The new compounds LiMHC6H5O7 (M = Li, Na, K, Rb) have been prepared from the metal carbonates and citric acid in solution. The crystal structures have been solved and refined using laboratory (Mo Kα) X-ray powder diffraction data, and optimized using density functional techniques. The compounds crystallize in triclinic space group P-1, and are nearly isostructural. The Na occupies a different lattice site than the other M. The structure is lamellar, with the layers in the ab plane. The boundaries of the layers consist of hydrophobic methylene groups and very strong intermolecular O-H...O hydrogen bonds between un-ionized terminal carboxylic acid and ionized terminal carboxylate groups. The O...O distances range from 2.666 Å for M = Li to 2.465 Å for M = Rb; the graph set is R1,1(8) and the ring includes M. The hydroxy group acts as a hydrogen bond donor, forming R1,1(6) hydrogen bonds; the ring includes the Li. The Li-O bonds exhibit significant covalent character (as indicated by the Mulliken overlap populations), while the heavier M-O bonds are ionic. The Li are 4, 5, or 6-coordinate, while the coordination numbers of the larger cations are higher: 8 for Na and 9 for K and Rb. For M = Na, K, and Rb, the hydroxyl group, the ionized terminal carboxylate, and the central ionized carboxylate triply chelate to M. For M = Li, the triple chelation involves the hydroxyl group, the ionized central carboxylate, and the terminal carboxylic acid. Other trends in chelation will also be discussed. For M = Li and Na, the terminal ionized carboxylate chelates to a Li. The citrate occurs in the trans,trans conformation, one of two low-energy conformations of an isolated citrate anion.

Dilithium (citrate) crystals and their relatives

253rd National Meeting (2017)

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