V₂O₂F₄(H₂O)₂·H₂O: a new V⁴⁺ layer structure related to VOF₃

Our recent work has focused on the preparation of V^IV oxyfluorides due to their inter­est as frustrated magnets. We have found that the successful synthesis of two-dimensionally connected vanadium(IV) oxyfluoride structures, for example, kagome lattices, requires the use of ionic liquids as solvents (Aidoudi et al., 2011a,b, 2014; Clark et al., 2015). On the other hand, there have been several previous reports of solvothermally synthesized vanadium compounds incorporating selenium, which are generally two- and three-dimensional in nature, and can attribute this high-dimensional structural property to the incorporation of selenium-based anionic linkers (Nguyen et al., 2011; Glor et al., 2011). Reduced dimensionally-extended vanadium compounds can also be prepared by these routes, for example, VO(SeO₂OH)₂ (Kim et al., 2010) and (4,4'-bi­pyridine)V₂Se₂O₈ (Dai et al., 2003). During solvothermal synthesis experiments aimed at producing two- and three-dimensional vanadium(IV) selenites with triangular lattices, a new V⁴⁺ layer structure, V₂O₂F₄(H₂O)₂.H₂O, (I), was discovered. This is apparently the first example of a fully reduced V⁴⁺ layered compound synthesized by solvothermal (rather than ionothermal) techniques. The structure is comprised of VOF₄(H₂O) corner- and edge-sharing o­cta­hedral building units. V₂O₂F₄(H₂O)₂.H₂O or VOF₂(H₂O).1/2H₂O, (I), is related structurally to previously synthesized vanadium oxyfluoride compounds, VOF₃ (Supel et al., 2007).

Vanadium trifluoride (VF₃, Alfa, 98%; 0.002 mol, 0.215 g) and ethyl­ene glycol (Fisher 99%+; 0.0032 mol, 0.2 ml) were added to a 40% solution of HF (0.5 ml, Alfa). Di­ethyl­amine [guanidine carbonate salt (0.001 mol, 0.09 g) was used in a different experiment in place of di­ethyl­amine and produced similar results] (Sigma, 99.5%; 0.001 mol, 0.5 ml) and selenious acid (Aldrich 99.999%; 0.001 mol, 0.129 g) were added to the resultant mixture before being sealed in a Teflon-lined stainless steel autoclave. This was heated at 393 K for 4 d, then allowed to cool to ambient temperature. The final product was filtered, washed in water and allowed to dry overnight at 333 K. The crystals obtained were blue blocks, indicating V⁴⁺, of typical dimension 0.06 × 0.04 × 0.03 mm. These were separated from the bulk product, a brown powder, under an optical microscope. The crystals recovered were too scarce to be analysed using powder X-ray diffraction or to be sent for elemental analysis.

Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms were located from Fourier maps and refined isotropically.

The VOF₄(H₂O) building unit of (I) (Fig. 1) exhibits a single vanadium site in a distorted o­cta­hedral environment (Fig. 2) and a single uncoordinated water molecule. The short V═O bond length along with the longer trans V—F and cis V—OH₂ bond lengths are comparable to those seen in the cis-building unit of [C₆H₄N₂₂][VOF₄(H₂O)]₂.H₂O, (II) (Aldous et al., 2007). Selected bond lengths for (I) and (II) are shown for comparison in Table 2. The o­cta­hedral units are linked via fluoride ligands to produce a new two-dimensional corner and edge-sharing layer compound of V⁴⁺ (Fig. 3a). Bond-valence-sum analysis (Brese & O'Keeffe, 1991) confirms the oxidation state of the vanadium centre as V⁴⁺, oxidized in situ from V³⁺ (Table 3). The layer in (I) resembles that of the VOF₃ structure, (III) (Supel et al., 2007), with both forming a layer comprised of corner- and edge-sharing o­cta­hedral units bridged via fluoride ligands (Fig. 3b). Aside from the different vanadium oxidation states, there are a number of significant differences between these two structures. The presence of uncoordinated water molecules in (I) results in longer inter-layer V···V distances, due to the hydrogen bonding from atom O3 to the layer [O3···H2 = 1.86 (4) Å and H2—O2 = 0.86 (4) Å], compared to those in (III) (Table 4). The elongated trans-fluoride ligand, F1, in (I) is involved in the edge-sharing V—F bonds, while in (III), the equivalent ligand is involved in the corner-sharing V—F bond. This results in a significant difference in the intra-layer V···V distances (Table 4). The layers of (I) can be seen to stack in a `staggered" manner (Fig. 4a.), while the layers in (III) stack in an `eclispsed' manner (Fig. 4b) when both are viewed along [010]. There are many previous examples of layered V⁴⁺-containing compounds that have synthesized solvothermally (Nguyen et al., 1995; Massa et al., 2002); however, these compounds owe their two-dimensional structural properties to phosphate and sulfate linkers. More recently, there have been examples of V⁴⁺-containing layers, synthesized via ionothermal methods (Aidoudi et al., 2011a,b, 2014). These compounds exhibit two-dimensional layers comprised fully of V⁴⁺ corner-sharing o­cta­hedral units. This appears to make compound (I) the first example of a purely V⁴⁺ layered compound produced via solvothermal techniques. From the presented synthesis route, compound (I) appears as a minor phase, together with a majority unidentified phase(s). Several further attempts to prepare (I) as the majority phase were unsuccessful. The synthetic method will need to be refined in order to produce (I) as the bulk phase, but this could lead to a simpler more cost-effective route, compared to ionothermal techniques, to obtain layered V⁴⁺ compounds for physical property investigation.