Barium chlorite hydrate, Ba(ClO₂)₂·3.5H₂O

The structure of barium chlorite hydrate, Ba(ClO₂)₂·3.5H₂O, has been determined by single-crystal X-ray analysis at 150 K. The structure is monoclinic, space group C2/c, with Z = 8. It contains layers of Ba²⁺ cations coordinated by ClO₂^- anions and water mol­ecules. There are also solvate water mol­ecules involved only in hydrogen bonding of the layers. Three solvate water O atoms are on sites of twofold symmetry, while all other atoms are in general positions. The full coordination environment of the Ba²⁺ cation consists of ten O atoms belonging to six chlorites and three water mol­ecules, forming a bicapped square antiprism.

S1. Comment

The anhydrous alkaline earth metal chlorites, Ca(ClO₂)₂, Sr(ClO₂)₂ and Ba(ClO₂)₂, were first investigated crystallographically by Riganti & Garrini (1960). Because these compounds were obtained as microcrystalline powders, only powder diffraction patterns were obtained, which were not indexed. These authors proposed that Ca(ClO₂)₂ and Sr(ClO₂)₂ are isomorphic and have pseudo-cubic cells with similar parameters. However, the complete structures of all anhydrous alkaline- arth metal chlorites are still unknown. They also reported that barium chlorite may form a hydrate with 3.5 H₂O molecules (unlike Ca and Sr chlorites), but its crystal structure was not determined because of its very low stability.

The purpose of the present investigation was to obtain structural data for the title hydrate. As the crystal structures of only some chlorites have been determined to date, a new contribution to the crystal chemistry of these simple compounds seemed useful. The present paper continues our research on chlorous acid salts, which has included LiClO₂ and KClO₂ (Smolentsev & Naumov, 2005a) and the redetermined structure of NH₄ClO₂ (Smolentsev & Naumov, 2005b).

In the structure of Ba(ClO₂)₂·3.5H₂O, the coordination sphere of the Ba²⁺ cation includes ten O atoms (Fig. 1). Seven of these belong to the chlorite anions and the other three to the H₂O molecules. The resultant coordination is a distorted bicapped square antiprism. By sharing edges, the antiprisms form layers parallel to the bc plane (Fig. 2). The layers are stacked in such way that adjacent layers are shifted along the b axis by 1/2 a unit translation.

The structure contains both H₂O molecules coordinated to the Ba²⁺ cations and solvate water molecules, which are involved only in hydrogen bonding of the layers. Non-coordinated water of crystallization has not been found in other hydrated chlorites studied so far, viz. NaClO₂·3H₂O (Tarimci & Schempp, 1975), Mg(ClO₂)₂·6H₂O (Okuda et al., 1990; Marsh, 1991), Zn(ClO₂)₂·2H₂O (Pakkanen, 1979) and La(ClO₂)₃·3H₂O (Castellani Bisi, 1984), in which all H₂O molecules are coordinated to the metal cations. In this regard, Mg(ClO₂)₂·6H₂O is outstanding: the water molecules form the complex cation [Mg(H₂O)₆]²⁺, which is then hydrogen bonded to ClO₂⁻ anions. A layered structure is found in Zn(ClO₂)₂·2H₂O, while the sodium salt contains chains, and both magnesium and lanthanum salts have three-dimensional frameworks. Unlike these other hydrated chlorites, the title compound has one more feature, i.e. two independent ClO₂⁻ groups with symmetry non-equivalent O atoms. Anions of the first type (O11—Cl1—O12) serve as tetradentate bridging ligands between the Ba²⁺ cations and are not included in the hydrogen-bond system. The O atoms of the second anion type (O21—Cl2—O22) have different environments. They are both coordinated to the same Ba²⁺ cation and involved in hydrogen bonding. However, atom O21 is also coordinated to the neighbouring cation and forms a hydrogen bond with only one coordinated H₂O molecule within the layer, while atom O22 is hydrogen-bonded to two interlayer H₂O molecules.

Due to the non-equivalence of the ClO₂⁻ anions, their geometric parameters are slightly different. Nevertheless, they are in good agreement with the values determined for other chlorites. Comparison can be made with the average Cl—O distance and O—Cl—O angle calculated from the corresponding values reported previously (Tarimci & Schempp, 1975; Tarimci et al., 1976; Pakkanen, 1979; Castellani Bisi, 1984; Okuda et al., 1990; Marsh, 1991; Smolentsev & Naumov, 2005a,b), which are 1.571 (14) Å and 110 (2)°, respectively.

S2. Experimental

The title compound appears as a thin crystalline film on the surface of a concentrated aqueous solution of barium chlorite during evaporation at room temperature. Thus, the main difficulty resides in selection of a crystal suitable for X-ray diffraction. It is also necessary to note that this hydrate is very unstable and without the mother solution it completely decomposes to the anhydrous salt, even in a few minutes. In contrast, at low temperatures the crystals are quite stable. Anhydrous Ba(ClO₂)₂ was previously synthesized by reacting an aqueous suspension of BaO₂ with chlorine dioxide, followed by precipitation from solution by adding a 3:1 mixture of ethanol and diethyl ether. Unlike the hydrate, it is one of the most stable salts of chlorous acid.

S3. Refinement

The H atoms were located in difference electron-density maps and refined with O—H distances constrained to 0.82 (1) Å.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Bruker, 2004); program(s) used to refine structure: SHELXTL; molecular graphics: BS (Ozawa & Kang, 2004) and POV-RAY (Cason, 2002); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The coordination environment of the Ba^2+ cation. Displacement ellipsoids are plotted at the 50% probability level. [Symmetry codes: (i) 1/2 − x, 1/2 − y, 1 − z; (ii) x, y, 1 + z; (iii) x, −y, 1/2 + z; (iv) 1/2 − x, 1/2 − y, 2 − z.]
	Fig. 2. A perspective packing diagram of the Ba(ClO2)2·3.5H2O structure, viewed along the c axis. Thin lines represent hydrogen bonds.

Barium chlorite hydrate

Crystal data

Ba(ClO2)2·3.5H2O	F(000) = 1256
Mr = 335.30	Dx = 2.797 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 3491 reflections
a = 18.4801 (5) Å	θ = 2.3–33.5°
b = 13.5715 (4) Å	µ = 5.65 mm^−1
c = 6.6904 (2) Å	T = 150 K
β = 108.364 (1)°	Plate, colourless
V = 1592.52 (8) Å^3	0.15 × 0.13 × 0.02 mm
Z = 8

Data collection

Bruker Nonius X8Apex CCD area-detector diffractometer	2821 independent reflections
Radiation source: fine-focus sealed tube	2321 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.022
Detector resolution: 25 pixels mm^-1	θmax = 32.6°, θmin = 2.3°
ϕ scans	h = −27→27
Absorption correction: multi-scan (SADABS; Bruker, 2004)	k = −20→12
Tmin = 0.484, Tmax = 0.895	l = −9→8
7807 measured reflections

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)] = 0.020	Hydrogen site location: difference Fourier map
wR(F^2) = 0.051	H atoms treated by a mixture of independent and constrained refinement
S = 1.05	w = 1/[σ^2(Fo^2) + (0.0234P)^2] where P = (Fo^2 + 2Fc^2)/3
2821 reflections	(Δ/σ)max = 0.002
118 parameters	Δρmax = 1.55 e Å^−3
16 restraints	Δρmin = −0.54 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
Ba1	0.255500 (6)	0.124940 (7)	0.754469 (15)	0.00965 (4)
Cl1	0.35850 (3)	0.12962 (3)	0.34553 (8)	0.01455 (10)
Cl2	0.09431 (2)	0.03281 (3)	0.35618 (7)	0.01376 (9)
O11	0.30632 (8)	0.19708 (9)	0.4337 (2)	0.0153 (3)
O12	0.30879 (8)	0.05939 (10)	0.1679 (2)	0.0159 (3)
O21	0.17568 (7)	0.04695 (10)	0.3315 (2)	0.0152 (3)
O22	0.08660 (8)	0.11048 (10)	0.5240 (2)	0.0171 (3)
O1W	0.33040 (8)	0.30117 (10)	0.9767 (2)	0.0160 (3)
H11	0.3767 (6)	0.2946 (16)	1.039 (3)	0.019*
H12	0.3287 (14)	0.3467 (14)	0.892 (3)	0.019*
O2W	0.41673 (9)	0.11535 (10)	0.8783 (3)	0.0170 (3)
H21	0.4392 (13)	0.1641 (13)	0.852 (3)	0.020*
H22	0.4322 (13)	0.0721 (13)	0.817 (3)	0.020*
O3W	0.5000	0.25403 (15)	0.7500	0.0186 (4)
H3	0.5267 (12)	0.2910 (14)	0.841 (3)	0.022*
O4W	0.5000	0.03007 (16)	0.2500	0.0187 (4)
H4	0.4741 (13)	0.0624 (15)	0.149 (3)	0.022*
O5W	0.5000	0.30284 (17)	0.2500	0.0280 (5)
H5	0.4802 (15)	0.3359 (17)	0.321 (4)	0.034*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba1	0.01155 (6)	0.00886 (7)	0.00868 (6)	−0.00021 (3)	0.00339 (4)	−0.00003 (3)
Cl1	0.01172 (19)	0.0168 (2)	0.0147 (2)	0.00043 (15)	0.00347 (16)	−0.00231 (16)
Cl2	0.01144 (18)	0.0153 (2)	0.01436 (19)	−0.00079 (15)	0.00388 (15)	−0.00230 (17)
O11	0.0201 (7)	0.0116 (6)	0.0165 (7)	0.0035 (5)	0.0092 (5)	0.0004 (5)
O12	0.0200 (7)	0.0145 (7)	0.0128 (6)	−0.0017 (5)	0.0047 (5)	−0.0026 (5)
O21	0.0099 (6)	0.0182 (7)	0.0183 (7)	−0.0018 (5)	0.0056 (5)	−0.0023 (6)
O22	0.0169 (7)	0.0191 (7)	0.0157 (7)	0.0022 (5)	0.0059 (6)	−0.0041 (6)
O1W	0.0136 (6)	0.0159 (7)	0.0175 (7)	−0.0005 (5)	0.0034 (5)	0.0010 (6)
O2W	0.0155 (7)	0.0162 (8)	0.0198 (8)	−0.0003 (5)	0.0063 (6)	0.0008 (6)
O3W	0.0217 (10)	0.0134 (10)	0.0187 (10)	0.000	0.0036 (8)	0.000
O4W	0.0195 (10)	0.0206 (11)	0.0138 (10)	0.000	0.0023 (8)	0.000
O5W	0.0353 (13)	0.0255 (13)	0.0328 (13)	0.000	0.0247 (11)	0.000

Geometric parameters (Å, º)

Ba1—O12^i	2.7741 (14)	Cl2—O21	1.5753 (13)
Ba1—O11	2.7781 (13)	Cl2—O22	1.5787 (15)
Ba1—O11^ii	2.7960 (13)	O1W—H11	0.829 (9)
Ba1—O12^iii	2.8148 (13)	O1W—H12	0.831 (12)
Ba1—O2W	2.8330 (15)	O2W—H21	0.828 (12)
Ba1—O21^iii	2.8918 (13)	O2W—H22	0.817 (12)
Ba1—O1W	2.9241 (14)	O3W—H3^v	0.822 (12)
Ba1—O1W^iv	2.9254 (14)	O3W—H3	0.822 (12)
Ba1—O21	2.9450 (14)	O4W—H4^vi	0.823 (12)
Ba1—O22	3.0210 (15)	O4W—H4	0.823 (12)
Cl1—O11	1.5728 (13)	O5W—H5^vi	0.820 (12)
Cl1—O12	1.5737 (14)	O5W—H5	0.820 (12)
O12^i—Ba1—O11	141.55 (4)	O11—Ba1—O21	64.73 (4)
O12^i—Ba1—O11^ii	133.80 (4)	O11^ii—Ba1—O21	82.42 (4)
O11—Ba1—O11^ii	62.44 (4)	O12^iii—Ba1—O21	65.46 (4)
O12^i—Ba1—O12^iii	83.27 (3)	O2W—Ba1—O21	115.25 (4)
O11—Ba1—O12^iii	85.92 (4)	O21^iii—Ba1—O21	75.65 (3)
O11^ii—Ba1—O12^iii	142.66 (4)	O1W—Ba1—O21	142.76 (4)
O12^i—Ba1—O2W	71.55 (4)	O1W^iv—Ba1—O21	120.62 (4)
O11—Ba1—O2W	70.22 (4)	O12^i—Ba1—O22	118.25 (4)
O11^ii—Ba1—O2W	114.47 (4)	O11—Ba1—O22	100.07 (4)
O12^iii—Ba1—O2W	67.38 (4)	O11^ii—Ba1—O22	67.44 (4)
O12^i—Ba1—O21^iii	66.69 (4)	O12^iii—Ba1—O22	102.09 (4)
O11—Ba1—O21^iii	137.53 (4)	O2W—Ba1—O22	165.61 (5)
O11^ii—Ba1—O21^iii	128.18 (4)	O21^iii—Ba1—O22	62.33 (4)
O12^iii—Ba1—O21^iii	63.31 (4)	O1W—Ba1—O22	124.07 (4)
O2W—Ba1—O21^iii	117.35 (4)	O1W^iv—Ba1—O22	70.28 (4)
O12^i—Ba1—O1W	78.04 (4)	O21—Ba1—O22	50.36 (4)
O11—Ba1—O1W	83.32 (4)	O11—Cl1—O12	110.77 (8)
O11^ii—Ba1—O1W	65.36 (4)	O21—Cl2—O22	107.22 (8)
O12^iii—Ba1—O1W	133.74 (4)	Ba1—O1W—H11	114.5 (15)
O2W—Ba1—O1W	66.62 (4)	Ba1^iv—O1W—H11	113.9 (15)
O21^iii—Ba1—O1W	139.06 (4)	Ba1—O1W—H12	110.6 (17)
O12^i—Ba1—O1W^iv	65.67 (4)	Ba1^iv—O1W—H12	104.6 (17)
O11—Ba1—O1W^iv	137.32 (4)	H11—O1W—H12	103 (2)
O11^ii—Ba1—O1W^iv	75.92 (4)	Ba1—O2W—H21	116.9 (17)
O12^iii—Ba1—O1W^iv	136.38 (4)	Ba1—O2W—H22	112.9 (17)
O2W—Ba1—O1W^iv	124.09 (4)	H21—O2W—H22	101 (2)
O21^iii—Ba1—O1W^iv	76.17 (4)	H3^v—O3W—H3	105 (3)
O1W—Ba1—O1W^iv	70.64 (4)	H4^vi—O4W—H4	116 (3)
O12^i—Ba1—O21	139.10 (4)	H5^vi—O5W—H5	113 (4)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H11···O5W^i	0.83 (1)	2.27 (1)	3.0913 (14)	168 (2)
O1W—H12···O21^ii	0.83 (1)	2.06 (1)	2.8929 (19)	176 (2)
O2W—H21···O3W	0.83 (1)	1.92 (1)	2.735 (2)	166 (2)
O2W—H22···O4W^vii	0.82 (1)	2.01 (1)	2.798 (2)	161 (2)
O3W—H3···O22^viii	0.82 (1)	1.91 (1)	2.730 (2)	173 (2)
O4W—H4···O2W^ix	0.82 (1)	1.92 (1)	2.7352 (18)	168 (2)
O5W—H5···O22^ii	0.82 (1)	1.98 (1)	2.7851 (17)	166 (3)

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