Effect of vanadium valence state on the solution chemistry and the stability of vanadium substituted polyoxometalates

Abstract

The reactions of lacunary Keggin-type silicotungstates ({SiW_12−x}, x = 1–3) with vanadyl sulfate (V^IVOSO₄) as the tetravalent vanadium source were monitored in real-time by ESI-MS in conjunction with DPV. Different phenomena from the pentavalent vanadium counterpart (NaV^VO₃) were found: the substitution products are unexpectedly stable whereas spontaneous transformations from mono- into di- and eventually tri-vanadium substituted products ({SiW₁₁V} → {SiW₁₀V₂} → {SiW₉V₃}) were observed when the substituted vanadium is of +5 valence state. DFT calculations were utilized to interpret the intrinsic structural reasons for distinctive stability of the vanadium-substituted POMs with different valences and with different substitution degrees. The effect of the vanadium valence states on the solution chemistry and stability of the substituted POMs were thereby unraveled.

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Introduction

Transition metal substituted polyoxometalates (TMSPs) represent the largest subclass of polyoxometalates (POMs) with a rich array of potential applications in the areas of catalysis,¹ magnetism² and so on due to their variable structures tuned by the incorporated TMs. One method to obtain TMSPs is to use preformed lacunary polyoxotungstate (LPOT) starting materials to react with TMs. A variety of TMSPs with different architectures has been synthesized so far.³ LPOTs are reactive species⁴ and are known to reorganize in solution which helps to introduce structural diversity. However, the complexity behind such ‘simple’ binary reaction is not stopped at the LPOT part. It was found that TMs also play an important role in defining the final structures of TMSPs. Our previous works⁵ have presented the systematic investigation of the reactions of a family of lacunary Keggin-type silicotungstates {SiW_12−x} (x = 1–3) with sodium metavanadate and found out that spontaneous transformations from mono- into di- and eventually tri-substituted products ({SiW₁₁V} → {SiW₁₀V₂} → {SiW₉V₃}) were observed at room temperature when the substituted vanadium is of +5 valence state and the conversion rate is in direct proportion with temperature and concentration of reactants. This prompts us to investigate further about effect of the valence states of the substituted TMs on the chemistry of TMSPs.

From the analytical technology perspective, fast and sensitive detection of the reaction solution in time (better in situ) to monitor the chemical changes in the reaction mixture is desperately needed with the development of POM chemistry. The advent of electrospray mass spectrometry (ESI-MS) has transformed the paths of new cluster discovery and the self-assembly mechanism via directly probing the reaction solution, thus allowing the formation process to be accomplished in unprecedented details that facilitate new cluster discovery and the elucidation of the self-assembly mechanism of POM clusters in solution. ESI-MS has become a powerful technology in POM chemistry because of its high sensitivity, rapid analysis, and low sample consumption.⁶ The superior power of ESI-MS to sensitively detect the subtle speciation changes of reaction mixture in real time lays it irreplaceable position in the mechanistic study.^3a–c,7

Herein, we report the reactions of the lacunary silicotungstates ([SiW₁₁O₃₉]⁸⁻, [SiW₁₀O₃₆]⁸⁻ and [SiW₉O₃₄]¹⁰⁻) with vanadyl sulfate (V^IVOSO₄), as an extension of our previous study on the same reactions with sodium metavanadate (NaV^VO₃).⁵ The objective of this study is to investigate the effect of vanadium valence state on the solution chemistry and the stability of vanadium-substituted POMs.

Results and discussion

The reaction of lacunary silicotungstates with sodium metavanadate (NaV^VO₃) was well addressed in our previous work.⁵ In this work, we use vanadyl sulfate (V^IVOSO₄) as the tetravalent vanadium source to react with lacunary Keggin silicotungstate. Fig. 1–3 are the ESI-MS spectra monitored in real time for the three reactions ({SiW₁₁} + VOSO₄ (I), {SiW₁₀} + 2VOSO₄ (II) and {SiW₉} + 3VOSO₄ (III)) at both low and high concentration regimes (5.0 × 10⁻⁴ M and 1.7 × 10⁻² M), respectively. It should be noted that the three vanadium-substituted products, {SiVW₁₁}, {SiV₂W₁₀} and {SiV₃W₉}, were observed by ESI-MS immediately after mixing up the two reactants accompanied with changes of solution colors (lavender → plum for I, olive green → dark green for II and brown → dark brown for III, respectively). The representative MS peaks indicated by the respective mass spectra are [H₂SiV^IVW₁₁O₄₀]⁴⁻ (m/z 685, 100%), [HSiV^IVW₁₁O₃₉]³⁻ (m/z 908, 13%) for reaction system I (Fig. 1), [H₂SiV^IV₂W₁₀O₃₉]⁴⁻ (m/z 648, 100%), [H₂KSiV^IV₂W₁₀O₃₉]³⁻ (m/z 878, 24%) for reaction system II (Fig. 2), and [H₄SiV^IV₃W₉O₃₉]⁴⁻ (m/z 616, 48%), [H₃SiV^IV₃W₉O₃₈]³⁻ (m/z 815, 100%), [H₅NaKSiV^IV₃W₉O₄₀]³⁻ (m/z 847, 31%) for reaction system III (Fig. 3). The most distinctive feature for the reactions with tetravalent vanadium source is that the substitution products are unexpectedly stable whereas spontaneous transformations from mono- into di- and eventually tri-substituted products ({SiW₁₁V} → {SiW₁₀V₂} → {SiW₉V₃}) were observed during the reaction course for the pentavalent vanadium source. Also, the conversion rate is in direct proportion with temperature and concentration of reactants.^5b This phenomenon is very interesting since it states that the valence state of vanadium can substantially affect the stability of the substituted products. Before addressing this point further, two experiments were performed to verify this result: one is to monitor the reaction processes at an elevated temperature (80 °C) by ESI-MS (Fig. S1 in the ESI†); the other is to analyze the reaction solutions as function of reaction time by a different approach called differential pulse voltammetry (DPV, Fig. 4). It can be seen from Fig. S1† that the V⁴⁺-substituted products are indeed very stable even at 80 °C. And the electrochemical data (Fig. 4) also confirm our previous findings by ESI-MS (Fig. 1–3). The presence of the DPV peak located at 0.512 V, 0.360 V and 0.232 V corresponds to the one-electron reductions of [SiV^IVW₁₁O₄₀]⁶⁻ to [SiV^IIIW₁₁O₄₀]⁷⁻, [SiV^IV₂W₁₀O₄₀]⁸⁻ to [SiV^IIIV^IVW₁₁O₄₀]⁹⁻ and [SiV^IV₃W₉O₄₀]¹⁰⁻ to [SiV^IIIV^IV₂W₉O₄₀]¹¹⁻, respectively.

Fig. 1 Real-time ESI-MS monitoring of the reaction mixture 1 comprising of K₈[β-SiW₁₁O₃₉]·14H₂O and 1 equivalent amounts of VOSO₄ at different time intervals ((a) 0 h, (b) 1 h, (c) 2 h, (d) 24 h) at room temperature. (a)–(d) in the left panel and (a′)–(d′) in the right panel represent the low-concentration (5 × 10⁻⁴ M) and the high-concentration reactions (1.7 × 10⁻² M), respectively. The middle color bars represent the corresponding solution colors.

Fig. 2 Real-time ESI-MS monitoring of the reaction mixture 2 comprising of K₈[γ-SiW₁₀O₃₆]·12H₂O and 2 equivalent amounts of VOSO₄ at different time intervals ((a) 0 h, (b) 1 h, (c) 2 h, (d) 24 h) at room temperature. (a)–(d) in the left panel and (a′)–(d′) in the right panel represent the low-concentration (5 × 10⁻⁴ M) and the high-concentration reactions (1.7 × 10⁻² M), respectively. The middle color bars represent the corresponding solution colors.

Fig. 3 Real-time ESI-MS monitoring of the reaction mixture 3 comprising of Na₉[β-HSiW₉O₃₄]·23H₂O and 3 equivalent amounts of VOSO₄ at different time intervals ((a) 0 h, (b) 1 h, (c) 2 h, (d) 24 h) at room temperature. (a)–(d) in the left panel and (a′)–(d′) in the right panel represent the low-concentration (5 × 10⁻⁴ M) and the high-concentration reactions (1.7 × 10⁻² M), respectively. The middle color bars represent the corresponding solution colors.

Fig. 4 Effect of time on the voltammogram of 0.2 mM reaction solutions 1, 2 and 3 with equivalent amounts of vanadium salts in 1.0 M sodium sulfate at pH = 3.0.

The clearly distinctive solution stabilities of the vanadium substituted products, {SiVW₁₁}, {SiV₂W₁₀} and {SiV₃W₉}, resulting from different valence states stimuli us to pursue further about the intrinsic structural reason. DFT calculations were utilized to investigate the relative stability of vanadium-substituted Keggin-type POMs with different vanadium valence states. All mono-, di-, and tri-vanadium-substituted POMs are fully optimized under C_s, C_2v, and C_3v, respectively and the important parameters are given in Table S1 in the ESI.† In all V^IV-containing POMs, the single electrons are mainly localized over the V(d_xy) type orbitals (Fig. 5).

Fig. 5 Spin density distributions for V^IV-containing mono-, di-, and tri-vanadium-substituted Keggin-type POMs.

To verify the received Gibbs free energies, the redox potential of mono-vanadium-substituted POMs is estimated for the process: β-[SiW₁₁V^VO₄₀]⁵⁻ + e⁻ = β-[SiW₁₁V^IVO₄₀]⁶⁻.^5a Since in electrochemical data, the normal hydrogen electrode (NHE) is taken as the zero on relative scale, thus the theoretical values need to be referred to an absolute theoretical zero.¹⁸ For the NHE reaction H⁺ + 1e⁻ → H, Cramer and co-workers reported the calculated absolute zero value is −4.28 eV for the free energy change.¹⁹ Thus the absolute theoretical Ag/AgCl electrode is −4.06 V.²⁰ According to the Nernst equation ΔE° = −ΔG°/nF, the calculated redox potential of β-[SiW₁₁V^IVO₄₀]⁶⁻/β-[SiW₁₁V^VO₄₀]⁵⁻ is 0.53 V, which agrees well with the experimental data (0.54 V).^5a

For all mono-, di-, and tri-vanadium-substituted Keggin anions, the calculation results show that the relative stability order is V^V-containing POMs < V^IV-containing POMs (Fig. 6). The reason of more stable V^IV-containing POMs may be attributed to V⁴⁺ (3d¹) has an additional stabilization from Jahn–Teller distortion, often encountered in octahedral complexes of the transition metals.²¹ Besides, previous DFT studies shown that the charge plays a critical role of influencing the relative stability toward the α/β isomer of transition metal substituted β-Keggin POMs.²² Thus in agreement with relative stability previously reported, the larger the charge discrepancy between the V^V- and V^IV-containing POMs, the larger the difference in relative stability. As a result, the largest relative stability discrepancy is found for the pair of β-[SiW₉V^IV₃O₄₀]¹⁰⁻/β-[SiW₉V^V₃O₄₀]⁷⁻ with ΔG of 182.5 kcal mol⁻¹, while the least relative stability discrepancy corresponds to the least charge-discrepancy, β-[SiW₁₁V^IVO₄₀]⁶⁻ and β-[SiW₁₁V^VO₄₀]⁵⁻ with ΔG of 105.8 kcal mol⁻¹ (Fig. 6). In a word, the mono-, di-, and tri-V^IV-substituted Keggin anions are more stable than the corresponding V^V-containing Keggin anions, which agrees well with our experimental observation.

Fig. 6 The energy discrepancy between the mono-, di-, and tri-vanadium-substituted Keggin anions.

In addition, we try to find out the theoretical base to account for the in-solution transformations of the V^V-substituted products in our previous observation,⁵ {SiV^VW₁₁} → {SiV^V₂W₁₀} → {SiV^V₃W₉}. To quantify the relative stability of the three systems, the following two reactions are proposed (eqn (1) and (2)).

β-[SiV^VW₁₁O₄₀]⁵⁻ + [VO₃(OH)]²⁻ → β-[SiV^V₂W₁₀O₄₀]⁶⁻ + [WO₃(OH)]⁻ (1)

β-[SiV^V₂W₁₀O₄₀]⁶⁻ + [VO₃(OH)]²⁻ → β-[SiV^V₃W₉O₄₀]⁷⁻ + [WO₃(OH)]⁻ (2)

Cronin and co-workers suggested that formation of POMs involves the addition of WO₄ tetrahedron which is present in the form of hydrogentungstate anion [WO₃(OH)]⁻ in aqueous solution, acting as an essential building block in POM synthesis.²³ In our case, the vanadium- and tungsten-oxide units are also used in the forms of [VO₃(OH)]²⁻ and [WO₃(OH)]⁻,²⁴ the geometries of which are fully optimized, and the solution has been considered with IEF-PCM instead of explicit water molecules around. Since the different chemical composition of molecules, we cannot directly compare the computed absolute energies of systems. Hence, we use reaction energy that fulfils conservation of matter to compare these three POMs.²⁵ The energies of the above two processes, ΔG₁ and ΔG₂, determine the relative stability of mono-, di-, and tri-vanadium-substituted Keggin anions. The calculation results of ΔG₁ = −18.6 kcal mol⁻¹ and ΔG₂ = −7.8 kcal mol⁻¹ suggest that the solution stability of the three V^V-substituted POMs decreases in the order of {SiV₃W₉} > {SiV₂W₁₀} > {SiVW₁₁}, in keeping good record with our previous observation that spontaneous transformation (β-[SiW₁₁V^VO₄₀]⁵⁻ → β-[SiV^V₂W₁₀O₄₀]⁶⁻ → β-[SiV^V₃W₉O₄₀]⁷⁻) took place when standing for the solutions of {SiVW₁₁} and {SiV₂W₁₀} for a period of time, respectively. It is worthwhile to mention that the stability order of {SiV_xW_12−x} is opposite to that of lacunary precursors {SiW_12−x} in solution,⁵ namely {SiW₁₁} > {SiW₁₀} > {SiW₉}. That is to say that the lacunary POMs with more vacant sites that can be readily functionalized by TMs are less stable and the as-formed TMSPs are more stable. This stability trend may help synthetic chemists to carefully choose the proper starting material to synthesize the target product. Finally, the gas-phase stability order of the three V^V-substituted POMs was also evaluated for a comparison to the solution-state stability order. Experimentally, this is realized by measuring the E_1/2 value (defined by the point at which half of the isolated parent ion was dissociated) in the dissociation curve obtained by energy-variable CID experiments. It is interesting to find that the V₃-substituted POM {SiV₃W₉} becomes the least stable species with the stability order of {SiV₃W₉} < {SiVW₁₁} < {SiV₂W₁₀} in the gas phase (Fig. S2 in the ESI†). The partially inverse stability order between the gas and solution states is tentatively attributed to the solvation effect, which can substantially lower the energy of the tri-vanadium substituted anion particularly. This manifests the important role of solvation effect in stabilizing vanadium-rich polyoxoanions in solution.

Conclusion

In summary, the reactions of lacunary Keggin-type silicotungstates ({SiW_12−x}, x = 1–3) with vanadyl sulfate (V^IVOSO₄) as the tetravalent vanadium source were successfully monitored in real-time by ESI-MS in conjunction with DPV as a comparison to those with the pentavalent vanadium source. It is concluded that the vanadium valence state can substantially affect the solution-state stabilities of the vanadium-substituted products, the relative solution-phase stabilities decrease in the order of {SiV^IV_xW_12−x} > {SiV^V_xW_12−x}. The DFT calculations explain the intrinsic structural reason as such that V⁴⁺ (3d¹) has an additional stabilization from Jahn–Teller distortion for all V^IV-containing POMs; the larger the charge discrepancy between the V^V- and V^IV-containing POMs, the larger the difference in relative stability. Furthermore, the spontaneous transformations of the V^V-substituted products, {SiV^VW₁₁} → {SiV^V₂W₁₀} → {SiV^V₃W₉}, were quantitatively interpreted by DFT calculation using reaction energy (ΔG) as a criterion to evaluate the relative energies of discrete polyanions. The calculation results of ΔG₁ = −18.6 kcal mol⁻¹ and ΔG₂ = −7.8 kcal mol⁻¹ suggest that the solution stability of the three V^V-substituted POMs decreases in the order of {SiV₃W₉} > {SiV₂W₁₀} > {SiVW₁₁}, in keeping good record with our previous observation. However, the V^V₃-substituted species {SiV₃W₉} becomes the least stable species with the stability order of {SiV₃W₉} < {SiVW₁₁} < {SiV₂W₁₀} in the gas phase based on the E_1/2 values measured by the MS/MS experiments. The partially inverse stability order between the gas and solution states manifests the important role of solvation effect in stabilizing vanadium-rich polyoxoanions in solution. This study may provide a guide for synthetic chemists who intend to prepare variable-structured TMSPs via the reaction of lacunary POMs with transition metals.

Experimental

Sample synthesis

The LPOM salts were synthesized by literature procedures as follows: K₈[β-SiW₁₁O₃₉]·14H₂O,⁸ K₈[γ-SiW₁₀O₃₆]·12H₂O,⁸ Na₉[β-HSiW₉O₃₄]·23H₂O.⁸ All other reagents were obtained from commercial sources and used as received. HPLC grade solvents were generally used.

Solution preparations

(1) Reaction mixtures (1–3) for ESI-MS were prepared by mixing solutions of LPOMs with VOSO₄ in stoichiometric ratios (1 : 1, 1 : 2 and 1 : 3 for mono-, di- and tri-LPOMs : VOSO₄, respectively), timing started at this point for the subsequent reactivity studies upon standing the mixed solutions at room temperature. An immediate color change of the resulting mixed solutions was observed. An aliquot (20 μL) was removed at every 1 hour during the reaction course and immediately diluted with 1 mL water to produce the solution (ca. 10⁻² to 10⁻⁴ M) suitable for ESI-MS analysis.

(2) Reaction mixtures (1–3) for the parallel DPV analyses were prepared by simultaneous addition of solutions of LPOMs and VOSO₄ under stoichiometric conditions to 5 mL of 1 M Na₂SO₄ solution (pH 3.0) to reach a final concentration of 0.2 mM for the analytes, timing started at this point for the subsequent electrochemical tests upon standing the mixed solutions at room temperature.

Mass spectrometry

Mass spectra were recorded in the negative mode on an Agilent 6520 Q-TOF LC/MS mass spectrometer. The m/z values refer to the highest peak in the complex isotopic envelope given by W-containing clusters. The dual spray ionization source conditions were as follows: V_cap 3500 V; skimmer 65 V; drying and nebulizer gas N₂; nebulizer 30 psi; drying gas flow 10 L min⁻¹; drying gas temperature 300 °C; fragmentor 80 V; scan range acquired 50–3000 m/z; injection volume 0.1 μL. Sample solutions were transferred to the electrospray source via an autosampler with a flow rate of 0.2 mL min⁻¹. CID experiments were performed using N₂ as the target gas. The desired multiply charged cluster was isolated and subjected to energy-variable CID in which the applied collision energy was raised incrementally. Plots of relative abundance of the parent ion versus applied collision energy were generated with Microcal Origin 8.0 (Microcal Software, Inc., Northampton, MA, USA) to determine E_1/2 values. The dissociation curves were measured in triplicate for each cluster. All data were collected and processed using MassHunter (Agilent Technologies (China) Co., Ltd) workstation software.

Differential pulse voltammetry (DPV)

Differential pulse voltammetric experiments were performed in a conventional three-electrode system controlled by a CHI 660D electrochemical workstation (Chenhua Instruments, Shanghai, China). The working electrode was a modified glassy carbon (GC) electrode (3 mm diameter). A saturated Ag/AgCl electrode and a platinum electrode were used as the reference electrode and the counter electrode, respectively. All the measurements were made at ambient temperature (25.0 ± 0.1 °C) in the range of potentials extending from +1.0 V to −0.6 V. The voltammogram for each solution was determined in triplicate.

Computational details

All calculations were performed using Gaussian 09 A.02 software package.⁹ The geometry optimizations were carried out by means of hybrid B3LYP exchange–correlation.^10–12 For V and W atoms, LanL2DZ pseudo-potential was added.¹³ The 6-31g(d, p) basis set was used for H, Si and O atoms.^14–16 Vibrational frequency calculations were performed at the same level to verify that an energy minimum was attained. The solvent H₂O was considered using the IEF-PCM¹⁷ and a tight convergence (10⁻⁸ au) criterion was employed in all DFT calculations.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21371025), the 111 Project (B07012) and the Fundamental Research Grant (20121942006) by Beijing Institute of Technology, the Foundation for University Key Teacher of Heilongjiang Province of China (1253G005), the Doctoral Scientific Research Foundation of Daqing normal university (11ZR01) calculations.

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Footnote

† Electronic supplementary information (ESI) available: Additional ESI-MS spectra of the reaction mixtures at 80 °C and dissociation curves for the V-substituted POMs. See DOI: 10.1039/c6ra24432g

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