Layered alkali propanoates M +(C2H5COO)−; M + = Na+, K+, Rb+, Cs+

The structures of four alkali propionates, M +(C2H5COO)−; M + = Na+, K+, Rb+, Cs+, have been determined. All of them show close structural similarity, which is manifested by the coordination of the cation by six oxygen atoms in a chessboard motif, forming a bilayer. This bilayer is situated between hydrophobic layers composed of dangling ethyl chains. The structures are built up by stacking of these sandwiches.

The title alkali propanoates poly[( 5 -propanoato)alkali(I)], M + (C 2 H 5 COO) 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. This bilayer is situated between hydrophobic layers composed of dangling ethyl chains from the carboxylate 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 carboxylate anion, while each of the other four oxygen atoms belongs to different carboxylate groups, which are in a bridging monodentate mode. Despite the close similarity, each of the studied alkali propanoates crystallizes in a different space group. The atoms are in general positions, except for the cation in K + (C 2 H 5 COO) À , 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 + propanoates, in contrast to the Rb + and Cs + propanoates. 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 + propanoate, a disorder of the methyl H atoms is observed.

Chemical context
The structures of the alkali propanoates, M + (C 3 H 5 O 2 ) À , with exception of Li + (C 3 H 5 O 2 ) À (Martínez Casado et al., 2009), have not been determined so far, despite their assumed simplicity. The structure of the chemically related compound Tl + (C 3 H 5 O 2 ) À was determined by Martínez Casado et al. (2010).
On the other hand, the physical properties of some alkali propanoates, together with related alkanoates, have been studied. Phase transitions were studied in alkali propanoates ISSN 2056-9890 together with alkali formates, acetates and butyrates by Ferloni et al. (1975) employing differential scanning calorimetry. The lowest-temperature phase transitions in Li, Na, K, Rb and Cs propanoates take place at 533, 470, 258 (AE2), 317 (AE2) and 314 K, respectively. Cingolani et al. (1979) determined the phase-transition temperatures in Li + (C 3 H 5 O 2 ) À , Na + (C 3 H 5 O 2 ) À and K + (C 3 H 5 O 2 ) À by conductometric measurements. The determined phase-transition temperatures corresponded well with those reported by Ferloni et al. (1975), except for Li + (C 3 H 5 O 2 ) À where the phase transition was detected at 553 K. Martínez Casado et al. (2009) determined the phasetransition temperature for the Li compound at 549.1 (AE0.7) K in the virgin sample. The temperature of this phase transition varied during repeated cooling and heating.
The unit-cell parameters of the title structures have been determined in the past. In addition, Martínez Casado et al. (2009) determined the unit-cell parameters of lithium propanoate by single-crystal X-ray diffraction at 100, 160 and 298 K. Massarotti & Spinolo (1979) determined the unit-cell parameters for three phases of sodium propanoate by powder X-ray diffraction. Entry No. 00-042-1901 in the powder diffraction file (PDF-4; Gates-Rector & Blanton, 2019) is derived from the latter powder data collection at 298 K. 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. Entries No. 00-042-1856-00-042-1859 in PDF-4 (Gates-Rector & Blanton, 2019) are derived from the data collection of the latter authors. Massarotti & Spinolo (1980) also determined two phases of Rb propanoate by powder X-ray diffraction above 317 K, but not phase III existing below this temperature.
No match regarding caesium propanoate has been found in the Cambridge Structural Database (Groom et al., 2016; version 5.41 from November 2019); however, there is an entry (No. 00-049-2031 in PDF-4;Gates-Rector & Blanton, 2019) that is attributed to this compound. The corresponding unitcell volume V = 1355.38 Å 3 is close to that observed recently at room temperature in Cs + (C 2 H 5 COO) À ÁH 2 O with V = 1334.25 (4) Å 3 (Samolová & Fá bry, 2020). Therefore, it can not be excluded that the reported phase in PDF-4 is in fact a hydrate. It should be emphasized that for each particular compound, their reported unit-cell parameters correspond to each other while multiplication of the unit-cell volume takes place in some cases.
Pretransitional phenomena have been observed in some of the title structures, which indicates a complicated structural rearrangement taking place before melting [see also the study of Li + (C 3 H 5 O 2 ) À , Na + (C 3 H 5 O 2 ) À and K + (C 3 H 5 O 2 ) À by Cingolani et al. (1979), and the study of Li + (C 3 H 5 O 2 ) À by Martínez Casado et al. (2009)]. Such phenomena are more prominent in the structures with longer hydrophobic chains, e.g. in butyrates (Duruz & Ubbelohde, 1972).
It should be mentioned that the crystals in the current study were cooled down instantly from room temperature to 240 K by putting them into a stream of a cooling gas. On the other hand, the measurement was carried out at temperatures not far from the thermodynamic equilibrium in which the roomtemperature-grown crystals are assumed to exist. Our experience has shown that cooling down the crystals to very low temperatures does not necessarily mean a better resolution or better quality of the measured data.
An important structural feature of alkali alkanoates M + C n H 2n+1 COO À (n > 2) seems to be their layered arrangement. For example, a layered structure has been observed in Li(C 3 H 5 O 2 ) (Martínez Casado et al., 2009) as well as in Li 2 Cd(C 2 H 5 COO) 4 (Griffith & Amma, 1992), despite the fact that Li + is coordinated by four oxygen atoms in contrast to the six oxygens in the title structures. On the other hand, the layered structure of Tl(C 3 H 5 O 2 ) (Martínez Casado et al., 2010) is more complicated because it contains three independent Tl + cations. Two of them (Tl1 and Tl3) are situated in a similar coordination to that in the title structures while Tl2 is situated in a roughly octahedral coordination. The presence of more than one symmetry-independent Tl + cation even in simple structures is quite common. This is the case, for example, in a high-temperature phase of Tl 2 MoO 4 (Friese et al., 1999) or in Tl 2 WO 4 (Okada et al., 1979) where three unique cations are present. Another example of a layered structure where the metal-oxygen sheet is surrounded by hydrophobic organic layers is potassium palmitate KC 16 H 31 O 2 (Dumbleton & Lomer, 1965).  (Brese & O'Keeffe, 1991) of the cations are 1.062 (2), 1.156 (3), 1.109 (5) and 1.042 (4) valence units for the Na, K, Rb, Cs compounds, respectively. The motifs shown in Fig. 1a-d are quite similar to those observed in potassium acrylate and potassium methacrylate (Heyman et al., 2020) while the unitcell parameters of the latter compounds also show a close correspondence to those of the title structures.

Structural commentary
The common prominent feature of the title structures is the presence of an oxygen-metal bilayer that is sandwiched by ethyl chains. The layers are aligned parallel to (001), and packing of these layers forms the title structures. The structural motifs in all of the title structures are quite similar, and the structures can be considered as homeotypic (Lima de Faria et al., 1989). Because of their similarity, overall packing views are given only for the potassium and rubidium propanoates ( Fig. 2a,b) because they represent the structures with positionally disordered and ordered methyl groups, respectively. The unit-cell parameters of the depicted structures are of similar size in contrast to the other structures.
The positional disorder observed in the Na and K propanoates (not in Rb and Cs propanoates) is worth being discussed in detail. Table 1 lists the distances between neigh-bouring carbon atoms of the methylene and methyl groups. In Rb + (C 3 H 5 O 2 ) À and Cs + (C 3 H 5 O 2 ) À , these distances are larger than in Na + (C 3 H 5 O 2 ) À and K + (C 3 H 5 O 2 ) À . This means that shorter distances between the methyl groups seem to be correlated with the observed positional disorder of the methyl groups. The disordered methyl groups are situated in rows, which are aligned parallel to the b axis in Na + (C 3 H 5 O 2 ) À and K + (C 3 H 5 O 2 ) À . However, the assumed switching by rotation from one disordered position to another should also affect neighbouring rows in the ab plane. A correlated ordering of the ethyl groups is thus expected to take place. This situation is analogous to that observed in BaCa 2 (C 3 H 5 O 2 ) 6 where the methyl carbon atoms get as close as 4.05 (2) Å (Stadnicka & Glazer, 1980). Table 2 shows that in the case of Na + (C 3 H 5 O 2 ) À and K + (C 3 H 5 O 2 ) À , the positional disorder can bring these groups even as close as 2.609 (8) and 2.651 (9) Å , respectively, a value that clearly indicates the impossibility of simultaneous occupation of these sites by both groups. This short value also indicates the presence of thermal fluctuations. These fluctuations would provoke revolution of the ethyl chain to the other, i.e. the disordered site, while causing a domino effect by forcing the other ethyl chains to revolve in order to remove as much repulsion as possible. The torsion angles O1/O2-C1-C2-C3, which are listed in Table 2, also throw some light on the observed disorder. They are close to 0 or 180 for the disordered Na and K title compounds in contrast to the ordered Rb and Cs title compounds. The disordered methyl groups are situated in energetically similar or even identical positions in the Na and K compounds, respectively, in contrast to the the Rb and Cs compounds.
In the studied crystal of Na + (C 3 H 5 O 2 ) À , the refined occupational parameters of the disordered methyl group converged to 0.808 (4) for one and 0.192 (4) for the other orientation. A hypothetical structure of Na(C 3 H 5 O 2 ) where no positional disorder occurs would be described in a unit cell with a halved unit-cell parameter a relative to the title structure. The space group of such a hypothetical structure would be P2 1 instead of P2 1 /a. The common structural motifs in the title structures: (a) Na Displacement ellipsoids are shown at the 30% probability level. The cations, O, C and H atoms are shown as green, red, grey ellipsoids and as tiny light-grey spheres, respectively.

Figure 2
The crystal packing of (a) K + (C 3 H 5 O 2 )-and (b) Rb + (C 3 H 5 O 2 ) À . K or Rb, O and C atoms are shown as green, red and grey spheres, respectively. H atoms are omitted for clarity. unit cell can also be carried out according to equation (1) in section 4]. Halving of the unit-cell parameter a would also be caused by a positional disorder in the ratio 0.50:0.50, provided that the blocks with the ordered molecules are sufficiently small. The space-group type of such a hypothetical structure would be P2 1 /m, which is equal to that of the reported structure of K(C 3 H 5 O 2 ). The unit-cell parameter a of the title structure Na(C 3 H 5 O 2 ) can be halved and transformed into the one that was reported by Massarotti & Spinolo (1979) or Cingolani et al. (1979) for phase III, the known lowesttemperature existing phase (determined by a powder diffraction study). In other words, it seems that the occupational parameters of the disordered ethyl groups can vary in different crystals of Na + (C 3 H 5 O 2 ) À . More probably, because of the repulsion of the methyl groups, the phases, which had been subjected to powder diffraction experiments, rather correspond to the structures described in the space groups P2 1 /m.

Synthesis and crystallization
The title compounds were prepared by dissolution of the pertinent alkali carbonates with propionic acid in the respective molar ratio of 1:2 in water. The pH of the solution was adjusted to 6-7 by addition of propionic acid. The solutions were filtered and the excessive amount of water was evaporated at 313 K. Prior to crystallization, which started on the surface of the solution, a more viscous layer seemed to develop. This layer was optically isotropic (no extinction in polarized light), in agreement with the observations for Li(C 3 H 5 O 2 ), Na(C 3 H 5 O 2 ), and K(C 3 H 5 O 2 ) (Cingolani et al., 1979). During the course of the concentration of the solution, crystals also grew at the bottom of the beaker.
For the preparation of Na(C 3 H 5 O 2 ), 1.49 g of Na 2 CO 3 and 1.04 g of propionic acid were used before adjustment of the pH to 6-7 by propionic acid; for the preparation of K(C 3 H 5 O 2 ), 1.49 g of K 2 CO 3 Á1.5H 2 O and 0.67 g of propionic acid were used before adjustment of the pH to 6-7 by propionic acid; for the preparation of Rb(C 3 H 5 O 2 ), 1.50 g of Rb 2 CO 3 and 0.48 g of propionic acid were used before adjustment of the pH to 6-7 by propionic acid; for the  Table 2 Torsional angles ( ) for the propanoate fragments in M + (C 2 H 5 COO) À ; M + = Na + , K + , Rb + , Cs + .
preparation of Cs(C 3 H 5 O 2 ), 1.50 g of Cs 2 CO 3 and 0.34 g of propionic acid were used before adjustment of the pH to 6-7 by propionic acid. All of the title compounds are hygroscopic. Crystals of Cs(C 3 H 5 O 2 ) turned out to be deliquescent, and from the resulting solution the monohydrate Cs(C 2 H 5 COO)ÁH 2 O crystallized after some time (Samolová & Fá bry, 2020). Rb(C 2 H 5 COO) also turned out to be deliquescent. K(C 2 H 5 COO) was hygroscopic and the hygroscopicity of Na(C 2 H 5 COO) (Massarotti & Spinolo, 1979) was confirmed as well.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Methyl hydrogen atoms were constrained: C methyl -H methyl = 0.96 Å while U iso (H methyl ) = 1.5U eq (C methyl ). Attached methylene hydrogen atoms were situated at calculated positions and refined under the constraints C methylene -H methylene = 0.97 Å and U iso (H methylene ) = 1.2U eq (C methylene ). Na(C 3 H 5 O 2 ): It turned out that the ethyl groups are disordered over two positions. The occupational parameters of the methyl groups were refined under the constraint that their sum equal unity, resulting in a 0.808 (4): 0.192 (4) ratio for the methyl groups C3 and C3a. The large unit cell can be transformed into the small unit cell that corresponds to that of K(C 3 H 5 O 2 ) by the transformation [a, b, c] small = [a, b, c] large [1/2 0 1/2 / 0 1 0 / 0 0 1] [equation (1)]. K(C 3 H 5 O 2 ): The ethyl groups are disordered over two positions due to the crystal symmetry, with occupancies equal to 1/2. The positions of the methyl hydrogens were discerned from the difference electron-density map. Rb(C 3 H 5 O 2 ): The positions of the methyl hydrogen atoms were discerned from the difference electron-density map. Cs(C 3 H 5 O 2 ): The methyl hydrogen atoms are equally disordered over two positions. The refined value of the Flack parameter [0.10 (9)] and its standard uncertainty did not enable the absolute structure to be determined reliably (Flack & Bernardinelli, 2000).

Poly[(µ 5 -propanoato)sodium(I)] (I)
Crystal data  . The positions of the methyl hydrogens were discerned in the difference electron density map. The methyl hydrogens were constrained: Cmethyl-Hmethyl = 0.96?Å while Uiso(Hmethyl) = 1.5Ueq(Cmethyl). The methylene carbons were considered to be superimposed possessing the same positional as well as anisotropic displacement parameters with the overall occupational parameter equal to 1. The distances C2-C3 and C2-C3a were restrained to be equal. The attached methylene hydrogens were situated into the calculated positions with the corresponding occupational parameters and refined under the constraints Cmethylene-Hmethylene = 0.97?Å and Uiso(Hmethylene) = 1.2Ueq(Cmethylene). Their occupancies equalled to those of the occupanices of the pertinent methyl.