Crystal structure of bis{4-[(4-methylbenzyl)oxy]-N′-(4-methylbenzylidene)benzohydrazidato}nickel(II)

In the title compound, the mononuclear nickel(II) complex exhibits point group symmetry .

In the title complex, [Ni(C 23 H 21 N 2 O 2 ) 2 ], the central Ni II atom is located on an inversion centre and exhibits a slightly distorted square-planar N 2 O 2 coordination environment. A trans-configuration of the N,O chelating ligands results from the imposed site symmetry of the central Ni II atom. In the crystal, individual molecules stack along the a axis through weakstacking interactions between the phenyl rings.

Chemical context
Variously substituted hydrazone ligands have attracted special attention because of their chelating capabilities and structural properties, such as the degree of rigidity, a conjugatedsystem and an N-H unit that readily participates in hydrogen bonding and may be easily deprotonated. The corresponding nickel(II) complexes are of considerable interest since they exhibit a broad spectrum of physiological and pharmacological activities (Yang et al., 2020;Al-Qadsy et al., 2021;Neethu et al., 2021;Krishnamoorthy et al., 2012), most of which are structure-dependent properties.
We report here the synthesis and crystal structure of another Ni II complex with a derivatized hydrazone ligand.

Structural commentary
The central metal Ni II atom of the title complex is located on an inversion center. Hence, the asymmetric unit comprises half a molecule (Fig. 1). The enolizable O atom and the azomethine N atom of the ligand coordinate to the Ni II atom to form a five-membered chelate ring. The Ni II atom exhibits a slightly distorted square-planar coordination environment with the deprotonated ligands in a trans configuration imposed by the crystal symmetry. The Ni-N1 and Ni-O1 bond lengths are 1.8677 (12) and 1.8363 (10) Å , respectively, with a chelating angle of 83.47 (5) . These data are in agreement with previously reported crystal structures of related complexes (Yang et al., 2020;Al-Qadsy et al., 2021;Neethu et al., 2021;Krishnamoorthy et al., 2012), irrespective of the substituents present in the ligand.
As expected, the C9-O1 bond length of 1.3009 (18) Å lies between a C-O single bond (1.43 Å ; Allen et al., 1987) and a C O double bond (1.21 Å ; Allen et al., 1987). The bond lengths N1-C8 of 1.2977 (19) Å and N2-C9 of 1.3145 (18) Å are close to the value of a typical C N bond (1.30 Å ; Allen et al., 1987). These data reveal that the -CH N-N C-O fragment of the ligand remains a conjugated system even after the loss of a H atom from its enolized carbonyl O atom. The complex is stabilized by weak intramolecular C8-H8Á Á ÁO1, C3-H3Á Á ÁN2 and C11-H11Á Á ÁO1 hydrogen bonds involving phenyl and methylene donor groups and the coordinating atoms as acceptor groups (Table 1). The benzylidene ring is tilted by 26.06 (6) with respect to the N 2 O 2 coordination plane, while the phenyl rings of the ether moiety form a dihedral angle of 83.29 (5) .
The bond-valence sum (BVS) calculated for the Ni II atom present in the complex, using the parameters of Brese & O'Keeffe (1991), indicate a higher value (2.97 valence units) than expected for a formal ionic charge of +2. The calculated high value can be reasonably attributed to a very pronounced covalent bonding associated with the Ni-O and Ni-N bonds. As a matter of fact, a set of new optimized r 0 parameters to be used for the BVS calculation for model compounds involving Ni II -O, Ni II -S, Ni II -N interactions has been proposed (Liu & Thorp, 1993). By using these values, the BVS calculation for this complex gives a value of 2.36 valence units.

Supramolecular features
Individual molecular complexes are weakly packed along the a axis through -ring interactions involving the phenyl rings, with centroid-to-centroid distances of 4.6914 (2) Å and a slippage of ca 3.0-3.3 Å , as shown in Fig. 2. In addition, the five-membered chelate rings of neighbouring complexes have even shorter distances [3.4555 (2) Å with a slippage of 0.96 Å ].

Database survey
A search in the Cambridge Crystal Structure Database (CSD, version 5.43, update June 2022); Groom et al., 2016) retrieved more than twenty bis-chelated square-planar nickel(II) complexes with hydrazone-based ligands also bearing bulky ferrocenyl groups (Krishnamoorthy et al., 2012), 2,2 0 -bithiophenyl (Yang et al., 2020) or 9-anthrylmethylene fragments (Mondal et al., 2014). However, no species comprising a long benzyl-phenyl ether chain has been reported so far. It is worth noting that all characterized Ni II complexes exhibit a transconfiguration of ligands, where the -CH N-N C-O fragment is chelating, and the coordination Ni-O and Ni-N bond lengths do not appear to be significantly affected by the electronic or steric properties of groups present on the ligands.

Figure 2
Crystal packing of individual complexes showing the -ring interactions as dotted lines.

Figure 1
Molecular structure of the centrosymmetric nickel(II) complex, drawn with displacement ellipsoids at the 50% probability level. [symmetry code for primed atoms: Àx + 2, Ày, Àz + 2.] (0.12 g, 1 mmol) was added and the mixture was refluxed for half an hour. A solution of nickel(II) acetate tetrahydrate (0.13 g, 0.5 mmol in 5 ml of ethanol) was then added and refluxing was continued for 2 h. The obtained orange precipitate was filtered off and washed three times with hot ethanol. The product was recrystallized from a mixture of chloroform and acetonitrile (5:

Bis{4-[(4-methylbenzyl)oxy]-N′-(4-methylbenzylidene)benzohydrazidato}nickel(II)
Crystal data 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.