Two Rh^III-substituted polyoxoniobates and their base-induced transformation: [H₂RhNb₉O₂₈]⁶⁻ and [Rh₂(OH)₄Nb₁₀O₃₀]⁸⁻

Abstract

Two new rhodium-substituted polyoxoniobates, [H₂RhNb₉O₂₈]⁶⁻ (RhNb₉) and [Rh₂(OH)₄Nb₁₀O₃₀]⁸⁻ (Rh₂Nb₁₀) are reported. The two distinct Rh^III-substituted niobate clusters behave differently when the pH is raised with TMAOH: the Rh₂Nb₁₀ is stable until pH ∼ 12.7, but RhNb₉ dissociates to form RhNb₅ and RhNb₁₀, similar to some of our other metal-substituted niobates, such as the MNb₉ ions (M = Cr or Mn), which transform to MNb₁₀ when the solution pH is raised.

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Transition-metal (TM) – substituted polyoxometalates are an important class of materials. The TM-substitution can add catalytic function to the cluster and the polyoxometalate framework adds great redox properties, especially for polyoxo-molybdates and -tungstates.¹ In the polyoxoniobate system, a series of TM-substituted decaniobate-type [MNb₉O₂₈]^x− (MNb₉, M = Cr–Ni) have been synthesized recently² and add to the Ti-, V- and Cu-substituted polyoxoniobates that were previously known.³ In these studies, the substitution is limited to the early transition metals. For the heavier transition metals, there have been structures reported for Re(CO)₃-, CpRh- or Pt-coordinated (capped) hexaniobates (Nb₆).⁴ However, substitution of 2nd- or 3rd-row transition metals in the polyoxoniobates as atoms internal to the structure, rather than as capping atoms, has not yet been reported to our knowledge, although among the group V polyoxometalates, [H₂Pt^IVV₉O₂₈]⁵⁻ is known in the polyoxovanadate system.⁵

Here we report the synthesis, structure, characterization and photocatalytic H₂ evolution study of two Rh^III-substituted niobates. The two compounds reported here have the stoichiometry: [H₂RhNb₉O₂₈]⁶⁻ (RhNb₉) and [Rh₂(OH)₄Nb₁₀O₃₀]⁸⁻ (Rh₂Nb₁₀) as tetramethylammonium (TMA) salts (Fig. 1). The structures of these clusters resemble those of two Cr^III-substituted niobates, [H₂CrNb₉O₂₈]⁶⁻ (CrNb₉) and [Cr₂(OH)₄Nb₁₂O₃₀]⁸⁻ (Cr₂Nb₁₀) that we previously described.^2,6

Fig. 1 Ball-and-stick model of RhNb₉ (left) and Rh₂Nb₁₀ (right) (Nb: gray, Rh: gold, O: red). Intramolecular hydrogen bonds are shown with dashed line in Rh₂Nb₁₀.

Substitution of Rh^III in the polyoxoniobate structure was challenging. Our previous methods employed for MNb₉ (M = Cr–Ni) generally showed low yield for rhodium substitution (less than 1%).² The low yield might be a result from the notoriously slow reaction rate of ligand substitutions at the Rh^III center.⁷ When we attempted to circumvent the slow kinetics with temperature, we found that some Rh^III was reduced to Rh⁰ as a gray or black powder mixed with the crude product at the hydrothermal reaction conditions when the temperature was higher than 120 °C. In order to solve this problem, we added hydrogen peroxide in the reaction mixture to prevent reduction of Rh^III. Correspondingly the yields were improved when H₂O₂ was added (40% and 7% for Rh₂Nb₁₀ and RhNb₉, respectively). Hydrogen peroxide also might have helped to dissociate the rather stable Nb₆ or Nb₁₀ ions and facilitate the formation of Rh^III-substituted structures.⁸

The solution after hydrothermal reaction was typically a mixture of Nb₁₀, Nb₆, RhNb₉ and Rh₂Nb₁₀ ions, as found by electrospray-ionization mass spectra (ESI-MS). We took advantage of the slightly different solubility of each compound to facilitate separation and purification of the Rh-substituted molecules. After washing with isopropanol, the product was extracted with ethanol. The ethanol extract was a mixture of Rh₂Nb₁₀ and Nb₆ ions, and the precipitate that remained after ethanol extraction was a mixture of Nb₁₀ and RhNb₉. The RhNb₉ was separated from Nb₁₀ by extraction with ethanol/methanol mixture. Mild heating of the ethanol extract for a few hours caused condensation of more soluble Nb₆ into less soluble Nb₁₀ precipitate. The ethanolic orange solution that remained after this heating step consisted of mostly Rh₂Nb₁₀. The crystalline products of Rh₂Nb₁₀ and RhNb₉ were obtained after solvent evaporation.

In the crystal structure of RhNb₉, Rh^III is substituted at the central metal site so that it does not possess a terminal oxo group, as we also observed in the MNb₉O₂₈ (M = Cr–Ni) series.² The Rh^III metal is disordered among the two central sites due to the centrosymmetry, and the sum of Rh^III occupancy in those two sites is 1.12, which agrees with stoichiometry of RhNb₉. Bond valence sum (BVS) calculation of the Rh site is (3.03), indicating the oxidation state of Rh^III. The BVS values of two Rh–μ₂-O–Nb (1.37 and 1.38) are much lower than other bridging oxygen atoms, which suggest that those are protonated, similarly to the substituted MNb₉ (M = Cr–Ni).² The structure of Rh₂Nb₁₀ is similar to Cr₂Nb₁₀, and it can be described as two RhNb₅ Lindqvist-type clusters fused by two μ₄-O atoms linking two Rh^III and two μ₃-O atoms linking Rh^III and Nb^V. The oxidation state of rhodium in Rh₂Nb₁₀ is also Rh^III, as determined by BVS calculation (2.95). The Rh–O bond lengths in Rh₂Nb₁₀ are longer and more regular (2.0245(16)–2.0605(16) Å) than Cr–O bonds in Cr₂Nb₁₀ (1.9428(13)–2.0131(12) Å). In the structure of Rh₂Nb₁₀, four protons are found on the four Rh–μ₂-O–Nb, like in the structure of Cr₂Nb₁₀.⁶ Those protons form intramolecular hydrogen bonds to the neighbouring Nb=O (H⋯O distances of 2.309 and 2.386 Å). The ESI-MS spectra of the RhNb₉ and Rh₂Nb₁₀ agree with their assigned stoichiometries (Fig. S1†).

The pH-dependent stability of the Rh^III-substituted niobate clusters were studied by using ESI-MS. When titrated with TMAOH, the golden yellow color of Rh₂Nb₁₀ solution did not change until highly basic conditions (pH ∼ 12.9, Fig. S2†), and most of the Rh₂Nb₁₀ clusters remained intact for months at this strongly basic condition, as checked by ESI-MS (Fig. S3†). When the solution of RhNb₉ was titrated with TMAOH to this condition, the solution color slowly changed from orange to faint yellow overnight (Fig. S2†). The ESI-MS spectra of the solution after one day (Fig. 2) indicated dissociation of RhNb₉ to RhNb₅ and Nb₆. Also, formation of a new RhNb₁₀ was detectable via ESI-MS, which could have formed by self-assembly of dissociated fragments (Fig. 2). It is most likely that this RhNb₁₀ would have a similar structure of previously reported [H₂Mn^IVNb₁₀O₃₂]⁸⁻ (MnNb₁₀), in view of their similar ESI-MS pattern.⁹ This observation spurred us to further investigate other TM-substituted polyoxoniobate clusters. We added 50 mg of TMAOH·5H₂O to each aqueous solution containing 30 mg of MNb₉ (M = Ti, Cr–Ni) and Cr₂Nb₁₀ clusters to make pH ∼ 12.6 and monitored the solution by using ESI-MS (Fig. S4–S10†). The Cr₂Nb₁₀ clusters were stable at this pH for a long period of time, like Rh₂Nb₁₀ (Fig. S4†). The TiNb₉, CrNb₉ and MnNb₉ clusters changed in a few days at this high-pH condition. Some Ti₂Nb₈ clusters,^3a,c,d formed after a week when the pH of TiNb₉ was increased (Fig. S5†). Considerable amount of CrNb₁₀ formed after a few days from the CrNb₉ solution at high pH (Fig. S6†). This result shows that Cr^III- and Rh^III-substituted polyoxoniobates are not only structurally similar, but also transform via similar pathways at high pH (Scheme 1). The color of MnNb₉ solution changed from purple to brown with time, suggesting oxidation of Mn^III, and ESI-MS spectra after 19 days showed formation of small amount of Mn^IVNb₁₀ (Fig. S7†). However, species such as MnNb₅ or CrNb₅ was not detectable, which suggests that they are unstable. Other MNb₉ clusters (M=Fe–Ni) were relatively stable at the high pH, but small amount of Nb₆ as a decomposition product was detected (Fig. S8–S10†). Thus M₂Nb₁₀ (M = Rh^III or Cr^III) seems to be more stable than MNb₉ at high pH. This higher stability of M₂Nb₁₀ is partly attributable to the existence of intramolecular hydrogen bonds, which hold the structure together, perhaps making it less susceptible to base hydrolysis.

Fig. 2 Change of ESI-MS spectra of RhNb₉ when the solution pH was adjusted to 12.9.

Scheme 1 Base-induced transformation of MNb₉ and M₂Nb₁₀ (M = Rh, Cr).

When titrated with acid, Rh₂Nb₁₀ was evident in the ESI-MS spectra until pH 4.0, and RhNb₉ was stable until pH 4.5 (Fig. S11 and S12†), although we recognize that the kinetics of dissociation may be suppressed by inclusion of the Rh^III. Both solutions formed hydrous niobium-oxide precipitate below those pH values, which could form without dissociating the structures. On the other hand, we note that the stability window of Rh₂Nb₁₀ (4 < pH < 13) is similar to Cr₂Nb₁₀.⁶ The RhNb₉ exhibited a wider stability range (4.5 < pH < 12) than other MNb₉ clusters (M = Cr–Ni) in general.

The UV-Vis spectra of RhNb₉ and Rh₂Nb₁₀ are shown in Fig. 3. The Rh₂Nb₁₀ shows about twice the absorption of visible light relative to the RhNb₉ ion, as expected from the stoichiometry of the clusters. The absorption band of Rh₂Nb₁₀ (440 nm) is more blue shifted compared to that of RhNb₉ (475 nm), which is responsible for the slightly different colors of the solutions of Rh₂Nb₁₀ (golden yellow) and RhNb₉ (orange-red). These absorption bands correspond to ¹A_1g to ¹T_1g or ¹T_2g transition of Rh^III.¹⁰ The RhNb₉ and Rh₂Nb₁₀ clusters were also characterized by using FT-IR (Fig. S13†). The FT-IR spectrum of Rh₂Nb₁₀ show similar feature to that of Cr₂Nb₁₀ and that of RhNb₉ is similar to those of MNb₉, which reflect their structural similarity.

Fig. 3 UV-Vis spectra of 2 mM solution of Rh₂Nb₁₀ and RhNb₉ without background electrolyte.

TM-substituted polyoxometalates, including polyoxoniobates, have recently been actively studied for use in the water-splitting reaction to generate H₂ and/or O₂ for energy applications.¹¹ We studied H₂ evolution from RhNb₉ and Rh₂Nb₁₀ ions as a continuation of our previous H₂-evolution study of the MNb₉ ions (M = Cr–Ni). When irradiated with visible light, similarly to the MNb₉ ions, neither RhNb₉ nor Rh₂Nb₁₀ solutions (20% v/v methanol/water) evolved H₂. Methanol was used as a sacrificial oxidant. The solution did not evolve H₂ without methanol. When a full spectrum from Xe lamp (without UV filter) was employed, however, H₂ evolution was observed (50 μmol g⁻¹ h⁻¹ and 43 μmol g⁻¹ h⁻¹, for RhNb₉ and Rh₂Nb₁₀, Fig. 4). After irradiation, the originally orange-red or yellow solution of RhNb₉ and Rh₂Nb₁₀ turned greenish brown with small amount of colloids, which probably indicate partial reduction of Rh^III to Rh⁰. We have found such colloids in our previous work and are not surprised by them.² The Rh^II is known to exhibit a green color.¹⁰ We do not know the amount of Rh^III that has been reduced, but comparison of the peak intensities in ESI-MS spectra of the solution before and after irradiation indicated that most of the clusters remained intact (Fig. S14 and S15†). The UV-Vis spectra of Rh₂Nb₁₀ before and after the irradiation also did not change considerably, but absorption of RhNb₉ solution increased after irradiation, undoubtedly due to the presence of the colloids mentioned above (Fig. S16 and S17†). In the previous H₂ evolution study of the MNb₉ ions (M = Cr–Ni),² we found that a significant amount of MNb₉ decomposed into Nb₆ and Nb₁₀, with corresponding changes in the UV-Vis spectrum due to the colloid formation. Among them, formation of colloids from NiNb₉ and CoNb₉ positively affected H₂ evolution, while colloid formation from other MNb₉ (M = Cr–Fe) did not increase the amount of H₂ evolution. In our previous work we have found that the colloids were mixed TM–niobium oxide and they were amorphous, as determined by XRD, TEM and EDX. The relative lack of Nb₆ and Nb₁₀ decomposition products s after irradiation in the present RhNb₉ and Rh₂Nb₁₀ suggests that these rhodium RhNb₉ and Rh₂Nb₁₀ clusters are more stable, perhaps only kinetically so, under the irradiation of intense light when compared to MNb₉ (M = Cr–Ni). We also compared H₂-evolution activity of the clusters when H₂PtCl₆ was added as a cocatalyst. In the existence of Pt, Nb₁₀ solution showed ∼20 fold increase in the H₂ evolution (1385 μmol g⁻¹ h⁻¹) and amount of precipitate was negligible, but H₂ evolution from RhNb₉ and Rh₂Nb₁₀ after adding H₂PtCl₆ showed only about 3 fold increases (167 and 152 μmol g⁻¹ h⁻¹, respectively) and conspicuous gray-black precipitate formed in the solution (Fig. 4). This presumed Pt–Rh–NbOx precipitate must be responsible for the slight increase of the H₂ evolution, but we did not attempt to characterize it further. The different precipitation behavior of Nb₁₀ and RhNb₉/Rh₂Nb₁₀ after photocatalytic reaction might be due to their different stabilities upon addition of acidic H₂PtCl₆. And the lower H₂ evolution activity of RhNb₉/Rh₂Nb₁₀ compared to Nb₁₀ is likely due to the reduced amount of dissolved clusters in solution caused by precipitation, as seen in ESI-MS (Fig. S14 and S15†).

Fig. 4 Comparison of H₂-evolution activity from the methanol/water solutions (20% v/v) of RhNb₉, Rh₂Nb₁₀ and Nb₁₀, with and without H₂PtCl₆.

Conclusions

Two types of new rhodium-substituted polyoxoniobates were synthesized and isolated. The evidences of base-promoted transformation of RhNb₉ to RhNb₅ and RhNb₁₀ suggest a new synthetic strategy for new polyoxoniobates. Such a reaction can be a useful post-synthetic pathway for new polyoxoniobates, instead of commonly employed hydrothermal reaction in the polyoxoniobate chemistry. The transformation of MnNb₉ and CrNb₉ at high pH shows that the stabilities of each TM-substituted decaniobate are different, even if they form similar dissociation products at high pH. The Rh^III-substituted polyoxoniobates discussed here are important because these can serve as feedstock to make even more TM-substituted polyoxoniobates, such as RhNb₅ which might have terminal Rh–OH groups.

We thank Jiarui Wang and Prof. Frank E. Osterloh for H₂ evolution measurement. This work was supported by an NSF CCI grant through the Center for Sustainable Materials Chemistry, number CHE-1102637.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, ESI-MS, FT-IR, UV-Vis data. CCDC 1417431 and 1417432. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt03797b

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