English

• 1. INTRODUCTION

  The past few decades have witnessed the increasing interests in the metal organic hybrid materials not only because of their potential applications in adsorption, magnetism, catalysis, and photoluminescence sensitivity^[1-3], but also motivated by the structurally aesthetic topology such as inter-or self-penetration, rotaxane, catenane, etc^[4-7]. Developing new functional materials and understanding the formation mechanism are challenging tasks because the assembly of desirable ultimate structures is subtly affected by the coordination geometry of the central metals, the backbone characteristics of organic ligands, the metal and ligand ratio, the counterions and the pH of the reaction system^{[8, 9]}. Especially, a rational choice of organic ligands has a significant role in the assembly for functional materials. Compared with that rigid ligands, the flexible aromatic carboxylic acids with different space lengths can provide more structural possibility by bending the backbone or rotating the C–C bond to meet the coordination geometry of the central metal ions^[10]. Some interesting entangled structures have been successfully prepared through organic molecules with the flexible backbones containing O-X-O (X = CH₂, C₂H₄, C₃H₆, terephthalylidene, etc) or N-X-N (terephthalylidene)^[11-15]. Their fluorescent properties like detecting explosive nitro compounds or recognizing pollutant metal ions were studied and great efforts were focused on understanding the elusive host-guest interactions in these structures.

  The structures and functionalities can be well tuned in useful ways by the deliberate, controlled introduction of N-heterocycles neutral molecules into the crystal structure. 1, 6-bis(imidazole-1-yl)hexane, as a long flexible tethering ligand, can provide more possibility for the construction of unusual coordination frameworks^{[16, 17]}. As our continuous work and an attempt to understand the correlation between the structure and property, complexes 1 and 2 were successfully prepared under hydrothermal conditions with 4, 4΄-(methylenebis(oxy))dibenzoic acid and 1, 6-bis(imidazole-1-yl)hexane as coligands.

  2. EXPERIMENTAL

  2.1 Synthesis of complex 1

  CdCl₂⋅2.5H₂O (0.3 mmol, 0.068 g), H₂L (0.1 mmol, 0.044 g) and bimh (0.1 mmol, 0.028 g) were dissolved in DMF (6 mL). The mixture was then transferred into a 23 mL Teflon-lined stainless-steel vessel and heated to 140 ℃ for 2 days under autogenous pressure. After the mixture was cooled to room-temperature, colorless crystals suitable for X-ray diffraction analysis were obtained in pure phase, washed with ethanol, and dried at room temperature (75% yield based on Cd). Elem. Anal. Calcd. (%): C, 57.55; H, 5.01; N, 9.94. Found (%): C, 56.99; H, 5.28; N, 9.88.

  2.2 Synthesis of the complex 2

  Complex 2 was synthesized by the same procedure as described for 1 except the addition of CoCl₂∙6H₂O (0.3 mmol, 0.089 g) instead of CdCl₂⋅2.5H₂O (0.3 mmol, 0.068 g). Purple block-shaped crystals were obtained and collected in the same way as for complex 1. Elem. Anal. Calcd. (%): C, 57.55; H, 5.01; N, 9.94. Found (%): C, 57.48; H, 4.99; N, 9.91.

  2.3 Determination of the crystal structures

  The crystal structures were determined by single-crystal X-ray diffraction. Reflection data were collected on a Bruker SMART CCD area-detector diffractometer (Mo-Kα radiation, graphite-monochromator) at room temperature with an ω-scan mode. The structures were solved by direct methods using the SHELX structure solution program incorporated into Olex2 and refined by full-matrix least-squares on F² using the SHELXL program package^[18]. Empirical absorption corrections were applied with SADABS^[19]. All non-hydrogen atoms were refined with anisotropic displacement coefficients. The hydrogen atoms bound to carbon were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached. The selected bond lengths and bond angles for the title compound are listed in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1 and 2

  DownLoad: CSV

  1
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cd(1)–O(6)#1	2.227(4)		Cd(1)–N(3)	2.230(4)		Cd(1)–N(1)	2.238(4)
  Cd(1)–O(2)	2.278(4)		Cd(1)–O(1)	2.446(4)		Cd(1)–O(5)#1	2.535(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(6)#1–Cd(1)–N(3)	102.63(17)		O(6)#1–Cd(1)–N(1)	132.70(15)		N(3)–Cd(1)–N(1)	100.25(14)
  O(6)#1–Cd(1)–O(2)	106.12(15)		N(3)–Cd(1)–O(2)	131.64(15)		N(1)–Cd(1)–O(2)	87.81(14)
  O(6)#1–Cd(1)–O(1)	97.34(14)		N(3)–Cd(1)–O(1)	84.26(14)		N(1)–Cd(1)–O(1)	125.83(14)
  O(2)–Cd(1)–O(1)	54.36(14)		O(6)#1–Cd(1)–O(5)#1	53.17(13)		N(3)–Cd(1)–O(5)#1	94.21(14)
  N(1)–Cd(1)–O(5)#1	84.47(13)		O(2)–Cd(1)–O(5)#1	134.14(14)		O(1)–Cd(1)–O(5)#1	149.51(13)
  2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–O(6)#2	1.992(7)		Co(1)–O(2)	1.993(4)		Co(1)–N(1)	2.028(4)
  Co(1)–N(3)	2.040(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(6)#2–Co(1)–O(2)	104.7(2)		O(6)#2–Co(1)–N(1)	122.6(3)		O(2)–Co(1)–N(1)	108.53(19)
  O(6)#2–Co(1)–N(3)	93.6(2)		O(2)–Co(1)–N(3)	125.19(19)		N(1)–Co(1)–N(3)	103.22(17)
  Symmetry codes: (#1) –x+1, y–1/2, –z+1/2; (#2) –x+1, y+1/2, –z+1/2

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure description

  Single-crystal structural analysis reveals that complex 1 crystallizes in the orthorhombic crystal system, space group Pbca with 8 asymmetrical units in one unit cell. Each asymmetrical unit consists of one central Cd atom, one protonated L^2- ligand and one bimh ligand. As shown in Fig. 1, the central Cd atom has an octahedral coordination sphere, which consists of four carboxylate oxygen atoms from two L^2- ligands and two nitrogen atoms from two bimh ligands. The Cd–O/N bond lengths are in the range of 2.227(4)~2.535(4) Å, which are all within normal ranges^{[20, 21]}. The two carboxylate groups of L^2- ligand adopt a bidentate chelation mode to links two Cd centres, giving a Cd···Cd distance of 13.588 Å. The two benzene rings of the L^2- ligand are crossed with a dihedral angle of 77.29°. Notably, the twisted V-shaped L^2- links two neighbouring Cd atoms to form two kinds of right- and left-helical chains with a pitch of 17.313 Å along the b axis, which are further inter-connected with the tethering bimh ligand into a two-dimensional (2D) layer paralleled to the bc plane, as depicted in Fig. 2. On the basis of the above analysis, this 2D layer can be simplified as a 4-connected sql net with a (4⁴·6²) point symbol based on topos analysis (Fig. S1)^[22]. The potential voids formed by a 2D net incorporate with another two identical networks, thus giving a 3-fold interpenetrating nets (Fig. 3).

  Figure 1

  

  Figure 1.  Asymmetric unit of compound 1 (non-hydrogen atoms shown with 30% ellipsoid probability)

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  Figure 2

  

  Figure 2.  View of the 2D layer containing right- and left-helical chains

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  Figure 3

  

  Figure 3.  View of the three-fold interpenetrating architecture

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  3.2 IR spectrum

  IR spectra for 1 and 2 show a similar characterization (Fig. S2). The strong absorption peaks at 3126 and 3059 cm^-1 correspond to the =C-H stretching vibration of the phenyl or imidazole ring. The methylene -CH₂- asymmetrical and symmetrical stretching vibration peaks are found at 2931 and 2858 cm^-1. The absorption peaks at 1602, 1556, 1417, 1392 and 1354 cm^-1 can be ascribed to the asymmetrical and symmetrical stretching vibration of COO^- or C=C group.

  3.3 Thermal stability

  The two complexes are stable under ambient atmosphere at room temperature and insoluble in common polar and nonpolar solvents. The TGA measurement for 1 and 2 was carried out in the range of 30~650 ℃, as shown Fig. 4. Compounds 1 and 2 are stable up to 305 and 275 ℃, and then start to gradually lose their ligands as a result of thermal decomposition. The ultimate residuals are presumed to be CdO for 1 (calculate 20.82%, found 21.64%) and CoO for 2 (calculate 12.63%, found 13.24%), respectively.

  Figure 4

  

  Figure 4.  Thermogravimetric patterns in the temperature range of 30~650 ℃ for complexes 1 and 2

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  3.4 Luminescence behavior and sensing property

  The d¹⁰ configuration complexes have been attracting more attention because of their potential applications in chemical sensors and photochemical catalysis. Herein, the solid-state luminescent property of 1 was investigated and its emission spectra were recorded at room temperature. The bimh has a maximum emission peak at 451 nm when excited at 387 nm. As displayed in Fig. 5(a), the emission peak of H₂L ligand occurs at 365 nm. The maximum emission peak of 1 is found at 453 nm under 377 nm excitation, a significant red shift compared with that of the constituent ligands. This result indicates that the fluorescence of 1 may be attributed to the mixture effects of intraligand charge transmission and LMCT from carboxylate groups to Cd(II) ions^[23].

  Figure 5

  

  Figure 5.  (a) the Emission spectra of complex 1 excited at 377 nm. (b) Emission intensity of 1 with the addition of different volume of nitrobenzene

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  Inspired by the preceding work on luminescent sensing of us and other groups, we examined the fluorescent quenching behavior of nitrobenzene on complex 1. The emulsion was prepared by adding 9 mg fine grinding sample of 1 into 3 mL of DMF. As displayed in Fig. 5(b), the largest emission peak is shifted a small extent to 430 nm. The emission intensity of the suspension decreases steeply with the addition of nitrobenzene in the range of 0~40 uL. The quenching efficiency of nitrobenzene was obtained based on the Stern-Volmer equation I₀/I = 1 + K_sv[M], in which I₀ and I are the luminescence intensities of the DMF solvent of compound 1 before and after adding nitrobenzene, and [M] is the concentration of nitrobenzene^{[24, 25]}. As shown in

  Fig. S3, the quenching efficiency exhibits good linear correlations with the concentration of nitrobenzene, and the value of K_sv is estimated as 113 M^-1. The luminescence quenching behavior is probably attributed to the charge transfer from the electron-donating framework of 1 to the electron-withdrawing-NO₂ group in nitrobenzene^[26].

  4. CONCLUSION

  In summary, we successfully prepared two three-fold interpenetrating 3D transition metal coordination polymers with right- and left-helical chains. The photoluminescent property shows that complex 1 can serve as a promising fluorescent functional material.

  
  
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• 

  Figure 1  Asymmetric unit of compound 1 (non-hydrogen atoms shown with 30% ellipsoid probability)

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  Figure 2  View of the 2D layer containing right- and left-helical chains

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  Figure 3  View of the three-fold interpenetrating architecture

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  Figure 4  Thermogravimetric patterns in the temperature range of 30~650 ℃ for complexes 1 and 2

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  Figure 5  (a) the Emission spectra of complex 1 excited at 377 nm. (b) Emission intensity of 1 with the addition of different volume of nitrobenzene

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1 and 2

  1
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cd(1)–O(6)#1	2.227(4)		Cd(1)–N(3)	2.230(4)		Cd(1)–N(1)	2.238(4)
  Cd(1)–O(2)	2.278(4)		Cd(1)–O(1)	2.446(4)		Cd(1)–O(5)#1	2.535(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(6)#1–Cd(1)–N(3)	102.63(17)		O(6)#1–Cd(1)–N(1)	132.70(15)		N(3)–Cd(1)–N(1)	100.25(14)
  O(6)#1–Cd(1)–O(2)	106.12(15)		N(3)–Cd(1)–O(2)	131.64(15)		N(1)–Cd(1)–O(2)	87.81(14)
  O(6)#1–Cd(1)–O(1)	97.34(14)		N(3)–Cd(1)–O(1)	84.26(14)		N(1)–Cd(1)–O(1)	125.83(14)
  O(2)–Cd(1)–O(1)	54.36(14)		O(6)#1–Cd(1)–O(5)#1	53.17(13)		N(3)–Cd(1)–O(5)#1	94.21(14)
  N(1)–Cd(1)–O(5)#1	84.47(13)		O(2)–Cd(1)–O(5)#1	134.14(14)		O(1)–Cd(1)–O(5)#1	149.51(13)
  2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–O(6)#2	1.992(7)		Co(1)–O(2)	1.993(4)		Co(1)–N(1)	2.028(4)
  Co(1)–N(3)	2.040(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(6)#2–Co(1)–O(2)	104.7(2)		O(6)#2–Co(1)–N(1)	122.6(3)		O(2)–Co(1)–N(1)	108.53(19)
  O(6)#2–Co(1)–N(3)	93.6(2)		O(2)–Co(1)–N(3)	125.19(19)		N(1)–Co(1)–N(3)	103.22(17)
  Symmetry codes: (#1) –x+1, y–1/2, –z+1/2; (#2) –x+1, y+1/2, –z+1/2

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•