Synthesis, crystal structure and characterization of alkali metal hydroxoantimonates

Inorganica Chimica Acta 378 (2011) 24–29

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Synthesis, crystal structure and characterization of alkali metal hydroxoantimonates Alexey A. Mikhaylov a, Elena A. Mel’nik a, Andrei V. Churakov a, Vladimir M. Novotortsev a, Judith A.K. Howard b, Sergey Sladkevich c, Jenny Gun c, Subramanian Bharathi c, Ovadia Lev c,⇑, Petr V. Prikhodchenko a,c,⇑ a b c

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii Prosp. 31, Moscow 119991, Russia Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK The Casali Institute of Applied Chemistry, The Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

a r t i c l e

i n f o

Article history: Received 16 May 2011 Received in revised form 1 August 2011 Accepted 3 August 2011 Available online 23 August 2011 Keywords: Antimonite Antimony Hydroxo complex Crystal structure Mass spectrometry

a b s t r a c t Several alkali metal hydroxoantimonates, K2[Sb(O)(OH)5], Na[Sb(OH)6], Cs[Sb(OH)6] and Cs2[Sb2(lO)2(OH)8] were isolated from aqueous solutions and characterized by single crystal and powder X-ray diffraction studies and by FTIR and thermal analysis. Crystal structures involving [Sb(O)(OH)5]2 were never anticipated before, and this is also the first disclosure of a dinuclear antimonate [Sb2(l-O)2(OH)8]2 . Aqueous antimonate solutions of different pH were studied by high resolution electrospray mass spectrometry showing pH indifferent spectra and predominance of the mono and dinuclear antimonate species at pH 4–10. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Aqueous antimonates have attracted considerable scientific attention recently because antimony, a group V element, can form metal vacancies in doped metal oxides (e.g. zinc oxide [1] and antimony doped tin oxide, ATO [2]), can regulate metal oxide crystal growth and alter its morphology [3], and is an emerging and promising photocatalyst [4]. Since antimony is prevalent in the Earth’s crust there is also growing interest in its geological and environmental chemistry [5], particularly since its concentration steadily rises in the environment due to its industrial usefulness [6]. However, despite the growing interest in aqueous antimonates there is little solid evidence regarding the structure of Sb(V) in aqueous solutions and in solids. Deciphering the aqueous chemistry of antimonates is especially demanding. Sb-NMR is practically unavailable and the computational chemistry is still a challenge, which makes X-ray diffraction studies of soluble species one of the major tools to mirror aqueous speciation. However, the only

⇑ Corresponding authors at: Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii Prosp. 31, Moscow 119991, Russia. Fax: +7 4959541279 (P.V. Prikhodchenko), tel.: +972 26584191; fax: +972 26586155 (O. Lev). E-mail addresses: [email protected] (J.A.K. Howard), ovadia@vms. huji.ac.il (O. Lev), [email protected] (P.V. Prikhodchenko). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.08.007

published crystal structures of antimonates isolated from aqueous solution pertain to mopungite, sodium hexahydroxoantimonate, which was first described by Asai [7] and later with higher accuracy by Palenik et al. [8], and to the polyoxoanion salt K6[Mo4Sb6(OH)12O24]
6H2O [9]. Surprising as it may sound there is no reported successful attempt to crystallize other alkali metal hydroxoantimonates. In fact, due to the lack of analytical speciation tools for antimonates, it is often assumed [5,6,10] that high pH aqueous antimonates are present only as [Sb(OH)6] , similar to tin chemistry [11]. Our findings illuminate the difference between the aqueous chemistries of the two elements. Attempts to isolate crystalline alkali metal hydroxoantimonates were frustrated by the formation of amorphous precipitates. Admittedly this was the reason why Nakano et al. [12] resorted to the study of nonaqueous media, and in fact Palenik et al. [8] obtained their sodium antimonate crystals as a byproduct in the synthesis of Sb(III) tartrate. An octanuclear antimonate species was isolated by Nakano et al. [12], and this species reacted with alkylsilanol to give a hexa-silylated tetraantimonate crystalline structure [13]. Here we report the synthesis of Na, K and Cs hydroxoantimonates and characterize their structures by single crystal and powder X-ray diffractometry, and by FTIR. The sodium antimonate aqueous solutions were studied by high resolution electrospray ionization mass spectrometry as a function of pH.

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2. Experimental 2.1. Materials Antimony(V)chloride, ammonium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide were purchased from Aldrich–Sigma. 2.2. Preparation of the complexes 2.2.1. K2[Sb(O)(OH)5] (1) Antimony(V)chloride (5.78 g, 19.3 mmol) was dissolved in water and hydrolyzed by ammonia at pH 5–6. The precipitate of antimony hydroxide was separated by centrifugation, washed four times by water and then introduced to 15 ml of 5 M potassium hydroxide aqueous solution (75.0 mmol). The resulting mixture was heated for about 2 h. After complete dissolution of the precipitate, the filtered solution was evaporated at 90 °C under nitrogen flow. The resulting colorless crystals were separated from residual mother liquor by filtration, washed three times with 40% aqueous ethanol and once with diethyl ether and dried in vacuum. Yield 66% (based on Sb). Thermal analysis revealed decomposition and water loss at 175 °C. Anal. Calc. for H5K2O6Sb (1): K, 25.98; Sb, 40.45. Found: K, 25.20; Sb, 41.02%. 2.2.2. Na[Sb(OH)6] (2) Compound 2 was obtained by exchange reaction of NaCl with compound 1 in aqueous media as described by Asai [7]. Thermal analysis revealed decomposition and water loss at 185 °C. Anal. Calc. for H6Na1O6Sb (2): Sb, 49.34. Found: Sb, 48.85%. 2.2.3. Cs[Sb(OH)6] (3) Antimony(V)chloride (4.65 g, 15.6 mmol) was dissolved in water and hydrolyzed by ammonia at pH 5–6. The precipitate of antimony hydroxide was washed four times with water and dissolved in 37.3 g of 50% cesium hydroxide aqueous solution (124.4 mmol). Resulting solution was evaporated at 90 °C under nitrogen flow. The resulting colorless crystals were separated from residual mother liquor by filtration, washed three times with 40% aqueous ethanol and once with diethyl ether and dried in vacuum. Yield 55% (based on Sb). Anal. Calc. for Cs1H6O6Sb (3): Cs, 37.26; Sb, 34.13. Found: Cs, 36.50; Sb 35.22%. 2.2.4. Cs2[Sb2(l-O)2(OH)8] (4) The procedure is similar as described for 3 except for the lower molar ratio (about 3) between cesium hydroxide and antimony(V)chloride precursor. Required amounts of reagents were 3.6 g (12.0 mmol) of SbCl5 and 10.8 g of 50% cesium hydroxide aqueous solution (36.0 mmol). Yield 68% (based on Sb). Thermal analysis revealed decomposition and water loss at 205 °C. Anal. Calc. for Cs2H8O10Sb2 (4): Cs, 39.24; Sb, 35.95. Found: Cs, 38.84; Sb 35.67%. 2.3. Crystal structure determinations and refinements X-ray powder diffraction measurements were performed on a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with a goniometer radius of 217.5 mm, Göbel Mirror parallel-beam optics, 2° Sollers slits, and a 0.2 mm receiving slit. The powder samples were filled into low background quartz sample holders. The specimen weight was 0.5 g. XRD patterns in the range 2° to 75° 2h were recorded at room temperature using Cu Ka radiation (k = 1.5418 Å) under the following measurement conditions: Tube voltage of 40 kV, tube current of 40 mA, step scan mode with a step

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size 0.02° 2h, and counting time of 1s/step. XRD patterns were processed by DiffracPlus software. FTIR studies were conducted using an Alpha model spectrometer, equipped with a single reflection diamond ATR sampling module, manufactured by Bruker Optik GmbH (Ettlingen, Germany), with 50 scans, at 25 °C. Differential thermal analysis, DTA was performed on differential scanning calorimeter, DSC 822 Mettler, Toledo in the range 40–400 °C under nitrogen flow at a heating rate of 5°/min. Electrospray ionization mass spectrometry (ESI-MS) analysis was conducted using an Agilent 6520 high accuracy Quadrupole Time of Flight (Q-TOF) mass spectrometer. We used 5 mL injection volume. Acetonitrile was used as the mobile phase at 0.2 ml/min flow rate. The analysis was conducted in both negative and positive mode using Agilent G3251A Dual ESI source. Nebuliser pressure was set to 40 psi, drying gas flow was 10 L/min, drying gas temperature 250 °C, capillary voltage potential was 4000 V for both positive and negative mode and 0 V was set for nozzle voltage. The fragmentor voltage was set at 145 V and skimmer voltage was 65 V for positive as well as negative mode. Scan range was 110– 1000 m/z and acquisition rate time was 0.72 spectra/s. The other MS parameters remained at autotune conditions. 1 mM solution of sodium antimonate for mass spectrometric analysis was prepared by dissolving crystalline 2 in water. The pH of this solution was altered in the range 4–10 by addition of 1 M hydrochloric acid (pH 10 was obtained without adding any acid). Potassium and cesium abundances were determined by gravimetric analysis as potassium and cesium tetraphenylborates, respectively [14]. Antimony content was estimated by iodometry [14]. Crystal data and details of single crystal X-ray analysis are given in Table 1. The crystals were extracted from mother liquor and covered immediately with inert oil to prevent contact with atmospheric moisture. After that they were rapidly mounted on the top of plastic fibre and transferred to a cold nitrogen stream. All experimental datasets were collected using graphite monochromatized Mo Ka radiation (k = 0.71073 Å) in x-scan mode. Absorption corrections based on measurements of equivalent reflections were applied [15]. The structures were solved by direct methods and refined by full matrix least-squares on F2 with anisotropic thermal parameters for all non-hydrogen atoms [16]. In the structures 2 and 3 hydrogen atoms were not included in the model. In 1 and 4, all hydrogen atoms were found from difference Fourier synthesis and refined using a riding model. The crystal of 2 represented merohedral twinning nature. The introduction of matrix 1 0 0 0 1 0 0 0 1 resulted in decrease of R1 value from approx. 10% down to approx. 3%. The possibility of higher symmetry space group (P42/nmc) for 2 was checked and not confirmed. Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49 7247 808 666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/ request_for_deposited_data.html) on quoting the CSD numbers 422765–422768.

3. Results and discussion 3.1. Crystal structures The structure of K2[Sb(O)(OH)5] (1) comprises K+ cations and slightly distorted octahedral [Sb(O)(OH)5]2 anions with cis O– Sb–O ranging within 82.74(9)°–95.91(9)° (Fig. 1). As expected, the Sb@O distance is significantly shorter than Sb–OH bond

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Table 1 X-ray structure determination summary. Compound

1

2

3

4

Empirical formula M Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z F(0 0 0) l (mm 1) T (K) Diffractometer

H5K2O6Sb1 300.99 triclinic  P1

H6Cs1O6Sb1 356.71 triclinic  P1

H8Cs2O10Sb2 677.38 triclinic  P1

5.2362(4) 6.5252(5) 9.1255(8) 84.824(2) 77.865(2) 85.929(2) 303.15(4) 2 284 5.884 120

H6Na1O6Sb1 246.79 tetragonal P42/n 8.0033(10) 8.0033(10) 7.8639(19) 90 90 90 503.70(15) 4 464 5.507 173

5.9868(11) 6.0085(11) 9.1004(17) 81.407(2) 71.379(2) 86.784(2) 306.73(10) 2 320 10.302 173

6.4725(18) 6.5510(18) 7.493(2) 109.201(4) 109.201(4) 112.790(4) 272.71(13) 1 300 11.562 173

SMART APEX

SMART APEX

II 4008 550 (0.0229) 2.54–26.94 42 0.0263 0.0864 1.124 0.752/ 0.562

II 2626 1321 (0.0202) 2.38–26.97 74 0.0225 0.0575 1.122 0.931/ 1.172

II 1796 951 (0.0188) 2.92–25.09 77 0.0195 0.0476 1.067 0.759/ 0.793

Data collected Unique data (Rint) h Range (°) Number of variables R1 [I > 2r(I)] wR2 (all data) Goodness of-fit-on (GOF) Dqmax (min/e Å 3)

Fig. 1. The structure of [Sb(O)(OH)5]2 drawn at 50% probability level.

2170 1377 (0.0136) 3.14–28.00 83 0.0188 0.0489 1.207 0.982/ 0.879

anion in 1. Displacement ellipsoids are

lengths (1.903(2) Å versus 1.983(2)–2.023(2) Å). All hydrogen atoms are involved in inter-anion hydrogen bonding resulting in an intricate 3D-network. The oxo O = atoms accept two hydrogen bonds, O4, O5 and O6 atoms accept one bond, and O(1) and O(2) do not accept H-bonds at all. Coordination number of both crystallographically independent K atoms is equal to nine and K–O distances vary from 2.685(2) Å to 3.184(2) Å. Crystal structure of Na[Sb(OH)6] (2) consists of sodium cations and nearly octahedral centrosymmetrical [Sb(OH)6] anions with cis O–Sb–O ranging within 88.5(1)°–91.5(1)°. The Sb–O distances range from 1.977(3) to 1.995(3) Å. These values are close to those previously reported for this compound (1.972(9)–1.999(9) Å [7,8]) and for [M(H2O)6][Sb(OH)6] (1.961(4)–1.994(4) Å; M = Mg, Co [17]). The coordination environment of the sodium atom is also a nearly regular octahedron with Na–O distances varying from 2.361(2) Å to 2.391(3) Å. In the crystal, the six oxygen atoms surrounding the Na cation belong to six different SbO6 octahedra.

SMART APEX

SMART APEX

Unfortunately, hydrogen atoms were not located from the experimental data. Moreover, the topological analysis of the structure based on inter-anion O


O distances and Sb–O


O angles does not result in unambiguous assignment of H atom positions. Recently the structure of 2 was determined with all hydrogen atoms found from difference Fourier synthesis [8]. The authors reported atoms H1 and H2 being fully occupied, while H3 is disordered over two positions H3A and H3B with occupation factors equal to ½. However, the analysis of the supplementary cif file for NaSb(OH)6 in [8] shows that this assignment of the location of the H atoms seems to be incorrect: half occupied atom H3B forms short inter-anion contact (1.22 Å) with fully occupied atom H1 of the adjacent Sb(OH)6 unit. At the same time, another component of disorder, namely H3A, forms short inter-anion contact with its own symmetry equivalent (0.89 Å). Thus, the location of hydrogen atoms in the structure of mopungite still remains unclear. Crystal structure of Cs[Sb(OH)6] (3) consists of cesium cations and nearly octahedral [Sb(OH)6] anions with cis O–Sb–O ranging within 87.5(2)°–92.6(2)° (Fig. 2). The Sb–O bond lengths lie within 1.971(4)–1.985(4) Å. The coordination number of the Cs atom is 10 and Cs–O distances vary in the range 3.157(4)–3.497(4) Å. As for 2, H atoms were not located from experimental data and the

Fig. 2. Crystal packing in Cs[Sb(OH)6]. Cs atoms are shown as green circles.

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attempts of unambiguous assignment of proton positions using a topological method failed. The structure of Cs2[Sb2(l-O)2(OH)8] (4) comprises Cs+ cations and dimeric [Sb2(l-O)2(OH)8]2 anions lying on the symmetry centers (Fig. 3). The coordination polyhedron of the Sb atom represents a distorted octahedron with cis O–Sb–O ranging within 80.78(16)°–93.32(16)° (Fig. 3). As expected, the minimal angle was observed for bridging oxygen atoms O1–Sb1–O1A. All trans O–Sb–O angles are nearly linear (173.61(15)°–174.68(17)°). The central Sb2(l-O)2 fragment is planar; Sb–O–Sb angle is 99.22(16)°. The terminal equatorial bonds Sb–O5 and Sb–O4 are noticeably shorter (1.957(4) and 1.965(4) Å) than four other Sb– O bonds (axial and bridging, 1.983(4)–2.012(4) Å). The same effect was observed for the isoelectronic anions [Te2(l-O)2(O)2(OH)6]2 [18] and [I2(l-O)2(O)4(OH)4]2 [19]. All terminal oxygen atoms are protonated. The same feature was observed previously for polynuclear complexes [(n-C4H9)4N]4[Sb8O12(OH)20] [12] and [(nC4H9)4N]2[Sb4O6(OH)4{OSi(CH3)2(t-C4H9)}6 [13]. The coordination number of the Cs atom is 9 and Cs–O distances vary in the range 3.043(4)–3.459(4) Å. All hydrogen atoms are involved in interanion hydrogen bonding resulting in a 3D-network.

3.2. XRD, FTIR and ESI/MS X-ray powder diffractograms of 1–4 along with the fit to the calculated diffractograms based on the single crystal structures are depicted in the supplementary materials (Fig. S1). The XRD diffractograms of 2 (Fig. S1, B) and 4 (Fig. S1, D) are clean with no amorphous phase and no crystalline impurities, which shows that they precipitate in a single phase which corresponds to the single crystal structures of 2 and 4. The XRD of the potassium antimonate precipitate (Fig. S1, A) contains an additional crystalline phase that could not be attributed to reported (carbonate, chloro- or hydroxo-) structures. Interestingly, the powder XRD of Cs[Sb(OH)6] (Fig. S1, C) is less pure, it contains a large amorphous phase (and its Scherer crystalline size was considerably smaller, 40 nm). Based on these findings, and particularly the striking difference between the purity of the dinuclear phase compared to the mononuclear phase, it is not clear at all that hexahydroxoantimonate is the dominant, let alone the only antimonate anion at very high pH. Fig. 4 depicts the FTIR spectra of the three different (relatively pure according to the powder XRD studies) crystalline phases. The corresponding assignment table is presented in the Table S1 of the supplementary material. The vibration spectra of all three antimonates are rather similar showing three distinct energy ranges: 400–800 cm 1 assigned to m(Sb–O) stretching vibrations,

Fig. 3. The structure of [Sb2(l-O)2(OH)8]2 anion in 4. Displacement ellipsoids are drawn at 50% probability level.

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Fig. 4. FTIR spectra of K2[SbO(OH)5] (A); Na[Sb(OH)6] (B); and Cs2[Sb2O2(OH)8] (C).

bands in the range 900–1200 cm 1 are attributed to d(SbOH) deformations, and the range 3000–3400 cm 1 is assigned to m(O– H) stretching vibrations, similar to previously reported spectra [20]. The spectrum of 1 contains additional absorbance bands at 1367 and 1467 cm 1which are attributed to Sb@O vibrations. The powder X-ray diffractograms and the formation of an additional cesium dinuclear phase were rather surprising, and we wanted to gain some more insight into the chemistry of the aqueous solution. It was also surprising for us that our attempts to precipitate crystalline phases from precursors of different pH (using different ratio between antimony hydroxide to the alkali hydroxide) were futile. Electrospray mass spectrometry is a gentle ionization process which has been successfully used for speciation of metallo-organic compounds [21]. The electrospray MS studies of antimonates were reported before [22], but we found only a single report of organic ligand-free antimonate methanol–water solution at an unspecified pH. The study revealed only monomeric species and it was conducted at low resolution MS [23]. Since the powder XRD diffraction of 2 showed only a single phase and the FTIR spectrum was clean, we have chosen solutions of 2 for in detail pH dependence analysis by ESI-MS. Fig. 5 depicts the full scan negative mode spectra of 1 mM sodium antimonate solution at pH 4, 7 and 10 (corresponding to frames A–C, respectively). The dominant ESI species are the mononuclear and dinuclear species, throughout the whole (studied) pH range, though some two orders of magnitude lower intensity peaks of trinuclear and even higher oligomers were also present in all spectra. Due to the high mass accuracy of the equipment used in this study unambiguous peak assignment was possible. The lower row in Fig. 5 depicts the spectra of the mononuclear, dinuclear and trinuclear species (obtained at pH 10). The peaks cluster corresponding to the mononuclear species included [Sb(OH)6] and mononuclear species after a loss of 1, 2 and 3 water molecules. The most dominant set of peaks were 186.8997 ± 3 ppm and 188.9004 ± 3 ppm assigned to [Sb(OH)6 2H2O] whose dominant theoretical peaks are 186.8991 and 188.8995, constituting a mass error of less than 3 ppm, well within the accuracy of the instrument. The spectral cluster of the dinuclear species (which was crystallized only from the cesium antimonate solution) was also pronounced in the ESI-MS spectra. It appeared as [Sb2(O)2(OH)82 + Na+ + nH2O] , where n = 0–3 (n = 3 being the most dominant species with peaks 484.8060 ± 3 ppm, 486.8068 ± 3 ppm and 468.8065 ± 3 ppm) and agreed well with the theoretical isotopic pattern. The trinuclear species, assigned to [Sb3H25O21Na] also agreed well with the theoretical pattern. Fig. 2S (in the electronic supplementary materials) shows a spectrum of positive mode analysis of a solution of 2 at pH 10. The most dominant species with peaks at 300.9474 and 302.9480

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insensitive to the minor dissolved species (dinuclear antimonate in this case). It is interesting to compare the basic aqueous speciation of Sb(V) with its isoelectronic neighbours Sn(IV) and Te(VI)). Tin(IV) appears predominantly as [Sn(OH)6]2 at high pH and neither we nor others have been successful in detection of non-monomeric Sn(IV) forms in strong basic solutions [11,25]. In contradistinction Te(VI) has richer speciation in aqueous solutions under basic conditions. Many crystalline water soluble hydroxotellurate crystals were reported including mono-, di- and oligonuclear anions reflecting equilibria between all these species in the solution [15,26]. The current study shows that antimonate behaviour is more similar to tellurates with mononuclear and dinuclear species predominating even at high pH, though our ESI/MS studies show that some oligonuclear species are also present in the solutions. Acknowledgments

Fig. 5. High-resolution, negative mode electrospray mass spectra of 1 mM aqueous sodium antimonate solutions at pH 10 (A), 7 (B), and 4 (C). Lower row presents the spectra of the mononuclear, dinuclear and trinuclear antimonate species at pH 10.

We thank the Israel Ministry of Science, the Russian Foundation for Basic Research (Grants 11-03-00551, 11-03-12131-OFI-M-2011 and 11-03-92478), the Council on Grants of the President of the Russian Federation (NSh-8503.2010.3), the Ministry of Education and Science of the Russian Federation (Federal Target program ‘‘Scientific and Pedagogical Staff of Innovating Russia 2009– 2013’’, State Contract No. 16.740.11.0428). The authors acknowledge financial support of the infrastructure research program of Israel Ministry of Science and the Israel National Fund. Appendix A. Supplementary material

+

+ +

in this case was assigned to [Sb(OH)6 + Na + 3H2O + H ] . Similar to the negative mode of analysis, the positive ESI-MS spectra were practically indifferent to pH. The ESI-MS spectra explain why a pH change did not affect strongly the precipitation, and we were not successful in crystallizing alkali metal antimonates by changing the pH of the solution. Solution speciation is little affected by the pH in the studied range.

4. Conclusions Three new alkali metal antimonate structures are reported in this paper. Sodium and cesium hexahydroxoantimonate were in line with our previous expectation regarding the prevalence of [Sb(OH)6] in aqueous solutions and we have also found a potassium salt of [Sb(O)(OH)5]2 . Although polyantimonates have been anticipated and the first octanuclear antimonate was crystallized from organic solvent, and tetra- and hexa-antimonate crystalline polyanions were isolated by reaction with silane and molybdate, respectively, antimonate dinuclear species were never crystallized from any solvent. As far as we know, the [Sb2(l-O)2(OH)8]2 anion is the first homonuclear nonmonomeric antimonate that was isolated from aqueous solutions. Even for soluble crystalline precipitates there is always uncertainty in extrapolating from the precipitate to the solution speciation. Although only indirect, we believe that the ESI/MS studies and the fit between the X-ray powder and single crystal data point out the relevance of the solid structures to aqueous chemistry. The electrospray mass spectrometry points to the equilibrium between mono and dinuclear species in aqueous solutions. Only at exceedingly alkaline solutions was it possible to precipitate predominantly mononuclear hydroxoantimonate crystals without dinuclear crystalline phase. This study does not contradict the Pourbaix (Eh-pH) diagram of antimony [24] which shows only a single Sb(V) form, [Sb(OH)6] . The Pourbaix diagram shows only the most predominant species, it is limited to pH < 12, and it is

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