English

• 0.   Introduction

  The design and synthesis of functional coordination polymers (CPs)^[1‐2] have become a rapidly developing field in crystal engineering and functional materials in the past two decades, which is due to their fascinating structural diversities together with their potential applications in magnetism, fluorescent sensing, gas adsorption and separation, catalysis, and so forth^[3‐16]. Tremendous achievements were made in constructing CPs with intriguing topologies and functional properties. However, it is still a great challenge to control the final products, which is due to the resulting structures influenced by many factors such as organic ligands, metal ions, metal and ligand ratio, pH, solvent, counterions, and temperature^[17‐21]. Among these influencing factors, the organic ligands play significant roles in the construction of CPs. Accordingly, the rational selection of organic ligands has become a critical step in the construction of CPs^[22‐23]. Among these organic ligands, the aromatic carboxylic acids, owing to their strong coordination ability, diversity of the coordination modes, high structural stability, and good flexibility, have been extensively employed in constructing CPs^[24‐25]. Our group have previously reported some CPs based on the carboxyl derivatives including 3, 3´, 5, 5´‐benzene‐1, 3‐biyl‐tetrabenzoic acid^[26‐27], terphenyl‐2, 2´, 4, 4´‐tetracarboxylic acid^[28], 5 ‐ (3´, 4´‐ dicarboxylphenoxy)isophthalic acid^[29], 2, 2´ ‐ oxybis(benzoic acid)^[30], and 5‐(3, 5‐dicarboxybenzyloxy)isophthalic acid^[31].

  In connection with our interest in developing and constructing some coordination polymers with intriguing network topologies and potential functionality, very recently, we have disclosed the Ni(Ⅱ)‐ CP based on 1‐(3, 5‐dicarboxybenzyl)‐1H‐pyrazole‐3, 5‐dicarboxylic acid (H₄L) as the main ligand and an auxiliary N‐donor ligand^[32]. In continuing our ongoing project on CPs, therefore, in this contribution, we herein report the synthesis and crystal structure of a new Co(Ⅱ)‐CPs, namely [Co₅(L) ₂ (μ₃‐OH)₂(H₂O)₈]_n (1). Moreover, the thermogravimetric analysis (TGA), magnetic, and fluorescence sensing properties of complex 1 have been also investigated systematically.

  1.   Experimental

  1.1   Materials and methods

  All synthetic reagents and solvents used were commercially available and applied directly without further purification. The real water samples were derived from the Yanhe River in Yan´an. The crystal structure was determined by a BRUKER SMART APEX‐Ⅱ CCD single crystal diffraction. The C, H, and N elemental analyses were performed by means of a PerkinElmer PE‐2400 elemental analyzer. The FT ‐ IR spectrum (500 ‐ 4 000 cm^-1) was documented on a Nicolet 170SX FT ‐ IR spectrophotometer. TGA was conducted with a NETZSCH STA 449F3 thermal gravimetric analyzer in flowing nitrogen at a heating rate of 10 ℃·min^-1. The magnetic susceptibility data were investigated by using Quantum Design MPMS SQUID VSM instrument in a range of 2‐300 K. The fluorescence sensing experiments were conducted on a Hitachi F ‐ 7000 fluorescence spectrophotometer. The powder X ‐ ray diffraction (PXRD) measurement was conducted using a Shimadzu XRD‐7000 diffractometer operating at 40 kV and 40 mA with Cu Kα radiation (λ =0.154 18 nm) at a scanning rate of 2 (°)·min^-1 from 5° to 50°.

  1.2   Synthesis of [Co₅(L)₂(μ₃⁃OH)₂(H₂O)₈]_n (1)

  A mixture of H₄L (16.7 mg, 0.05 mmol), Co(NO₃)₂·6H₂O (29.1 mg, 0.10 mmol), NaOH (8.00 mg, 0.20 mmol), and H₂O (13 mL) was stirred for 30 min at room temperature and then placed in a 25 mL Teflon‐lined stainless ‐ steel vessel and heated at 160 ℃ for 4 d. Purple crystals of 1 were obtained. Yield: 23% (based on Co). Anal. Calcd. for C₂₈H₃₀N₄O₂₆Co₅(%): C 29.68; H 2.67; N 4.94; Found(%): C 29.72; H 2.76; N 4.86. IR (KBr, cm^-l): 3 130(w), 1 610(s), 1 520(s), 1 440(w), 1 350(s), 1 230(w), 1 090(m), 1 010(w), 944(w), 749(s), 658(s).

  1.3   X⁃ray crystallographic studies

  The diffraction intensity data were corrected by semi‐empirical absorption using the SADABS program. The crystal structure was analyzed by the direct method using the SHELXS ‐ 2014 program and refined by the full‐matrix least‐squares on F² using the SHELXL‐2014 program. The coordinates and anisotropy parameters of all non‐hydrogen atoms were corrected to convergence by the least ‐squares method. The coordinates of hydrogen atoms were obtained by geometric calculation. However, the hydrogen atoms from eight coordinated water molecules in complex 1 were hard to add. In addition, the large residual peaks in the CIF file were meaningless. The detailed crystallographic data of complex 1 are collected in Table 1. Selected bond lengths and bond angles of complex 1 are presented in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement for complex 1

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  Parameter	1		Parameter	1
  Empirical formula	C28H30N4O26Co5		μ / mm-1	2.067
  Formula weight	1 133.21		F(000)	561
  Crystal system	Triclinic		θ range / (°)	2.694‐25.499
  Space group	P1		Limiting indices	-8 ≤ h ≤ 9, -12 ≤ k ≤ 12, -16 ≤ l ≤ 16
  a / nm	0.797 98(16)		Rint	0.011 5
  b / nm	1.031 4(2)		Reflection collected	5 260
  c / nm	1.398 5(3)		Unique reflection	3 777
  α/(°)	108.996(3)		Parameter	283
  β/(°)	100.859(3)		Completeness to θ / %	98.5
  γ/(°)	100.559(3)		Goodness of fit on F2	1.031
  V / nm3	1.031 1(4)		Final R indices [I > 2σ(I)]*	R1=0.041 4, wR2=0.124 8
  Z	1		R indices (all data)	R1=0.045 3, wR2=0.127 6
  Dc / (g·cm-3)	1.812		Largest diff. peak and hole / (e·nm-3)	2 231 and -744
  *R1=∑||Fo|-|Fc||/∑|Fo|; wR2=[∑w(Fo2-Fc2)2/∑(Fo2)2]1/2.

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (°) for complex 1

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  Co1—O1	0.205 6(3)	Co2—O8#2	0.207 8(3)	Co3—O11	0.203 7(2)
  Co1—O1#1	0.205 6(3)	Co2—O5	0.204 2(3)	Co3—O4#4	0.210 4(3)
  Co1—O9	0.211 3(7)	Co2—O7#2	0.223 4(3)	Co3—O12	0.238 6(3)
  Co1—O9#	0.211 3(7)	Co2—O11	0.204 1(2)	Co3—O13	0.207 1(3)
  Co1—O10	0.212 4(6)	Co2—O11#3	0.209 8(2)	Co3—O6	0.201 3(3)
  Co1—O10#	0.212 4(6)	Co2—O12	0.216 6(3)	Co3—N2#4	0.214 1(3)
  O1#1—Co1—O1	180.0	O5—Co2—O11#3	85.15(10)	O6—Co3—O11	94.33(10)
  O1—Co1—O9	89.5(2)	O8#2—Co2—O11#3	94.89(11)	O6—Co3—O13	92.05(12)
  O1—Co1—O9#1	90.5(2)	O11—Co2—O12	84.48(10)	O6—Co3—O4#4	170.69(12)
  O9—Co1—O9#1	180.0	O11#3—Co2—O12	165.94(10)	O11—Co3—N2#4	165.70(11)
  O1#1—Co1—O10	91.11(19)	O5—Co2—O7#2	166.15(11)	O13—Co3—N2#4	95.15(13)
  O1—Co1—O10	88.89(19)	O8#2—Co2—O7#2	60.01(10)	O4#4—Co3—N2#4	76.98(11)
  O9—Co1—O10	92.6(3)	O12—Co2—O7#2	93.84(11)	O11—Co3—O12	79.12(9)
  O9#1—Co1—O10	87.4(3)	O11—Co2—O5	99.51(10)	O13—Co3—O12	172.21(12)
  O10—Co1—O10#1	180.0(2)	O11—Co2—O8#2	154.18(10)	O4#4—Co3—O12	82.32(11)
  Symmetry codes: #1: -x+1, -y+1, -z; #2: x, y+1, z; #3: -x+2, -y+2, -z+1; #4: -x+1, -y+1, -z+1.

  1.4   Fluorescence sensing experiments

  A prepared crystal powder sample (30 mg) was dispersed in 100 mL of aqueous solution, sonicated for 1 h and let it stand for 3 d. The obtained supernatant containing 1 mmol·L^-1 of M(NO₃)_x (M^x+=Y³⁺, Dy³⁺, Nd³⁺, Sm³⁺, Ba²⁺, Pb²⁺, Co²⁺, Eu³⁺, Sr²⁺, Ca²⁺, Zn²⁺, Ni²⁺ and Ag⁺) or HgCl₂ was used to investigate the fluorescence sensing studies at room temperature.

  1.5   Real sample test

  The real water sample was first centrifuged. Then the obtained supernatant was diluted to 5 mmol·L^-1 with distilled water and used to test the fluorescence intensity. The experiments were conducted three times under the same conditions.

  2.   Results and discussion

  2.1   Description of crystal structure

  Single‐crystal X‐ray diffraction analysis of complex 1 indicates that it crystallizes in the triclinic system with a space group of P1. The composition of complex 1 consists of five Co(Ⅱ) ions, two L^4- ions, two hydroxyl groups, and eight coordinated water molecules. As depicted in Fig. 1, Co(Ⅱ) ions are six‐coordinated in a slightly distorted octahedron geometry. Co1 center is coordinated by two oxygen atoms (O1 and O1#1) from two different L^4- ions and four oxygen atoms (O9, O9#1, O10, and O10#1) from four water molecules. Co2 is coordinated by two oxygen atoms (O11 and O12) from hydroxyl groups, three oxygen atoms (O5, O7#2, and O8#2) from three different L^4- ions, and one oxygen atom (O11#3) from a water molecule. Co3 is coordinat‐ ed by one nitrogen atom (N2#4) from one L^4- ion, two oxygen atoms (O6 and O4#4) from two individual L^4-ions, and two oxygen atoms (O11 and O12) from two hydroxyl groups as well as one oxygen atom (O13) from one water molecule. The bond lengths of Co—O range from 0.201 3(3) to 0.238 6(3) nm and the Co—N bond length is 0.214 1(3) nm (Table 2).

  Figure 1

  

  Figure 1.  Coordination environment of Co(Ⅱ) in complex 1

  All hydrogen atoms are omitted for clarity; 50% ellipsoid probability; Symmetry codes: #1:-x+1, -y+1, -z; #2: x, y+1, z; #3:-x+2, -y+2, -z+1; #4: -x+1, -y+1, -z+1

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  The coordination mode of L^4- in complex 1 indicates the L^4- ligand is linked to four Co(Ⅱ) ions (Fig. 2). As illustrated in Fig. 3, the carboxyl groups from L^4- ions are connected to each other to Co(Ⅱ) ions, forming a similar"stepped"2D structure. On one hand, one side containing the benzene ring of H₄L constitutes the stepxs horizontal direction. On the other hand, the side containing the imidazole ring forms a vertical direction of the step from the directions of the a‐axis and the b‐axis. As shown in Fig. 4, the"step‐like"structure is connected by nitrogen atoms at both ends of imidazole to form a 3D structure from the directions of the a‐axis and the b‐axis.

  Figure 2

  

  Figure 2.  Coordination mode of L^4- in 1

  50% ellipsoid probability; All hydrogen atoms are omitted for clarity

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  Figure 3

  

  Figure 3.  Two‐dimensional network of 1

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  Figure 4

  

  Figure 4.  Three‐dimensional framework of 1

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  2.2   Thermal stability

  The thermal stability of complex 1 was investigated by TGA. As shown in Fig. 5, the first weight loss of 8.91% for complex 1 was observed with increasing the temperature from room temperature to 150 ℃, corresponding to the loss of eight‐coordinated water (Calcd. 12.71%). The framework began to decompose and collapse after further heating to 305 ℃, and the remaining weight of 33.18% may be CoO (Calcd. 33.33%).

  Figure 5

  

  Figure 5.  TGA curve of 1

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  2.3   PXRD analysis of complex 1

  In order to determine the purity of complex 1, the PXRD experiment was carried out, and the experimental and simulation results were shown in Fig. 6. It can be seen from Fig. 6 that the peak positions of the experimental and simulated patterns were basically in accordance, indicating the high phase purity of complex 1. The difference in peak strength may be attributed to the different cell orientations of the crystals.

  Figure 6

  

  Figure 6.  PXRD patterns of complex 1

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  2.4   Magnetic properties

  The variable‐temperature magnetic susceptibility measurement of complex 1 was conducted from 2 to 300 K with an applied magnetic field intensity of 10 kOe^[33]. The curves of χ_M‐T and χ_MT‐T (χ_MT represents the molar susceptibility) of complex 1 are shown in Fig. 7. At room temperature (300 K), the χ_MT of complex 1 was 4.748 cm³·mol^-1·K, which was higher than the theoretical value of 3.75 cm³·mol^-1·K for the corresponding two spin‐Co(Ⅱ) (S=3/2). With the decrease in temperature, the χ_MT decreased slowly. When the temperature was 2 K, the χ_MT decreased to 1.319 cm³·mol^-1·K, indicating that there is an antiferromagnetic interaction between Co(Ⅱ) ions in complex 1. According to the Heisenberg model (Hamiltonian $ \hat H = - J\sum {\widehat {{S_1}}} \widehat {{S_2}}$, where J is the magnetic exchange coupling constant, S₁=S₂=3/2), the empirical expression was used to fit the experimental data^[34]. The least‐squares fit to the experimental data was observed with g=2.03, J=-0.246 cm^-1 and the agreement factor R=3.144×10^-3. The negative value of J in complex 1 further demon‐ strates an antiferromagnetic interaction between Co(Ⅱ) ions.

  Figure 7

  

  Figure 7.  Plots of χ_M and χ_MT as functions of T for complex 1

  Red solid lines represent the fitted curve

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  2.5   Fluorescent sensing performances of complex 1

  2.5.1   Fluorescent sensing performances of complex 1 for metal ions

  The fluorescence spectra of complex 1 for various metal ions in aqueous solutions were recorded (Fig. 8). As depicted in Fig. 8, complex 1 possessed different effects on the fluorescence intensity response for different metal ions. It was worth noting that complex 1 exhibited a prominent fluorescence quenching effect on Hg²⁺ compared with other metal ions, indicating that there might be selective fluorescence sensing for Hg²⁺.

  Figure 8

  

  Figure 8.  Fluorescent intensity of complex 1 towards various metal ions

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  In order to further elucidate the fluorescence sensing of complex 1 for Hg²⁺, the Hg²⁺ titration experiments were carried out (Fig. 9). With the continuous dropwise addition of Hg²⁺, the fluorescence peaks decreased rapidly. Through the Stern‐Volmer equation of I₀/I =K_svc_Hg²⁺+1, where I₀ denotes the initial fluorescence intensity, I denotes the fluorescence intensity when Hg²⁺ was added, c_Hg²⁺ denotes the concentration of Hg²⁺, and K_sv denotes the quenching constant, the fluorescence sensing efficiency was calculated^[35]. At low concentrations, the Hg²⁺ concentration had a nearly linear relationship with I₀/I (R²=0.991 7). As shown in the Inset of Fig. 9, the calculated K_sv was 1.23×10⁶ L·mol^-1 and the limit of detection (LOD) for Hg²⁺ was 0.037 4 μmol·L^-1 according to 3σ/K_sv (σ=standard error).

  Figure 9

  

  Figure 9.  (a) Fluorescence titration curves of complex 1 after adding different amounts of Hg²⁺; (b) Fluorescence response of complex 1 to different concentrations of Hg²⁺

  Inset: linear response of complex 1 to Hg²⁺ in a low concentration range of 0‐60 μmol·L^-1

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  2.5.2   Anti‐interference test

  The anti‐interference experiments of complex 1 towards Hg²⁺ were also investigated in the presence of other metal ions. As shown in Fig. 10, the fluorescence intensity of complex 1 was just a slight decrease in the presence of other metal ions. However, the fluorescence intensity was immediately quenched after adding Hg²⁺. Accordingly, it can be concluded that complex 1 performed high selective recognition ability towards Hg²⁺ in the presence of other interfering metal ions.

  Figure 10

  

  Figure 10.  Comparison of fluorescent intensity of complex 1 in the presence of various other interfering metal ions

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  2.5.3   Practical application in real Yanhe River water samples

  In order to verify the practical applicability of complex 1 for the fluorescent sensing detection of Hg²⁺, the real water samples of Yanhe River were assayed via a spiked recovery experiment. The test results are shown in Table 3. As can be seen from Table 3, the spiked recoveries at different concentrations were calculated ranging from 94.3% to 108.2% with the relative standard deviation (RSD) of 1.61%‐2.54%, indicating the feasibility and accuracy of complex 1 for detection of Hg²⁺ in actual water samples.

  Table 3

  

  Table 3.  Recovery test of Hg²⁺ spiked in Yanhe River water samples

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  Spiked /(mmol·L-1)	Detected /(mmol·L-1)*	Recovery / %	RSD/%
  0	Not detected	—	—
  5	4.1	94.3	2.54
  10	7.1	105.8	1.61
  15	13.2	108.2	2.12
  * Average value (n=3).

  3.   Conclusions

  In summary, a novel Co(Ⅱ) coordination polymer based on a pyrazole carboxylic acid ligand (H₄L) was successfully synthesized using hydrothermal conditions. The crystallographic analysis of complex 1 reveals that it possesses a three ‐ dimensional network framework. All three cobalt atoms in the molecule adopt a six‐coordination mode, forming a slightly distorted octahedral coordination configuration. Magnetic studies indicate that there is an antiferromagnetic interaction between Co(Ⅱ) ions in complex 1. Additionally, the fluorescence sensing experiments suggest that complex 1 exhibits good sensitivity via an intense fluorescence quenching effect and highly selective detection of Hg²⁺ in the presence of various interference metal ions. The fluorescence results indicate that complex 1 can be a potential candidate for fluorescent chemical sensors with high sensitivity and selection to Hg²⁺. Moreover, it also can detect Hg²⁺ in real water samples of Yanhe River through the standard recovery experiment.

  
  
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  Figure 1  Coordination environment of Co(Ⅱ) in complex 1

  All hydrogen atoms are omitted for clarity; 50% ellipsoid probability; Symmetry codes: #1:-x+1, -y+1, -z; #2: x, y+1, z; #3:-x+2, -y+2, -z+1; #4: -x+1, -y+1, -z+1

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  Figure 2  Coordination mode of L^4- in 1

  50% ellipsoid probability; All hydrogen atoms are omitted for clarity

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  Figure 3  Two‐dimensional network of 1

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  Figure 4  Three‐dimensional framework of 1

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  Figure 5  TGA curve of 1

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  Figure 6  PXRD patterns of complex 1

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  Figure 7  Plots of χ_M and χ_MT as functions of T for complex 1

  Red solid lines represent the fitted curve

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  Figure 8  Fluorescent intensity of complex 1 towards various metal ions

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  Figure 9  (a) Fluorescence titration curves of complex 1 after adding different amounts of Hg²⁺; (b) Fluorescence response of complex 1 to different concentrations of Hg²⁺

  Inset: linear response of complex 1 to Hg²⁺ in a low concentration range of 0‐60 μmol·L^-1

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  Figure 10  Comparison of fluorescent intensity of complex 1 in the presence of various other interfering metal ions

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  Table 1.  Crystal data and structure refinement for complex 1

  Parameter	1		Parameter	1
  Empirical formula	C28H30N4O26Co5		μ / mm-1	2.067
  Formula weight	1 133.21		F(000)	561
  Crystal system	Triclinic		θ range / (°)	2.694‐25.499
  Space group	P1		Limiting indices	-8 ≤ h ≤ 9, -12 ≤ k ≤ 12, -16 ≤ l ≤ 16
  a / nm	0.797 98(16)		Rint	0.011 5
  b / nm	1.031 4(2)		Reflection collected	5 260
  c / nm	1.398 5(3)		Unique reflection	3 777
  α/(°)	108.996(3)		Parameter	283
  β/(°)	100.859(3)		Completeness to θ / %	98.5
  γ/(°)	100.559(3)		Goodness of fit on F2	1.031
  V / nm3	1.031 1(4)		Final R indices [I > 2σ(I)]*	R1=0.041 4, wR2=0.124 8
  Z	1		R indices (all data)	R1=0.045 3, wR2=0.127 6
  Dc / (g·cm-3)	1.812		Largest diff. peak and hole / (e·nm-3)	2 231 and -744
  *R1=∑||Fo|-|Fc||/∑|Fo|; wR2=[∑w(Fo2-Fc2)2/∑(Fo2)2]1/2.

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  Table 2.  Selected bond lengths (nm) and bond angles (°) for complex 1

  Co1—O1	0.205 6(3)	Co2—O8#2	0.207 8(3)	Co3—O11	0.203 7(2)
  Co1—O1#1	0.205 6(3)	Co2—O5	0.204 2(3)	Co3—O4#4	0.210 4(3)
  Co1—O9	0.211 3(7)	Co2—O7#2	0.223 4(3)	Co3—O12	0.238 6(3)
  Co1—O9#	0.211 3(7)	Co2—O11	0.204 1(2)	Co3—O13	0.207 1(3)
  Co1—O10	0.212 4(6)	Co2—O11#3	0.209 8(2)	Co3—O6	0.201 3(3)
  Co1—O10#	0.212 4(6)	Co2—O12	0.216 6(3)	Co3—N2#4	0.214 1(3)
  O1#1—Co1—O1	180.0	O5—Co2—O11#3	85.15(10)	O6—Co3—O11	94.33(10)
  O1—Co1—O9	89.5(2)	O8#2—Co2—O11#3	94.89(11)	O6—Co3—O13	92.05(12)
  O1—Co1—O9#1	90.5(2)	O11—Co2—O12	84.48(10)	O6—Co3—O4#4	170.69(12)
  O9—Co1—O9#1	180.0	O11#3—Co2—O12	165.94(10)	O11—Co3—N2#4	165.70(11)
  O1#1—Co1—O10	91.11(19)	O5—Co2—O7#2	166.15(11)	O13—Co3—N2#4	95.15(13)
  O1—Co1—O10	88.89(19)	O8#2—Co2—O7#2	60.01(10)	O4#4—Co3—N2#4	76.98(11)
  O9—Co1—O10	92.6(3)	O12—Co2—O7#2	93.84(11)	O11—Co3—O12	79.12(9)
  O9#1—Co1—O10	87.4(3)	O11—Co2—O5	99.51(10)	O13—Co3—O12	172.21(12)
  O10—Co1—O10#1	180.0(2)	O11—Co2—O8#2	154.18(10)	O4#4—Co3—O12	82.32(11)
  Symmetry codes: #1: -x+1, -y+1, -z; #2: x, y+1, z; #3: -x+2, -y+2, -z+1; #4: -x+1, -y+1, -z+1.

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  Table 3.  Recovery test of Hg²⁺ spiked in Yanhe River water samples

  Spiked /(mmol·L-1)	Detected /(mmol·L-1)*	Recovery / %	RSD/%
  0	Not detected	—	—
  5	4.1	94.3	2.54
  10	7.1	105.8	1.61
  15	13.2	108.2	2.12
  * Average value (n=3).

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