Catena-Poly[[(acetato-κ2 O,O′)triaquabarium(II)]- μ3-acetato-κ4 O:O,O′:O′]

Article abstract of DOI:10.1107/S0108270108006057

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(Oaqua)3(Oacetate)6 mono-mer 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 mol-ecule is halved by a mirror plane bis-ecting the Ba centre, one water mol-ecule and one acetate ligand, while containing the other acetate ligand. The chains are inter-connected 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.

Full text of DOI:10.1107/S0108270108006057

metal-organic compounds
˚
those in the less hydrated forms [4.330 A in partial hydrate
˚
(IV) and 4.338 A in anhydrate (III)].
Acta Crystallographica Section C
Crystal Structure
Communications
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.
ISSN 0108-2701
catena-Poly[[(acetato-j2O,O0)triaqua-
barium(II)]-l3-acetato-j4O:O,O0:O0]
Eleonora Freirea and Ricardo Baggiob*
a
´
´
´
´
Departamento de Fısica, Comision Nacional de Energıa Atomica and CONICET,
b
´
´
Buenos Aires, Argentina, and Departamento de Fısica, Comision Nacional de
´
´
Energıa Atomica, Buenos Aires, Argentina
Correspondence e-mail: baggio@cnea.gov.ar
Received 27 February 2008
Accepted 4 March 2008
Online 15 March 2008
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(Oaqua)3(Oacetate)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—Hꢀ ꢀ ꢀ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.
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 acetate O atoms and 0.206 for
water O atoms.
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
noncolliding way, the local symmetry must be lowered to 1, as
shown in Fig. 2. The possible centrosymmetric H-atom arrays
give rise to noncolliding H-atom distributions and sensible
hydrogen-bonding schemes (Table 2), while providing an
Comment
Barium acetate is known to present a variety of hydration
states, the X-ray crystal structures having been reported
(Cambridge Structural Database, Version 5.29 of November
2007; Allen 2002) for a monohydrate, Ba(C2H3O2)2ꢀH2O, (II)
(Groombridge et al., 1985), an anhydrous form, Ba(C2H3O2)2,
(III) (Gautier-Luneau & Mosset, 1988), and a partially
hydrated form, [Ba(C2H3O2)2]6ꢀ3.5H2O, (IV) (Leyva et al.,
2007). We report here the crystal structure of the title tri-
hydrate, Ba(C2H3O2)2ꢀ3H2O, (I), which although character-
ized by a thorough vibrational study 20 years ago (Maneva &
Nikolova, 1988) has not been studied so far from a crystal-
lographic point of view.
Fig. 1 shows a schematic view of the (linear) polymeric
structure of (I), built up by a Ba centre, three water molecules
and two acetate ligands, one (acetate 2) acting in a simple
chelating mode and the second (acetate 1) in a ꢀ3-ꢁ4O,O0
chelating double-bridging mode. This leads to a Baꢀ ꢀ ꢀBa
Figure 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, ꢂy + 1, z; (ii) ꢂx + 12, ꢂy + 21, ꢂz; (iii) ꢂx + 21, y + 21,
ꢂz.]
˚
distance along the chain of 4.608 (1) A, comparable with the
˚
separation in monohydrate (II) (4.586 A), but longer than
m164 # 2008 International Union of Crystallography
doi:10.1107/S0108270108006057
Acta Cryst. (2008). C64, m164–m166

metal-organic compounds
Figure 2
A schematic representation of one of the possible locally centrosym-
metric water H-atom 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 (12, 21, 0), giving rise to the
symmetry operation (i). [Symmetry codes: (i) ꢂx + 1, ꢂy + 1, ꢂz; (ii)
ꢂx + 1, y, ꢂz; (iii) x, ꢂy + 1, z; (iv) ꢂx + 1, ꢂy + 1, ꢂz + 1; (v) x,y,1 + z;
(vi) ꢂx + 12, ꢂy + 21, ꢂz; (vii) ꢂx + 21, y + 21, ꢂz; (viii) x + 12, ꢂy + 21, z; (ix) x + 12,
y + 12, z.]
Figure 3
A schematic packing view of (I), projected down b, the chain direction.
Disordered H atoms have been omitted for clarity. Short Oꢀ ꢀ ꢀO contacts
resulting from the hydrogen bonding are shown as dashed lines.
suddenly. 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 within a few hours, losing crystalline character; if left in a
drop of mother liquor, they are digested and the stable monohydrate
grows.
‘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
water–water/acetate O—Hꢀ ꢀ ꢀO hydrogen bonds, where all
water H atoms take part. The first entry in Table 2 corresponds
to an intrachain contact, while the remaining entries 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 coordi-
nated water molecules has the effect of reducing the covalent
links between the Ba polyhedra, thus ‘opening’ the three-
dimensional structure into the one-dimensional structure
displayed by (I), a fact presumably associated with its intrinsic
instability.
Crystal data
3
˚
V = 1005.6 (4) A
Z = 4
[Ba(C2H3O2)2(H2O)3]
Mr = 309.48
Monoclinic, C2=m
Mo Kꢃ radiation
ꢀ = 3.95 mmꢂ1
T = 263 (2) K
0.28 ꢃ 0.18 ꢃ 0.14 mm
˚
a = 16.020 (3) A
b = 7.4892 (15) A
˚
˚
c = 9.1211 (18) A
ꢂ = 113.23 (3)ꢁ
Data collection
Rigaku AFC-6 diffractometer
Absorption correction: scan
(North et al., 1968)
Tmin = 0.39, Tmax = 0.58
2492 measured reflections
1069 independent reflections
1006 reflections with I > 2ꢄ(I)
Rint = 0.066
3 standard reflections
every 150 reflections
intensity decay: <2%
Experimental
Refinement
R[F2 > 2ꢄ(F2)] = 0.032
wR(F2) = 0.085
S = 1.12
1069 reflections
85 parameters
15 restraints
H atoms treated by a mixture of
independent and constrained
refinement
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 warmed to 268 K afterwards. After 10–15 d, and with the
solution almost dry, very large colourless blocks of (I) appeared
ꢂ3
˚
Áꢅmax = 2.00 e A
ꢂ3
˚
Áꢅmin = ꢂ1.75 e A
ꢄ
Acta Cryst. (2008). C64, m164–m166
Freire and Baggio [Ba(C2H3O2)2(H2O)3] m165

metal-organic compounds
Table 1
Selected bond lengths (A).
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 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
˚
Ba1—O11i
Ba1—O22
Ba1—O11
2.722 (3)
2.810 (4)
2.833 (3)
Ba1—O12
Ba1—O2W
Ba1—O1W
2.845 (5)
2.866 (5)
2.918 (3)
1
1
Symmetry code: (i) ꢂx þ ; y þ ; ꢂz.
2
2
Table 2
Hydrogen-bond geometry (A, ).
ꢁ
˚
˚
sets. O—H distances were restrained to 0.85 (2) A. In all cases,
Uiso(H) values were set at 1.5Ueq(C,O).
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
Data collection: MSC/AFC Diffractometer Control Software
(Molecular Structure Corporation, 1988); cell refinement: MSC/AFC
Diffractometer Control Software; data reduction: MSC/AFC
Diffractometer Control Software; 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 and PLATON (Spek, 2003).
O1W—H1WBꢀ ꢀ ꢀO22ii
O1W—H1WCꢀ ꢀ ꢀO1Wiii
O1W—H1WAꢀ ꢀ ꢀO2Wiv
O2W—H2WAꢀ ꢀ ꢀO1Wiii
O2W—H2WBꢀ ꢀ ꢀO12v
0.85 (2)
0.85 (2)
0.85 (2)
0.85 (2)
0.85 (2)
1.90 (2)
2.02 (3)
1.97 (2)
2.04 (2)
2.02 (2)
2.744 (4)
2.763 (6)
2.800 (5)
2.800 (5)
2.705 (7)
174 (5)
145 (5)
164 (5)
147.4 (19)
137 (3)
1
2
1
2
Symmetry codes: (ii) ꢂx þ ; ꢂy þ ; ꢂz; (iii) ꢂx þ 1; y; ꢂz; (iv) ꢂx þ 1; ꢂy þ 1; ꢂz; (v)
ꢂx þ 1; ꢂy þ 1; ꢂz þ 1.
Table 3
Water content versus density for all known Ba(C2H3O2)2(H2O)n com-
pounds.
We acknowledge the Spanish Research Council (CSIC) for
providing us with a free-of-charge licence to the Cambridge
Structural Database (Allen, 2002), and Professor Donka
Stoilova for introducing us to the elusive Ba(C2H3O2)2ꢀ3H2O
problem, finally solved herein.
Structure
n
ꢅ (Mg mꢂ3
)
Structure type
Room-temperature
stability
(III)
(IV)
(II)
(I)
0
0.583
1
3
2.53
Three-dimensional
Three-dimensional
Three-dimensional
One-dimensional
Stable
Stable
Stable
Unstable
2.328
2.26
2.044
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: SQ3131). Services for accessing these data are
described at the back of the journal.
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 space group
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 gener-
ated by the full space-group symmetry, generated around each OW
atom an almost perfect CH3-like umbrella which was sterically
incompatible with their neighbours.
References
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An analysis of the possible hydrogen-bonding interactions and the
scheme which they suggested led to an acceptable centrosymmetric
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ꢄ
m166 Freire and Baggio
[Ba(C2H3O2)2(H2O)3]
Acta Cryst. (2008). C64, m164–m166