Diammonium potassium citrate, (NH4)2KC6H5O7

The crystal structure of diammonium potassium citrate has been solved and refined using laboratory X-ray powder diffraction data and optimized using density functional theory techniques.

The crystal structure of diammonium potassium citrate, 2NH 4 + ÁK + ÁC 6 H 5 O 7 3À , has been solved and refined using laboratory X-ray powder diffraction data and optimized using density functional theory. The KO 7 coordination polyhedra are isolated. The ammonium cations and the hydrophobic methylene sides of the citrate anions occupy the spaces between the coordination polyhedra. Each hydrogen atom of the ammonium ions acts as a donor in a charge-assisted N-HÁ Á ÁO, N-HÁ Á Á (O,O) or N-HÁ Á Á(O,O,O) hydrogen bond. There is an intramolecular O-HÁ Á ÁO hydrogen bond in the citrate anion between the hydroxide group and one of the terminal carboxylate groups.

Structure description
A systematic study of the crystal structures of Group 1 (alkali metal) citrate salts has been reported by Rammohan & Kaduk (2018). The study was extended to ammonium citrates by Wheatley & Kaduk (2019). The title compound represents a further extension to mixed ammonium Group 1 citrates, specifically diammonium potassium citrate, (NH 4 ) 2 KC 6 H 5 O 7 .
The structure of (NH 4 ) 2 KC 6 H 5 O 7 was solved and refined from powder X-ray 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 nonhydrogen citrate atoms in the Rietveld-refined and DFT-optimized structures is 0.108 Å (Fig. 2). The maximum deviation is 0.211 Å , at O14. The r.m.s. displacement of the potassium ions is 0.054 Å . The r.m.s. displacements of the ammonium ions N19 and N20 are 0.111 and 0.151 Å respectively. The good agreement between the two structures is strong 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 data reports citrate anion occurs in the trans,trans-conformation (about C2-C3 and C3-C4), which is one of the two low-energy conformations of an isolated citrate anion (Rammohan & Kaduk, 2018). The central carboxylate group and the hydroxyl group exhibit a small twist [O16-C6-C3-O17 torsion angle = 7.0 ] from the normal planar arrangement. The Mulliken overlap populations indicate that the K-O bonds are ionic.
The citrate anion doubly chelates to K21 through the hydroxyl group O17 and the terminal carboxylate group (atom O11). The anion doubly chelates to another potassium cation through the hydroxyl group and the other terminal carboxylate group (atom O14). Each oxygen atom bonds to a single potassium cation. As a result, K21 is seven-coordinate (capped trigonal prismatic), with a bond-valence sum of 0.98.
The Bravais-Friedel-Donnay-Harker (Bravais, 1866;Friedel, 1907;Donnay & Harker, 1937) method suggests that we might expect block morphology for diammonium potassium citrate. A 2nd order spherical harmonic preferred orientation model was included in the Rietveld refinement; the texture index was 1.179, indicating that preferred orientation was significant for this rotated flat sheet specimen.
The KO 7 coordination polyhedra are isolated (  (Table 1). There is an intramolecular hydrogen bond between the hydroxide group and one of the terminal carboxylate groups. The N-HÁ Á ÁO hydrogen-bond energies were calculated by the correlation of Wheatley & Kaduk (2019), and the O-HÁ Á ÁO hydrogen bond energy was calculated by the correlation of Rammohan & Kaduk (2018).
Details of the comprehensive literature search for citrate structures are presented in Rammohan & Kaduk (2018). The Comparison of the refined and optimized structures of (NH 4 ) 2 KC 6 H 5 O 7 . The refined structure is in red, and the DFT-optimized structure is in blue.

D-HÁ
3À and C6H13KN2O 7 3 of 4 powder pattern of (NH 4 ) 2 KC 6 H 5 O 7 was indexed using N-TREOR (Altomare et al., 2013). A reduced-cell search of the cell of diammonium potassium citrate in the Cambridge Structural Database (Groom et al., 2016) resulted in no hits.

Synthesis and crystallization
Diammonium potassium citrate was synthesized by dissolving 1.1217 g diammonium hydrogen citrate (Fisher Lot #995047) and 0.3279 g potassium carbonate (Sigma-Aldrich Lot #098 K0064) in $5 ml of deionized water. The clear solution was dried at 363 K for two days to yield a white solid.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. A Rietveld plot is presented in Fig. 6. The structure was solved using Monte Carlo simulated annealing techniques with FOX (Favre-Nicolin & Č erný 2002) using a citrate anion, one K + cation and two ammonium cations as fragments. The structure was refined by the Rietveld method using GSAS-II (Toby & Von Dreele, 2013). The hydrogen atoms were included in fixed positions, which were recalculated during the course of the refinement using Materials Studio (Dassault Systems, 2019). All C-C and C-O bond distances and all bond angles were restrained based on a Mercury/Mogul Geometry Check (Sykes et al., 2011;Bruno et al., 2004) of the molecule. The U iso values of the atoms in the central and outer portions of the citrate were constrained to be equal, and the U iso values of the hydrogen atoms were Overlay of the crystal structures of diammonium potassium citrate and triammonium citrate, showing that they are isostructural.

Figure 6
Rietveld plot for (NH 4 ) 2 KC 6 H 5 O 7 . The blue crosses represent the observed data points, and the green line is the calculated pattern. The cyan curve is the normalized error plot. The vertical scale has been multiplied by a factor of 10Â for 2 > 50.0 . The row of blue tick marks indicates the calculated reflection positions. The red line is the background curve.

Figure 5
Comparison of the X-ray powder diffraction patterns of diammonium potassium citrate (black) and triammonium citrate (green). data reports constrained to be 1.3Â those of the atoms to which they are attached. A Chebyschev background function with three coefficients was used to model the background. A ten-term diffuse scattering function was used to describe the scattering from the capillary and any amorphous component. 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 k-points and the B3LYP functional, and took $5 days.