cl4N6O18Tb2
Formula weight
T (K)
1965.74
298(2)
Crystal system
Space group
a (Å)
Triclinic
P\bar1
73.63(19)
10.6911(12)
12.4497(13)
15.7119(10)
85.623(1)
71.148(1)
74.672(1)
1909.6(3)
1
167.6(2)
b (Å)
c (Å)
a (ꢂ)
b (ꢂ)
c (ꢂ)
1246s, 1213s, 1108 w, 1078 w, 1012 w, 927 w, 896 w, 845 w,
V (Å3)
742 w, 676 m, 657 w, 588 w, 545 w, 459 w. UV–vis kmax/nm
(1.07 ꢁ 10ꢀ5 mol Lꢀ1
, MeOH; e, L
molꢀ1 cmꢀ1): 230
(20600), 296 (8135), 373 (7620), 425 (7250).
Z
Dcalc (g cm–3
)
1.709
2.588
968
m (Mo Ka) (mm–1
F(000)
)
Measured reflections
Unique reflections
Observed reflections (I ꢃ 2r(I))
Parameters
10713
6966
5099
462
Crystal structure determinations
A
suitable
single
crystal
with
dimensions
of
Restraints
0
0.11 ꢁ 0.08 ꢁ 0.08 mm3 for the complex was selected for sin-
gle-crystal X-ray diffraction analysis. Crystallographic data
were collected at a temperature of 298(2) K on a Bruker
Apex II CCD diffractometer with graphite-monochromated
Mo Ka radiation (k ¼ 0.71073 Å). Data processing was
accomplished with the SAINT processing program.[18] The
structure was solved by direct methods and refined on F2 by
full-matrix least squares using SHELXTL.[19] The locations
of the heaviest atoms (Cd, Tb) were easily determined, and
the O, N, Cl, and C atoms were subsequently determined
from the difference Fourier maps. The non-H atoms were
refined isotopically. The crystal data and refinement parame-
ters are listed in Table 1. Selected bond lengths and angles
of the complex are summarized in Table 2.
Goodness of fit on F2
R1, wR2 [I ꢃ 2r(I)]a
1.008
0.0596, 0.1497
0.0843, 0.1696
R1, wR2 (all data)a
aR1 ¼ Fo – Fc/Fo, wR2 ¼ [ w(Fo – Fc2)/ w(Fo2)2]1/2
P
P
2
Synthesis of [Cd2Tb2L4(CH3COO)2(DMF)2]
A solution of triethylamine (0.65 mL, 0.30 mmol) was added
to the methanolic solution (10 mL) of H2L (29 mg,
0.10 mmol). The reaction mixture was stirred for 30 min.
Then, solid terbium nitrate hexahydrate (45 mg, 0.10 mmol)
was added to the above mixture, and the resulting solution
was stirred for 1 h. Solid nickel acetate tetrahydrate (25 mg,
0.10 mmol) was added and further stirred for 1 h. A total of
5 mL N,N-dimethylformamide was subsequently added, and
the reaction mixture was filtered and then left unperturbed
to allow the slow evaporation of the solution. Deep green
block single crystals, suitable for X-ray diffraction analysis,
were obtained after 12 days. These crystals were collected by
filtration, washed with cold methanol, and dried in air.
Yield: 1.1 g (76%). Elemental analysis: Calcd for
C70H68Cd2Cl4N6O18Tb2: C, 42.77; H, 3.49; N, 4.28. Found:
C, 42.96; H, 3.61; N, 4.15. IR (KBr, cmꢀ1): 3587 w, 3531 w,
Antibacterial assay
Qualitative determination of antibacterial activity was done
using the disk diffusion method. Suspensions in sterile pep-
tone water from 24 h culture of microorganisms were
adjusted to 0.5 McFarland. Muller–Hinton Petri dishes of
90 mm were inoculated using these suspensions. Paper disks
(6 mm in diameter) containing 10 lL of the substance to be
1642s, 1598s, 1543 m, 1480s, 1433s, 1381 m, 1310 m, 1282 m, tested (at a concentration of 2048 lg/mL in DMSO) were

INORGANIC AND NANO-METAL CHEMISTRY
3
Figure 1. Molecular structure of the complex, showing the crystallographic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level and
H atoms are omitted for clarity. Atoms labeled with the suffix A and those unlabeled are related to the symmetry operation 1 – x, 2 – y, – z.
placed in a circular pattern in each inoculated plate. broad band (kmax ¼ 345 nm) in the electronic absorption
spectrum that extends into the visible region.[16]
The reaction of H2L with terbium nitrate hexahydrate
and nickel acetate tetrahydrate in the presence of triethyl-
amine in a mixture of DMF and MeOH resulted in the
green solution of the complex. The crystals of the com-
plex were obtained by slow evaporation of the solution
in air.
Incubation of the plates was done at 37 ꢂC for 18–24 h.
DMSO impregnated disks were used as negative controls.
Toxicity tests of the solvent, DMSO, showed that the con-
centrations used in antibacterial activity assays did not inter-
fere with the growth of the microorganisms. Reading of the
results was done by measuring the diameters of the inhib-
ition zones generated by the test substance. Penicillin was
used as a reference.
Determination of MIC was done using the serial dilutions
in liquid broth method. The materials used were 96-well
plates, suspensions of microorganism, Muller-Hinton broth
and stock solutions of each substance to be tested (2048 lg/
mL in DMSO). The following concentrations of the substan-
ces to be tested were obtained in the 96-well plates: 1024,
512, 256, 128, 64, 32, 16, 8, 4, 2, and 1 lg/mL. After incuba-
tion at 37 ꢂC for 18–24 h, the MIC for each tested substance
was determined by microscopic observation of microbial
growth. It corresponds to the well with the lowest concen-
tration of the tested substance where microbial growth was
clearly inhibited.
Structure description of [Cd2Tb2L4(CH3COO)2(DMF)2]
The molecular structure of the complex is shown in Figure
1. The complex crystallizes in the triclinic space group
P\bar1, and with Z ¼ 1 such that the molecule has centro-
symmetric site symmetry in the crystal. The complex is
composed of four metal atoms, four Schiff base ligands, two
acetate bridging ligands, and two DMF terminal ligands.
The Cd atom has adopted a slightly distorted octahedral
geometry with an O5N donor set, while the Tb atom has a
distorted square antiprismatic geometry with an O7N donor
set. The Cd atom is chelated by the imino N and two phe-
nolate atoms of one of the two independent Schiff base
ligands in the structure, with these two phenolate O atoms
each forming l2-bridges from the chelated Cd to different
Tb atoms. The other independent Schiff base ligand coordi-
nates rather differently, with one phenolate O atom chelat-
ing a Tb atom. The vanillin-derived phenolate O atom
coordinates to the Tb atom, and not coordinates to other
metal atoms. The amino phenolate O atom, by contrast,
forms a l3-bridge over a Cd2Tb triangle. Two such triply
bridging oxygens coordinate to each Cd atom, with an acet-
ate ligand coordinating through oxygen completing the O5N
Results and discussion
Chemistry
To the best of our knowledge, although some complexes
derived from similar Schiff bases as H2L have been
reported,[13–17] the crystal structure of the present complex
is new. The Schiff base H2L was obtained as orange product
from the condensation reaction of 2-amino-4-chlorophenol
with 3-ethoxysalicylaldehyde. The orange color arises from a environment of each Cd center. Each Tb atom is ligated by

4
S.-S. QIAN
Figure 2. The molecular packing structure of the complex, viewed along the a axis.
the ONO donor set of one Schiff base ligand, and the OO IR and UV–vis spectra
donor set of the other independent Schiff base ligand from
The IR spectrum of H2L and the complex provide informa-
opposite directions, filling five coordination sites in a dis-
torted pentagonal planar geometry. The sixth coordination
site is occupied by a l2-phenolate O atom from a third
Schiff base ligand, and the seventh and eighth sites are filled
by the other O atom of the acetate ligand, and the O atom
of the DMF ligand. The core of the complex can be
described in terms of the well-known defect-dicubane or
butterfly topology with two Cd atoms in the body positions
and each Tb atom in the wing positions.[14] The metal
atoms of Cd2Tb2 core are each bridged by l2-phenolate O
atoms from two symmetry-related Schiff base ligands, while
the two other Schiff base ligands form l3-bridges over the
two Cd2Tb triangles with O2 atom displaced by 0.925(2) Å
out of the Cd2Tb plane, on opposite sides of the strictly pla-
nar Cd2Tb2 core. Similar tetranuclear topology can be found
in some nickel or nickel and rare earth metal com-
plexes.[20–22] Each acetate ligand bridges the terminal metal
and central metal in a syn-syn fashion.
The crystal structure is shown as Figure 2. There are
weak pꢄꢄꢄp interactions among the molecules with centroid
to centroid distances of 3.02–4.83 Å [Cg1ꢄꢄꢄCg4 ¼ 3.024(4) Å,
Cg1ꢄꢄꢄCg9 ¼ 4.826(4) Å, Cg4ꢄꢄꢄCg4iv ¼ 4.824(4) Å, Cg4ꢄꢄꢄCg7v
¼ 3.491(4) Å, Cg4ꢄꢄꢄCg9vi ¼ 3.853(4) Å, Cg6ꢄꢄꢄCg8 ¼ 3.645(4)
Å, Cg7ꢄꢄꢄCg9 ¼ 3.657(4) Å, Cg9ꢄꢄꢄCg9vii ¼ 4.390(4) Å; Cg1,
Cg4, Cg6, Cg7, Cg8 and Cg9 are the centroids of Cd1-O2-
Cd1A-O2A, Cd1-O4-C24-C23A-C22A-N2A, C1-C2-C3-C4-
C5-C6, C8-C9-C10-C11-C12-C13, C16-C17-C18-C19-C20-
C21 and C23-C28-C27A-C26A-C25A-C24A rings, respect-
ively; symmetry codes: iv: – x, 2 – y, – z; v: 1 – x, 2 – y, –
z; vi: ꢀ1 þ x, y, z; vii: 2 – x, 2 – y, – z].
tion about the metal-ligand bonding. The assignments are
based on the typical group frequencies. The weak and broad
absorptions in the range of 3400–3600 cmꢀ1 can be assigned
to the O-H vibrations of phenol groups or solvent mole-
cules. The spectra of the compounds show several weak
bands corresponding to aromatic and aliphatic C-H stretch-
ing in the range of 2810–3015 cmꢀ1. The strong absorption
band at 1634 cmꢀ1 in the spectrum of H2L is assigned to the
azomethine group, ꢀ(C ¼ N). The band is observed at
1642 cmꢀ1 in the spectrum of the complex, indicating the
coordination through the imino N atom. The Ar-O stretch-
ing band at 1245 cmꢀ1 in the spectrum of the free Schiff
base H2L shift to lower frequency by 32 cmꢀ1 in the com-
plex (1213 cmꢀ1), upon complexation with metals.[23,24] The
weak bands at 588, 545, and 459 cmꢀ1 for the complex can
be assigned to ꢀ(M-O).[23,25] The difference between
ꢀasym(COO) (1598 cmꢀ1
)
and ꢀsym(COO) (1433 cmꢀ1
)
(Dꢀ ¼ 165 cmꢀ1), which reflects the bidentate bridging
coordination mode of the acetate ligand.[24,26]
ꢅ
In the UV–vis spectrum of H2L, there are three p–p
transitions at 263 nm, 298 nm, and 345 nm. For the spec-
trum of the complex, a new band at 425 nm can be ascribed
to LMCT excitation. The bands at 230 and 296 nm due to
intra-ligand transitions are observed.[20]
1H and 13C NMR spectra of H2L
The 1H NMR spectrum of H2L displays triplet signal at
1.35 ppm, which is assigned to protons of the CH3 group,
and the quartet signal at 4.07 ppm assigned to protons of

INORGANIC AND NANO-METAL CHEMISTRY
5
Table 3. Antimicrobial activities of the compounds as MIC values (lg/mL).
Disclosure statement
H2L
the complex
Penicillin
No potential conflict of interest was reported by the authors.
Escherichia coli
Pseudomonas aeruginosa
Salmonella typhi
Staphylococcus aureus
32
16
32
64
8
8
4
230
65
250
Supplementary data
32
12
CCDC 2006772 contains the supplementary crystallographic data for
this paper. 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: (þ44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
the CH2 group. The signals at 6.86–7.49 ppm are assigned to
the aromatic protons. The singlet signal at 8.98 ppm is
assigned to CH ¼ N group. The singlet signals at 13.70 and
10.01 ppm are assigned to the OH groups. The 13C NMR
spectrum of H2L displays signal at 14.77 ppm, which is
assigned to the carbon of the CH3 group, and the signal at
64.09 ppm assigned to the carbon of the CH2 group. The
signal at 162.96 ppm is assigned to the carbon of the
CH ¼ N group. All the other signals are ascribed to the car-
bon atoms of the aromatic rings.
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