Poly[[tetradecakis(μ-propionato)heptabarium] propionic acid monosolvate tetrahydrate]

The structure of the title compound is held together by Ba—O—Ba bonds as well as by O—H⋯O hydrogen bonds of moderate strength. There is an occupationally as well as a positionally disordered molecule of propionic acid in the structure. Its occupation deviates from a potentially full occupation while the disordered molecules occupy two positions related by a twofold rotation axis.


Chemical context
A relatively low number of structurally determined metal propionates with divalent cations are known so far, as manifested by comparison of the numbers of propionates, acetates and formates with alkaline-earth cations which were retrieved from the Cambridge Structural Database (Groom et al., 2016;version 5.40 from November 2018). Their numbers are 8, 60 and 70, respectively. One of the reasons for such a low number of determined structures might be associated with the tendency for difficult crystallization in case of some propionates. As an example of a difficult crystallization of a propionate salt from aqueous solution, Ca(propionate) 2 and Cd(propionate) 2 in a 2:1 molar ratio (Fá bry, 2020) can be given.
Among the propionate salts, the most studied compounds are the isostructural salts Ca 2 Pb(propionate) 6 and Ca 2 Sr (propionate) 6 . In the latter compounds, ferroelectric phases occur (see a short review by Nakamura & Deguchi, 1992). Structurally related Ca 2 Ba(propionate) 6 shows interesting structural properties such as positional disorder of propionate chains in the room-temperature phase with symmetry Fd3m (Stadnicka & Glazer, 1980). This disorder is a reason for diffuse streaks in the diffraction pattern, indicating correlated occurrence of the disordered propionate molecules. The latter compound undergoes low-temperature phase transitions to phases with suggested orthorhombic symmetry (Gesi, 1993).
The title compound was prepared serendipitously. A few crystals of it were isolated from a batch of seemingly cubic crystals (they showed no extinction under polarized light) that grew from aqueous solutions of propionic acid (molar proportion > 30) with the amounts of BaCO 3 and 4MgCO 3 ÁMg(OH) 2 Á4H 2 O in a molar ratio of 5:2; the pH of the solution was about 6. The motivation for the synthesis was a planned preparation of an analogue of Ca 2 Ba(propionate) 6 where Ca 2+ and Ba 2+ are overbonded and slightly underbonded (Brese & O'Keeffe, 1991), respectively. For example, in the above-mentioned room-temperature phase of Ca 2 Ba-(propionate) 6 , the bond-valence sums (Brese & O'Keeffe, 1991) of Ca 2+ and Ba 2+ amount to 2.78 (1) and 1.93 (1) valence units, respectively [see the refinement/model 'A' given in the article by Stadnicka & Glazer (1980) who discussed strong bonding of Ca 2+ in this structure]. It was therefore hoped that a hypothetical structure 'Ba 2 Mg(propionate) 6 ' might be isostructural to Ca 2 Ba(propionate) 6 or related to it despite an expected lowering of the bond-valence sum by smaller Mg 2+ cations. Indeed, alongside a few crystals of the title compound, cubic crystals were obtained, the structure determination of which is ongoing at present.

Structural commentary
A view of the crystal structure is given in Fig. 1. There are four independent Ba 2+ cations that are all coordinated by oxygen atoms stemming either from the carboxylate or carboxylic groups or from water molecules. The latter molecules coordinate exclusively to Ba2 2+ . Ba3 2+ is coordinated by the carboxylic group of an occupationally and positionally disordered propionic acid molecule. Ba4 2+ is situated on a twofold Figure 1 A view of the crystal structure along the b axis. Ba, O, C and water H atoms are shown as green, red, dark gray and tiny gray spheres, respectively. It can readily be seen from Fig. 1 that the cohesion within the crystal structure is mostly provided by a three-dimensional network of Ba-O-Ba bonds. This network is shown in more detail in Fig. 4, which also includes O-HÁ Á ÁO hydrogen bonds of moderate strength (Gilli & Gilli, 2009). The corresponding donor groups are water molecules while the acceptors are carboxylate oxygen atoms. Numerical details of hydrogenbonding interactions are provided in Table 2, excluding the O propionic acid -HÁ Á ÁO propionate hydrogen bond along O16Á Á ÁO4 vii [2.706 (13) Å ; symmetry code: (vii) Àx + 1, Ày, Àz + 1] that is donated by the free propionic acid molecule. This molecule is disordered over two positions related by (Àx + 1, y, Àz + 1 2 ) about a twofold rotation axis (Wyckoff position c). The low occupancy is probably the reason why the bridging hydrogen atom of the O16Á Á ÁO4 vii hydrogen bond could not be located in the difference electron density map. However, the angle C22-O16Á Á ÁO4 vii , which measures 110.8 (8) , is close to the tetrahedral angle and is in agreement with the assumed presence of a hydrogen bond. The longer C22-O16 bond [1.303 (18) Å ] in comparison with the C22-O15 bond [1.187 (12) Å )] indicates that the bridging hydrogen atom is attached to O16. Table 2 also lists a weak C-HÁ Á ÁO interaction between a methyl group and the carboxylic O atom of the propionic acid molecule. The numerical parameters conform to the criteria for a weak hydrogen bond (Desiraju & Steiner, 1999). Fig. 5a shows a detailed view of the disordered propionic acid molecule over two positions associated with the abovementioned twofold rotation. The refined occupation of the molecule of propionic acid converged to 0.473 (4) (full occupation of the site corresponds to 0.5). SQUEEZE, a func-  Molecular structures of the propionate molecules, with displacement ellipsoids shown at the 50% probability level. (a) molecules with the carboxylate atom C1, (b) molecules with the carboxylate atom C4, (c) molecules with the carboxylate atom C7, (d) molecules with the carboxylate atom C10, (e) molecules with the carboxylate atom C13, (f) molecules with the carboxylate atom C16 and (g) molecules with the carboxylate atom C19.

Table 2
Hydrogen-bond geometry (Å , ). Symmetry codes: (iii) Àx + 1, Ày + 1, Àz + 1; (iv) x + 1 2 , Ày + 1 tionality included in PLATON (Spek, 2015), yielded a value of 0.431. This means that the occupation of the disordered molecule is not full; however, analysis of the bond-valence sum for Ba3 2+ still points to a slight overbonding ( In addition to the occupational disorder of the propionic acid molecule, its methyl group was found to be disordered over two positions. One of these positions (the methyl C24b x atom) is very close to atom C22 (Fig. 5b). The occupational parameters of the disordered methyl groups split into C24a and C24b converged to 0.30 (2) and 0.17 (1); methyl hydrogen atoms were not found. The displacement parameters of the methyl group C24a (Fig. 5a) are quite large and indicate an intense libration. The displacement parameter of C24b was constrained to that of C22 (Fig. 5a).
Reported structures comprising propionate anions and/or propionic acid molecules were retrieved from the Cambridge Structural Database (Groom et al., 2016;version 5.40 from November 2018). Fig. 6 shows a scattergram of the shorter C-O (or C O) and longer C-O (or C-OH) distances in the carboxylate or carboxylic group, respectively. Corresponding distances in the title structure are normal although those pertinent to the carboxylates are on the verge of the region where both C-O distances are about the same. Interestingly, there is no large difference between these parameters in the carboxylate (black squares) and the carboxylic groups (red circles) in the propionate or propionic acid molecules, respectively. There seem to be a clustering of points at about 1.21 and 1.35 Å , which manifest different bonding types in these molecules.   [Symmetry codes: (i) x À 1 2 , Ày + 1 2 , Àz + 1; (vii) Àx + 1, Ày, Àz + 1; (viii) Àx + 3 2 , Ày + 1 2 , z À 1 2 ; (ix) x, Ày, z À 1 2 ]. The disordered atoms are related by a symmetry operation (Wyckoff position c) (x) Àx + 1, y, Àz + 1/29. For colours, see caption for

Figure 6
Scattergram of the distances for the shorter and the longer C O bonds in the carboxylate groups in propionates (black squares) as well as of C O bonds and C-OH bonds in propionic acid molecules (red circles). The corresponding values for the propionates and the propionic acid molecule present in the title structure are shown as green and blue triangles, respectively.

Synthesis and crystallization
1 g of BaCO 3 and 0.95 g of basic magnesium carbonate [Aldrich, product number 13118, the powder diagram of which corresponded best to that of the powder diffraction file 01-070-0361 of PDF-4 (International Centre for Diffraction Data, 2019)], i.e. 4MgCO 3 ÁMg(OH) 2 Á4H 2 O], were dissolved in an aqueous solution of 2.28 g of propionic acid. These masses correspond to molar ratios of 5:2:30. The majority of the solid dissolved in the acid solution and a few ml of propionic acid (100%) were added to the solution, maintaining its pH between 6 and 7. The solution was then filtered through a sintered disk. The filtrate was concentrated by evaporation at 323 K until colourless crystals appeared. A prevalent majority of the crystals were of cubic form with a typical size of 1 mm. Under a polarizating microscope, these crystals did not show extinction, i.e. they were optically isotropic. However, among these crystals a few crystals that showed extinction were found. They were isolated and one of them was chosen for single crystal X-ray structure determination.

Structure determination and refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.
The structure can be divided into a non-disordered part composed of the Ba 2+ cations, propionate anions and water molecules, and the disordered molecule of propionic acid. The refinement of the non-disordered structure part was straightforward, with methylene hydrogen atoms calculated and their parameters constrained to C-H = 0.99 Å and U iso (H) = 1.2U eq (C). The methyl hydrogen atoms of the propionate molecules were discernible in the difference electron density map. They were constrained with C-H = 0.98 Å and U iso (H) = 1.5U eq (C). The water hydrogen atoms were also discernible in the difference electron density map. Their positional parameters were restrained in such a way that O-H distances were set to 0.82 (1) Å , with U iso (H) = 1.5U eq (O). The residual maxima in the difference electron density map after the refinement of the non-disordered part of the structure conformed to the expected shape of the non-hydrogen atoms of a propionic acid molecule (see Fig. 5a,b). The functionality of SQUEEZE included in PLATON (Spek, 2015) indicated 138 electrons corresponding to the symmetry-related regions with the disordered molecule present in the unit cell. Since a propionic acid molecule has 40 electrons, the expected occupational parameter for the disordered molecule is 138/160 = 0.8625 or 0.4313 for the occupancy considering the special position (twofold rotation axis) in its vicinity. The value of the expected occupancy is in fair agreement with the refined value of 0.473 (4) for the molecule of propionic acid where four hydrogen atoms remained undetermined (the methyl as well as the hydroxy hydrogen atoms). This disorder results in a statistical distribution of the molecule about the twofold rotation axis, indicating that vacancies without the molecule of propionic acid are likely to be present in the crystal structure.
Reliability factors of a trial refinement with assumed full occupation of the disordered molecule converged with negligibly worse values and are collated in the refine_special_ details section of the CIF. The respective electron densities of the peaks that were assigned to the atoms O15, O16, C22, C23 and C24a are 1.22, 0.97, 0.96, 0.82 and 0.31 e À Å À3 . The independently refined occupational parameters of the atoms of the disordered molecule converged to the following values: O15: 0.410 (7); O16: 0.362 (7); C22: 0.571 (11); C23: 0.391 (9); C24: 0.184 (12), pointing to another type of occupational disorder, in particular regarding the distribution of the methyl group, which may partly overlap with atom C22 (Fig. 5b). Treatment of these atoms after localization of all nonhydrogen atoms of the disordered propionic acid molecule is described in detail in the refine_special_details section of the CIF.
43 reflections were discarded from the refinement because |I obs À I calc |/(I obs ) > 10. They are listed in the refine_ special_details section of the CIF, together with the results of an alternative refinement with SHELXL (Sheldrick, 2015b) where the contributions of the disordered propionic acid molecule were removed using the SQUEEZE option in PLATON (Spek, 2015

Poly[[tetradecakis(µ-propionato)heptabarium] propionic acid monosolvate tetrahydrate]
Crystal data 2) Details of the disorder in the proprionic acid molecule: The overall occupational parameter of the disordered propionic acid molecule was determined by refinement of the molecular part comprising of the atoms O15, O16 and C23 which seemed to be the ones least-affected by disorder or overlapping. This refined value has then been used as a value to which the sum of partial occupational parameters of the methyl atoms C24a and C24b should equal while refining the occupational parameter of C24b. The displacement parameter of C24b due to its proximity was supposed to be equal to that of C22 which was refined. The positions of the methylene hydrogen atoms of C23a and C23b H1C23a, H2C23a; H1C23b, H2C23b) were calculated, with occupational parameters constrained to be equal to the occupational parameters of C24a and C24b, respectively, and with C-H = 0.99Å, U iso (H) = 1.2U eq (C). The distance C22-C23 was restrained to 1.52 (1) Å while the distances C23-C24a and C23-C24b were restrained to 1.50 (1) Å.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (