Mutual Lewis Acid–Base Interactions of Cations and Anions in Ionic Liquids

Abstract

Solute properties are known to be strongly influenced by solvent molecules due to solvation. This is due to mutual interaction as both the properties of the solute and of the solvent strongly depend on each other. The present paper is based on the idea that ionic liquids are cations solvated by anions and anions solvated by cations. To show this (in this system strongly pronounced) interaction the long time established donor–acceptor concept for solvents and ions in solution by Viktor Gutmann is extended to ionic liquids. A number of solvent parameters, such as the Kamlet–Abboud–Taft and the Dimroth–Reichardt ET scale for ionic liquids neglect this mutual influence, which, however, seems to be in fact necessary to get a proper description of ionic liquid properties. It is shown how strong such parameters vary when the influence of the counter ion is taken into account. Furthermore, acceptor and donor numbers for ionic liquids are presented.

On the Use of a Protic Ionic Liquid with a Novel Cation To Study Anion Basicity

“…[33] Anion basicity is also factored in equations to describe the polarity [34,35] of solvents in general and ionic liquidsi np articular. [38,39] An umber of qualitative and quantitative spectroscopic scalesa re also reported which are mostly basedo nU V/ Vis, [40][41][42][43][44][45] IR [46,47] and NMR [14,[48][49][50][51][52][53][54][55] spectroscopies. [38,39] An umber of qualitative and quantitative spectroscopic scalesa re also reported which are mostly basedo nU V/ Vis, [40][41][42][43][44][45] IR [46,47] and NMR [14,[48][49][50][51][52][53][54][55] spectroscopies.…”

A Hybrid Anion for Ionic Liquid and Battery Electrolyte Applications: Half Triflamide, Half Carbonate

“…[a] Donor numbers measured for respective anions using tetrabutylammonium as the counter cation Ref. and in parentheses for bmim salts . All ionic conductivities obtained for 1 m solutions of salt in 1:1 solvent mixtures except for last entry.…”

“…[3,4] As ac onsequence,t he nature of the anion should be weakly coordinating in order to maximise the amount of charge carriers and to lower the lattice energy for ease of solubility [3][4][5] and, in the case of ionic liquid based electrolytes,lower the barrier for de-solvation at the electrode electrolyte interface.W eakly coordinating anions (WCA) [5][6][7] are typically characterised by the presence of electron withdrawing groups positioned to affect av ery high degree of charge delocalisation across the molecule and, what is often neglected in discussions,alow donor number (DN). [4,5,8,9] Apart from the beneficial coordination properties as econd and equally important aspect of highly charge delocalised WCAsi st heir superior oxidative stability, [4,5] allowing these salts to be used in conjunction with high-voltage electrode materials to pursue high-energy density batteries.T oillustrate the concept of WCAs,k ey properties for selected examples are listed in Table 1.Although LiPF 6 is not the best choice in addressing the above mentioned criteria, it is still the most popular salt in commercial lithium-ion batteries (LIBs) because it presents ag ood compromise,i np articular as ar esult of its ability to form as table solid electrolyte interface layer (SEI) on graphite anodes and ap rotective layer on the aluminium current collector.V ery prominent amongst alternative anions are sulfonyl substituted bis-imides,s uch as bis(trifluoromethanesulfonyl)imide (TFSI) and its smaller analogue bis(fluorosulfonyl)imide (FSI), which have relatively good ionic conductivities and stabilities.N otably,t hese are also the anions of choice to stabilise metallic Li anodes during extensive charge discharge cycling, which is ap rerequisite to enable high energy density Li-technology. [16,17] Despite being crucial, the development of new anions is moving at av ery slow pace.…”

“…[3,4] As ac onsequence,t he nature of the anion should be weakly coordinating in order to maximise the amount of charge carriers and to lower the lattice energy for ease of solubility [3][4][5] and, in the case of ionic liquid based electrolytes,lower the barrier for de-solvation at the electrode electrolyte interface.W eakly coordinating anions (WCA) [5][6][7] are typically characterised by the presence of electron withdrawing groups positioned to affect av ery high degree of charge delocalisation across the molecule and, what is often neglected in discussions,alow donor number (DN). [4,5,8,9] Apart from the beneficial coordination properties as econd and equally important aspect of highly charge delocalised WCAsi st heir superior oxidative stability, [4,5] allowing these salts to be used in conjunction with high-voltage electrode materials to pursue high-energy density batteries.T oillustrate the concept of WCAs,k ey properties for selected examples are listed in Table 1.…”

Solvent Effects on Chemical Reactivity

“…[27][28][29][30] For example Cu(tmen)(acac)ClO 4 , where tmen = N,N,N',N'-tetramethylethylendiamine and acac = acetylacetonate, has be successfully used as a Lewis-basicity indicator, and Fe(phen) 2 (CN) 2 , where phen = 1,10-phenanthroline, as a Lewis-acidity indicator. The physical origin of the underlying color changes is sketched in the Figure 12.1.3, as modified from ref.…”