Gold Thione Complexes

The reaction of the ligand Et4todit (4,5,6,7-Tetrathiocino-[1,2-b:3,4-b']diimidazolyl-1,3,8,10-tetraethyl-2,9-dithione) with gold complexes leads to the dinuclear gold(I) complexes [{Au(C6F5)}2(Et4todit)] and [Au(Et4todit)]2(OTf)2, which do not contain any gold-gold interactions, or to the gold(III) derivative [{Au(C6F5)3}2(Et4todit)]. The crystal structures have been established by X-ray diffraction studies and show that the gold centers coordinate to the sulfur atoms of the imidazoline-2-thione groups.


Results and Discussion
The Et 4 todit ligand was prepared from the corresponding imidazoline-2-thione-4,5-dione by reaction with the Lawesson's reagent, as it was described previously [9,13,14].The HOMO-1 and HOMO calculated at DFT level for Et 4 todit are built up of the in-phase and out-of-phase combinations of the non-bonding lone pairs localized on the sulfur atoms, perpendicular to the imidazole plane, respectively, with remarkably negative NBO charges [9].The terminal thiocarbonyl groups are potential donors not only towards molecular acids, [8][9][10] but also towards transition metal ions [11,12].We have, therefore, studied the coordination properties of this ligand towards gold(I) and gold(III) derivatives.Scheme 1 represents the synthesized complexes.The reaction of Et 4 todit with [Au(C 6 F 5 )(tht)] (tht = tetrahydrothiophene, molar ratio = 1:2) leads to the dinuclear derivative [{Au(C 6 F 5 )} 2 (Et 4 todit)] (1) (Scheme 1).It is a white solid that has been characterized by 1 H-NMR and 19 F-NMR spectroscopy and mass spectrometry.Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of hexane in a solution of 1 in 1,2-dichloroethane, and the molecular structure is depicted in Figure 1.The two five-membered rings of the bis(thiocarbonyl) donors are twisted about the C-C bond by 71.19°, similarly to what found in the crystal structure of Et 4 todit (69.90°) and its derivatives, such as Et 4 todit•2Br 2 , where the four independent molecules of the asymmetric units show torsion values ranging between 67.5° and 79.7° [9,14].The two gold centers present a distorted linear geometry and are bonded to the C=S groups of the thioimidazol rings and to the pentafluorophenyl groups.The S(1)-Au(1)-C(1) angle (174.8(1)°) and the Au(1)-C(1) (2.022(4) Å) and Au(1)-S(1) (2.333(1) Å) bond distances found in the solid structure are in good agreement with those reported in the literature [4,5].No Au(I)-Au(I) interactions have been observed in the crystal structure.The complex is not luminescent in solid state or in solution.A selection of bond lengths and angles for complex 1 are summarized in Table 1.

Distances (Å) Angles (°)
Au( 1  A selection of bond lengths and angles for complex 2 are summarized in Table 2.With the gold(III) complex [Au(C 6 F 5 ) 3 (tht)] (molar ratio = 1:2) the dinuclear derivative [{Au(C 6 F 5 ) 3 } 2 (Et 4 todit)] (3) was obtained (Scheme 1).The yellow solid is soluble in dichloromethane, acetone or diethyl ether, but insoluble in hexane.Complex 3 has been characterized by 1 H-NMR and 19 F-NMR spectroscopy and mass spectrometry.Single crystals were obtained by slow diffusion of hexane in a solution of complex 3 in 1,2-dichloroethane.Crystal structure has been elucidated and the proposed stoichiometry was confirmed, although (crystal system, triclinic; space group, P-1; cell: a, 12.6131; b, 13.2567; c, 18.3299 Å; α, 78.727; β, 74.371; γ, 82.645°; V, 2885.41;Z, 2) the quality of the data was not good.Thus, no comparison of bond distances and angles with other complexes can be made, although a molecular diagram is shown in Figure 3.The two gold centers present a distorted square-planar geometry and each one is bonded to the C=S groups of the thioimidazol rings and to three pentafluorophenyl groups.

General Procedure for the Synthesis of the Complexes 1-3
Synthesis of [{Au(C 6 F 5 )} 2 (Et 4 todit)] (1).A solution of [Au(C 6 F 5 )(tht)] (83.7 mg, 0.18 mmol) in dichloromethane (5 mL) was added to a solution of Et 4 todit (40.4 mg, 0.09 mmol) in dichloromethane (5 mL).The mixture was stirred for two hours at room temperature and the solution was filtered through Celite.The complex 1 was obtained as white crystals by slow diffusion of hexane in the reaction mixture (92.8 mg, yield, 86.7%). 1   (2).[Ag(tht)(OTf)] (51.2 mg, 0.15 mmol) was added to a solution of [AuCl(tht)] (47.6 mg, 0.15 mmol) in dichloromethane (25 mL).The mixture was stirred for three hours at room temperature and the AgCl precipitated was filtered off through Celite.To the filtered mixture a solution of Et 4 todit (58.4 mg, 0.13 mmol) in dichloromethane (10 mL) was added dropwise and stirred for one hour at room temperature.The slightly yellow solution was concentrated under reduced pressure and solid was precipitated with hexane.The product was collected by filtration and recrystallized from dichloromethane/hexane (78.7 mg, yield: 75%). 1   3).[Au(C 6 F 5 ) 3 (tht)] (95.4 mg, 0.12 mmol) was added to a solution of Et 4 todit (26.5 mg, 0.06 mmol) in dichloromethane (8 mL) and the mixture was stirred for two hours at room temperature.The slightly yellow solution was concentrated under reduced pressure and solid was precipitated with hexane.The yellow product was collected by filtration and recrystallized from dichloromethane/hexane (51.0 mg, yield: 46%).

Cristallography
Crystals were mounted in inert oil on glass fibers and transferred to the cold gas stream of Xcalibur (Agilent Technologies, Waldbronn, Germany) Oxford Diffraction (3) diffractometer equipped with a low-temperature attachment.Data were collected using monochromated Mo Kα radiation (λ = 0.71073 Å).Scan type ω.Absorption correction based on multiple scans were applied using spherical harmonics implemented in SCALE3 ABSPACK [18] scaling algorithm.The structures were solved by direct methods and refined on F 2 using the program SHELXL-97 [19], All non-hydrogen atoms were refined anisotropically.Refinements were carried out by full-matrix least-squares on F 2 for all data.Further details of the data collection and refinement are given in Table 3. CCDC-1006288 (1) and 1006289 (2) contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre [20].

Conclusions
The reaction of the ligand Et 4 todit with some gold(I) or gold(III) complexes leads to the dinuclear [{Au(C 6 F 5 )} 2 (Et 4 todit)] (1), [Au(Et 4 todit)] 2 (OTf) 2 (2) or [{Au(C 6 F 5 ) 3 } 2 (Et 4 todit)] (3).They represent some of the few examples of thione gold derivatives, and because the stability of the complexes the bond Au-S within these complexes may be strong.They do not contain intra-or inter-molecular gold-gold interactions, probably because steric effects of the ligand, and they are not luminescent in solid state or in solution.
support.Massimiliano Arca, Vito Lippolis and Francesco Caddeo also thank University of Cagliari for financial support.

Figure 1 .
Figure 1.Molecular structure of the compound 1, with the atom labeling scheme (hydrogen atoms omitted for clarity).
Au(1)-C(1)-C(2) 121.3(3)Au(1)-C(1)-C(6) 124.1(3)The ligand Et 4 todit also reacts with [Au(tht) 2 ]OTf (OTf = triflate, CF 3 SO 3 ) to give the dinuclear [Au(Et 4 todit)] 2 (OTf) 2 (2) (Scheme 1) as a yellow solid.It is soluble in dichloromethane and acetone and insoluble in diethyl ether and hexane.It has been characterized by 1 H-NMR spectroscopy and mass spectrometry.Single crystals suitable for X-ray diffraction analysis were obtained by slow diffsion of hexane in a solution of 2 in 1,2-dichloroethane, and the molecular structure is depicted in Figure2.The two five-membered rings of the bis(thiocarbonyl) donors are twisted about the C-C bond by 72.65°.The two gold centers present a distorted linear geometry and are bonded to two C=S groups of the thioimidazole of different Et 4 todit ligands.The S(1)-Au(1)-S(2) angle (174.8(1)°) and the Au(1)-S(1) (2.289(1) Å) and Au(1)-S(2) (2.294(1) Å) bond distances found in the solid structure are in good agreement with those reported in the literature[4,5].No Au(I)-Au(I) intra-or inter-molecular interactions have been observed in the crystal structure.The complex is not luminescent in solid state or in solution.

Figure 2 .
Figure 2. Molecular structure of the compound 2, with the atom labeling scheme (hydrogen atoms omitted for clarity).

Figure 3 .
Figure 3. Molecular structure of the compound 3 (hydrogen atoms omitted for clarity).

Table 3 .
Details of Data Collection and Structure Refinement for Complexes 1 and 2.