English

• 1. INTRODUCTION

  Coordination polymers (CPs) containing poly-nuclear lanthanide (Ln) clusters have been deeply concerned in recent years owing to their rich architectures and wide applications in kinds of fields such as optics, adsorption, magnetism, catalysis and so on^{[1, 2]}. Compared to TM (transition metal) cations, lanthanide cations always exhibit high and variable coordination numbers and flexible coordination geometry, which tend to generate unexpected coordination polymers with new structural features^{[3, 4]}. However, only a few CPs containing poly-nuclear Ln clusters (tetra-, penta- and hexa-cluster) with different dimensions have been reported up to now^[5]. Therefore, the preparations of new CPs containing poly-nuclear Ln clusters are still meaningful and promising work. Previous studies have shown that the choice of appropriate linkers including OH^–, NO₃^–, SO₄^2–, CO₃^2–, PO₄^3– anions and various organic ligands are the key to synthesize such CPs^[6]. As we know, the poly-nuclear Ln clusters often own high positive charge and they are less stable in aqueous solutions, which makes it more difficult to obtain poly-nuclear rare earth polymers. At this level, the preparation of poly- nuclear rare earth polymers is also a difficult task. Based on the above analysis, we have the following considerations for the synthesis of such polymers: firstly, we used OH^- anion as a structural extender to produce poly-nuclear Ln clusters, and then employed organic ligands containing -N/O to connect the resulted poly-nuclear clusters, and finally obtained a CP containing poly-nuclear Ln clusters with novel structure. As part of our ongoing research^[7-13], herein, by using OH^– anions, Ln cations and two kinds of organic ligands (isonicotinate anions and linear azide anions), we successfully synthesized a novel tetra-nuclear Nd³⁺ polymer Nd₄(µ₃-OH)₄(µ₂-H₂O)₂(C₆H₄NO₂)₆(N₃)₂(H₂O)₂ (1) under hydrothermal conditions.

  2. EXPERIMENTAL

  2.1 Instruments and reagents

  Crystal diffraction data were collected on a Bruker SMART APEX-II CCD diffractometer equipped with a graphite- monochromated Mo-Kα radiation (λ = 0.71073 Å) at 296(2) K. Elemental analyses of C, H and N were measured on a Perkin Elmer 240 analyzer. IR spectrum was studied on a Nicolet 5700 FT-IR spectrometer using KBr pellets between 400~4000 cm^–1. The TG analysis was investigated on a Perkin-Elmer 7 thermal analysis instrument in flowing N₂ at a heating rate of 10 ℃/min in the range of 30~800 ℃. Near-infrared luminescent spectra were investigated on a FLS-980 fluorescent spectrometer. All chemical reagents were obtained directly from commercial resources and used without further purification.

  2.2 Synthesis of Nd₄(µ₃-OH)₄(µ₂-H₂O)₂(C₆H₄NO₂)₆(N₃)₂ (H₂O)₂ 1

  1 was synthesized using hydrothermal method. A mixture of Nd(NO₃)₃·6H₂O (0.11 g, 0.25 mmol), isonicotinic acid (0.08 g, 0.66 mmol), NaN₃ (0.07 g, 0.5 mmol), NaOH (0.08 g, 2 mmol) and H₂O (3.0 mL, 166.7 m·mol) was stirred for half an hour. Then it was transferred to a Teflon-lined stainless- steel autoclave (50 mL) and kept at 160 ℃ for 7 days. The resulting pure phase as pink block crystals were obtained and collected, washed with distilled water, and dried at an ambient temperature, giving ca. 55% yield based on Nd(NO₃)₃·6H₂O. It is worth noting that if NaOH is not used during the synthesis process, we cannot get the final desired polymer 1. According to the presence of OH^– anions in the structure of polymer 1, we conclude that NaOH not only serves as a raw material, but also provides an alkaline environment. Elemen- tal analyses of C, H and N were measured on a Perkin-Elmer 240 analyzer. Analysis for C₃₆H₃₆N₁₂Nd₄O₂₀, calculated: C, 28.17; H, 2.35; N, 10.95%. Found: C, 27.94; H, 2.68; N, 10.71%.

  2.3 X-ray crystallographic determination

  A suitable crystal of 1 (0.28mm × 0.26mm × 0.22mm) was selected for structural analysis. Intensity data were collected on a Bruker APEX-II CCD detector at 296(2) K using an ω-φ scan mode and Mo-Kα radiation (λ = 0.71073 Å) in the range of 2.0 ≤ θ ≤ 26.0º with –10 ≤ h ≤ 9, –13 ≤ k ≤ 15 and –15 ≤ l ≤ 15. Of the 6339 reflections collected, 4452 were unique (R_int = 0.0192). Absorption correction was used by the SADABS^[14]. The structure was solved by direct methods and refined by full-matrix least-squares on F² through OLEX2 program package^[15]. All of the non-hydrogen atoms were refined anisotropically. The H atoms of C atoms form isonicotinate anions were added in geometrically idealized positions and treated using a riding-model approximation, with C–H = 0.93 Å and U_iso(H) = 1.2U_eq(C). The H atoms of O(1W), O(2W), O(7) and O(8) were obtained from difference Fourier peaks. Selected bond lengths and bond angles of 1 are given in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Nd(1)–N(1)	2.553(4)		Nd(1)–O(7)A	2.415(4)		Nd(2)–O(4)B	2.394(4)
  Nd(1)–O1(W)	2.622(4)		Nd(1)–O(8)	2.441(4)		Nd(2)–O(6)B	2.384(4)
  Nd(1)–O2(W)	2.445(4)		Nd(2)–N(1)	2.523(5)		Nd(2)–O(7)	2.447(4)
  Nd(1)–O(3)	2.384(4)		Nd(2)–O(1)	2.385(4)		Nd(2)–O(8)A	2.408(4)
  Nd(1)–O(5)	2.421(4)		Nd(2)–O(1W)	2.582(4)		Nd(2)–O(8)B	2.460(4)
  Nd(1)–O(7)	2.393(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(3)–Nd(1)–O(1W)	117.26(13)		O(7)–Nd(1)–O(1W)	69.06(12)		O(6)–Nd(2)–N(1)A	113.52(16)
  O(3)–Nd(1)–O(2W)	145.02(14)		O(7)–Nd(1)–O(2W)	78.27(12)		O(6)–Nd(2)–O(1W)A	74.62(14)
  O(3)–Nd(1)–O(5)	75.92(15)		O(7)–Nd(1)–O(5)	141.18(13)		O(6)–Nd(2)–O(4)A	76.72(16)
  O(3)–Nd(1)–O(7)	136.08(13)		O(7)–Nd(1)–O(8)	112.06(12)		O(6)–Nd(2)–O(7)A	138.39(14)
  O(3)–Nd(1)–O(8)	83.05(13)		O(1)–Nd(2)–O(1W)	77.05(13)		O(6)–Nd(2)–O(8)B	149.32(13)
  O(5)–Nd(1)–N(1)	111.81(15)		O(1)–Nd(2)–O(4)A	147.79(15)		O(6)–Nd(2)–O(8)A	81.04(14)
  O(5)–Nd(1)–O(1W)	76.49(13)		O(1)–Nd(2)–O(6)A	79.82(16)		O(8)–Nd(2)–N(1)A	151.23(14)
  O(5)–Nd(1)–O(2W)	76.71(14)		O(1)–Nd(2)–O(7)	73.12(13)		O(8)–Nd(2)–N(1)B	82.84(15)
  O(5)–Nd(1)–O(8)	89.64(13)		O(1)–Nd(2)–O(8)B	108.38(14)		O(8)–Nd(2)–O(7)B	70.89(12)
  O(7)–Nd(1)–N(1)	68.73(13)		O(1)–Nd(2)–O(8)A	71.60(13)		O(8)–Nd(2)–O(8)B	74.17(13)
  Symmetry codes: (A) –x + 1, −y, −z; (B) x + 1, y, z

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure

  X-ray structural analyses show that 1 belongs to the triclinic crystal system with space group P$ \overline 1 $. The molecular structural unit consists of four Nd³⁺ cations, four µ₃-OH^– anions, two µ₂-H₂O, six C₆H₄NO₂^– anions, two linear azide anions and two coordinated water molecules (Fig. 1). Within its molecular structural unit, four Nd³⁺ cations are eight- coordinated with the coordination geometry of a distorted dodecahedron. Nd(1) and Nd(1A) (A: 1–x, –y, –z) are coordinated by three O atoms from µ₃-OH^– anions, one N atom from azide anion, two O atoms from carboxylate anions and two coordinated water molecules, while Nd(2) and Nd(2A) (A: 1–x, –y, –z) are coordinated by three O atoms from µ₃-OH^– anions, one N atom from azide anion, three O atoms from carboxylate anions and one coordinated water molecule. It is worth noting that the C₆H₄NO₂^– anions display two different coordination modes. Four C₆H₄NO₂^– anions chelate to Nd³⁺ cations with two carboxyl oxygen atoms, while the others coordinate to Nd³⁺ cations with one oxygen atom. The bond lengths of Nd–O and Nd–N fall in the ranges of 2.384(4)~2.622(4) and 2.553(4)~2.523(5) Å, respectively. Selected bond lengths of Nd–O and Nd–N are listed in Table 1. Compared with the previously related polymers^{[1, 5]}, these bond distances of Nd–O and Nd–N are within their normal ranges. Moreover, in order to further determine the valence of Nd sites in 1, the bond valence sum calculations^[16] were applied. The results show that the valences of Nd(1) (+3.34: 0.47, 0.46, 0.43, 0.40, 0.51, 0.40, 0.43 and 0.24) and Nd(2) (+3.44: 0.47, 0.45, 0.40, 0.47, 0.44, 0.38, 0.55 and 0.28) sites are positively trivalent in the polymer.

  Figure 1

  

  Figure 1.  Molecular structural unit of 1. Displacement ellipsoids are drawn at the 30% probability level

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  The Nd(1) and Nd(2) cations are linked together to form a binuclear unit by µ₃-OH, µ₂-OH₂ bridges and N atoms from azide anions. The binuclear unit is further extended into a tetra-nuclear Nd³⁺ molecular structural unit via µ₃-OH. In the {Nd4} cluster (Fig. 2), the four Nd³⁺ cations are almost on the same plane and located at the four vertices of the parallelogram. The distances of Nd³⁺···Nd³⁺ cations are 3.8507(16), 3.8907(12), 3.9409(11) and 6.7513(26) Å, respectively. The neighbor molecular structural units are further linked with each other through N atoms from linear azide anions, O atoms from carboxylate ligands, µ₃-OH and µ₂-OH₂ bridges, forming a 1-D infinite chain structure (Fig. 3). The connection modes between the Ln³⁺ cations in 1 are com- parable with those in polymers {[Eu₄(Mimda)₄(Tfpa)₂·4H₂O]}_n and {[Eu₄(Pimda)₄(Tfpa)₂·4H₂O]}_n (Mimda = 2-methyl- 1H-imidazole-4, 5-dicarboxy acid, Pimda = 2-propyl-1H- imidazole-4, 5-dicarboxy acid, Tfpa = (3, 4, 5, 6-tetrafluoro-1, 2-phthalate acid)^[17]. In the previously reported polymers [Eu₄(Mimda)₄(Tfpa)₂·4H₂O]}_n and {[Eu₄(Pimda)₄(Tfpa)₂·

  Figure 2

  

  Figure 2.  Structure of tetra-nuclear Nd³⁺ structural unit

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  Figure 3

  

  Figure 3.  View of the structure for 1-D infinite chain

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  4H₂O]}_n, the Eu³⁺ cations are connected by the carboxylate ligands, while in polymer 1, the Nd³⁺ cations are linked by the four types of bridges: azide anions, O atoms from carboxylate ligands, µ₃-OH and µ₂-OH₂ bridges. The O–H···O and O–H···N hydrogen bonds were observed in its packing structure. The detailed parameters of hydrogen bonds are provided in Table 2. Moreover, the π-π stacking interactions could be found in the packing structure. The detailed information of π-π stacking interactions is as follows: (i) Adjacent pyridine rings (N(4)/C(2)~C(6)) are parallel to each other with the centroid-to-centroid distances of 3.315(3) and 3.148(3) Å; (ii) Adjacent pyridine rings (N(5)/C(8)~C(12)) are parallel to each other with the centroid-to-centroid distances of 3.287(4) and 2.699(4) Å; (iii) Adjacent pyridine rings (N(6)/C(14)~C(18)) are parallel to each other (centroid-to-centroid distance 3.415(4) Å). The adjacent chains were further assembled by means of these hydrogen bonds and π-π stacking interactions into a 3-D supramolecular structure (Fig. 4).

  Table 2

  

  Table 2.  Hydrogen Bond Lengths (Å) and Bond Angles (°)

  DownLoad: CSV

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
  O(7)–H(7)···O(1)	0.82	2.45	2.879(6)	114
  O(8)–H(8)···O(1)D	0.83	2.24	2.835(5)	128
  O(8)–H(8)···O(2W)	0.83	2.58	2.988(5)	112
  O(1W)–H(1WA)···N(5)E	0.97	1.77	2.712(7)	162
  O(1W)–H(1WB)···O(2)	0.97	1.81	2.667(5)	145
  O(2W)–H(2WA)···N(4)F	0.84	1.93	2.761(7)	170
  O(2W)–H(2WB)···N(6)G	0.83	1.95	2.764(7)	167
  Symmetry codes: (D) x − 1, y, z; (E) −x + 1, −y + 1, −z; (F) −x + 2, −y, −z + 1; (G) −x + 1, −y + 1, −z + 1

  Figure 4

  

  Figure 4.  Three-dimensional supramolecular structure of 1. Dashed lines represent the hydrogen bonds

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  3.2 Infrared spectroscopy

  In the IR spectrum of polymer 1 (Fig. 5, left), the absorp- tion peaks at 3397, 3605 and 1622 cm^–1 are assigned to O–H vibrations from OH^– groups and water molecules. The peaks of 2118 and 1345 cm^–1 belong to the azide anions. The peaks at 1544 and 1223 cm^–1 are ascribed to the asymmetric and symmetric stretching vibrations of carboxylic groups, revealing the presence of chelating coordination modes in 1. These peaks are comparable with those of reported known carboxylate-containing polymers. A series of absorption peaks between 1144 and 683 cm^–1 (1144, 1101, 1031, 857, 762 and 683 cm^–1) should be attributed to the pyridine ring derived from organic ligands^{[1, 2]}.

  Figure 5

  

  Figure 5.  Left: IR spectrum of 1; right: thermal stability of 1

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  3.3 Thermal stability

  In order to further verify the existence of bridging hydro- xide, bridging and coordination water molecules in its structure, we have tested the thermal stability of 1 between 30 and 800 ℃. The compound has a two-step weight loss process (Fig. 5, right). The weight loss in the first step is 2.48% between 345 and 396 ℃, which corresponds to the release of two coordinated water molecules (theoretical value: 2.35%), and that in the second step is 54.05% in the range of 419~724 ℃ due to the departure of bridging hydroxyl, bridging water molecules and organic ligands in the structure (theoretical value: 53.77%). According to the experimental results, we can infer that the residue after final burning should be neodymium oxide. These results are in good agreement with those from single-crystal structural analysis.

  3.4 Near-infrared fluorescent properties

  The structure of polymer 1 containing Nd³⁺ cations promotes us to investigate its near-infrared emissions at room temperature. Polymer 1 was dissolved in DMF solution with a concentration of 10^–3 mol/L. 1 exhibits three characteristic peaks at 882, 1055 and 1328 nm upon the excitation at 354 nm, belonging to the ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2 transitions of Nd³⁺ cations, respectively^{[1, 3]}. The emission peaks of 882 and 1328 nm are weaker and the peak of 1055 and 1328 nm is the strongest (Fig. 6, red curve), which is similar with those of known polymers containing Nd³⁺ cations^[18-20]. Although there are slight differences in the positions of emission peaks in these polymers due to the fact that their coordination environments around the Nd³⁺ cations, the connection modes and their structural dimensions are different, the near-infrared emissions are all from the Nd³⁺ cations. Previous studies have shown that Cu²⁺ cations acting as quenching agents can lead to fluorescence quenching ^[21-22]. In order to study the fluorescence quenching of 1 for Cu²⁺ cation, the strongest peak of 1055 nm is selected to verify the fluorescence quenching effect. That is, the fluorescence quenching effect can be judged by observing the intensity changes of emission peak (1055 nm). Here, we employed the different concentrations of Cu²⁺/DMF solutions (CuCl₂·2H₂O was 1 immersed in DMF solutions with the following concentrations: 10^–5, 10^–4, 10^–3 and 10^–2 mol/L) to evaluate its fluorescence quenching for Cu²⁺ cation. The results indicated that there are obvious effects on the emission intensities. According to the results, we found that the intensities of emission peaks were decreased with increasing the concentration of Cu²⁺ cation (10^–5~10^–2 mol/L). The NIR emissions under different concentrations of Cu²⁺ cation are provided in Fig. 6. The results further showed that Cu²⁺ cations can act as a NIR fluorescence quenching agent of 1.

  Figure 6

  

  Figure 6.  NIR emission of 1 in DMF solution (red curve) and the NIR emissions of 1 immersed in DMF solutions with different concentrations of Cu²⁺ (10^–5~10^–2 mol/L, excited at 354 nm)

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  4. CONCLUSION

  In summary, the tetra-nuclear Nd(III) coordination polymer containing two types of organic ligands (isonico- tinate and linear azide anions) has been synthesized by using the hydrothermal method. The structure of 1 was well established by single-crystal X-ray diffraction and the C, H and N elemental analyses, IR spectrum, thermal stability and near-infrared luminescent properties were also fully studied. The NIR luminescent properties showed that 1 displays three characteristic peaks at 882, 1055 and 1328 nm, belonging to the ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2 transitions of Nd³⁺ cations, respectively. More importantly, the NIR fluore- scence quenching of 1 for Cu²⁺ cation was well investigated. The successful discovery of 1 enriches the family of poly- nuclear rare earth polymers and provides an effective and convenient way for the preparation of more such polymers in the future.

  
  
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• 

  Figure 1  Molecular structural unit of 1. Displacement ellipsoids are drawn at the 30% probability level

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  Figure 2  Structure of tetra-nuclear Nd³⁺ structural unit

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  Figure 3  View of the structure for 1-D infinite chain

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  Figure 4  Three-dimensional supramolecular structure of 1. Dashed lines represent the hydrogen bonds

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  Figure 5  Left: IR spectrum of 1; right: thermal stability of 1

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  Figure 6  NIR emission of 1 in DMF solution (red curve) and the NIR emissions of 1 immersed in DMF solutions with different concentrations of Cu²⁺ (10^–5~10^–2 mol/L, excited at 354 nm)

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Nd(1)–N(1)	2.553(4)		Nd(1)–O(7)A	2.415(4)		Nd(2)–O(4)B	2.394(4)
  Nd(1)–O1(W)	2.622(4)		Nd(1)–O(8)	2.441(4)		Nd(2)–O(6)B	2.384(4)
  Nd(1)–O2(W)	2.445(4)		Nd(2)–N(1)	2.523(5)		Nd(2)–O(7)	2.447(4)
  Nd(1)–O(3)	2.384(4)		Nd(2)–O(1)	2.385(4)		Nd(2)–O(8)A	2.408(4)
  Nd(1)–O(5)	2.421(4)		Nd(2)–O(1W)	2.582(4)		Nd(2)–O(8)B	2.460(4)
  Nd(1)–O(7)	2.393(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(3)–Nd(1)–O(1W)	117.26(13)		O(7)–Nd(1)–O(1W)	69.06(12)		O(6)–Nd(2)–N(1)A	113.52(16)
  O(3)–Nd(1)–O(2W)	145.02(14)		O(7)–Nd(1)–O(2W)	78.27(12)		O(6)–Nd(2)–O(1W)A	74.62(14)
  O(3)–Nd(1)–O(5)	75.92(15)		O(7)–Nd(1)–O(5)	141.18(13)		O(6)–Nd(2)–O(4)A	76.72(16)
  O(3)–Nd(1)–O(7)	136.08(13)		O(7)–Nd(1)–O(8)	112.06(12)		O(6)–Nd(2)–O(7)A	138.39(14)
  O(3)–Nd(1)–O(8)	83.05(13)		O(1)–Nd(2)–O(1W)	77.05(13)		O(6)–Nd(2)–O(8)B	149.32(13)
  O(5)–Nd(1)–N(1)	111.81(15)		O(1)–Nd(2)–O(4)A	147.79(15)		O(6)–Nd(2)–O(8)A	81.04(14)
  O(5)–Nd(1)–O(1W)	76.49(13)		O(1)–Nd(2)–O(6)A	79.82(16)		O(8)–Nd(2)–N(1)A	151.23(14)
  O(5)–Nd(1)–O(2W)	76.71(14)		O(1)–Nd(2)–O(7)	73.12(13)		O(8)–Nd(2)–N(1)B	82.84(15)
  O(5)–Nd(1)–O(8)	89.64(13)		O(1)–Nd(2)–O(8)B	108.38(14)		O(8)–Nd(2)–O(7)B	70.89(12)
  O(7)–Nd(1)–N(1)	68.73(13)		O(1)–Nd(2)–O(8)A	71.60(13)		O(8)–Nd(2)–O(8)B	74.17(13)
  Symmetry codes: (A) –x + 1, −y, −z; (B) x + 1, y, z

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  Table 2.  Hydrogen Bond Lengths (Å) and Bond Angles (°)

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
  O(7)–H(7)···O(1)	0.82	2.45	2.879(6)	114
  O(8)–H(8)···O(1)D	0.83	2.24	2.835(5)	128
  O(8)–H(8)···O(2W)	0.83	2.58	2.988(5)	112
  O(1W)–H(1WA)···N(5)E	0.97	1.77	2.712(7)	162
  O(1W)–H(1WB)···O(2)	0.97	1.81	2.667(5)	145
  O(2W)–H(2WA)···N(4)F	0.84	1.93	2.761(7)	170
  O(2W)–H(2WB)···N(6)G	0.83	1.95	2.764(7)	167
  Symmetry codes: (D) x − 1, y, z; (E) −x + 1, −y + 1, −z; (F) −x + 2, −y, −z + 1; (G) −x + 1, −y + 1, −z + 1

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