Crystal structure of bis{2-[(E)-(4-fluorobenzyl)iminomethyl]phenolato-κ2 N,O}nickel(II)

In the square-planar [Ni(C14H11FNO)2] complex, weak C—H⋯F and C—H⋯π interactions play an important role in the molecular self-assembly, resulting in the formation of 2D molecular sheets which are stacked along the b axis.


Chemical context
Schiff base ligands are well-known and important compounds because of their wide range of biological activities and uses in industrial systems (Feng et al., 2013;Kumar et al., 2010;Liu et al., 2005) as well as being versatile ligands for transition metals. Transition metal complexes with Schiff base ligands, especially those of Pd II and Ni II , have been shown to display a variety of structural features and, in some cases, exhibit interesting reactivity. In particular they can be photoluminescent (Guo et al., 2013a) and are used as catalysts for many organic reactions such as Heck and Suzuki crosscoupling reactions (Kumari et al., 2012;Teo et al., 2011).

Structural commentary
The asymmetric unit of (1) contains one-half of the molecule with the Ni II cation lying on an inversion centre and the Schiff base anion acting as an N,O-bidentate chelate ligand (Fig. 1). The cation binds to the N and the O atoms of two symmetryrelated Schiff base ligand such that the N and O atoms are mutually trans. The N 2 O 2 donor sets of the two chelating Schiff base ligands in the equatorial plane around Ni1 adopt a slightly distorted square planar coordination geometry with the angles O1-Ni1-N1 = 92.56 (4) and O1-Ni1-N1 i = 87.44 (4) [symmetry code: (i) 1 À x, Ày, 1 À z]. As expected under inversion symmetry, the trans angles (N11-Ni1-N1 i and O1-Ni1-O1 i ) are found to be linear. The Ni1-N1 and Ni1-O1 distances in the N 2 O 2 coordination plane are 1.9242 (10) Å and 1.8336 (9) Å , respectively. These compare well with those observed in the two other closely related Ni II complexes with N 2 O 2 coordinating Schiff base ligands (Bahron et al., 2011;. The Ni1/O1/C1/C6/C7/N1 ring adopts an envelope conformation with the Ni1 atom displaced by 0.3885 (5) Å from the O1/C1/C6/C7/N1 plane, with the puckering parameters Q = 0.2429 (10) Å , = 65.3 (3) and ' = 4.0 (3) . Other bond lengths and angles observed in the structure are also normal. The fluorophenyl ring (C9-C14) makes a dihedral angle of 82.98 (7) with the phenolate ring (C1-C6). The molecular structure of (1), showing 50% probability displacement ellipsoids and the atom-numbering scheme. The labelled atoms are related to the unlabelled atoms of the Schiff base ligands by the symmetry code: 1 À x, Ày, 1 À z. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Screw chains of molecules of (1) linked by C-HÁ Á ÁF contacts drawn as dashed lines.

Figure 3
C-HÁ Á Á contacts for (1) drawn as dotted lines with ring centroids shown as coloured spheres. Cg1 and Cg2 are the centroids of the C1-C6 and C9-C14 rings, respectively.

Supramolecular features
In the crystal packing, the molecules are linked into screw chains by weak C2-H2AÁ Á ÁF1 interactions ( Fig. 2, Table 1). C-HÁ Á Á interactions involving both the fluorophenyl and the phenolate rings, C5-H5AÁ Á ÁCg1 and C13-H13AÁ Á ÁCg2, connect the molecules into chains along the c-axis direction (Fig. 3, Table 1). They also contribute to the formation of sheets parallel to the ac plane, which are further stacked along the b axis as shown in Fig. 4.

Database survey
A search of the Cambridge Database (Version 5.35, November 2013 with 3 updates) revealed a total of 1191 Ni II complexes with an NiN 2 O 2 coordination sphere. No fewer than 333 of these had the Ni atom chelated by two 3-(iminomethyl)phenolate residues. No corresponding structures with a benzyl or substituted benzyl unit bound to the imino N atom were found. However extending the search to allow additional substitution on the phenolate ring resulted in eight discrete structures including the two closely related structures mentioned previously (Bahron et al., 2011(Bahron et al., , 2014, and several other related complexes (see, for example Guo et al. 2013a,b;Senol et al. 2011;Chen et al. 2010).

Synthesis and crystallization
An ethanolic solution of 4-fluorobenzylamine (4 mmol, 0.5010 g) was added to salicylaldehyde (4 mmol, 0.4970 g), dissolved in absolute ethanol (2 ml), forming a bright-yellow solution. The mixture was heated under reflux for an hour to produce the ligand, (E)-2-[(4-fluorobenzylimino)methyl]phenol. Nickel(II) acetate tetrahydrate (2 mmol, 0.4983 g) was dissolved separately in absolute ethanol (10 ml) and added to a flask containing the cooled ligand solution. The mixture was stirred and refluxed for 3 h upon which a dark-green solid formed. This was filtered off, washed with ice-cold ethanol and air-dried at room temperature.
The infrared spectra of the title complex revealed a strong band of 1612 cm À1 in the spectrum assignable to C=N stretching frequency upon complexation (Nair et al., 2012). The appearance of new bands at 451 and 597 cm À1 in the spectrum of the title complex attributable to Ni-O and Ni-N vibrations, respectively, supports the suggestion above of the participation of the N atom of the imine group and O atom of the phenolic group of the ligand in the complexation with Ni II cation (Ouf et al., 2010). Accordingly, it can be deduced that the ligand binds to the Ni II cation in an N,O-bidentate fashion in 2:1 ratio. An antibacterial activity investigation of the title complex against B. subtilis, S. aureus and E. coli showed very mild or no inhibition with clear inhibition diameters of 7-8 mm at the highest concentration of 50 M. The negative control of a 9:1 mixture of DMSO:acetone and the positive control of 30 U of chloramphenicol showed inhibition diameters of 6 mm and 20 mm, respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 The packing of (1) viewed along the b axis showing molecular sheets of the Ni II complex. atoms. The U iso values were constrained to be 1.2U eq of the carrier atoms.

Bis{2-[(E)-(4-fluorobenzyl)iminomethyl]phenolato-K 2 N,O 1 }nickel(II)
Crystal data  (Cosier & Glazer, 1986) operating at 100.0 (1) K. 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. 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 > 2sigma(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.