catena-Poly[[tetraaquanickel(II)]-μ-(9,10-dioxo-9,10-dihydroanthracene-1,4,5,8-tetracarboxylato)-κ2 O 1:O 8-[tetraaquanickel(II)]-μ-4,4′-bipyridine-κ2 N:N′]

In the crystal of the title polymeric complex, [Ni2(C18H4O10)(C10H8N2)(H2O)8]n, each NiII cation is coordinated by four water molecules in the equatorial plane, and is further bridged by an 9,10-dioxo-9,10-dihydroanthracene-1,4,5,8-tetracarboxylate anion and a 4,4′-bipyridine ligand in the axial directions, forming a distorted octahedral geometry. The 9,10-dioxo-9,10-dihydroanthracene-1,4,5,8-tetracarboxylate anion is centrosymmetric with the centroid of the quinone ring located about an inversion center; the 4,4′-bipyridine ligand is also centrosymmetric with the mid-point of the C—C bond linking two pyridine rings located about another invertion center. The 9,10-dioxo-9,10-dihydroanthracene-1,4,5,8-tetracarboxylate anion and bypiridine ligand bridge the NiII cations, forming a polymeric chain along the b axis. π–π stacking is observed between pyridine and benzene rings [centroid–centroid distance = 3.705 (2) Å]. All of the coordinating water molecules are involved in O—H⋯O hydrogen bonding.


Related literature
For the synthesis, see: Liu et al. (2010).
The nickel atom is in a normal octahedral geometry with O atoms which from the coordinating water molecules on the equatorial plane and O atom from the carboxylate group, N atom from the 4,4-bipyridine in the axial position. Every nickel atom is coordinated with one 9,10-dioxo-9,10-dihydroanthracene-1,4,5,8-tetracarboxylate ligand and one 4,4-bipyridine ligand. And every 9,10-dioxo-9,10-dihydroanthracene-1,4,5,8-tetracarboxylic acid ligand coordinates with two nickel atoms and they form a one-dimensional nickel chain along b axis. There are two nickel chains which cross each other in the crystal structure.

Experimental
The title compound was obtained unintentionally as the product of an attempted synthesis of a polymeric network Ni complex with 9,10-dioxo-9,10-dihydroanthracene-1,4,5,8-tetracarboxylic acid and 4,4-bipyridine (Liu et al., 2010), the pH value of the mixture was adjusted to 8.0 with NaOH solution, and then solution was heated at 393 K for 3 d. After the mixture was slowly cooled to room temperature, green crystals of the title compound were obtained.

Refinement
Water H atoms were located in a difference map and refined with distance restraints of O-H = 0.85 Å, other H atoms were placed in calculated positions with C-H = 0.93 Å and refined in riding mode. U iso (H) = 1.2U eq (C,O).

Figure 2
The packing of (I), viewed down the a axis. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.86 e Å −3 Δρ min = −0.49 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.