Crystal structure of dicesium hydrogen citrate from laboratory single-crystal and powder X-ray diffraction data and DFT comparison

The crystal structure of dicesium hydrogen citrate has been solved using laboratory X-ray single-crystal diffraction data, refined using laboratory powder 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 experimentally determined and in the DFT-optimized structures is 0.098 Å (Fig. 2). The largest differences are 0.13 Å , at Cs19 and O11. This good agreement provides strong ISSN 2056-9890 evidence that the experimentally determined structure is correct (van de Streek & Neumann, 2014). The following discussion uses the DFT-optimized structure.
Most of the bond lengths, bond angles, and torsion angles fall within the normal ranges indicated by a Mercury Mogul geometry check (Macrae et al., 2008). The C1-C2-C3 angle of 114.1 is flagged as unusual (average = 104.0 (32), Z-score = 3.1). The Cs + cation is 9-coordinate, with a bond-valence sum of 0.92 valence units. The location of the citrate anion on a mirror plane and the coordination of all seven oxygen atoms to Cs + cations presumably are the source of the slight distortion. The citrate anion occurs in the trans,trans conformation, which is one of the two low-energy conformations of an isolated citrate moiety. The citrate anion triply chelates to two Cs + cations through O12, O17, and O15. The citrate also chelates through O12/O16, O15/O17, and O15/O16. The Mulliken overlap populations and atomic charges indicate that the metal-oxygen bonding is ionic. The Bravais-Friedel-Donnay-Harker (Bravais, 1866;Friedel, 1907;Donnay & Harker, 1937) morphology is blocky, with {020} as major faces. A 4th-order spherical harmonic model was included in the refinement. The texture index was 1.016, indicating that preferred orientation was slight in the rotated flat-plate specimen.

Supramolecular features
The CsO 9 coordination polyhedra share edges and corners to form a three-dimensional framework (Fig. 3). The central hydroxy/carboxylate O-HÁ Á ÁO hydrogen O17-H18Á Á ÁO16 is short, and (unusually) intermolecular. The centrosymmetric end-end O12-H20-O12 hydrogen bond (with H20 situated on an inversion center) is exceptionally short and strong (Table 1). By the correlation of Rammohan & Kaduk (2017a), these hydrogen bonds contribute 16.5 and 21.7 kcal mol À1 to the crystal energy. The hydrophobic methylene groups occupy pockets in the framework (Fig. 3 The asymmetric unit of the title compound, with the atom numbering. The atoms are represented by 50% probability spheroids.

Figure 2
Comparison of the refined and optimized structures of dicesium hydrogen citrate. The refined structure is in red, and the DFT-optimized structure is in blue. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
Citric acid monohydrate, H 3 C 6 H 5 O 7 (H 2 O), (2.0796 g, 10.0 mmol) was dissolved in 20 ml deionized water. Cs 2 CO 3 (3.2582 g, 10.0 mmol, Sigma-Aldrich) was added to the citric acid solution slowly with stirring. The resulting clear colourless solution was evaporated to dryness in a 333 K oven. Single crystals were isolated from the colourless solid.

Refinement
A single crystal was mounted in inert oil and transferred to the cold gas stream of a Bruker Kappa APEX CCD area detector system equipped with a Cu K sealed tube with MX optics. Despite suggestions from multiple programs that the space group was Pnma, all attempts to refine the structure in this space group yielded unreasonable disorder and non-positivedefinite displacement coefficients. Presumably the poor crystal quality and/or twinning were the source of the problems. The best refinement using single crystal data was obtained using space group P2 1 2 1 2 1 . A portion of the sample was ground in a mortar and pestle, and blended with NIST SRM 640b silicon internal standard. The powder pattern indicated that the sample contained about 24 wt% CsHC 6 H 5 O 7 (Rammohan & Kaduk, 2017f), which was included as phase 2 in the refinement. The Si internal standard was included as phase 3.
Initial Rietveld refinements used the single crystal P2 1 2 1 2 1 model, but were unstable. The ADDSYM module of PLATON (Spek, 2009) suggested the presence of an addi-tional center of symmetry, and that the correct space group was Pnma (with a transformation of axes). Refinement in the higher-symmetry space group was uneventful. Pseudo-Voigt profile coefficients were as parameterized in Thompson et al. (1987) with profile coefficients for Simpson's rule integration of the pseudo-Voigt function according to Howard (1982). The asymmetry correction of Finger et al. (1994) was applied, and microstrain broadening by Stephens (1999). The structure was refined by the Rietveld method using GSAS/EXPGUI (Larson & Von Dreele, 2004;Toby, 2001). All C-C and C-O bond lengths were restrained, as were all bond angles. The hydrogen atoms were included at fixed positions, which were recalculated during the course of the refinement using Materials Studio (Dassault Systemes, 2014). The limited resolution of the powder data precluded refining displacement coefficients, which were fixed at typical values for alkali metal citrates. Diffraction data are displayed in Fig. 4. Crystal data, data collection and structure refinement details are summarized in Table 2.

DFT calculations
After the Rietveld refinement, a density functional geometry optimization (fixed experimental unit cell) was carried out using CRYSTAL14 . The basis sets for the C, H, and O atoms were those of Peintinger et al. (2012), and the basis set for Cs was that of Sophia et al. (2014). The calculation was run on eight 2.1 GHz Xeon cores (each with 6 Gb RAM) of a 304-core Dell Linux cluster at IIT, used 8 kpoints and the B3LYP functional, and took about 13 h. The U iso values from the Rietveld refinement were assigned to the optimized fractional coordinates. Crystal structure of Cs 2 HC 6 H 5 O 7 , viewed down the a-axis. CsO 9 polyhedra are green.

Figure 4
Rietveld plot for the refinement of Cs 2 HC 6 H 5 O 7 . The red crosses represent the observed data points, and the green line is the calculated pattern. The magenta curve is the difference pattern, plotted at the same scale as the other patterns. The row of black tick marks indicates the reflection positions, the row of red tick marks indicates the positions of the CsH 2 C 6 H 5 O 7 impurity peaks, and the blue tick marks indicate the Si internal standard peaks.  Step Step 2 values ( ) 2 min = 5.042 2 max = 70.050 2 step = 0.020 2 min = 5.042 2 max = 70.050 2 step = 0.020 The same symmetry and lattice parameters were used for the DFT calculation. Computer programs: DIFFRAC.Measurement (Bruker, 2009), SHELXT (Sheldrick, 2015), GSAS (Larson & Von Dreele, 2004), DIAMOND (Crystal Impact, 2015), publCIF (Westrip, 2010