Aggregation Behavior of Octyl Viologen Di[bis ... - ACS Publications

16566

J. Phys. Chem. B 2008, 112, 16566–16574

Aggregation Behavior of Octyl Viologen Di[bis(trifluoromethanesulfonyl)amide] in Nonpolar Solvents Ester Marotta,† Federico Rastrelli,† and Giacomo Saielli*,‡ Dipartimento di Scienze Chimiche dell’UniVersita`, Via Marzolo, 1 - 35131 PadoVa, Italy, and Istituto per la Tecnologia delle Membrane del CNR, Sezione di PadoVa, Via Marzolo, 1 - 35131, PadoVa, Italy ReceiVed: September 18, 2008; ReVised Manuscript ReceiVed: October 21, 2008

The aggregation behavior in nonpolar solvents of the octyl viologen (OV) salt with the hydrophobic anion bis(trifluoromethanesulfonyl)amide (Tf2N-) has been investigated. 1H and 19F NMR, ESI-MS and DFT calculations suggest that large aggregates are formed in toluene, benzene and chloroform, where the salt is highly soluble. The lifetime of the aggregates is long enough to be detected as independent species by 1H and 19 F NMR spectroscopy, together with the smaller neutral OV(Tf2N)2 cluster. This behavior is quite at variance with usual NMR detected equilibria where only average signals are generally observed. Large aggregates are also observed in ESI-MS spectra of toluene and chloroform solutions despite the well-known low-coordinating ability of Tf2N-. It is suggested that the structure of the large aggregates mimics the thermotropic smectic phase that this system exhibits near room temperature. Introduction Ion-pairing phenomena in solution have significant consequences in several branches of chemistry and materials science; see ref 1 for a recent review on the subject. Among the many successful exploitations of ion-pairing we mention the recent transfer of chirality information from a chiral anion to the TS of the adduct with the pro-chiral cation thus obtaining the final product with a large enantiomeric excess.2 Viologen salts (salts of the 1,1′-dialkyl-4,4′-bipyridinium dications) have received a great deal of attention because of their interesting redox properties, particularly methyl viologen.3 For example, their three oxidation states may be used to trigger some kind of motion in molecular machines.4 To this end, however, the synthesis of large supramolecular structures containing viologens has to face with the complex ion-pairing equilibria of the latter in weakly and nonpolar solvents.5 Viologens with relatively long alkyl chains have recently attracted some interest also because they display liquidcrystalline behavior at, or near, room temperature.6 Generally, ionic liquid crystals exhibit smectic phases, particularly the SmA phase,7 because the driving force to the transition is the microphase segregation of the hydrophobic alkyl chains with respect to the ionic core. Thus, no specific role of the counteranion is assumed in this qualitative view, except to guarantee electroneutrality. However, the role played by the anion in the stability of the liquid and liquid crystalline phases of viologen salts is quite important: 1,1′-dialkyl-4,4′-bipyridinium dibromides and diiodides, for example, usually have stable crystal phases up to the decomposition temperature, which is generally in the range 250-300 °C, although some high temperature liquid crystal phases for some long chain viologen dihalides have been reported.8 In contrast, when the halide anion is replaced with bis(trifluoromethanesulfonyl)amide (Tf2N-, also known as bistriflimide), a significant increase in the polymorphism and a * Corresponding author. E-mail: [email protected]. Fax: +39049-8275239. † Universita` di Padova. ‡ ITM-CNR, Padova Section.

greater stability of the liquid crystalline phases is found. Smectic phases (normally SmA) appear at low temperature and they are stable over a broad temperature range of several tens of degrees. The isotropic liquid phase is also generally observed at a lower temperature than for the corresponding dihalide salt.6 In particular, the title compound, 1,1′-dioctyl-4,4′-bipyridinium di[bis(trifluoromethanesulfonyl)amide], OV(Tf2N)2 hereafter, shows a large temperature range of a smectic A phase, from 37 to 136 °C, after which an isotropic liquid is found.6a The peculiar behavior of the Tf2N- anion is not unusual: (CF3SO2)N- is known to be a noncoordinating, hydrophobic anion, which usually depresses the melting point of common ionic liquids based on imidazolium9 and quaternary ammonium10 cations. Also, although neutron diffraction studies provide evidence of an ordered local structure of several ionic liquids, similar to their crystal phase, namely [bmim][PF6],11 [dmim][Cl]12a and [dmim][PF6],12b such local order seems to be absent in the salt [dmim][Tf2N].12c Analogously, recent X-ray diffraction studies showed a different organization of the liquid phase of [emim][Tf2N] compared to the crystal phase.13 On the other hand, mass spectrometry investigations of common ionic liquids also suggested that the behavior of Tf2Ndiffers from that one of other widespread anions found in ionic liquids. ESI experiments conducted in polar solvents revealed a large degree of clustering of imidazolium cations with several anions except Tf2N-.14 To understand more in detail the interaction of viologen dications with Tf2N-, and the role of the latter in stabilizing liquid crystalline phases, we have investigated the ion clustering of OV(Tf2N)2 by NMR (1H and 19F) in several solvents of different polarity, ranging from DMSO to benzene, and mass spectrometry in MeOH, CHCl3 and toluene, together with DFT calculations. We will show that stable and long-lived aggregates are formed in weak and nonpolar solvents that probably mimic the structure of the smectic phase observed for the pure system. In the next section we will first present the experimental results obtained by NMR spectroscopy, mass spectrometry and DFT calculations followed by a general discussion of the data before the Conclusion.

10.1021/jp808306r CCC: $40.75  2008 American Chemical Society Published on Web 11/21/2008

Octyl Viologen Di[bis(trifluoromethanesulfonyl)amide] Experimental Section 1,1′-Di-n-octyl-4,4′-bipyridinium Dibromide (OVBr2). A 500 mg (3.20 mmol) sample of 4,4′-bipyridine was added to a mixture of 1.5 mL of bromooctane and 6 mL of acetonitrile, and the mixture was refluxed for 24 h. The yellow precipitate was filtered off and washed with cold acetone (0 °C) and then recrystallized from water/acetone 15:85 v/v. Yield: 80%. 1H NMR (400 MHz, CD3OD): δ ) 9.32 (d, 3JHH 6.5 Hz, 4 H, H2, H6, H2′, H6′), 8.71 (d, 3JHH 6.5 Hz, 4 H, H3, H5, H3′, H5′), 4.78 (t, 3JHH 7.6 Hz, 4 H, N-CH2-R), 2.12 (m, 4H, N-CH2-CH2-R), ∼1.44 (m, 20 H), 0.91 (m, 6 H, R-CH3) ppm. 13C NMR (100 MHz, CD3OD): δ ) 150.3 (C4, C4′), 146.1 (C2, C6, C2′, C6′), 127.3 (C3, C5, C3′, C5′), 62.3 (N-CH2-R), 31.9, 31.6 (N-CH2-CH2-R), 29.2, 29.1, 26.3, 22.7, 13.4 (R-CH3) ppm. 14N NMR (28.9 MHz, CD3OD): -164.2 ppm. ESI-MS: m/z ) 191 M2+, 269 [M - C8H17]+. Elemental analysis: found C 57.57%, H 7.97%, N 5.22%; calcd C 57.57%, H 7.80%, N 5.16%. 1,1′-Di-n-octyl-4,4′-bipiridinium Di[bis(trifluoromethanesulfonyl)amide] (OV(Tf2N)2). A 760 mg sample of OVBr2 (1.4 mmol) was dissolved in 20 mL of MeOH. A solution of 1.21 g of LiN(SO2CF3)2 (4.1 mmol) in 30 mL of MeOH was slowly added and stirred overnight. The solution was evaporated in vacuum and the solid washed several times with water. The white crystalline solid obtained was dried several days in vacuum. Yield: 95%. 1H NMR (400 MHz, CD2Cl2): δ ) 8.91 (d, 3JHH 6.0 Hz, 4 H, H2, H6, H2′, H6′), 8.56 (d, 3JHH 6.0 Hz, 4 H, H3, H5, H3′, H5′), 4.64 (t, 3JHH 7.4 Hz, 4 H, N-CH2-R), 2.08 (m, 4H, N-CH2-CH2-R), ∼1.35 (m, 20 H), 0.88 (m, 6 H, CH3-R) ppm. 13C NMR (100 MHz, CD2Cl2): δ ) 150.5 (C4, C4′), 145.7 (C2, C6, C2′, C6′) 128.2 (C3, C5, C3′, C5′), 120.1 (q, 1JCF 318 Hz, (CF3SO2)2N-), 63.6, 32.0, 31.7, 29.2, 29.1, 26.4, 22.9, 14.1 ppm. 19F NMR (282.4 MHz, CD2Cl2): -80.85 ((CF3SO2)2N-) ppm. ESI-MS: m/z ) 191 M2+, 269 [M - C8H17]+, 280 (CF3SO2)2N-. Elemental analysis: found C 38.54%, H 4.20%, N 5.80%, S 13.82; calcd C 38.21%, H 4.49%, N 5.94%, S 13.60%. NMR Spectroscopy. NMR spectra were acquired on three different Bruker spectrometers, namely an AVANCE 300 equipped with a 5 mm BBO z-grad probe (53 G/cm max gradient strength), an AVANCE DRX 400 equipped with a 5 mm BBI z-grad probe (53.5 G/cm max gradient strength) and an AVANCE DMX 600 equipped with a 5 mm TXI xyz-grad inverse probe (68 G/cm max z gradient strength). ppm are given with respect to 1H and 13C of TMS, 14N of CH3NO2 and 19F of CFCl3 and measured through the lock signal. 1H DOSY experiments were run at 300, 400, and 600 MHz, whereas 19F DOSY experiments were run at 282 MHz. Diffusion-ordered spectra were obtained by means of a stimulated-echo pulse sequence (STE) with bipolar gradient pulses (BPP) featuring an additional longitudinal eddy-current delay (LED).15 Typical employed parameters were 1-2 ms for the encoding gradient length (δ) 300-900 ms for the diffusion delay (∆) and 5 ms for the LED. Care was taken to quench possible temperature gradients (and the resulting convection) within the sample by increasing the refrigerating air flow to 800 L/h. Preliminary calibration was done using neat fluorobenzene.16 All the diffusion coefficients were estimated by fitting the experimental data to the model equation of ref 15. MS Spectrometry. Mass spectra were performed with an MSD SL Trap mass spectrometer equipped with ESI source (Agilent Technologies, Palo Alto, CA). Solutions of OV(Tf2N)2 10-4 M in toluene, chloroform and methanol were directly introduced into the spectrometer via a syringe pump at a flow

J. Phys. Chem. B, Vol. 112, No. 51, 2008 16567 SCHEME 1: Violgen Salts Investigated (X- ) (CF3SO2)2N-, Br-)

TABLE 1: 1H and 19F Chemical Shifts, ppm, of OV(Tf2N)2 and OVBr2 in Various Solvents with Dielectric Constant ε0 OV(Tf2N)2 solvent a

benzene toluenea CDCl3a CD2Cl2 acetone MeOD acetonitrile DMF DMSO

OVBr2

ε0

δ(1Ho)b

δ(1Hm)b

δ(19F)

δ(1Ho)b

δ(1Hm)b

2.2 2.4 4.9 8.9 20.7 32.6 36.6 38.3 46.7

8.32 8.26 8.89 8.88 9.53 9.25 8.89 9.63 9.35

7.70 7.76 8.53 8.63 8.92 8.64 8.36 9.00 8.74

-83.50 -83.68 -83.88 -84.39 -84.90 -85.67 -85.21 -84.69 -83.71

9.59 9.56 9.64 9.28 9.01 9.69 9.37

9.11 9.19 9.09 8.67 8.48 9.06 8.77

a

Only the sharp signals (see text) are reported. All systems were ca. 10-3 M or less. b Ho ) ortho protons, H2, H2′, H6, H6′; Hm ) meta protons, H3, H3′, H5, H5′.

rate of 20 µL/min. The spectra were acquired, in both positive and negative polarities, in normal mass range mode with m/z from 100 to 2200 and in extended mass range mode setting m/z from 200 to 4000. The desolvation gas was nitrogen. The nebulizer pressure was set at 15 psi and the dry gas at 4 L/min with a temperature of 325 °C. The capillary voltage was 4000 V for the solution in toluene and 3500 V for those in chloroform and in methanol. The target mass was changed in the range m/z 280-3800. MS/MS experiments were obtained by resonance activation of preselected species. DFT Calculations. Geometries were optimized at the B3LYP/ 6-311G** level of theory.17 Minimized structures (except of the larger cluster C2A4; see Results and Discussion) were checked by frequency calculations to be true minima. Calculations were run using the software package Gaussian03.18 Results and Discussion NMR Spectroscopy. Chemical shifts of the bipyridinium ring protons of the octyl viologen salts investigated (see Scheme 1) show large effects due to the solvent, the concentration and the counteranion. In contrast, carbon and nitrogen chemical shifts, as well as the alkyl chain proton resonances, do not show significant differences, as the solvent and the counteranion are varied. A useful indicator of the involvement of the octyl viologen dication in aggregation processes is the chemical shift value of some representative protons: if ion pairs or larger clusters are formed, then chemical shifts will change, being very sensitive probes of the local structure, and these changes will also be dependent on the anionic partners of OV2+. Therefore, in Table 1, we compare the results obtained for our system (Tf2N- salt) with those obtained for the reference bromide salt. In polar solvents a single set of resonances is observed; among these, in the more polar solvents (DMSO to methanol) the resonances of the ring protons are only slightly affected by the counteranion, suggesting that ion clustering does not take place. In the less polar solvents like acetone and dichloromethane the effect of the counteranion is larger, although we still observe only one set of sharp signals in the NMR spectra. This suggests that ion clustering occurs but it is fast on the NMR time scale so that only the average signal of free and complexed dication is detected.

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Figure 1. (Left) 1H and (right) 19F spectra of OV(Tf2N)2 in benzene, 5.3 × 10-3 M.

In contrast, in weakly and nonpolar solvents, like chloroform, benzene and toluene, we observe two sets of signals: one set is sharp, as found in polar solvents (the 3JHH doublet is clearly resolved), and the second set is rather broad, particularly in benzene and toluene. This indicates that a significant clustering of ions occurs and that these aggregates are long-lived species. An approximate estimate of the lower limit of their lifetime can be obtained considering the difference, in Hz, between the resonances of the two set of signals: because this is on the order of 102 Hz the aggregates are expected to exchange at a lower rate, which means their lifetime is at least on the order of 1 ms.19 We note that it is quite rare to have the opportunity to observe different ion clusters in solutions through NMR because usually the exchange is fast enough that only an average resonance is detected.20 19 F spectra confirm these results: in Figure 1 we compare 1H (aromatic region) and 19F spectra of OV(Tf2N)2 in benzene. We note that the two spectra have exactly the same shape and relative area of the signals. Because 1H and 19F NMR only see the dication and anion’s resonances, respectively, we can exclude the existence of stable, long-lived charged species, as the CA+ ion pair (C for the octyl viologen dication and A for the bistriflimide anion): in such a case there would be two different signals in the 19F spectrum, one for the CA+ species and the second one for the free A- anion, whereas there would be still one single set of signals in the 1H spectrum of the cation (CA+). The analogous spectra obtained in toluene and chloroform are reported in the Supporting Information, Figures S2 and S3. The chemical shift of the aromatic protons (for nonpolar solvents we only consider the set of sharp resonances) does not follow a clear trend as a function of the solvent polarity; see Figure 2. In fact, we expect the chemical shift of the cation to be strongly influenced by specific solvation effects, particularly in solvents with high donor number. Nevertheless, with the exception of methanol and acetonitrile, the maximum deshielding is observed for solvents of medium polarity. Such a trend is, instead, clearly visible in the chemical shift of fluorine, although it goes in the opposite direction: δ(19F) first decreases (fluorine atoms are shielded), as the polarity of the solvent is reduced, reaching a minimum for methanol and then it increases again (fluorine atoms are deshielded) in low-polar solvents. The more regular behavior of fluorine’s chemical shift is certainly due to the low coordination ability of (CF3SO2)N- for which strong and specific interactions with the solvent are unlikely.

Figure 2. (Top) 1H and (bottom) 19F chemical shifts, ppm, of OV(Tf2N)2 in the same solvents as in Table 1. For benzene, toluene and chloroform we only report the sharp set of signals; see text. All systems were ca. 10-3 M.

These results suggest that two opposite effects are in action, one being predominant in polar solvents the other one in nonpolar solvents, respectively. Although it might be reasonably argued that the solvation of a dicationic and an anionic species should be very different, it is important not to overinterpret the above results: as already mentioned, the chemical shift is strongly influenced by the local structure and it is rather difficult to extract detailed information on the solvation shell simply from the 1H (or 19F) chemical shifts in such complex systems.20 In solution several equilibria, as in eq 1, may be established

nC2+ + mA- ) (CnAm)(2n-m)+ Keq ) [CnAm(2n-m)+]/[C2+]n[A-]m

(1)

recalling that long-lived species can only be formed with m ) 2n, at least for small values of n and m. The broad signals, observed both in the 1H and in the 19F spectrum, are likely to be the superposition of several resonances

Octyl Viologen Di[bis(trifluoromethanesulfonyl)amide]

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Figure 3. (Left) 1H and (right) 19F spectra of OV(Tf2N)2 in benzene (5.3 × 10-3 M): (top) just after mixing; (bottom) after 1 h. The equilibrium spectra (after 24 h) are reported in Figure 1.

from clusters of different size. To support this hypothesis, we attempted a Lorentian/Gaussian line shape fitting of the broad and narrow signals appearing in the NMR spectra, because possible non-Lorentian contributions to the line shape can result from an envelope of many different chemical shifts. In this respect, 19F NMR spectra were selected as the best candidates for the line shape fitting because the broad signals display a lower asymmetry if compared to their 1H counterparts. For the case shown in Figure 2 we found that the sharp signal at -83.50 ppm has a pure Lorentian line shape, whereas the broad signal at -83.94 ppm is best fit by a 73% Gaussian and 27% Lorentian line shape (see Figure S4 in the Supporting Information). This result indeed supports the conjecture that the broad signal may actually be an envelope of different resonances. However, it must be noted that a Gaussian distribution of clusters of different sizes will not necessarily produce a corresponding Gaussian envelope of chemical shifts and the above result just provide a qualitative picture which must be further investigated. The equilibria of eq 1 are established in a relatively long time scale, depending on the analytical concentration, and their kinetics can be followed using NMR. In Figure 3 we show the dependence of the 1H and 19F spectra taken at different times after the salt has been dissolved in benzene. We note, at the beginning, a third set of signals which slowly disappear. These resonances can be reasonably attributed to an intermediate cluster which evolves to the equilibrium species. Even if its transient nature hampers a complete characterization, its detec-

tion highlights the ability of NMR to observe several species in a dynamic system in the low exchange regime. To better understand the ion clustering equilibrium that is established in nonpolar solvents, we have acquired spectra at different temperatures; see Figure 4. Clearly, the sharp doublets are largely affected by T, increasing in intensity as the temperature rises. This suggests that they correspond to the smallest clusters present in solution, assuming that increasing T shifts the equilibria of eq 1 to the left. Also, the resonance of the meta protons, δ(Hm), the most shielded ones, is significantly affected by T, in contrast to that one of the ortho protons, δ(Ho). The dependence of the chemical shift, particularly that of the meta protons, on the temperature (see Figure 5) suggests a fast equilibrium on the NMR time scale.21 This should involve at least four species, as already discussed, the free ions, C2+ and A-, and the clusters CA+ and CA2.

C2+ + A ) CA+

(2a)

CA+ + A- ) CA2

(2b)

Therefore, the observed sharp resonances in the 1H and 19F spectra may be attributed to the average signals obtained from the four species.

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Figure 4. 1H spectra of OV(Tf2N)2 in benzene (1.7 × 10-2 M) at (left) 278 K and (right) 335 K. The singlet at 7.53 ppm is one of the 13C satellite peaks of deuterated benzene.

Figure 5. Aromatic 1H chemical shifts of OV(Tf2N)2 in benzene vs temperature: (solid square) δ(Ho) broad; (empty squares) δ(Ho) sharp; (solid circles) δ(Hm) broad; (empty circles) δ(Hm) sharp.

The effect of the concentration has been investigated as well, for solutions in benzene and chloroform: on dilution the broad signals decrease their intensity, eventually leaving only the sharp doublets in the spectra; see Figure S5 in the Supporting Information for an example. Again, these results suggest that the set of sharp signals correspond to the smallest clusters (CA+, CA2 and the free ions in fast equilibrium), assuming that dilution shifts the equilibria of eq 1 to the left. On the other hand, increasing the concentration beyond a critical value leads to a phase separation: when 8.5 mg is added to 0.6 mL of deuterated chloroform, a pale yellow top layer with a high salt concentration is separated from the diluted CDCl3 solution. The color is probably due to a weak intermolecular charge-transfer transition between anion and cation as often found for viologen salts.3,22 Similarly, when 52 mg is added to 0.75 mL of C6D6 a yellow bottom layer is separated from the diluted top solution. In dichloromethane a single set of signals is observed, in contrast to what is found in solvents of lower polarity; however, because also this solvent is not highly polar, we believe that clustering of ions occurs to a large extent, although it might be fast on the NMR time scale. In Figure 6 we report the aromatic 1 H chemical shifts of the bipyridinium ring as a function of the concentration. The sigmoidal curve is an indication of a fast equilibrium between two species in solution. We also note that the meta protons are more affected by ion clustering than the ortho protons.

Figure 6. Aromatic 1H chemical shifts of OV(Tf2N)2 in CD2Cl2 as a function of concentration. The dot line is a sigmoidal fit; see Figure S6 in the Supporting Information.

Useful insights can be gained by measuring the diffusion coefficients of the species observed by DOSY spectroscopy. This can be done independently for the cation and the anion using 1H and 19F NMR, respectively. In Table 2 we report the data obtained. Two sets of 1H data obtained at 600 and 400 MHz, respectively, for the benzene solution, are in very good agreement with each other: the benzene self-diffusion coefficient is in good accord with experimental data (2.21 × 10-9 m2/s)23 as well as that of water in benzene (6.3 × 10-9 m2/s).24 The sharp signals correspond to a fast diffusing species: because the diffusion coefficient is 3-4 times smaller than that of benzene, it is compatible with an aggregate not larger than CA2. We also note that the diffusion coefficients of the cation and anion are not the same. The broad signals, instead, correspond to a species, or distribution of species, whose diffusion coefficient is about 20 times smaller. In this case, the diffusion coefficients obtained from 1H and 19F NMR are very close as it might be expected for large aggregates. Essentially the same results are obtained for the chloroform solution. The measured self-diffusion coefficient of the solvent is in good accord with literature data (2.45 × 10-9 m2/s)25 whereas the slow and fast species diffuse similarly to those in the benzene solution, suggesting that they represent the same kind of aggregate. The situation is different in dichloromethane, where there is only a single set of sharp resonances. Again, the residual solvent

Octyl Viologen Di[bis(trifluoromethanesulfonyl)amide]

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TABLE 2: Diffusion Coefficient, D, (10-10 m2/s) Obtained from the Sharp and Broad Signals Observed in Benzene and Chloroform and from the Only Sharp Signal Observed in Dichloromethane and in Methanol 1

H

a

benzene

benzeneb

CDCl3c CD2Cl2d CD2Cl2e MeOHf

broad sharp solvent H2O broad sharp solvent H2O broad sharp solvent sharp solvent sharp solvent sharp solvent

0.20 7.5 24 62 0.18 6.1 22 63 0.23 4.4 26 10.5 35 6.9 30 11 29

19

F

0.24 3.9

11 4.7 15

a 2.6 × 10-3 M, 301 K, 1H 600 MHz, 19F 282 MHz. b 2.6 × 10-3 M, 299 K, 1H 400 MHz. c 3.5 × 10-3 M, 298 K, 1H 400 MHz. d 2.0 × 10-3 M, 301 K, 1H 300 MHz, 19F 282 MHz. e 1.1 × 10-1 M, 301 K, 1H 300 MHz, 19F 282 MHz. f 2.0 × 10-3 M, 301 K, 1H 300 MHz, 19F 282 MHz.

self-diffusion coefficient is in good agreement with literature data (3.5 × 10-9 m2/s).25 In diluted CD2Cl2 solutions diffusion coefficients measured from 1H and 19F agree very well, both being about 1 × 10-9 m2/s. They are also very close, particularly in the concentrated solution, to those corresponding to the fast diffusing species in benzene and chloroform. Increasing the concentration by 2 orders of magnitude slightly increases the viscosity, as indicated by the slower diffusion coefficient of the residual solvent. However, the species observed by NMR are slowed down more than what can be accounted for by the viscosity; therefore, an association process takes place at higher concentration. Also, the anion signal (19F) appears to be affected by the concentration slightly more than the signal of the cation (1H). All these findings are consistent with the aggregation process described by eq 2. Finally, the relatively high values of diffusion coefficients found for both the anion and the cation in methanol (a polar solvent with self-diffusion coefficient of 2.4 × 10-9 m2/s at 25 °C)26 support the conjecture of isolated charged molecules diffusing freely without any aggregation. This situation is very close to that obtained in diluted dichloromethane solution, thus confirming that in dichloromethane the aggregation is strongly dependent on the salt concentration. The difference in the diffusion coefficients of the small and large aggregates in nonpolar solvents, suggests that the size of the large aggregates is on the order of hundreds of CA2 units.20 Considering, however, the relative intensity of the two signals, which is about 1:10, depending on temperature and concentration, we understand that the number of large aggregates in solution should be rather small. MS Spectrometry. ESI-MS may reveal interesting insights on the aggregates formation in solution,14,27-30 especially when specifically designed instruments are used.31 As mentioned in the Introduction, the Tf2N- anion does not show significant clustering with imidazolium cations in the ESI-MS spectra in polar solvents.14 However, because our NMR data suggest that large clusters are present in nonpolar solvents as stable species, we expect to observe them also in ESI-MS experiments. The

Figure 7. ESI-MS spectra in (top) toluene, (middle) chloroform and (bottom) methanol, negative mode. (n,m) indicates the clusters CnAmand the doubly charged cluster C7A162-. The inset in the middle panel is the MS/MS spectrum of the (7,16) peak. See text for experimental details.

solvents generally used for electrospray ionization are highly polar, but recently good quality ESI-MS spectra of ionic liquids obtained in nonpolar solvents have been reported.32 Ionic liquids containing bistriflimide have also been proposed as additives for allowing electrospray ionization of analytes dissolved in hexane, benzene and toluene.33 On the basis of these findings, we performed the ESI-MS spectra of OV(Tf2N)2 in toluene and compared them with those obtained in chloroform and in methanol. In Figure 7 the spectra obtained in the three solvents in negative ion mode are reported. The spectrum in toluene is the average obtained by varying the target mass parameter from m/z 3000 to 4000. Note the rather low intensity of the signal. The peaks can be easily assigned to the singly charged clusters A-, m/z of 280, CA3-, m/z of 1222, C2A5-, m/z of 2164, and C3A7-, m/z of 3106, where C is the OV2+ dication and A is the Tf2N- anion. The next cluster in the series, C4A9-, has an m/z ratio outside the range of detection. More interesting, an intense peak is observed at m/z of 3577. This corresponds to the doubly charged C7A162- cluster. Essentially the same pattern is observed in chloroform. The spectrum in Figure 7 is obtained with a target mass m/z of 3700. In this case we also show in the insert the fragmentation pattern obtained from the collisional activation (MS/MS spectrum) of the peak at m/z of 3577: we obtain the same peaks observed in the ESI-MS spectrum, that is, CA3-, C2A5-, and C3A7-, confirming the attribution of the signal at m/z of 3577 to the doubly charged C7A162- cluster. In striking contrast, the spectrum acquired in methanol, still with a target mass of m/z 3700, only shows the free anion peak at m/z of 280. By varying the target mass parameter some other peaks appear; however, we have no evidence of the presence of C7A162- and also C3A7- always appears very weak. This is in agreement with the results reported in ref 14, indicating a low coordination ability of the Tf2N- anion in polar solvents.

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Figure 8. Gas-phase optimized structures of CA+ clusters: (left) model A; (middle) model B; (right) model C.

The dependence from the solvent in the detection of ionic liquids aggregates by ESI-MS was previously evidenced by Dorbritz et al.27 The authors reasonably attributed the aggregates formation in the less polar solvents to the need of minimizing the charge density within the ions. In our case the relation between clustering and charge stabilization is evident also for doubly charged aggregates as the smallest observed is C7A162-. On the other hand, if no higher clusters are observed this is due to the limited mass range of the mass spectrometer (m/z of C8A182- would be 4048). In the positive detection mode the difference between polar and nonpolar solvents does not appear so evident, see Figure S7 in Supporting Information. In fact, in chloroform we observe several singly charged clusters (C2+, CA+, C2A3+, C3A5+, and C4A7+) plus a doubly charged one, C7A122+, as confirmed by its fragmentation pattern. A rather similar peaks distribution is found in methanol. However, the intensity of the high m/z clusters is rather low compared to the first two ions. DFT Calculations. We have optimized gas-phase geometries of model systems composed of methyl viologen dication and bistriflimide anion. The purpose of these calculations was to investigate whether or not the Tf2N- anion has specific favorable interactions with the bipyridinium ring, besides the obvious electrostatic interaction, which may justify the strong association observed in nonpolar solvents. We note, in fact, that the analogous iodide and bromide salts have a very low solubility in chloroform and they are insoluble in benzene.22 In the gas phase three stable ion pairs CA+ have been found; although the search on the potential energy surface was not exhaustive these three minima represent very different structures. In all cases Tf2N- is arranged in the most stable trans conformation.34 They are reported in Figure 8. In the first case, model structure A, the anion is located on top of one pyridinium ring; a short distance is observed between one oxygen atom and a meta proton of the other pyridinium ring. In the second case, model structure B, Tf2N- is stretched along the bipyridinium axis. In this case we can see an almost perfect match between the trifluoromethyl groups and the two nitrogens as well as between the oxygens and the meta protons. In the last case, model structure C, Tf2N- lies on one side of the bipyridinium ring and a short distance is observed between the nitrogen of the anion and the meta protons of the dication. The interaction energies are calculated as the difference between the SCF energy of the complex and that of the constituent ions. They are reported in Table 3. We note that they are very close, for the three complexes, the difference being below the limit of accuracy of DFT computational protocols. For the larger cluster, CA2, we have minimized three structures similar to those found for the CA+ ion pair; they are reported in Figure 9. The interaction energies, reported in Table 3, now indicate that the most stable structure is conformer C, by 3 and 8 kcal/ mol, respectively. Although the calculations are run in the gas

TABLE 3: Energies (B3LYP/6-311G**) of the Model Systems of Figures 8 and 9 MV+ Tf2NCA+ CA2

a

A B C A B C

E, hartree

∆E, kcal/mola

-574.793211 -1827.570114 -2402.579545 -2402.579135 -2402.577287 -4230.27952 -4230.27231 -4230.284432

-135.7 -135.4 -134.3 -217.2 -212.6 -220.3

Ecomplex - EMV - 2ETf2N.

phase they are expected to be representative of a nonpolar solvent as benzene. It is noteworthy that in the most stable arrangement, model C, the meta protons experience a strong interaction with the anion, thus accounting for the dependence of their chemical shift on temperature and concentration, considering the set of sharp resonances. However, a closer inspection of the structures B and C reveals that is it not easy to replicate them to build a larger aggregate without having close contacts between anions and/or between cations. In contrast, geometry A can be taken as the building block of a large layered superstructures obtained by replicating it along the direction perpendicular to the bipyridinium axis. A cluster C2A4 of this type has been minimized to check whether such aggregate would be stabilized by cation-anion attractive interactions or destabilized by anion-anion and cation-cation repulsive interactions. The minimized geometry features a close packing of the ions (see Figure 10), with an interaction energy of -463 kcal/mol that is significantly larger than twice the stabilization energies of model geometry CA2. General Discussion. The system investigated clearly shows a large degree of aggregation in nonpolar solvents similar to what can be expected for common surfactants and lyotropic liquid crystals. As for the case of thermotropic ionic liquid crystals, microsegregation is also invoked to explain the stability of lyotropic liquid crystalline phases, typically formed by amphiphilic molecules having a polar head attached to a long alkyl chain. Well-known examples are cetyltrimethylammonium bromide and sodium dodecyl sulfate. When dissolved in water, micelles, or other phases, are observed. Less common is the case of so-called inverse micelles formed when the amphiphilic molecule is dissolved in a nonpolar solvent. In the latter case, however, the stability of the system is enhanced by the presence of water as a third constituent of the mixture so that the inverse micelle is assembled around the water droplet avoiding the strong electrostatic repulsion that would be generated if too many polar heads were forced in a small space. In all these cases the role of the counterion is often simply to guarantee the electro-neutrality of the mixture whereas the phase stability and structure is almost entirely determined by the shape and

Octyl Viologen Di[bis(trifluoromethanesulfonyl)amide]

J. Phys. Chem. B, Vol. 112, No. 51, 2008 16573

Figure 9. Gas-phase optimized structures of CA2 clusters: (left) model A; (middle) model B; (right) model C.

nonpolar solvents at relatively low concentrations. These aggregates are long-lived species that can be detected by NMR together with the smaller CA2 clusters, an occurrence that is rarely found in NMR spectroscopy. DOSY experiments allowed us to measure the diffusion coefficients of the small and large aggregates. These are compatible with a simple CA2 cluster and larger aggregates with an average diffusion coefficient more than an order of magnitude smaller. ESI-MS spectra in nonpolar solvents confirm the presence of several clusters in solution of nonpolar solvents, despite the well-known low coordination ability of Tf2N-. Despite the fact that the amphiphilic molecule is the viologen dication, the counteranion is responsible for the formation of the large aggregates and the high solubility of the salt in nonpolar solvents. We suggest that these systems could have interesting applications because the redox properties of the viologen would allow us to induce lyotropic phase transitions by tuning the redox potential, similarly to what reported by Rosslee and Abbott for a ferrocenyl-based surfactant.36 Figure 10. Minimized geometry of the C2A4 model cluster. The local arrangement of the ions is as in structures A of CA+ and CA2.

chemical nature of the amphiphilic molecule. The most common shapes of supramolecular aggregates formed by amphiphilic molecules are ellipsoidal (the surface is curved in both dimensions, with the sphere being a particular case), cylindrical (the surface is curved only in one dimension) and planar (no curvature of the surface).35 In the present case, however, significant differences are encountered. First, the shape of the molecule (the cation), which is not made of a polar head attached to a long alkyl chain. Rather, there is a rigid ionic central core and two medium-length side chains, the three moieties having about the same length. Second, the counteranion, Tf2N-, is essential in driving the aggregation because the bromide salt, as well as the iodide salt (not discussed here) are not soluble in benzene and have a very low solubility in chloroform.22 It is unlikely that the aggregates observed in nonpolar solvents are micelles-like or cylindrical, because the shape of the viologen does not allow an easy packing in a curved surface. In contrast it is possible that layered aggregates are formed. In fact, as mentioned in the Introduction, this octyl viologen salt shows a large temperature range of stability of a smectic phase, behaving also as a thermotropic ionic liquid crystal. The results of DFT calculations suggest that an arrangement similar to that of Figure 10 is sufficiently stable to exist in solution. Conclusion We have presented experimental evidence of supramolecular aggregates formed by octyl viologen bistriflimide salt in

Acknowledgment. Calculations were run on the Linux cluster of the Laboratorio Interdipartimentale di Chimica Computazionale (Department of Chemical Sciences) of the University of Padova. Supporting Information Available: NMR and ESI-MS spectra of OV(Tf2N)2 and lists of optimized geometries. This material is available free of charge via the Internet at http:// pubs.acs.org References and Notes (1) Marcus, Y.; Hefter, G. Chem. ReV. 2006, 106, 4585. (2) Schulz, P. F.; Mueller, N.; Boesmann, A.; Wasserscheid, P. Angew. Chem., Int. Ed. 2007, 46, 1293. (3) Monk, P. M. The Viologens; John Wiley & Sons: Chichester, U.K., 1998. (4) (a) Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.; Venturi, M. Acc. Chem. Res. 2001, 34, 445. (b) Credi, A.; Balzani, V.; Langford, S. J.; Stoddard, J. F. J. Am. Chem. Soc. 1997, 119, 2679. (5) (a) Huang, F.; Jones, J. W.; Selbodnik, C.; Gibson, H. W. J. Am. Chem. Soc. 2003, 125, 14458. (b) Arduini, A.; Calzavacca, F.; Pochini, A.; Secchi, A. Chem. Eur. J. 2003, 9, 793. (c) Credi, A.; Dumas, S.; Silvi, S.; Venturi, M.; Arduini, A.; Pochini, A.; Secchi, A. J. Org. Chem. 2004, 69, 5881. (6) (a) Bhowmik, P. K.; Han, H.; Cebe, J. J.; Burchett, R. A.; Acharya, B.; Kumar, S. Liq. Cryst. 2003, 30, 1433. (b) Bhowmik, P. K.; Han, H.; Nedeltchev, I. K.; Cebe, J. J. Mol. Cryst. Liq. Cryst. 2004, 419, 27. (7) Binnemans, K. Chem. ReV. 2005, 105, 4148. (8) Yu, L.-P.; Samulski, E. T. In Oriented Fluids and Liquid Crystals; Griffin, A. C., Johnson, J. F., Eds.; Plenum: New York, 1984; Vol. 4, p 697. (9) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (10) Sun, J.; Forsyth, M.; MacFarlane, D. R. J. Phys. Chem. B 1998, 102, 8858.

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