The di(thiourea)gold(I) complex [Au{S=C(NH₂)₂}₂][SO₃Me] as a precursor for the convenient preparation of gold nanoparticles

2.1 Solid-state structure

The molecular structure of 1 in the solid state has been determined by single-crystal X-ray diffraction (Fig. 1). Selected bond distances (Å) and angles (deg) are summarized in the caption of Fig. 1. Suitable single crystals were obtained by slow cooling of a concentrated ethanol solution from 60°C to ambient temperature.

Fig. 1:

Ortep plot (displacement ellipsoids at the 50% probability level; hydrogen atoms as spheres with arbitrary radii) of the molecular structure of 1 with atom numbering scheme. Selected bond lengths (Å), angles and torsion angles (deg): Au1–S1 2.2774(14), Au1–S2 2.2727(14), C1=S1 1.726(6), C2=S2 1.728(5), C1–N1 1.314(7), C1–N2 1.308(6), C2–N3 1.302(7), C2–N4 1.314(7); S1–Au1–S2 179.50(5), C1–S1–Au1 106.17(18), C2–S2–Au1 107.31(19), N1–C1–N2 119.3(5), N3–C2–N4 119.7(5), S1–C1–N1 117.8(4), S1–C1–N2 123.0(4), S2–C2–N3 118.0(4), S2–C2–N4 122.3(4); C1–S1–S2–C2 113.2(3).

Complex 1 crystallizes in the monoclinic space group C2/c with one formula unit in the asymmetric unit. The gold(I) ion is coordinated by two thiourea ligands (Au1–S1 2.2774(14) Å and Au1–S2 2.2727(14) Å) in a linear fashion (S1–Au1–S2 179.50(5)°), similar to other bis(thiourea) gold complexes, e.g. [Au{S=C(NH₂)₂}₂][Br] and [Au{S=C(NH₂)₂}₂][Cl] [91], [92].

The C=S entity is rotated by 106.17(18) / 107.31(19)° (Au1–S1/2–C1/2) towards the linear S1–Au1–S2 unit (179.50(5)°), which also agrees with previous findings (see above) [91], [92]. The coordination of the C=S moiety increases the C=S bond length from, e.g. 1.688 Å [93] for non-coordinated thiourea to herein 1.726(6) / 1.728 (5) Å, which corresponds to data of similar complexes of this type [94]. The C–N distances of 1.302(7)–1.314(7) Å are not affected by the coordination [93], lying in the range between C=N double (1.279 Å) and N–C_sp3 single bonds (1.469 Å), revealing a significant amount of delocalization [95].

The planar thiourea entities (rmsd=0.0055/0.0056 Å) [96] show a leaflet structure with a C1–S1–S2–C2 torsion angle of 113.2(3)°. A CSD database survey (see Fig. S4, Supporting Information available online) shows that this leaflet angle can range from 0 to 180°, which is rather unspecific and predominantly affected by the hydrogen bond network and the allocation of the counter ions.

Although aurophilic interactions are not observed in the crystals, short intermolecular contacts smaller than the sum of their van der Waals radii consolidate the packing of the ions. S···C(NH₂)₂ interactions between S2 and C1 of adjacent cations result in a dimer [1₂]²⁺ with a chair conformation (Fig. 2). Further interactions are based on hydrogen bonds that exclusively occur between the nitrogen atoms of the cationic fragment and the oxygen atoms of the mesylate anions (Table 1). Thereby, N1, N3 and N4 act as hydrogen bond donors towards two oxygen atoms of different anions, except for N2, where only one interaction is observed. Each cation is thus connected to 6 anions (Fig. 3) and the dication in total interacts with 10 mesylate anions. Consequently, each mesylate accepts hydrogen bonds from 6 cations, whereby O1 and O3 interact with two and O2 with three nitrogen atoms (Fig. 4). The combination of all these interactions results in a three-dimensional network, involving all ionic components.

Fig. 2:

Ortep plot (50% probability level) of [1₂]²⁺ formed by intermolecular S···C interactions (orange). Anions are omitted for clarity. Geometric interaction properties: S2···C1=3.400(5) Å (van der Waals radii: Σ_C,S=3.50 Å), Au1–S2···C1A=85.4°, S2···C1A–S1A=101.1°; symmetry code: (A) 1/2−x, 1/2−y, −z.

Fig. 3:

Ball-and-stick model of 1 showing the cation and its hydrogen bond interactions (blue) towards the mesylate anions. Geometric details and symmetry operations are summarized in Table 1.

Fig. 4:

Ball-and-stick model of 1 showing the mesylate anion and its hydrogen bond interactions (blue) towards the cation. Geometric details and symmetry operations are summarized in Table 1.

2.2 Thermal behavior

The thermal behavior of 1 was studied by TG, TG-MS and DSC methods to gain information on the thermal decomposition mechanism of the complex.

TG measurements were performed in atmospheres of nitrogen (gas flow of 20 mL min⁻¹) and oxygen (gas flow of 20 mL min⁻¹) in the temperature range of 40–800°C with a heating rate of 10 K min⁻¹. Additionally, a continuous nitrogen carrier gas flow of 40 mL min⁻¹ was used. The decomposition of 1 occurs in four steps with an over-all weight loss of 56.3% in the range of T=200–650°C (Fig. 5).

Fig. 5:

TG traces of 1 under nitrogen (solid) and oxygen (dashed), (gas flow 20 mL min⁻¹ (each), heating rate 10 K min⁻¹).

The thermal behavior of 1 under an atmosphere of nitrogen and oxygen is virtually identical, the last decomposition step under oxygen being finished at slightly lower temperature (N₂: 650°C; O₂: 616°C). The residues at T=800°C of both measurements are with 43.8% close to the calculated value for elemental gold (44.3%). The formation of gold was proven by powder X-ray diffraction (PXRD) studies [ICDD 03-065-2870] (Fig. 6).

Fig. 6:

PXRD pattern (black) of the TG residue of 1 and the characteristic reflections of crystalline Au (red) ([ICDD 03-065-2870]).

The first decomposition step of 1 shows an onset of T=207°C and a weight loss of 19.2%. Within this step multiple overlaying processes occur, which are associated with the release of one of the two thiourea ligands (calculated: 17.1%) and the start of the anion decomposition. The 2^nd step observed at an onset of 311°C and the 3^rd one at 391°C show a mass loss of 27.6% and 3.5%, respectively, indicating the further decomposition of the thiourea ligand as well as of the counter anion. The as-formed residue of 49.7% suggests the formation of Au₂S, which gradually decomposes to elemental gold in the final step (weight loss 5.9%, T=450–650°C).

DSC studies under an atmosphere of nitrogen and oxygen show the melting point of 1 as an endothermic peak at 164°C (Fig. 7). Under inert conditions each decomposition step shows an endothermic peak (E₂=−140.6 J g⁻¹, E₃=−245.2 J g⁻¹), whereas under oxidative atmosphere the 1^st and 2^nd step are both split into an endothermic and exothermic peak (1^st step: E₂=−74.6 J g⁻¹, E₃=19.2 J g⁻¹, 2^nd step: E₄=29.9 J g⁻¹, E₅=−107.3 J g⁻¹), due to the oxidation of gaseous decomposition products. The final step under oxygen shows an exothermic peak (E₇=80.7 J g⁻¹), which fits well with the decomposition of Au₂S by the release and oxidation of sulfur to SO₂ under formation of elemental gold [97].

Fig. 7:

TG (solid) and DSC (dashed) traces of 1 under nitrogen (left) and oxygen (right), (gas flow 20 mL min⁻¹, heating rate 10 K min⁻¹).

To confirm the thermal decomposition behavior of 1, TG-MS coupling experiments were performed under argon (gas flow 20 mL min⁻¹, heating rate 5 K min⁻¹). In Fig. 8 the TG traces and their first derivative as well as their respective mass-to-charge ratio (m/z) are depicted.

Fig. 8:

TG-MS traces and their 1^st derivative (top) of 1 (argon, gas flow 20 mL min⁻¹, heating rate 5 K min⁻¹). Ion current (bottom) of m/z=15 [CH₃]⁺, 16 [NH₂]⁺, 32 [N₂H₄]⁺, 44 [CS]⁺, 48 [SO]⁺, 64 [SO₂]⁺ and 76 [CS₂]⁺.

TG-MS coupling experiments show that the first mass decay starting at T=200°C is caused by the release and decomposition of one of the two thiourea ligands as evidenced by the appearance of fragments with m/z=16 [NH₂]⁺, 32 [N₂H₄]⁺, 44 [CS]⁺ and 76 [CS₂]⁺. Fragments of m/z=15 [CH₃]⁺, 48 [SO]⁺ and 64 [SO₂]⁺ further indicate that the process is overlaid by the degradation of the anion. In the next step the decomposition of the 2^nd thiourea ligand can be observed by typical fragments such as m/z=16, 32 and 44, as well as the anion fragmentation (m/z=48, 64). The last two steps are characterized by the decomposition of the remaining organics and the as-formed Au₂S by detection of fragments such as m/z=16, 44, 48 and 64.

2.3 Nanoparticle formation

The gold(I) complex 1 was used as a precursor for gold nanoparticle formation. With reference to the decomposition temperature of 1 (Fig. 5), 1-hexadecylamine (b. p. 330°C) was used as solvent. This amine is also expected to prevent the agglomeration of the Au NPs, due to its coordination properties [98].

The Au NPs were synthesized by heating a 1-hexadecylamine solution of 1 (4.0 mmol L⁻¹) first to 200°C, followed by heating to 330°C, to ensure complete decomposition, at a constant heating rate of 25 K min⁻¹. NP formation was observable by a typical color shift from yellow to deep red between 200 and 250°C. This observation shows that 1 decomposes in solution at about the same temperature as found for the solid state (Fig. 5; onset: 207°C). The solution was then cooled to 50°C and hexane was added to prevent solidification of 1-hexadecylamine. The Au NPs were precipitated by addition of ethanol and separated by centrifugation. After washing with ethanol, the Au NPs were re-dispersed in hexane for analysis.

The Au NPs have been characterized by transmission electron microscopy (TEM), electron diffraction and UV/Vis spectroscopy. The UV/Vis spectrum in hexane showed a broad absorption at 528 nm, due to the characteristic surface plasmon resonance (SPR) of the Au NPs (Fig. 9) [99].

Fig. 9:

UV/Vis spectrum of Au NPs in hexane formed by thermal decomposition of 1 in 1-hexadecylamine (c=4.0 mmol L⁻¹) at 330°C.

TEM was used to determine the size and size distribution of the Au NPs. Mainly spherical particles with a mean diameter (d) of 14.5 nm and a standard deviation of σ=3.9 nm (size variation c_v=27%) were obtained (Fig. 10). Electron diffraction studies confirmed the formation of Au NPs (see Fig. S6, Supporting Information available online).

Fig. 10:

TEM image and size distribution of Au NPs obtained by thermal decomposition of 1 (c=4.0 mmol L⁻¹, d=14.5 nm, σ=3.9 nm) at T=330°C.

4.1 General

Chemicals were purchased from commercial suppliers and were used without any further purification. The synthesis of Au NPs was carried out by a modified methodology published by Adner et al. [100], [101] and was performed under an atmosphere of argon using standard Schlenk techniques. All solvents used for Au NPs formation were degassed by sonication under vacuum using the sonication bath SONOREX RK 100 by BANDELIN electronic GmbH & Co. KG. Hexane was dried using a M. Braun SBS-800 purification system (stationary drying with molecular sieve columns 3 Å). 1-Hexadecylamine (98%) was melted at 60°C and degassed in a sonication bath (5 cycles). Methanesulfonic acid (BASF SE, 5 volume-% in water) was used for the wet chemical and electrochemical synthesis of 1. Gold (99.99% purity), as well as gold and platinum electrodes, has been produced by Saxonia Edelmetalle GmbH. A Nafion™ membrane was used for the electrolysis.

4.2 Instruments

NMR spectra (500.3 MHz for ¹H and 125.8 MHz for ¹³C{¹H}) were recorded using a Bruker Avance III 500 spectrometer operating at 293 K. Chemical shifts δ are reported in ppm (parts per million) downfield from tetramethylsilane in (CD₃)₂SO (¹H NMR δ=2.50 ppm; ¹³C{¹H} NMR δ=39.52 ppm). The infrared spectrum was recorded between 500 and 4000 cm⁻¹ with a Thermo Nicolet 200 FT-IR spectrometer. Elemental analysis was performed with a Thermo FLASHEA 1112 Series instrument. The high resolution ESI-TOF mass spectrum was measured with a Bruker micrOTOF QII equipment. The melting point was determined by using a Gallenkamp MFB 595 010 M melting point apparatus. TG/DSC experiments were performed with a Mettler Toledo TGA/DSC1 1100 system with an UMX1 balance. The TG-MS experiment was performed with a Mettler Toledo TGA/DSC1 1600 system with a MX1 balance coupled with a Pfeiffer Vacuum MS Thermostar GSD 301 T2 mass spectrometer. For PXRD measurements the STOE powder diffractometer STOE Stadi P with Ge(111) monochromator CuKα radiation (λ=0.15406 nm, 40 kV, 40 mA) was applied. Transmission electron microscopy (TEM) was performed with a PHILIPS CM 20 FEG operated at 200 kV. Sample preparation for TEM investigations were made by drop coating of the Au NPs dispersion in hexane on a carbon-coated copper grid (CARBON FILM 300 MESH, COPPER, Electron Microscopy Sciences). UV/Vis spectra were recorded with a Genesys 6 instrument (Thermo Electron Corporation) in quartz cuvettes (Company Roth, 1.0 cm optical path length) between 400 and 700 nm (measurement interval 0.2 nm).

4.3 Synthesis of [Au{S=C(NH₂)₂}₂][SO₃Me] (1)

The title complex was prepared by two different synthetic methodologies, which were both performed under aerobic conditions.

4.3.2 Electrochemical synthesis

To an electrolysis cell separated by an ion selective Nafion™ membrane, methanesulfonic acid was added. Thiourea was dissolved in the anode half-chamber to reach concentrations in the range 35–55 g L⁻¹. The gold anode and a platinated titanium cathode were added to the cell (Scheme 2). Application of a voltage of smaller than 7.0 V led to a stepwise dissolution of the anode. The solid product appearing in the anolyte was isolated by evaporation of water. After recrystallization of the as-obtained residue from ethanol at 60°C and subsequent vacuum filtration, complex 1 could be isolated as a colorless solid. Yield: 31.00 g (69.8 mmol, 85% based on the gold losses at the anode).

Scheme 2:

Schematic illustration of an electrolysis cell for the formation of 1 (1=platinated titanium, 2=gold, 3=Nafion™ membrane, 4=methanesulfonic acid, 5=methanesulfonic acid and thiourea).

M. p. 164°C. – ¹H NMR (500.3 MHz, (CD₃)₂SO): δ=2.41 (s, 3 H, CH₃), 8.18 (s, 4 H, NH₂), 8.47 (s, 4 H, NH₂) ppm. – ¹³C{¹H} NMR (125.8 MHz, (CD₃)₂SO): δ=39.8 (–CH₃), 175.3 (C(=S)) ppm. – IR (KBr): ν=3384 (m), 3290 (m), 3198 (m), 3165 (m), 1631 (s), 1616 (s), 1495 (m), 1437 (s), 1241 (s), 1393 (s), 1193 (s), 1058 (s), 785 (m), 772 (m), 710 (m), 563 (m), 536 (m), 526 (m) cm⁻¹. – HRMS ((+)-ESI): m/z=348.9864 (calcd. 348.9856 for [Au{S=C(NH₂)₂}₂]⁺). – C₃H₁₁N₄O₃S₃Au (444.31): calcd. C 8.11, H 2.50, N 12.61; found C 8.29, H 2.38, N 12.43.

4.4 Single-crystal X-ray diffraction analysis

Data of 1 were collected with an Oxford Gemini S diffractometer at 120 K with graphite-monochromatized MoKα radiation (λ=0.71073 Å) (Table 2). The structure was solved by Direct Methods using Shelxs-13 [103] and refined by full-matrix least-squares procedures on F² using Shelxl-13 [104]. All non–hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced at calculated positions and treated as riding on their parent atoms using AFIX 33 for the methyl group. The NH₂ group was treated as a terminal =CH₂ moiety (AFIX 93), due to its intense double bond character and in order to simplify the hydrogen network properties (vide supra). C–H distances were fixed at 0.96 Å (CH₃) and 0.86 (NH₂) with assigned temperature factors of H_iso(H)=1.5U_eq (CH₃) and 1.2U_eq (NH₂). Graphics of the molecular structure were created by using Ortep and the Shelxtl program package [105].

CCDC 1952839 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.