Synthesis,structure and properties of two novel metallohelical compounds

2.1 Materials and methods

The bis(benzimidazole) ligand were synthesized follow the literature ways [14]. All chemicals used in the synthesis were of A.R. grade(≥99%) and used without further purification. Distilled water was used for all procedures. The IR spectra were measured on a Shimazu IR 435 spectrometer adopting KBr pellets in the scale of 400–4000 cm^-1. Powder XRD patterns were collected on a Philips X-pert X-ray diffractometer at a scanning rate of 4° min^-1 in the 2θ range from 6 to 53° with graphite monochromatized Cu-Kα radiation (λ= 0.15418 nm) with an X’ Celerator detector. UV-vis diffuse reflectance spectra (DRS) were recorded with the aid of a Cary 5000 UV-vis infrared spectrophotometer. Photoluminescent measurements of compounds 1-2 in the solid state were conducted on a HITACHI F-4600 spectrophotometer and the data were collected at room temperature.

2.2 Compounds syntheses

{[L₁Cd₂(SCN)₂(CH₂)₄(COO)₂(CH₃OH)₂]·2CH₃OH} (1). A mixture of ligand L₁ (0.0108 g), KSCN (0.003 g), Cd(NO₃)₂·4H₂O (0.0092 g) and adipic acid (0.0044 g) were dissolved in 4 mL methanol. Added ligand L₁ to the solution of metal salt, stired for 20 minutes, if there is precipitation, then filtered into the vial, sealed the bottle with plastic wrap, and placed in a quiet place. The rod crystals yield is 4%. Anal. Calc. (%) for 1: C, 52.79; H, 4.75; N, 10.99%. Found: C, 52.81; H, 4.71; N, 11.01%. FW = 1274.09. IR(KBr): 3168.4(s), 2099.4(s), 1545(m), 1457(s), 1281(m), 1146.8(w), 1014.8(m), 748.6(m) cm^–1.

{[L₁Cd(CH₂)₄(COO)₂]·CH₃CH₂OH}_n (2). ligand L₁ (0.0108 g), Cd(NO₃)₂·6H₂O (0.0154 g) and sodium acetate (0.0073 g) were dissolved in 3 mL methanol. Added ligand L₁ to the solution of metal salt, stired for 2 minutes, remove the magneton, then sealed the bottle with plastic wrap, and placed in a quiet plac. Anal. Calc. (%) for 2: C, 56.21; H, 5.03 N, 8.74%. Found: C, 56.02; H,4.99; N, 8.76%. FW = 641.01. IR(KBr): 3428.4(m), 2077.4(m), 1545(s), 1457(s), 1281(m), 1155.6(m), 1030.2(w), 748.6(s), 557.2(w) cm^–1.

2.3 X-ray crystallography study

Crystallographic data for the compounds were collected at 100(2) K on a Bruker APEX-II area-detector diffractometer equipped with graphite-monochromatized Mo-Kα radiation (wavelength: 0.71073Å). The structures were refined with full-matrix least-squares techniques on F² using the OLEX-2 program package. The main bond lengths of the crystallographic data of compounds 1-2 are shown in Tables 1-2.

We researched the adsorption property of compounds 1-2 on used the same organic dyes (MB MO RHB). The steps were carried out following in the literature methods [15].

2.5 Photocatalytic measurements of compounds 1-2

We researched the photocatalytic degradation property of compounds 1-2 on used the same organic dyes (MB MO RHB). The steps were carried out following in the literature methods [16].

3.1 Description of crystal structure

{·2CH₃OH} (1). The compound 1 crystallizes in the triclinic space group P-1 and the unit photograph contains two Cd(II) atoms, one L₁, two SCN anionics, one adipate ions and two methanol molecules (Fig. 1a). The unit image is like two dancing butterflies. Figure 1(b) displays the hydrogen bond diagram of compound 1, the lengths of N₄-H_4A…N₅ is 2.104 Å N₂-H_2A…O₄ is 1.970 Å, O₄-H_4B…O₂ is 1.943 Å. The central Cd atom coordinates with two N atoms (N₁, N₃), three O atoms (O₁, O₂, O₃) and one S₁ atoms to exhibit a contorted octahedron configuration. Hence 2D flat structure can be formed through hydrogen bonding (Fig. 1c). The key Cd-N bond length is in the range of 2.239Å–2.296Å, the important Cd-S bond is 2.682 Å, the vital Cd-O has length in the range of 2.384–2.362 Å, which comes into being a single spiral structure, as shown in Fig. 1(d).

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Fig.1

(a) The unit of compound 1. (b) Hydrogen bonding drawing in compound 1. (c) 2D Stacked picture of compound 1. (d) The single spiral stacked diagram of compound 1.

{[L₁Cd(CH₂)₄(COO)₂]·CH₃CH₂OH}_n (2) The compound 2 comprises one Cd(II) atom, one adipate ions, one L₁ and one lattice ethyl alcohol molecule(Fig. 2(a)). The unit cell parameters were a = 11.6609 (7) Å, b = 14.9534(11) Å, c = 16.2444(9) Å, and α= 90° β= 90° γ= 90°. Compound 2 is a 1D chain structure and compound 1 is a double nuclears structure, both of which can form 2D network structure through hydrogen bond, as shown in Fig. 1(c) and Fig. 2(c). Among the structure, Cd is presents in a six-coordination mode. The lengths of the Cd-N bonds rangs from 2.273 to 2.276 Å. The distances of the Cd-O bonds is within the range from 2.302 to 2.506 Å. Thus the coordinate pattern of Cd is the irregular octahedron in the unit picture. A compound most similar to 2 is compound 3. Compounds 2 and 3 are 1D chain structure. Complex 3 contains one Zn(II) atom, one L₁, and two terephthalate ion in the Fig. 2(e). The crystal system of compound 3 is orthorhombic and Pbc21 space group [14]. Zn atoms form irregular tetrahedrons with nitrogen atoms and oxygen atoms. Stacked picture of compound 3 in the Fig. 2(f). Comparison of some important parameters of compounds 2 and 3 are presented in the Table 3. Figure 2(b) and Fig. 2(c) shows the hydrogen bond picture for compound 2, the lengths of N₂-H_2A…O₅ is 1.884 Å, the lengths of N₄-H_4A…O₁ is 1.907 Å, the lengths of O₅-H₅…O₄ is 1.926 Å, which forms 2D network structure via hydrogen bonds liking many tug-of-war kids. A stacked view of compound 2 as in shown in Fig. 2(d).

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Fig.2

(a) Structural unit diagram of compound 2. (b) Hydrogen bond drawing of compound 2. (c) 2D structure formed by hydrogen bonds. (d) Stacked diagram of compounds 2. (e) Structural unit diagram of compound 3. (f) Stacked diagram of compounds 3.

Powder X-ray diffraction (PXRD) patterns were recorded using Cu-Kα radiation on a PAN analytical X’ Pert PRO diffractometer (Fig. 3). The purity of compounds 1-2 was studied by X-ray powder diffraction (PXRD). The as-synthesized PXRD patterns closely match the simulated patterns generated from the results of the single-crystal diffraction data, indicative of pure products.

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Fig.3

Power X-ray Diffraction of compounds 1-2.

The solid state UV-vis absorption spectra of compounds 1-2 are shown in Fig. 4. The electron spectra of 1-2 show similar absorbance with distinct peaks in the 250–300 nm range, due to O, N⟶Cd charge transition (LMCT) and inorganic to organic partial charge transition. Their absorption peaks are slightly different; It may be due to its different coordination environment.

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Fig.4

The UV absorption spectrum of compounds 1-2.

We researched the photoluminescence emission spectra of compounds 1-2 (at room temperature) (Fig. 5). the compound 1 shows two main strong peak at 407 nm and 428 nm upon excitation at 306 nm. It could be that the molecule contains p-p conjugated double bonds, which are able to emit strong fluorescence. The luminescence of compound 2, which displays two main peak at 409 and 429 nm during excitation at 306 nm; These emisions are likely to put down to φ*-φ transition of ligand [17]. Whereas, the intensity of compound 2 is lower than that compound 1 the probable causes is that long-distance hydrogen bonding in the molecular increases the distance of the electron transition, causing the energy to dissipate and resulting in a decrease in fluorescence degree [18, 19].

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Fig.5

Emission spectra of compounds 1-2.

The adsorption spectra of compounds 1-2 are showed in Fig. 6-7. MB can be adsorption by compound 1 effectively, RHB second, it is possible that KSCN ion forms a hyperconjugated structure with benzene ring, which enhances electron delocalization. Compound 2 has better adsorption property to organic dye MO, What matters most is that these compounds have pore structure and reinforce their adsorption nature. Compound 1 show relatively better decomposition performance for MB, MO and RHB.

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Fig.6

UV absorption spectrum curves of compound 1 blank control test in organic dyes solution for various adsorption times. (a-b) adsorption of MB solution, (c-d) MO solution, (e-f) RhB solution.

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Fig.7

UV absorption spectrum curves of compound 2 and blank control test in organic dyes solution for various adsorption times. (a-b) adsorption of MB solution, (c-d) MO solution, (e-f) RhB solution.

Figure 8 shows the UV-vis absorption spectra of compounds 1-2. For these compounds, the higher energy absorption peaks are 272 nm, which should be initially rooted in the absorption peak of intraligand. It also manifests that the peaks are generated by φ-φ* transition of the ligand. For the sake of study the conductivity of the five compounds, their band gap (Eg) are acquired by the measurements of diffuse reflectance. The band gap energies were evaluated to be 1.70, 1.683 eV for 1-2. They are display that 1-2 has feaible semiconductor peculiarity as underlying photocatalysts [20]. In addition, they have the latent capacity for election sunlight harvesting for the maximum photon flux of sunlight is similar and small [21, 22].

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Fig.8

Band gap diagram of compounds 1-2.

It can be seen from the Fig. 9-10 that compound 1 has better catalytic effect on MB, compound 2 has better catalytic performance on MO and RHB.

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Fig.9

The insets are photos of the corresponding solutions at various times. (a-c) photocatalysis of MB solution (a), MO solution (b) and RhB solution (c) with the use of compound 1 and the control experiment without any catalyst.

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Fig.10

The insets are photos of the corresponding solutions at various times. (a-c) photocatalysis of MB solution (a) MO solution (b) and RhB solution (c) with the use of compound 2 and the control experiment without any catalyst.