Crystal structure of ammonium divanadium(IV,V) tellurium(IV) heptaoxide

In the title layered, mixed-valence ammonium vanadium tellurite, the VV atom are tetrahedrally coordinated and the VIV atoms adopt distorted octahedral coordination geometries. The presumed TeIV lone pairs of electrons are directed inwards into lacunae in the double polyhedral layers.


Chemical context
An important feature of the crystal chemistry of tellurium(IV), electron configuration [Kr]4d 10 5s 2 , is the stereochemical activity of the 5s 2 lone-pair of electrons presumed to reside on the Te atom (Wells, 1962). This leads to distorted and unpredictable coordination polyhedra for the Te IV atom in the solid state (Zemann, 1968;Weber & Schleid, 2000), and its inherent asymmetry may promote the formation of noncentrosymmetric crystal structures with potentially interesting physical properties (Nguyen et al., 2011). As part of our studies in this area (Johnston & Harrison, 2007), we now describe the synthesis and structure of the title mixed-valence compound, (NH 4 )(V IV O 2 )(V V O 2 )(TeO 3 ), (I). Some of the starting vanadium(V) was unexpectedly reduced, perhaps accompanied by oxidation of some of the ammonia.

Structural commentary
The polyhedral building units of (I) are shown in Fig. 1. Atom V1 is bonded to four O-atom neighbours (O3 i , O4, O6 and O7; ISSN 1600-5368

Figure 1
The asymmetric unit of (I) (50% displacement ellipsoids) expanded to show the coordination polyhedra of the V and Te atoms; see Table 1 for symmetry codes. mean = 1.711 Å ) in a distorted tetrahedral arrangement (see Table 1 for symmetry codes) The mean O-V1-O bond angle is 109.2 , although the O7-V1-O3 i [124.1 (7) ] and O3 i -V1-O4 [97.0 (7) ] bond angles diverge considerably from the ideal tetrahedral value. The bond-valence-sum (BVS) values (in valence units) for V1, as calculated by the Brown & Altermatt (1985) formalism, using parameters appropriate for V IV and V V , are 4.96 and 5.22, respectively. Both clearly indicate a pentavalent state for this atom.
Te1 is three-coordinated by oxygen atoms (O1, O2 and O3) in a distorted trigonal-pyramidal arrangement [mean Te-O = 1.867 Å ; BVS(Te1) = 3.98]. The O-Te-O bond angles are all less than 95 , suggesting that a treatment of the bonding about Te involving sp 3 hybrid orbitals and a lone pair (as in ammonia) may be too simple (Wells, 1962). As is typical (Feger et al., 1999) of the crystal chemistry of tellurium(IV), its environment includes further O atoms much closer than the Bondi (1964) van der Waals radius sum of 3.65 Å for Te and O. In particular, there is a fourth O atom within 2.70 Å [Te1-O7 vii = 2.695 (7) Å (vii) = 1 2 À x, 1 2 + y, 1 2 + z], which results in an overall distorted folded-square arrangement about Te1.
Assuming the presence of V V and V IV in equal amounts in the structure, the charge-balancing criterion indicates that N1 must be part of an ammonium ion (which is obviously consistent with the use of significant quantities of ammonia in the synthesis), although no H atoms could be located from the present diffraction data. However, short NÁ Á ÁO contacts in the crystal structure (vide infra) are indicative of hydrogen bonding. The presence of NH 4 + ions is also supported by the IR spectrum of (I). The alternative possibilities of neutral ammonia molecules or water molecules and a different distribution of vanadium oxidation states seem far less likely to us.

Packing features
The connectivity of the VO 4 , VO 6 and TeO 3 polyhedra in (I) leads to a layered structure. The building blocks share vertices via V-O-V and V-O-Te bonds; conversely, there are no Te-O-Te links, which can occur in tellurium-rich compounds (Irvine et al., 2003). Each anionic layer in (I) is constructed from two infinite (100) sheets of composition built up from a network of cornersharing four-and six-membered rings (Fig. 2). The fourmembered rings are built from one TeO 3 , one V1O 4 tetrahedron and two V2O 6 octahedra, whilst the six-membered rings are constructed from two of each different polyhedra. It is interesting to note the V-O-V inter-polyhedral angles (mean = 154.1 ) are much more obtuse than the Te-O-V angles (mean = 124.0 ). The two sheets within each layer are linked through V2-O6-V1 bonds and are orientated so that the four-membered rings of one sheet are aligned with the six-membered rings of the other, and the lone-pair electrons of the Te IV species point into the centre of the layer. These 'lone-pairs sandwiches' represent a novel way of accommodating the Te IV lone-pairs, which may be compared to self-contained 'tubes' in BaTe 3 O 7  (7) Te1-O1-V2 iv 125.5 (9) Table 2 Hydrogen-bond geometry (Å ).

Figure 2
View approximately down [100] of part of a polyhedral layer in (I).
Colour key: V1O 4 tetrahedra orange, V2O 6 octahedra yellow, O atoms red. The TeO 3 pyramids are shown as green pseudo-tetrahedra with the presumed lone-pair of electrons shown as a white sphere.

Synthesis and crystallization
0.7276 g (4 mmol) of V 2 O 5 and 0.3249 g (3 mmol) TeO 2 were placed in a 23 ml capacity Teflon-lined stainless steel autoclave. Added to this were 7 ml of a 1.3 M NH 3 solution and 8 ml of H 2 O (pre-oven pH = 8.5). The autoclave was sealed and heated in an oven at 438 K for three days, followed by cooling to room temperature over a few hours. The resulting solid products, consisting of dark-red needles of (I), transparent chunks of TeO 2 and an unidentified yellow powder, were recovered by vacuum filtration and washing with water and acetone. IR data (KBr disk) were collected using a handpicked sample of (I): broad bands at $3400 and 3000 cm À1 can be ascribed to the symmetric and asymmetric stretches of the tetrahedral ammonium ion (Balraj & Vidyasagar, 1998). The doublet at 1440 and 1411 cm À1 is indicative of H-N-H bending modes; the presence of a doublet is in itself interesting, suggesting there may be some disorder associated with the H atoms of the ammonium cation. This phenomenon may also contribute to the difficulty in locating the H-atom positions from the X-ray data. The large number of overlapping bands in the 1000-400 cm À1 range can be attributed to framework V O, V-O, Se-O and O-Se-O modes.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms could not be located in difference maps, neither could they be geometrically placed. The crystal studied was found to be a racemic twin.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 R-factors(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )