Coordination of bis(pyrazol-1-yl)amine to palladium(II): influence of the co-ligands and counter-ions on the molecular and crystal structures1

The crystal structures for five PdII complexes containing the tridentate ligand bis[2-(3,5-dimethylpyrazol-1-yl)ethyl]amine (pza) are reported. The co-ligand completing the square-planar coordination of the PdII centre influences the conformation of the pza ligand.

tetrakis(thiocyanato-S)palladate, [Pd(pza)NCS] 2 [Pd(SCN) 4 ], (5), the [Pd(pza)X] + complex cation displays a square-planar coordination geometry, and the pza ligand is twisted, approximating twofold rotation symmetry. Although the pza ligand is found with the same conformation along the series, the dihedral angle between pyrazole rings depends on the co-ligand X. This angle span the range 79.0 (3)-88.6 (1) for the studied complexes. In (3), two complex cations, two I À anions and one water molecule of crystallization are present in the asymmetric unit. In (5), the central amine group of pza is disordered over two positions [occupancy ratio 0.770 (18):0.230 (18)]. The complex [Pd(SCN) 4 ] 2À anion of this compound exhibits inversion symmetry and shows the Pd 2+ transition metal cation likewise in a square-planar coordination environment. Compound (5) is also a rare occurrence of a non-polymeric compound in which the pseudohalide ligand NCS À behaves both as thiocyanate and isothiocyanate, i.e. is coordinating either through the N atom (in the cation) or the S atom (in the anion).

Chemical context
The coordination chemistry of transition metals having a d 8 shell is clearly dominated by the square-planar geometry, which gives strong crystal field stabilization, because filled orbitals d z2 and degenerated orbitals (d xz d yz ) do not interact directly with orbitals of the ligands. This holds true for group 10 metal complexes, for which the tetrahedral geometry is considered as an oddity (Alvarez et al., 2005).
We synthesized a series of such square-planar complexes, with general formula n[Pd(pza)X]YÁmH 2 O, in which pza is the ISSN 2056-9890 tridentate ligand bis-[2-(3,5-dimethylpyrazol-1-yl)ethyl]amine, and X, Y are halide, pseudohalide, nitrate, or a complex anion. This series was first considered within a larger project related to a systematic study of modifications of cis-platin, obtained through the substitution of NH 3 ligands by N-heterocyclic systems, like imidazole-and pyrazole-based ligands. The Pd II synthetic chemistry may be easily transferred to Pt II , with the advantage that Pd II starting materials are somewhat cheaper than their Pt II analogues. On the other hand, regarding the chemical crystallography, Pd II complexes are almost always isostructural to their Pt II analogues. Finally, any new Pd II complex is also of potential interest for studies about the fundamental aspects of the catalysis of the Heck reaction type.
We thus focused our efforts on the crystallographic characterization of the Pd II complexes obtained as single crystals, with the hope of rationalizing the effect of the co-ligand X and counter-ion Y on the molecular and crystal structures of the complex [Pd(pza)X] + cations. An earlier report of the crystal structure of the starting material, [Pd(pza)Cl]ClÁ2H 2 O has been given (Mendoza et al., 2006), and we now report on the characterization of [Pd (

Structural commentary: molecular and crystal structures
Complex (1) is a result of the substitution of the counter-ion Y = Cl À in the starting material, i.e. in the dihydrate [Pd(pza)Cl]ClÁ2H 2 O by a nitrate, but crystallizes as an anhydrous species, [Pd(pza)Cl]NO 3 (Fig. 1). As expected, the square-planar coordination of the metal cation is retained, and the conformation of the pza ligand is not affected by the counter-ion substitution. The cation conformation may be characterized by the dihedral angle between the pyrazole mean planes, 85.1 (3) versus 87.62 (11) in the chloride salt (Mendoza et al., 2006). A least-squares fit between the [Pd(pza)Cl] + cations in the chloride and nitrate salts gives an r.m.s. deviation of 0.124 Å . However, the crystal structures are different because the water molecules in the chloride dihydrate are determinant for the supramolecular arrangement through hydrogen-bonding and intermolecular contacts. In (1), the nitrate ion interacts with the central amine group of the pza ligand, with a N10-H10Á Á ÁO1 separation of 1.98 Å . Other inter-ion contacts beyond the asymmetric unit are unexceptional, and the observed crystal structure is basically a consequence of Coulombic interactions rather than hydrogen bonds (Table 1).
Complex (2), with X = Br À and Y = NO 3 À is isostructural with the X = Cl À analogue (1). However, a slight relaxation of the folded pza ligand is observed, with a dihedral angle between pyrazole rings of 83.6 (2) . An overlay between cations in (1) and (2) gives a small deviation of 0.049 Å (Fig. 1,  inset). The nitrate anion interacts with the complex cation in (2) with a distance N10-H10Á Á ÁO1 = 1.98 Å (Table 2). Thus, the nature of the halogen co-ligand X in [Pd(pza)X]NO 3 seems to be unimportant for the resulting crystal structure.
compound (2), is amplified with X = I À : the angle between the pyrazole rings is now 79.0 (3) and 83.3 (3) , for the Pd1 and Pd2 cations, respectively. There seems to be a regular trend for [Pd(pza)X] + cations: the smaller the ionic radius of the coligand X, the closer the angle between the pyrazole rings is to 90 . A possible rationalization of this observation is that methyl groups substituting pyrazole rings at position 3 interact with the co-ligand X. This destabilizing steric interaction favors the twisting of pza, which in general adopts a noncrystallographic twofold rotation symmetry. However, the large iodide anion forces the separation between methyl groups, compared to the small chloride ion. In order to keep the coordination geometry around Pd II as planar as possible, the heterocycles in pza then make a slight rotation motion, which is reflected in the deviation from orthogonality of these terminal rings. In other words, the combined twisting and folding motions of the pza ligand lead to as planar as possible a coordination environment for Pd II . Counter-ions Y and lattice water molecules have only slight influences, if any, on the cation conformation. In the case of (3), the water molecule behaves both as a donor and acceptor group for hydrogen bonding. O-HÁ Á ÁI bonds are formed with the non-coordinating iodide anions, and the central amine group of pza forms a N-HÁ Á ÁO bond with the same water molecule (Table 3). However, as for previous complexes (1) and (2) View of the molecular structure of complex (3), corresponding to X = Y = I À , with displacement ellipsoids for non-H atoms drawn at the 30% probability level.

Figure 4
View of the molecular structure of complex (5), corresponding to X = NCS À and Y = [Pd(SCN) 4 ] 2À , with displacement ellipsoids for non-H atoms at the 30% probability level. Only one position for the disordered amine group in the cation has been retained (N10A). In the anion, unlabelled atoms are generated by symmetry code (Àx + 1, Ày + 2, Àz + 2).
Using the pseudohalide X = Y = azide, compound (4) was crystallized as an hydrate, [Pd(pza)N 3 ]N 3 ÁH 2 O (Fig. 3). The nitrogen atoms in the coordinating N 3 À ligand are not sterically demanding as the iodide ion in (3) and, as a consequence, the pyrazole rings come back in a more orthogonal arrangement, identical to that observed in [Pd(pza)Cl] + . The dihedral angle between pyrazole rings is 87.3 (1) in (4). The strongest hydrogen bond is found between the amine group of pza and the free azide ion, the N10-H10Á Á ÁN32 separation being 1.95 Å and the angle for the contact 171 (Table 4).
Finally, in the fifth compound (5), the counter-ion Y is a complex anion, namely [Pd(SCN) 4 ] 2À . The formula for (5) is 2[Pd(pza)NCS][Pd(SCN) 4 ], and the anion is located about an inversion centre, while the cation is in a general position (Fig. 4). The pza ligand in [Pd(pza)NCS] + , in contrast to previous compounds, has the amine group N10 disordered over two positions, N10A and N10B, with occupancies 0.770 (18) and 0.230 (18), respectively. The same type of disorder was previously reported for an Au III complex (Segapelo et al., 2011). In spite of this disorder, the general conformation of pza is identical to that observed in compounds (1)-(4), approximating the non-crystallographic twofold rotation symmetry. The co-ligand X = NCS À coordinates through its N atom, and the local environment of the metal is very similar to that resulting from azide coordination in complex (4). The dihedral angle between pyrazole rings should thus be close to 90 . The actual value is 88.6 (1) . The anion [Pd(SCN) 4 ] 2À is also square-planar, but with the ligands coordinating in a S-fashion, while in the cation, the NCS ligand is bound in a N-fashion to the metal cation. If complexes with bridging thiocyanate ligands are not considered, very few structures are known in which the ambidentate ligand NCS À is bonded in two modes (S-and N-) to the same transition metal. In the case of Pd II , classified as a soft acid in the Pearson's HSAB concept, the soft base SCN À should have a preference for the S-coordination. Apparently, only a few non-polymeric crystal structures have been reported including both coordination modes of SCN À to this metal (e.g. Paviglianiti et al., 1989;Chang et al., 2005). In the crystal structure, weak hydrogen bonds between the disordered amino group and the NCS groups of neighbouring cations and anions are observed (Table 5).
The conformation observed for pza is determined by the coordination number of the metal centre. For example, hexacoordinated transition metals like Ni II or Cd II favor the facial coordination of pza, which is then found in a folded conformation, while coordination numbers 5 and 4 promote some defolding. The ligand pza with the dihedral angle between pyrazole rings very close to 0 has been observed in Co II complexes (Massoud et al., 2012a(Massoud et al., , 2013. A conformation for pza close to that observed in (1)-(5) has been reported with Mg II (Lian et al., 2007b) and Au III (Segapelo et al., 2011).
Synthesis of (1). [Pd(pza)Cl]ClÁ2H 2 O (1 mmol) was dissolved in CH 3 CN, and a solution of AgNO 3 (1 mmol in CH 3 CN) was added slowly. The mixture was stirred for 1 h at room temperature. After elimination by filtration of the white precipitate of AgCl, the mixture was further stirred for 1 h. Evaporation of the solvent afforded complex (1) as a brownyellow solid, in 82% yield, and crystals were obtained by recrystallization from CH 3 CN.
Synthesis of (2). [Pd(pza)Cl]ClÁ2H 2 O (1 mmol) was dissolved in CH 3 CN, and a solution of AgNO 3 (2 mmol in CH 3 CN) was added slowly. The mixture was stirred for 2 h at room temperature, and the precipitated AgCl was removed by filtration. An aqueous solution of NaBr (1 mmol) was then added, and NaNO 3 precipitates, which was removed by filtration. The solution was further stirred for 5 h. Evaporation of the solvent afforded complex (2) as a yellow solid, in 76% yield, and crystals were obtained by recrystallization from CH 3 CN.
Synthesis of (3). [Pd(pza)Cl]ClÁ2H 2 O (1 mmol) was dissolved in CH 3 CN (5 ml) and a solution of 2 mmol of NaBF 4 in CH 3 CN was added slowly. After elimination of NaCl by filtration, a solution of 2 mmol of NEt 4 I in CH 3 CN was added slowly, and the mixture, which turned red, was stirred for 6 h at room temperature. Evaporation of the solvent afforded complex (3) as a red solid, in 82% yield, and crystals were  Table 5 Hydrogen-bond geometry (Å , ) for (5). Synthesis of (4). [Pd(pza)Cl]ClÁ2H 2 O (1 mmol) was dissolved in CH 3 CN. A solution of NaN 3 (2 mmol, CH 3 CN/ H 2 O mixture 4:1, v/v) was added slowly. The formed precipitate of NaCl was eliminated by filtration, and the mixture was further stirred at room temperature for 10 h. Evaporation of the solvent afforded complex (4) as a yellow solid, in 61% yield, and crystals were obtained by recrystallization from CH 3 CN.
[Pd(pza)Cl]ClÁ2H 2 O (1 mmol) was dissolved in H 2 O, and an aqueous solution of 2 mmol of KNCS was added slowly. The mixture was stirred for 10 h at room temperature. The formed pink solid, (5), was separated by filtration and dried in reduced pressure at 313 K. Yield: 48%. Crystals were obtained by recrystallization from a mixture of CH 3 CN and CH 2 Cl 2 (2:1, v/v).