Crystal structure of caesium dihydrogen citrate from laboratory X-ray powder diffraction data and DFT comparison

The crystal structure of caesium dihydrogen citrate has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques.

The crystal structure of caesium dihydrogen citrate, Cs + ÁH 2 C 6 H 5 O 7 À , has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. The coordination polyhedra of the nine-coordinate Cs + cations share edges to form chains along the a-axis. These chains are linked by corners along the c-axis. The un-ionized carboxylic acid groups form two different types of hydrogen bonds; one forms a helical chain along the c-axis, and the other is discrete. The hydroxy group participates in both intra-and intermolecular hydrogen bonds.

Chemical context
In the course of a systematic study of the crystal structures of Group 1 (alkali metal) citrate salts to understand the anion's conformational flexibility, ionization, coordination tendencies, and hydrogen bonding, we have determined several new crystal structures. Most of the new structures were solved using powder diffraction data (laboratory and/or synchrotron), but single crystals were used where available. The general trends and conclusions about the 16 new compounds and 12 previously characterized structures are being reported separately (Rammohan & Kaduk, 2017a). Ten of the new structures -NaKHC 6 H 5 O 7 , NaK 2 C 6 H 5 O 7 , Na 3 C 6 H 5 O 7 , NaH 2 C 6 H 5 O 7 , Na 2 HC 6 H 5 O 7 , K 3 C 6 H 5 O 7 , Rb 2 HC 6 H 5 O 7 , Rb 3 C 6 H 5 O 7 (H 2 O), Rb 3 C 6 H 5 O 7 , and Na 5 H(C 6 H 5 O 7 ) 2 -have been published recently (Rammohan & Kaduk, 2016a,b,c,d,e, 2017bRammohan et al., 2016), and two additional structures -KH 2 C 6 H 5 O 7 and KH 2 C 6 H 5 O 7 (H 2 O) 2 -have been communicated to the CSD (Kaduk & Stern, 2016a,b). ISSN 2056-9890

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 Rietveld-refined and DFT-optimized structures is 0.387 Å (Fig. 2). This agreement is at the upper end of the range of correct structures as discussed by van de Streek & Neumann (2014). Re-starting the Rietveld refinement from the DFToptimized structure led to higher residuals (R wp = 0.1287 and 2 = 26.43). Accurate determination of the positions of C and O atoms in the presence of the heavy Cs atoms using X-ray powder data might be expected to be difficult. This 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), but the torsion angles involving the central carboxylate and hydroxyl group are flagged as unusual; the central portion of the molecule is less-planar than usual. In the refined structure, the O8-C1 and O10-C6 bonds, as well as the C3-C2-C1 angle, were flagged as unusual. The citrate anion occurs in the trans,trans conformation, which is one of the two low-energy conformations of an isolated citrate. The central carboxylate O10 and the terminal carboxylate O12 atoms chelate to the Cs + cation. 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 suggests that we might expect a platy morphology for cesium dihydrogen citrate, with {020} as the principal faces. A 4th-order spherical harmonic texture model was included in the refinement. The texture index was 1.183, indicating that preferred orientation was significant for this rotated flat-plate specimen.

Supramolecular features
The nine-coordinate Cs + cation (bond-valence sum 0.96) share edges to form chains along the a axis (Fig. 3). These chains are linked by corners along the c axis. The O7-H20Á Á ÁO8 hydrogen bonds (Table 1) form a helical chain along the c axis, and the O11-H21Á Á ÁO10 hydrogen bonds are discrete. The Mulliken overlap populations in these hydrogen bonds are 0.064 and 0.095 e, respectively. By the correlation in Rammohan & Kaduk (2017a), these hydrogen bonds contribute 13.8 and 16.8 kcal mol À1 to the crystal energy. The hydroxy group O13-H16 acts as a donor in two hydrogen bonds. The one to O10 is intramolecular, with a graph-set symbol S(5). The one to O9 is intermolecular, with a graph set symbol S(7). These hydrogen bonds are weaker, contributing 11.2 and 9.1 kcal mol À1 to the crystal energy. The asymmetric unit, with the atom numbering. The atoms are represented by 50% probability spheroids.

Figure 3
Crystal structure of CsH 2 C 6 H 5 O 7 , viewed down the c-axis.

Database survey
Details of the comprehensive literature search for citrate structures are presented in Rammohan & Kaduk (2017a). A reduced-cell search of the cell of cesium dihydrogen citrate in the Cambridge Structural Database (Groom et al., 2016) (increasing the default tolerance from 1.5 to 2.0%) yielded 60 hits, but combining the cell search with the elements C, H, Cs, and O only yielded no hits.

Synthesis and crystallization
H 3 C 6 H 5 O 7 (H 2 O) (2.0766 g, 10.0 mmol) was dissolved in 10 ml deionized water. Cs 2 CO 3 (1.6508 g, 5.0 mmol, Sigma-Aldrich) was added to the citric acid solution slowly with stirring. A white precipitate formed in about two minutes, and the colourless solution was evaporated to dryness at ambient conditions.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. The powder pattern (Fig. 4) (2), b = 20.5351 (2), c = 5.1682 (5) Å , V = 927.17 (9) Å 3 , and Z = 4. The peak list from a Le Bail fit in GSAS was imported into Endeavour 1.7b (Putz et al., 1999), and used for structure solution. The successful solution used a citrate, a Cs atom, and two oxygen atoms from water molecules. Initial Rietveld refinements moved the oxygens close to the Cs site, so they were deleted from the refinement.
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. The C-C bonds were restrained at 1.54 (1) Å , and the C3-O13 bond at 1.42 (2) Å . The C-O bonds in the carboxylate groups were restrained at 1.26 (2) Å . All angles were also restrained; the restraints were 109 (3) for the angles around tetrahedral carbon atoms, and 120 (3) for the angles in the planar carboxylate groups. The restraints contributed 3.0% to the final 2 . The hydrogen atoms were included at fixed positions, which were recalculated during the course of the refinement using Materials Studio (Dassault Systè mes, 2014).

DFT calculations
A density functional geometry optimization (fixed experimental unit cell) 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

Figure 4
Rietveld plot for the refinement of CsH 2 C 6 H 5 O 7 . The vertical scale is not the raw counts but the counts multiplied by the least squares weights. This plot emphasizes the fit of the weaker peaks. 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.