catena-Poly[[(acetato-[kappa]²O,O')triaquabarium(II)]-[mu]₃-acetato-[kappa]⁴O:O,O':O']

The present form of barium acetate, formulated as [Ba(C₂H₃O₂)₂(H₂O)₃]_n, is the largest reported hydrate of the salt and this leads to a distinct structural behaviour setting it apart from the rest of the family. The compound is a linear polymer with a nine-coordinate Ba(O_aqua)₃(O_acetate)₆ mono­mer unit. The non-H part of the structure is ordered according to C2/m symmetry, while the disordered water H atoms only abide by this symmetry in a statistical sense. Each mol­ecule is halved by a mirror plane bis­ecting the Ba centre, one water mol­ecule and one acetate ligand, while containing the other acetate ligand. The chains are inter­connected by a disordered water-water/acetate O-H...O hydrogen-bonding network involving all water H atoms. The structure and stability of this phase are compared with the other known acetates of barium which differ in the degree of hydration.

S1. Comment

Barium acetate is known to present a variety of hydration states. For three of them, their X-ray crystal structures have been reported (Cambridge Structural Database, Version?; Allen 2002), viz. a monohydrate, Ba(C₂H₃O₂)₂·H₂O, (II) (Groombridge et al. 1985), an anhydrous form, Ba(C₂H₃O₂)₂, (III) (Gautier-Luneau & Mosset, 1988), and a partially hydrated form, [Ba(C₂H₃O₂)₂]₆·3.5H₂O, (IV) (Leyva et al., 2007). We report here the crystal structure of the title trihydrate, Ba(C₂H₃O₂)₂·3H₂O, (I), which although characterized by a thorough vibrational study 20 years ago (Maneva & Nikolova, 1988) has not been studied so far from a crystallographic point of view.

Fig. 1 shows a schematic view of the (linear) polymeric structure of (I), built up of a Ba centre, three water molecules and two acetate ligands, one of them (acetate 2) acting in a simple chelating mode and the second (acetate 1) in a µ₃κ⁴O,O' chelating double-bridging mode. This leads to a Ba···Ba distance along the chain of 4.608 (1) Å, comparable with the separation in the monohydrate, (II) [4.586 Å], but longer than those in the less hydrated forms [4.330 Å in the semihydrate, (III) [Should this be (IV) as defined in first paragraph?], and 4.338 Å in the anhydrate, (IV) [Should this be (III) as defined in first paragraph?]].

Each Ba(C₂H₃O₂)₂(H₂O)₃ unit in (I) is halved by a mirror plane which passes through the Ba1 centre and one water molecule (O2W), while bisecting both perpendicular acetate ligands: acetate 2, through all four non-H atoms (C12, C22, O12 and O22, which thus lie on special positions in the mirror plane), and acetate 1, through atoms C11 and C21.

Due to chelation, the BaO₉ polyhedron is rather deformed, with a wide range of coordination angles [45.69 (11)–152.83 (14)°], which makes the resulting geometry difficult to describe in terms of a regular model. The water molecules are more loosely bound to Ba than are the carboxylate O atoms, as inferred from the coordination distances (Table 1). Total bond valence (Brown & Altermatt, 1985) on Ba1 amounts to 2.283, with a mean value of 0.277 for O_ac and 0.206 for O_water.

As explained in the Refinement section, only the non-H part of the structure follows a strict C2/m symmetry. The H atoms follow it only on average, and to accommodate them in a non-colliding way, the local symmetry must be lowered to 1, as shown in Fig. 2. The possible centrosymmetric H-atom arrays give rise to non-colliding H-atom distributions and sensible hydrogen-bonding schemes (Table 2), while providing an `average' model compatible with the electron-density map. The polymeric structure consists of ribbons (Fig. 1) which run along b and are in turn interconnected by the disordered set of (O—H)_water···O_water/ac hydrogen bonds, where all H_water atoms take part. The first entry in Table 2 corresponds to an intrachain contact, while the remaining ones are the interchain interactions along the a and c axis directions (Fig. 3)

As expected, the four structurally characterized barium acetates present a predictable inverse relationship between water content and crystal density (Table 3). In addition, all but (I) present a stable three-dimensional structure at room temperature. It might be concluded that, in the particular case of the structure reported here, the large number of coordinated water molecules has the effect of reducing the covalent links between the Ba polyhedra, thus `opening' the three-dimensional structure into the one-dimensional one displayed by (I), a fact presumably associated with its intrinsic instability.

Related literature

For related literature, see: Allen (2002); Brown & Altermatt (1985); Gautier-Luneau & Mosset (1988); Groombridge et al. (1985); Leyva et al. (2007); Maneva & Nikolova (1988).

S2. Experimental

Crystals of (I) were obtained through a two-step low-temperature recrystallization procedure. A concentrated aqueous solution of the as-purchased monohydrated salt was initially left at 255 K until chilled, and taken to 268 K afterwards. After 10–15 d, and with the solution almost dry, very large colourless blocks of (I) appeared all of a sudden. Thermogravimetric analysis suggested a water content of three molecules per formula unit, a fact confirmed by the structural analysis. The specimens are unstable at room temperature: if dry, they decompose in a few hours, losing crystalline character; if left in a drop of mother liquor, they are digested and the stable monohydrate grows.

S3. Refinement

Crystals of (I) are unstable at room temperature and decompose easily in the X-ray beam. After several unsuccessful attempts, a complete data set was finally collected on a single specimen under a soft N₂ cooling stream (ca 260 K). The non-H part of the structure could be easily solved and refined in centrosymmetric C2/m, with the molecule halved by a mirror plane, but H-atom assignment posed a problem because the positions from the Fourier synthesis were unacceptable due to collision/superposition, e.g. the electron-density maxima, and their symmetry-related images generated by the full space-group symmetry, generated around each OW atom an almost perfect CH₃-like umbrella which was sterically incompatible with their neighbours.

An analysis of the possible hydrogen-bonding interactions and the scheme which they suggested led to an acceptable centrosymmetric H-atom configuration, not abiding by the full space-group symmetry (see Fig. 2 for a typical example), but which could explain the lost `2' and `m' symmetries and the electron-density maps, when overlapped. Thus, C2/m should be considered as the `non-H' space group, but when H atoms are taken into account this is true only in an `average' sense. The refinement was accordingly performed in C2/m with this disordered H-atom distribution. Acetate methyl H atoms were included at calculated positions, with O—C—C—H torsion angles left free to refine, a procedure which also resulted in rotationally disordered CH₃ sets. O—H distances were restrained to 0.85 (2) Å. In all cases, U_iso(H) was set to 1.5U_eq(X), where X = C or O, as appropriate.

Computing details

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1988); cell refinement: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1988); data reduction: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1988); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL-NT (Sheldrick, 2008); software used to prepare material for publication: SHELXTL-NT (Sheldrick, 2008) and PLATON (Spek, 2003).

Figures

	Fig. 1. A molecular diagram of (I), showing the way in which chains are formed along the b axis. Independent atoms are drawn as octant shaded displacement ellipsoids at the 40% probability level, connected by solid bonds, while symmetry-related atoms are shown as open ellipsoids connected by hollow bonds. H atoms have been omitted for clarity. [Symmetry codes: (i) x, -y + 1, z; (ii) -x + 1/2, -y + 1/2, -z; (iii) -x + 1/2, y + 1/2, -z.]
	Fig. 2. A schematic representation of one of the possible locally centrosymmetric Hwater dispositions leading (on average) to a C2/m distribution compatible with the heavy-atom space group. The figure is built up around the symmetry centre at (1/2, 1/2, 0), giving rise to the symmetry operation (i). [Symmetry codes: (i) -x + 1, -y + 1, -z; (ii) -x + 1, y, -z; (iii) x, -y + 1, z; (iv) -x + 1, -y + 1, -z + 1; (v) x,y,1 + z; (vi) -x + 1/2, -y + 1/2, -z; (vii) -x + 1/2, y + 1/2, -z; (viii) x + 1/2, -y + 1/2, z; (ix) x + 1/2, y + 1/2, z.]
	Fig. 3. A schematic packing view of (I), projected down b, the chain direction. Disordered H atoms have been omitted for clarity. Short O···O contacts resulting from the hydrogen-bonding are shown as dashed lines.

catena-Poly[[(acetato-κ²O,O')triaquabarium(II)]- µ₃-acetato-κ⁴O:O,O':O']

Crystal data

[Ba(C2H3O2)2(H2O)3]	F(000) = 592
Mr = 309.48	Dx = 2.044 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 25 reflections
a = 16.020 (3) Å	θ = 10–15°
b = 7.4892 (15) Å	µ = 3.95 mm^−1
c = 9.1211 (18) Å	T = 263 K
β = 113.23 (3)°	Block, colourless
V = 1005.6 (4) Å^3	0.28 × 0.18 × 0.14 mm
Z = 4

Data collection

Rigaku AFC-6 diffractometer	1006 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.066
Graphite monochromator	θmax = 26.0°, θmin = 2.4°
ω/2θ scans	h = −3→19
Absorption correction: ψ scan (North et al., 1968.)	k = −9→9
Tmin = 0.39, Tmax = 0.58	l = −11→10
2492 measured reflections	3 standard reflections every 150 reflections
1069 independent reflections	intensity decay: <2%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.032	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.085	w = 1/[σ^2(Fo^2) + (0.0521P)^2 + 0.0465P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max < 0.001
1069 reflections	Δρmax = 2.00 e Å^−3
85 parameters	Δρmin = −1.75 e Å^−3
15 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0061 (8)

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.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Ba1	0.338393 (19)	0.5000	0.09767 (3)	0.0232 (2)
C11	0.1413 (4)	0.5000	−0.1858 (7)	0.0255 (11)
C21	0.0396 (5)	0.5000	−0.2807 (12)	0.056 (2)
H21A	0.0255	0.4301	−0.3757	0.084*	0.50
H21B	0.0190	0.6203	−0.3094	0.084*	0.50
H21C	0.0099	0.4496	−0.2173	0.084*	0.50
O11	0.1820 (2)	0.3532 (4)	−0.1458 (4)	0.0354 (7)
C12	0.2630 (4)	0.5000	0.3771 (7)	0.0296 (12)
C22	0.2239 (6)	0.5000	0.5024 (10)	0.054 (2)
H22A	0.1694	0.5705	0.4662	0.081*	0.50
H22B	0.2673	0.5497	0.5995	0.081*	0.50
H22C	0.2099	0.3797	0.5214	0.081*	0.50
O12	0.3471 (3)	0.5000	0.4152 (6)	0.0423 (11)
O22	0.2069 (3)	0.5000	0.2300 (5)	0.0319 (9)
O1W	0.40795 (19)	0.2802 (5)	−0.0933 (4)	0.0352 (7)
H1WC	0.4585 (18)	0.234 (6)	−0.033 (3)	0.053*	0.50
H1WB	0.3697 (18)	0.199 (4)	−0.140 (5)	0.053*
H1WA	0.416 (4)	0.346 (3)	−0.163 (4)	0.053*	0.50
O2W	0.5308 (3)	0.5000	0.2786 (6)	0.0374 (11)
H2WB	0.542 (2)	0.5000	0.3781 (16)	0.056*
H2WA	0.5541 (19)	0.4070 (14)	0.255 (3)	0.056*	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba1	0.0235 (3)	0.0145 (2)	0.0305 (3)	0.000	0.00954 (18)	0.000
C11	0.035 (3)	0.019 (2)	0.026 (3)	0.000	0.015 (3)	0.000
C21	0.033 (4)	0.056 (5)	0.062 (5)	0.000	0.000 (4)	0.000
O11	0.0393 (16)	0.0193 (13)	0.0415 (17)	0.0011 (13)	0.0095 (14)	0.0008 (13)
C12	0.029 (3)	0.025 (3)	0.032 (3)	0.000	0.009 (3)	0.000
C22	0.041 (4)	0.086 (6)	0.037 (4)	0.000	0.017 (4)	0.000
O12	0.033 (2)	0.061 (3)	0.030 (2)	0.000	0.009 (2)	0.000
O22	0.028 (2)	0.0280 (18)	0.036 (2)	0.000	0.0077 (19)	0.000
O1W	0.0341 (17)	0.0301 (13)	0.0401 (18)	−0.0062 (13)	0.0133 (16)	−0.0046 (13)
O2W	0.035 (2)	0.040 (3)	0.033 (2)	0.000	0.010 (2)	0.000

Geometric parameters (Å, º)

Ba1—O11^i	2.722 (3)	C21—H21B	0.9600
Ba1—O11^ii	2.722 (3)	C21—H21C	0.9600
Ba1—O22	2.810 (4)	C12—O12	1.252 (8)
Ba1—O11	2.833 (3)	C12—O22	1.287 (8)
Ba1—O11^iii	2.833 (3)	C12—C22	1.504 (10)
Ba1—O12	2.845 (5)	C22—H22A	0.9600
Ba1—O2W	2.866 (5)	C22—H22B	0.9600
Ba1—O1W	2.918 (3)	C22—H22C	0.9600
Ba1—O1W^iii	2.918 (3)	O1W—H1WC	0.85 (2)
C11—O11^iii	1.257 (4)	O1W—H1WB	0.85 (2)
C11—O11	1.257 (4)	O1W—H1WA	0.85 (2)
C11—C21	1.513 (9)	O2W—H2WB	0.85 (2)
C21—H21A	0.9600	O2W—H2WA	0.85 (2)
O11^i—Ba1—O11^ii	152.70 (15)	O2W—Ba1—O1W^iii	76.50 (10)
O11^i—Ba1—O22	76.87 (7)	O1W—Ba1—O1W^iii	68.68 (13)
O11^ii—Ba1—O22	76.87 (7)	O11^iii—C11—O11	122.0 (6)
O11^i—Ba1—O11	112.22 (7)	O11^iii—C11—C21	119.0 (3)
O11^ii—Ba1—O11	67.90 (11)	O11—C11—C21	119.0 (3)
O22—Ba1—O11	75.86 (11)	C11—C21—H21A	109.5
O11^i—Ba1—O11^iii	67.90 (11)	C11—C21—H21B	109.5
O11^ii—Ba1—O11^iii	112.22 (7)	H21A—C21—H21B	109.5
O22—Ba1—O11^iii	75.86 (11)	C11—C21—H21C	109.5
O11—Ba1—O11^iii	45.68 (11)	H21A—C21—H21C	109.5
O11^i—Ba1—O12	78.27 (7)	H21B—C21—H21C	109.5
O11^ii—Ba1—O12	78.27 (7)	C11—O11—Ba1^ii	144.6 (3)
O22—Ba1—O12	46.11 (13)	C11—O11—Ba1	94.9 (3)
O11—Ba1—O12	118.10 (11)	Ba1^ii—O11—Ba1	112.10 (11)
O11^iii—Ba1—O12	118.10 (11)	O12—C12—O22	121.4 (6)
O11^i—Ba1—O2W	94.44 (7)	O12—C12—C22	120.9 (6)
O11^ii—Ba1—O2W	94.44 (7)	O22—C12—C22	117.6 (6)
O22—Ba1—O2W	124.81 (13)	C12—C22—H22A	109.5
O11—Ba1—O2W	150.35 (9)	C12—C22—H22B	109.5
O11^iii—Ba1—O2W	150.35 (9)	H22A—C22—H22B	109.5
O12—Ba1—O2W	78.70 (15)	C12—C22—H22C	109.5
O11^i—Ba1—O1W	137.97 (10)	H22A—C22—H22C	109.5
O11^ii—Ba1—O1W	69.29 (10)	H22B—C22—H22C	109.5
O22—Ba1—O1W	141.58 (7)	C12—O12—Ba1	95.9 (4)
O11—Ba1—O1W	74.99 (9)	C12—O22—Ba1	96.6 (3)
O11^iii—Ba1—O1W	100.31 (9)	Ba1—O1W—H1WC	109.3 (19)
O12—Ba1—O1W	136.96 (8)	Ba1—O1W—H1WB	109.6 (18)
O2W—Ba1—O1W	76.50 (10)	H1WC—O1W—H1WB	110 (3)
O11^i—Ba1—O1W^iii	69.29 (10)	Ba1—O1W—H1WA	108.9 (19)
O11^ii—Ba1—O1W^iii	137.97 (9)	H1WC—O1W—H1WA	109 (3)
O22—Ba1—O1W^iii	141.58 (7)	H1WB—O1W—H1WA	109 (3)
O11—Ba1—O1W^iii	100.31 (9)	Ba1—O2W—H2WB	110 (2)
O11^iii—Ba1—O1W^iii	74.99 (9)	Ba1—O2W—H2WA	109.4 (19)
O12—Ba1—O1W^iii	136.96 (8)	H2WB—O2W—H2WA	109 (2)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H1WB···O22^ii	0.85 (2)	1.90 (2)	2.744 (4)	174 (5)
O1W—H1WC···O1W^iv	0.85 (2)	2.02 (3)	2.763 (6)	145 (5)
O1W—H1WA···O2W^v	0.85 (2)	1.97 (2)	2.800 (5)	164 (5)
O2W—H2WA···O1W^iv	0.85 (2)	2.04 (2)	2.800 (5)	147 (2)
O2W—H2WB···O12^vi	0.85 (2)	2.02 (2)	2.705 (7)	137 (3)

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