Crystal structure of poly[(2,2′-bipyridine-κ2 N,N′)tetrakis(μ-cyanido-κ2 N:C)dinickel(II)]

The binuclear coordination polymer consists of two nickel cations with different coordination environments. One has a square-planar environment whereas the other has an octahedral environment. Cyanide ligands bridge the cations into a polymeric layer structure.

The polymeric title complex, [Ni 2 (CN) 4 (C 10 H 8 N 2 )] n , was obtained serendipitously under hydrothermal conditions. The asymmetric unit consists of one half of an [Ni(CN) 4 ] 2À anion with the Ni 2+ cation situated on an inversion centre, and one half of an [Ni(2,2 0 -bpy)] 2+ cation (2,2 0 -bpy is 2,2 0 -bipyridine), with the second Ni 2+ cation situated on a twofold rotation axis. The two Ni 2+ cations exhibit different coordination spheres. Whereas the coordination of the metal in the anion is that of a slightly distorted square defined by four C-bound cyanide ligands, the coordination in the cation is that of a distorted octahedron defined by four N-bound cyanide ligands and two N atoms from the chelating 2,2 0 -bpy ligand. The two different Ni 2+ cations are alternately bridged by the cyanide ligands, resulting in a two-dimensional structure extending parallel to (010). Within the sheets,interactions between pyridine rings of neighbouring 2,2 0 -bpy ligands, with a centroid-to-centroid distance of 3.687 (3) Å , are present. The crystal packing is dominated by van der Waals forces. A weak C-HÁ Á ÁN interaction between adjacent sheets is also observed.

Chemical context
Coordination metal complexes have been the subject of intensive investigation not only due to their potential application to material science as catalytic, conductive, luminescent, magnetic, porous, chiral or non-linear optical materials (Janiak et al., 2003), but also because of their intriguing structural diversity (Kong et al., 2008). The assembly of functional molecular building blocks into crystalline polymeric materials through coordination bonds or other weak interactions has many advantages over traditional stepwise syntheses and was demonstrated to be an effective approach to fabricating new materials (Kopotkov et al., 2014). Using this approach, numerous materials with interesting structures and properties have been prepared through the reactions of cyanidometallate building blocks (Cui et al., 2011;Zhang & Lachgar, 2015). These compounds show novel functionalities due to strong interactions mediated by the linear cyanide bridges. The probably oldest and most interesting example is the Prussian blue framework, Fe 4 [Fe(CN) 6 ] 3 Á14H 2 O, and its analogues derived from the assembly of hexacyanidometalate anions [M(CN) 6 ] n and transition-metal ions (Li et al., 2008). For instance, cyanide-bridged bimetallic assemblies were obtained from K 3 [Fe(CN) 6 ] as a source of cyanidometalate anions, metal cations, and aromatic nitrogen-containing ligands. These compounds show interesting magnetic and other properties that can be affected through the careful choice of the building-block components (Shen et al., 2014). ISSN 2056-9890 Our own efforts are focused to assemble metallic complexes and the achievement of tuning their properties by crystal engineering of the terminal/bridging ligands. However, the hydrothermal reaction of Ni(acetate) 2 , 2,2 0 -bipyridine and K 3 [Fe(CN) 6 ] did not yield the expected bimetallic system analogous to coordinated iron ions which were reported in literature (Colacio et al., 2003), but to the serendipitous formation of the polymeric complex (I), [Ni 2 (CN) 4 (C 10 H 8 N 2 )] n , the crystal structure of which is reported here.

Supramolecular features
Within a sheet,interactions between pyridine rings with a centroid-to-centroid distance of 3.687 (3) Å are present. The adhesion of the sheets in the crystal packing is dominated by van der Waals forces. However, a weak non-classical C-HÁ Á ÁN interaction (Table 2)  The principal building units of complex (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity. For symmetry codes, see text.  (2) Symmetry code: (i) x À 1; y; z.

Synthesis and crystallization
Ni(acetate) 2 (0.159 g, 0.64 mmol), 2,2 0 -bipyridine (0.047 g, 0.3 mmol) and K 3 [Fe(CN) 6 ] (0.21 g, 0.64 mmol) dissolved in aqueous solution of 1M NaCl (8 ml) were added to a 15 ml Teflon-lined autoclave and heated at 433 K for 3 d. The autoclave was then cooled to room temperature. Green blockshaped crystals of (I) deposited on the wall of the container and were collected and air-dried.

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.