Bis{S-benzyl 3-[(6-methylpyridin-2-yl)methylidene]dithiocarbazato}nickel(II)

The asymmetric unit of the title compound, [Ni(C15H14N3S2)2], consists of two independent molecules with similar configurations. Each Ni2+ cation is coordinated in a cis-mode by two tridentate N,N′,S-chelating Schiff base ligands, creating a distorted octahedron [the smallest angle being 77.57 (7)° and the widest being 168.97 (7)° for one molecule, and 78.04 (7) and 167.55 (7)° for the second molecule]. The dihedral angle between the mean coordination planes of the two ligands is 86.76 (7)° for one and 89.99 (7)° for the second molecule. π–π interactions between neighbouring pyridine rings with plane-to-plane distances of 3.540 (1) and 3.704 (1) Å are observed.

The asymmetric unit of the title compound, [Ni(C 15 H 14 -N 3 S 2 ) 2 ], consists of two independent molecules with similar configurations. Each Ni 2+ cation is coordinated in a cis-mode by two tridentate N,N 0 ,S-chelating Schiff base ligands, creating a distorted octahedron [the smallest angle being 77.57 (7) and the widest being 168.97 (7) for one molecule, and 78.04 (7) and 167.55 (7) for the second molecule]. The dihedral angle between the mean coordination planes of the two ligands is 86.76 (7) for one and 89.99 (7) for the second molecule.interactions between neighbouring pyridine rings with planeto-plane distances of 3.540 (1) and 3.704 (1) Å are observed.

Comment
The title compound, [Ni(C 15 H 14 N 3 S 2 ) 2 ], was preferentially formed by elimination of the saccharinate anion from nickel(II) saccharinate during an attempt to introduce S-benzyl-beta-N-(6-methylpyrid-2-yl) methylenedithiocarbazate (for details see experimental part). More background on the coordination chemistry of hydrazine carbodithioates is given by Ravoof et al. (2010).
In the structure of the title compound two independent molecules are in the asymmetric unit with similar configurations.
The Ni 2+ cations are coordinated by two tridentate ligands in form of distorted NiN 4 S 2 octahedra with angles varying from 77.57 (7)° to 168.97 (7)°. The ligands are coordinating to the Ni 2+ ions in their deprotonated mercaptide forms via the pyridine nitrogen atoms, the azomethine nitrogen atoms and the thiolate sulfur atoms. The deprotonation of the ligands is accompanied by their tautomerism to the iminothiolate forms. While coordinating in the iminothiolate form, the negative charge generated upon deprotonation is delocalized in the C-N-N-C system as observed by their intermediate bond complex of the same 6-methylpyridine-2-aldehyde Schiff base with C-N bonds ranging from 1.281 (3) Å to 1.358 (3) Å and N-N bonds ranging from 1.379 (2) to 1.386 (2) Å (Omar et al., 2012) The two ligands coordinate to the nickel(II) ion in a cis mode (Fig. 1). Similar configurations have been observed in other bis-ligand metal complexes of related NNS-tridentate ligands (Ali et al., 1997(Ali et al., , 1999. The angle between the planes defined by S205-C204-N203-N202-C201-C214-N215 (minimum deviation from the mean plane: 0.003 Å and maximum deviation 0.04 Å), and S105-C104-N103-N102-C101-C114-N115 (0.001 and 0.094 Å) is 86.76 (7)°.
The angle between the corresponding planes in the molecule containing Ni2 is perfectly orthogonal with a value of 89.99 (7)°.  (Omar et al., 2012;Ali et al., 1997Ali et al., , 1999. None of the bond angles in the complex conform to the ideal values expected of a regular octahedral geometry, a trend that was observed in other related complexes indicating that distortion from ideal octahedral geometry is a common phenomenon in six-coordinate metal complexes of Schiff base ligands derived from dithiocarbazic acid and thiosemicarbazone ligands (Ali et al., 1997). The packing of the structure (Fig. 2) is dominated by π-π interactions between neighbouring pyridine rings with planeto-plane distances of 3.540 (1) and 3.704 (1) Å.

Experimental
Nickel(II) saccharinate, [Ni(sac) 2 (H 2 O) 4 ] . 2H 2 O was prepared using a similar procedure used for the synthesis of Cu(II) saccharinate] (Ravoof et al., 2004). The 6-methyl-2-pyridine carboxaldehyde Schiff base of S-benzyldithiocarbazate was synthesized following the procedure given by Ali et al. (1997). [Ni(sac) 2 (H 2 O) 4 ] . 2H 2 O (0.001 mol) was dissolved in ethanol (25 ml) and was mixed with a solution of the appropriate Schiff base (0.001 mol) in ethanol (50 ml). The resulting mixture was heated on a water bath until the volume reduced to 30 ml. On standing overnight, the mixture yielded crystals which were filtered off, washed with ethanol and dried in a desiccator over anhydrous silica gel. Upon recrystallization from acetonitrile, the solution yielded blackish green crystals suitable for X-ray analysis.

Refinement
The H atoms were all located in a difference map, but those attached to C atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H in the range 0.93-0.98, N-H in the range 0.86-0.89 Å) and U iso (H)(in the range 1.2-1.5 times U eq of the parent atom), after which the positions were refined with riding constraints.

Computing details
Data collection: CrysAlis PRO (Agilent, 2011); cell refinement: CrysAlis PRO (Agilent, 2011); data reduction: CrysAlis PRO (Agilent, 2011); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS (Betteridge et al., 2003).  The title compound with displacement ellipsoids drawn at the 50% probability level. Only one of the molecules in the asymmetric unit is shown.  Molecular packing diagram of the title compound viewed along the a axis. Hydrogen atoms are omitted for clarity.