Crystal structure of bis(μ2-4-bromo-2-[({2-[({2-[(5-bromo-2-oxidobenzylidene)amino]ethyl}sulfanyl)sulfonyl]ethyl}imino)methyl]phenolato)dicopper(II) dimethylformamide disolvate

The crystal structure of the dinuclear copper(II) complex with dianionic Schiff base derived from 5-bromosalicylic aldehyde and cysteamine prepared by direct synthesis is reported.


Chemical context
Schiff bases and their metal complexes have been studied extensively over the past few decades and represent one of the most widely used organic compounds due to their synthetic flexibility and wide range of applications (Mitra et al.,1997;Bera et al.,1998;Prabhakaran et al., 2004). Spontaneous selfassembly of Schiff base ligands appears to be an extremely powerful tool for the construction of novel polynuclear compounds. Such complexes having sulfur-containing ligands are of considerable interest because of their diverse coordination modes and bridging ability. The formation and cleavage of disulfide bonds are known to be important for the biological activity of several sulfur-containing peptides and proteins (Gilbert et al.,1999;Jacob et al., 2003). It has been shown earlier that copper(II) complexes containing ligands having thioalkyl moieties are efficient DNA-cleaving agents on treatment with either a reducing agent or on photo-irradiation (Dhar et al., 2005). In these studies, we continued our investigations in the field of direct synthesis -an efficient method to obtain novel mixed-valence  and heterometallic complexes with polynuclear (Vassilyeva et al., 1997;Kovbasyuk et al., 1998;Semenaka et al., 2010) and polymeric [Nesterova (Pryma) et al., 2004, Nesterova et al., 2005, 2008 structures. The conditions of direct synthesis influence the spontaneous self-assembly process enabling preparation of coordination compounds with commonly simple ligands e.g. aminoalcohols (Vassilyeva et al., 1997;Kovbasyuk et al.,1998;Semenaka et al., 2010), ethylenediamine or related compounds [Kokozay & Sienkiewicz, 1995;Nesterova (Pryma) et al., 2004, Nesterova et al., 2005, 2008. The title compound was isolated in an attempt to prepare a heterometallic Cu/Mn complex with a Schiff base ligand, a product of condensation between 5-bromsalicylaldehyde and cysteamine, formed in situ in a methanol/dimethylformamide (DMF) mixture starting from zero-valent Cu and MnCl 2 . We were unable to obtain the heterometallic complex, nevertheless we suppose that in this system MnCl 2 catalysed conversion of disulfides to thiosulfonates. Synthesis from the same starting materials with the same conditions without MnCl 2 leads to a Cu II complex whose structure is very similar to that of the already published compound (CSD refcode FEDCIB; Dhar et al., 2005).

Structural commentary
The title compound is a discrete dinuclear complex that lies across an inversion center (Fig. 1). The formula unit also contains two DMF molecules of crystallization. The Schiff base acts as a tetradentate bridging ligand with each Cu II ion bonded to four donor sites of the ligand. Each Cu II ion in the complex has a distorted square-planar CuN 2 O 2 environment.
The ligand fragments coordinated to Cu II ions are twisted, as defined by the dihedral angle of 22.6 (2) between the mean planes of atoms O1/N1/C1/C7 and O2/N2/C8/C14. The thiosulfonate moiety is not involved in a metal-ligand interaction. The coordination geometry around the Cu II ion is comparable to that found in the aforementioned Cu II complex with a very similar ligand that results from the condensation between salicylaldehyde and cysteamine hydrochloride (CSD refcode FEDCIB; Dhar et al., 2005). The separation between the two symmetry-related Cu II ions in the title complex is 4.6533 (15) Å . In general, all bonding parameters and the dimensions of the angles in the title complex are in good agreement with those encountered in related complexes (Dhar et al., 2005;Zhang et al., 2010). A fairly short intramolecular C-HÁ Á ÁO hydrogen bond is observed (Table 1).

Figure 1
The molecular structure of the title compound with ellipsoids drawn at the 50% probability level. Cu1A and unlabelled atoms are generated by the symmetry code (Àx + 1, Ày + 2, Àz + 1). For the sake of clarity, the DMF solvent molecules are not shown.

Figure 2
The crystal packing of the title compound viewed along the c axis. Short SÁ Á ÁBr contacts and weak C-HÁ Á ÁO hydrogen bonds are shown as dashed lines. Only selected H atoms are shown.

Synthesis and crystallization
A solution of KOH (0.12 g, 2 mmol) in minimum amount of methanol was added to a solution of aminoethanethiol hydrochloride (0.23g, 2 mmol) in methanol (5 ml) and stirred in an ice bath for 10 min. The white precipitate of solid KCl was removed by filtration and 5-bromosalicylaldehyde (0.402 g, 2 mmol) in dimethylformamide (10 ml) were added to the filtrate and stirred magnetically for 40 min. Copper powder (0.064 g, 1 mmol) and MnCl 2 Á4H 2 O (0.198 g, 1 mmol) were added to the yellow solution of the Schiff base formed in situ, and the resulting deep green-brown solution was stirred magnetically and heated in air at 323-333 K for 2 h, resulting in a deep-brown precipitate. Crystals suitable for crystallographic study were grown from a saturated solution in DMF after successive addition of CH 2 Cl 2 . The crystals were filtered off, washed with dry i-PrOH and finally dried at room temperature (yield: 18%). The IR spectrum of the title compound (as KBr pellets) is consistent with the structural data. It shows all the characteristic functional group peaks in the range 4000-400 cm À1 : (CH) due to aromatic C-H stretching at 3000-3100 cm À1 , the aromatic ring vibrations in the 1600-1400 cm À1 region, weak S-S absorptions at 500-540 cm À1 as well as absorbance at 1630 cm À1 assigned to the azomethine (C N) group and (SO) at 1330cm À1 . Analysis calculated for C 42 H 46 Br 4 Cu 2 N 6 O 10 S 4 : C 36.83, H 3.38, N 6.14, S 9.36%; found: C 37.1, H 3.4, N 6.0, S 9.4%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were placed in calculated positions and refined in a riding-model approximation.: C-H = 0.95-0.99 Å with U iso (H) = 1.5U eq (C-methyl) and = 1.2U eq (C) for other H atoms.    program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2016/4 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.006 Δρ max = 0.68 e Å −3 Δρ min = −0.77 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.