English

• 0.   Introduction

  Metal-organic cages are a class of molecular containers formed via coordination-driven self-assembly^[1]. Upon external stimulation^[2-3], the binding and release of guest molecules inside the cavity due to structure changes, have been widely used for molecular recognition^[4], chirality sensing^[5], and catalysis^[6]. Spin crossover (SCO) phenomena^[7-10], not only enable the reversibly interconverted between high-spin (HS) and low-spin (LS) states in complexes but also lead to changes in color^[11], volume^[12], and electrical conductivity^[13-14]. Given this characteristic, incorporating SCO units into coordination cages, since the spin state change will be accompanied by a huge volume change, the spin state can be regulated through external stimulation to achieve the binding and release of guest molecules^[15]. Therefore, the development of metal-organic cages based on SCO has become a research hotspot^[16-18].

  To date, numerous studies have reported on different structural configurations, predominantly focusing on the SCO behavior of iron^[19-22]. For instance, McConnell′s group^[23] achieved the transition from high to low spin states in the Fe^Ⅱ centers of Fe^Ⅱ₄L₄ cages through subcomponent exchange. Li′s group^[24] successfully prepared a mixed-spin [Fe₄L₆]⁸⁺ tetrahedral SCO cage, with VT-XPS (variable temperature X-ray photoelectron spectroscopy), reproducing a regular SCO dependence. Recently, Sun′s group^[25] demonstrated that the spin transition temperature (T_1/2) of the SCO host can be adjusted through the encapsulation of multiple guests. Despite many reports on various SCO complexes about irons, however, the construction of Co-based SCO cages remains unreported. Since organic cages based on cobalt have demonstrated significant research value in the field of electrocatalysis, therefore, the construction of SCO-type Co cage will help further explore the control of electrocatalytic performance by regulating spin states.

  In this regard, we designed three bidentate pyridylamine ligands and synthesized three new complexes with cobalt perchlorate: [Co₂(L₁)₃](ClO₄)₄·2CH₃CN (1), [Co₂(L₂)₃](ClO₄)₄·2CH₃OH (2), and [Co₂(L₃)₃](ClO₄)₄·2CH₃OH (3), where L₁: bis-[4-(2-pyridylmethyleneamino)-phenyl]thioether, L₂: bis-[4-(2-pyridylmethyleneamino)-phenyl]ether, L₃: bis-[4-(2-pyridylmethyleneamino)-phenyl]methane. Under solvated conditions, complex 3 exhibited SCO behavior that was different from complexes 1 and 2. At the same time, we investigated the impact of different ligand fields on the SCO behavior of the complexes^{[9, 17-18]}. Electron-donating groups, such as —CH₂, enhanced the ligand field strength; in contrast, electron-withdrawing groups, like —O and —S, favored the reverse bonding capability between Co^Ⅱ ions and ligands, thereby weakening the ligand field strength and predisposing the complexes towards a high-spin state^{[16, 26-27]}. This dynamic illustrates the nuanced interplay between ligand structure and electronic properties in determining the spin states of coordination complexes, highlighting the crucial role of ligand design in the modulation of magnetic properties in metal complexes^[28-30].

  1.   Experimental

  1.1   Materials and synthesis of the complexes

  The chemical reagents used are all analytical reagents sold on the market. If there is no special explanation in this paper, it is directly used. The ligands L₁, L₂, and L₃ were synthesized according to the procedure described in the literature^{[18, 31-32]}. Detailed synthesis procedures of the ligands are described in Scheme S1 (Supporting information).

  Complex 1 was synthesized by the solution volatilization method. At room temperature, ligand L₁ (0.06 mmol) was dissolved with 5 mL of acetonitrile solution, and 10 mL of a methanol solution containing Co(ClO₄)₂·6H₂O (0.04 mmol) was added to the former solution. The massive red crystals were obtained after three weeks. Based on Co(ClO₄)₂·6H₂O, its yield was about 40%. Anal. Calcd. for C₇₆H₆₀Cl₄Co₂N₁₄O₁₆S₃(%): C, 51.20; H, 3.37; N, 11.00. Found(%): C, 51.13; H, 3.32; N, 10.88.

  The synthesis method of complex 2 was the same as complex 1. Red block crystals were obtained after three weeks. Based on Co(ClO₄)₂·6H₂O, its yield was about 36%. Anal. Calcd. for C₇₅H₆₆Cl₄Co₂N₁₂O₂₂(%): C, 51.52; H, 3.78; N, 9.62. Found(%): C, 51.47; H, 3.73; N, 9.57.

  The synthesis method of complex 3 was the same as complex 1. Red block crystals were obtained after four weeks. Yield: 30% based on Co(ClO₄)₂·6H₂O. Anal. Calcd. for C₇₇H₆₈Cl₄Co₂N₁₂O₁₈(%): C, 54.06; H, 3.98; N, 9.83. Found(%): C, 53.98; H, 3.92; N, 9.76.

  1.2   Physical measurement

  Crystals suitable for single-crystal X-ray diffraction, covered with a thin layer of hydrocarbon oil, were placed under a cold nitrogen stream using glass fibers connected to a copper needle. Single-crystal X-ray diffraction data were collected on a Bruker D8 VENTURE CMOS-based diffractometer (Bruker AXS Company, Karlsruhe, Germany) (Mo Kα radiation, λ=0.071 073 nm) using the SMART and SAINT programs. Final unit cell parameters were based on all observed reflections from integrating all frame data. Through the ShelXT structure parsing program implanted in Olex2, the crystal structure was analyzed by the direct method, and the data was refined by the full matrix least square method using the ShelXL program. For all complexes, all non-hydrogen atoms were treated with anisotropy, and hydrogen atoms were determined utilizing electron cloud or theoretical hydrogenation.

  Thermogravimetric analysis (TGA) was performed using TG/DTA Q600 (Mettler Toledo) instruments in a nitrogen atmosphere at a heating rate of 10 K·min^-1 from ambient temperature to 1 085 K. Elemental analysis was performed on a Vario EL Ⅲ element analyzer (Elementar Company, Hanau, Germany). Powder X-ray diffraction (PXRD) data were obtained in a Bruker AXS D8 Advance X-ray powder diffractometer using the voltage of 45 kV and the test current of 200 mA in a range of 5°-50° (Cu Kα radiation, λ=0.154 18 nm). Magnetic measurements were carried out using the polycrystalline sample with the Quantum Design PPMS. The variable-temperature magnetization data were corrected for diamagnetic contribution from the sample holder and diamagnetic contributions from the molecule using Pascal′s constants. The UV-Vis absorption spectra of the complexes were measured by HITACHI HU4150 spectrophotometer.

  2.   Results and discussion

  2.1   Crystal structure

  The synthesis and growth of red crystals of complexes 1, 2, and 3 were conducted by the slow diffusion of the methanol solution of Co(ClO₄)₂·6H₂O into the acetonitrile solution of ligands in test tubes. The solid structures of three dinuclear complexes 1, 2, and 3 were determined by single-crystal X-ray diffraction. Detailed crystallographic data are listed in Table 1.

  Table 1

  

  Table 1.  Crystallographic data for complexes 1-3 at different temperatures

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  Parameter	1	2	3
  Temperature / K	120	120	120	400
  Formula	C76H60Cl4Co2N14O16S3	C75H66Cl4Co2N12O22	C77H68Cl4Co2N12O18	C75H60Cl4Co2N12O16
  Formula weight	1 781.22	1 747.05	1 709.09	1 645.01
  Crystal system	Monoclinic	Monoclinic	Triclinic	Monoclinic
  Space group	Pn	C2/c	P1	P21/n
  a / nm	1.358 57(15)	2.951 75(12)	1.050 30(16)	2.515 35(10)
  b / nm	1.330 72(14)	1.026 54(4)	1.536 2(2)	1.050 94(5)
  c / nm	2.191 3(2)	2.480 22(11)	2.473 5(4)	2.854 46(10)
  α / (°)			87.248(5)
  β / (°)	107.335(4)	91.102 0(10)	88.417(5)	92.009(3)
  γ / (°)			70.555(5)
  V / nm3	3.781 7(7)	7.513 9(5)	3.758 6(10)	7.541 1(5)
  Z	2	4	2	4
  Dc / (g·cm-3)	1.564	1.544	1.510	1.449
  F(000)	1 824.0	3 592.0	1 760.0	3 376.0
  Rint	0.096 9	0.036 9	0.083 9	0.027 0
  GOF on F2	1.059	1.034	1.054	1.028
  R1 [I≥2σ(I)]a	0.061 7	0.053 6	0.078 2	0.074 5
  wR2 [I≥2σ(I)]b	0.155 9	0.143 8	0.214 1	0.201 8
  a R1=∑||Fo|-|Fc||/∑|Fo|; b wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

  All complexes are isostructural whose assembly mode was characterized by an [M₂L₃] pattern (Fig. 1), incorporating a dinuclear structure and four isolated perchlorate anions to maintain electrical neutrality. The two Co^Ⅱ centers adopt a distorted octahedral coordination environment, bound to three imine and three pyridine nitrogen donors from different ligands.

  Figure 1

  

  Figure 1.  Major structure of complexes 1 (a), 2 (b), and 3 (c)

  The H atoms and anions are omitted for clarity; Color code: Co, purple; O, red; C, gray; S, yellow; N, blue.

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  The asymmetric unit of complex 1 contains two CH₃CN solvent molecules and two types of metal Co, while both complexes 2 and 3 contain two CH₃OH solvent molecules. Complex 2 has only one type of metal Co compared to the two types of metal Co of complexes 1 and 3. TGA shows that complex 1 slowly lost solvent at 414 K and stabilized at 481 K, followed by a sharp decrease in the curve due to the decomposition of the complex after 600 K. The lost mass of 5.4% is approximately equal to that of two CH₃CN molecules (Fig.S1). Compared with complex 1, complex 2 was very stable below 600 K (Fig.S2). For complex 3, as the temperature increased, complex 3 began to slowly lose a small amount of solvent, which lost 2% of its mass, approximately equivalent to the mass of two CH₃OH molecules (1.87%) and then decomposed at 600 K (Fig.S3).

  Complexes 1 and 2 crystallize in the space group of Pn and C2/c, respectively. At the measured temperature of 120 K, the average bond length of Co1—N and Co2—N is 0.214 7(4) and 0.214 8(5) nm for complex 1, and 0.214 3(3) nm for complex 2, indicating that the Co^Ⅱ centers are in the HS state (Table 2). Owing to the loss of the solvent in complex 3, the space group has changed from P1 at 120 K to P2₁/n at 400 K. At the same time, the main skeleton remains unchanged, and the average bond lengths for Co1—N and Co2—N increase from 0.213 7(3) and 0.214 1(3) nm to 0.215 2(3) and 0.215 4(3) nm, respectively (Table 2), which represents the occurrence of the SCO process. In addition, the hydrogen bonding interactions between perchlorate ions and methanol molecules were found in the lattice with hydrogen bonding distances of 0.197 4 nm for complex 2 (Fig.S4), 0.198 8 and 0.204 6 nm for complex 3 (Fig.S5). The hydrogen bonding has no significant effect on the environment for the formation of the unique dinuclear structure.

  Table 2

  

  Table 2.  Structural parameters for complexes 1-3

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  Complex		dCo—Na / nm	Σb / (°)	CShMc	dCo…Cod / nm	Φe / (°)
  1-120 K	Co1	0.214 7(4)	94.20	1.509	1.133	102.563
  	Co2	0.214 8(5)	90.45	1.353
  2-120 K	Co	0.214 3(3)	80.75	1.444	1.135	116.427
  3-120 K	Co1	0.213 7(3)	83.49	1.342	1.150	114.745
  	Co2	0.214 1(3)	79.01	1.364
  3-400 K	Co1	0.215 4(3)	85.37	1.452	1.139	114.516
  	Co2	0.215 2(3)	82.26	1.487
  a The average Co—N bond length; b Defined as the sum of the deviation from 90° of the 12 cis N—Co—N angles of the [CoN6] octahedron; c The CShM value represents the deviation degree of the ideal octahedron; d The distance between the central cobalt atom; e Refers to the angle on the ligand that is centered on the center of symmetry of the ligand′s center-connecting unit.

  2.2   Spectroscopic analysis

  To further investigate the electronic structure, the solid-state variable-temperature UV-Vis absorption spectra of complex 3 were examined within a range of 300-1 500 nm. As shown in Fig. 2, two broad absorption bands near 350 and 950 nm observed at 250 K can be attributed to the metal-to-ligand charge transfer (MLCT) or d-d transitions of LS-Co^Ⅱ ions (Fig. 2). The peak was around 950 nm which may be attributed to d-d transitions for high spin Co^Ⅱ in the pseudo-octahedral environment. The intensity of these bands gradually decreased with the temperature increased, which was consistent with the thermally induced spin crossover from LS-Co^Ⅱ to HS-Co^Ⅱ states.

  Figure 2

  

  Figure 2.  Temperature-dependent UV-Vis absorption spectra of complex 3 in the solid state

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  2.3   Magnetic property

  The phase purities of all three complexes were confirmed by PXRD patterns (Fig.S6-S8). The variable-temperature magnetic susceptibility of the above complexes was measured at a scanning speed of 3 K·min^-1 and under a DC magnetic field of 1 kOe. Upon warming to 395 K, the χ_MT values of complexes 1 and 2 continuously increased to 6.41 and 6.16 cm³·mol^-1·K (Fig. 3a and 3b), significantly larger than the saturated value of 5.0 cm³·mol^-1·K for two Co^Ⅱ in the HS state (S=3/2). Curie-Weiss fits the magnetization curves for the temperature interval 2-400 K yielded a Curie constant C of 6.63 cm³·mol^-1·K, and a Weiss temperature θ of -16.82 K were obtained for complex 1 (Fig.S9). A Curie constant C of 6.37 cm³·mol^-1·K and a Weiss temperature θ of -12.72 K were obtained for complex 2 (Fig.S10). Negative Weiss temperatures provide further evidence that complexes 1 and 2 exhibit antiferromagnetic interactions.

  Figure 3

  

  Figure 3.  Plots of the temperature dependence of χ_MT under 1 kOe DC field for complexes 1 (a), 2 (b), and 3 (c)

  Black: heating mode; Red: cooling mode.

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  For complex 3, as the temperature increased to 300 K, the χ_MT value gradually increased and stabilized at 6.13 cm³·mol^-1·K. Upon further heating to 395 K, the χ_MT value rose to 6.76 cm³·mol^-1·K, indicating the occurrence of incomplete SCO behavior. After the sample was continuously maintained at 395 K for 1 h to ensure complete desolvation, the magnetic data of the 3-desolvated phase were recorded over the temperature range of 2-395 K. With the reduction of temperature, it was observed that the χ_MT value decreased to 3.9 cm³·mol^-1·K at 2 K similar to complexes 1 and 2. Curie-Weiss fits the magnetization curves for complex 3 gave a Curie constant C of 7.08 cm³·mol^-1·K and a Weiss temperature θ of -17.31 K (Fig.S11), demonstrating that the desolvated phase of complex 3 undergoes antiferromagnetic interactions similar to complexes 1 and 2. The above results suggest that complex 3 undergoes an irreversible SCO behavior induced by the loss of solvent.

  2.4   Magnetostructural relationship

  Electron-withdrawing groups facilitate the reverse bonding ability between the Co^Ⅱ ion and the ligand, thereby weakening the ligand field strength and tending the complex to a high spin state, so complexes 1 and 2 are in the high spin state. However, electron-donating groups, such as —CH₂, enhanced the ligand field strength. Consequently, complex 3 possesses SCO behaviors that are different from complex 1 and complex 2. Therefore, it is proved different ligand fields have a great influence on the SCO behavior of complexes. To further analyze the structural distortions among the SCO behaviors, the structural parameters are listed including d_Co—N (nm), Σ (°), CShM, Co…Co distance (nm), and Φ (°), which are shown in Table 2. The values of these parameters indicate that there are different degrees of geometrical distortion in the coordination spheres of the three complexes. The Co…Co distance between complexes 1 and 2 is 1.133 and 1.135 nm, respectively, allowing the dinuclear structure to compress horizontally, resulting in a large distortion of the N6 environment involved in the coordination of metal Co, and with distortion parameters of 94.20°, 90.45°, and 80.75°, respectively, which makes the central metal cobalt more inclined to the HS. At the same time, the Co…Co distance of complex 3 decreases from 1.150 nm at 120 K to 1.139 nm at 400 K close to complexes 1 and 2 due to solvent loss, which exacerbates the compression of the crystal cell in the horizontal direction and leads to an increase in the distortion of the center metal from 83.49°, 79.01° to 85.37°, 82.26°. Therefore, the Co…Co distance not only reflects the degree of distortion of the center metal but also indirectly indicates the SCO behavior of complex 3. Besides this, we also explored the symmetry angles of different ligands which are 102.56° and 116.427° for complexes 1 and 2 at 120 K respectively, the symmetry angle of complex 3 decreases from 114.745° to 114.516° with increasing temperature. Since the ligand configuration has no obvious change during the transformation process, so we speculate that the ligand intercalation angle has a weak effect on the SCO of the complexes.

  In addition to the first coordination sphere of the metal center, subtle crystal packing effects also affect the SCO behavior in the solid state. Close examination of intermolecular interactions reveals additional insights into structure-function relationships. As shown in Fig. 4, complex 1 exhibits the π…π intermolecular interaction between the benzene rings of the Schiff base ligands in different units, characterized by a distance of 0.376 9 nm. Besides, there are two types of C—H…π intermolecular interactions between the Schiff base ligands in complex 1, with a distance of 0.348 7 and 0.353 0 nm (Fig. 5a). These multiple intermolecular interactions facilitate a denser spatial arrangement of complex 1, leading to an increased rigidity in the coordination environment of the central metal. This, in turn, enhances the stability of the complex in the HS state. Unlike complex 1 which possesses intermolecular interactions, complexes 2 and 3 only exhibit two kinds of C—H…π intramolecular interactions (Fig. 5b-5d). The two different C—H…π intramolecular interactions originate from the benzene or pyridine rings in the ligand and the C—H of the same ring on the adjacent ligand, whereas the distance of 0.360 5 and 0.368 1 nm for complex 2, with 0.377 7 and 0.378 2 nm for complex 3, respectively. Although complexes 2 and 3 exhibit the same type of intramolecular interactions, the difference in the strength of these interactions leads to a vertically more compressed dinuclear structure in complex 2 compared to complex 3, thus increasing the rigidity within the molecule stabilizes complex 2 in the HS state. For complex 3, the C—H…π intramolecular interaction distances decrease from 0.377 7 to 0.374 0 nm with the temperature increase. And the enhancement of intramolecular forces keeps the desolvated phase of complex 3 in the HS state which is consistent with complex 2. At the same time, this trend aligns with the results from magnetization measurements. Therefore, it can be concluded that intermolecular or intramolecular forces exert a significant influence on the magnetic properties of the complex.

  Figure 4

  

  Figure 4.  Illustration of intramolecular π…π stacking within the complex 1

  The red dashed lines represent π…π interaction.

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  Figure 5

  

  Figure 5.  C—H…π interaction for the complexes: (a) 1 at 120 K, (b) 2 at 120 K, (c) 3 at 120 K, and (d) 3 at 400 K

  The orange dashed lines represent the C—H…π interaction.

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  3.   Conclusions

  In summary, three dinuclear Co^Ⅱ complexes [Co₂(L₁)₃](ClO₄)₄·2CH₃CN (1), [Co₂(L₂)₃](ClO₄)₄·2CH₃OH (2), and [Co₂(L₃)₃](ClO₄)₄·2CH₃OH (3) were synthesized utilizing three imine-based ligands and cobalt perchlorate. Among these, complex 3, bearing the electron-donating —CH₂ substituent, demonstrated solvent-induced SCO behavior, unlike complexes 1 and 2 with electron-withdrawing substituents —S and —O, which exhibit antiferromagnetic behaviors. Through magneto-structure relationship analysis, substituent-mediated ligand field and intramolecular or intermolecular stacking interactions can significantly influence the SCO behavior in dinuclear complexes. In turn, this allows for the adjustment of cavity volume within metal-organic cages to accommodate guest molecules. These insights will contribute to the development of more Co-based cages exhibiting SCO behavior.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Major structure of complexes 1 (a), 2 (b), and 3 (c)

  The H atoms and anions are omitted for clarity; Color code: Co, purple; O, red; C, gray; S, yellow; N, blue.

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  Figure 2  Temperature-dependent UV-Vis absorption spectra of complex 3 in the solid state

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  Figure 3  Plots of the temperature dependence of χ_MT under 1 kOe DC field for complexes 1 (a), 2 (b), and 3 (c)

  Black: heating mode; Red: cooling mode.

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  Figure 4  Illustration of intramolecular π…π stacking within the complex 1

  The red dashed lines represent π…π interaction.

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  Figure 5  C—H…π interaction for the complexes: (a) 1 at 120 K, (b) 2 at 120 K, (c) 3 at 120 K, and (d) 3 at 400 K

  The orange dashed lines represent the C—H…π interaction.

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  Table 1.  Crystallographic data for complexes 1-3 at different temperatures

  Parameter	1	2	3
  Temperature / K	120	120	120	400
  Formula	C76H60Cl4Co2N14O16S3	C75H66Cl4Co2N12O22	C77H68Cl4Co2N12O18	C75H60Cl4Co2N12O16
  Formula weight	1 781.22	1 747.05	1 709.09	1 645.01
  Crystal system	Monoclinic	Monoclinic	Triclinic	Monoclinic
  Space group	Pn	C2/c	P1	P21/n
  a / nm	1.358 57(15)	2.951 75(12)	1.050 30(16)	2.515 35(10)
  b / nm	1.330 72(14)	1.026 54(4)	1.536 2(2)	1.050 94(5)
  c / nm	2.191 3(2)	2.480 22(11)	2.473 5(4)	2.854 46(10)
  α / (°)			87.248(5)
  β / (°)	107.335(4)	91.102 0(10)	88.417(5)	92.009(3)
  γ / (°)			70.555(5)
  V / nm3	3.781 7(7)	7.513 9(5)	3.758 6(10)	7.541 1(5)
  Z	2	4	2	4
  Dc / (g·cm-3)	1.564	1.544	1.510	1.449
  F(000)	1 824.0	3 592.0	1 760.0	3 376.0
  Rint	0.096 9	0.036 9	0.083 9	0.027 0
  GOF on F2	1.059	1.034	1.054	1.028
  R1 [I≥2σ(I)]a	0.061 7	0.053 6	0.078 2	0.074 5
  wR2 [I≥2σ(I)]b	0.155 9	0.143 8	0.214 1	0.201 8
  a R1=∑||Fo|-|Fc||/∑|Fo|; b wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

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  Table 2.  Structural parameters for complexes 1-3

  Complex		dCo—Na / nm	Σb / (°)	CShMc	dCo…Cod / nm	Φe / (°)
  1-120 K	Co1	0.214 7(4)	94.20	1.509	1.133	102.563
  	Co2	0.214 8(5)	90.45	1.353
  2-120 K	Co	0.214 3(3)	80.75	1.444	1.135	116.427
  3-120 K	Co1	0.213 7(3)	83.49	1.342	1.150	114.745
  	Co2	0.214 1(3)	79.01	1.364
  3-400 K	Co1	0.215 4(3)	85.37	1.452	1.139	114.516
  	Co2	0.215 2(3)	82.26	1.487
  a The average Co—N bond length; b Defined as the sum of the deviation from 90° of the 12 cis N—Co—N angles of the [CoN6] octahedron; c The CShM value represents the deviation degree of the ideal octahedron; d The distance between the central cobalt atom; e Refers to the angle on the ligand that is centered on the center of symmetry of the ligand′s center-connecting unit.

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