A Hydronium Solvate Ionic Liquid: Ligand Exchange Conduction Driven by Labile Solvation

We observed a new class of proton conduction in a hydronium (H 3 O + ) solvate ionic liquid (IL). The IL is described as [H 3 O + · 18C6]Tf 2 N, composed of 18-crown-6-ether (18C6)-coordinated H 3 O + cation (solvate) and bis(triﬂuoromethylsulfonyl)amide (Tf 2 N – ; Tf = CF 3 SO 2 ) anion. Self-diffusion coefﬁcient measurements revealed that protons of H 3 O + (solute ion) move faster than 18C6 ligand (solvent), marking the ﬁrst observation of ligand exchange conduction not only among solvate ILs but also among protic ILs without free neutral molecules. The presence of ligand exchange conduction without inducing external electric ﬁeld indicates that solvation of H 3 O + by 18C6 is kinetically labile, i.e. ligand exchange is very fast for the reaction H 2 O + 18C6 + HTf 2 N (cid:2)(cid:3) [H 3 O + · 18C6]Tf 2 N, while the equilibrium shifts to the right. The fundamental ﬁndings regarding the hydronium solvate IL will help in the design of proton conductors.


Experimental
The synthesis of [H 3 O + • 18C6]Tf 2 N is described in our previous report. 18The pulsed-field gradient spin echo NMR (PGSE-NMR) measurements were performed at 75 • C and 85 • C using JNM-ECA600 NMR spectrometer (JEOL Ltd.).The 1 H and 19 F spectra were measured.Other basic experimental conditions were the same as for conventional 1 H NMR spectroscopy.The self-diffusion coefficients were measured using a simple Hahn spin-echo sequence and analyzing on the basis of the Stejskal equation ln E = −Dγ 2 g 2 δ 2 ( -δ/3), 24 where E is the echo signal attenuation, D is the self-diffusion coefficient, γ is the gyromagnetic ratio, g is the amplitude of the gradient pulses, δ is the duration of the gradient pulses, and is the interval between the leading edges of the gradient pulses.The g values used were in the range of 20 mT m -1 -300 mT m -1 at 75 • C and 20 mT m -1 -250 mT m -1 at 85 • C, δ was 8 ms, and the value of was 100 ms.Separately, we prepared an equimolar mixture of H 2 O and HTf 2 N, a colorless transparent liquid at room temperature, and 1 H NMR was measured at room temperature to check the chemical state of HTf

plots have linear relationships. Table I lists the estimated values of self-diffusion coefficients for [H
The values for the hydronium solvate IL are 10 -7 cm 2 s -1 , almost the same order of magnitude as those for analogous lithium solvate ILs. 8 The ionicity i.e. the degree of cation-anion dissociation of [H 3 O + • 18C6]Tf 2 N has been estimated to be imp / ideal = 0.44 from the Walden plot (molar conductivity imp vs. fluidity η -1 ). 18The ionicity of [H 3 O + • 18C6]Tf 2 N can also be estimated as imp / NMR using the PGSE-NMR results.NMR can be calculated from the ionic self- 6.0 3.9 4.0 diffusion coefficients of cations (D + = D H3O+ ) and anions (D − = D Tf2N-) measured by PGSE-NMR using the Nernst−Einstein equation NMR = F 2 /RT(D + + D − ) where F is the Faraday constant, R is the gas constant, and T is the absolute temperature. 9Using the data listed in Table I, we obtain NMR = 2.0 S cm 2 mol -1 at 75 • C and 3.1 at 85 • C. Since the molar conductivity imp is 1.05 S cm 2 mol -1 at 75 • C and 1.40 S cm 2 mol -1 at 85 • C, 18 the ionicity imp / NMR is 0.53 at 75 • C and 0.45 at 85 • C. The value is similar to imp / ideal = 0.44, 18 and also comparable to some lithium solvate ILs. 9,10n Figs. 2 and 3 the gradients of the fitted lines, which according to the Stejskal equation are proportional to diffusion coefficients, 24 are almost the same for 18C6 and Tf 2 N -, while that for H 3 O + is significantly larger.The ratio of the diffusion coefficient D H3O+ /D 18C6 is 1.5, indicating that protons of hydronium ions move faster than 18C6 ligands, indicating ligand exchange conduction of solute ions.Notably, the activation energy estimated from Arrhenius plots 18 is 28.0(6) kJ mol -1 for ionic conduction, certainly lower than 28.9(1) kJ mol -1 for viscous flow.Thus, we consider that a ligand exchange conduction mechanism exists although vehicle mechanism is dominant in the hydronium solvate IL.In solvate ILs, by contrast, metal ions (such as lithium ions) and the ligands move together: a vehicle mechanism has been suggested because the ratio is identical within the experimental error, i.e.][10] Therefore, a line should be drawn between the hydronium solvate IL and the other reported solvate ILs.At this point, however, we cannot conclude whether H + or H 3 O + moves since we have not measured diffusion coefficients of oxygen (D O ) of H 3 O + .If H + diffuse (much) faster than oxygen which is the mass center of the hydronium (D H3O+ /D O >> 1), we can say the proton has hopping conduction where the H 3 O + cation unit would relay one of its protons to an adjacent molecule upon acceptance of a proton.Given D H3O+ /D O ∼ 1, migration of H 3 O + as a whole (vehicular diffusion of proton) should occur, just like the proton in acidic solution as observed by Kreuer et al. 25 Clearly, the presence of hydrogen bonding, which should be related to the observed ligand exchange conduction, differentiates the hydronium solvate IL from all metal ion solvate ILs previously reported.In protic ILs, Grotthuss-like H + transfer (i.e.proton hopping) takes place in the presence of neutral molecules.For example, a mixture of HTf 2 N and excess (more than equimolar) imidazole molecules shows proton hopping conduction, revealed by the fact that protons move faster than imidazole molecules. 5,16,17Although the conductivity is high in the HTf 2 N-imidazole mixture, the volatility of neutral imidazole molecules prevents their use at high temperature.Notably, in the equimolar mixture of HTf 2 N-imidazole, a protic IL composed of [ImH + ] cation and Tf 2 N -anion, hopping conduction does not occur (diffusion coefficient ratio D H+ /D Im = 1).In our hydronium solvate IL, however, the faster transfer of H 3 O + ions as a whole (or faster transfer of H + ) than 18C6 ligands occurs with negligible number of neutral molecules.In other words, an experimentally "equimolar" mixture can contain tiny-less than 1%-amount of excess neutral molecules, but such possible neutral molecules did not cause a hopping conduction for HTf 2 N-imidazole but for H 2 O-18C6-HTf 2 N.
It is noteworthy that before and after the PGSE-NMR measurements the water content of the [H 3 O + • 18C6]Tf 2 N sample was same, thus the sample composition did not change (neither contamination of extra water nor sublimation of HTf 2 N component was detected) during the measurements, strongly indicating that the ligand exchange conduction is intrinsic to stoichiometric [H 3 O + • 18C6]Tf 2 N. The estimated transference number of cation t +, using the value of D H3O+ and D Tf2N-, was as large as 0.58 at 75 • C and 0.60 at 85 • C. The obtained t + is comparable to those of some solvate ILs (0.44-0.61) 9,10 and slightly smaller than those of the Im-HTf 2 N protic ILs (0.66-0.78). 17he proton conductivity, i.e., the product of t + and ionic conductivity, can be obtained as 1.37 mS cm -1 at 75 • C and 1.83 mS cm -1 at 85 • C.
The ligand exchange conduction without intrinsically volatile neutral molecules is fascinating because the concept could enable high temperature application with high conductivity.Consequently, in terms of the ligand exchange conduction mechanism, the hydronium solvate IL should be distinguished from all protic ILs previously reported.The conduction relates to the evolution of a hydrogen bonding network in [H 3 O + • 18C6]Tf 2 N.In the hydronium solvate IL there are three O-H bonds in H 3 O + and three hydrogen bonds with 18C6 (Fig. 1a), while in [ImH + ]Tf 2 N there seems to be only one N-H bond.Thus, we consider that the dense hydrogen bonding network causes the unusual conduction mechanism, which does not need the help of neutral molecules.
A further insight for possible origin of the foregoing ligand exchange conduction of the hydronium solvate IL was speculated from the thermogravimetric analysis result. 18The temperature of a 5% mass loss for the hydronium solvate IL is 132 • C at a heating rate of 5 K min -1 .Although the ligand (i.e., crownether and glyme) and/or coordinated cation (Li + , Na + , K + , etc) is different, this temperature seems much lower than those for previous solvate ILs with Tf 2 N -anion (190-220 • C; heating rate: 10 K min -1 ). 9,11In this respect, although several chemical characterization results including infrared spectra revealed that neutral 18C6 molecules are negligible, 18 i.e. [H 3 O + • 18C6] solvate is thermodynamically "stable" as well as the previous solvate ILs, the hydronium solvate IL is kinetically "labile".In other words, the kinetic constant for ligand exchange i.Since the inert solvate ILs prohibit ligand exchange conduction and allow only vehicle conduction, we propose that it is the labile solvation of the hydronium solvate IL that enables the ligand exchange conduction.Additionally, if the solvation ability of hydronium solvate IL were inert, the active proton of H 3 O + could not move out of the ligand.Also, the strong acidity of [H 3 O + • 18C6]Tf 2 N, which we reported previously, 18 would not appear.
We also note that, without 18C6, i.e. an equimolar mixture of H 2 O and HTf 2 N shows very different nature from that of [H 3 O + • 18C6]Tf 2 N. It has been suggested by infrared experiments (for 1:1 H 2 O and HTf 2 N in an organic solution) and ab initio calculations (for one H 2 O and one HTf 2 N molecules in vacuum) that, the proton of HTf 2 N remains associated in equimolar mixture of H 2 O and HTf 2 N. 18,26,27 However, bulk system of 1:1 H 2 O and HTf 2 N without any solvents has not experimentally studied so far.We have checked 1 H NMR of the equimolar mixture of H 2 O and HTf 2 N (without 18C6 or any other solvents).As a result, only one singlet was observed at 8.18 ppm (shown in Fig. 4), evidencing that neutral H 2 O -usually at 3.3 ppm for pure water-is absent and H 2 O has reacted with HTf 2 N.However, the obtained chemical shift was much lower than those of molten [H 3 O + • 18C6]Tf 2 N (10.85 ppm) 18 and HTf 2 N dissolved in acetone (10.42 ppm). 28The value is very close to the reported value of HTf 2 N dissolved in CFCl 3 , (7.92 ppm). 28Since acetone can dissociate HTf 2 N and CFCl 3 cannot, the obtained results for 1:1 mixture of H 2 O and HTf 2 N may suggest that HTf 2 N protonates H 2 O to be  18 Also, the peak area ratio was 12:1, in good agreement with the stoichiometric one (see Fig. 5b).These results clarify that the solvating and un-solvating 18C6 cannot be detected independently within the NMR chemical shift timescale, which strongly suggest the fast kinetic exchange of the solvating and un-solvating 18C6, or labile solvation of [H 3 O + • 18C6]Tf 2 N.
Similar 1 H NMR results-undistinguishable peaks for solvated and un-solvated ligands within the NMR chemical shift timescale-have been reported for a LiTf 2 N-glyme system, which also revealed a very quick ligand exchange between solvating and un-solvating glymes. 10n this case, however, a ligand exchange conduction mechanism is not proposed for the bulk conduction of solvate IL [Li + • glyme]Tf 2 N, but is proposed only when an external electric field is applied to induce interfacial electrochemical reactions of [Li + • glyme] cations.Therefore, we propose that the difference between coordinated cation (Li + and H 3 O + ) may allow the bulk ligand exchange conduction for [H 3 O + • 18C6]Tf 2 N without any external electric field, which can be attributed to the labile solvation.We also note that the labile solvation should explain why [H 3 O + • 18C6]Tf 2 N is highly acidic as we reported previously, 18 despite the large stability constants for [H 3 O + • 18C6]. 23

Conclusions
We revealed that [H 3 O + • 18C6]Tf 2 N, the first example of hydronium solvate IL, is highly proton-conductive (proton transference number t proton = 0.60) and shows a ligand exchange conduction mechanism.Ligand exchange conduction without inducing external electric field has not been reported for known metal-cation solvate ILs.The ligand exchange conduction occurs without sizable number of neutral molecules, suggesting a hopping mechanism different from those of protic ILs that require sizable number of neutral molecules.Further study to determine if the carrier is H + or H 3 O + , by measuring diffusion coefficients of oxygen (D O ) of H 3 O + , is of special interest.

Figures 2 and 3 Figure 1 .
Figures 2 and 3 shows plots of echo signal attenuation on the basis of the Stejskal equation for 18C6 (red circles), Tf 2 N -(blue circles), and H 3 O + (black circles) for [H 3 O + • 18C6]Tf 2 N at 75 • C and 85 • C, which was obtained reproducibly in different runs.As shown, the

Figure 2 .
Figure 2. Plots of echo signal attenuation on the basis of the Stejskal equation for 18C6 (red circles), Tf 2 N -(blue circles), and H 3 O + (black circles) at 75 • C.

Figure 3 .
Figure 3. Plots of echo signal attenuation on the basis of the Stejskal equation for 18C6 (red circles), Tf 2 N -(blue circles), and H 3 O + (black circles) at 85 • C.

Figure 4 . 1 H
Figure 4. 1 H NMR spectrum for 1:1 mixture of H 2 O and HTf 2 N obtained at 30 • C, showing only one singlet due to absence of neutral H 2 O (see text for details).
e., H 2 O + 18C6 + HTf 2 N [H 3 O + • 18C6]Tf 2 N is very fast, while the equilibrium shift to the right.Thus, the lability of solvation in H 2 O + 18C6 + HTf 2 N [H 3 O + • 18C6]Tf 2 N could help the fast(er) transfer of H 3 O + or H + , and accelerate mass loss at lower temperature due to the volatile nature of H 2 O, 18C6, and HTf 2 N compared to the previous solvate ILs.