{1-[1-(2-Hydroxyphenyl)ethylidene]-2-(pyridin-2-yl-κN)hydrazine-κ2 N′,O}{1-[1-(2-oxidophenyl)ethylidene]-2-(pyridin-2-yl-κN)hydrazine-κ2 N′,O}nickelate(II) nitrate hemihydrate

The asymmetric unit of the title complex comprises a supramolecular dimer composed of the Δ(−) and Λ(−) optical isomers, which are linked by strong hydrogen-bonds, two nitrate anions and one water molecule. In the crystal, the dimers are joined by numerous hydrogen bonds, forming a three-dimensional framework.

The 2-hydrazinopyridine precursor has been widely used to prepare ligands of various kinds by condensation with carbonyl compounds. These types of ligands are suitable for synthesizing novel transition metal (II) complexes with interesting magnetic properties. In this context we have synthesized the ligand 1-(2-hydroxyphenyl-2-ethylidene)-2-(pyridin-2-yl)hydrazine (HL) which was used in the preparation of the mononuclear title complex, [Ni(C 13 H 12 N 3 O)-(C 13 H 13 N 3 O)]NO 3 Á0.5H 2 O. As a result of the presence of HL and L in the [{Ni(HL)(L)}] + unit, the complex appears to be a supramolecular dimer composed of the Á(À) and Ã(À) optical isomers, which are linked by strong hydrogen-bonds. As well as the dimer generated by two mononuclear [{Ni(HL)(L)}] + cations, the asymmetric unit also contains two nitrate anions and one water molecule. Each Ni atom is coordinated to two ligand molecules by a nitrogen atom of the pyridine ring, an imine nitrogen atom and a phenolic oxygen atom of one of the ligand molecules and a phenolate oxygen atom of the other organic molecules. The environment around the cation is a distorted octahedron. The basal planes are defined by the two nitrogen atoms of the pyridine rings and the two phenolic oxygen atoms of the ligand, the apical positions being occupied by the azomethine atoms. The O atoms of one of the nitrate ions are disordered over two sets of sites in a 0.745 (9):0.255 (9) ratio. In the crystal, the dimers are linked by numerous hydrogen bonds, forming a threedimensional framework.

Chemical context
Organic ligands derived from salicylaldehyde containing N and O donor atoms are widely used in coordination chemistry (Wang et al., 2006;Gü veli & Ü lkü seven, 2011;Liu et al., 2018). Indeed, these derivatives can give very different structures depending on the type of metal used and the reaction medium (Mahapatra et al., 2016). The coordination chemistry of transition metals continues to be widely explored by researchers because of the wide variety of structures (Bhattacharya & Mohanta, 2015) and applications of these derivatives in different fields (El-Sayed et al., 2016;Donga et al., 2016). The growing interest in the use in coordination chemistry of ligands containing a hydrazino unit (Drożdżewski & Kubiak, 2009;Mukherjee et al., 2013;Guhathakurta et al., 2017) is due to the presence of N donor atoms, allowing them to act as multidentate ligands to generate supramolecular structures (Konar, 2015;Chavan et al., 2014) that have inter-esting catalytic properties (Nassar et al., 2017) or biological activities (Singh et al., 2013). In this context we have synthesized the ligand 1-(2-hydroxyphenyl-2-ethylidene)-2-(pyridin-2-yl)hydrazine (HL), which was used in the preparation of the title compound. We combined 2-hydroxyacetophenone and 2-hydrazino pyridine to prepare a ligand with four potential donor sites (N, O) that acts as a tridentate ligand. In trying to coordinate the 1-(2-hydroxyphenyl-2-ethylidene)-2-(pyridin-2-yl)hydrazine ligand to the first series of transition metals in ethanol, we obtained a nickel(II) complex. Fig. 1 shows the structure of the complex. The asymmetric unit contains a dimer generated by two mononuclear [{Ni(HL)(L)}] + cations, which are strongly hydrogen bonded, two nitrate anions and one water molecule. The O atoms of one of the nitrate ions are disordered over two sets of sites in a 0.745 (9):0.255 (9) ratio. As a result of the presence of HL and L in the [{Ni(HL)(L)}] + unit, the complex is chiral. The dimer is formed by the Á(À) and Ã(À) optical isomers because of the clockwise and anti-clockwise arrangement of the ligands around the Ni 2+ ion. The two optical isomers of the dimer are linked by strong O-HÁ Á ÁO hydrogen bonds between the phenoxo oxygen atoms and the phenolic hydrogen atoms (O1-H1OÁ Á ÁO4 and O3-H3OÁ Á ÁO2) with a mean HÁ Á ÁA distances of 1.64 Å .

Structural commentary
In both complex molecules, the Ni 2+ ion is hexacoordinated in an octahedral environment. Each Ni 2+ ion is bonded to a ligand molecule, whose phenolic function is deprotonated and to a second neutral ligand molecule. The basal plane of the octahedron around each Ni 2+ ion is occupied by two nitrogen atoms from the pyridine moieties, a phenolic oxygen atom and a phenolate oxygen atom. The apical positions are occupied by the nitrogen atoms of the imine functions. The angles (Table 1) in the basal plane of the octahedron are in the range 84.34 (6)-102.46 (7) for Ni1 and 84.32 (6)-103.78 (7) (Jana et al., 2017). The diagonal basal angles (N1-Ni1-O1, N5-Ni1-O2, N8-Ni2-O3 and N11-Ni2-O4) and the apical angles (N2-Ni1-N4 and N7-Ni2-N10) deviate significantly from the ideal values of 180 . The angles N2-Ni1-O1 and N2-Ni1-N1 are very different. This can be explained by the rings formed by the ligand by binding in a tridentate fashion to the Ni 2+ ion. The first angle is derived from a six-membered ring whereas the second one is derived from a five-membered ring. The flexibility of the six-membered ring compared to the fivemembered ring implies that the angles should be larger in the six-membered ring than in the five-membered ring. The same behavior is observed for the angles around Ni1 with the second ligand molecule. These observations are also noticed for the second molecule in the asymmetric unit.

Supramolecular features
In the crystal, the complex appears as a dimer composed by the Á(À) and Ã(À) optical isomers, which are linked by strong hydrogen bonds (

Figure 1
An ORTEP view of the title compound, showing the atom-numbering scheme and intramolecular contacts ( different intermolecular hydrogen bonds, OW-HÁ Á ÁONO 2 , N-HÁ Á ÁONO 2 , N-HÁ Á ÁOW and C-HÁ Á ÁONO 2 , involving the complex molecule, the non-coordinating water molecule and the uncoordinated nitrate groups (Fig. 2). These intermolecular and intramolecular hydrogen bonds stabilize and link the components into a three-dimensional network.

Synthesis and crystallization
A mixture of 2-hydrazinopyridine (1 mmol) and 2-hydroxyacetophenone (1 mmol) in ethanol (10 mL) was stirred under reflux for 60 min. On cooling, a yellow precipitate was obtained. After filtration, the resulting solid was dried in a  , 107, 116, 117, 118, 119, 120, 127, 130, 138, 149, 156, 158. A mixture of NiCl 2 Á6H 2 O (1 mmol) in ethanol (10 mL) was added to a solution of HL (2 mol) in 10 mL of ethanol. The mixture was stirred for 60 min and the resulting greenish solution was filtered. The filtrate was kept at 298 K and after six days, green crystals suitable for X-ray analysis appeared and were collected by filtration.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms of OH and OH 2 groups were located in difference-Fourier maps and refined using a riding model with U iso (H) = 1.5U eq (O). Other H atoms (CH, NH and CH 3 groups) were geometrically optimized (C-H = 0.93-0.96 Å , Å N-H = 0.86 Å ) and refined as riding with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for all other H atoms. High thermal motion for the O atoms of one of the nitrate group was noted, indicating some disorder in their Molecular representation of the title compound, showing the intermolecular hydrogen-bond contacts (Table 2) as dotted lines. Table 2 Hydrogen-bond geometry (Å , ).    program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b).  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.