Crystal and molecular structures of dichloridopalladium(II) containing 2-methyl- or 2-phenyl-8-(diphenyphosphanyl)quinoline

The steric requirement resulting from the substituted methyl or phenyl group at the ortho-position of the coordinating quinoline-N atom in 8-(diphenylphosphanyl)quinoline enforces the square-planar coordination geometry of the dichloridopalladium(II) center to be highly distorted.


Chemical context
8-Quinolylphosphanes are competent ligands for various functional coordination compounds, because they consist of a strongly -donating phosphane donor group and a -conjugated quinoline moiety and form a stable planar fivemembered chelate ring on coordination to a metal center (Cai et al., 2018;Hopkins et al., 2019;Scattolin et al., 2017). In one of our previous studies, it was revealed that 8-(diphenylphosphanyl)quinoline (PQ H ) in the simplest dichloridopalladium(II) complex, [PdCl 2 (PQ H )] (3), exhibits a strong trans influence of the diphenylphosphanyl donor group and an intermolecularstacking interaction between the quinoline ring systems (Suzuki et al., 2015). Also, in the bis(PQ H )type Ni II , Pd II and Pt II (M II ) complexes, [M II (PQ H ) 2 ]X 2 (X = ClO 4 , BF 4 or CF 3 SO 3 ), the cis(P,P)-isomers are preferably formed due to the above-mentioned trans influence, but the mutually cis-positioned quinoline groups give a steric congestion between them, causing the coordination environ-ment around M II to be distorted (Suzuki, 2004;. When a methyl or phenyl group substituted at the ortho-position of the quinoline-N atom of PQ H is used for complexation, for example in 2-methyl-8-(diphenylphosphanyl)quinoline (PQ Me ) (1) or 2-phenyl-8-(diphenylphosphanyl)quinoline (PQ Ph ) (2), a much larger steric hindrance would be expected at the cis-position of the coordinating quinoline-N donor site. In the present study we reveal the characteristic structural features of the simplest PdCl 2 complexes bearing PQ Me (1) and PQ Ph (2) chelate ligands.

Structural commentary
In the crystals of (1) and (2), the quinolylphosphane moiety coordinates to a Pd II center in a bidentate 2 P,N mode, and the coordination environment around the Pd II center is roughly square-planar with two additional chlorido ligands. The analogous PQ H complex (3) has a typical planar environment (Suzuki et al., 2015); the 4 value (Yang et al., 2007) indicating the tetrahedral distortion around the four-coordi-nate Pd II center is here only 0.0552 (4). The coordination plane (defined by the central Pd and four donor atoms) and the quinoline plane are almost co-planar, with their dihedral angle being 8.58 (3) . The Pd1-P1 and Pd1-N1 bond lengths in the structure of (3) are 2.2026 (6) and 2.065 (2) Å , respectively, and the P1-Pd1-N1 chelate bite angle is 84.75 (6) . The Pd-Cl1 (trans to P1) and Pd1-Cl2 (trans to N1) bond lengths are 2.3716 (7) and 2.2885 (8) Å , respectively, indicating a strong trans influence of the Ph 2 P-donor group.
In the PQ Me complex (1) (Fig. 1), the coordination environment around the Pd II center is apparently distorted; the 4 value is 0.1555 (4). The steric requirement from the 2-methyl substituent of the coordinating quinoline group causes the Cl ligand in the cis-position to be pushed away (Fig. 2). Thus, the Cl1 atom is considerably displaced from the Pd II coordination plane (defined by Pd1, Cl2, P1 and N1) by 0.554 (1) Å , and the P1-Pd1-Cl1 bond angle is 166.74 (2) , as compared to the N1-Pd1-Cl2 angle of 171.32 (5) . More importantly, the PQ Me chelate ring is no longer planar. The Pd1 atom is displaced by 0.755 (2) Å from the chelating ligand plane (defined by P1, C8, C9 and N1), and the dihedral angle ' C between the plane [Pd1,P1,N1] and the quinoline plane Perspective (a) top and (b) side views of a dimeric unit of (1), showing the distortion of the coordination environment around Pd II and the intermolecularstacking interaction between the quinoline ring systems. Color code: Pd, blueish purple; P, orange; N, blue; C, black and H, gray.
(defined by N1 and C1-C9) is 25.35 (3) . Thus, an envelopetype deformation of the chelate ring is observed. This distortion would weaken the Pd-N bond, because the direction of the lone-pair electrons on the N atom does not match with the Pd II acceptor d-orbital. In fact, the Pd1-N1 bond length of 2.0971 (17) Å in (1) is slightly longer than that in (3). Other coordination bonds and angles are collated in Table 1 and are comparable to those in (3).

Supramolecular features
In the crystal structure, the molecular PQ H complex (3) forms a dimer by an intermolecularstacking interaction between the quinoline ring systems. A similar stacking interaction is observed in the crystal structure of the PQ Me complex (1) (Fig. 2). The shortest intermolecular contact distance is 3.322 (3) Å for C2Á Á ÁC6 i [symmetry code: (i) Àx + 1, Ày + 1, Àz). By contrast, the PQ Ph complex (2) does not show a similar stacking interaction to the above examples, because the twist motion of the attached phenyl group prohibits a full stacking interaction between the molecules (Fig. 4). The shortest intermolecular contact distance in (2) is 3.549 (3) Å for C2Á Á ÁC3 ii [symmetry code (ii) Àx + 1, Ày + 2, Àz + 1]. There are no other obvious supramolecular features in the crystal structures of (1) and (2) (Figs. 5 and 6).

Figure 4
A perspective side view of (2), showing the distortion of the coordination environment around Pd II . Color code: Pd, blueish purple; P, orange; N, blue; C, black and H, gray.

Synthesis and crystallization
The ligands, PQ Me and PQ Ph , were prepared according to the methods reported previously  tals of (1) were obtained by recrystallization from an acetonitrile solution by diffusion of diisopropyl ether. Complex (2) was prepared as follows: under a nitrogen atmosphere, a dichloromethane solution (10 ml) of PQ Ph (0.071 g, 0.18 mmol) was added under stirring to a dichloromethane solution (10 ml) of [PdCl 2 (PhCN) 2 ] (0.070 g, 0.18 mmol), and the mixture was stirred overnight at room temperature. The resulting brown precipitate was filtered off, and the filtrate was concentrated under reduced pressure. Diethyl ether was added under stirring to the concentrate, giving a yellow precipitate, which was collected by filtration, washed with diethyl ether (10 ml), and dried in vacuo. Yield: 0.041 g (40%). Analysis found: C, 56.23; H, 3.50; N, 2.56%. Calculated for C 27 H 20 Cl 2 NPPd: C, 57.22; H, 3.56; N, 2.47%. Yellow block-like crystals of (2) were obtained by recrystallization from an acetonitrile solution by diffusion of diisopropyl ether.

Dichlorido[8-(diphenylphosphanyl)-2-methylquinoline-κ 2 N,P]palladium(II) (complex1)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.46 e Å −3 Δρ min = −0.34 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.

Dichlorido[8-(diphenylphosphanyl)-2-phenylquinoline-κ 2 N</i.,P]palladium(II) (complex2)
Crystal data 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (3)