Tricaesium citrate monohydrate, Cs3C6H5O7·H2O: crystal structure and DFT comparison

The crystal structure of tricaesium citrate monohydrate has been solved and refined using laboratory X-ray single-crystal diffraction data, and optimized using density functional techniques.


Structural commentary
The asymmetric unit of the title compound is shown in Fig. 1. The root-mean-square deviation of the non-hydrogen atoms in the experimental and DFT-optimized structures is 0.123 Å (Fig. 2). The largest difference is 0.200 Å , at O1W. This good agreement provides strong evidence that the experimental structure is correct (van de Streek & Neumann, 2014). Almost all of the bond lengths, bond angles, and torsion angles in the experimentally determined structure fall within the normal ranges indicated by a Mercury Mogul geometry check (Macrae et al., 2008). Only the O8-C1-C2 angle of 118.0 is flagged as unusual [average = 119.8 (4) , Z-score = 4.2). The Z-score is the result of the exceptionally low uncertainty on the average of this bond angle. In the DFT-optimized structure, the O7-C1-C2 angle of 115.9 is flagged as unusual [average = 120.3 (12) , Z-score = 3.6]. The citrate anion occurs in the trans,trans conformation, which is one of the two low-energy conformations of an isolated citrate. The three Cs + cations are eight-, eight-, and seven-coordinate, with bond-valence sums of 0.91, 1.22, and 1.12 valence units. There is extensive chelation of the citrate anion to Cs + cations: O12(end)/ O13(OH) to Cs1, O8(end)/O10(central) to Cs2, O11(end)/ O10(central) to Cs2, C11(end)/O9(central) to Cs2, O7(end)/ O13(OH) to Cs2, O8(end)/O9(central) to Cs3, and O11(end)/ O11(central) to Cs3. The carboxylate group O11/O12 also acts as a bidentate ligand to Cs1. The Mulliken overlap populations and atomic charges indicate that the metal-oxygen bonding is ionic.

Supramolecular features
The coordination polyhedra link into a three-dimensional framework (Fig. 3). The hydrophobic methylene groups occupy pockets in the framework. The hydroxy group forms the usual S(5) hydrogen bond with the central carboxylate group, and the water molecule acts as a donor in two strong hydrogen bonds (2.686 and 2.662 Å ). By the correlation between the square root of the Mulliken overlap population and hydrogen-bond energy derived in Rammohan & Kaduk (2017a), these hydrogen bonds contribute 14.4, 14.1, and 14.1 kcal mol À1 , respectively, to the crystal energy. Numerical details of the hydrogen bonds in the experimentally determined and DFT-optimized structures are given in Tables 1 and 2, respectively. Comparison of the refined and optimized structures of tricaesium citrate monohydrate. The refined structure is in red and the DFT-optimized structure is in blue.

Table 2
Hydrogen-bond geometry (Å , ) for the DFT-optimized structure.  (Carrell et al., 1987) and Rb + (Rammohan & Kaduk, 2017c) compounds with the same formula, but the previously-reported structure of K 3 C 6 H 5 O 7 ÁH 2 O has to be transformed from setting P2 1 /a to P2 1 /n to make the similarities clear (Table 3).

Synthesis and crystallization
H 3 C 6 H 5 O 7 ÁH 2 O (2.0774 g, 10.0 mmol, Sigma-Aldrich) was dissolved in 8 ml deionized water. Cs 2 CO 3 (4.9324 g, 15.1 mmole, Sigma-Aldrich) was added to the citric acid solution slowly with stirring. The resulting clear colorless solution was evaporated to dryness in a oven at 333 K. Single crystals were isolated from the white product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The hydrogen atoms were freely refined with isotropic ADPs. The lattice parameters at 300 K (derived from a Le Bail fit of the powder pattern) are a = 7.8851 (4)

DFT calculations
After the Rietveld refinement, a density functional geometry optimization (fixed experimental unit cell at 100 K) was carried out using CRYSTAL09 (Dovesi et al., 2005). The basis sets for the C, H, and O atoms were those of Gatti et al. (1994), and the basis set for Cs was that of Prencipe (1990  References: (a) Carrell et al. (1987); (b) Rammohan & Kaduk (2017c); (c) this work.

Figure 3
Crystal structure of Cs 3 C 6 H 5 O 7 ÁH 2 O, viewed down the b axis. The same symmetry and lattice parameters were used for the DFT calculations. Computer programs: APEX2 and SAINT (Bruker, 2008), XM (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and OLEX2 (Dolomanov et al., 2009 Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00244 (9) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.