Iodido{4-phenyl-1-[1-(1,3-thiazol-2-yl-κN)ethylidene]thiosemicarbazidato-κ2 N′,S}{4-phenyl-1-[1-(1,3-thiazol-2-yl)ethylidene]thiosemicarbazide-κS}mercury(II)

In the title compound, [Hg(C12H11N4S2)I(C12H12N4S2)], the Hg atom is in a distorted square-pyramidal coordination, defined by the iodide ligand, by the S atom of the neutral ligand in the apical position, and by the N atom of the thiazole ring, the thioureido N and the S atom of the deprotonated ligand. The deprotonated ligand intramolecularly hydrogen bonds to the thiazole ring N atom, while the deprotonated ligand forms an intermolecular hydrogen bond to the thiolate S atom. The deprotonation of the tridentate ligand and its coordination to Hg via the S atom strikingly affects the C—S bond lengths. In the free ligand, the C—S bond distance is 1.685 (7) Å, whereas it is 1.749 (7) Å in the deprotonated ligand. Similarly, the Hg—S bond distance is slightly longer to the neutral ligand [2.6682 (18) Å] than to the deprotonated ligand [2.5202 (19) Å]. The Hg—I distance is 2.7505 (8) Å.

In the title compound, [Hg(C 12 H 11 N 4 S 2 )I(C 12 H 12 N 4 S 2 )], the Hg atom is in a distorted square-pyramidal coordination, defined by the iodide ligand, by the S atom of the neutral ligand in the apical position, and by the N atom of the thiazole ring, the thioureido N and the S atom of the deprotonated ligand. The deprotonated ligand intramolecularly hydrogen bonds to the thiazole ring N atom, while the deprotonated ligand forms an intermolecular hydrogen bond to the thiolate S atom. The deprotonation of the tridentate ligand and its coordination to Hg via the S atom strikingly affects the C-S bond lengths. In the free ligand, the C-S bond distance is 1.685 (7) Å , whereas it is 1.749 (7) Å in the deprotonated ligand. Similarly, the Hg-S bond distance is slightly longer to the neutral ligand [2.6682 (18)

Comment
Studies on thiosemicarbazones and their metal complexes has remained a fertile field of research for more than three decades due to their significant impacts in biology and chemistry (Ali & Livingstone, 1974;Campbell, 1975;Pellerito & Negy, 2002). Thiosemicarbazones are known to coordinate to many metals, potentially as tridentate ligands. The ligation can occur with the metals as a neutral molecule or, after deprotonation, as an anionic ligand (Campbell, 1975;Raper, 1985;Casas et al., 2000). Further, the metal-chelating ability is attributed to the thione-thiol tautomerism exhibited by these molecules. Some of the metal complexes are found to exhibit enhanced biological activity compared to their basic ligand (Blanz & French, 1968).
Among the metals studied with thiosemicarbazones, mercury and organomercury compounds are scarce (Grecu & Neamtu, 1967;Lobana et al.,1998;Bermejo et al., 1999;Akinchan et al., 2002;Bermejo et al., 2003). Removal and remediation of many mercury species from environmental samples are very important. We report here the synthesis and structure of a mercury(II) iodide complex of the phenyl derivative of 2-acetylthiazole-3-thiosemicarbazone. The title complex is a result of interaction between two neutral ligand molecules and mercury(II) iodide in methanol. In this complex, Hg(II) is chelated by two 2-acetylthiazole-3-phenylthiosemicarbazone ligands, forming a distorted square pyramidal geometry (Fig 1). One of the two ligands is deprotonated at N3, and tridentate through its N, N, S atoms. Along with the iodide atom, it forms the square planar base. The other ligand is neutral and apical to the Hg atom, binding through its S atom. The proton NMR also provides the evidence of ligand deprotonation during metal chelation. The sharp resonance signal due to N-NH proton at 11.12 p.p.m. disappears in the spectrum of the complex. The N4 signal (at ~8.64 p.p.m.) in the ligand undergoes a downfield shift that is marked in the mercury (II) complexes, indicating coordination via the S atom. The protonated and deprotonated ligands have different conformations, differing primarily by the N-N-C-N torsion angle, which is antiperiplanar in the deprotonated ligand (torsion angle N2-N3-C6-N4 179.6 (5)°) and synperiplanar in the neutral ligand (torsion angle N6-N7-C18-N8 9.3 (9)°). Hydrogen bonding details are given in Table 1.

Experimental
To a solution of 2-acetyl thiazole thiosemicarbazone (Venkatraman et al. 2009) (1.38 g, 5 mmol) in warm methanol (50 ml) was added an equimolar methanol solution (50 ml) of mercury(II) iodide (1.36 g. 5 mmol). The mixture was stirred for about 24 h, after which time the yellow solid obtained was filtered and vacuum dried (yield ~75%). Crystals suitable for diffraction studies were obtained from the mother liquor at room temperature after a week.

Refinement
All H atoms were placed in calculated positions, guided by difference maps, with C-H bond distances 0.95-0.98 Å, N-H 0.88 Å, and thereafter refined as riding with U iso = xU eq , where x = 1.5 for methyl H and 1.2 for all other H atoms. The highest peak in the final difference map was 1.29 Å from the Hg position.  Fig. 1. The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.