Low Temperature Activation of Tellurium and Resource‐Efficient Synthesis of AuTe₂ and Ag₂Te in Ionic Liquids

Abstract

The low temperature syntheses of AuTe₂ and Ag₂Te starting from the elements were investigated in the ionic liquids (ILs) [BMIm]X and [P₆₆₆₁₄]Z ([BMIm]⁺=1‐butyl‐3‐methylimidazolium; X = Cl, [HSO₄]⁻, [P₆₆₆₁₄]⁺ = trihexyltetradecylphosphonium; Z = Cl⁻, Br⁻, dicyanamide [DCA]⁻, bis(trifluoromethylsulfonyl)imide [NTf₂]⁻, decanoate [dec]⁻, acetate [OAc]⁻, bis(2,4,4‐trimethylpentyl)phosphinate [BTMP]⁻). Powder X‐ray diffraction, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy revealed that [P₆₆₆₁₄]Cl is the most promising candidate for the single phase synthesis of AuTe₂ at 200 °C. Ag₂Te was obtained using the same ILs by reducing the temperature in the flask to 60 °C. Even at room temperature, quantitative yield was achieved by using either 2 mol % of [P₆₆₆₁₄]Cl in dichloromethane or a planetary ball mill. Diffusion experiments, ³¹P and ¹²⁵Te‐NMR, and mass spectroscopy revealed one of the reaction mechanisms at 60 °C. Catalytic amounts of alkylphosphanes in commercial [P₆₆₆₁₄]Cl activate tellurium and form soluble phosphane tellurides, which react on the metal surface to solid telluride and the initial phosphane. In addition, a convenient method for the purification of [P₆₆₆₁₄]Cl was developed.

Ag₂Te and AuTe₂ were obtained from the elements in the ionic liquid [P₆₆₆₁₄]Cl at room temperature. The reaction is driven by a dissolved tellurium species that reacts with the noble metal. In commercial [P₆₆₆₁₄]Cl, tellurium is reduced by trialkylphosphane impurities. A purification protocol was developed to suspend the influence of the trialkylphosphane. In the purified medium, a different tellurium species appears.

1. Introduction

Metal tellurides are narrow bandgap semiconductors. Numerous studies in recent decades revealed additional physical properties with potential for application, such as their efficient thermoelectric power conversion and photocatalytic activity under radiation ranging from ultraviolet (UV) to near‐infrared (NIR).[ ¹ , ² ] Gold ditelluride AuTe₂, naturally found as the mineral calaverite, is discussed as a candidate for thermoelectric applications and is typically synthesized by reacting the elements at 600 °C, followed by annealing at 400 °C.[ ³ , ⁴ ] Alternatively, the compound can be accessed at 250 °C by wet‐chemical hot‐injection using solutions of highly reactive starting materials TeCl₄ and HAuCl₄ ⋅ 2H₂O in dioctylether/oleylamine. ^[5]

Silver telluride, Ag_2–δTe, in its different phases is an ion conductor, a thermoelectric, a topological insulator and has a large magnetoresistance that can be exploited in magnetic field sensors.[ ⁶ , ⁷ , ⁸ , ⁹ ] The textbook example for the synthesis of Ag₂Te is the reaction of solid silver with tellurium vapor at 475 °C. ^[10] Thin films are accessible by vacuum deposition of tellurium on chlorine activated silver films. ^[11] For the synthesis of Ag₂Te nanomaterials, which are especially interesting for thermoelectric applications, solvothermal methods are used.[ ⁶ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ ] Silver telluride nanowires have been synthesized by the composite‐hydroxide‐mediated (CHM) method at 180 to 225 °C, starting from AgNO₃ and elemental tellurium in NaOH/KOH with hydrazine hydrate and ethylenediamine (en) in a Teflon vessel. ^[6] Ag₂Te−Ag nanocomposites are accessible at room temperature by reacting AgCl and tellurium in an aqueous NaOH solution with NaBH₄ as reducing agent. The addition of NaCl enhances the solubility of AgCl, which leads to smaller silver grains. ^[12] Na₂TeO₃ is often used as a tellurium source because of its good solubility. In a solvothermal co‐reduction approach using a Teflon‐lined stainless steel autoclave at 240 °C, more than 20 μm long nanowires formed from an aqueous solution of AgNO₃ with the disodium salt of ethylenediaminetetraacetic acid (Na₂(EDTA) ⋅ 2H₂O), ethylene glycol (EG), and NaOH. ^[13] Crooked nanowires were synthesized in the microwave with EG as the solvent, starting from AgNO₃, Na₂TeO₃ and NaOH at 190 °C within 15 min. ^[14] Nanowires are also accessible through an ammonium‐hydroxide‐buffered hydrothermal synthesis by using AgNO₃ as silver source and hydrazine hydrate (N₂H₄⋅H₂O) as reducing agent. ^[18] Many more hydrothermal approaches with AgNO₃ have been published. Nanowires or nanorods can be synthesized by starting with tellurium nanowires (TeNWs) and N₂H₄ as reducing agent at room temperature or nanotubes with NaTeO₃ instead of a tellurium template and NH₃ at 120 °C.[ ¹⁵ , ¹⁶ , ¹⁷ ] A broad variety of water‐free syntheses were also reported. Sonochemical synthesis of nanocrystals is possible in en from AgNO₃ and tellurium. ^[19] Nanocrystalline rod‐like crystals of Ag₂Te were synthesized at room temperature starting with AgCl and tellurium in en/hydrazine hydrate mixtures. ^[20] Dendritic nanostructures can be synthesized at room temperature using AgNO₃, TeO₂, and N₂H₄⋅H₂O in en without stirring or hollow microspheres forming under the same conditions with continuous stirring. ^[21] Thin films are accessible by electrodeposition in DMSO from NaNO₃, AgNO₃, and TeCl_4. Trialkylphosphans (R ₃P) like trioctylphosphane (TOP) are often used surfactants for the activation of tellurium or Ag. For example, TOP−Te (solution of tellurium in TOP) is used as the tellurium source in a hot injection method at 170 °C with dodecanethiol as the ligand and oleylamine as the reducing agent in order to gain nanoparticles. ^[1] A modification with additional Ag−TOP, extract of silver with TOP from an aqueous AgNO₃ solution, as the silver source is also possible. ^[22] Recently, a surfactant‐free synthesis in an aqueous solution of AgNO₃ was also developed using NaHTe as the active tellurium species. ^[23] A synthesis with elemental silver and tellurium as the starting materials at room temperature (RT) is also possible in liquid NH₃ at 7 atm using a Teflon‐in‐glass sealable thick‐walled Schlenk tube. ^[24]

These versatile methods still have some disadvantages that can be circumvented using ionic liquids (IL). The ability of ILs to dissolve hard‐to‐dissolve elements can reduce the energy consumption for telluride syntheses in comparison to high temperature vapor deposition methods and avoids the use of toxic and/or expensive chemicals. ^[25] Because of their negligible vapor pressure there is no need for autoclaves, which are necessary for the solvothermal synthesis. In recent studies, it was discovered that almost insoluble chalcogens (Ch = Se, Te) can be activated by ionic liquids. Zhang et al. demonstrated that Ch react with the phosphonium cation at 220 °C and form PR ₃−Ch (R = C₄, C₆, C₁₄ alkyl chains). These intermediate trialkylphosphane chalcogens can be employed as precursors for the synthesis of metal chalcogenides, e. g. Ag₂Te, with silver dicyanamide (Ag[DCA]) as the silver source (yield 51 %). ^[26] Unfortunately, the decomposition of the ionic liquid prohibits its recycling for multiple reactions and necessitates the use of an autoclave for the reaction. In this study, we investigated the activation of tellurium and the formation of gold telluride AuTe₂ and silver tellurides (Ag₂Te and Ag_5‐δTe₃) in [BMIm]X (X = Cl⁻, [HSO₄]⁻) and [P₆₆₆₁₄]Z (Z = Cl⁻, Br⁻, [DCA]⁻, [NTf₂]⁻, [dec]⁻, [OAc]⁻,[BTMP]⁻). For Ag₂Te we describe a convenient one‐pot synthesis at temperatures down to RT that preserves the IL without the need for an autoclave.

2. Results and Discussion

2.1. Synthesis of Gold and Silver Tellurides in ILs

At first, the influence of different ILs on the formation of AuTe₂ was analyzed at 200 °C. We used commercially available, easy to synthesize ILs that have the potential for upscaling the process. Two cations were tested, [BMIm]⁺ and [P₆₆₆₁₄]⁺, which both can form room temperature ionic liquids (RTILs) and have proved as suitable media for inorganic synthesis. ^[25] We then focused on phosphonium based ILs because they show higher thermal stability than the imidazolium salts. ^[27] Anions for [P₆₆₆₁₄]⁺ were Cl⁻, Br⁻, [DCA]⁻, [NTf₂]⁻, [dec]⁻, [OAc]⁻, [BTMP]⁻ and for [BMIm]⁺, Cl⁻, and [HSO₄]⁻. Tellurium heated for 16 h to 200 °C in ILs with the anions [dec]⁻, [OAc]⁻, [DCA]⁻, and [BTMP]⁻ showed a purple reaction solution and, after separation of the ILs and addition of protic solvents like EtOH, a black precipitate of tellurium (powder X‐ray diffraction, PXRD, in the supporting information, S1) was formed. When a mixture of tellurium and gold powder were treated in the same way, no significant yield of AuTe₂ could be obtained in these ILs (Figure 1). However, the synthesis of single phase (according to PXRD) AuTe₂ was achieved in ILs with chloride anions.

Figure 1.

PXRD patterns of the solid products after the reaction of Au and Te in various ILs for 16 h at 200 °C. Single phase synthesis of AuTe₂ was achieved in [BMIm]Cl and [P₆₆₆₁₄]Cl. Bottom: Pattern of AuTe₂ simulated on the basis of the Inorganic Crystal Structure Database No. ICSD‐30905, pattern of Au simulated on the basis of the Inorganic Crystal Structure Database No. ICSD‐52700 and pattern of Te simulated on the basis of the Inorganic Crystal Structure Database No. ICSD‐40041.

In scanning electron microscope (SEM) studies of the product from [P₆₆₆₁₄]Cl, the particles appear to be porous agglomerates of a nanocrystalline powder (Figure 2). Energy‐dispersive X‐ray (EDX) spectroscopy confirmed that small particles have a homogenous distribution of gold and tellurium in the molar ratio of 1 : 2. Some of the larger particles, however, enclose aggregates of unreacted gold grains in the core. Their diameters (S2) fit to the starting gold material (0.3 to 0.8 microns). Possible reasons for this could be wetting problems of gold by the IL or passivation of gold by the formed shell of AuTe₂, combined with insufficient mechanical crushing by the magnetic stirring bar.

Figure 2.

EDX map of the solid reaction products from the synthesis in [P₆₆₆₁₄]Cl after 16 h at 200 °C. Left: Au+2 Te; particle with unreacted gold in a shell of AuTe₂. Right: 2 Ag+Te; even large particles show homogenous distribution of silver and tellurium in the molar ratio 2 : 1.

To shed more light on this, passivation experiments were carried out, in which a gold flake and tellurium powder were placed in [P₆₆₆₁₄]Cl and heated to 200 °C without stirring. After 16 h, the gold flake was covered with a dark and hard layer, which withstands mechanical scratching with a needle (S3). SEM reveals a rough surface, in contrast to the unchanged smooth surface of a gold flake which was exposed to the IL without tellurium. EDX shows the formation of a telluride layer with the molar ratio Au : Te = 1 : 2 (S4). This indicates that elemental tellurium is activated and forms a mobile species in the chloride IL, which then reacts on the surface of the gold particles.

To test whether only tellurium is forming mobile species, diffusion experiments were performed. Powders of tellurium or gold were placed at opposite ends of a H‐shaped tube with central fritted glass filter. Both were carefully covered with [P₆₆₆₁₄]Cl, until the horizontal connection between the two sides was completely filled and the two elements were connected via the liquid phase. The tube was heated in an oil bath at 200 °C. After several hours, the IL on the tellurium side turned yellow, indicating the formation of a mobile tellurium species. Over time, the coloring expanded to the gold side. After three days, the gold had lost its metallic luster (S5). After one week, the solids were separated from the IL and washed with dichloromethane (DCM). The PXRD pattern of the brown powder that formed on the gold side showed only the reflections of gold. Obviously, the layer of AuTe₂ on the gold surface was not thick enough to be detected with PXRD. The powder on the tellurium side was unreacted tellurium.

The results of the synthesis and the passivation and diffusion experiments can be merged into a model of the reaction. Wetting problems lead to an aggregation of the gold particles when gold powder is used. The chloride IL activates tellurium. The mobile tellurium species reacts on the surface of the gold particles forming a AuTe₂ layer. Further reaction necessitates diffusion through this solid shell. If the agglomerates of gold are large, the reaction remains incomplete at the given reaction time and temperature.

When silver was used in the diffusion experiment, the same color change of the IL occurred, but the color on the silver side became never as deep as for the gold system. After two days, the surface of the silver powder had turned black and after one week, black needles had grown on the silver surface into the liquid phase (S6). These needles consisted of pure Ag₂Te, as shown by PXRD. In the passivation experiment, the silver flake largely disintegrates within 16 hours to form a dendritic dark grey material (S7). The phase formation proceeds much faster than in the gold system. Seemingly, there are no passivation effects, which might be attributed to the mobility of silver cations in Ag₂Te.

Starting from the silver powder, the influence of different ILs on the formation of Ag₂Te was examined. The ILs are the same as in the gold system but the systems showed a completely different behavior. Ag₂Te forms in the halide ILs, as well as in all other used ILs at 200 °C, yet the chloride ILs worked best. EDX showed a homogeneous distribution of silver and tellurium (molar ratio Ag : Te = 2 : 1) as shown in Figure 2, in even larger particles as for the gold system. In [BMIm][HSO₄], the tellurium‐richer Ag_5–δTe₃ phase formed as main product. In [OAc]⁻ or [DCA]⁻ containing ILs, the formed material contained residuals of unreacted silver. Reaction mixtures that contained [OAc]⁻, [DCA]⁻, or [dec]⁻ showed an intense red to purple color, indicating a different mobile tellurium species as observed in the other ILs.

At 200 °C, the used ILs decompose under formation of trialkylphoshane species (R ₃P). ^[27] The Lewis bases R ₃P are known to react with tellurium.[ ²⁶ , ²⁸ ] Reducing the reaction temperature to 105 °C led to single phase telluride in [P₆₆₆₁₄]Cl and [BMIm]Cl as well as to an extension of the reaction time necessary for full conversion from 4 h to 16 h. In [P₆₆₆₁₄]Cl, full conversion to Ag₂Te is achieved even at 60 °C (Figure 3). This result is independent of the typical (phosphane) impurities of commercially available [P₆₆₆₁₄]Cl. Below this temperature, the increasing viscosity of the IL becomes an issue as stirring and diffusion are hindered and agglomeration of metal particles occurs, which results in residuals of unreacted silver. Addition of 5 mol % [P₆₆₆₁₄]Cl to [P₆₆₆₁₄][NTf₂], which has a lower viscosity but gives no reaction products at room temperature, resulted in only partial conversion of the silver. Using a mixture of 2 mol % of [P₆₆₆₁₄]Cl in the inert solvent dichloromethane (DCM), however, yielded single phase Ag₂Te after 16 h at RT (Figure 3).

Figure 3.

PXRD patterns of Ag₂Te from 2 Ag+Te with [P₆₆₆₁₄]Cl. 60 °C is the lowest temperature at which the reaction with a magnetic stirrer in neat IL was successful. At RT, dilution the IL with the inert solvent DCM or using a planetary ball mill was successful in the synthesis of single phase Ag₂Te. Bottom: Pattern of Ag₂Te simulated on the basis of the Inorganic Crystal Structure Database No. ICSD‐73230.

In an alternative approach, we charged a planetary ball mill with the two elements and [P₆₆₆₁₄]Cl. A planetary ball mill was chosen because the high viscosity of the IL prevents the ball's movement in most vibrating ball mills at the lowest frequency at which the ball mills work steadily. Over the entire reaction time, the maximum temperature was 38.6 °C. The product was single phase Ag₂Te (Figure 3). Since the width of the X‐ray reflections does not differ significantly for the products of all three methods, their crystalline domains appear to be of a similar size.

2.2. Mechanism of the Activation of Tellurium in [P₆₆₆₁₄]Cl at 60 °C

As shown by the single phase synthesis of silver telluride and the diffusion experiments, [P₆₆₆₁₄]Cl activates tellurium even at room temperature. We used in‐situ liquid‐phase ³¹P and ¹²⁵Te‐NMR spectroscopy to gain further insight into the activation mechanism.

In a first step, we investigated the impurities in the commercially available [P₆₆₆₁₄]Cl with a specified purity of > 95 % with ³¹P‐NMR and mass spectrometry, as impurities could play a major role in the tellurium activation. Our results are consistent with a similar analysis by Deferm et al. ^[27] In the ³¹P‐NMR spectrum (Figure 4), [P₆₆₆₁₄]⁺ yields a signal at 33.29 ppm. However, several further signals indicate the presence of other phosphorus‐containing species. Tetraalkylphosphonium salts are usually synthesized in a quaternization reaction of tertiary phosphanes by the nucleophilic addition of haloalkanes RX (X = Cl, Br, I). ^[29] The ³¹P spectrum provides evidence for tertiary phosphanes at −32.21 ppm and secondary phosphanes at −69.52 ppm. The mass spectrum confirms the presence of trihexylphosphane as protonated molecules (M+Z) (m/z: 287.286 [M+H]) and dihexyltetradecylphosphane (m/z: 399.411 [M+H]) as well as hexyltetradecylphosphane (m/z: 315.317 [M+H]). It is not possible to distinguish between trihexylphosphane and dihexyltetradecylphosphane solely by ³¹P‐NMR spectroscopy, because the influence of the alkyl chain length on the chemical shift of the central phosphorus is negligible when the number of carbon atoms exceeds four. ^[30] Tertiary phosphanes can be provided by a free radical addition of gaseous phosphane to α‐olefins. ^[31] Although the addition at the α‐carbon atom is preferred, the phosphane can also react with the β‐carbon atom, giving a branched alkyl phosphane like 1‐methylpentyl‐di‐n‐hexylphosphane, which was also detected at −18.90 ppm. When the branched phosphane is present during the quaternization, a branched phosphonium cation like 1‐methylpentyl‐di‐n‐hexyl(tetradecyl)phosphonium can be expected as a side product. The resonance of the branched phosphonium cation is indeed found at 37.11 ppm. Due to their high reactivity, phosphanes are sensitive to oxygen. A broad signal at 45.38 ppm indicates the presence of phosphane oxides. Due to the broadening, it is not possible to distinguish between different phosphane oxides. However, mass spectroscopy was able to detect the sodium adducts of trihexylphosphane oxide (M+Na) (m/z: 325.26 [M+Na]) and dihexylphosphane oxide (m/z: 241.169 [M+Na]). The signal at 12.03 ppm splits into a doublet when no proton decoupling is used during the measurement (J = 482 Hz). The chemical shift and coupling constant indicate that the resonance originates from the protonated tertiary phosphanes. ^[30]

Figure 4.

Section of the ³¹P‐NMR spectrum of commercial [P₆₆₆₁₄]Cl. The frame is zoomed in on one of the weak signals of the impurities. The signal at 33.29 ppm is the resonance of the [P₆₆₆₁₄]⁺ cation.

It is known that tertiary phosphanes (R ₃P) can be oxidized by elemental tellurium and form organophosphorus(V) tellurides. ^[28] After the treatment of [P₆₆₆₁₄]Cl with tellurium for 16 h at 60 °C, the resonances of all tertiary and secondary phosphanes disappeared in the ³¹P‐NMR spectrum. In parallel, two new signals show up at 10.50 ppm and −7.58 ppm. Next to the latter one, two satellites were visible that might be an overlapping doublet, indicating the spin‐spin coupling between phosphorus and tellurium with a coupling constant of 1672 Hz. It is plausible that the new signals originate from the oxidation products of phosphane impurities by the elemental tellurium. Because of the low natural abundance of the NMR‐sensitive isotope of tellurium ¹²⁵Te and the low concentration of the dissolved tellurium species, it is difficult to find the corresponding signal in the ¹²⁵Te‐NMR spectrum. When ¹²⁵Te‐enriched tellurium is used, the signals in the ³¹P‐NMR spectrum at 10.50 ppm and −7.58 ppm completely split into doublets (Figure 5).

Figure 5.

³¹P‐NMR spectra of the IL (a) of [P₆₆₆₁₄]Cl after the reaction with tellurium, and (b) after the synthesis of Ag₂Te.

The ¹²⁵Te NMR spectrum also reveals two doublets (Figure 6). The first one at −829.87 ppm has the same coupling constant of 1672 Hz as the doublet at −7.58 ppm in the ³¹P‐NMR spectrum. This proves the covalent P−Te bond and the formation of trihexylphosphane telluride. The second doublet in the ¹²⁵Te‐spectrum at −903.34 ppm has a coupling constant of 1705 Hz. This is identical to the coupling constant of the doublet at 10.50 ppm in the ³¹P spectrum. These resonances originate from the oxidation product of the branched trialkylphosphonium cation with elemental telluride. These ³¹P‐ and ¹²⁵Te‐NMR results reveal that the oxidation products of tertiary phosphanes are present as mobile tellurium species in tetraalkylphosphonium based ionic liquids. They probably act as intermediates in the formation of metal tellurides.

Figure 6.

¹²⁵Te‐NMR spectrum of the IL [P₆₆₆₁₄]Cl after the reaction with tellurium.

After the reaction of equivalent amounts of tellurium and silver in [P₆₆₆₁₄]Cl, the IL was again analyzed with ³¹P‐NMR spectroscopy. The resonances of the tertiary, secondary and protonated phosphane reoccur in the spectrum (Figure 5 b). A possible reaction mechanism might be the initial reduction of elemental tellurium by the phosphane impurities in the IL. The trialkylphosphane telluride then reacts with the elemental silver to form Ag₂Te and restores the phosphanes. Since the phosphanes are restored after the reaction, only catalytic amounts are necessary.

To test whether the tellurium activation is based only on the phosphane impurities or further activation processes are involved, we repeated our experiment with purified [P₆₆₆₁₄]Cl. Most of the purification protocols in literature rely on liquid extraction with hexane or heptane.[ ²⁷ , ³² ] Using these protocols, we always experienced a partial blend of the IL and the extraction solvent, and furthermore, incomplete extraction of the phosphane impurities. We thus developed a protocol to purify the IL by adsorption on silica gel and stepwise elution. First, the impurities are mobilized by the almost nonpolar solvent diethyl ether. Then the phosphonium cations and the chloride ions are remobilized by the polar solvent acetonitrile. After the eluent had been removed, purified [P₆₆₆₁₄]Cl remained with a yield of 30 to 40 %. The organic phase of the first step can be reused to repeat the process.

The ³¹P spectra of several batches of purified [P₆₆₆₁₄]Cl showed no evidence for secondary or tertiary phosphanes in the region from 10 ppm to −150 ppm (S8). However in some of the batches, the resonance of 1‐methylpentyl‐di‐n‐hexyl(tetradecyl) phosphonium (next to the main signal of [P₆₆₆₁₄]Cl) as well as resonances at 44.7 ppm and 53.4 ppm are still present in the spectra. These resonances have chemical shifts similar to phosphane oxides, although no phosphane oxides were found in mass spectra. The additional signals might originate from other phosphonium cations with more than one branched alkyl group that are formed by the addition of phosphane to the β‐carbon of α‐olefins. This would also explain the similarity of their elution behavior with that of [P₆₆₆₁₄]⁺.

The reaction with tellurium and the synthesis of Ag₂Te were repeated with purified [P₆₆₆₁₄]Cl at 100 °C and 60 °C. After the treatment with elemental tellurium, no visible change of the IL is noticed and ¹²⁵Te and ³¹P‐NMR spectra gave no evidence for phosphane tellurides in the IL. The ³¹P‐NMR spectrum of the IL remained unchanged, even after the (successful) synthesis of Ag₂Te. This means, the phosphonium cation remains inert during the syntheses at these moderate temperatures. An alternative mobile tellurium species must exist since a quantitative yield of Ag₂Te was nevertheless obtained in the purified IL at 60 °C. The lack of resonances in the ¹²⁵Te‐NMR spectrum leads to the assumption that their concentration is lower than the detection limit.

3. Conclusions

In this article, we demonstrated the activation of tellurium and the formation of silver tellurides in [BMIm]X (X = Cl⁻, [HSO₄]⁻) and [P₆₆₆₁₄]Z (Z = Cl⁻, Br⁻, [DCA]⁻, [NTf₂]⁻, [dec]⁻, [OAc]⁻, [BTMP]⁻) at low temperatures. [P₆₆₆₁₄]Cl has shown to be the best choice for the quantitative synthesis of AuTe₂ at 200 °C and Ag₂Te down to 60 °C. In comparison to the results of Zhang et al., the described method has decreased the reaction temperature for the synthesis of Ag₂Te even down to room temperature. No evidence for a decomposition of the IL or a reaction between the IL cation and the elemental tellurium is found. This is essential for the recyclability of the IL. We found that one possible reaction mechanism is the catalytic tellurium activation by phosphane impurities in commercially available [P₆₆₆₁₄]Cl. The phosphanes reduce elemental tellurium and form phosphane tellurides. The latter react with solid silver, whereby Ag₂Te forms and the phosphane is recovered. The unexpected finding that phosphane‐free [P₆₆₆₁₄]Cl also allows the quantitative synthesis of Ag₂Te at 60 °C implies an additional activation mechanism independent from phosphane, which is yet unknown.

Experimental Section

The starting materials and the products were handled in an argon‐filled glove‐box (MBraun UNILab) with p(O₂)/p ⁰<1 ppm and p(H₂O)/p ⁰<1 ppm. Tellurium (Chempur, 99.9999 %, bar) was distilled and reduced in a hydrogen flow at 400 °C. Gold powder (ABCR, 99.8 %, spherical, 0.3 to 0.5 microns), gold flakes (Chempur 99.99 %), silver powder (−200 mesh), and Ag flakes (Roth, 99 %, 0.1 mm thickness) were used without further treatment. DCM (Fisher Scientific, HPLC grade, amylene stabilized) and EtOH (Fisher Scientific, absolute 99.8 %, Certified AR for Analysis) were degassed and dried in an MBraun SPS. Enriched ¹²⁵Te (99.7 %) for NMR experiments was purchased from Coretechnet. Ionic liquids were purchased from IoLiTec and dried at 110 °C for 16 h at about 10⁻³ mbar. Further purification (see below) was applied only for special purpose. [P₆₆₆₁₄]Cl>95 %, [P₆₆₆₁₄]Br>95 %, [P₆₆₆₁₄][DCA]>95 %, [P₆₆₆₁₄][BTMP]>90 %, [P₆₆₆₁₄][NTf₂] 99 %, [P₆₆₆₁₄][dec]>95 % and [BMIm]Cl 99 %, [BMIm][HSO₄] 98 %.

Synthesis of [P₆₆₆₁₄][OAc]. [P₆₆₆₁₄][OAc] was synthesized by anion metathesis using potassium acetate and undried [P₆₆₆₁₄]Cl. 20.8 g of the chloride IL (40 mmol) was dissolved in 20 mL absolute EtOH, combined under vigorous stirring with 50 mL ethanolic solution of (excess) potassium acetate (4.9 g, 50 mmol) and stirred for 16 h at room temperature. Precipitated potassium chloride was filtered off. After removal of the solvent on a rotary evaporator, during which more salt precipitated, the IL was filtered again and afterwards dried under dynamic vacuum (about 10⁻³ mbar).

Purification of P₆₆₆₁₄Cl. The first step of P₆₆₆₁₄Cl purification was a neutralization of the hydrogen chloride impurities. The IL was stirred with an equivalent volume of 0.1 m NaOH solution for 2 h. Following the separation, the IL phase was washed several times with water. This step might cause foaming during the mixing of IL and water. For the following column chromatography, the IL was adsorbed on silica (Macherey‐Nagel, Silica 60 M, 0.04–0.063 mm), which then was dispersed in diethyl ether and filled in the chromatography column. The column (diameter 34 mm) contained 5 cm of the same silica gel dispersed in diethyl ether. The prepared column was eluted with 100 mL diethyl ether. The obtained eluate was rinsed again on the column, which was then eluted again with 200 mL diethyl ether, giving the fraction with the impurities. The subsequent elution with acetonitrile give the fraction with the purified IL. The acetonitrile was removed by vacuum evaporation in a rotary evaporator, and the remaining IL was dried at 110 °C for 16 h under dynamic vacuum (about 10⁻³ mbar).

Synthesis setup. In general, the synthesis of the metal tellurides was carried out in 5 mL round‐bottom flasks equipped with a magnetic stirring bar (PTFE lined, cylindrical, 12.5 mm×4.5 mm). The flasks were filled with the elements and 1 g of IL under argon atmosphere in a glovebox, sealed with a glass stopper and high‐vacuum silicon grease and placed in an oil bath. After the mixture was stirred (minimum 350 rpm is necessary) for 16 h at various temperatures, the flask was removed from the bath and allowed to cool to room temperature. After the reaction the samples were brought into a glovebox again followed by the manual separation of the IL by a pipette. The samples were washed with dried DCM three times, except [dec]⁻ and [HSO₄]⁻ ILs, which are insoluble in DCM and were thus washed with EtOH. Afterwards the powders were dried in a vacuum drying chamber at room temperature outside the glovebox.

Synthesis of AuTe₂. Gold powder (0.2212 mmol, 43.56 mg) and tellurium (2 equiv. 0.4423 mmol, 56.44 mg) were freshly ground in an agate mortar before use. The black precipitate resulting from the reaction at 200 °C in [P₆₆₆₁₄]Cl or [BMIm]Cl was pure AuTe₂ (PXRD). SEM/EDX shows that larger particles may contain enclosures of unreacted gold. Quantitative yield (100 mg) of single phase AuTe₂ was achieved.

Passivation experiment. A gold flake (0.7077 mmol, 139.40 mg) was flattened mechanically in an agate mortar. The gold flake and tellurium (1.4155 mmol, 180.61 mg) were overlayed with 1 g of [P₆₆₆₁₄]Cl in a 5 mL glass flask. Reaction without stirring leads to an opaque brown‐greyish surface layer on the gold flake without any change in morphology of the flake.

Diffusion experiment. For diffusion experiments H‐shaped tubes with central fritted glass filter, to prevent mixing of the solid starting materials and to assure connection only via the liquid phase, was used. Gold (0.33 mmol, 65.34 mg) and tellurium (0.66 mmol, 84.66 mg) were placed in opposite ends of the H‐shaped tubes. [P₆₆₆₁₄]Cl was then carefully added on both sides till the horizontal connection is completely filled and both parts are only connected via the liquid phase. The apparatus was kept upright in a glass beaker and mounted in an oil bath, were it was kept at 200 °C for one week. Afterwards the apparatus was moved inside a glovebox were the IL was separated on both sides from the solids. The residues were washed with DCM.

Synthesis of Ag₂Te. Silver powder (0.58 mmol, 62.83 mg) and tellurium (0.2913 mmol, 37.17 mg) were freshly ground in an agate mortar before use. The black precipitate resulting from the reaction at 200 °C was pure Ag₂Te in [P₆₆₆₁₄]Cl, [P₆₆₆₁₄][NTf₂], [P₆₆₆₁₄][BTMP], [P₆₆₆₁₄][dec], or [BMIm]Cl. In [P₆₆₆₁₄]Cl, or [BMIm]Cl quantitative yield (100 mg) was achieved.

At a reaction temperature of 105 °C only in [P₆₆₆₁₄]Cl, or [BMIm]Cl quantitative yield (100 mg) was achieved.

At reaction temperatures below 60 °C only [P₆₆₆₁₄]Cl can be used due to the melting point of [BMIm]Cl (T _m = 65 °C).

Passivation experiment. For passivation experiments the starting materials were silver flakes which were cut out of a silver foil (stored in air; 0.1205 mmol, 13 mg) by a scissor and 0.5 equiv. of tellurium (0.06 mmol, 7.69 mg). Both were placed in a 5 mL glass flask that was sealed after addition of the IL and heated to 200 °C for 1 h, 2 h, 4 h and 16 h.

Diffusion experiment. The procedure is the same as for diffusion experiments in the gold system, with silver (1.17 mmol, 125.68 mg) and tellurium (0.58 mmol, 74.34 mg) in [P₆₆₆₁₄]Cl at 200 °C for one week.

DCM with 2 mol % [P₆₆₆₁₄]Cl: Silver (1.164 mmol, 125.6 mg), tellurium (0.582 mmol, 74.33 mg) freshly ground in an agate mortar, were placed in an 25 mL Schlenk tube equipped with a magnetic stirring bar with 0.5 g (0.963 mmol) of [P₆₆₆₁₄]Cl, sealed with a septa and placed on a magnetic stirring plate. After the addition of 3 mL (46.979 mmol) dry DCM, under dynamic argon flow, by using a syringe in the Schlenk technique and turning on the stirring (350 rmp), the reaction mixture turned to dark grey after 1 min. The reaction mixture was left stirring for 16 h at room temperature. The black precipitate was washed three times with DCM and characterized by PXRD. Single phase Ag₂Te in quantitative yield was obtained.

Planetary Ball Mill: Silver (1.164 mmol, 125.6 mg), tellurium (0.582 mmol, 74.33 mg) and 6 mL of [P₆₆₆₁₄]Cl were filled into a planetary ball mill container (Fritsch Pulverisette 7 premium line; 20 mL container and ten 10 mm balls of yttrium‐stabilized zirconia) inside the glovebox. Afterwards the sealed ball mill containers were placed in the ball mill, mounted in a laboratory fume hood outside the glovebox. The milling program comprised 135 cycles (almost 16 h), each with 2 min. at 200 rpm followed by 5 min. break for cooling the system. Temperature was measured by a sensor (Fritsch MillControl), which was in direct contact with the reaction mixture (T _max = 38.6 °C). The obtained dark grey suspension was washed three times with 10 mL of DCM and centrifuged with 4000 rpm for 5 min in a glovebox. 200 mg (quantitative yield) of pure Ag₂Te were obtained.

Powder X‐ray Diffraction. Characterization of solid reaction products were done by powder X‐ray diffraction (PXRD) at 296(1) K on an X'Pert Pro MPD diffractometer (PANalytical), equipped with a curved Ge(111) monochromator, using Cu Kα ₁ radiation (λ = 154.056 pm).

SEM/EDX Measurements. Powder samples for SEM/EDX were fixed in two compound epoxy polymer pellet in which graphite was dispersed. The pellets were wet‐ground with a MetaServ 250 (Buehler, silicon carbide grinding paper) and subsequently polished with a VibroMet 2 (Buehler, MasterPrep alumina suspension 0.05 μm). The polished pellets were fixed on the sample holders with a graphite pad and sputtered with carbon. Needle shaped crystals and flakes were glued to a carbon pad. Scanning electron microscopy (SEM) was performed by using an SU8020 instrument (Hitachi) with a triple detector system for secondary and low‐energy backscattered electrons (U _e = 3 kV). The compositions of the samples were determined by quantitative EDX (U _e = 20 kV) analysis using an SU8020 instrument (Hitachi) equipped with a Silicon Drift Detector X–MaxN (Oxford). In light of the preparation method, the elements C and O (both part of the polymer) and Al (from polishing suspension) were omitted in EDX quantifications.

NMR Spectroscopy. For the liquid‐state NMR experiments a Bruker Avance Neo 300 MHz spectrometer with a 5 mm high‐resolution probe was used. All samples were prepared and sealed with polytetrafluoroethylene caps in low pressure/vacuum 5 mm NMR tubes from Deutero in the glove box under argon atmosphere. The samples of the ionic liquid after the solvation of tellurium were filtered, using syringe filters with the pore width 0.45 μm. ¹²⁵Te enriched tellurium (>99.7 %, Cortechnet) was used as starting material in the synthesis and experiments that were analyzed by ¹²⁵Te‐NMR‐spectroscopy. The filtered ILs were not diluted with a deuterated solvent, instead capillaries filled with DMSO‐d₆ were added to the IL inside the NMR tube to ensure field‐frequency lock. The ³¹P‐NMR spectra were recorded at the transmitter frequency of 121.49 MHz using 128 scans, a relaxation delay of 10 s and a pulse length of 8.3 μs. The chemical shifts in the ³¹P‐NMR spectra were referenced relative to H₃PO₄. The ¹²⁵Te‐NMR spectra were recorded at the transmitter frequency 94.65 MHz using 16000 to 32000 scans. In order to shorten the relaxation time and increase the excitation bandwidths, a 30° instead of a 90° pulse with a pulse length of 3.33 μs and relaxation delay of 5 s was used. To scan a large chemical shift range from 7000 to −4000 ppm the transmitter frequency offset was varied in steps of 104159.35 Hz. The chemical shift was measured relative to Me₂Te (δ = 0 ppm). For calibration the secondary external standard Te(OH)₆ (δ = 707 ppm) was used ^[33]

HPLC‐MS. For the HPLC‐MS measurement a 1260 infinity HPLC coupled with 6538 UHD Accurate Mass Q‐TOF LC/MS was used. The samples were prepared in the glove box in glass vials and dissolved in dry acetonitrile after withdrawal. For separation, a Zorbax Extend‐C18 1.8 μm (2.1×50 mm) column was used at 25 °C. After injection of 1.00 μl the elution started with a mixture of 40 % acetonitrile for mobile phase A and 60 % water for mobile phase B at a flow rate of 0.5 ml/min from 0 to 10 min. Afterwards a linear gradient was applied from 40 % to 100 % A, from 10 to 20 min, hold from 20 to 50 min and back from 100 % to 40 %, from 50 to 51 min. These conditions were maintained for another 5 min. For the electrospray ionization we used the following parameter: nebulizer pressure 50 psig, fragmentor voltage 200 V, capillary voltage 4 kV, drying gas temperature 350 °C and gas flow 10 L/min.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

M. A. Grasser, T. Pietsch, E. Brunner, T. Doert, M. Ruck, ChemistryOpen 2021, 10, 117.