Redetermination of ammonium metavanadate

The crystal structure of ammonium metavanadate, NH₄VO₃, a compound widely used as a starting material for the synthesis of vanadium and polyoxidovanadate compounds, had been determined twice using single-crystal X-ray data [Syneček & Hanic (1954). Czech. J. Phys. 4, 120–129 (Weissenberg data); Hawthorne & Calvo (1977). J. Solid State Chem. 22, 157–170 (four-circle diffractometer data)]. Its structure is now redetermined at higher resolution using Ag Kα radiation, and the result is compared with the former refinements. Structural data for the polymeric [VO₃]_∞ chain remain unchanged, while more accurate parameters are obtained for the ammonium cation, improving the description of hydrogen-bonding inter­actions in the crystal structure.

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Structure description

Ammonium metavanadate, NH₄VO₃ [IUPAC name: ammonium trioxidovanadate(V)], is a widely used compound at the laboratory scale, for example for the preparation of V₂O₅, polyoxidovanadates, catalysts, or as an analytical reagent. Its crystal structure has been known for a long time. The first study (Lukesh, 1950) reported the correct space group, Pbcm, and a hypothetical arrangement for the [VO₄] tetra­hedra that turned out to be correct as revealed by subsequent studies. Weissenberg data allowed the determination of the atomic coordinates for all non-H atoms (Syneček & Hanic, 1954), and a least-squares refinement using the same experimental intensities was published subsequently (Evans, 1960).

The crystal structure of NH₄VO₃ is quite simple (Fig. 1). Vertice-sharing [VO₄] tetra­hedra form infinite [VO₃]_∞ chains along [001], and given that the crystal lattice is ortho­rhom­bic, all chains are parallel; NH₄⁺ cations are situated in the voids between these chains. Both the tetra­hedral anionic and cationic groups are sited on mirror planes parallel to (001) in space group Pbcm, and thus have point group symmetry C_s. Finally, the polymeric character of the vanadate anion stems from the special position of the O atom positioned away from the mirror plane. This atom (O3) lies on the twofold rotation axis parallel to [100].

Figure 1 Part of the crystal structure of ammonium metavanadate, viewed approximately down [100], using a polyhedral representation for the vanadate chains and spheres of arbitrary radii for ammonium cations (Boudias & Monceau, 1998). The inset shows non-H atoms with displacement ellipsoids at the 80% probability level. The V1′ and O3′ sites are generated by the symmetry operations x,  − y, −z and x, y,  − z, respectively.

The most precise structure report of NH₄VO₃ up to date is probably that by Smrčok et al. (2009), based on laboratory/synchrotron temperature-variable X-ray powder diffraction data combined with solid state DFT calculations. However, these authors commented that they made `unsuccessful attempts to grow a single-crystal suitable for single-crystal neutron diffraction'. Indeed, only one single-crystal study based on four-circle diffractometer data may be retrieved from the literature (Hawthorne & Calvo, 1977). This long-standing X-ray study reported the structure at a standard resolution, d = 0.84 Å, and, as mentioned by Smrčok et al. (2009), `the positions [of the hydrogen atoms] are not sufficiently accurate to be directly used in calculations of vibrational spectra'.

We now have redetermined the crystal structure of NH₄VO₃, using room-temperature X-ray diffraction data collected at 0.61 Å resolution. As expected, the bond lengths and angles for the vanadate chain are identical, within experimental uncertainties, to those obtained from the 1977 and 2009 articles (see comparison in Table 1). However, differences occur for the ammonium moiety. Our data allowed us to rationalize the distortion of the tetra­hedral coordination sphere for the cation. The short N1—H3 bond length, 0.77 (2) Å, compared to the long N1—H1 and N1—H2 bonds, 0.85 (2) Å, results from the involvement of the latter in strong hydrogen bonds, while the former gives much weaker and bifurcated hydrogen bonds (Table 2). In the same way, the largest H—N—H angle, 116 (4)°, involves H atoms participating in weak hydrogen bonds, while the acute angle of 104 (2)° is H1—N—H2, where H1 and H2 are engaged in stronger hydrogen bonds. The framework of hydrogen bonds is, however, essentially identical in the three structure determinations. Each cation in the crystal inter­acts with four symmetry-related [VO₄] tetra­hedra, forming two strong and two weak hydrogen bonds (Table 2). Weak contacts with H3 as donor form R₂²(6) ring motifs with the vanadate chains. Such rings connect the cation with two vertices of a [VO₄] tetra­hedron, and the same donor groups are connected to the neighbouring vanadate chain in the crystal, forming larger R₂²(10) ring motifs (Fig. 2). Strong contacts with H1/H2 as donors also connect neighbouring chains. The strength of the hydrogen bonds is reflected in the displacement parameters for the ammonium H atoms: H3 refines with the highest displacement parameter, U_iso = 0.069 (6) Ų, because it is not stabilized on its site through a strong inter­action with the vanadate chain. Smrčok et al. (2009) included in their description two additional very bent contacts. However, the N—H⋯O angles are then below 110°, and these contacts should thus contribute to the crystal stabilization with an energy approaching 0 kJ mol⁻¹ (Wood et al., 2009).

Table 1
Bond lengths (Å) and angles (°) in NH₄VO₃ determined in this work compared to those reported in previous studies

The labelling scheme for atomic sites is identical in the three studies.

	1977 study^a	2009 study^b	This work
Vanadate anion
V1—O1	1.640 (4)	1.655	1.6388 (10)
V1—O2	1.647 (4)	1.679	1.6538 (9)
V1—O3 (×2)	1.803 (1)	1.808	1.8021 (3)
O1—V1—O2	107.3 (1)	107.2	107.40 (5)
O1—V1—O3 (×2)	109.7 (1)	109.0	109.75 (3)
O2—V1—O3 (×2)	111.1 (1)	112.1	111.08 (2)
O3—V1—O3	107.9 (1)	107.2	107.77 (3)
Ammonium cation
N1—H1	0.82 (8)	1.05	0.85 (2)
N1—H2	0.94 (8)	1.05	0.85 (2)
N1—H3 (×2)	0.97 (5)	1.04	0.77 (2)
H1—N1—H2	111 (6)	112	104 (2)
H1—N1—H3 (×2)	106 (3)	109	111.7 (15)
H2—N1—H3 (×2)	107 (3)	110	105.8 (15)
H3—N1—H3	119 (5)	108	116 (4)
References: (a) Hawthorne & Calvo (1977); (b) Smrčok et al. (2009), with DFT-optimized parameters rounded to the precision of the experimentally determined values.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1⋯O2^i	0.85 (2)	2.09 (3)	2.9400 (16)	172 (2)
N1—H2⋯O2^ii	0.85 (2)	1.99 (2)	2.8364 (15)	171 (2)
N1—H3⋯O3	0.77 (2)	2.48 (2)	2.9690 (12)	122.9 (17)
N1—H3⋯O1^iii	0.77 (2)	2.49 (2)	3.2141 (7)	156 (2)
Symmetry codes: (i) ; (ii) ; (iii) .

Figure 2 Part of the crystal structure of ammonium metavanadate, showing hydrogen bonds as dashed lines. Four vanadate chains are represented with different colours, and the asymmetric unit is indicated with black labels. Hydrogen bonds are labelled (1)–(4), corresponding to entries in Table 2, and the supra­molecular rings linking the anions and cations in the crystal are coloured yellow. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x + 1, y + , −z + ; (ii) −x + 2, y + , −z + ; (iii) x + 1, −y + , −z.]

Synthesis and crystallization

Warning! Ammonium metavanadate is a haza­rdous material classified with health code 4 in the NFPA 704 standard system: very short exposure could cause death or major residual injury. The single crystal used in the present study was harvested as a recrystallized unreacted material from the crude reaction in which we attempted to obtain a V^IV derivative of tris(metforminium)²⁺ deca­vanadate (Treviño et al., 2015), using acetyl­salicylic acid both as a source of protons and as a reducing agent. Typically, metformin hydro­chloride (0.850 g), aspirin (0.5 g) and NH₄VO₃ (0.5 g) were stirred at room temperature for one hour, and the remaining solids were filtered off by gravity. Good-quality single crystals of the main product (orange crystals; Sánchez-Lombardo et al., 2014) and NH₄VO₃ (colourless crystals) were obtained by fractional crystallization, allowing the solvent to evaporate under ambient conditions.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. The three H atoms in the asymmetric unit were refined with free coordinates and free isotropic displacement parameters.

Table 3
Experimental details

Crystal data
Chemical formula	NH4^+·VO3^−
Mr	116.98
Crystal system, space group	Orthorhombic, Pbcm
Temperature (K)	295
a, b, c (Å)	4.9045 (3), 11.7945 (5), 5.8231 (2)
V (Å^3)	336.84 (3)
Z	4
Radiation type	Ag Kα, λ = 0.56083 Å
μ (mm^−1)	1.41
Crystal size (mm)	0.32 × 0.09 × 0.09
Data collection
Diffractometer	Stoe Stadivari
Absorption correction	Multi-scan (X-AREA; Stoe & Cie, 2018)
Tmin, Tmax	0.295, 0.656
No. of measured, independent and observed [I > 2σ(I)] reflections	17208, 846, 754
Rint	0.027
(sin θ/λ)max (Å^−1)	0.823
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.014, 0.042, 1.13
No. of reflections	846
No. of parameters	40
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.33, −0.35
Computer programs: X-AREA (Stoe & Cie, 2018), SHELXT2018/2 (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b), Mercury (Macrae et al., 2008) and CaRIne Crystallography (Boudias & Monceau, 1998) and publCIF (Westrip, 2010).

Structural data

Computing details

Data collection: X-AREA (Stoe & Cie, 2018); cell refinement: X-AREA (Stoe & Cie, 2018); data reduction: X-AREA (Stoe & Cie, 2018); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008) and CaRIne Crystallography (Boudias & Monceau, 1998); software used to prepare material for publication: publCIF (Westrip, 2010).

Hydroxy(dioxido)vanadium ammoniate

Crystal data

NH4^+·VO3^−	Dx = 2.307 Mg m^−3
Mr = 116.98	Ag Kα radiation, λ = 0.56083 Å
Orthorhombic, Pbcm	Cell parameters from 39692 reflections
a = 4.9045 (3) Å	θ = 2.7–33.0°
b = 11.7945 (5) Å	µ = 1.41 mm^−1
c = 5.8231 (2) Å	T = 295 K
V = 336.84 (3) Å^3	Prism, colourless
Z = 4	0.32 × 0.09 × 0.09 mm
F(000) = 232

Data collection

Stoe Stadivari diffractometer	846 independent reflections
Radiation source: Sealed X-ray tube, Axo Astix-f Microfocus source	754 reflections with I > 2σ(I)
Graded multilayer mirror monochromator	Rint = 0.027
Detector resolution: 5.81 pixels mm^-1	θmax = 27.5°, θmin = 3.3°
ω scans	h = −8→8
Absorption correction: multi-scan (X-AREA; Stoe & Cie, 2018)	k = −19→19
Tmin = 0.295, Tmax = 0.656	l = −9→8
17208 measured reflections

Refinement

Refinement on F^2	0 constraints
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.014	All H-atom parameters refined
wR(F^2) = 0.042	w = 1/[σ^2(Fo^2) + (0.0248P)^2 + 0.023P] where P = (Fo^2 + 2Fc^2)/3
S = 1.13	(Δ/σ)max < 0.001
846 reflections	Δρmax = 0.33 e Å^−3
40 parameters	Δρmin = −0.35 e Å^−3
0 restraints

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N1	0.9355 (2)	0.41286 (10)	0.250000	0.0254 (2)
H1	0.796 (5)	0.4562 (18)	0.250000	0.047 (6)*
H2	1.071 (4)	0.4581 (16)	0.250000	0.023 (5)*
H3	0.947 (3)	0.379 (2)	0.137 (4)	0.069 (6)*
V1	0.46440 (4)	0.17437 (2)	0.250000	0.01270 (5)
O1	0.1305 (2)	0.16927 (8)	0.250000	0.02460 (18)
O2	0.57698 (19)	0.04220 (7)	0.250000	0.02260 (17)
O3	0.58198 (18)	0.250000	0.000000	0.02029 (16)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0219 (5)	0.0176 (4)	0.0366 (6)	−0.0022 (4)	0.000	0.000
V1	0.01423 (9)	0.01304 (8)	0.01082 (7)	−0.00023 (6)	0.000	0.000
O1	0.0158 (4)	0.0255 (4)	0.0324 (5)	0.0002 (3)	0.000	0.000
O2	0.0227 (4)	0.0160 (4)	0.0291 (4)	0.0031 (3)	0.000	0.000
O3	0.0235 (4)	0.0236 (4)	0.0139 (3)	0.000	0.000	0.0050 (3)

Geometric parameters (Å, º)

N1—H1	0.85 (2)	V1—O1	1.6388 (10)
N1—H2	0.85 (2)	V1—O2	1.6538 (9)
N1—H3	0.77 (2)	V1—O3	1.8021 (3)
N1—H3^i	0.77 (2)	V1—O3^i	1.8021 (3)
H1—N1—H2	104 (2)	O1—V1—O3	109.75 (3)
H1—N1—H3	111.7 (15)	O2—V1—O3	111.08 (2)
H2—N1—H3	105.8 (15)	O1—V1—O3^i	109.75 (3)
H1—N1—H3^i	111.7 (15)	O2—V1—O3^i	111.08 (2)
H2—N1—H3^i	105.8 (15)	O3—V1—O3^i	107.77 (3)
H3—N1—H3^i	116 (4)	V1—O3—V1^ii	142.68 (5)
O1—V1—O2	107.40 (5)
O1—V1—O3—V1^ii	1.91 (3)	O3^i—V1—O3—V1^ii	−117.57 (3)
O2—V1—O3—V1^ii	120.53 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O2^iii	0.85 (2)	2.09 (3)	2.9400 (16)	172 (2)
N1—H2···O2^iv	0.85 (2)	1.99 (2)	2.8364 (15)	171 (2)
N1—H3···O3	0.77 (2)	2.48 (2)	2.9690 (12)	122.9 (17)
N1—H3···O1^v	0.77 (2)	2.49 (2)	3.2141 (7)	156 (2)

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