Crystal structures of catena-poly[[μ-aqua-di-aqua(μ3-2-methyl-propano-ato-κ4 O: O, O': O')calcium] 2-methyl-propano-ate dihydrate], catena-poly[[μ-aqua-di-aqua-(μ3-2-methyl-propano-ato-κ4 O: O, O': O')strontium] 2-methyl-propano-ate dihydrate] and catena-poly[[μ-aqua-di-aqua-(μ3-2-methyl-propano-ato-κ4 O: O, O': O')(calcium/strontium)] 2-methyl-propano-ate dihydrate]

Abstract

The crystal structures of catena-poly[[μ-aqua-di-aqua-(μ₃-2-methyl-propano-ato-κ⁴ O:O,O':O')calcium] 2-methyl-propano-ate dihydrate], {[Ca(C₄H₇O₂)(H₂O)₃](C₄H₇O₂)·2H₂O} _n , (I), catena-poly[[μ-aqua-di-aqua-(μ₃-2-methyl-propano-ato-κ⁴ O:O,O':O')strontium] 2-methyl-propano-ate dihydrate], {[Sr(C₄H₇O₂)(H₂O)₃](C₄H₇O₂)·2H₂O} _n , (II), and catena-poly[[μ-aqua-di-aqua-(μ₃-2-methyl-propano-ato-κ⁴ O:O,O':O')(calcium/strontium)] 2-methyl-propano-ate 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 hydro-philic sheets containing the cations, carboxyl-ate groups as well as water mol-ecules. The hydro-phobic layers, which consist of 2-methyl-propano-ate chains, surround the hydro-philic 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 inter-actions in which just one symmetry-independent 2-methyl-propano-ate anion is included, together with three water mol-ecules. These mol-ecules participate in an irregular coordination polyhedron composed of eight O atoms around the cation. Additional water mol-ecules as well as the second 2-methyl-propano-ate anion are not part of the coordination sphere. These mol-ecules 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 mol-ecules and both anions are situated on a mirror plane with a concomitant positional disorder of the 2-methyl-propyl groups; the non-coordinating water mol-ecule also shows positional disorder of its hydrogen atom.

Keywords: carboxyl­ates; hydrogen bonding; layered crystal structure; solid solution; the Cambridge Structural Database.

Figure 1

(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).

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.

Figure 3

View of the hydrogen-bonded substructures in (a) (II) and (b) (III). The symmetry codes correspond to those given in Tables 5 ▸ and 6 ▸, respectively. Colour code is as in Fig. 1 ▸.