Angewandte Reviews Chemie How to cite: Ionic Liquids International Edition: doi.org/10.1002/anie.202104035 German Edition: doi.org/10.1002/ange.202104035 Inorganic Synthesis Based on Reactions of Ionic Liquids and Deep Eutectic Solvents Tao Zhang, Thomas Doert, Hui Wang, Suojiang Zhang,* and Michael Ruck* Keywords: chemical reactivity · deep eutectic solvents · inorganic materials · ionic liquids · ionothermal synthesis Angewandte Chemie &&&& 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60,2–20 Ü Ü These are not the final page numbers! Angewandte Reviews Chemie Ionic liquids and deep eutectic solvents are of growing interest as From the Contents solvents for the resource-efficient synthesis of inorganic materials. This Review covers chemical reactions of various deep eutectic solvents and 1. Introduction 3 À types of ionic liquids, including metal-containing ionic liquids, [BF4] - 2. Reactions of Metal-Containing À or [PF6] -based ionic liquids, basic ionic liquids, and chalcogen- Ionic Liquids 5 containing ionic liquids. Cases in which cations, anions, or both are À À incorporated into the final products are also included. The purpose of 3. Reactions of [BF4] - or [PF6] - this Review is to raise caution about the chemical reactivity of ionic Based Ionic Liquids 7 liquids and deep eutectic solvents and to establish a guide for their 4. Reactions of Basic Ionic Liquids 8 proper use. 5. Reactions of Chalcogen- Containing Ionic Liquids 1. Introduction (Including Reactions of Ionic Liquids with Chalcogens) 9 Ionic liquids (ILs), first reported by Paul Walden in 1914,[1] are defined as molten salts with melting points below 6. Reactions of Ionic Liquids 1008C. Nowadays, ILs are widely applied in a broad variety of Whose Cations, Anions, or Both fields, including catalysis, separations, synthesis, and many Are Incorporated into the Final others.[2–5] Compared to the broad applications of ILs in Products 10 organic chemistry, which have already been explored for about 40 years, inorganic syntheses in ILs, especially the so- 7. Reactions of Deep Eutectic called ionothermal syntheses,[6–8] have been actively inves- Solvents 14 tigated only since the early 2000s. Since then, various inorganic compounds (e.g. metals and non-metals, metal 8. Other Types of Reactions of oxides and chalcogenides, metalates and framework com- Ionic Liquids 15 pounds) have been prepared using, or in the presence of, ILs.[9–14] 9. Summary and Outlook 16 ILs provide several unique properties for the preparation of inorganic materials. For example, ILs can facilitate the dissolution of versatile precursors, including both inorganic and organic compounds, which is fundamental for the syn- as the more accessible DESs are increasingly being used in the thesis of most materials.[15,16] ILs create a special microphasic synthesis of inorganic materials. separation of the hydrophilic and hydrophobic fragments, [17] typically with imidazolium ILs with their long alkyl chains. [*] Prof. T. Zhang, Prof. H. Wang, Prof. S. J. Zhang This heterogeneity of ILs provides the ability to control Beijing Key Laboratory of Ionic Liquids Clean Process nucleation and growth rates, particle sizes, and morphologies CAS Key Laboratory of Green Process and Engineering in materials synthesis.[17] Furthermore, some other character- Institute of Process Engineering istics, such as good thermal stability in ionothermal synthesis, Chinese Academy of Sciences high polarizability for microwave synthesis, or wide electro- 100190 Beijing (China) E-mail: [email protected] chemical windows and high conductivity for electrodeposi- tion, make ILs an attractive alternative to conventional Prof. T. Zhang, Prof. H. Wang, Prof. S. J. Zhang Innovation Academy for Green Manufacture organic solvents for the synthesis of inorganic materials as Chinese Academy of Sciences well as to high-temperature reactions in melts or the solid Beijing 100190 (China) state. Prof. T. Doert, Prof. M. Ruck In 2003, the concept “deep eutectic solvent” was first Faculty of Chemistry and Food Chemistry coined by Abbott et al.[18] Deep eutectic solvents (DESs) are Technische Universitt Dresden acknowledged as a new class of IL-analogue solvents and 01062 Dresden (Germany) share many characteristics of traditional ILs, such as low E-mail: [email protected] vapor pressure, high polarity, and tunable chemical proper- Prof. M. Ruck ties. However, DESs are easier to access synthetically. In most Max Planck Institute for Chemical Physics of Solids 01187 Dresden (Germany) cases, a DES is obtained by mixing a quaternary ammonium or phosphonium salt with a hydrogen-bond donor (HBD), The ORCID identification numbers for the authors of this article can be found under: https://doi.org/10.1002/anie.202104035. thereby generating a new liquid phase with a melting point 2021 The Authors. Angewandte Chemie International Edition below that of either individual component.[19,20] No purifica- published by Wiley-VCH GmbH. This is an open access article under tion is usually needed. Furthermore, most DESs are quite the terms of the Creative Commons Attribution License, which inexpensive because of the low cost of their constituents, such permits use, distribution and reproduction in any medium, provided as urea and choline chloride. Therefore, IL analogues as well the original work is properly cited. Angew. Chem. Int. Ed. 2021, 60, 2 – 20 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org &&&& These are not the final page numbers! Ü Ü Angewandte Reviews Chemie The thermal and chemical stabilities of ILs are usually new strategies for designing sustainable ILs for advanced highlighted as advantageous for inorganic synthesis. Previous inorganic synthesis. investigations have indicated that the actual degradation However, many ILs contain reactive moieties in either the temperature of ILs is overestimated by the onset decom- cation or anion. Thus, the ILs themselves can take part in position temperature (Tonset) derived from the ramped tem- reactions. For example, an IL can be tailored for a specific perature in thermogravimetric analysis.[21–23] Therefore, the task, such as to release one component of the desired product concept of long-term thermal stability is utilized to obtain upon its decomposition. Thus, the IL acts as solvent, template, more accurate information on the decomposition of ILs at and reactant, thereby simplifying the reaction system signifi- high temperature. The thermal stability of ILs, including the cantly. In other circumstances, the IL cation and anion can characterization methods, mechanism of decomposition, and separate during the reactions, thereby leading to incorpora- kinetics of thermal degradation, has been extensively tion of the IL cation or anion in the final products. Such explored by several research groups,[24–29] and is not within reactions are often used in the synthesis of some framework the scope of this Review. compounds (e.g. zeolites, MOFs, and polycationic/polyanion- ILs are readily accessible as inert reaction media for ic compounds).[13,33,34] The IL cation or anion serves as inorganic synthesis. Dai and co-workers demonstrated that a counterion to balance the charge of the framework as well À several imidazolium ILs containing [NTf2] anions could be as a template. IL decomposition or cation/anion separation used as the flux medium for the direct recycling of spent during the ionothermal synthesis may cause a change in the cathode materials or as effective structure-directing templates properties of the IL (e.g. viscosity, conductivity, and dissolving for the synthesis of advanced catalysts and anode materials capacity) and further influence the formation of the target because of their good thermal and chemical stability.[30–32] In product.[35] Thus, the reaction mechanism of ILs should be À all cases, the [NTf2] -containing imidazolium ILs can be considered. Furthermore, the ionothermal approach, in readily reused and recycled after the reaction, thus providing particular, which exploits the chemical reactivity of ILs or Tao Zhang obtained his PhD in inorganic Suojiang Zhang received his PhD from chemistry from the Technische Universitt Zhejiang University in 1994 with Prof. Dresden in 2018 under the supervision of Shijun Han. He has been a professor at the Prof. Michael Ruck. He then joined to the Institute of Process Engineering of the Chi- Institute of Process Engineering, Chinese nese Academy of Sciences since 2001, and Academy of Sciences in 2019 as an Associ- was elected as a member of the Chinese ate Professor. His research interests are con- Academy of Sciences in 2015. He is cur- cerned with the synthesis of functional mate- rently Director General of the Institute of rials in ionic liquids. Process Engineering and Fellow of the Royal Society of Chemistry. His research interests focus on the designed synthesis of ionic liquids and their applications in green chemistry and green process engineering. Thomas Doert received his PhD from the Michael Ruck received his PhD from the University of Dsseldorf in 1994 with Prof. University of Stuttgart in 1991 with Prof. Peter Bçttcher. He then moved to the Arndt Simon. After his habilitation at the Technische Universitt Dresden, where he is University of Karlsruhe in 1997, he was now Adjunct Professor. He was visiting scien- a Heisenberg fellow of the German Science tist at Stockholm University as well as Guest Foundation. Since 2000 he has been Full Professor at Strasbourg University and Professor of Inorganic Chemistry at the Shanghai University of Engineering Science. Technische Universitt Dresden, where he His research interests cover crystallographic has served as Dean of the Faculty of Science aspects of solid-state chemistry, solid-state and Vice Rector. He has repeatedly been chemistry of chalcogenides, layered materi- appointed a Fellow of the Max Planck als, and frustrated magnets. Society and awarded the Steinhofer Research Award and the Will Kleber Com- memorative Coin. His research interests include solid-state chemistry and Hui Wang is a professor at the Institute of sustainable material syntheses.