Crystal structure and Hirshfeld surface analysis of tris(2,2′-bipyridine)nickel(II) bis(1,1,3,3-tetracyano-2-ethoxypropenide) dihydrate

The title compound crystallizes as a racemic mixture in the monoclinic space group C2/c. In the crystal, the 1,1,3,3-tetracyano-2-ethoxypropenide anions and the water molecules are linked by O—H⋯N hydrogen bonds, forming chains running along the [010] direction. The bpy ligands of the cation are linked to the chain via C—H⋯π(cation) interactions involving the CH3 group.


Chemical context
The use of polynitrile anions as ligands, either alone or in combination with neutral co-ligands, is a very promising and appealing strategy to obtain molecular architectures with different topologies and dimensionalities owing to their ability to coordinate and bridge metal ions in many different ways (Miyazaki et al., 2003;Atmani et al., 2008;Benmansour et al., 2007Benmansour et al., , 2008Yuste et al., 2009;Gaamoune et al., 2010;Addala et al., 2015;Setifi et al., 2010Setifi et al., , 2013aSetifi et al., ,b, 2014aSetifi et al., ,b, 2015Setifi et al., , 2016Setifi et al., , 2017. The presence of several potentially coordinating nitrile groups, their rigidity and their electronic delocalization, allows the synthesis of original magnetic high-dimensional coordination polymers with transition-metal ions (Benmansour et al., 2010).
In view of the possible roles of these versatile polynitrile ligands, we have been interested in using them in combination ISSN 2056-9890 with other chelating or bridging neutral co-ligands to explore their structural and electronic characteristics in the field of molecular materials exhibiting interesting magnetic exchange coupling. During the course of attempts to prepare such complexes with 2,2-dipyridyl, we isolated the title compound, whose structure is described herein along with the Hirshfeld surface analysis.

Supramolecular features
As shown in Fig. 2, there are four [Ni(bpy) 3 ] 2+ cationic units within the unit cell of the compound, charge-balancing the1,1,3,3-tetracyano-2-ethoxypropenide anions. These, together with the hydration water, define planar and zigzag hydrogen-bonded chains, in which anions and water molecules alternate, running along the [010] direction, as shown in Fig. 3

Figure 2
Disposition of Á (red) and Ã (blue) stereoisomers in the unit cell.

Hirshfeld surface analysis
The fingerprint plots (Fig. 4) of the intermolecular contacts were computed using program CrystalExplorer (McKinnon et al., 2007;Wolff et al., 2012). The short contacts spikes are due to the NÁ Á ÁH hydrogen bonds (outer spikes) and to the NiÁ Á ÁN coordination bonds (inner spikes). The proportions of the different contacts and their enrichment  Table 3) were computed with program MoProViewer (Guillot et al., 2014). The enrichment ratios E xy are obtained from the actual contacts between the different chemical species (x, y) and equi-probable proportions computed from the surface chemical content . They allow contacts that are favored (over-represented) and which are likely to be the crystal driving force to be highlighted.
The Hirshfeld surface was computed for all the entities present in the crystal -the (tcnoet) À anion, the [Ni(bpy) 3 ] 2+ complex and the water molecule -in order to analyze the crystal contacts. Moieties not in contact with each other were selected in the crystal packing in order to obtain integral surfaces.
The nickel cation does not contribute to the molecular surface, as it is coordinated by six nitrogen atoms within the [Ni(bpy) 3 ] 2+ complex. Nearly three quarters of the Hirshfeld surface is of hydrophobic in nature, constituted by atoms C and Hc. The most abundant contact is of the CÁ Á ÁHc type as a result of the extensive C-HÁ Á Á interactions involving the aromatic rings. The second major contact is NÁ Á ÁHc, which is due to the abundance of the N and Hc chemical types and to the significant enrichment of this favorable weak hydrogen bond. The third major contact is of the CÁ Á ÁC type and is due research communications Hirshfeld surface fingerprint plot for the title compound showing the CÁ Á ÁC, CÁ Á ÁH, HÁ Á ÁO, HÁ Á ÁH, HÁ Á ÁN and NiÁ Á ÁN contacts in detail.

Figure 3
Partial crystal packing diagram showing the alternating zigzag (tcnoet) Àwater chains defined by O-HÁ Á ÁN hydrogen bonds running along the [010] direction. Symmetry code: (ii) Àx + 1 2 , y À 1 2 , Àz + 1 2 . to stacking between the [Ni(bpy) 3 ] 2+ aromatic rings and the C(C(C N) 2 ) 2 group of the (tcnoet) À anion. The other significantly over-represented contacts are the strong hydrogen bonds NÁ Á ÁH-O (E = 2.5) between the water molecule and two nitrile groups. These are the hydrogen bonds with shortest distance d(N5Á Á ÁH25) = 2.11 Å and d(N4Á Á ÁH26) = 2.10 Å (Table 1). There is a deficit of strong hydrogen-bond donors compared to acceptors in this crystal structure. As a result, weak hydrogen bonds to H-C groups are formed. NÁ Á ÁH-C weak hydrogen bonds occur and are slightly enriched. The oxygen atoms form only weak OÁ Á ÁH-C hydrogen bonds, which are quite favored at E = 1.8. Globally there are two O-HÁ Á ÁN strong hydrogen bonds, six C-HÁ Á ÁN and two C-HÁ Á ÁO weak hydrogen bonds ( Table 2). The two major hydrophobic contacts, CÁ Á ÁHc and CÁ Á ÁC, are both slightly enriched. If all hydrophobic contacts (within C and Hc atoms) are considered together, they are globally slightly under-represented with an enrichment ratio E = 0.92 because of the avoidance of the less favorable HcÁ Á ÁHc contacts. All contacts between charged atoms (O, Ho, N) are absent except for the attractive NÁ Á ÁHo hydrogen bond. The cross hydrophilic/hydrophobic contacts are slightly overrepresented at E = 1.16 because of the occurrence of many weak OÁ Á ÁHc and NÁ Á ÁHc hydrogen bonds, which result from an unbalanced number of strong hydrogen-bond acceptors versus donors.

Database survey
The Cambridge Structural Database (CSD, Version 5.39, update August 2018, Groom et al., 2016) includes a few structures involving polycyanopropide counter-ions, of which only 16 entries are hexacyano derivatives and four have (tcnoet) À anions. There are no significant differences in C-N and C-C bond lengths between the hexacyano derivatives and (tcnoet) À anions. However, the C21-C20-C16-C17 torsion angles in (tcnoet) À anion (15.78 ) are slightly smaller than the analogous torsion angle in other anions (16.32-21.68 ). This difference can be explained by this compound and its isostructural structure featuring two hydrogen bonds, O2-H25Á Á ÁN5 and O2-H25Á Á ÁN4 ii . These interactions orient the cyano groups toward to coplanarity with respect to other (tcnoet) À molecules that exhibit fewer hydrogen bonds. Finally, this compound has been used for the synthesis of lowdimensional metal-organic frameworks employing Mn II , Cu II , Co II and Fe II ions because the half cyano groups interact by hydrogen bonding with the metal aqua complexes, avoiding the formation of high-dimensional frameworks (Thé tiot et al., 2003).

Synthesis and crystallization
The title compound was synthesized solvothermally under autogenous pressure from a mixture of Ni(NO 3 ) 2 Á6H 2 O (29 mg, 0.1 mmol), 2,2-dipyridyl (16 mg, 0.1 mmol) and K(tcnoet) (45 mg, 0.2 mmol) in water-ethanol (4:1 v/v, 20 cm À3 ). This mixture was sealed in a Teflon-lined autoclave and held at 423 K for three days, and then cooled to ambient temperature at a rate of 10 K h À1 (yield: 54%). Light-green blocks of the title compound suitable for single-crystal X-ray diffraction were selected directly from the synthesized product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All H atoms were located in difference-Fourier maps. C-bound H atoms were then treated as riding atoms: C-H = 0.95 Å (aromatic), 0.98 Å (CH 3 ) or 0.99 Å (CH 2 ), and with U iso (H) = kU eq (C), where k = 1.5 for the methyl groups, which were permitted to rotate but not to tilt, and 1.2 for all others. H atoms bonded to the water O atom were permitted to ride at the positions located in the difference map, with U iso (H) = 1.5U eq (O).  Table 4 Experimental details.

Tris(2,2′-bipyridine)nickel(II) bis(1,1,3,3-tetracyano-2-ethoxypropenide) dihydrate
Crystal data [Ni(C 10  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.