What can electrospray mass spectrometry of paratungstates in an equilibrating mixture tell us?

Abstract

An acidified aqueous solution of Na₂WO₄ at nearly neutral conditions, in which paratungstates A ([W₇O₂₄]⁶⁻) and B ([H₂W₁₂O₄₂]¹⁰⁻) were coexistent in an equilibrating mixture, was firstly investigated by a combination of ESI-MS, ¹⁸³W NMR and Raman spectroscopy. [W₇O₂₄]⁶⁻ was proved to be the main species in the equilibrating mixtures at pH ≤ 7 by NMR and Raman spectroscopies, however, it failed to be detected by ESI-MS due to its limited stability in the gas phase. The distinctive composition of the precipitate from its precipitating solution suggests that the product isolated in its crystalline form may not necessarily be the one with the highest abundance in solution. This is the first representative example to demonstrate non-compliant structures derived from ESI-MS and NMR studies, illustrative of the limitation of ESI-MS analysis for labile POMs, and to show different speciation in the solid and its corresponding solution.

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Introduction

Polyoxometalates (POMs) are a vast class of anionic oxide clusters of the early transition metals which have been extensively studied due to tunable structures and virtually unmatched range of physical properties.¹ A family of isopolyoxotungstates, e.g. [W₆O₁₉]²⁻,² [W₆O₂₂]⁸⁻,³ [W₇O₂₄]⁶⁻,⁴ [W₁₀O₃₂]⁴⁻,⁵ [H₂W₁₂O₄₀]⁶⁻,⁶ [H₂W₁₂O₄₂]¹⁰⁻,⁷ [H₄W₁₉O₆₂]⁶⁻,⁸ [H₄W₂₂O₇₄]¹²⁻,⁹ [H₁₀W₃₄O₁₁₆]¹⁸⁻,⁹ [H₁₂W₃₆O₁₂₀]¹²⁻,¹⁰ has been synthesized using a routine “one-pot” reaction by acidification of an aqueous tungstate solution to different pH values. However, the complex solution speciation processes behind the simple method arising from multiple equilibria are still not completely understood.

Electrospray ionisation mass spectrometry (ESI-MS) has been used to analyze a range of polyoxoanions, as well as complex mixtures of oxoanions.^11,12 The approach has recently been extended to directly probe the reaction solution, thereby simplifying the conventional crystallization-based procedure to cluster discovery.^13–15 However, people seldom scrutinize this method. There are cases to show failure on detection of intact POMs by ESI-MS¹⁶ and subtle speciation changes due to ESI-induced reactions.^17–20

It is known that paratungstate A ([W₇O₂₄]⁶⁻) is rapidly formed when sodium tungstate is acidified to nearly neutral pH. It will then convert to thermodynamically stable paratungstate B ([H₂W₁₂O₄₂]¹⁰⁻) slowly. The kinetics to establish the equilibrium between the two is quite slow.^21,22 Usually the salt based on [H₂W₁₂O₄₂]¹⁰⁻, not [W₇O₂₄]⁶⁻, crystallizes upon standing of the equilibrating solution²³ unless special measures (i.e., the use of organic amine or transition-metal-complex cations^4d,24–28) were taken. The difficulty in the crystallization of heptatungstate is still a great challenge in POM synthesis. Yamase and coworkers²⁹ characterized a pH 6.8 Na₂WO₄ solution by ¹⁸³W NMR spectroscopy in which three peaks (δ = 268.3, −178.0 and −91.8 ppm) with an intensity ratio of 1 : 2 : 4 can be assigned to paratungstate A anion as a main species. This result tells us the equilibrium between [W₇O₂₄]⁶⁻ and [H₂W₁₂O₄₂]¹⁰⁻ is largely shifted to [W₇O₂₄]⁶⁻ at the pH. Our recent work on the MS of an unusual compound Na[Cu(en)₂]₂[HW₇O₂₄]·5H₂O (en = ethanediamine)^4d indicated that the main species is [W₆O₁₉]²⁻ (W₆), not [W₇O₂₄]⁶⁻ (W₇). The contradictory result from NMR and MS measurements based on the same system begs a question: Is the W₇ anion itself not stable in solution or it is simply not survival the ionization process?

Herein we report a complete investigation on the speciation of an aqueous Na₂WO₄ solution at nearly neutral pH by a combination of ESI-MS, ¹⁸³W NMR and Raman spectroscopy. This work is highlighted by the characterization of paratungstates in an equilibrating mixture in solution and solid phases. The validity of ESI-MS method is also addressed by taking this system as a test case.

Experimental

Sample preparation

Na₂WO₄·2H₂O and formic acid were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd and used without further purification. The measurements of MS, NMR and Raman spectroscopy were conducted in triplicate for each sample. A Hanna HI 8521 pH meter (Hanna Instruments, Italy) was used to measure pH values. All aqueous solutions of tungstate for MS analysis were prepared with a concentration of 10⁻³ M using HPLC-grade water. After adjusting the pH to about 6, 7 and 8 with formic acid, the solutions were allowed to equilibrate for different lengths of time, which were then analyzed. The precipitate from the aqueous solution of tungstate was obtained by adding aqueous solution of tetrabutylammonium bromide with stirring. The resultant precipitates were filtered and washed with water and dried under the ambient environment. The tungstate solutions used for Raman spectroscopy were prepared with concentrations of 0.1 M, 0.5 M, 1 M, 4 M, respectively. TBA₂W₆O₁₉ and Na[Cu(en)₂]₂[HW₇O₂₄]·5H₂O were prepared according to ref. 2 and 4d. The solutions used for ¹⁸³W NMR were prepared by adjusting pH values of the 0.6 M Na₂WO₄ (1 g Na₂WO₄ dissolved in 5 mL of 1 : 1 H₂O : D₂O) to 5.8, 6.8, 7.8 with H₂SO₄ (33 wt%).

Methods

Mass spectra were recorded on an Agilent 6520 Q-TOF LC/MS in the negative ion mode. Each scan or data point on the total ion chromatogram is an average of 5000 transients, producing a scan every 500 ms. Spectra were created by averaging the scans across each peak. The dual-spray electrospray ionization source conditions: V_cap: 3500 V; skimmer: 65 V; OCT R_fV: 750 V; nebulizer: 30 psi; drying gas flow: 10 L min⁻¹; drying gas temperature: 300 °C; fragmentor: 80 V; scan range 100–3000 m/z. Sample solutions were made to approximately 10⁻³ M in water (solutions) or in acetonitrile (precipitates) and analyzed by direct injection using an automatic sampler with a flow rate of 0.2 mL min⁻¹. CID experiments were performed using N₂ as the target gas. Collisional energy voltage was 40 V for [W₆O₁₉]²⁻ precursors. ¹⁸³W NMR spectra of the Na₂WO₄ solution (in H₂O : D₂O = 1 : 1) in nearly neutral condition at 20 °C were obtained on a 400 MHz JEOL ECX NMR system (equipped with 10 mm multinuclear probe head) at 12.5 MHz using external Na₂WO₄ (2.0 mol L⁻¹ in D₂O) as reference. The spectra were recorded over the region between −300 and 500 ppm. Raman spectra were recorded on a RM (Renishaw) spectrophotometer equipped with a liquid-nitrogen-cooled CCD detector. The argon line at 514.5 nm was used for excitation. The measurements were carried out at room temperature (25 ± 3 °C) three times for each sample solution after preparation. The spectra were recorded over the region between 500 and 1500 cm⁻¹. The grating grooves were 1800 L mm⁻¹ and the exposure time was 30 s.

Results and discussion

Analyses of the acidified solutions of Na₂WO₄ at nearly neutral condition (pH = 5.8, 6.8) by ¹⁸³W NMR spectroscopy (Fig. 1) show three groups of W centers, which correspond to a single tungsten center (a) and two types of tungsten centers (consisting of four “cap” (b) and two “belt” (c) W atoms) typically found in a paratungstate A anion [W₇O₂₄]⁶⁻, related to three peaks at δ = 270.7, −90.6, −177.5 ppm with an intensity ratio of 1 : 4 : 2, respectively. This result suggests that paratungstate A anion [W₇O₂₄]⁶⁻ is a main species in the solutions (pH = 5.8, 6.8), consistent with that of pH 6.8 reported in the literature.²⁹ However, this anion starts a noticeable dissociation (into [WO₄]²⁻) at pH 7.8, indicated by the abundant peak of [WO₄]²⁻ in Fig. 1(c).

Fig. 1 ¹⁸³W NMR spectra of Na₂WO₄ solution (in 1 : 1 H₂O : D₂O) as a function of pH (equilibrating time = 4 h): (A), pH 5.8; (B), pH 6.8; (C), pH 7.8.

Raman spectra of the Na₂WO₄ solutions show the characteristic peaks for [W₇O₂₄]⁶⁻ at 962 and 903 cm⁻¹ and for [WO₄]²⁻ at 931 cm⁻¹, respectively, indicating that [W₇O₂₄]⁶⁻ is the dominant species at pH values of 5.8 and 6.8 except for 7.8 in which [WO₄]²⁻ is prevalent (Fig. 2). The characteristic Raman frequencies of isopolytungstates determined to be 931 cm⁻¹ (intense) for [WO₄]²⁻, 962 cm⁻¹ (intense) for [W₇O₂₄]⁶⁻, and 996 cm⁻¹ (intense) for [W₆O₁₉]²⁻ (Fig. S1 in the ESI†), respectively, via using a few compounds with known structures (e.g. an aqueous solution of Na₂WO₄ with adjusted and unadjusted pH, Na[Cu(en)₂]₂[HW₇O₂₄]·5H₂O and TBA₂[W₆O₁₉] in DMF) were in excellent agreement with those reported in the literature.^30,31 Also, it was found that the relative abundances of [W₇O₂₄]⁶⁻ were in direct proportion to its concentrations in solution (Fig. S2 in the ESI†).

Fig. 2 Raman spectra of Na₂WO₄ solution as a function of pH (equilibrating time = 4 h): (A), pH 5.8; (B), pH 6.8; (C), pH 7.8.

The ESI-MS mass spectra of the Na₂WO₄ solutions at pH 5.8, 6.8 and 7.8 after equilibrating for 4 hours were shown in Fig. 3, where the major species changes noticeably with the pH. At pH values of 5.8 and 6.8, [W₆O₁₉]²⁻ exist as main species which were assigned on the basis of the exact mass and isotopic distribution of the clusters. At a slightly basic pH (7.8), however, [W₆O₁₉]²⁻ was barely detectable and [HWO₄]⁻ became the dominant species. The reason leading to the conflicting results between NMR, Raman and MS is the stability of [W₇O₂₄]⁶⁻. Two points should be addressed with regard to the limited stability of [W₇O₂₄]⁶⁻: (1) [W₇O₂₄]⁶⁻ has a lower symmetry in the structure than the Lindqvist analogue [W₆O₁₉]²⁻; (2) it is very difficult to crystallize heptatungstate from solution because paratungstates A and B are formed simultaneously during the acidification of [WO₄]²⁻ at pH 6–8 and establishment of the equilibrium is very slow. To date, only five crystals based on [W₇O₂₄]⁶⁻ have been successfully isolated, Na₆[W₇O₂₄]·21H₂O,^4a (C₅H₁₀NH₃)₆[W₇O₂₄],^4b [(t-C₄H₉)NH₃]₆[W₇O₂₄]·2H₂O,^4c Na[Cu(en)₂]₂[HW₇O₂₄]·5H₂O (en = ethanediamine),^4d and [Co(en)₃]₂[H₂W₇O₂₄]·8H₂O.^4e The limited stability of [W₇O₂₄]⁶⁻ during ESI process was firstly reported by our group for the compound of Na[Cu(en)₂]₂[HW₇O₂₄]·5H₂O,^4d in which the quasi-molecular ion, [H₅W₇O₂₄]⁻ (m/z 1675.8376, 67%) partially dissociates into [W₆O₁₉]²⁻ (m/z 703.8175, 100%). The subsequent CID experiments on [W₆O₁₉]²⁻ generated from two precursors (Fig. 4), one is TBA₂[W₆O₁₉] (dissolved in pure CH₃CN) and the other is an aqueous Na₂WO₄ solution at pH 6.8, confirms the [W₆O₁₉]²⁻ anions generated from different sources have the same Lindqvist structure based on the identical CID spectra.

Fig. 3 Negative-ion ESI mass spectra of Na₂WO₄ solution as a function of pH (equilibrating time = 4 h): (A), pH 5.8; (B), pH 6.8; (C), pH 7.8.

Fig. 4 CID mass spectra of [W₆O₁₉]²⁻ generated from different precursors: (A) TBA₂W₆O₁₉ in CH₃CN; (B) Na₂WO₄ solution at pH 6.8 (equilibrating time = 4 h). Collision energy = 40 eV, isolation width = 4. The parent ion (denoted by a diamond) is shown in a blue square box in each spectrum.

The complete absence vs. partial presence of intact molecular ion [W₇O₂₄]⁶⁻ in the mass spectra of aqueous solutions of Na₂WO₄ at pH 6.8 and Na[Cu(en)₂]₂[HW₇O₂₄]·5H₂O may be attributed to the nature of cations. It was mentioned by many studies that cations affect the solution chemistry and also the gas-phase dissociation chemistry of polyoxoanions.^32–34 That the copper-complex cation (Cu(en)₂²⁺) stabilizes [W₇O₂₄]⁶⁻ to a greater extent than Na⁺ does allows a lot more portion of intact molecular ion to be detected in presence of Cu(en)₂²⁺. As for the same alkali metal cations, Li⁺ has a stronger stabilizing effect on the polyanion than Na⁺. Howarth et al.^35,36 studied aqueous Li₂WO₄ solution at pH ca. 6.0 and showed by ESI-MS and ¹⁸³W NMR spectroscopy to be very largely paratungstate B ([H₂W₁₂O₄₂]¹⁰⁻). This result states that the equilibrium between paratungstates A and B is substantially dependent on the cations. The Li⁺ cation, having the least ionic radius, can effectively shift the balance from paratungstate A to paratungstate B.

The MS analyses of Na₂WO₄ and Na[Cu(en)₂]₂[HW₇O₂₄]·5H₂O in aqueous solutions suggest that the heptatungstate anion can hardly retain its structural integrity in the gas phase, which can be explained by the following reasons. First, the intrinsic structural weakness of the asymmetric anion [W₇O₂₄]⁶⁻. Second, it was reported by a few papers^17–20 that the electrospray ionization process can induce a series of reactions including protonation/desolvation and fragmentation, which make the species in solution more or less different from those detected in the mass spectrum. The ESI-induced dissociation of [W₇O₂₄]⁶⁻ might result from collisions between the analyte and nitrogen gas during the desolvation of electrospray ionization process. This is a special case of non-compliant structures derived from solution and gas phases for the same system. By monitoring the whole equilibrium process of an aqueous Na₂WO₄ solution at pH 6.8, we found out that the W₆ cluster started to be generated in large quantity at 1 hour and kept in growth for longer equilibrium time. No abundant polymerization products other than W₆ clusters were detected during the whole equilibrium period (Fig. S3 in the ESI†).

Finally, we tempted to analyze the constitution of the precipitate isolated from the aqueous Na₂WO₄ solution by adding tetrabutylammonium bromide (TBABr) into the solution at final equilibrating time (48 h). The resultant precipitate was redissolved in CH₃CN for ESI-MS analysis after filtration and a few washings with water. By comparing the mass spectrum of the precipitate with that of the corresponding pH 5.8 aqueous solution of Na₂WO₄ (Fig. 5), it can be seen that a range of W₁₂ species appeared in addition to W₆ clusters, e.g., [TBANa_xH_8−xW₁₂O₄₂]³⁻ (x = 1 and 2, m/z 1049.8479 and 1057.1756, 30% and 77%), [TBA₂Na₄H₃W₁₂O₄₂]³⁻ (m/z 1152.5854, 54%), [TBANa_xH_9−xW₁₂O₄₂]²⁻ (x = 2 and 3, m/z 1586.2871 and 1597.2804, 19% and 23%), [TBA₂Na_xH_8−xW₁₂O₄₂]²⁻ (x = 4 and 5, m/z 1729.3899 and 1740.3744, 13% and 25%) [TBA₃Na₆W₁₂O₄₂]²⁻ (m/z 1871.9984, 12%) and [H₄W₁₂O₄₀]⁴⁻ (m/z 712.3118, 100%), [Na₃HW₁₂O₄₀]⁴⁻ (m/z 728.7865, 91%), [Na_xH_5−xW₁₂O₄₀]³⁻ (x = 0–4, m/z 950.0780, 957.4084 and 964.7362, 972.1154 and 979.3961, 17%, 24%, 33%, 3% and 16%); [W₆O₁₉]²⁻ (m/z 703.8073, 12%). This result manifest a fact that most often the product isolated in crystalline form may not necessarily be the one with highest abundance in solution, suggesting that the crystallisation process itself may cause the selective formation of a particular product out of vast library of candidates available in solution. At this end, it is summarized that [W₇O₂₄]⁶⁻ is the main species in the aqueous Na₂WO₄ solution at pH 5.8 and 6.8, however, this anion can not be isolated from solutions unless special measures were taken. Usually [W₁₂O₄₂]¹²⁻ (mostly in a high degree of cationized and/or dehydrated form due to the instability of highly charged polyoxoanions) is the solid form isolated from the solution. The results in the study are summarized in Scheme 1.

Fig. 5 Comparison of negative-ion ESI-MS mass spectra of (A) Na₂WO₄ solution at pH 5.8 (equilibrating time = 48 h); (B) precipitate isolated from (A).

Scheme 1 The equilibrium between paratungstates A and B in an aqueous solution of Na₂WO₄ at pH ≈ 7 and the actual species detected by ESI-MS.

In summary, this study highlights a combined use of ESI-MS, ¹⁸³W NMR and Raman spectroscopy to address the speciation in an aqueous solution of Na₂WO₄ at nearly neutral conditions. In general, [W₇O₂₄]⁶⁻ is proved to be the main species in an equilibrating mixture at pH ≤ 7 by NMR and Raman spectroscopies. This fragile anion, however, fails to be detected due to its ESI-induced dissociation into Lindqvist [W₆O₁₉]²⁻. The distinctive composition of the precipitate from its precipitating solution suggests that the product isolated in crystalline form may not necessarily be the one with the highest abundance in solution. This study also shows that ESI-MS is applicable mostly to stable polyanions. [W₇O₂₄]⁶⁻ polyanion is a representative of unstable polyanions whose structures are labile, cautions must be taken when correlating the MS data with the solution speciation in this case.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21173021, 21231002, 21276026, 21371025), 973 Program (2014CB932103), the 111 Project (B07012) and the Fundamental Research Grant (20121942006) by Beijing Institute of Technology. We thank Dr Zhengguo Lin for performing the W-NMR analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18059g

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