Controlled Synthesis of Polyions of Heavy Main-Group Elements in Ionic Liquids

Ionic liquids (ILs) have been proven to be valuable reaction media for the synthesis of inorganic materials among an abundance of other applications in different fields of chemistry. Up to now, the syntheses have remained mostly “black boxes”; and researchers have to resort to trial-and-error in order to establish a new synthetic route to a specific compound. This review comprises decisive reaction parameters and techniques for the directed synthesis of polyions of heavy main-group elements (fourth period and beyond) in ILs. Several families of compounds are presented ranging from polyhalides over carbonyl complexes and selenidostannates to homo and heteropolycations.


Introduction
Ionic liquids (ILs)-Often defined as salts with melting points below 100 • C-have actually been known for quite a long time. In the decades after the description of the first representative ethylammonium nitrate by Paul Walden in 1914 [1], this valuable family of compounds fell into oblivion. Despite pioneering works [2], there had only been approximately 20 articles per year on ILs until 1995 [3]. Beginning with the late 1990s, the annual number of publications rose tremendously to about 9200 in the year 2015, according to "Web of Science", indicating the huge interest of different research communities.
Usually, ILs are constituted from bulky organic cations and (often) polyatomic anions, which can be selected in order to tune the properties of the IL. Typical cations range from simple quaternary ammonium or phosphonium ions over substituted imidazolium or pyridinium rings to more complex cations, such as the so-called TAAILs (Tunable Aryl Alkyl Ionic Liquids) [4]. A similar variety can be found among the anions including, for example, simple halides, complex organic anions, or halogenidometalates [5]. Among others, the latter can be utilized to introduce additional physical properties like magnetic moments [6].
Despite the abundance of inorganic compounds yielded by IL approaches, the syntheses remain a "black box" in several cases. Not only are the mechanisms of product formation barely examined but the overall role of the IL might also be vague. The latter can range from being a mere lubricant for solid state reactions via acting as solvent for (a part of) the starting materials to crucial directive properties leading to tunable products. In addition, the interplay of ILs with additives or directing agents has to be investigated further. Therefore, either in situ reaction monitoring [60][61][62] or comparisons of several syntheses can elucidate the influence of ILs. The present review article aims to highlight a variety of reaction parameters and techniques for the directed synthesis of polyions of heavy main-group elements (fourth period and beyond) in ILs (cation and anion abbreviations in Table 1) in order to provide researchers with an insight into promising synthetic approaches. We will summarize or deduce crucial reaction parameters and techniques and-in several cases-demonstrate the benefit of utilizing an IL in comparison to classic synthetic routes. The review is subdivided into sections on polyanions and polycations, and further into subsections about specific subgroups with their decisive reaction parameters. 1-propyl-2,3-dimethylimidazolium [C 4 MPyr] butyl-methylpyrrolidinium [C 10 MPyr] decyl-methylpyrrolidinium [N(n-Bu) 3

Polyanions
Several polyanions, among them also polyoxometalates [63], have been synthesized in ILs. Herein, we focus on heavy polyhalides, carbonyl clusters, and selenidostannates. Owing to the abundance of synthesized compounds, crucial reaction parameters as well as the influence or benefit, respectively, of the ILs can be deduced.

Starting Materials with High Vapor Pressure
The ability of ILs to decrease the vapor pressure of delicate volatile compounds and, simultaneously, to provide access to dissolved species has been utilized for the synthesis of several heavy main-group element polyanions. In order to demonstrate the potential of this method, we discuss the utilization of halogens and metal carbonyls in IL-based syntheses.
The interplay between ILs and gases has been studied intensively for many years. Some gases, for example CO 2 and SO 2 , proved to feature a remarkable solubility in some ILs [16,[64][65][66]. The interplay between gaseous species and ILs can be manifold [64]. For CO 2 , interactions with the anions of ILs have been observed [67], and the gases PH 3 and BF 3 are dissolved via chemical complexation [68].

Polyhalides
Intriguing examples for the utilization of halogens are the polybromides synthesized in ILs [43,[69][70][71][72]. While polyiodides were known with up to 29 iodine atoms [73], the size of the corresponding polybromides was limited to a maximum of 10 atoms [74,75] before new approaches including ILs (Feldmann group) were developed [76]. This disparity is most likely due to the vapor pressure of bromine (10 kPa at 2.5 • C, b.p.: 58.8 • C) [77], which is considerably higher than for iodine (10 kPa at 108 • C, b.p.: 184.4 • C) [77]. Moreover, the reactivity of the lighter halogens necessitates a chemically stable environment, which can be provided by ILs due to their high redox stability [78]. interplay between gaseous species and ILs can be manifold [64]. For CO2, interactions with the anions of ILs have been observed [67], and the gases PH3 and BF3 are dissolved via chemical complexation [68].

Polyhalides
Intriguing examples for the utilization of halogens are the polybromides synthesized in ILs [43,[69][70][71][72]. While polyiodides were known with up to 29 iodine atoms [73], the size of the corresponding polybromides was limited to a maximum of 10 atoms [74,75] before new approaches including ILs (Feldmann group) were developed [76]. This disparity is most likely due to the vapor pressure of bromine (10 kPa at 2.5 °C, b.p.: 58.8 °C) [77], which is considerably higher than for iodine (10 kPa at 108 °C, b.p.: 184.4 °C) [77]. Moreover, the reactivity of the lighter halogens necessitates a chemically stable environment, which can be provided by ILs due to their high redox stability [78].   [71]. Br-Br distances with d ≤ 320 are drawn as solid lines to emphasize the [Br24] 2− unit. Additional Br-Br distances up to 370 pm (i.e., twice the van der Waals distance [71]; dashed broken-off bonds) indicate the network character.
All IL-based syntheses of polybromides rely on one strategy: Bromide anions dissolved in an IL act as electron donors to bromine molecules. The characteristic red-brown vapor of bromine is missing above the IL at moderate temperatures [69] indicating the high solubility in and strong interactions with the IL. The same effect was also observed for iodine in the course of the synthesis of phosphorus iodides [60]. For convenient isolation of the solid product, the reaction mixture has to be liquid at room temperature or, in some cases, even below, which can be achieved by different means: either a pure room-temperature ionic liquid (RTIL), for example [HMIm][Br], is used [70] [69]. To achieve crystallization of all other compounds (with larger anions), deeper cooling was necessary. In the case of [C4MPyr]2[Br20], the liquid was initially cooled to −15 °C, which lead to the crystallization of the target compound and the IL itself. By reheating to +5 °C, only crystals of the polybromide remained [43]. It seems worth noting  [71]. Br-Br distances with d ≤ 320 are drawn as solid lines to emphasize the [Br 24 ] 2− unit. Additional Br-Br distances up to 370 pm (i.e., twice the van der Waals distance [71]; dashed broken-off bonds) indicate the network character.
All IL-based syntheses of polybromides rely on one strategy: Bromide anions dissolved in an IL act as electron donors to bromine molecules. The characteristic red-brown vapor of bromine is missing above the IL at moderate temperatures [69] indicating the high solubility in and strong interactions with the IL. The same effect was also observed for iodine in the course of the synthesis of phosphorus iodides [60]. For convenient isolation of the solid product, the reaction mixture has to be liquid at room temperature or, in some cases, even below, which can be achieved by different means: either a pure room-temperature ionic liquid (RTIL), for example [HMIm][Br], is used [70] or auxiliaries, like (2-bromophenyl)diphenylphosphine, are added to the RTIL [N(n-Bu) 3 [69]. In further investigations, the structure of the IL cation was identified to be the most important parameter with respect to the bromine content of the product. Therefore, further optimizing the cation-bromine interactions might lead to polybromides with even higher bromine contents [72].

Utilization of Metal Carbonyls
Handling metal carbonyls can be cumbersome due to their high vapor pressure and toxicity. Thus, introducing a reaction medium for saver storage and handling of carbonyls would be highly beneficial. First investigations by Brown 4 ] did not show any mass loss, even when stored under vacuum for 48 h, indicating the high stability of this liquid salt and its ability to provide "dissolved" carbon monoxide [79].
The first IL-based synthesis of carbonyl clusters of heavy main group elements were reported by the Feldmann group (Table 2). Mn 2 (CO) 10 or Fe(CO) 5 were reacted with metalloid iodides in different ILs at 130 • C. Similar to the aforementioned case of polybromides, the reaction temperature is remarkably high considering the vapor pressure and boiling point of e.g., Fe(CO) 5 (10 kPa at 44 • C, b.p.: 103 • C) [77]. Some of the CO ligands remain bonded to the transition metal atom in the product. It is noteworthy that gaseous CO itself is just sparingly soluble in ILs [64], which matches the observation of an excess pressure of the reaction by-product CO while opening the sealed ampules [80]. Therefore, metal carbonyl fragments from the starting material exist under the chosen reaction conditions and are thus dissolved and available for further reactions.  [82,83].
In conclusion, the utilization of ILs allows safe handling of compounds with high vapor pressure like bromine or metal carbonyls at reaction temperatures above their boiling point due to their exceptional solubility in chosen ILs. Furthermore, dissolved species such as metal carbonyl fragments are available for reactions with, e.g., compounds of heavy main-group elements. In the case of the polybromides, the structure of the IL cation proved to be the dominant parameter for product-selective synthesis.

Amine-Assisted Syntheses of Selenidostannates
Structure-directing properties of ILs for the synthesis of inorganic materials are known since the initial studies of the Morris group [84][85][86]. They also noticed that the addition of small amounts of molecular solvents, e.g., H2O, has a strong effect on the formation of the products if combined with ILs [87]. This feature was further explored during the ionothermal synthesis of molecular sieves with auxiliary amines [61,62,88,89]. The groups of Dehnen and Huang reported similar findings for clusters of heavy main-group elements. They noticed that the addition of amines had strong impact on the phase formation of selenidostannates synthesized from the elements or prereacted species in ILs [51,[90][91][92][93][94][95][96][97][98]. More than 25 different selenidostannates were synthesized following this approach, which does not only illustrate the diversity of this class of compounds but also demonstrates the capability of the method. More details are given in a recent review about the synthesis and structure of selenidostannates by Dehnen et al. [51].

Promoting Phase Formation
There are many examples in which products have exclusively been obtained in the presence of an amine including the famous "zeoball"-type selenidostannates [BMMIm]24[Sn36Ge24Se132] (ZBT-1) and [BMIm]24[Sn32.5Ge27.5Se132] (ZBT-2), which feature the largest known discrete polyanion of main-group elements ( Figure 3) [91]. Both compounds were synthesized from [K4(H2O)3][Ge4Se10] and SnCl4•5H2O in tetrafluoridoborate ILs in the presence of DMMP (DMMP = 2,6-dimethylmorpholine). Noteworthy, only uncharacterized Ge/Se-containing powder precipitated if the reactions were conducted in the absence of the amine, whereas an excess of it led to microcrystalline SnSe2. In another reaction, SnCl4•5H2O was replaced with [K4(H2O)4][SnSe4], which contains a preformed binary unit of tin and selenium atoms. This led to the formation of ZBT-2 in the absence of DMMP. This strongly evidences that the amine is involved in the formation of initial binary (or higher) species of tin and selenium. In further experiments, [K4(H2O)3][Ge4Se10] and SnCl4•5H2O were reacted with en (en = ethylenediamine) instead of DMMP, which led to the formation of ZBT-1 at a remarkably lower amine to IL ratio, probably due to the higher base strength of en [92].

Amine-Assisted Syntheses of Selenidostannates
Structure-directing properties of ILs for the synthesis of inorganic materials are known since the initial studies of the Morris group [84][85][86]. They also noticed that the addition of small amounts of molecular solvents, e.g., H 2 O, has a strong effect on the formation of the products if combined with ILs [87]. This feature was further explored during the ionothermal synthesis of molecular sieves with auxiliary amines [61,62,88,89]. The groups of Dehnen and Huang reported similar findings for clusters of heavy main-group elements. They noticed that the addition of amines had strong impact on the phase formation of selenidostannates synthesized from the elements or prereacted species in ILs [51,[90][91][92][93][94][95][96][97][98]. More than 25 different selenidostannates were synthesized following this approach, which does not only illustrate the diversity of this class of compounds but also demonstrates the capability of the method. More details are given in a recent review about the synthesis and structure of selenidostannates by Dehnen et al. [51].   (Table 3). According to these experiments, the basicity of the amine has strong influence on the phase formation [92]. With increasing basicity, the overall tendency of the Ge/Se and Sn/Se subunits to aggregate is decreased and the incorporation of tin into the anionic substructure is favored (Table 3). In particular, products could be precisely targeted by switching the amine from DMMP to en. Adding larger amounts of en led exclusively to the formation of a 3D network, whereas the layered (2D) and the cluster (0D) compounds are formed exclusively at lower concentrations [92]. Table 3. Influence of the amine content on the product distribution of ternary selenidostannates under otherwise identical reaction conditions. A dash indicates the absence of an identifiable product. The structural connectivity of the anionic part is indicated by 0D (cluster), 2D (layer), or 3D (framework  [90,94]. It becomes evident that tuning only the amount of amine does not necessarily determine the dimensionality of  (Table 3). According to these experiments, the basicity of the amine has strong influence on the phase formation [92]. With increasing basicity, the overall tendency of the Ge/Se and Sn/Se subunits to aggregate is decreased and the incorporation of tin into the anionic substructure is favored (Table 3). In particular, products could be precisely targeted by switching the amine from DMMP to en. Adding larger amounts of en led exclusively to the formation of a 3D network, whereas the layered (2D) and the cluster (0D) compounds are formed exclusively at lower concentrations [92]. Table 3. Influence of the amine content on the product distribution of ternary selenidostannates under otherwise identical reaction conditions. A dash indicates the absence of an identifiable product. The structural connectivity of the anionic part is indicated by 0D (cluster), 2D (layer), or 3D (framework  [90,94]. It becomes evident that tuning only the amount of amine does not necessarily determine the dimensionality of the crystallized anion. Increased addition of amines led to two-dimensional anions in the case of binary anions and to 3D-networks for ternary ones (Table 3). This is most likely because other factors, e.g., temperature or the shape of the cation, also play an important role [90,93,94]. Therefore, if an undesired dimensionality results from a synthesis with auxiliary amines, increasing as well as decreasing of the amine content should be considered. Table 4. Impact of the amine content on the product distribution of binary selenidostannates under otherwise identical reaction conditions. Sn (1 mmol), Se (2.5 mmol), hydrazine hydrate, and [PMMIm]Cl (1 g) were annealed at 160 • C for 5 days and subsequently cooled to ambient temperature [90].

Crystal Growth
As mentioned above, it was possible to synthesize the "zeoball" compound ZBT-2 without addition of amines. However, it was noted that crystal quality and yield were poor compared to the amine-assisted synthesis [91]. Therefore, the amines apparently play a role during crystal growth.
This phenomenon has also been observed in NMR experiments during the synthesis of molecular sieves [61]. In their experiments, Xu et al. found that imidazolium cations form hydrogen bonds with the amine molecules during crystallization. These intermediates were identified as structure directing agents for the forming solid. The imidazolium cations themselves are pore-filling agents during crystal growth [61,62]. The influence of hydrogen bonding in the course of crystallization was also observed for the transformation of 2D-[BMMIm] 16  , however, is almost quantitative after two days. Lin et al. supposed that the protonation of DMMP and the resulting DMMPH cation is advantageous for the formation of the 1D structure, which is otherwise seemingly disfavored. Additionally, the transformation can be reversed by reacting any of the 1D-compounds with en. It was pointed out that in this case, en might play an important role as auxiliary agent that forms intermediate selenium-hydrogen bonds [93]. Indeed, hydrogen bridges between en and selenium were later recognized in selenenidostannates with metal-amine complexes (MACs) acting as cations [95][96][97][98]. For example, in (BMMIm) 3 [Ni(en) 3 ] 2 [Sn 9 Se 21 ]Cl, hydrogen bonds have been found between en and the selenium atoms of the 2 ∞ [Sn 3 Se 7 ] 2− layers. However, this compound also demonstrates the complexity and diversity of different interactions between the MAC, the imidazolium cation, the chloride anion, and the selenium atoms of the selenidostannate layer ( Figure 4) [98].
In conclusion, the combination of amines and ILs enabled the synthesis of a variety of selenidostannates. The introduction of auxiliary amines provides several additional reaction parameters. The amount, structure, and basicity of the amine can influence the dimensionality of the polyanions as well as their crystallization. The reaction mechanism includes formation of hydrogen bonds between the amines and dissolved starting materials as well as the cations of the ILs. Thus, utilization of amines or comparable auxiliaries could be beneficial for many other syntheses. [98]. The disorder and the anion except for one selenium atom each is omitted for clarity. Hydrogen bonds are pictured as green-grey dashed lines and anticipated anion-π interactions as orange dashed lines. Hydrogen atoms are only depicted if participating in bonds toward chlorine atoms. The figure was developed according to [98].

Homopolycations-Adjustments via Redox Potential and Starting Materials
While polyanions are generally synthesized in (Lewis-)basic media, polycations of heavy main group elements are usually obtained from (Lewis-)acidic solutions such as oleum or more generally in systems with only weakly coordinating anions or solvent molecules such as Na[AlCl4] melts or liquid SO2. Weakly coordinating anions (e.g., weak Lewis bases) are necessary since strong Lewis bases destabilize polycations. The introduction of ILs has increased the convenience of polycation syntheses and enabled substitution of toxic substances such as benzene, SO2, or AsF5 [99]. Commonly used Lewis-acidic ILs are combinations of alkylimidazolium halides with more than equimolar amounts of aluminum or gallium trihalides (MX3). The excess of trihalides is beneficial for several reasons: Free MX3 or their condensation products with [MX4] − anions such as [M2X7] − act as scavengers for halide ions or other Lewis bases. This leads to increased solubility for metalloid halides (e.g., BiX3 dissociates into BiX2 + and X − ) and partly self-drying ILs (if the Lewis base is water) protecting the formed polycations from hydrolysis. In addition, the concentration of the different halogenidometalate species can be tuned by temperature [100], mole fraction of employed MX3 [5], and by adding additional free X − anions [5100]. The latter has been utilized by Ruck et al. to overcome hindered crystallization of products by adding small amounts of NaCl after completion of the reaction to increase the mole fraction of [AlCl4] − [23,26,101].

Bismuth Homopolycations
Bismuth is famous for its ability to form ligand-free homopolyanions and especially homopolycations. Three of the latter could be synthesized in Lewis-acidic ILs: Bi5 3+ [99,104], Bi8 2+ [26], and Bi9 5+ [26,27]. In general, IL-based syntheses of bismuth polycations are advantageous compared to classic syntheses by increasing purity and yield in addition to lowering the reaction temperature [26].
The first synthesis of Bi5 3+ by reacting elemental bismuth with BiCl3 (molar ratio 3:1) in [BMIm]Cl•1.3AlCl3 at room temperature was established by Ahmed et al. in 2009 [99]. As continuative experiments in our group have shown, the resulting Bi5[AlCl4]3 is strongly favored in this system as long as redox reactions involving bismuth might occur and especially during reactions at elevated temperature and in highly Lewis-acidic ILs. In fact, Bi5[AlCl4]3 can be regarded  [98]. The disorder and the anion except for one selenium atom each is omitted for clarity. Hydrogen bonds are pictured as green-grey dashed lines and anticipated anion-π interactions as orange dashed lines. Hydrogen atoms are only depicted if participating in bonds toward chlorine atoms. The figure was developed according to [98].

Homopolycations-Adjustments via Redox Potential and Starting Materials
While polyanions are generally synthesized in (Lewis-)basic media, polycations of heavy main group elements are usually obtained from (Lewis-)acidic solutions such as oleum or more generally in systems with only weakly coordinating anions or solvent molecules such as Na[AlCl 4 ] melts or liquid SO 2 . Weakly coordinating anions (e.g., weak Lewis bases) are necessary since strong Lewis bases destabilize polycations. The introduction of ILs has increased the convenience of polycation syntheses and enabled substitution of toxic substances such as benzene, SO 2 , or AsF 5 [99]. Commonly used Lewis-acidic ILs are combinations of alkylimidazolium halides with more than equimolar amounts of aluminum or gallium trihalides (MX 3 ). The excess of trihalides is beneficial for several reasons: Free MX 3 or their condensation products with [MX 4 ] − anions such as [M 2 X 7 ] − act as scavengers for halide ions or other Lewis bases. This leads to increased solubility for metalloid halides (e.g., BiX 3 dissociates into BiX 2 + and X − ) and partly self-drying ILs (if the Lewis base is water) protecting the formed polycations from hydrolysis. In addition, the concentration of the different halogenidometalate species can be tuned by temperature [100], mole fraction of employed MX 3 [5], and by adding additional free X − anions [5100]. The latter has been utilized by Ruck et al. to overcome hindered crystallization of products by adding small amounts of NaCl after completion of the reaction to increase the mole fraction of [AlCl 4 ] − [23,26,101]. A variety of homoatomic polycations of group 15 or 16 elements has been synthesized in ILs [26,27,31,99,[101][102][103][104] and several influencing reaction parameters were deduced.

Bismuth Homopolycations
Bismuth is famous for its ability to form ligand-free homopolyanions and especially homopolycations. Three of the latter could be synthesized in Lewis-acidic ILs: Bi 5 3+ [99,104], Bi 8 2+ [26], and Bi 9 5+ [26,27]. In general, IL-based syntheses of bismuth polycations are advantageous compared to classic syntheses by increasing purity and yield in addition to lowering the reaction temperature [26].

Tellurium Homopolycations
Tellurium forms a large variety of polycations. Apart from Te 4 2+ , which proves a dominance in Lewis-acidic AlCl 3 -containing ILs similar to Bi 5 3+ , several other tellurium polycations ( Figure 5) were accessed in ILs [26,27,31,[101][102][103]. Thereby, the utilization of an IL as reaction medium can lead to extraordinary properties of the reaction product as the following example demonstrates: Te 4 [Bi 0.67 Cl 4 ] obtained from a gas-phase transport reaction is an conventional semiconductor. However, the closely related Te 4 [Bi 0.74 Cl 4 ], obtained from an IL-based low-temperature synthesis, proved to be a one-dimensional metal and type-I superconductor [31].
as an omnipresent (and inconvenient) side product which has to be prevented from crystallization. Reducing BiCl3 with transition metals or other moderate reducing agents typically leads to Bi5[AlCl4]3. In order to suppress Bi5[AlCl4]3 and to obtain other homopolycation compounds, several strategies have proven to be viable. One possibility is the introduction of bromine, which seems to disfavor the crystallization of Bi5 3+ : By reacting equimolar amounts of Bi and BiBr3 in [BMIm]Cl•2AlCl3 at room temperature, pure Bi6Br7, which includes Bi9 5+ polycations, is accessible. A second approach on targeting the type of crystallized polycations is by adjusting the reducing agent: The Bi8 2+ polycation is accessible as Bi8[AlCl4]2 by reduction of BiCl3 with elemental sodium (molar ratio 1:2.8) in [BMIm]Cl•3.6AlCl3 at 140 °C with bismuth, Na[AlCl4], and Bi5[AlCl4]2 as byproducts. Lowering the reaction temperature to 80 °C and using a less Lewis-acidic IL with 1.3 equivalents of AlCl3 leads to Bi8[AlCl4]2 with Na[AlCl4] and bismuth as the only by products [26]. Utilizing indium, however, changes the precipitating cation to Bi9 5+ . Bi6Cl7 can be obtained at room temperature by the reduction of BiCl3 with indium (molar ratio 3:2) in [BMIm]Cl•2AlCl3 [26]. The latter result might also be attributed to the oxidation of indium into (probably) InCl3, which should interact with the IL as additional Lewis acid and could tune the reaction not (only) via the redox potential.
Kloo et al. explored the formation of the Bi5 3+ cation by reducing BiCl3 with elemental gallium in different ILs with GaCl3 as Lewis acid in different ratios [104].

Tellurium Homopolycations
Tellurium forms a large variety of polycations. Apart from Te4 2+ , which proves a dominance in Lewis-acidic AlCl3-containing ILs similar to Bi5 3+ , several other tellurium polycations ( Figure 5) were accessed in ILs [26,27,31,[101][102][103]. Thereby, the utilization of an IL as reaction medium can lead to extraordinary properties of the reaction product as the following example demonstrates: Te4[Bi0.67Cl4] obtained from a gas-phase transport reaction is an conventional semiconductor. However, the closely related Te4[Bi0.74Cl4], obtained from an IL-based low-temperature synthesis, proved to be a one-dimensional metal and type-I superconductor [31].   [26]. The distinctive directing parameter of these room temperature reactions has proven to be the ratio of the starting materials: If elemental tellurium, TeCl 4 , and BiCl 3 are utilized in a proportion according to the respective composition, each compound is yielded as phase-pure product. In the case of the super-conductor Te 4 [Bi 0.74 Cl 4 ], the amount of BiCl 3 can be increased to 3 equivalents and still the sole crystallization of the desired product does occur.
Omitting any bismuth-containing starting materials leads to the formation of pure Te 4 [AlCl 4 ] 2 under the same conditions [101]. Te 4 [Al 2 Cl 7 ] 2 has been synthesized during an attempt to access phosphorus-tellurium polycations by reacting equimolar amounts of tellurium, TeI 4 , and red phosphorus in the same IL with n = 4.8 at 80 • C [26]. The crystallization of the identical polycation but with [Al 2 Cl 7 ] − as anion can be rationalized with the higher content of AlCl 3 resulting in a virtual absence of dissolved [AlCl 4 ] − [5].
Introducing other redox agents and simultaneously forming anions has shown to alter the obtained tellurium homopolycation. Ahmed

Antimony-Selenium Heteropolycations-Auxilaries, Temperature, and Influence of Halogens
In recent years, we synthesized several binary or ternary antimony-selenium heteropolycations ( Figure 6) in Lewis-acidic ILs [BMIm]X·nAlX 3 (X = Cl, Br; n = 1. 2-5.2) [30,34,35,38]. Thereby, we deduced two main decisive reaction parameters for the controlled synthesis of a desired polycation in this system: temperature and the utilization of different auxiliaries. In all reactions, elemental antimony and grey selenium were employed as main starting materials. If

Bismuth-Tellurium Heteropolycations-Adjustments via Starting Materials
In a fashion similar to the aforementioned case of antimony and selenium, heteropolycations of bismuth and tellurium (and bromine) can be synthesized in Lewis-acidic ILs. In this system, however, the choice of starting materials seems to have the dominant influence on the obtained polycation.

Bismuth-Tellurium Heteropolycations-Adjustments via Starting Materials
In a fashion similar to the aforementioned case of antimony and selenium, heteropolycations of bismuth and tellurium (and bromine) can be synthesized in Lewis-acidic ILs. In this system, however, the choice of starting materials seems to have the dominant influence on the obtained polycation.
Starting  [105], which has initially been synthesized by Beck et al. in a NaCl·11AlCl 3 melt at 130 • C, was synthesized by reacting bismuth telluride and bismuth trichloride in [BMIm]Cl·4.7AlCl 3 at 100 • C. Thereby, the yield could be significantly improved compared to the original approach [30].

Manipulating the Stacking Order of Layered Compounds
Recently, we proved that the choice of starting materials can even influence the crystallized polytype of a layered compound, namely Cu 2 Bi 2 S 3 [AlCl 4 ] 2 [32]. In its structures, Bi 2 S 3 molecules are connected by copper ions forming cationic layers that are separated by [AlCl 4 ] − tetrahedra. Upon dissolving Cu 3 Bi 2 S 3 Br 2 [106] in [BMIm]Cl·3.6AlCl 3 or [EMIm]Cl·3.6AlCl 3 at 80 to 200 • C, approximately half of the precursor recrystallizes as Cu 2 Bi 2 S 3 [AlCl 4 ] 2 with a rhombohedral structure. In addition, small amounts of Bi 5 [AlCl 4 ] 3 (vide supra) and a hexagonal polytype are found. The compound is also accessible from CuCl, Bi 2 S 3 , and AlCl 3 at 200 • C either by solvent-free reaction or by ionothermal synthesis in [BMIm]Cl·3.6AlCl 3 . Omitting the IL results in a larger portion of Bi 5 [AlCl 4 ] 3 while the rhombohedral polytype is again the main (but only microcrystalline) product. In contrast, reacting the binary starting materials in the IL strongly favors the crystallization of the hexagonal polytype. Substitution of CuCl with Cu 2 S and the corresponding amount of Bi 2 S 3 with BiCl 3 results in the crystallization of similar amounts of both polytypes from the IL.

Influence of Concentration and Ion Specification
Lewis-acidic ILs feature a remarkable solubility for metalloids and their halides, which extends to ternary compounds [29,107]  The concentration-and/or temperature-driven preference for one of the two compounds can be rationalized with the anion specification in the IL or similar salt melts [5,100,110]. At high contents of the aluminum halide (i.e., strongly Lewis-acidic), adducts [Al n X 3n+1 ] − prevail. Their lower charge density and symmetry compared to [AlX 4 ] − , however, strongly disfavors their incorporation into crystal structures. Increasing temperature breaks the adducts and increases the concentration of isolated tetrahedra [100]. A lower amount of dissolved [Pd@Bi 10 ] 4+ polycations leads apparently to the crystallization of tetrahedra pairs owing to the higher excess of pairs.

Conclusions
The scope of this review is to summarize and deduce decisive reactions parameters for controlled syntheses of polyions of heavy main-group elements in ILs. Without covering all possibilities, we want to demonstrate several (unexpected) pathways to tune reactions in ILs in order to synthesize new compounds. Aside from intuitive parameters such as temperature, concentration of starting materials, or their stoichiometry, syntheses in ILs offer additional ways to control the reaction products such as by the shape and charge-density of the IL cation. For instance, the choice of the starting material can influence the yielded polyion or its polymorph. Auxiliary compounds can be additionally introduced, which for instance subtly adjust the redox potential or influence the dimensionality of polyanions. In addition, general advantages of utilizing ILs are demonstrated such as easier handling of delicate volatile components, provision of dissolved species for further reactions, or substitution of toxic compounds. We would like to encourage readers to explore the abilities of ILs in synthesis because the variety of decisive reaction parameters promises an abundance of possible new compounds. We believe that many discussed reaction principles can be transferred to other classes of compounds, especially in main group-element chemistry.