Absolute Brønsted Acidities and pH Scales in Ionic Liquids

Abstract

Although receiving large interest over the last years, some fundamental aspects of Brønsted acidity in ionic liquids (ILs) have up to now been insufficiently highlighted. In this work, standard states, activity, and activity coefficient definitions for IL solvent systems were developed from general thermodynamic considerations and then extended to a general mixed solvent standard state. By using the bromide/bromoaluminate systems as representative ILs, formulae for thermodynamically consistent pH scales for ILs with simple (Br(-) ) and complex ([Aln Br3n+1 ](-) ) anions were derived on the basis of the chemical potential of the proton. Supported by quantum chemical [ccsd(t)/MP2/DFT/COSMO-RS] calculations, Gibbs solvation energies of the proton were calculated, which allowed the ILs to be ranked in absolute acidity, that is, pHabs or μabs (H(+) , IL), and additionally allowed their acidity to be compared with molecular Brønsted acid systems. It was shown that bromoaluminate ILs are suited for reaching superacidic conditions. The complexity of autoprotolysis processes in C6 MIM(+) [AlBr4 ](-) (C6 MIM=1-hexyl-3-methylimidazolium) with or without the addition of basic (i.e. Br(-) ) or acidic (AlBr3 and/or HBr) solutes was examined in detail by model calculations, and they indicated a large thermodynamic influence of small deviations from the exact stoichiometric composition.

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

“…We also recognize the possibility of using ag asphase reference state as suggested by Krossinga nd co-workers, [23] to quantify the acidities of these strong acids in ionic liquid media. We also recognize the possibility of using ag asphase reference state as suggested by Krossinga nd co-workers, [23] to quantify the acidities of these strong acids in ionic liquid media.…”

“…[8][9][10][11][12][13] Their basicities can to some extent be inferred from the equilibrium constant of their corresponding acids' dissociation at low concentrations in water [2,14] (pK a values) but the accuracyo fs uch deduction is limited by solvent (medium)a nd concentratione ffects. [20][21][22] Most recenti st he development of an absolute pH scale based on the gas-phases tandard state of the proton, [23] which has already found applicationi nm ixed-solvent liquid chromatography. [20][21][22] Most recenti st he development of an absolute pH scale based on the gas-phases tandard state of the proton, [23] which has already found applicationi nm ixed-solvent liquid chromatography.…”

“…[36,37] Kamlet-Taft multiparameter equationisaw ell-known example. In the case of Guttmann DN measured by 23 Na NMR spectroscopy,t he broadness of sodiuml ines is another source of error.T here is also as ubtle point here worth making that the basicity of anionst owards sodium ion is not necessarily the same as their basicity towards proton.T his is in addition to the fact that the cation has an on-innocent effect on 23 Na chemical shift since it competes with the sodium ion for the anion andt he result of this competition affects the chemicalshift. Most of these methodsa re based on spectroscopic characterization of ap robe, dissolved in ILs at low concentrations, the response of which is found to be correlated to the acidity/basicity of the medium.T he main disadvantage of this approach is the disproportionatee ffect of impurities consequento nt he low concentration of the probe.…”

Basic Remarks on Acidity

“…Fors ystems in which both species are in equilibrium, the situation is somewhat more complicated. [41] Them ajor advantage of such pH S definitions for ILs is that they allow the establishment of the hydrogen electrode and, thus,e lectrochemical standard potentials of arbitrary redox systems in ILs.T his approach was exemplarily probed for some systems in bromide and bistriflimide ILs. [42] ILs with Complex Anions,w hich Decompose upon Protonation.…”

“…Thedegrees of protonation of aromatic compounds with HBr/ AlBr 3 /bromoaluminate IL systems-a HSAB-soft environment-were extensively examined in our group,accompanied by quantum chemical modeling studies and the experimental determination of the protonation degrees by NMR spectroscopy. [41,74] Theresults were compared with those obtained for other, more traditional systems.I nt he soft bromoaluminate system, acidities of 170 to 169 in pH abs terms are required to stabilize the mesitylenium cation, similar to the tert-butyl cation (pH abs = 171). In the hard solvents SO 2 or classical superacids,the acidity has to be quite abit higher to stabilize this cation compared with the soft bromoaluminate ILs.T his HSAB effect can be attributed to the stabilization of the soft carbocations through the soft-soft interactions with the medium.…”