Poly[tetra-μ-aqua-hexa­aquadi-μ₃-malon­ato-dinitratodibarium(II)nickel(II)]

There has been considerable interest in the design and synthesis of coordination polymers based on assemblies of transition metal ions and α,ω-dicarboxylate ligands, because carboxylates can give rise to different coordination modes with metal ions in the construction of coordination polymers with specific architectures (Rodriguez-Martin et al., 2002), and there is increaseing recognition of the role of metals in biological systems (Maji et al., 2003). The malonate ligand, with two neighbouring carboxylate groups, is very flexible. Its basic coordination mode is as a chelate via two distal carboxylate O atoms to form a six-membered ring, and the coordinating ability of the nonchelating O atoms makes the formation of polymeric networks possible (Djeghri et al., 2005). With its versatile coordinating ability as a monodentate, chelated bidentate or bridging ligand, malonate can create various patterns (Delgado et al., 2004; Gil de Muro et al., 2000; Djeghri et al., 2006; Wang et al., 2004). Our interest in heterobimetallic malonate complexes involving transition and alkaline-earth metals stems from their potential application in molecular-based magnetism, catalysis, supramolecular chemistry and materials science (Gil de Muro et al., 1998, 2004). We have recently reported the crystal structure of poly[tetraaquo-di-µ₄-malonato-barium(II)zinc(II)] (Guo & Guo, 2006). Interestingly, when nickel chloride was used, we expected a structure similar to or isotypic with that of the zinc compound. Instead, a completely different crystal structure was obtained. Here, we report the structure of the title bimetallic malonate complex, (I).

The asymmetric unit in the structure of (I) comprises half an Ni atom, one Ba atom, one nitrate anion, one complete malonate dianion and five nonequivalent water molecules, and is shown in Fig. 1 in a symmetry-expanded view, which displays the full coordination geometry of the Ba and Ni atoms. Selected geometric parameters are given in Table 1.

The Ni atom, lying on a crystallographic centre of symmetry, is octahedrally coordinated, with atoms O1 and O3 of two malonate anions in a planar arrangement [Ni—O 2.047 (2) and 2.021 (2) Å] and two O8 atoms [Ni—O 2.098 (2) Å] from two water molecules forming the opposing apices of the octahedron. The Ni—O_water bonds are slightly longer, and the Ni—O_malonate bonds are somewhat shorter, than those in the [CaNi(mal)₂(H₂O)₄]·2H₂O complex (Gil de Muro et al., 2000). All the cis O—Ni—O bond angles are close to 90° [in the range 86.77 (10)–93.23 (10)°] and, because of the site symmetry of Ni, all the trans angles are exactly 180°. Thus, the coordination octahedra of the Ni atoms can be visualized as having a slightly elongated axial distortion.

In the present structure, the variability of the malonate ligand can be clearly seen (Fig. 1). Monodentate, bidentate-chelating, chelated six-membered and bridging bonding modes are all present. Atom O1 of the carboxylate group O1/C1/O2 adopts a monodentate mode. Atoms O3 and O4 of the carboxylate group O3/C3/O4 have both a bidentate 1,2-chelating mode to connect with the Ba atom and a bridging bonding mode, atom O3 being coordinated to one Ni atom and one Ba atom, while atom O4 is connected to two Ba atoms. Furthermore, the whole molecule chelates the Ni atom to form a six-membered ring. The resulting six-membered chelate ring Ni/O1/C1/C2/C3/O3 has a boat conformation, with atoms Ni and C2 lying 0.4769 and 0.4176 Å, respectively, out of the O1/C1/C3/O3 mean plane. The O—C—O angle for the monodentate carboxylate group is 123.2 (3)°, slightly larger than the value of 121.9 (3)° for the chelating carboxylate group. The two C—O bond distances (O1—C1 and O2—C1) of the monodentate carboxylate group are 1.255 (4) and 1.260 (4) Å, respectively, while the two C—O bond distances (O4—C3 and O3—C3) of the chelating carboxylate group are 1.240 (4) and 1.277 (4) Å, respectively. This indicates that the mesomeric effect for the monodentate carboxylate group is larger than that of the chelating carboxylate group.

The Ba atom is coordinated by two nitrate group O atoms, five water molecules and three malonate O atoms. The Ba—O distances for Ba—O_nitrate range from 2.886 (3) to 2.902 (3) Å, those for Ba—O_water range from 2.805 (3) to 2.897 (2) Å and those for Ba—O_malonate range from 2.752 (2) to 2.941 (2) Å. Of these Ba—O distances, Ba—O3 and Ba—O5 are the longest, and the increase of these bond lengths is related to the requirements of the conformation of the fused ring system. These Ba—O distances correspond well with the sum of the ionic radii [1.21 + 1.66 = 2.87 Å for ten-coordinated Ba²⁺ ions; Bauer et al., 2005] and are comparable with the values reported for barium malonate (Hodgson & Asplund, 1991). The Ba polyhedra share edges to form zigzag BaO₈ polyhedral chains propagating along [100] via pairs of water molecules, O10 and O10ⁱⁱ atoms or bridging malonate O4 and O4ⁱ atoms (see Table 1 for symmetry codes). This results in Ba1···Ba1ⁱ and Ba1···Ba1ⁱⁱ distances of 4.6867 (7) and 4.7877 (8) Å, respectively, and a Ba1ⁱ—Ba1—Ba1ⁱⁱ angle of 97.422 (16)°. Each Ni atom is connected to two Ba atoms through atoms O3 and O3ⁱⁱⁱ of the carboxylate group in the c direction, defining infinite chains of BaO₈ polyhedra. The Ni···Ba distance is 4.6731 (9) Å, and the Ba—Ni—Ba angle is 180°. Thus, a complete two-dimensional polymeric layer is formed in the direction of the ac plane.

In the crystal structure of (I), symmetry-related molecules are linked by hydrogen bonds (Table 2). Hydrogen bonding plays an important role in the stabilization of the extended structure. The structure consists of alternating layers in the b direction. Neighbouring layers are linked together via these hydrogen bonds to build up a three-dimensional network (Fig. 2).