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]

Abstract

The crystal structures of catena-poly[[μ-aqua-diaqua(μ3-2-methylpropanoato-κ4 O:O,O′:O′)calcium] 2-methylpropanoate dihydrate], {[Ca(C4H7O2)(H2O)3](C4H7O2)·2H2O} n , (I), catena-poly[[μ-aqua-diaqua(μ3-2-methylpropanoato-κ4 O:O,O′:O′)strontium] 2-methylpropanoate dihydrate], {[Sr(C4H7O2)(H2O)3](C4H7O2)·2H2O} n , (II), and catena-poly[[μ-aqua-diaqua(μ3-2-methylpropanoato-κ4 O: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.