The title complex, [CaCu(C(5)H(6)O(4))(2)(H(2)O)(2)](n), is the first heterobimetallic complex based on a substituted malonate dianion. The Cu(II) cation and two independent 2,2-dimethylmalonate (or 2,2-dimethylpropanedioate) dianions build up a robust dianionic [Cu(C(5)H(6)O(4))(2)](2-) complex, which acts as a building block to coordinate to four Ca(2+) cations. Each Cu(II) centre is in a four-coordinate square plane of dimethylmalonate O atoms, while each Ca(II) atom is in an eight-coordinate distorted bicapped trigonal-prismatic environment of six O atoms from four different dimethylmalonate groups and two water molecules. This arrangement creates a two-dimensional layer connectivity of the structure. The dianionic [Cu(C(5)H(6)O(4))(2)](2-) units are involved in different intermolecular hydrogen-bonding interactions with water molecules via the formation of hydrogen-bonded rings of graph sets R(1)(2)(8) and R(6) within this layer. The crystal was nonmerohedrally twinned by rotation about [011] with a major twin volume fraction of 0.513 (3).
The coordination mode of the dimethylmalonate ligand in the two title Cu(II) complexes, {[Cu(C(5)H(3)O(4))(H(2)O)]·H(2)O}(n), (I), and [Cu(C(5)H(3)O(4))(H(2)O)](n), (II), is the same, with chelated six-membered, bis-monodentate and bridging bonding modes. However, the coordination environment of the Cu(II) atoms, the connectivity of their metal-organic frameworks and their hydrogen-bonding interactions are different. Complex (I) has a perfect square-pyramidal Cu(II) environment with the aqua ligand in the apical position, and only one type of square grid consisting of Cu(II) atoms linked via carboxylate bridges to three dimethylmalonate ligands, with weak hydrogen-bond interactions within and between its two-dimensional layers. Complex (II) has a coordination geometry that is closer to square pyramidal than trigonal bipyramidal for its Cu(II) atoms with the aqua ligand now in the basal plane. Its two-dimensional layer structure comprises two alternating grids, which involve two and four different dimethylmalonate anions, respectively. There are strong hydrogen bonds only within its layers.
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