Reactions of noble-metal oxides in ionic liquids near room temperature

Abstract

The reaction of Ag₂O, Au₂O₃, and HgO with CuCl, CuI, AgCl, AgI, AuCl, and AuI in ionic liquids ([EMIm]Cl, [BMIm]Cl) near room temperature (20–80 °C) is evaluated and results in the new compounds (C₈H₁₄N₂)CuCl, (C₈H₁₄N₂)AgI, (C₆H₁₀N₂)AuCl, [(C₈H₁₄N₂)₂Hg][CuCl₃], [(C₈H₁₄N₂)₂Hg][AgCl₃], and [EMIm][Ag₂I₂Cl]. Thereof, (C₈H₁₄N₂)CuCl, (C₈H₁₄N₂)AgI, (C₆H₁₀N₂)AuCl, [(C₈H₁₄N₂)₂Hg][CuCl₃], and [(C₈H₁₄N₂)₂Hg][AgCl₃] are NHC complexes (NHC: N-heterocyclic carbene) with M–C bonds (M: Cu, Ag, Au, Hg). Whereas (C₈H₁₄N₂)CuCl and (C₈H₁₄N₂)AgI crystallize as single molecules, (C₆H₁₀N₂)AuCl is dimerized via aurophilic interactions. [(C₈H₁₄N₂)₂Hg][CuCl₃] and [(C₈H₁₄N₂)₂Hg][AgCl₃] exhibit Hg atoms with two Hg–C bonds. Moreover, (C₈H₁₄N₂)AgI shows intense green fluorescence at room temperature with a quantum yield of 44%, whereas all other compounds do not show any emission at room temperature. Finally, [EMIm][Ag₂I₂Cl] is not an NHC compound but contains _∞¹[AgI_1/2I_2/4Cl_1/2]⁻ chains with infinite d¹⁰–d¹⁰ interaction of the silver atoms. The title compounds are characterized by single-crystal structure analysis, infrared spectroscopy, thermogravimetry, and fluorescence spectroscopy.

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Introduction

Ionic liquids have significantly advanced the synthesis of inorganic compounds and, in the meantime, resulted in several unique compounds that most probably would have not been obtainable in conventional solvents.¹ The polychloride [Et₄N]₂[(Cl₃)₂·Cl₂], the intermetalloid [CuBi₈]³⁺ cluster cation in [CuBi₈][AlCl₄]₂[Al₂Cl₇], the heavy-metal porphyrin-analogue [Hg₄Te₈(Te₂)₄]⁸⁻ in [DMIm]₈[Hg₄Te₈(Te₂)₄] ([DMIm]⁺: 1-decyl-3-methylimidazolium), the ligand-stabilized [Ga₅]⁵⁺ pentagon, the linear uranyl-type [N≡U≡N] complex and supertetrahedral selenido germanate clusters are some exemplary compounds.² We could contribute the synthesis of the three-dimensional polybromide [C₄MPyr]₂[(Br)₂·(Br₂)₉], the highly coordinated Sn(II)I₈ subunit in the carbonyl [SnI₈{Fe(CO)₄}₄][Al₂Cl₇]₂, or crown-ether coordination compounds like Mn₂I₄(18-crown-6) with unique fluorescence features such as 100% quantum yield, strong second-harmonic generation (SHG) and intense orange emission via SHG-driven excitation in the infrared spectral range.³ Often, ionic-liquid-based synthesis can be performed near room temperature (≤100 °C), which is ideal to realize new metastable compounds with kinetic control (i.e. at low activation energy).⁴ Key features of ionic liquids, in this regard, relate to the high thermal and chemical stability and the weakly coordinating properties.^1,5

Until now, the synthesis of metastable compounds in ionic liquids was mainly focused on metal halides, metal selenides/tellurides or metal clusters.^1–5 The synthesis of new metal oxides and compounds using metal oxides as starting materials, in contrast, was barely addressed, which can be attributed to the high lattice energy of many metal oxides and their low solubility in ionic liquids. Several studies, however, have successfully shown options to dissolve metal oxides in ionic liquids.⁶ In the course of our studies on ionic-liquid-based syntheses and the optional use of metal oxides as starting materials, we have reacted Ag₂O, Au₂O₃, and HgO with CuCl, CuI, AgCl, AgI, AuCl, and AuI in every combination of the respective metal oxides and metal halides, whereof many lead to new compounds. As results, the novel NHC complexes (NHC: N-heterocyclic carbene) (C₈H₁₄N₂)CuCl (1), (C₈H₁₄N₂)AgI (2), (C₆H₁₀N₂)AuCl (3), [(C₈H₁₄N₂)₂Hg][CuCl₃] (4), [(C₈H₁₄N₂)₂Hg][AgCl₃] (5), as well as [EMIm][Ag₂I₂Cl] (6) with an infinite _∞¹[AgI_1/2I_2/4Cl_1/2]⁻ chain showing d¹⁰–d¹⁰ interaction of the silver atoms.

Results and discussion

Ionic-liquid-based synthesis

All reactions were performed using the metal oxides Ag₂O, Au₂O₃, and HgO as well as the metal halides CuCl, CuI, AgCl, AgI, AuCl, and AuI as the starting materials. [EMIm]Cl and [BMIm]Cl ([EMIm]: 1-ethyl-3-methylimidazolium, [BMIm]: 1-butyl-3-methylimidazolium) are introduced to form ionic liquids after reaction with the aforementioned metal halides. All compounds and sample handling needs to be performed with inert conditions in order to avoid hydrolysis. Interestingly, the reactions already start at room temperature (20 °C). For 1 and 4, leaving the ampoule at room temperature for 4 days is sufficient to grow single crystals with a yield of 40–60%. For 2, 3, 5, and 6, the reactions also start at room temperature but were accelerated and completed by sleight heating to 40–80 °C. After 4 days, moisture-sensitive, colourless crystals were formed with good yield (40–60%) (Fig. 1). The synthesis of the title compounds 1–6 can be rationalized based on the following reactions:

2C₈H₁₅N₂Cl + Ag₂O + 2CuCl → 2(C₈H₁₄N₂)CuCl (1) + 2AgCl + H₂O

2C₆H₁₁N₂Cl + Ag₂O + 2AuI → 2(C₆H₁₀N₂)AgI (2) + 2AuCl + H₂O

3C₆H₁₁N₂Cl + 0.5Au₂O₃ + 3AuCl → 3(C₆H₁₀N₂)AuCl (3) + AuCl₃ + 1.5H₂O

2C₈H₁₅N₂Cl + HgO + CuCl → [(C₈H₁₄N₂)₂Hg][CuCl₃] (4) + H₂O

2C₈H₁₅N₂Cl + HgO + AgCl → [(C₈H₁₄N₂)₂Hg][AgCl₃] (5) + H₂O

[EMIm]Cl + 2 AgI* → [EMIm][Ag₂I₂Cl] (6) (*Note that Ag₂O was also present in this reaction)

Fig. 1 Scheme to illustrate the reactions of noble-metal oxides and halides in ionic liquids near room temperature to obtain the title compounds 1–6.

In regard of a reaction of Ag₂O, Au₂O₃, and HgO, the metal chlorides CuCl, AgCl, and AuCl turned out to be most reactive, which can be ascribed to the more ionic character in comparison to the respective metal iodides. Although all possible combinations of the respective metal oxides and metal halides were evaluated, some did not result in the formation and crystallization of new compounds, which can be also related to the solubility of the respective metal halide in the ionic liquid. The reaction of Ag₂O with AgI is particularly different from all other reactions since the presence of Ag₂O was necessary to obtain [EMIm][Ag₂I₂Cl] although it is not part of the aforementioned reaction. Obviously, the argentophilic interaction in 6 leads to a thermodynamically more stable compound than the formation of a NHC complex. Nevertheless, the Ag⁺ excess originating from Ag₂O is crucial and promotes the argentophilic interactions. However, the dominating reaction relates to the formation of NHC complexes 1–5. Generally, NHC ligands serve as strong sigma-donating ligands and result in a rich coordination chemistry with almost all transition metals.⁷ Some NHC complexes show promising properties and applications, including homogeneous catalysis with second-generation Grubbs catalysts as one of the most prominent examples.⁸ Cu–NHC complexes have become popular due to their ability to activate C–H bonds.⁹ Furthermore, Au– and Pt–NHC complexes received specific attention due to their potential suitability as anti-cancer agents.¹⁰ Among the NHC complexes, the ligands essentially contain an imidazole or a derivative as central unit, which is usually modified by sterically demanding groups, such as in 1,3-dimesityl-imidazol-4,5-dihydro-2-ylidene (SIMes).^7,11 After the discovery of the very first NHC complex by Wanzlick et al. in 1968,¹² NHC complexes were prepared by different approaches, most often using transmetallation strategies.^7,13 In addition to these conventional approaches, few examples also show the synthesis of NHC complexes in ionic liquids,^6b,14 including electrochemical reduction in ionic liquids.¹⁵

Structural characterization

(C₈H₁₄N₂)CuCl (1) crystallizes in the form of colourless rhombuses in the orthorhombic, non-centrosymmetric and chiral space group P2₁2₁2₁ (ESI: Table S1 and Fig. S1†). Herein, a deprotonated imidazole is connected to a CuCl unit via Cu–C bonding (Fig. 2a). The C–Cu–Cl arrangement is almost linear (173.7(2)°) and with a Cu–C distance of 186.9(5) pm (Table 1). Both is in good agreement with literature data (186.6–195.3 pm, 174–180°).¹⁶ Similarly, the Cu–Cl distance (210.9(2) pm) is well in agreement with comparable compounds (e.g. 208.8 pm in (C₄₁H₃₂N₂O₂)CuCl).^16a In fact, the composition of 1 was already claimed in a patent to improve bleaching agents. However, its structure and composition were not characterized.¹⁷ With [Ag(EMIm-ylide)₂][X] (X: SCN, CN₂, OAc), Binnemans et al. also showed the synthesis of comparable Ag–NHC complexes in ionic liquids for the first time,^6b which were obtained by reaction of Ag₂O and the ionic liquid [EMIm][X] (X: SCN, CN₂, OAc) only. Despite of comparable experimental conditions, the reaction of Ag₂O with CuCl in [BMIm]Cl in our case did not result in an Ag–NHC compound but the Cu–NHC compound 1, which can be ascribed to the presence of CuCl as well as of Ag₂O, whereas Binnemans et al. examined the reaction of Ag₂O only in the ionic liquid [EMIm][X].^6b Taking the reaction mechanism proposed by Binnemans et al. into account,^6b we suggest a modified two-step reaction for the formation of 1 (Fig. 2d). First of all, Ag₂O as a base subtracts H⁺ from [BMIm]⁺ and Cl⁻ coordinates to Ag⁺. Secondly, the intermediate decomposes with cleavage of the C–H bond and coordination of the carbene to Cu⁺, followed by the formation of (C₈H₁₄N₂)CuCl, AgCl, and H₂O (Fig. 2d).

Fig. 2 Molecular structures of the NHC complexes (C₈H₁₄N₂)CuCl (1) (a), (C₈H₁₄N₂)AgI (2) (b) (C₆H₁₀N₂)AuCl (3) (c), and scheme of the reaction mechanism illustrated for 1 (d).

Table 1 Comparison of selected distances in (C₈H₁₄N₂)CuCl (1), (C₆H₁₀N₂)AgI (2), (C₆H₁₀N₂)AuCl] (3), [(C₈H₁₄N₂)₂Hg][CuCl₃] (4), and [(C₈H₁₄N₂)₂Hg][AgCl₃] (5) with literature data

Atoms	Compound	Distance/pm
Cu–Cl	1	210.9(2)
	(C41H32N2O2)CuCl^16a	208.8
	CuCl^27	238.2
Cu–C	1	186.9(5)
	(C41H32N2O2)CuCl^16a	186.9
	(C21H36N2)CuCl^16c	195.3
	(C27H38N2)CuCl^16b	189.6
Ag–I	3	332.3(2)
	(C21H26N2)AgI^18a	256.6
	(C27H36N2)AgI^18b	254.7
	(C27H38N2)AgI^18c	257.4
	AgI AgI4	281.0
Ag–C	3	210.0(2)
	(C21H26N2)AgI^18a	211.3
Au–Cl	2	227.5(3), 228.1(2)
	(C7H12N2)AuCl^19a	228.8
	(C5H8N2)AuCl^20	228.8
	AuCl^19b	230.0
Au–C	2	196.2(11), 197.7(9)
	(C7H12N2)AuCl^19a	198.6
	(C5H8N2)AuCl^20	197.9
Hg–Cl	4	299.4(2)
	[S4N3][HgCl3]^26	234.5, 243.3, 248.5, 302.2, 320.4
	HgCl2 ^32	228.4, 230.1
Hg–C	4	208.0(5)
	[(C15H12N2)2Hg][ClO4]2 ^25a	206.0
	(C16H18N4)Hg(PF6)2 ^34	206.7
Cu–Cl	4	222.2(3), 227.5(2)
	CuCl^27	238.2
Hg–Cl	5	304.4(1)
	[S4N3][HgCl3]^26	234.5, 243.3, 248.5, 302.2, 320.4
	HgCl2 ^33	228.4, 230.1
Hg–C	5	206.8(3)
	[(C15H12N2)2Hg][ClO4]2 ^25a	206.0
	(C16H18N4)Hg(PF6)2 ^34	206.7
Ag–Cl	5	247.0(1), 250.4(1)
	CsAgCl2 ^28a	249.2, 266.7, 287.2
	AgCl^28b	277.3

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Since we did not observe any crystalline Ag–NHC compound upon reacting of Ag₂O with AgCl and AgI, we have performed syntheses with Ag₂O, Au₂O₃, AuCl, and AuI. Here, the compounds (C₈H₁₄N₂)AgI (2) and (C₆H₁₀N₂)AuCl (3) were obtained and crystallize in the space group P1̄ and Cc (ESI: Table S1, Fig. S2 and S3†). The structure of 2 compares to 1 with a deprotonated imidazole connected to a AgI unit via Ag–C bonding (Fig. 2b). The Ag–C distance is 210.0(2) pm, the Ag–I distance is (332.3(2) pm) (Table 1). Both are well in agreement with literature data (Ag–C: 206–211 pm, Ag–I: 256–257 pm).¹⁸ The C–Ag–I arrangement is again almost linear (175.2(5)°). The Au–NHC compound 3 also exhibits structural features similar to 1 and 2, however, with molecular units dimerized via aurophilic interactions (Fig. 2c). Here, Au–C bonding is observed with distances of 196.6(11) and 197.6(9) pm (Table 1) and almost linear C–Au–Cl arrangement (176.8(3)–179.2(3)°). Distance and angle compare to literature data (Au–C: 198 pm, 174–180°).¹⁹ Similarly, the Au–Cl distances (227.1(3), 228.1(2) pm) are well in agreement with comparable compounds, such as (C₇H₁₂N₂)AuCl (228.8 pm) or AuCl (230.0 pm).^19a,20 Finally, short intermolecular Au⋯Au distances (348.2(1) pm in 3) are observed that are only slightly larger than the doubled van der Waals radius (332 pm) (Fig. 2c).²¹ Such aurophilic interactions are characteristic for Au(I) and have been frequently observed.²² A direct single-step formation of Au–NHC compounds such as for 3, to the best of our knowledge, has not been shown in literature before and has yet performed by transmetallation of Ag–NHC compounds in a two-step synthesis.²³

Beside using Ag₂O and Au₂O₃ as a base to obtain NHC compounds, we also have evaluated the reaction of CuCl and AgCl with HgO as a base. In this context, Pelz et al. already pointed to the feasibility of acid–base reactions of imidazolium cations and HgO.²⁴ We have reacted HgO with CuCl or AgCl in [BMIm]Cl as the ionic liquid, which resulted in colourless crystals of [(C₈H₁₄N₂)₂Hg][CuCl₃] (4) and [(C₈H₁₄N₂)₂Hg][AgCl₃] (5). Both crystallize in the monoclinic space group C2/c (ESI: Table S2, Fig. S4 and S5†). As the most relevant building unit, a central Hg²⁺ cation bridges two BMIm-ylide units. Furthermore, trigonal planar [(Cu,Ag)Cl₃]²⁻ units coordinate with two Cl atoms to the central Hg²⁺. The third Cl atom of each [(Cu,Ag)Cl₃]²⁻ unit is bridging to another Hg²⁺, so that infinite –[Cl₂(Cu,Ag)Cl]^–⋯[(C₈H₁₄N₂)₂Hg²⁺]–[Cl₂(Cu,Ag)Cl]⁻⋯ strands are formed (Fig. 3).

Fig. 3 Molecular structures of the NHC complexes [(C₈H₁₄N₂)₂Hg][CuCl₃] (4) (a) and [(C₈H₁₄N₂)₂Hg][AgCl₃] (5) (b).

In regard of the distances, Hg–C distances of 208.0(5) pm (4) and 206.8(3) pm (5) fit well with known Hg–NHC compounds (204.6(5)–209.2(5) pm).²⁵ The Hg–Cl distances are 227.6(1) and 299.5(2) pm in 4 as well as 250.4(1) pm and 304.4(1) in 5 for the μ₂-binding [(Cu,Ag)Cl₃]²⁻ units, which compares to [S₄N₃][HgCl₃] (234.5–320.4 pm).²⁶ μ₁-Binding is accompanied with longer Hg⋯Cl distances of 322.4(1) pm in 4 and 322.5(1) pm in 5 (Table 1). The Cu–Cl distances in the [CuCl₃]²⁻ unit of 4 with 222.2(3)–227.5(2) pm and are slightly shorter than in CuCl (238.2 pm),²⁷ which reflects the threefold coordination in the title compound as compared to a tetrahedral coordination of Cu⁺ cation in CuCl. For similar reasons, the Ag–Cl distances of the [AgCl₃]²⁻ unit in 5 with 247.0(1)–250.4(1) pm are slightly shorter than in CsAgCl₂ (249.2–287.2 pm) or AgCl (277.3 pm).²⁸ Interestingly, for 5 a comparably short Hg⋯Ag distance of 334.5(1) pm is observed, which is slightly above the van der Waals distance (327 pm) but still points to a weak interaction.²¹ Finally, it needs to be noticed that the butyl chain of the BMIm-ylide in 4 is disordered, which was tackled by split positions for all carbon atoms with a probability of 33 and 67%. Although Hg–NHC complexes are amongst the first described for this class of compounds,^7,13,25 the here observed structure and coordination, to the best of our knowledge, are described for the first time. Thus, a coordination of trigonal planar [CuCl₃]²⁻/[AgCl₃]²⁻ units in a Hg–NHC complex is new. Trigonal planar [CuCl₃]²⁻/[AgCl₃]²⁻ units are rarely observed in general and typically require sterical stabilization by sterically demanding organic ligands,²⁹ or hydrogen bonds.³⁰ In the case of Ag⁺, such trigonal planar arrangement is known from [PPh₄]₂[Ag₂Cl₄], but only with a strongly distorted [Ag₂Cl₄]²⁻ unit,³¹ or from the rigid network in the Zintl phase Li₁₇Ag₃Sn₆.³²

In difference to the formation of NHC complexes by the reaction of Ag₂O, Au₂O₃, HgO with CuCl, AgCl, AuCl, the reaction of Ag₂O and AgI in [EMIm]Cl did not result in an NHC compound but in the formation of [EMIm][Ag₂I₂Cl]. 6 crystallizes with colourless needles in the triclinic space group P1̄ (ESI: Table S2 and Fig. S6†). The compound is composed of infinite anionic _∞¹[AgI_2/4I_1/2Cl_1/2]⁻ chains, which are separated by [EMIm]⁺ cations (Fig. 4). In these chains, the Ag atoms are coordinated by three I and one Cl atom to form distorted Ag(1)I₃Cl and Ag(2)I₃Cl tetrahedra with two crystallographically independent Ag sites. All AgI₃Cl tetrahedra are connected via edge-sharing over all edges to four adjacent AgI₃Cl tetrahedra (Fig. 4a). Such edge-sharing is rare and in difference, for instance, to the corner-sharing AgI₄ tetrahedra in the binary sphalerite-type AgI.³⁵ Most remarkably, two I atoms in 6 belong to four different AgI₃Cl tetrahedra. The Ag–I distances in 6 range from 277.9(1) to 291.9(1) pm, which is comparable to AgI (281.4 pm). Due to the edge-sharing of tetrahedra in 6, the longer Ag–I distances in comparison to AgI with corner-sharing tetrahedra are to be expected. In contrast, the Ag–Cl distances (261.0(2), 261.1(2) pm) in 6 are slightly shorter than in AgCl (277.2 pm),³⁵ which can be attributed to the lower coordination in comparison to NaCl-type AgCl.

Fig. 4 _∞¹[AgI_2/4I_1/2Cl_1/2]⁻ chain with d¹⁰–d¹⁰ interaction in [EMIm][Ag₂I₂Cl] (6).

Due to the edge-sharing AgI₃Cl tetrahedra, moreover, short Ag–Ag distances are observed in the _∞¹[AgI_2/4I_1/2Cl_1/2]⁻ chains of 6 (Fig. 4b). Thus, the Ag–Ag distances are at 299.7(1), 308.0(1), and 319.3(1) pm. All of them are below 330 pm, which was considered as attractive d¹⁰–d¹⁰ interaction by Jansen.³⁶ Hence, an infinite zig-zag chain of attractive Ag–Ag distances is present in 6. These Ag–Ag distances are of course longer than in elemental silver (288.9 pm), which reflects the positive charge of the Ag⁺ cations. In principle, compounds with chain-like [Ag₂I₃]⁻ anions were already described in the literature.³⁷ However, a formation of infinite Ag⁺ chains with attractive d¹⁰–d¹⁰ interaction was only discussed for [C₈H₉N₂][Ag₂I₃] by Zhang.³⁸ This compound has a comparable connectivity of AgI₄ tetrahedra as in 6 but only contains iodine and exhibits significantly longer Ag–Ag distances (322.9–334.9 pm) than in 6. Moreover, this compound requires a synthesis in concentrated HI.³⁸ A combination of chlorine and iodine in chain-like halido argentates (I) is generally rare and does not result in Ag chains with attractive d¹⁰–d¹⁰ interactions.³⁹ In sum, a compound like 6 with infinite _∞¹[AgI_2/4I_1/2Cl_1/2]⁻ chains and zig-zag-shaped Ag–Ag chains was obtained for the first time and prepared by ionic-liquid-based synthesis.

Material properties

Beside the structural properties, chemical composition, thermal stability and fluorescence properties of the title compounds were examined by Fourier-transformed infrared (FT-IR) spectroscopy, thermogravimetry (TG), and photoluminescence spectroscopy (PL). FT-IR spectra of the NHC compounds 1, 3–5 are dominated by the imidazolium cations with characteristic vibrations of ν(C–H): 3300–2800 cm⁻¹, ν(C=N): 1650–1600 cm⁻¹, as well as the fingerprint area at 1500–500 cm⁻¹ (ESI: Fig. S7†). Moreover, the Hg–C stretching vibration at 720–680 cm⁻¹ is observed in the spectra of 4 and 5. Similarly, the characteristic vibrations of the [EMIm]⁺ cation are observed for 6 (ESI: Fig. S8†). The absence of O–H-related vibrations at 3600–3000 cm⁻¹ confirms the absence of moisture.

TG analysis was performed, on the one hand, to examine the thermal stability of the title compounds, and on the other hand, to confirm the chemical composition via total organic combustion analysis (Fig. 5). All title compounds show thermal decomposition with 1 to 3 decompositions steps, which are summarized in Table 2. Except for 4, which starts to decompose already slightly above room temperature, the inset of thermal decomposition is at 150 to 200 °C and is initiated by the decomposition of the EMIm/BMIm-ylides of 1 and 3–5 as well as the [EMIm]⁺ cation in 6. At higher temperature, HCl, I₂, CuCl, AgCl, or HgCl₂ evaporate (Table 2). The decomposition ends with the metals (Cu, Ag, Au) or metal halides (CuCl, AgCl) as solid residues. The observed mass loss fits well with the calculated values, and thus, also confirms the composition and purity of the title compounds.

Fig. 5 TG analysis of the compounds (C₈H₁₄N₂)CuCl (1), (C₆H₁₀N₂)AuCl] (3), [(C₈H₁₄N₂)₂Hg][CuCl₃] (4), [(C₈H₁₄N₂)₂Hg][AgCl₃] (5), and [C₆H₁₁N₂][Ag₂I₂Cl] (6).

Table 2 TG analysis of the compounds 1, 2, 4, 5 and 6

Compound	Temperature range/°C	Mass loss/%	Calculated mass loss/%
(C8H14N2)CuCl (1)	140–450	42
	450–800	18	58 (C8H13N2)
	800–960	14	15 (HCl)
		26 (residue)	27 (Cu)
(C6H10N2)AuCl (2)	200–400	42	42 (C6H10N2)
		58 (residue)	58 (Au)
[(C8H14N2)2Hg][CuCl3] (4)	20–270	23	22 (2C5H10)
	270–420	47	42 (HgCl2)
	500–750	16	21 (2C3H4N2)
		14 (residue)	15 (CuCl)
[(C8H14N2)2Hg][AgCl3] (5)	250–500	77	79 (2C8H14N2, HgCl2)
		23 (residue)	21 (AgCl)
[C6H11N2][Ag2I2Cl] (6)	260–420	29	24 (C6H10N2, HCl)
	800–1100	41	41 (I2)
		30 (residue)	35 (2Ag)

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Whereas the compounds 1 and 3–6 do not show any PL at room temperature, intense yellow emission is observed for (C₈H₁₄N₂)AgI (2), which is visible for the naked eye under UV-excitation (Fig. 6). Excitation and emission spectra indicate strong absorption below 400 nm and intense emission at 400–550 nm with a maximum at 572 nm. The efficiency of the photoluminescence processes was quantified with an absolute quantum yield of 44%, following the method given by Friend et al.⁴⁰ A comparable luminescence was frequently observed for coinage metal complexes but predominately for Cu(I) and Au(I) compounds.⁴¹ In contrast, a luminescence of molecular Ag(I) complexes was rarely reported and is usually limited to quantum yields <20% at room temperature.⁴²

Fig. 6 PL spectra with excitation and emission of (C₈H₁₄N₂)AgI (2) as well as a photo of the sample.

Conclusions

The noble metal oxides Ag₂O, Au₂O₃, and HgO are reacted with the respective noble metal halides CuCl, CuI, AgCl, AgI, AuCl, and AuI in ionic liquids ([EMIm]Cl, [BMIm]Cl) near room temperature (20–80 °C). Most often, the reactions start already at room temperature. Certain heating accelerates and completes the reaction. As a result, the NHC complexes (C₈H₁₄N₂)CuCl, (C₈H₁₄N₂)AgI, (C₆H₁₀N₂)AuCl, [(C₈H₁₄N₂)₂Hg][CuCl₃] and [(C₈H₁₄N₂)₂Hg][AgCl₃] are formed as new compounds. They contain EMIm- and BMIm-ylides with Cu–C, Ag–C, Au–C, and Hg–C bonds. (C₆H₁₀N₂)AuCl is dimerized via aurophilic interactions. [(C₈H₁₄N₂)₂Hg][CuCl₃] and [(C₈H₁₄N₂)₂Hg][AgCl₃] contain central Hg atoms with two Hg–C bonds of BMIm-ylides. Furthermore, trigonal planar [CuCl₃]⁻ and [AgCl₃]⁻ connect the [(C₈H₁₄N₂)₂Hg]⁺ cations via alternating μ₁- and μ₃-binding to infinite chains. The reaction of Ag₂O with AgI in [EMIm]Cl does not result in an NHC compound but yields [EMIm][Ag₂I₂Cl], which exhibits _∞¹[AgI_1/2I_2/4Cl_1/2]⁻ chains with infinite d¹⁰–d¹⁰ interaction of silver atoms. Structure, composition, and thermal properties of all compounds were examined by single-crystal structure analysis, infrared spectroscopy, and thermogravimetry. Finally, (C₈H₁₄N₂)AgI shows intense green fluorescence at room temperature with a quantum yield of 44%. Metal oxides, in general, were rarely used as starting materials in ionic-liquid-based synthesis until now, which can be related to their low solubility and reactivity. The here presented reactions and compounds, however, show that metal oxides nevertheless can be used for reactions already at room temperature, which offers various options for chemical synthesis and the formation of new compounds.

Experimental

Synthesis

General. All reactions and sample handling were carried out under dried argon atmosphere using standard Schlenk techniques or glove boxes. Reactions were performed in Schlenk flasks and glass ampoules. The starting materials AgCl (99.9%, Alfa Aesar), Ag₂O (≥99%, Alfa Aesar), AuCl (99.9%, Sigma Aldrich), Au₂O₃ (99%, abcr), CuCl (≥99%, Alfa Aesar), and HgO (99%, Alfa Aesar) were used as received. The ionic liquid [BMIm]Cl (99.9%, IoLiTec) was dried at 130 °C at reduced pressure (1 × 10⁻³ mbar) for 3 days. All compounds were handled and stored in argon-filled glove boxes (MBraun Unilab, c(O₂, H₂O) < 0.1 ppm).

(C₈H₁₄N₂)CuCl (1). 100.0 mg (0.573 mmol) of [BMIm]Cl, 57.0 mg (0.573 mmol) of CuCl, and 50.0 mg (0.216 mmol) of Ag₂O were mixed and filled in a glass ampoule. After 4 days at room temperature, colourless rod-like crystals were obtained with a yield of about 40%. The compound is sensitive to moisture and must be handled under inert gas conditions.

(C₆H₁₀N₂)AgI (2). 100 mg (0.374 mmol) of [BMIm]Cl, 121 mg AuI (0.376 mmol), and 50 mg of Ag₂O (0.216 mmol) were mixed and filled in a glass ampoule. This mixture was heated to 80 °C, and the temperature was kept for 96 h. After cooling to room temperature with a rate of 1 K per h, colourless crystals were obtained with a yield of about 40%. These crystals are stable in air.

(C₆H₁₀N₂)AuCl (3). 100 mg (0.682 mmol) of [EMIm]Cl, 158 mg (0.682 mmol) of AuCl, and 80 mg (0.181 mmol) of Au₂O₃ were mixed and filled in a glass ampoule. This mixture was heated to 60 °C, and the temperature was kept for 96 h. After cooling to room temperature with a rate of 1 K per h, colourless, moisture-sensitive crystals were obtained with a yield of about 40%.

[(C₈H₁₄N₂)₂Hg][CuCl₃] (4). 100.0 mg (0.573 mmol) of [BMIm]Cl, 57.0 mg (0.576 mmol) of CuCl, and 50.0 mg of (0.231 mmol) of HgO were mixed and filled into a glass ampoule. Again, the reaction occurred already at room temperature. After 4 days, rhombic, colourless crystals were obtained with a yield of about 60%. The crystals are highly moisture sensitive and must be handled under inert gas atmosphere.

[(C₈H₁₄N₂)₂Hg][AgCl₃] (5). 100.0 mg (0.573 mmol) of [BMIm]Cl, 82.0 mg (0.573 mmol) of AgCl, and 50 mg (0.231 mmol) of HgO were mixed and filled in a glass ampoule. The reaction again started at room temperature. The growth of suitable single crystals was promoted by heating to 80 °C. The ampoule was kept at 80 °C for 96 h and then cooled to room temperature with a rate of 1 K per h. Colourless, moisture-sensitive, rhombic crystals were obtained with a yield of about 50%.

[EMIm][Ag₂I₂Cl] (6). 100.0 mg (0.682 mmol) of [EMIm]Cl, 160.0 mg (0.682 mmol) of AgCl and 50 mg (0.32 mmol) of Ag₂O were mixed in a glass ampoule. The ampoule was heated to 45 °C and kept at this temperature for 96 h. Subsequently, the ampoule was cooled to room temperature with a rate of 1 K per h. Crystals of 5 were obtained as colourless needles with a yield of about 40%. The compound is sensitive to moisture and needs to be handled under inert gas conditions.

Analytical equipment

Detailed information on the analytical equipment can be obtained from the ESI.† Further details of the crystal structure investigation may be obtained from the joint CCDC/FIZ Karlsruhe deposition service on quoting the depository number CSD-No. 2104796–2104800 and 2220642.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the Deutsche Forschungsgemeinschaft (DFG). Furthermore, the authors thank Prof. Dr Dieter Fenske, Prof. Dr Peter Roesky, and Dr Michael Garmer for data collection on the gallium-jet Stoe StadiVari diffractometer as well as on Stoe Stadi Vari diffractometer.

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Footnote

† Electronic supplementary information (ESI) available: Additional data related to the analytical techniques, the crystallographic data, and the unit cells of the title compounds 1–6. CCDC 2104796–2104800 and 2220642. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3ra00892d

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