Inclusion of methylviologen in cucurbit[7]uril

Abstract

The inclusion behavior of methylviologen (N,N′-dimethyl-4,4′-bipyridinium, MV) dication in cucurbit[7]uril (CB[7]) has been studied by using various spectroscopic and electrochemical methods. The inclusion complex of MV dication in CB[7] is stable thermodynamically and kinetically. The electrochemical study reveals that unlike β-cyclodextrin, CB[7] prefers the charged species, MV dication (MV²⁺), and cation radical (MV^+⋅) to the fully reduced neutral (MV⁰ species as guests. Dimerization of MV^+⋅ is suppressed effectively by forming a stable complex with CB[7] in aqueous solution as confirmed by spectroelectrochemical experiments. Furthermore, the first redox process (MV²⁺/MV^+⋅) of the MV²⁺–CB[7] complex occurs predominantly via the direct electron transfer pathway, whereas the second redox process (MV^+⋅/MV⁰) occurs via both the direct and indirect pathway because of the low affinity of the fully reduced species MV⁰ to CB[7].

To uncover the principles governing the interplay between redox events and molecular recognition is important not only for fundamental understanding of biological systems such as redox enzymes (1) but also for designing molecular devices (refs. 2–5 and ref. 6, entire issue). Thus, a number of host–guest systems involving redox-active guests have been studied to elucidate interdependence of redox processes and molecular recognition (7–16). Following the seminal work of Evans, Osa, and coworkers (10) on the binding and voltammetric behavior of ferrocene carboxylate in β-cyclodextrin (β-CD), other host–guest systems such as CD-viologen (11, 12), CD-cobaltocene (13), and CD-ferrocene derivatives (14), and calixarene-viologen (15) and calixarene-ferrocene derivatives (16) have been studied. Through these extensive studies, it has been well established that the stability of host–guest complexes can be affected markedly by the redox state of the redox-active guest molecules. Another interesting result from these studies is that the electrochemical redox processes in the presence of the hosts occur only on the free guests. No direct electron transfer to and from inclusion complexes has been observed with an exception of hemicacerand-ferrocene system (17), in which the redox-active guest is locked in the host cavity.

Cucurbituril (cucurbit[6]uril, CB[6]), a macrocyclic cavitand comprising six glycoluril units, has a cavity of ≈5.5 Å in diameter that is accessible through two identical carbonyl-fringed portals (18–20). Similar to CDs, the cavity can hold small organic molecules through hydrophobic interaction. Unlike CDs, however, the carbonyl groups at the portals allow CB[6] to bind ions and molecules through charge-dipole as well as hydrogen-bonding interactions. The rigid structure and capability of forming complexes with molecules and ions make CB[6] very attractive as a synthetic receptor (18–20) and as a building block for the construction of supramolecular architectures (21–29). Our recent synthesis (30) of new CB homologues (CB[n], n = 5, 7, and 8) containing five, seven, and eight glycoluril units, respectively, has opened up new opportunities in supramolecular chemistry (31, 32). For example, the largest member of the CB family, CB[8], with a cavity comparable to that of γ-CD can accommodate two aromatic guest molecules to form 1:2 host–guest complexes (30), or 1:1:1 ternary complexes (31) and even include macrocycles and their transition metal complexes in the cavity (32). We also learned that CB[8] exclusively forms a 1:1 host–guest complex with methylviologen dication (MV²⁺) (31) even though the host has a cavity large enough to accommodate two MV²⁺ molecules. In terms of size, an MV²⁺ molecule would fit better into the cavity of CB[7], the size of which is comparable to that of β-CD. Indeed, we now discovered that CB[7] forms a very stable 1:1 inclusion complex with MV²⁺, although it is well known that β-CD does not bind MV²⁺ appreciably. Furthermore, direct electron transfer to and from the inclusion complex is observed. Here we report the inclusion of MV²⁺ and its reduced species in CB[7] and the electron transfer behavior in this host–guest system.

Materials and Methods

Materials.

MV dichloride was purchased from Aldrich, and CB[7] was prepared according to our previous report (30). Unlike other CB homologues, CB[7] has a reasonable solubility in water (≈10⁻² M), which is comparable to that of β-CD. The concentrations of MV²⁺ in solution were calculated from UV-visible absorption data by using the molar absorption coefficient at 262 nm (ɛ = 21,000 M⁻¹⋅cm⁻¹; ref. 33). The binding stoichiometries between MV²⁺ and CB[7] were determined by ¹H NMR spectroscopy by using a Bruker (Billerica, MA) DPX-500 spectrometer.

Isothermal Titration Calorimetry.

The formation constant and thermodynamic parameters for the inclusion of MV²⁺ in CB[7] were measured by the titration calorimetry method by using a VP-ITC instrument from MicroCal (Amherst, MA). All solutions were prepared in a Tris-buffer solution (0.05 M, pH 7). A solution (1.0 mM) of CB[7] was placed in the sample cell. As a 20 mM solution of MV dichloride was added in a series of 50 injections (5 μL), the heat evolved was recorded at 25°C. The heat of dilution was corrected for by injecting the MV²⁺ dichloride solution into a neat buffer solution and subtracting this data from that of the host–guest titration. The data were analyzed and fitted by using ORIGIN software (MicroCal).

Selective Inversion-Transfer Experiment.

The measurement of the life-time for the inclusion complex of MV²⁺–CB[7] was carried out by the selective inversion-transfer method (34). The resonance of the complexed MV²⁺ was inverted selectively by using the DANTE pulse sequence (35), and the carrier was set on the resonance to be inverted. Mixing times ranging from 50 μs to >5 T₁ were used; T₁ values were estimated by the inversion-recovery method. Inversion-transfer spectra were measured at 18 delays. The analysis of data was performed by the reported method (36) using nonlinear least-square fitting.

Electrochemical Experiments.

The electrochemical experiments were performed with a Princeton Applied Research model-273 multipurpose instrument interfaced to a personal computer. A glassy carbon working electrode (0.07 cm²), a Pt counter electrode, and a saturated calomel electrode as a reference electrode separated with a fine glass frit were used in a single-compartment cell. The surface of the working electrode was polished with 0.05 μm alumina/water slurry on a felt surface and rinsed with purified water before electrochemical experiments. The experiments were conducted in 0.1 M phosphate buffer solutions (pH 7.0) prepared with purified water (Milli-Q, Millipore). All solutions were deoxygenated by purging with argon gas and maintained under an inert atmosphere during the electrochemical experiments. The voltammetric data were analyzed by digital simulations carried out with the DIGI-SIM 2.1 software package (Bioanalytical Systems, West Lafayette, IN).

Spectroelectrochemical Experiments.

A spectroelectrochemical cell was assembled with a piece of glass sheet and a piece of indium tin oxide-coated glass and a spacer film (thickness, 100 μm). The ball-shaped polyethylene supporters located between the indium tin oxide working electrode and cell wall were used to maintain a reproducible light-path length. Electrolysis of MV²⁺ to generate one-electron-reduced species was achieved by applying a potential (−0.8 V) to the indium tin oxide working electrode. UV-visible absorption spectra were recorded on a Hewlett–Packard 8453 diode array spectrophotometer. The solution before electrolysis was used as a blank reference.

Results and Discussion

Inclusion of MV²⁺ in CB[7].

MV²⁺ readily forms a stable 1:1 host–guest complex with CB[7] in water as evidenced by ¹H NMR and mass spectrometry (Scheme S1). The chemical shift of the β-proton of the bipyridinium moiety is shifted significantly to higher field after the complex formation, whereas the signal of the methyl group is shifted to lower field, and that of the α-proton of the bipyridinium moiety is almost unchanged (Fig. 1). The NMR data clearly indicate that the resulting host–guest complex between MV²⁺ and CB[7] has a simple pseudorotaxane structure in which the bipyridinium moiety resides inside CB[7], and the two methyl groups are located outside. The parent peak {(MV-CB[7])²⁺ = 674.2)} corresponding to the 1:1 host–guest complex was observed also by mass spectrometry. The formation constant and thermodynamic parameters associated with the inclusion phenomenon have been determined by titration calorimetry: log K = 5.30, ΔH° = −3.4 kcal⋅mol⁻¹, and ΔS° = 13.0 cal⋅mol⁻¹⋅K⁻¹. It is interesting to note that the inclusion process is accompanied by a positive entropy change that rarely is observed in host–guest systems (37, 38). Most importantly, however, the remarkably strong binding of MV²⁺ to CB[7] is in sharp contrast to the fact that β-CD, which has a hydrophobic cavity with a comparable size, does not bind MV²⁺ appreciably because of the +2 charge (11, 12). The strong binding of MV²⁺ by CB[7] can be explained by the favorable ion-dipole interaction between the positive charge of the guest and the portal oxygen atoms of CB[7] in addition to the hydrophobic effects.

Scheme 1.

Schematic illustration of the inclusion of MV²⁺ in CB[7].

Figure 1.

¹H NMR spectra in deuterium oxide of MV²⁺ in the absence of CB[7] (a), after the addition of 0.5 equivalents of CB[7] (b), and after the addition of 1 equivalent of CB[7] (c). ○, CB[7]; ▴, included MV²⁺; ▵, free MV²⁺.

We also investigated the kinetic lability of the inclusion complex. As shown in Fig. 1b, two sets of signals are observed for MV²⁺, one for included MV²⁺ and the other for free, which implies that the exchange between the two is slow on the NMR time scale. To obtain quantitative information, selective inversion-transfer experiments (34) were performed on a 1:2 mixture of CB[7] and MV²⁺. Analysis of the data gives a life-time of 5.3 ± 0.5 ms for the inclusion complex of MV²⁺ in CB[7], from which the dissociation rate constant is estimated to be 1.9 × 10² s⁻¹. This dissociation rate constant is much smaller than those (≈10⁴ s⁻¹) for the host–guest complexes formed between β-CD and ferrocene carboxylate (10), cobaltocene (13), and fully reduced viologen derivatives (11, 12), which are known to be bound strongly to the host. We therefore conclude that the inclusion complex of MV²⁺ in CB[7] is thermodynamically much more stable and kinetically much less labile compared with those of the β-CD complexes.

Electrochemistry of MV in the Presence of CB[7].

Typical cyclic voltammograms of MV obtained in the absence and presence of CB[7] are shown in Fig. 2. The corresponding half-wave potentials (E_1/2) and the diffusion coefficients (D) measured are given in Table 1. As well documented, MV²⁺ undergoes two reversible one-electron reductions in the absence of CB[7]. However, the addition of CB[7] produced a pronounced effect on the cyclic voltammogram of MV²⁺: noticeable shifts in the peak potentials and a marked decrease in the current levels. The first reduction wave retains the reversible shape but shifts a more negative potential (ΔE ≈ −22 mV) in the presence of equimolar CB[7]. The second reduction wave is shifted to a much more negative potential (ΔE ≈ −111 mV) and becomes quasireversible. This negative shift is more pronounced as the amount of CB[7] increases. For example, the shift of the half-wave potential for the second reduction increases to ΔE ≈ −150 mV after the addition of 3 equivalents CB[7], but the voltammetric response for the first redox process is little affected.

Figure 2.

Cyclic voltammograms (0.1 V⋅s⁻¹) of 0.5 mM MV²⁺ in the presence of 3 equivalents of CB[7] (solid line) and absence of CB[7] (dashed line). SCE, saturated calomel electrode.

Table 1.

Voltammetric parameters for MV²⁺ in the absence of CB[7] and presence of 3 equivalents of CB[7] in 0.1 M phosphate buffer (pH 7.0) solution at 25°C

	E(ΔEp)*	E(ΔEp)†	D (cm2⋅s−1)‡
MV2+	−0.704 (64)	−1.014 (64)	6.6  × 10−6
MV2+/CB[7]	−0.726 (66)	−1.164 (122)	1.8  × 10−6

^*Half-wave potential for the first reduction process expressed in volts vs. saturated calomel electrode. The values in parentheses represent the potential difference in mV between the cathodic and anodic peak potentials measured at 0.1 V⋅s⁻¹. 

^† For the second reduction process. 

^‡ Diffusion coefficient determined from the plot of i_p vs. scan rate^1/2. 

The shift in half-wave potentials reflects relative binding affinities of the guest of different redox states to the host. The small negative shift in the E_1/2 value of the first reduction (MV²⁺/MV^+⋅) in the presence of CB[7] indicates that the cation radical form interacts slightly less strongly with CB[7] compared with the initial, dication form. On the other hand, the much larger negative shift observed in the half-wave potential for the second reduction (MV^+⋅/MV⁰) reveals that the binding affinity of the fully reduced, neutral species to CB[7] is considerably reduced. Therefore, the voltammeric results clearly demonstrate that CB[7] prefers the charged species such as MV²⁺ and MV^+⋅ to the neutral species MV⁰ as a guest. The formation constants between the reduced MV species (MV^+⋅ and MV⁰) and CB[7] estimated by the potential shifts are listed and compared with those of the corresponding β-CD complexes in Table 2. It is interesting to point out that the relative binding affinities of CB[7] toward MV in the three different redox states exhibit an exactly opposite trend to those of β-CD. It is well known that β-CD binds the neutral species MV⁰ much more strongly than the charged species MV^+⋅ and MV²⁺ (11, 12). Although CB[7] and β-CD have similar hydrophobic cavities, the different nature of interaction at the cavity entrances results in the large difference in their host–guest behavior. The carbonyl oxygens at the portal of CB[7] allow additional ion-dipole interaction with positively charged guests, which makes CB[7] favor MV²⁺ and MV^+⋅ with much higher affinities compared with β-CD.

Table 2.

Formation constants (M⁻¹) for the inclusion of MV species in CB[7] and comparison with those of β-CD^*

Host	Constant			Ref.
	K1	K2	K3
CB[7]	2.0  × 105	8.5  × 104	2.5  × 102	This work
β-CD	0	30	1.4  × 103	11

^*See Scheme S2. 

The electrochemistry of viologen in aqueous solutions is complicated often by dimerization of the cation radical forms (V^+·) as well as precipitation of the cation radical and/or fully reduced species (V⁰). The cyclic voltammogram of MV²⁺ in the presence of CB[7] indicates that CB[7] effectively suppresses the dimerization of MV^+⋅ and precipitation of MV⁰. A spectroelectrochemical study confirms that the dimerization of MV^+⋅ is suppressed largely by the addition of 1 equivalent of CB[7] as MV^+⋅ generated in the presence of CB[7] exhibits a UV-visible absorption spectrum (Fig. 3) that is essentially identical to that of the monomeric MV^+⋅ (39–41). Moreover, the suppression of dimerization of MV^+⋅ by CB[7] is much more efficient than that by β-CD, because a large excess (at least 15-fold) of the latter is required to produce the same effect (39–41). The voltammogram of MV²⁺, particularly at a high concentration of MV²⁺ (>1 mM) in the absence of CB[7] often shows an adsorption wave on the reverse scan caused by the significantly decreased solubility of MV⁰ in aqueous solution. However, the addition of just one equivalent of CB[7] results in a remarkable decrease of the adsorption wave, presumably caused by the improved solubility of MV⁰ by forming a complex with CB[7].

Figure 3.

Absorption spectra of one-electron-reduced species of MV²⁺ (1.5 mM) in phosphate buffer solutions (pH 7.0) using an indium tin oxide glass electrode taken at its potential of −0.8 V. Solid and dashed lines represent spectra in the presence of 1 equivalent of CB[7] and absence of CB[7], respectively.

Investigation of Electron Transfer Pathways.

The thermodynamically and kinetically high stability of the inclusion complex of CB[7] with MV²⁺ allows us to investigate the electron transfer behavior of this host–guest system. The electron transfer between inclusion complexes of redox-active guests and the electrode surface can proceed by the two different pathways: (i) direct electron transfer to and from inclusion complexes, and (ii) indirect electron transfer after dissociation of the complexes. Scheme S2 illustrates possible electrochemical and chemical equilibria involved in the reduction of MV²⁺ guest in the presence of CB[7] host.

Scheme 2.

Electrochemical and chemical equilibria involved in the reduction of MV²⁺ guest in the presence of CB[7] host.

Digital simulations were performed to examine whether the electron transfer occurs directly or indirectly. In the presence of 3 equivalents of CB[7] at a scan rate of 0.1 V⋅s⁻¹, the direct electron transfer pathway affords satisfactory simulation that reproduces the observed voltammetric behavior, whereas the simulated voltammogram based on the indirect pathway does not provide a good agreement with experimental data as shown in Fig. 4. The rather poor agreement between the experimental and simulated currents in the potential region negative from −1.0 V presumably is caused by a contribution from solvent reduction, which is not considered in the simulations.

Figure 4.

Cyclic voltammogram at 0.1 V⋅s⁻¹ for a 0.5 mM solution of MV²⁺ after the addition of 3 equivalents of CB[7] (solid line) in pH 7.0, 0.1 M phosphate buffer and simulated voltammograms based on direct (dashed line) and indirect (dotted line) electron transfer pathway. Parameters used in the simulations: k_s/(D_free)^1/2 are equal to 15.6 s^−1/2 and 7.8 s^−1/2 for the first and second reductions, respectively. The formation constants were taken from those in Table 2. The association rate constants for the three complexations were taken as k_a = 4 × 10⁷ (MV²⁺), 1 × 10⁵ (MV^+⋅), and 7 × 10² (MV⁰ M⁻¹⋅s⁻¹). D_complex/D_free = 0.28. SCE, saturated calomel electrode.

We further investigated the voltammetric behavior of the inclusion complex of MV in CB[7] at faster scan rates to obtain more concrete evidences for the direct electron transfer. By increasing the scan rate, the time scale of the voltammetric experiments approaches the life-time of the inclusion complexes. Thus, if the inclusion complex is capable of direct electron transfer, the voltammetric behavior should not be affected much. On the other hand, if complex dissociation must precede the electron transfer, a substantial current decrease is expected, because the availability of the electroactive species at the electrode surface is limited by the kinetics of the dissociation reaction. The solid line in Fig. 5 shows the voltammetric response recorded at a scan rate of 3.0 V⋅s⁻¹ with MV²⁺ in the presence of 3 equivalents of CB[7]. Notice that the voltammetric response for the first redox process (MV²⁺/MV^+⋅) is not affected, which supports the direct electron transfer pathway. The simulated voltammograms at the same scan rate further support that the first redox process (MV²⁺/MV^+⋅) in the presence of CB[7] occurs predominantly via the direct electron transfer pathway. The dashed line voltammogram in Fig. 5 would be obtained if the inclusion complex reacts directly at the electrode surface, whereas the dotted line voltammogram is expected if the inclusion complex does not engage directly in electrochemical processes. As shown in Fig. 5, the voltammetric behavior observed at a fast scan rate is very close to the simulated one represented by the dashed line, thus supporting the direct electron transfer pathway. We also estimated the apparent standard rate constant for heterogeneous electron transfer (k_s) of MV²⁺ by Nicholson's method (42): 0.04 cm⋅s⁻¹ for the free MV²⁺ and 0.006 cm⋅s⁻¹ for the MV²⁺ complexed by CB[7]. The reduction of the electron transfer rate can be explained by the fact that the electron transfer is hindered largely because of the strong complexation of MV²⁺ by CB[7]. A similar retardation of the electron transfer also was reported for the complexes of ferrocene bound inside hemicarcerands, which are stable kinetically and thermodynamically (17).

Figure 5.

Experimental voltammogram (solid line) at 3 V⋅s⁻¹ for a 0.5 mM solution of MV²⁺ in the presence of 3 equivalents of CB[7] and simulated voltammograms based on direct (dashed line) and indirect (dotted line) electron transfer pathway. Parameters used in the simulations are similar to those given in Fig. 4. SCE, saturated calomel electrode.

On the other hand, the electron transfer pathway for the second redox process is not decisive, because the binding affinity of MV⁰ to CB[7] is not large enough to assure the direct electron transfer pathway. In fact, we have observed a voltammeric response corresponding to free MV^+⋅/MV⁰ in addition to that of the complexed MV^+⋅/MV⁰ in the presence of equimolar CB[7] at a scan rate of 3.0 V⋅s⁻¹ (Fig. 6). We therefore conclude that the direct electron transfer to and from the inclusion complex is predominant in the first redox process (MV²⁺/MV^+⋅), but in the second redox process (MV^+⋅/MV⁰) the electron transfer seems to occur via both the direct and indirect pathway because of the low affinity of the fully reduced species MV⁰ to CB[7].

Figure 6.

Experimental voltammogram (solid line) at 3 V⋅s⁻¹ for a 0.5 mM solution of MV²⁺ in the presence of equimolar CB[7] and simulated voltammograms based on direct electron transfer pathway (dashed line). Parameters used in the simulations are similar to those given in Fig. 4. SCE, saturated calomel electrode.

Conclusions

We have investigated the inclusion behavior of MV in CB[7] by using various spectroscopic and electrochemical methods. The MV²⁺–CB[7] complex is stable thermodynamically and kinetically. The electrochemical study demonstrates that CB[7] prefers the charged species such as MV²⁺ and MV^+⋅ to the neutral species MV⁰ as guests, which is exactly opposite to β-CD. The ion-dipole interaction between positively charged guests and carbonyl oxygens at the portal of CB[7] seems to be responsible for the unique inclusion behavior. The dimerization of the cation radical MV^+⋅ is suppressed effectively by forming a stable complex with CB[7]. Furthermore, direct electron transfer to and from the MV²⁺–CB[7] complex is observed, a rare event in host–guest complexes. These findings not only contribute to a deeper understanding of interplay between redox events and molecular recognition but also provide an insight to the design of novel molecular devices such as electrochemically controllable molecular machines.

Abbreviations

CD

cyclodextrin

CB[n]

cucurbit[n]uril

MV

methylviologen