Refinement

The crystal structure of the title compound, [Ni(C(3)H(4)N(2))(6)][Ni(C(3)H(4)N(2))(3)(H(2)O)(3)](C(12)H(6)O(4))(2), contains uncoordinated naphthalene-dicarboxyl-ate dianions and two kinds of Ni(II) complex cations, both assuming distorted octa-hedral geometries. One Ni(II) ion is located on an inversion center and is coordinated by six imidazole mol-ecules, while the other Ni(II) ion is located on a twofold rotation axis and is coordinated by three water mol-ecules and three imidazole mol-ecules in a mer-NiN(3)O(3) arrangement. The naphthalene-dicarboxyl-ate dianion links both Ni(II) complex cations via O-H⋯O and N-H⋯O hydrogen bonding, but no π-π stacking is observed between aromatic rings in the crystal structure. One imidazole ligand is equally disordered over two sites about a twofold rotation axis; one N atom and one water O atom have site symmetry 2.

The crystal structure of the title compound, [Ni(C 3 H 4 N 2 ) 6 ]-[Ni(C 3 H 4 N 2 ) 3 (H 2 O) 3 ](C 12 H 6 O 4 ) 2 , contains uncoordinated naphthalenedicarboxylate dianions and two kinds of Ni II complex cations, both assuming distorted octahedral geometries. One Ni II ion is located on an inversion center and is coordinated by six imidazole molecules, while the other Ni II ion is located on a twofold rotation axis and is coordinated by three water molecules and three imidazole molecules in a mer-NiN 3 O 3 arrangement. The naphthalenedicarboxylate dianion links both Ni II complex cations via O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonding, but nostacking is observed between aromatic rings in the crystal structure. One imidazole ligand is equally disordered over two sites about a twofold rotation axis; one N atom and one water O atom have site symmetry 2.
The crystal structure contains uncoordinated naphthalenedicarboxylate dianions and two independent Ni II complex cations ( Fig. 1). Both Ni II complexes assume distorted octahedral geometry. The Ni1 atom is located in an inversion center and coordinated by six imidazole ligands, while the Ni2 atom is located on a twofold axis and coordinated by three water and three imidazole ligands. In the Ni2-containing complex cation, the O2W and N9 atoms are also located on the twofold axis, but the other atoms of the disordered N9-imidazole ring do not lie on the twofold axis and the N9-imidazole ring is tilted to the twofold axis by an angle of 11.9 (5)°, similar to 14.2 (3)° found in the Mn II analogue (Li et al., 2008).
The coordination bond distances (Table 1) Table 2). Two carboxyl groups are twisted with respect to the naphthalene ring system by dihedral angles of 56.4 (5)° and 50.4 (5)°, which are larger than those found in the structure of free naphthalenedicarboxylic acid (ca 40°; Derissen et al., 1979). No π-π stacking is observed between aromatic rings in the crystal structure.

S2. Experimental
A water-ethanol solution (16 ml, 1:3 v/v) of naphthalene-1,4-dicarboxyllic acid (0.108 g, 0.5 mmol) and sodium carbonate (0.053 g, 0.5 mmol) was refluxed for 0.5 h, then nickel chloride hexahydrate (0.118 g, 0.5 mmol) was added to the above solution. The reaction mixture was refluxed for a further 6.5 h, then imidazole (0.102 g, 1.5 mmol) was added to the above solution and the reaction mixture was refluxed for another 0.5 h. After cooling to room temperature the solution was filtered. Green prisms of (I) were obtained from the filtrate after 4 d.

S3. Refinement
The N9-containing imidazole molecule is disordered over two sites, close to a twofold rotation axis, but N9 atom is located on the twofold axis and is not disordered. The disordered components were refined with a half site occupancy and bond-length restraints were used to stabilise the refinement.
The water H atoms were located in a difference Fourier map and refined as riding in as-found relative positions with  The molecular structure of (I) with 30% probability displacement (arbitrary spheres for H atoms). One of the disordered imidazole components has been omitted for clarify. Dashed lines indicate hydrogen bonding [symmetry codes: (i) -x + 3/2, -y + 3/2, z; (ii) -x + 3/2, -y + 1/2, z + 1].

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (