Anderson-Type Polyoxotungstate with Central Ruthenium, [Ru^IVW₆O₂₄]^8–: Synthesis, Structure, Redox Studies, and Electrochemical Water Oxidation Activity

Abstract

The reaction of Na₂WO₄·2H₂O with [(benzene)­RuCl₂]₂ at temperatures above 70 °C produced an Anderson-type ruthenotungstate, Na₈[Ru^IVW₆O₂₄], together with (benzene)­Ru-tungstates, [{(benzene)­Ru}₄W₂O₁₀] and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀]. The first ruthenium-containing Anderson-type polyoxometalate was isolated by recrystallization and characterized using single-crystal X-ray diffraction (XRD), elemental analysis, powder XRD, infrared (IR) spectroscopy, thermogravimetry-differential thermal analysis (TG-DTA), X-ray absorption fine structure (XAFS), and cyclic voltammetry. Notably, Na₈[Ru^IVW₆O₂₄] could only be synthesized using [(benzene)­RuCl₂]₂; other ruthenium sources, such as RuCl₃, Ru­(DMSO)₄Cl₂, Ru­(acac)₃, and K₂RuCl₅(H₂O), did not yield Na₈[Ru^IVW₆O₂₄] under the same conditions. Cyclic voltammetry measurements revealed that the Ru^IV ions in Na₈[Ru^IVW₆O₂₄] undergo a quasi-reversible one-electron redox process, Ru^IV/III. Furthermore, Na₈[Ru^IVW₆O₂₄] demonstrates catalytic performance in water oxidation reactions, significantly outperforming the catalytic activity of Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] and [{(benzene)­Ru}₄W₂O₁₀] complexes, highlighting its potential as an efficient catalyst for energy conversion processes.

Introduction

Polyoxometalates (POMs) are a distinctive class of negatively charged molecular clusters made up of early transition-metal atoms such as tungsten (W^VI), molybdenum (Mo^VI), and vanadium (V^V) coordinated by oxygen atoms. − These clusters display a broad spectrum of structural and compositional variations, which have garnered interest across multiple scientific fields including materials science, catalysis, − sensing, biomedical science, , magnetism, coordination chemistry, and staining reagents. −

In particular, ruthenium-containing polyoxometalates have emerged as a major focus of research because of the distinctive redox and catalytic characteristics of Ru, leading to numerous applications in catalysis. , Inspired by the elegant work of Prof. Pope on [PW₁₁O₃₉Ru­(H₂O)]^4–, we have been engaged in the synthesis of Ru-containing polyoxometalates, with particular emphasis on elucidating their redox properties and reactivity with organic substrates. Noteworthy examples include the development of mono-Ru-substituted Keggin-type and Dawson-type heteropolytungstates, tetra-Ru-sandwiched silicotungstate, and (benzene)­Ru-containing polyoxometalates, , and we are interested in making a new one.

Despite numerous reports on Ru-containing polyoxometalates and the ability of most transition-metal cations, including noble metals such as Ir^IV, Rh^III, and Pd^IV, to form Anderson-type POMs, the formation of Ru-containing Anderson-type POM remains unreported.

The Anderson-type polyoxoanion displays a planar structure comprising six edge-sharing octahedra, such as MoO₆ or WO₆, which form an encircling framework around a central heteroatom (XO₆) in an octahedral configuration and closely approach D _3d symmetry (Figure ). − The general formula is given as [H _y (XO₆)­M₆O₁₈]^n–, where y ranges from 0 to 6, n spans from 2 to 8, M stands for the addenda atoms (Mo^VI or W^VI), and X represents the central heteroatom (Figure ). The physical and chemical characteristics of Anderson-type polyoxometalates are intricately influenced by the specific properties of the heteroatom, the choice of countercation, and the nature of the organic functionalization, each contributing to their distinct behavior and functionality. , They have been extensively applied in a variety of fields, with notable uses in the development of nanomaterials, catalysis, ,, photocatalysis, electrochemical sensors, , magnetism, and biomaterial properties. ,

1.

(a) Ellipsoidal (probability 50%) and (b) polyhedral representation of Anderson-type [Ru^IVW₆O₂₄]^8–, and (c) polyhedral representation of a unit cell. Green, pink, red, and gray balls represent W, Ru, O, and Na atoms, respectively. The green and pink octahedra represent the W–O₆ and Ru–O₆ octahedra.

In our recent work, we reported the synthesis of several polyoxotungstates containing [(benzene)­Ru]²⁺ moieties, such as [{(benzene)­Ru}₄W₂O₁₀] and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀]. These compounds were successfully synthesized by reacting [(benzene)­RuCl₂]₂ with Na₂WO₄·2H₂O in water at room temperature or 80 °C. Herein, we present an Anderson-type compound with a central Ru atom, Na₈[Ru^IVW₆O₂₄], which was prepared by heating the reaction mixture to a temperature above 70 °C until all the solvents were removed, followed by recrystallization (Scheme ). This represents the first example of an Anderson-type molecule incorporating Ru atoms, which was thoroughly characterized using single-crystal X-ray diffraction (XRD), powder XRD, infrared (IR) spectroscopy, thermogravimetric differential thermal (TG-DTA) analysis, elemental analysis, X-ray absorption fine structure (XAFS), and cyclic voltammetry. We also demonstrated that [(benzene)­RuCl₂]₂ is the exclusive Ru source required to generate the targeted [Ru^IVW₆O₂₄]^8–. Furthermore, we demonstrate that Na₈[Ru^IVW₆O₂₄] exhibits catalytic activity for water oxidation.

1. Formation of Anderson-Type Na₈[Ru^IVW₆O₂₄], [{(benzene)­Ru}₄W₂O₁₀], and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] by the Reaction of Na₂WO₄ and [(benzene)­RuCl₂]₂ .

Experimental Section

Materials and Methods

All chemicals were commercially available reagent-grade compounds and were used as received without any additional purification. Deionized water, obtained from a homemade Elix Essential UV3 system (Merck), was used in all of the experiments.

Synthesis of Na₈[Ru^IVW₆O₂₄], [{(benzene)­Ru}₄W₂O₁₀], and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀]

Na₂WO₄·2H₂O (60 mg, W: 0.181 mmol) and [(benzene)­RuCl₂]₂ (5.9 mg, Ru: 0.024 mmol) were dissolved in a 20 mL vial with a screw cap (SV-20, Nichiden-Rika Glass Co. Ltd., Japan) containing a mixed solution of 0.50 mL of H₂O and 0.50 mL of C₂H₅OH. The mixture was heated at 90 °C for 24 h in a metal bath with a closed cap. Subsequently, the resulting deep-brown solution was left overnight at 90 °C without a cap to allow the solvents to evaporate. The solid was then cooled to room temperature, and 0.30 mL of H₂O was added, resulting in a light-turbid solution. The solution was allowed to stand until it became clear, and the final pH was 8.0. After 10 min, brown plate-shaped crystals of [{(benzene)­Ru}₄W₂O₁₀] were filtered out, washed with C₂H₅OH and H₂O, and dried in open air for 2 h. The yield of [{(benzene)­Ru}₄W₂O₁₀] was 2.0 mg (27% based on Ru content). The structure of [{(benzene)­Ru}₄W₂O₁₀] was confirmed by IR and powder XRD.

The clear deep-brown solution was left to crystallize in an open vial, and dark-green block-shaped crystals of Na₈[Ru^IVW₆O₂₄] appeared after 1 day. The Na₈[Ru^IVW₆O₂₄] crystals were promptly filtered off on the same day, washed with a small amount of H₂O, and then dried in open air for 2 h. The final yield of Na₈[Ru^IVW₆O₂₄] was 16.5 mg (31% based on the Ru content).

The remaining deep-brown filtrate was then left to crystallize further in an open vial. After 2 days, yellow needle-shaped crystals of Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] were formed. The crystals were promptly filtered off on the same day, washed with C₂H₅OH, and air-dried for 2 h. The final yield of Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] was 11 mg (38% based on the Ru content). The structure of Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] was confirmed by IR and powder XRD.

Na₈[Ru^IVW₆O₂₄]·27H₂O, IR spectrum (KBr pellet, cm^–1): 3439 (s, br.), 2126 (w, br.), 1649 (m), 908 (s), 859 (s), 716 (w, br), 630 (s), 575 (s), 459 (s). Cyclic voltammogram (CV): E _1/2 (Ru^IV/III) = 361 mV and ΔE (Ru^IV/III) = 253 mV vs Ag/AgCl (3 M NaCl) in 0.1 M KOAc buffer solution (0.1 M of KOAc and 0.1 M of AcOH were mixed, pH 4.0) at a scan rate of 25 mV s^–1. Elemental Anal. calcd for Na₈[Ru^IVW₆O₂₄]·27H₂O: Na 8.14; Ru 4.48; W 48.8; H 2.4; C 0; Cl 0%. Found: Na 8.15; Ru 4.66; W 48.8; H 2.4; C 0.11; Cl < 0.1%. TG analysis revealed a total weight loss of 19% up to 1100 °C, corresponding to the loss of 23 water molecules (Figure S1).

Characterization

Cyclic voltammetry was performed using a CHI620D potentiostat (BAS Inc.) under ambient conditions. The electrochemical cell was equipped with a 3 mm diameter glassy carbon working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode (with a potential of 197 mV relative to the normal hydrogen electrode at 25 °C in a 3 M NaCl solution, BAS Inc.). Prior to the measurements, the glassy carbon electrode was polished with alumina powder. The formal potential values (E _1/2) and peak separation (ΔE) were extracted by analyzing the cathodic and anodic peak potentials. pH measurements were performed using a HORIBA pH/Cond Meter 0–54 (Model HORIBA, Japan). Fourier-transform infrared (FT-IR) spectra were obtained using a JASCO 4X FTIR spectrometer in KBr pellet form at room temperature. Thermogravimetric (TG) and differential thermal analyses (DTA) were conducted using a Hitachi SII TG/DTA7300, with a constant nitrogen flow rate of 200 mL/min and a heating rate of 10 °C/min. Elemental analysis was performed by the Mikroanalytisches Labor Pascher (Remagen, Germany). Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D2 Phaser using Cu Kα radiation (λ = 1.54184 Å) and a 1D Lynxeye detector at room temperature. Powder XRD simulation was done using CCDC Mercury program with standard simulation parameters including Cu Kα radiation wavelength (λ = 1.54056 Å), degree with a steep (0.02 degree), the broadness of the peak measured as fwhm (0.1 degree). X-ray absorption fine structure (XAFS) measurements at the Ru K-edge were performed at the BL01B1 beamline in SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan). The XAFS spectrum of Na₈[Ru^IVW₆O₂₄] was collected in transmission mode at 10 K to suppress thermal vibrations, with the sample pellets mounted in a copper holder attached to a cryostat. The XAFS data of the reference samples (Ru foil, Ru­(acac)₃, RuO₂, and SrRuO₃) were measured at room temperature. Data reduction was performed using xTunes software. X-ray absorption near-edge structure (XANES) spectra were obtained by normalizing the XAFS data at the edge height. The k ³-weighted extended X-ray absorption fine structure (EXAFS) in the range of 3–16 Å was Fourier-transformed. The local structures of the samples were analyzed by curve-fitting analysis in the range of 1.2 ≤ r ≤ 1.8 Å for Ru–O bonds and 2.6 ≤ r ≤ 3.5 Å for Ru–W bonds.

In Situ Yield Estimation Using Cyclic Voltammograms

Cyclic voltammetry measurements of Na₈[Ru^IVW₆O₂₄] in a 0.1 M KOAc solution at pH 4.0 revealed a quasi-reversible redox couple at 361 mV, whereas [{(benzene)­Ru}₄W₂O₁₀] and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] did not show redox signals under the same conditions (Figure S2). Furthermore, only Ru-substituted polyoxometalates showed well-defined redox couples at more than 200 mV. − , To calculate the in situ yield of [Ru^IVW₆O₂₄]^8–, the oxidation peak current of Ru­(IV/III) in the reaction mixture (roughly 66 mg of solid) dissolved in 5 mL of 0.1 M KOAc (pH 4.0) was compared to that of Ru­(IV/III) in the isolated [Ru^IVW₆O₂₄]^8– (approximately 2 mg) dissolved in 1 mL of 0.1 M KOAc (pH 4.0). It was assumed that the anodic peak current (I _pa) was linearly related to the concentration of the solution, as outlined in eq :

$${\mathit{I}}_{\mathrm{pa}}=2.69\times {10}^5{\mathrm{n}}^{3/2}\mathrm{A}\mathrm{C}{\mathrm{D}}^{1/2}{\mathrm{v}}^{1/2}$$ 1

where n is the number of electrons (n = 1), A is the electroactive surface area of the electrode (A = 0.07 cm²), C and D are the concentration and diffusion coefficient of [Ru^IVW₆O₂₄]^8–, respectively, and v is the scan rate (0.025 V s^–1).

Procedure for Electrocatalytic Water Oxidation

Carbon paste (35 mg, BAS, Carbon Paste Oil Base) was finely ground with Na₈[Ru^IVW₆O₂₄] (6 mg, 15 wt %, 0.0027 mmol), and the resulting paste was placed into a cylindrical electrode (diameter: 3 mm) and polished with glassine weighing paper until a smooth, even surface was achieved. [{(benzene)­Ru}₄W₂O₁₀]/carbon paste, Na₆[{(benzene)­Ru}₂H₂W₈O₃₀]/carbon paste, Cs₅[SiW₁₁O₃₉Ru­(H₂O)]/carbon paste, and RuO₂/carbon paste were prepared using a constant molar quantity of 0.0027 mmol of Ru.

The electrolyte was a potassium phosphate buffer prepared by mixing 0.5 M KH₂PO₄ and 0.5 M KOH at pH 8.0. The potential scan was carried out toward the positive potential from −300 mV vs Ag/AgCl at a rate of 25 mV s^–1, and the overpotential was measured at 10 mA cm^–2. Controlled-potential electrolysis studies were conducted at a constant potential of +1200 mV (vs Ag/AgCl) in undivided and divided cells (VB12A, EC Frontier, Japan). The potentials, recorded with respect to Ag/AgCl, were adjusted to the reversible hydrogen electrode (RHE) using the Nernst equation: E _RHE = E _Ag/AgCl + 0.0591 × pH + 0.197. The headspace gases of the undivided cell were analyzed by gas chromatography (Shimadzu, GC-8A, TCD detector, column: Molecular Sieve-5A, 60–80 mesh) to estimate the amount of O₂ and H₂ generated.

X-ray Crystallography

A suitable single crystal was suspended in mineral oil and mounted on a goniometer head under a constant nitrogen flow. Intensity data were collected at −150 °C on a Bruker SMART APEXII diffractometer equipped with a CCD area detector and Mo Kα radiation (λ = 0.71073 Å) monochromated by multilayered confocal mirrors. Data reduction and integration were carried out using the Bruker APEX3 suite, and multiscan absorption correction (SADABS) was applied. The initial structure was solved with SHELXT and was subsequently refined with the SHELXL program running on a ShelXle interface. Hydrogen atoms of water molecules were located from the difference map; the O–H distances and the H···H distance (i.e., the H–O–H bond angle) were restrained to 0.84 Å with a DFIX command and 1.34 Å with a DANG command, respectively. All non-hydrogen atoms were refined in an anisotropic manner. The detailed crystallographic data are summarized in Table S1 in the Supporting Information.

Bond Valence Sum (BVS) Calculations

The BVS values were calculated using equation for the variation of the length r _ij of a bond between two atoms i and j in the observed crystal with valence V _i

$$V_i=\sum _j\mathit{exp}\left(\frac{{r^{\prime }}_0-r_{ij}}{B}\right)$$ 2

where B is a constant equal to 0.37 Å and r′₀ is the bond valence parameter for a given atom pair; r′₀ = 1.921 Å for W⁶⁺–O^2– and 1.834 Å for Ru⁴⁺–O^2– pairs were used to calculate BVS.

Results and Discussion

Formation and Characterization of Na₈[Ru^IVW₆O₂₄]

We have previously demonstrated that mixing Na₂WO₄·2H₂O with [(benzene)­RuCl₂]₂ in a 5:1 W:Ru ratio in water at room temperature or 90 °C led to the formation of two (benzene)­Ru-containing polyoxotungstates, namely, [{(benzene)­Ru}₄W₂O₁₀] and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀]. We discovered that dark-green, block-shaped crystals of Na₈[Ru^IVW₆O₂₄] were produced when the mixture was heated to 80 °C for solvent evaporation and subsequent crystallization (Scheme ). We found that [{(benzene)­Ru}₄W₂O₁₀] was removed first, then the desired Na₈[Ru^IVW₆O₂₄] was crystallized, and finally, Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] was crystallized (experimental).

Single-crystal X-ray diffraction analysis revealed that the dark-green crystal possessed an Anderson-type structure, Na₈[Ru^IVW₆O₂₄]·26H₂O, which crystallized in the monoclinic space group P2₁/n (14) (Table S1). This structure is similar to that of the Pt^IV complex, Na₈[Pt^IVW₆O₂₄]·26H₂O (P2₁/n, a = 10.8730(10) Å, b = 11.7850(10) Å, c = 18.1160(10) Å, β = 93.560(10)°, V = 2316.87 ų, Z = 2) and the Ir^IV complex Na₈[Ir^IVW₆O₂₄]·26H₂O (P2₁/n, a = 10.7874(2) Å, b = 11.7736(2) Å, c = 17.9983(3) Å, β = 93.574(2)°, V = 2281.46(7) ų, Z = 2). The individual molecule is composed of a ring of six edge-shared WO₆ octahedra encircling the central RuO₆ unit (Figure ), and we found eight Na⁺ without disorder (Figure S3). Na₈[Ru^IVW₆O₂₄]·26H₂O is the first Anderson-type heteropolytungstate with an octahedral Ru cation at its core, with Ru–O bond lengths varying between 1.991(2) and 2.009(3) Å (Table S2). These Ru–O distances match those of the Ru^IV-containing polyoxotungstate [(PW₁₁O₃₉)₂{(HO)­Ru^IV–O–Ru^IV(OH)}]^10–, with Ru–O distances ranging from 1.965(15) to 2.014(14) Å, as well as [(SiW₁₁O₃₉Ru)₂O]^13– (Ru–O distances: 1.815(12)–2.126(12) Å) and with central metal–O bond distances of Anderson-type tungstates, Na₆[H₃Cr^IIIW₆O₂₄], Na₈[Mn^IVW₆O₂₄], and Na₈[Ir^IVW₆O₂₄] (Table S2). In Na₈[Ru^IVW₆O₂₄]·26H₂O, the W–O distances are grouped into three distinct ranges: terminal WO_t bonds, which measure between 1.753(2) and 1.765(3) Å; bridging W–O_b bonds between two W, spanning from 1.938(3) to 1.974(3) Å; and W–O_c bonds, where O_c is bound to two W and one Ru, with lengths ranging from 2.140(2) to 2.180(3) Å. The W–O bond lengths in Na₈[Ru^IVW₆O₂₄]·26H₂O closely resemble those found in other [XW₆O₂₄]^n– compounds (X = Pt^IV, Ir^IV, Mn^IV, Te^VI, n = 8; X = Sb^V, n = 7; X = Cr^III, n = 6). The metallic centers in [Ru^IVW₆O₂₄]^8– are aligned in a single plane, where a central Ru atom is symmetrically enclosed by six tungsten atoms in a hexagonal arrangement. The W–W and Ru–W bond lengths ranged from 3.223(6) to 3.246(6) Å and 3.214(5) to 3.247(5) Å, respectively.

We found that eight Na⁺ countercations encircle each polyanion together with water molecules in the solid-state structure of Na₈[Ru^IVW₆O₂₄]·26H₂O. (Figure S3, Figure S4, and Table S3), and the crystal water molecules and Na⁺ counter cations assemble into continuous zigzag chains, with each Na1 and Na2 ion linking two neighbor polyanions (Figure S3).

Bond valence sum (BVS) indicated that the oxidation states of W, Ru, and O were 6+, 4+, and 2–, respectively (Table S4). There are no protonated oxygen atoms in the Anderson molecule, [Ru^IVW₆O₂₄]^8–.

Elemental analysis of the isolated solids confirmed the formula Na₈[Ru^IVW₆O₂₄]·27H₂O. This is in close agreement with the 26 H₂O molecules observed in the single-crystal X-ray analysis.

It can be seen from the TG curve of Na₈[Ru^IVW₆O₂₄] that 23 water molecules evaporate (found 19% mass loss and calculated 19.0% mass loss) up to ca. 100 °C, with an endothermic peak (DTA) at T _max = 51.3 °C corresponding to the DTG peak at T _max = 50.0 °C (Figure S1). The mass loss of 23 water molecules was very close to the elemental analysis and single-crystal X-ray diffraction determinations.

The powder X-ray diffraction pattern (XRD) of the isolated Na₈[Ru^IVW₆O₂₄] was consistent with the theoretical patterns generated from single-crystal structure data. The powder XRD pattern of the isolated Na₈[Ru^IVW₆O₂₄] (Figure a) was similar to the simulated patterns generated from single-crystal XRD data (Figure b). Slight peak shifts between the observed and simulated pattern might be due to the unit cell length difference caused by the temperature difference, where the single-crystal XRD was performed at −150 °C, whereas powder XRD was performed at room temperature (ca. 23 °C). Furthermore, no signals corresponding to possible impurities, Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] and [{(benzene)­Ru}₄W₂O₁₀] (Figure b and c), were observed, thereby confirming the phase purity of the isolated solid.

2.

(a) Observed powder XRD of Na₈[Ru^IVW₆O₂₄], and simulated powder XRD from single-crystal XRD results of (b) Na₈[Ru^IVW₆O₂₄], (c) [{(benzene)­Ru}₄W₂O₁₀], and (d) Na₆[{(benzene)­Ru}₂H₂W₈O₃₀].

The IR spectrum of the isolated Na₈[Ru^IVW₆O₂₄] shows characteristic bands of the Anderson-type heteropolytungstate, which are similar to those of Na₈[Ir^IVW₆O₂₄]·26H₂O and Na₆[H₃Cr^IIIW₆O₂₄]·22H₂O (Figure ). The antisymmetric stretching vibrations of the terminal WO_t bonds and Ru–O_c–W bridges are represented by strong bands at approximately 908 and 859 cm^–1, respectively. The antisymmetric stretching of W–O_b–W bridges is responsible for the band at approximately 630 cm^–1, whereas the bending vibrations of W–O_b–W bridges are linked to the band at 575 cm^–1. The bending vibrations of the Ru–O_c–W bridges are the source of the noticeable band at approximately 459 cm^–1. Furthermore, four bands from cocrystallized H₂O molecules, strong band at roughly 3439 cm^–1, weak broad band at 2000–2300 cm^–1, medium band at 1649 cm^–1, and weak broad band at 800–600 cm^–1, which decreased in intensity upon drying (Figure S5), were observed. These four bands were assignable to stretching vibration, combination of bending and libration, bending vibration, and libration of water, respectively (Figure S5). , The broad weak peaks at 2000–2300 cm^–1 and at 800–600 cm^–1 are characteristic of hydrogen-bonded water which was confirmed by single-crystal XRD. Furthermore, no bands corresponding to possible impurities, Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] and [{(benzene)­Ru}₄W₂O₁₀], were observed, confirming the purity of the isolated Na₈[Ru^IVW₆O₂₄].

3.

IR spectra of (a) Na₈[Ru^IVW₆O₂₄]·27H₂O, (b) [{(benzene)­Ru}₄W₂O₁₀], and (c) Na₆[{(benzene)­Ru}₂H₂W₈O₃₀].

The oxidation state and local structure of Ru in Na₈[Ru^IVW₆O₂₄] were also characterized by Ru K-edge XANES and EXAFS. Figure shows the Ru K-edge XANES spectra of Na₈[Ru^IVW₆O₂₄] together with those of Ru foil, Ru^III(acac)₃, Ru^IVO₂, and SrRu^IVO₃. The edge energy of Na₈[Ru^IVW₆O₂₄] is close to those of Ru^IVO₂ and SrRu^IVO₃, confirming that the oxidation state of Ru is 4+. FT-EXAFS curve-fitting analysis (Figure S6) indicated two peaks corresponding to the Ru–O distance (r = 1.99 ± 0.03, coordination number = 5.5 ± 0.2) and the Ru–W distance (r = 3.24 ± 0.02, coordination number = 5.5 ± 0.2). These distances and coordination numbers are very close to those obtained by single-crystal X-ray structure analysis (Ru–O, r = 1.991(2)–2.009(3), and Ru–W, r = 3.214(5)–3.247(5)).

4.

(a) XANES spectra at the Ru K-edge of (red) Ru foil, (blue) Ru­(acac)₃, (black) Na₈[Ru^IVW₆O₂₄], (green) RuO₂, and (pink) SrRuO₃, and (b) FT-EXAFS of Na₈[Ru^IVW₆O₂₄].

Electrochemical Study

Cyclic voltammograms demonstrated a quasi-reversible redox couple (Figure ), with a peak potential (E _1/2) of 361 mV (potential difference between the anodic and cathodic peak, ΔE = 253 mV) in a 0.1 M KOAc buffer solution (pH 4.0), where the open-circuit potential was +500 mV. We believe that the irreversibility, the ΔE = 253 mV was larger than 59/n mV (n = number of electrons) for reversible redox couple at room temperature, is due to slow electron transfer between [Ru^IVW₆O₂₄]^8– and electrode. The irreversibility and instability presented below made it difficult to estimate the exact number of electrons of this redox couple. Based on the open-circuit potential, we tentatively assigned that the observed waves correspond to the Ru^IV/III redox transition in [Ru^IVW₆O₂₄]^8–. The tungsten reduction peak of [Ru^IVW₆O₂₄]^8– was observed at a more negative potential of approximately −504 mV.

5.

Cyclic voltammograms of 1.78 mM [Ru^IVW₆O₂₄]^8– in 0.1 M KOAc at pH 4.0, with a scan rate of 25 mV s^–1. The red line was scanned between +880 and −200 mV, and the blue line was scanned between +880 and −774 mV. The potential scan began at +500 mV and proceeded in a negative direction.

Figure S6 shows the cyclic voltammograms of [Ru^IVW₆O₂₄]^8– in a 0.1 M KOAc buffer solution at pH 4.0 with different scan rates. As the square root of the scan rate (v ^1/2) increased, the peak currents for both forward and reverse scans also rose proportionally, indicative of diffusion-controlled electron transfer (Figure S7). The ratios of the cathodic (I _pc) to anodic (I _pa) peak currents were consistently close to 1.0 across a range of scan rates. The ΔE value increased with increasing scan rate, implying a slower electron transfer between [Ru^IVW₆O₂₄]^8– and the electrode (Table S5).

Cyclic voltammograms in solution with different pH values showed that the potential shifted to more negative potentials with increasing pH, indicating that reduction was accompanied by protonation (Figure S8). However, it was difficult to obtain a reasonable relationship between the pH and redox potential (Pourbaix diagram). We believe that this is because reasonable redox potentials could not be obtained owing to the slow electron transfer rate.

Furthermore, we observed that the redox current decreased when the solution was kept for more than 100 min, indicating that decomposition occurred (Figure S9). This instability also makes it difficult to study detailed redox behavior. Further research to understand the detailed redox process by finding a suitable buffer solution is currently ongoing in our laboratory.

Optimization of the [Ru^IVW₆O₂₄]^8– Synthesis Procedure

Heating Na₂WO₄·2H₂O and [(benzene)­RuCl₂]₂ in a mixed H₂O–ethanol solution produced a mixture of Na₈[Ru^IVW₆O₂₄], [{(benzene)­Ru}₄W₂O₁₀], and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀]. We successfully identified the recrystallization conditions required to separate the three complexes. After the mixture of Na₂WO₄·2H₂O and [(benzene)­RuCl₂]₂ was heated in closed and open reactors to remove the solvent, the obtained solid was mixed with H₂O, and the insoluble [{(benzene)­Ru}₄W₂O₁₀] was removed. Keeping the filtrate at room temperature in an open sample tube for 1 day, Na₈[Ru^IVW₆O₂₄] was crystallized and filtered off. Further crystallization produced Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] solid within 2 days (experimental). The effect of reaction temperature on the isolated and in situ yields is summarized in Table . The in situ yield of [Ru^IVW₆O₂₄]^8– in the reaction mixture was estimated by cyclic voltammetry (experimental). The estimated in situ yields were similar to the isolated yields (Table , Nos. 1, 2, 3, 4, and 7), confirming that the cyclic voltammetry method is suitable for the detection of [Ru^IVW₆O₂₄]^8– in the reaction mixture.

1. Synthetic Conditions and Yield of Anderson [Ru^IVW₆O₂₄]^8–

	Reaction		Yield/%
	Close	Open	In situ	Isolated
No.	Temp. (hours)	Temp. (hours)	RuW6	RuW6	Ru4W2	Ru2W8
	Na2WO4 + [(benzene)RuCl2]2
1	70 °C (24)	70 °C (24)	7	6	68	14
2	80 °C (24)	80 °C (24)	11	9	54	24
3	85 °C (24)	85 °C (24)	24	22	41	31
4	90 °C (24)	90 °C (24)	30	31	27	38
5	-	90 °C (24)	6
6	90 °C (24)	r.t. (72)	21
7	100 °C (6)	90 °C (24)	35	35	n.d	52
	Na2WO4 + Ru(acac)3
8	90 °C (24)	90 °C (24)	n.d.
9	100 °C (6)	90 °C (24)	n.d.
	Na2WO4 + K2RuCl5(H2O)
10	90 °C (24)	90 °C (24)	n.d.
11	100 °C (6)	90 °C (24)	n.d.
	Na2WO4 + RuCl3
12	90 °C (24)	90 °C (24)	n.d.
13	100 °C (6)	90 °C (24)	n.d.
	Na2WO4 + RuCl2(DMSO)4
14	90 °C (24)	90 °C (24)	n.d.
15	100 °C (6)	90 °C (24)	n.d.

^aNa₂WO₄·2H₂O (60 mg, 0.181 mmol) and [(benzene)­RuCl₂]₂ (5.9 mg, Ru: 0.024 mmol) (W/Ru = 8) were heated at different temperatures in a mixed solvent of 0.5 mL of H₂O and 0.5 mL of C₂H₅OH in a 20 mL sample tube with a closed cap, and the solution was left at the same temperatures without the cap (open) to evaporate the solvents.

^bThe reaction mixture (ca. 66 mg) was mixed with a 0.1 M KOAc buffer solution (pH 4.0, 5.0 mL), and the in situ yields were estimated using cyclic voltammetry.

^cThe reaction mixture (ca. 66 mg) was mixed with 0.3 mL of H₂O and stirred for 10 min, and the insoluble brown plate crystals, [{(benzene)­Ru}₄W₂O₁₀] (Ru₄W₂), were filtered off. After 1 day of standing of the filtrate, green block crystals of Na₈[Ru^IVW₆O₂₄] (RuW₆) were filtered off. After standing for two more days, yellow needle crystals of Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] (Ru₂W₈) were filtered off.

^dNot detected.

By increasing the reaction temperature (Nos. 1, 2, 3, 4, and 7), the yields of Na₈[Ru^IVW₆O₂₄] and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] increased, whereas the yield of [{(benzene)­Ru}₄W₂O₁₀] decreased. The Ru atom in [(benzene)­RuCl₂]₂ is coordinated by η⁶-benzene ligand, and higher temperatures are required to cleave the Ru-benzene coordination bond. If the reaction mixture was immediately evaporated, the yield was lower (No. 5), indicating that heating in solution was necessary. If the reaction mixture was evaporated at room temperature, the yield was lower (No. 6), indicating that evaporation at a higher temperature is required to obtain a higher yield.

The effect of reaction time on the yield at various temperatures was examined (Figure S10). With the same heating time of 24 h, the in situ yield increased from 70 to 90 °C but decreased at 100 °C. Extending the reaction time at lower temperatures enhanced the in situ yield, particularly at 85 °C, where a high in situ yield of 34% was achieved after 48 h. Moreover, by increasing the reaction time from 0 to 96 h at a reaction temperature of 90 °C, the in situ yields increased up to 24 h and decreased with further heating. At reaction temperatures of 100 °C or higher, shorter reaction times were sufficient. A high in situ yield of 35% was achieved at 100 °C with a reaction time of 6 h.

The influence of different Na₂WO₄·2H₂O/[(benzene)­RuCl₂]₂ ratios (ranging from 1:1 to 1:9) on the in situ yield is summarized in Figure S11. At a W/Ru ratio of 2, the formation of [Ru^IVW₆O₂₄]^8– was not observed. The in situ yield increased with an increasing Na₂WO₄·2H₂O ratio, while the weight of [(benzene)­RuCl₂]₂ was kept constant, reaching a maximum at a Ru/W ratio of 1:8 before declining.

The influence of different solvents on the in situ yield of [Ru^IVW₆O₂₄]^8– is summarized in Table S6. The findings show that 1:1 mixtures of ethanol–water and dimethylformamide–water are the most effective solvents for achieving high in situ yields of [Ru^IVW₆O₂₄]^8– at 90 °C for 24 h. Pure water and ethanol decreased the in situ yield, and mixed solvents of CH₃CN, dimethylacetamide, methanol, ethyl acetate, tetrahydrofuran, 1-propanol, 2-propanol, dimethyl sulfoxide, dichloromethane, and acetic acid with water decreased the in situ yield.

The in situ yields of [Ru^IVW₆O₂₄]^8– with various Ru sources are presented in Table . Other Ru sources, Ru­(acac)₃, K₂RuCl₅(H₂O), RuCl₂(DMSO)₄, and RuCl₃, did not yield [Ru^IVW₆O₂₄]^8– under the same conditions. Prof. Neumann has reported that heating a mixture of RuCl₂(DMSO)₄ with 6 equiv of Na₂WO₄·2H₂O in an aqueous solution (pH 5.5) at 90 °C produced not Anderson-type [Ru^IVW₆O₂₄] compound but one-dimensional nonatungstate (K­[HW₉O₃₃(Ru­(DMSO)₃)₂]^6–)_n compound. [(benzene)­RuCl₂]₂ is the only Ru source that produces the desired [Ru^IVW₆O₂₄]^8– species.

Electrocatalytic Water Oxidation

Water-insoluble POM compounds have recently garnered significant attention as heterogeneous catalysts in electrochemical research on water oxidation, notably those that include active Ru metal. , Oxo species of higher-valent metals are present in most of these catalysts, such as Ru^V(O), which act as active centers for water oxidation. ,, Anderson-type polyoxometalates demonstrate electrocatalytic activity and have been utilized as water redox catalysts. , [Ru^IVW₆O₂₄]^8– is soluble in water and buffer solutions with a pH range of 1–7 but becomes insoluble at pH values greater than 7. The cyclic voltammetry of Na₈[Ru^IVW₆O₂₄] mixed with carbon paste showed a redox couple at ca. 1050 mV and 1480 mV vs RHE (ca. 380 mV and 810 mV vs Ag/AgCl, respectively) in phosphate buffer solution (0.5 M KH₂PO₄ + 0.5 M KOH (pH 8.0)), which might be assigned to Ru^IV/III and Ru^V/IV redox, respectively (Figure S12). Further scanning to a more positive potential increased the oxidation current corresponding to the catalytic oxidation of water (Figure a). When the same amount of Ru was present in the carbon paste electrode, the activity of Na₈[Ru^IVW₆O₂₄] (overpotential at a current density of 10 mA cm^–2: 665 mV) was lower than that of RuO₂·H₂O (overpotential at a current density of 10 mA cm^–2: 615 mV), but higher than those of other Ru-containing heteropolytungstates, [{(benzene)­Ru}₄W₂O₁₀] (overpotential at a current density of 10 mA cm^–2: 830 mV), Na₆[{(benzene)­Ru}₂H₂W₈O₃₀] (overpotential at a current density of 10 mA cm^–2: 770 mV), and Cs₅[SiW₁₁O₃₉Ru­(H₂O)] with the same amount of Ru loaded on the electrode.

6.

(a) Linear sweep voltammetry at a scan rate of 25 mV s^–1 and (b) current–time plots of electrolysis at +1200 mV vs Ag/AgCl (3 M NaCl) in a single cell of (orange) RuO₂·H₂O, (brown) Na₈[Ru^IVW₆O₂₄], (green) Na₆[{(benzene)­Ru}₂H₂W₈O₃₀], (blue) [{(benzene)­Ru}₄W₂O₁₀], (red) Cs₅[SiW₁₁O₃₉Ru­(H₂O)], and (black dotted) carbon paste in 0.5 M KH₂PO₄ + 0.5 M KOH (ca. pH 8.0). Na₈[Ru^IVW₆O₂₄] (15 wt %) was mixed with carbon paste, and the same amount of Ru was mixed with carbon paste for the other Ru complexes.

The controlled potential electrolysis of the carbon paste mixtures of Na₆[{(benzene)­Ru}₂H₂W₈O₃₀], [{(benzene)­Ru}₄W₂O₁₀], Na₈[Ru^IVW₆O₂₄], and RuO₂·H₂O at +1200 mV vs Ag/AgCl (+1869 mV vs RHE) showed a stable current density, where Na₈[Ru^IVW₆O₂₄] and RuO₂·H₂O achieved the highest current densities of 4.5 and 8.3 mA cm^–2, respectively, outperforming [{(benzene)­Ru}₄W₂O₁₀] and Na₆[{(benzene)­Ru}₂H₂W₈O₃₀], which were maintained for 2 h (Figure b). Fluctuations in Na₈[Ru^IVW₆O₂₄] and RuO₂·H₂O were observed, suggesting fast gas bubble generation (Figure S13a). We also confirmed that hydrogen and oxygen were generated at a ca. 2:1 ratio using gas chromatography, although the faradaic efficiency was ca. 70% using a divided cell (Figure S13b and c). Because the current density was stable, Na₈[Ru^IVW₆O₂₄] exhibited high stability under electrolysis conditions. Furthermore, the FT-IR spectrum of Na₈[Ru^IVW₆O₂₄] recovered after electrolysis was the same as that of the original Na₈[Ru^IVW₆O₂₄], confirming the stability of Na₈[Ru^IVW₆O₂₄] under electrolysis conditions (Figure S14). Further research on the development of higher electrocatalysis based on Na₈[Ru^IVW₆O₂₄] is currently underway in our group.

Conclusion

In summary, we demonstrated that the reaction of Na₂WO₄·2H₂O with [(benzene)­RuCl₂]₂ at elevated temperatures affords a ruthenium-containing Anderson-type polyoxometalate, Na₈[Ru^IVW₆O₂₄], together with (benzene)Ru tungstate complexes. Structural and spectroscopic analyses unambiguously confirmed the formation of the first isolated Ru-centered Anderson-type polyoxometalate. The selective formation of Na₈[Ru^IVW₆O₂₄] exclusively from [(benzene)­RuCl₂]₂ highlights the critical role of organometallic precursors in directing cluster assembly. Electrochemical studies revealed a quasi-reversible Ru^IV/III redox couple, indicating accessible electron-transfer properties within the framework. Importantly, Na₈[Ru^IVW₆O₂₄] exhibits superior catalytic activity for water oxidation compared to related (benzene)­Ru–tungstate species, underscoring the advantage of the Anderson-type structure in promoting catalytic efficiency. These findings provide new insights into the design of ruthenium-functionalized polyoxometalates and establish a promising platform for developing efficient catalysts for energy conversion.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.6c02109.

• Figure S1: TG-DTA and DTG curves; Figure S2: cyclic voltammograms of [{(benzene)­Ru}₄W₂O₁₀], Na₆[{(benzene)­Ru}₂H₂W₈O₃₀], and Na₈[Ru^IVW₆O₂₄]; Figure S3: solid-state structure of Na₈[Ru^IVW₆O₂₄]·26H₂O showing the polyanions being surrounded by Na⁺ counter cations; Figure S4: structure of Na₈[Ru^IVW₆O₂₄]·26H₂O showing the atom labeling; Figure S5: FT-IR spectrum of the isolated [Ru^IVW₆O₂₄]^8– after heating; Figure S6: FT-EXAFS curve-fitting analysis; Figure S7: cyclic voltammograms of [Ru^IVW₆O₂₄]^8– at various scan rates; Figure S8: cyclic voltammograms of [Ru^IVW₆O₂₄]^8– in solutions at different pH values; Figure S9: cyclic voltammograms of [Ru^IVW₆O₂₄]^8– at different time intervals; Figure S10: reaction temperature and time dependency of the in situ yield of [Ru^IVW₆O₂₄]^8–; Figure S11: W/Ru ratio effect on the in situ yield of [Ru^IVW₆O₂₄]^8–; Figure S12: cyclic voltammograms using Na₈[Ru^IVW₆O₂₄] mixed with a carbon paste electrode; Figure S13: pictures of the bulk electrolysis and plots of charge and amount of H₂ and O₂ vs electrolysis time; Figure S14: FT-IR spectra of Na₈[Ru^IVW₆O₂₄] recovered from the carbon paste mixture after electrolysis; Table S1: crystal data and structure refinement for Na₈[Ru^IVW₆O₂₄]·26H₂O; Table S2: comparison of bond lengths in Na₈[Ru^IVW₆O₂₄]·26H₂O with other reported values; Table S3: hydrogen bonds in the structure of Na₈[Ru^IVW₆O₂₄]··26H₂O; Table S4: bond valence sum values; Table S5: values of redox peak potentials and currents depending on the scan rate for [Ru^IVW₆O₂₄]^8–; Table S6: solvent effect on the yield of Anderson [Ru^IVW₆O₂₄]^8–; Table S7: product in situ yields of [Ru^IVW₆O₂₄]^8– under different solvents; and Table S8: product yields of [Ru^IVW₆O₂₄]^8– for different Ru sources (PDF)

S.E.E. performed optimization of reaction conditions and measured powder XRD, IR, and electrochemical analysis. S. found this complex and performed single-crystal X-ray analysis. S.K., S.Y., and T.M. measured and analyzed XAFS. M.S. organized and managed this research topic. The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

The authors declare no competing financial interest.