Ionothermal Synthesis of Selenido Metalates in Tunable Aryl Alkyl Ionic Liquids (TAAILs)

Abstract

We report on the first reactions of selenido germanates in tunable aryl alkyl ionic liquids (TAAILs), namely, the treatment of [K₄(H₂O)₃]­[Ge₄Se₁₀] with SnCl₄·5H₂O in (Ph_4‑BrC₆Im)­[NTf₂], (Ph_4‑BrC₈Im)­[NTf₂], (Ph_4‑BrC₁₂Im)­[NTf₂], (Ph_2‑MeC₈Im)­[NTf₂], and (Ph_4‑OMeC₈Im)­(NTf₂), in the presence of 2,6-dimethylmorpholine (DMMP) as a basic auxiliary agent (Ph_Y‑X denotes a Ph group in the 1-position of the imidazolium cation (Im); the numeral Y specifies the ortho (2) or para (4) position of a substituent X at the Ph ring; and C _n denotes a linear alkyl substituent with n C atoms in the 3-position of the Im ring; NTf₂ = bis­(trifluoromethan)­sulfonimide). The use of different TAAILs allowed the observation of different products: (Cat)₂₄[Ge₂₄Sn₃₆Se₁₃₂] (1), (Ph_2‑MeC₈Im)₂​(DMMPH)₂​[Ge_1.4Sn_4.6Se₁₄] (2), (Cat)₆​[GeSn₈Se₂₁] (3), (Cat)₂[Sn₃Se₇] (4), and (DMMPH)₄[Ge₄Se₁₀] (5). The compounds comprise cations from the corresponding TAAIL or (DMMPH)⁺, which is the monoprotonated derivative of DMMP that forms in situ (note that in some cases, extreme disorder of the cations did not allow their localization on the difference Fourier map; these are referred to as ‘Cat’ in their formula). We discuss the anionic substructures in comparison to those of known compounds based on Ge, Sn, and Se atoms and report the effect of the different architectures on optical absorption properties.

1. Introduction

Syntheses of chalcogenido metalate compounds have been the subject of multifaceted research endeavors over the last few decades. Chalcogenido metalate compounds exhibit exciting chemical and physical properties: fast ion conductivity, − photocatalytic activity, − and ion-exchange capability, − which could lead to potential applications in several areas, such as energy storage. −

Ionic liquids with their unusual properties, e.g., negligible vapor pressure and high thermal stability, can be used as alternatives to conventional solvents. They are useful reaction media due to their high variability, e.g., regarding cation/anion composition or pH value. − As compared to reactions in classical solvents as reaction media, chalcogenido metalate compounds prepared in ionic liquids oftentimes possess different compositions that can come along with very unusual structural motifs of their underlying anionic substructures. − The tunability of ionic liquids, through the virtually unlimited choice of cations and anions, allows for a precise control of the reaction conditions and, consequently, the synthesis of previously inaccessible compounds. One means of defining the properties of the respective reaction medium is the appropriate choice of the cation, which can also serve as a structural template for the anionic substructures. , Most frequently, imidazolium-based ionic liquids of the type (C _l C _m C _n Im)­[An] have been used in corresponding reactions (l, m, n: number of carbon atoms in the linear alkyl chains attached to the Im ring positions 1, 2, 3, respectively; [An]⁻: [BF₄]⁻ or a halide ion). ,

Recently, the selective partial methylation of chalcogenido metalate anions was achieved by transfer of methyl groups from such ionic liquid cations onto the terminal chalcogenide ligands of the clusters. It turned out that a selective methyl transfer takes place if at least one of the two alkyl substituents attached to the N atoms of the imidazolium cation is a methyl group. A selectively butylated supertetrahedral sulfido-oxo stannate cluster anion could eventually be accessed by using a doubly butylated ionic liquid, hence not displaying any methyl substituent at all. Salts of this cluster are soluble in common solvents (e.g., CH₃CN) due to the reduced overall charge and the lower polarity owing to the presence of butyl groups. Notably, the small number of organic groups leaves the optical absorption energy nearly unaffected, as confirmed by experimental studies as well as time-dependent density functional theory (TD-DFT) calculations. This is in clear contrast to the notable widening of the optical band gap of metal chalcogenide clusters that are fully shielded by organic groups. The findings were subsequently expanded to corresponding clusters bearing propyl, pentyl, or hexyl groups at the terminal S atoms. Imidazolium-based ionic liquids can also be used for the controlled and limited aggregation of the anionic molecules into oligomers, thereby again reducing the negative charge per cluster unit and thus allowing for better solubility in common solvents like DMF.

Tunable aryl alkyl ionic liquids (TAAILs) − differ from the mentioned imidazolium-based ionic liquids by replacement of the alkyl substituent at the N-site in the 1-position of the imidazolium ring with a phenyl ring, which in addition carries a substituent in para- or ortho-position. This substituent enables additional fine-tuning of the chemical and physical properties of the ionic liquid’s cation, which is the basis for the denomination as ‘TAAILs’. Their physicochemical properties have been evaluated concerning thermal properties, viscosities, and electrochemistry. By using DFT calculations, it could be shown that the distribution of the electron density between standard ionic liquids and TAAILs is significantly different. For the previously known alkyl-alkyl systems like 1,3-dimethylimidazolium (C₁C₁Im) or 1,3-butylmethylimidazolium (C₄C₁Im), the positive charge is mostly located on the alkyl groups, while TAAILs carry most of the charge (≈ 70%) at the imidazolium core. To date, TAAILs have been used in transition-metal catalysis, metal extraction, and metal nanoparticle synthesis. −

We were interested in exploring the effect of using TAAILs on the product spectrum during the formation of chalcogenido metalate compounds. We studied the reactions with regard to the following aspects: (a) transformations of chalcogenido germanates and synthesis of chalcogenido metalates and (b) a variety of (new) structural motifs. Herein, we present our findings and discuss and compare them with respect to previous studies on common ionic liquids.

2. Experimental Section

2.1. General

All reactions (except the syntheses of the ionic liquids) and measurements were carried out in a dry argon atmosphere under strong exclusion of oxygen and moisture using standard Schlenk techniques or a glovebox (type Unilab plus; MBraun). The starting material [K₄(H₂O)₃]­[Ge₄Se₁₀] was prepared in a way similar to a synthesis protocol described in the literature, , by reacting stoichiometric amounts of K₂Se, Ge, and Se (1:2:4) in an evacuated quartz ampule. The mixture was heated to 850 °C and maintained at this temperature for 32 h before allowing the mixture to cool down to room temperature. The obtained yellow powder was stirred in 50 mL of degassed water at 45 °C for 30 min, dried under vacuum, and stored under an argon atmosphere. K₂Se was synthesized by reacting stoichiometric amounts of K and Se in liquid ammonia according to the literature. Ge (powder, Sigma-Aldrich, 99.999%), Se (powder, Sigma-Aldrich > 99.5%), and K lumps (Acros Organics, 98%) were used as received. (Ph_4‑BrC₆Im)­[NTf₂] (1-(4-bromphenyl)-3-hexyl-1H-imidazolium bis­(trifluoromethan)­sulfonimide), (Ph_4‑BrC₁₂Im)­[NTf₂] (1-(4-bromphenyl)-3-dodecyl-1H-imidazolium bis­(trifluoromethan)­sulfonimide), (Ph_4‑OMeC₈Im)­[NTf₂] (1-(4-methoxyphenyl)-3-octyl-1H-imidazolium bis­(trifluoromethan)­sulfonimide), and (Ph_2‑MeC₈Im)­[NTf₂] (1-(2-methylphenyl)-3-octyl-1H-imidazolium-bis­(trifluoromethan)­sulfonimide) were prepared according to a literature method. 2,6-Dimethylmorpholine (DMMP, Sigma-Aldrich, 99.8%) was distilled and stored over a molecular sieve (3 Å). For a list of used chemicals, see Table S1.

The synthetic procedure was the same for compounds 1–5. 56 mg (0.043 mmol) of [K₄(H₂O)₃]­[Ge₄Se₁₀] and 60 mg (0.17 mmol) of SnCl₄·5H₂O were weighed into a borosilicate glass ampule with a volumetric capacity of 7 mL. Tunable aryl alkyl ionic liquids and auxiliary were added. The ampule was sealed under vacuum. The ampule was then heated in an oven (Nabertherm) to 150 °C at a heating rate of 30 °C/h, kept at 150 °C for 72 h, and then cooled down to room temperature at a cooling rate of 5 °C/h. Details of the composition of the reaction mixtures are given below. “Cat” indicates the presence of cations that could not be determined in full from the difference Fourier map. The crystals were manually selected under a microscope after unit-cell determination, washed with acetonitrile, dried in dynamic vacuum, and subsequently weighed. The yield was calculated based on the initial amount of SnCl₄·5H₂O used for the reaction. Besides the formation of compounds 1–5, SnSe, GeSe, and elemental Sn were identified as byproducts. It should be noted that the cation (DMMPH)⁺, which represents the monoprotonated derivative of DMMP, forms in situ.

2.1.1. Synthesis of (Cat)₂₄[Ge₂₄Sn₃₆Se₁₃₂] (1)

500 mg of (Ph_4‑OMeC₈Im)­[NTf₂] and 37 μL of DMMP were added. After cooling, 1 was obtained in the form of red block-like crystals in approximately 62% yield.

2.1.2. Synthesis of (Ph_2‑MeC₈Im)₂(DMMPH)₂[Ge_1.4Sn_4.6Se₁₄] (2)

500 mg of (Ph_2‑MeC₈Im)­[NTf₂] and 100 μL of DMMP were added. After cooling, 2 was obtained in the form of orange, plate-shaped crystals in approximately 47% yield.

2.1.3. Synthesis of (Cat)₆[GeSn₈Se₂₁] (3)

500 mg of (Ph_4‑BrC₆Im)­[NTf₂] and 100 μL of DMMP were added. After cooling, 3 was obtained in the form of orange needles in approximately 26% yield.

2.1.4. Synthesis of (Cat)₂[Sn₃Se₇] (4)

500 mg of (Ph_4‑BrC₁₂Im)­[NTf₂] and 100 μL of DMMP were added. After cooling, 4 was obtained in the form of orange, block-shaped crystals in approximately 76% yield.

2.1.5. Synthesis of (DMMPH)₄[Ge₄Se₁₀] (5)

500 mg of (Ph_4‑BrC₈Im)­[NTf₂] and 100 μL of DMMP were added. After cooling, 5 was obtained in the form of orange, block-shaped crystals in approximately 81% yield.

2.1.6. Single-Crystal X-ray Diffraction

Crystals suitable for X-ray diffraction analyses of compounds 1 and 4 were investigated with Ga–Kα radiation (λ = 1.34143 Å) at T = 180 K on the area detector system STOE StadiVari. Compounds 2 and 5 were investigated with a Bruker D8 Quest with a microfocus source emitting Mo–Kα radiation (λ = 0.71073) and a Photon 100 detector at T = 100 K. Compound 3 was investigated with a STOE StadiVari diffractometer at 100 K, using Cu–Kα radiation (λ = 1.54186) from an X-ray micro source with X-ray optics and a Pilatus 300 K Si hybrid pixel array detector. Structure solution by dual space methods and full-matrix least-squares refinement against F ² were carried out using SHELXT15, SHELXL15, and OLEX2 software. All nonhydrogen atoms were refined using anisotropic displacement parameters. H atoms were calculated by using a riding model. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-2483737 (2), CCDC-2483738 (3), CCDC-2483739 (4), and CCDC-2483740 (5).

2.1.7. UV–Visible Spectroscopy

The optical absorption properties of compounds 1–5 and of [K₄(H₂O)₃]­[Ge₄Se₁₀] were measured under inert conditions employing a Varian Cary 5000 UV/vis/NIR spectrometer from Agilent, equipped with a Praying Mantis accessory for solid-state samples. To determine the optical band gaps, the raw data were transformed from reflectance R to absorption according to the Kubelka-Munk function: ,

$$F(R)=\frac{k}{s}=\frac{(1-R_{\infty }{)}^2}{2R_{\infty }}$$

where k is the K-M absorption coefficient, R _∞ is the diffuse reflection, and s is the K-M scattering coefficient. , Tauc plots were generated by plotting

$$(F(R_{\infty })\times h\nu {)}^{1/\gamma }$$

as a function of the photon energy hν. The power coefficient can be $\gamma =\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.,\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$ , 2, or 3, depending on the nature of the transition, which corresponds to direct allowed, direct forbidden, indirect allowed, or indirect forbidden transitions, respectively. E _g is estimated from the intercept with the x axis of the linear fit from the corresponding region. Since the transition in the region of interest was far more pronounced when choosing γ = 2, we assume that all analyzed compounds feature an indirect band gap. −

2.1.8. Fourier-Transform Infrared Spectroscopy

Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet Summit X FTIR Spectrometer (ThermoFisher Scientific) in the wavenumber range 4000–400 cm^–1. The instrument is located inside a glovebox, to enable measurements of air- and moisture-sensitive solid samples.

3. Results and Discussion

In Scheme (top), we give an overview of the products of reactions of [K₄(H₂O)₃]­[Ge₄Se₁₀] with SnCl₄·5H₂O in the presence of DMMP in TAAILs, which were obtained as single crystals (Figure S6). As mentioned in Section , cations are indicated explicitly in the sum formulas, if they could be localized on the difference Fourier map and could hence be identified, or as “Cat” in cases where disorder prohibited their localization. It should be noted that this is a rather typical finding in such systems. The new compounds are given as (Cat)₂₄[Ge₂₄Sn₃₆Se₁₃₂] (1), (Ph_2–MeC₈Im)₂​(DMMPH)₂​[Ge_1.4Sn_4.6Se₁₄] (2), (Cat)₆[GeSn₈Se₂₁] (3), (Cat)₂[Sn₃Se₇] (4), and (DMMPH)₄[Ge₄Se₁₀] (5). For comparison, products of corresponding reactions undertaken in the common ionic liquid (C₄C₁Im)­[BF₄] under otherwise similar reaction conditions are also shown in Scheme (bottom). Although some of the anionic frameworks have also been accessible in common ionic liquids, like the salts comprising the ‘zeoball’ anion [Ge₂₄Sn₃₆Se₁₃₂] − or the 2D-{[Sn₃Se₇]^2–} substructure (middle row), the overview in Scheme indicates that the reaction medium mostly has a notable impact on the product spectrum.

1. Illustration of the Anions of the Different Reaction Products Emerging from Reactions of [K₄(H₂O)₃]­[Ge₄Se₁₀] and SnCl₄·5H₂O in the Presence of 2,6-Dimethylmorpholine (DMMP) in TAAILs (1–4; Top, Black Lines), or in the Common Ionic Liquid (C₄C₁Im)­[BF₄] (I and II; Bottom; Gray Lines), under Otherwise Same Conditions; the Anionic Structure of Compound 5 Is Not Drawn, as It Is the Same as That of the Precursor .

^a All atoms are drawn in ball-and-stick mode, Se: red, Sn: dark gray, Ge: light blue. Only anionic substructures are illustrated, counter ions are omitted for clarity.

The compositions of compounds 1–5 were confirmed by micro-X-ray fluorescence spectroscopy (μ-XFS) analysis (see Figures S7–S10 and Tables S2–S5). Their structures were determined by means of single-crystal X-ray diffraction and are described below. Selected structural parameters are summarized in Table . More details of the X-ray diffraction analyses are provided in Tables S6–S10, Figures S11 and S12. We further demonstrate the effect of the anionic architecture on the optical absorption properties.

1. Selected Structural Parameters of Compounds 2–5 .

parameter	2	3	4	5
Ge-(μ3-Se)	2.3774(12)–2.3793(13)
Sn-(μ3-Se)	2.953(11)–2.9770(11)	2.8413(23)	2.7586(7)
(Sn/)Ge-(μ-Se)in	2.4058(11)–2.4254(11)	2.4466(22)		2.3771(9)– 2.3963(12)
Sn-(μ-Se)in	2.5136(12)–2.5272(10)	2.5114(18)–2.5592(19)	2.5583(9)
(Sn/)Ge–Seex	2.3076(13)–2.3134(13)	2.3241(37)–2.3687(35)		2.2796 (16)
Sn-(μ-Se)ex	2.4938(10)–2.6460(11)	2.4818(25)–2.7734(21)	2.5175(9)– 2.6872(9)
Se–Sn–Se	95.044(36)–124.020(39)	87.422(60)–124.579(64)	87.525(21)– 122.083(26)
Sn–Se–Sn	87.087(33)–101.619(35)	85.008(60)–95.710(64)	86.046– 94.735(25)
(Sn/)Ge–Se–Sn	89.542(38)–93.449(39)	92.886(79)
Se–Ge–Se	103.924(43)–119.711(49)	102.037(90)– 120.391(89)		108.935(7)– 112.080(5)
Ge–Se–Ge				105.889(6) 106.300(2)

^aDistances are given in Å, and angles are given in °. Indices refer to internal (“in”) bonds within the {T₄Se₃} defect-heterocubane units (T = Ge, Sn) or to external (“ex”) bonds to μ-bridging Se atoms between the different subunits, respectively.

The reaction in (Ph_4‑OMeC₈Im)­[NTf₂] in the presence of 100 μL DMMP affords single crystals of compound 1 in yields of 42–63%, depending on details of the procedure (detailed information regarding all synthetic procedures is given in the Supporting Information). The compound crystallizes in the monoclinic crystal system, space group C2/m, with two formula units per unit cell. It represents a new salt of the ‘zeoball’ anion [Ge₂₄Sn₃₆Se₁₃₂]^24–. Owing to the limited scattering power of the crystals, the number of single-crystal data did not allow for a refinement of the many parameters of this structure, but the data were sufficient to unambiguously identify the very prominent anion, which we present here as a structural model; an explanation of the situation and basic crystallographic data including the unit cell parameters are provided in Table S6. It consists of two subunits, {Ge₃Se₉} and {Sn₆Se₁₈}, which are connected via μ-Se atoms (Scheme ). Owing to heavy disorder, which contributes to the limited data quality, the (Ph_4‑OMeC₈Im)⁺ cations of 1 could only be partially localized on the difference Fourier map. As found for other salts of the [Ge₂₄Sn₃₆Se₁₃₂]^24– anion, ,, they are situated in the voids between the anions and can alsopartially or fullyoccupy the inner cavity of the anionic spheres.

2. Subunits of the [Ge₂₄Sn₃₆Se₁₃₂]^24– Anion .

^a The inner cavity has a diameter of 15.4 Å by the smallest center-to-center distance of opposite Se atoms and 11.6 Å (illustrated as a sphere) upon consideration of van der Waals radii.

Employing a TAAIL with a methyl group in the ortho-position of the Ph ring instead of a methanolate substituent in para-position affords single crystals of compound 2 in approximately 47% yield. The compound crystallizes in the monoclinic crystal system, space group type P2₁/n, with four formula units per unit cell. The anionic structure consists of {Ge_0.7Sn_2.3Se₇}^2– defect-heterocubane-type building units that are connected by μ-Se bridges to form alternating one-dimensional strands along ⟨010⟩. However, the {Ge_0.7Sn_2.3Se₇}^2– units exhibit a mixed occupancy of Ge and Sn atoms on one of the group 14 atom positions, as confirmed both through the refinement of the single-crystal X-ray diffraction data and μ-XFS analysis (percentages found for Ge, Sn, Se = 7.62%, 23.13%, 69.24%, see Table S3). This is reflected by differences in bond lengths and angles: definite Sn–Se bond lengths range from 2.4938(10) to 2.9533(11) Å, while the Sn/Ge–Se bond lengths are shorter on average, 2.3076(14)–2.4246(11) Å. The definite Se–Sn–Se angles within one building unit exhibit values from 114.394(38) to 116.439(39)°; values including the μ-Se atoms between the building units are 95.044(36)–124.020(39)°. The Se–Sn/Ge–Se angles range from 103.924(43) to 119.711(49)°. The Sn–Se–Sn angle within the defect-heterocubane cages is 101.619 (35)°, and the Sn/Ge–Se–Sn angles amount to 89.542(38) and 93.449(39)°. Sn–Se–Sn angles including the μ-Se bridges between the building units range from 87.087(33) to 87.817(34)° (Table ).

The negative charge is compensated by two ionic liquid cations and two protonated DMMP molecules per formula unit of compound 2. Half of the channels that extend between the anionic strands in the ⟨010⟩ direction are filled with (DMMPH)⁺ cations, and the other half is filled with (Ph_4‑MeC₈Im)⁺ cations. Notably, the two types of cations remain separate in their respective channels.

The closest Se···Se distance between atoms from two adjacent strands in the c direction is 5.2525(13) Å, while the closest Se···Se distances in the a direction range from 6.4566(14) to 7.0775(14) Å. As expected, the smaller cation (DMMPH)⁺ fills the voids between the strands. Figure illustrates the crystal structure of compound 2.

1.

Crystal structure of 2. (a) Ternary building unit of the anion in 2. (b) Asymmetric unit including cations. (c) View of the section of one anionic strand. (d) Packing of cations and anions in the crystal, viewed along the crystallographic b axis. Thermal ellipsoids of Ge, Sn, and Se atoms are drawn at the 50% probability level. Cations are shown in wire representation. Se: red, Sn: dark gray, Ge/Sn: light blue, C: gray, N: blue, and O: red. H atoms have been omitted for the sake of clarity.

The use of (Ph_4‑BrC₆Im)­[NTf₂], in which the substituent in para-position is a Br atom and the alkyl part is a hexyl group, results in the formation of yellow crystals of compound 3 in approximately 26% yield. 3 crystallizes in the monoclinic crystal system, space group type P2₁/m, with four formula units per unit cell. The anionic structure consists of {Sn₃Se₇}^2– and {GeSn₂Se₇}^2– defect-heterocubane-type building units in a 2:1 ratio that are connected via μ-Se atoms into five-membered macrocycles. The latter are fused via sharing of two connected {Sn₃Se₇}^2– units each into a one-dimensional strand running along ⟨010⟩. Sn–Se bond lengths range from 2.4818(25) to 2.8413(23) Å, while the Ge–Se bond lengths are naturally slightly shorter, 2.3241(37)–2.4466(22) Å (Table ). All bond lengths are in good agreement with reported values. − The Se–Sn–Se angles within one building unit range from 87.422(60) to 119.287(63)°, while the values including the μ-bridging Se atoms are in the range from 87.422(60) to 125.494(79)°. The Se–Ge–Se angles exhibit values from 102.037(90) to 120.391(89)°.

The Sn–Se–Sn angles within the defect-heterocubane cages range from 85.008 (60) to 95.710 (64)°, the Sn–Se–Sn angles including the μ-Se bridges range from 85.904(67) to 88.971(71)°, and the Ge–Se–Sn angle is 92.886(79)° (Table ).

The cations could not be localized on the difference Fourier map, which is a very typical finding for reactions under solvothermal or ionothermal conditions. However, the cations are assumed to be (DMMPH)⁺ molecules according to the number of electrons (720 e^– in a volume of 3979Å^–3 in one void per unit cell) found in the voids between the anions, which accords with the presence of three (DMMPH)⁺ molecules per asymmetric unit or six per unit cell. The Se···Se distances between two layers range from 5.1125(27) to 5.7264(44) Å. Figure illustrates the crystal structure of compound 3.

2.

Crystal structure of 3. (a) Binary building unit of the anion in 3. (b) Ternary building unit of the anion in 3. (c) Top view of the fused five-membered rings. (d) Packing of anions in the crystal, viewed along the crystallographic b axis. Thermal ellipsoids of Ge, Sn, and Se atoms are drawn at the 50% probability level. Se: red, Sn: dark gray, Ge: light blue. Disorder positions are shown in semitransparent mode.

The anionic substructures of compounds 2 and 3 are rare examples of a ternary Ge/Sn/Se architecture. Notably, all ternary anions with Ge/Sn/Se architecture reported so far were obtained from ionothermal syntheses. Besides various salts of the ternary ‘zeoball’ anion, , only four further types of ternary Ge/Sn/Se anions have been known to date: [Ge_0.83Sn_3.17Se_9.06]^2– (II), [Cs@Sn^II ₄(Ge^IV ₄Se₁₀)₄]^7– (III), and [Sn^II(Ge^IV ₄Se₁₀)]^2– (IV and V; see Scheme and Figure ). For comparison, we briefly describe the architectures here: compound II is built of one {Sn₆Se₁₀} subunit and two {GeSe₄} tetrahedra to form two-dimensional corrugated layers (Scheme ). Compound III is based on four {Ge₄Se₁₀} subunits, which are connected via four Sn^II atoms to form a supertetrahedral [Sn^II ₄(Ge^IV ₄Se₁₀)₄]^8– cluster anion embedding an endohedral Cs⁺ cation (Figure a). The anionic substructures of compounds IV and V are also built of {Ge₄Se₁₀} subunits, but here, the subunits are linked by sharing μ-Se atoms with {Sn^IISe₃} units into one-dimensional double strands; both architectures differ in the mode of linkage of the two subunits (Figure b,c). While each {Sn^IISe₃} unit bridges between two {Ge₄Se₁₀} subunits in IV, the {Sn^IISe₃} units in V connect three {Ge₄Se₁₀} moieties each.

3.

Examples of ternary anionic Ge/Sn/Se architectures. (a) [Cs@Sn^II ₄(Ge^IV ₄Se₁₀)₄]^7– in III. (b) [Sn^II(Ge^IV ₄Se₁₀)]^2– in IV. (c) [Sn^II(Ge^IV ₄Se₁₀)]^2– in V. All atoms are drawn in ball-and-stick mode; Se: red, Sn: dark gray, Ge: light blue, Cs: turquoise. ,

Compound 4 was obtained from treatment of the precursors in (Ph_4‑BrC₁₂Im)­[NTf₂]. The only change compared to the synthesis of compound 3 is the length of the alkyl group of the TAAIL, which in the synthesis of compound 4 is a dodecyl group. Compound 4 crystallizes as orange crystals in approximately 76% yield in the trigonal crystal system, space group P $\stackrel{\circledR }{3}$ , with two formula units per unit cell. The motif of the anion in 4 is already known from other salts. − Notably, the [K₄(H₂O)₃]­[Ge₄Se₁₀] starting material served only as a source of Se^2– to form the binary selenido stannate network in the reaction with SnCl₄·5H₂O. As a consequence, exclusively {Sn₃Se₇}^2– defect-heterocubane-type building units are connected in 4 via μ-Se atoms. The resulting pattern is a honeycomb-type _∞ {[Sn₃Se₇]^2–} network of six-membered macrocycles that are annealed in the ab plane. The Se···Se distances between two honeycomb networks range from 6.0212(10) to 7.3947(13) Å.

The Sn–Se bond lengths within the anionic architecture range from 2.5175(9) to 2.7586(7) Å. These values are again in good agreement with reported lengths. − The Se–Sn–Se angles within one building unit show values from 87.525(21) to 115.674(25)°, while the values including the μ-bridging Se atoms are in the range 91.028(21)–122.083(26)°. The Sn–Se–Sn angles within the defect-heterocubane cages range from 86.046(12) to 94.735(25)°, and the Sn–Se–Sn angle including the μ-Se bridges is 89.713(26)° (Table ).

While the imidazolium skeletons of the cations were localized on the difference Fourier map, the substituents were not localizable owing to inherent disorder. However, the imidazole rings clearly indicate that they are (Ph_4‑BrC₁₂Im)⁺ molecules. All of the cations are situated in the voids between the anions and arranged in layers. Compared to compound 3, the distance between two layers increased with the longer alkyl chainevidencing the TAAIL cations’ roles as templating agents in these reactions. Figure illustrates the structural features of compound 4.

4.

Crystal structure of 4: (a) binary building unit of the anion in 4. (b) Top view of a section of one of the honeycomb-type _∞ {[Sn₃Se₇]^2–} layers. (c) 2 × 2 × 2 supercell of 4 viewed along the crystallographic b axis. Thermal ellipsoids of Sn and Se atoms are drawn at the 50% probability level. Fragments of the cations are shown in wire representation (disorder and H atoms omitted). Se: red, Sn: dark gray, C: gray, N: blue.

A different situation was reported regarding the role of imidazolium-based ionic liquids comprising alkyl chains of different lengths in the case of tellurido mercurates. In the compounds (C₁₀C₁Im)₄[Hg₄Te₁₂], (C₁₀C₁Im)₆​[Hg₆Te₁₀​(TeDec)₂] (Dec = decyl), (C₁₀C₁Im)₆​[Hg₆Te₁₀​(TeDec)­(TeMe)] (Me = methyl), (C₈C₁Im)₈​[Hg₈Te₁₆], or (C₁₀C₁Im)₈​[Hg₈Te₁₆], the alkyl chains assembled in a way that led to a lamellar arrangement of the tellurido mercurate anions in the crystal structures. Here, the tellurido mercurate units are always surrounded by the imidazolium rings of the cations, with the long alkyl chains directing outward and undergoing van der Waals interactions with other alkyl chains of neighboring cations. ,

The use of (Ph_4‑BrC₈Im)­[NTf₂], in which the substituent in para-position is a Br atom and the alkyl chain is an octyl group, results in the formation of yellow crystals of compound 5. It crystallizes in the tetragonal system in the space group type I4₁/amd. Compound 5 comprises {Ge₄Se₁₀}^4– anions exhibiting a T2-type supertetrahedral structure, like the anion of the starting material (therefore not shown in Scheme ). The charge of 4– is compensated for by four (DMMPH)⁺ cations. Thus, the K⁺ cations from the starting material are simply replaced with the organic cation (DMMPH)⁺ that forms in situ from the DMMP auxiliary and protons from crystal water in the starting compounds. The Ge–Se bond lengths in the anion range from 2.2796(16) to 2.3963(12) Å, with the external Ge–Se bonds (2.2796(16) Å) being shorter than the internal Ge–Se bonds (2.3771(9)–2.3963(12) Å), as typical and in agreement with reported values. The Se–Ge–Se angles within one defect-heterocubane unit adopt values from 110.668(4)° to 112.080(5)°, while the angles including the external Se atoms are at 108.935(7)°. In the unit cell, every [Ge₄Se₁₀]^4– cluster anion is surrounded by four (DMMPH)⁺ cations such that the N atom always points in the direction of the cluster, thus allowing for optimal anion···cation charge compensation. Figure illustrates the crystal structure of 5.

5.

Crystal structure of 5. (a) [Ge₄Se₁₀]^4– anion with four closest (DMMPH)⁺ cations. (b) Extended unit cell of 5 viewed along the crystallographic c axis. Thermal ellipsoids of Ge and Se atoms are drawn at the 50% probability level. Cations are shown in wire representation (H atoms omitted). Se: red, Ge: light blue, C: gray, N: blue, O: red.

3.1. Optical Properties

Although the compositions and underlying structural units of compounds (Cat)₂₄[Ge₂₄​Sn₃₆​Se₁₃₂] (1), (Ph_2‑MeC₈Im)₂​(DMMPH)₂​[Ge_1.4Sn_4.6Se₁₄] (2), (Cat)₆​[GeSn₈Se₂₁] (3), (Cat)₂​[Sn₃Se₇] (4), and (DMMPH)₄[Ge₄Se₁₀] (5) are not identical, their optical absorption properties and visible colors fall within a comparable range, suggesting that subtle variations in the structures can affect their electronic properties. UV-visible spectra were recorded to quantify the optical absorption properties (see Figures and S13–S18). According to the Tauc plots, all compounds are indirect semiconductors, with band gaps between 1.76 and 2.25 eV. Changing the cation from K⁺ to an organic cation like (DMMPH)⁺ in salts of the binary [Ge₄Se₁₀]^4– anion results in a minimal blue-shift of the absorption and a corresponding slight widening of the band gap (from 1.76 eV in [K₄(H₂O)₃]­[Ge₄Se₁₀] to 1.80 eV in 5). Smaller molecular units [Sn₃Se₇]^2–, although being based on Sn/Se atoms and assembled into two-dimensional layers, also cause a slight blue-shift of the band gap in compound 4 (1.91 eV). The three ternary compounds show even larger band gapsindicating the influence of admixing Ge to the compounds. However, the Ge:Sn ratio alone does not define the band gap, as this is a result of both the Ge:Sn:Se composition and the extension and dimensionality of the network. The nearly identical gaps in compound 1 (1.96 eV) and compound 2 (1.99 eV) indicate that the larger relative amount of Sn atoms in 2 is overcompensated by the overall larger amount of metal atoms in 1. The composition of compound 3 is closely related to that of compound 4; yet one out of nine Sn atoms is replaced with Ge. In addition, instead of a two-dimensional layer-type arrangement of six-membered macrocycles, the subunits assemble into five-membered macrocycles forming a one-dimensional strand. Hence, another widening of the gap to 2.13 eV seems understandable. Overall, the differences in optical absorption properties of the new compounds 1–5 are small, with all compounds forming yellow to orange crystals, in accordance with their optical absorption properties. Obviously, the different characteristics of the anionic substructures regarding compositions, structural motifs, and dimensionalities, all of which affect the optical absorption properties, and the properties of the cations acting as separators between the anionic architectures seem to compensate each other in a way that allows for very similar electronic properties to result for the crystalline solids.

6.

(a) UV-vis spectra of single crystals of [K₄(H₂O)₃]­[Ge₄Se₁₀] and 1–5. (b) Tauc plots generated by plotting (F­(R _∞) × hν)^1/γ as a function of the photon energy hν, with γ = 2 of [K₄(H₂O)₃]­[Ge₄Se₁₀] and 1–5. The color codes are the same in (a) and (b).

4. Conclusions

We reported the successful application of five different tunable aryl alkyl ionic liquids (TAAILs) for the ionothermal treatment of [K₄(H₂O)₃]­[Ge₄Se₁₀] with SnCl₄·5H₂O and the synthesis of a series of new selenido metalate salts. Subtle changes of the nature of the TAAIL cations, that is, the substituent in the ortho or para-position of the Ph group attached to the N1-position of the imidazolium ring, led to different reaction products. We presented the crystal structures of the products that either represent salts of famous anionsthe “zeoball” anion in 1, the _∞ {[Sn₃Se₇]^2–} honeycomb-type layer in 4, or a new salt of the [Ge₄Se₁₀]^4– anionor comprise novel types of ternary anion typesone-dimensional strands of ternary Ge/Sn/Se defect-heterocubane-type building units in 2, or alternating five-membered macrocycles built from two different defect-heterocubane-type building units in 3. These findings illustrate that the dimensionality of the products can be tuned with the help of TAAIL cations. Although a direct correlation between the properties of a given cation and the structure of the product cannot be determined at this stage, the fact that some of the compounds were only accessible this way refers to the notable structure-directing effect of TAAILs. The optical absorption properties of the new compounds span a narrow range (1.76–2.13 eV), indicating a subtle compensation of effects based on the composition, structural motifs, and dimensionality of the chalcogenido metalate architecture, as well as the separation of anionic architectures by the cations.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c05009.

• Syntheses, light microscopy, micro-X-ray fluorescence spectroscopy, X-ray diffraction, optical absorption spectroscopy, and FT-IR spectroscopy (PDF)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.