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Fe(III) of a Zn(II) coordination polymer
Journal Pre-proof Syntheses, structure and luminescent sensing for Cr(VI)/Fe(III) of a Zn(II) coordination polymer Liansheng Cui, Yonggang Li, Yongle Gan, Qun Feng, Jinqiao Long PII:
S0022-2860(19)30888-9
DOI:
https://doi.org/10.1016/j.molstruc.2019.07.044
Reference:
MOLSTR 26797
To appear in:
Journal of Molecular Structure
Received Date: 23 March 2019 Revised Date:
28 June 2019
Accepted Date: 10 July 2019
Please cite this article as: L. Cui, Y. Li, Y. Gan, Q. Feng, J. Long, Syntheses, structure and luminescent sensing for Cr(VI)/Fe(III) of a Zn(II) coordination polymer, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.07.044. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Syntheses, structure and luminescent sensing for Cr(VI)/Fe(III) of a Zn(II) coordination polymer
Liansheng Cui*, Yonggang Li, Yongle Gan, Qun Feng, Jinqiao Long* Guangxi Colleges and Universities Key Laboratory of Regional Ecological Environment Analysis and Pollution Control of West Guangxi, College of Chemistry and Environmental Engineering, Baise University, Baise, Guangxi 533000, China
Abstract
A Zn(II) coordination polymer formulated as {[Zn(L)(bimb)]·2H2O)}n
from 4-mercaptobenzoic acid (HL) and 1,4-bis(imidazol-1-ylmethyl)benzene) (bimb), was successfully constructed under the condition of solvothermal and characterized by
X-ray
single-crystal
diffraction,
elemental
analysis,
IR
spectra
and
thermogravimetric (TG) analysis. X-ray single-crystal diffraction analysis exhibits that the complex possesses a uninodal 4-connected 2D (two-dimensional) sql topology framework with the Schläfli symbol (44·62). Luminescence studies demonstrate that complex 1 has high selectivity and sensitivity for pollutant Cr(VI) (CrO42-/Cr2O72-) anions and Fe(III) cations.
Key words: Zn(II) coordination polymer, crystal structure, luminescence sensing, fluorescence quenching.
*
Corresponding author.
E-mail address: [email protected], [email protected] Telephone/fax: +867762848132 (Dr. L.S. Cui, Prof. J.Q. Long)
1. Introduction With the rapid development of society, industry has enjoyed unprecedented prosperity. The environmental pollution has become more and more serious. Heavy metal ion pollution is one of the most serious environmental pollution [1]. Cr(VI) anions are potent carcinogenic ions and have been classified as serious pollutants by the U.S. Environmental Protection Agency [2-3]. Meanwhile, Fe(III) cations are recognized as an industrial pollutant and play a significant role in living organisms. Both a depressed and elevated concentration of Fe(III) cations may result in a host of serious diseases such as agrypnia, skin diseases and decreased immunity [4-6]. Therefore, selective recognition and detection of them are extremely important for human health. In recent years, the design and synthesis of coordination polymers (CPs) have become the focus in the field of supramolecular chemistry and crystal engineering. Owing to their diversity of appealing skeleton structure and topological novelty [6-9], this kind of material has tremendous potential applications in gas storage and separation [10-11], electrical conduction [12-13], luminescence materials [14-16], molecular magnets [17-19], heterogeneous catalysts [20-23], and so on. Particularly, CPs were known as an excellent chemical sensor because of their higher selectivity, sensitivity, and operability [24-27]. For instance, the Suresh group reported a neutral two-dimensional adeninebased luminescent coordination polymer with Zn(II) metal nodes, which exhibited a high sensitivity to Hg2+ and 2,4,6-trinitrophenol [28]. Cd-CPs reported by Yang group showed an enhanced sensitivity to Cu2+ ions [29]. Fan
group
has
studied
a
series
of
Zn/Cd-CPs
based
on
“V”-shaped
3,5-di(4′-carboxyl-phenyl)benzene acid, which demonstrated highly promoted sensitivity to Fe3+, Cu2+ and Cr2O72− ions in wastewater [30]. In synthesis process of such materials, the structure of CPs is mainly influenced by the coordination geometry of the central metal ion, ligand structure, solvents [31-32], pH [33-34], metal-ligand ratio [35-36], counterions [37-38] and temperature [39-40]. At present, synthesis of coordination polymers mainly used multidentate ligands such as polycarboxylate and N-heterocyclic ligands. This is because, on the
one hand, the coordination centers that can flexibly twist around to meet different coordination environment. On the other hand, when coordinate with metal ions, the polycarboxylic groups that can partially or fully deprotonated to adopt various coordination modes. Besides, the N-heterocyclic ligands are good molecular building blocks and co-ligands for constructing CPs with interesting structures and properties. These two types of ligands contain multiple N and O atoms, which have strong coordination ability and rich coordination mode. In this paper, concerned above, a 2D structure Zn(II) coordination polymer {[Zn(L)(bimb)]·2H2O}n, was synthesized and characterized by X-ray single-crystal diffraction. The luminescence studies show that complex 1 maybe acted as highly selective bifunctional luminescent sensor toward Cr(VI) (CrO42-/Cr2O72-) anions and Fe(III) cations.
2. Experimental section 2.1 Materials and physical measurements 4-mercaptobenzoic acid (HL) and 1,4-bis(imidazol-1-ylmethyl)benzene) (bimb) were purchased from Jinan Henghua Sci. Tec. Co. Ltd., Zn(NO3)2·6H2O and other solvents were purchased from the local reagent company. Infrared spectra were recorded with the Varian 640FT-IR by using KBr pellets. Elemental analysis of C, H, N were performed in the model 2400 PerkinElmer analyzer. Thermogravimetric (TG) analyses were measured on a Perkin-Elmer TGA-7 thermogravimetric analyzer under nitrogen conditions from room temperature to 800 ℃ with a heating rate of 10 ℃ min-1. Topological analysis were performed and confirmed by the Topos program and the Systre software [41-42]. 2.2 Synthesis of {[Zn(L)(bimb)]·2H2O}n (1) A mixture of Zn(NO3)2·6H2O (0.060 g, 0.2 mmol), bimb (0.024 g, 0.1 mmol), HL (0.015 g, 0.1 mmol), H2O (5.0 mL), DMF (5.0 mL) were stirred for 0.5 hour in air. And then the solution was transformed into the Teflon-lined stainless steel vessel (20 mL), sealed, and heated to 130℃ for 3 days. Subsequently, the vessel was cooled to the room temperature at the degree of 5 ℃ h-1. Colorless column crystals were collected. Anal. Calcd for C21H22ZnN4O4S (%) : C, 51.28; H, 4.51; N, 11.39. Found: C, 51.30; H, 4.49;
N, 11.40. IR (KBr disk, cm−1): 3410 (m), 3125 (w), 1661 (w), 1624 (m), 1563 (s), 1517 (s), 1453 (m), 1395 (s), 1364 (s), 1307(m), 1243 (w), 1053 (m), 818(m), 775(m), 734 (m), 652 (w). 2.3 X-ray crystal structure determination Single-crystal X-ray diffraction for the suitable crystal of the complex was obtained
on
a
Bruker
Apex
Smart
CCDC
diffractometer,
using
graphite-monochromated Mo-kα radiation (λ = 0.71073 Å) at 293 K. The structure was solved by direct methods using SHELXS-97 [43]. The non-hydrogen atoms were defined by the Fourier synthesis method. Positional and thermal parameters were refined by the full matrix least-squares method (on F2) to convergence [44]. Crystallographic data for complex 1 is given in Table 1. Selected bond lengths and angles for 1 are listed in Table S1. CCDC number for complex 1 is 1585413. Table 1 2.4 Luminescence Sensing Experiments Complex 1 (3 mg) was dispersed in different potassium salts solution (4 mL, 0.01 mol L-1) with different kinds of anions (I-, Cl-, OH-, IO3-, NO3-, ClO3-, OH-, SCN-, H2PO4-, CO32-, Cr2O42-, SO42-, Cr2O72- and CrO42-). The mixture was stirred for 30 minutes in the dark, then the luminescence spectra of the mixture was recorded. In addition, the same procedure was performed for determining the sensor ability of 1 for the metal cations (4 mL, 0.01 mol L-1) of AgNO3 or MClx (M= Na+, K+, Ca2+, Mg2+, Mn2+, Co2+, Zn2+, Cd2+, Ni2+, Cu2+ and Fe3+). 3. Results and discussion 3.1 Structure description of {[Zn(L)(bimb)]·2H2O}n (1) As shown in Fig. S1, the peaks at 1690 cm-1 (C=O) and 1294 cm-1 (C-O) were remarkably reduced and shifted to lower frequencies in the IR spectra of 1 compared to the IR spectra of HL, which suggested that the carbonyl group and O atom in the carboxyl moiety of HL were involved in the 1. X-ray singal-crystal structural analysis reveals that 1 crystallizes in the monoclinic space group P21. In Fig. 1, Zn(II) cations with O atom, S atom from L- ligands and
two N atoms from bimb ligands exhibits tetrahedral coordination sphere. Zn(II) occupies the center and O, S, two N atoms lie in four vertices of the tetrahedron. One-dimensional (1D) waveform chain structure [Zn-bimb]n and 1D threadiness structure [Zn-L]n are formed by Zn(II) cations connecting bimb ligands and L- ligands (Fig.
2a-2b).
Furthermore,
[Zn-bimb]n
and
[Zn-L]n
produce 2D network
([bimb-Zn-L]n) (Fig. 2c). Consequently, neighbouring [bimb-Zn-L]n make 3D framework through O-H···O hydrogen bond from dissociative water molecules and Lligands (Fig. 2d). A topological analysis reveals that Zn(II) ions link each other and serve as 4-connected node. Thus, the 2D framework of 1 is a 4-connected network, building a sql topological framework with the point symbol (44.62), determined by TOPOS program (Fig. 2e). Fig. 1 Fig. 2 a-e 3.2 Thermal analyses Thermogravimetric (TG) analysis of the complex 1 was performed in an nitrogen atmosphere with a heating rate of 10 °C min 1 and the results are shown in Fig. S2. −
First weight loss about 4% in the temperature range of 100-150 °C is consistent with the removal of the dissociative water molecules. The second weight loss about 60% in the temperature range of 350-500 °C corresponds to the loss of the organic ligands. Thermogravimetric analysis reveals that the complex 1 is stable from 150°C to 350 °C. 3.3 Luminescent properties
The coordination center Zn (II) ions possess d10 electron orbital configuration and has good optical properties. The solid-state excitation and emission spectra of HL, bimb and complex 1 were measured at room temperature (Fig. S3). Intense emission was observed with a peak at 406 nm (λex = 355 nm) for HL, while bimb exhibit relatively weak emissions at 474 nm (λex = 395 nm). Complex 1 gives emission at 490 nm (λex = 405 nm). Compared with the HL and bimb ligands, the emission bands of complexes 1 can probably be attributed to the bimb ligand fluorescence emission and show a 16 nm blue-shift. Zn (II) is difficult to be oxidized or reduced during the
coordination process, mainly because the N-heterocyclic ligand forms the coordination bond with the coordination center Zn (II) ions, and it is also affected by the d10 orbital. The fluorescence properties of complex 1 should be mainly attributed to ligand-to-ligand charge transfer [45] from the bimb ligand and ligand-to-metal charge transfer [46]. 3.4 Pollutant-ion sensing As depicted in Fig. 3, it is clearly seen that the majority of anions led to some degree of weakening on the luminescence intensity for complex 1. While, CrO42-/Cr2O72- afford the most striking quenching effect. The result show that complex 1 can be highly effective and selective luminescent sensor for Cr(℃) anions. To further determine the sensitivity of complex 1 for Cr(℃) anion, CrO42- was taken as an example and the emission intensities were recorded by adding a series of concentrations of CrO42-. As shown in Fig. 4, following the concentration reduce of CrO42-, the fluorescence quenching rate of complex 1 is gradually decreased, and at the concentration of 5×10-3 mol·L-1, the luminescence is completely quenched. Stern-Volmer Equation (I0/I-1=Ksv[M]) was used to calculated the corresponding quenching coefficient quantitatively, in which the value I0 is the initial intensity and I is the intensity at the corresponding concentration ([M]) of CrO42-, and Ksv is the quenching constant [47]. As shown in Fig. S4, the Ksv values are calculated to be 4.94×104 M. From the slope and standard error of the fitting lines, the detection limits [48] are found to be 0.61 µM for 1 according to the equation 3σ/k (σ: standard error; k: slope).
Fig. 3 Fig. 4
As depicted in Fig. 5, it clearly shows that the most metal ions cannot cause a significant fluorescence intensity reduction of complex 1, nevertheless, Fe(III) cations can
lead
to
complete
fluorescence
quenching
of
complex
1.
The
concentration-dependent test was also performed. As shown in Fig. 6, the luminescence intensities reduced monotonically with the Fe3+ concentration
increasing from 0 mM to 0.5 mM. Meanwhile, the result displayed that the concentration-depend quenching coefficient also match well with Stern-Volmer equation. As shown in Fig. S5, the Ksv value and the linear correlation coefficient R are 1.66×105 M-1 and 0.99661, respectively. By the ratio of 3σ/k, the detection limit of Fe3+ is 0.18 µM for 1. Fig. 5 Fig. 6
In addition to evaluating its high sensitivity, the antiinterference ability of 1 as sensor is crucially significant. The effects caused by various inorganic anions and metal ions on anti-interference ability of 1 are examined. As shown in Fig. 7 and Fig. 8, the fluorescence intensities of 1 were not reduced distinctly even with the introduction of a relatively higher concentration of analytes (10 mM). However, with the introduction of 0.5 mM of CrO42- or Fe3+ in the parallel tests, the fluorescence intensities decreased sharply and presented obvious quenching phenomenon. All of the above results imply that complex 1 can effectively and selectively sense CrO42-/Cr2O72- and Fe3+. Fig. 7 Fig. 8
3.5 Fluorescence quenching mechanism In order to examine the sensing mechanism of 1 toward CrO42-/Cr2O72- and Fe3+, further experiments were measured. As depicted in Fig. S6, the PXRD patterns of 1 before and after immersing in the solution of CrO42-/Cr2O72- and Fe3+ were recorded and almost the same patterns before and after the soaking indicate the maintaining of their structures, which excludes the possibility of collapse of the structure [49-50]. In addition, the UV-Vis absorption spectra showed that the Cr2O72- and CrO42- ions exhibit two broad absorption bands from 230 to 450 nm, and Fe3+ ions exhibited a large overlap from 250 to 500 nm (Fig. S7), all of them hindered the absorption of complex 1 and caused photoluminescence attenuation of the complex. In this type of environment, a competition between Cr2O72-/CrO42-/Fe3+ and 1, led to the luminescence quenching [51-53].
4. Conclusion In this paper, a Zn(II) coordination polymer {[Zn(L)(bimb)]·2H2O}n was synthesized and X-ray single-crystal diffraction analysis showed that the complex belongs to monoclinic crystal system, space group P21 and possesses a uninodal 4-connected 2D sql topology framework with the Schläfli symbol (44·62). Systematic investigation of the luminescence test of complex 1 for Cr(VI) anion and Fe(III) cation demonstrated that complex 1 as a good candidate for chemical sensor, can rapidly and selectively detect two types of ions. Acknowledgments This research was supported by the Guangxi Natural Science Foundation to P. F. Yao (2018GXNSFBA281197, 2018GXNSFAA294060) and the university-level scientific research project of Baise University in 2018 to L. S. Cui (2018KN17).
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Table 1 Summary of crystal data and structure refinement parameters for complex 1. Empirical formula
C21H19ZnN4O2S·H2O
Formula weight
491.85
Crystal system
Monoclinic
Space group
P21
a (Å)
7.4316 (6)
b (Å)
15.268 (1)
c (Å)
9.8101 (6)
α (°)
90
β (°)
101.603(7)
γ(°)
90
V (Å3) Z
1090.35 (13)
2
Dcalcd(Mg m−3)
1.498
µ (mm−1)
1.26
Reflections collected Data/parameters
17131
4509/280
F(000)
508
T (K)
293
Rint
0.029
Final R indices [I> 2σ(I)]
R1 = 0.0448 wR2 = 0.1117
R indices (all data)
R1 = 0.0510 wR2 = 0.1150
Gof
1.04
Fig. 1 Coordination environment of Co(II) in 1 (All the H atoms are omitted for clarity). Symmetry codes: (i) x, y, z+1; (ii) −x+2, y+1/2, −z+2; (iii) x, y, z−1; (iv) −x+2, y−1/2, −z+2.
a)
Zn
d)
b)
c)
e)
Fig. 2 a) 1D waveform chain structure [Zn-bimb]n; b) 1D threadiness structure [Zn-L]n; c) 2D network ([bimb- Zn -L]n) by Co(II) cations connecting bimb ligands and L- ligands; d) 3D framework of 1 through O-H···O hydrogen bond from dissociative H2O and L-; e) 2D topology structure of 1
Fig. 3 The photoluminescent spectra intensities for complex 1 in aqueous solution with various inorganic anions.
Fig. 4 Emission spectra for complex 1 in aqueous solutions of different CrO42- concentrations.
Fig. 5 The photoluminescent spectra intensities for complex 1 in aqueous solution with various metal ions.
Fig. 6 Emission spectra for complex 1 in aqueous solutions of different Fe3+ concentrations.
Fig. 7 Fluorescence intensity of 1 in aqueous solution with the introduction of diverse other inorganic anions (red) and introduction of CrO42- (blue).
Fig. 8 Fluorescence intensity of 1 in aqueous solution with the introduction of diverse other metal ions (red) and introduction of Fe( ) (blue).
1. A Zn(II) coordination polymer was successfully constructed under the condition
of solvothermal. 2. Luminescence studies demonstrate that complex 1 has high selectivity and sensitivity for pollutant Cr(VI) (CrO42-/Cr2O72-) anions and Fe(III) cations.
S0022-2860(19)30888-9
DOI:
https://doi.org/10.1016/j.molstruc.2019.07.044
Reference:
MOLSTR 26797
To appear in:
Journal of Molecular Structure
Received Date: 23 March 2019 Revised Date:
28 June 2019
Accepted Date: 10 July 2019
Please cite this article as: L. Cui, Y. Li, Y. Gan, Q. Feng, J. Long, Syntheses, structure and luminescent sensing for Cr(VI)/Fe(III) of a Zn(II) coordination polymer, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.07.044. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Syntheses, structure and luminescent sensing for Cr(VI)/Fe(III) of a Zn(II) coordination polymer
Liansheng Cui*, Yonggang Li, Yongle Gan, Qun Feng, Jinqiao Long* Guangxi Colleges and Universities Key Laboratory of Regional Ecological Environment Analysis and Pollution Control of West Guangxi, College of Chemistry and Environmental Engineering, Baise University, Baise, Guangxi 533000, China
Abstract
A Zn(II) coordination polymer formulated as {[Zn(L)(bimb)]·2H2O)}n
from 4-mercaptobenzoic acid (HL) and 1,4-bis(imidazol-1-ylmethyl)benzene) (bimb), was successfully constructed under the condition of solvothermal and characterized by
X-ray
single-crystal
diffraction,
elemental
analysis,
IR
spectra
and
thermogravimetric (TG) analysis. X-ray single-crystal diffraction analysis exhibits that the complex possesses a uninodal 4-connected 2D (two-dimensional) sql topology framework with the Schläfli symbol (44·62). Luminescence studies demonstrate that complex 1 has high selectivity and sensitivity for pollutant Cr(VI) (CrO42-/Cr2O72-) anions and Fe(III) cations.
Key words: Zn(II) coordination polymer, crystal structure, luminescence sensing, fluorescence quenching.
*
Corresponding author.
E-mail address: [email protected], [email protected] Telephone/fax: +867762848132 (Dr. L.S. Cui, Prof. J.Q. Long)
1. Introduction With the rapid development of society, industry has enjoyed unprecedented prosperity. The environmental pollution has become more and more serious. Heavy metal ion pollution is one of the most serious environmental pollution [1]. Cr(VI) anions are potent carcinogenic ions and have been classified as serious pollutants by the U.S. Environmental Protection Agency [2-3]. Meanwhile, Fe(III) cations are recognized as an industrial pollutant and play a significant role in living organisms. Both a depressed and elevated concentration of Fe(III) cations may result in a host of serious diseases such as agrypnia, skin diseases and decreased immunity [4-6]. Therefore, selective recognition and detection of them are extremely important for human health. In recent years, the design and synthesis of coordination polymers (CPs) have become the focus in the field of supramolecular chemistry and crystal engineering. Owing to their diversity of appealing skeleton structure and topological novelty [6-9], this kind of material has tremendous potential applications in gas storage and separation [10-11], electrical conduction [12-13], luminescence materials [14-16], molecular magnets [17-19], heterogeneous catalysts [20-23], and so on. Particularly, CPs were known as an excellent chemical sensor because of their higher selectivity, sensitivity, and operability [24-27]. For instance, the Suresh group reported a neutral two-dimensional adeninebased luminescent coordination polymer with Zn(II) metal nodes, which exhibited a high sensitivity to Hg2+ and 2,4,6-trinitrophenol [28]. Cd-CPs reported by Yang group showed an enhanced sensitivity to Cu2+ ions [29]. Fan
group
has
studied
a
series
of
Zn/Cd-CPs
based
on
“V”-shaped
3,5-di(4′-carboxyl-phenyl)benzene acid, which demonstrated highly promoted sensitivity to Fe3+, Cu2+ and Cr2O72− ions in wastewater [30]. In synthesis process of such materials, the structure of CPs is mainly influenced by the coordination geometry of the central metal ion, ligand structure, solvents [31-32], pH [33-34], metal-ligand ratio [35-36], counterions [37-38] and temperature [39-40]. At present, synthesis of coordination polymers mainly used multidentate ligands such as polycarboxylate and N-heterocyclic ligands. This is because, on the
one hand, the coordination centers that can flexibly twist around to meet different coordination environment. On the other hand, when coordinate with metal ions, the polycarboxylic groups that can partially or fully deprotonated to adopt various coordination modes. Besides, the N-heterocyclic ligands are good molecular building blocks and co-ligands for constructing CPs with interesting structures and properties. These two types of ligands contain multiple N and O atoms, which have strong coordination ability and rich coordination mode. In this paper, concerned above, a 2D structure Zn(II) coordination polymer {[Zn(L)(bimb)]·2H2O}n, was synthesized and characterized by X-ray single-crystal diffraction. The luminescence studies show that complex 1 maybe acted as highly selective bifunctional luminescent sensor toward Cr(VI) (CrO42-/Cr2O72-) anions and Fe(III) cations.
2. Experimental section 2.1 Materials and physical measurements 4-mercaptobenzoic acid (HL) and 1,4-bis(imidazol-1-ylmethyl)benzene) (bimb) were purchased from Jinan Henghua Sci. Tec. Co. Ltd., Zn(NO3)2·6H2O and other solvents were purchased from the local reagent company. Infrared spectra were recorded with the Varian 640FT-IR by using KBr pellets. Elemental analysis of C, H, N were performed in the model 2400 PerkinElmer analyzer. Thermogravimetric (TG) analyses were measured on a Perkin-Elmer TGA-7 thermogravimetric analyzer under nitrogen conditions from room temperature to 800 ℃ with a heating rate of 10 ℃ min-1. Topological analysis were performed and confirmed by the Topos program and the Systre software [41-42]. 2.2 Synthesis of {[Zn(L)(bimb)]·2H2O}n (1) A mixture of Zn(NO3)2·6H2O (0.060 g, 0.2 mmol), bimb (0.024 g, 0.1 mmol), HL (0.015 g, 0.1 mmol), H2O (5.0 mL), DMF (5.0 mL) were stirred for 0.5 hour in air. And then the solution was transformed into the Teflon-lined stainless steel vessel (20 mL), sealed, and heated to 130℃ for 3 days. Subsequently, the vessel was cooled to the room temperature at the degree of 5 ℃ h-1. Colorless column crystals were collected. Anal. Calcd for C21H22ZnN4O4S (%) : C, 51.28; H, 4.51; N, 11.39. Found: C, 51.30; H, 4.49;
N, 11.40. IR (KBr disk, cm−1): 3410 (m), 3125 (w), 1661 (w), 1624 (m), 1563 (s), 1517 (s), 1453 (m), 1395 (s), 1364 (s), 1307(m), 1243 (w), 1053 (m), 818(m), 775(m), 734 (m), 652 (w). 2.3 X-ray crystal structure determination Single-crystal X-ray diffraction for the suitable crystal of the complex was obtained
on
a
Bruker
Apex
Smart
CCDC
diffractometer,
using
graphite-monochromated Mo-kα radiation (λ = 0.71073 Å) at 293 K. The structure was solved by direct methods using SHELXS-97 [43]. The non-hydrogen atoms were defined by the Fourier synthesis method. Positional and thermal parameters were refined by the full matrix least-squares method (on F2) to convergence [44]. Crystallographic data for complex 1 is given in Table 1. Selected bond lengths and angles for 1 are listed in Table S1. CCDC number for complex 1 is 1585413. Table 1 2.4 Luminescence Sensing Experiments Complex 1 (3 mg) was dispersed in different potassium salts solution (4 mL, 0.01 mol L-1) with different kinds of anions (I-, Cl-, OH-, IO3-, NO3-, ClO3-, OH-, SCN-, H2PO4-, CO32-, Cr2O42-, SO42-, Cr2O72- and CrO42-). The mixture was stirred for 30 minutes in the dark, then the luminescence spectra of the mixture was recorded. In addition, the same procedure was performed for determining the sensor ability of 1 for the metal cations (4 mL, 0.01 mol L-1) of AgNO3 or MClx (M= Na+, K+, Ca2+, Mg2+, Mn2+, Co2+, Zn2+, Cd2+, Ni2+, Cu2+ and Fe3+). 3. Results and discussion 3.1 Structure description of {[Zn(L)(bimb)]·2H2O}n (1) As shown in Fig. S1, the peaks at 1690 cm-1 (C=O) and 1294 cm-1 (C-O) were remarkably reduced and shifted to lower frequencies in the IR spectra of 1 compared to the IR spectra of HL, which suggested that the carbonyl group and O atom in the carboxyl moiety of HL were involved in the 1. X-ray singal-crystal structural analysis reveals that 1 crystallizes in the monoclinic space group P21. In Fig. 1, Zn(II) cations with O atom, S atom from L- ligands and
two N atoms from bimb ligands exhibits tetrahedral coordination sphere. Zn(II) occupies the center and O, S, two N atoms lie in four vertices of the tetrahedron. One-dimensional (1D) waveform chain structure [Zn-bimb]n and 1D threadiness structure [Zn-L]n are formed by Zn(II) cations connecting bimb ligands and L- ligands (Fig.
2a-2b).
Furthermore,
[Zn-bimb]n
and
[Zn-L]n
produce 2D network
([bimb-Zn-L]n) (Fig. 2c). Consequently, neighbouring [bimb-Zn-L]n make 3D framework through O-H···O hydrogen bond from dissociative water molecules and Lligands (Fig. 2d). A topological analysis reveals that Zn(II) ions link each other and serve as 4-connected node. Thus, the 2D framework of 1 is a 4-connected network, building a sql topological framework with the point symbol (44.62), determined by TOPOS program (Fig. 2e). Fig. 1 Fig. 2 a-e 3.2 Thermal analyses Thermogravimetric (TG) analysis of the complex 1 was performed in an nitrogen atmosphere with a heating rate of 10 °C min 1 and the results are shown in Fig. S2. −
First weight loss about 4% in the temperature range of 100-150 °C is consistent with the removal of the dissociative water molecules. The second weight loss about 60% in the temperature range of 350-500 °C corresponds to the loss of the organic ligands. Thermogravimetric analysis reveals that the complex 1 is stable from 150°C to 350 °C. 3.3 Luminescent properties
The coordination center Zn (II) ions possess d10 electron orbital configuration and has good optical properties. The solid-state excitation and emission spectra of HL, bimb and complex 1 were measured at room temperature (Fig. S3). Intense emission was observed with a peak at 406 nm (λex = 355 nm) for HL, while bimb exhibit relatively weak emissions at 474 nm (λex = 395 nm). Complex 1 gives emission at 490 nm (λex = 405 nm). Compared with the HL and bimb ligands, the emission bands of complexes 1 can probably be attributed to the bimb ligand fluorescence emission and show a 16 nm blue-shift. Zn (II) is difficult to be oxidized or reduced during the
coordination process, mainly because the N-heterocyclic ligand forms the coordination bond with the coordination center Zn (II) ions, and it is also affected by the d10 orbital. The fluorescence properties of complex 1 should be mainly attributed to ligand-to-ligand charge transfer [45] from the bimb ligand and ligand-to-metal charge transfer [46]. 3.4 Pollutant-ion sensing As depicted in Fig. 3, it is clearly seen that the majority of anions led to some degree of weakening on the luminescence intensity for complex 1. While, CrO42-/Cr2O72- afford the most striking quenching effect. The result show that complex 1 can be highly effective and selective luminescent sensor for Cr(℃) anions. To further determine the sensitivity of complex 1 for Cr(℃) anion, CrO42- was taken as an example and the emission intensities were recorded by adding a series of concentrations of CrO42-. As shown in Fig. 4, following the concentration reduce of CrO42-, the fluorescence quenching rate of complex 1 is gradually decreased, and at the concentration of 5×10-3 mol·L-1, the luminescence is completely quenched. Stern-Volmer Equation (I0/I-1=Ksv[M]) was used to calculated the corresponding quenching coefficient quantitatively, in which the value I0 is the initial intensity and I is the intensity at the corresponding concentration ([M]) of CrO42-, and Ksv is the quenching constant [47]. As shown in Fig. S4, the Ksv values are calculated to be 4.94×104 M. From the slope and standard error of the fitting lines, the detection limits [48] are found to be 0.61 µM for 1 according to the equation 3σ/k (σ: standard error; k: slope).
Fig. 3 Fig. 4
As depicted in Fig. 5, it clearly shows that the most metal ions cannot cause a significant fluorescence intensity reduction of complex 1, nevertheless, Fe(III) cations can
lead
to
complete
fluorescence
quenching
of
complex
1.
The
concentration-dependent test was also performed. As shown in Fig. 6, the luminescence intensities reduced monotonically with the Fe3+ concentration
increasing from 0 mM to 0.5 mM. Meanwhile, the result displayed that the concentration-depend quenching coefficient also match well with Stern-Volmer equation. As shown in Fig. S5, the Ksv value and the linear correlation coefficient R are 1.66×105 M-1 and 0.99661, respectively. By the ratio of 3σ/k, the detection limit of Fe3+ is 0.18 µM for 1. Fig. 5 Fig. 6
In addition to evaluating its high sensitivity, the antiinterference ability of 1 as sensor is crucially significant. The effects caused by various inorganic anions and metal ions on anti-interference ability of 1 are examined. As shown in Fig. 7 and Fig. 8, the fluorescence intensities of 1 were not reduced distinctly even with the introduction of a relatively higher concentration of analytes (10 mM). However, with the introduction of 0.5 mM of CrO42- or Fe3+ in the parallel tests, the fluorescence intensities decreased sharply and presented obvious quenching phenomenon. All of the above results imply that complex 1 can effectively and selectively sense CrO42-/Cr2O72- and Fe3+. Fig. 7 Fig. 8
3.5 Fluorescence quenching mechanism In order to examine the sensing mechanism of 1 toward CrO42-/Cr2O72- and Fe3+, further experiments were measured. As depicted in Fig. S6, the PXRD patterns of 1 before and after immersing in the solution of CrO42-/Cr2O72- and Fe3+ were recorded and almost the same patterns before and after the soaking indicate the maintaining of their structures, which excludes the possibility of collapse of the structure [49-50]. In addition, the UV-Vis absorption spectra showed that the Cr2O72- and CrO42- ions exhibit two broad absorption bands from 230 to 450 nm, and Fe3+ ions exhibited a large overlap from 250 to 500 nm (Fig. S7), all of them hindered the absorption of complex 1 and caused photoluminescence attenuation of the complex. In this type of environment, a competition between Cr2O72-/CrO42-/Fe3+ and 1, led to the luminescence quenching [51-53].
4. Conclusion In this paper, a Zn(II) coordination polymer {[Zn(L)(bimb)]·2H2O}n was synthesized and X-ray single-crystal diffraction analysis showed that the complex belongs to monoclinic crystal system, space group P21 and possesses a uninodal 4-connected 2D sql topology framework with the Schläfli symbol (44·62). Systematic investigation of the luminescence test of complex 1 for Cr(VI) anion and Fe(III) cation demonstrated that complex 1 as a good candidate for chemical sensor, can rapidly and selectively detect two types of ions. Acknowledgments This research was supported by the Guangxi Natural Science Foundation to P. F. Yao (2018GXNSFBA281197, 2018GXNSFAA294060) and the university-level scientific research project of Baise University in 2018 to L. S. Cui (2018KN17).
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Table 1 Summary of crystal data and structure refinement parameters for complex 1. Empirical formula
C21H19ZnN4O2S·H2O
Formula weight
491.85
Crystal system
Monoclinic
Space group
P21
a (Å)
7.4316 (6)
b (Å)
15.268 (1)
c (Å)
9.8101 (6)
α (°)
90
β (°)
101.603(7)
γ(°)
90
V (Å3) Z
1090.35 (13)
2
Dcalcd(Mg m−3)
1.498
µ (mm−1)
1.26
Reflections collected Data/parameters
17131
4509/280
F(000)
508
T (K)
293
Rint
0.029
Final R indices [I> 2σ(I)]
R1 = 0.0448 wR2 = 0.1117
R indices (all data)
R1 = 0.0510 wR2 = 0.1150
Gof
1.04
Fig. 1 Coordination environment of Co(II) in 1 (All the H atoms are omitted for clarity). Symmetry codes: (i) x, y, z+1; (ii) −x+2, y+1/2, −z+2; (iii) x, y, z−1; (iv) −x+2, y−1/2, −z+2.
a)
Zn
d)
b)
c)
e)
Fig. 2 a) 1D waveform chain structure [Zn-bimb]n; b) 1D threadiness structure [Zn-L]n; c) 2D network ([bimb- Zn -L]n) by Co(II) cations connecting bimb ligands and L- ligands; d) 3D framework of 1 through O-H···O hydrogen bond from dissociative H2O and L-; e) 2D topology structure of 1
Fig. 3 The photoluminescent spectra intensities for complex 1 in aqueous solution with various inorganic anions.
Fig. 4 Emission spectra for complex 1 in aqueous solutions of different CrO42- concentrations.
Fig. 5 The photoluminescent spectra intensities for complex 1 in aqueous solution with various metal ions.
Fig. 6 Emission spectra for complex 1 in aqueous solutions of different Fe3+ concentrations.
Fig. 7 Fluorescence intensity of 1 in aqueous solution with the introduction of diverse other inorganic anions (red) and introduction of CrO42- (blue).
Fig. 8 Fluorescence intensity of 1 in aqueous solution with the introduction of diverse other metal ions (red) and introduction of Fe( ) (blue).
1. A Zn(II) coordination polymer was successfully constructed under the condition
of solvothermal. 2. Luminescence studies demonstrate that complex 1 has high selectivity and sensitivity for pollutant Cr(VI) (CrO42-/Cr2O72-) anions and Fe(III) cations.