Experimental and computational exploration of the UV-visible properties of hexaniobate and hexatantalate ions

Abstract

The UV-visible properties of hexaniobate (H_xNb₆O₁₉^x−8) and hexatantalate (H_xTa₆O₁₉^x−8) ions were investigated experimentally and by DFT calculations. Due to the huge discrepancies among the reported values found in prior studies, the extinction coefficients of hexaniobate ions were determined from multiple samples in various media. A simple and low-cost method was then developed for the determination of the niobium content of both synthetic and industrial samples. Taking advantage of the UV spectra of the hexaniobate ions, the first protonation constant of Nb₆O₁₉⁸⁻ could be determined experimentally (pK_a = 13.4 ± 0.1 at 25 °C, Δ_rH = −95 kJ mol⁻¹ in 3 M KOH/KCl) and is in accordance with the reported values previously extrapolated from potentiometric measurements (pK_a = 13.3 ± 0.6). UV batch titrations performed at different ionic strengths suggest that the equilibrium between the monoprotonated and the deprotonated forms of the Lindqvist ion is accompanied by an exchange of potassium ions and that the cluster is easier to deprotonate in Na⁺ media. This study highlights the importance of the ion-pairing on the chemistry of these polyoxometalates. The tremendous difference between the UV spectrum of Nb₆O₁₉⁸⁻ and Ta₆O₁₉⁸⁻ was investigated by DFT computations. The UV spectra were reproduced and show a good agreement with the experimental data. Moreover this study revealed some insights into the evolution of the spectra together with the pH of the medium by studying the nature of the transitions involved in this spectral domain.

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Introduction

Niobium (Z = 41) and tantalum (Z = 73) are attracting increasing attention because they play a critical role in alloys and electronic industries. However, the development of efficient, economic and environmentally friendly processes to recover them from ores is still a challenge. Although classical routes to recover and purify niobium and tantalum require highly acidic and fluoride containing solutions,¹ the chemistry of both valuable metals has been extensively studied in alkaline media over the past decades. Indeed, high solubility of Nb(V) and Ta(V) can be reached in alkaline solutions along with separation from impurities such as Fe, Mn and Ti.² The chemistry of Nb(V) and Ta(V) at basic pH is dominated by the Lindqvist type ions H_xM₆O₁₉^x−8 (M = Nb, Ta; 0 ≤ x ≤ 3).³ The structure of the cluster M₆O₁₉ has been widely investigated and consists of a super octahedron of six edge-sharing octahedra MO₆ yielding three different types of oxygen atoms: one central atom μ₆–O, six terminal oxygens η–O and twelve bridging oxygens μ₂–O (Fig. 1).

Fig. 1 Ball-and-stick representation of the Lindqvist ion M₆O₁₉ (red: oxygens. Blue: niobium or tantalum) which contains 1 central oxygen (μ₆–O), 6 terminal oxygens (η–O) and 12 bridging oxygens (μ₂–O).

Contrasting with the renewed interest in alkaline solutions of Nb and Ta, methods to determine niobium and tantalum content in alkaline samples are still limited to Inductively Coupled Plasma spectrometry. In this regard, the UV-visible properties of the hexaniobate and hexatantalate ions could help to implement new and cheap methods for the determination of Nb and Ta content in samples. UV-based methods could also be developed to probe the solution thermodynamics of H_xM₆O₁₉^x−8 ions, including pK_a determination and stability of these polyoxometalates. Surprisingly, the studies published on the absorbance properties of hexaniobate and hexatantalate ions are scarce and took place in the early development of polyoxoniobates and polyoxotantalates chemistry when the existence of the hexameric species was still controversial. In the early 60's, Jander and Ertel⁴ first performed a spectrophotometric study on hexaniobate ions showing a strong absorbance band below 300 nm with a maximum lying between 250 and 230 nm depending on the pH. The only hexatantalate UV spectrum was reported three years later by Nelson and Tobias⁵ highlighting the low absorbance properties of H_xTa₆O₁₉^x−8 ions compared to their niobium counterparts. Knowing that Nb(V) and Ta(V) have identical ionic radii⁶ and very similar electronic configurations, resulting in quasi-identical geometry for Nb₆O₁₉ and Ta₆O₁₉ clusters,⁷ the striking difference between their UV properties challenges our understanding and needed to be investigated.

The present study revisits the UV properties of hexaniobate and hexatantalate ions. The absorbance properties of hexaniobate ions allowed us to implement a fast and convenient method for the determination of Nb concentration in various samples including those found in industrial processes used for Nb and Ta recovery. Taking advantage of the UV absorbance of hexaniobate ions, the first protonation constant and its reaction enthalpy have been determined experimentally for the first time. Finally, the tremendous difference between the UV properties of Nb and Ta Lindqvist ions has been investigated by DFT calculations and enriches our understanding on Nb and Ta chemistry in alkaline media.

Experimental section and computational details

Regents

All stock solutions were prepared with deionized water (R > 18.2 × 10⁶ Ω) degassed by boiling for 1 h while being purged under N₂. All stock solutions were kept in a glove box under N₂ atmosphere. Nb₂O₅ 99.9% (Sigma Aldrich), Nb(HC₂O₄)₅, nH₂O (Alfa Aesar, 19.2 wt% Nb) and NbCl₅ 99% (Alfa Aesar) were used without further purification. Industrial alkaline solutions of Nb(V) were provided by Eramet Research company (France). These samples were slightly contaminated by Ta(V); Nb/Ta = 80 mol mol⁻¹. Stock solutions of NaCl (VWR), KCl (VWR), CsOH, H₂O (Alfa Aesar) and potassium hydrogen phthalate (99.98%, Fluka) were obtained by direct dissolution of weighed samples in CO₂-free deionized water. The KOH stock solution was purchased and opened under N₂ atmosphere (Alfa Aesar, Selectipur®, 30 wt% KOH. An analytical certificate provided by the manufacturer gives a CO₂ contamination of 0.3 wt% K₂CO₃). The concentrated KOH stock solution was standardized by titrating against potassium hydrogen phthalate under positive N₂ pressure. The titration was repeated four times and yielded a KOH concentration of 6.761 ± 0.020 mol L⁻¹. The NaOH stock solution was prepared by dissolving NaOH pellets in CO₂-free deionized water. The NaOH solution was further purged with N₂ for 1 h and titrated against potassium hydrogen phthalate ([NaOH] = 4.866 ± 0.020 mol L⁻¹). For the synthesis of K₈Nb₆O₁₉, 16H₂O, NbCl₅ was hydrolyzed at pH 4 to produce Nb₂O₅, nH₂O which was then used to produce the potassium hexaniobate salt as reported by Nyman et al.⁸ K₈Nb₆O₁₉, 16H₂O was identified by powder XRD (ICDD PDF 01-077-4938), Raman spectroscopy and thermogravimetric analysis (TGA). Raman (cm⁻¹): 875; 827; 745; 529; 460; 376; 288; 217. TGA: %H₂O found = 19.6 wt% (theoretical: 19.7). Na₇HNb₆O₁₉, 15H₂O was synthesized by alkaline fusion (5 h at 450 °C, NaOH/Nb₂O₅ = 10 mol mol⁻¹), washed and recrystallized in water. The product was also identified by powder XRD (ICDD PDF 04-016-4410), Raman spectroscopy and TGA. Raman (cm⁻¹): 904; 837; 769; 524; 485; 458; 380; 278; 215; 172. TGA: %H₂O found = 21.5 wt% (theoretical: 20.9).

Materials

A Vista Pro spectrometer (Varian) was used for ICP-AES titrations. Quantitative analyses were performed at 295.088 nm spectral emission line for Nb. All UV spectra were measured on a Cary 100-Scan spectrometer used in double beam mode. Each spectrum was acquired in Quartz Suprasil® cuvette (path length of 10.00 mm) and recorded against the corresponding blank sample. The cuvette lixiviation in high alkaline media was checked to be very limited in order to avoid silicate contamination (provider's data: dissolution of 0.27 μg cm⁻² h⁻¹ with KOH 30 wt%). The alkaline samples were hold in the cuvettes less than 5 min and then discarded.

UV batch titrations

Each acid–base titration contained, at least, 8 samples prepared under N₂ atmosphere by diluting the stock solutions of KCl, KOH and K₈Nb₆O₁₉, 16H₂O or NaCl, NaOH and Na₇HNb₆O₁₉, 15H₂O with CO₂-free deionized water. Polyethylene vessels were used to avoid contamination by silicates. The hydroxide concentration was varied from 0.01 mol L⁻¹ to the maximum value (which equals the ionic strength of the series) while the niobium concentration and the ionic strength were kept constant. The total niobium concentration was typically 2.4 × 10⁻⁴ mol L⁻¹ i.e. ∑[H_xNb₆O₁₉^x−8] = 4 × 10⁻⁵ mol L⁻¹. The corresponding blank samples were prepared similarly without niobium. After being thermostated under N₂ atmosphere for 24 h, the UV spectrum (220–350 nm – 260 data points) of each sample was recorded against its corresponding blank.

Data treatment

For each batch titration, the UV spectra, the extinction coefficient of the deprotonated species, the total hexaniobate concentration and the estimated pH were imported into the refinement program HypSpec⁹ and analyzed by nonlinear least-squares refinement. The determination of equilibrium constants by this software has proven its efficiency elsewhere.^10–12 The protonation constant given is this work refers to the following equation:

Nb₆O_19(aq)⁸⁻ + H₃O⁺ = HNb₆O_19(aq)⁷⁻ + H₂O (1)

Due to the low concentration of hexaniobate compared to hydroxide ions in the samples ([HNb₆O₁₉⁷⁻] + [Nb₆O₁₉⁸⁻] = 4 × 10⁻⁵ mol L⁻¹ compared to 0.01 ≤ [HO⁻] ≤ 3.00 mol L⁻¹), the hydroxide concentration in each sample was evaluated from the volume of standardized stock solutions. The corresponding pH was calculated using eqn (2).

The K_w values in KCl/KOH media at the different ionic strengths and temperatures were taken from Sweeton et al.¹³ The K_w values in NaCl/NaOH media were taken from Kron et al.¹⁴ The whole procedure (samples preparation, UV batch titration and data treatment) was duplicated for 3 different ionic strengths and 2 different temperatures so that the uncertainty of the method was determined to be ±0.1 log unit.

Computational details

All calculations were carried out using DFT-based methods, and more specifically the PBE0 global hybrid functional.^15,16 The Gaussian 09 software was used throughout.¹⁷ For optimizations, oxygen was represented by the Dunning basis set¹⁸ while Nb and Ta were described by the Stuttgart–Dresden ECP and its associated basis set.¹⁹ For thermodynamics and UV-vis properties, single points were processed on the optimized structured using the larger Def2-TZVP basis set for the three elements.²⁰ Optimizations were carried out without any symmetry constraint using standard algorithms and the nature of the minima was checked by vibrational analysis. The solvent was taken into account using an implicit model, namely the Polarizable Continuum Model (PCM)²¹ as implemented in Gaussian 09.

Electronic transitions were computed in the time-dependent variant of DFT (TD-DFT) within the adiabatic approximation taking into account the 250 lowest-lying excited states. To produce spectra, each transition was convoluted using a Gaussian-centered function with a 3000 cm⁻¹ FWHM and all components were added.

Results

UV properties of hexaniobate ions

The UV spectrum of hexaniobate solutions displays a large absorbance band below 300 nm with a maximum lying between 250 and 240 nm depending on the hydroxide concentration (Fig. 2). Upon increasing of the pH, the absorbance wavelength of the charge transfer band shifts from 241.5 nm in presence of 0.01 M of KOH to 247.5 nm in 3.00 M KOH. Measurements performed with a KOH concentration ranging from 3 M to 9 M did not show further variations of the UV spectra (Fig. 3 and S1†). The red shift observed in the UV spectrum is characteristic of the deprotonation of the hexaniobate ion HNb₆O₁₉⁷⁻ and suggests that a hydroxide concentration higher than 3 M is required to fully deprotonate the hexaniobate cluster. The isosbestic point observed at 240 nm indicates the presence of an equilibrium between the deprotonated and the monoprotonated clusters. Taking into account these observations, the extinction coefficient of the fully deprotonated cluster Nb₆O₁₉⁸⁻ was determined experimentally.

Fig. 2 UV spectra of hexaniobate solutions with hydroxide concentration varying from 0.01 M to 3.00 M. I = 3 M (KCl/KOH). T = 25 °C. [Nb₆O₁₉]_total = 6.3 × 10⁻⁵ M. Path length = 10.00 mm. Inlet: absorbance change at 247.5 nm (triangles), 240.0 nm (circles) and 230.0 nm (squares) with the corresponding HypSpec fits.

Fig. 3 Variation of the wavelength at maximal absorbance for H_xNb₆O₁₉^x−8 as a function of hydroxide concentration. I = 3 M (KCl/KOH) for samples with [HO⁻] ≤ 3 M. Ionic strength not maintained for samples with [HO⁻] > 3 M. T = 25 °C. [Nb₆O₁₉]_total = 6.3 × 10⁻⁵ M.

Due to huge discrepancy in the literature data,^4,22,23 special care was taken in order to determine the molar absorptivity of Nb₆O₁₉⁸⁻. More than thirty independent samples with various media (Fig. S2†) were diluted in 4 M KOH, their UV spectrum was recorded and their niobium content was determined by conventional Inductively Coupled Plasma spectrometric titration (ICP-AES). The extinction coefficient ε_Nb6O19 = 15 900 ± 600 L mol⁻¹ cm⁻¹ was determined for Nb₆O₁₉⁸⁻. In order to validate this value, samples of niobium oxalate were prepared by dissolution of commercial Nb(HC₂O₄)₅, nH₂O in water, the samples were then diluted in 4 M KOH and their UV spectra were recorded after 10 min equilibration time. The same procedure was implemented for solutions of commercial NbCl₅ dissolved in HCl. The Nb concentration of each sample was calculated using Beer–Lambert's law and the extinction coefficient value previously determined.

The theoretical Nb concentrations and those obtained by the UV titration are in good agreement for both niobium chloride and niobium oxalate samples as displayed in Fig. S3.† The UV-based method was also used to determine the Nb content of 17 independent industrial solutions of hexaniobate. The concentrations measured by UV titration and by ICP-AES are in good agreement (Fig. S4†) confirming the value of 15 900 L mol⁻¹ cm⁻¹ ε_Nb6O19. The linear range of the method was found to be 5 × 10⁻⁶ to 10⁻⁴ M of Nb₆O₁₉⁸⁻. The influence of the ionic media on the UV spectrum of Nb₆O₁₉⁸⁻ was checked by diluting hexaniobate samples in 4 M NaOH, KOH and CsOH. The nature of the alkali ion does not influence the shape of the charge transfer band but the wavelength at maximum absorbance (λ_max) is slightly shifted from 246.5 in NaOH to 247.5 in KOH and 249 nm in CsOH (Fig. 4).

Fig. 4 Absorbance spectra of Nb₆O₁₉⁸⁻ measured in 4 M NaOH (orange), 4 M KOH (black) and 4 M CsOH (blue).

UV properties of hexatantalate ions

Compared to their niobium counterparts, solutions of hexatantalate ions H_xTa₆O₁₉^x−8 (x ≤ 3) exhibit a relatively low absorbance in the UV region. Variation of the hydroxide concentration from 0.01 M to 9 M has no influence on the UV spectra suggesting that the hexatantalate ion is fully deprotonated at pH ≥ 12. At 247.5 nm, which is the wavelength of maximum absorbance for Nb₆O₁₉⁸⁻, the molar absorptivity of Ta₆O₁₉⁸⁻ is 80 times lower than that of its niobium equivalent (Fig. 5). At the isosbestic point of 240 nm observed for hexaniobate, the ratio ε_Nb6O19/ε_Ta6O19 is still 20.

Fig. 5 Molar absorptivity of Ta₆O₁₉⁸⁻ (orange) and Nb₆O₁₉⁸⁻ (blue) determined experimentally and molar absorptivity of HNb₆O₁₉⁷⁻ (purple) calculated from UV batch titrations.

pK_a determination

The variation observed on the hexaniobate UV spectrum while varying the hydroxide concentration was large enough to accurately determine equilibrium constant. Nonlinear least-squares fits of the spectra included the fully deprotonated species Nb₆O₁₉⁸⁻ and the monoprotonated species HNb₆O₁₉⁷⁻ (Fig. 5). The refinement process also enabled us to determine the extinction coefficient of the monoprotonated species HNb₆O₁₉⁷⁻.

Spectrophotometric batch titrations were performed in KOH/KCl and NaOH/NaCl media at different ionic strengths and temperatures. The equilibrium constant decreases when the ionic strength increases. At a given ionic strength and temperature the pK_a HNb₆O₁₉⁷⁻/Nb₆O₁₉⁸⁻ was found to be lower in NaCl/NaOH media than in KCl/KOH media. The results are summarized in Fig. 6.

Fig. 6 Protonation constant of Nb₆O₁₉⁸⁻ measured at 25 °C in KCl/KOH media (square) and NaCl/NaOH media (circle) and values extrapolated by Neumann²⁴ (triangle), Spinner et al.²⁵ (crosses) and Etxebarria et al.²⁶ (lozenge). The error bars correspond to ±0.1 log unit.

For an ionic strength of 3 M (KOH/KCl), the equilibrium constant was determined at different temperatures (Fig. 7). The refinement of the titration data showed that the reaction is exothermic. Using Van't Hoff's equation, the reaction enthalpy was determined and yielded a Δ_rH value of −95.1 kJ per mole of hexaniobate.

Fig. 7 Effect of temperature on the protonation constant of Nb₆O₁₉⁸⁻. I = 3 M (KCl/KOH). The error bars correspond to ±0.1 log unit.

Discussion

Even though hexaniobate and hexatantalate ions have been known for a long time,^4,5 their UV-vis properties are still poorly investigated. To our knowledge, the only published data mentioning the UV properties of Nb and Ta Lindqvist ions are graphical.^5,22–24,27 Extrapolation of the molar absorption coefficient from these pioneer studies revealed a huge discrepancy as shown in Table 1. Thus, a precise determination of the extinction coefficients was needed in order to develop UV-based methods for probing the hexaniobate and hexatantalate solution thermodynamics. The strong discrepancy between literature data is surprising but might be imputable to the titration methods used for niobium at the time like ignition and weighting of the sample as Nb₂O₅ which can be easily biased in presence of alkali ions (A) due to the formation of ANbO₃.

Table 1 Reported values for the extinction coefficient of hexaniobate and hexatantalate ions

pH of measurement	Species	λmax (nm)	ε^a (L mol^−1 cm^−1)	Reference
a Value calculated from the metal concentration, path length and absorbance published in the original articles. Absorbance values and wavelengths were determined with PlotDigitizer software.
9.9–11.2	—	∼233	∼25 000	4
13	—	∼243	∼24 000	4
14	—	∼248	∼29 000	4
13.5	—	242	∼10 900	22
10.2–11.5	H3Nb6O19^5−	∼235	3200	23
10.2–11.5	H2Nb6O19^6−	∼232	3000	23
10.2–11.5	HNb6O19^7−	∼235	2400	23
	Nb6O19^8−	∼248	3000	23
8.5	—	230	∼12 500	27
12–14.6	HNb6O19^7−	240.5	14 300 ± 400	This work
12–14.6	Nb6O19^8−	247.5	15 900 ± 600	This work
12.2	Ta6O19^8−	—	3600 at 230 nm	5
12–15	Ta6O19^8−	—	4000 ± 150 at 230 nm	This work

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The strong absorbance band observed for H_xNb₆O₁₉^x−8 ions varies with the pH due to the protonation or deprotonation. The strongest absorbance is observed at hydroxide concentration higher than 3 M where the cluster is fully deprotonated. Therefore, the isolation of Nb₆O₁₉⁸⁻ under high alkaline conditions allows the direct determination of its molar absorptivity. In a prior study, Spinner and co-workers²³ reported that after 3 days and at pH ≥ 14, hexaniobate ions are transformed into the tetrameric species Nb₄O₁₂(OH)₄⁸⁻ and Nb₄O₁₆¹²⁻. This assumption was based on spectrophotometric measurements. We did not observe significant variations on the UV spectra over time, even after several days in solutions containing up to 9 M KOH (Fig. S1†). It should be noted that hexaniobate salts can be synthesized in very basic conditions such as alkaline fusion so it would be very unlikely that the Lindqvist ion is unstable at high pH. The same group also reported extinction coefficient values for H_xNb₆O₁₉^x−8 ions largely underestimated as highlighted in Table 1. In one hand, the formation of tetrameric species in solution was never confirmed by another group but Spinner's study was mentioned in few literature reviews.^26,28,29 In the other hand, Nb₆O₁₉⁸⁻ was observed in solutions at pH higher than 14 by multiple techniques (Raman, SAXS, NMR).^30–32 Consequently, the existence of tetrameric niobates seems rather unlikely.

The strong absorbance that exhibits Nb₆O₁₉⁸⁻ offers an easy and cheap way of titrating niobium solutions. Several UV-based methods have been developed previously for Nb(V) containing samples but they all require expensive or toxic complexing agents and none of them were applied to alkaline samples.^33–36 Moreover, these methods often contain a liquid–liquid extraction step prior to the spectrophotometric measurement. After determining precisely the extinction coefficient of Nb₆O₁₉⁸⁻ ion, acidic samples of niobium oxalate and niobium chloride could be titrated by their dilution in 4 M KOH. In chloride media, Nb(V) forms chloro–oxo ions [NbO_m(OH)_nCl_l]^{5−2m−n−l} whereas solutions of commercial Nb(HC₂O₄)₅, nH₂O contains both [NbO(C₂O₄)₃]³⁻ and [NbO(H₂O)(C₂O₄)₃]⁻.^30,37 [NbO_m(OH)_nCl_l]^{5−2m−n−l}, [NbO(C₂O₄)₃]³⁻ and [NbO(H₂O)(C₂O₄)₂]⁻ ions are readily transformed into hexaniobate ions since (i) the strong absorbance band at 247.5 nm is observed after dilution of niobium chloride or niobium oxalate solutions in alkaline media, and (ii) the Nb contents determined by spectrophotometric titration match those determined by ICP-AES. This could open ways for new synthetic methods of hexaniobate salts under mild aqueous conditions and using commercially available materials since it is not the case for Nb₂O₅, nH₂O used in many synthesis. The method developed (dilution in 4 M KOH and UV titration) could also be useful for the determination of Nb content in hydrous niobium oxide Nb₂O₅, nH₂O which is known to be soluble in alkaline solutions⁸ and used in several niobium studies.³⁸ Our UV-based method was also applied to industrial samples of hexaniobates since alkaline media are catching growing attention in Nb and Ta recovery processes. The Nb samples were slightly contaminated by tantalum (Nb/Ta = 80 mol mol⁻¹) but due to the low absorbance properties of Ta₆O₁₉⁸⁻ compared to Nb₆O₁₉⁸⁻ (ε_Nb6O19/ε_Ta6O19 = 80 at 247.5 nm), the niobium concentration can still be determined by UV spectrophotometry. Thus, UV titrations of niobium can be helpful in the development of hydrometallurgical processes that aim at recovering or purifying niobium from ores.

Previous studies revealed that Nb₆O₁₉⁸⁻ and Ta₆O₁₉⁸⁻ clusters can be protonated. The protonation constants reported for these ions are given in Table 2. All reported data on hexaniobate ions are derived from potentiometric titrations, and due to the high value of the first protonation constant (>12.5), the authors had to extrapolate it. At an ionic strength of 3 M KCl/KOH, the value determined in the present study is in the range of the previously extrapolated values. It should be underlined that the pK_a reported by Spinner²⁵ is about 1 log unit lower than those reported by Neumann, Etxebarria et al. and this work.

Table 2 Protonation constants of hexaniobate and hexatantalate ions

i	log βi for reaction Nb6O19^8− + iH^+ = HiNb6O19^i−8
a Potentiometric measurements.
1	13.8 ± 0.2	12.6 ± 0.1	13.63 ± 0.04	13.4 ± 0.1
2	24.68 ± 0.05	23.46 ± 0.15	23.55 ± 0.04	—
3	—	32.85 ± 0.17	32.90 ± 0.07	—
	3 M KCl, T = ?	3 M KCl	3 M KCl	3 M KCl
	10.7 ≤ pH ≤ 12.4	25 °C	25 °C	25 °C
		8 ≤ pH ≤ 12.1	9.7 ≤ pH ≤ 13.1	12 ≤ pH ≤ 14.5
	Pot.^a	Pot.^a	Pot.^a	UV-vis
Ref.	24	25	26	This work

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i	log βi for reaction Ta6O19^8− + iH^+ = HiTa6O19^i−8
1	<10	12.68 ± 0.1	13.89 ± 0.02	11.5
2	—	23.49 ± 0.2	25.91 ± 0.01	20.8
3	—	32.77 ± 0.25	—	—
	0.5 M KCl	1 M KCl	3 M KCl	3 M KCl
	25 °C	25 °C	25 °C	25 °C
	10 ≤ pH ≤ 13	8.5 ≤ pH ≤ 12	12.4 ≤ pH ≤ 13	10.4 ≤ pH ≤ 14.5
	Pot.^a	Pot.^a	Pot.^a	^17O NMR
Ref.	5	43	42	7

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The protonation constant measured at different ionic strengths in K⁺ containing media shows a decrease of this constant when the potassium concentration increases (Fig. 6). The trend is opposed to what was observed by Spinner et al. but, as explained above, their pioneer studies seem doubtful. Interactions between hexaniobate and K⁺, Rb⁺ or Cs⁺ ions at very high pH values have been highlighted by the Nyman's group based on small-angle X-ray scattering experiments.^31,39 The authors reported the formation of the solvent-shared ion-pairs K₈Nb₆O₁₉ and [K₁₀Nb₆O₁₉]²⁺ in 3 M potassium hydroxide solutions. In the solid state, the eight K⁺ ions are associated by the cluster faces and edges through the bridging oxygens (Fig. 1).⁸ It has also been proved by multinuclear NMR studies that the protonation of Nb₆O₁₉⁸⁻, in both the solid state and solution, occurs at the bridging oxygens and not at the terminal oxygens.⁴⁰ This was also checked by our DFT computations: while the addition of one proton on a terminal oxygen leads to structures that are very difficult to stabilize, the binding of a proton on a bridging oxygen is easily realized and leads to a structure consistent with expectations. Therefore, high alkali concentration should facilitate the formation of non-protonated Nb₆O₁₉⁸⁻ yielding a lower protonation constant.

Due to low solubility of sodium hexaniobate salts and the alkaline error in Na⁺ media, the previous potentiometric studies were not performed in such media (Table 2). The low Nb concentration required by UV titration ([Nb₆O₁₉] = 1 to 5 × 10⁻⁵ M) allowed us to investigate the protonation of Nb₆O₁₉⁸⁻ in Na⁺ media. The measurements in NaCl/NaOH media yielded lower protonation constants than in KOH/KCl media (Fig. 6). It is well-known that ion-pairing between alkali and hexaniobate ions follows the trend Cs > Rb > K > Na > Li.^3,39 Contrary to Li⁺ and Na⁺ media, the strong ion-pairing effect in K⁺, Rb⁺ and Cs⁺ media allows the stabilization of protonated clusters as it is visible in the solid state. Indeed, di-protonated and mono-protonated hexaniobate salts can be synthesized with potassium, rubidium and cesium^8,41 whereas di-protonated sodium hexaniobate and protonated lithium hexaniobate have never been isolated. Thus, H_xNb₆O₁₉^x−8 ions are easier to deprotonate in Li⁺ and Na⁺ than in K⁺, Rb⁺ and Cs⁺ media. This implies that proton equilibria for hexaniobate clusters should be seen as an exchange between alkali ions and protons rather than a simple proton release. Nonetheless, the stoichiometry and formation constants of the ion-pairs formed between K⁺ and HNb₆O₁₉⁷⁻ are still unknown so that the current acid–base model cannot take into account the ion-pairing effect.

For H_xTa₆O₁₉^x−8, no changes where observed on the UV spectra at pH ≥ 12 indicating that the tantalum cluster is fully deprotonated under these conditions. The published values on the first protonation constant of Ta₆O₁₉⁸⁻ are largely dispersed compared to those for Nb₆O₁₉⁸⁻ (Table 2). Our observations are in agreement with the most recent study performed by Balogh et al.⁷ Knowing that a protonated hexatantalate salt has never been isolated by aqueous synthesis, the pK_a values of 12.68 and 13.89, extrapolated from potentiometric measurements by Arana et al.⁴² and Spinner and Kheddar,⁴³ respectively, seem overestimated. To support this conclusion, the proton transfer was studied by DFT computations. More precisely, the thermodynamic quantities of reaction (3) were computed.

HNb₆O_19(aq)⁷⁻ + Ta₆O_19(aq)⁸⁻ = Nb₆O_19(aq)⁸⁻ + HTa₆O_19(aq)⁷⁻ (3)

Interestingly, Δ_rG^o = 7.1 kJ mol⁻¹ is obtained (as explained in computational details, the energetic contribution to this quantity is computed at the Def2-TZVP level while the entropic contribution is computed with a smaller model). For the sake of clarity, Δ_rG^o should be expressed in terms of pK_a difference under the form pK_a(HNb₆O₁₉⁷⁻/Nb₆O₁₉⁸⁻) − pK_a(HTa₆O₁₉⁷⁻/Ta₆O₁₉⁸⁻) = 1.3. Considering the precision of the methods used, this value is in good agreement with the pK_a's determined experimentally in this work and by Balogh et al. whose value is 1.9 (Table 2), thus, confirming the more difficult protonation of the hexatantalate versus the hexaniobate. These computational results are also consistent with those obtained in previous works.⁷

The effect of temperature on the protonation of Nb₆O₁₉⁸⁻ was evaluated (Fig. 7). The protonation reaction was found exothermic with a reaction enthalpy of Δ_rH = −95.1 kJ per mole of hexaniobate. Consequently, the non-protonated species is favored when the temperature increases. The protonation constant was shifted by almost 1 log unit between 20 °C and 40 °C highlighting the importance of this parameter during the synthesis of hexaniobate salts and for separation processes. The temperature effect could also explain the slightly higher value reported in Neumann's study which was performed at room temperature (Table 2).

The tremendous difference between the UV spectra of the isostructural ions Nb₆O₁₉⁸⁻ and Ta₆O₁₉⁸⁻ was investigated by DFT calculations. The UV spectra were computed for each species and are depicted in Fig. 8. A good agreement is obtained between the computed and experimental spectra as shown by the comparison between Fig. 5 and 8 (the wavelengths domain is more extended on the computed spectra than on the experimental one). Especially, the computed absorption is much lower in the case of hexatantalate from 230 nm. It should be noticed than the convoluted bands originates from several transitions; to give more insights on this aspect, the nature of the main transitions are reported in Table 3.

Fig. 8 Computed transitions (orange) and convoluted spectra (blue) for Nb₆O₁₉⁸⁻ (a) and Ta₆O₁₉⁸⁻ (b) ions. The scale is the same for both spectra.

Table 3 Main electronic transitions (f > 0.03) computed for each species, oscillator strength and nature of the transition on the 200–300 nm domain

Species	Transition wavelength (nm)	Oscillator strength	Nature of the transition
a Several transitions are computed around 199 nm that justifies the general features of the convoluted spectrum for hexatantalate.
Nb6O19^8−	248.3	0.135	n(μ2O) → π*(Nb–μ2O)
	248.1	0.136	n(μ2O) → π*(Nb–μ2O)
	248.1	0.136	n(μ2O) → π*(Nb–μ2O)
	222.8	0.140	n(O (all)) → n(μ2O)
	222.7	0.136	n(O (all)) → n(μ2O)
	222.7	0.136	n(O (all)) → n(μ2O)
Ta6O19^8−^a	215.4	0.118	n(μ2O) → π*(Ta–μ2O)
	215.3	0.118	n(μ2O) → π*(Ta–μ2O)
	215.2	0.115	n(μ2O) → π*(Ta–μ2O)

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The precise investigation of the transitions explains two important aspects of the measured spectra. First, the apparent lack of absorption of the hexatantalate is simply explained by a blue-shift of the transitions whose lowest absorption bands emerge from. Indeed, the transitions displace electron density from bridging oxygens to π* orbitals mainly localized on the metal atom. The energetic level being higher in Ta(V) than in Nb(V) (the latter is above than the former in the periodic table), the transition in consistently blue-shifted in Ta vs. Nb. Second, the effect of the protonation of hexaniobate (reported in Fig. 5) consists mainly in a blue-shift of the measured band. Once again it should be explained while considering the nature of the transition: the protonation of μ₂O (justified above) leads to a lower possibility for the electron density around those atoms to be implied in transitions, then to a blue-shift of the absorption band.

Conclusions

The hexaniobate ions, HNb₆O₁₉⁷⁻ and Nb₆O₁₉⁸⁻, exhibit a strong absorbance band in the UV region 230–250 nm. Taking advantage of the different UV-vis properties of Nb₆O₁₉⁸⁻ and Ta₆O₁₉⁸⁻, a cheap and fast method was developed to determine the Nb content of both synthetic and industrial samples. For the first time, the protonation constant of Nb₆O₁₉⁸⁻ has been determined experimentally. The nature of the electrolyte and the temperature were found to play a major role on the protonation of Nb₆O₁₉⁸⁻ highlighting the need for further investigations on the ion-pairing between alkali ions and polyoxometalates. Due to the large uncertainties on previously reported studies, the absorbance properties and the protonation of the isostructural ions Ta₆O₁₉⁸⁻ and Nb₆O₁₉⁸⁻ were investigated by quantum-chemistry-based methods and corroborate the experimental findings. The UV properties of the hexaniobate ions could help to develop UV-based methods for probing the solution thermodynamics of Nb(V) in alkaline media. We are currently exploiting the spectral properties of H_xNb₆O₁₉^x−8 and H_xTa₆O₁₉^x−8 to study alkaline solutions of Nb(V) and Ta(V) by capillary electrophoresis.

Acknowledgements

This work was granted access to the HPC resources of CINES under the allocation 2014086933 made by GENCI (Grand Equipement National de Calcul Intensif). We also thank the CRI (Centre de Ressources Informatiques) of the Université de Lille for providing computing time for part of the theoretical calculations. The experimental part of this work was financed by Eramet Research (France).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14866e

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