Crystal structure of (μ-N,N′-dibenzyldithiooxamidato-κN,S:N′,S′)bis[(η3-crotyl)palladium(II)]

In the centrosymmetric dinuclear title compound, [Pd2(C4H7)2(C16H14N2S2)], the metal atom is η3-coordinated by three C atoms of a crotyl ligand [Pd—C = 2.147 (4), 2.079 (5) and 2.098 (5) Å], the longest distance influenced by the steric interaction with the benzyl substituents of the dibenzyldithiooximidate (DTO) ligand. The Pd—N and Pd—S bonds to this ligand are 2.080 (3) and 2.3148 (9) Å, respectively, completing a square-planar coordination environment for PdII. The benzyl groups are oriented so as to maximize the interaction between a benzylic H atom and an S atom, resulting in a dihedral angle of 77.1 (2)° between the benzene rings and the metal complex plane. In the crystal, no inter-complex hydrogen-bonding interactions are present.


S1. Comment
η 3 -Allyl palladium complexes are very widely studied because they can act as precursors or intermediates in different catalytic processes (Jalòn et al., 2005). In the dimeric title η 3 -allyl-palladium(II) complex [Pd 2 (C 4 H 6 ) 2 (S 2 N 2 C 16 H 14 )] with the dibenzyldithiooximidate (DTO) ligand, the asymmetric unit consists of a half molecule lying across an inversion center ( Fig. 1). The DTO ligands adopt a binucleating role through the S and N atoms bridging two equivalent palladium centres with Pd-N6 and Pd-S bond distances of 2.080 (7) Å and 2.3148 (9) Å, respectively. The C4-C4 i bond length within the DTO ligand [1.517 (5) Å] is typical of values found in planar trans-dithiooxamides. The benzyl groups are oriented so as to maximize the intramolecular interaction between a benzylic H-atom and a sulfur atom [C5-H5···S i , 3.005 (4) Å: C-H···S, 116°], resulting in a dihedral angle of 77.1 (2)° between the benzene rings and the metal complex plane. With the allyl ligand, the η 3 asymmetric coordination as well as its orientation with respect to the palladium centers are mainly determined by steric hindrance between crotyl and benzyl fragments placing the latter on the opposite side of the methyl group. The Pd-C distances for the three C-atoms of the crotyl ligand (C1, C2, C3) are 2.147 (4), 2.079 (5) and 2.098 (5) Å, respectively. Although the secondary dithiooxamide is chelating the metal via the N···S sites in this complex, structural parameters around the Pd II are in good agreement with those in similar compounds in which the planar DTO bridge is chelating the "hard" palladium(II) via N,N′ atoms (Lanza et al., 2000;Lanza et al., 2003). The overall molecular packing, as shown in Fig. 2, no inter-complex hydrogen-bonding interactions are present.
The title compound consists of two halves each of which is made by two chiral planes: the one containing palladium and the other perpendicular to it, i.e. the η 3 -linked crotyl plane. The compound is a mesoform: in fact either chiral crotyl plane and palladium square plane of one half-molecule have opposite configurations with respect to the corresponding planes of the other half (Fig. 3). The compound might exist in different diastereomeric forms: one, represented here, is a mesoform having crotyl CH 3 cis to sulfur; the other is a racemate having crotyl CH 3 cis to sulfur and crotyl cusps oriented toward the same side of palladium molecular square planes. A more detailed description of the stereochemistry of the title compound has been done by us previously (Lanza et al., 2000;Lanza et al., 2011). Both the other mesoform and the racemate having the crotyl CH 3 cis to the nitrogen probably cannot be formed because of close contact between the crotyl CH 3 and the DTO benzyl substituents.

S3. Refinement
Although the hydrogen atoms could be clearly identified in a difference Fourier synthesis, all were idealized and refined at calculated positions riding on the carbon atoms with C-H distances of 0.96 Å (methyl), 0.97 Å (methylene), 0.93 Å (aromatic) and 0.97 Å for C1 and 0.98 Å for C2 and C3. Carbon atoms of the η 3 -linked crotyl fragment show large anisotropic displacement parameters giving rise to unusual C-C bond lengths and short H-H separations.

Figure 1
A perspective view of the centrosymetric title complex, with atom numbering and non H-atoms represented as 40% probability displacement ellipsoids. Symmetry code (i): -x, -y, -z + 1.

Figure 2
A packing diagram of the title complex viewed along the b axis with C-H···S interactions shown by dotted lines.

Figure 3
Enantiomeric symmetry-related C2-pairs in the title compound.

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 > 2σ(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