Dichlorido(4-methylaniline-κN)[N-(4-methylphenyl)-1-(thiophen-2-yl)methanimine-κN]palladium(II)

The structure of a mono-amine PdII complex is reported in which the Pd—NH2 length is slightly shorter than the observed mean value for other complexes involving Pd attached to the nitrogen of an aniline derivative.


Chemical context
The chemistry of monodentate mono-amine Pd II compounds with amine ligands is of interest because the hydrogen bond between the amine and the catalyst plays a key role in the catalytic transformation of simple, easily accessible amines into highly substituted, biologically important aminecontaining molecules and pharmaceutical agents (Calleja et al., 2015). While mono-amine Pd II complexes are generally unstable and are formed as intermediates during the reaction, the corresponding bis(amine) Pd II complex is stable and ultimately hampers the utility of these compounds in the C-H activation reaction. Probably because of this, well-characterized mono-amine Pd II complexes are relatively rare. In this article we report a well-characterized and room-temperaturestable mono-amine Pd II complex.

Structural commentary
In the Schiff base ligand HL1 used [HL 1 = (E)-1-(thiophen-2yl)-N-(p-tolyl)methanimine and L 2 = p-toluidine], we expected that the ortho proton of the tolyl ring of HL 1 could be acidic and thus could be employed for a metallation reaction. Ding and coworkers (Ding et al., 1992) have reported a series of mercuration reactions on similar Schiff base ligands through electrophilic substitution reactions. On the basis of these observations, we also envisaged that a palladation reaction should take place at the ortho position of the tolyl ring in the Schiff base ligand. To investigate this C-H activation step, we attempted to prepare the complex PdL 1 L 2 Cl. However, when we treated 2-thiophenecarboxaldehyde with two equivalents of p-toluidine in the presence of Na 2 PdCl 4 in ethanol solvent at 343 K, none of the expected palladated molecules, PdL 1 Cl or PdL 1 L 2 Cl were observed, and instead we directly isolated the corresponding mono-amine Pd II complex Pd(HL 1 )L 2 Cl 2 ,1, as red needles in good yield along with a small amount of a yellow solid. The isolated solid was not soluble in common organic solvents. The filtrate of the reaction mixture was allowed to evaporate at room temperature and afforded red needles of a mono-amine Pd II complex. In the FTIR spectra, the C N stretching frequency in the Pd complex shifts to lower values (1611 cm À1 ) with somewhat weaker intensity in comparison to those of the corresponding free ligand (1615 cm À1 ). Two singlets were also observed at 3777 and 3696 cm À1 for the asymmetric and symmetric N-H stretching frequencies, respectively, in the Pd complex. Both frequencies shift to longer wavelengths with weaker intensity in comparison to free p-toluidine (3421 and 3338 cm À1 for N-H) as a result of the presence of strong N-HÁ Á ÁCl hydrogenbonding interactions. This observation was further supported by single-crystal X-ray studies.
Not only does this result contrast with those found for other Schiff base compounds (Dubey et al., 2019), which readily form a stable palladated complex, but this reaction is also a relatively rare example of a mono-amine Pd II complex. A search of the Cambridge Structural Database (CSD, version 5.43, update of November 2021; Groom et al., 2016) for structures containing a Pd(NH 2 -phenyl derivative)Cl 2 fragment gave 51 hits, of which 30 were bis(amine)PdCl 2 moieties and among these was the complex Pd(p-toluidine) 2 Cl 2 (YOYWOB; Tay, 2019) which is relevant for comparison with the title compound. Of the remaining 21, 11 contained the NH 2 group as part of a chelate ring and only 10 contained a monodentate mono-amine PdCl 2 complex (BOCTIX, Hadzovic et al., 2008;HIPDEP, Vicente et al., 1998;KASNAU, Asma et al., 2005;OCATEV, OCATIZ, Xia et al., 2021;OCEPOE, Asma & Kaminsky, 2017;XEKFEZ, Randell et al., 2006;XIYLOG, Liu et al., 2002;XORVIM, Hu et al., 2019;and YELMOS, Asma et al., 2006). One of these structures (HIPDEP; Vicente et al., 1998) is particularly relevant as it contains an sp 2 C donor attached to a PdCl 2 (o-toluidine) fragment where the major difference with the present structure is the substitution of the sp 2 C for sp 2 N.
An ORTEP view of the molecular structure of Pd(HL 1 )L 2 Cl 2 , 1, is shown in Fig. 1 and selected bond lengths and bond angles are given in Table 1. This mono-amine Pd II complex crystallizes in the triclinic space group, P1. The primary geometry around the Pd II atom closely resembles square planar ( 4 0 = 0.069, where 0 = square planar and 1 = tetrahedral; Okuniewski et al., 2015). In the (E)-1-(thiophen-2yl)-N-(p-tolyl)methanimine ligand, the phenyl and thiophene rings are not coplanar because of the steric clash of the hydrogen atoms attached to C5 and C7, exhibiting a dihedral angle of 38.5 (1) . In addition, the coordination plane (Pd1, Cl1, Cl2, N1, and N2) is almost perpendicular to both the planes of the  Table 1 Selected geometric parameters (Å , ).

Figure 1
Molecular structure of Pd(HL 1 )L 2 Cl 2 showing the atom-numbering scheme. Atomic displacement parameters are at the 30% probability level.
A related structure (HIPDEP; Vicente et al., 1998) contains an sp 2 C donor attached to a PdCl 2 (o-toluidine) fragment where the major differences with the present structure are the substitution of the sp 2 C atom for sp 2 N, and the fact that there are cis Cl donors, which leads to a substantial trans effect involving the Pd-Cl distances. In this structure, the Pd-NH 2 distance is 2.076 (2) Å . The other related structure (Tay, 2019) is trans-Pd(o-toluidine) 2 Cl 2 in which the Pd-NH 2 distance is 2.050 (3) Å .

Supramolecular features
The molecules display an interesting supramolecular synthon in the crystal. This synthon is based on reciprocal intermolecular N-HÁ Á ÁCl hydrogen-bonding interactions (Table 2) of the p-toluidine amine fragment and results in centrosymmetric dimeric units (Fig. 2). These units are further linked by intermolecular C-HÁ Á ÁCl interactions, resulting in chains in the c-axis direction where the mean-planes of the repeating fragment are oriented in the (110) plane.

Synthesis and crystallization
A solution of 2-thiophenecarboxaldehyde (0.50 ml, 5 mmol) and 2 equivalent of p-toluidine (1.07 g, 10 mmol) in 20 ml of freshly distilled ethanol was allowed to stir at room temperature for 1 h. Then Na 2 PdCl 4 (1.47 g, 10 mmol) was added. The reaction mixture was refluxed under stirring at 343 K for 2 h. A small amount of yellow solid gradually separated during the reaction. After stirring for 3 h the solid was filtered off and the filtrate underwent slow evaporation at room temperature to give red needles of (p-CH 3 C 6 H 4 NH 2 )SbHPdCl 2 ; yield: 0.80 g, 33%, m.

Figure 2
Packing diagram for Pd(HL 1 )L 2 Cl 2 showing the intermolecular N-HÁ Á ÁCl hydrogen-bonding interactions of the p-toluidine amine fragment resulting in centrosymmetric dimeric units that are further linked by intermolecular C-HÁ Á ÁCl interactions, resulting in chains in the c-axis direction where the mean planes of the repeating fragment are oriented in the (110) plane.

Dichlorido(4-methylaniline-κN)[N-(4-methylphenyl)-1-(thiophen-2-yl)methanimine-κN]palladium(II)
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.