Crystal structures of diiodidobis[(1S,5S)-4-mesityl-1,2,8,8-tetramethyl-2,4-diazabicyclo[3.2.1]octan-3-ylidene-κC 3]palladium(IV) and dichlorido[(1S,5S)-4-mesityl-1,2,8,8-tetramethyl-2,4-diazabicyclo[3.2.1]octan-3-ylidene-κC 3](triphenylphosphane-κP)palladium(IV)

The PdII atoms in two N-heterocyclic carbene(NHC)/halogenide complexes show distorted square-planar coordination environments. In one complex, two NHC ligands are present, and the second complex contains an auxiliary triphenylphosphane ligand.


Chemical context
Six-membered N-heterocyclic carbene (NHC) ligands differ from the extensively reported five-membered analogues in several aspects. As a result of the increased N-C-N angle, the N-substituents exhibit a larger proximity to the metal atom, which can be an advantage for the transfer of chirality from the ligand to the product during a catalytic reaction or for the reductive elimination during the catalytic cycle (Cavallo et al., 2005). The increased -donor ability of sixmembered NHC ligands in comparison with their fivemembered analogues can be helpful for catalytic applications or for the discovery of new catalytic reactions (Drö ge & Glorius, 2010). Furthermore, NHC-metal complexes are less sensitive to dissociation, oxygen or elevated temperature compared to similar phosphane-metal complexes (Nolan, 2006). Notably, (NHC) 2 Pd complexes are known for their synthetic and catalytic applications (Schneider et al., 2006;Tü rkmen & Cetinkaya, 2006). Structures of related biscarbene complexes are known from Dunsford & Cavell (2014), Mayr et al. (2004) and Poulten et al. (2014).

Structural commentary
The molecular structures of the title compounds, (I) and (II), are shown in Figs. 1 and 2, respectively. The structure of (I) ISSN 2056-9890 shows a distorted square-planar coordination environment around the Pd II atom by the two N-heterocyclic carbene (NHC) and two iodido ligands. The deviation of the Pd II atom from the I 2 C 2 best plane is 0.206 (1) Å . The iodide ligands are trans-arranged and enclose an I-Pd-I angle of 163.275 (13) Å , whereas the C-Pd-C angle measures 178.32 (12) . Pd-X bond lengths for X = C1, C20, I1, I2 are 2.070 (3), 2.079 (3), 2.6334 (4) and 2.6360 (4) Å , respectively. Other selected X-Pd-X angles are listed in Table 1. The mesityl ring planes make a dihedral angle of 32.7 (2) .
The structure of (II) also shows the Pd II atom to be in a slightly distorted square-planar coordination by one NHC, one phosphine and two chlorido ligands. The deviation of Pd II from the PCl 2 C best plane is only 0.052 (1) Å . The Cl ligands are also trans-arranged and enclose a Cl-Pd-Cl angle of 173.53 (9) whereas the C-Pd-P angle measures 177.6 (2) . Pd-X bond lengths for X = C1, P1, Cl1, Cl2 are 2.048 (7), 2.355 (2), 2.309 (2) and 2.311 (2) Å , respectively. Other selected X-Pd-X angles are listed in Table 2. The more pronounced deviation from planarity of the iodido complex is caused by the sterically more demanding iodido and the requirements of the mesityl-NHC ligands, respectively. In general, the NHC ligands in the structures of (I) and (II) exhibit no unexpected geometries.

Supramolecular features
The crystal packing of (I) shows weak intermolecular C5-H5AÁ Á ÁI1 hydrogen bonds that link molecules into zigzag chains extending parallel to [100] ( The molecular structure of (I), with anisotropic displacement ellipsoids drawn at the 50% probability level.

Figure 2
The molecular structure of (II), with anisotropic displacement ellipsoids drawn at the 50% probability level.

Figure 3
The crystal packing of (I), viewed approximately along [010], with intermolecular hydrogen bonds shown as dashed lines. H atoms not involved in the hydrogen bonding have been omitted.

Figure 4
The crystal packing of (II), viewed along [100], with intermolecular hydrogen bonds shown as dashed lines. H atoms not involved in the hydrogen bonding have been omitted. Table 4 Hydrogen-bond geometry (Å , ) for (II). with pentane. Yellow crystals of Pd(NHC) 2 I 2 (I) and colourless crystals of Pd(NHC)(PPh 3 )Cl 2 (II) suitable for X-ray diffraction were obtained after several days.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 5. Hydrogen atoms were clearly located from difference Fourier maps and then refined at idealized positions riding on the carbon atoms with U iso (H) = 1.2U(C eq ) or 1.5U(C eq ) (-CH 3 ) and C-H 0.95-1.00 Å . All CH 3 hydrogen atoms were allowed to rotate but not to tip.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'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 > σ(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.