(+)-trans-Chlorido{2-[(R p)-2-(methylsulfanyl)ferrocenyl]-2,5,6,7-tetrahydropyrrolo[1,2-c]imidazol-3-ylidene}bis(triphenylphosphane-κP)palladium(II) hexafluoridophosphate dichloroform disolvate

A solvated palladium(II) complex bearing a planar chiral ferrocenyl pyrroloimidazolylidene (NHC) ligand, synthesized by oxidative addition of a chloroimidazolium salt to Pd(PPh3)4, features a pendant thioether group that is not involved in coordination to Pd.


Chemical context
N-Heterocyclic carbenes (NHCs), such as imidazolylidenes, are electron-rich -donor ligands that may be electronically and sterically fine-tuned by changing the substituents on the azole ring (Clavier, 2006). These ligands exhibit weak -backbonding, resulting in increased electron density at the metal atom. Their overall electron-donating capacity is similar to that of trialkylphosphane ligands and is a main reason for interest in imidazolylidenes as ancillary ligands for transitionmetal complexes with potential applications in catalysis (Hopkinson et al., 2014). In general, higher electron density at transition metal atomshas been shown to promote oxidative addition steps in catalytic cycles (Peris, 2007). The selective synthesis of homochiral NHC ligands has been investigated concurrently with achiral forms. Particular attention has been paid to developing NHC ligands derived from planar chiral ferrocenes owing to the commercial importance of chiral ferrocene ligands, e.g. Josiphos (Schultz et al., 2005), Xyliphos (Spindler et al., 1990) and PhTRAP (Kuwano et al., 2000). Some early examples of complexes bearing chiral ferrocenyl NHCs include Chung's iridium complex 1, in which the thioether group is not involved in metal ligation (Seo et al., 2003) (Fig. 1). In contrast, bidentate 2 (Debono et al., 2010) or tridentate pincer-like ferrocenyl NHC-phosphane ligands 3 (Gischig & Togni, 2004) have been prepared, which feature seven-membered palladacycles. Complex 2 has been shown to catalyze asymmetric Suzuki-Miyaura coupling of aryl bromides with naphthylboronic acids in up to 42% ee (Debono et al., 2010). The preceding chiral ferrocenyl NHC ISSN 2056-9890 ligands were prepared by initial diastereoselective lithiation of Ugi's amine (complexes 1 and 3) (Marquarding et al., 1970) or Kagan's ferrocenyl acetal (complex 2) (Riant et al., 1993). We have recently reported that an iridium complex bearing a monodentate imidazolinylidene ligand catalyzes the hydrogenation of 2-substituted quinolines in up to 80% ee (John et al., 2015). This ligand was prepared by diastereoselective lithiation of a ferrocene containing a new pyrroloimidazolone chiral auxiliary in which the N atom was directly attached to the cyclopentadienyl (Cp) ring. The pyrroloimidazolone functionality doubled as a precursor to the NHC. In this sense, the NHC ligand in 4 is distinct from those in complexes 1-3, which have 'pendant' imidazolylidenes. In this paper, we have extended this synthetic approach to prepare an unsaturated pyrroloimidazolylidene analogue of the ligands in complexes 1-3 to study its coordination behaviour with palladium. The crystal structure of the title compound, 8, is presented herein.

Structural commentary
The molecular structure of the title compound, 8, is shown in Fig. 2. The Pd II ion is coordinated in a slightly distorted square-planar coordination geometry, with the Cl atom trans to the pyrroloimidazolylidene ligand. The ligand is monodentate, with an R p absolute configuration of the ferrocene moiety (Schlö gl, 1967). The Schlö gl convention has been used to assign planar chirality (R p or S p ) for consistency with our prior ferrocene work. As in iridium complex 1, the thioether group is not involved in coordination to the metal atom in the title complex. The triphenylphosphane ligands are in slightly different chemical environments, an observation that is consistent with the non-equivalency of their P atoms by 31 P NMR spectroscopy. The cyclopentadienyl (Cp) rings of the ferrocenyl group are tilted slightly, by 2.75 (14) , with respect to each other. The dihedral angle between the fused imidazole ring and the Cp ring to which it is attached is 46.1 (2) . The fused pyrrolidine ring is in an envelope conformation, with atom C3 forming the flap. Atom C3 is disordered over two sites, with refined occupancies of 0.77 (4) and 0.23 (4). Within the cation, there are siginficant intramolecularstacking interactions, with centroid-centroid distances less than 4 Å namely, Cg1Á Á ÁCg6 = 3.712 (3) Å , Cg2Á Á ÁCg5 = 3.861 (8)   Coordination complexes with chiral ferrocenyl NHC ligands.

Figure 2
The molecular structure of the cation of the title compound, shown with 30% probabilty displacement ellipsoids. H atoms have been omitted for clarity. The minor disorder component is not shown. Table 1 Hydrogen-bond geometry (Å , ).

Supramolecular features
In the crystal, weak C-HÁ Á ÁF and C-HÁ Á Á interactions connect the components of the structure, forming chains propagating along [110] (

General
The stereoselective synthesis of planar chiral ferrocene 6 by diastereoselective lithiation has been reported previously (Metallinos et al., 2012(Metallinos et al., , 2013. Thus, sequential deprotonation of imidazolone 5, followed by elecrophile quenching with dimethyl disulfide and subsequent acid-induced elimination of silanol, gave the chiral unsaturated urea 6. Heating urea 6 in neat phosphorus oxychloride in a sealed tube at 323 K resulted in the formation of chloroimidazolium salt 7, which was isolated as the hexafluoridophosphate salt upon salt Part of the crystal structure of 8, with weak C-HÁ Á Á interactions shown as dashed lines. The centroids Cg1, Cg2 and Cg3, and the symmetry code are defined in Table 1. Only H atoms involved in weak interactions are shown.

Figure 4
Part of the crystal structure of 8, with weak C-HÁ Á ÁF interactions shown as dashed lines. Only H atoms involved in weak interactions are shown. metathesis. Chloride 7 readily underwent oxidative addition with Pd(PPh 3 ) 4 according to the method of Fü rstner et al. (2003) to give the title palladium complex 8 in 67% yield. Recrystallization of 8 from chloroform solution containing a small amount of pentane gave the product as small yellow crystals that were suitable for X-ray diffraction. The reaction scheme is shown in Fig. 5.

Preparation of
A mixture of imidazolone 6 (147 mg, 0.42 mmol) in neat POCl 3 (0.5 ml, 5.36 mmol) was heated at 323 K for 16 h. The resulting solution changed progressively from orange to black during this period. After cooling to room temperature, the volatiles were removed under high vacuum. The black residue obtained was dissolved in CH 2 Cl 2 (10 ml) and treated with a saturated solution of KPF 6 in H 2 O/MeOH (2 ml). The mixture was stirred for 15 min at room temperature, resulting in a colour change from black to deep red. Water was added (10 ml), resulting in a biphasic mixture from which the organic layer was isolated, washed with water, dried over anhydrous Na 2 SO 4 , filtered and concentrated under reduced pressure. The crude product was taken up in CH 2 Cl 2 (2 ml) and added to an ice-cooled Et 2 O solution in an ice bath.

Figure 5
The reaction scheme.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were placed in calculated positions, with C-H = 0.95-1.00 Å , and included in a ridingmodel approximation, with U iso (H) = 1.5U eq (C) for methyl H atoms or 1.2U eq (C) otherwise. The flap atom, C3, of the fused pyrrolidine ring system was refined as disordered over two sites, with final occupancies of 0.77 (4) and 0.23 (4).