Synthesis, structural characterizations and spectroscopic properties of binuclear CoIII complex and its Schiff ligand as a chemosensor for fuorescent recognition of ZnII

Article abstract of DOI:10.1016/j.ica.2021.120581

Binuclear Co^III complex [Co₂(L)₃(DMF)₃] containing a Schiff ligand H₂L was synthesized and analyzed by elemental analysis, single crystal X-ray crystallography, FT-IR, UV–Vis absorption spectroscopy,

Full text of DOI:10.1016/j.ica.2021.120581

Inorganica Chimica Acta 527 (2021) 120581
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structural characterizations and spectroscopic properties of
binuclear CoIII complex and its Schiff ligand as a chemosensor for
fuorescent recognition of ZnII
Yin-Xia Sun*, Juan Li, Geng Guo, Yi-Gang Sun, Wen-Min Ding
School of Chemistry and Chemical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, China
A R T I C L E I N F O
A B S T R A C T
Binuclear CoIII complex [Co2(L)3(DMF)3] containing a Schiff ligand H2L was synthesized and analyzed by
elemental analysis, single crystal X-ray crystallography, FT-IR, UV–Vis absorption spectroscopy, Hirshfeld sur-
face analysis and DFT calculation. The CoIII complex was a 3:2 ((L)2ꢀ : CoIII) binuclear structure. The spatial
configuration of central CoIII ions was six-coordinated slightly twisted octahedron. The 3D supramolecular
Keywords:
Coumarin Schiff-base
Cobalt complex
Crystal structure
ZnII recognition
Test paper
–
network structure was linked by intermolecular hydrogen bonds, C H⋅⋅⋅π and π⋅⋅⋅π stacking interactions. The
Schiff ligand H2L exhibits weak luminescence, but this weak fluorescence is turned on after coordination with the
ZnII ion. Furthermore, H2L could as an “OFF-ON” chemosensor for fluorescent recognition of ZnII.
1. Introduction
synthesized and characterized by elemental analyses, single crystal X-
ray crystallography, FT-IR, UV–Vis absorption spectroscopy, Hirshfeld
Schiff base compounds and their derivatives are usually used for
chelating ligands because they can provide a good coordination envi-
ronment for metal ions [1,2]. In the last few years, they have been
widely used in organic synthesis intermediates, industrial catalysis,
electrochemistry, luminescence and magnetic materials [3–5]. Mean-
while, Cobalt complexes have excellent biological activity and catalytic
properties. Previous reports have shown that Schiff base cobalt com-
plexes have been used in the study of DNA interaction and antitumor
activity [6,7]. On the one hand, Schiff base ligands and their complexes
surfaces analyses and DFT calculation. It is noteworthy that the ligand
H2L could as an “OFF-ON” chemosensor for fluorescent recognition of
ZnII.
2. Experimental
2.1. Materials and methods
The 4-hydroxycoumarin (99%) and salicylic aldehyde (98%) used in
the experiment were purchased from Alfa Aesar (New York, USA). The
remaining reagents and solvents are of analytical grade, provided by
Tianjin Chemical Reagent Factory, and can be used without further
purification. Melting points were measured by an X-4 microscopic
melting point apparatus made by Beijing Taike Instrument Limited
Company and it was not calibrated before use [17]. German Vario EL
V3.00 automatic element analyzer was used for the analysis of C, H, and
N elements. The single crystal X-ray structure data is measured by
Bruker Smart 1000 CCD area detector. Prepare the sample into KBr (400
~ 4000 cmꢀ 1) microspheres, and perform FT-IR spectroscopy on the
VERTEX70 FT-IR spectrophotometer. Ultraviolet–visible absorption
spectrum is measured by Hitachi UV-3900 spectrometer [18]. With
tetramethylsilane (δ = 0.00) as the internal standard, 1H NMR spectra
–
–
have excellent optical properties. For example, the structure of C C,
–
–
C
O and lactone ring in coumarin compounds can effectively increase
the conjugation degree of molecules and make them have strong fluo-
rescence [8,9]. In the process of designing fluorescent probe com-
pounds, it is an ideal candidate for fluorescent groups [10–13]. On the
other hand, Zinc ion has been considered as an important biological
element since ancient times, it is a kind of human metal ion with strong
biocompatibility. Zinc ions deficiency can cause diseases such as slow
growth, decline of immune function and loss of appetite [14–16].
Therefore, it will be of great use to develop efficient and sensitive zinc
ion fluorescence detection probes. In this paper, a dinuclear CoIII com-
plex [Co2(L)3(DMF)3] based on coumarin ligand 4-hydroxy-3-[(2-hy-
droxy-methylenebenzene)-iminomethyl]-benzopyran-2-one (H2L) was
* Corresponding author.
E-mail address: sun_yinxia@163.com (Y.-X. Sun).
https://doi.org/10.1016/j.ica.2021.120581
Received 31 May 2021; Received in revised form 28 July 2021; Accepted 13 August 2021
Available online 19 August 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

Y.-X. Sun et al.
Inorganica Chimica Acta 527 (2021) 120581
Scheme 1. Synthetic routes of H2L.
excitation wavelength of the fluorescence spectrum is 350 nm, the
incident slit is 5 nm, and the exit slit is 5 nm.
Table 1
Crystal date and structure parameters description of CoIII complex.
Compound
[Co2(L)3]⋅(DMF)3
2.2. Preparation of H2L
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C60H50Co2N9O15
1254.95
H2L was synthesized according to the following synthetic routes
shown in Scheme 1. 3-Aldehyde-4-hydroxyl-coumarin was synthesized
in the light of the formerly reported method [20,21]. Yield: 63.8%. m.p.
114 ~ 115 ℃. Anal. Calcd. for C10H6O4 (%): C, 63.16; H, 3.18. Found
(%): C, 62.56; H, 3.57. The ligand H2L was synthesized in a similar way
[22].
296
0.71073
Triclinic
P–1
13.6074(19)
b (Å)
13.9185(17)
c (Å)
15.837(2)
(◦)
81.196(4)
3-Aldehyde-4-hydroxyl-coumarin (1.71 g, 9 mmol) and O-hydrox-
ybenzaldehyde (1.0 mL, 9 mmol) were dissolved in anhydrous ethyl-
alcohol, added drop-wise added hydrazine hydrate (0.6 mL, 9 mmol) at
70 ◦C, and then continued heating and refluxing for 5–6 h. The mixed
solution was cooled overnight, and a light-yellow solid was precipitated,
and then washed with absolute ethanol to obtain 1.44 g of H2L (Figs. S1
and S2). Yield: 71.2 %. m.p. 269 ~ 270 ℃, Anal. Calcd. for C17H12N2O4
(%): C, 66.23; H, 3.92; N, 9.09. Found (%): C, 66.57; H, 3.22; N, 9.25
(Table S4). 1H NMR (500 MHz, DMSO‑d6) δ 13.89 (s, 1H), 10.32 (s, 1H),
9.06 (s, 1H), 8.73 (s, 1H), 7.97 (dd, J = 7.8, 1.6 Hz, 1H), 7.75–7.67 (m,
2H), 7.40–7.32 (m, 3H), 6.94 (dd, J = 17.8, 7.9 Hz, 2H).
α
β (◦)
86.607(4)
γ (◦)
81.665(4)
V (Å3)
Z
2930.7(7)
2
Dc (g cmꢀ 3
)
1.422
μ
(mmꢀ 1
)
0.642
F (000)
1294.0
Crystal size (mm)
0.22 × 0.26 × 0.28
2.21 to 26.06
ꢀ 17 ≤ h ≤ 17; ꢀ 18 ≤ k ≤ 18; ꢀ 15 ≤ l ≤ 20
26810, 13,597
13,597
θ range (◦)
Index ranges
Reflections collected
Independent reflections
Rint
0.035
Completeness %
Date / restraints / parameters
GOF
98.3%
2.3. Syntheses of CoIII complex
13,597 / 0 / 780
0.888
R1, wR2 [I > 2
σ
(I)]
0.0564 / 0.2010
0.1037 / 0.1612
ꢀ 0.340 / 0.490
A solution of Co(OAc)2⋅4H2O (1.0 mg, 0.004 mmol) in methanol (2.0
mL) was added dropwise to a solution of H2L (1.8 mg, 0.006 mmol) in
DMF (2.5 mL). The mixture solution immediately turns green. Then it
was stirred at room temperature for 30 min. Subsequently, filtered and
placed at room temperature for about two weeks. The solvent evapo-
rated partially, and pale black single crystals of CoIII complex suitable for
X-ray crystallographic analysis were obtained [23]. Anal. Calcd. for
R1, wR2 indices (all data)
Largest differences Peak and hole (e
Åꢀ 3
)
were recorded in DMSO‑d6 solution using Bruker AV instrument at room
temperature. Hirshfeld surfaces analyses and two-dimensional inger-
print plots were calculated using using CrystalExplorer 3.1. The B3LYP
function is used as the basis of geometric optimization, and the Gaussian
09 software program is used for DFT calculation [19]. The luminescence
spectra of the solution were measured and recorded on Hitachi F-7000
spectrometer. When ligand H2L was used to recognize thirteen metal
cations (CaII, CrIII, BaII, MnII, NiII, CoII, CuII, CdII, MgII, AlIII, FeIII, ZnII,
HgII), nitrate was used as raw material to prepare cation solution. The
C
60H50Co2N9O15 (%): C, 57.42; H, 4.02; N, 10.05. Found (%): C, 57.87;
H, 4.12; N, 10.02.
2.4. Crystal structure determinations of CoIII complex
A single crystal with a size of approximately 0.22 mm × 0.26 mm ×
0.28 mm was placed on the Bruker Smart 1000 CC. The reflections were
Fig. 1. (a) Molecular structure of CoIII complex; (b) coordination configuration of CoIII ions.
2

Y.-X. Sun et al.
Inorganica Chimica Acta 527 (2021) 120581
π
interactions of CoIII complex.
–
Fig. 2. Intermolecular hydrogen bonding and C H⋅⋅⋅
3. Results and discussion
3.1. Crystal structures and related properties of CoIII complex
3.1.1. Crystal structures of CoIII complex
The molecular structure of CoIII complex is shown in Fig. 1, and
selected bond lengths and angles are listed in (Table S1). The CoIII
complex was crystallized in a triclinic crystal system, P-1 space group.
The CoIII complex was a 3:2 ((L)2ꢀ : CoIII) binuclear structure, and
composed of two CoIII ions, three deprotonated (L)2ꢀ units, and three
crystallizing DMF solvent molecules (Fig. 1a). The spatial configuration
of two central CoIII ions were all six-coordinated slightly twisted octa-
hedron, and both of the CoIII ion were coordinated by three imine N
atoms and three phenolic oxygen atoms (Fig. 1b).
The CoIII complex existed five weak intermolecular non-classical
hydrogen bond, C11-H11⋅⋅⋅O15, C13-H13⋅⋅⋅O2, C28-H28⋅⋅⋅O14, C41-
H41⋅⋅⋅O13 and C58-H58A⋅⋅⋅O2 (Fig. 2 and Table S2). Two crystallizing
DMF solvent molecules was bound to the CoIII complex main units
through C28-H28⋅⋅⋅O14 and C11-H11⋅⋅⋅O15 hydrogen bonding interac-
tion. The remaining DMF molecule connected two adjacent CoIII com-
plex molecules to form a dimer unit by C41-H41⋅⋅⋅O13 and C58-
H58A⋅⋅⋅O2 hydrogen bonds. This dimer units were further linked by
C13-H13⋅⋅⋅O2 hydrogen bonding interaction to obtain an infinite 1D
banded supramolecular structure along the a-axis direction. Simulta-
Fig. 3. The supramolecular structure of CoIII complex.
collected using graphite-monochromatized Mo Kα radiation (λ =
–
neously, this linkage was further stabilized by the
(C52–H52A⋅⋅⋅Cg1) interaction. In addition, this 1D banded supramo-
⋅⋅⋅
C
H⋅⋅⋅
π
0.71073 Å) at 296 K [24,25]. Furthermore, in this experiment, LP
correction is suitable for SAINT program, SADABS scheme adopts semi
empirical correction and the direct method was used to solve the crystal
structure (shelss-2014) [26]. The crystal data and experimental pa-
rameters of the CoIII complex are listed in Table 1.
lecular structures were further linked by the
π π (Cg2⋅⋅⋅Cg3 and
Cg3⋅⋅⋅Cg3) stacking interactions between the coumarin rings into an
infinite 2D layer supramolecular structure parallel to the ac plane (Fig. 3
and Table S3).
3.1.2. Hirshfeld surface analyses
The solid crystal structure of any complexes is steadied by a large
number of supramolecular interactions, so these interactions need to be
Fig. 4. The Hirshfeld surface of the CoIII complex mapped with dnorm, shape index and curvedness.
3

Y.-X. Sun et al.
Inorganica Chimica Acta 527 (2021) 120581
Fig. 5. Fingerprint plots (de vs di) of the CoIII complex showing atoms interactions.
considered [27]. Hirsfield surface analysis is an effective tool for
researching the interaction between molecular crystals. The combina-
tion of de and di in the form of two-dimensional fingerprint shows the
contact area between molecules in the crystal [28]. The surfaces ana-
lyses of the CoIII complex are given in Fig. 4 mapping with dnorm, shape
index and curvedness. The dnorm mapping on the surface of Hirshfeld
molecules proves that they are in contact with each other, the visible
HOMO and LOMO (ΔE = ELUMO - EHOMO), and the smaller the E value,
the higher the activity of the molecule [37].
3.1.4. IR spectra analyses
The IR spectra of H2L and its CoIII complex show different charac-
teristic stretching vibration bands in the scope of 400 ~ 4000 cmꢀ 1 [38].
ꢀ 1
–
As shown in Fig. S4, the O H stretching band is observed at 3471 cm
–
bright red areas indicate the presence of C H⋅⋅⋅O hydrogen bond con-
in H2L, which belongs to phenolic hydroxyl group. However, in the CoIII
complex, a new stretching vibration band (wider peak) was watched at
3436 cmꢀ 1, corresponding to crystalline water [39]. The absorption
band of free ligand H2L at 1645 cmꢀ 1 is attributed to the stretching vi-
tacts on the surface [27–30]. Some white dots on the surface are caused
by the H⋅⋅⋅H interaction and the blue areas on the surface indicate that
there is no interaction. Shape exponents and curvature can be used to
identify stack interactions. For the CoIII complex, red or blue triangle
(bow tie pattern) and flat surface patches are observed in the shape
–
–
–
–
bration of the C O double bond of the coumarin ring, while the C
O
stretching vibration of CoIII complex occurs at about 1621 cmꢀ 1. In
–
index diagram, which indicates the existence of
π
⋅⋅⋅
π
stacking interac-
addition, the ligand H L showed characteristic tensile bands of C N
–
2
tion. The 2D fingerprint shows the proportion of all in termolecular
interactions (Fig. 5) [31]. The proportion of O⋅⋅⋅H/H⋅⋅⋅O, C⋅⋅⋅H/H⋅⋅⋅C
and H⋅⋅⋅H interactions comprising 21.7%, 26.8% and 44.0% of the total
Hirshfed surfaces for each molecule of the CoIII complex, respectively.
groups at 1685 cmꢀ 1, and its corresponding CoIII complex was watched
at 1694 cmꢀ 1. Compared with the ligand H L, the C N stretching fre-
–
–
quency of CoIII complex shifted about 9 cmꢀ 12to higher frequency, which
indicates that the coordination between the metal atom and the amino
–
–
nitrogen lone pair increases the frequency of the C N bond. Compared
3.1.3. DFT computation
with the contraction band of the H2L at 1213 cmꢀ 1, the Ar-O contraction
band of the complex at 1200 cmꢀ 1 moved about 13 cmꢀ 1 to the low
frequency, which indicates that Mꢀ O bond is formed between the metal
ions and the oxygen atoms of the phenolic groups [40].
The electronic structure of the CoIII complex was further studied by
DFT calculation [32]. The singlet molecular geometry was optimized at
the level of B3LYP through Gaussian 09, and metal atoms were pro-
cessed through the B3LYP/LanL2DZ basis set [33]. Fig. S3 shows the
orbital energies of the molecular orbitals of the CoIII complex, mainly
from LUMO + 1 to HOMO-1. As everyone knows, HOMO means the
ability to give electrons, and LUMO means the ability to accept elec-
trons. The diagram of the HOMO of CoIII complex shows that these
HOMOs are delocalized and mainly distributed on the Co1 ion orbitals.
Compared with HOMO, LUMO is mainly distributed in the fragments of
Schiff base ligands and CoIII ions [34–36]. The molecular orbital en-
ergies of EHOMO, ELUMO, EHOMO-1 and ELUMO+1 of CoIII complex are
respectively ꢀ 5.686 eV, ꢀ 2.813 eV, ꢀ 5.708 eV, ꢀ 2.773 eV. The energy
3.1.5. UV–Vis absorption spectra and fluorescence spectrum analysis
The electronic absorption spectra of ligand H2L and its CoIII complex
were detected in DMSO solution. As shown in Fig. 6a, it can be clearly
observed that free H2L has two absorption peaks at 296 nm and 356 nm,
they are the
π
⋅⋅⋅
π
* transition of aromatic ring and the internal ligand
III
–
transition of n⋅⋅⋅
π
* (C N), respectively [39–41]. While that of the Co
–
complex has a strong absorption peak at 258 nm and a weak absorption
peak at 405 nm. Compared with the free H2L, the absorption peak of
CoIII complex blue shifted from 296 nm to 258 nm, which indicated the
coordination between H2L and CoIII ions. And the latter at 356 nm dis-
appeared in the complex, but a new weak absorption peak at 405 nm
was observed, which was attributed to the d-d transition of CoIII ions
[42].
gaps between them are ΔE (ELUMO - EHOMO) = 2.873 eV, ΔE (ELUMO+1
-
E
HOMO-1) = 2.936 eV, respectively. The molecular orbitals energies of
CoIII complex are all negative, indicating the good stability of CoIII
complex molecular. The complex has a lower energy gap between
Fig. 6. (a) The ultraviolet spectrum of ligand H2L and CoIII complex; (b) Fluorescence spectra of ligand H2L and CoIII complex (c = 4.0 × 10ꢀ 5 mol/L).
4

Y.-X. Sun et al.
Inorganica Chimica Acta 527 (2021) 120581
Fig. 8. Fluorescence spectra of the H2L in DMSO solution with increasing
concentration of ZnII (0–1.0 equiv).
ions, the emission peak strength has no obvious enhancement except
ZnII ions. Furthermore, the emission peak was red-shifted from 456 nm
to 519 nm and the intensity increased by 8 times upon addition of ZnII
ions [45]. Under the UV lamp, the solution of other metal ions was
added to DMSO solution of H2L in turn, a strong bright yellow fluores-
cence is produced only added ZnII ions, and other metal cations has
basically no obvious effect, the fluorescence remains unchanged or
quenched (Fig. 7b). It can be proved that H2L could selectively identify
ZnII among other measured metal ions in DMSO solution. It is helpful to
study the high sensitivity of H2L to ZnII ions by anti-interference
experiment. In Fig. 7c, after adding ZnII to H2L solutions containing
different metal ions, other metal cations had no markedly effect on the
fluorescence recognition to ZnII ions except for the slight quenching of
CuII ion. Therefore, even if there are other disturbing ions, probe H2L has
a high selectivity for ZnII in DMSO system [46].
Fig. 7. (a) Fluorescence response of H2L to various metal cations in DMSO
solution; (b) The images of color changes upon different cationic added to the
H2L in DMSO solution under 365 nm UV lamp; (c) Fluorescence emission
spectra (λex = 350 nm) of the H2L in the presence of ZnII and various cationic in
DMSO solutions.
3.2.2. Titration experiment of H2L to ZnII
As shown in Fig. 8, the fluorescence titration experiment was carried
out by the gradual addition of ZnII ions to the H2L solution [47]. The H2L
exhibited a weak emission peak at 456 nm. When the concentration of
ZnII ions gradually increased, the weak emission peak of H2L at 456 nm
gradually weakened and change, and a new peak emission present to
519 nm and its intensity gradually increased when the concentration of
ZnII ion increases continuously. And when the content of ZnII reached at
1.0 equivalent, the fluorescence emission intensity reaches the
maximum at 519 nm, indicating that the optimal binding ratio of ZnII to
probe H2L is 1:1. The titration results were brought into Benesi-
Hildebrand equation [48], as Eq (1), and the binding constant is
calculated Ka = 0.45 × 104 Mꢀ 1 (Fig. S6a), Where, F0 is the fluorescence
intensity without ZnII, F is the fluorescence intensity at any given ZnII
concentration, and Fmax is the fluorescence intensity after titration
The fluorescence spectra of ligand H2L and its CoIII complex were
detected in DMSO solution. As can be seen from the Fig. 6b, under 350
nm excitation, the free ligand H2L has a strong emission peak at 456 nm,
which is caused by the π⋅⋅⋅π transition. The fluorescence intensity of H2L
is almost quenched upon coordination with CoIII ions, indicating the
CoIII ions coordinated with the N and O atoms of H2L and a photo-
induced electron transfer process occurs [43].
3.2. Fluorescence recognition of metals cations by ligand H2L
3.2.1. Fluorescence response of ZnII
saturation. The detection limit calculated using LOD = 3σ/slope is 3.26
The selectivity of H2L to thirteen different metal cations (CaII, CrIII,
BaII, MnII, NiII, CoII, CuII, CdII, MgII, AlIII, FeIII, ZnII, HgII) were studied by
fluorescence spectra at room temperature [44]. Firstly, the solvation
effect of ligand H2L was studied (Fig. S5). The fluorescence intensity of
H2L was measured in different solvent systems, it was observed that two
weak emission peaks appeared in other organic solvent systems at 432
nm and other different wavelengths. In DMSO solution system, the
fluorescence intensity of H2L at 519 nm was significantly enhanced. In
addition, the fluorescence intensity decreased significantly in the
mixture of DMSO and H2O. As shown in Fig. 7a, the H2L and cations
stock solutions were prepared in DMSO solution at a concentration of
5.0 × 10ꢀ 5 mol/L and 1.0 × 10ꢀ 2 mol/L. The H2L showed a weak
fluorescence emission peak at 456 nm, after the addition of other metal
× 10ꢀ 7 mol/L (Fig. S6b), and the calculation result is lower than the
limit of WHO drinking water (7.0 × 10ꢀ 6 mol/L) [49,50]. Among,
σ is
the standard deviation of five successive measurements of the fluores-
cence strength by ligand H2L, and the slope is obtained by the rela-
tionship between the emission intensity and concentration. At the same
time, the time response experiment is to verify the rapid detection
performance of probe H2L for ZnII. As shown in (Fig. S5), when ZnII was
added to the ligand H2L solution, the response time was 30 s.
1
1
1
1
=
+
×
(1)
(F ꢀ F0) (Fm ax ꢀ F0) Kd[C] (Fm ax ꢀ F0)
5

Y.-X. Sun et al.
Inorganica Chimica Acta 527 (2021) 120581
Fig. 9. 1H NMR titration of H2L with different concentrations of ZnII in d6-DMSO.
Fig. 10. Plausible mechanism for ZnII recognition by H2L.
3.2.3. The detection mechanism of ZnII ion by H2L
fluorescence enhancement recognition mechanism of ZnII by H2L may be
mainly due to fact that the presence of ZnII hinders the light-induced
electron transfer (PET) effect of H2L, leading to fluorescence quench-
ing. Due to the existence of imino group (–CH = N) in H2L and contin-
uous rotation, there is a weak fluorescence at 456 nm in H2L solution.
The addition of ZnII ions can coordinate with the ligand H2L, inhibit the
free rotation of the imino of the molecules and produce chelation
enhanced fluorescence (CHEF) effect. At the same time, strong fluores-
cence emission occurs at 425 ~ 675 nm, which leads to the enhancement
of bright yellow fluorescence [53,54].
1H NMR titration experiments in DMSO‑d6 were performed to
investigate the recognition mechanism of the H2L to ZnII. In Fig. 9, when
ZnII was added in the same amount from 0 to 1.0 equivalent, the protons
of phenol hydroxyl group H1 (δ = 10.32 ppm) and H2 (δ = 13.89 ppm),
which indicated that ZnII induced deprotonation of hydroxyl group [51],
and hydroxyl O atoms participated in the coordination. For complex L-
ZnII, the proton signal becomes more extensive with the addition of
1
metal ions, and the H NMR titration data supports the metal coordi-
nation ratio of 1:1. In addition, the 1H NMR of zinc ion complex powder
was characterized and compared with the 1H NMR data of adding 1.0
equivalent zinc ion, which further showed that the binding mechanism
was consistent with the above experimental results (Fig. S8). The anal-
ysis of Job plot further confirmed the stoichiometric ratio between them
(Fig. S9). The mole fraction of ZnII ions added reached the maximum
value at 0.5, indicating that the binding ratio of ZnII ions to SW was 1:1
[52]. After binding with ZnII, the pristine peak of the complex was found
at 391.28 (calcd for [H2L/ZnII + H2O) (Fig. S10), which further
confirmed the binding mode of probe H2L with ZnII. As shown in Fig. 10,
after 365 nm excitation, H2L shows weak fluorescence emission in the
range of 417 ~ 528 nm, and reaches the maximum at 456 nm. The
3.2.4. DFT calculation
In order to understand the interaction and coordination properties of
H2L with ZnII, the geometric optimization of H2L and L-ZnII complexes
was carried out by using B3LYP / 6-31G and LanL2DZ basis sets [55],
respectively. As shown in Fig. 11, the L-ZnII complex is coordinated by a
ZnII ion, one H2L molecule and one water molecule. Among them, the
coordination atoms are phenolate oxygen and imine nitrogen on the
salicylaldehyde group in the ligand H2L, hydroxyl oxygen on the
coumarin group and the oxygen of the water molecule. In the HOMO and
LUMO orbitals of ligand H2L, the electron density is mainly delocalized
6

Y.-X. Sun et al.
Inorganica Chimica Acta 527 (2021) 120581
Fig. 11. Molecular structure and orbital diagram of the optimized ligand H2L and complex L-ZnII.
Fig. 12. Tests paper experiment of probe H2L. (a) The color changes after dropping different concentrations of ZnII ions; (b) The experiment of paper test after adding
different metal ions (1.0 × 10ꢀ 2 mol/L).
on the conjugated skeleton of the whole ligand molecule, and the LUMO
and HOMO energies of H2L were ꢀ 2.298 eV and ꢀ 5.898 eV, respec-
tively. The energy gap difference ΔE (ELUMO - EHOMO) between them is
3.60 eV, which is consistent with the weak fluorescence of H2L. When
ZnII is coordinated with H2L, LUMO and HOMO energies were ꢀ 2.772
eV and ꢀ 6.042 eV, respectively. Correspondingly, the energy gap was
3.270 eV, and the electron cloud density of HOMO of L-ZnII is mainly
concentrated on the salicylaldehyde group, whereas the electron cloud
density of HOMO is mainly delocalized on the whole Schiff group. The
HOMO-LUMO transition shows the fluorophore-metal charge transfer,
indicating that with the PET effect, the excited electrons are readily
averted to metal ions [56]. The decrease of ΔE indicates that ZnII and
H2L have good bonding ability and form a stable coordination
environment.
3.2.5. Test papers of H2L for ZnII
In practical application, it is of great practical value to develop a
simple, convenient and cheap paper sensor by changing the wavelength
of the fluorescent probe [57]. The probe H2L was designed as a paper
sensor for ZnII ion recognition. Firstly, the filter paper was soaked in
DMSO solution of probe H2L for 3 h, then it was taken out and dried.
Then it was placed under a 365 nm UV lamp [58], and different
7

Y.-X. Sun et al.
Inorganica Chimica Acta 527 (2021) 120581
concentrations of ZnII ions or other different metal ions were added, and
the response signals were observed. As shown in Fig. 12a, the sensitivity
and color change of H2L to ZnII were observed by naked eye. In addition,
the selectivity of H2L to different metal ions was also studied (Fig. 12b).
When ZnII was added to the H2L strip, the color of the strip changed from
blue to yellow. When other metal ions were added, the strip had almost
no obvious color change. The results show that the probe H2L has high
selectivity for the recognition of ZnII as a paper sensor.
[10] J.F. Wang, R.Y. Li, P. Li, W.K. Dong, Inorg. Chim. Acta 518 (2021) 120247.
[11] T. Hu, J.P. Cheng, L.J. Li, Y.F. Zhan, W.J. Li, Z.D. Chang, Inorg. Chim. Acta 498
(2019) 119131.
[12] A.K. Adak, B. Dutta, S.K. Manna, C. Sinha, ACS Omega 4 (2019) 18987–18995.
[13] Z.L. Wei, J.F. Wang, W.T. Guo, Y. Zhang, W.K. Dong, Spectrochim. Acta A 228
(2020) 117775.
[14] C.Q. Li, X. Li, Q.Y. Zhang, Spectrochim. Acta A 243 (2020) 118763.
[15] L. Wang, Z.L. Wei, M. Yu, Y.Q. Pan, Y. Zhang, W.K. Dong, Chin. J. Inorg. Chem. 35
(2019) 1791–1804.
[16] X.X. An, Q. Zhao, H.R. Mu, W.K. Dong, Crystals 9 (2019) 101.
[17] H.R. Jia, J. Li, J.Q. Guo, B. Yu, L. Xu, Crystals 7 (2017) 247.
[18] J. Li, H.J. Zhang, H.R. Jia, Y.X. Sun, Y.Q. Huang, Crystals 8 (2018) 176.
[19] M.M. Lu, S.Y. Qiu, S.Q. Cui, S.Z. Pu, J. Phys. Org. Chem. 33 (2020) e4113.
[20] J. Chang, S.Z. Zhang, Y. Wu, H.J. Zhang, Y.X. Sun, Trans. Met. Chem. 45 (2020)
279–293.
4. Conclusions
In this paper, we designed and synthesized the ligand 4-hydroxy-3-
[(2-hydroxy-methylenebenzene)-iminomethyl]-benzopyran-2-one
(H2L) with absolute ethanol as the medium, and combined it with Co
(CH3COO)2⋅4H2O was cultured in DMF/CH3OH solvent system by nat-
ural volatilization to obtain the complex [Co2(L)3(DMF)3]. The spatial
configuration of CoIII complex was determined by X-ray diffraction
method and a series of structural characterization were carried out. The
CoIII complex was a 3:2 ((L)2ꢀ : CoIII) binuclear structure and the spatial
configuration of central CoIII ions were six-coordinated slightly twisted
octahedron. At the same time, H2L had a specific fluorescence response
to ZnII in the DMSO system and it can be used as a fluorescent chemo-
sensor for recognition of ZnII. And the detection limits on colorimetric
and fluorescence response of the H2L to ZnII is down to 3.26 × 10ꢀ 7 mol/
L. The binding ratio of probe H2L to ZnII was 1:1 by 1H NMR, MS analysis
and theoretical calculation. In addition, the probe SW can be used for the
detection of ZnII in DMSO solution under 365 nm ultraviolet light, and
the tast paper detection of ZnII in DMSO, which has a potential appli-
cation prospect.
[21] J. Chang, H.J. Zhang, H.R. Jia, Y.X. Sun, Chin. J. Inorg. Chem. 34 (2018)
2097–2107.
[22] Q. Zhao, X.X. An, L.Z. Liu, W.K. Dong, Inorg. Chim. Acta 490 (2019) 6–15.
[23] R.Y. Li, Z.L. Wei, L. Wang, Y. Zhang, J.X. Ru, Microchem. J. 162 (2021) 105720.
[24] S.Z. Zhang, H.J. Zhang, J. Chang, Y.X. Sun, Y. Wu, Y.B. Wang, Chin. J. Inorg. Chem.
36 (2020) 503–514.
[25] G. Guo, L. Juan, Y. Wu, W.M. Ding, S.Z. Zhang, Y.X. Sun, J. Chin, Inorg Chem. 37
(2021) 1113–1124.
[26] Q.P. Kang, X.Y. Li, Z.L. Wei, Y. Zhang, W.K. Dong, Polyhedron 165 (2019) 38.
[27] L. Wang, Z.L. Wei, Z.Z. Chen, C. Liu, W.K. Dong, Y.J. Ding, Microchem. J. 155
(2020) 104801.
[28] Y.J. Li, R.N. Bian, P. Li, K.F. Xie, W.K. Dong, Polyhedron 192 (2020) 114867.
[29] B. Yu, C.Y. Li, Y.X. Sun, H.R. Jia, J.Q. Guo, J. Li, Spectrochim. Acta A 184 (2017)
249–254.
[30] T. Anand, S.K. Ashok, S.K.S. Kumar, Chem. Select. 2 (2017) 7570–7579.
[31] Y.X. Sun, R.E. Lu, X.R. Li, Y.Y. Zhao, C.Y. Li, Chin. J. Inorg. Chem. 31 (2015)
1055–1062.
[32] T. Feng, L.L. Li, Y.J. Li, W.K. Dong, Acta Cryst. B 77 (2021) 168–181.
[33] L. Wang, Z.L. Wei, C. Liu, W.K. Dong, J.X. Ru, Spectrochim. Acta A 239 (2020)
118496.
[34] X.Y. Dong, Q.P. Kang, X.Y. Li, J.C. Ma, W.K. Dong, Crystals 8 (2018) 139.
[35] L. Wang, Y.Q. Pan, J.F. Wang, Y. Zhang, Y.J. Ding, J. Photoch. Photobio. A 400
(2020) 112719.
[36] X. Xu, R.N. Bian, S.Z. Guo, W.K. Dong, Y.J. Ding, Inorg. Chim. Acta 513 (2020)
119945.
Declaration of Competing Interest
[37] X.X. An, C. Liu, Z.Z. Chen, K.F. Xie, W.K. Dong, Crystals 9 (2019) 602.
[38] Y. Wu, W.M. Ding, J. Li, G. Guo, S.Z. Zhang, H.R. Jia, Y.X. Sun, J. Fluoresc. 31
(2021) 437–446.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[39] J.Q. Guo, Y.X. Sun, B. Yu, J. Li, H.R. Jia, Chin. J. Inorg. Chem. 33 (2017)
1481–1512.
[40] X. Xu, Y.J. Li, T. Feng, W.K. Dong, Y.J. Ding, Luminescence 36 (2020) 1–11.
[41] J.F. Wang, R.N. Bian, T. Feng, K.F. Xie, L. Wang, Y.J. Ding, Microchem. J. 160
(2021) 105676.
Acknowledgments
[42] S.Z. Zhang, G. Guo, W.M. Ding, J. Li, Y. Wu, Y.X. Sun, J. Mol. Struct. 1230 (2021)
129627.
This work was supported by the Youth Doctoral Fund Project of
colleges and universities in Gansu Province (2021QB-050), which is
gratefully acknowledged.
[43] T. Anand, S.K. Kumar A, S.K. Sahoo, Photochem. Photobiol. 17 (2018) 414–422.
[44] M. Yu, Y. Zhang, Y.Q. Pan, L. Wang, Inorg. Chim. Acta 509 (2020) 119701.
[45] M. Sudipa, S.M. Mandal, O. Durbadal, C. Debprasad, S. Chittaranjan, Polyhedron
172 (2019) 28–38.
[46] Y.D. Peng, Y. Zhang, J.Y. Li, Z.L. Ren, F. Wang, L. Wang, J. Fluoresc. 22 (2020)
1053–1509.
Appendix A. Supplementary data
[47] J. Li, S.Z. Zhang, G. Guo, H.R. Jia, Y.X. Sun, Chem. Pap. 75 (2021) 4697–4706.
[48] Q. Su, Q. Zhao, X.X. An, Y.B. Wang, X.Y. Li, W.K. Dong, Chin. J. Inorg. Chem. 35
(2019) 524–536.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120581.
[49] R.N. Bian, X. Xu, T. Feng, W.K. Dong, Inorg. Chim. Acta 516 (2021) 120098.
[50] M. Shyamal, P. Mazumdar, S. Maity, H. Samanta, G.P. Sahoo, A. Misra, ACS
Sensors 6 (2016) 739–747.
References
[51] W.M. Ding, Y. Wu, S.Z. Zhang, J. Li, Luminescence 36 (2021) 1306–1316.
[52] B.C. Han, J. Sun, K. Chen, Z.Y. Chen, M.H. Huang, Z.Z. Gao, Tetrahedron 74 (2018)
7193–7197.
[1] W.L. Zhang, Y. Zhang, Y.F. Cui, M. Yu, W.K. Dong, Inorg. Chim. Acta 506 (2020)
119534.
[53] P. Mahapatra, S. Ghosh, D.K. SKadam, V. Rane, R. Kadam, Inorg. Chem. 56 (2017)
5105–5121.
[2] Y. Zhang, M. Yu, X.Y. Li, Q.P. Kang, W.K. Dong, Chin. J. Inorg. Chem. 35 (2019)
1283–1294.
[54] L.Z. Liu, L. Wang, M. Yu, Q. Zhao, Y. Zhang, Y.X. Sun, W.K. Dong, Spectrochim.
Acta A 222 (2019) 117209.
[3] Y. Zhang, L.Z. Liu, Y.D. Peng, N. Li, W.K. Dong, Trans. Metal Chem. 4 (2019) 1.
[4] Y.Q. Pan, Y. Zhang, M. Yu, Y. Zhang, L. Wang, Appl. Organomet. Chem. 34 (2020)
e5441.
[55] Y. Zhang, Y.Q. Pan, M. Yu, X. Xu, W.K. Dong, Appl. Organomet. Chem. 33 (2019)
e5240.
[56] H.J. Zhang, J. Chang, H.R. Jia, Y.X. Sun, Chin. J. Inorg. Chem. 34 (2018)
2261–2270.
[5] B. Yu, Y.X. Sun, C.J. Yang, J.Q. Guo, J. Li, Anorg. Allg. Chem. 643 (2017) 689–698.
[6] H.R. Jia, J. Chang, H.J. Zhang, J. Li, Crystals 8 (2018) 272.
[7] Y.X. Sun, Y.Q. Pan, X. Xu, Y. Zhang, Crystals 9 (2019) 607.
[8] Q.P. Kang, X.Y. Li, Q. Zhao, J.C. Ma, W.K. Dong, Appl. Organomet. Chem. 32
(2018) e4379.
[57] H.S. Ke, W. Wei, Y.S. Yang, H.P. Wu, Y.Q. Zhang, G. Xie, S.P. Chen, Inorg. Chem.
21 (2020) 24–39.
[58] H.P. Diao, L.X. Guo, W. Liu, L.H. Feng, Spectrochim. Acta A 196 (2018) 274–280.
[9] C. Liu, X.X. An, Y.F. Cui, K.F. Xie, W.K. Dong, Appl. Organomet. Chem. 34 (2020)
e5272.
8