Abstract
The title alkali propano-ates poly[(μ5-propano-ato)alkali(I)], M +(C2H5COO)-, with alkali/M + = Na+, K+, Rb+ and Cs+, show close structural similarity, which is manifested by the coordination of the cations by six oxygen atoms in a chessboard motif, forming a bilayer. This bilayer is situated between hydro-phobic layers composed of dangling ethyl chains from the carboxyl-ate groups. Stacking of these two-dimensional sandwiches, which are parallel to (001), forms the title structures. Each metal cation is coordinated by six O atoms in the form of a distorted trigonal prism. One pair of these oxygen atoms belongs to a bridging, bidentately coordinating carboxyl-ate anion, while each of the other four oxygen atoms belongs to different carboxyl-ate groups, which are in a bridging monodentate mode. Despite the close similarity, each of the studied alkali propano-ates crystallizes in a different space group. The atoms are in general positions, except for the cation in K+(C2H5COO)-, which is situated on a mirror plane. Positional disorder of the methyl groups that are disordered over two positions is present in the Na+ and K+ propano-ates, in contrast to the Rb+ and Cs+ propano-ates. In the Na+ compound, the occupational parameters of the disordered methyl groups are different compared to the K+ compound where they are equal. This difference results in doubling of the a unit-cell parameter of the Na+ compound with respect to that of the K+ compound, otherwise the structures are homeotypic. In Cs+ propano-ate, a disorder of the methyl H atoms is observed.
Highlights
The title alkali propanoates poly[(5-propanoato)alkali(I)], M+(C2H5COO)À, with alkali/M+ = Na+, K+, Rb+ and Cs+, show close structural similarity, which is manifested by the coordination of the cations by six oxygen atoms in a chessboard motif, forming a bilayer
Each metal cation is coordinated by six O atoms in the form of a distorted trigonal prism. One pair of these oxygen atoms belongs to a bridging, bidentately coordinating carboxylate anion, while each of the other four oxygen atoms belongs to different carboxylate groups, which are in a bridging monodentate mode
Positional disorder of the methyl groups that are disordered over two positions is present in the Na+ and K+ propanoates, in contrast to the Rb+ and Cs+ propanoates
Summary
The structures of the alkali propanoates, M+(C3H5O2)À, with exception of Li+(C3H5O2)À (Martınez Casado et al, 2009), have not been determined so far, despite their assumed simplicity. Massarotti & Spinolo (1979) determined the unit-cell parameters for three phases of sodium propanoate by powder X-ray diffraction. Cingolani et al (1979) determined the unit-cell parameters for three phases of sodium propanoate and for phases I and II of potassium propanoate by powder X-ray diffraction but not for the lowest-temperature existing phase III of the latter compound. Massarotti & Spinolo (1980) determined the unitcell parameters for phases I and II of potassium propanoate but not for the lowest-temperature existing phase III either. The title structures were determined at 240 K, i.e. in the stability region of the known lowesttemperature phases This means that the temperature regions in which the phases III of K+(C3H5O2)À and Rb+(C3H5O2)À (Cingolani et al, 1979) exist have been measured. Another example of a layered structure where the metal–oxygen sheet is surrounded by hydrophobic organic layers is potassium palmitate KC16H31O2 (Dumbleton & Lomer, 1965)
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