Inorganic Synthesis Based on Reactions of Ionic Liquids and Deep Eutectic Solvents

Abstract Ionic liquids and deep eutectic solvents are of growing interest as solvents for the resource‐efficient synthesis of inorganic materials. This Review covers chemical reactions of various deep eutectic solvents and types of ionic liquids, including metal‐containing ionic liquids, [BF4]−‐ or [PF6]−‐based ionic liquids, basic ionic liquids, and chalcogen‐containing ionic liquids. Cases in which cations, anions, or both are incorporated into the final products are also included. The purpose of this Review is to raise caution about the chemical reactivity of ionic liquids and deep eutectic solvents and to establish a guide for their proper use.


Introduction
Ionic liquids (ILs), first reported by Paul Walden in 1914, [1] are defined as molten salts with melting points below 100 8 8C. Nowadays,ILs are widely applied in abroad variety of fields,i ncluding catalysis,s eparations,s ynthesis,a nd many others. [2][3][4][5] Compared to the broad applications of ILs in organic chemistry,w hich have already been explored for about 40 years,i norganic syntheses in ILs,e specially the socalled ionothermal syntheses, [6][7][8] have been actively investigated only since the early 2000s.S ince then, various inorganic compounds (e.g. metals and non-metals,m etal oxides and chalcogenides,m etalates and framework compounds) have been prepared using, or in the presence of, ILs. [9][10][11][12][13][14] ILs provide several unique properties for the preparation of inorganic materials.F or example,I Ls can facilitate the dissolution of versatile precursors,i ncluding both inorganic and organic compounds,w hich is fundamental for the synthesis of most materials. [15,16] ILs create aspecial microphasic separation of the hydrophilic and hydrophobic fragments, typically with imidazolium ILs with their long alkyl chains. [17] This heterogeneity of ILs provides the ability to control nucleation and growth rates,particle sizes,and morphologies in materials synthesis. [17] Furthermore,s ome other characteristics,such as good thermal stability in ionothermal synthesis, high polarizability for microwave synthesis,o rw ide electrochemical windows and high conductivity for electrodeposition, make ILs an attractive alternative to conventional organic solvents for the synthesis of inorganic materials as well as to high-temperature reactions in melts or the solid state.
In 2003, the concept "deep eutectic solvent" was first coined by Abbott et al. [18] Deep eutectic solvents (DESs) are acknowledged as an ew class of IL-analogue solvents and share many characteristics of traditional ILs,s uch as low vapor pressure,h igh polarity,a nd tunable chemical properties.However,DESs are easier to access synthetically.Inmost cases,aDES is obtained by mixing aq uaternary ammonium or phosphonium salt with ah ydrogen-bond donor (HBD), thereby generating an ew liquid phase with am elting point below that of either individual component. [19,20] No purification is usually needed. Furthermore,m ost DESs are quite inexpensive because of the low cost of their constituents,such as urea and choline chloride.T herefore,ILanalogues as well as the more accessible DESs are increasingly being used in the synthesis of inorganic materials.
Ionic liquids and deep eutectic solvents are of growing interest as solvents for the resource-efficient synthesis of inorganic materials.This Review covers chemical reactions of various deep eutectic solvents and types of ionic liquids,including metal-containing ionic liquids, [BF 4 ] Àor [PF 6 ] À -based ionic liquids,basic ionic liquids,a nd chalcogencontaining ionic liquids.Cases in whichc ations,anions,o rboth are incorporated into the final products are also included. The purpose of this Review is to raise caution about the chemical reactivity of ionic liquids and deep eutectic solvents and to establish ag uide for their proper use. Thet hermal and chemical stabilities of ILs are usually highlighted as advantageous for inorganic synthesis.Previous investigations have indicated that the actual degradation temperature of ILs is overestimated by the onset decomposition temperature (T onset )d erived from the ramped temperature in thermogravimetric analysis. [21][22][23] Therefore,t he concept of long-term thermal stability is utilized to obtain more accurate information on the decomposition of ILs at high temperature.T he thermal stability of ILs,i ncluding the characterization methods,m echanism of decomposition, and kinetics of thermal degradation, has been extensively explored by several research groups, [24][25][26][27][28][29] and is not within the scope of this Review.
ILs are readily accessible as inert reaction media for inorganic synthesis.D ai and co-workers demonstrated that several imidazolium ILs containing [NTf 2 ] À anions could be used as the flux medium for the direct recycling of spent cathode materials or as effective structure-directing templates for the synthesis of advanced catalysts and anode materials because of their good thermal and chemical stability. [30][31][32] In all cases,t he [NTf 2 ] À -containing imidazolium ILs can be readily reused and recycled after the reaction, thus providing new strategies for designing sustainable ILs for advanced inorganic synthesis.
However,many ILs contain reactive moieties in either the cation or anion. Thus,t he ILs themselves can take part in reactions.F or example,a nI Lc an be tailored for as pecific task, such as to release one component of the desired product upon its decomposition. Thus,the IL acts as solvent, template, and reactant, thereby simplifying the reaction system significantly.I no ther circumstances,t he IL cation and anion can separate during the reactions,t hereby leading to incorporation of the IL cation or anion in the final products.S uch reactions are often used in the synthesis of some framework compounds (e.g.zeolites,MOFs,and polycationic/polyanionic compounds). [13,33,34] TheI Lc ation or anion serves as ac ounterion to balance the charge of the framework as well as at emplate.I Ld ecomposition or cation/anion separation during the ionothermal synthesis may cause ac hange in the properties of the IL (e.g.viscosity,conductivity,and dissolving capacity) and further influence the formation of the target product. [35] Thus,t he reaction mechanism of ILs should be considered. Furthermore,t he ionothermal approach, in particular,w hich exploits the chemical reactivity of ILs or DESs,p rovides new options for the synthesis of inorganic materials.
Several inspiring reviews on this young and fast-growing subject of inorganic synthesis in ILs or DESs have been published, with an emphasis on selected themes. [7][8][9][10][11][36][37][38][39][40] However,most of them lack acomprehensive understanding of the chemical reactivity of the ILs or DESs in the reactions. To date,t here are only af ew case studies on the detailed reaction mechanisms. [16,[41][42][43][44][45] In comparison, the chemical reactivity of ILs in organic synthesis has been discussed in several reviews. [35,[46][47][48] Herein, we attempt to systematically and comprehensively summarize this fascinating research area from the point of view of inorganic synthesis based on the chemical reactions of ILs or DESs.Itincludes reactions of metal-containing ILs, fluorine-containing ILs,b asic ILs,c halcogen-containing ILs, and DESs.Moreover,reactions of ILs whose cations,anions, or both are incorporated into the final products are also included. Table 1s hows as ummary of all the abbreviations used in this Review.T he decomposition and reaction mechanism of some IL/DES-based reactions are discussed. This Review aims to illustrate apromising synthetic approach based on the reactivity of ILs/DESs and to provide ab etter understanding of the fundamental chemistry of ILs/DESs in the reactions.

Reactions of Metal-Containing Ionic Liquids
An overview of all the discussed studies reporting the synthesis of inorganic materials using,o ri nt he presence of, ILs is presented in Table 2.
In this Review,m etal-containing ionic liquids (M-ILs) represent asubclass of ILs that contain ametal atom as part of the cation and/or anion. In addition to the general fluidic properties of ILs,t he incorporated metal ions endow M-ILs with some new functions,s uch as luminescent, catalytic, or magnetic properties.Recently,M-ILs have gained increasing research attention in av ariety of fields (e.g.c atalysis,optical devices,and magnetic components). [49] M-ILs that serve as metal sources in inorganic synthesis have been widely investigated. In 2004, Taubert reported that CuCl nanoplatelets were synthesized from aCu-containing IL ( Figure 1a)and 6-O-palmitoyl ascorbic acid (Figure 1b). [50] It was found that the mixture of the two compounds forms thermotropic liquid crystals with lamellar self-assembled structures.T he layered structures then template the formation of CuCl nanoplatelets as the temperature is increased (Figure 1c). TheCu-containing IL can be regarded as an "allin-one" IL because it acts as solvent, reactant, as well as template.After this study,anumber of M-ILs were designed and applied for the synthesis of inorganic nanomaterials, including metal oxides and metal sulfides.
Dai and co-workers reported that hierarchical ZnO structures with diverse morphologies were obtained under an ionothermal synthesis when employing the Zn-containing  IL Zn(L) 4 (NTf 2 ) 2 (L = alkylamine,N Tf 2 = À N(SO 2 CF 3 ) 2 )a s both the solvent and Zn source. [51] Thesolvent properties can be tailored by varying the IL ligand structures,t hereby resulting in various morphologies of ZnO.I na ddition, CuO nanorods can be prepared in the presence of the Cu-IL [C 16 MIm] 2 [CuCl 4 ]u nder solvothermal conditions. [52] Zheng and co-workers demonstrated that three-dimensional (3D) hierarchical CuS microspheres assembled from nanosheets were produced from the Cu-containing IL precursor [BMIm] 2 [Cu 2 Cl 6 ]b yasolvothermal method. [53] The Cu-IL plays as ignificant role in directing the final CuS structures.O no ne hand, the crystal growth along the [001]  direction is inhibited because the [BMIm] + prefers to adsorb onto the (001) facets of CuS.Onthe other hand, the assembly of CuS microspheres is influenced by the alkyl chain of the Cu-IL. At ight hierarchical CuS structure is the preferred form when as hort-chain IL is used. ZnS quantum dots were prepared using the Zn-containing IL [C n MIm][ZnCl 3 ](n = 4, 8, and 16) as ap recursor,t emplate,a nd solvent. [54] FeS 2 microspheres wrapped by N-doped reduced graphene oxide were synthesized from the Fe-based IL [C 12 MMIm][ FeCl 4 ]. [55] TheF e-IL can be used as the metal and nitrogen source,ana ssembly medium, and surfactant.
Another major application of M-ILs is for the lowtemperature electrodeposition of various metals and alloys. [56][57][58][59] One well-studied example involves the electrodeposition of Al in the IL-AlCl 3 system, whose Lewis acidity depends on the molar ratio of the organic salts to metal halides. [56] In recent years,I Ls with metal-containing cations have been developed for the electrodeposition of metals at high current densities because of the easy access of cationic metal complexes to the electrode surface. [60] Investigations into the electrodeposition of metals (e.g. Ni, Co,Cu, Al, and rare earth metals) by various cationic metal-containing ILs have been reported by aseries of research groups. [60][61][62][63][64][65]

Reactions of [BF 4 ] À -or[ PF 6 ] À -Based Ionic Liquids
Inorganic metal fluorides are well-studied for their applications in photonics,c atalysis,b iosensing, lubricants, electrochemical energy storage,and high-temperature superconductor devices. [66,67] In traditional syntheses of metal fluorides,t he toxic and harmful HF,N aF,o rN H 4 Fisu sually utilized as afluorine source.R ecent studies have shown that fluorine-containing ([BF 4 ] À or [PF 6 ] À )I Ls can be used as fluorine sources,t hereby opening as afe pathway to prepare metal fluorides with novel morphologies and functions.T he hydrolysis of the [BF 4 ] À anion occurs in the presence of as mall amount of residual water in the IL or the water of crystallization in metal salts upon heating, thereby forming BF 3 ·H 2 Oand F À .The reaction of fluoride ions with metal ions under the given conditions contributes to the in situ crystallization of metal fluorides. [67] Similarly,[ PF 6 ] À may also decompose to release F À under specific conditions. [68] Wena nd co-workers synthesized an anostructured MnF 2 by using Mn(CH 3 COO) 2 ·4 H 2 Oa sam anganese source and [BMIm][BF 4 ]a safluorine source (Figure 2a). [69] Ther esulting MnF 2 nanoparticles could be promising anode materials for lithium batteries with al ong cycle life.L ie tal. reported that av ariety of hydrated Fe-based fluoride nanoparticles could be successfully synthesized using Fe(NO 3 ) 3 ·9 H 2 Oa nd [BMIm][BF 4 ]a st he precursors (e.g. Figure 2b). [70][71][72][73] In the process,[ BMIm][BF 4 ]s erves as the solvent, template,a nd fluorine source.The as-synthesized iron-based fluoride and its composite materials can be used as cathodes for lithium or sodium batteries.
Wickleder and co-workers synthesized aseries of ternary fluoridosilicates A 2 SiF 6 (A = Li, Na, K, Rb,a nd Cs) with particle sizes of afew tens of nanometers ( Figure 2c)byusing [BMIm][PF 6 ]a sb oth the solvent and fluoride source in am icrowave-assisted ionothermal synthesis at low temperatures. [68] This approach is very simple,e nergy-efficient, and time-saving since it avoids the use of harmful and toxic HF or its derivatives.T he nanoparticles of this series of ternary fluoridosilicates A 2 SiF 6 are regarded as possible host materials for future LEDs.
Rare-earth fluorides are an interesting family of compounds because of their optical properties,and their syntheses in ILs have been widely investigated. Yana nd co-workers reported that novel spherical NaYF 4 6 ]with the assistance of microwave radiation. [74] ILs serve as reaction solvents, fluorine sources,and microwave absorbents.T hey found that some NaF particles were formed when [BMIm][PF 6 ]was used to prepare NaYF 4 nanocrystals.This result is probably due to the easier breaking of the P À Fb ond in [ 6 ]m ight prevent the assembly and aggregation of small nanoparticles,w hich affects the final morphology.F inally,N aYF 4 nanoparticles doped with lanthanide ions (Ln 3+ )d isplay superior upconversion properties.
Interestingly,itis found that the degraded components of [BF 4 ] À anions can be utilized in the presence of moisture or heat as aboron precursor in the reaction. Tu rbostratic boron nitride nanoflakes (t-BN) were obtained on using [BMMIm]-[BF 4 ]asthe boron source. [81] Thehydrogen-bond-co-p-p stack mechanism is responsible for the self-assembly of the [BMMIm][BF 4 ]I Lf or the formation of the flake-like t-BN.

Reactions of Basic Ionic Liquids
Conventional inorganic bases,s uch as NaOH, KOH, or Na 2 CO 3 ,h ave many disadvantages such as being corrosive and producing waste.B asic ILs,w hich combine the advantages of inorganic bases and ILs,h ave great potential to replace them. They are noncorrosive,n onvolatile,f lexible, and immiscible with many organic solvents.T herefore,b asic ILs are often applied in some base-catalyzed processes in organic chemistry [82] and nanomaterial preparation. In inorganic synthesis,b asic IL anions provide the required basic environment of the reactions,w hile the organic cations may play an important role in the crystal nucleation and growth.
Li et al. designed av ariety of tetraalkylammonium hydroxide ILs to synthesize metal oxides of various sizes and shapes.F or example,s everal unusual ZnO nanostructures,i ncluding flower-like particles," lotus-leaf-like" ZnO plates,a nd porous ZnO plates,c an be produced from tetrabutylammonium hydroxide (TBAH). [83] In these reactions,T BAHs erves as an efficient IL precursor for the synthesis of ZnO nanoparticles with controlled sizes and morphologies. [84,85] Interestingly,n ot only regular nanocrystals,b ut also special hollow ZnO mesocrystals can be obtained in TBAH by varying the concentration of the zinc acetate precursor (Figure 3a). [86] Theresulting tubular microstructures are composed of smaller nanosized ZnO primary particles with ahigh degree of order,which classifies them as mesocrystals.I ti sa ssumed that the large tetrabutylammonium cation of TBAH reverses the polarity of the negatively charged surfaces of the small particles,thus preventing further growth of these small primary particles and aiding their aggregation into larger structures.Awide variety of uniform metal (hydr)oxide particles (Figure 3b-e) was successfully prepared from water/TBAH liquid precursor mixtures simply by using metal acetates (M(OAc) 2 ,M= Fe,C o, Mn, Ni, and Cu) as the metal sources. [87,88] Moreover,some other types of hydroxide-based IL precursors,i ncluding tetraethylammonium hydroxide (TEAH) and benzyltrimethylammonium hydroxide (BTMAH), were used instead of TBAH for the efficient synthesis of ZnO particles with controlled sizes and morphologies. [89,90] Ruck and co-workers successfully extended the use of TBAH for the fabrication of ap erovskite-type oxide SrTiO 3 (Figure 3f). [91] Hierarchical desert-rose-like SrTiO 3 microstructures with ah igh surface area of up to 186 m 2 g À1 are obtained by using TBAH as the alkali under solvothermal conditions mediated by ethylene glycol. It was found that TBAH or tetrabutylphosphonium hydroxide (TBPH) can replace the ethylene glycol and act as both the solvent and reactant to yield polyhedral SrTiO 3 nanoparticles.  [86] Copyright2 008, Wiley-VCH. b-d) g-Fe 2 O 3 /Fe 3 O 4 cubes and spheres, b-Ni(OH) 2 plates, and Co-(OH) 2 plates, respectively. Reproducedwith permission. [87] Copyright 2008, Wiley-VCH.e)CuO nanoplates. Reproducedwith permission. [88] Copyright 2008, AmericanC hemical Society.f)Hierarchically structured SrTiO 3 microparticles. Reproducedwith permission. [91] Copyright 2017, Royal Society of Chemistry.

Reactions of Chalcogen-Containing Ionic Liquids (Including Reactions of Ionic Liquids with Chalcogens)
This section covers reactions of imidazolium or phosphonium salts with chalcogens (S,S e, or Te )t og enerate the corresponding imidazole-2-chalcogenones or trialkylphosphane chalcogenides,r espectively.T he reaction mechanisms are also discussed. Theu tilization of chalcogenones or trialkylphosphane chalcogenides as chalcogen sources for the synthesis of metal chalcogenides is also summarized in this section.
It has been recognized that the C2-position of the 1,3dialkylimidazolium cation contains an acidic proton. Deprotonation of this position leads to the formation of as table carbene,w hich is often used as an intermediate in organic synthesis. [92] It is found that reactions of inorganic reactants (e.g. chalcogens) with imidazolium salts at their C2-position can also take place,e specially in the presence of ab ase. Rogers and co-workers reported that 1,3-dialkylimidazolium acetates can react with elemental So rS et oa fford the corresponding imidazole-2-chalcogenones directly,e ven in the absence of an additional base (Scheme 1). [93] Investigations show that the imidazolium acetate IL serves as both the carbene source and base to generate,i nsitu, the carbenes, which then react with Stof orm the thione.
Theuse of refluxing methods has also been reported. For example,W asserscheid and co-workers found that 1,3-dialkylimidazolium halide salts can react with elemental Si n refluxing methanol in the presence of NaOMe to afford 1-alkyl-3-methylimidazolium-2-thiones. [104] Tian et al. prepared avariety of selenones by refluxing the respective imidazolium salts with Se and Na 2 CO 3 in water. [105] In addition, Inesi and co-workers developed an efficient combined electrochemical and ultrasound method for the synthesis of imidazole-2-thiones. [106] In this reaction, the imidazolium IL is first electrochemically reduced to the corresponding carbene,w hich then reacts with elemental S under ultrasound irradiation to give the target thiones in high yields.L ei and co-workers reported that reactions of imidazolium salts with potassium thioacetate/thiocyanate as the S source yield imidazole-2-thiones rapidly and efficiently under microwave radiation. [107] In contrast to thiones and selenones,t he synthesis of tellurones derived from their corresponding imidazolium salts is much more difficult because of the relatively weak C À Te bond compared to C À S/Se bond. Singh and co-workers developed an ew approach for the high-yielding synthesis of benzimidazolin-2-tellurones by the reaction of Te nucleophiles Na 2 Te/Na 2 Te 2 with various benzimidazolium salts. [108] Compared to Te powder,the stronger Te 2À /Te 2 2À nucleophiles facilitate chemical reactions with benzimidazolium salts under mild conditions.
In comparison to the popularly investigated imidazolium ILs,p hosphonium ILs have been less studied. Despite their higher thermal and chemical stability,p hosphonium ILs are not completely inert, and decomposition can occur under certain conditions. [35] Ruck and co-workers showed that quaternary phosphonium cations of ILs can undergo decomposition in the presence of Se/Teabove 220 8 8C. [110,111] Aseries of dissolution tests,i nw hich the solute Se/Tes pecies were tracked by nuclear magnetic resonance (NMR) spectroscopy, was applied to systematically investigate the decomposition mechanisms.These studies indicate that one alkyl substituent of the quaternary phosphonium cations is eliminated through an S N 2decomposition pathway,leading to dissolution of Se/Te through the formation of the corresponding trialkylphosphane selenides/tellurides ( Figure 4). However,t he decomposition mechanism of the phosphonium IL in the presence of Te is much more complicated than that in the presence of Se. The 1 J PTe coupling,which indicates aP ÀTe bond is formed, is only observed in the NMR spectra when as ufficient amount of Te (e.g. Te /IL = 1:1) is present (Figure 4d). Theu se of smaller amounts of Te results in the 125 Te satellites in the 31 PNMR spectra disappearing and the doublets in the 125 Te NMR spectra collapsing to one broad doublet (Figure 4c)orasingle line (Figure 4b). In addition, the existence of ap arallel, competitive IL decomposition route to the S N 2 Scheme 1. Reactionso fthe imidazolium acetate ionic liquid with chalcogens. [93] .

Angewandte Chemie
Reviews reaction is regarded as the side reaction for the dissolution of Te.T his may at least partially explain the relatively lower solubility of Te compared to Se in phosphonium-based ILs.
These preformed trialkylphosphane selenides/tellurides can serve as Se/Ter eservoirs for the preparation of nanostructured metal selenides/tellurides,such as octahedral NiSe 2 particles,Z nSe nanocrystal aggregates,o r3 Di ntergrown Bi 2 Te 3 crystals (Figure 5a,b,e,f). Additionally,T es ingle crystals and various Te microstructures,including 3D hierarchical fern-leaf-like Te structures,3 DT ef usiform assemblies,a nd 3D aloe-like Te microarchitectures,a re obtained when using areactive Te solution in dried commercial [P 66614 ]Cl as the Te precursor (Figure 5c,d,g-i). [111,112] These IL-based synthetic methods provide convenient and efficient strategies for the preparation of Se/Te-based materials compared to the conventional complicated solution methods which typically need areductant (e.g. NaBH 4 )inthe presence of asurfactant (e.g. polyvinylpyrrolidone or cetyltrimethylammonium bromide).
Some examples also document the direct synthesis of chalcogenides from chalcogen-containing ILs.T his case is similar to the use of metal-containing ILs as metal sources,as mentioned above,since both the chalcogen and metal species can be incorporated in the IL anion or cation. Wu and coworkers reported that the thiocyanate IL [BMIm][SCN] can serve as both the solvent and sulfur source for the preparation of CdS nanocomposites. [113] Zheng and co-workers designed the Se-containing IL [BMIm][SeO 2 (OCH 3 )] as an ovel Se source. [114] TheI La nion ([SeO 2 (OCH 3 )] À ion) shows similar reactivity as the commonly used Na 2 SeO 3 system. However, precipitates form in some systems from the reaction of Na 2 SeO 3 with metal ions.T he use of the [SeO 2 (OCH 3 )] À anion avoids this precipitation problem, because of its weaker polarizing capability,a nd the metal ions exist as free ions in the solutions.Moreover,particle growth is influenced by the adsorption of the [BMIm] + cation on the formed crystal surfaces,leading to nanoparticles with diverse shapes. As ac onsequence of these distinct characteristics of this Secontaining IL precursor,various metal selenides with special morphologies,i ncluding CuSe nanoflakes, [115] Cu 2Àx Se nanocrystals, [115] ZnSe hollow nanospheres, [116] and CdSe nanospheres and nanodendrites, [117] have been successfully prepared. Selenium can also be incorporated into the cation of the ILs.J aniak and co-workers synthesized several selenoether-functionalized ILs with the [NTf 2 ] À anion, and these were used as both the reaction media and Se reagents for the preparation of ZnSe nanoparticles under irradiation with microwaves at 220 or 250 8 8C. [118] It is assumed that the proximity of the in situ generated carbene precursor complex to the Zn 2+ ions leads to an interaction and the formation of an intramolecular coordinative ZnÀSe bond. Thed ecomposition of these NHC(Se)-Zn complexes under microwave heating yields the corresponding ZnSe nanoparticles.F urthermore,the same group reported the synthesis of avariety of metal selenide (e.g. CdSe,P bSe,a nd Pd 17 Se 15 )n anoparticles by decomposing the corresponding metal-Se-based molecular complexes in ILs. [119][120][121]

Reactions of Ionic Liquids Whose Cations, Anions, or Both Are Incorporated into the Final Products
ILs allow the creation of several types of open-framework materials such as zeolites or metal-organic frameworks (MOFs) as well as polyanionic/polycationic compounds.I n many reactions,t he IL cation or anion, as the counterion, is incorporated into the final structures as aresult of the charge of the framework. In this section, reactions of ILs whose cations,a nions,o rb oth are incorporated into the final products are summarized.  77 Se NMR spectrum of the reaction solution with amolar ratio of Se/[P 66614 ][decanoate] = 1:4at220 8 8Cu nder Ar.Reproduced with permission. [110] Copyright 2017, Royal Society of Chemistry. 125 Te NMR spectra of Te solutions with amolar ratio of Te/[P 66614 ] [decanoate] = 1:7.6 (b), 1:2(c), and 1:1(d) at 220 8 8Cunder Ar. Reproducedw ith permission. [111] Copyright2 018, Wiley-VCH. Figure 5. SEM and TEM images of Se/Te-based nano-/microparticles obtained in phosphoniumILs. a,b) NiSe 2 and ZnSe nanoparticle aggregates. Reproduced with permission. [110] Copyright 2017, Royal Society of Chemistry.c)Leaf-like Te microstructure. d) Te single crystal. e) Bi 2 Te 3 nanoplate.f)Flower-like Bi 2 Te 3 particle. Reproducedwith permission. [111] Copyright 2018, Wiley-VCH.g -i)3Dcomplex Te microstructures. Reproducedwith permission. [100] Copyright 2020, Royal Society of Chemistry.

Metal Halide Compounds
As new types of halide sources,h alide-based ILs exhibit distinctive features compared to the commonly used halide sources.Z heng and co-workers reported that various BiOCl nanostructures,s uch as ultrathin BiOCl nanoflakes,c urved nanoplates,a nd nanoplate arrays,c ould be successfully prepared using [C 16 MIm]Cl as the solvent, template,a nd chloride source. [122] The[ C 16 MIm] + cation with its long alkyl chain tends to adsorb on the (001) plane of BiOCl, and crystal growth along the c-axis direction is inhibited, which leads to the formation of thin BiOCl nanoflakes.The obtained BiOCl nanoplates show potential applications for the removal of heavy metal ions in the field of wastewater treatment.
Ruck and co-workers investigated the ionothermal synthesis of several borate halide compounds using quaternary phosphonium halide ILs as both the solvent and halide source.T he Li-ion-conductive polycrystalline Li 3 ]t riangles,f orms interpenetrating channels,w ithin which Li + and Cl À are trapped (Figure 6a).
Similarly,Li 4 B 7 O 12 Br can also be synthesized using [P 66614 ]Br as the bromide source.T hree nanostructured borate halides of the A 2 B 5 O 9 Xt ype (A = Sr, Ba, X = Cl, Br)-Sr 2 B 5 O 9 Cl nanorods,S r 2 B 5 O 9 Br nanoneedles,a nd Ba 2 B 5 O 9 Cl nanosheets-were synthesized by the reaction of acetates A-(OAc) 2 and boric acid B(OH) 3 in am ixture of [P 66614 ]X and LiNTf 2 . [124] As shown in Figure 6b,t he [BO 4 ]t etrahedra and [BO 3 ]t riangles form the anionic [B 5 O 9 ] 3À framework with large channels.H alide anions,f rom the phosphonium halide ILs,a nd metal cations are trapped in these channels.I nvestigations further show that the reactivity of [P 66614 ]X is promoted by adding the metal salt LiNTf 2 to the reaction system, as it weakens the cation-anion interactions of the ILs. However,m icrocrystalline Pb 2 B 5 O 9 X( X= Cl, Br), with an average diameter of 1 mm, was obtained in [P 66614 ]X without adding LiNTf 2 . [125] Both Pb 2+ and X À are incorporated into the channels of Pb 2 B 5 O 9 Xcompounds.These borate halides show efficient second harmonic generation, even as microcrystalline powders.

Zeolites
Zeolites are af amily of porous materials that are widely applied in adsorption and catalysis. [126] As an example,t he industrially significant aluminosilicate zeolite frameworks are comprised of corner-sharing [SiO 4/2 ]and [AlO 4/2 ] À tetrahedra linked through bridging oxygen atoms.T hus,t he overall framework bears an egative charge caused by the negatively charged [AlO 4/2 ] À units.Awide range of cations,such as Na + , K + ,C a 2+ ,M g 2+ ,a nd others,c an be accommodated in the zeolite cavities as counterions to balance the anionic charge. If az eolite is prepared in an IL, the organic cations are incorporated into the zeolite channels to balance the charge and also act as structural templates. [34] Thefirst IL-based synthesis of zeolites in ILs was reported by Morris and co-workers in 2004. [6] Several aluminophosphates (e.g.S IZ-1, SIZ-3, SIZ-4, and SIZ-5, Figure 7a)a re produced in [EMIm]Br.I nS IZ-1 (Figure 7a), the hexagonal prismatic units are joined to form layers,and the neighboring layers are linked by four tetrahedral centers into a3 D framework. Thef ramework is negatively charged due to the presence of terminal P À Ob onds.

Angewandte Chemie
Reviews present in the pores,b alancing the negative framework charges as well as templating the formation of the zeolite structure.
Since then, various framework types have been successfully obtained for aluminophosphates,i ncluding AEL, [127][128][129] LTA, [130,131] CHA, [132] and layered structures, [133] by the ionothermal synthetic route.I nt hese reactions,t he organic cations of the IL have been demonstrated to be effective templates that often reside in the cavities of the obtained zeolites to compensate for the negative charges of the frameworks.V ery recently,L in and co-workers designed the multifunctional IL [MIm][H 2 PO 4 ](MIm = N-methylimidazolium) for the ionothermal synthesis of crystalline metal phosphates (metal = Be,A l, Zn, and Fe). [134,135] Interestingly, [MIm][H 2 PO 4 ]p rovides the phosphorus source to build the framework unit without phosphoric acid, as well as being at emplate and solvent. Theo btained aluminum phosphate has a2 Ds tructure with 8-membered-ring windows and the beryllium phosphate contains extra-large 24-membered-ring channels.T he [MIm] + cations are located within the large channels in the case of beryllium phosphate or within the interlayer region in the case of aluminum phosphate.
Despite much success in the ionothermal synthesis of aluminophosphates,the application of ILs in the preparation of silica-based zeolites still faces am ajor obstacle because silica is poorly soluble in ILs.Morris and co-workers designed at ask-specific IL [BMIm]OH 0.65 Br 0.35 for the first ionothermal synthesis of siliceous zeolites. [136] Theh ydroxide component in the IL anion leads to ab etter dissolution of silica, while the cation templates the formation of the MFI framework. Fluoride is added to promote the dissolution of silicate precursors and the crystallization of the zeolite.T he fluorine atoms are present in the final product as part of SiO 4 Fwithin the pentasil units.T he [BMIm] + cations are incorporated inside the pores to balance the negative charge of the fluoride.
TheI L-templating effects were also used by Dai and coworkers to synthesize porous transition-metal oxides. [30,31] Unlike the incorporation of the IL cation or anion into the zeolite framework, the IL template (e.g. [BMIm][NTf 2 ]) used for the fabrication of porous transition-metal oxides can be easily extracted and removed by organic solvents.B ased on this [BMIm][NTf 2 ]t emplating method, well-defined nanoporous TiNb 2 O 7 and mesoporous MnCeO x have been synthesized that exhibit superior performance for fast-rechargeable lithium-ion batteries and high activity for the selective oxidation of hydrocarbons at low temperature (100-120 8 8C), respectively.

Metal-Organic Frameworks (MOFs)
MOFs,asagroup of porous crystalline materials consisting of metal ions (clusters) coordinated to organic ligands, have received much research interest in many applications (e.g. gas storage,g as separation, catalysis,d rug delivery,a nd sensing) because of their diverse structures,high porosity,and controllable chemical structures. [137] In an ionothermal synthesis of MOFs,t he IL cation, anion, or both may be incorporated in the open cavities of MOFs.
Theanions of an IL can also be incorporated in the voids of the MOF as charge-compensating species.H uang and coworkers reported that three Cd 3 F-based compounds with cationic frameworks,n amely [Cd 3 F(ina) 4 4 ]. [149] The[ BF 4 ] À anion serves as ac harge-balancing unit located in the voids of the frameworks.H owever,t he F À ions formed by in situ hydrolysis of the [BF 4 ] À anion are trapped within the MOF framework through the formation of a[ Cd 3 F] 5+ unit. Thus, the [BMIm][BF 4 ]I Ls erves as solvent, fluoride source,a nd structure-directing agent.

(OH)Br] in [PMIm]Br and [BMIm][Zn 2 (btc)(OH)I] in
[BMIm]I by using Zn(NO 3 ) 2 ·6 H 2 Oa nd H 3 btc as starting materials. [151,152] In both cases,t he IL anions (bromide or iodide) form Zn À X(X= Br or I) bonds to become part of the anionic frameworks.T he imidazolium cations are incorporated in the channels and appear to show strong interactions with the frameworks.

Polyanionic/Polycationic Compounds
Polyanions and polycations constitute an interesting class of compounds as ac onsequence of their diverse structures and chemical bonding.T he ionothermal approach is promis-ing for the formation of new polyanionic and polycationic compounds with unique structures that are not accessible using well-established hydro-/solvothermal techniques. Recently,t he application of room-temperature ILs for the preparation of polycationic or polyanionic cluster compounds was extensively explored by the groups of Ruck, Dehnen, Kanatzidis,F eldmann, and Riedel. [13,36,[153][154][155][156][157] To date,s everal types of polyanionic compounds,such as polyhalides, [158][159][160] metal carbonyl cluster anions, [161][162][163] and anionic chalcogenide frameworks, [36,164,165] have been synthesized in ILs.F or example,polybromides were usually limited to am aximum of 10 atoms because of their increasing vapor pressure and reactivity before the new IL-based approaches were developed. [13] Theuse of ILs has resulted in several new polybromides.F eldmann and co-workers reported the first 3D bromine-rich polybromide network, namely [ ( Figure 8a). [158] [DMPyr]Br acts as a" bromide donor" to bromine molecules.
[ BMPyr]-[OTf] is used as a" liquifier" to form ae utectic mixture with [DMPyr]Br,thereby establishing aliquid state of the mixture at or even below room temperature for convenient isolation of the solid product. Later,M aschmeyer and co-workers discovered that the higher-order polybromide [ ( Figure 8b). [159] Investigations show that the IL cation is the dominant factor in the product-selective synthesis.The large number of close H···Br interactions between the butyl chains of [P 4444 ] + and the [Br 24 ] 2À are assumed to stabilize and direct the formation of this high-nuclearity species.I n[ P 4444 ] 2 [Br 24 ], the "central" bromine atom is fivecoordinate,w hereas that of [BMPyr] 2 [Br 20 ]i ss ix-coordinate ( Figure 8). This arises primarily from the required space for an octahedral building block being occupied by abutyl chain of [P 4444 ] + ,w hich prevents coordination of as ixth dibromine molecule.
In another case reported by Dehnen and co-workers,two polyanionic compounds [BMMIm] 24 4 ], respectively) in the presence of DMMP (DMMP = 2,6-dimethylmorpho-line). [164] These two compounds contain the largest known discrete polyanion [Sn 36Àx Ge 24+x Se 132 ] 24À (x = 0or3.5) with an outer diameter of 2.83 nm and an inner diameter of 1.16 nm. TheILcations [BMMIm] + and [BMIm] + serve as counterions that surround the anions and partially penetrate them. This research further shows that the addition of asmall amount of an amine promotes phase formation and phase selectivity of the products.
Polycationic compounds are commonly carried out in Lewis-acidic ILs,w hich usually combine alkylimidazolium halides with more than equimolar amounts of aluminum or gallium trihalides (MX 3 ). [13] This leads to the high solubility of metals (e.g.S e, Te,S b, and In) and their metal halides in highly polar Lewis-acidic systems.M oreover,t he self-drying ILs protect the formed polycations from hydrolysis.T he structures of the polycationic compounds can be tuned by using different Lewis acids.

Reactions of Deep Eutectic Solvents
DESs exhibit some similar physical and chemical properties as ILs.T hus,t hey also lead to significant successes in inorganic synthesis,e specially in large-scale applications because of their inexpensive constituents and easy preparation. This DES-based synthetic strategy opens up many new opportunities for the synthesis of zeolites and other inorganic nanomaterials.Inmany cases,however,one or more components can decompose in the DESs upon moderate heating. This section summarizes some examples of the preparation of inorganic materials by utilizing the unstable properties of DESs.Anoverview is given in Table 3.
To date,t he 1:2m ixture of choline chloride and urea (ChCl/urea, melting point 12 8 8C) is the most widely investigated DES in the literature.Investigations have shown that heating at 125-225 8 8Cu sually leads to decomposition of the urea in ChCl/urea, thereby resulting in the reactivity of ChCl/ urea at high temperatures. [171] Thus,ChCl/urea is usually used as areactive reagent for the preparation of various inorganic compounds with unusual structures.I n2 004, Morris and coworkers first reported that an ovel zeolite-type framework (SIZ-2, Al 2 (PO 4 ) 3 ·3 NH 4 )w as produced in aC hCl/urea eutectic mixture (Figure 7b). [6] Thea mmonia stemming from the partial decomposition of the urea templates the structure and balances the charge of the framework that forms the interrupted structure of SIZ-2. Similarly,anew zinc organophosphate was synthesized in aC hCl/urea mixture by Liao et al. and, again, the ammonia acts as at emplate. [172] Later, Morris and co-workers studied several eutectic mixtures based on quaternary ammonium halides (e.g. choline chloride and tetraethylammonium bromide) and urea derivatives (e.g.1 ,3-dimethylurea, ethylene urea, and N,N'-trimethyleneurea) as the reaction media for the ionothermal synthesis of new zeolites. [173] As expected, the breakdown of the various urea derivatives of the DESs at high temperatures gives rise to the corresponding organic species (e.g.m ethylammonium, ethylene diammonium, and propylene diammonium), which serve as templates and enable controlled delivery to the reaction mixture.N ine aluminophosphate materials including five unknown compounds have been prepared in this way.
Metal phosphate MPO 4 (M = Ga, Zr, Co,F e, and Mn) frameworks can also be successfully synthesized using various DESs. [174][175][176] Theorganic template is delivered to the reaction mixture by decomposition of one or more components of the DES mixture.S ome new metal phosphate frameworks have been produced using the unique and flexible properties of DESs.
Clearly,i nt hese ChCl/urea-based reaction systems,i ti s the urea portion that provides the better template.I nf act, choline itself can be av ery attractive template.T oa void the  [187] competition between these two ammonium cations as templates,M orris and co-workers synthesized as eries of DESs based on choline chloride/carboxylic acid for the ionothermal synthesis of cobalt aluminophosphate (CoAlPO) materials, [177] including an unusual layered zeolite material ([Al 3 CoClP 4 O 16 ][C 5 H 13 NOH] 2 ,S IZ-13). Thec holine cations fill the interlayer space without significant chemical modification. However,the chloride ions from choline chloride are incorporated into the structure of SIZ-13 through the formation of covalent Co À Cl bonds.S uch metal-chlorine bonds have not been found in the hydrothermal synthesis because of their sensitivity to hydrolysis,b ut the water in DESs tends to be less reactive because of strong interactions with anions of the DESs. [178,179] Similarly,H arrison reported that the compound C 5 H 14 NO·ZnCl(HPO 3 ), which contains covalent Zn À Cl bonds,w as obtained by reacting choline chloride with Zn 2+ and hydrogen phosphite precursors in ChCl/urea. [180] DESs are also applied as reactive reagents for the preparation of various functional materials such as metal hydroxides,m etal oxides,m etal chalcogenides,a nd organicinorganic hybrids. [39] Gu and co-workers synthesized various nanostructured transition-metal complexes and layered transition-metal hydroxides,a sw ell as their derivatives in ChCl/ urea through an ionothermal strategy at ar elatively high temperature (120-210 8 8C). Octahedral [Ni(NH 3 ) 6 ]Cl 2 crystals with an open structure can be obtained when the Ni 2+ :ChCl/ urea solution is heated in as ealed vessel (Figure 10 a). [181] Nanosheet-like NiCl 2 is produced by annealing the [Ni-(NH 3 ) 6 ]Cl 2 precursor.W hen the Ni 2+ :ChCl/urea solution is thermally treated under an open system, however,t he ammonia released from the urea is removed from the reaction solution, which leads to the formation of the flower-like a-Ni(OH) 2 when as mall amount of water is added to the solution under heating (Figure 10 b). NiO with the same flower-like morphology is synthesized through annealing the as-obtained a-Ni(OH) 2 (Figure 10 c). Similarly, a-Co(OH) 2 and Co 3 O 4 can also be accessed by this water injection method. [182] In the case of Fe,however,itisthe Fe-oxide phase Fe 2 O 3 that is directly obtained from aF e 3+ :ChCl/urea solution. [183] If aM n 2+ :ChCl/urea solution is heated in aclosed system, acalcite-type MnCO 3 phase is produced. [171] Moreover,a"two-stage water injection" strategy is applied to synthesize the cobalt iron layered double hydroxide (CoFe LDH) with different interlayer spacings. [184] Investigations have shown that the volume of injected water at each stage plays an important role in determining the structure of the CoFeLDH. When asmall amount of water is injected in the first stage,the ChCl-DES maintains its superstructure.Inthis case,C oFeL DH with an expanded interlayer spacing of 11.3 in its (003) plane is obtained. Further calcination of the CoFeL DHs results in porous CoFeo xide nanosheets with alarge specific surface area of 79.5 m 2 g À1 . [185] In addition to the well-studied ChCl/urea-based DES, other types of DESs may also be used as reactive reagents for the formation of inorganic materials.Afamily of binary metal sulfides,s uch as Bi 2 S 3 ,S b 2 S 3 ,C uS,Z nS,P bS,A g 2 S, and CdS, have been successfully synthesized by Ruck and co-workers by using aD ES based on ChCl/thioacetamide (TAA;F igure 10 d-j). [186] Thep roposed reaction mechanism consists of:1)ametal salt is dissolved or dispersed in the ChCl/TAAbased DES and the corresponding metal-DES is formed; 2) the final sulfide is formed by thermal decomposition of the metal-DES complex. This ChCl/TAA-based DES serves as both the solvent and sulfur source,providing an ideal "all-inone" reaction medium for the efficient synthesis of sulfide nanoparticles.
Li and co-workers found that aF e-based organic-inorganic hybrid-a hierarchical 3D iron alkoxide of glycerolwas synthesized using Fe(NO 3 ) 3 ·9 H 2 Oa sastarting material in aC hCl/glycerol (1:2) DES (Figure 10 k). [187] TheC hCl/ glycerol DES serves not only as abenign reaction medium but also as ar eactant. Fe 3+ is chelated by glycerol and is integrated into the DES matrix through coordinative bonding and hydrogen bonding.T hus,t he reactants are effectively brought together by the DES through aprestructuring effect, which results in the formation of the 3D hierarchical seedlike iron alkoxide of glycerol nanospheres.The as-synthesized Febased organic-inorganic hybrid exhibits an enhanced oxygen evolution reaction performance,w ith al ow overpotential of 280 mV at ac urrent density of 10 mA cm À2 .

Other Types of Reactions of Ionic Liquids
In some IL-based reactions,t he IL itself may undergo complete decomposition. As mentioned in Section 5, phosphonium ILs are partly decomposed into trialkylphosphanes, and these intermediates then react with Se/Tet of orm the corresponding trialkylphosphane selenides/tellurides.I nterestingly,phosphonium ILs can be further used as aphosphorus source for the synthesis of metal phosphides through com- Figure 10. a) SEM image of the Ni[NH 3 ] 6 Cl 2 octahedronw ith exfoliated facets. b,c) TEM image and corresponding SEAD patterns of a-Ni-(OH) 2 and NiO. Reproducedwith permission. [39,181] Copyright 2013 and 2017, Royal Society of Chemistry.d -j) SEM images of ZnS, CuS, Bi 2 S 3 ,Sb 2 S 3 ,PbS, Ag 2 S, and CdS, respectively, which are obtained from ChCl/TAA-based DESs. Reproducedwith permission. [186] Copyright 2017, Wiley-VCH.k)SEM image of the as-synthesized iron glycerate hybrid in ChCl/glycerol based DES. Reproducedw ith permission. [187] Copyright 2019, AmericanC hemical Society. plete decomposition of the quaternary phosphonium cations at higher temperatures (> 350 8 8C). Li and co-workers reported that nanostructured Ni 2 Pa nd Ni 12 P 5 nanoparticles were fabricated using [P 4444 ]Cl as both the phosphorus source and reaction medium upon microwave heating at 350 8 8Cf or 1-2 minutes. [188] Theas-synthesized Ni 2 Pnanocrystals show an enhanced electrocatalytic hydrogen evolution performance in an acidic medium. Moreover,p hosphonium ILs can be designed to be metal-containing ILs and can thus be utilized as both the metal and phosphorus source for the preparation of metal phosphides.For example,[P 66614 ] 2 [CoCl 4 ]was used to synthesize Co 2 Pb yaone-step phosphidation at 400 8 8C without adding other reagents. [189,190] Theo btained Co 2 P/ carbon nanotube (CNT) composite shows high activity for the hydrogen evolution reaction. This strategy based on phosphonium ILs provides ar emarkable advantage for the efficient synthesis of metal phosphides.Ruck and co-workers reported the synthesis of copper-deficient Cu 3Àx P( 0.1 < x < 0.7) from elemental precursors in halide ILs (e.g. [P 66614 ]Cl). [42,191] Investigations have shown that the halide anions drastically promote the reactivity of red or white phosphorus and kinetically suppress the formation of Cu 2 P by-products.B ased on mechanistic studies,i tw as found that chloride ions act as strong nucleophiles that attack the phosphorus network, thereby resulting in degradation of the phosphorus.Atahigh concentration of chloride ions,the PÀP bonds are sufficiently activated, leading to ad rastic increase in the formation of the Cu 3Àx Pphase.
Recently,t here has been al ot of interest in using ILs (including DESs) as versatile carbon precursors,r ather than conventional polymers,for the preparation of carbon materials because of the unique properties of ILs,such as negligible vapor pressure,carbon-rich nature,and structural diversity.In the carbonization processes,ILs are completely decomposed, and the corresponding ILs are converted into carbon residues. They ield, properties,a nd structure of the obtained carbons depend on the structure of the IL precursors.T he relevant studies have been comprehensively reviewed elsewhere, [5,39,192,193] and will not be the focus of this Review.

Summary and Outlook
Great developments have been made in inorganic syntheses using, or in the presence of,I Ls,w hich has led to the formation of diverse compounds with interesting properties. However,the chemical reactivity of the ILs and DESs in the reactions is usually neglected or not explicitly discussed. This Review gives an overview of the chemical reactions of ILs or DESs in inorganic synthesis.
Metal-containing ILs represent apromising class of metal sources.T he IL properties can be tailored by varying the ligand structures and the incorporated metal ions,w hich enables the formation of inorganic nanoparticles with diverse sizes and morphologies.B asic ILs are usually employed to replace the traditional bases,s uch as NaOH, KOH, or Na 2 CO 3 ,t op rovide the required basic environment for the production of metal (hydr)oxide particles.M oreover,t he organic cations of the basic ILs can act as at emplate to control the crystal nucleation and growth. Theh ydrolysis of [BF 4 ] À or [PF 6 ] À anions gives af luorine source for the synthesis of metal fluorides,thereby avoiding the use of toxic and harmful HF,N aF,o rN H 4 F. TheC 2-position of the 1,3dialkylimidazolium cation shows reactivity towards chalcogens (e.g.S ,Se, and Te )for the formation of the corresponding imidazole-2-chalcogenones,e specially in the presence of ab ase.I na ddition, phosphonium ILs can react with chalcogens at high temperatures to generate the corresponding trialkylphosphane chalcogenides.T hese formed imidazole-2-chalcogenones or trialkylphosphane chalcogenides can be further applied as chalcogen sources for the preparation of metal chalcogenides.W hens ome porous materials such as zeolites,MOFs,and polycationic/polyanionic compounds are synthesized by an ionothermal approach, either the cation or anion of the IL can be incorporated in the channels of the products to balance the charged frameworks and can also serve as structural templates.Inthe case of DESs,one or more of their components may decompose to form in situ templates for the formation of metal phosphate frameworks or act as reactive species for the synthesis of functional nanoparticles (e.g. metal hydroxides,metal oxides,metal chalcogenides,and organic-inorganic hybrids).
Thec hemical reactivity of ILs and DESs in inorganic synthesis should not be underestimated. Furthermore,t he reactive properties of the ILs and DESs in the reactions can be used as asynthetic tool to prepare inorganic materials that are difficult or even impossible to obtain by traditional synthetic routes.I nt his regard, it is important to have an indepth understanding of the interaction of ILs or DESs with reactants and solutes,a nd thus fully understand the reaction mechanism for the directed use of ILs and DESs for the preparation of inorganic materials.