Crystal structure of bis(2-{1-[(E)-(4-fluorobenzyl)imino]ethyl}phenolato-κ2 N,O)palladium(II)

In the square planar [Pd(C15H13FNO)2] complex, weak C—H⋯O and π–π interactions play important roles in the molecular self-assembly, resulting in the formation of two-dimensional molecular sheets which are further stacked along the b axis.

The asymmetric unit of the title complex, [Pd(C 15 H 13 FNO) 2 ], contains one half of the molecule with the Pd II cation lying on an inversion centre and is coordinated by the bidentate Schiff base anion. The geometry around the cationic Pd II centre is distorted square planar, chelated by the imine N-and phenolate O-donor atoms of the two Schiff base ligands. The N-and O-donor atoms of the two ligands are mutually trans, with Pd-N and Pd-O bond lengths of 2.028 (2) and 1.9770 (18) Å , respectively. The fluorophenyl ring is tilted away from the coordination plane and makes a dihedral angle of 66.2 (2) with the phenolate ring. In the crystal, molecules are linked into chains along the [101] direction by weak C-HÁ Á ÁO hydrogen bonds. Weakinteractions with centroid-centroid distances of 4.079 (2) Å stack the molecules along c.

Chemical context
Schiff bases represent one of the most widely utilized classes of ligands in coordination chemistry and the chemistry of Schiff bases is still an area of increasing interest (Canali & Sherrigton, 1999). The Pd II and Ni II complexes of Schiff bases have attracted much attention as they play important roles in bioinorganic chemistry and may provide the basis for models of active sites of biological systems (Malik et al., 2013) or act as catalysts (Shahnaz et al., 2013). The title compound, [Pd(C 15 H 13 FNO) 2 ], is related to the previously reported compound bis{2-[(E)-(4-fluorobenzyl)iminomethyl]phenolato-2 N,O 1 }nickel(II)  in terms of the coordination geometry around the central metal. In this article, we report the synthesis of the title Schiff base-Pd II complex and its characterization by spectroscopy and elemental analysis. The X-ray structure (Fig. 1), confirms the binding mode of the 4-fluorobenzyl(iminoethyl)phenolate ligand to the Pd II cation.
The title compound (1) was screened for catalytic activity in the Suzuki cross-coupling reaction between phenylboronic acid and iodobenzene with a catalyst loading of 1 mmol%. The conversion of iodobenzene was found to occur with a yield of 52%.

Structural commentary
The asymmetric unit of (1) contains one-half of the molecule with the Pd II cation lying on an inversion centre and the Schiff base anion acting as an N,O bidentate chelate ligand (Fig. 1).
The Pd II cation binds to the N and the O atoms of two symmetry-related 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 Pd1 adopt a slightly distorted square-planar coordination geometry. The Pd1-N1 and Pd1-O1 distances ( Table 1) are typical of square-planar Pd II complexes, and compare well with those observed in other closely related Pd II complexes (Adrian et al., 2008;Bahron et al., 2014;Wan Ibrahim & Shamsuddin, 2012). The bite angle of the iminomethylphenolate chelate, N1-Pd1-O1 is 88.48 (8) , which is also similar to that in a related Pd II complex . The ring Pd1/N1/C8/C9/C10/O1 adopts an envelope conformation with Pd1 displaced by 0.270 (2) Å from the plane of the other ring atoms, and with puckering parameters Q = 0.525 (2) Å , = 112.8 (3) and ' = 206.9 (3) . Other bond lengths and angles observed in the structure are also normal. The fluorophenyl ring (C1-C6) makes a dihedral angle of 66.2 (2) with the phenolate ring (C9-C14).

Figure 1
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, 2 À y, 2 À z.

Synthesis and crystallization
The ligand, (E)-2-(1-(4-fluorobenzylimino)ethyl)phenol (Mohd Tajuddin et al., 2012b) (2 mmol, 0.4877 g) was dissolved in CH 3 CN (30 mL) in a round-bottomed flask. Palladium(II) acetate (1 mmol, 0.2251 g) was dissolved separately in CH 3 CN (20 mL) and was then added into the flask containing the ligand solution. The mixture was stirred and refluxed for 5 h upon which a turmeric yellow solid was formed. The solid was filtered off, washed with ice-cold CH 3 CN and air dried at room temperature. The solid product was recrystallized from chloroform, yielding yellow crystals (yield 48.5%). 1 H NMR, 13 C NMR and IR spectral bands have been studied and agree well with the structure obtained from the values of the CHN analyses and X-ray structure determination.
Melting point 508-510 K. Analytical data for C 30  The infrared spectrum of (1) exhibits a strong band at 1598 cm À1 assignable to the C N stretching frequency of the azomethine moiety. Weak bands at 556 and 450 cm À1 attributable to Pd-N and Pd-O vibrations, respectively (Ouf et al., 2010), are due to the participation of the nitrogen of the azomethine group and the oxygen of the phenolate ring in the complexation of the palladium(II) centre by the Schiff base ligands. From the NMR results, the free 4-fluorobenzyl-(iminoethyl)phenolate ligand shows a multiplet at around 6.80-7.57 p.p.m. assignable to the aromatic protons. A corresponding multiplet appears in almost the same position in the spectrum of the Pd II complex (compound 1) as that observed by Gupta et al. (2013). Singlets for aliphatic methylene (-CH 2 ) and methyl (-CH 3 ) protons appear at 5.11 and 2.32 p.p.m., respectively. The 13 C chemical shift for the imine carbon (C N) is found at 169.8 p.p.m., agreeing with data reported by Ş enol et al. (2011).
The title compound was screened for catalytic activity in the Suzuki cross-coupling reaction between phenylboronic acid with iodobenzene. The reaction was carried out under nitrogen at 373 K in dimethylacetamide with a catalyst loading of 1 mmol%. The conversion of iodobenzene was monitored using GC-FID after 24 hours of reaction time. This resulted in a 52% conversion of iodobenzene in the reaction.

Refinement
Crystal data, data collection and crystal structure refinement details are summarized in Table 3 contacts for (1) drawn as dotted lines with ring centroids shown as coloured spheres. Cg4 is the centroid of the C9-C14 ring. H atoms are omitted for clarity.

Figure 4
The packing of (1) viewed approximately along the b axis showing molecular sheets of the Pd II complex. Only H atoms involved in C-HÁ Á ÁO interactions are shown for clarity.

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. 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.