Poly[diaqua[μ-1,4-bis(1H-imidazol-1-yl)benzene-κ2 N 3:N 3′](μ-fumarato-κ2 O 1:O 4)nickel(II)]

In the title compound, [Ni(C4H2O4)(C12H10N4)(H2O)2]n, the NiII ion has a distorted octahedral coordination geometry. The asymmetric unit is composed of an Ni2+ ion, located on a twofold rotation axis, one half of a 1,4-bis(1H-imidazol-1-yl)benzene (BIMB) ligand and one half of a fumarte (fum2−) dianion, both ligands being located about inversion centers, and a coordinating water molecule. The NiII ions are linked by two BIMB ligands and two fum2− dianions, forming a four-connected layered structure parallel to (010) with a 44-sql topology. Within each layer, there are rhombic grids with dimensions of ca 13.5 × 9.0 Å and approximate angles of 109 and 70°. The crystal packing features a two-dimensional → two-dimensional parallel/parallel interpenetration in which one undulating layer is catenated to another equivalent one, forming a new bilayer. Moreover, the entangled two-dimensional layers are connected by O—H⋯O and C—H⋯O hydrogen bonds, generating a three-dimensional structure.

Polycatenation as a type of interesting networks of entangled systems has attracted much attention for their potential application in energy of electron transfer and drug delivery (Harriman & Sauvage, 1996;Raymo & Sauvage, 1999).
Herein, we report on the crystal structure of a Ni II coordination polymer built from linear BIMB and fum 2ligands, which features a two-dimensional → two-dimensional parallel/parallel polycatenation network.
The asymmetric unit of the title compound contains half a Ni II ion located on a two-fold rotation axis, half a fum 2dianion and half a BIMB ligand both located about inversion centers, and a coordinated water molecule. Each Ni II ion is coordinated by two water molecules, two different carboxylate O atoms from two different fum 2dianions and by two N atoms from two different BIMB ligands, and has a distorted octahedral geometry (Fig. 1).
It is interesting to note that the maleic acid (hydrolysis product of maleic anhydride) is converted into fumaric acid on the self-assembly of the title compound. This is probably because trans-fumaric has a higher thermal stability than cismaleic acid.
In the crystal, each Ni II ion is connected by two BIMB ligands and two fum 2ligands to form an infinite twodimensional puckered sheet with rhombic grids (Fig. 2). Within each layer, the rhombic grids have dimensions of ca. 13.5 Å × 9.0 Å with angles of of ca. 109.60 and 70.40° (defined by Ni···Ni distances and Ni···Ni···Ni angles). The large size of the grids in two adjacent layers allow a two-dimensional → two-dimensional parallel/parallel polycatenation to occur ( Fig. 3). From a topological perspective, each Ni II ion can be regarded as a four-connected node, thus this twodimensional network can be assigned to the 4 4 -sql topology.
Moreover, the entangled two-dimensional layers are further connected by O-H···O hydrogen bonds to generate a threedimensional structure (Fig. 4).
The structure of a similar Ni II coordination polymer assembled by BIMB ligand and adipic acid has been described by (Chen et al., 2010). However, compared with the title compound, the adipic acid is a longer spacer length and more flexible, and crystallizes in the lower symmetry triclinic space group P1 rather than orthorhombic space group Pbcn for the title compound with the short fumarate spacer.
Another relevant example reported by (Bu et al., 2004) is a Zn II coordination polymer (Li et al. 2012). Like the title complex, it is also built from BIMB and fum 2ligands. However, the difference in the metal center results in an interesting 5-fold interpenetrated three-dimensional framework based on a diamondoid topology.

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Acta Cryst. (2012). E68, m1286-m1287 In summary, we have synthesized a Ni II coordination polymer by the hydrothermal reaction of Ni(NO 3 ) 2 with H 2 fum and BIMB ligands, which features a two-dimensional → two-dimensional parallel/parallel polycatenation network. On comparing with two relevant complexes based on the BIMB ligand, we found that the coordination geometry of the central metal ions and the flexibility of the auxiliary carboxylate ligands indeed have a significant effect on the architecture of the target complexes. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 2010); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and publCIF (Westrip, 2010).   A view of the two-dimensional undulated 4 4 -sql layer of the title compound.

Figure 3
A view of the two-fold parallel polycatenation of the two-dimensional layers in the crystal structure of the title compound.

Figure 4
A view of the entangled two-dimensional layers that extended to a three-dimensional structure via O-H···O hydrogen bonds in the crystal structure of the title compound.