Crystal structure of dilithium potassium citrate, Li2KC6H5O7 determined from powder diffraction data and DFT calculations

The crystal structure of dilithium potassium citrate has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. The KO7 coordination polyhedra share edges, forming chains parallel to the a axis. These chains share edges with one tetrahedral Li, and are bridged by edge-sharing pairs of the second tetrahedral Li, forming layers parallel to the ac plane.


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 lithium metal hydrogen citrates in Cigler & Kaduk (2018), to sodium metal hydrogen citrates in Cigler & Kaduk (2019a), and to sodium dirubidium citrates in Cigler & Kaduk (2019b). We now describe the synthesis and structure of the title compound, Li 2 KC 6 H 5 O 7 , which represents a further extension to the family of known lithium potassium citrates. Only one mixed lithium potassium citrate has been reported previously: the double salt LiK 2 (HC 6 H 5 O 7 )(H 2 CH 5 O 7 )(H 2 O) [CSD (Groom et al., 2016) refcode LATPOL; Zacharias & Glusker, 1993].

Structural commentary
The structure of Li 2 KC 6 H 5 O 7 was solved and refined from powder data and optimized by density functional theory (DFT) calculations (see Experimental Section) and is illustrated in Fig. 1. The root-mean-square Cartesian displacement of the non-hydrogen atoms in the refined and optimized structures is 0.24 Å (Fig. 2). The largest differences (0.3-0.4 Å ) are in the K19 coordination sphere. The general good agree-

Supramolecular features
The KO 7 coordination polyhedra share edges, forming chains parallel to the a-axis direction (Fig. 3). These chains share edges with Li20, and are bridged by edge-sharing pairs of Li21, forming layers lying parallel to the ac plane (Figs. 4 and 5).
The only traditional hydrogen bond is an intramolecular O17-H18Á Á ÁO11 link between the hydroxyl group and one of the terminal carboxylate groups (Table 1). By the correlation of Rammohan & Kaduk (2018), this hydrogen bond contributes about 13.9 kcal mol À1 to the crystal energy. There is also a weak intramolecular C-HÁ Á ÁO hydrogen bond (Table 1).

Figure 1
The asymmetric unit of Li 2 KC 6 H 5 O 7 with the atom numbering and 50% probability spheres.

Figure 2
Comparison of the refined and optimized structures of Li 2 KC 6 H 5 O 7 . The refined structure is in red, and the DFT-optimized structure is in blue.

Figure 3
Crystal structure of Li 2 KC 6 H 5 O 7 , viewed down the a-axis direction.

Figure 4
Crystal structure of Li 2 KC 6 H 5 O 7 , viewed down the b-axis direction.

Database survey
Details of the comprehensive literature search for citrate structures are presented in Rammohan & Kaduk (2018). The pattern of Li 2 KC 6 H 5 O 7 was indexed using N-TREOR (Altomare et al., 2013), and the cell was reduced using the tools in the PDF-4+ database (Fawcett et al., 2017). A reduced cell search in the Cambridge Structural Database (Groom et al., 2016) yielded no hits.

Synthesis and crystallization
0.7412 g Li 2 CO 3 (10.0 mmol, Sigma-Aldrich) and 0.6910 g K 2 CO 3 (5.0 mmol, Sigma-Aldrich) were added to a solution of 2.0175 g citric acid (10.0 mmol, Sigma-Aldrich) monohydrate in 10 ml water. After the fizzing subsided, the clear solution was dried at 338 K to yield a clear glass. The glass was heated at 423 K for 30 min to yield a slightly hygroscopic white solid.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. A Rietveld plot is shown in Fig. 6. The structure of Li 2 KC 6 H 5 O 7 was solved using Monte Carlosimulated annealing techniques as implemented in EXPO2014 (Altomare et al., 2013). A citrate anion, a K cation, and two Li cations were used as fragments. The positions of H7-H10 were calculated using Materials Studio (Dassault, 2018). The position of the active (ionizable) hydrogen atom H18 was deduced from the potential intramolecular hydrogen-bonding pattern.
The Li positions were unreasonable, so they were deleted from the model. Potential Li positions were identified by using Materials Studio to search for voids in the structure, with a Connelly radius of 0.9 Å . Pseudo-Voigt profile coefficients were as parameterized in Thompson et al. (1987) and the asymmetry correction of Finger et al. (1994) was applied and microstrain broadening by Stephens (1999). The hydrogen atoms were included in fixed positions, which were re-calculated during the course of the refinement using Materials Studio. The U iso values of C2, C3, and C4 were constrained to be equal, and those of H7, H8, H9, and H10 were constrained to be 1.3Â that of these carbon atoms. The U iso values for C1, C5, C6, and the oxygen atoms were constrained to be equal, and that of H18 was constrained to be 1.3Â this value. The U iso values of Li20 and Li21 were fixed. An 11-term diffuse scattering function was used to describe the scattering from the capillary and the significant fraction of amorphous material.
A density functional geometry optimization (fixed experimental unit cell) was carried out using CRYSTAL14 (Dovesi et al., 2014). The basis sets for the H, C, and O atoms were those of Gatti et al. (1994), and the basis sets for Li and K were those of Peintinger et al. (2013). The calculation was run on eight 2.1 GHz Xeon cores (each with 6 GB RAM) of a 304core Dell Linux cluster at Illinois Institute of Technology, using 8 k-points and the B3LYP functional, and took 18 h. Rietveld plot for Li 2 KC 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 vertical scale has been multiplied by a factor of five for 2 > 20.0 . The row of black tick marks indicates the reflection positions for this phase.