Tris[triphenylantimony(V)]hexa(μ-oxido)tellurium(VI): a molecular complex with six Te—O—Sb bridges

The structure of (C18H15Sb)3TeO6, contains a [TeO6] octahedral unit linked to three trigonal–bipyramidal [SbC3O2] units via pairs of bridging O atoms to form a discrete molecular unit. The packing of the units is dominated by C—H⋯O hydrogen bonding and weak dispersion forces, with a minor contribution from C—H⋯π bonds and π–π stacking interactions.


Chemical context
Organoantimony(V) species readily form covalent derivatives with a range of organic and inorganic oxo-ions and these can be used in the construction of metal-oxide clusters (Nicholson et al., 2011). Unlike the series of molecular fivefold-coordinated tetraphenylantimony(V) compounds, which easily dissociate in solution to yield tetraphenylstibonium cations, [Ph 4 Sb] + (Domasevitch et al., 2000), the derivatives of triphenylantimony(V) are much more chemically robust and they are well suited for the preparation of covalent oxide materials. The interactions between the Ph 3 Sb 2+ cations and oxoanions are particularly important as they potentially control the assembly of these units into either discrete oxoclusters or polymers. For example, one-dimensional covalent chains of oxo-bridged Ph 3 Sb 2+ moieties were identified as a possible motif for amorphous [Ph 3 SbO] n formation (Carmalt et al., 1996). In addition, there are a few complexes known in which singly charged oxoanions form molecular five-coordinate structures with terminal [ReO 4 ] À (Wirringa et al., 1992) or [PhSO 3 ] À (Rü ther et al., 1986) groups or bridging [Ph 2 PO 2 ] À groups (Srungavruksham & Baskar, 2013), while insoluble derivatives with tetrahedral dianions, such as SO 4 2À , SeO 4 2À and CrO 4 2À , are likely to be polymeric .
At the same time, Ph 3 Sb 2+ units may coordinate to the O atoms of octahedral oxoanion species to form discrete mol- ISSN 2056-9890 ecules: one can anticipate using Ph 3 Sb 2+ for the functionalization of inorganic metal-oxide octahedra with the generation of doubly bridged M(-O) 2 Sb motifs. The latter are formally similar to 1,2-benzenediolate chelates, which have been observed in molecular organoantimony compounds (Hall & Sowerby, 1980). Such double bridges are well suited for covalent immobilization of triorganoantimony moieties at the developed metal-oxide surfaces of polyoxometalates. The coordination behaviour of such systems, however, does not appear to have been considered so far. In this context, we have examined a structurally simple and attractive inorganic oxoanion, namely octahedral hexaoxidotellurate(VI). In the present contribution, we crystallize this unit with Ph 3 Sb 2+ units and report the crystal structure of the title compound, (C 18 H 15 Sb) 3 TeO 6 , which features the formation of discrete clusters, [Te{(-O) 2 SbPh 3 } 3 ].

Structural commentary
The title compound crystallizes in the monoclinic space group, C2/c, and contains the discrete molecular unit shown in Fig. 1 The molecular structure of the title compound with displacement ellipsoids drawn at the 40% probability level. Hydrogen atoms are represented by small circles of arbitrary radius. Table 1 Selected geometric parameters (Å , ).  (Addison et al., 1984). These values are closer to unity, the value expected for a perfect trigonal-bipyramidal geometry, than to zero, which is expected for a square-based pyramidal geometry.
In each of the three Sb-based trigonal bipyramids, the axial Sb-O ax bonds, Sb1-O2, Sb2-O4 and Sb3-O5, are slightly longer [in the range 2.087 ( The distribution of the Te-O ax Sb and Te-O eq Sb bonds indicates that the coordination octahedron around the Te atom has the merconfiguration (Fig. 2). This is consistent with the mer-octahedral geometry adopted in the previously examined trisubstituted tellurates, e.g. mer-[(Bu 3 SnO) 3 Te(OH) 3 ] (Beckmann et al., 2002).

Supramolecular features
The relatively loose packing of the title compound is dominated by weak dispersion forces, with the calculated packing index of 67.5 approaching the lower limit of the 65-75% range expected for organic solids (Dunitz, 1995). For comparison, the perceptibly denser packing of more symmetrical polyphenyl substituted species, e.g. 1,3,5,7-tetraphenyladamantane, supporting a complex framework of aromatic interactions, has a packing index of 70.4 (Boldog et al., 2009). In the absence of stronger bonding, the present supramolecular array is mediated by a series of C-HÁ Á ÁO and C-HÁ Á Á hydrogen bonds with a minor contribution from / stacking interactions.

Figure 3
One-dimensional chains running along the b-axis direction, in which translation-related molecules of the title compound are linked by a series of weak C-HÁ Á ÁO hydrogen bonds (shown as dashed blue lines).

Hirshfeld analysis
Supramolecular interactions in the title structure were further accessed and visualized by Hirshfeld surface analysis (Spackman & Byrom, 1997;McKinnon et al., 2004;Hirshfeld, 1977;Spackman & McKinnon, 2002) performed using Crys-talExplorer17 (Turner et al., 2017). The two-dimensional fingerprint plots (Fig. 5) suggest that the major contributors to the Hirshfeld surface are HÁ Á ÁH (58.0%) and HÁ Á ÁC/CÁ Á ÁH (32.6%) contacts, while the HÁ Á ÁO/OÁ Á ÁH contacts contribute only 7.8%. The latter are identified by a pair of short and very diffuse spikes, at ca 2.6 Å , which are actually superimposed upon the regions for the HÁ Á ÁC/CÁ Á ÁH interactions (the shortest of which is ca 2.9 Å ). These results are consistent with the weakness of the C-HÁ Á ÁO bonds in the structure. It is evident that only a few of the HÁ Á ÁC/CÁ Á ÁH contacts correspond to C-HÁ Á Á bonding. Therefore, the HÁ Á ÁC/CÁ Á ÁH plot represents a rather diffuse collection of points between the pair of poorly resolved features and there no 'wings' at the upper left and lower right, which are characteristic of C-HÁ Á Á interactions (Spackman & McKinnon, 2002). The fraction of CÁ Á ÁC contacts is particularly low (1.6%), indicating only very minor significance of the stacking interactions. In fact, with the exception of the onestack noted above, this kind of interaction is irrelevant to the title structure.

Synthesis and crystallization
In previously reported syntheses, a range of silver salts were used in ion-exchange reactions to form Ph 3 SbCl 2  and Ph 4 SbBr (Goel, 1969) derivatives cleanly and in high yields. Our attempts to prepare tellurate(VI) analogues of such compounds led to dearylation and the formation of mixtures. The title compound was prepared in low yield by reacting the silver salt, Ag 3 H 3 TeO 6 , with tetraphenylantimony(V) bromide as follows: The starting material, Ag 3 H 3 TeO 6 , was synthesized according to the method of Gospodinov (1992). 0.220 g (0.4 mmol) of Ag 3 H 3 TeO 6 were added to a solution containing 0.612 g (1.2 mmol) of Ph 4 SbBr in 20 mL of acetonitrile. The mixture was stirred for 3 h and then the AgBr precipitate removed by filtration. Evaporation of the solution yielded a colourless glassy material, which was then dissolved in 10 mL Crystal packing of the title compound, viewed down the b axis, showing how the C-HÁ Á ÁO bonded chains (which are orthogonal to the drawing plane) are connected into layers by means of C-HÁ Á Á and slippedstacking interactions. The blue and grey colours indicate two separate bilayers, which lie parallel to the bc plane. [Symmetry codes: (iv) x, Ày + 1, z + 1 2 ; (v) Àx, Ày + 1, Àz.]
of a 1:1 v/v mixture of benzene and butyl acetate. Slow evaporation of the solution to a volume of 2-3 mL afforded 0.138 g (27%) of the product in the form of long colourless prisms. The crystals were filtered and dried in air. Analysis

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All the hydrogen atoms were located in difference-Fourier maps and then refined as riding with C-H = 0.95 Å and U iso (H) = 1.2U eq (C).

Funding information
This work was supported by the Ministry of Education and Science of Ukraine (project No. 19BF037-05).  (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012).

Hexa-µ-oxido-1:2κ 4 O:O;1:3κ 4 O:O;1:4κ 4 O:O-nonaphenyl-2κ 3 C,3κ 3 C,4κ 3 C-triantimony(V)tellurium(VI)
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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (7)