Bis[(dimethylphosphoryl)methanaminium] tetrachloridopalladate(II)

In the crystal structure of the title compound, (C3H11NOP)2[PdCl4], (dimethylphosphoryl)methanaminium (dpmaH+) cations are connected head-to-tail by strong N—H⋯O hydrogen bonds, forming inversion-related cyclic dimers. The square-planar [PdCl4]2− counter-dianion is located about a center of inversion. The dications and the [PdCl4]2− dianions are connected by medium–strong N—H⋯Cl hydrogen bonds, forming zigzag chains parallel to [001]. Somewhat weaker N—H⋯Cl hydrogen bonds connect the chains into a three-dimensional network.


Comment
The synthesis of (dimethylphosphoryl)methanamine (dmpa) and its use as a bidentate N,O-ligand has been proved (Kochel, 2009 and literature therein). Furthermore, two transition metal complexes are reported in which the dpmaH + cation acts as a monodentate ligand (Reiss, 2013b,c). Recently, a series of dmpaH + salts have been synthesized and structurally characterized. In all cases hydrogen bonds strongly affect the set-up of the crystal structures. In many cases the head-to-tail connection of two and more dpmaH + tectons leads to hydrogen-bonded polymeric structures with the counter anions only very weakly attached (Buhl et al., 2013;Lambertz et al. 2013;Reiss, 2013a), but there are structures which show the ability of the dpmaH + tecton to form medium-strong hydrogen bonds with the surrounding cations and anions as well (Reiss & Jörgens, 2012;Reiss, 2013d).
Imidazolium tetrachloridopalladate(II) salts have recently attracted much attention as potent precursors for preparation of catalytic metal nanoparticles (Serpell et al., 2013), as solids that show thermochromism (Hardacre et al., 2001) and may form liquid crystalline phases (Lee et al., 2004). Furthermore, it should be mentioned that corresponding tetrachloridopalladate(II) salts have recently been used as pre-catalyst for the Suzuki reaction (Song et al. 2012).
The asymmetric unit of the structure of the title compound, (C 3 H 11 NOP) 2 [PdCl 4 ], consists of one dpmaH + cation and one half [PdCl 4 ] 2anion with the Pd(II) atom located on an inversion center (Wyckoff site: 2a). The bond lengths and angles of both ions are all in the expected ranges. Pairs of dpmaH + cations are connected head-to-tail around centers of inversion (Wyckoff site: 2 d) forming cyclic dimers (Fig. 1). The cyclic dimers and the centrosymmetric [PdCl 4 ] 2anions are connected by medium-strong and charge-supported N-H···Cl hydrogen bonds (H···Cl = 2.41 (2) Å; Table 1), forming chains along [001]. The hydrogen bonding connection of the dicationic cycle can be classified by the first level graph-set descriptor R 2 2 (10) (Grell et al. 2002), whereas the connection along the chain is represented by the second level graph-set descriptor C 3 3 (11) (Fig. 1). Taking into account also weak bifurcated N-H···Cl hydrogen bonds (H···Cl: 2.73 (2) and 2.83 (2) Å), a three-dimensional network is obtained.
According to the occupation of two different centers of inversion by cyclic dimers and [PdCl 4 ] 2anions, respectively, the arrangement of each of them can be described as a body-centered sublattice (Fig. 2). Secondary Pd···Cl interactions perpendicular to the plane of the [PdCl 4 ] 2anion, known for related salt structures (e.g. Willett & Willett, 1977;Vranec et al., 2012) are ruled out by the afore discussed packing scheme. In the title structure two methyl groups of two different (dpmaH) 2 2+ dications roughly occupy and block these axial positions at the Pd(II) atom (Fig. 1).

Experimental
In a typical experiment 0.25 g PdCl 2 (1.4 mmol) were dissolved in 4 ml concentrated hydrochloric acid (37%) and 0.3 g (2.8) dpma was added to this yellow solution. Within a few days yellow crystals grow at the bottom of the vessel.

Refinement
All hydrogen atoms were identified in difference Fourier syntheses. The methyl groups were idealized and refined using rigid groups allowed to rotate about the P-C bond (AFIX 137 option of the SHELXL-2013 program (Sheldrick, 2008)) with the U iso (H) = 1.5U eq (C). The hydrogen atoms at the CH 2 -group were idealized and treated as riding with U iso (H) = 1.2U eq (C). The coordinates of the hydrogen atoms at the NH 3 -group were refined freely simultaneously with individual U iso values.

Figure 1
The cyclic dimer, built up from two dpmaH + cations, is hydrogen-bonded to neighbouring [PdCl 4 ] 2anions. Ellipsoids are drawn at the 50% probability level. The graph-set descriptors R 2 2 (10) and C 3 3 (11) are indicated by green and red numbers.  In the packing of the title structure the cations and anions occupy different centers of inversion. Therefore, the arrangement of cations and anions, respectively, represent body-centered sublattices. Ellipsoids are drawn at the 40% probability level; the methyl groups are omitted and the methylene group is shown in a wireframe style for clarity.