Synthesis, characterization and properties of ruthenium-substituted polyoxometallic acid H 6 Ru(H 2 O)FeW 11 O 39 · 18H 2 O with Keggin structure

The ruthenium-substituted polyoxometallic acid H 6 [Ru(H 2 O)FeW 11 O 39 ] · 18H 2 O was synthesized by stepwise acidification and stepwise addition of solutions of the component elements, and an ion-exchange-cooling method. The product was characterized using inductively coupled plasma spectrometry (ICP), Infrared Spectroscopy (IR), Ultraviolet Spectroscopy (UV), and X-ray diffraction (XRD). The results show that this complex has the Keggin structure. The determination of the thermal stability and proton conductivity of this polyoxometallic acid was carried out by a thermogravimetric-differential thermal analysis (TG-DTA) and electrochemical impedance spectroscopy (EIS).

In recent years, polyoxometallic acids and their salts, which have a unique molecular structure and are easy to design and assemble, have been widely used as new and efficient catalysts, supramolecular materials, magnetic materials, photochromic materials, electrochromic materials, high-quality sub-conductors, selective electrodes, recording materials, and in gas equipment, drugs, and fuel cells [1][2][3][4][5][6][7][8][9]. Complexes containing the precious metal ruthenium have excellent reactivity and selectivity in many catalytic oxidations of organic compounds. Ruthenium complexes with anti-tumor activity have low toxicity. Few studies have investigated polyoxometallic acids and their salts containing ruthenium and have only recently attracted much interest [10], Therefore, research on the synthesis, characterization, properties, and applications of new polyoxometallic acids and their salts containing ruthenium is a major challenge. We report the synthesis, characterization, and properties of the new ruthenium-substituted polyoxometallic acid H 6 Ru(H 2 O)FeW 11 -O 39 ·18H 2 O in this paper.

Instruments and reagents
The purity of the copper powder was 200 meshes. RuCl 3 · nH 2 O with Ru ≥ 37% was used，purchased from Xi'an catalyst chemical Co., Ltd. All other reagents were of reagent grade，purchased from China medicine (Group) Shanghai chemical reagent company.
IR spectra were recorded on a Nicolet Nexus 470 spectrometer (America Thermo Nicolet Company) in the range 400 -4000 cm −1 using KBr pellets. Elemental analyses were carried out using an 8410 inductively coupled plasma (ICP) spectrometer (Australia Labtam Company). UV spectra were measured on a UV-2550 spectrophotometer (Japan Shimadzu Company) in the range 190 -400 nm. TG-DTA was carried out on a STA-409 thermal analyzer (Germany NETZSCH Instruments Manufacturing Co., Ltd) in a dynamic nitrogen atmosphere with a temperature increase rate of 10°C min −1 . Impedance measurements were performed on a VMP2 electrochemical impedance analyzer (America Princeton Applied Research Company) with copper electrodes over the frequency 0.01 Hz to 99.9 kHz. H 6 [Ru(H 2 O)FeW 11 O 39 ]·18H 2 O was pressed into tablets 10 mm in diameter and 3.74 mm in thickness under a pressure of 15 MPa at room temperature (22°C) and a relative humidity of 75%. A copper sheet was attached to each side of the tablet. The proton conductivity was measured using a copper | sample | copper cell.

Preparation and processing of raw materials
(1) Preparation of RuCl 3 solution. RuCl 3 ·nH 2 O (5.46 g) was dissolved in water in a 100 mL flask and diluted to the scale to give a 0.02 mol L −1 aqueous solution of RuCl 3 .
(2) Treatment of cation-exchange resin. The cationexchange resin was washed several times with deionized water until the pH value was the same as that of the deionized water, and then bottled. ( 055 mol) was dissolved in 100 mL of water and the pH of the solution was adjusted to 6.3 with acetic acid. The solution was then heated to boiling and a solution of Fe(NO 3 ) 3 ·9H 2 O (2.02 g; 0.005 mol) in 30 mL of hot water was added dropwise with stirring. After 30 min, 50 mL of a solution of RuCl 3 ·nH 2 O (1.37 g; 0.005 mol) was added dropwise. The pH was readjusted to 5.0 and stirring was continued for 1.5 h. After cooling, grayish brown oil was obtained by adding absolute ethanol. The oily product was extracted three times by the dissolving-cooling method. The obtained oil was dissolved in 80 mL of water and the solution was passed through an Amberlite IR-120 cationexchange column in H + form until the pH was less than 1. Finally, the solid acid was separated by the cooling method. The final yield was about 60.9%, based on W.

Elemental analysis
Ruthenium, iron, and tungsten were analyzed using ICP spectrometry. The water content was determined by thermogravimetry. Found: Ru, 3 [11].
In the IR spectrum of  [12].
Comparing the 4 characteristic bands of the IR spectra of H 6  shift to a higher wavenumber as the ionic radius increases from Al Ⅲ (0.05 nm) to Ru Ⅲ (0.077 nm) to In Ⅲ (0.081 nm).
In the high wavenumber region, there are two absorption peaks at 3448.16 cm −1 (v OH modes) and 1624.80 cm −1 (δ OH modes) associated with the OH modes (the stretching vibration v and bending vibration δ ) of the acidic hydroxyls and water molecules; the latter absorption peak indicates the presence of H 3 O + (Brønsted acid).

UV spectrum
Crystals of the 1:11 series of polyoxometallic acids H n Ru-   [13].
In the UV spectrum of the target complex (Figure 2), there are two characteristic bands: 200.00 nm, O d →W, and 262.00 nm, O b /O c →W.

X-ray powder diffraction
The X-ray powder diffraction pattern of H 6 [Ru(H 2 O)-FeW 11 O 39 ]·18H 2 O is shown in Figure 3, and X-ray powder diffraction data are listed in Table 2.
In each of the four 2θ ranges, i.e., 7°-10°, 16°-22°, 25°-30°, and 33°-38°, the patterns were similar to those of complexes with the Keggin structure [1]. The X-ray diffraction data, combined with IR and UV spectra, confirm that H 6 [Ru(H 2 O)FeW 11 O 39 ]·18H 2 O has the Keggin structure ( Figure 4).      the total percentage weight loss was 10.47%, which indicates that each product molecule has 19 molecules of water, and that there are three weight-loss steps. The first step is the loss of 16 molecules of hydration water, the second step is the loss of two molecules of protonated water, and the third step is the loss of one molecule of structural water.

Thermal analysis
Thus, the precise molecular formula of the product is (H 5

Conductivity
Electrochemical impedance spectroscopy is a frequency domain measurement method which measures the impedance over a very wide frequency range to study electrode systems. This is a better method than other conventional electrochemical methods because more information can be obtained. We recorded the results of complex impedance measurements for the polyoxometallic acid (the frequency range was 0.01 Hz to 99.9 kHz) at room temperature (Figure 6). The resistance of the product can be obtained by electrochemical impedance spectroscopy analysis of an equivalent circuit, and then converting to the corresponding conductivity: σ = L /(R × S) (L is thickness, S is cross-sectional area) [15].
The calculation shows that at room temperature (22°C), the conductivity of H 6 [Ru(H 2 O)FeW 11 O 39 ]·18H 2 O is 4.51 × 10 −3 S/cm, this shows that the product is a solid high-proton conductor.