Crystal structure of Sr5Te4O12(OH)2, the first basic strontium oxotellurate(IV)

In the crystal structure of the basic strontium oxotellurate(IV), Sr5Te4O12(OH)2, the principal building blocks, namely SrOx polyhedra (x = 7 or 8) and trigonal–pyramidal TeO3 units are linked into a framework structure delimiting channels parallel to [010].


Chemical context
The peculiar feature of the crystal chemistry of oxotellurates(IV) (Christy et al., 2016) is the presence of the 5s 2 lone electron pair, denoted E. In the majority of cases, the lone electron pair E is stereochemically active, making oxotellurates(IV) interesting for crystal engineering, e.g. in terms of the synthesis of compounds with non-centrosymmetric structures or structures with polar directions. Next to the influence of the (metal) cation on the physico-chemical characteristics of oxotellurates(IV), physical and underlying structural properties of such compounds can also be varied by incorporation of other oxoanions into the oxotellurate(IV) framework, e.g. by p-block oxoanions such as nitrate (Stö ger & Weil, 2013) or selenate (Weil & Shirkanlou, 2015), or by d-block oxoanions such as vanadate (Weil, 2015).
In this context we attempted the hydrothermal synthesis of new oxotellurate phases in the system Sr-Te-Se-O-(H). In comparison with typical solid-state reactions using open crucibles under atmospheric conditions, this method is more feasible because Te IV then tends not to be oxidized or to be evaporated during the reaction process. However, a clear disadvantage of the hydrothermal method is the high(er) number of adjustable parameters (pressure, concentration, temperature, time, filling degree, solvent etc), which often makes the products of these experiments difficult to predict or even to reproduce, accompanied by formation of several solid phases in one batch. This was also the case for the present study. Instead of a strontium oxoselenatotellurate, several oxotellurate phases were obtained without incorporation of selenium. Amongst these phases, the title compound, Sr 5 Te 4 O 12 (OH) 2 , a hitherto unknown strontium oxotellurate, was isolated and structurally determined by single crystal X-ray diffraction.

Structural commentary
The asymmetric unit of Sr 5 Te 4 O 12 (OH) 2 comprises three Sr, two Te and seven O atoms (H atoms were not included in the final model, see Section 5 and discussion below). Except one Sr atom (Sr2) that is located on a twofold rotation axis, all atoms are in general positions.
The coordination numbers of the Sr atoms are 7 (for Sr1 and Sr3) and 8 (for Sr2) if Sr-O distances < 3.0 Å are considered as relevant for the first coordination sphere. The corresponding polyhedra are considerably distorted, with Sr-O bond lengths ranging from 2.393 (11) to 2.960 (11) Å (Table 1) and might be described as monocapped octahedra for Sr1 and Sr3, and as a bicapped trigonal prism for Sr2. The SrO 8 and the two SrO 7 polyhedra share corners and edges, thereby constructing a three-dimensional framework structure encapsulating channels that propagate along [010]. Each of the two Te atoms connect to the outer oxygen atoms of the framework in a very similar trigonal-prismatic configuration (Table 1), with the 5s 2 lone electron pair E being stereochemically active, i.e. pointing towards the empty space of the channels (Fig. 1)

Figure 1
Projection of the crystal structure of Sr 5 Te 4 O 12 (OH) 2 along [010], with displacement ellipsoids drawn at the 74% probability level. The trigonalpyramidal TeO 3 groups are given in red; the O atom representing the OH group is given in yellow, all other O atoms are colourless. Bond-valence calculations (Brown, 2002) clearly reveal the presence of an OH group for atom O7 (Table 2), also required by charge neutrality. Atom O7 is bonded to four Sr atoms (Table 1, Fig. 1) and has also four possible oxygen acceptor atoms for hydrogen bonding of medium to weak strength ( Table 3). The situation of four possible acceptor atoms is displayed in Fig. 2 and makes it appear likely that the corresponding H atom of the OH group is positionally disordered and thus could not be located during the present study.
In the sense of a crystal-chemically more detailed formula, the title compound may alternatively be formulated as 4SrTeO 3 ÁSr(OH) 2 and represents the first basic strontium oxotellurate(IV), viz. with the presence of an OH functionality. In comparison with the other strontium oxotellurates(IV) compiled in Section 3, all Sr-O and Te-O lengths are in similar ranges.

Figure 2
The vicinity of the OH group emphasizing the different possibilities for OÁ Á ÁO hydrogen bonding (green lines). Sr-(OH) bonds have been omitted for clarity. Symmetry operators refer to those of Table 3; displacement ellipsoids are the same as in Fig. 1. crystal forms clearly discernible. According to X-ray powder diffraction of the bulk material, the following phases could be identified: -TeO 2 (Lindqvist, 1968), SrTe 2 O 5 (Redman et al., 1970), SrTe 3 O 8 (Barrier et al., 2006;Weil & Stö ger, 2007) and SrTe 5 O 11 (Burckhardt & Trö mel, 1983). Solid reaction products containing Se-phases were not detected. Platy Sr 5 Te 4 O 12 (OH) 2 crystals were present in only minor amounts, and were manually separated for structure determination from the other solid products.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Some of the O atoms showed physically unreasonable behaviour when refined with anisotropic displacement parameters. Hence, for the final model all O atoms were refined with individual isotropic displacement parameters. The H atom of the OH group (or positionally disordered parts) could not be located and thus was not included in the model. Twinning by inversion was also taken into account, with a contribution of the minor twin component of about 6%. The maximum and minimum remaining electron densities are found 2.34 and 0.96 Å , respectively, from Sr3. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Crystal data
Sr 5  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.