Dibromido(di-2-pyridyl disulfide-κ2 N,N′)zinc(II)

The molecular structure of the title compound, [ZnBr2(C10H8N2S2)], contains a seven-membered chelate ring in which the zinc atom is coordinated by two bromide ions and by the two pyridyl N atoms of a single 2,2′-dipyridyldisulfide (dpds) ligand within a slightly distorted tetrahedron. As is usual for this type of complex, the disulfide group does not participate in zinc coordination. The chelate complexes are connected via weak intermolecular C—H⋯Br hydrogen bonding into chains, which extend in the [010] direction.


Comment
In our ongoing investigation on the synthesis, structures and properties of new coordination polymers based on zinc(II) halides and N-donor ligands (Bhosekar et al. 2007), we have startet systematic investigation of their thermal behavior because we have demonstrated that new ligand-deficient coordination polymers can be conveniently prepared by thermal decompisition of suitable ligand-rich precursur compounds (Näther et al. 2003;Näther & Jess, 2006). In further investigations we have reacted zinc(II) bromine with 2,2'-bipyridyldisulfide (dpds). In this reaction the title chelate-complex has been formed by accident.
The versatile coordination properties of dpds enables a series of different chelate-complexes and coordination polymers.
It can act in N,N'-bidentate (Kinoshita et al., 2003;Kadooka et al. 1976& Pickardtet al. 2005 or bridging (Kubo et al. 1998& Kinoshita et al. 2003) coordination modes toward many metals. When dpds is connected to the metal atom as a chelate ligand, a seven-membered ring is formed.
The title compound is isotypic to that of the corresponding chloride compound reported by Pickardt et al. in 2005. In the crystal structure the coordination geometry about the Zn(II) ion is almost tetrahedral with bonds being formed to two bromine ions and the two pyridyl nitrogen atoms of a single dpds ligand (Fig. 1). These latter interactions result in the formation of a seven-membered chelate ring. As usual for this type of complexes, the disulfide group does not participate in  (15) to 119.06 (4)°, the largest being Br-Zn-Br (Tab. 1). The structural parameters in the dpds molecule are quite regular. In particular the C-S bond, 1.784 (7)-1.783 (6) Å, is in good agreement with those expected for C(sp 2 )-S bonds (1.77 Å). The S-S bond length, 2.050 (3) Å, is somewhat longer than that found in the structure of the free ligand, 2.016 (2) Å (Raghavan & Seff, 1977).

Experimental
ZnBr 2 and dpds was obtained from Alfa Aesar and methanol was obtained from Fluka. 0.125 mmol (28.15 mg) zinc(II) bromine, 0.125 mmol (27.5 mg) dpds and 3 ml of methanol were transfered in a test-tube, which were closed and heated to 110 °C for four days. On cooling colourless block-shaped single crystals of (I) were obtained.

Refinement
All H atoms were located in difference map but were positioned with idealized geometry and were refined isotropic with U eq (H) = 1.2 U eq (C) of the parent atom using a riding model with C-H = 0.95 Å.  Fig. 1. : Molecular structure of the title compund with labelling and displacement ellipsoids drawn at the 50% probability level.  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 > σ(F 2 ) is used only for calculating Rfactors(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.