Crystal structure of potassium orthoselenate(IV), K₂SeO₃

Crystal structure data for potassium orthoselenate(IV), K₂SeO₃, are reported for the first time. Colorless, block-shaped crystals were grown in a potassium hydro­flux, i.e. under ultra-alkaline conditions, within 10 h. K₂SeO₃ crystallizes isostructural with Na₂SO₃ and K₂TeO₃ in the trigonal space group P with lattice parameters a = 6.1063 (4) Å and c = 6.9242 (4) Å at 100 (1) K. In the trigonal–pyramidal, C_3v-symmetric [SeO₃]^2– anion, the bond length is 1.687 (1) Å, and the bond angle is 103.0 (1)°. Two of the three unique potassium cations exhibit a coordination number of six, and the third a coordination number of nine.

1. Chemical context

Ternary alkali metal selenates(IV) are a long-known but poorly studied class of compounds. After the discovery of the first salts of selenic acid by Berzelius, comprehensive studies on these salts were not carried out until the beginning of the 1930s, when Janitzki reported the syntheses of sodium and potassium salts of selenic acid (Janitzki, 1932). Moreover, the composition and solubility of hydrates and anhydrates of these selenates(IV) were determined. However, only two crystal structures of ternary alkali metal selenates(IV) are known to date, viz. K₂Se₂O₅ (Rider et al., 1985) and Na₂SeO₃ (Helmholdt et al., 1999; Wickleder, 2002). The latter compound was synthesized by annealing a mixture of Na₂O and SeO₂ at 773 K.

In this communication, we report on the synthesis and crystal structure of potassium orthoselenate(IV), K₂SeO₃. The title compound was synthesized using the hydro­flux approach, an ultra-alkaline reaction medium consisting of an approximately equimolar mixture of water and alkali metal hydroxide (Bugaris et al., 2013; Chance et al., 2013). Advantages of the hydro­flux method are the good solubility of oxides and hydroxides, the fast and simple reaction at moderate temperatures, and the formation of single-crystals suitable for X-ray diffraction. Moreover, the high hydroxide concentration within the hydro­flux reduces the activity of water, leading to the unexpected fact that water-sensitive products can be isolated, e.g. K₂[Fe₂O₃(OH)₂] (Albrecht et al., 2019), Tl₃IO (Albrecht et al., 2020), or K₂Te₃ (Albrecht & Ruck, 2021).

2. Structural commentary

Five atoms represent the asymmetric unit of K₂SeO₃, one selenium atom (site symmetry 3.., Wyckoff position 2d), three potassium atoms (K1: .., 1a; K2: .., 1b; K3: 3.., 2d) and one oxygen atom (1, 6g). The unit cell of K₂SeO₃ is depicted in Fig. 1. The selenium atom is bound to three oxygen atoms with a Se—O bond length of 1.687 (1) Å and a bond angle O—Se—O of 103.0 (1)°. The pyramidal shape of the C_3v-symmetric [SeO₃]^2– anion can be attributed to the electron lone pair of the selenium(IV) atom. This oxidation state is supported by the bond-valence sum calculation (Brese & O'Keeffe, 1991) for selenium ν(Se) = ∑exp [(R_SeO – d_SeO)/b)] = 3 · exp [(1.811 Å – 1.687 (1) Å) / 0.37 Å)] = 4.2 valence units. The potassium cations K1 and K2 are octa­hedrally coordinated by oxygen atoms with K—O distances of 2.631 (1) and 2.771 (1) Å, respectively. K3 has nine oxygen neighbors at distances of 2.792 (1), 3.020 (1) Å, and 3.474 (1) Å (Fig. 2).

Figure 1 Crystal structure of K2SeO3 at 100 K, with displacement ellipsoids drawn at the 99% probability level; the unit cell is outlined.

Figure 2 Coordination polyhedra of the potassium atoms, with displacement ellipsoids drawn at the 99% probability level.

It is noted that the X-ray powder diffraction pattern of ground K₂SeO₃ crystals (Fig. 3) differs significantly from previously published data (Hanawalt et al., 1938; Klushina et al., 1968).

Figure 3 Powder X-ray diffractogram and Rietveld refinement of ground K2SeO3 crystals measured in a capillary at room temperature [a = 6.1114 (1) Å, c = 6.9938 (1) Å; Rp = 0.056, wRp = 0.057, gof = 1.21].

3. Database survey

K₂SeO₃ crystallizes isostructural with Na₂SO₃ (Zachariasen & Buckley, 1931; Larsson & Kierkegaard, 1969) and K₂TeO₃ (Andersen et al., 1989). On a more general level, the structure of K₂SeO₃ can be related to the Ni₂In type in space group P6₃/mmc (Laves & Wallbaum, 1942), with the K⁺ ions on the Ni positions and [SeO₃]^2– anions occupying the positions of the In atoms. The orientation of the selenate(IV) groups is responsible for the symmetry reduction to P; the higher pseudo-symmetry is mirrored in the respective twin laws.

4. Synthesis and crystallization

Potassium orthoselenate(IV), K₂SeO₃, was synthesized in a potassium hydroxide hydro­flux with a molar water-base ratio of 1.7. The reaction was carried out in a PTFE-lined 50 mL Berghof-type DAB-2 stainless steel autoclave to prevent evaporation of water. The starting material SeO₂ (4 mmol, abcr, 99.8%) was dissolved in 3 ml of water before adding 6.3 g of KOH (Fischer Scientific, 86%). After closing the autoclave, the reaction mixture was heated to 473 K at a rate of 2 K min⁻¹ and, after 8 h, cooled to room temperature at a rate of −1 K min⁻¹. The solid reaction product was washed twice with 2 ml of methanol on a Schlenk frit under inert conditions to remove adherent hydro­flux. The colorless, block-shaped crystals of K₂SeO₃ (Fig. 4) dissolve readily in water, but dissolve in methanol a little slower than the hydro­flux. Scanning electron microscopy showed that the surface of the crystals was etched by the washing process (Fig. 5). Due to its hygroscopicity, the product was dried in dynamic vacuum and stored under argon. Pure K₂SeO₃ was obtained with a yield of about 50%. Energy-dispersive X-ray spectroscopy on selected crystals confirmed the chemical composition within the limits of the method.

Figure 4 Photograph of K2SeO3 crystals.

Figure 5 Scanning electron microscopy image after the washing process.

For the Rietveld refinement, the program JANA2006 was used (Petříček et al., 2014). Scanning electron microscopy was performed using a SU8020 (Hitachi) with a triple detector system for secondary and low-energy backscattered electrons (U_a = 5 kV). The composition of selected single crystals was determined by semi-qu­anti­tative energy dispersive X-ray analysis (U_a = 15 kV) using a Silicon Drift Detector X–MaxN (Oxford Instruments). The data were processed applying the AZtec software package (Oxford Instruments, 2013).

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The investigated crystal was found to be a fourfold twin: twinning by merohedry plus twofold rotation along [001]. The crystal, thus, partially conserves the hexa­gonal (pseudo-)symmetry of the Ni₂In type.

Table 1
Experimental details

Crystal data
Chemical formula	K2SeO3
Mr	205.2
Crystal system, space group	Trigonal, P
Temperature (K)	100
a, c (Å)	6.1063 (2), 6.9242 (4)
V (Å^3)	223.59 (2)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	10.11
Crystal size (mm)	0.05 × 0.05 × 0.02
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.539, 0.747
No. of measured, independent and observed [I > 3σ(I)] reflections	12526, 790, 785
Rint	0.021
(sin θ/λ)max (Å^−1)	0.858
Refinement
R[F > 3σ(F)], wR(F), S	0.009, 0.033, 1.05
No. of reflections	790
No. of parameters	24
Δρmax, Δρmin (e Å^−3)	0.77, −1.51
Computer programs: APEX2 (Bruker, 2016), SAINT (Bruker, 2016), SUPERFLIP (Palatinus & Chapuis, 2007), JANA2006 (Petříček et al., 2014), DIAMOND (Brandenburg, 2021), and publCIF (Westrip 2010).

Supporting information

Computing details

Data collection: APEX2 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: Superflip (Palatinus & Chapuis, 2007); program(s) used to refine structure: JANA2006 (Petříček et al., 2014); molecular graphics: DIAMOND (Brandenburg, 2021); software used to prepare material for publication: publCIF (Westrip 2010).

Dipotassium orthoselenate(IV)

Crystal data

K2SeO3	Dx = 3.047 Mg m^−3
Mr = 205.2	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P3	Cell parameters from 7032 reflections
Hall symbol: -P 3	θ = 2.9–37.6°
a = 6.1063 (2) Å	µ = 10.11 mm^−1
c = 6.9242 (4) Å	T = 100 K
V = 223.59 (2) Å^3	Block, colourless
Z = 2	0.05 × 0.05 × 0.02 mm
F(000) = 192

Data collection

Bruker APEXII CCD diffractometer	790 independent reflections
Radiation source: X-ray tube	785 reflections with I > 3σ(I)
Graphite monochromator	Rint = 0.021
ω– and φ–scans	θmax = 37.6°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −10→10
Tmin = 0.539, Tmax = 0.747	k = −10→10
12526 measured reflections	l = −11→11

Refinement

Refinement on F^2	Primary atom site location: chargeflipping
R[F > 3σ(F)] = 0.009	Secondary atom site location: difference Fourier map
wR(F) = 0.033	Weighting scheme based on measured s.u.'s w = 1/(σ^2(I) + 0.000576I^2)
S = 1.05	(Δ/σ)max = 0.001
790 reflections	Δρmax = 0.77 e Å^−3
24 parameters	Δρmin = −1.51 e Å^−3
0 restraints	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
0 constraints	Extinction coefficient: 570 (40)

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

	x	y	z	Uiso*/Ueq
Se	0.666667	0.333333	0.338432 (15)	0.00495 (4)
K1	0	0	0	0.00741 (8)
K2	0	0	0.5	0.00764 (8)
K3	0.333333	0.666667	0.14233 (5)	0.01091 (6)
O	0.38608 (13)	0.25027 (14)	0.23422 (10)	0.0135 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Se	0.00527 (5)	0.00527 (5)	0.00432 (6)	0.00263 (3)	0	0
K1	0.00743 (9)	0.00743 (9)	0.00735 (14)	0.00372 (5)	0	0
K2	0.00816 (9)	0.00816 (9)	0.00658 (13)	0.00408 (5)	0	0
K3	0.01248 (8)	0.01248 (8)	0.00778 (10)	0.00624 (4)	0	0
O	0.0064 (3)	0.0208 (4)	0.0126 (3)	0.0062 (2)	−0.0022 (2)	0.0004 (2)

Geometric parameters (Å, º)

Se—O	1.6865 (8)	K2—K3	4.3084 (4)
Se—O^i	1.6865 (12)	K2—K3^xiii	4.3084 (4)
Se—O^ii	1.6865 (7)	K2—K3^xiv	4.3084 (3)
K1—K2^iii	3.4621 (4)	K2—K3^xv	4.3084 (4)
K1—K2	3.4621 (4)	K2—O	2.7708 (7)
K1—K3^iv	3.6606 (3)	K2—O^ix	2.7708 (10)
K1—K3^v	3.6606 (1)	K2—O^x	2.7708 (8)
K1—K3	3.6606 (3)	K2—O^xiii	2.7708 (7)
K1—K3^vi	3.6606 (3)	K2—O^xvi	2.7708 (10)
K1—K3^vii	3.6606 (1)	K2—O^xvii	2.7708 (8)
K1—K3^viii	3.6606 (3)	K3—K3^vii	4.0391 (4)
K1—O	2.6307 (7)	K3—K3^viii	4.0391 (3)
K1—O^ix	2.6307 (10)	K3—K3^xviii	4.0391 (4)
K1—O^x	2.6307 (8)	K3—O	2.7915 (10)
K1—O^vi	2.6307 (7)	K3—O^xix	2.7915 (8)
K1—O^xi	2.6307 (10)	K3—O^xx	2.7915 (12)
K1—O^xii	2.6307 (8)	K3—O^viii	3.0203 (8)
K2—K3^iv	4.3084 (4)	K3—O^xxi	3.0203 (10)
K2—K3^v	4.3084 (3)	K3—O^xii	3.0203 (8)
O—Se—O^i	103.03 (4)	O—K2—O^x	80.69 (2)
O—Se—O^ii	103.03 (4)	O—K2—O^xiii	180
O^i—Se—O^ii	103.03 (5)	O—K2—O^xvi	99.31 (2)
O—K1—O^ix	85.98 (3)	O—K2—O^xvii	99.31 (2)
O—K1—O^x	85.98 (2)	O^ix—K2—O^x	80.69 (3)
O—K1—O^vi	180	O^ix—K2—O^xiii	99.31 (2)
O—K1—O^xi	94.02 (3)	O^ix—K2—O^xvi	180
O—K1—O^xii	94.02 (2)	O^ix—K2—O^xvii	99.31 (3)
O^ix—K1—O^x	85.98 (3)	O^x—K2—O^xiii	99.31 (2)
O^ix—K1—O^vi	94.02 (3)	O^x—K2—O^xvi	99.31 (3)
O^ix—K1—O^xi	180	O^x—K2—O^xvii	180
O^ix—K1—O^xii	94.02 (3)	O^xiii—K2—O^xvi	80.69 (2)
O^x—K1—O^vi	94.02 (2)	O^xiii—K2—O^xvii	80.69 (2)
O^x—K1—O^xi	94.02 (3)	O^xvi—K2—O^xvii	80.69 (3)
O^x—K1—O^xii	180	O—K3—O^xix	114.97 (3)
O^vi—K1—O^xi	85.98 (3)	O—K3—O^xx	114.97 (2)
O^vi—K1—O^xii	85.98 (2)	O—K3—O^viii	92.04 (2)
O^xi—K1—O^xii	85.98 (3)	O—K3—O^xxi	132.80 (3)
O—K2—O^ix	80.69 (2)	O—K3—O^xii	82.84 (2)

Symmetry codes: (i) −y+1, x−y, z; (ii) −x+y+1, −x+1, z; (iii) x, y, z−1; (iv) x−1, y−1, z; (v) x, y−1, z; (vi) −x, −y, −z; (vii) −x, −y+1, −z; (viii) −x+1, −y+1, −z; (ix) −y, x−y, z; (x) −x+y, −x, z; (xi) y, −x+y, −z; (xii) x−y, x, −z; (xiii) −x, −y, −z+1; (xiv) −x, −y+1, −z+1; (xv) −x+1, −y+1, −z+1; (xvi) y, −x+y, −z+1; (xvii) x−y, x, −z+1; (xviii) −x+1, −y+2, −z; (xix) −y+1, x−y+1, z; (xx) −x+y, −x+1, z; (xxi) y, −x+y+1, −z.