English

• Metal-organic coordination polymers (MOCPs) have attracted considerable attention, due to their exceptional structural versatility, tunable porosity, and ability to incorporate functional metal centres and organic linkers^[1-4]. These properties have made these materials extremely suitable for a wide range of applications such as catalysis, gas storage, drug delivery, and chemical sensing^[5-7]. Materials with tailored physicochemical properties can be designed to interact selectively with specific analytes through precise control of their crystalline architecture^[8-9]. Among these, transition metal-based coordination networks offer unique advantages, including enhanced stability, reactivity, and response to external stimuli^[10-13]. Their modularity and adaptability hold great promise in addressing complex industrial and environmental challenges, such as developing high-performance sensors, energy storage systems, and smart functional materials^[14-15].

  Aluminum ion (Al³⁺) plays a vital role in various industrial and biological systems, but over-accumulation presents significant environmental and health risks, including neurotoxicity and potential links to Alzheimer's disease^[16-18]. There is a need to develop innovative sensing strategies as traditional detection techniques often face challenges such as low specificity and complex sample preparation^[19-21]. Because of their strong binding affinity, turn-on fluorescence response, and high selectivity in aqueous environments, fluorescent coordination polymers and metal-organic supramolecular sensors have emerged as promising tools for Al³⁺ detection^[22-24]. These materials are valuable for applications in environmental safety, healthcare, and industrial quality control, not only because of their excellent sensitivity and detection limits, but also because they offer rapid and cost-effective solutions for real-time monitoring^[25-29].

  Co(Ⅱ)-based coordination polymers are of great interest because of their diverse structural topology, redox activity, and magnetic properties, which make these polymers valuable for catalytic, sensing, and energy storage applications^[30-34]. The flexible coordination geometry and variable oxidation states of Co enable enhanced electronic communication within supramolecular architectures, contributing to advanced functional materials^[35-37]. Further extending their applicability, these complexes also exhibit robust stability and tunable properties^[38-40]. To enhance their performance in multifunctional materials applications, future research should focus on increasing the structural diversity, optimising stability, and designing novel ligand frameworks^[41-43].

  Single crystal X-ray diffraction analysis plays a key role in the structural intricacies of metal-organic frameworks and coordination polymers^[44-47]. Rational design of materials with desired properties can be achieved by accurately determining bond lengths, coordination environments, and intermolecular interactions^[48-49]. In this study, two complexes [Co(hfacac)₂(L¹)]_n (1) and [Co(hfacac)(L²)]_n (2) (Hhfacac=hexafluoroacetylacetonate) were synthesized by the reaction between Co(hfacac)₂ and ligands 3, 6-di(pyridin-4-yl)-9H-carbazole (L¹) and 9-methyl-3, 6-di(pyridin-4-yl)-9H-carbazole (L²), respectively. A 1D chain structure stabilized by hydrogen bonding and π-π interactions has been characterized in the novel Co(Ⅱ)-based coordination polymer 2. Upon interaction with Al³⁺ ions, complex 1 exhibits remarkable Al³⁺ sensing capabilities, with high fluorescence enhancement. These findings highlight the potential for environmental monitoring and sensing applications and contribute to the growing field of functional coordination materials.

  1.   Experimental

  1.1   Materials and instruments

  All chemicals used in this study were obtained from commercial sources and applied directly without further purification. The ligands L¹ and L² were synthesized according to previously reported procedures^[50]. Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=0.154 nm), operating at 40 kV and 40 mA, over a 2θ range of 5°-50° at a scanning rate of 5 (°)·min^-1. IR spectra were recorded on a Bruker INVENIO-S FT-IR spectrometer in a range of 4 000-400 cm^-1 using KBr pellets. Fluorescence measurements were performed using a Hitachi F7000 spectrofluorometer, while UV-Vis absorption spectra were acquired with a UH-4150 spectrophotometer. Electrospray ionization mass spectra (ESI-MS) were conducted on a JEOL Accu-TOF system. Single-crystal X-ray diffraction. Data collection and processing were carried out using APEX2 software. Thin-layer chromatography (TLC) analysis was performed on silica gel plates (200-300 mesh size).

  1.2   Synthesis

  1.2.1   Synthesis of complex 1

  A methanol solution (1.0 mL) containing Co(hfacac)₂·6H₂O (3.3 mg, 0.007 mmol) was gradually introduced dropwise into a 2.0 mL methanol solution of L¹ (3.2 mg, 0.014 mmol) under continuous stirring at 25 ℃. The reaction was maintained for 2 h, leading to the gradual formation of a white precipitate. The mixture was left undisturbed for 24 h to ensure complete crystallization. The resulting solid was isolated by filtration, thoroughly washed with cold methanol (3×2 mL), and subsequently dried under vacuum at 60 ℃ for 12 h. Crystals were obtained by slow evaporation of an acetonitrile/methanol (1∶1, V/V) solution at ambient temperature over 10 d. ESI-MS analysis revealed a peak at m/z=908.20 for [Co(hfacac)₂(L¹)+2C₂H₅OH+H₂O+2H]⁺ (Calcd. 908.60) (Fig.S1, Supporting information). Elemental analysis Calcd. for C₃₂H₁₇CoF₁₂N₃O₄(%): C, 48.38; H, 2.16; N, 5.29. Found(%): C, 48.37; H, 2.15; N, 5.31.

  1.2.2   Synthesis of complex 2

  A 1.0 mL methanol solution of Co(hfacac)₂·6H₂O (3.3 mg, 0.007 mmol) was slowly added to a 2.0 mL methanol solution of L² (3.9 mg, 0.014 mmol) under continuous stirring at 25 ℃. The reaction proceeded with constant agitation for 2 h, leading to the gradual emergence of a fine white precipitate. The mixture was then allowed to stand undisturbed for 24 h to facilitate complete crystallization. The solid product was collected via filtration, thoroughly washed with chilled methanol (3×2 mL), and dried under vacuum at 60 ℃ for 12 h.

  For single-crystal formation, the obtained solid was dissolved in an acetonitrile/methanol solution (1∶1, V/V) and left to undergo slow evaporation at room temperature over 10 d, yielding well-defined crystals suitable for X-ray diffraction analysis. ESI-MS confirmed the product with a signal at m/z=936.66 for [Co(hfacac)₂(L²)+2C₂H₅OH+CH₃OH+2H⁺] (Calcd. 936.23) (Fig.S2). Elemental analysis Calcd. for C₃₃H₁₉CoF₁₂N₃O₄(%): C, 49.03; H, 2.37; N, 5.20. Found(%): C, 49.06; H, 2.36; N, 5.18.

  2.   Results and discussion

  2.1   ESI-MS spectra of the complexes

  ESI-MS spectra of complexes 1 and 2 in C₂H₅OH verified the formation of the supramolecular chains, as depicted in Fig.S1 and S2. Analysis of 1 and 2, namely [Co(hfacac)₂(L¹)]_n and [Co(hfacac)₂(L²)]_n, by ESI-MS, showed predominant signals at m/z=908.20 and 936.66 for [Co(hfacac)₂(L¹)](C₂H₅OH)₂(H₂O)(2H⁺) and [Co(hfacac)₂(L²)](C₂H₅OH)₂(CH₃OH)(2H⁺), respectively. The available evidence clearly confirms the presence of organometallic chains in solution. This was further confirmed by single-crystal X-ray diffractometry of 2, which revealed that the [Co(hfacac)₂(L)]_n of 2 is highly symmetric.

  2.2   Single-crystal X-ray structure of complex 2

  The structural information about complex 1 is not displayed and discussed because of the low quality of the single crystal samples. The detailed crystallographic data of complex 2 are presented in Table 1. Selected bond lengths and bond angles are listed in Table 2. Crystallographic studies reveal that complex 2 exhibits six-coordinated [CoO₄N₂] octahedral geometries (Fig.1a and 1b). Co(Ⅱ) in complex 2 is coordinated by four oxygen atoms from two hfacac^- ligands and two nitrogen atoms from two L² ligands, forming a distorted octahedral geometry. The Co—O bond lengths range from 0.204 to 0.206 nm, and the Co—N bond lengths are 0.216 and 0.218 nm. The N(carbazole)—Co—N(carbazole) bond angle is 180°, with O—Co—O bond angles ranging from 89.48° to 180°. The structure forms a 1D sinusoidal chain with a Co…Co distance of 1.594 nm, where the dihedral angles between the pyridyl rings and the carbazole core are 6.25° and 2.63°. This arrangement leads to an extended wavelike chain. Inter-chain π-π stacking interactions (plane…plane distances of 0.363-0.414 nm) reinforce the stability of complex 2 (Fig.2).

  Table 1

  

  Table 1.  Crystal data and structure refinement for complex 2

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  Parameter	2		Parameter	2
  Empirical formula	C36H37F12N3O5Co		Dc / (g·cm-3)	1.487
  Formula weight	863.31		μ / mm-1	0.906
  Temperature / K	198		F(000)	886
  Crystal system	Triclinic		2θ range / (°)	4.69-65.50
  Space group	P1		Index ranges	-13 ≤ h ≤ 13, -22 ≤ k ≤ 21, -24 ≤ l ≤ 24
  a / nm	0.877 9(5)		Independent reflection	13 128
  b / nm	1.496 2(9)		Rint	0.087 5
  c / nm	1.653 8(10)		Rσ	0.061 2
  α / (°)	67.422(2)		Goodness-of-fit on F2	1.086
  β / (°)	74.707(2)		Final R indexes [I≥2σ(I)]	R1=0.057 9, wR2=0.164 8
  γ / (°)	80.109(2)		Final R indexes (all data)	R1=0.088 4, wR2=0.187 2
  Volume / nm3	1.928 7(2)		Largest peak and hole / (e·nm-3)	1 350 and -900
  Z	2

  Table 2

  

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

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  Co1—O1	0.206	Co1—N1	0.218	Co2—O4	0.206
  Co1—O2	0.204	Co2—O3	0.205	Co2—N2	0.216
  O1—Co1—O2	89.35	O2ⅰ—Co1—O2ⅱ	180	O4—Co2—N2	91.03
  O1—Co1—O2ⅰ	90.65	N1ⅰ—Co1—N1ⅱ	180	O3ⅰ—Co2—O3ⅱ	180
  O1—Co1—N1	88.85	O4—Co2—O3	89.48	O4ⅰ—Co2—O4ⅱ	180
  O2—Co1—N1	90.29	O4ⅰ—Co2—O3	90.52	N2ⅰ—Co2—N2	180
  O1ⅱ—Co1—O1ⅰ	180	O3—Co2—N2	88.16
  Symmetry codes: ⅰ 3-x, -y, 1-z; ⅱ -x, 1-y, 2-z.

  Figure 1

  

  Figure 1.  (a) ORTEP drawing of the asymmetric unit of complex 2 with ellipsoids at the 50% probability level; (b) Partial packing diagram of 2 showing the 1D chain structure formed through bridging ligands along the crystallographic axis

  Hydrogen atoms are omitted for clarity; Symmetry codes: ^ⅰ 3-x, -y, 1-z; ^ⅱ-x, 1-y, 2-z.

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  Figure 2

  

  Figure 2.  π-π stacking between adjacent molecules of complex 2

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  2.3   Characterization of complexes 1 and 2

  2.3.1   UV-Vis absorption spectra

  The UV-Vis absorption spectra of ligands L¹, L², and their corresponding complexes 1 and 2 were recorded in acetonitrile at room temperature (20 μmol·L^-1), as shown in Fig.3. As depicted in Fig.3a and 3b, free ligands L¹ and L² exhibited intense absorption bands in a range of 260-280 nm, which are attributable to allowed π→π* transitions of the ligands. Additional absorption bands within 300-320 nm are assigned to n→π* transitions, due to the presence of heteroatoms such as nitrogen and oxygen within the ligand framework. Upon coordination with the metal ion (Fig.3c and 3d), new broad absorption bands emerged in the 330-400 nm region, which are ascribed to ligand-to-metal charge transfer (LMCT) transitions. The appearance of these peaks confirms the successful formation of complexes 1 and 2.

  Figure 3

  

  Figure 3.  UV-Vis absorption spectra of ligands L¹ (a) and L² (b), and complexes 1 (c) and 2 (d)

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  2.3.2   IR spectra

  Further evidence for successful coordination is provided by IR spectral analysis (Fig.4a). Both complexes displayed a prominent vibration band in the 1 650-1 550 cm^-1 region, indicative of C=O stretching vibrations from the pyridine moiety participating in metal coordination. Additionally, the spectral region between 1 000 and 500 cm^-1 exhibited several peaks and valleys, which can be assigned to stretching and bending vibrations involving metal-ligand (M—N, M—O) bonds. These results suggest that the metal centers bind to both nitrogen and oxygen donors, forming multidentate coordination environments. The complexes thus possess rigid frameworks and intricate coordination geometries, as further confirmed by the following assignments: 800 cm^-1 (—M—N_pyridine), 1 640 cm^-1 (—M—O=C).

  Figure 4

  

  Figure 4.  IR spectra (a) and PXRD patterns (b) of complexes 1 and 2

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  2.3.3   PXRD patterns

  The crystalline nature and long-range order of complexes 1 and 2 were assessed via PXRD analysis (Fig.4b). Both complexes exhibited sharp and intense diffraction peaks within a 2θ range of 10°-30°, suggesting high crystallinity and regular crystal packing. The pronounced diffraction features highlight the structural rigidity and robust stacking of the complexes. These observations, in conjunction with the UV-Vis and IR data, further support the successful synthesis and coordination of ligands L¹ and L² with their respective metal centers to form two distinct complexes.

  2.4   Fluorescence response of complexes 1 and 2 toward metal ions

  Selectivity experiments were conducted on complexes 1 and 2 to evaluate their fluorescence response to a series of metal ions in an aqueous solution (20 μmol·L^-1) at room temperature under 360 nm excitation. As shown in Fig.5a, after adding Al³⁺, the fluorescence intensity of complex 1 increased significantly. For complex 1, compared with the initial value, from 1.51 to 974.4, there was an increase of 644.8 times, as shown in Fig.5b. At the same time, as the emission intensity at 487 nm increased sharply, the fluorescence color also changed from dark to bright blue, and the fluorescence intensity did not change significantly when other cations were added.

  Figure 5

  

  Figure 5.  (a) Fluorescence spectra of complex 1 after adding different metal ions in H₂O; (b) Fluorescence enhancement of 1 at 487 nm; (c) Image of the fluorescence color changes of 1 after adding different metal ions

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  These results indicate that complex 1 exhibits a highly selective response to Al³⁺ compared to other metal ions. Upon interaction with Al³⁺, a stable chelate, complex 1-Al³⁺, was formed, which enhanced the rigidity of the molecular structure, leading to the chelation-enhanced fluorescence (CHEF) effect^[51-52]. Under the same conditions, the fluorescence spectrum of complex 2 was also tested, and there were no significant changes. These results indicate that complexes 1 are highly selective sensors for Al³⁺.

  Next, fluorometric titration experiments were carried out in H₂O to study the fluorescent response of complexes 1 to Al³⁺. As shown in Fig.6a and 6b, complex 1 exhibited a weak emission peak upon excitation at 370 nm. Upon adding different equivalents of Al³⁺ to complex 1, a new emission peak was observed at 487 nm. With the addition of 8.0 equivalents of Al³⁺ ($ c_{\mathrm{Al}^{3+}} / c_\boldsymbol{1}$=8), the emission peak reached its maximum, and the fluorescence intensity increased from the original value of 1.51 to 974.4, representing a 644.8-fold enhancement. At this point, the fluorescence color changed from colorless to bright blue. In addition, it was shown that the fluorescence exhibited a good linear relationship with Al³⁺ within a range of 0-8.0 equivalents. This suggests that complex 1 is suitable for the quantitative detection of Al³⁺ ions.

  Figure 6

  

  Figure 6.  (a) fluorometric titration of Al³⁺ with complex 1 in H₂O; (b) Plot of fluorescence intensity of 1 at 487 nm $c_{\mathrm{Al}^{3+}} / c_\boldsymbol{1}$

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  Anti-interference performance is an important criterion for testing sensors. Competition experiments were conducted to examine the immunity of complex 1 in detecting Al³⁺ ions in aqueous solution when different interfering metal ions coexist in equivalent ratios. Under the same experimental conditions, the changes in the emission peaks and fluorescence of complex 1, induced by various metal ions (2.5 equivalents) in H₂O at 380 nm excitation, were further studied, as shown in Fig.7.

  Figure 7

  

  Figure 7.  Anti-interference performance of complex 1 for the detection of Al³⁺

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  When only Al³⁺ was added, the fluorescence intensity was significantly enhanced, accompanied by a fluorescence color change from dark to bright blue. No obvious interference in fluorescence was observed when Al³⁺ (8.0 equivalents) was added alongside other metal ions (8.0 equivalents). These results indicate that complex 1 can serve as a remarkable selective fluorescent chemosensor for the detection of Al³⁺. This suggests that complex 1 exhibits high selectivity for detecting Al³⁺ ions over other metal ions.

  2.5   Limit of detection of Al³⁺ in H₂O

  Fluorometric titration was performed to investigate the sensitivity of complexes 1 toward Al³⁺. The limit of detection (LOD) was estimated using the equation LOD=3σ/k, where σ is the standard deviation of the blank sample, and k is the slope of the linear correlation curve between fluorescence intensity and Al³⁺ concentration. Remarkably, a linear relationship was observed within the concentration range of 0-20 μmol·L^-1. The LOD for complex 1 detecting Al³⁺ was calculated to be 51.3 nmol·L^-1 (Fig.8).

  Figure 8

  

  Figure 8.  Linear relationship between fluorescence intensity of complex 1 at 487 nm and Al³⁺ concentration in aqueous solution

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  2.6   Application of fluorescent test strip

  Due to the rapid response and high selectivity of complex 1 toward Al³⁺, test strips were developed to enable the convenient application of this sensor. The test strips were prepared following a method reported in the literature^[53-54]. After drying, the strips were immersed in various ion solutions. As expected, the fluorescence color changed from dark to bright blue under UV light only when the strips were treated with Al³⁺ solutions, as shown in Fig.9. These results demonstrate that complex 1 can function effectively as a fluorescent agent integrated into filter paper.

  Figure 9

  

  Figure 9.  Photograph of test papers treated with complex 1 in response to different cations (0.1 mol·L^-1) under UV light

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  3.   Conclusions

  In summary, two different Co(Ⅱ)-based and long-chain complexes 1 and 2 were designed, synthesized, and spectroscopically analyzed. Structural analysis by single-crystal X-ray diffraction revealed the long-chain structure of 2. Complex 1 can serve as an effective fluorescence turn-on sensor for the detection of Al³⁺ ions in H₂O, exhibiting high selectivity and sensitivity, emitting at a wavelength distinctly different from that of the identified cation, and for on-site analysis using a test paper. This study provides new insights into the future development of chemosensors based on metal coordination polymers.

  
  Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No.21906002), the Beijing Natural Science Foundation of China (Grant No.2212002), and the Beijing Municipal Science and Technology Project (Grant No.KM202010005010). Author contributions: All authors have approved the final version of the manuscript.
  Conflict of interest statement: There are no conflicts of interest to declare.
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  (a) ORTEP drawing of the asymmetric unit of complex 2 with ellipsoids at the 50% probability level; (b) Partial packing diagram of 2 showing the 1D chain structure formed through bridging ligands along the crystallographic axis

  Hydrogen atoms are omitted for clarity; Symmetry codes: ^ⅰ 3-x, -y, 1-z; ^ⅱ-x, 1-y, 2-z.

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  Figure 2  π-π stacking between adjacent molecules of complex 2

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  Figure 3  UV-Vis absorption spectra of ligands L¹ (a) and L² (b), and complexes 1 (c) and 2 (d)

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  Figure 4  IR spectra (a) and PXRD patterns (b) of complexes 1 and 2

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  Figure 5  (a) Fluorescence spectra of complex 1 after adding different metal ions in H₂O; (b) Fluorescence enhancement of 1 at 487 nm; (c) Image of the fluorescence color changes of 1 after adding different metal ions

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  Figure 6  (a) fluorometric titration of Al³⁺ with complex 1 in H₂O; (b) Plot of fluorescence intensity of 1 at 487 nm $c_{\mathrm{Al}^{3+}} / c_\boldsymbol{1}$

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  Figure 7  Anti-interference performance of complex 1 for the detection of Al³⁺

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  Figure 8  Linear relationship between fluorescence intensity of complex 1 at 487 nm and Al³⁺ concentration in aqueous solution

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  Figure 9  Photograph of test papers treated with complex 1 in response to different cations (0.1 mol·L^-1) under UV light

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  Table 1.  Crystal data and structure refinement for complex 2

  Parameter	2		Parameter	2
  Empirical formula	C36H37F12N3O5Co		Dc / (g·cm-3)	1.487
  Formula weight	863.31		μ / mm-1	0.906
  Temperature / K	198		F(000)	886
  Crystal system	Triclinic		2θ range / (°)	4.69-65.50
  Space group	P1		Index ranges	-13 ≤ h ≤ 13, -22 ≤ k ≤ 21, -24 ≤ l ≤ 24
  a / nm	0.877 9(5)		Independent reflection	13 128
  b / nm	1.496 2(9)		Rint	0.087 5
  c / nm	1.653 8(10)		Rσ	0.061 2
  α / (°)	67.422(2)		Goodness-of-fit on F2	1.086
  β / (°)	74.707(2)		Final R indexes [I≥2σ(I)]	R1=0.057 9, wR2=0.164 8
  γ / (°)	80.109(2)		Final R indexes (all data)	R1=0.088 4, wR2=0.187 2
  Volume / nm3	1.928 7(2)		Largest peak and hole / (e·nm-3)	1 350 and -900
  Z	2

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

  Co1—O1	0.206	Co1—N1	0.218	Co2—O4	0.206
  Co1—O2	0.204	Co2—O3	0.205	Co2—N2	0.216
  O1—Co1—O2	89.35	O2ⅰ—Co1—O2ⅱ	180	O4—Co2—N2	91.03
  O1—Co1—O2ⅰ	90.65	N1ⅰ—Co1—N1ⅱ	180	O3ⅰ—Co2—O3ⅱ	180
  O1—Co1—N1	88.85	O4—Co2—O3	89.48	O4ⅰ—Co2—O4ⅱ	180
  O2—Co1—N1	90.29	O4ⅰ—Co2—O3	90.52	N2ⅰ—Co2—N2	180
  O1ⅱ—Co1—O1ⅰ	180	O3—Co2—N2	88.16
  Symmetry codes: ⅰ 3-x, -y, 1-z; ⅱ -x, 1-y, 2-z.

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