English

• 0.   Introduction

  Coordination chemistry of transition-metal complexes containing halide or pseudohalide ions X^- (Cl^-, HCOO^-, NO₃^-, N₃^-, etc.) is interesting from the structural point of view because of different coordination modes of the ligands to the metal ions^[1-5]. Many anions can link the metal center either in an end-to-end or in an end-on mode, both possibilities being structurally characterized for metal complexes^[6-8]. Such variable structures provide never-ending opportunities and challenges to install them into crystalline frameworks with unique topologies and properties, such as magnetism^[9-10], catalysis^[11-12], and luminescence^[13-14]. Usually, a traditional synthetic strategy is to control the anion species of the metal salt or select suitable ligands^[15-16]. In our previous research, we report a series of transition-metal complexes (Zn-L_X, X=Cl^-, N₃^-, NO₃^-) suitably designed from amino acid-derived links^[17-18]. However, compared with the largely explored anions of metal salt-based complex systems, the anions of ligands have rarely been observed in complexes, possibly due to limitations for the synthesis complication of ligands^[19-20]. Only a handful of isostructural complexes with anions of ligands have been reported in the literature. The ligand's backbone and coordinated anions greatly modulate the crystallization of the complexes^[21]. In other words, transition-metal complexes functionalization accomplished via ligands or anions has imparted stability as well as other abilities^[22-23].

  Recently, the extremely toxic heavy metal ions released into the ecological environment from industrial wastewater or other pathways have brought about severe global environmental problems and biological health problems due to their toxic effects that may accumulate in the food chain^[24-25]. Specifically, Cr(Ⅵ) ion is a carcinogenic pollutant that can also give rise to environmental pollution and human diseases: lung cancer, liver, kidney, and gastric damage, and DNA mutagenesis^[26-27]. For this reason, Cr(Ⅵ) ion, which has been classified as extremely toxic pollution by the United States Environmental Protection Agency^[28]. Up to now, various methods have been introduced for the removal or recognition of Cr(Ⅵ) ion via separation^[29], adsorbents^[30], and photocatalytic degradation^[31], of which, the effective detection of Cr(Ⅵ) ion has attracted extensive attention, and several chemical sensors of Cr(Ⅵ) ion have been reported^[32]. Nevertheless, some flaws remain in these materials, for example, poor sensing performance, time-consuming, and poor chemical stability. Therefore, it is necessary and important to design and synthesize novel materials with high solvent stability that can detect Cr(Ⅵ) ion in aqueous solution^[33].

  Based on the aforementioned contents, we have successfully synthesized two novel isostructural Zn(Ⅱ) complexes [Zn(L)(HCOO)]_n (Zn-L_For) and [Zn(L)Cl]_n (Zn-L_Cl) under hydrothermal reaction conditions (Fig.1), where HL=2-{[(pyridin-4-yl)methyl]amino}-3, 3-dimethylbutanoic acid. Single-crystal X-ray diffraction, FTIR, elemental analyses, and powder X-ray diffraction (PXRD) were employed for characterizing the synthesized products. The fluorescence sensing performance of Zn-L_For was further discussed, and it was found that Zn-L_For showed special sensing and recognition to Cr(Ⅵ) ion in aqueous solution.

  Figure 1

  

  Figure 1.  Systematic scheme of Zn-L_For and Zn-L_Cl

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  1.   Experimental

  1.1   Materials and methods

  All of the chemicals were purchased and used without purification. Distilled water was prepared in the laboratory and was used throughout the work. FTIR spectra were measured in a range of 4 000-400 cm^-1 using KBr pellets on a PerkinElmer spectrometer. PXRD patterns were obtained using crushed single crystals on an Empyrean Panalytical apparatus with Cu Kα radiation (λ=0.154 178 nm, 40 kV, 30 mA, 2θ=5°-50°). Elemental analysis (C, H, N) was performed on an Elementar Vario EL Ⅲ elemental analyzer. The fluorescence spectra were measured on a Hitachi F-4600 fluorescence spectrometer at room temperature. The UV-Vis absorption spectra were measured using a spectrophotometer (Agilent Technologies Cary5000) in a range of 200-800 nm. NMR spectra of the ligands were recorded on a Bruker Avance Ⅲ after being dissolved in D₂O.

  1.2   Synthesis of the ligands

  To an aqueous solution (20.00 mL) of L-tert-leucine (15 mmol) and Na₂CO₃ (9.4 mmol), 4-pyridinecarboxaldehyde (15 mmol) was added slowly in MeOH (10 mL). The solution was stirred for 10 min at 80 ℃. The mixture was spontaneously cooled to room temperature and then subjected to an ice bath. NaBH₄ was added slowly to the reaction mixture at 0 ℃ and stirred for 7 h. Formic acid (98%) was then added to neutralize the excess of sodium carbonate until pH=5.0-6.0. The reaction mixture was further stirred for 1 h and then evaporated to dryness. The solid was extracted in hot and dry MeOH (100 mL×3), and the filtrate was evaporated to get a white powder (HL·Na(HCOO)). Yield: 2.60 g, 72.83%. IR (KBr, cm^-1): 3 551 (vs), 3 087 (m), 2 965 (w), 2 832 (w), 2 792 (w), 1 627 (s), 1 436 (m), 1 361 (s), 1 233 (w), 1 176 (m), 996 (w), 810 (w), 758 (m), 648 (w), 561 (w), 504 (w) (Fig. S1, Supporting information). Then, the ligand HL was crystallized from the aqueous solution of the white powder. Crystals were collected and utilized for their characterization. Anal. Calcd. for C₁₂H₁₈N₂O₂(%): C, 64.86; H, 8.11; N, 12.62. Found(%): C, 64.71; H, 8.22; N, 11.66. ¹H NMR (D₂O, 400 MHz): δ 8.57 (d, J=6.2 Hz, 2H), 7.50 (d, J=6.2 Hz, 2H), 4.34 (d, J=13.7 Hz, 1H), 4.17 (d, J=13.8 Hz, 1H), 3.13 (s, 1H), 0.99 (s, 9H). ¹³C NMR (101 MHz, D₂O): δ 171.85, 149.30, 140.36, 125.40, 71.26, 49.62, 32.18, 25.96 (Fig.S2 and S3).

  HL·NaCl was prepared exactly as HL·Na(HCOO), except HCl was used instead of HCOOH for pH adjustment. Yield: 2.80 g, 78.43%. IR (KBr, cm^-1): 3 412 (vs), 3 267 (m), 2 954 (m), 2 908 (m), 2 861 (w), 2 722 (w), 2 595 (w), 1 594 (s), 1 477 (m), 1 419 (w), 1 390 (s), 1 332 (w), 1 227 (w), 1 176 (w), 1 065 (m), 1 002 (m), 864 (w), 793 (m), 723 (m), 637 (m), 619 (m), 561 (s), 515 (m).

  1.3   Synthesis of Zn-L_For and Zn-L_Cl

  To an aqueous solution (2 mL) of HL·Na(HCOO) (0.2 mmol), Zn(CH₃COO)₂·2H₂O (0.1 mmol) was added and sonicated for 2 min. The clear solution was heated at 90 ℃ for 40 h, and then cooled down to room temperature at a rate of 2 ℃·h^-1. Rod-shaped transparent crystals, which were insoluble in water or common organic solvents, were collected by filtration, washing with deionized water, and finally dried at ambient temperature. Yield of Zn-L_For: 18 mg (64%, based on metal salt). Elemental analysis Calcd. for C₁₃H₁₈N₂O₄Zn(%): C, 47.13, H, 5.44, N, 8.46; Found(%): C, 47.03, H, 5.38, N, 8.43. IR (KBr, cm^-1): 3 452 (vs), 3 145 (m), 2 960 (m), 2 849 (w), 1 616 (s), 1 569 (s), 1 477 (m), 1 430 (m), 1 315 (s), 1 287 (w), 1 222 (m), 1 065 (w), 1 025 (m), 984 (m), 926 (m), 805 (m), 735 (m), 631 (w), 567 (w).

  A similar synthetic protocol was adopted, like Zn-L_For, only HL·NaCl was used instead of HL· Na(HCOO). Yield: 23 mg (69%, based on metal salt). Elemental analysis Calcd. for C₁₂H₁₇ClN₂O₂Zn(%): C, 44.72, H, 5.28, N, 8.70; Found(%): C, 44.69, H, 5.23, N, 8.63. IR (KBr, cm^-1): 3 365 (vs), 3 261 (s), 3 174 (m), 3 070 (m), 3 035 (m), 2 960 (s), 2 902 (m), 2 809 (w), 1 604 (s), 1 477 (m), 1 430 (m), 1 372 (m), 1 297 (m), 1 268 (w), 1 222 (m), 1 060 (w), 1 019 (m), 972 (m), 915 (m), 799 (s), 741 (m), 619 (m), 579 (w).

  1.4   Crystal structure determination

  Crystallographic data were collected at a temperature of 293(2) K on a Bruker Apex Ⅱ CCD diffractometer with graphite monochromated Mo Kα radiation (λ=0.071 073 nm). The structures were solved by direct methods and refined using the SHELXTL suite of programs^[34-35]. All non-hydrogen atoms were refined anisotropically. The H atoms were introduced in calculated positions and refined with fixed geometry with respect to their carrier atoms. The details of crystal data and refinement for Zn-L_For and Zn-L_Cl are given in Table 1. Selected bond lengths and angles for Zn-L_For and Zn-L_Cl are listed in Table 2, S1, and S2.

  Table 1

  

  Table 1.  Crystallographic data and refinement parameters of Zn-L_For and Zn-L_Cl

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  Parameter	Zn-LFor	Zn-LCl
  Empirical formula	C13H18N2O4Zn	C12H17Cl N2O2Zn
  Formula weight	331.66	322.10
  Crystal system	Orthorhombic	Orthorhombic
  Space group	P212121	P212121
  a / nm	0.920 50(10)	0.919 56(4)
  b / nm	0.943 73(10)	1.036 02(7)
  c / nm	1.624 50(17)	1.491 90(6)
  Volume / nm3	1.411 2(3)	1.421 31(13)
  Z	4	4
  Dc / (g·cm-3)	1.561	1.505
  Absorption coefficient / mm-1	1.755	1.911
  F(000)	688.0	664.0
  θ range for data collection / (°)	3.09-25.01	2.96-28.40
  Reflection collected	43 843	88 824
  Independent reflection	2 476 (Rint=0.031 7)	3 550 (Rint=0.021 9)
  Completeness to θ=25.00° / %	99.4	99.4
  Data, Nres, Npar*	2 476, 0, 184	3 552, 0, 166
  Goodness-of-fit on F 2	1.100	1.129
  Flack parameter	0.037(12)	0.018(7)
  Final R indices [I > 2σ(I)]	R1=0.020 6, wR2=0.051 4	R1=0.017 9, wR2=0.047 5
  R indices (all data)	R1=0.021 4, wR2=0.052 2	R1=0.018 2, wR2=0.047 7
  * Nres=number of restraints, Npar=number of parameters.

  Table 2

  

  Table 2.  Selected bond lengths (nm) and trigonality index values of Zn-L_For and Zn-L_Cl

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  Complex	Bond length / nm					Trigonality index (τ5)
  	Zn—Namine	Zn—Npyridine	Zn—Cl/OHCOO	Zn—O1carboxylate	Zn—O2carboxylate
  Zn-LFor	0.212 30(2)	0.209 44(19)	0.202 02(17)	0.215 02(17)	0.200 24(17)	0.089
  Zn-LCl	0.214 74(11)	0.209 17(12)	0.225 43(4)	0.206 19(11)	0.216 57(11)	0.144

  1.5   Fluorescence sensing experiment

  Detailed procedures of fluorescence experiments can be found in the Supporting information.

  2.   Results and discussion

  2.1   Crystal structure of the complexes

  As shown in Fig.2a and 2b, two complexes were essentially isomorphous, having distorted square pyramidal structures, and they mainly differ in the terminal ligands (HCOO^-, Cl^-) around the Zn(Ⅱ) ions. In the representative molecular structure of Zn-L_For, which consists of one crystallographically Zn(Ⅱ) ion, one L^- ligand and one HCOO^- ligand, forming an open channel with side chains in space. The Zn(Ⅱ) center is five-coordinated by two nitrogen (N_pyridyl, N_amine) atoms from the L^- ligand, two oxygen (O3, O4) atoms deriving from carboxyl atoms of the L^- ligand, and one oxygen atom (O1) from the HCOO^- ligand, and the trigonality factor (τ₅) is 0.089. The average bond distance of Zn—N is 0.210 9 nm, and the average bond distance of Zn—O is 0.205 8 nm; the O—Zn—O and N—Zn—N angles are 157.738° and 152.454°. The adjacent Zn(Ⅱ) ions are bridged by one carboxylate group, one pyridyl group, and one N_amine atom of the bridging ligand L^-, extending the structure into a 2D framework (Fig.2c and 2d).

  Figure 2

  

  Figure 2.  (a, b) Structures of square pyramidal geometries of complexes Zn-L_For (a) and Zn-L_Cl (b); View of the layer structures of Zn-L_For (c) and Zn-L_Cl (d)

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  Compared with the crystal structure of Zn-L_For, in the asymmetric unit of Zn-L_Cl, the Zn(Ⅱ) center is five-coordinated by O_carboxylate, N_amine, N_pyridyl, and Cl^-. The τ₅ value (0.144) is different from that of Zn-L_For. The average bond distance of Zn—N is 0.212 0 nm, and the average bond distance of Zn—O is 0.211 4 nm; the O—Zn—O and N—Zn—N angles are 155.718° and 147.107°, respectively.

  Based on the simplification principle, the Zn(Ⅱ) center is simplified to a 3-connected node, the L^- ligands act as 3-connected linkers, and the whole network can be simplified as a {6³} topology, as shown in Fig.3.

  Figure 3

  

  Figure 3.  Simplified structure with the (6³) topology of the complexes

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  2.2   Fluorescence properties

  The fluorescence properties of two complexes and free ligands were investigated in aqueous solution at room temperature, as given in Fig.4. It was found that HL·Na(HCOO) and HL·NaCl displayed a broad emission band at 397 nm upon excitation at 294 nm. Interestingly, complex Zn-L_For exhibited fluorescence with the maximum emission at 406 nm under the same experimental conditions. Compared to the ligands, the complex indeed gave rise to much higher emission energy and a red shift of 9 nm for maximum emission, which can be derived from the charge transition between the ligands and Zn²⁺ metal ion^{[27, 36]}. Therefore, complex Zn-L_For is suitable for fluorescence sensor applications.

  Figure 4

  

  Figure 4.  Fluorescence spectra of the complexes and ligands in aqueous solution

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  2.3   Fluorescence detection of Cr₂O₇^2-

  To explore the influence of various anions on the fluorescence of Zn-L_For, 4.8 mg sample of Zn-L_For was dispersed in 2.0 mL of aqueous solution to form a suspension by ultrasound method, and 70 μL of potassium salts solution (5 mmol·L^-1) (K_nX, X^n-=Br^-, S₂O₈^2-, F^-, NO₃^-, Cl^-, I^-, Cr₂O₇^2-, respectively) was lowly dropped into the above solutions to form suspension. Interestingly, the fluorescence of Zn-L_For was reduced vastly in the case of Cr₂O₇^2-, leading to the fluorescence quenching, while other anions had no obvious effect on the emission (Fig.5). This fluorescence exhibited by Zn-L_For motivates us to explore its potential utility in the detection of Cr₂O₇^2-. Then, concentration titration experiments were carried out, and it was observed that the fluorescence emission intensity was quenched with the increase of Cr₂O₇^2- concentration. Upon the addition of 130 μL of a 5 mmol·L^-1 Cr₂O₇^2- solution to the suspension of Zn-L_For in aqueous solution (2 mL), the fluorescence was quenched by 94.92% (Fig.6).

  Figure 5

  

  Figure 5.  Intensities of the fluorescence (406 nm) of Zn-L_For dispersed in different anion aqueous solutions

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  Figure 6

  

  Figure 6.  Fluorescence spectra of Zn-L_For upon the gradual addition of 5 mmol·L^-1 Cr₂O₇^2- solution

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  The fluorescence quenching efficiency of Cr₂O₇^2- can be quantitatively investigated using the Stern-Volmer (SV) equation: I₀/I=1+K_SVc, where I₀ and I are the maximum luminescent intensity of Zn-L_For before and after addition of the analyte, c is the molar concentration of analyte, and K_SV is the Stern-Volmer quenching constant. By calculation, the K_SV value was 2.23×10⁴ L·mol^-1, which was comparable to those of reported complexes for detecting Cr₂O₇^2- (Table 3). Zn-L_For exhibited an excellent linear relationship with a linear correlation coefficient of 0.989 02, suggesting that Zn-L_For is a potential quantitative sensor for Cr₂O₇^2-. Accordingly, the detection limit (LOD) was calculated to be 1.03 μmol·L^-1 (at 3σ/k level, where σ is the standard deviation, k is the slope) (Fig.7 and 8).

  Table 3

  

  Table 3.  K_SV values and LOD values of the reported complexes for detecting Cr₂O₇^2- in H₂O and DMF suspension

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  Complex	KSV / (L·mol-1)	LOD / (μmol·L-1)	Solvent	Ref.
  Zn-LFor	2.23×104	1.03	H2O	This work
  [Cd(PLIA)(TIB)]n	1.92×104	0.463	H2O	[37]
  [Zn(tpbpc)2]	1.13×104	0.676	H2O	[27]
  [Zn7(TPPE)2(SO4)7](DMF·H2O)	1.09×104	n.d.*	H2O	[38]
  [Zn(btz)]n	3.19×103	10	H2O	[33]
  [Zn2(ttz)H2O]n	2.35×103	20	H2O	[33]
  {[Eu2L1.5(H2O)2EtOH]·DMF}n	1.53×103	10	DMF	[39]
  {[Tb4(BPDC)4(μ3-OH)4(H2O)8]·11H2O}n	1.13×104	0.1	H2O	[40]
  {[Tb4Mn-(BPDC)3(μ3-OH)4(HCOO)1.5(H2O)4]·2.5OH·8H2O}n	5.0×103	0.1	H2O	[40]
  {[Cd(TIPE)(Cl)2·Cd(TIPE)(Cl)2]·5H2O}n	6.40×104	3.1	H2O	[41]
  Zr-MOF	4.32×104	n.d.	H2O	[42]
  {[Cd2(2F-bpdc)2(tib)]}n	7.23×103	n.d.	H2O	[43]
  [Th6(OH)4(O)4(H2O)6](BCTPE)6·(DMF)18·(H2O)9	4.63×105	0.004 6	H2O	[44]
  [Th6(OH)4(O)4(H2O)6](BCTPE)5(HCOO)2·(DMF)26·(H2O)32	2.22×105	0.094	H2O	[44]
  {[(CH3)2NH2]3(In3L4)}·(solvent)x	1.32×104	n.d.	H2O	[45]

  Figure 7

  

  Figure 7.  Stern-Volmer plot for the sensing of Cr₂O₇^2- with Zn-L_For in aqueous solution

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  Figure 8

  

  Figure 8.  Emission intensity evolution of Zn-L_For at different concentrations

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  What is more, to further find out the unique selective detection of Cr₂O₇^2-, the anion interference experiments were conducted under the same conditions. As shown in Fig.9, the fluorescence intensity in the anions except for Cr₂O₇^2- has no significant change; nevertheless, it was nearly quenched upon adding Cr₂O₇^2-. The fluorescence measurements demonstrated that most anions had no marked effects on the fluorescence spectra of Zn-L_For, while it experienced a significant luminescent response in the presence of the Cr₂O₇^2- ion.

  Figure 9

  

  Figure 9.  Comparison of the quenching efficiency of Zn-L_For for different interference anions

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  Based on all the above observations, the fluorescence quenching mechanism was further studied by testing the UV-Vis absorption spectra of Cr₂O₇^2- in aqueous solution (Fig.10). PXRD analysis revealed that the crystallinity and frameworks of Zn-L_For can essentially remain unchanged after soaking in Cr(Ⅵ) for 24 h (Fig.S4). The fluorescence excitation spectrum of Zn-L_For showed a strong overlap with the absorption spectrum of Cr₂O₇^2-. This appearance indicates that during Zn-L_For excitation, the detected anions (Cr₂O₇^2-) competitively absorb the excitation wavelength of light, resulting in the fluorescence quenching of Zn-L_For, which belongs to the energy competition absorption mechanism.

  Figure 10

  

  Figure 10.  UV-Vis spectrum of Cr₂O₇^2- solution with the excitation and emission spectra of Zn-L_For

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  3.   Conclusions

  In summary, two isostructural coordination polymers have been synthesized and characterized. It is confirmed that they have the square pyramidal units and high-purity phases through various characterizations. The luminescent investigations suggest that Zn-L_For can sensitively detect Cr₂O₇^2- in aqueous solution by luminescent quenching effect. The mechanisms of Zn-L_For for detecting Cr₂O₇^2- have been preliminarily elucidated. The results show that Zn-L_For can be used as a chemical sensor with high sensitivity for detecting Cr₂O₇^2-.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Systematic scheme of Zn-L_For and Zn-L_Cl

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  Figure 2  (a, b) Structures of square pyramidal geometries of complexes Zn-L_For (a) and Zn-L_Cl (b); View of the layer structures of Zn-L_For (c) and Zn-L_Cl (d)

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  Figure 3  Simplified structure with the (6³) topology of the complexes

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  Figure 4  Fluorescence spectra of the complexes and ligands in aqueous solution

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  Figure 5  Intensities of the fluorescence (406 nm) of Zn-L_For dispersed in different anion aqueous solutions

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  Figure 6  Fluorescence spectra of Zn-L_For upon the gradual addition of 5 mmol·L^-1 Cr₂O₇^2- solution

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  Figure 7  Stern-Volmer plot for the sensing of Cr₂O₇^2- with Zn-L_For in aqueous solution

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  Figure 8  Emission intensity evolution of Zn-L_For at different concentrations

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  Figure 9  Comparison of the quenching efficiency of Zn-L_For for different interference anions

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  Figure 10  UV-Vis spectrum of Cr₂O₇^2- solution with the excitation and emission spectra of Zn-L_For

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  Table 1.  Crystallographic data and refinement parameters of Zn-L_For and Zn-L_Cl

  Parameter	Zn-LFor	Zn-LCl
  Empirical formula	C13H18N2O4Zn	C12H17Cl N2O2Zn
  Formula weight	331.66	322.10
  Crystal system	Orthorhombic	Orthorhombic
  Space group	P212121	P212121
  a / nm	0.920 50(10)	0.919 56(4)
  b / nm	0.943 73(10)	1.036 02(7)
  c / nm	1.624 50(17)	1.491 90(6)
  Volume / nm3	1.411 2(3)	1.421 31(13)
  Z	4	4
  Dc / (g·cm-3)	1.561	1.505
  Absorption coefficient / mm-1	1.755	1.911
  F(000)	688.0	664.0
  θ range for data collection / (°)	3.09-25.01	2.96-28.40
  Reflection collected	43 843	88 824
  Independent reflection	2 476 (Rint=0.031 7)	3 550 (Rint=0.021 9)
  Completeness to θ=25.00° / %	99.4	99.4
  Data, Nres, Npar*	2 476, 0, 184	3 552, 0, 166
  Goodness-of-fit on F 2	1.100	1.129
  Flack parameter	0.037(12)	0.018(7)
  Final R indices [I > 2σ(I)]	R1=0.020 6, wR2=0.051 4	R1=0.017 9, wR2=0.047 5
  R indices (all data)	R1=0.021 4, wR2=0.052 2	R1=0.018 2, wR2=0.047 7
  * Nres=number of restraints, Npar=number of parameters.

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  Table 2.  Selected bond lengths (nm) and trigonality index values of Zn-L_For and Zn-L_Cl

  Complex	Bond length / nm					Trigonality index (τ5)
  	Zn—Namine	Zn—Npyridine	Zn—Cl/OHCOO	Zn—O1carboxylate	Zn—O2carboxylate
  Zn-LFor	0.212 30(2)	0.209 44(19)	0.202 02(17)	0.215 02(17)	0.200 24(17)	0.089
  Zn-LCl	0.214 74(11)	0.209 17(12)	0.225 43(4)	0.206 19(11)	0.216 57(11)	0.144

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  Table 3.  K_SV values and LOD values of the reported complexes for detecting Cr₂O₇^2- in H₂O and DMF suspension

  Complex	KSV / (L·mol-1)	LOD / (μmol·L-1)	Solvent	Ref.
  Zn-LFor	2.23×104	1.03	H2O	This work
  [Cd(PLIA)(TIB)]n	1.92×104	0.463	H2O	[37]
  [Zn(tpbpc)2]	1.13×104	0.676	H2O	[27]
  [Zn7(TPPE)2(SO4)7](DMF·H2O)	1.09×104	n.d.*	H2O	[38]
  [Zn(btz)]n	3.19×103	10	H2O	[33]
  [Zn2(ttz)H2O]n	2.35×103	20	H2O	[33]
  {[Eu2L1.5(H2O)2EtOH]·DMF}n	1.53×103	10	DMF	[39]
  {[Tb4(BPDC)4(μ3-OH)4(H2O)8]·11H2O}n	1.13×104	0.1	H2O	[40]
  {[Tb4Mn-(BPDC)3(μ3-OH)4(HCOO)1.5(H2O)4]·2.5OH·8H2O}n	5.0×103	0.1	H2O	[40]
  {[Cd(TIPE)(Cl)2·Cd(TIPE)(Cl)2]·5H2O}n	6.40×104	3.1	H2O	[41]
  Zr-MOF	4.32×104	n.d.	H2O	[42]
  {[Cd2(2F-bpdc)2(tib)]}n	7.23×103	n.d.	H2O	[43]
  [Th6(OH)4(O)4(H2O)6](BCTPE)6·(DMF)18·(H2O)9	4.63×105	0.004 6	H2O	[44]
  [Th6(OH)4(O)4(H2O)6](BCTPE)5(HCOO)2·(DMF)26·(H2O)32	2.22×105	0.094	H2O	[44]
  {[(CH3)2NH2]3(In3L4)}·(solvent)x	1.32×104	n.d.	H2O	[45]

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