Layered alkali propano-ates M +(C2H5COO)-; M + = Na+, K+, Rb+, Cs

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.

Keywords: carboxyl­ates; crystal structure; hydrogen bonding; occupational disorder; positional disorder; the Cambridge Structural Database.