Crystal structure of a dicationic PdII dimer containing a 2-[(diisopropylphosphanyl)methyl]quinoline-8-thiolate pincer ligand

A dicationic PdII dimer containing a [2-(methylenphosphanyl)-8-thiolatequinoline] pincer ligand was isolated and its crystal structure determined.


Chemical context
The stereoelectronic properties of transition-metal complexes can be finely modulated thanks to the ligands introduced on the metal coordination sphere, and this plays a fundamental role in organometallic chemistry. Over the past two decades, impressive developments have been achieved with pincer complexes, which nicely illustrate how the properties and reactivity of a complex can be adjusted through ligand modifications (Morales-Morales, 2018). In pincer complexes, the central M-X bond is enforced by the coordination of two peripheral donor groups (D), and the chelating rigid nature of the monoanionic DXD pincer ligand bestows a unique balance between stability and reactivity. This has led to spectacular catalytic developments, including with pincer complexes based on Pd, a transition metal that occupies a central place in organometallic catalysis. As far as Pd is concerned, the main topology of the used monoanionic pincer ligands consists of an aryl central moiety featuring two coordinating side arms, as illustrated in Fig. 1 (model I). These complexes have been successfully applied to C-C or C-X bond-forming catalytic transformations. The impact of the side groups (coordinating atom and linker) on the catalytic performances has been explored (Selander et al., 2011). We have developed new models of Pd pincer complexes varying the aromatic central ring, introducing indenyl and indolyl moieties (model II in Fig. 1). The nature of the central ring was found to significantly ISSN 2056-9890 impact the catalytic activity of the Pd complexes in the allylation of amines (Lisena et al., 2013).
Seeking to further modify the structure of the Pd pincer complexes so that the catalytic activity can be modulated, we now aim to incorporate an extended -system as the central moiety (so that rigidity is increased). We have thus designed and prepared a pincer PNS Pd complex based on a 8-thiolatequinoline featuring a methylenephosphine side arm (model III in Fig. 1). We report herein that when cationizing the corresponding chloro palladium pincer complex 1 with AgSbF 6 , a dimeric dicationic species 2 crystallized with a tight S-bridging assembling of the two quinoline-based PNS Pd pincer fragments. The structural features are discussed. It is worth noting that we have previously reported S-bridged homo and hetero polymetallic species derived from Pd pincer complexes of type II (Nebra et al., 2011(Nebra et al., , 2012.

Structural commentary
X-ray diffraction of the yellow crystals obtained from 2(SbF 6 ) 2 revealed a dimeric structure, composed of two cationic PNSPd fragments, that crystallizes in the orthorhombic system and Pbca space group (Figs. 2 and 3; selected bond lengths and bond angles are given in Table 1). The dicationic nature of the structure is confirmed by the presence of two SbF 6 À units per dimer. The two PNSPd fragments are connected to each other by two bridging S atoms. The S donor atom of each PNSPd fragment completes the coordination sphere of the other, forming a Pd 2 S 2 diamond core.
For each PNSPd fragment, besides the two bridging S atoms, the Pd atom is coordinated by one N atom and one P atom, completing a tetracoordinate sphere that deviates The molecular structure of the title compound with the atom numbering. Displacement ellipsoids are drawn at the 50% probability level.  Figure 1 Schematic representation of Pd pincer complexes I-III

Figure 3
Detail of the molecular structure of 2 2+ , showing the main atomnumbering scheme and displacement ellipsoids at the 50% probability level. H atoms and i Pr groups have been omitted for clarity. As for the Pd 2 S 2 core, the two Pd-S bond lengths for each Pd atom are slightly different and, interestingly, the bonds between the Pd atoms and the bridging S atom of the other fragment are shorter [2.3149 (16) and 2.3184 (16) for Pd1-S2 and Pd2-S1, respectively] than the bonds between the Pd atoms and the chelating S atom of the pincer ligand [2.3657 (17) and 2.3602 (17) for Pd1-S1 and Pd2-S2, respectively]. This is most likely due to the rigidity of the 8thio-quinoline moiety (the C3-C4-S1 and C26-C27-S2 angles deviate from 120 by less than 2 ). The two S atoms are noticeably pyramidalized (AES = 287 and 290 for S1 and S2, respectively). The hinge angle of the core unit (involving the two [S,Pd,S] planes) has a value of 108.0 (1) , which is in fact the lowest value reported for such kind of dicationic species with a Pd 2 S 2 core (see the Database survey section). This results in a rather short Pd1-Pd2 distance of 2.8425 (7) Å , which is significantly shorter than the sum of van der Waals radii (4.10 Å ; Batsanov et al., 2001) and exceeds the sum of the covalent radii (2.78 Å ; Cordero et al., 2008) by only 2%.

Supramolecular features
The crystal packing of the title compound, illustrated in Fig. 4, involves weak intramolecular C-HÁ Á ÁCg contacts, and intermolecular C-HÁ Á ÁF contacts between the cations and anions, which link the components in a three-dimensional network (Table 2, Figs. 5 and 6). No classical hydrogen-bonding interactions were found.

Figure 5
C-HÁ Á ÁF hydrogen bonds (blue dotted lines). C-H bonds from five different dicationic units, while the other one (containing Sb2), interacts weakly with C-H bonds from three dicationic units and from a CH 2 Cl 2 solvent molecule. Finally, an intramolecular C-HÁ Á ÁCg short contact is observed between one of the CH 3 of the i Pr groups of one PNSPd pincer fragment (Pd2) and the benzo ring of the quinoline moiety of the other fragment [C16Á Á ÁCg1 = 3.701 (8) Å , associated with a H16AÁ Á ÁCg1 distance of 2.93 Å ] (Fig. 6). It should be noted that a significantly longer distance (H28BÁ Á ÁCg2 of 3.2 Å ) is observed for the other part of the unit (CH 3 group of the Pd2 fragment with the benzo ring of the other), indicating a non-symmetrical organization of the dimer.

Bis{2-[(diisopropylphosphanyl)methyl]quinoline-8-thiolato}palladium(II) bis(hexafluoridoantimonate) dichloromethane monosolvate)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.50 e Å −3 Δρ min = −1.07 e Å −3 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.