English

• 0.   Introduction

  Coordination polymers (CPs) as a new class of inorganic and organic hybrid materials are constructed by inorganic components (metal ions or metal clusters) and organic components (ligands)^[1-2]. Owing to the wide selection of components and the variability of reaction conditions, CPs can provide an almost unlimited number of crystalline materials with definite structures^[3-6]. With such a large number of candidates, CPs have many applications in the fields of catalytic reactions, material separation, magnetism, molecular ferroelectricity, gas adsorption, optical materials, etc^[7-22]. In the synthesis of CPs, researchers still hope to obtain target materials with specific structures via controlling reaction conditions. However, many reaction parameters including solvent system, temperature, pH of the reaction medium, and the composition of the reactants have important effects on the final structures of CPs^[23-25]. Faced with so many factors, it is very difficult to screen the synthetic conditions required to reach the target product. Fortunately, the organic building block (ligand) is the key point and plays a structural guiding role in the self-assembly process of CPs^[26-28]. Thus, researchers can develop different CPs with controllable structures and properties by matching different organic modules.

  In the field of coordination chemistry, carboxylic acids and N-heterocyclic compounds are two important classes of ligands. Among carboxylic acids, aromatic polycarboxyl compounds are considered the most popular synthons for their strong coordination capabilities and rich coordination models^[29-31]. N‑heterocyclic ligands, such as imidazole, triazole, tetrazole, pyridine, and their derivatives, can also construct functional CPs via the coordination of the metal center and N atoms^[32-33]. Aromatic acids and N-heterocyclic ligands not only can be used separately in the synthesis of CPs (single ligand method) but also can assemble with metal ions in the same reaction system (mixed ligand method). The latter is an important synthetic tactic for the reason that the coordination configuration and coordination number of metal centers can be changed when carboxylic acid and N-heterocyclic ligands react with metal ions in the same system^[34-36]. Under mixed ligand strategy, a series of CPs with similar structures can be obtained when one ligand is fixed and the others are homologous derivatives^[37-38]. In this way, researchers can break through the “synthetic barrier” and achieve control for the synthesis of CPs.

  Based on this consideration, we selected an ether-bridged dicarboxylic aromatic acid, namely 4-(1-carboxyethoxy)benzoic acid (H₂cba), to explore its application for preparing Ni(Ⅱ)-based CPs in the presence of similar N-heterocyclic ligands. As a result, under hydrothermal reaction conditions, H₂cba and Ni(Ⅱ) ion assembled with large volume imidazole derivatives 1, 4-di(1H-imidazol-1-yl)benzene (1, 4-dib) and 4, 4′-di(1H-imidazol-1-yl)-1, 1′-biphenyl (4, 4′-dib) to obtain complexes {[Ni(cba)(1, 4-dib)(H₂O)_0.5]·0.5H₂O}_n (HU21) and {[Ni(cba)(4, 4′-dib) (H₂O)_0.5]·0.5H₂O}_n (HU22) (Scheme 1). Owing to ligand-controlled effects, both HU21 and HU22 indicate 3D structures with six-connected topological nets. As semiconductor materials, they have strong absorption capacity for UV-Vis light and outstanding hydro‑stability. Furthermore, magnetic experiments revealed that HU21 and HU22 possess antiferromagnetic properties.

  Scheme 1

  

  Scheme 1.  Synthetic routes for complexes HU21 and HU22

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  1.   Experimental

  1.1   Reagents and physical measurements

  H₂cba was prepared according to the previously reported synthesis process^[39]. Other reagents and solvents are analytically pure, purchased commercially, and can be used directly without further purification. Powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) were performed on the Rigaku MiniFlex600 powder diffractometer (Voltage: 40 kV, Current: 15 mA, Cu Kα radiation, λ=0.154 06 nm, 2θ=5°-50°) and the Netzsch STA 449F5 thermogravimetric analyzer under nitrogen atmosphere from RT to 800 ℃, respectively. The infrared spectral curves were obtained via an FTIR650 infrared spectrometer. The contents of C, H, and N from the complexes were decided on a Perkin-Elmer 240C element analyzer. UV-Vis absorption spectra were collected by a Shimadzu UV-3600 Plus spectrophotometer. Variable temperature magnetic data were acquired on an MPMS-XL magnetic system.

  1.2   Synthesis of HU21

  The mixture of Ni(NO₃)₂·6H₂O (44 mg, 0.15 mmol), H₂cba (21 mg, 0.10 mmol), 1, 4-dib (21 mg, 0.10 mmol), Na₂CO₃ (16 mg, 0.15 mmol) was added to 23 mL of teflon reactor tank. After stirring at RT for about 15 min and placing at 120 ℃ for 48 h, the blue block crystals were filtered and washed with ethanol, and the yield after drying was 45% (based on H₂cba, Fig.S1 in the Supporting information). C₄₄H₄₀N₈O₁₂Ni₂(%): C 53.37, H 4.07, N 11.32; Found(%): C 53.64, H 4.12, N 11.46. IR (KBr, cm^-1): 3 425(m), 3 117(w), 1 668(s), 1 604(s), 1 528(s), 1 400(s), 1 307(w), 1 245(m), 1 133(w), 1 103(w), 1 063(m), 958(w), 935(w), 830(w), 778(w), 760(w), 655(w), 539(w), 475(w) (Fig.S2).

  1.3   Synthesis of HU22

  The synthesis of HU22 was similar to that of HU21, except that ligand 1, 4-dib was replaced by 4, 4′-dib. Blue block crystals of HU22 were collected with a yield of 50% (based on H₂cba, Fig.S3). C₅₆H₄₈N₈O₁₂Ni₂(%): C 58.88, H 4.24, N 9.81; Found(%): C 57.12, H 4.15, N 9.82. IR (KBr, cm^-1): 3 437(m), 3 140(w), 1 618(m), 1 610(m), 1 511(s), 1 389(s), 1 307(m), 1 237(m), 1 219(w), 1 167(w), 1 127(w), 1 063(m), 958(w), 934(w), 818(m), 783(m), 752(w), 665(m), 533(w) (Fig.S4).

  1.4   Crystal structure determination

  The single diffraction data of complexes HU21 and HU22 were obtained on the Rigaku 003 single crystal diffractometer by using Mo Kα ray (λ=0.071 073 nm) as the radiation source. Then, Rigaku OD 2015 software was used to restore the original data, and the final structures were solved by the SHELXT-2017 program and further refined via full-matrix least-squares techniques on Olex2-1.2 software with SHELXL-2017. All non-hydrogen atoms and their thermal parameters were refined anisotropically. Their crystallographic data are shown in Table 1, and some selected bond lengths and bond angles are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement for HU21 and HU22

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  Parameter	HU21	HU22
  Formula	C44H40N8O12Ni2	C56H48N8O12Ni2
  Formula weight	990.26	1 142.41
  Temperature/K	295.2	295.2
  Crystal system	Monoclinic	Monoclinic
  Space group	C2/c	C2/c
  a/nm	1.411 71(5)	1.587 64(10)
  b/nm	1.529 98(4)	1.794 16(11)
  c/nm	2.216 53(6)	2.060 16(12)
  β/(°)	107.551(3)	108.555(6)
  Volume/nm3	4.564 6(2)	5.563 3(6)
  Z	4	4
  Dc/(g·cm-3)	1.441	1.363
  μ/mm-1	0.895	0.743
  F(000)	2 048.0	2 328.0
  2θ range/(°)	7.714-60.684	7.332-60.372
  Reflection collected	58 031	35 170
  GOF	1.032	1.073
  Independent reflection	6 468	7 611
  Goodness-of-fit on F 2	1.032	1.073
  R1a [I>2σ(I)]	0.047 6	0.051 9
  wR2b [I>2σ(I)]	0.120 7	0.137 8
  Final R1 values (all data)	0.052 8	0.070 2
  Final wR2 values (all data)	0.122 7	0.147 5
  Largest diff. peak and hole/(e·nm-3)	970, -440	500, -280
  a R1=∑||Fo|-|Fc||∑|Fo|; b wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

  Table 2

  

  Table 2.  Selected bond length (nm) and angles (°) for HU21 and HU22

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  HU21
  Ni1—O1W	0.209 4(2)	Ni1—O2a	0.205 36(19)	Ni1—O1	0.204 82(19)
  Ni1—O4b	0.210 23(19)	Ni1—N1	0.209 0(2)	Ni1—N3	0.209 0(2)
  O1W—Ni1—O4a	89.86(7)	O2b—Ni1—O1W	94.41(7)	O2b—Ni1—O4a	175.73(8)
  O2b—Ni1—N(1)	89.39(8)	O2b—Ni1—N3	87.74(9)	O1—Ni1—O1W	88.84(7)
  O1—Ni1—O2b	91.78(9)	O1—Ni1—O4a	88.48(9)	O1—Ni1—N1	84.55(9)
  O1—Ni1—N	176.81(9)	N1—Ni1—O1W	172.48(7)	N1—Ni1—O4a	86.39(8)
  N1—Ni1—N3	92.29(9)	N3—Ni1—O1W	94.35(8)	N3—Ni1—O4a	91.76(8)
  Ni1b—O1W—Ni1	117.90(12)
  HU22
  Ni1—O2a	0.213 48(19)	Ni1—O5b	0.209 01(19)	Ni1—O1W	0.210 59(15)
  Ni1—O4	0.201 7(2)	Ni1—N1	0.207 0(2)	Ni1—N3	0.206 7(2)
  O5a—Ni1—O2b	176.12(8)	O5a—Ni1—O1W	93.86(7)	O1W—Ni1—O2b	89.57(7)
  O4—Ni1—O2b	85.68(8)	O4—Ni1—O5a	96.27(9)	O4—Ni1—O1W	87.25(7)
  O4—Ni1—N1	89.83(9)	O4—Ni1—N3	175.63(10)	N1—Ni1—O2b	86.76(8)
  N1—Ni1—O5a	89.89(9)	N1—Ni1—O1W	175.48(7)	N3—Ni1—O2b	90.33(9)
  N3—Ni1—O5a	87.82(9)	N3—Ni1—O1W	90.97(8)	N3—Ni1—N1	91.70(10)
  Symmetry codes: a: 1.5-x, 1.5-y, 1-z; b: 1-x, y, 0.5-z for HU21; a: 1-x, y, 0.5-z; b: 3/2-x, 3/2-y, 1-z for HU22.

  CCDC: 2382486, HU21; 2382485, HU22.

  2.   Results and discussion

  2.1   Crystal structure of complexes HU21 and HU22

  The single crystal structure analysis confirmed that HU21 crystallizes in a monoclinic C2/c space group, and each asymmetric unit is comprised of a Ni(Ⅱ) center, a deprotonated cba^2- anion, two half 1, 4-dib fragments, half a coordination water molecule, and half a guest water molecule. Herein, the cba^2- anion and coordinated water molecule adopt as μ₃- and μ₂-conformation to connect three and two Ni(Ⅱ) centers, respectively. With such coordination patterns, a binuclear [Ni₂(CO₂)₂(H₂O)]²⁺ unit is formed, where Ni(Ⅱ) center adopts an octahedral pattern to coordinate with three carboxyl O atoms from cba^2- anions, two imidazole N atoms from 1, 4-dib ligands and water molecule (Fig. 1). As a result, each binuclear [Ni₂(CO₂)₂(H₂O)]²⁺ unit is linked by four cba^2- anions and four 1, 4-dib ligands.

  Figure 1

  

  Figure 1.  Coordination environment of Ni(Ⅱ) in complex HU21

  Ellipsoid probability level: 50%; Symmetry codes: a: 2-x, 1-y, 1-z; b: 1.5-x, 1.5-y, 1-z; c: 1-x, y, 0.5-z; d: 1.5-x, 1.5-y, -z.

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  Considering that compound HU21 is constructed from two distinct ligands cba^2- and 1, 4-dib, it is divided into two parts to simplify this complicated structure. As shown in Fig. 2a, Ni(Ⅱ) ions are connected by coordination water molecules and 1, 4-dib ligands to form a large right-handed helical chain along b-axis, and the opposite left-handed chain also exist in HU21 (Fig. 2b). Further analysis showed that the pitch length from helical chains is identical to the b-axis length and each round of helical chains consist of six Ni(Ⅱ) ions, four 1, 4-dib ligands and two water molecules. According to the ratio of 1∶1, the adjacent left- and right-handed chains are joined together along the b-axis to form a 3D 1, 4-dib-Ni-(H₂O)_0.5-framework with large channels and the width of the channels is up to 1.6 nm (Fig. 2c). In such an empty 1, 4-dib-Ni-(H₂O)_0.5-framework, structural interpenetration can be inevitable. The whole 1, 4-dib-Ni-(H₂O)_0.5-framework is a 3-fold interpenetration structure (Fig. 2d). Herein, the Ni(Ⅱ) center is a three‑ connected node, and the 1, 4-dib and H₂O are only simple linkers, thus the 1, 4-dib-Ni-(H₂O)_0.5-framework can be simplified into a 3-connected ths net with point symbol (10³). Moreover, cba^2- anions, Ni(Ⅱ) ions, and H₂O molecules are linked together to obtain a 1D [Ni₂(H₂O)(cba)₂]_n-chain along the c-axis (Fig. 2e). The above [Ni₂(H₂O)(cba)₂]_n-chains were further filled into the 1, 4-dib-Ni-(H₂O)_0.5-framework to construct a 3D dense structure of HU21 (Fig. 2f). From a topological view, [Ni₂(CO₂)₂(H₂O)]²⁺ unit is a 6-connected node and the final structure can be reduced into a 6-connected net with point symbol (4⁴.6¹¹).

  Figure 2

  

  Figure 2.  Schematic illustrations of complex HU21: (a) right-handed helical chain; (b) left-handed helical chain; (c) 3D 1, 4-dib-Ni-(H₂O)_0.5-framework with helical nano-channels; (d) 3-fold-interpenetrating structure;(e) [Ni₂(H₂O)(cba)₂]_n-chain; (f) 3D structures based 1, 4-dib-Ni-(H₂O)_0.5-framework and [Ni₂(H₂O)(cba)₂]_n-chains

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  Complex HU22 also crystallizes in a monoclinic system with a C2/c space group. Each asymmetric unit from HU22 contains a Ni(Ⅱ) center, a cba^2- anion, two half 4, 4′-dib fragments, and half a coordination water molecule. Because disorder guest molecules can not be further identified, their corresponding single crystal diffraction contributions had to be deleted by using the solvent mask strategy. Nevertheless, half a water as a guest molecule should be present in the asymmetric unit according to the synthesis condition, elemental analysis result, and TGA curve. Just like HU22, binuclear [Ni₂(CO₂)₂(H₂O)]²⁺ units are also formed owing to μ₃-pattern from cba^2- anion and μ₂-pattern from coordination water molecule (Fig. 3).

  Figure 3

  

  Figure 3.  Coordination environment of Ni(Ⅱ) in complex HU22

  Ellipsoid probability level: 50%; Symmetry codes: a: 1-x, y, 1.5-z; b: 1.5-x, 1.5-y, 1-z; c: 1-x, y, 0.5-z; d: -x, 2-y, -z.

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  Based on the same considerations, complex HU22 is divided into two parts to better describe its complicated structure according to different types of ligands. Right-handed and left-handed helical chains are constructed by Ni(Ⅱ) centers, 4, 4′-dib ligands, and H₂O molecules along the a-axis (Fig. 4a and 4b). The pitch lengths of helical chains are equal to the a-axis length, and each round of helical chain contains six Ni(Ⅱ) centers, three 4, 4′-dib ligands, and three H₂O molecules. In complex HU21, adjacent helical channels are connected together resulting in a 3D framework, but above right-handed and left-handed helical chains in HU22 only offer a 2D 4, 4′-dib-Ni-(H₂O)_0.5-layer (Fig. 4c). On the other hand, binuclear [Ni₂(CO₂)₂(H₂O)]²⁺ units are also bridged by cba^2- anion to build a [Ni₂(H₂O)(cba)₂]_n-chain along the c‑axis (Fig. 4d). In the presence of [Ni₂(H₂O)(cba)₂]_n-chains, adjacent 4, 4′-dib-Ni-(H₂O)_0.5- layers further form a 3D framework in HU22 (Fig. 4e). In the framework, each [Ni₂(CO₂)₂(H₂O)]²⁺ units are linked by four cba^2- anions and four 4, 4′-dib ligands. Thus, [Ni₂(CO₂)₂(H₂O)]²⁺ units should be simplified as a six-connected node, and cba^2- anion and 4, 4′-dib ligand are only simple linkers. Based on this analysis, the whole framework of HU22 can be considered as a six-connected net with a point symbol of (4⁸.6⁷) (Fig. 4f).

  Figure 4

  

  Figure 4.  Structural illustrations of complex HU22: (a) right-handed helical chain; (b) left-handed helical chain;(c) 2D helical layer constructed by Ni(Ⅱ) centers, 4, 4′-dib ligands, and H₂O molecules; (d) 1D chain based on cba^2- anions, Ni(Ⅱ) centers, and H₂O molecules; (e) 3D framework; (f) six-connected topology network

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  The above structure descriptions show that Ni(Ⅱ) ions resulted in a binuclear [Ni₂(CO₂)₂(H₂O)]²⁺ unit with the help of coordinated water molecule and carboxyl groups from cba^2- anions, acting as a six-connected node to build the 3D frameworks in HU21 and HU22 in presence of 1, 4-dib and 4, 4′-dib. Imidazole derivatives 1, 4-dib and 4, 4′-dib have similar double coordination patterns, resulting in helical structures with water molecules and Ni(Ⅱ) ion centers in HU21 and HU22, respectively. It should be noted that the length of 1, 4-dib is 0.98 nm, while that of 4, 4′-dib is 1.41 nm. Herein, the obvious structural difference between 1, 4-dib and 4, 4′-dib leads to the different shape and composition of helical structures. For this reason, the helical chains based on 1, 4-dib ligands construct a 3D framework, while the helical chains based on 4, 4′-dib only build a 2D layer. In addition to these structural differences, complexes HU21 and HU22 have the same metal coordination patterns, crystal systems, space groups, and six-connected net, which are decided by the same carboxylic acid ligands and similar coordination modes from heterocyclic ligands. Therefore, ligand structure plays a decisive role in the synthesis of HU21 and HU22, and the structure of complexes can be regulated through the selection of ligands.

  2.2   PXRD, TGA, and UV-Vis absorption spectrum

  To examine the phase purity and water stability of HU21 and HU22, PXRD was performed on the synthesized samples. The results of PXRD showed that the experimental patterns were in good agreement with simulated ones, confirming that the synthesized products are all pure phase. Moreover, the further experiment indicated that the PXRD patterns of HU21 and HU22 still did not change much even if they were soaked in water over 24 h. Thus, complexes HU21 and HU22 have better water stability (Fig. 5a and 5b). During TGA, HU21 had about 2% weight loss from RT to 105 ℃, which should be attributed to the release of guest water molecules (Calcd. 1.8%). Above 310 ℃, sharp weightlessness appeared and the whole framework of HU21 began to collapse (Fig. 5c). The TGA curve of HU22 also indicated a slight weight loss of 2% between RT and 105 ℃, confirming that an asymmetric unit contains half a guest water molecule (Calcd. 1.6%). Difference from HU21, the TGA curve of HU22 can reveal obvious weightlessness from coordinated water molecules in the temperature range of 170-230 ℃ (Obsd. 1.8%, Calcd. 1.6%). Above 320 ℃, a sharp weight loss shows that the framework of HU22 has started to collapse (Fig. 5d).

  Figure 5

  

  Figure 5.  PXRD patterns (a, b) and TGA curves (c, d) of complexes HU21 and HU22

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  Solid-state UV-Vis absorption spectra for complexes HU21 and HU22 were also determined to check their light absorption capacities. As shown in Fig. 6a and 6c, HU21 and HU22 had similar UV-Vis absorption spectra with two strong absorption bands at 200-450 nm and 550-800 nm, respectively. Thus, HU21 and HU22 have strong light absorption capacity in the ultraviolet and visible regions. To further understand the UV-Vis absorption spectra of HU21 and HU22, solid UV-Vis absorption spectra for the ligands H₂cba, 1, 4-dib, and 4, 4′-dib were also determined (Fig.S5-S7). The test results showed that although the three ligands absorbed strongly in the ultraviolet region, they did not have significant absorption peaks in the visible region. Therefore, the UV-Vis absorption spectra of HU21 and HU22 should be attributed to π-π* electron transfer between the ligands and the metal centers. Additionally, HU21 and HU22 belong to semi-conducting materials with band gaps of 2.32 and 2.30 eV based on the Kubelka-Munk (K-M) method, respectively (Fig. 6b and 6d).

  Figure 6

  

  Figure 6.  UV-Vis absorption spectra (a, c) and bandgaps based on the K-M method (b, d) for complexes HU21 and HU22

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  2.3   Magnetic characteristic

  Considering that many Ni(Ⅱ)-based complexes have good magnetic behaviors, samples of complexes HU21 and HU22 were selected to do magnetic tests. Their variable temperature magnetic curves were obtained in a range of 2-300 K under a 1 000 Oe magnetic field, respectively (Fig. 7a and 7b). For HU21 and HU22, two Ni(Ⅱ) centers (Ni…Ni separations: 0.359 and 0.357 nm, respectively) are bridged by two carboxyl groups and a water molecule to form a dinuclear [Ni₂(CO₂)₂(H₂O)]²⁺ unit, which is further connected by cba^2- anion, 1, 4-dib and 4, 4′-dib ligands to construct a 3D framework, respectively. In these frameworks, the closest distances between the two dinuclear units [Ni₂(CO₂)₂(H₂O)]²⁺ are up to 1.696 and 2.06 nm, respectively, showing that the binuclear units are very isolated. The final magnetic results revealed that the χ_MT values of [Ni₂(CO₂)₂(H₂O)]²⁺ units were 2.04 cm³·mol^-1·K for HU21 and 2.02 cm³·mol^-1·K for HU22 at RT, respectively. The test values were very close to the theoretical values of the two uncoupled nickel ions (2 cm³·mol^-1·K)^[40-41]. While the temperature decreased, the χ_MT values remained roughly constant from 300 to 50 K for HU21 and 300 to 30K for HU22. And then, they decreased rapidly as the temperature decreased further. The minimum values at 2 K were 0.18 cm³·mol^-1·K for HU21 and 0.75 cm³·mol^-1·K for HU22. In the range of 10-300 K and 2-300 K, the magnetic susceptibility of HU21 and HU22 fitted well with the Curie-Weiss law. The fitting results showed that Curie constants for HU21 and HU22 were 2.13 and 2.08 cm³·mol^-1·K, and the corresponding Weiss temperatures were -5.25 and -0.79 K, respectively. The negative Weiss constants confirm that complexes HU21 and HU22 have antiferromagnetic features^[42]. Herein, similar magnetic properties between HU21 and HU22 are attributed to their similar structural features.

  Figure 7

  

  Figure 7.  Temperature dependence of χ_MT for HU21 (a) and HU22 (b)

  Inset: the Curie-Weiss fit curves.

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  3.   Conclusions

  Dicarboxylic acid ligand H₂cba and Ni(Ⅱ) ions assembled with 1, 4-dib and 4, 4′-dib to obtain two 3D complexes {[Ni(cba)(1, 4-dib)(H₂O)_0.5]·0.5H₂O}_n (HU21) and {[Ni(cba)(4, 4′-dib)(H₂O)_0.5]·0.5H₂O}_n (HU22), respectively. Both complexes HU21 and HU22 have structural characteristics of binuclear units, helical structures, and a six-connected framework, which are decided by the same carboxylic acid ligand and similar nitrogen heterocyclic ligands. Meanwhile, the difference in length from 1, 4-dib and 4, 4′-dib brings about some different structural details between HU21 and HU22. In addition, HU21 and HU22 have similar absorption capacity to light and anti-ferromagnetic behavior. This work further shows that the structure of the complex can be controlled through the selection of ligands and the expected functionalized CPs can be obtained.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Scheme 1  Synthetic routes for complexes HU21 and HU22

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  Figure 1  Coordination environment of Ni(Ⅱ) in complex HU21

  Ellipsoid probability level: 50%; Symmetry codes: a: 2-x, 1-y, 1-z; b: 1.5-x, 1.5-y, 1-z; c: 1-x, y, 0.5-z; d: 1.5-x, 1.5-y, -z.

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  Figure 2  Schematic illustrations of complex HU21: (a) right-handed helical chain; (b) left-handed helical chain; (c) 3D 1, 4-dib-Ni-(H₂O)_0.5-framework with helical nano-channels; (d) 3-fold-interpenetrating structure;(e) [Ni₂(H₂O)(cba)₂]_n-chain; (f) 3D structures based 1, 4-dib-Ni-(H₂O)_0.5-framework and [Ni₂(H₂O)(cba)₂]_n-chains

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  Figure 3  Coordination environment of Ni(Ⅱ) in complex HU22

  Ellipsoid probability level: 50%; Symmetry codes: a: 1-x, y, 1.5-z; b: 1.5-x, 1.5-y, 1-z; c: 1-x, y, 0.5-z; d: -x, 2-y, -z.

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  Figure 4  Structural illustrations of complex HU22: (a) right-handed helical chain; (b) left-handed helical chain;(c) 2D helical layer constructed by Ni(Ⅱ) centers, 4, 4′-dib ligands, and H₂O molecules; (d) 1D chain based on cba^2- anions, Ni(Ⅱ) centers, and H₂O molecules; (e) 3D framework; (f) six-connected topology network

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  Figure 5  PXRD patterns (a, b) and TGA curves (c, d) of complexes HU21 and HU22

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  Figure 6  UV-Vis absorption spectra (a, c) and bandgaps based on the K-M method (b, d) for complexes HU21 and HU22

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  Figure 7  Temperature dependence of χ_MT for HU21 (a) and HU22 (b)

  Inset: the Curie-Weiss fit curves.

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  Table 1.  Crystal data and structure refinement for HU21 and HU22

  Parameter	HU21	HU22
  Formula	C44H40N8O12Ni2	C56H48N8O12Ni2
  Formula weight	990.26	1 142.41
  Temperature/K	295.2	295.2
  Crystal system	Monoclinic	Monoclinic
  Space group	C2/c	C2/c
  a/nm	1.411 71(5)	1.587 64(10)
  b/nm	1.529 98(4)	1.794 16(11)
  c/nm	2.216 53(6)	2.060 16(12)
  β/(°)	107.551(3)	108.555(6)
  Volume/nm3	4.564 6(2)	5.563 3(6)
  Z	4	4
  Dc/(g·cm-3)	1.441	1.363
  μ/mm-1	0.895	0.743
  F(000)	2 048.0	2 328.0
  2θ range/(°)	7.714-60.684	7.332-60.372
  Reflection collected	58 031	35 170
  GOF	1.032	1.073
  Independent reflection	6 468	7 611
  Goodness-of-fit on F 2	1.032	1.073
  R1a [I>2σ(I)]	0.047 6	0.051 9
  wR2b [I>2σ(I)]	0.120 7	0.137 8
  Final R1 values (all data)	0.052 8	0.070 2
  Final wR2 values (all data)	0.122 7	0.147 5
  Largest diff. peak and hole/(e·nm-3)	970, -440	500, -280
  a R1=∑||Fo|-|Fc||∑|Fo|; b wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

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  Table 2.  Selected bond length (nm) and angles (°) for HU21 and HU22

  HU21
  Ni1—O1W	0.209 4(2)	Ni1—O2a	0.205 36(19)	Ni1—O1	0.204 82(19)
  Ni1—O4b	0.210 23(19)	Ni1—N1	0.209 0(2)	Ni1—N3	0.209 0(2)
  O1W—Ni1—O4a	89.86(7)	O2b—Ni1—O1W	94.41(7)	O2b—Ni1—O4a	175.73(8)
  O2b—Ni1—N(1)	89.39(8)	O2b—Ni1—N3	87.74(9)	O1—Ni1—O1W	88.84(7)
  O1—Ni1—O2b	91.78(9)	O1—Ni1—O4a	88.48(9)	O1—Ni1—N1	84.55(9)
  O1—Ni1—N	176.81(9)	N1—Ni1—O1W	172.48(7)	N1—Ni1—O4a	86.39(8)
  N1—Ni1—N3	92.29(9)	N3—Ni1—O1W	94.35(8)	N3—Ni1—O4a	91.76(8)
  Ni1b—O1W—Ni1	117.90(12)
  HU22
  Ni1—O2a	0.213 48(19)	Ni1—O5b	0.209 01(19)	Ni1—O1W	0.210 59(15)
  Ni1—O4	0.201 7(2)	Ni1—N1	0.207 0(2)	Ni1—N3	0.206 7(2)
  O5a—Ni1—O2b	176.12(8)	O5a—Ni1—O1W	93.86(7)	O1W—Ni1—O2b	89.57(7)
  O4—Ni1—O2b	85.68(8)	O4—Ni1—O5a	96.27(9)	O4—Ni1—O1W	87.25(7)
  O4—Ni1—N1	89.83(9)	O4—Ni1—N3	175.63(10)	N1—Ni1—O2b	86.76(8)
  N1—Ni1—O5a	89.89(9)	N1—Ni1—O1W	175.48(7)	N3—Ni1—O2b	90.33(9)
  N3—Ni1—O5a	87.82(9)	N3—Ni1—O1W	90.97(8)	N3—Ni1—N1	91.70(10)
  Symmetry codes: a: 1.5-x, 1.5-y, 1-z; b: 1-x, y, 0.5-z for HU21; a: 1-x, y, 0.5-z; b: 3/2-x, 3/2-y, 1-z for HU22.

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