Sr5(VIVOF5)3F(H2O)3 refined from a non-merohedrally twinned crystal

The title compound, pentastrontium tris[pentafluoridooxidovanadate(IV)] fluoride trihydrate, was obtained under hydrothermal conditions. Its crystal structure has been refined from intensity data of a non-merohedrally twinned crystal. Two domains in almost equal proportions are related by a −180° rotation along the reciprocal [101]* vector. The structure may be considered as a derivative of the fluorite structure type, adopted here by SrF2. In the title compound, fluorite-like large rods are recognized, built up from a group of 16 Sr atoms of which 6 are substituted by V atoms, leading to [Sr10V6]∞ units. These rods extend infinitely along the b axis and are interconnected by the three water molecules. Each of the water molecules is shared by two different Sr atoms belonging to two different rods. The rods are also interconnected by an ‘independent’ F atom in a distorted triangular [FSr3] coordination and by hydrogen-bonding interactions via donor water molecules. The acceptors are either F atoms or the O atoms of the vanadyl ion, VO2+, that is part of the [VOF5] isolated octahedron.

The title compound, pentastrontium tris[pentafluoridooxidovanadate(IV)] fluoride trihydrate, was obtained under hydrothermal conditions. Its crystal structure has been refined from intensity data of a non-merohedrally twinned crystal. Two domains in almost equal proportions are related by a À180 rotation along the reciprocal [101]* vector. The structure may be considered as a derivative of the fluorite structure type, adopted here by SrF 2 . In the title compound, fluorite-like large rods are recognized, built up from a group of 16 Sr atoms of which 6 are substituted by V atoms, leading to [Sr 10 V 6 ] 1 units. These rods extend infinitely along the b axis and are interconnected by the three water molecules. Each of the water molecules is shared by two different Sr atoms belonging to two different rods. The rods are also interconnected by an 'independent' F atom in a distorted triangular [FSr 3 ] coordination and by hydrogen-bonding interactions via donor water molecules. The acceptors are either F atoms or the O atoms of the vanadyl ion, VO 2+ , that is part of the [VOF 5 ] isolated octahedron.

Comment
The title compound is the first hydrated strontium vanadium oxy-fluoride characterized crystallographically. It is built up from a network of SrO x F y polyhedra (x + y = 9 -12), connected by faces, edges and vertices. Isolated (V IV OF 5 ) 3octahedra with a short V IV ═O bond (1.596 (4)-1.691 (4) Å) characteristic of a vanadyl ion, VO 2+ , (Crosnier-Lopez et al., 1994) are inserted into this network (Fig. 1). One of the fluorine atoms (F1) is shared exclusively by three strontium atoms (Sr1, Sr2, Sr3) in a triangular [FSr 3 ] coordination, and will be named 'independent' according to previous descriptions (such a structure unit is also present in Sr 5 Zr 3 F 22 (Le Bail, 1996); it shows the same A 5 B 3 X 22 formula as the title compound). The three water molecules coordinate to these three (Sr1, Sr2, Sr3) strontium atoms, all of which have an overall ninefold coordination ( Fig. 2), that is best described by a distorted tri-capped trigonal prism. The two remaining strontium atoms are exclusively coordinated by F atoms. The distorted [Sr(5)F 12 ] cuboctahedron is connected to the [Sr(4)F 11 ] polyhedra (best described as a defect cuboctahedron, lacking one vertex) by a square face (Fig. 3).
Any strong relation with the fluorite structure (adopted by SrF 2 ) seems to be ruled out by the absence of F atoms in tetrahedral coordination [FSr 4 ]. However, most F atoms are forming [FSr 3 V] distorted tetrahedra. One may consider that four strontium atoms (two Sr(4)F 11 and two Sr(5) 12 polyhedra sharing faces) represent a small fluorite structure relic around which half of the expected 12 Sr atoms are replaced by V atoms (forming [Sr 10 V 6 ] blocks), which leads to the [FSr 3 V] distorted tetrahedra. Indeed, these blocks form infinite rods along the b axis, with formulation [Sr 5 V 3 ] ∞ . The oxygen atoms of the six [VOF 5 ] octahedra are part of the vanadyl V IV ═O double bond and are all directed externally to these rods. This is well seen on the crystal structure projection (Fig. 4) where the water molecules are also placed in the rod interstices together with the F1 atom. Both types of ligands play a role in the interconnections between the rods. The relation of Sr 5 (V IV OF 5 ) 3 F(H 2 O) 3 with the SrF 2 fluorite structure is provided in Fig. 5. (Table 1), participating in the interconnection of the [Sr 10 V 6 ] ∞ rods. One of these hydrogen bonds is clearly bifurcated (O1-H11···O4/O5). The distinction between F and O atoms was evident from the valence bond analysis (Table 2) according to the empirical expression given by Brown & Altermatt (1985), using parameters from Brese & O'Keeffe (1991). The valence bond analysis allows also to recognize the water molecules. The short vanadium-oxygen bond is characteristic of a vanadyl V IV ═O double bond.

The hydrogen bonding involves both O and F atoms through Ow-H···O/F interactions
Thermal analysis (TGA) measurement from selected crystals shows a mass loss starting close to 573 K, without any clear stop for the expected 3H 2 O release; the corresponding 5.42% mass loss is attained at 693 K, then the mass loss accelerates and attains 17% up to 873 K. The X-ray powder diffraction pattern of the final product is similar to fluorite-type SrF 2 , but the real composition is more probably corresponding to a fluorite solid solution with formula close to Sr 5 V 3

Refinement
From a first data collection on a conventional four-circle diffractometer, the structure could be solved in spite of the twinning, removing a lot of reflections that belong to two domains, or that were partly overlapping. However, the final data/parameter ratio was so poor that a second data collection was performed (years later), using a Bruker SMART APEX system. The distinction between F and O atoms was clear from the valence bond analysis, allowing also to recognize the water molecules. The short vanadium-oxygen bond is characteristic of a vanadyl V IV =O double bond.
In the final Fourier map the highest peak is 0.85 Å from atom Sr1 and the deepest hole is 0.82 Å from atom Sr2. Fig. 1. ORTEP-3 view of the [V IV OF 5 ] 3octahedra showing the off-centered V position with short V=O distances (1.596-1.691 Å) and long opposite V-F distances (2.056-2.105 Å). Displacement ellipsoids are drawn at the 50% probability level.   sup-4 Fig. 5. Comparison between the [Sr 10 V 6 ] ∞ rods in the title compound (bottom) and the corresponding Sr 16 block in the SrF 2 fluorite-type structure (top), obtained by doubling its c axis and reversely replacing V by Sr atoms. The heights of the z coordinates are indicated by the fractions×100.

Special details
Geometry. All e.s. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  The valence deficit observed on O4, O5 and O6, as well as on F4, F7, F13 and F16, is expected to be compensated by hydrogen bonding, since they behave as acceptors.