Synthesis, crystal structure and Hirshfeld surface analysis of bis{2-[(pyridin-2-yl)amino]pyridinium} tetracyanonickelate(II)

The title structure consists of [Ni(CN)4]2– square-planar anions separated by layers of (C10H10N3)+ cations. The crystal packing features N—H⋯N hydrogen bonds, which generate [101] chains.

In the title molecular salt, (C 10 H 10 N 3 ) 2 [Ni(CN) 4 ], the dihedral angle between the pyridine rings in the cation is 1.92 (13) and the complete anion is generated by a crystallographic centre of symmetry. An intramolecular N-HÁ Á ÁN hydrogen bond occurs in the cation, which closes an S(6) ring. In the crystal, the components are linked by N-HÁ Á ÁN and weak C-HÁ Á ÁN hydrogen bonds, which generate chains propagating in the [101] direction. Weak aromaticstacking interactions are also observed. A Hirshfeld surface analysis and twodimensional fingerprint plots indicate that the most important contact types in the crystal packing are NÁ Á ÁH/HÁ Á ÁN, CÁ Á ÁH/HÁ Á ÁC and HÁ Á ÁH with contributions of 37.2, 28.3 and 21.9%, respectively.
We have been interested in using the tetracyanonickelate(II) anion in combination with other chelating or bridging neutral co-ligands to explore their structural features ISSN 2056-9890 and properties relevant to the field of molecular materials exhibiting the spin-crossover phenomenon (Setifi et al., 2013(Setifi et al., , 2014Kucheriv et al., 2016). During the course of attempts to prepare such complexes with 2,2 0 -dipyridylamine (dpa), we isolated the title molecular salt, (I), whose molecular and supramolecular structure is described herein.

Structural commentary
The asymmetric unit of (I) contains one (C 10 H 10 N 3 ) + cation and one half of a [Ni(CN) 4 ] 2À anion (Fig. 1). The C-N and C-C bonds lengths in the cation vary from 1.340 (3) to 1.383 (3) Å and from 1.346 (4) to 1.402 (3) Å , respectively. The C-N-C bond angles range from 117.8 (2) to 129.7 (2) and the N-C-C angles range from 119.0 (2) to 123.4 (2) . The dihedral angle between the C3-C7/N4 and C8-C12/N5 rings is 1.92 (13) . These data are comparable to those found for other compounds containing dpa as an organic template (Bowes et al., 2003;Willett, 1995). In the cation, the pyridyl nitrogen atoms are arranged on both sides of the central N3 atom and assume a cis conformation (Fig. 1). The (C 10 H 10 N 3 ) + cation is monoprotonated at the pyridyl-N4 atom, which leads to the the formation of a short and presumably strong intramolecular N4-H4AÁ Á ÁN5 hydrogen bond (Table 1), which generates an S(6) ring (Fig. 2).
The Ni 2+ ion of the anion is located on a crystallographic inversion center and coordinates four terminal (non-bridging) cyanide ligands, exhibiting a square-planar geometry. The bond lengths and angles in the anion are in good agreement with those found in other [Ni(CN) 4 ] 2À salts (Paharová et al., 2003;Karaag aç et al., 2013). The pyridinium N4 atom in the cation, as well as forming the intramolecular hydrogen bond described above, acts as donor to the cyanate N atom in the anion, in an N4-H4AÁ Á ÁN1 ii [symmetry code: (ii) Àx + 1, Ày + 1, Àz + 1) link (Table 1). The secondary amino group (N3H) forms a strong N3-H3AÁ Á ÁN2 hydrogen bond with a cyano group acceptor and the H3AÁ Á ÁN2 distance is 2.0 Å . Fig. 3 shows the parallel offset -stacking contacts between pyridyl groups [centroid-centroid distance of 4.3421 (16) Å ] and parallel face-centred -stacking interactions between the S(6) centroids and pyridyl groups [centroid-centroid distance of 3.487 (2) Å ].

Figure 2
Offset and parallel --stacking interactions (broken lines) in the cationcation chains.

Figure 1
The molecular structure of (I) with displacement ellipsoids drawn at the 50% probability level. Symmetry code: (i) Àx + 1, Ày, Àz in Figs. 4 and 5, respectively. The red spots on the Hirshfeld surface represent strong interaction through N-HÁ Á ÁN and C-HÁ Á ÁN hydrogen bonding, whereas the blue color represents a lack of interaction. The presence ofstacking interactions is indicated by adjacent red and blue triangles on the shape-index surface (Fig. S1a (Willett, 1995;CSD refcode ZAMCEV), which crystallizes in the same space group of P1. In this compound the cation is diprotonated and the pyridyl nitrogen atoms are in a cis conformation and the pyridine rings are significantly twisted away from coplanarity. The tetrachlorocuprate anion takes on a squashed tetrahedral geometry.

Synthesis and crystallization
The title compound was synthesized solvothermally under autogenous pressure using a mixture of iron(II) sulfate heptahydrate (28 mg, 0.10 mmol), 2,2 0 -dipyridylamine (17 mg, 0.10 mmol) and potassium tetracyanonickelate(II) (24 mg, 0.10 mmol) in mixed solvents of water/ethanol (3:1 v/v, 20 ml). The mixture was sealed in a Teflon-lined autoclave and held at 423 K for 3 d, and then cooled to room temperature at a rate of 10 K per hour (yield 27%). Pale-yellow plates of (I) suitable for single-crystal X-ray diffraction analysis were selected.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned geometrically in idealized positions and constrained to ride on their parent atoms, with C-H = 0.93 or N-H = 0.86 Å , and with U iso (H) = 1.2U eq (C,N).  Hirshfeld surface of (I) mapped over d norm .   ; program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

(I)
Crystal data (C 10 (17) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.