H 0 in Various ILs CATION EFFECT ANION EFFECT

1. Hammett Acidity Scale in Ionic Liquids : An Indication of Their Weak Dissociating Character ? Robert T.*, Magna L.**, Olivier-Bourbigou H.**, Gilbert B*. T.Robert@ulg.ac.be; *University of Liège, Sart-Tilman, B6c, B-4000 Liège, Belgium ** Institut Français du Pétrole, BP3-69360 Solaize, France Introduction Solvating power, for instance towards the proton, is a fundamental property of solvents. Therefore, we are interested to investigate the acid-base properties in ionic liquids (ILs) in order to ultimately establish a correlation with the acidic catalysis activity. To determine the accessible acidity levels in these new “green” ionic solvents, we suggest the colorimetric Hammett acidity function H0.1 Following the Hammett proposition, this UV-Vis spectroscopic method is based on the protonation equilibrium for a family of coloured indicator (CI) with pKa’s assumed as solvent independent,. Hammett Acidity Function Experimental Theory Because the ILs and the acids (HOTf and HNTf2) are very hygroscopic and/or moisture sensitive, storage, preparation of all solutions and UV-cells filling are performed in a glove box.  Spectroscopic measurements are made with a Perkin Elmer Lamda 14P spectrometer in quartz cuvettes. References are the same IL with the same acid concentration but without indicator. Basic form, I Acid concentration increases Batch solution of indicator: IL + I Acidic batch solution of indicator: IL + acid + HI+ Acidic form, HI+ + acid Hammett assumption Intermediate solutions: IL + acid + HI+/I In practice Figure 1: Evolution of the 2,4-dichloro -6-nitroaniline spectrum in BMImNTf2 with the addition of HNTf2 with X, the ratio of the unprotonated form of the indicator determined by computer Results and Discussion Use of 2 Indicators … H0 in Various ILs CATION EFFECT ANION EFFECT The Hammett acidity function is evalu-ated in several ILs to study the effect of the ILs nature towards the proton acidity. Very acidic level, -2 to -8, can be reached in common ILs. For a same concentration of a same added acid; a difference of H0 is observed when two different CI are used in a given IL (▼, ○, △). This is typical of a non totally dissociating media. Then, the Hammett acidity function becomes an apparent function underestimating the real acidity level. formation constant of IH+X- unitary slope The cation effect is investigated in NTf2- based ILs with HNTf2 as spiking acid. No significant difference is observed for a same CI (○,□,∆): BMIm+ = BMImH+ = HNEt3+ HOTf vs. HNTf2 The comparison of two common acids, HOTf and HNTf2, is investigated in BMImOTf, BMImNTf2 and BMImBF4. In a given IL, the comparison is made with the same CI. IL autoprotolysis: BMIm+A-⇌ BMIm• + HA BMImOTf no difference of H0 (CI -3,32: ▼,▽)  HNTf2 is strong HNTf2 + BMImOTf → HOTf + BMImNTf2 (leveling effect) BMImNTf2 HOTf (●) is weaker than HNTf2 (○) (CI – 4.53) HOTf + BMImNTF2 HNTf2 + BMImOTf BMImBF4 HNTf2 (◇) is stronger than HOTf (◆) but both are weak acids BMImBF4 + HNTf2 HBF4 + BMImNTf2 BMImBF4 + HOTf HBF4 + BMImOTf HNTf2≠ HOTF high acidity level the proton linked on the IL cation has no influence on the accessible acidity because its acidity is masked by HNTf2 In this case, the IL anion nature plays a dominant role on the accessible acidity. The accessible acidity level follows : OTf- < NTf2- < BF4- < PF6- A higher acidity corresponds to a lower solvating power of the IL anion towards the proton. Figure 2: Comparison of the apparent Hammett acidity with addition of (filled symbol) HOTf or (empty symbols) HNTf2 in (▼▽) BMImOTf, (●○) BMImNTf2, (▲△) HNEt3 NTf2, (∎) BHImNTf2, (◆◇) BMImBF4 and ()BMImPF6. Conclusions The acidity increases when acids are added in ILs and high acidity levels can be reached. The Hammett acidity function allows the evaluation of the acidity level in our ILs which ranges from -2 to -8. Nevertheless, because the solvent affects the dissociation equilibrium, the Hammett function must be understood as an apparent function, underestimating the real acidity. Changing the IL cation does not affect the accessible acidity level: BMIm = BHIm = HNEt3. On the contrary, clear variations of solvating power towards the proton are observed between several IL anions. Accessible acidity levels can be classified with the following anion order: Differences of acidity are observed between HOTf and HNTf2 in BMImNTf2 and much more in BMImBF4. This is in agreement with high acidity levels and the proposed acidity order: the higher is the acidity level, the more strong acids can be differentiated.  All these conclusions are confirmed by our electrochemical measurements with the hydrogen electrode.2 H2OOTf-NTf2-BF4-PF6- References: (1) T. Robert, L. Magna, H. Olivier-Bourbigou and B. Gilbert, J. Electrochem. Soc., 156, 115 (2009) (2) C. Malherbe, T. Robert, L. Magna, H. Olivier-Bourbigou and B. Gilbert, ECS Trans., 16, 3 (2009) LEUVEN 2010