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

The monovanadium-substituted β-octamolybdate [VMo7O26]5− was prepared by a one-pot approach using peroxido complexes of vanadium. 51V NMR spectroscopy confirmed the high selectivity of the synthesis.


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, 2015Molitor et al., 2017;Fu et al., 2015;Bijelic et al., 2018Bijelic et al., , 2019. Particularly interesting are photoactive POMs with applications in water splitting, the photooxidation of organic pollutants, photoreductive CO 2 activation and H 2 generation (Streb et al., 2019). Vanadium-containing POMs are a promising subgroup of photocatalysts. A V V centre acts as a more efficient light absorber in comparison to Mo VI /W VI ; moreover, it may easily promote a photoredox reaction via its photoreduction to a V IV 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, [Mo 6 O 19 ] 2À , 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 [VMo 5 O 19 ] 3À (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 -octamolybdate structure [Mo 8 O 26 ] 4À (Fig. 1) is one of the main components in H + /OH À /MoO 4 2À systems under acidic conditions, yet only the disubstituted vanadium derivative [V 2 Mo 6 O 26 ] 6À 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 Mo C . Very recently, a monovanadium-substituted derivative was prepared as H 4 K 2 Na 2 (H 2 O) 4 (C 12 H 12 (Zhao et al., 2018). In this case, the V atom was claimed to be statistically distributed in all positions of the parent -octamolybdate anion.
In the current work, we present a regioselective synthesis of a new isomer of [VMo 7 O 26 ] 5À in which the V atom occupies only Mo C positions of the parent -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 [H x V 10 O 28 ] (6-x)- (Jahr et al., 1963;Nakamura & Ozeki, 2001) (Krivosudský et al., 2014).

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

Synthesis and crystallization
For the preparation of K 5 [VMo 7 O 26 ]Á6H 2 O (VMo 7 ), K 2 MoO 4 (1.67 g, 7 mmol) and VOSO 4 ÁnH 2 O (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, H 2 O 2 (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 K 5 Mo 7 -VO 32 H 12 (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% HNO 3 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 À1 (Merck, Ultra Scientific and Analytika Prague).

IR spectroscopy
VMo 7 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 À1 .
2.4. 51 V NMR spectroscopy 51 V nuclear magnetic resonance spectroscopy measurements of aqueous solutions (with 10% of D 2 O used for locking, at 20 C) were taken on a Bruker AV II+ 500 MHz instrument operating at 131.60 MHz for the 51 V nucleus (2000 scans, accumulation time 0.05 s, relaxation delay 0.01 s). Chemical shift values are given with reference to VOCl 3 (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 corre-  sponding U ij components are of low quality and were forced by the restrained ISOR to affect the standard deviation and approximate the U ij components to isotropic behaviour.

Results and discussion
Upon reaction of the initial H x VO 4 (3-x)and H x MoO 4 (2-x)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 [  (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 7 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 7 contains one half of the [VMo 7 O 26 ] 5À 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     approximately twice as large as the ellipsoids of the Mo A and Mo B 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). [VMo 7 O 26 ] 5À consists of eight {Mo/VO 6 } 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 hydrogenbond network. The water molecules complete an irregular coordination polyhedra around the potassium cations, forming a rich polymeric network based on electrostatic interactions (Fig. 4).
The IR spectrum of VMo 7 (Fig. 5)  We employed 51 V NMR spectroscopy to inspect the synthesis and hydrolytic stability of VMo 7 (Fig. 6). All chemical shifts of the major species were assigned according to a very thorough speciation study based on NMR spectroscopy ( 51 V, 95 Mo and 17 O) and potentiometric data (Howarth et al., 1991). In the crystallization solution one day after the synthesis (Fig. 4a), the [VMo 7 O 26 ] 5À anion (À534.2 ppm, 95% of V V ) is dominant, accompanied by a monovanadium-substituted hexamolybdate [VMo 5 O 19 ] 3À (À505.0 ppm) and some minor species. After two days (Fig. 4b), when about 33% of the product has crystallized out, roughly 73% of V V is still The crystal packing in VMo 7 , viewed along the a axis. Colour code: {Mo/VO 6 } yellow octahedra and {KO 8 } violet distorted square antiprism. H atoms of water molecules have been omitted for clarity.

Pentapotassium [hexaikosaoxido(heptamolybdenumvanadium)]ate hexahydrate
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.   (5)