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

The polyhedral building blocks of the layered inorganic network in the mixed-valence title compound, (NH₄)(V^IVO₂)(V^VO₂)(TeO₃), are vertex-sharing V^VO₄ tetra­hedra, distorted V^IVO₆ octa­hedra and TeO₃ pyramids, which are linked by V—O—V and V—O—Te bonds, forming double layers lying parallel to (100). The presumed Te^IV lone-pairs of electrons appear to be directed inwards into cavities in the double layers. The charge-balancing ammonium cations lie between the layers and probably inter­act with them via N—H⋯O hydrogen bonds.

1. Chemical context

An important feature of the crystal chemistry of tellur­ium(IV), electron configuration [Kr]4d¹⁰5s², is the stereochemical activity of the 5s² 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 non-centrosymmetric 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, (NH₄)(V^IVO₂)(V^VO₂)(TeO₃), (I). Some of the starting vanadium(V) was unexpectedly reduced, perhaps accompanied by oxidation of some of the ammonia.

2. Structural commentary

The polyhedral building units of (I) are shown in Fig. 1. Atom V1 is bonded to four O-atom neighbours (O3ⁱ, O4, O6 and O7; 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—O3ⁱ [124.1 (7)°] and O3ⁱ—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.

Table 1
Selected geometric parameters (Å, °)

V1—O7	1.631 (7)	V2—O1^iii	1.973 (16)
V1—O6	1.656 (5)	V2—O7^i	2.053 (7)
V1—O3^i	1.770 (5)	V2—O6	2.311 (5)
V1—O4	1.788 (9)	Te1—O1	1.748 (14)
V2—O5	1.612 (5)	Te1—O3	1.921 (5)
V2—O2	1.935 (15)	Te1—O2	1.931 (14)
V2—O4^ii	1.961 (7)
Te1—O1—V2^iv	125.5 (9)	V1—O4—V2^vi	145.3 (4)
Te1—O2—V2	120.3 (8)	V1—O6—V2	167.7 (6)
V1^v—O3—Te1	131.3 (2)	V1—O7—V2^v	149.9 (4)
Symmetry codes: (i) ; (ii) ; (iii) x, y, z-1; (iv) x, y, z+1; (v) ; (vi) .

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.

The coordination polyhedron about atom V2 is a distorted octa­hedron. O5 is bonded to V2 by a short `vanad­yl' V=O double bond [1.612 (5) Å], whilst O1, O4, O7 and O2 occupy the equatorial positions with V—O bond lengths between 1.93 and 2.06 Å. O6 is located trans to O5 [O5—V2—O6 = 176.1 (11)°] and is consequently much farther away from the metal ion [2.311 (5) Å] than the other O atoms. This octa­hedral distortion mode is characteristic of both vanadium(IV) and vanadium(V) and may be theoretically analysed in terms of a second-order Jahn–Teller distortion (Kunz & Brown, 1995). The O—V2—O bond angles also show a broad spread [cis: 73.8 (5) to 104.2 (8)°, trans: 157.0 (6) to 176.1 (11)°]. BVS calculations for V2 yield values of 4.20 (V^IV parameters) and 4.42 (V^V parameters), which both indicate vanadium(IV).

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 sp³ 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) =  − x,  + y,  + 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 N⋯O contacts in the crystal structure (vide infra) are indicative of hydrogen bonding. The presence of NH₄⁺ 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.

3. Packing features

The connectivity of the VO₄, VO₆ and TeO₃ 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 [(V^IVO₂)(V^VO₂)(TeO₃)]⁻, built up from a network of corner-sharing four- and six-membered rings (Fig. 2). The four-membered rings are built from one TeO₃, one V1O₄ tetra­hedron and two V2O₆ 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°).

Figure 2 View approximately down [100] of part of a polyhedral layer in (I). Colour key: V1O4 tetra­hedra orange, V2O6 octa­hedra yellow, O atoms red. The TeO3 pyramids are shown as green pseudo-tetra­hedra with the presumed lone-pair of electrons shown as a white sphere.

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<sup>IV</sup> lone-pairs, which may be compared to self-contained `tubes' in BaTe₃O₇ and BaTe₄O₉ (Johnston & Harrison, 2002) or large 12-ring channels in Mg_0.5ZnFe(TeO₃)₃·4.5H₂O (Miletich, 1995).

The layers stack in the [100] direction, with the ammonium cations occupying the inter-layer regions (Fig. 3). Connectivity between the layers is presumably mediated by N—H⋯O hydrogen bonds, with N1 having eight O-atom neighbours within 3.4 Å (four in each layer). The N⋯O distances are listed in Table 2.

Table 2
Hydrogen-bond geometry (Å)

D—H⋯A	D⋯A	D—H⋯A	D⋯A
N1⋯O5^vii	2.820 (7)	N1⋯O5^viii	3.15 (3)
N1⋯O1^iii	2.89 (2)	N1⋯O1^viii	3.20 (2)
N1⋯O2	2.95 (2)	N1⋯O5^ix	3.20 (3)
N1⋯O2^viii	2.96 (2)	N1⋯O3^iii	3.39 (3)
Symmetry codes: (iii) x, y, z-1; (vii) x, y-1, z; (viii) ; (ix) .

Figure 3 View approximately down [001] of the crystal structure of (I) showing the (100) polyhedral layers inter­spersed by ammonium ions. Colour key: N atoms blue, other atoms as in Fig. 2.

4. Database survey

A search of the Inorganic Crystal Structure Database (ICSD, 2014; web version 2.2.2) revealed three compounds containing ammonium ions, vanadium, tellurium and oxygen: (NH₄)₂(VO₂)[TeO₄(OH)]·H₂O (Kim et al., 2007) contains V^VO₄ tetra­hedra and Te^VIO₅(OH) octa­hedra, which link together into infinite chains. (NH₄)₂(VO₂)₂[TeO₄(OH₂)] (Yun et al., 2010) is a layered structure containing unusual V^VO₅ square pyramids and Te^VIO₄(OH₂) octa­hedra. (NH₄)₉K(Mo₁₂V₁₂TeO₆₉)(TeO₃)₂·27H₂O (Corella-Ochoa et al., 2011) is a complex polyoxidometallate containing V^V, V^IV and Te^IV atoms.

5. Synthesis and crystallization

0.7276 g (4 mmol) of V₂O₅ and 0.3249 g (3 mmol) TeO₂ were placed in a 23 ml capacity Teflon-lined stainless steel autoclave. Added to this were 7 ml of a 1.3 M NH₃ solution and 8 ml of H₂O (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 TeO₂ and an unidentified yellow powder, were recovered by vacuum filtration and washing with water and acetone. IR data (KBr disk) were collected using a hand-picked sample of (I): broad bands at ∼3400 and 3000 cm⁻¹ can be ascribed to the symmetric and asymmetric stretches of the tetra­hedral ammonium ion (Balraj & Vidyasagar, 1998). The doublet at 1440 and 1411 cm⁻¹ 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⁻¹ range can be attributed to framework V=O, V—O, Se—O and O—Se—O modes.

6. 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.

Table 3
Experimental details

Crystal data
Chemical formula	(NH4)(VO2)(VO2)(TeO3)
Mr	359.52
Crystal system, space group	Orthorhombic, Pna21
Temperature (K)	293
a, b, c (Å)	18.945 (2), 7.0277 (8), 5.4402 (6)
V (Å^3)	724.29 (14)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	6.52
Crystal size (mm)	0.17 × 0.02 × 0.02
Data collection
Diffractometer	Bruker SMART1000 CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2000)
Tmin, Tmax	0.404, 0.881
No. of measured, independent and observed [I > 2σ(I)] reflections	7528, 2368, 1595
Rint	0.047
(sin θ/λ)max (Å^−1)	0.756
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.040, 0.082, 0.98
No. of reflections	2368
No. of parameters	101
No. of restraints	1
Δρmax, Δρmin (e Å^−3)	0.99, −1.13
Absolute structure	Flack (1983), 1201 Friedel pairs
Absolute structure parameter	0.5 (1)
Computer programs: SMART and SAINT (Bruker, 2000), SHELXS97 and SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 2012) and ATOMS (Dowty, 1999).

Supporting information

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and ATOMS (Dowty, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Ammonium divanadium(IV,V) tellurium(IV) heptaoxide

Crystal data

(NH4)(VO2)2(TeO3)	Dx = 3.297 Mg m^−3
Mr = 359.52	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pna21	Cell parameters from 5060 reflections
a = 18.945 (2) Å	θ = 2.2–32.5°
b = 7.0277 (8) Å	µ = 6.52 mm^−1
c = 5.4402 (6) Å	T = 293 K
V = 724.29 (14) Å^3	Rod, dark red
Z = 4	0.17 × 0.02 × 0.02 mm
F(000) = 660

Data collection

Bruker SMART1000 CCD diffractometer	2368 independent reflections
Radiation source: fine-focus sealed tube	1595 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.047
ω scans	θmax = 32.5°, θmin = 2.2°
Absorption correction: multi-scan (SADABS; Bruker, 2000)	h = −28→26
Tmin = 0.404, Tmax = 0.881	k = −10→10
7528 measured reflections	l = −6→8

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: notdet
R[F^2 > 2σ(F^2)] = 0.040	H-atom parameters not defined
wR(F^2) = 0.082	w = 1/[σ^2(Fo^2) + (0.0318P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.98	(Δ/σ)max < 0.001
2368 reflections	Δρmax = 0.99 e Å^−3
101 parameters	Δρmin = −1.13 e Å^−3
1 restraint	Absolute structure: Flack (1983), 1201 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.5 (1)

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq
N1	0.0293 (3)	0.2583 (8)	0.225 (5)	0.031 (2)
V1	0.31266 (5)	0.53486 (13)	0.2408 (12)	0.0245 (2)
V2	0.12159 (5)	0.75777 (13)	0.2385 (9)	0.0238 (2)
Te1	0.164135 (17)	0.50920 (5)	0.7434 (5)	0.02034 (10)
O1	0.1064 (8)	0.561 (3)	0.985 (2)	0.056 (5)
O2	0.0970 (8)	0.575 (3)	0.490 (2)	0.048 (4)
O3	0.1367 (2)	0.2463 (7)	0.728 (4)	0.053 (2)
O4	0.3316 (4)	0.4539 (9)	−0.0638 (13)	0.0355 (15)
O5	0.0453 (3)	0.8594 (7)	0.231 (5)	0.055 (2)
O6	0.2292 (2)	0.6033 (8)	0.224 (4)	0.043 (2)
O7	0.3270 (3)	0.3621 (9)	0.4347 (13)	0.0332 (14)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.020 (2)	0.021 (3)	0.053 (6)	−0.004 (2)	0.000 (7)	0.005 (7)
V1	0.0150 (4)	0.0096 (4)	0.0488 (7)	0.0017 (3)	−0.003 (2)	0.003 (3)
V2	0.0200 (4)	0.0128 (4)	0.0386 (6)	−0.0008 (3)	−0.0089 (16)	−0.003 (2)
Te1	0.01691 (14)	0.01447 (16)	0.02964 (19)	0.00084 (13)	0.0000 (9)	−0.0019 (7)
O1	0.017 (5)	0.099 (10)	0.052 (8)	−0.022 (5)	0.018 (4)	−0.054 (7)
O2	0.018 (5)	0.088 (9)	0.040 (8)	−0.011 (5)	−0.006 (4)	0.038 (6)
O3	0.028 (2)	0.018 (2)	0.114 (7)	0.003 (2)	−0.037 (7)	−0.004 (8)
O4	0.043 (4)	0.029 (3)	0.035 (3)	0.012 (3)	0.005 (3)	−0.002 (3)
O5	0.030 (2)	0.020 (2)	0.115 (6)	0.007 (2)	−0.025 (9)	−0.005 (11)
O6	0.018 (2)	0.040 (3)	0.070 (6)	0.0099 (19)	−0.008 (6)	−0.021 (7)
O7	0.036 (3)	0.027 (3)	0.037 (4)	0.003 (3)	0.002 (3)	0.010 (3)

Geometric parameters (Å, º)

V1—O7	1.631 (7)	V2—O6	2.311 (5)
V1—O6	1.656 (5)	Te1—O1	1.748 (14)
V1—O3^i	1.770 (5)	Te1—O3	1.921 (5)
V1—O4	1.788 (9)	Te1—O2	1.931 (14)
V2—O5	1.612 (5)	O1—V2^iv	1.973 (16)
V2—O2	1.935 (15)	O3—V1^v	1.770 (5)
V2—O4^ii	1.961 (7)	O4—V2^vi	1.961 (7)
V2—O1^iii	1.973 (16)	O7—V2^v	2.053 (7)
V2—O7^i	2.053 (7)
O7—V1—O6	114.3 (6)	O1^iii—V2—O7^i	75.9 (6)
O7—V1—O3^i	124.1 (7)	O5—V2—O6	176.1 (11)
O6—V1—O3^i	105.7 (3)	O2—V2—O6	85.7 (6)
O7—V1—O4	109.2 (4)	O4^ii—V2—O6	87.1 (4)
O6—V1—O4	103.4 (8)	O1^iii—V2—O6	77.0 (6)
O3^i—V1—O4	97.0 (7)	O7^i—V2—O6	73.8 (5)
O5—V2—O2	95.6 (9)	O1—Te1—O3	93.7 (9)
O5—V2—O4^ii	96.2 (5)	O1—Te1—O2	94.2 (3)
O2—V2—O4^ii	100.8 (6)	O3—Te1—O2	91.2 (7)
O5—V2—O1^iii	99.2 (8)	Te1—O1—V2^iv	125.5 (9)
O2—V2—O1^iii	89.7 (3)	Te1—O2—V2	120.3 (8)
O4^ii—V2—O1^iii	160.3 (5)	V1^v—O3—Te1	131.3 (2)
O5—V2—O7^i	104.2 (8)	V1—O4—V2^vi	145.3 (4)
O2—V2—O7^i	157.0 (6)	V1—O6—V2	167.7 (6)
O4^ii—V2—O7^i	88.6 (3)	V1—O7—V2^v	149.9 (4)

Symmetry codes: (i) −x+1/2, y+1/2, z−1/2; (ii) −x+1/2, y+1/2, z+1/2; (iii) x, y, z−1; (iv) x, y, z+1; (v) −x+1/2, y−1/2, z+1/2; (vi) −x+1/2, y−1/2, z−1/2.

Hydrogen-bond geometry (Å)

D—H···A	D···A
N1···O5^vii	2.820 (7)
N1···O1^iii	2.89 (2)
N1···O2	2.95 (2)
N1···O2^viii	2.96 (2)
N1···O5^viii	3.15 (3)
N1···O1^viii	3.20 (2)
N1···O5^ix	3.20 (3)
N1···O3^iii	3.39 (3)

Symmetry codes: (iii) x, y, z−1; (vii) x, y−1, z; (viii) −x, −y+1, z−1/2; (ix) −x, −y+1, z+1/2.