Crystal structures of catena-poly[[μ-aqua-diaqua(μ3-2-methylpropanoato-κ4 O:O,O′:O′)calcium] 2-methylpropanoate dihydrate], catena-poly[[μ-aqua-diaqua(μ3-2-methylpropanoato-κ4 O:O,O′:O′)strontium] 2-methylpropanoate dihydrate] and catena-poly[[μ-aqua-diaqua(μ3-2-methylpropanoato-κ4 O:O,O′:O′)(calcium/strontium)] 2-methylpropanoate dihydrate]

The crystal structures of [Ca(C4H7O2)(H2O)3](C4H7O2)·2H2O, space group Pbca, (I), and [Sr(C4H7O2)(H2O)3](C4H7O2)·2H2O space group Cmce (II) are homeotypic; [(Ca,Sr)(C4H7O2)(H2O)3](C4H7O2)·2H2O, space group Pbca, (III), is an Sr-containing solid solution of (I) with Ca2+ and Sr2+ occupationally disordered in the ratio 0.794 (2):0.206 (2).

are homeotypic with different space groups of Pbca and Cmce, respectively. All the title crystal structures are composed of hydrophilic sheets containing the cations, carboxylate groups as well as water molecules. The hydrophobic layers, which consist of 2-methylpropanoate chains, surround the hydrophilic sheets from both sides, thus forming a sandwich-like structure extending parallel to (001). The cohesion forces within these sheets are the cation-oxygen bonds and O-HÁ Á ÁO hydrogen bonds of moderate strength. Stacking of these sandwiches along [001] is consolidated by van der Waals forces. The structures contain columns defined by the cation-oxygen interactions in which just one symmetryindependent 2-methylpropanoate anion is included, together with three water molecules. These molecules participate in an irregular coordination polyhedron composed of eight O atoms around the cation. Additional water molecules as well as the second 2-methylpropanoate anion are not part of the coordination sphere. These molecules are connected to the above-mentioned columns by O-HÁ Á ÁO hydrogen bonds of moderate strength. In (II), the Sr 2+ cation, two of the coordinating water molecules and both anions are situated on a mirror plane with a concomitant positional disorder of the 2-methylpropyl groups; the noncoordinating water molecule also shows positional disorder of its hydrogen atom.
In the crystal structure of catena-[tetrakis( 2 -formato)tetraaquadimagnesium], MGFORD03 (Coker et al., 2004), no hydrophobic organic chain is present. In the other member of this series, bis bis(acetic acid)diaquatrimagnesium acetic acid solvate, NAGQOC [Coker et al. (2004), see also the redetermination of this structure by Scheurell et al. (2012), NAGQOC02], there are sheets within the structure separated into hydrophilic parts (composed of the cations and oxygen atoms) and hydrophobic parts (composed of methyl groups). The remaining free acetic acid molecules are bound by O acetic -HÁ Á ÁO hydrogen bonds between the layers. NAGQUI is an example of a structure where the hydrophilic part is surrounded by a hydrophobic layer. The same holds for hexakis[bis( 2 -3,3-dimethylbutanato)(3,3-dimethylbutanoic acid)magnesium], NAGRET (Coker et al., 2004), as well as for bis(pivalato)tetrakis(pivalic acid)magnesium, VAMCUI01 [Coker et al. (2004), see also VAMCUI determined by Troyanov et al. (2002)]. Thus, the longer the organic chain, the more important the van der Waals forces become for molecular cohesion in structures with carboxylate anions. The different cohesion forces in the hydrophilic and the hydrophobic parts are the reason for the formation of layer-like structures or structures where an organic part completely surrounds a hydrophilic metaloxygen sheet or a hydrophilic cluster. Likewise, the longer the hydrophobic chains, the larger is the probability of inclusion of non-coordinating water molecules into the structure because the latter can provide binding bridges between the carboxylate anions, which would otherwise be isolated. Such a situation is realised in VIQTOG where the water molecules complete a column substructure that is defined by the cation-oxygen bonds stemming from the carboxylate groups and water molecules. The growing complexity of water substructures with a growing number of carbon atoms in carboxylate anions has also been observed in the salts of the first five dicarboxylic acids with 4,6-diaminopyrimidine (Matulková et al., 2017).
The present study was undertaken to prepare dicalcium strontium hexakis(2-methylpropanoate) with the intention that the resulting crystal structure might be related to dicalcium strontium hexakis(propionate) (CASRPP06; Mishima, 1984), which exhibits interesting structural and physical properties (e.g. Itoh, 1992). However, the synthesis attempt resulted in one of the title structures, catena-poly [[-aquadiaqua( 3 -2-methylpropanoato-4 (III). We then also prepared the pure Ca and Sr compounds, i.e. (I) and (II), the crystal structures of which are also reported here.

Structural commentary
The structures have the same features and are composed of the respective cation, two carboxylate molecules and additional water molecules. One of the carboxylate anions and three water molecules coordinate to the cation, the remaining molecules form a substructure interconnected by hydrogen bonds only. Compound (III) is an Sr-containing solid solution of (I), and the two structures are crystal-chemically isotypic. The refined ratio of the occupationally disordered cation site is Ca:Sr = 0.7936 (16):0.2064 (16). The crystal structures of (I)/ (III) and (II) are homeotypic (Lima-de-Faria et al., 1990), with similar lattice parameters and crystal-chemical features, but different space-group types.
There are three main cohesion forces present in the title structures: The first cohesion force regards the cation-oxygen interactions. For each of the crystal structures, there are eight oxygen atoms in the coordination sphere, defined by one carboxylate molecule in a bidentate bridging mode. (In VIQTOG there are two carboxylate anions coordinating in a monodentate mode and bridging to other Mg 2+ cations.) In the title structures, the cation-coordinating atoms are symmetryequivalent atoms O1 in (II), and O1 and O2 in (I) and (III), respectively. Other coordinating O atoms are the water O atoms O2, O3 and O4 in (II), and the water O atoms O3, O4 and O5 in (I) and (III). [The Sr 2+ cation in (II) is located on a mirror plane (Wyckoff position 8f)]. Numerical values of the cation-oxygen bonds are listed in Tables 1, 2 and 3 for structures (I), (II) and (III), respectively. The coordination polyhedra form columns oriented parallel to the a axis. Because of the similarity of the three structures, (III) was chosen as a representative (Fig. 1a,b).
The second type of a cohesion force in the title structures originates from O-HÁ Á ÁO hydrogen bonds of moderate strength (Gilli & Gilli, 2009) that link the above mentioned columns into hydrophilic sheets parallel to (001) (Fig. 2a,b). Within a sheet, the coordinating water molecules are solely engaged as donor groups whereas the non-coordinating water molecules (Ow1 and Ow2 in (I) and (III), and Ow1 in (II)) have the functions both as donor and acceptor groups. The carboxylate acceptor atoms O6 and O7 in the structure of (I) and (III) and the pair of equivalent atoms O5 (x, y, z and 1 À x, y, z) in the structure of (II) stem from the second, noncoordinating carboxylate anion. Each of these carboxylate oxygen atoms is an acceptor of three hydrogen bonds that are donated by two coordinating and by one non-coordinating water molecules. Numerical values of these interactions are collated in Tables 4, 5 and 6 for structures (I), (II) and (III)), respectively. Fig. 3a,b depict the hydrogen-bonded substructures in (II) and (III). The graph-set motifs are R 5 5 (10) (  Symmetry codes: (i) Àx þ 3 2 ; y; Àz þ 1 2 ; (ii) x À 1 2 ; y; Àz þ 1 2 ; (iii) Àx þ 1; y; z. Symmetry codes: (i) x þ 1 2 ; y; Àz þ 1 2 ; (ii) x À 1 2 ; y; Àz þ 1 2 .

Figure 2
View of the unit-cell content of (a) (II) and (b) (III). Hydrogen bonds are shown as yellow dashed lines; colour code as in Fig. 1. The substructures with the hydrophilic sheets and hydrogen-bonded system, which connects the columns and water molecules, are clearly discernible from the hydrophobic part of the structure composed of 2-methylethyl chains. (a) View of the columns along the a axis in the crystal structure of (III). The columns depicted are formed by (Ca1/Sr1) (green) and O atoms (red); the latter are also depicted with bonds to carbon C atoms (grey) and H atoms (light-grey spheres of arbitrary radius). Displacement ellipsoids are shown at the 30% probability level. (b) Perspective view of the columns in (III).

Synthesis and crystallization
For (III), two molar equivalents of CaCO 3 and one molar equivalent of SrCO 3 were neutralized by six molar equivalents of 2-methylpropionic acid (using 0.76 g of CaCO 3 , 0.56 g of SrCO 3 and about 2.50 g of 2-methylpropionic acid). The solution was heated at 343 K, an excess of the acid was then added until the pH was between 5 and 6. The solution was filtered and then heated at 313 K until needle-like colourless crystals appeared. The pure Ca compound, (I), and the pure Sr compound, (II), were prepared for the sake of comparison. 0.85 g of CaCO 3 were neutralized by 1.5 g of 2-methylpropionic acid and 1.26 g of SrCO 3 were neutralized by 1.5 g of 2-methylpropionic acid, respectively; in each case these values correspond to the molar ratio of 1:2. The solutions were heated at 343 K, an excess of the acid was then added until the pH was between 5 and 6. The solutions were filtered and then heated at 313 K until needle-like colourless crystals appeared.
We have also tried to prepare magnesium 2-methylpropanoate and barium 2-methylpropanoate in a similar way as for (I)-(III). However, it turned out that the obtained crystals of the former compound correspond to VIQTOG, while the crystal structure of the latter compound is modulated and is being solved at present. Provided that we obtain a satisfactory model, the results will be published elsewhere.

Structure determination and refinement
Crystal data, data collection and structure refinement details are summarized in Table 7. In all structures, the methanetriyl hydrogen atoms were placed in calculated positions and refined with C methanetriyl -H methanetriyl = 1.00 Å , U iso (H methanetriyl ) = 1.2U eq (C methanetriyl ). Methyl hydrogen atoms were discernible in difference electron-density maps and were refined with C methyl -H methyl = 0.98 Å , U iso (H methyl ) = 1.5U eq (C methyl ). Finally, difference electron density maps revealed the water hydrogen atoms, which were refined with restraints of O water -H water = 0.840 (1) Å .