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

Andrew J Cigler,James A Kaduk

doi:10.1107/s2056989019002809

Abstract

The crystal structure of poly[μ-citrato-dilithium(I)potassium(I)], [Li2K(C6H5O7)] n , has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. The citrate anion triply chelates to the K+ cation through the hydroxyl group, the central carboxyl-ate, and the terminal carboxyl-ate. The KO7 coordination polyhedra share edges, forming chains parallel to the a axis. These chains share edges with one tetra-hedral Li ion, and are bridged by edge-sharing pairs of the second tetra-hedral Li ion, forming layers parallel to the ac plane.

Highlights

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, 2008)
The citrate anion occurs in the trans,trans-conformation, which is one of the two low-energy conformations of an isolated citrate (Rammohan & Kaduk, 2018)
The citrate anion triply chelates to K19 though the hydroxyl group O17, the central carboxylate group, and the terminal carboxylate (O12)

Summary

Structural commentary

The root-mean-square Cartesian displacement of the non-hydrogen atoms in the refined and optimized structures is 0.24 A (Fig. 2). 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, 2008). The citrate anion triply chelates to K19 though the hydroxyl group O17, the central carboxylate group (atom O16), and the terminal carboxylate (O12). K19 is seven-coordinate (irregular), with a bond-valence sum of 1.15. Li20 and Li21 are tetrahedral, with bond-valence sums of 0.95 and 1.08, respectively. A 2nd order spherical harmonic preferred orientation model was included in the refinement; the texture index was 1.012, indicating that preferred orientation was very slight for this rotated capillary specimen

Supramolecular features

Database survey

Synthesis and crystallization

Refinement