Tribarium dicitrate pentahydrate, [Ba3(C6H5O7)2(H2O)4]·H2O

The crystal structure of tribarium dicitrate pentahydrate, [Ba3(C6H5O7)2(H2O)4](H2O), has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques.


Chemical context
A systematic study of the crystal structures of Group 1 (alkali metal) citrate salts has been reported in Rammohan & Kaduk (2018). The study was extended to mixed Group 1 citrates and to alkali/ammonium citrates in a series of papers, to magnesium citrates in Kaduk (2020a), and to calcium citrates in Kaduk (2018) and Kaduk (2020b). This paper represents a further extension to barium citrates and describes the synthesis and structure of the title compound, (I).

Structural commentary
The crystal structure of tribarium dicitrate pentahydrate, [Ba 3 (C 6 H 5 O 7 ) 2 (H 2 O) 4 ](H 2 O), has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques (Fig. 1). The rootmean-square Cartesian displacements of the non-H atoms in the Rietveld-refined and DFT-optimized structures of the two crystallographically distinct citrate anions are 0.155 and 0.093 Å (Fig. 2). The absolute differences in the positions of the three unique Ba 2+ cations are 0.075, 0.345, and 0.081 Å . ISSN 2056-9890 The good agreement between the structures is evidence that the experimental structure is correct (van de Streek & Neumann, 2014). The rest of the discussion will emphasize the DFT-optimized structure. Almost all of the citrate bond distances, bond angles, and torsion angles fall within the normal ranges indicated by a Mercury Mogul Geometry Check (Macrae et al., 2020). The O13-C5-O14 bond angle of 122.1 is flagged as unusual [average = 123.8 (4) , Z-score = 3.3]. The standard uncertainty on this average is exceptionallysmall, inflating the Z-score. The C22-C23-C24-C25 torsion angle is flagged as unusual; it lies on the tail of a minor gauche population in a mainly trans distribution of similar torsion angles. Citrate anion 1 (atoms C1-H18) occurs in the trans,trans-conformation (about C2-C3 and C3-C4), which is one of the two low-energy conformations of an isolated citrate anion (Rammohan & Kaduk, 2018), while citrate anion 2 (C21-H38) is the in the trans, gauche conformation, which is the other low-energy arrangement. For the larger Group 1 cations, the trans,trans conformation is more typical. The central carboxylate groups and the hydroxyl groups exhibit significant twists of À20 and À24 from the normal planar arrangement.
The three barium cations Ba19, Ba20, and Ba39 are ten-, nine-and ten-coordinate, respectively. Ba19 is coordinated to one water molecule, eight carboxylate oxygen atoms and one hydroxyl group. Ba20 is coordinated to three water molecules and six carboxylate oxygen atoms. Ba39 is coordinated to one water molecule, seven carboxylate oxygen atoms and two hydroxyl groups. Water molecule O40 is uncoordinated. Comparison of the refined and optimized structures of the citrate anions in (I). The refined structure is in red, and the DFT-optimized structure is in blue. Citrate ion 1 (C1-H18) is on the left, and citrate ion 2 (C21-H38) is on the right. The asymmetric unit of (I) with the atom numbering and 50% probability spheres.

Supramolecular features
The Ba coordination polyhedra share edges and corners to form a three-dimensional framework (Fig. 3). The framework contains edge-sharing layers propagating in the ab plane. These layers share corners to form the framework. All of the active hydrogen atoms act as donors in O-HÁ Á ÁO hydrogen bonds: most of the acceptors are carboxylate oxygen atoms, but there are also waterÁ Á Áwater hydrogen bonds (Table 1). Both of the hydroxyl groups form intramolecular hydrogen bonds to terminal carboxyl groups. Two weak C-HÁ Á ÁO hydrogen bonds also contribute to the packing.

Database survey
Details of the comprehensive literature search for citrate structures are presented in Rammohan & Kaduk (2018 (Kaduk & Mueller, 2020). A search of the Powder Diffraction File (Gates-Rector & Blanton, 2019) for barium citrates yielded only entry 00-001-0009 for barium citrate heptahydrate (Hanawalt et al., 1938), one of the compounds in the first group of entries in the PDF. This powder pattern differs from that of the current compound.

Synthesis and crystallization
Tribarium dicitrate pentahydrate was synthesized by dissolving 2.0818 g (10.0 mmol) of citric acid monohydrate in 25 ml of water, and adding 2.9615 g (15.0 mmol) of BaCO 3 to the clear solution. After slow fizzing, some solid remained, so the slurry was heated to boiling, and additional fizzing occurred. The slurry was filtered and dried at room temperature to yield the title compound as a white powder.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. A laboratory pattern, measured using Cu K radiation, was indexed using DICVOL06 (Louë r & Boultif, 2007) on a primitive monoclinic cell with a = 11.4741, b = 13.7366, c = 15.0626 Å , = 107.944 , V = 2258.62 Å 3 , and Z = 4. After attempts to solve the structure using the laboratory data were unsuccessful, the powder pattern was measured at beamline 11-BM at the Advanced Photon Source, Argonne National Laboratory using a wavelength of 0.413891 Å and was indexed on a similar cell (Fig. 4). The structure was solved using Monte Carlo simulated annealing techniques as implemented in DASH (David et al., 2006). Three Ba atoms and two citrate anions were used as fragments. Oxygen atoms of water molecules were placed in voids located by Mercury (Macrae et al., 2020). Approximate positions of the hydrogen atoms were determined by analysis of potential hydrogen-bonding patterns.
The structure was refined by the Rietveld method using GSAS-II (Toby & Von Dreele, 2013). The initial refinement clarified the presence of extra peaks, which were identified as witherite, BaCO 3 , which was added as a second phase; its contribution refined to 9.2 wt%. All non-H bond distances and angles in the citrate anions were subjected to restraints, based on a Mercury Mogul Geometry Check (Sykes et al., 2011;Bruno et al., 2004); the Ba-O distances were not restrained. The Mogul average and standard deviation for each quantity were used as the restraint parameters. The restraints contributed 1.5% to the final 2 . The hydrogen atoms were included in calculated positions, which were recalculated

Figure 4
Comparison of the synchrotron (black) and laboratory X-ray powder diffraction patterns of (I). The laboratory pattern (measured using Cu K radiation) was converted to the synchrotron wavelength of 0.413891 Å using JADE Pro (MDI, 2020).
during the refinement using Materials Studio (Dassault Systems, 2020). The U iso values (Å 2 ) were grouped by chemical similarity; the U iso for the H atoms were fixed at 1.3 Â the U iso of the heavy atoms to which they are attached. Attempts to refine the U iso of the C and O atoms of the citrate anions led to values very close to zero, so these were fixed at reasonable values based on experience. The generalized microstrain model was used to describe the peak profiles. A 4th-order spherical harmonics preferred orientation model was included; the texture index refined to 1.006. The background was described by a six-term shifted Chebyshev polynomial, with a peak at 5.60 to describe the scattering from the Kapton capillary and any amorphous component. The largest errors in the fit (Fig. 5) are in the positions and shapes of some of the strong low-angle peaks, and suggest that the specimen changed during exposure to the X-ray beam. A density functional geometry optimization (fixed experimental unit cell) was carried out using CRYSTAL09 (Dovesi et al., 2018). The basis sets for the H, C and O atoms were those of Gatti et al. (1994), and the basis set for Ba was that of Piskunov et al. (2004). The calculation used 8 k-points and the B3LYP functional, and took $10.5 days on a 2.4 GHz PC. Rietveld plot for (I). The blue crosses represent the observed data points, and the green line is the calculated pattern. The cyan curve is the normalized error plot. The vertical scale has been multiplied by a factor of 20Â for 2 > 12.0 . The row of blue tick marks indicates the calculated reflection positions, and the red tick marks indicate the peak positions for the BaCO 3 impurity. The red line is the background curve.