Unexpected formation and crystal structure of tetrakis(1H-pyrazole-κN 2)palladium(II) dichloride

In the title salt, the Pd2+ cation is located on an inversion centre and has a square-planar coordination sphere defined by four N atoms of four neutral pyrazole ligands. The two chloride anions are not coordinating to Pd2+ but are connected to the complex cations through N—H⋯Cl hydrogen bonds. C—H⋯Cl hydrogen bonds lead to a three-dimensional linkage of cations and anions.


Chemical context
Transition metal complexes containing pyrazole or substituted pyrazoles as ligands are of current interest due to their supramolecular arrangements (Lumme et al., 1988;Takahashi et al., 2006;Casarin et al., 2007;Alsalme et al., 2013). In the course of an investigation on the coordination chemistry of various azolyl-nitrochloroalkanes , we have previously studied the reaction of copper(II) perchlorate hexahydrate with equimolar amounts of 1-chloro-1-nitro-2,2,2-tris(pyrazolyl)ethane, Cl(NO 2 )CH-C(C 3 H 3 N 2 ) 3 ( Fig. 1) in methanol solution (Edelmann et al., 2008). Quite unexpectedly, a complete degradation of the starting material took place during the course of this reaction. As a result, the dark-blue compound trans-bis(perchlorato)-tetrakis-(pyrazole)copper(II), [Cu(C 3 H 4 N 2 ) 4 (ClO 4 ) 2 ], was isolated. The formation of free pyrazole could only be explained by a solvolytic degradation of the starting material. This degradation must have taken place to a large extent as the isolated yield was 64% (Edelmann et al., 2008).
We have now carried out a closely related reaction of 1chloro-1-nitro-2,2,2-tris(pyrazolyl)ethane with palladium(II) dichloride in methanol solution. Structure determination of the yellow reaction product using X-ray analysis surprisingly again revealed the presence of a homoleptic pyrazole complex. The structure of the resultant title compound, [Pd(C 3 H 4 N 2 ) 4 ]Cl 2 is presented here. An elemental analysis of the title compound was also in very good agreement with the composition C 12 H 16 Cl 2 PdN 8 . In this case, too, the yield was fairly high (56%), indicating a far-reaching decomposition of the starting material. Apparently, the ligand degradation of azolyl-nitrochloroalkanes in the presence of transition metal salts is a more common phenomenon than originally anticipated.

Structural commentary
In the crystal structure of the title compound, the Pd 2+ ion is located on an inversion centre and is bonded to four neutral pyrazole ligands within a square-planar coordination environment (Fig. 2). The average Pd-N distance in the [Pd(pyrazole) 4 ] 2+ cation is 2.000 (2) Å . This is exactly the same value as found for the Cu-N distance in trans-bis(perchlorato)tetrakis(pyrazole)copper(II) [2.000 (1) Å ; Edelmann et al., 2008]. The two chloride anions are not coordinating to the Pd 2+ cation. This is in marked contrast to the analogous copper(II) complex [Cu(pyrazole) 4 Cl 2 ] (Xing et al., 2006), in which the Cu 2+ ion is six-coordinated by four N atoms from four pyrazole ligands and two Cl À ions. The same octahedral coordination has also been reported for the manganese(II) analog [Mn(pyrazole) 4 Cl 2 ] (Lumme, 1985).

Supramolecular features
In the title compound, the crystal packing is stabilized by two N-HÁ Á ÁCl hydrogen bonds (Table 1) between the complex cations and the Cl À counter-anions (Fig. 3). Weaker C-HÁ Á ÁCl hydrogen bonds are also observed, stabilizing a threedimensional network. The crystal structures of the formally analogous complexes [M(pyrazole) 4 Cl 2 ] show related features. In the structures with M = Mn and Cu and an octahedral coordination of the metal cation, the crystal structures likewise exhibit N-HÁ Á ÁCl and C-HÁ Á ÁCl hydrogen bonds which, in combination, yield three-dimensional networks.

Figure 2
The coordination sphere of Pd 2+ and the Cl À counter-ions in the title compound. Displacement ellipsoids represent the 50% probability level.

Relation with other compounds
Various closely related homoleptic metal-pyrazole complexes are known from the literature (Misra et al., 1998;Reedijk, 1969;Sastry et al., 1986). Analogous complexes of composition [M(pyrazole) 4 Cl 2 ] have previously been reported for M = Mn, Fe, Co, Ni, and Cu (Daugherty & Swisher, 1968;Bagley et al., 1970;, 1971Lumme, 1985;Sun et al., 2001;Xing et al., 2006). Generally, these compounds are prepared in a more straightforward manner by treatment of the transition metal dichlorides with four equivalents of pyrazole in suitable solvents such as methanol. While the analogous nickel(II) complex has been studied frequently (Daugherty & Swisher, 1968;, to the best of our knowledge neither the title compound nor the platinum homologue [Pt(pyrazole) 4 ]Cl 2 have ever been reported.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms attached to carbon were included using a riding model, with C-H = 0.95 Å , and with U iso (H) = 1.2U eq (C). The hydrogen atoms attached to nitrogen were refined with a restrained distance N-H = 0.88 (2) Å and with U iso (H) = 1.2U eq (N).

Tetrakis(1H-pyrazole-κN 2 )palladium(II) dichloride
Crystal data [Pd(C 12  Hydrogen site location: inferred from neighbouring sites H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.049P) 2 + 4.2172P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.79 e Å −3 Δρ min = −1.73 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

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