Hybrid supramolecule of azolium-linked cyclophane: Synthesis,structure and thermostability

2.1 Reagents

All chemicals and solvents are of A.R. grade (≥99%) and used without further purification.

2.2 Materials and methods

L1 was synthesized according to the reported procedure [5–7, 12–14]. The IR spectra was measured on a Shimazu IR 435 spectrometer adopting KBr pellets in the scale of 400–4000 cm^–1. Element analyses of C, H and N were performed using a Perkin-Elmer 240 elemental analyzer.

2.3 Supramolecular syntheses and conformation behavior

Synthesis of {L1·(NH₄) [Mo₈O₂₆]·(H₂O)} (1)

Compound 1 was prepared by hydrothermal synthesis method. For compound 1, (NH₄)₆Mo₇O₂₄·4H₂O (0.062 g, 0.05 mmol), L1·Br₃ (0.1071 g, 0.15 mmol), CdI₂ and 20 mL H₂O were mixed well at room temperature, the mixture was stirred with small magnets for 1 hour. The mixture was constantly stirred and reacted at 120°C for 96 hours and then cooled to room temperature within 18 hours. Colorless transparent massive crystals of 1 were obtained with a yield of 30 %. IR (KBr, cm^–1): 3530.69(w), 3125.29(m), 3066.93(m), 1553.40(s), 1462.86(m), 1143.76(s), 725.82(s), 618.71(s), 570.73(w), 474.03(w), Anal. Calc: C, 52.19; H, 4.05; N, 18.04; Found: C, 52.3; H, 4.07; N, 18.1%.

2.4 X-ray crystallography study

The X-ray single crystal diffraction data of compound 1 was recorded on the Bruckner SMART CCD diffractometer with graphite-monochromatic Cu-Kα radiation (λ = 0.71073Å) at 293K. Data reduction and absorption correction were done using the SADABS software package. After absorption correction, employ SHELXTL-97, OLEX-2 and other packages for analysis. The main bond lengths of the crystallographic data of compound 1 are shown in Tables 1 and 2. The purity of compound 1 was studied by X-ray powder diffraction (PXRD).

3.1 Description of crystal structures

Crystal structure of compound {L1·(NH ₄ )[Mo ₈ O ₂₆ ]·2(H ₂ O)} (1) The conformation of compound 1 was fixed and were determined by single-crystal X-ray diffraction analyses. X-ray single crystal diffraction shows that compound 1 (Fig. 1a-b) was crystallized in monoclinic system with space group C2/c. Compound 1 consists of three parts: the organic cation L1³⁺, anion [-Mo₈O₂₆]^4–, NH⁺, and H₂O. As shown in Fig. 1a, the [Mo₈O₂₆]^4– anion is composed of eight [MoO₆] octahedran. As shown in Fig. 1b, the unit of [Mo₄O₁₃]^2– is approximately C_2h symmetrical, and each [Mo₄O₁₃]^2– unit is formed by four MoO₆ octahedrons through co-edges. As shown in Fig. 1a, the oxygen atoms of the [-Mo₈O₂₆]^4– can be classified into four types according to the form of bonding with Mo atoms: terminal oxygen, two bridge oxygen, triple bridge oxygen and Five bridge oxygen, the four types of Mo-O bond length range are: 1.696–1.714 Å, 1.752–2.271 Å, 1.951–2.357 Å, 2.119–2.452 Å. These bond length data indicate slight distortion of the MoO₆ octahedron. As shown in Fig. 1c-d, compare compound 1 with L1·Br₃·2H₂O¹, the structure of L1³⁺ is different from L1·Br₃·2H₂O (Table 3) [13e, 14e]. The angle between the two benzene rings in compound 1 is 0.731°. The dihedral angles between the imidazole rings are 26°, 51.95° and 26°. The distances between the C2 atoms of the imidazolium are 4.524, 4.864, and 4.524 Å. These findings indicate that the cations of compound 1 and L1·Br₃·2H₂O are not exactly the same, that is, their conformations are different and alterable to some extent. The complexation of L1 with polyoxometalate results in a cation shrinkage (∼0.03Å) in compound 1, which can be similarly described as a “breathing process”. A compound most similar to 1 is {L1·Ag(SCN)₄·(CH₃CN)·2H₂O} (2) (Fig. 1e-f) [1a]. Its smallest repeating unit consists of the organic cation L1, anion [Ag(SCN)₄]^3–, H₂O and CH₃CN. The distance of the benzene ring in the cation is 5.095 Å. The three imidazole rings are asymmetrically arranged, the angles between them are 52.41°, 75.18° and 52.41°, and the C2 distances on the three imidazole rings are 4.592 Å, 4.546 Å and 4.546 Å. The complexation of L1 with AgSCN results in a cation shrinkage (∼0.06Å) in compound 1, which can be similarly described as a “breathing process”. The complexation of metal halides and polymetallic with L1 leads to contraction of the cations, causing breathing behavior.

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Fig.1

(a) Structural unit of compound 1; (b) anion ([-Mo₈O₂₆]^4–) of compound 1; (c-d) the unit of compound 1 (All H atoms were omitted for clarity); (e) Structure unit diagram of 2. The H atoms were omitted for clarity; (f) The hydrogen bonding in compound 2.

Powder X-ray diffraction has been used to confirm the purity of the samples in the solid state. For compound 1 the as-synthesized PXRD patterns closely match the simulated patterns generated from the results of the single crystal diffraction data, indicative of pure products which were shown in Fig. 2. Simulation of the PXRD spectra was carried out by the single-crystal data and diffraction-crystal module of the Mercury (Hg) program available free of charge via the internet at http://www.iucr.org.

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Fig.2

Power X-ray Diffraction of compound 1.

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Fig.3

TG plot of compound 1.

To investigate thermostability of compound 1, the thermogravimetric analyses (TGA) experiments were performed up to 900°C in a flowing N₂ atmosphere, as shown in Fig. 2. Compound 1 is stable up to the temperature of around 315°C, indicating that it is a heat-resistant hybrid [15]. The TG curve of compound 1, which is mainly undergoing one stage of weightlessness. Compound 1 has the largest mass loss at 310–730°C. The biggest reason may be the volatile decomposition of organic cations. Therefore, the stability of compound 1 is relatively good. The stability of compound 1 may be due to the presence of intermolecular Van der Waals forces.