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Article

The Synthesis and Characterisation of Ru(III)-Substituted Keggin-Type Phosphomolybdates

by
Max Papajewski
†,
Jan-Christian Raabe
†,
Hamid Anwari
,
Dorothea Voß
,
Jakob Albert
and
Maximilian J. Poller
*
Institute for Technical and Macromolecular Chemistry, Universität Hamburg, Bundesstraße 45, 20146 Hamburg, Germany
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work and share first authorship.
Inorganics 2025, 13(6), 176; https://doi.org/10.3390/inorganics13060176
Submission received: 29 April 2025 / Revised: 15 May 2025 / Accepted: 20 May 2025 / Published: 23 May 2025
(This article belongs to the Special Issue State-of-the-Art Inorganic Chemistry in Germany)
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Abstract

:
Polyoxometalates are a promising family of compounds for the development of new catalyst materials, although up to now they have mainly been applied in acid catalysis and oxidative processes. In this study, we present the synthesis and characterisation of two new Keggin-type phosphomolybdates, H6[PRuMo11O40] and H9[PRu2Mo10O40]. The successful synthesis was confirmed with ICP-OES (elemental composition) and infrared spectroscopy (structure). Furthermore, the molecular structure of H6[PRuMo11O40] was determined by electron diffraction. The new compounds were comprehensively characterised using 31P-NMR spectroscopy, UV-Vis spectroscopy, and electrochemical methods. Square-Wave-Voltammetry revealed an additional RedOx peak for the Ru-substituted POMs compared to the unsubstituted phosphomolybdate at around 825 mV. In a test reaction, the new compounds showed promising catalytic activity for the hydrogenation of lactic acid.

Graphical Abstract

1. Introduction

Polyoxometalates (POMs) are a highly versatile class of inorganic molecules. They exhibit huge structural variety and contain a large diversity of elements. While their framework usually consists of Mo or W, many other transition elements, and to a lesser extent also main group elements, can be incorporated [1,2,3,4,5,6,7,8,9,10]. Applications of POMs range from the biomedical field [11,12,13,14,15] to molecular magnetism and spintronics [16,17] and catalysis [1,18,19,20,21,22,23,24,25,26,27]. The most prominent application of transition metal-substituted POMs (with different transition metals in the framework position) is the catalysis of selective biomass oxidation [22,27,28,29,30]. This catalytic activity is usually achieved through the incorporation of V, a transition metal known for its activity in oxidation reactions [27,29,31,32,33,34]. On the other hand, POMs show promising applications in biomedicine based on their antibiotic, antiviral, and anti-tumour effects. However, such applications are restricted due to the toxicity effects found in cells [35].
In comparison, examples for the use of transition metal-substituted POMs catalysing reductive reactions, such as hydrogenations, are quite rare. The reason for this is the fact that there are few POMs containing transition metals such as Pt, Pd, or Ru [36]. Most of the known examples are difficult to synthesize in larger quantities, preventing their relevant use in widespread applications, with reported yields in the range of a few milligrams [37,38]. The first investigations in this area of research were carried out using Os, Rh, Pd, Pt, Ag, and Au as noble metals in the heteroelement and framework/foreign element position of POMs [39]. In contrast, the synthesis of V-substituted Keggin-type phosphomolybdates, which are employed for oxidation catalysis, is easily scalable to the multi-gram scale [22].
In the present study, we therefore focussed our efforts on the introduction of Ru, a metal often used in hydrogenation reactions [40,41,42,43,44], into the Keggin-type phosphomolybdate’s structure, with the goal of expanding the scope of synthetic strategies for the synthesis of POM catalysts for hydrogenation catalysis. A particular challenge in this endeavour was the large difference in oxidation states between the original framework element (MoVI) and RuIII. The increasing degree of substitution led to an increasing negative charge of the POM anion, which ultimately threatened its stability. While substitution degrees of up to 6, i.e., half of the framework elements were replaced, have been reported for substitution with V+V [5], to the best of our knowledge, the highest negative charge reported for a Keggin-type anion is -14 in H14[PV3Mn2Mo7O40] [4]. Based on these prior findings, we predicted that a maximum of three RuIII atoms could be incorporated into the Keggin-type phosphomolybdate’s structure, resulting in a charge of −12.

2. Results and Discussion

To achieve this incorporation of Ru(III) into the phosphomolybdate structure, we employed the well-established self-assembly approach in which all precursor compounds (molybdenum(VI) oxide, phosphoric acid, and ruthenium(III) chloride) were mixed in an aqueous solution in the targeted stoichiometric ratios [4,45,46,47]. While the reaction proceeded as planned for the mono- and di-substitution, forming a dark clear solution, the attempt to create a tri-substituted POM yielded an unexpected precipitate.
After isolating the products, they were analysed using AAS/ICP-OES and thermogravimetric analysis (TGA) to determine their composition (Supplementary Material Section S1.3.1, Table 1) and with vibrational spectroscopy to determine their molecular structure (Figure 1).
The AAS/ICP-OES data (Supplementary Material Section S1.3.1) confirm the target stoichiometry of the mono- and di-substituted POMs (Table 1). The precipitate formed in the attempt to produce the tri-substituted H12[PRu3Mo9O40] was identified as Ru2(MoO4)3 (Table 1, Supplementary Material Section S1.3.1 and Figure S3).
Using the self-assembly approach, we were not able to synthesise the desired POMs with substitution degrees higher than two. An attempt to incorporate three or more Ru(III) ions into the phosphomolybdate structure resulted in the precipitation of a dark, poorly soluble solid, which was identified by AAS/ICP-OES (Table 1) and vibrational spectroscopy as a mixed salt of ruthenium(III) molybdate (Ru2(MoO4)3) and ruthenium(III) phosphate (RuPO4). We hypothesized that the -12 charge of a three-fold Ru(III)-substituted Keggin POM is too large and the resulting coulomb repulsion prevents the formation of a stable molecule.
In order to confirm the successful formation of the Keggin-type structure, infrared spectra were recorded (Figure 1). From the vibrational spectroscopic data, the characteristic vibrational bands of a Keggin-type structure can be identified [2,3,48,49]. In the IR spectra of the newly synthesised Ru-substituted POMs, these characteristic bands are easily observed.
Ru and Mo have similar masses, and therefore, the typical shoulder formation or peak splitting of TMSPOMs is not observed [5]. However, it is noticeable that the vibrational bands are much broader in the two-fold substituted Ru-POM. This observation can be explained by the presence of different positional isomers, which differ in their substitution pattern. For the two-fold substituted HPRu2Mo, five different isomers are possible [50]. Each positional isomer exhibits slightly different vibrational frequencies, which results in peak broadening. Therefore, the broader bands of the substituted POMs can be regarded as an indication of successful element substitution.
In combination with the elemental analysis, the IR spectra prove the successful synthesis of H6[PRuMo11O40] (HPRu1Mo) and H9[PRu2Mo10O40] (HPRu2Mo).
Since it is well established that POMs are usually accompanied by a significant amount of hydration water, we performed thermogravimetric analysis to determine the exact amount of hydration water in our new compounds. The TGA results show a significant mass loss upon heating the compounds to 350 °C (Supplementary Material Figure S1), which corresponds to the loss of 11 (HPRu1Mo) and 12 (HPRu2Mo) equivalents of hydration water.
Attempts to grow single crystals of HPRu1Mo and HPRu2Mo for single-crystal X-ray diffraction were unsuccessful. POM acids, especially transition metal-substituted POMs, are very difficult to crystallise. As they are multi-anionic clusters, the POM anions repel each other in the solid-state structure. Protons are too small to sufficiently separate the large anionic charges in the organised solid-state structure. Therefore, electron diffraction (ED) experiments were employed in cooperation with Rigaku Europe SE to elucidate the molecular structure from nanocrystalline powder. HPRu2Mo exhibited insufficient crystallinity, but we were able to successfully determine the solid-state molecular structure of HPRu1Mo (Figure 2).
The solid-state structure was refined in the cubic space group P423 2 (208) [51,52]. Refinement of the model in the alternative space group Pn-3m (224) did not result in a satisfactory model. Here, the asymmetric unit (AU) contains a total of seven atoms: five oxygen atoms (including one molecule of hydration water), one metal position (shared by Mo and Ru), and one phosphorus atom. Hydrogen atoms were not modelled. As in our previous work in which the POMs were analysed by X-ray diffraction, no specific location for the Ru atom could be determined as it is statistically distributed over all twelve metal positions [4,22,53]. Therefore, the model was refined with partial occupation of the metal positions (11/12 Mo, 1/12 Ru) in accordance with the elemental analysis results.
The bonds of the Keggin-type structure can be classified as follows: the P-Oa bond, the Oa-M bond (the oxygen atom of the PO4 tetrahedron coordinating with the metals), M-Ob-M (metal–metal bridging oxo ligands), and M=Ot (terminal oxo ligands). The lengths of these bonds were compared with the bond lengths of the unsubstituted POM species HPMo (Table 2) [4].
The direct comparison of the bond lengths of HPMo and HPRu1Mo shows that the P-Oa bond length in HPRu1Mo is 0.02 Å longer than in HPMo. The Oa-M bond length is extended by 0.013 Å in HPRu1Mo, while the M-Ob-M bond length is shortened by 0.149 Å in HPRu1Mo. This observation can be explained by the smaller ionic radius of Ru(III) (1.25 Å) compared to Mo(VI) (1.38 Å) [55]. In contrast, the M=Ot bond length in HPRu1Mo is shortened by 0.005 Å. These findings support the conclusion that Ru(III) was successfully incorporated into the phosphomolybdate Keggin structure. A summary of the crystallographic results can be found in the Supplementary Material Table S2. Figure S2 in the Supplementary Material shows the powder XRD (pXRD) data in comparison with the simulated diffractogram of the ED data for compound HPRu1Mo. A short discussion can be found in the ESI. In agreement with our experience, pXRD is not a suitable method for analysing POMs (see Supplementary Material).
Note: In general, the refined data sets of ED experiments show high R values. The reason for this lies in the ED method itself: instead of “large” single crystals, micro-/nanocrystalline powered is used. The result of the ED is therefore based on the measurement of small crystalline domains of a few nanometers in size. However, the entire bulk material exhibits inhomogeneities and many crystal structure defects, resulting in high R values after the refinement process [56].
In addition to characterisation in the solid state, the behaviour of HPRu1Mo and HPRu2Mo in solution was studied by 31P-NMR spectroscopy (Figure 3). The 31P-NMR spectra were recorded in an acidic aqueous solution (pH 1).
As expected for TMSPOMs, the spectra of HPRu1Mo and HPRu2Mo exhibit multiple signals (Figure 3). In the case of HPRu2Mo, this is in part due to the different positional isomers. Furthermore, both of the Ru-substituted POMs undergo dissociation reactions, which is a well-documented phenomenon [5,50,57,58]. Regarding the presented POMs, these dissociation reactions can be described as follows:
[PRuMo11O40]6− → [PMo11O39]9− + Ru3+
2 [PRuMo11O40]6− → [PRu2Mo10O40]9− + [PMo12O40]3−
In addition to the dissociation and the positional isomers, some of the less intense peaks can be attributed to the β-isomers of the Keggin structure. In this structural isomer, one group of three MO6 octahedra is rotated by 60° relative to the α-isomer [59,60,61,62,63]. In general, we observed a low signal-to-noise ratio and comparatively broad peaks, which can be attributed to the paramagnetic influence of Ru(III) (d5). Similar effects were also observed in our previous study of Mn(II)-substituted phosphomolybdates [4].
Furthermore, aqueous solutions of HPRu1Mo and HPRu2Mo were analysed with UV-Vis spectroscopy (Figure 4). In the UV-Vis spectra, ligand to metal charge transfer (LMCT) bands can be observed, which can be assigned to O→Mo(VI) (200 to 300 nm) and Ru(III) (300 to 500 nm). The Ru(III) LMCT bands are comparatively weak in intensity, which is in part due to the lower Ru content (1 or 2 Ru vs. 11 or 10 Mo) but can also be explained by the stability of the +III oxidation state of Ru, as the LMCTs are formally a reduction (the transfer of an electron from the ligands to the metal). It is generally known that each LMCT band formally consists of two bands, namely one LMCT of the bridging oxo ligands and a slightly different LMCT of terminal oxo ligands to the metals. The latter LMCT contributes to shoulder formation of the individual LMCT bands towards higher wavelengths, as is observed in the case of the Mo(VI) LMCT bands [64,65,66,67]. In the case of Ru(III), this leads to the formation of a particularly broad band that extends in the 300 to 500 nm wavelength range. Two bands can also be identified in the spectrum of a RuCl3 solution, which we attributed to the LMCT of the chlorido and aqua ligands in an aqueous solution.
During our UV-Vis analysis, we also determined the extinction coefficients of the LMCT bands according to Beer–Lambert’s law. The results are summarized in the Supplementary Material, Table S3 and Figures S4–S6.
The RedOx behaviour of our TMSPOMs was investigated by using electrochemical methods (cyclic voltammetry—CV, and square-wave voltammetry—SWV) [68]. The CV data of the Ru(III)-substituted POMs (Supplementary Material Figure S7) are difficult to interpret due to the multitude of RedOx reactions. However, the overall shape of the CV graphs leads to the conclusion that these RedOx reactions are, at least in part, irreversible. For further electrochemical characterisation, the SWV data (Figure 5, Supplementary Material Table S4) were primarily considered.
In general, the peak potentials for the Ru(III)-substituted POMs are very similar to H3[PMo12O40] (see Table S4). However, the incorporation of Ru(III) results in the formation of a new peak potential at 830 mV for HPRu1Mo and 820 mV for HPRu2Mo. Peak potentials in this range have not yet been found for TMSPOM systems and represent a characteristic property of Ru(III)-substituted phosphomolybdates [3,4,5,22]. Previously reported values for the RedOx potentials of Ru at pH 0 are 249 mV for Ru(III) to Ru(II), 623 mV for Ru(III) to Ru(0), and 810 mV for Ru(II) to Ru(0) [69]. Our measurements were performed at a similarly acidic pH value (pH 1), and therefore, it can be assumed that the peak potentials are comparable. From the direct comparison of the SWV data with those of RuCl3, it is noticeable that the RedOx potential of Ru(III) to Ru(II) was found in the range of 205 mV (HPRu1Mo) to 250 mV (RuCl3), coinciding with a RedOx reaction of the unsubstituted POM at 205 mV. The potential for the reaction of Ru(III) to Ru(0) was identified at 645 mV for RuCl3 but does not appear for the Ru-substituted POMs. The reduction of Ru(II) to metallic Ru(0) was observed at a potential of 770 mV for RuCl3. Therefore, the peak potentials of 830 mV and 820 mV in the TMSPOMs can be assigned to the reduction to metallic Ru(0), which explains the irreversibility observed in the CV data.
The results presented here show that the incorporation of up to two Ru(III) atoms into the Keggin-type phosphomolybdate structure was successful. In particular, the electrochemical data show interesting properties which have not yet been observed for other TMSPOMs [3,4,5]. In order to assess their potential as a reduction catalyst, we proceeded to test these new TMSPOMs as catalysts for the hydrogenation of lactic acid, which is a promising biogenic platform chemical that can be reduced to 1,2-propanediol (PD) or propionic acid (PA).
The test reaction was carried out in 21 mL stainless steel pressure reactors using a 10 mL aqueous solution of lactic acid (0.555 mol/L), 0.05 mmol of catalyst, and 31.5 bar of hydrogen (at ambient temperature). The reactors were heated to 200 °C for 20 h, after which the contents of the gas and liquid phase were analysed by GC and HPLC, respectively. A detailed description of the experimental procedure can be found in Section S2.2 of the Supplementary Material (Figures S8–S10, Table S5 and Equations (S1)–(S4)).
The results are summarized in Table 3. For the control reaction containing no catalyst, no conversion was observed at all. In the control experiment with unsubstituted phosphomolybdic acid (H3[PMo12O40]), a low conversion of only 7 % was observed. A low PA yield of 9 % was determined, with an estimated error of ±2 %, which accounts for the entire conversion resulting in a 100 % selectivity. RuCl3, as a benchmark for an acidic Ru3+-species on the other hand, shows a slightly higher conversion of 20 % and both a low PD (9%) and PA yield (6%).
HPRu1Mo significantly increases both lactic acid conversion (33 %) and PA yield (24 %), while only a low PD yield (4%) was observed, leading to a high PA selectivity of 73%. HPRu2Mo shows similar conversion of lactic acid (36%) but exhibits a significantly lower PA yield (15%) and a higher PD yield (12%), leading to a decrease in product selectivity. Further details of the catalytic tests can be found in the Supplementary Material. Overall, the incorporation of more than one Ru atom in the Keggin-type phosphomolybdate does not improve catalytic activity, but shifts the selectivity from PA to PD. One possible reason for this could be the aforementioned dissociation of the POM leading to free Ru(III) ions. As shown in the reaction with RuCl3, this favours the formation of PD.

3. Materials and Methods

All experimental procedures and more detailed information about the analytical methods are found in the Supplementary Material.

3.1. Chemicals

Catalyst synthesis:
The chemicals were purchased from the following suppliers:
  • H3[PMo12O40] HPMo from Sigma Aldrich, St. Louis, MO, USA.
  • Molybdenum trioxide from Alfa Aesar, Haverhill, MA, USA.
  • Phosphoric acid from Grüssing, Westoverledingen, Germany, as 85 % solution in water (was further diluted with deionized water to a concentration of 36.9 %).
  • Ruthenium(III) chloride hydrate was purchased from Sigma Aldrich, St. Louis, MO, USA (38.0 to 42.0 % Ru basis); product number: 84050; batch number: BCCK1103 (40 % Ru according to ICP). The specifications of the supplier correspond to a trihydrate.
  • Deionized water was always used as the solvent.
Hydrogenation of lactic acid (catalysis):
The chemicals were purchased from the following suppliers:
  • Lactic acid (LA, 90.3 wt.-% aqueous solution) was purchased from Sigma-Aldrich, St. Louis, MO, USA.
  • Propanoic acid (PA, 99.9 %) was purchased from Sigma-Aldrich, St. Louis, MO, USA.
  • Propane-1,2-diol (PD, 99.5 %) was purchased from Thermo Fischer Scientific, Waltham, MA, USA.
  • Hydrogen (H2, 5.0 grade) was purchased from Linde, Dublin, Ireland.
  • Nitrogen (N2, 5.0 grade) was purchased from Linde, Dublin, Ireland.
All of these materials were used without further purification. Deionized water was used for catalytic experiments.

3.2. General Procedure for Synthesizing the Catalysts

General synthetic procedure:
For the synthetic procedures, a precise stoichiometry of P/Ru/Mo (1:1:11 for HPRuMo and 1:2:10 for HPRu2Mo) was required. All weights of the precursor compounds are listed in Table S1 in the Supplementary Material.
Molybdenum trioxide was suspended in water and a 36.9 % solution of phosphoric acid in water was added. The mixture was heated to reflux until a clear yellow solution was obtained. Ruthenium(III) chloride was dissolved in water and ethanol, and the dark solution was added to the reaction mixture. As a result, the colour changed from yellow to dark. In the next step, the reaction solution was concentrated under reduced pressure (rotary evaporator 85 °C oil bath and 200 mbar pressure) to yield a dark solid.
Synthesis of ruthenium molybdate y hydrate:
Ruthenium(III) molybdate y hydrate was obtained via the addition of ruthenium(III) chloride x hydrate to a solution of sodium molybdate dihydrate in water as a dark precipitate. The precipitate was collected by filtration and dried under reduced pressure in a desiccator over orange gel.
All precise weights and information can be found in Table 4.

3.3. Characterization of the Catalysts

H6[PRuMo11O40]: 3.075 g of a dark solid was obtained (92.52 % referred to H3PO4).
ICP-OES: Calculated for H6[PRuMo11O40] ∙ 11 H2O: 1.525 % P, 4.975 % Ru, 51.947 % Mo. Found for H6[PRuMo11O40] ∙ 11 H2O: 1.72 % P, 5.17 % Ru, 47.97 % Mo. Data normalized to molybdenum. P/Ru/Mo ratio: 1.22/1.12/11.
H9[PRu2Mo10O40]: 3.363 g of a dark solid were obtained (92.54 % referred to H3PO4).
ICP-OES: Calculated for H9[PRu2Mo10O40] ∙ 12 H2O: 1.505 % P, 9.823 % Ru, 46.624 % Mo. Found for H9[PRu2Mo10O40] ∙ 12 H2O: 1.71 % P, 9.48 % Ru, 43.12 % Mo. Data normalized to molybdenum. P/Ru/Mo ratio: 1.22/2.08/10.
Ru2(MoO4)3:
ICP-OES: Calculated for Ru2(MoO4)3 ∙ 6 H2O: 25.586 % Ru, 36.431 % Mo. Found for Ru2(MoO4)3 ∙ 6 H2O: 26.04 % Ru, 33.75 % Mo, 1.00 % P. Data normalized to molybdenum. Ru/Mo ratio: 2.20/3.00.

3.4. Hydrogenation of Lactic Acid (Catalysis)

All catalytic experiments were conducted at a 10-fold plant which consists of 21 mL high-pressure stainless steel (1.4571) vessels. The reactors were equipped with FKM o-ring gaskets and PTFE-coated magnetic stirring bars. All reactors were placed inside a heating block controlled by two independent PT-100 thermocouples, and stirring was performed with an IKA stirrer.
For the reaction, the vessels were filled with 10 mL of an aqueous solution containing 5 wt.-% of LA and the equivalent amount of catalyst for a constant ruthenium mass of 5 mg. Afterwards, the vessels were closed and purged with N2 three times to remove any residual oxygen, two times at 20 bar and the third time at 50 bar coupled with a leak test. Then, the reactors were purged with H2 twice and lastly set to a pressure of approximately 31 bar H2. The heating block was then set to a temperature of 200 °C. While heating, the stirrer was set to a speed of 300 rpm. When the heating block reached 200 °C, the stirrer was set to 500 rpm and the reaction time started. At this point, the H2 pressure inside the reactors increased to approximately 50 bar.
After the desired reaction time, the reaction was stopped by placing the vessels inside a fume hood, enabling them to cool down to room temperature in approximately 30 min. The vessel temperature and pressure were noted, and the reactor pressure was released into a gas bag for gas chromatography analysis. Subsequently, the reactor was opened, and the liquid was filtered to separate the stirrer and any precipitation. For HPLC analysis, this filtrate was again filtered through a 0.2 µm syringe filter. The remaining filtrate was used for any other analysis.

3.5. Product Analysis

NMR analysis:
For the qualitative determination of products, the samples were measured on a Bruker AVANCEI 500 MHz spectrometer. All samples were prepared as follows: 0.5 mL of filtrate and 0.1 mL of water-D2O was added as deuterated solvent.
The spectra were recorded using a standard 13C{1H}-NMR program with 4k (=4096) scans covering a range from −20 ppm to 220 ppm. The analysis of the spectra was performed with MestReNova®. The data were subsequently exported in .csv format and plotted in OriginPro 2019b for visualization.
Product quantification:
Gas-phase products (propane, ethane, and methane) were quantified by gas chromatography. For this, a Varian 450-GC equipped with a Restek 2 m, 0.53 mm ID, Shin Carbon ST 80/100, and a flame ionization detector were used. The injection temperature was 220 °C, and the oven temperature started at 40 °C for 2.5 min, ramping up at 15 °C/min for 6 min and, lastly, staying at 140 °C for 3 min. As a carrier gas, argon was used.
For the liquid-phase product, an HPLC system from Shimadzu was utilized. It was equipped with an Aminex HPX-87H 300 mm × 7.8 mm BIORAD column and a refractive index detector. The eluent was an aqueous sulfuric acid solution (5 mmol/L). All measurements were conducted at 45 °C and at a flow rate of 0.5 mL/min.

4. Conclusions

In summary, we present the synthesis and characterization of different Ru(III)-substituted phosphomolybdates using the ab initio self-assembly method. Using this method, we were able to incorporate an electron-rich d5 element into the Keggin-type structure by substituting an electron-poor d0 element. We were able to show that the maximum substitution of two Mo(VI) with Ru(III) is possible. Successful synthesis was confirmed by elemental analysis (ICP-OES) and IR spectroscopy. Although no single crystal was obtained, we were able to elucidate the molecular structure of HPRu1Mo via electron diffraction performed by Rigaku Europe SE. The resulting transition metal-substituted POMs were further characterised in an aqueous solution using 31P NMR spectroscopy, UV-Vis spectroscopy, and electrochemical methods. Finally, we explored their catalytic potential in the hydrogenation of lactic acid, in which the new POMs outperformed both RuCl3 and the unsubstituted phosphomolybdate.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics13060176/s1, Table S1: Precise weights of all precursor compounds for synthesizing Ru(III)-substituted phosphomolybdates; Figure S1: TGA data of investigated Ru(III)-substituted TMSPOMs; Table S2: Crystal data and structure refinement for HPRu1Mo; Figure S2: The pXRD data of all POMs investigated in this work. The diffractogram of POM HPRu1Mo was simulated from the ED data (orange line); Figure S3: FT-IR analysis of the precipitate resulting from the attempt to incorporate more than two Ru(III) atoms into the Keggin-type phosphomolybdate structure (bottom). A comparison with self-synthesized ruthenium(III) molybdate (top); Figure S4: The calibration line for determining the extinction coefficient for the Mo(VI) LMCT band of compound HPMo; Figure S5: The calibration line for determining the extinction coefficient for the Mo(VI) LMCT band of compound HPRuMo; Figure S6: The calibration line for determining the extinction coefficient for the Mo(VI) LMCT band of compound HPRu2Mo; Table S3: Wavelength maxima and extinction coefficients determined by UV-Vis spectroscopy in aqueous solution; Figure S7: CV data of all investigated TMSPOMs in comparison to HPMo and RuCl3. All measurements were performed in aqueous, diluted hydrochloric acid solution at pH 1 (concentration: 1 mmol/L); Table S4: Peak potentials in mV found in SWV data for investigated TMSPOMs; Figure S8: Product determination for the hydrogenation reaction of lactic acid to propionic acid, propane-1,2-diol, and acetone by 13C-NMR measured according to Supplementary Materials Section S2.3.1; Figure S9: A suggested reaction network for the hydrogenation of lactic acid to either propanoic acid or propane-1,2-diol including the over-hydrogenation and carbon–carbon cleavage to the undesired gaseous products propane, ethane, and methane; Table S5: Conversion of lactic acid and product yields of catalyst screening; Figure S10: The carbon balance of catalyst screening including gas-phase products (methane, ethane, and propane). Experimental conditions: 200 °C, 50 bar H2, 20 h, 1000 rpm, and 10 mL stock solution (0.555 mol/L of lactic acid): (a) 0.05 mmol of ruthenium; (b) 0.05 mmol of polyoxometalate clusters. Reference [70] is cited in the supplementary materials.

Author Contributions

J.-C.R.: methodology, investigation, visualization, writing the original draft, and writing—review and editing. M.P.: methodology, investigation, visualization, writing the original draft, and writing—review and editing. J.-C.R. and M.P. contributed equally to this study and therefore share first authorship. H.A.: investigation. D.V.: supervision and writing—review and editing. J.A.: resources and writing—review and editing. M.J.P.: project administration, conceptualization, supervision, resources, writing the original draft, and writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding. The APC was waived by MDPI.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank Rigaku Europe SE, especially Christian Schürmann and Felix Hennersdorf, for the electron diffraction measurement of HPRu1Mo. Furthermore, we thank the Central Element Analysis Department (ZEA), headed by Dirk Eifler, for measuring numerous AAS/ICP-OES samples, Ute Gralla for measuring the Raman samples, the NMR service led by Thomas Hackl for measuring the NMR samples, and Thomas Marx from the research group of Peter Burger, for providing the equipment to perform the electrochemical analysis. Finally, we gratefully acknowledge Andreas Pawlig for his assistance with some experiments.

Conflicts of Interest

The authors declare no conflicts of interest. The company ‘Rigaku Europe SE’ was not involved in the study design, any data collection or interpretation beyond the electron diffraction measurements, the writing of this article, or the decision to submit it for publication.

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Inorganics 13 00176 g001
Figure 1. Vibrational spectra (IR) of HPMo in comparison to investigated TMSPOMs.
Figure 1. Vibrational spectra (IR) of HPMo in comparison to investigated TMSPOMs.
Inorganics 13 00176 g001
Inorganics 13 00176 g002
Figure 2. The solid-state structure of compound HPRu1Mo, determined by ED. The space group was determined to P423 2 (208) R1: 23.79 %, wR2: 54.26 %, Rint: 38.95 %, and GooF: 2.343. Colour code: purple—phosphorus, blue—metals (Mo, Ru), and red—oxygen. The full .cif file is available through the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service (deposition number: 2447574).
Figure 2. The solid-state structure of compound HPRu1Mo, determined by ED. The space group was determined to P423 2 (208) R1: 23.79 %, wR2: 54.26 %, Rint: 38.95 %, and GooF: 2.343. Colour code: purple—phosphorus, blue—metals (Mo, Ru), and red—oxygen. The full .cif file is available through the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service (deposition number: 2447574).
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Inorganics 13 00176 g003
Figure 3. 31P-NMR spectroscopic analysis of the investigated TMSPOMs in comparison to H3[PMo12O40], measured in 90 % of a diluted, aqueous hydrochloric acid solution (pH 1) and 10 % acetone-d6.
Figure 3. 31P-NMR spectroscopic analysis of the investigated TMSPOMs in comparison to H3[PMo12O40], measured in 90 % of a diluted, aqueous hydrochloric acid solution (pH 1) and 10 % acetone-d6.
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Inorganics 13 00176 g004
Figure 4. UV-Vis spectra of the Ru(III)-substituted POMs in comparison with H3[PMo12O40] and RuCl3, measured in an aqueous solution (0.02667 g/L).
Figure 4. UV-Vis spectra of the Ru(III)-substituted POMs in comparison with H3[PMo12O40] and RuCl3, measured in an aqueous solution (0.02667 g/L).
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Inorganics 13 00176 g005
Figure 5. SWV data of HPRu1Mo and HPRu2Mo in comparison to H3[PMo12O40] and RuCl3. All measurements were performed in an aqueous hydrochloric acid solution at pH 1 (concentration: 1 mmol/L). The data for HPRu1Mo, HPRu2Mo, and RuCl3 were smoothed in Ivium software using Savitzky Golay smoothing with 25 points.
Figure 5. SWV data of HPRu1Mo and HPRu2Mo in comparison to H3[PMo12O40] and RuCl3. All measurements were performed in an aqueous hydrochloric acid solution at pH 1 (concentration: 1 mmol/L). The data for HPRu1Mo, HPRu2Mo, and RuCl3 were smoothed in Ivium software using Savitzky Golay smoothing with 25 points.
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Table 1. Compositional analysis of the investigated POMs determined by AAS/ICP-OES and TGA.
Table 1. Compositional analysis of the investigated POMs determined by AAS/ICP-OES and TGA.
CompoundTarget StoichiometryP/Ru/Mo Ratio aHydration Water Content
[mol/mol-POM] b
HPRu1Mo cH6[PRuMo11O40]1.22/1.12/1111
HPRu2Mo dH9[PRu2Mo10O40]1.22/2.08/1012
PrecipitateRu2(MoO4)3-/2.20/3.006
a normalised to targeted Mo content. b Hydration water content was determined by TGA. c Yield: 92.52 %. d Yield: 92.54 %.
Table 2. Bond lengths of compound HPRu1Mo compared to literature values of HPMo [54].
Table 2. Bond lengths of compound HPRu1Mo compared to literature values of HPMo [54].
POMP-Oa [Å]Oa-M [Å]M-Ob-M [Å]M=Ot [Å]
HPMo1.5342.4391.9161.674
HPRu1Mo1.5542.4521.7671.699
Table 3. Conversion of lactic acid and product yields of catalyst screening.
Table 3. Conversion of lactic acid and product yields of catalyst screening.
CatalystConversion
[mol%]		Yield [mol%]Selectivity [%]
PDPAPDPA
Control00000
H3[PMo12O40] b7090100
RuCl3 a20964731
HPRu1Mo a,b334241273
HPRu2Mo b3612153542
Experimental conditions: 200 °C, 50 bar H2, 20 h, 1000 rpm, and 10 mL stock solution (0.555 mol/L of lactic acid): (a) 0.05 mmol of ruthenium and (b) 0.05 mmol of polyoxometalate clusters. PD: Propane-1,2-diol. PA: propionic acid. Estimated error: ±2 mol %.
Table 4. Precise weights of all precursor compounds for synthesizing Ru(III)-substituted phosphomolybdates.
Table 4. Precise weights of all precursor compounds for synthesizing Ru(III)-substituted phosphomolybdates.
CompoundMolybdenum Trioxide36.9 % Phosphoric Acid in WaterVolume of Water [mL]Ruthenium(III) Chloride x HydrateVolume of Water [mL]Volume of Ethanol [mL]
HPRuMo2.590 g
17.99 mmol
11 equiv.		0.435 g
1.636 mmol
1 equiv.		800.414 g
1.638 mmol Ru
1 equiv.		1515
HPRu2Mo2.353 g
16.35 mmol
10 equiv.		0.469 g
1.766 mmol
1.08 equiv.		800.825 g
3.265 mmol Ru
1.99 equiv.		1515
Product
m [g]M [g/mol] *Lattice watern [mmol]n H3PO4 [mmol] **Yield [%]
HPRuMo3.0752031.5755111.5141.63692.52
HPRu2Mo3.3632057.7446121.6341.76692.54
equiv. = equivalent(s); m: mass of the product; M: molar mass of the product, including lattice/hydration water content; n: amount of substance product; n PO43−: amount of substance PO43−; * including the hydration water content, determined by TGA; ** component with less amount of substance.
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Papajewski, M.; Raabe, J.-C.; Anwari, H.; Voß, D.; Albert, J.; Poller, M.J. The Synthesis and Characterisation of Ru(III)-Substituted Keggin-Type Phosphomolybdates. Inorganics 2025, 13, 176. https://doi.org/10.3390/inorganics13060176

AMA Style

Papajewski M, Raabe J-C, Anwari H, Voß D, Albert J, Poller MJ. The Synthesis and Characterisation of Ru(III)-Substituted Keggin-Type Phosphomolybdates. Inorganics. 2025; 13(6):176. https://doi.org/10.3390/inorganics13060176

Chicago/Turabian Style

Papajewski, Max, Jan-Christian Raabe, Hamid Anwari, Dorothea Voß, Jakob Albert, and Maximilian J. Poller. 2025. "The Synthesis and Characterisation of Ru(III)-Substituted Keggin-Type Phosphomolybdates" Inorganics 13, no. 6: 176. https://doi.org/10.3390/inorganics13060176

APA Style

Papajewski, M., Raabe, J.-C., Anwari, H., Voß, D., Albert, J., & Poller, M. J. (2025). The Synthesis and Characterisation of Ru(III)-Substituted Keggin-Type Phosphomolybdates. Inorganics, 13(6), 176. https://doi.org/10.3390/inorganics13060176

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