Crystal structures of catena-poly[μ-aqua-di­aqua(μ3-2-methyl­propano­ato-κ4O:O,O′:O′)calcium] 2-methyl­propano­ate dihydrate], catena-poly[μ-aqua-di­aqua­(μ3-2-methyl­propano­ato-κ4O:O,O′:O′)strontium] 2-methyl­propano­ate dihydrate] and catena-poly[μ-aqua-di­aqua­(μ3-2-methyl­propano­ato-κ4O:O,O′:O′)(calcium/strontium)] 2-methyl­propano­ate dihydrate]

The crystal structures of catena-poly[μ-aqua-di­aqua­(μ3-2-methyl­propano­ato-κ4O:O,O′:O′)calcium] 2-methyl­propano­ate dihydrate], {[Ca(C4H7O2)(H2O)3](C4H7O2)·2H2O}n, (I), catena-poly[μ-aqua-di­aqua­(μ3-2-methyl­propano­ato-κ4O:O,O′:O′)strontium] 2-methyl­propano­ate dihydrate], {[Sr(C4H7O2)(H2O)3](C4H7O2)·2H2O}n, (II), and catena-poly[μ-aqua-di­aqua­(μ3-2-methyl­propano­ato-κ4O:O,O′:O′)(calcium/strontium)] 2-methyl­propano­ate dihydrate], {[(Ca,Sr)(C4H7O2)(H2O)3](C4H7O2)·2H2O}n, (III), are related. (III) can be considered as an Sr-containing solid solution of (I), with Ca2+ and Sr2+ occupationally disordered in the ratio 0.7936 (16):0.2064 (16). (I)/(III) and (II) 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—H⋯O 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 symmetry-independent 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—H⋯O hydrogen bonds of moderate strength. In (II), the Sr2+ 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 non-coordinating water mol­ecule also shows positional disorder of its hydrogen atom.