Synthesis, structures, magnetic and fluorescent sensing properties of 1D Co(Ⅱ)/Ni(Ⅱ) coordination polymers

Coordination polymers (CPs) are infinite arrays of extended structures in which the inorganic nodes and linkers are connected through coordination bonds. They are"coordination compounds with repeating coordination entitles extending in one, two, or three dimensions (1D, 2D, or 3D)"^[1-2]. The areas of CPs have experienced recent exponential growth in research activity. In the last few decades, over 70 000 CPs have been reported in the literature. Due to their novel and diverse structures and unique properties, they have been used in heterogeneous catalysis^[3-6], gas storage and separation^[7-9], drug delivery^[10], sensing^[11-12], energy storage^[13], and conductivity^[14].

The design and synthesis of CPs with specific structures have always been the focus of scientific and technological workers. For a long time, coordination polymer materials based on pyridine carboxylic acid ligands have attracted much attention. This is because pyridine carboxylic acid ligands are a common bifunctional ligand, which combines the advantages of carboxylic acid ligands and nitrogen-containing heterocyclic ligands, and has the active sites of carboxylic acid O atoms and heterocyclic N atoms. More importantly, at different pH values, carboxylic acids will lead to additional coordination modes due to different degrees of protonation. At the same time, the potential pyridine nitrogen atom can be used as a functional center to recognize Lewis acidic metal ions, which has a certain synergistic effect with oxygen atoms when participating in coordination.

Inspired by these ideas, two CPs were constructed with 1, 4-bis(2, 6-dimethyl-3, 5-dicarboxypyridine) benzene (H₄L) as the main ligand, 1, 4-bis(1-imidazolyl) benzene (1, 4-bib) as the auxiliary ligand as well as Co(Ⅱ) ion and Ni(Ⅱ) ion. In addition, the magnetic and fluorescent sensing properties of 1 and 2 were studied in detail.

1.   Experimental

1.1   Material and methods

All synthetic reagents and solvents employed were commercially available and used directly without further purification. Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker D8 ADVANCE diffractometer operating at 40 kV and 40 mA using Cu Kα radiation (λ=0.154 18 nm) at a scanning rate 2 (°)·min^-1 from 5° to 50°. The C, H, and N elemental analyses were conducted with a PerkinElmer PE-2400 elemental analyzer. The FT-IR spectrum (400-4 000 cm^-1) was recorded on a Nicolet 170SX FT-IR spectrophotometer. Thermal gravimetric analysis (TGA) was performed with a NETZSCH STA 449F3 thermal gravimetric analyzer in flowing nitrogen at a heating rate of 10 ℃·min^-1. In a range of 2-300 K, the magnetic susceptibility data were investigated by using a Quantum Design MPMS SQUID VSM instrument. Fluorescence experiments were carried out on the Hitachi F-7100 Fluorescence Spectrophotometer.

1.2   Synthesis of [Co(L)_0.5(1, 4-bib)(H₂O)₃]_n (1)

A mixture of Co(NO₃)₂·6H₂O (29.1 mg, 0.10 mmol), H₄L (23.0 mg, 0.05 mmol), 1, 4-bib (10.5 mg, 0.05 mmol), NaOH (14.40 mmol·L^-1) and 13 mL H₂O was stirred for 30 min and then placed in a 25 mL Teflon-lined stainless-steel vessel, which was heated to 160 ℃ and kept constant for 120 h. Finally, purple block crystals were obtained. Yield: 34% (based on Co). Anal. Calcd. for C₂₄H₂₄N₅O₇Co(%): C, 52.08; H, 4.37; N, 12.65. Found(%): C, 52.10; H, 4.33; N, 12.56. IR (KBr, cm^-1): 3 119 (w), 2 365 (w), 2 345 (w), 1 560 (s), 1 535 (s), 1 352 (w), 1 310 (s), 1 258 (m), 1 121 (m), 1 063 (w), 961 (s), 831 (m), 766 (s), 650 (w), 534 (w), 501 (w).

1.3   Synthesis of [Ni(L)_0.5(1, 4-bib)(H₂O)₃]_n (2)

The synthesis method of 2 was the same as that of 1, and only the metal salt was replaced with Ni(NO₃)₂·6H₂O. Finally, green crystals were obtained with a yield of 36% (based on Ni). Anal. Calcd. for C₂₄H₂₄N₅O₇Ni (%): C, 52.11; H, 4.37; N, 12.65. Found(%): C, 52.14; H, 4.30; N, 12.54. IR (KBr, cm^-1): 3 117 (w), 2 365 (w), 2 345 (w), 1 560 (s), 1 533 (s), 1 352 (w), 1 310 (s), 1 258 (m), 1 124 (m), 1 063 (w), 961 (s), 831 (m), 766 (s), 652 (w), 536 (w), 501 (w).

1.4   X-ray crystallographic studies

The crystals with regular shape and moderate size were selected and placed on BRUKER SMART APEX-Ⅱ CCDX ray single crystal diffractometer. The unit cell parameters and diffraction data were measured by ω-2θ scanning with Mo Kα (λ =0.071 073 nm) ray monochromated by graphite monochromator. The diffraction data were corrected by semi-empirical absorption using the SADABS program. By using the SHELXS-97 and SHELXL-97 programs, the structures were solved by direct methods and refined by full-matrix least squares on F². All non-hydrogen atoms were refined anisotropically. The crystallographic data of complexes 1 and 2 are shown in Table 1. Selected bond lengths and bond angles are listed in Table 2 and 3.

Table 1

Table 1.  Crystal data and structure refinement parameters for complexes 1 and 2

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Parameter	1	2
Empirical formula	C24H24N5O7Co	C24H24N5O7Ni
Formula weight	553.41	553.19
Crystal system	Triclinic	Triclinic
Space group	P1	P1
Temperature/K	293(2)	293(2)
a/nm	0.761 51(19)	0.760 89(11)
b/nm	1.238 7(3)	1.238 24(19)
c/nm	1.395 1(3)	1.396 4(2)
α/(°)	107.152(4)	106.430(2)
β/(°)	99.896(4)	100.127(2)
γ/(°)	105.738(4)	106.240(2)
V/nm3	1.164 0(5)	1.164 2(3)
Z	2	2
Dc/(g·cm-3)	1.579	1.578
Reflection collected	6 145	6 067
Unique reflection	4 287	4 282
θ range/(°)	1.94-25.50	1.58-25.50
F(000)	572	574
Goodness-of-fit on F2	1.060	1.079
R1, wR2 [I > 2σ(I)]*	0.049 7, 0.138 7	0.050 8, 0.143 8
R1, wR2 (all data)	0.065 7, 0.149 1	0.058 4, 0.151 2
* R1=∑||Fo|-|Fc||/∑|Fo|, wR2={∑[w(Fo2-Fc2)2]/∑w(Fo2)2}1/2.

Table 2

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

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Co1—O1	0.211 2(2)	Co1—O6	0.214 4(3)	Co1—N2	0.210 8(3)
Co1—O5	0.209 8(3)	Co1—O7	0.207 4(7)	Co1—N5A	0.211 3(3)
O7—Co1—O5	94.81(19)	O7—Co1—N5A	87.0(2)	N2—Co1—O6	90.91(13)
O7—Co1—N2	173.91(19)	O5—Co1—N5A	92.26(12)	O1—Co1—O6	90.35(10)
O5—Co1—N2	90.88(12)	N2—Co1—N5A	94.97(12)	N5A—Co1—O6	90.11(12)
Symmetry code: A: x-1, y-1, z.

Table 3

Table 3.  Selected bond lengths (nm) and bond angles (°) for complex 2

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Ni1—O1	0.207 8(2)	Ni1—O6	0.206 2(7)	Ni1—N2	0.206 3(3)
Ni1—O5	0.208 9(3)	Ni1—O7	0.211 2(3)	Ni1—N5A	0.206 9(3)
O6—Ni1—N2	175.0(2)	N2—Ni1—O5	91.77(12)	O5—Ni1—O7	176.53(11)
O6—Ni1—N5A	87.4(2)	N5A—Ni1—O5	92.08(12)	O6—Ni1—O6A	9.5(3)
N2—Ni1—N5A	94.09(12)	O1—Ni1—O5	86.18(11)	N2—Ni1—O6A	174.25(19)
Symmetry code: A: x-1, y-1, z.

2.   Results and discussion

2.1   Description of crystal structures

X-ray single crystal diffraction analysis shows that complexes 1 and 2 are heterogeneous isomorphisms, so we only describe the structure of 1. Complex 1 belongs to the triclinic system and P1 space group. The asymmetric unit of complex 1 comprises a Co(Ⅱ) ion, a half L^4- ion, one 1, 4-bib molecule, and three coordinated water molecules. As shown in Fig. 1, Co(Ⅱ) ions belong to the six-coordinated mode, O1 from L^4-, N2 and N5 from 1, 4-bib and O5, O6, O7 atoms of three H₂O molecules. All the Co—O (0.220 6(7)-0.207 4(7) nm) and Co—N (0.210 8(3)-0.211 3(3) nm) bond lengths fall into the normal ranges (Table 2). L^4- adopts the bridged coordination mode in complex 1, and the carboxyl group adopts monodentate coordination mode, connecting two Co(Ⅱ) ions. The 1D chain structure is formed by the metal connection of L^4- and 1, 4-bib (Fig. 3).

Figure 1

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

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Figure 2

Figure 2.  Coordination environment of the Ni(Ⅱ) in 2

Symmetry code: A: x-1, y-1, z

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Figure 3

Figure 3.  One-dimensional chain framework of 1 or 2

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2.2   PXRD analysis

To confirm the purity of complexes 1 and 2, PXRD patterns of complexes 1 and 2 were measured. The experimental and simulated patterns were presented in Fig. 4. The peak positions of complexes 1 and 2 were consistent with those of single crystal structure simulation, indicating that they are pure phases.

Figure 4

Figure 4.  PXRD patterns of complexes 1 and 2

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2.3   Thermal stability analysis

To determine the thermal stability of complexes 1 and 2, TGA experiments were conducted on the crystal sample. As shown in Fig. 5, the weight loss of complex 1 was about 10.9% due to the loss of three coordination water molecules from room temperature to 246 ℃. When the temperature was increased to 310 ℃, the framework of complex 1 began to decompose. Complex 2 had the same thermal stability as complex 1.

Figure 5

Figure 5.  TGA curves for complexes 1 and 2

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2.4   Magnetic properties

The temperature dependencies of the magnetic susceptibility (χ_M) of complexes 1 and 2 were measured in a range of 2-300 K under a static field of 10 and 1 kOe, respectively. Fig. 6 shows plots of χ_M and χ_MT vs T for complex 1. Here, we refer to the data of the literature^[15] to calculate the diamagnetic correction factor (χ_D) for [Co(L)_0.5(1, 4-bib)(H₂O)₃]:

Figure 6

Figure 6.  Plots of χ_M and χ_MT vs T for complex 1

The red lines represent the fitted curve

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The χ_M of complex 1 was determined by equation (χ_M=χ_meas-χ_D), where χ_meas is the measured magnetic susceptibility.

For 1, As the temperature decreased, the value of χ_MT decreased slowly. The value of χ_MT decreased from 2.921 4 cm³·mol^-1·K at 300 K to 1.359 cm³·mol^-1·K at 2 K. The experimental value of one Co(Ⅱ) ion at room temperature was 2.921 4 cm³·K·mol^-1, which was larger than that calculated for the spin-only case (1.875 cm³·mol^-1·K), revealing a significant orbital contribution. The 1/χ_M vs T plot was linear, and the least-squares fitting of the data to the Curie-Weiss law gave C=2.928 cm³·mol^-1·K and θ=-8.917 K. The negative value of θ may be attributed to antiferromagnetic interactions between Co(Ⅱ) ions. Assuming an isotropic exchange between Co(Ⅱ) ions, the magnetic susceptibility of one Co(Ⅱ) ion can be expressed as^[16]:

The best fit (N, β, and g have their usual meanings) of the experimental data to the equation yielded J=-3.47 cm^-1, g=2.22, with an agreement factor R=∑(χ_obsd-χ_cacld)²/∑ χ_obsd²=3.21×10^-4. It seems that the spin-only model gave quite a good fit.

Fig. 7 shows plots of χ_M and 1/χ_M vs T for complex 2. As we can see, it was very difficult to get a suitable theoretical model to fit this system. Fortunately, the given plot of 1/χ_M vs T showed a straight line down to 2 K, which suggests that it can be fit approximately by the Curie-Weiss law. The relationship between reciprocal susceptibility and T obeys the Curie-Weiss Law with C=1.943 cm³·mol^-1·K and θ=-9.236 K, demonstrating the antiferromagnetic interaction between Ni(Ⅱ) ions.

Figure 7

Figure 7.  Plots of χ_M and 1/χ_M vs T for complex 2

The red lines represent the fitted curve

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2.5   Fluorescent sensing properties

As shown in Fig. 8, the UV-Vis absorption spectra of complexes 1 and 2 in a range of 250-600 nm were determined. The maximum absorption peak of complexes 1 and 2 was at 250 and 252 nm, respectively. We took them as the optimal excitation wavelength to find the maximum emission wavelength repeatedly. Finally, complexes 1 and 2 exhibited the maximum emission at 330 nm.

Figure 8

Figure 8.  UV-Vis absorption spectra and emission spectra of 1 and 2

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Due to the abuse of antibiotics, the water system and the environment have caused great pollution. Therefore, it is very urgent to find a simple and efficient method to detect antibiotics^[17]. A variety of antibiotics including lincomycin hydrochloride (LIN), metronidazole (MDZ), ornidazole (ODZ), tetracycline (TET), roxithromycin (ROX), chloramphenicol (CAP), gentamicin sulfate (GEN), azithromycin (AZM), cefixime (CEF) and penicillin sodium (PEN) was selected to evaluate the sensing ability of 1. As shown in Fig. 9a, the maximum quenching was caused by CEF. The anti-interference experiments were carried out to investigate the selectivity of CEF to other antibiotics. In the absence of CEF, the fluorescence intensity of other antibiotics decreased slightly, but the fluorescence intensity was greatly quenched when CEF was added to the above solution (Fig. 9b). Then the concentration gradient experiment was performed. When the concentration of CEF increased, the intensity of 1 gradually decreased (Fig. 9c). The I₀/I-1 value was almost linearly related to the low concentration of CEF (Fig. 9d).

Figure 9

Figure 9.  (a) Fluorescence intensities of different antibiotics added to the suspension of 1; (b) Luminescence intensity of 1 with the addition of CEF to different antibiotics; (c) Emission spectra of 1 in the presence of CEF with different concentrations; (d) Linear relationship of I₀/I-1 with CEF concentration for the detection of CEF by 1

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At the same time, we also carried out antibiotics tests for complex 2. As shown in Fig. 10a, the maximum quenching was caused by TET. The anti-interference experiments were carried out to investigate the selectivity of TET to other antibiotics. In the absence of TET, the fluorescence intensity of other antibiotics decreased slightly, but the fluorescence intensity was greatly quenched when TET was added to the above solution (Fig. 10b). In addition, the quenching effect of 2 on the concentration of TET in a range of 0-250 μmol·L^-1 was also studied. With the increase of TET concentration, the intensity of 2 gradually decreased (Fig. 10c), I₀/I-1 value was also linearly correlated with low concentration TET (Fig. 10d). The detection limit was calculated using 3σ/k (σ: standard deviation, k: slope) and the detection limit for 1 towards CEF was 0.86 μmol·L^-1 and for 2 towards TET was 0.49 μmol·L^-1.

Figure 10

Figure 10.  (a) Fluorescence intensities of different antibiotics added to the suspension of 2; (b) Luminescence intensity of 2 with the addition of TET to different antibiotics; (c) Emission spectra of 2 in the presence of TET with different concentrations; (d) Linear relationship of I₀/I-1 with TET concentration for the detection of TET by 2

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To explain the fluorescence quenching effect of CEF and TET on complexes 1 and 2 respectively, the UV-Vis absorption spectra of antibiotics were recorded. The UV-Vis absorption of CEF partially overlapped with the emission spectrum of 1. TET exhibited absorption in a range of 300-400 nm, which overlapped the emission spectrum of 2. This means that the fluorescence quenching of 1 and 2 should be caused by energy competition and absorption.

Figure 11

Figure 11.  UV-Vis absorption spectra of CEF and TET and emission spectra of 1 and 2

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3.   Conclusions

In summary, two novel Co(Ⅱ)/Ni(Ⅱ) coordination polymers based on a main pyrazole carboxylic acid ligand (H₄L) and an auxiliary N-donor ligand (1, 4-bib) were successfully synthesized according to the hydrothermal method. Structural analysis shows that complexes 1 and 2 are heterogeneous isomorphisms and 1D structures. Magnetic studies reveal that there is an antiferromagnetic interaction between Co(Ⅱ)/Ni(Ⅱ) ions in complexes 1 and 2. In addition, 1 and 2 can also detect cefixime and tetracycline by fluorescence quenching, respectively.

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