**1.2 The choice of boron cations and anions in the formation of ionic liquids**

The physicochemical characteristics of all ILs, not just boron-related ILs, depend on the nature of cations and anions and their combination. Generally, when designing ILs, properties such as melting point, conductivity, viscosity, air and moisture stability, hydrophobicity, miscibility with water, and density must be adjusted and optimized for the application to be utilized. According to Plechkova and Seddonna et al. [5], the cation is responsible for the physical properties of an IL, for example, density, viscosity, and melting point, while the anion is accountable for the chemical properties and reactivity. For example, halides as anions with hydrogen in position 2 (CH ··· X−) lead to higher melting points. For this reason, it can be said anions play a critical role in higher melting and lower melting points, as well as cations [34]. The anion type has a significant effect on the hydrophobicity and hydrophilic tuning tunable of IL. Fluoridation of the anion leads to promoted hydrophobicity and weaker hydrogen

**Figure 1.**

*Modification of BBILs for energy-related applications.*

bonding, and hence lower melting temperatures; it also increases thermal and electrochemical stability. Besides halides, the inorganic anions [BF4] − and [PF6]− are often utilized in designing ILs. The modification of the cation by N-alkylation is critical in adjusting ILs. Increasing cation size and chain length lowers IL melting values while increasing viscosity. Also, the cation's branched alkyl chains induce a greater melting temperature in IL. In this context, melting points appear to be more connected to cation asymmetry [13] because this asymmetry causes packing inefficiency and hence prevents crystallization. Asymmetric cation salts with just C1 symmetry, for example, have lower melting temperatures than identical salts containing C2v symmetry cations [36]. It has been noted that the absence of strong hydrogen bonding correlates to lower IL melting temperatures [36].

The most commonly used cations in the formation of ILs are N,N′-dialkylated imidazolium ions, due to their ease of synthesis and favorable physical and chemical properties. Cations of imidazolium impart low melting points, high conductivity, and low viscosity properties to ILs [34]. Quaternary ammonium is also used as the cation for some EAs. The use of ILs using 1,3-dialkyl imidazolium cations in high-energy electrochemical devices such as lithium-ion batteries is not possible due to the electrochemical instability of the cation. Therefore, ILs containing quaternary ammonium ions are preferred as they are more resistant to reduction and oxidation [37, 38].

Inorganic anions with which the ILs decompose endothermically, while organic anions lead to exothermic thermal decomposition [39]. The thermal stability of halides is substantially lower (300°C). Many of these imidazolium salts display supercooling and are liquid at room temperature. More symmetric cations lead to higher melting points and vice versa. A longer alkyl chain (propyl as compared to ethyl) lowers the melting points and branching of the chain (i-propyl) raises the melting point [39]. Considering the aforementioned features, ILs synthesized by changing the boron anion and cation are potential candidates for the field of energy.

Boron anion and boron cation-based ILs are described in further depth in the next section.

#### *1.2.1 Boron anions based ILs*

Over the past few decades, boron anion families of ILs have been prominent in the field of energy due to their high thermal stability, low flammability, negligible vapor pressure, and wide electrochemical window [8, 40–53]. One of the liquid salts borates is the most commonly used negatively charged boron compound as anions in BBILs [22, 23, 35–37]. **Figure 2** illustrates examples of tetrahedral boron (borate) anions in ILs. The negative charge of ILs produced with tetrahedral boron anions is

**Figure 2.** *Tetrahedral boron anions (adapted from Refs [22, 31, 35]).*

compensated for by positively charged groups such as ammonium. Mixed borates containing [BF4] − as an anion can react with trimethylsilyl ethers of acids to generate IL, as shown in the figure.

The [BF4] − anion is uncoordinated in an aqueous solution; therefore, they are called weakly coordinated anions. A modest negative charge and strong charge delocalization are some desirable features for weakly coordinated anions. Nonnucleophilicity and the minimally base surface of the anion also lead to poor coordination [8, 21, 40].

Anions should not be divided into small moieties in ILs, such as [BF4] − which loses a fluoride when attacked by a nucleophile. Therefore, kinetic and thermodynamic stability are also significant points. Anions must also resist oxidation because the corresponding electrophilic cation often acts as an oxidizing agent [8].

As an alternative to Lewis acid/base pair anions that function in a combination of a Lewis acidic boron component and a Lewis base, derivatives of polyhedral anions such as 1-carba-closo-dodecaborate ([CB11H12] − )are used (see **Figure 3**) [1, 8, 40].

Polyhedral borane clusters and three-dimensional carboranes have attracted attention in recent years with an increasing interest in ILs as materials and building blocks because of their great chemical and thermal stability [35].

Because of their inertness, dipolarity, and high symmetry, carboranes (most known: closo ([C2BnH*n* + 2]), nido ([C2BnH*n* + 4)]), arachno ([C2BnH*n* + 6])) as a new class of weakly coordinating anions can be considered three-dimensional analogs of benzene. Carboranes are promising candidates in ILs as anions due to their size, spherical shape, remarkable chemical stability, and only weakly coordinated B-H groups [41]. Carboranes are electron-delocalized organometallic clusters composed of carbon (C), boron (B), and hydrogen (H).

Carboranes are promising candidates in ILs as anions due to their size, spherical shape, remarkable chemical stability, and only weakly coordinated B-H groups [41]. Carboranes are electron-delocalized organometallic clusters composed of carbon (C), boron (B), and hydrogen (H). The general formula of carboranes is represented by C2BnH*n* + *m*, in which n is an integer; *n* range from 3 to 10. Carboranes are synthesized by adding one-carbon reagents (i.e., cyanide, isocyanides, and formaldehyde) to boron hydride clusters. For example, monocardiodecaborate ([CB11H12] − ) is produced from decaborane and formaldehyde, followed by the addition of borane

#### **Figure 3.**

*Closo carborane (a green circle represents a C–H unit or a C in the cases where a charge is specified. And another corner one B-H unit closo-Carborane. Adapted from Refs [41, 42, 45]).*

dimethylsulfide [42–45]. Monocarboranes are precursors of poorly coordinated anions [41, 42]. The resulting poor coordination is established in a charge distribution over the entire 12-cornered carborane anion. The 10-sided anion [CB9H10] − and its halogenated and methylated derivatives coordinate more strongly than the 12-sided carboranes. Due to their specific properties, these boron molecules are some of the most inert and least nucleophilic anions currently known.

Cyanoborate anions ([B(CN)]<sup>−</sup> ) (see **Figure 4**) have become an important class of building blocks in materials science, especially for ILs that are used as components of electrolytes for electrochemical devices. Also, in recent years, low viscosity room temperature cyanoborate based ILs have been studied in EAs, especially dye-sensitized solar cells (DSSCs) and as a fuel additive for hyperbolic liquids. Cyanoborate chemistry has been popularized as an important topic in the last years, beginning with two independent research on the successful synthesis of the tetracyanoborate anion [B(CN)4] − in 2000 [53–56].

A wealth of [B(CN)4] − with different substituents in addition to CN group (s) bonded to boron, such as hydrogen, halogen, alkyl, and alkoxy, have been developed, which makes possible the tuning of features of compounds. The easily accessible alkali metals are convenient starting materials for the preparation of cyanoborates with various organic, and inorganic compounds.

Furthermore, cyanoborates are promising starting compounds for the synthesis of other boron species, for example, the weakly coordinating tetrakis(trifluoromethyl) borate anion [B(CF3) 4 ] − and the boron-centered nucleophile B(CN)3 2−. Detailed studies on this subject will be described in more detail in the next section.

Moreover, recent studies have shown that orthoborate anion-based ionic liquid combinations possess outstanding electrolyte characteristics and are attractive solvents for lithium-ion battery solvents. **Figure 5** depicts the simplest orthoborate ion, [BO3] 3−. Orthoborate is an anion derived from orthoboric acid (B(OH)3). It is a very weak monobasic that functions solely by hydroxyl-ion acceptance rather than proton donation. Many organic molecules have boron-oxygen bonds (B-O), europium borate (Eu(BO2)3), chromium borate (CrBO3), beta barium borate (β-BaB2O4), gadolinium

**Figure 5.** *Simplest orthoborate anion.*

orthoborate (GdBO3), and polyborate [B3O9]9 − ion as anions are examples of such compounds [54, 55].

#### *1.2.2 Boron cation based ILs*

Studies indicate boron has a significantly higher electropositive charge than most of the donor elements in the ligand (L). After the discovery of cationic boron species, the developed ILs are hard-to-find, highly electrophilic species with high Lewis acidity and reactivity, which are essential in boron chemistry [28, 47]. Boron cation-based ILs, which have high melting points, are used as electrolytes that must be liquid at sub-zero temperatures for electric vehicle applications that must be liquid at ambient temperature battery applications and electrochemical devices due to their relatively low viscosity. Furthermore, the integration of cationic boron centers in organic heterocycles or transition-metal metallocenes is also providing opportunities for the discovery of novel redox-active and optical materials [28].

In recent years, tetracoordinate borocations have had relative stability, which arises from a filled octet and a complete coordination sphere. Because of their larger electronic deficiency and coordinative unsaturation, they are also known to be more reactive than neutral borates [23, 28, 41]. In literature, commonly boron cations are divided into three main groups [28]: Borinium (two-coordinate), borenium (three-coordinate), and boronium (four-coordinate) cation ions (see figure, where: L is a Lewis base; R is substituents, based on the coordination number at boron) [28, 34–47] (see **Figure 6**).

Borinium cations are typically bi-coordinate species bonded by two R that can compensate for the electron deficiency in boron via p donation. Since these species can only have two valence electrons, an additional electron pair must be ensured by an electron donor L (i. e., N or O). That is, at least one L capable of p-bonding must be present to stabilize these cations [46]. In addition, borinium compounds are quite reactive compared to borenium and boronium.

On the other hand, borenium cations are tri-coordinate species containing two bonded R, one L and a third coordination site [28].

The boroniums are the third and most prevalent class of boron cations. The simplicity with which the boronium ions are manufactured and the structural diversity embodied in them provide an essential pragmatic basis for formulating IL. In 2010, Rüther et al. [48] brought out boronium-cation-based RTILs as novel electrolytes for rechargeable lithium batteries. These boronium-containing RTILs exhibited good conductivities and electrochemical windows (4.3–5.8 V). They are stable up to 238

**Figure 6.** *Boron cation species (adapted from Ref. [44].)*

and 335°C, respectively, and have enabled reversible charge-–discharge cycles in batteries with high-capacity retention [48]. Among the boron cations, borinium ions have the most electron deficiencies and are the most chemically unstable species. Thus, while extensive research has been conducted on three-and four-coordinate boron cations, there are only limited examples of borinium ions. Borinium ions generated by electron ionization of borane or borate precursors have been reported to react aggressively and selectively with organic substrates in several cases. However, only a few ionic products are produced, which give valuable structural information on the substrates [46]. Donor ILs in boronium and borinium often serve to extinguish the boron's positive charge. Borenium cations, on the other hand, are better ideal for energy investigations because of the additional stability and electron density offered by donor L. This is evident in the number of studies that describe the production of species or use them as intermediates in various chemical processes [46–48]. Finally, cationic boron compounds are an uncommon but important species in boron chemistry. Emerging research shows that cationic boron compound chemistry is on the verge of a quantum leap in activity; we hope that this review will motivate additional effort in this interesting area of EA.
