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.
Highlights
The crystal structures of catena-poly[[-aqua-diaqua(3-2-methylpropanoato4O:O,O0:O0)calcium] 2-methylpropanoate dihydrate], {[Ca(C4H7O2)(H2O)3](C4H7O2)Á2H2O}n, (I), catena-poly[[-aqua-diaqua(3-2-methylpropanoato4O:O,O0:O0)strontium] 2-methylpropanoate dihydrate], {[Sr(C4H7O2)(H2O)3](C4H7O2)Á2H2O}n, (II), and catena-poly[[-aqua-diaqua(3-2-methylpropanoato-4O:O,O0:O0)(calcium/strontium)] 2-methylpropanoate dihydrate], {[(Ca,Sr)(C4H7O2)(H2O)3](C4H7O2)Á2H2O}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
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
Summary
A search of the Cambridge Structural Database (Groom et al, 2016; version 5.41 with updates until August 2020) for crystal structures containing solely alkaline earth cations and 2-methylpropanoate (or isobutyrate) anions revealed hexakis[bis(2-2-methylpropanoato)(2-methylpropanoic acid)mag-. E76, 1684–1688 research communications nesium], refcode NAGQUI (Coker et al, 2004) and catenapoly[[triaqua(isobutyrato-kO)magnesium]--isobutyrato2O:O0] monohydrate, refcode VIQTOG (Malaestean et al, 2013) Limited to these two examples, some basic structural features of these compounds can be inferred from other simple carboxylate salts. 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. In the other member of this series, bis(2-acetato-O,O,O0)-tetrakis(2-acetato-O,O0)-
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