Gold Thione Complexes

Abstract

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.

1. Introduction

The coordination chemistry of organosulfur compounds featuring two or more 3-alkylimidazole-2-thione groups has not been studied in depth in spite of their potential structural coordination possibilities. The most intensely studied ligands are the derivatives that contain two 3-methylimidazoline-2-thione groups bounded by a –(CH₂)_n– chain [n = 1 (Mbit), 2 (Ebit), 4 (Bbit)] and some complexes with nickel(II), cobalt(II) [1], rhodium(III) and iridium(III) [2], antimony(III), bismuth(III) [3], tin(IV) [4], lead(II) [5], silver(I) [6,7] and group 11 elements [7] have been described in the literature with these types of ligands.

Tetrathiocino-diimidazolyl species also contains two C=S groups that can behave as donors towards Lewis acids [8,9,10]. In particular, the ligand Et₄todit (4,5,6,7-tetrathiocino-[1,2-b:3,4-b']-diimidazolyl-1,3,8,10-tetraethyl-2,9-dithione) (Scheme 1) features very distant positions of the thiocarbonyl groups that make this molecule capable to act as bidentate bridging ligand. Some polymeric complexes with stoichiometry [M(Et₄todit)X₂] (M = Zn, Cd, Hg; X = Cl, Br) [11] or [Cu(Et₄todit)I] [12] have been reported. This notwithstanding, no one gold complex with this type of ligand has been obtained to date.

In this paper we report the synthesis and structural characterization of the dinuclear gold(I) complexes [{Au(C₆F₅)}₂(Et₄todit)] and [Au(Et₄todit)]₂(OTf)₂ and of the gold(III) derivative [{Au(C₆F₅)₃}₂(Et₄todit)].

2. Results and Discussion

The Et₄todit 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 Et₄todit 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.

Scheme 1. (i) 2[Au(C₆F₅)(tht)], (ii) [Au(tht)₂]OTf, (iii) 2[Au(C₆F₅)₃(tht)].

Scheme 1. (i) 2[Au(C₆F₅)(tht)], (ii) [Au(tht)₂]OTf, (iii) 2[Au(C₆F₅)₃(tht)].

The reaction of Et₄todit with [Au(C₆F₅)(tht)] (tht = tetrahydrothiophene, molar ratio = 1:2) leads to the dinuclear derivative [{Au(C₆F₅)}₂(Et₄todit)] (1) (Scheme 1). It is a white solid that has been characterized by ¹H-NMR and ¹⁹F-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 Et₄todit (69.90°) and its derivatives, such as Et₄todit·2Br₂, 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.

Figure 1. Molecular structure of the compound 1, with the atom labeling scheme (hydrogen atoms omitted for clarity).

Figure 1. Molecular structure of the compound 1, with the atom labeling scheme (hydrogen atoms omitted for clarity).

A selection of bond lengths and angles for complex 1 are summarized in Table 1.

Table 1. Selected bond lengths (Å) and angles (°) for 1.

	Distances (Å)		Angles (°)
Au(1)–S(1)	2.333(1)	S(1)–Au(1)–C(1)	174.8(1)
Au(2)–S(6)	2.309(1)	Au(1)–S(1)–C(7)	97.8(1)
Au(2)–C(21)	2.028(4)	Au(2)–S(6)–C(15)	113.7(1)
		Au(1)–C(1)–C(2)	121.3(3)
		Au(1)–C(1)–C(6)	124.1(3)

Table 1. Selected bond lengths (Å) and angles (°) for 1.

	Distances (Å)		Angles (°)
Au(1)–S(1)	2.333(1)	S(1)–Au(1)–C(1)	174.8(1)
Au(2)–S(6)	2.309(1)	Au(1)–S(1)–C(7)	97.8(1)
Au(2)–C(21)	2.028(4)	Au(2)–S(6)–C(15)	113.7(1)
		Au(1)–C(1)–C(2)	121.3(3)
		Au(1)–C(1)–C(6)	124.1(3)

The ligand Et₄todit also reacts with [Au(tht)₂]OTf (OTf = triflate, CF₃SO₃) to give the dinuclear [Au(Et₄todit)]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 ¹H-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 Figure 2. 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 Et₄todit 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. Molecular structure of the compound 2, with the atom labeling scheme (hydrogen atoms omitted for clarity).

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

A selection of bond lengths and angles for complex 2 are summarized in Table 2.

Table 2. Selected bond lengths (Å) and angles (°) for 2.

	Distances (Å)		Angles (°)
Au(1)–S(1)	2.289(1)	S(1)–Au(1)–S(2)	174.8(1)
S(2)–C(10)	1.729(3)	Au(1)–S(1)–C(1)	111.5(1)
S(2)–Au(1)	2.294(1)
S(1)–C(1)	1.709(2)

Table 2. Selected bond lengths (Å) and angles (°) for 2.

	Distances (Å)		Angles (°)
Au(1)–S(1)	2.289(1)	S(1)–Au(1)–S(2)	174.8(1)
S(2)–C(10)	1.729(3)	Au(1)–S(1)–C(1)	111.5(1)
S(2)–Au(1)	2.294(1)
S(1)–C(1)	1.709(2)

With the gold(III) complex [Au(C₆F₅)₃(tht)] (molar ratio = 1:2) the dinuclear derivative [{Au(C₆F₅)₃}₂(Et₄todit)] (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 ¹H-NMR and ¹⁹F-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.

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

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

3. Experimental Section

3.1. Instrumentation

Mass spectra were recorded on a BRUKER (Bremen, Germany) ESQUIRE 3000 PLUS, with the electrospray (ESI) technique and on a BRUKER (Bremen, Germany) MICROFLEX (MALDI-TOF), with a Dithranol or a T-2-(3-(4-tbutyl-phenyl)-2-methyl-2-propenylidene)malononitrile matrix. Room-temperature NMR spectra were recorded with a Bruker (Karlsruhe,, Germany) ARX 400 spectrometer (¹H, 400 MHz and ¹⁹F, 376.5 MHz). The chemical shifts are reported in ppm relative to the residual solvent peak [¹H (CD₃)₂CO: 2.05] and CFCl₃, respectively.

3.2. Starting Materials

Et₄todit [13,14], [Au(C₆F₅)(tht)] [15], [Au(tht)₂]OTf [16,17] and [Au(C₆F₅)₃(tht)] [15], were prepared according to literature procedures. Other starting materials and solvents were purchased from commercial suppliers and used as received unless otherwise stated.

3.3. General Procedure for the Synthesis of the Complexes 1–3

Synthesis of [{Au(C₆F₅)}₂(Et₄todit)] (1). A solution of [Au(C₆F₅)(tht)] (83.7 mg, 0.18 mmol) in dichloromethane (5 mL) was added to a solution of Et₄todit (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%). ¹H-NMR (CD₃COCD₃, 400 MHz): δ 1.33 (6H, t, CH₃, J(HH) 7.1 Hz), 1.59 (6H, t, CH₃, J(HH) 7.1 Hz), 4.21 (2H, m, CH₂), 4.82 (4H, dt, CH₂, J(HH) 14.0 and 7.1 Hz), 4.98 (2H, m, CH₂). ¹⁹F-NMR (CD₃COCD₃, 377 MHz): δ −165.6 (4F, m, m-F), −163.3 (2F, t, p-F, J(FF) 19.8 Hz), −117.8 (4F, m, o-F). MS (MALDI⁺): m/z 1163.3 [(M)⁺ 0.8%, calculated: 1163.9], 1101.4 [(M − 2S + 2H)⁺ 4.0%], 933.3 [(M − 2S − C₆F₅)⁺ 36.8%], 569.2 [(M − 2S − C₁₂F₁₀Au)⁺ 100%]. Analytical data: C₂₆H₂₀Au₂F₁₀N₄S₆ (1163.92) requires C, 26.81; H, 1.73; N, 4.81; S, 16.52; found C, 26.72; H, 1.71; N, 4.66; S, 16.22.

Synthesis of [Au(Et₄todit)]₂(OTf)₂ (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 Et₄todit (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%). ¹H-NMR (400 MHz, CD₃COCD₃) δ 4.56 (4H, m, CH₂), 3.91 (4H, m, CH₂), 1.45 (6H, t, CH₃, J(HH) 7.1 Hz), 1.19 (6H, t, CH₃, J(HH) 7.1 Hz). MS (MALDI⁺): m/z 1414.9 [(M − CF₃SO₃)⁺ 4%] calculated: 1414.89, 1266 [(M − 2CF₃SO₃)⁺ 10%], calculated: 1265.94. Analytical data: C₃₀H₄₀Au₂F₆N₈O₆S₁₄ (1563.84) requires C, 23.02; H, 2.58; N, 7.16; S, 28.67; found C, 23.14; H, 2.44; N, 7.01; S, 28.17.

Synthesis of [{Au(C₆F₅)₃}₂(Et₄todit)] (3). [Au(C₆F₅)₃(tht)] (95.4 mg, 0.12 mmol) was added to a solution of Et₄todit (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%). ¹H-NMR (400 MHz, CD₃COCD₃) δ 4.90 (2H, m, CH₂), 4.59 (4H, m, CH₂), 4.26 (2H, m, CH₂), 1.44 (12H, m, CH₃). ¹⁹F-NMR (377 MHz, CD₃COCD₃) δ −122.42 (4F, m, o-F), −122.57 (4F, m, o-F), −124.17 (4F, m, o-F), −159.42 (4F, t, J(FF) 19.7 Hz), −160.53 (2F, t, J(FF) 19.5 Hz), −163.04 (4F, m, m-F), −164.67 (8F, m, m-F). MS (MALDI⁺): m/z 1263.3 [(M − 3C₆F₅ − 2S)⁺ 10.1%] calculated: 1267.0, 1101.2 [(M − 4C₆F₅ − 2S)⁺ 8.7%], 933.3 [(M − 2S − 5C₆F₅)⁺ 11.1%], 736.2 [(M − 3S − 6C₆F₅ + 2H)⁺ 26.3%]. Analytical data: C₅₀H₂₀Au₂F₃₀N₄S₆ (1831.89) requires C, 32.76; H, 1.10; N, 3.06; S, 10.50; found C, 32.34; H, 1.44; N, 3.01; S, 10.17.

3.4. 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 F² using the program SHELXL-97 [19], All non-hydrogen atoms were refined anisotropically. Refinements were carried out by full-matrix least-squares on F² 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].

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

Compound	1	2·2CH2Cl2
Chemical Formula	C26H20Au2F10N4S12	C32H44Au2Cl4F6N8O6S14
Appearance	Colorless plate	Colorless needle
Crystal size/mm	0.42 × 0.23 × 0.06	0.20 × 0.08 × 0.06
Crystal system	Triclinic	Triclinic
Space group	P-1	P-1
a/Å	9.97740(10)	9.7616(2)
b/Å	12.1758(2)	12.9096(3)
c/Å	14.0916(2)	13.0537(3)
α/°	73.7730(10)	63.202(2)
β/°	82.3940(10)	77.979(2)
γ/°	88.3680(10)	76.346(2)
U/Å3	1629.16(4)	1416.97(5)
Z	2	1
Dc/g cm−3	2.374	2.034
M	1164.75	1735.33
F(000)	1096	844
T/°C	−173	−173
2θmax/°	51	51
µ(Mo-Kα)/mm−1	9.465	5.943
Transmission	0.6005, 0.1092	0.7169, 0.3828
No. of reflections measured	29873	27048
No. of unique reflections	6032	5254
Rint	0.038	0.018
Ra (F > 4σ(F))	0.024	0.045
wR2 (F2, all refl.)	0.066	0.175
No. of reflections used	6032	5254
No. of parameters	437	329
S	1.059	1.041
Max. ∆ρ/eÅ−3	1.77	0.90

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

Compound	1	2·2CH2Cl2
Chemical Formula	C26H20Au2F10N4S12	C32H44Au2Cl4F6N8O6S14
Appearance	Colorless plate	Colorless needle
Crystal size/mm	0.42 × 0.23 × 0.06	0.20 × 0.08 × 0.06
Crystal system	Triclinic	Triclinic
Space group	P-1	P-1
a/Å	9.97740(10)	9.7616(2)
b/Å	12.1758(2)	12.9096(3)
c/Å	14.0916(2)	13.0537(3)
α/°	73.7730(10)	63.202(2)
β/°	82.3940(10)	77.979(2)
γ/°	88.3680(10)	76.346(2)
U/Å3	1629.16(4)	1416.97(5)
Z	2	1
Dc/g cm−3	2.374	2.034
M	1164.75	1735.33
F(000)	1096	844
T/°C	−173	−173
2θmax/°	51	51
µ(Mo-Kα)/mm−1	9.465	5.943
Transmission	0.6005, 0.1092	0.7169, 0.3828
No. of reflections measured	29873	27048
No. of unique reflections	6032	5254
Rint	0.038	0.018
Ra (F > 4σ(F))	0.024	0.045
wR2 (F2, all refl.)	0.066	0.175
No. of reflections used	6032	5254
No. of parameters	437	329
S	1.059	1.041
Max. ∆ρ/eÅ−3	1.77	0.90

4. Conclusions

The reaction of the ligand Et₄todit with some gold(I) or gold(III) complexes leads to the dinuclear [{Au(C₆F₅)}₂(Et₄todit)] (1), [Au(Et₄todit)]₂(OTf)₂ (2) or [{Au(C₆F₅)₃}₂(Et₄todit)] (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.