trans-Bis[1-(2-benzamidoethyl)-3-(2,4,6-trimethylphenyl)imidazol-2-ylidene]dichloridopalladium(II)

In the title compound, [PdCl2(C21H23N3O)2], the PdII atom is located on an inversion centre and is coordinated in a slightly distorted square-planar environment by the chloride and N-heterocyclic carbene (NHC) ligands in mutual trans positions. There are several hydrogen-bonding interactions, the most significant of which is a hydrogen bond between the amide moiety of the NHC and the chloride ligand. These hydrogen-bond interactions form a three-dimensional network.

In the title compound, [PdCl 2 (C 21 H 23 N 3 O) 2 ], the Pd II atom is located on an inversion centre and is coordinated in a slightly distorted square-planar environment by the chloride and Nheterocyclic carbene (NHC) ligands in mutual trans positions. There are several hydrogen-bonding interactions, the most significant of which is a hydrogen bond between the amide moiety of the NHC and the chloride ligand. These hydrogenbond interactions form a three-dimensional network.

trans-Bis[1-(2-benzamidoethyl)-3-(2,4,6-trimethylphenyl)imidazol-2-ylidene]dichloridopalladium(II) Stefan Warsink and Andreas Roodt Comment
Palladium complexes bearing NHC ligands are well documented in literature (Hahn & Jahnke, 2008), even before the first free NHC was crystallographically characterized (Arduengo et al., 1991). As part of our focus on ligand manipulation for palladium complexes (Meij et al., 2005), we concentrated on the development of complexes bearing NHC ligands. NHCs are well-known for being very good σ-donors, but because of the empty p-orbital on the carbene carbon, complexes with high electron density are not necessarily destabilized by the presence of an NHC (Warsink et al., 2010). Several examples exist where an NHC is present on an electron-rich palladium(0) atom (Warsink et al., 2009), or where more than one NHC is present on palladium(II) (Fu et al., 2010).
With the addition of two NHCs to palladium(II), two possible isomers can result. Both have been prepared, with reaction conditions normally favouring the kinetic trans-product. The cis-product can be obtained by performing the reaction under thermodynamic control. When this type of complex is prepared from the silver(I) NHC complex (Wang & Lin, 1998), transfer of the carbene ligand usually takes place in minutes, even when two NHC moieties are transferred.
The precipitation of the silver salt ensures the reaction goes to completion.
The geometric parameters of the title compound, [PdCl 2 (C 21 H 23 N 3 O) 2 ], (I), show that the complex is square-planar, with bond lengths between palladium and its ligands being in the expected range. The Pd 2+ cation lies on an inversion centre, generating half of the molecule by symmetry. The C1-Pd1-Cl1 angle is 87.55 (4) °, slightly distorting the geometry of the complex. The NHC is twisted out of the coordination plane to alleviate the steric bulk induced by the mesitylsubstituent; the dihedral angle between the carbene core and the coordination plane is 72.37 (13) °.
There are several hydrogen bonding interactions, both inter-and intramolecular. The most significant of these is a hydrogen bond between the amide H atoms and the chlorido ligands (Table 2

Special details
Experimental. The intensity data was collected on a Bruker X8 ApexII 4 K Kappa CCD diffractometer using an exposure time of 5 s/frame. A total of 1386 frames was collected with a frame width of 0.5° covering up to θ = 28.31° with 99.7% completeness accomplished. Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s 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 > 2σ(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.