A polyoxometalate-based inorganic–organic hybrid material: synthesis, characterization structure and photocatalytic study

2.1 Crystal structure of the title compound

Single crystal X-ray structural analysis reveals that the compound consists of one [SiW₁₂O₄₀]⁴⁻ (abbreviated to SiW₁₂) polyoxoanion, two Ag cations and three bib molecules (Fig. 1). Similar to other Keggin anions, the SiW₁₂ polyoxoanion includes a SiO₄ tetrahedron surrounded by 12 WO₆ octahedra, which are grouped into four triads {W₃O₁₃} in edge-sharing mode. Oxygen atoms in the polyoxoanion are divided into four groups according to the different coordination environments: Ot (terminal oxygen atoms connecting to one W atom), Ob (oxygen atoms located in the corners shared by two W₃O₁₃ units), Oc (oxygen atoms connecting edge-sharing WO₆ octahedra in the W₃O₁₃ unit) and Oa (oxygen atoms connecting the center Si and W atoms of a W₃O₁₃ unit). The Si–O distances and relevant W–O bonds all fall in the normal ranges. The Ag centers adopt a linear coordination mode with two N atoms (N1 and N4) of different bib molecules. The bond lengths are 2.14(1) and 2.15(1) Å for Ag–N1 and Ag–N4, respectively. As a result, an isolated [Ag₂(bib)₃] subunit is formed (Fig. 1).

Fig. 1:

Ball-and-stick representation of the asymmetric unit of the title compound. All H atoms are omitted for clarity.

The Ag centers and POMs link each other forming chains (POM-Ag₂)_n with much longer and, hence, weaker contacts Ag–O14 3.02 Å and Ag–O20 3.00 Å as shown in Fig. 2. Further, the chains are thus connected via [Ag₂(bib)₃] subunits forming an inorganic–organic hybrid layer (Fig. 3). Finally, the layers are linked together along the b axis via short interactions between Ag and N4 forming the 3D supramolecular frameworks. From the topology view, if we assign the Ag centers and {SiW₁₂} clusters as 4-connected nodes, and the bib molecules as connectors, the structure can be rationalized as a 4-connected 2-nodal framework with mog topology (Fig. 4).

Fig. 2:

The inorganic chain formed by Ag centers and SiW₁₂ clusters.

Fig. 3:

Structure of a layer via Ag ions of inorganic chains from different finite units.

Fig. 4:

Topology of the 3D supramolecular structure in the title compound (yellow = Ag; orange = POM).

2.2 IR spectrum and TG analysis

The IR spectrum is shown in Fig. 5. Characteristic bands at 968, 922 and 782 cm⁻¹ are attributed to ν(W=O), ν(Si–O) and ν(W–O–W) vibrations, respectively. Bands in the region of 1635–1299 cm⁻¹ are attributed to the organic ligands. The TG curve (Fig. 6) exhibits a weight loss in the temperature range of 40–800 °C. The weight loss of 16.9 % at 40–740 °C corresponds to the loss of bib, agreeing well with the calculated value of 16.9 % for three bib molecules per empirical formula unit. This result further confirms the constitution of compound 1.

Fig. 5:

The IR spectrum of the title compound.

Fig. 6:

The TG curve of the title compound.

2.3 Photocatalysis properties

The photocatalytic properties of POMs have attracted much attention aiming at potential applications in purifying air and water [34, 35]. The introduction of transition-metal complexes as functional groups into POMs can enrich their potential applications. Herein, we investigated the photocatalytic activities of the new compound as a catalyst. The photodecomposition of rhodamine-B was evaluated under UV light irradiation in a typical process: 50 mg of the powder was distributed in 100 mL of a 2.0 × 10⁻⁵ mol L⁻¹ (C₀) rhodamine-B solution in a beaker by ultrasonic dispersion for 10 min. The mixture was continuously stirred in the dark for 0.5 h till it reached the surface-adsorption equilibrium on the particles. The suspension was stirred continuously under ultraviolet irradiation from a 250 W high-pressure Hg lamp. After 0, 30, 60, 90, 120, 150 and 180 min, 3 mL samples were taken out and subjected to several centrifugations to remove the catalyst. This way clear solutions were obtained which were subjected to UV/Vis analysis.

As shown in Fig. 7, after irradiating compound 1 for 180 min, the photocatalytic decomposition rate, defined as 1 − C/C₀, is 56.9 %. In contrast, the photocatalytic decomposition rate using insoluble (NBu₄)₄[SiW₁₂O₄₀] as a catalyst is 39.3 % after UV light irradiation of 180 min, which illustrates that the formation of an organic–inorganic hybrid compound based on POMs improves the photocatalytic performance of (NBu₄)₄[SiW₁₂O₄₀]. The enhanced photocatalytic activity may be due to the interaction between the [Ag₂(bib)₃] subunit and the SiW₁₂ clusters acting as a photosensitizer and promote the transition of electrons to the POMs. In order to demonstrate that the title compound is stable enough for catalytic cycles, the catalyst was separated from the reaction mixture after the first catalytic run, washed several times with alcohol and water to remove physisorbed molecules, dried and reused in another catalytic cycle. A series of catalytic experiments (56.9 %, 55.8 %, 56.0 %, 55.2 % for 1–4 rounds, respectively) has suggested that the title compound, as a catalyst, is stable and retains its catalytic activity. The result indicates that the title compound presents better degradation activity and may be a potential photocatalyst to decompose organic dyes.

Fig. 7:

Evolution of UV/Vis absorption spectra after 180 min of illumination for the photodegradation of rhodamine-B by (NBu₄)₄[SiW₁₂O₄₀] (left) and compound 1 (right).

3.1 General procedures

All reagents were purchased commercially and used without further purification. H4SiW12O40, AgNO3, NH4VO3, NaOH (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China), bib (Jinan Henghua Sci. & Technology, Co., Ltd, Shandong, China) were used as supplied. Elemental analyses were performed on a Perkin-Elmer 2400 CHN Elemental Analyzer (C, H and N) and a Leaman inductively coupled plasma spectrometer (Ag). The IR spectra were obtained on an Alpha Centaurt FT/IR spectrometer (Bruker corporation, Germany) with a KBr pellet in the 400–4000 cm−¹region. The TG analyses were performed on a Perkin-Elmer TGA7 instrument (PerkinElmer, USA) in flowing N2 with a heating rate of 10 °C min−¹. The UV/V is absorption spectra were recorded on a 756 CRT spectrophotometer (Shanghai Youke Instrument CO., Ltd., China).

3.2 Preparation of [Ag₂(bib)₃][H₂SiW₁₂O₄₀] (1)

A mixture of H₄SiW₁₂O₄₀ (298 mg, 1 mmol), AgNO₃ (150 mg, 1 mmol), bib (30 mg, 1.5 mmol), NH₄VO₃ (24 mg, 2.0 mmol) and H₂O (10 mL) was stirred for 1 h in air. After the pH value was adjusted to about 3.5 with 1 m NaOH, the mixture was sealed into a 20 mL Teflon-lined autoclave and heated to 160 °C in 100 min. The autoclave was kept at 160 °C for 4 days and then slowly cooled to room temperature. Red block-shaped crystals of 1 were filtered, washed with water and dried at room temperature (32 % yield based on W). – Elemental analysis for C₃₆H₃₂N₁₂Ag₂SiW₁₂O₄₀ (3722): calcd. C 11.60, H 0.85, N 4.51, Ag 5.80; found C 11.57, H 0.93, N 4.49, Ag 5.79 %.

3.3 X-ray structure determination

Crystal data for the title compound were collected on a Bruker SMART-CCD diffractometer with monochromatized MoK_α radiation (λ = 0.71069 Å) at 293 K. The structure was solved by the direct methods and refined by full-matrix least-squares methods on F² using the Shelxtl crystallographic software package [36, 37]. All non-hydrogen atoms were refined anisotropically. During the refinement, the command ISOR was used to restrain the non-H atoms with anisotropy displacement parameters (ADP) and non-positive definite (NPD) problems, which led to 135 restraints for 1. The command ISOR was used to refined atoms O7, O9, O21 and O22. Additionally, the restraint command DELU was used to average the displacement parameters of atoms with the same environments along the bond. In this way the atoms C1, C2, C3, C16, C17, C18, N1, N2 and N5 were refined. Finally, the restraint command SIMU was used to average the displacement parameters of atoms with the same environments (atoms C1, C2, C3, C16, C17, C18, N1, N2 and N5). The positions of hydrogen atoms on carbon atoms were calculated theoretically. Crystallographic data are given in Table 1.

CCDC 1034377 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.