Diaquabis(ethylenediamine-κ2 N,N′)copper(II) 2,2′-dithiodinicotinate sesquihydrate

In the title compound, [Cu(C2H8N2)2](C12H6N2O4S2)·1.5H2O, there are two half-molecules of the cationic complex in the asymmetric unit. The Cu2+ ions lie on inversion centres and are octahedrally coordinated by two ethylenediamine (en) and two aqua ligands in a typical Jahn–Teller distorted environment with the water O atoms in the axial positions. Two 2-mercaptonicotinate units (mnic) are linked by a disulfide bridge. All the ethylenediamine N—H and O—H groups form intermolecular hydrogen bonds with acceptor O and N atoms, giving rise to a three-dimensional network. One of the uncoordinated water molecules has a site occupation factor of 0.5.

In the title compound, [Cu(C 2 H 8 N 2 ) 2 ](C 12 H 6 N 2 O 4 S 2 )Á1.5H 2 O, there are two half-molecules of the cationic complex in the asymmetric unit. The Cu 2+ ions lie on inversion centres and are octahedrally coordinated by two ethylenediamine (en) and two aqua ligands in a typical Jahn-Teller distorted environment with the water O atoms in the axial positions. Two 2mercaptonicotinate units (mnic) are linked by a disulfide bridge. All the ethylenediamine N-H and O-H groups form intermolecular hydrogen bonds with acceptor O and N atoms, giving rise to a three-dimensional network. One of the uncoordinated water molecules has a site occupation factor of 0.5.

Comment
As is well known, many oxidizing agents, such as nitric acid, hydrogen peroxide, oxygen, dimethyl sulfoxide and potassium ferricyanide, can oxidize thiols to disulfides (Yiannos & Karaninos, 1963). In several cases, the thiol-to-disulfide conversion can also be quickly completed via oxygen in the presence of certain metal ions (Chowdhury et al., 1994;Yamamoto & Sekine, 1984). In the present case, the formation of the mnic-mnic (mnic: 2-mercaptonicotinate) dianion may be due to the oxidation of mnic via oxygen in the presence of Cu(II). It was of interest to determine the structure of the title compound, as there are a limited number of documented metal-organic disulfide salts (Briansó et al., 1981;Casals et al., 1987). Here, we report the crystal structure of the title compound, (I).
The asymmetric unit of compound (I) contains two crystallographically independent half-complexes in which the ethylenediamine (en) ligands, aqua ligands, 2-mercaptonicotinate anions and water molecules occupy general positions, whereas the Cu(II) ions are located on centres of inversion. In the crystal structure of the title compound, (I), the Cu(II) ions are coordinated by four N atoms of en ligands, forming a slightly distorted square plane. The Cu-N distances of 2.005 (2), 2.016 (2), 2.025 (2), and 2.015 (2)Å are comparable to those in other ethylenediamine-copper(II) complexes, such as trans-Bis(ethylenediamine)bis(p-nitrobenzoxasulfamato)copper(II) (Kazak et al., 2004), Diaquabis(ethylenediamine) copper(II) bis(4-nitrobenzoate) (Harrison et al., 2007), The coordination sphere of the Cu(II)ions is completed by two longer contacts to two symmetry equivalent aqua ligands located above and below the tetragonal plane. The Cu-Ow distances of 2.702 (2)Å (Cu1-O1) and 2.499 (2)Å (Cu2-O2) are strongly elongated due to Jahn-Teller distortion and the coordination polyhedra around the Cu(II) ions can be described as significantly distorted octahedral.
The crystal packing of (I) is formed via interesting intermolecular hydrogen bonding interactions. It can be seen from  (Table   2), which lead to three dimensional extended network in the unitcell.
Experimental 2-mercaptonicotinic acid (0.31 g, 2 mmol) (HMNA) was added into a solution of Cu(II)Cl 2 .2H 2 O (0.17 g, 1 mmol) in ethanol (40 ml). After stirring for 30 min, ethylenediamine (0.12 g, 2 mm l) was added into solutions of these compounds, under stirring, and mixtures were allowed to stand at room temperature. After a few days, well formed purple crystals were selected for X-ray studies.