English

• 1. INTRODUCTION

  In recent years, organic ligands and metal ions self-assembly design and synthesis of complexes with special functions have drawn great attention. These materials have not only obtained various novel structures, but also have been applied to the fields of catalysts^[1], gas adsorption and separation^{[2, 3]}, magnets^{[4, 5]}, fluorescence^{[6, 7]}, biological^[8], hydrogen storage^[9], and so forth. Pyridine is an important aromatic six-membered heterocyclic. The lone pair electrons on the ring that can be combined with protons does not participate in the formation of a conjugated system on the ring. Therefore, pyridine has a certain alkalinity, and is often used as ligands to construct complexes with metal ions. On account of this property, pyridine ligands have a wide range of applications, such as fluorescence materials^{[10, 11]}, pesticides^[12], molecular reservoirs^[13], catalysts^[14], life sciences^[15], and so on.

  In the previous work, our group reported the crystal material 2-carboxylic acid-4-nitropyridine-1-oxide (POA)^[16] with the second-order nonlinear optical (NLO). The structure is modified by introducing a substituent to a different position of the pyridine. It is well-known that -COOH is a planar configuration and has a plurality of coordination sites. If the 4-nitropyridine-1-oxide in the 2 position is substituted by -COOH, the molecular coplane will be formed by the -COOH group with 4-nitropyridine-1-oxide and the steric interaction will be avoided between -COOH and -NO₂. Furthermore, it is possible that the molecular conjugation will be enhanced and crystal centrosymmetry will be removed by adding -COOH in the 2-position of 4-nitropyridine-1-oxide. The properties of excited states were calculated by using the time-dependent density functional theory (TDDFT). The Kurtz powder SHG measurement shows that POA exhibits powder SHG efficiency approximately 4.6 and 9.8 times as large as that of KH₂PO₄ (KDP). It is an excellent second-order nonlinear optical material.

  As part of this series of research work, we report the synthesis, crystal structure and fluorescence properties of three compounds [Zn(POA)₂(H₂O)₂] (1), [Zn(POA)₂(H₂O)₂]·2H₂O (2) and [Zn(POA)₂]_n (3) with different structures in which POA serves as a ligand with transition metal Zn(Ⅱ) ions by different methods.

  2. EXPERIMENTAL

  2.1 General procedures

  All the reagents and chemicals in this work were of analytical reagent grade, commercially obtained and used without further purification. Elemental analyses (C, H and N) were performed on an Elmentar Vario EL elemental analyzer. Powder X-ray diffraction (PXRD) patterns were carried out on a Rigaku MiniFlex 600 using CuKα (λ = 1.54056 Å) at a rate of 5 ℃/min in the range of 5°~50°. IR spectra were measured on a NICOLET iS50 of a Fourier transform infrared spectrometer using KBr pellets with the frequency wavelength range of 4000~400 cm^-1. TG measurements were performed under a flow of argon atmosphere from room temperature to 1000 ℃ at a heating rate of 10 ℃/min by using a Shimadzu DTG-60H thermo-gravimetric analyzer. Single-crystal data for the three complexes were collected on an Agilent GeminiE single-crystal diffractometer from Agilent, Poland. The fluorescence spectra were performed on an Edinburgh FS5 fluorescence spectrometer in the solid state at room temperature.

  2.2 Synthesis of POA

  2-Carboxylic acid-4-nitropyridine-1-oxide (POA) was synthesized according to literature^[16]. The preparation process is shown in Scheme 1 in 68% yield. m.p.: 148 ℃. Anal. Calcd. for C₆H₄N₂O₅ (%): C, 39.14; H, 2.17; N, 15.21. Found (%): C, 39.18; H, 2.23; N, 15.15. IR (KBr, cm^-1): 3121 (w), 3096 (w), 2655 (w), 1724 (s), 1615 (m), 1540 (m), 1497 (m), 1437 (m), 1352 (s), 1281 (s), 845 (s), 774 (s).

  Scheme 1

  

  Scheme 1.  Preparation of ligand POA

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  2.3 Synthesis of 1

  Complex 1 was prepared by mixing Zn(Ac)₂·2H₂O (1 mmol, 219 mg), POA (2 mmol, 368 mg) and 10 mL distilled water into a 25 mL Teflon-lined stainless-steel autoclave. The autoclave was heated at 413 K for 2 days and then powered off. When the autoclave was naturally cooled down to room temperature, orange block-like crystals were obtained and used for the collection of single-crystal X-ray data. The yield was 58% based on POA. Anal. Calcd. for C₁₂H₁₀N₄O₁₂Zn (%): C, 30.82; H, 2.14; N, 11.98. Found (%): C, 31.57; H, 2.12; N, 12.08. IR (KBr, cm^-1): 3445 (s), 3108 (w), 3076 (w), 1624 (m), 1529 (m), 1459 (m), 1431 (m), 1347 (s), 1262 (s), 857 (s), 840 (s), 749 (s).

  2.4 Synthesis of 2

  Zn(Ac)₂·2H₂O (1.1 mmol, 241 mg) and POA (2.2 mmol, 405 mg) in distilled water (20 mL) were refluxed for 8 h and then cooled to room temperature. The precipitates were filtered and the filtrate was transferred to the flask that was naturally volatilized in the air. Two days later, the yellow block-like crystals were obtained and used for the collection of single-crystal X-ray data. The yield was 45% based on POA. Anal. Calcd. for C₁₂H₁₄N₄O₁₄Zn (%): C, 28.61; H, 2.78; N, 11.13. Found (%): C, 28.54; H, 2.61; N, 11.75. IR (KBr, cm^-1): 3601 (s), 3508 (m), 3354 (s), 3154 (w), 3092 (w), 1647 (m), 1533 (m), 1463 (m), 1351 (s), 1235 (s), 857 (s), 842 (s), 752 (s).

  2.5 Synthesis of 3

  Zn(Ac)₂·2H₂O (5 mmol, 1095 mg) was dissolved in 15 mL of absolute methanol solution and POA (2 mmol, 368 mg) was dissolved in 10 mL of acetone, then they were mixed and stirred for 3 h. The precipitates were filtered and naturally evaporated for about 3 days to obtain brown columnar-like crystals, and used for the collection of single-crystal X-ray data. The yield was 65% based on POA. Anal. Calcd. for C₁₂H₆N₄O₁₀Zn (%): C, 33.39; H, 1.39; N, 12.98. Found (%): C, 33.14; H, 1.46; N, 12.57. IR (KBr, cm^-1): 3154 (w), 3093 (w), 1655 (m), 1533 (s), 1463 (m), 1355 (s), 1235 (s), 857 (s), 841 (s), 751 (s), 715 (s).

  2.6 X-ray structural determination

  The single crystals of complexes 1 (0.03mm × 0.04mm × 0.05mm), 2 (0.15mm × 0.20mm × 0.30mm) and 3 (0.11mm × 0.13mm × 0.14mm) were selected for X-ray diffraction analysis. The data were collected on an Agilent GeminiE single-crystal diffractometer equipped with a graphite-monochro-matic MoKα radiation (λ = 0.71073 Å) by using a φ-ω scan mode at 293(2) K. The empirical absorption was applied to the intensity data. A total of 7766 reflections were collected in the range of 3.38 < θ < 27.47° (–28≤h≤29, –9≤k≤9, –13≤l≤12), of which 1873 were independent (R_int = 0.0277) and 1632 were observed with I > 2σ(I) for 1. A total of 2492 reflections were collected in the range of 2.67 < θ < 25.81° (–9≤h≤9, –8≤k≤9, –7≤l≤10), of which 1654 were independent (R_int = 0.0162) and 1602 were observed with I > 2σ(I) for 2. A total of 2340 reflections were collected in the range of 3.65 < θ < 25.00° (–5≤h≤5, –9≤k≤14, –13≤l≤8), of which 1207 were independent (R_int = 0.0432) and 1013 were observed with I > 2σ(I) for 3. All the structures were solved by direct methods with SHELXTL-97 program^[17] and refined by full-matrix least-squares method on F² with anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms were theoretically added. For complex 1, the final R = 0.0321, wR = 0.0913 (w = 1/[σ²(F_o²) + (0.0741P)² + 0.1407P], where P = (F_o² + 2F_c²)/3), (Δ/σ)_max = 0.000, S = 0.965, (Δρ)_max = 0.625 and (Δρ)_min = –0.239 e/ų. For 2, the final R = 0.0295, wR = 0.0760 (w = 1/[σ²(F_o²) + (0.0372P)² + 0.3778P], where P = (F_o² + 2F_c²)/3), (Δ/σ)_max = 0.000, S = 1.087, (Δρ)_max = 0.278 and (Δρ)_min = –0.384 e/ų. For complex 3, the final R = 0.0409, wR = 0.0980 (w = 1/[σ²(F_o²) + (0.0563P)² + 0.0000P], where P = (F_o² + 2F_c²)/3), (Δ/σ)_max = 0.004, S = 1.023, (Δρ)_max = 0.654 and (Δρ)_min = –0.613 e/ų. The selected bond distances and bond angles are presented in Tables 1~3, while the hydrogen bonding interactions are shown in Tables 4 and 5.

  Table 1

  

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

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	2.0620(16)		Zn(1)–O(2)	2.1375(15)		Zn(1)–O(1)i	2.0620(16)
  Zn(1)–O(2)i	2.1375(15)		Zn(1)–O(6)	2.0426(17)		Zn(1)–O(6)i	2.0426(17)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O2	79.97(6)		O(2)–Zn(1)–O(1)i	97.71(6)		O(1)i–Zn(1)–O(6)	94.57(7)
  O(6)–Zn(1)–O(1)	87.48(7)		O(2)i–Zn(1)–O(6)i	165.58(6)		O(2)–Zn(1)–O(2)i	82.11(9)
  O(2)i–Zn(1)–O(1)	97.71(6)		O(2)i–Zn(1)–O(6)	92.65(7)		O(2)i–Zn(1)–O(1)i	79.97(6)
  Symmetry code: i –x, y, –z+1/2 for 1

  Table 2

  

  Table 2.  Selected Bond Lengths (Å) and Bond Angles (°) for Complex 2

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	2.0625(16)		Zn(1)–O(2)	2.0406(15)		Zn(1)–O(1)i	2.0625(16)
  Zn(1)–O(2)i	2.0406(15)		Zn(1)–O(6)	2.1355(17)		Zn(1)–O(6)i	2.1355(17)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O(2)	83.63(7)		O(2)–Zn(1)–O(1)i	96.37(7)		O(1)i–Zn(1)–O(2)i	83.63(7)
  O(2)i–Zn(1)–O(1)	96.37(7)		O(6)i–Zn(1)–O(1)	91.55(7)		O(6)i–Zn(1)–O(2)	87.08(7)
  O(6)i–Zn(1)–O(1)i	88.45(7)		O(6)i–Zn(1)–O(2)i	92.92(7)		O(6)–Zn(1)–O(6)i	180.00
  Symmetry code: i –x+2, –y+1, –z+1 for 2

  Table 3

  

  Table 3.  Selected Bond Lengths (Å) and Bond Angles (°) for Complex 3

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	2.040(2)		Zn(1)–O(2)	2.025(2)		Zn(1)–O(1)ii	2.040(2)
  Zn(1)–O(2)ii	2.025(2)		Zn(1)–O(3)iii	2.190(2)		Zn(1)–O(3)iv	2.190(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O(2)	88.61(9)		O(2)–Zn(1)–O(1)ii	91.39(9)		O(1)ii–Zn(1)–O(2)ii	88.61(9)
  O(2)ii–Zn(1)–O(1)	91.39(9)		O(3)iii–Zn(1)–O(2)ii	88.15(9)		O(3)iii–Zn(1)–O(1)ii	93.86(9)
  O(3)iii–Zn(1)–O(2)	91.85(9)		O(3)iii–Zn(1)–O(1)	86.14(9)		O(3)iii–Zn(1)–O(3)iv	180.00
  Symmetry codes: ii –x, –y+1, –z; iii x–1, y, z; iv –x+1, –y+1, –z for 3

  Table 4

  

  Table 4.  Hydrogen Bonding Lengths (Å) and Bond Angles (°) for Complex 1

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  D–H…A	d(D–H)	d(H…A)	d(D…A)	∠DHA
  O(6)–H(6A)…O(3)ii	0.854	1.901	2.733	164.40
  Symmetry code: ii –x, y–1, –z+1/2 for 1

  Table 5

  

  Table 5.  Hydrogen Bonding Lengths (Å) and Bond Angles (°) for Complex 2

  DownLoad: CSV

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
  O(6)–H(6A)···O(3)ii	0.811	2.122	2.920	168.02
  O(6)–H(6B)···O(0AA)	0.815	1.959	2.768	171.56
  O(0AA)–H(0AA)···O(5)iii	0.850	2.465	3.310	172.94
  O(0AA)–H(0AA)···O(4)iii	0.850	2.559	3.174	130.09
  O(0AA)–H(0AB)···O(3)iv	0.850	1.971	2.818	174.23
  Symmetry codes: ii x, y+1, z; iii –x+1, –y, –z+1; iv –x+2, –y+1, –z for 2

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of complex 1

  The single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in monoclinic system, space group C2/c with Z = 4, and the crystal structure is shown in Fig. 1a. The selected bond lengths and bond angles are listed in Table 1. Complex 1 is composed of one Zn(Ⅱ) ion, two deprotonated POA ligands, and two coordination water molecules. The entire molecular configuration likes a butterfly, in which two POA ligands are the wings of a butterfly.

  Figure 1

  

  Figure 1.  Structure of complex 1: (a) Molecular structure; (b) 1D structure connected by hydrogen bonds. Ellipsoid probability level: 30%. Symmetry codes: ⁱ –x, y, –z+1/2, ⁱⁱ –x, y–1, –z+1/2

  DownLoad: Full-Size Img PowerPoint

  The Zn(Ⅱ) ion is located on the C₂ symmetry axis andcoordinated with two oxygen atoms O(6) and O(6)ⁱ of the two coordination water molecules, respectively. In addition to the carboxyl O(2) and O(2)ⁱ of a pair of POA ligands, the oxygen atoms O(1) and O(1)ⁱ on the nitrogen of two pyridine rings are coordinated in a chelating form to form a distorted octahedral configuration.

  O(1), O(2), O(1)ⁱ and O(6) form an octahedral equatorial plane. ∠O(1)–Zn(1)–O(2) = 79.97(6)°, ∠O(2)–Zn(1)–O(1)ⁱ = 97.71(6)°, ∠O(1)ⁱ–Zn(1)– O(6) = 94.57(7)° and ∠O(6)–Zn(1)–O(1) = 87.48(7)°, with their sum to be 359.73°, which is close to 360°. The distance deviation R_ms between the O(1), O(2), O(1)ⁱ, O(6) and Zn(1) atoms from the least-squares plane is 0.0544 Å. Zn(Ⅱ) ion is almost coplanar with the equatorial plane. O(2)^Ii and O(6)ⁱ are located at the two poles of the equator. The bond angle of O(2)ⁱ–Zn(1)–O(6)ⁱ is 165.58(6)°, deviating from the poles by 14.42°.

  The Zn–O bonds of the complex are 2.0426(17) and 2.1375(15) Å, which belong to normal Zn–O bond lengths^{[18, 19]}. The Zn(1)–O(2) bond length is 2.1375(15) Å, and O(2) is derived from monodentate chelated acetate. The C(6)–O(2) bond length of carbonyl is 1.26 Å, which is slightly longer than the carbonyl bond length (1.206(4) Å)^[16] before coordination. The double bond becomes longer, indicating that the O(2) of the carbonyl is involved in coordination. The bond length of N(1)–O(1) is 1.311(2) Å, which is slightly shorter than the bond length before coordination (1.318(4) Å). The ligand forms a conjugated system with Zn(Ⅱ) ion. The bond angles of O(2)–Zn(1)–O(2)ⁱ and N(2)–Zn(1)–N(2)ⁱ are 82.11(9)° and 98.213(29)°, respectively, indicating that the overall configuration of complex 1 is V-shaped.

  As shown in Fig. 1b, there is an obvious intermolecular hydrogen bond between the coordination water and the carboxyl of POA in the molecule (Table 4). The oxygen atom O(6) of coordination water in molecule I generates hydrogen bond (O(6)···O(3)ⁱⁱ 0.2733 nm 2.733 Å, symmetry code: ⁱⁱ –x, y–1, –z+1/2) with the oxygen atom O(3)ⁱⁱ on the carboxyl in molecule Ⅱ. These intermolecular hydrogen bonds connect molecules into a one-dimensional chain structure along the b-axis.

  3.2 Crystal structure of complex 2

  Single-crystal X-ray diffraction analysis reveals that complex 2 crystallizes in triclinic system, space group P$ \overline 1 $ with Z = 1, and the crystal structure is shown in Fig. 2a. The selected bond lengths and bond angles are listed in Table 2. The asymmetric unit of complex 2 contains half Zn(Ⅱ) ion, one deprotonated POA ligand, one coordination water and one crystal water molecules. Different from 1, complex 2 is a symmetrical chair structure. The two deprotonated POA ligands take Zn(Ⅱ) ion as the symmetric center. The dihedral angle of the plane formed by two pyridine rings is 0.00(0.13)°, and the two pyridine rings are parallel to each other. The octahedral configuration of ZnO₆ is composed of two carboxyl oxygen atoms (O(2), O(2)ⁱ) on two deprotonated POA ligands, together with oxygen atoms O(1) and O(1)ⁱ on the nitrogen of two pyridine rings and two coordination water molecules. O(1), O(2), O(1)ⁱ and O(2)ⁱ form the equatorial plane of the octahedron.∠O(1)–Zn(1)–O(2) = 83.63(7)°, ∠O(2)–Zn(1)–O(1)ⁱ = 96.37(7)°, ∠O(1)ⁱ–Zn(1)– O(2)ⁱ = 83.63(6)° and ∠O(2)ⁱ–Zn(1)–O(1) = 96.37(6)°, with the sum being 360°. Distance deviation R_ms is 0.0000 Å between O(1), O(2), O(1)ⁱ, O(2)^Ii and Zn(1) atoms from the least-squares plane. Zn(Ⅱ), O(1), O(2), O(1)^Ii and O(2)ⁱ atoms are coplanar. O(6) and O(6)ⁱ are at the poles of the equator. The bond angles of O(6)ⁱ–Zn(1)–O(1), O(6)ⁱ–Zn(1)–O(2), O(6)ⁱ–Zn(1)–O(1)^Ii and O(6)ⁱ– Zn(1)–O(2)ⁱ between the polar axis and the equatorial plane are 91.55(7)°, 87.08(7)°, 88.45(7)° and 92.92(7)°, respectively. This indicates that the polar axis is not completely perpendicular to the equatorial plane, and the octahedron is slightly distorted. The angle between the polar axis and the equatorial plane (∠O(6)–Zn(1)–O(6)ⁱ = 180.0°) forms a right angle.

  Figure 2

  

  Figure 2.  Structure of complex 2: (a) molecular structure; (b) 2D structure connected by hydrogen bonds Ellipsoid probability level: 50%. Symmetry codes: ⁱ –x+2, –y+1, –z+1; ⁱⁱ x, y+1, z; ⁱⁱⁱ –x+1, –y, –z+1; ^iv –x+2, –y+1, –z

  DownLoad: Full-Size Img PowerPoint

  The bond lengths of Zn–O are in a wide range of 2.0406(15)~2.1355(17) Å. O(2) is derived from a monodentate chelated acetate, where the Zn(1)–O(2) bond length is 2.0406(15) Å, which is shorter than the Zn(1)–O(2) bond length of complex 1. Moreover, the bond angles of O(2)–Zn(1)–O(2)ⁱ and N(2)– Zn(1)–N(2)ⁱ are 180.00(6)° and 180.00°, indicating that complex 2 is a central symmetric structure. The bond length of carbonyl C(6)–O(2) is 1.249(3) Å, which is also slightly longer than uncoordinated (1.206(4) Å)^[16]. The N(1)–O(1) bond length is 1.308(2) Å, which is slightly shorter than the bond length before coordination (1.318(4) Å).

  As shown in Fig. 2b, complex 2 contains coordinated water molecules, crystal water molecules, carboxyl, and nitro, and these cause intermolecular hydrogen bonding of complex 2 (Table 5). The oxygen atoms in coordinated water and the carboxyl oxygen atoms of the neighboring complex molecules (O(6)···O(3)ⁱⁱ 2.92 Å, symmetry codes: ⁱⁱ x, y+1, z) as well as the crystal water oxygen atoms (O(6)···O(0AA) 2.768 Å) for hydrogen bonds. O(6) of coordination water generate hydrogen bonds (O(6)···O(3)ⁱⁱ 2.92 Å, symmetry code: ⁱⁱ x, y+1, z) with O(3)ⁱⁱ on the carboxyl on adjacent molecule and (O(6)···O(0AA) 2.768 Å) with O(0AA) on the crystal water. The oxygen atom in the crystal water acts as a hydrogen donor and two nitroxide atoms in the adjacent complex molecules (O(0AA)···O(4)ⁱⁱⁱ 3.174 Å, O(0AA)···O(5)ⁱⁱⁱ 3.31 Å, symmetry codes: ⁱⁱⁱ –x+1, –y, –z+1) and the carboxyl oxygen atoms of adjacent molecules (O(0AA)···O(3)^iv 2.818 Å, symmetry codes: ^iv –x+2, –y+1, –z) form hydrogen bonds, which are joined to form a two-dimensional network structure.

  3.3 Crystal structure of complex 3

  The single-crystal X-ray diffraction analysis shows that complex 3 is a one-dimensional chain coordination polymer based on [Zn^Ⅱ(POA)₂]. It crystallizes in monoclinic system with space group P2₁/c, and its asymmetric unit is shown in Fig. 3a. The Zn(Ⅱ) ion coordinates with the surrounding six oxygen atoms to form a ZnO₆ octahedron. Among them, two carboxyl oxygens (O(2), O(2)ⁱⁱ) from one unit, and O(1) and O(1)ⁱ on the nitrogen atom of two pyridine rings constitute an octahedral equatorial plane. The other two carboxyl oxygen atoms (O(3)ⁱⁱⁱ, O(3)^iv) from adjacent units are in the axial positions of the octahedron. O(1), O(2), O(1)ⁱⁱ and O(2)ⁱⁱ form the equatorial plane of the octahedron. ∠O(1)–Zn(1)– O(2) = 88.61(9)°, ∠O(2)–Zn(1)–O(1)ⁱⁱ = 91.39(9)°, ∠O(1)ⁱⁱ–Zn(1)–O(2)ⁱⁱ = 88.61(9)° and ∠O(2)ⁱⁱ– Zn(1)–O(1) = 91.39(9)°, and the sum of them is 360°. Distance deviation R_ms is 0.0000 Å between O(1), O(2), O(1)ⁱⁱ and O(2)ⁱⁱ from the least-squares plane. The Zn(Ⅱ), O(1), O(2), O(1)ⁱⁱ and O(2)ⁱⁱ atoms are coplanar. The angles of O(3)ⁱⁱⁱ–Zn(1)–O(2)ⁱⁱ, O(3)ⁱⁱⁱ–Zn(1)–O(1)ⁱⁱ, O(3)ⁱⁱⁱ–Zn(1)–O(2) and O(3)ⁱⁱⁱ– Zn(1)–O(1) between the polar axis and the equatorial plane are 88.15(9)°, 93.86(9)°, 91.85(9)° and 86.14(9)°, respectively. The polar axis is not perpendicular to the equator, but slightly warped. The angle between the polar axis and the equatorial plane (∠O(3)ⁱⁱⁱ–Zn(1)–O(3)^iv = 180.0°) forms a right angle.

  Figure 3

  

  Figure 3.  Structure of complex 3: (a) Part of the molecular structure; (b) 1D chain structure; (c) 3D supramolecular structure. Hydrogen atoms are omitted for clarity; Ellipsoid probability level: 50%; Symmetry codes: ⁱ x+1, y, z; ⁱⁱ –x, –y+1, –z; ⁱⁱⁱ x–1, y, z; ^iv –x+1, –y+1, –z

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  In the octahedral geometry, the Zn–O bond lengths are between 2.025(2) and 2.190(2) Å. O(2) and O(3)ⁱⁱⁱ are from two different units of acetate. The Zn(1)–O(2) bond is 2.025(2) Å, which is shorter than Zn(1)–O(2) in complex 2. The bond of Zn(1)–O(3)ⁱⁱⁱ is 2.190(2) Å and longer than Zn(1)–O(2) of 2, both belonging to the normal Zn–O bond. Moreover, ∠O(2)–Zn(1)–O(2)ⁱ = 180.00(12)° and ∠N(2)– Zn(1)–N(2)ⁱ = 180.00°, and the monomer also has a central symmetrical structure. The carbonyl C(6)–O(2) bond length is 1.247(4) Å and C(6)–O(3)ⁱⁱ is 1.254(4) Å, which are also slightly longer than the uncoordinated ones (1.206(4) Å)^[16].

  As shown in Fig. 3b, the Zn(Ⅱ) ions are connected by two carboxyl oxygen atoms on the deprotonated POA ligands, and the [Zn^Ⅱ(POA)₂]²⁺ units are connected along the a-axis into a one-dimensional chain structure. Wherein, the central Zn(Ⅱ) ion is collinear, and two adjacent pyridine rings of different basic units form two conjugate planes. The dihedral angle of the two conjugate planes is 0.00(0.21)°. The distance between the two conjugate planes is 3.3535 Å. It can be seen that after the formation of the coordination polymer, the deprotonated POA ligands are parallel to each other, and thus the one-dimensional chain constitutes a "stepped" linear chain. The three-dimensional stacked structure composed of one-dimensional chains is a parallel structure of each two molecules in the projection direction of the a-axis (Fig. 3c).

  3.4 Thermal analysis and PXRD patterns

  Thermogravimetric analyses (TGA) were carried out under Ar atmosphere to examine the thermal stability of complexes 1~3 (Fig. 4). The TG curve of complex 1 indicates that the first weight loss of 8.13% from 135 to 171 ℃ corresponds to the removal of two coordinate water molecules (Calcd. 7.70%). The decomposition of the residue occurs in the range of 182~540 ℃, and the curve is close to flat. The remaining weight of 17.58% should be the final product ZnO (Calcd. 17.41%). The TG curve of complex 2 is also mainly divided into two segments. The first weight loss of 13.91% occurs in the range of 102~151 ℃, indicating the removal of two crystal water molecules and two coordination water molecules (Calcd. 14.30%). Then the TG curve dropped rapidly with obvious weight loss, and became stable until 542 ℃. The final residue may be ZnO (Obsd. 15.55%, Calcd. 16.16%). The TG curve of complex 3 indicates that the weight loss of 82.01% takes place between 145 and 603 ℃ due to the collapse of the one-dimensional chain skeleton. Above 603 ℃, the curve tends to be flat. The sample is decomposed completely, and the final residue may be ZnO (Obsd. 17.73%, Calcd. 18.86%).

  Figure 4

  

  Figure 4.  TG curves of complexes 1~3

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  X-ray power diffraction (PXRD) patterns of 1~3 were recorded at room temperature. As shown in Fig. 5, the peak positions of simulated and experimental patterns are in good agreement with each other, suggesting the phase purity of the products. The differences in intensity may be due to the preferred orientation of the crystalline powder samples.

  Figure 5

  

  Figure 5.  PXRD patterns of complexes 1~3

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  3.5 Fluorescence properties

  The solid fluorescence spectra of 1~3 at room temperature are shown in Fig. 6. POA displays one strong peak at maximum 340 nm under excitation at 237 nm. Using a wavelength of 300 nm as the excitation light, complexes 1, 2 and 3 show maximum fluorescence emission peaks at 446, 469 and 456 nm, respectively. The wavelengths of the maximum emission peaks of the three complexes are significantly red-shifted (106, 129, 116 nm, respectively) compared to POA. This may be the carbonyl bond length on the carboxyl becoming shorter after the ligand forms complexes with the Zn(Ⅱ) ions, and the N←O bond becomes shorter due to the resonance donating effect. The complexes form a conjugated system, and the delocalized electron increases. On the other hand, the ligand-to-metal-change-transfer (LMCT) occurs^{[20, 21]}, and the rigidity of the complex is enhanced, the rotation of the molecule is limited, and the energy loss is reduced, thereby making the emission band obviously red-shifted.

  Figure 6

  

  Figure 6.  Fluorescence emission spectra of the ligand and complexes 1~3 (POA: λ_ex = 237 nm; 1, 2 and 3: λ_ex = 300 nm)

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  4. CONCLUSION

  In this paper, 2-carboxylic acid-4-nitropyridine-1-oxide(POA) was self-assembled with Zn(Ⅱ) ions by different assembly methods to obtain three complexes with different structures: [Zn(POA)₂(H₂O)₂] (1), [Zn(POA)₂(H₂O)₂]·2H₂O (2) and [Zn(POA)₂]_n (3). The structures of complexes 1~3 are determined by single-crystal X-ray analysis, XRD powder diffraction analysis, infrared spectroscopy and thermal stability analysis method. The fluorescent properties of the three complexes were investigated at room temperature. Fluorescence analysis showed that the complexes formed conjugated system and the ligand-to-metal-change-transfer (LMCT) occurred. The coordination of POA and Zn(Ⅱ) ions have a large red shift (106, 129, 116 nm, respectively). These three complexes have good fluorescence properties and will become potential fluorescent materials.

  
  
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• 

  Scheme 1  Preparation of ligand POA

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  Figure 1  Structure of complex 1: (a) Molecular structure; (b) 1D structure connected by hydrogen bonds. Ellipsoid probability level: 30%. Symmetry codes: ⁱ –x, y, –z+1/2, ⁱⁱ –x, y–1, –z+1/2

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  Figure 2  Structure of complex 2: (a) molecular structure; (b) 2D structure connected by hydrogen bonds Ellipsoid probability level: 50%. Symmetry codes: ⁱ –x+2, –y+1, –z+1; ⁱⁱ x, y+1, z; ⁱⁱⁱ –x+1, –y, –z+1; ^iv –x+2, –y+1, –z

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  Figure 3  Structure of complex 3: (a) Part of the molecular structure; (b) 1D chain structure; (c) 3D supramolecular structure. Hydrogen atoms are omitted for clarity; Ellipsoid probability level: 50%; Symmetry codes: ⁱ x+1, y, z; ⁱⁱ –x, –y+1, –z; ⁱⁱⁱ x–1, y, z; ^iv –x+1, –y+1, –z

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  Figure 4  TG curves of complexes 1~3

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  Figure 5  PXRD patterns of complexes 1~3

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  Figure 6  Fluorescence emission spectra of the ligand and complexes 1~3 (POA: λ_ex = 237 nm; 1, 2 and 3: λ_ex = 300 nm)

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

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	2.0620(16)		Zn(1)–O(2)	2.1375(15)		Zn(1)–O(1)i	2.0620(16)
  Zn(1)–O(2)i	2.1375(15)		Zn(1)–O(6)	2.0426(17)		Zn(1)–O(6)i	2.0426(17)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O2	79.97(6)		O(2)–Zn(1)–O(1)i	97.71(6)		O(1)i–Zn(1)–O(6)	94.57(7)
  O(6)–Zn(1)–O(1)	87.48(7)		O(2)i–Zn(1)–O(6)i	165.58(6)		O(2)–Zn(1)–O(2)i	82.11(9)
  O(2)i–Zn(1)–O(1)	97.71(6)		O(2)i–Zn(1)–O(6)	92.65(7)		O(2)i–Zn(1)–O(1)i	79.97(6)
  Symmetry code: i –x, y, –z+1/2 for 1

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  Table 2.  Selected Bond Lengths (Å) and Bond Angles (°) for Complex 2

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	2.0625(16)		Zn(1)–O(2)	2.0406(15)		Zn(1)–O(1)i	2.0625(16)
  Zn(1)–O(2)i	2.0406(15)		Zn(1)–O(6)	2.1355(17)		Zn(1)–O(6)i	2.1355(17)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O(2)	83.63(7)		O(2)–Zn(1)–O(1)i	96.37(7)		O(1)i–Zn(1)–O(2)i	83.63(7)
  O(2)i–Zn(1)–O(1)	96.37(7)		O(6)i–Zn(1)–O(1)	91.55(7)		O(6)i–Zn(1)–O(2)	87.08(7)
  O(6)i–Zn(1)–O(1)i	88.45(7)		O(6)i–Zn(1)–O(2)i	92.92(7)		O(6)–Zn(1)–O(6)i	180.00
  Symmetry code: i –x+2, –y+1, –z+1 for 2

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  Table 3.  Selected Bond Lengths (Å) and Bond Angles (°) for Complex 3

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	2.040(2)		Zn(1)–O(2)	2.025(2)		Zn(1)–O(1)ii	2.040(2)
  Zn(1)–O(2)ii	2.025(2)		Zn(1)–O(3)iii	2.190(2)		Zn(1)–O(3)iv	2.190(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O(2)	88.61(9)		O(2)–Zn(1)–O(1)ii	91.39(9)		O(1)ii–Zn(1)–O(2)ii	88.61(9)
  O(2)ii–Zn(1)–O(1)	91.39(9)		O(3)iii–Zn(1)–O(2)ii	88.15(9)		O(3)iii–Zn(1)–O(1)ii	93.86(9)
  O(3)iii–Zn(1)–O(2)	91.85(9)		O(3)iii–Zn(1)–O(1)	86.14(9)		O(3)iii–Zn(1)–O(3)iv	180.00
  Symmetry codes: ii –x, –y+1, –z; iii x–1, y, z; iv –x+1, –y+1, –z for 3

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  Table 4.  Hydrogen Bonding Lengths (Å) and Bond Angles (°) for Complex 1

  D–H…A	d(D–H)	d(H…A)	d(D…A)	∠DHA
  O(6)–H(6A)…O(3)ii	0.854	1.901	2.733	164.40
  Symmetry code: ii –x, y–1, –z+1/2 for 1

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  Table 5.  Hydrogen Bonding Lengths (Å) and Bond Angles (°) for Complex 2

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
  O(6)–H(6A)···O(3)ii	0.811	2.122	2.920	168.02
  O(6)–H(6B)···O(0AA)	0.815	1.959	2.768	171.56
  O(0AA)–H(0AA)···O(5)iii	0.850	2.465	3.310	172.94
  O(0AA)–H(0AA)···O(4)iii	0.850	2.559	3.174	130.09
  O(0AA)–H(0AB)···O(3)iv	0.850	1.971	2.818	174.23
  Symmetry codes: ii x, y+1, z; iii –x+1, –y, –z+1; iv –x+2, –y+1, –z for 2

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•