3680
J. Phys. Chem. B 2008, 112, 3680-3683
Proton Conducting Ionic Liquid Organization as Probed by NMR: Self-Diffusion Coefficients and Heteronuclear Correlations Patrick Judeinstein,*,† Cristina Iojoiu,‡ Jean-Yves Sanchez,‡ and Bernard Ancian§ R.M.N. en Milieu Oriente´ , ICMMO, UMR CNRS 8182, Baˆ timent 410, UniVersite´ Paris-Sud, 91405 Orsay Cedex, France, LEPMI, UMR CNRS 5631-INPG, UniVersite´ Joseph Fourier, BP.75. 38402 Saint-Martin-d’He` res Cedex, France, and UFR Chimie, Tour 44-54, UniVersite´ Paris-Diderot, 75251 Paris Cedex, France ReceiVed: NoVember 29, 2007; In Final Form: January 12, 2008
The structure and local organization of new proton conducting ionic liquids (PCILs) obtained by reacting alkylamine with various acids were deciphered by complementary 1- and 2-D heteronuclear NMR experiments. One the one hand, PFG NMR yielded the self-diffusion coefficients of the PCIL components (and thus information on their possible concerted translational motions), while on the other hand, 13C, 1H, and 15N, 1H correlation and intermolecular Overhauser experiments gave insight into the nature of protonic species and ion-pairing behavior.
Introduction Ionic liquids are considered as high-tech new media with emerging applications as solvents for organic reactions or as electrolytes.1 The organization degree in these complex fluids is actually very challenging since it dictates microscopic as well as macroscopic properties. Especially, contact ion-pairing association, or even larger supramolecular aggregation stages, has strong consequences concerning the properties of such roomtemperature liquids.2 Tentative conclusions are generally deduced from macroscopic data (thermodynamic, viscosity, conductivity, etc.).3,4 Some further information on the molecular structure and dynamics of these intriguing liquids also was provided by X-ray and neutron scattering;5 NMR,6 Raman,7 and dielectric8 spectroscopies; light scattering;9 or molecular modeling.7,10,11 Until now, most of these studies were concerned with imidazolium-based salts, and these measurements led to the estimation of the dissociation constant relative to the equilibrium [anion + cation/anion - cation]. The influence of external factors also was investigated by heteronuclear 19F{1H} NMR.12,13 Protic ionic liquids are another class of ionic liquids with interesting properties and applications.14 Their protonic conductivity is essentially used in fuel cell membrane electrolytes (PEM).1-4 We recently focused more specifically on a series of protonic conducting ionic liquids (PCIL) obtained by reacting alkylamine with various acids.15 The main advantages of these PCILs arise from their anhydrous proton conductivity and from their high thermal stability. Their blending with appropriate polymers allows PEM fuel cells (PEMFC) to operate at temperatures above 120 °C without gas humidification. Therefore, this would improve both thermal management and Pt electrocatalyst poisoning. Several of these ionic liquids (A‚‚‚H‚‚‚N(R)3) present a good ionic conductivity above their melting point and stability over a wide range of temperatures, and their properties can be easily tuned thanks to the versatility of the synthesis scheme. However, improving their properties * Corresponding author. E-mail:
[email protected]. † Universite ´ Paris-Sud. ‡ Universite ´ Joseph Fourier. § Universite ´ Paris-Diderot.
Figure 1. NMR of the NH group in PCIL: 1H chemical shifts vs 15N chemical shifts [δ(15N) ) 0 ppm for MeNO2] [AA: CH3COOH; TFA: CF3COOH; MS: CH3SO3H; Trif: CF3SO3H; and TFSi: HN(SO2CF3)2]]. Inset: 15N spectra of the NH grouping in AA-TEA and TFSiTEA.
also requires the control of their intimate structure to determine the mobile species and, more specifically, the nature of the proton carrier species: proton H+, ammonium HN+(R)3, or neutral ion pairs A- HN+(R)3. For example, a key question is the role of the anion in the dissociation scheme.4,15 As the contact ion pairs do not contribute to the ionic conductivity, one of the crucial points is the dissociation degree in PCILs. It is well-known that high ion-pair dissociation results from poor nucleophilic anions that are generally the conjugated bases of strong acids or superacids. In this paper, we emphasize the description of the local structure of these organic salts. Intra- and intermolecular interactions are probed by (heteronuclear) NMR methods to determine pair correlations between the different components of these systems, namely, the proton H+, the amine, and the anion. Experimental Procedures PCILs were obtained by reacting equimolar ratios of triethylamine and selected acids. Most of these materials were prepared
10.1021/jp711298g CCC: $40.75 © 2008 American Chemical Society Published on Web 03/06/2008
Proton Conducting Ionic Liquid Organization
J. Phys. Chem. B, Vol. 112, No. 12, 2008 3681
TABLE 1: NMR Results and Proposed Structure for PCILs
and purified according to procedures described in previous papers of our group.15 The PCILs based on acetic acid and trifluoroacetic acid were purified by distillation at, respectively, 150 and 210 °C. The NMR tubes were prepared and sealed in a glove box to avoid any contact with air moisture. NMR experiments were performed using a Bruker Avance NMR 400 spectrometer with a specific QXO probe (1H, 19F, 13C, and 15N) equipped with a z-axis 45 G cm-1 gradient coil. Experiments were performed at 310 K ((1 K). The π/2 pulse widths were calibrated to 18, 21, 6.5, and 21 µs, respectively, for 1H, 19F, 13C, and 15N nuclei. Chemical shifts were referred to the common reference samples (δ(1H) ) 0 ppm and δ(13C) ) 0 ppm for TMS and δ(15N) ) 0 ppm for MeNO2 and are positive downfield from the reference). They were measured on bulk samples by the external calibration method, and the values obtained for pure amine were similar to those already published.24 The determination of 1H and 19F self-diffusion coefficients was performed by pulsed field gradient (PFG) stimulated echo appended by the longitudinal eddy-current delay (LED) sequence and by using two spoil gradient pulses.15,25 In the HOESY experiments, evolution and mixing times were recycled twice to enhance the NOE as described previously.18,19 Results and Discussion Self-Diffusion Measurements. Three different PCILs obtained by reacting triethylamine ((CH3CH2)3N, abbreviated to TEA) with three different organic acids were mainly studied: a weak acid, acetic acid (CH3COOH, abbreviated as AA); a strong acid, trifluoroacetic acid (CF3COOH, abbreviated as TFA); and a superacid, trifluoromethyl-sulfonimide acid ((CF3SO2)2NH, abbreviated as TFSi). Even if the AA-TEA salt is not a good candidate for PCIL applications, the choice of these three different PCILs was guided by their different dynamical behavior as evidenced by the examination of their self-diffusion coefficients measured by PFG NMR (Table 1). Even if the various diffusion coefficients are in the same range, the most striking feature is the relative diffusion coefficients of H+, TEA, and the anion in these three different systems. They are all similar in TFA-TEA. In the TFSi-TEA ionic liquid, those of
H+ and of the amine are similar but larger than the one for the anion, while in AA-TEA, the three values markedly differ. Thus, these data provide by themselves the major trends concerning the behavior and the structure of the mobile species inside these ionic liquids: mostly ion-pairing association in TFA-TEA, partially dissociated ammonium cations and anions in TFSiTEA, and full dissociation for AA-TEA since, in that case, the three species (H+, amine, and anion) have different diffusion coefficients and therefore move separately in the liquid.16 Obviously, PFG NMR probes the structure of these liquids through the measurement of a dynamical property on a macroscopic space scale of 1-100 µm. Conversely, NMR measurements of spin-spin couplings afford structural information at the molecular level.17 These selective measurements involving any pair of specific nuclei are restricted to a spatial scale of a few bond lengths or internuclear distances shorter than 1 nm. Further, specific NMR techniques allow us to distinguish between through-bond scalar couplings and throughspace dipolar couplings, by using, respectively, (i) direct heteronuclear correlation spectroscopy (HETCOR; scalar couplings), (ii) inverse heteronuclear single quantum correlation (HSQC; scalar couplings), and (iii) 1- and 2-D heteronuclear Overhauser effect spectroscopy (HOESY; dipolar couplings) between 1H and 13C (or 15N) nuclei.18,19 Determination of the Amine versus Ammonium Character. Figure 1 presents typical 15N spectra of PCILs at room temperature. Without 1H decoupling, TFA-TEA exhibits a doublet (-324.6 ppm, 1JN-H coupling ∼72 Hz), AA-TEA a single broad peak (-325.8 ppm), and TFSi-TEA a doublet (-321.2 ppm, 1JN-H coupling ∼70 Hz) and a single line (-243 ppm, not shown in Figure 1), the latter corresponding to the nitrogen of the TFSi anion. 15N NMR signals of pure TEA and its hydrochloride salt (dissolved in water) were measured, respectively, at -333.8 and -319.6 ppm, and it can be seen that the chemical shifts of the three ionic liquids investigated here lie between these two extreme values. Meanwhile, chemical shifts are known to be related to the properties of the nitrogen atom in terms of geometry and/or averaged electronic density and reflect the amine/ammonium character of the ionic liquid
3682 J. Phys. Chem. B, Vol. 112, No. 12, 2008
Judeinstein et al.
Figure 2. 15N{1H} HOESY NMR spectroscopy: build-up curves for AA-TEA (nonbonded N-H) and TFSi-TEA (bonded N-H).
as well as the ionicity of the N-H bond.20,21 In this respect, 1H chemical shifts are complementary because they also provide information about the acidic character of the ammonium moiety such as the N-H bond length and hydrogen bonding with the anion. The correlation between these two sets of values appears clearly in Figure 1 (more TEA salts were also studied,15 and their 15N chemical shifts also are reported on this graph). A linear relationship occurs for these different TEA salts with the exception of the AA salt, which is also the only salt for which no N-H scalar coupling was observed and for which the inverse 15N{1H} HSQC experiment completely fails (as an obvious consequence). For all other salts, spin-spin NH couplings evidence the existence of N-H bonding with a lifetime longer than 1/1JN-H.22 These couplings also are observed in 15N{1H} HETCOR experiments and can be removed by 1H irradiation. The coherence between these two sets of data reveal a clear increase of ammonium acidity in the following series: TFSiTEA < CF3SO3-TEA< CH3SO3-TEA < CF3CO2-TEA. This increase of acidity is normally accompanied by a longer N-H bond. AA-TEA uses this rule of thumb, but this larger acidity corresponds also in addition to the breaking of the N-H bond and to the dissociation of the ammonium cation. These features are clearly evidenced by the three distinct diffusion coefficients obtained for the amine, the proton, and the anion (see Table 1). However, some changes of local geometry or torsional flexibility when going from amine and ammonium also may explain the fact that the AA-TEA compound is outside the straight line of Figure 1. Such an effect was also recently reported for hydrogen bonding between pyridine derivatives and carboxylic acids.21 The localization of the H+ protons inside this material can be detailed from 15N{1H} HOESY experiments. They were performed on the different compounds, and the spectra are similar for all of them. As 2-D experiments exhibit a single correlation peak between the nitrogen nucleus and the NH proton, build-up curves (for mixing times varying from 100 ms to 10 s) can be obtained more quickly from the 1-D experiments. The results for TFSi-TEA and AA-TEA are presented in Figure 2. A rather similar behavior was observed for these two compounds with the same initial slope and maximum effect occurring at a mixing time of 2 s. This result is rather puzzling because it reveals similar geometrical proximities in AA-TEA and other TFSi-TEA salts, while the first results do not present a stable N-H bond and the other does. In fact, these apparently contradictory measurements are related to the different time scales of these experi-
Figure 3. 13C{1H} HOESY NMR spectrum of AA-TEA and HOE build-up curves.
ments. PFG NMR probes the microdynamics of the species on the scale of the so-called diffusion interval (greater than 100 ms), the N-H coupling lifetime is comparable to 1/JN-H (i.e., ≈10 ms), while the time scale of the HOESY experiment is defined by long mixing times (e2 s) during which rapid concerted exchanges can occur. Thus, the specific character of AA is due to the very short lifetime of the ammonium form that arises from the amine/proton combination. Indeed, these different time scales illustrate the complementarity of the different techniques used here. Characterization of Ion Pairing. Figure 3 shows the 13C{1H} 2-D HOESY spectrum for AA-TEA and the build-up curves for the different cross-correlation signals. Intramolecular correlations corresponding to the carbon atoms and their attached protons are observed inside the TEA anion (cross-peaks 1 and 2) and acetate (peak 3). For the acetate moiety, a remote correlation between the methyl protons and the carbonyl carbons also is observed (peak 4). All these signals also were observed in the 13C{1H} HSQC and HETCOR experiments. Most interesting and remarkable is the cross-peak (peak 5) corresponding to a dipolar interaction between the carbonyl carbon and the proton H+. It reveals the spatial proximity between this proton and the AA anion, less than 3 Å, whereas the 1JN-H correlation is always absent in the relevant HSQC experiment. Similar to the N‚‚‚H experiments, the lack of long-time bonding between the H+ proton and the acetate is evidenced by the different values of the diffusion coefficients and the absence of correlations by scalar couplings (HETCOR; see Table 1). Further quantitative information can be obtained from the build-up curves (Figure 2), according to which the initial slope (dA/ dtmixing) is expressed as follows:
dA/dtmixing ) qNr-6
(1)
where A is the area of the cross-peak signal, N is the number of neighboring atoms, r is the C‚‚‚H distance, and q is the efficiency factor of the considered HOESY experiments that depends, among other factors, on the dynamics of the system and that can be estimated from the build-up of the C(carbonyl)‚‚‚H(methyl) correlation for which the geometry is well-known. According to this procedure, the distance of interest (H+‚‚‚C(carbonyl)) is estimated at 1.8 Å ((0.3 Å), which in good agreement with data reported in the literature from neutron diffraction experiments.23
Proton Conducting Ionic Liquid Organization
J. Phys. Chem. B, Vol. 112, No. 12, 2008 3683 The possibility to tune interactions between the different components is important for the optimization of other physical properties, especially proton conductivity. Acknowledgment. The work presented here was supported by ANR (Agence National de Recherche) in the framework of the PAN-H program. Mathieu Martinez is thanked for the preparation of some of the samples. The referees are acknowledged for helpful comments. References and Notes
Figure 4.
13
C{1H} HOESY NMR spectra of TFA-TEA.
Figure 4 shows the 13C{1H} 2-D HOESY spectrum for TFATEA. Here, the presence of many correlation peaks clearly demonstrates the structural complexity of this ionic liquid, with spatial correlations between H+ and TFA on one hand and TFA and TEA on the other hand. This result is in total agreement with the diffusion coefficient measurements and evidences the high level of ion pairing in this system in which the anion and cation are strongly maintained in close contact by electrostatic forces, so that all the constitutive parts diffuse as a unique entity. Unfortunately, the presence of the trifluoromethyl group splits the signals of the trifluoroacetate anions because of the C-F couplings (1JC-F ) 292 Hz and 2JC-F ) 32 Hz) and did not allow us to make quantitative measurements with a sufficient accuracy. Finally, no interionic 13C{1H} HOESY correlations were obtained for the TFSi-TEA salts (spectrum not presented here), in agreement with the different diffusion coefficients for the two ionic species in this PCIL. Conclusion Complementary NMR techniques permitted us to probe triethylamine based ionic liquids, providing dynamical and structural information. The main pieces of information are summarized in Table 1. Diffusion coefficients measured by PFG NMR are illustrative of the very different behavior of the three salts. Heteronuclear HOESY 15N{1H} and 13C{1H} experiments revealed interionic spatial correlations and led to qualitative and even semiquantitative data on ion association. Obviously, changing the anion induced strong effects on the following equilibrium scheme:
TEA‚‚‚H+‚‚‚A- / TEAH+ + A- / TEAH+, A- (2)
(1) (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; J. Wiley and Sons: New York, 2003. (b) Ohno, H. Electrochemical Aspects of Ionic Liquids; J. Wiley and Sons: New York, 2005. (2) Ohno, H. Bull. Chem. Soc. Jpn. 2006, 79, 1665-1680. (3) Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2001, 105, 4603-4610. (4) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 16593-16600. (5) Hardacre, C.; Holbrey, J. D.; Nieuwenhuyzen, M.; Youngs, T. G. Acc. Chem. Res. 2007, 40, 1146-1155. (6) Bankmann, D.; Giernoth, R. Prog. Nucl. Mag. Reson. Spectrosc. 2007, 51, 63-90. (7) Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayashi, Y.; Ishiguro, S. J. Phys. Chem. B 2006, 110, 8179-8183. (8) Weinga¨rtner, H.; Sasisanker, P.; Daguenet, C.; Dyson, P. J.; Krossing, I.; Slattery, J. M.; Schubert, T. J. Phys. Chem. B 2007, 111, 47754780. (9) Kuang, Q.; Zhang, J.; Wang, Z. J. Phys. Chem. B 2007, 111, 98589863. (10) Bagno, A.; D’Amico, F.; Saielli, G. J. Mol. Liquids 2007, 131132, 17-23. (11) Del Po´polo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 17441752. (12) Mele, A.; Tran, C. D.; De Paoli Lacerda, S. H. Angew. Chem., Int. Ed. 2003, 42, 4364-4366. (13) Nanma, D.; Kumar, P. G. A.; Pregosin, P. S.; Geldbach, T. J.; Dyson, P. J. Inorg. Chim. Acta 2006, 359, 1907-1911. (14) Greaves, T. L.; Drummond, C. J. Chem. ReV. 2008, 108, 206237. (15) Iojoiu, C.; Judeinstein, P.; Sanchez, J.-Y. Electrochim. Acta 2007, 53, 1395-1403. (16) Lucas, L. H.; Larive, C. K. Concepts Magn. Reson. 2004, 20A, 24-41. (17) Pregosin, P. S.; Kumar, P. G. A.; Ferna´ndez, I. Chem. ReV. 2005, 105, 2977-2998. (18) Chahinian, M.; Seba, H. B.; Ancian, B. Chem. Phys. Lett. 1998, 285, 337-345. (19) Seba, H. B.; Thureau, P.; Ancian, B.; The´vand, A. Magn. Reson. Chem. 2006, 44, 1109-1117. (20) Martin, G. J.; Martin, M. L.; Gouesnard, J.-P. In 15N-NMR Spectroscopy, In NMR Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1981, Vol. 18, Chapter 6, pp 75-185. (21) Limbach, H. H.; Pietrzak, M.; Sharif, S.; Tolstoy, P. M.; Shenderovich, I. G.; Smirnov, S. N.; Golubev, N. S.; Denisov, G. S. Chem.s Eur. J. 2004, 10, 5195-5204. (22) Jackmann, L. M.; Cotton, F. A. Dynamic Nuclear Magnetic Resonance Spectroscopy; Academic Press: New York, 1975. (23) Jo¨nsson, P.-G. Acta Crystallgor., Sect. B: Struct. Sci. 1971, 27, 893-898. (24) Duthaler, R. O.; Roberts, J. D. J. Am. Chem. Soc. 1978, 100, 38893895. (25) Altieri, S.; Hinton, D. P.; Byrd, R. A. J. Am. Chem Soc. 1995, 117, 7566-7567.
