English

• 1. INTRODUCTION

  Over the past few years, the design and synthesis of coordination complexes are currently attracting considerable attention not only because of their fascinating architectures and topologies but also due to their potential applications in a number of fields, such as magnetism, molecular adsorption, molecular recognition, asymmetric catalysis, biological medicine, electric conductivity and photoluminescence^[1-5]. It is well known that novel structures can be successfully synthesized under hydrothermal conditions from some subunits, whereby several factors like metal ion, organic ligands, reaction temperature and coordinate bonding are important roles for the self-assembly processes^[6-8]. For this reason, a great many of scientific investigations are encouraged to synthesize plenty of coordination complexes with fascinating structures and properties in this field^{[9, 10]}.

  It is well-known that imidazole and its derivatives have been shown to be good building blocks in the design and syntheses of compounds^[11-13]. So far, a great many compounds with imidazole molecules have been designed and characterized^[13]. Particularly, recent study indicates that 2,2'-H₂biimidazole (L) is a good nitrogen and hydrogen donor to construct supramolecular hybrid materials for the ability to build hydrogen bonds in the assembly of coordination polymers^{[14, 15]}. As we have seen, many reported complexes based on L ligand are mononuclear and via hydrogen bonding interactions and π-π interactions layer or network supramolecular architectures are generated^[14-16]. In this article, two new coordination complexes [Cd(L)₂(1,2,3-HBTC)]·3H₂O (1) and {[Zn(L)(1,2-BDC)]·H₂O}_n (2) with L ligand as the first ligand were reported. Furthermore, the luminescent property of 1 was also studied in the solid state.

  2. EXPERIMENTAL

  2.1 Generals

  The commercially available reagents were used without further purification. Elemental analysis was measured on a Perkin-Elmer 240 CHN elemental analyzer. The powder X-ray diffraction (PXRD) data of the samples were collected on a Rigaku Dmax 2000 X-ray diffractometer with graphite-monochromatized CuKα radiation (λ = 0.154 nm) and 2θ ranging from 5 to 50°. The photoluminescent properties were measured on a FLSP920 Edinburgh Fluorescence Spectrometer.

  2.2 Synthesis of [Cd(L)₂(1,2,3-HBTC)]·3H₂O (1)

  1 was synthesized hydrothermally in a 20-mL Teflon-lined autoclave by heating a mixture of 1,2,3-H₃BTC (0.12 mmol), Cd(CH₃COO)₂·2H₂O (0.08 mmol) and L (0.12 mmol) in 9 mL of water at 135 ℃ for 96 hours. After the mixture had been cooled to room temperature at a rate of 10 ℃·h^–1, yellow rod shape crystals of 1 were collected in 41% yield based on cadmium. Anal. Calcd. for C₂₁H₂₂CdN₈O₉ (%): C, 39.24; H, 3.45; N, 17.43. Found (%): C, 39.01; H, 3.36; N, 17.30.

  2.3 Synthesis of {[Zn(L)(1,2-BDC)]·H₂O}_n (2)

  The preparation of 2 was similar to that of 1 except that 1,2-H₂BDC and Zn(CH₃COO)₂·3H₂O (0.08 mmol) were used instead of 1,2,3-H₃BTC (0.12 mmol) and Cd(CH₃COO)₂·2H₂O of 1 as well as 130 ℃ for 2 rather than 135 ℃ for 1. Pale yellow rod shape crystals of 2 were collected in 22% yield based on zinc. Anal. Calcd. for C₁₄H₁₂ZnN₄O₅ (%): C, 44.06; H, 3.17; N, 14.68. Found (%): C, 43.78; H, 3.14; N, 14.61.

  2.4 X-ray structure determination

  Crystals of 1 and 2 suitable for X-ray diffraction were chosen and mounted on a Rigaku RAXIS-RAPID diffractometer equipped with a graphite-monochromatized MoKα (λ = 0.71073 Å) radiation by using an ω-φ scan method at 293(2) K. The crystal structures were solved by direct methods with SIR2014 (Burla et al., 2014)^[17] and refined with SHELXL2018/3 (Sheldrick, 2015)^[18] by full-matrix least-squares techniques on F². The non-hydrogen atoms of the complex were refined with anisotropic temperature parameters. All H atoms were positioned geometrically (C–H = 0.93 Å) and refined as riding with U_iso(H) = 1.2U_eq(carrier).

  3. ESULTS AND DISCUSSION

  3.1 Description of the crystal structure

  Selected bond lengths and bond angles of 1 and 2 are given in Table 1, and hydrogen bond lengths and bond angles of 2 are listed in Table 2. The assymetric unit of 1 consists of one Cd(II) atom, two L ligands, one unique 1,2,3-HBTC anion, and three free water molecules. As illustrated in Fig. 1, the Cd(II) atom is five-coordinated by four nitrogen atoms from two L ligands and one oxygen atom from one 1,2,3-HBTC anion in a slightly distorted square-pyramidal geometry. Atoms N(1), N(2), N(5) and N(6) constitute the equatorial plane, while the O(1) is located on the vertex of the square-pyramid. The Cd–N bond lengths range from 2.279(5) to 2.322(5) Å, and Cd–O distance is 2.190(4) Å. It is noteworthy that one carboxylate of the 1,2,3-HBTC anion coordinates to the Cd(II) atom, whereas the remaining carboxylates are free. The L ligand chelates to the Cd(II) atom adopting a bis-chelating coordination mode. Because of the presence of aromatic L ligands, adjacent [Cd(L)₂(1,2,3-HBTC)]·3H₂O molecules are linked into a bimolecular structure through π-π stacking between the two imidazole rings of the L ligand with the centroid-to-centroid distance of 3.691(4) Å, face-to-face distance of 3.304(3) Å and the dihedral angle between the two planes being about 0.8(4)^o (The two imidazole rings are composed of N(5)/C(16)/C(17)/N(7)/C(18) and N(6)ⁱⁱⁱ/C(19)ⁱⁱⁱ/N(8)ⁱⁱⁱ/C(20)ⁱⁱⁱ/C(21)ⁱⁱⁱ, respectively; symmetry code: ⁱⁱⁱ 2–x, 1–y, 2–z) (Fig. 2). Additionally, N(7)– H(7)⋯O(2)ⁱⁱ and N(8)–H(8)⋯O(2)ⁱⁱ hydrogen bonds further stabilize the bimolecular structure of 1 (symmetric code: ⁱⁱ x, y–1, z, as seen in Table 2). As shown in Fig. 2, the bimolecular structures are formed into a one-dimensional chain structure by the interaction of six free water molecules with the adjacent molecules via hydrogen bonds (O(6)– H(6)⋯O(2W), O(1W)–H(1A)⋯O(3W)^iv, O(1W)–H(1B)⋯O(4), O(2W)–H(2A)⋯O(1W)^v, O(2W)–H(2B)⋯O(1W)^vi, O(3W)– H(3A)⋯O(5), symmetric codes: ^iv –x+1, y–1/2, –z+3/2; ^v x, –y+3/2, z–1/2, as shown in Table 2). Furthermore, the chain structures are linked into a two-dimensional supramolecular layer structure by N–H⋯O (N(3)–H(3)⋯O(4)ⁱⁱ, N(4)–H(4)⋯O(3)ⁱⁱ, symmetric code: ⁱⁱ x, y–1, z, as illustrated in Table 2) hydrogen-bonding interactions between the adjacent chain structures (Fig. 2).

  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)–N(1)	2.322(5)		Cd(1)–N(5)	2.279(5)		Cd(1)–O(1)	2.190(4)
  Cd(1)–N(2)	2.296(5)		Cd(1)–N(6)	2.309(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Cd(1)–N(5)	112.88(18)		N(5)–Cd(1)–N(6)	74.86(18)		N(2)–Cd(1)–N(1)	73.89(18)
  O(1)–Cd(1)–N(2)	99.50 (18)		N(2)–Cd(1)–N(6)	97.03(17)		N(6)–Cd(1)–N(1)	146.90(18)
  N(5)–Cd(1)–N(2)	147.44(19)		O(1)–Cd(1)–N(1)	101.59(18)
  O(1)–Cd(1)–N(6)	111.36(18)		N(5)–Cd(1)–N(1)	95.57(17)
  2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–N(1)	2.042(5)		Zn(1)–O(1)	1.956(4)		Zn(1)–O(4)i	1.972(4)
  Zn(1)–N(2)	2.079(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O(4)iii	96.20(17)		O(4)iii–Zn(1)–N(1)	123.87(19)		O(4)iii–Zn(1)–N(2)	114.77(18)
  O(1)–Zn(1)–N(1)	124.46(19)		O(1)–Zn(1)–N(2)	115.89(19)		N(1)–Zn(1)–N(2)	82.91(18)
  Symmetry transformation: 2 (i) x–1, y, z; (ii) –x, –y, –z–1; (iii) x+1, y, z

  Table 2

  

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

  DownLoad: CSV

  Crystal	D–H∙∙∙A	d(D–H)	d(H∙∙∙A)	d(D∙∙∙A)	∠DHA
  1	N(3)–H(3)∙∙∙O(4)ii N(4)–H(4)∙∙∙O(3)ii N(7)–H(7)∙∙∙O(2)iii N(8)–H(8)∙∙∙O(2)iii O(6)–H(6)∙∙∙O(2W) O(1W)–H(1A)∙∙∙O(3W)iv O(1W)–H(1B)∙∙∙O(4) O(2W)–H(2A)∙∙∙O(1W)v O(2W)–H(2B)∙∙∙O(1W)vi O(3W)–H(3A)∙∙∙O(5)	0.86 0.86 0.86 0.86 0.82 0.85 0.85 0.72 0.87 0.85	1.91 1.81 2.05 1.99 1.80 2.03 1.88 2.16 1.89 2.12	2.744(6) 2.653(6) 2.846(7) 2.796(7) 2.582(7) 2.810(8) 2.696(7) 2.830(7) 2.751(8) 2.879(8)	165 166 153 155 160 154 160 156 175 148
  	D–H∙∙∙A	d(D–H)	d(H∙∙∙A)	d(D∙∙∙A)	∠DHA
  2	N(3)–H(3A)∙∙∙O(2)iv N(3)–H(3A)∙∙∙O(4)iv N(3)–H(3A)∙∙∙O(1W) N(4) H(4A)∙∙∙O(2)iv	0.86 0.86 0.86 0.86	2.10 2.66 2.42 2.09	2.864(6) 3.160(6) 3.02(2) 2.853(6)	148 119 127 147
  Symmetry transformation: 1 (i) –x+2, y–1/2, –z+3/2; (ii) x, y–1, z; (iii) –x+2, –y+1, –z+2; (iv) –x+1, y–1/2, –z+3/2; (v) x, –y+3/2, z–1/2; (vi) –x+1, y+1/2, –z+3/2. 2 (i) x–1, y, z; (ii) –x, –y, z; (iii) x+1, y, z; (iv) –x, –y, –z

  Figure 1

  

  Figure 1.  View of the molecular structure of 1

  DownLoad: Full-Size Img PowerPoint

  Figure 2

  

  Figure 2.  View of the layer structure of 1 formed by π-π and hydrogen-bonding interactions

  DownLoad: Full-Size Img PowerPoint

  As can be observed in Fig. 3, there are one Zn(II) atom, one L ligand, one 1,2-BDC anion, and one free water molecule in the asymmetric unit. Each Zn(II) cation is four-coordinated by two nitrogen atoms from one L ligand (Zn(1)–N(1) = 2.042(5), Zn(1)–N(2) = 2.079(5) Å), and two oxygen atoms from two different 1,2-BDC anions (Zn(1)–O(1) = 1.956(4), Zn(1)– O(4)ⁱ = 1.972(4) Å, symmetric code: ⁱ 2–x, 1/2–y, 3/2–z). As seen in Fig. 4, the 1,2-BDC anions bridge two neighboring Zn(II) atoms, yielding a one-dimensional chain structure with the Zn⋯Zn distance of 7.408 Å along the a axis. The L ligands are located on one side of the one-dimensional chain through chelating Zn(II) atoms. Obviously, the L ligands play an important role in the construction of one-dimensional double chain. The conjugated L ligands are stacked with those of an adjacent chain through π-π interactions to generate a one-dimensional double-chain structure (N(1)/C(9)~C(10)/N(3)/C(11), N(2)/C(12)/N(4)/C(13)~C(14) at (–x, –y, –z), centroid-to-centroid distance 3.741(5) Å and face-to-face distance 3.519(3) Å, and dihedral angle of 1.7(4)^o) (Fig. 4). More interestingly, the C–H⋯π interactions have been observed in 2. It is the C–H⋯π interactions between the C(10) atom of the L ligand and the benzene ring (C(2)~C(7) at (x, y, –1+z)) of 1,2-BDC anion (2.800 Å and angle of 6.83^o, Fig. 4). When these interactions are taken into account, the one-dimensional double chains become a two-dimensional supramolecular layer (Fig. 4). Additionally, there exist N–H⋯O hydrogen bonds in 2, which further stabilize the two-dimensional layer structure of 1 (Table 2).

  Figure 3

  

  Figure 3.  View of the coordination environment of 2

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  Figure 4

  

  Figure 4.  View of the layer structure of 2 formed by π-π and C–H⋯π interactions

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  3.2 PXRD patterns and luminescent properties

  As depicted in Fig. 5, the experimental PXRD patterns of 1 well correspond to the simulated ones, indicating the phrase purity. The solid-state photoluminescent properties of 1,2,3-H₃BTC and 1 have been investigated in solid state at room temperature (Fig. 6). The emission spectrum of 1,2,3-H₃BTC shows emissions at 447 (λ_ex = 325 nm), which are probably attributable to the π*→n or π*→π transition^[19]. The L ligand shows an emission peak at about 425 nm^[20]. The spectrum of 1 shows two emission peaks at 445 and 538 nm upon excitation at 325 nm, which are red-shifted relative to 1,2,3-H₃BTC and free L ligand^[19]. The red-shifts of 1 might be attributed to the complexation of L ligand and 1,2,3-BTC with the Cd(II) atom^[21].

  Figure 5

  

  Figure 5.  Experimental and simulated PXRD patterns of 1

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  Figure 6

  

  Figure 6.  Solid state emission spectra of 1,2,3-H₃BTC and 1

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  3.3 Theoretical calculations

  All calculations in this work were carried out by the Gaussian09 program and with the parameters of the molecular structure from the experimental data of 2. Natural bond orbital (NBO) analysis^[22] was performed by density functional theory with the B3LYP^[23] hybrid functional and the LANL2DZ basis set^[24].

  The selected natural atomic charges and natural electron configuration for 2 are shown in Table 3. It is indicated that the electronic configurations of Zn(II) ion, O and N atoms are 4s^0.453d^9.964p^0.515s^0.014d^0.015p^0.01, 2s^{1.67, 1.68}2p^5.03~5.053p^0.01 and 2s^{1.36, 1.37}2p^{4.16, 4.17}3p^0.02, respectively. Based on the above results, one can conclude that the Zn(II) ion coordination with O and N atoms is mainly on the 4s, 3d and 4p orbitals. O and N atoms supply electrons of 3s, 2p and 3p to the Zn(II) ion and form coordination bonds. Therefore, according to valence-bond theory the atomic net charge distribution in the compound shows obvious covalent interactions between the coordinated atoms and Zn(II) ion^[22].

  Table 3

  

  Table 3.  Selected Natural Atomic Charges and Natural Electron Configuration of 2

  DownLoad: CSV

  Atom	Net charge	Electron configuration
  Zn(1)	1.059	[core]4s(0.45)3d(9.96)4p(0.51)5s(0.01)4d(0.01)5p(0.01)
  O(1)	−0.728	[core]2s(1.67)2p(5.05)3p(0.01)
  O(4)iii	−0.722	[core2s(1.68)2p(5.03)3p(0.01)
  N(1)	−0.552	[core]2s(1.37)2p(4.16)3p(0.02)
  N(2)	−0.557	[core]2s(1.36)2p(4.17)3p(0.02)
  Symmetry transformation: (iii) x+1, y, z

  As can be seen from Fig. 7, the HOMO and HOMO-1 are mainly composed of the d orbital of Zn(II) ion, p orbitals of two O and two N atoms coordinated with Zn(II) ion. The HOMO-2 are located largely on the Zn(II) ion and L ligand. The LUMO, LUMO+1 and LUMO+2 are mainly consisted of the π orbital of the 1,2-BDC anion.

  Figure 7

  

  Figure 7.  Frontier molecular orbitals of 2

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  Two new coordination complexes, [Cd(L)₂(1,2,3-HBTC)]·3H₂O (1) and {[Zn(L)(1,2-BDC)]·H₂O}_n (2), have been successfully synthesized by the reaction of various carboxylate ligands and 2,2'-H₂biimidazole under hydrothermal conditions. The elemental analyses have been investigated for 1 and 2. The fluorescence emission of 1 shows that the coordination complex may be a good candidate for photoactive materials. In addition, NBO analysis of 2 shows obvious covalent interaction between the coordinated atoms and Zn(II) ion.

  
  
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• 

  Figure 1  View of the molecular structure of 1

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  Figure 2  View of the layer structure of 1 formed by π-π and hydrogen-bonding interactions

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  Figure 3  View of the coordination environment of 2

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  Figure 4  View of the layer structure of 2 formed by π-π and C–H⋯π interactions

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  Figure 5  Experimental and simulated PXRD patterns of 1

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  Figure 6  Solid state emission spectra of 1,2,3-H₃BTC and 1

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  Figure 7  Frontier molecular orbitals of 2

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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)–N(1)	2.322(5)		Cd(1)–N(5)	2.279(5)		Cd(1)–O(1)	2.190(4)
  Cd(1)–N(2)	2.296(5)		Cd(1)–N(6)	2.309(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Cd(1)–N(5)	112.88(18)		N(5)–Cd(1)–N(6)	74.86(18)		N(2)–Cd(1)–N(1)	73.89(18)
  O(1)–Cd(1)–N(2)	99.50 (18)		N(2)–Cd(1)–N(6)	97.03(17)		N(6)–Cd(1)–N(1)	146.90(18)
  N(5)–Cd(1)–N(2)	147.44(19)		O(1)–Cd(1)–N(1)	101.59(18)
  O(1)–Cd(1)–N(6)	111.36(18)		N(5)–Cd(1)–N(1)	95.57(17)
  2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–N(1)	2.042(5)		Zn(1)–O(1)	1.956(4)		Zn(1)–O(4)i	1.972(4)
  Zn(1)–N(2)	2.079(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O(4)iii	96.20(17)		O(4)iii–Zn(1)–N(1)	123.87(19)		O(4)iii–Zn(1)–N(2)	114.77(18)
  O(1)–Zn(1)–N(1)	124.46(19)		O(1)–Zn(1)–N(2)	115.89(19)		N(1)–Zn(1)–N(2)	82.91(18)
  Symmetry transformation: 2 (i) x–1, y, z; (ii) –x, –y, –z–1; (iii) x+1, y, z

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

  Crystal	D–H∙∙∙A	d(D–H)	d(H∙∙∙A)	d(D∙∙∙A)	∠DHA
  1	N(3)–H(3)∙∙∙O(4)ii N(4)–H(4)∙∙∙O(3)ii N(7)–H(7)∙∙∙O(2)iii N(8)–H(8)∙∙∙O(2)iii O(6)–H(6)∙∙∙O(2W) O(1W)–H(1A)∙∙∙O(3W)iv O(1W)–H(1B)∙∙∙O(4) O(2W)–H(2A)∙∙∙O(1W)v O(2W)–H(2B)∙∙∙O(1W)vi O(3W)–H(3A)∙∙∙O(5)	0.86 0.86 0.86 0.86 0.82 0.85 0.85 0.72 0.87 0.85	1.91 1.81 2.05 1.99 1.80 2.03 1.88 2.16 1.89 2.12	2.744(6) 2.653(6) 2.846(7) 2.796(7) 2.582(7) 2.810(8) 2.696(7) 2.830(7) 2.751(8) 2.879(8)	165 166 153 155 160 154 160 156 175 148
  	D–H∙∙∙A	d(D–H)	d(H∙∙∙A)	d(D∙∙∙A)	∠DHA
  2	N(3)–H(3A)∙∙∙O(2)iv N(3)–H(3A)∙∙∙O(4)iv N(3)–H(3A)∙∙∙O(1W) N(4) H(4A)∙∙∙O(2)iv	0.86 0.86 0.86 0.86	2.10 2.66 2.42 2.09	2.864(6) 3.160(6) 3.02(2) 2.853(6)	148 119 127 147
  Symmetry transformation: 1 (i) –x+2, y–1/2, –z+3/2; (ii) x, y–1, z; (iii) –x+2, –y+1, –z+2; (iv) –x+1, y–1/2, –z+3/2; (v) x, –y+3/2, z–1/2; (vi) –x+1, y+1/2, –z+3/2. 2 (i) x–1, y, z; (ii) –x, –y, z; (iii) x+1, y, z; (iv) –x, –y, –z

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  Table 3.  Selected Natural Atomic Charges and Natural Electron Configuration of 2

  Atom	Net charge	Electron configuration
  Zn(1)	1.059	[core]4s(0.45)3d(9.96)4p(0.51)5s(0.01)4d(0.01)5p(0.01)
  O(1)	−0.728	[core]2s(1.67)2p(5.05)3p(0.01)
  O(4)iii	−0.722	[core2s(1.68)2p(5.03)3p(0.01)
  N(1)	−0.552	[core]2s(1.37)2p(4.16)3p(0.02)
  N(2)	−0.557	[core]2s(1.36)2p(4.17)3p(0.02)
  Symmetry transformation: (iii) x+1, y, z

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