A new example of intramolecular C—H⋯Ni anagostic interactions: synthesis, crystal structure and Hirshfeld analysis of cis-bis[4-methyl-2-(1,2,3,4-tetrahydronaphthalen-1-ylidene)hydrazinecarbothioamidato-κ2 N 1,S]nickel(II) dimethylformamide monosolvate

The homoleptic nickel–thiosemicarbazonate complex shows structural features including an unusual cis-coordination and trans-anagostic Ni—H intramolecular interactions. In the crystal, complex and DMF solvate molecules build up a one-dimensional hydrogen-bonded polymer along [010].


Chemical context
One of the first reports on thiosemicarbazone chemistry can be traced back to the beginning of the 20th century in Germany (Freund & Schander, 1902). Initially, thiosemicarbazone derivatives were the products of the identification and characterization reactions of aldehydes and ketones, with thiosemicarbazide as reagent. In the 1940s it was reported that, in in vitro assays, thiosemicarbazones turned out to be very effective for the Mycobacterium tuberculosis growth inhibition (Domagk et al., 1946) while the synthesis of thiosemicarbazone metal complexes had already been investigated by the early 1950s (Kuhn & Zilliken, 1952). As a result of the main fragment, R N-N(H)-C( S)-NR 2 , thiosemicarbazone derivatives have a wide range of coordination modes and applications in inorganic chemistry. The hydrazinic H atom can be easily removed and the negative charge is then delocalized over the C-N-N-C-S backbone, which enables chemical bonding with many different metal ions (Lobana et al., 2009). However, a cis configuration of the ligated molecules is a rather uncommon coordination mode for mono-thiosemicarbazones and, as far as we know, there is only one Ni II mono-thiosemicarbazone complex reported in the literature, with N-phenyl-2-(1,2,3,4-tetrahydronaphthalen-1-ylidene)hydrazinecarbothiamide as ligand (for the ligand crystal structure, see: de Oliveira et al., 2014a; for the crystal structure of the complex, see: de Oliveira et al., 2014b). It can be suggested that the molecular symmetry decreases from a trans to a cis configuration, possibly by loss of inversion symmetry at the central metal cation, which is compensated for by HÁ Á ÁNi intramolecular interactions and hydrogen-bond formation with solvent molecules. In general, HÁ Á Ámetal ion interactions can show covalent or electrostatic character and are observed in some complexes with catalytic applications (Brookhart et al., 2007). As part of our research on the synthesis and structural studies of thiosemicarbazone derivatives, we report herein a new solvated nickel homoleptic complex with the 4-methyl-2-(1,2,3,4-tetrahydronaphthalen-1ylidene)hydrazinecarbothioamide ligand and dimethylformamide (DMF) as solvent.

Structural commentary
One molecule of the title complex and one dimethylformamide solvate comprise the asymmetric unit. The Ni II ion is fourfold coordinated in a distorted square-planar environment by two chelating thiosemicarbazonate ligands (Fig. 1).
The title complex shows two remarkable structural features, namely a cis coordination mode, which is rather uncommon for mono-thiosemicarbazone ligands, as well as two positioned   Symmetry code: (i) Àx þ 1; y À 1 2 ; Àz þ 1 2 .

Figure 1
The molecular structure of the title compound and the dimethylformamide solvate, with labelling and displacement ellipsoids drawn at the 40% probability level.

Supramolecular features and Hirshfeld surface analysis
In the crystal, the coordination entities are linked by DMF solvate molecules through N-HÁ Á ÁO interactions. The DMFoxygen atoms are hydrogen-bond acceptors, forming a bridging structure between two N-HÁ Á ÁO arrangements: N6-H6Á Á ÁO1 and N3-H3Á Á ÁO1 i [symmetry code: (i) Àx + 1, y À 1 2 , Àz + 1 2 ]. The molecules are linked into one-dimensional hydrogen-bonded polymers along [010] (Fig. 3, Table 1). Additional C-HÁ Á ÁO interactions are also present (Table 1). Hirshfeld (1977) analysis of the crystal structure suggests that the intermolecular HÁ Á ÁH interactions contribute 66.6% to the crystal packing, the HÁ Á ÁS interactions 12.3% and the HÁ Á ÁC interactions 10.9%. Other important intermolecular contacts for the cohesion of the molecules are HÁ Á ÁN = 4.5% and HÁ Á ÁO = 4.0%. The weak HÁ Á ÁNi interactions contribute by 0.20% to the crystal structure. All contributions to the crystal cohesion are shown as two-dimensional Hirshfeld surface fingerprint plots with cyan dots (Wolff et al., 2012). The d e (y axis) and d i (x axis) values are the closest external and internal distances (values in Å ) from given points on the Hirshfeld surface contacts (Fig. 4). Section of the crystal structure of the title compound viewed along [001], with hydrogen bonds shown as dashed lines (for details, see: Table 1). The figure is simplified for clarity.

Comparison with a related structure
For comparison with the title compound, a literature search revealed only one crystal structure of an Ni II -monothiosemicarbazone complex with cis configuration, viz. bis{cis-(2-(1,2,3,4-tetrahydronaphthalen-1-ylidene)-4-phenyl-hydrazinecarbothioamidate-2 N 1 ,S)}nickel(II) monohydrate bis(tetrahydrofurane) solvate (de Oliveira et al. 2014b). The graphical representation of the Hirshfeld surface was performed for the two complexes and suggests, represented in magenta colour, the locations of the strongest intermolecular contacts (Fig. 5). Both structures have the same main fragment for the ligand, the -tetralone-thiosemicarbazone, anagostic HÁ Á ÁNi intramolecular interactions and hydrogen bonding with the solvate molecules, suggesting the stabilization of the crystal packing, since the cis configuration implies a symmetry decrease with loss of the inversion center and appears to be energetically unfavourable.

Synthesis and crystallization
Starting materials were commercially available and were used without further purification. The synthesis of the ligand was adapted from a procedure reported previously (Freund & Schander, 1902) with 1-tetralone and 4-methylthiosemicarbazide. 2-(1,2,3,4-Tetrahydronaphthalen-1-ylidene)-4methyl-hydrazinecarbothioamide was dissolved in tetrahydrofuran (THF; 2 mmol / 40 ml) with stirring maintained for 30 min until the solution turned yellow. At the same time, a green solution of nickel acetate tetrahydrate in THF (1 mmol/ 40 ml) was prepared under continuous stirring. A dark coloured mixture of both solutions was maintained with stirring at room temperature for 6 h. A crude dark red material was obtained by evaporation of the solvent. Dark red crystals of the complex, suitable for X-ray analysis, were obtained by recrystallization of the solid from a dimethylformamide solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in difference maps but were positioned with idealized geometry and were refined using a riding model with U iso (H) = 1.2U eq (C and N) for the sp 2 -hybridized DMF C atom, the aromatic and the secondary C atoms, and for all N atoms, and with U iso (H) =     -bis[4-methyl-2-(1,2,3,4-tetra-hydronaphthalen-1-ylidene)hydrazinecarbothioamidato-κ 2 N 1 ,S]nickel(II) dimethylformamide monosolvate