Regioselective synthesis and characterization of monovanadium-substituted β-octamolybdate [VMo7O26]5.

The monovanadium-substituted polyoxometalate anion [VMo7O26]5-, exhibiting a β-octamolybdate archetype structure, was selectively prepared as pentapotassium [hexaikosaoxido(heptamolybdenumvanadium)]ate hexahydrate, K5[VMo7O26]·6H2O (VMo7), by oxidation of a reduced vanadomolybdate solution with hydrogen peroxide in a fast one-pot approach. X-ray structure analysis revealed that the V atom occupies a single position in the cluster that differs from the other positions by the presence of one doubly-bonded O atom instead of two terminal oxide ligands in all other positions. The composition and structure of VMo7 was also confirmed by elemental analyses and IR spectroscopy. The selectivity of the synthesis was inspected by a 51V NMR investigation which showed that this species bound about 95% of VV in the crystallization solution. Upon dissolution of VMo7 in aqueous solution, the [VMo7O26]5- anion is substantially decomposed, mostly into [VMo5O19]3-, α-[VMo7O26]4- and [V2Mo4O19]4-, depending on the pH. open access.

Introduction

Polyoxometalates (POMs) of W, Mo and V represent an important group of inorganic metal–oxide clusters (Pope, 1983 ▸) whose structural variability gives rise to an exceptionally wide range of applications in catalysis (Wang & Yang, 2015 ▸), magnetism (Clemente-Juan et al., 2012 ▸), redox processes (Gumerova & Rompel, 2018 ▸) and materials chemistry (Song & Tsunashima, 2012 ▸), as well as in biological chemistry (Bijelic & Rompel, 2015 ▸, 2017 ▸; Molitor et al., 2017 ▸; Fu et al., 2015 ▸; Bijelic et al., 2018 ▸, 2019 ▸). Particularly inter­esting are photoactive POMs with applications in water splitting, the photooxidation of organic pollutants, photoreductive CO2 activation and H2 generation (Streb et al., 2019 ▸). Vanadium-containing POMs are a promising subgroup of photocatalysts. A VV centre acts as a more efficient light absorber in comparison to MoVI/WVI; moreover, it may easily promote a photoredox reaction via its photoreduction to a VIV species. Substitution of some Mo or W atoms in molybdates and tungstates may therefore lead to enhanced photocatalytic properties (Streb, 2012 ▸). Substitution of one Mo atom in a Linqvist-type hexa­molybdate, [Mo6O19]2−, by vanadium leads to enhanced photocatalytic degradation of a model organic dye under both aerobic and anaerobic conditions caused by a low-energy O→V LMCT (ligand-to-metal charge transfer) transition in [VMo5O19]3− (Tucher et al., 2012 ▸). However, the controlled synthesis of mixed vanadomolybdates and vanadotungstates remains a serious challenge. Simple mixing of addenda-atom precursors leads to a complicated equilibria of several species (Pope, 1983 ▸; Howarth et al., 1991 ▸). The β-octa­molybdate structure [Mo8O26]4− (Fig. 1 ▸) is one of the main components in H+/OH−/MoO4 2− systems under acidic conditions, yet only the disubstituted vanadium derivative [V2Mo6O26]6− is commonly known in the literature and has been structurally characterized (Nenner, 1985 ▸; Fei et al., 2015 ▸; Li et al., 2011 ▸). The two V atoms occupy chemically equivalent positions, denoted MoC. Very recently, a monovanadium-substituted derivative was prepared as H4K2Na2(H2O)4(C12H12N4O2)[VMo7O26]·10H2O (Zhao et al., 2018 ▸). In this case, the V atom was claimed to be statistically distributed in all positions of the parent β-octa­molybdate anion.

Figure 1

Schematic representation of the structure of the β-octa­molybdate anion [Mo8O26]4−. Chemically non-equivalent Mo atoms are shown in different colours: MoA green, MoB orange, MoC blue and O red.

In the current work, we present a regioselective synthesis of a new isomer of [VMo7O26]5− in which the V atom occupies only MoC positions of the parent β-octa­molybdate structure. The regioselectivity was achieved by controlled stepwise synthesis via vanadium peroxido complexes as precursors. The peroxide-mediated synthesis route (Schwendt et al., 2016 ▸) has already been successfully utilized for the synthesis of several polyoxometalates, such as [HV10O28](6– (Jahr et al., 1963 ▸; Nakamura & Ozeki, 2001 ▸), [HPV14O42](9–, Keggin structures [H3+PMo12–VO40], and Wells–Dawson structures [H6+P2Mo18–VO62] (Odyakov et al., 2015 ▸) and [V12O30F4(H2O)2]4− (Krivosudský et al., 2014 ▸).

Experimental

All chemicals were purchased from Sigma–Aldrich (Austria) and used as received.

Synthesis and crystallization

For the preparation of K5[VMo7O26]·6H2O (VMo), K2MoO4 (1.67 g, 7 mmol) and VOSO4·nH2O (0.2 g, 1.22 mmol) were dissolved in distilled water (40 ml) by heating. HCl (0.7 ml of a 37% w/w solution) was added. When the temperature reached 80 °C, H2O2 (0.1 ml of a 30% w/w solution, 1 mmol) was added and the colour of the solution changed immediately from dark violet to orange. The solution was boiled for 1 min and the pH of the still hot solution was adjusted to 3.1 with 50% KOH solution. The clear-yellow solution was left to crystallize at 18 °C. Yellow block-shaped crystals were filtered off after 2 d, washed with water and ethanol and air-dried (yield 0.47 g, 33%, based on Mo). Elemental analysis (%) for K5Mo7VO32H12 (calculated): K 14.0 (13.6), Mo 46.6 (46.6), V 3.43 (3.53).

Elemental analysis

Elemental analyses were performed in aqueous solutions containing 2% HNO3 using inductively coupled plasma mass spectrometry (PerkinElmer Elan 6000 ICP MS) for Mo and V, and atomic absorption spectroscopy (PerkinElmer 1100 Flame AAS) for K. Standards were prepared from single-element standard solutions of concentration 1000 mg l−1 (Merck, Ultra Scientific and Analytika Prague).

IR spectroscopy

VMo was identified by IR measurement on a Bruker Vertex70 IR Spectrometer equipped with a single reflection diamond-ATR (attenuated total reflectance) unit in the range 4000–100 cm−1.

51V NMR spectroscopy

51V nuclear magnetic resonance spectroscopy measurements of aqueous solutions (with 10% of D2O used for locking, at 20 °C) were taken on a Bruker AV II+ 500 MHz instrument operating at 131.60 MHz for the 51V nucleus (2000 scans, accumulation time 0.05 s, relaxation delay 0.01 s). Chemical shift values are given with reference to VOCl3 (0 ppm) as the standard.

Refinement

Crystal data, data collection, structure refinement and software details are summarized in Table 1 ▸. No H atoms were inserted on the free water O atoms due to the disorder and instability of the model. In the case of the disordered groups, one bond was added to the connectivity array (O15S—K3). The disordered Mo4A/V4 atoms occupying the same position of the POM anion were treated with half occupancies. The O atoms of the solvent molecules (O16S, O17S, O18S and O19S) and the partially occupied K3 and K4 atoms and their corresponding U components are of low quality and were forced by the restrained ISOR to affect the standard deviation and approximate the U components to isotropic behaviour.

Table 1

Experimental details

Crystal data
Chemical formula	K5[VMo7O26]·6H2O
M r	1442.12
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	143
a, b, c (Å)	12.9356 (10), 16.1978 (10), 13.8618 (9)
β (°)	90.962 (4)
V (Å3)	2904.0 (3)
Z	4
Radiation type	Mo Kα
μ (mm−1)	4.06
Crystal size (mm)	0.1 × 0.1 × 0.1
Data collection
Diffractometer	Bruker D8 Venture
Absorption correction	Multi-scan (SADABS; Bruker, 2016 ▸)
T min, T max	0.512, 0.564
No. of measured, independent and observed [I > 2σ(I)] reflections	80344, 4259, 4028
R int	0.057
(sin θ/λ)max (Å−1)	0.705
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.027, 0.059, 1.26
No. of reflections	4259
No. of parameters	235
No. of restraints	37
H-atom treatment	H-atom parameters not defined
Δρmax, Δρmin (e Å−3)	0.64, −0.74

Computer programs: APEX3 (Bruker, 2018 ▸), SAINT (Bruker, 2016 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2018 (Sheldrick, 2015 ▸), OLEX2 (Dolomanov et al., 2009 ▸), DIAMOND (Brandenburg, 2006 ▸), PLATON (Spek, 2009 ▸) and ShelXle (Hübschle, et al., 2011 ▸).

Results and discussion

Upon reaction of the initial HVO4 (3– and HMoO4 (2– precursors and adjustment of the pH to a certain value, complicated reaction mixtures with several equilibrated species are formed (Howarth et al., 1991 ▸). It was therefore necessary to choose a different synthesis approach that would favour the formation of [VMo7O26]5−. We employed a VIV precursor (VOSO4) that forms with molybdate mixed-valence polynuclear deep-blue vanadomolybdates at pH ≃1.5 (Müller et al., 2005 ▸; Botar et al., 2005 ▸). Subsequent addition of hydrogen peroxide resulted in an orange solution formed by immediate oxidation with vanadium peroxido complexes (Schwendt et al., 2016 ▸). The crucial point of the synthesis was the adjustment of the pH of the hot solution to 3.1. This value represents a region where the β-octa­molybdate anion [Mo8O26]4− is the main species present in the simple molybdate solutions at c Mo = 0.1 mol dm−3 (Ozeki et al., 1988 ▸). We also obtained different products from solutions with the pH range 1.5–7.0; however, IR and ICP–MS analyses indicated that the products are mixtures and VMo can only be obtained as a pure product in the pH range 2.8–3.5. Adjustment of the pH of the cooled solution leads to the formation of precipitates and an obvious reduction of vanadium (formation of a green solution).

The asymmetric unit of VMo contains one half of the [VMo7O26]5− POM anion lying on a centre of symmetry (Fig. 2 ▸). The K+ cations which compensate the charge of the anion occupy four positions, one of them at full occupancy (K2) and one disordered over two positions (K3 and K4). The K1 atom is coordinated by two [VMo7O26]5− anions in an inter­esting fashion, forming an irregular twisted anti­prismatic coordination polyhedron, with K—O distances in the range 2.703 (3)–2.767 (3) Å (Fig. 3 ▸) and without the participation of water mol­ecules. The two square bases formed by oxido ligands O1, O2, O3 and O4 are twisted by approximately 10–13°. The [VMo7O26]5− anion adopts the expected β-con­formation (compare Figs. 1 ▸ and 2 ▸). The centrosymmetric anion was firstly refined as a pure [Mo8O26]4− cluster for localization of the V atoms. While the MoA and MoB positions showed initial occupancies very close to 1 (≃0.96), significantly lower occupancies of the Mo atoms at the MoC positions indicated the presence of V atoms which are equally distributed in both symmetrically equivalent positions. The positioning of the V atoms at 0.5 occupancies in the MoC positions led to a significant improvement of the model. Moreover, it can be seen from Fig. 1 ▸ that the MoC atoms differ significantly from the MoA and MoB atoms in the replacement of one Mo=O bond by a bridging Mo—O bond. Therefore, we consider it reasonable that the V atom occupies preferably position MoC which is not only crystallographically, but also chemically, markedly non-equivalent to the other positions in the octa­metalate. In this context, we should note for the structure of H4K2Na2(H2O)4(C12H12N4O2)[VMo7O26]·10H2O (Zhao et al., 2018 ▸) that the displacement ellipsoids at the MoC positions are approximately twice as large as the ellipsoids of the MoA and MoB atoms, indicating that the V atom occupies preferably this position also in this structure, although the model was constrained with a statistical distribution of the V atoms throughout the anion. However, the selectivity of vanadium substitution is not known and the existence of such a species might be theoretically possible despite the fact that it was not predicted by speciation (Howarth et al., 1991 ▸). [VMo7O26]5− consists of eight {Mo/VO6} face- and edge-sharing octa­hedra. Except for atoms V4/Mo4A, all other Mo atoms are coordinated by two terminal oxido ligands, with shorter Mo=O double bonds in the range 1.601 (5)–1.726 (3) Å, and bridging oxide ligands exhibiting longer bond distances of up to 2.430 (2) Å for the Mo2—O13 bond incorporating the penta­coordinated O atom (see the supporting information for further details). All water mol­ecules exhibit a certain degree of disorder and therefore we do not discuss the hydrogen-bond network. The water mol­ecules complete an irregular coordination polyhedra around the potassium cations, forming a rich polymeric network based on electrostatic inter­actions (Fig. 4 ▸).

Figure 2

The mol­ecular structure of [VMo7O26]5− in VMo, showing the atom-labelling scheme. Displacement ellipsoids are shown at the 50% probability level. Colour code: Mo black, V blue and O red.

Figure 3

The coordination of the potassium cation K1 by two [VMo7O26]5− ligands. Colour code: {Mo/VO6} yellow octa­hedra and {KO8} violet distorted square anti­prism.

Figure 4

The crystal packing in VMo, viewed along the a axis. Colour code: {Mo/VO6} yellow octa­hedra and {KO8} violet distorted square anti­prism. H atoms of water mol­ecules have been omitted for clarity.

The IR spectrum of VMo (Fig. 5 ▸) exhibits bands typical for a β-octa­molybdate structure and the respective bands corresponding to Mo—O or V—O vibrations are not distinguishable. The stretching vibrations of the terminal Mo/V=O units appear at 934 and 888 cm−1, whereas the peaks in the region from 470 to 840 cm−1 correspond to the anti­symmetric and symmetric deformation vibrations of the Mo—O—Mo and Mo—O—V bridging fragments. The crystallization water mol­ecules exhibit typical bands for valence O—H vibrations and deformation H—O—H vibrations at 1610 and 3540 cm−1, respectively.

Figure 5

The IR spectrum of VMo in the region 4000–100 cm−1.

We employed 51V NMR spectroscopy to inspect the syn­thesis and hydrolytic stability of VMo (Fig. 6 ▸). All chemical shifts of the major species were assigned according to a very thorough speciation study based on NMR spectroscopy (51V, 95Mo and 17O) and potentiometric data (Howarth et al., 1991 ▸). In the crystallization solution one day after the synthesis (Fig. 4 ▸ a), the [VMo7O26]5− anion (−534.2 ppm, 95% of VV) is dominant, accompanied by a monovanadium-substituted hexa­molybdate [VMo5O19]3− (−505.0 ppm) and some minor species. After two days (Fig. 4 ▸ b), when about 33% of the product has crystallized out, roughly 73% of VV is still consumed in the [VMo7O26]5− species. Thus, the NMR investigation confirms that a synthetic protocol starting from reduced mixed Mo/V polyoxometalates oxidized by H2O2 leads almost exclusively to the desired [VMo7O26]5− anion. The speciation work by Howarth et al. proposed that [VMo7O26]5− should be most stable around pH = 4.2. At this pH value (Fig. 4 ▸ c), we observed substantial decomposition of VMo into [VMo5O19]3− (−505.1 ppm) and α-[VMo7O26]4−, a structure that resembles the Anderson–Evans archetype polyoxometalate capped by one Mo=O and one V=O unit (−502.9 ppm). Increased pH (Figs. 4 ▸ d and 4 ▸ e) results in a profound decomposition of [VMo7O26]5− into hexa­metalate [V2Mo4O19]4− (−497.0 and −496.3 ppm). The results of the 51V NMR measurements showed that once the [VMo7O26]5− anion is selectively formed from suitable precursors, it stays relatively intact in the solution for at least 24 h. On the other hand, by dissolution of VMo in aqueous solution, hydrolysis takes place and the newly formed species, mostly [VMo5O19]3−, α-[VMo7O26]4− and [V2Mo4O19]4−, cannot give rise to the formation of [VMo7O26]5− even under conditions when the equilibrium of the POM species should be favoured (0.5 M NaCl, pH = 4.2).

Figure 6

(a)/(b) The 51V NMR spectra of the crystallization solution of VMo and (c)/(d)/(e) solutions obtained upon dissolution of crystallized VMo at different pH values. Conditions: (a) 175 mM MoVI, 25 mM VV, pH = 3.1, 24 h after the synthesis; (b) same conditions as (a) after another 24 h; (c)/(d)/(e) 10 mM VV solution prepared from VMo by dissolving it in 0.5 mM NaCl at pH values of (c) 4.0, (d) 5.2 and (e) 6.0. The pH was adjusted by the addition of a dilute KOH solution.

Crystal structure: contains datablock(s) global. DOI: 10.1107/S205322961900620X/ky3170sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S205322961900620X/ky3170Isup2.hkl

CCDC reference: 1898165