Poly[[aqua(μ5-1H-benzimidazole-5,6-dicarboxylato)(μ4-1H-benzimidazole-5,6-dicarboxylato)dibarium] 4.5-hydrate]

The polymeric title compound, {[Ba2(C9H4N2O4)2(H2O)]·4.5H2O}n, adopts a layer structure parallel to (001) in which adjacent BaII atoms are connected by two benzimidazole-5,6-dicarboxylate dianions, one functioning in a μ4-bridging mode and the other in a μ5-bridging mode. The Ba atom having water in its coordination environment as well as the Ba atom without water exist in a nine-coordinate polyhedron of O atoms; the geometry is difficult to derive. Lattice water molecules occupy the space between layers and interact with the layers through O—H⋯O, O—H⋯N and N—H⋯O hydrogen bonds. ne of the five lattice water molecules is equally disordered around an inversion centre and shows half-occupancy.

The polymeric title compound, {[Ba 2 (C 9 H 4 N 2 O 4 ) 2 (H 2 O)]Á-4.5H 2 O} n , adopts a layer structure parallel to (001) in which adjacent Ba II atoms are connected by two benzimidazole-5,6dicarboxylate dianions, one functioning in a 4 -bridging mode and the other in a 5 -bridging mode. The Ba atom having water in its coordination environment as well as the Ba atom without water exist in a nine-coordinate polyhedron of O atoms; the geometry is difficult to derive. Lattice water molecules occupy the space between layers and interact with the layers through O-HÁ Á ÁO, O-HÁ Á ÁN and N-HÁ Á ÁO hydrogen bonds. ne of the five lattice water molecules is equally disordered around an inversion centre and shows halfoccupancy.
The Ba atom having water in its coordination environment as well as the Ba atom without water exist in a nine-coordinate polyhedron of O atom; the geometry is undefined. Lattice water molecules occupy the space between layers, and interact with the layers through O-H···O and N-H···O hydrogen bonds to generate a three-dimensional network (Table 1).

Experimental
Barium chloride (0.0416 g, 0.20 mmol) and 1H-benzimidazole-5,6-dicarboxylic acid (0.0412 g, 0.20 mmol) were placed in water (35 ml); the reactants dissolved upon addition of several drops of dilute sodium hydroxide to a pH of 7. The solution was set aside for the growth of crystals over several weeks; yield 60% based on Ba.

Refinement
Hydrogen atoms were placed in calculated positions (C-H 0.93 Å) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C). The amino and water H atoms were placed in chemically sensible positions on the basis of hydrogen bonding interactions (O-H 0.84 Å and N-H 0.88 Å); their temperature factors were tied by a factor of 1.2-1.5 times. Positioning the H atoms lead to H12···H21 and H31···H42 distances of about 2 Å, which are regarded as being acceptable. The O5w molecule then forms only one hydrogen bond. Positioning the second H atom elsewhere led to too short interactions.
On this basis, the N1 and N4 atoms are atoms having a hydrogen connected to them whereas the N2 and N3 atoms do not.
One of the water molecules (O5w) is disordered about a center-of-inversion.
The anisotropic temperature factors of the free water molecules were restrained to be nearly isotropic.