Lithium dipotassium citrate monohydrate, LiK2C6H5O7(H2O)

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

The crystal structure of dilithium potassium citrate monohydrate, Li + Á2K + Á C 6 H 5 O 7 3À ÁH 2 O or LiK 2 C 6 H 5 O 7 ÁH 2 O, has been solved by direct methods and refined against laboratory X-ray powder diffraction data, and optimized using density functional techniques. The complete citrate trianion is generated by a crystallographic mirror plane, with two C and three O atoms lying on the reflecting plane, and chelates to three different K cations. The KO 8 and LiO 4 coordination polyhedra share edges and corners to form layers lying parallel to the ac plane. An intramolecular O-HÁ Á ÁO hydrogen bond occurs between the hydroxyl group and the central carboxylate group of the citrate anion as well as a charge-assisted intermolecular O-HÁ Á ÁO link between the water molecule and the terminal carboxylate group. There is also a weak C-HÁ Á ÁO hydrogen bond.

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 hydrogen citrates in Cigler & Kaduk (2018), to sodium hydrogen citrates in Cigler & Kaduk (2019a), to sodium dirubidium citrates in Cigler & Kaduk (2019b) and to dilithium potassium citrate (Cigler & Kaduk, 2019c). We now report the synthesis and structure of the title compound, LiK 2 C 6 H 5 O 7 (H 2 O), which represents a further extension to lithium dipotassium citrates.

Structural commentary
The structure of LiK 2 C 6 H 5 O 7 (H 2 O) 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 hon-hydrogen atoms in the refined and optimized ISSN 2056-9890 structures is 0.047 Å (Fig. 2). The excellent agreement between the structures is evidence that the experimental structure is correct (van de Streek & Neumann, 2014). 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 citrate anion occurs in the trans,trans-conformation (about C2-C3 and the symmetry-related atoms), which is one of the two low-energy conformations of an isolated citrate anion (Rammohan & Kaduk, 2018). Since C3, the central C6/O15/O16 carboxylate group and the O17-H18 hydroxy group lie on the mirror plane, they exhibit the normal planar arrangement. The Mulliken overlap populations indicate that both the Li-O and K-O bonds have some covalent character, but that the Li-O bonds are more covalent.
The C 6 H 5 O 7 3citrate anion doubly chelates to three different K19 ions though O11/O16, O11/O15 and O12/O17. Each citrate oxygen atom bridges multiple metal atoms. K19 is eight-coordinate (irregular), with a bond-valence sum (in valence units) of 1.04 and Li20 (site symmetry m) is tetrahedral with a bond-valence sum of 1.10. Atom O21 of the water molecule of crystallization also lies on a (100) mirror plane.
The Bravais-Friedel-Donnay-Harker (Bravais, 1866;Friedel, 1907;Donnay & Harker, 1937) method suggests that we might expect a blocky morphology for lithium dipotassium citrate monohydrate. A 2nd order spherical harmonic preferred orientation model was included in the refinement; the texture index was 1.000, indicating that preferred orientation was not present for this rotated capillary specimen.

Supramolecular features
The KO 8 and LiO 4 coordination polyhedra share edges and corners to form layers lying parallel to the ac plane (Fig. 3). The only traditional hydrogen bonds are an intramolecular O17-H18Á Á ÁO16 interaction between the hydroxyl group and the central carboxylate group (Table 1), and a charge-assisted hydrogen bond between the water molecule O21-H22 and O11. By the correlation of Rammohan & Kaduk (2018), these hydrogen bonds contribute 13.2 and 13.4 kcal mol À1 , respec- Comparison of the refined and optimized structures of LiK 2 C 6 H 5 O 7 (H 2 O). The refined structure is in red, and the DFToptimized structure is in blue.

Figure 1
The crystal structure of LiK 2 C 6 H 5 O 7 (H 2 O) with the atom numbering and 50% probability spheroids.

Database survey
Details of the comprehensive literature search for citrate structures are presented in Rammohan & Kaduk (2018). A reduced cell search in the Cambridge Structural Database (Groom et al., 2016) yielded two hits, but no citrate structures. A few weak unindexed peaks were identified as 2.0 wt% dilithium potassium citrate (Cigler & Kaduk, 2019c).

Synthesis and crystallization
Masses of 0.3777 g of Li 2 CO 3 (5.00 mmol, Sigma-Aldrich) and 1.3851 g of K 2 CO 3 (10.0 mmol, Sigma-Aldrich) were added to a solution of 2.0325 g of citric acid (10.0 mmol, Sigma-Aldrich) monohydrate in 15 ml of water. After the fizzing subsided, the clear solution was dried first at 450 K to yield a sticky solid. The solid was heated at 477 K to yield a white foam. Further heating at 505 K yielded additional expansion of the foam, and slight discoloration. This foam was amorphous. Storage of the foam under ambient conditions yielded a puddle. Heating this puddle to 394 K yielded a glassy solid. Adding two drops of water to this solid yielded a paste, which yielded the title compound as a crystalline white powder after heating to 394 K for 15 min.

Refinement
The pattern of LiK 2 C 6 H 5 O 7 (H 2 O) was indexed using Jade 9.8 (MDI, 2017). EXPO2014 (Altomare et al., 2013) suggested the space group Pmn2 1 , which was confirmed by successful solution and refinement of the structure. The structure of LiK 2 C 6 H 5 O 7 (H 2 O) was solved by direct methods as implemented in EXPO2014 (Altomare et al., 2013), which located all the non-hydrogen atoms including the lithium atom. The positions of H7 and H8 were calculated using Materials Studio (Dassault, 2018). The position of the active hydrogen atom H18 was deduced from the potential intramolecular hydrogenbonding pattern, and the position of H22 was deduced from the hydrogen-bonding pattern. Pseudo-Voigt profile coefficients were as parameterized in Thompson et al. (1987) and the asymmetry correction of Finger et al. (1994) was applied and the microstrain broadening model of Stephens (1999). The hydrogen atoms were included in fixed positions, which were re-calculated during the course of the refinement using Materials Studio. Crystal data, data collection and structure refinement ( Fig. 4) details are summarized in Table 2. The U iso values for C2 and C3 were constrained to be equal, and those of H7 and H8 were constrained to be 1.3Â that of these carbon atoms. The U iso of 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 background was modeled by a three-term shifted Chebyshev polynomial. A ten-term diffuse scattering function was used to describe the scattering from the capillary and any amorphous material. The structure of dilithium potassium citrate, Li 2 KC 6 H 5 O 7 (Cigler & Kaduk, 2019c), was included as a second phase in the Rietveld refinement but its atomic positional and displacement parameters were not refined. Observed, calculated, and difference patterns of LiK 2 C 6 H 5 O 7 (H 2 O). The red crosses represent the observed data points, the green solid line the calculated pattern, and the magenta line the difference (observedcalculated) pattern. The vertical scale is multiplied by a factor of 8 above 23 2. A density functional geometry optimization was carried out using CRYSTAL14 (Dovesi et al., 2014). The basis sets for the H, C, N, and O atoms were those of Gatti et al. (1994), and the basis set for K was that of Peintinger et al. (2013). 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, using 8 kpoints and the B3LYP functional, and took two hours.