Two alkali metal chlorites, LiClO₂ and KClO₂

The structures of tetragonal (P4₂/ncm) lithium chlorite, LiClO₂, and orthorhombic (Cmcm) potassium chlorite, KClO₂, have been determined by single-crystal X-ray analyses. In LiClO₂, the Li atom is at a site of symmetry, while in KClO₂, the K atom is at a site with 2/m symmetry. In both compounds, the unique Cl and O atoms are at sites with mm and m symmetry, respectively. The structure of LiClO₂ consists of layers of Li⁺ cations coordinated by ClO₂^- anions. In contrast, the structure of KClO₂ contains pseudo-layers of K⁺ and ClO₂^- ions containing four short K-O distances. The Li⁺ and K⁺ cations are surrounded by four and eight chlorite O atoms in tetrahedral and distorted cubic coordination environments, respectively.

S1. Comment

Previous crystal structure investigations of chlorites include NH₄ClO₂ (Levi & Scherillo, 1931; Gillespie et al., 1959), NaClO₂·3H₂O (Tarimci & Schempp, 1975), NaClO₂ (Tarimci et al., 1976), Zn(ClO₂)₂·2H₂O (Pakkanen, 1979), Mg(ClO₂)₂·6H₂O (Ferrari & Colla, 1937; Okuda et al., 1990; Marsh, 1991), AgClO₂ (Curti et al., 1957; Cooper & Marsh, 1961; Okuda et al., 1990), Pb(ClO₂)₂ (Okuda et al., 1990) and La(ClO₂)₃·3H₂O (Coda et al, 1965; Castellani Bisi, 1984). It is apparent that alkali metal chlorites have not been investigated systematically. The limited number of studies is probably due to the difficulty of preparation and investigation of these compounds, resulting from their low stability. A majority of chlorites quickly decompose to the corresponding chlorates and chlorides under the effect of temperature, sunlight or X-radiation. Besides this, the crystallization of chlorites is often difficult and additional research is required to determine appropriate conditions for single-crystal growth. This paper reports the results of our study of two alkali chlorites, namely LiClO₂ and KClO₂.

The structure of lithium chlorite contains separate layers, within which the Li⁺ cation is surrounded by four chlorite O atoms forming a tetrahedron which is squashed along a twofold axis (Fig. 1). These layers are parallel to the ab plane and are linked by van der Waals interactions, with the shortest interlayer Cl···Cl distance being 3.6339 (9) Å (Fig. 2). The layers are stacked in such way that adjacent layers are rotated by 90°. The ClO₂^- anions serve as tetradentate bridging ligands between the Li⁺ cations.

The structure of potassium chlorite contains pseudo-layers parallel to the ac plane and consisting of K⁺ and ClO₂⁻ ions with short K—O distances (Fig. 3). The K⁺ cations and Cl atoms are nearly coplanar (Fig. 4). The full coordination environment of the K⁺ cation involves eight O atoms forming a distorted cube. Four of these O atoms belong to one layer and the other four to the layers above and below.

The structures of LiClO₂ and KClO₂ show some similarities to those of the corresponding hypophosphites. In the case of LiClO₂, the role of ClO₂⁻ as a tetradentate ligand is identical to that of H₂PO₂⁻ in LiH₂PO₂ (Naumov et al., 2004). However, the structures of the layers are different in these compounds: by sharing edges, the Li⁺-centred tetrahedra form chains linked in different ways within a layer. In KClO₂, the layers and the immediate environment of the K⁺ cations are very similar to those in the K, Rb and Cs hypophospites (Naumova et al., 2004). The differences arise from the way the layers are joined together and from the K⁺ environment, which includes two H atoms in the hypophospites instead of two O atoms in KClO₂.

S2. Experimental

Lithium and potassium chlorites were synthesized by mixing aqueous solutions of barium chlorite, Ba(ClO₂)₂, and the corresponding alkali metal sulfates in an equimolar ratio. The reaction mixtures were filtered and the crystals were grown by slow evaporation. In the case of potassium chlorite, the compound decomposes at room temperature in a few hours and its crystal growth was carried out at 273–278 K over approximately 24 h. These conditions yielded crystals in the form of thin plates or needles with a maximum size of 0.5 mm, suitable for X-ray diffraction. The decomposition of lithium chlorite seems to occur more slowly, which allowed crystals to be grown at room temperature. The maximum crystal size was 0.3 mm with a plate morphology. The X-ray powder patterns show good agreement between the bulk products and the single crystals. However, in the case of potassium chlorite, additional peaks in the powder pattern indicate the presence of KCl and KClO₃ phases. The precursor used for the preparation of lithium and potassium chlorites [Ba(ClO₂)₂] was obtained by reacting an aqueous suspension of BaO₂ with chlorine dioxide and precipitation from solution by adding a 3:1 mixture of ethanol and diethyl ether. It was found that Ba(ClO₂)₂ is one of the most stable salts of chlorous acid and is a convenient starting material for preparing other chlorites.

Computing details

For both compounds, data collection: CD4CA0 (Enraf-Nonius, 1989); cell refinement: CD4CA0; data reduction: CADDAT (Enraf-Nonius, 1989); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: BS (Ozawa & Kang, 2004); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Projection of an (001) layer in LiClO2.
	Fig. 2. Packing diagram of the LiClO2 structure.
	Fig. 3. Projection of an (010) layer in KClO2.
	Fig. 4. Packing diagram of the KClO2 structure.

(I) lithium chlorate(III)

Crystal data

LiClO2	Dx = 2.152 Mg m^−3
Mr = 74.39	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P42/ncm	Cell parameters from 20 reflections
Hall symbol: -P 4ac 2ac	θ = 12.6–14.8°
a = 4.7223 (11) Å	µ = 1.30 mm^−1
c = 10.298 (3) Å	T = 293 K
V = 229.65 (10) Å^3	Plate, colourless
Z = 4	0.24 × 0.12 × 0.04 mm
F(000) = 144

Data collection

Enraf-Nonius CAD-4 diffractometer	87 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.017
Graphite monochromator	θmax = 25.9°, θmin = 4.0°
2θ/θ scans	h = 0→5
Absorption correction: empirical (using intensity measurements) (CADDAT; Enraf-Nonius, 1989)	k = 0→5
Tmin = 0.746, Tmax = 0.950	l = 0→12
134 measured reflections	3 standard reflections every 60 min
133 independent reflections	intensity decay: none

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.047	Secondary atom site location: difference Fourier map
wR(F^2) = 0.097	w = 1/[σ^2(Fo^2) + (0.0497P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.91	(Δ/σ)max < 0.001
133 reflections	Δρmax = 0.35 e Å^−3
13 parameters	Δρmin = −0.33 e Å^−3

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
Li	0.2500	0.7500	0.2500	0.028 (3)
Cl	0.7500	0.7500	0.06960 (18)	0.0256 (6)
O	0.5565 (6)	0.5565 (6)	0.1575 (4)	0.0248 (11)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Li	0.028 (4)	0.028 (4)	0.028 (6)	0.000	0.000	0.000
Cl	0.0256 (8)	0.0256 (8)	0.0256 (10)	0.0001 (18)	0.000	0.000
O	0.0196 (15)	0.0196 (15)	0.035 (2)	0.0022 (18)	0.0049 (15)	0.0049 (15)

Geometric parameters (Å, º)

Li—O^i	1.959 (2)	Cl—O^iv	1.578 (4)
Li—O^ii	1.959 (2)	Cl—O	1.578 (4)
Li—O	1.959 (2)	O—Li^v	1.959 (2)
Li—O^iii	1.959 (2)
O^i—Li—O^ii	103.67 (8)	O^ii—Li—O^iii	103.67 (8)
O^i—Li—O	103.67 (8)	O—Li—O^iii	103.67 (8)
O—Li—O^ii	121.82 (19)	O—Cl—O^iv	110.0 (3)
O^i—Li—O^iii	121.82 (19)

Symmetry codes: (i) −y+1, x+1/2, −z+1/2; (ii) −x+1/2, −y+3/2, z; (iii) y−1/2, −x+1, −z+1/2; (iv) −x+3/2, −y+3/2, z; (v) x+1/2, −y+1, −z+1/2.

(II) potassium chlorate(III)

Crystal data

KClO2	F(000) = 208
Mr = 106.55	Dx = 2.448 Mg m^−3
Orthorhombic, Cmcm	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2c 2	Cell parameters from 22 reflections
a = 6.1446 (9) Å	θ = 11.1–11.9°
b = 6.3798 (12) Å	µ = 2.48 mm^−1
c = 7.3755 (19) Å	T = 293 K
V = 289.13 (10) Å^3	Plate, colourless
Z = 4	0.40 × 0.24 × 0.16 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	168 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.059
Graphite monochromator	θmax = 27.5°, θmin = 5.4°
2θ/θ scans	h = 0→7
Absorption correction: empirical (using intensity measurements) (CADDAT; Enraf-Nonius, 1989)	k = −1→8
Tmin = 0.494, Tmax = 0.672	l = −1→9
243 measured reflections	3 standard reflections every 60 min
193 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.030	w = 1/[σ^2(Fo^2) + (0.0464P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.074	(Δ/σ)max < 0.001
S = 1.01	Δρmax = 0.32 e Å^−3
193 reflections	Δρmin = −1.03 e Å^−3
16 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.025 (7)

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
K	0.0000	0.0000	0.0000	0.0329 (4)
Cl	0.5000	−0.03139 (16)	0.2500	0.0261 (4)
O	0.2937 (4)	−0.1753 (4)	0.2500	0.0401 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K	0.0328 (6)	0.0358 (6)	0.0303 (6)	0.000	0.000	−0.0003 (4)
Cl	0.0226 (6)	0.0205 (6)	0.0352 (7)	0.000	0.000	0.000
O	0.0263 (11)	0.0290 (12)	0.0651 (17)	−0.0063 (10)	0.000	0.000

Geometric parameters (Å, º)

K—O^i	2.8120 (18)	K—O^vii	3.0493 (19)
K—O	2.8120 (18)	Cl—O	1.565 (2)
K—O^ii	2.8120 (18)	Cl—O^viii	1.565 (2)
K—O^iii	2.8120 (18)	O—K^ix	2.8120 (18)
K—O^iv	3.0493 (19)	O—K^x	3.0493 (19)
K—O^v	3.0493 (19)	O—K^xi	3.0493 (19)
K—O^vi	3.0493 (19)
O—K—O^i	180.00 (9)	O^i—K—O^vi	81.93 (2)
O^i—K—O^ii	79.86 (7)	O—K—O^vi	98.07 (2)
O—K—O^ii	100.14 (7)	O^ii—K—O^vi	113.15 (2)
O^i—K—O^iii	100.14 (7)	O^iii—K—O^vi	66.85 (2)
O—K—O^iii	79.86 (7)	O^iv—K—O^vi	49.12 (8)
O^ii—K—O^iii	180.0	O^v—K—O^vi	130.88 (8)
O^i—K—O^iv	113.15 (2)	O^i—K—O^vii	98.07 (2)
O—K—O^iv	66.85 (2)	O—K—O^vii	81.93 (2)
O^ii—K—O^iv	81.93 (2)	O^ii—K—O^vii	66.85 (2)
O^iii—K—O^iv	98.07 (2)	O^iii—K—O^vii	113.15 (2)
O^i—K—O^v	66.85 (2)	O^iv—K—O^vii	130.88 (8)
O—K—O^v	113.15 (2)	O^v—K—O^vii	49.12 (8)
O^ii—K—O^v	98.07 (2)	O^vi—K—O^vii	180.00 (10)
O^iii—K—O^v	81.93 (2)	O—Cl—O^viii	108.18 (18)
O^iv—K—O^v	180.0

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