Crystal structure of chlorido(2-{[2-(phenylcarbamothioyl)hydrazin-1-ylidene](pyridin-2-yl)methyl}pyridin-1-ium)gold(I) chloride sesquihydrate

Synthesis and structural characterization of a new gold(I) complex with di-2-pyridyl ketone phenylthiosemicarbazone, [AuCl(C18H16N5S)]Cl·1.5H2O


Chemical context
Thiosemicarbazones are generated from reactions of thiosemicarbazides with either an aldehyde or a ketone. They are compounds that can coordinate to transition metals and exhibit keto-enol tautomerism (Duan et al., 1996). Thiosemicarbazones are known to have diverse biological activity, including anti-malarial properties and antibacterial, antitubercular, antiviral and antitumor activity (Beraldo & Gambino, 2004, Casini et al., 2008, Khanye et al., 2010. The study of gold compounds with thiosemicarbazones has great importance: the literature reports that some compounds of this type have been shown to exhibit biological activity and have potential applications (Casini et al., 2008, Lessa et al., 2011. ISSN 2056-9890

Structural commentary
In the title complex ( Fig. 1), the di-2-pyridyl ketone phenylthiosemicarbazone ligand is protonated at the pyridine (py) nitrogen and only the sulfur donor atom is used to bond to the central metal ion. The thiosemicarbazone adopts the E conformation in relation to the C6 N3 and N4-C12 bonds.
The crystal structure data confirm reduction of gold(III) of the starting material [HPy] [AuCl 4 ] during the synthesis. Two solvent water molecules and an non-coordinating chloride ion complete the structural assembly and are hydrogen bonded to the cationic complex.
The gold(I) atom displays the expected linear geometry, with a Cl-Au-S coordination angle of 174.23 (5) , close to the ideal angle of 180 expected for sp hybridization of the metal.
The C12-S1 bond length reported for di-2-pyridyl ketone phenylthiosemicarbazone is 1.676 (2) Å and it is lengthened to 1.713 (4) Å on coordination to gold; this is typical of the ketone form with a concomitant shortening of the N3-N4 bond (Suni et al., 2006).

Supramolecular features
In the crystal, the chloride ion is linked to the complex molecule by N-HÁ Á ÁCl hydrogen bonds. The molecular structure is also stabilized by intermolecular O-HÁ Á ÁCl and O-HÁ Á ÁO hydrogen bonding involving the water molecules. Therefore, upon protonation of the ligand, hydrogen-bond formation with the chloride ion results in a stabilization of the conformation of the cationic gold complex, and hydrogen bonding plays an important role in the crystallization of the compound (Table 1 and Fig. 2).      (7) Symmetry codes: (i) Àx þ 1 2 ; Ày þ 3 2 ; Àz; (ii) x; Ày þ 1; z À 1 2 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms potentially involved in hydrogen-bonding interactions were located in difference electron-density maps and their positional and isotropic displacement parameters were refined. Hydrogen atoms of water molecules were refined with distance restraints, with an HÁ Á ÁH separation of 1.38 (2) Å , the H-O distance restrained to 0.82 (2) Å and with U iso = 1.5U eq (O). Other H atoms were included in the refinement at calculated positions and treated as riding with U iso (H) = 1.2U eq (C).   program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 ; molecular graphics: DIAMOND (Crystal Impact, 2014); software used to prepare material for publication: publCIF (Westrip, 2010).