Poly[tetraaqua-di-μ4-malonato-barium(II)cadmium(II)]

In the title complex, [BaCd(C3H2O4)2(H2O)4]n, the BaII atoms, located on crystallographic twofold axes, adopt slightly distorted square-antiprismatic coordination geometries, while the CdII atoms, which lie on crystallographic centres of symmetry, have a distorted octahedral coordination. Each malonate dianion binds two different CdII atoms and two different BaII atoms. This connectivity generates alternating layers along [100] in the structure, with one type containing CdII cations and malonate dianions, while the other is primarily composed of BaII ions and coordinated water molecules. The water molecules also participate in extensive O—H⋯O hydrogen bonding.

In the title complex, [BaCd(C 3 H 2 O 4 ) 2 (H 2 O) 4 ] n , the Ba II atoms, located on crystallographic twofold axes, adopt slightly distorted square-antiprismatic coordination geometries, while the Cd II atoms, which lie on crystallographic centres of symmetry, have a distorted octahedral coordination. Each malonate dianion binds two different Cd II atoms and two different Ba II atoms. This connectivity generates alternating layers along [100] in the structure, with one type containing Cd II cations and malonate dianions, while the other is primarily composed of Ba II ions and coordinated water molecules. The water molecules also participate in extensive O-HÁ Á ÁO hydrogen bonding.
We thank Tianjin Polytechnic University for financial support.

Comment
The malonate dianion, with two neighboring carboxylate groups, is a very flexible ligand. Its basic coordination mode is as a chelate via two distal carboxylate oxygen atoms to form a six-membered ring and the coordinating ability of the nonchelating oxygen atoms makes the formation of polymeric networks possible (Djeghri et al., 2005;Guo & Guo, 2006). On the other hand, malonate can also coordinate in monodentate, chelated bidentate and bridging modes to create various molecular architectures (Delgado et al., 2004). Herein, we report the structure of the title heterobimetallic malonate complex, (I). It and the chemically similar complex poly[tetraaqua-di-mu4-malonato-barium(II)zinc(II)] (Guo & Guo, 2006) are isotypic.
The asymmetric unit in the structure of (I) comprises half a Ba II cation, half a Cd II cation, a complete malonate dianion defined by C1-C3/O1-O4 and two independent water molecules involving O5 and O6. Fig. 1 shows a symmetry-expanded view which displays the full coordination of the Ba 2+ and Cd 2+ centers. Selected geometric parameters are given in Table 1.
The Ba 2+ cation, lying on a crystallographic twofold axis, is eight-coordinate, bonded to oxygen atoms of four different malonate groups and four water molecules with Ba-O distances ranging from 2.793 (9) to 2.878 (10) Å. The Ba polyhedra may be described as slightly distorted square antiprisms. They share edges to form chains propagating along c.
The Cd 2+ cations, lie on crystallographic centres of symmetry, and have distorted octahedral coordination, with O2 and O3 of two bidentate malonate anions at the equatorial sites and two O1 atoms from two other malonate anions at the apical sites.
Also evident in Fig. 1 is the variability of the coordination modes of the malonate dianion with monodentate (O1), bidentate chelating (O2 and O3) and bridging (O4) bonding modes all present.
The structure as a whole consists of two distinct types of layer, both parallel to (100) and stacked alternately in the direction of a. The first of these ( Fig. 2) is composed entirely of Cd II ions and malonate dianions and occurs at x = 0 and 1/2. The other type of layer, type 2, alternating with the first and centred on x = 1/4 and 3/4 contains, primarily, the Ba ions and the water molecules. Two forms of connectivity occur within the type 2 layers. First of all O4 atoms on the surfaces of the type 1 layers create chains of edge sharing Ba polyhedra propagating along c and at the same time link the two types of layer and complete the three-dimensional connectivity of the structure. The interlayer connectivity is further enhanced by the hydrogen bonds of the form O5-H5A···O2 iv and O6-H6A···O1 vi given in Table 2.
After filtration, slow evaporation over a period of a week at room temperature provided colorless plate-like crystals of (I).