Crystal structure of unsymmetrical α-diimine palladium(II) complex cis-[{ArN=C(Me)–(Et)C=NAr}PdCl2] [Ar = 2,6-(iPr)2C6H3]

The synthesis and crystal structure of palladium(II) complex, cis-[{ArN=C(Me)-(Et)C=NAr}PdCl2] (Ar = 2,6-iPr2C6H3), containing unsymmetrical α-diimine ligand, is reported.


Chemical context
-Diimines (or) 1,4-diaza-1,3-butadienes (DAD) are one of the most versatile classes of chelating nitrogen-donor ligands, and are well known to stabilize several transition metal complexes at various oxidation levels (Bart et al., 2005;Greene et al., 2014). Nickel and palladium complexes of -diimines are reported to be effective catalysts for various olefin polymerization and co-polymerization reactions (Ittel et al., 2000). Furthermore, the polymer properties, topology and stability of these catalysts can be tuned by altering the steric and electronic properties of the -diimine ligands (Gates et al., 2000). These observations have motivated the synthesis of several nickel and palladium complexes with -diimine ligands containing various substituents at the imine nitrogen atom (Nakamura et al., 2009). -Diimine ligands may be conveniently prepared by condensation reactions between alkyl or aryl amine with 1,2-diketones. Most of the reported -diimine ligands possess molecular C2 symmetry, while very few unsymmetrical -diimine ligands, obtained by varying the substituents on the nitrogen atom, have been reported (Jeon & Kim, 2008). We report herein the synthesis and spectroscopic characterization of the unsymmetrical -diimine ligand [ArN C(Et)-(Me)C NAr], (I), [Ar = 2,6-i(Pr) 2 C 6 H 3 ] and the corresponding palladium complex cis-[PdCl 2 {I}] (II), where the -diimine ligand backbone contains methyl and ethyl substituents. The crystal structure of compound (II) has been established using single-crystal X-ray diffraction.

Structural commentary
The molecular structure of Pd II complex (II), is presented in Fig. 1. Compound (II) crystallized along with a solvent molecule of 1,2-dichloroethane, which is disordered over the two crystallographic positions. The molecular structure of (II) revels the chelation of the -diimine ligand to the palladium(II) atom. The Pd1-N1 and Pd1-N2 distances are 2.0280 (19) and 2.0200 (18) Å , respectively, and are in the typical range for palladium -diimine complexes (Zou & Chen, 2016). The C1-C2 bond length is 1.492 (3) Å , which is slightly shorter than a standard C-C bond length (1.54 Å ; Chandrasekaran et al., 2014), and similarly minimal elongation of the C1-N1 and C2-N2 bonds confirms the slight delocalization of the double bonds. As expected, the palladium(II) atom is in a distorted square-planar geometry, with an N2-Pd1-N1 angle of 79.01 (8) . The coordination plane shows a slight tetrahedral distortion from square-planar, as indicated by the dihedral angle between the Cl1-Pd1-Cl2 and N1-Pd1-N2 planes of 4.19 (8) . The chelate ring is folded along the N1Á Á ÁN2 vector by 7.1 (1) . The aryl substituents at N1 and N2 are nearly perpendicular to the metal-ligand plane, subtending dihedral angles of 81.82 (2) (C6-C11 aryl ring) and 86.74 (2) (C18-C23 aryl ring). The aryl substituents in square-planar -diimine complexes are anticipated to lie perpendicular to the metal-ligand plane due to steric repulsion.

Supramolecular features
In the crystal lattice, the components are linked through weak C-HÁ Á ÁCl hydrogen-bonding interactions between the complex and solvent molecule 1,2-dichloroethane (Table 1, Fig. 2).

Figure 2
Hydrogen-bonding interactions in the crystal lattice.

Figure 1
Perspective view of palladium complex (II) with displacement ellipsoids drawn at the 50% probability level. Growing X-ray quality crystals of thw ligand by slow evaporation from various solvents such as hexane, diethyl ether, dicholoromethane and toluene was unsuccessful.

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, collected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 15 sec/frame. 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. Hatoms attached to carbon were placed in calculated positions (C-H = 0.95 -1.00 Å). All were included as riding contributions with isotropic displacement parameters 1.2 -1.5 times those of the attached atoms. The dichloroethane solvent molecule is disordered over two resolved sites in an 86:14 ratio. The minor component was refined with restraints that its geometry appoximate that of the major component.