Lithium dipotassium citrate monohydrate, LiK2C6H5O7(H2O).

The crystal structure of dilithium potassium citrate monohydrate, Li+·2K+·C6H5O7 3-·H2O or LiK2C6H5O7·H2O, 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 KO8 and LiO4 coordination polyhedra share edges and corners to form layers lying parallel to the ac plane. An intra-molecular O-H⋯O hydrogen bond occurs between the hydroxyl group and the central carboxyl-ate group of the citrate anion as well as a charge-assisted inter-molecular O-H⋯O link between the water mol-ecule and the terminal carboxyl-ate group. There is also a weak C-H⋯O hydrogen bond.

Structures of dipotassium rubidium citrate monohydrate, K2RbC6H5O7(H2O), and potassium dirubidium citrate monohydrate, KRb2C6H5O7(H2O), from laboratory X-ray powder diffraction data and DFT calculations.

The crystal structures of the isostructural compounds dipotassium rubidium citrate monohydrate, K2RbC6H5O7(H2O), and potassium dirubidium citrate monohydrate, KRb2C6H5O7(H2O), have been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. The compounds are isostructural to K3C6H5O7(H2O) and Rb3C6H5O7(H2O), but exhibit different degrees of ordering of the K and Rb cations over the three metal-ion sites. The K and Rb site occupancies correlate well to both the bond-valence sums and the DFT energies of ordered cation systems. The MO6 and MO7 coordination polyhedra share edges to form a three-dimensional framework. The water mol-ecule acts as a donor in two strong charge-assisted O-H⋯O hydrogen bonds to carboxyl-ate groups. The hydroxyl group of the citrate anion forms an intra-molecular hydrogen bond to one of the central carboxyl-ate oxygen atoms.

Crystal structures of two isostructural compounds: a second polymorph of dipotassium hydrogen citrate, K2HC6H5O7, and potassium rubidium hydrogen citrate, KRbHC6H5O7.

The crystal structures of a new polymorph of dipotassium hydrogen citrate, 2K+·HC6H5O72-, and potassium rubidium hydrogen citrate, K+·Rb+·HC6H5O72-, have been solved and refined using laboratory powder X-ray diffraction and optimized using density functional techniques. In the new polymorph of the dipotassium salt, KO7 and KO8 coordination polyhedra share corners and edges to form a three-dimensional framework with channels parallel to the a axis and [111]. The hydrophobic methylene groups face each other in the channels. The un-ionized carboxylic acid group forms a strong charge-assisted hydrogen bond to the central ionized carboxylate group. The hydroxy group forms an intermolecular hydrogen bond to a different central carboxylate group. In the potassium rubidium salt, the K+ and Rb+ cations are disordered over two sites, in approximately 0.72:0.28 and 0.28:0.72 ratios. KO8 and RbO9 coordination polyhedra share corners and edges to form a three-dimensional framework with channels parallel to the a axis. The un-ionized carboxylic acid group forms a strong charge-assisted hydrogen bond to an ionized carboxylate group. The hydroxy group forms an intermolecular hydrogen bond to the central carboxylate group. Density functional theory (DFT) calculations on the ordered cation structures suggest that interchange of K+ and Rb+ at the two cation sites changes the energy insignificantly.

Sodium dirubidium citrate, NaRb2C6H5O7, and sodium dirubidium citrate dihydrate, NaRb2C6H5O7(H2O)2.

The crystal structures of sodium dirubidium citrate {poly[µ-citrato-dirubi-dium(I)sodium(I)], [NaRb2(C6H5O7)] n } and sodium dirubidium citrate dihydrate {poly[di-aqua-(µ-citrato)dirubidium(I)sodium(I)], [NaRb2(C6H5O7)(H2O)2] n } have been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. Both structures contain Na chains and Rb layers, which link to form different three-dimensional frameworks. In each structure, the citrate triply chelates to the Na+ cation. Each citrate also chelates to Rb+ cations. In the dihydrate structure, the water mol-ecules are bonded to the Rb+ cations; the Na+ cation is coordinated only to citrate O atoms. Both structures contain an intra-molecular O-H⋯O hydrogen bond between the hy-droxy group and one of the terminal carboxyl-ate groups. In the structure of the dihydrate, each hydrogen atom of the water mol-ecules participates in a hydrogen bond to an ionized carboxyl-ate group.

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

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.

Sodium rubidium hydrogen citrate, NaRbHC6H5O7, and sodium caesium hydrogen citrate, NaCsHC6H5O7: crystal structures and DFT comparisons.

The crystal structure of sodium rubidium hydrogen citrate, NaRbHC6H5O7 or [NaRb(C6H6O7)] n , has been solved and refined using laboratory powder X-ray diffraction data, and optimized using density functional techniques. This compound is isostructural to NaKHC6H5O7. The Na atom is six-coordinate, with a bond-valence sum of 1.16. The Rb atom is eight-coordinate, with a bond-valence sum of 1.17. The distorted [NaO6] octa-hedra share edges to form chains along the a-axis direction. The irregular [RbO8] coordination polyhedra share edges with the [NaO6] octa-hedra on either side of the chain, and share corners with other Rb atoms, resulting in triple chains along the a-axis direction. The most prominent feature of the structure is the chain along [111] of very short, very strong hydrogen bonds; the O⋯O distances are 2.426 and 2.398 Å. The Mulliken overlap populations in these hydrogen bonds are 0.140 and 0.143 electrons, which correspond to hydrogen-bond energies of about 20.3 kcal mol-1. The crystal structure of sodium caesium hydrogen citrate, NaCsHC6H5O7 or [NaCs(C6H6O7)] n , has also been solved and refined using laboratory powder X-ray diffraction data, and optimized using density functional techniques. The Na atom is six-coordinate, with a bond-valence sum of 1.15. The Cs atom is eight-coordinate, with a bond-valence sum of 0.97. The distorted trigonal-prismatic [NaO6] coordination polyhedra share edges to form zigzag chains along the b-axis direction. The irregular [CsO8] coordination polyhedra share edges with the [NaO6] polyhedra to form layers parallel to the (101) plane, unlike the isolated chains in NaKHC6H5O7 and NaRbHC6H5O7. A prominent feature of the structure is the chain along [100] of very short, very strong O-H⋯O hydrogen bonds; the refined O⋯O distances are 2.398 and 2.159 Å, and the optimized distances are 2.398 and 2.347 Å. The Mulliken overlap populations in these hydrogen bonds are 0.143 and 0.133 electrons, which correspond to hydrogen-bond energies about 20.3 kcal mol-1.

Dilithium (citrate) crystals and their relatives.

New compounds of the type 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 powder X-ray diffraction data, and optimized using density functional techniques. The compounds crystallize in the triclinic space group P-1 and are nearly isostructural. The structures are 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. The O...O distances range from 2.666 Šfor M = Li to 2.465 Šfor M = Rb. The Li-O bonds exhibit significant covalent character, while the heavier M-O bonds are ionic. The Li atoms are four-, five-, or six-coordinate, while the coordination numbers of the larger cations are higher, i.e. eight for Na and nine for K and Rb. The citrate anion occurs in the trans,trans conformation, one of the two low-energy conformations of an isolated citrate anion. The crystal structure of LiRbHC6H5O7·H2O was also solved and refined. It consists of the same layers as in the anhydrous M = Rb compound, with interlayer water molecules and a different hydrogen-bonding pattern.