Caesium propanoate monohydrate

Caesium propanoate monohydrate is composed of two symmetry-independent Cs+ cations, which are situated on special position 4e, one symmetry-independent propanoate molecule in a general position and a pair of water molecules also situated on special position 4e. Two pairs of these symmetry-independent cations, four propanoate molecules and two pairs of symmetry-independent water molecules form a repeat unit, which gives rise to columns running along the c-axis direction. These columns are held together by intermolecular Ow—H⋯Op (w = water, p = propanoate) hydrogen bonds of moderate strength, and other pairs of moderate intramolecular hydrogen bonds reinforce the cohesion within the columnar unit.


Chemical context
No structure of a simple hydrated alkali propanoate has been determined until now. ('Simple' means a structure where the constituting cation belongs just to one chemical species.) This is in contrast to alkali formates and acetates where water-free alkali salts and complexes with parent acids as well as hydrates are known. These structures show different structural motifs: Some of them are layered, such as lithium acetate dihydrate, LiC 2 H 3 O 2 Á2H 2 O [refcode LIACET06 (Kearley et al., 1996) in the Cambridge Structural Database, version 5.41, update of November 2019 (Groom et al., 2016)], some are columnar including sodium dihydrogen triacetate, NaC 2 H 3 O 2 Á2C 2 H 4 O 2 (NADHAC; Perotti & Tazzoli, 1981) while the cations and anions surround each other in the structure of catena-[bis( 4acetato)tetrakis( 3 -acetato)bis( 2 -acetato)octaaquaoctalithium] (UVELAJ; Martínez Casado et al., 2011).
In the series of carboxylic acids with an increasing number of carbon atoms, it is propionic acid where the hydrophobic properties start to be prominent. Formates and acetates are definitely distinct from propanoates and other carboxylates related to the acids C n H 2n+1 CO 2 where n > 2. This is due to the longer and more voluminous organic chains in the latter compounds, which need space [cf. Duruz & Ubbelohde (1972) and Dumbleton & Lomer, 1965]. Cohesion is provided by van der Waals forces. Occasionally, positional disorder of these groups may take place. Dicalcium barium hexakis(propanoate) (Stadnicka & Glazer, 1980) can serve as an example.
The structural differences between alkali formates and acetates on one hand and simple alkali propanoates such as Li(C 3 H 5 O 2 ) (Martínez Casado et al., 2009) and M(C 3 H 5 O 2 ); M = Na, K, Rb, Cs (Fá bry & Samolová , 2020) on the other reflect the chemical differences between these two groups of compounds. The latter structures are characterized by stacking of layers that are composed of a metal-oxygen bilayer surrounded by hydrophobic layers comprising the ethyl groups. The cohesion forces between the hydrophobic layers hold these structures together. The structure of the chemically related compound Tl(C 3 H 5 O 2 ) (refcode WEWKAM; Martínez Casado et al., 2010) is also a layered structure with three symmetry-independent cations. Mirnaya et al. (1991) pointed out the tendency for various alkanoates to form hydrates. Such a case is reported in this study -see the Synthesis and crystallization section. It is of interest how strikingly different the title structure Cs(C 3 H 5 O 2 )ÁH 2 O is from Cs(C 3 H 5 O 2 ) (Fá bry & Samolová , 2020), despite the chemical similarity.

Structural commentary
The title structure confirms the tendency for various alkanoates to form hydrates, as noted by Mirnaya et al. (1991). Caesium propanoate monohydrate is composed of two symmetry-independent Cs + cations, which are situated on the special position 4e, i.e. on a symmetry plane, one symmetryindependent propanoate molecule in a general position and a pair of water molecules also situated on special position 4e of space group P42 1 m. Two pairs of these symmetry-independent cations, four propanoate molecules and two pairs of symmetry-independent water molecules form a repeat unit.   Packing of the title molecules in the unit cell (DIAMOND; Brandenburg, 2005). Displacement ellipsoids are shown at the 30% probability level: the cations and the O and C atoms are shown in green, red and grey, respectively. H atoms are shown as small light-grey spheres. The covalent bonds are represented by solid lines, Cs-O bonds by dashed black lines and hydrogen bonds by yellow dashed lines.
These units form columns along the c-axis direction (Fig. 1a). The length of the repeat unit along the c axis corresponds to this unit-axis length. Each column has mm2 symmetry ( Fig. 1b). There are four such columns passing through each unit cell (Fig. 2). The columns are interconnected by bifurcated three-centered O w -HÁ Á ÁO p (w = water, p = propanoate) hydrogen bonds (Jeffrey, 1995), whose lengths and angles are quite different, but which are still of moderate strength (Gilli & Gilli, 2009); the donor is O4 and the donated hydrogen is H1o4 (Table 1). Each column thus donates four three-centered bifurcated hydrogen bonds (Jeffrey, 1995) to its neighbours (Fig. 2). There are also intramolecular twocentered O w -HÁ Á ÁO p hydrogen bonds of moderate strength within each column in the structure; the donor is O3 and the donated atom is H1o3.
Cs1 is coordinated by six oxygen atoms (two of them are water O atoms and four are carboxylate O atoms) in a trigonal-prismatic geometry, while Cs2 is in a less regular tetragonal-prismatic coordination environment (by four water and four carboxylate oxygen atoms) (Fig. 2). The bondvalence sums (Brese & O'Keeffe, 1991) of Cs1 and Cs2 are 0.902 (3) and 0.997 (2) v.u., respectively. The fact that the cation with a smaller number of ligands that exhibits a regular coordination environment has a smaller bond-valence sum than that with the larger number of surrounding cations seems to be a peculiarity of the present structure, for example compared to -K 2 SO 4 compounds with two symmetry-independent cations (Fá bry & Pé rez-Mato, 1994). One has eleven ligands while the other has nine. The former has a more irregular coordination compared to the latter and its bondvalence sum is also lower than that of the latter cation. This example is a specific case that has been considered by Brown (1992): The larger coordination number usually results in the formation of a larger cavity around the cation. Stabilization of the cation causes the cation to shift towards some ligand. Such a shift contributes to irregularity of the coordination polyhedron with large numbers of ligands. Despite this stabilization, the bonding of a cation with a high coordination number tends to be lower than that of a cation with a low coordination number.
In contrast to the alkali propanoates, M(C n H 2n+1 COO), the methylene-methylene, methyl-methyl carbon atoms are not in close contact in the title structure. The closest contact C2Á Á ÁC3 vi , i.e. a methylene-methyl contact is 3.961 (4) Å ; symmetry code: (vi) Ày + 1, x, Àz + 1. This is related to the fact that no disorder of the ethyl groups is observed in the studied structure. At the same time, there are elongated voids in the c-axis direction that run parallel through the 4d positions and which are surrounded by the ethyl groups. The radius of the void is 1.381 Å while its height nearly corresponds to the c axis. The voids were calculated and depicted (see the supporting information) using Mercury 4.0 (Macrae et al., 2020).

Synthesis and crystallization
The crystals formed spontaneously in a droplet from dissolved deliquescent crystals of Cs(C 3 H 5 O 2 ) that otherwise have been grown from an aqueous solution of Cs 2 CO 3 with a little excess of propionic acid (Fá bry & Samolová , 2020).
refined under the constraints C-H = 0.96 Å and U iso (H) = 1.5U eq (C). The water hydrogen atoms were also found in the difference electron density maps. The O-H distances were restrained to 0.820 (1) Å with U iso (H) = 1.5U eq (O). When the water H atoms were refined, the O-H water distances converged to values of $0.78 Å . The structure was treated as an inversion twin. The Flack (1983) parameter is 0.03 (3).