Ni3Te2O2(PO4)2(OH)4, an open-framework structure isotypic with Co3Te2O2(PO4)2(OH)4

The crystal structure of Ni3Te2O2(PO4)2(OH)4 comprises a comparatively rare penta-coordinated TeIV atom, resulting in a [TeO3(OH)2] square-pyramidal coordination polyhedron.


Chemical context
The crystal chemistry of Te IV -containing compounds is very diverse and strongly influenced by the stereochemically active 5s 2 lone pair. The space requirement of the latter frequently results in one-sided and low-symmetric coordination spheres around Te IV , as surveyed recently for the vast family of oxidotellurates(IV) (Christy et al., 2016). The polarities and shapes of corresponding oxidotellurate(IV) anions can be utilized in the search for new compounds with non-centrosymmetric structures. The absence of an inversion centre is a precondition for a substance to have ferro-, pyro-or piezoelectric properties or to have non-linear optical properties (Ok et al., 2006). Combining oxidotellurates(IV) with additional (transition) metal cations often leads to open-framework structures because of the space required for the 5s 2 lone pair. This way, either channels can be integrated within threedimensional frameworks, or layers, chains or clusters of building blocks can be formed (Stö ger & Weil, 2013). Incorporating additional anions into transition-metal oxidotellurates(IV) increases the possibilities for structural diversification. Following this strategy, several mixed-anion oxotellurates (IV) Zimmermann et al., 2011].
In this communication we describe the synthesis and crystal structure analysis of Ni 3 Te 2 O 2 (PO 4 ) 2 (OH) 4 , which is isotypic with its cobalt(II) analogue. The two structures are quantitatively compared.

Structural commentary
Of the ten atoms in the asymmetric unit (2 Ni, 1 Te, 1 P, 5 O, 1 H), seven are situated on special positions. Ni1 is located on an inversion centre (Wyckoff position 4 f), Ni2 on a site with symmetry 2/m (2 b), Te1 and P1 both possess site symmetry m (4 i), and three of the oxygen sites (O1, O3, O4) are likewise located on a mirror plane (4 i) while the other two oxygen atoms and the hydrogen atom (O2, O5, H1) are located on general positions (8 j).
Both nickel atoms are coordinated octahedrally but otherwise show a different environment. Ni1 is surrounded by six oxygen atoms (O1, O2, O3 and their symmetry-related counterparts) and forms chains of edge-sharing [Ni1O 6 ] octahedra extending parallel to [010]. The 1 1 [Ni1O 4/2 O 2/1 ] chains are not entirely straight; the octahedra are tilted against each other with every second unit being oriented in the same direction. The three pairs of Ni1-O bond lengths are rather similar (Table 1), with an average length of 2.058 Å . The distance between neighbouring Ni1 atoms in a chains amounts to 2.9717 (11) Å . Ni2 is coordinated by two O atoms (O4 and its symmetry-related counterpart) in axial positions and by four hydroxide groups (O5 and its three symmetry-related counterparts) in the equatorial positions. The latter have a slightly longer bond than the former (Á = 0.032 Å ), with an average bond length of 2.084 Å for the six O atoms. The [Ni2O 2 (OH) 4 ] octahedra are isolated from each other and are also not linked to the 1 1 [Ni1O 4/2 O 2/1 ] chains. Te1 is coordinated by five oxygen atoms, two of them being hydroxide groups. The surrounding atoms form a distorted square pyramid (Fig. 1), a coordination polyhedron that is comparatively rare in the crystal chemistry of oxidotellur-626 Eder and Weil Ni 3 Te 2 O 2 (PO 4 ) 2 (OH) 4 Acta Cryst. (2020). E76, 625-628 research communications Table 1 Comparison of bond lengths (Å ) in the isotypic M 3 Te 2 O 2 (PO 4 ) 2 (OH) 4 compounds (M = Ni, Co a ).

Figure 2
The crystal structure of Ni 3 Te 2 O 2 (PO 4 ) 2 (OH) 4 in a projection along [010]. Displacement ellipsoids are drawn at the 90% probability level; hydrogen bonds are shown as orange lines.

Figure 1
The square-pyramidal [TeO 3 (OH) 2 ] polyhedron in the title compound. Displacement ellipsoids are drawn at the 90% probability level.
[Symmetry code: (i) x, Ày + 1, z.] ates(IV), with a trigonal pyramid (TeO 3 2-) as the most commonly observed type of anion (Christy et al., 2016). The Te1 atom is displaced from the basal plane of the pyramid by 0.1966 (2) Å . The two symmetry-related O2 atoms defining one side of the basal plane exhibit a significantly larger distance [2.3094 (18) 4 ] octahedra, as well as a corner of the phosphate tetrahedra. In this way, a threedimensional framework structure is obtained with channels running parallel to [010]. The free-electron pairs point into the smaller type of channels whereas the hydrogen atoms of the hydroxy group protrude into the larger type of channels. This results in hydrogen bonds of medium strength, with the OH groups linking to opposite O atoms ( Fig. 2; Table 2).
As a result of the similar ionic radii (Shannon, 1976) of sixcoordinated Ni 2+ (0.69 Å ) and Co 2+ (0.75 Å , assuming a highspin d 7 state), the comparable bond lengths in the two isotypic structures differ only marginally (Table 1). The two structures were also quantitatively compared using the program compstru (de la Flor et al., 2016). The absolute distances between paired atoms are 0 Å for Ni1/Co1, 0 Å for Ni2/Co2, 0.0213 Å for P1, 0.0289 Å for O1, 0.0289 Å for O2, 0.0342 Å for O3, 0.0192 Å for O4 and 0.0271 Å for O5. The degree of lattice distortion is 0.0072, the arithmetic mean of the distance between paired atoms is 0.0227 Å , and the measure of similarity is 0.011.

Synthesis and crystallization
Crystals of Ni 3 Te 2 O 2 (PO 4 ) 2 (OH) 4 were obtained under hydrothermal conditions. The starting materials, 0.1796 g (0.591 mmol) NiCO 3 Á2Ni(OH) 2 , 0.1870 g TeO 2 (1.172 mmol) and 0.16 g 85% H 3 PO 4 (1.4 mmol), were weighed into a small Teflon vessel with a volume of ca 3 ml. The reactants were mixed, and the vessel filled to about two thirds with deionized water. The reaction vessel was heated inside a steel autoclave at 483 K for 7 d; the autoclave was removed from the oven and allowed to cool to room temperature over about four hours. A bright-green solid besides small amounts of a pale-yellow powder was obtained as the reaction product. X-ray powder diffraction of the bulk revealed Ni 3 Te 2 O 2 (PO 4 ) 2 (OH) 4 as the main product and TeO 2 (corresponding to the pale-yellow powder) as a side product. A light-green block-shaped single crystal of Ni 3 Te 2 O 2 (PO 4 ) 2 (OH) 4 was selected for the diffraction experiment.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Atom labels and starting coordinates for refinement were adopted from the isotypic Co 3 Te 2 O 2 (PO 4 ) 2 (OH) 4 structure (Zimmermann et al., 2011). The hydrogen atom of the hydroxy group was located in a difference-Fourier map and was refined freely. The remaining maximum electron density of 3.6 e À Å À3 is located 0.71 Å from P1. Modelling the corresponding site as a minor disorder component lead to unrealistic P-O distances and physically non-reasonable displacement parameters. We therefore did not consider this site in the final model. Computer programs: APEX3 and SAINT (Bruker, 2016), SHELXL (Sheldrick, 2015), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010). Table 2 Hydrogen-bond geometry (Å , ).

Crystal data
Ni 3  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.