Article pubs.acs.org/Langmuir
Combining Proton and Electron Transfer to Modulate the Stability of Cucurbit[7]uril Complexes Wei Li and Angel E. Kaifer* Center for Supramolecular Science and Department of Chemistry, University of Miami, Coral Gables, Florida 33124-0431, United States S Supporting Information *
ABSTRACT: (Ferrocenylmethyl)methylammonium (1H+) is an excellent cationic guest for inclusion complexation by the cucurbit[7]uril host. However, deprotonation of the guest leads to a decrease of 14.8 kJ mol−1 in the overall stability of its inclusion complex, and one-electron oxidation of the guest’s ferrocene residue further diminishes the complex stability by another 12.3 kJ mol−1. Overall, guest deprotonation followed by its oxidation constitutes an accessible method to lower the equilibrium association constant for the CB7·1H+ complex by more than 4 orders of magnitude.
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INTRODUCTION One of the main reasons driving the increasing popularity of the cucurbit[n]uril (CBn) host family1−5 in supramolecular chemistry is that their inclusion complexes reach extraordinarily high stability with suitable guests. For instance, cucurbit[7]uril (CB7) exhibits extremely high binding affinities with ferrocene,6 adamantane,7 and structurally related8,9 derivatives. The association equilibrium constant (K) between CB7 and the dicationic guest 1,1′-bis(trimethylamoniomethyl)ferrocene is similar to that measured between avidin and biotin (K ≈ 1015 M−1).6 The ultra high thermodynamic stability of these complexes leads to extremely long complex lifetimes and very slow dissociation rates. It would be very desirable to develop accessible methods to modulate the complex binding affinities in order to facilitate their dissociation on demand. With this goal in mind, we decided to investigate the binding interactions of a simple ferrocene derivative, (ferrocenylmethyl)methylamine (1), with the CB7 host in aqueous media (see structures in Figure 1). The key idea behind the selection of this guest is that protonation/deprotonation of the amine nitrogen may add or remove ion−dipole attractive interactions
between the ammonium nitrogen and one of the host cavity portals, which are laced with carbonyl oxygens. This may result in sizable changes on the stability of the CB7·1 inclusion complex. Also, the accessible one-electron oxidation of the ferrocenyl group in this complex leads to the generation of a positively charged group inside the hydrophobic cavity of the host, another factor that is expected to lower the stability of the supramolecular host−guest complex. Therefore, we were particularly interested in determining the changes in binding affinities that can be brought about by amine protonation/ deprotonation reactions, as well as by the reversible oneelectron oxidation of the ferrocene residue in guest 1. We report here the results of this investigation.
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Compound 1 was prepared by the reductive amination of ferrocenecarboxaldehyde with methylamine as reported before.10 CB7 was prepared and isolated as previously reported.11 Cobaltocinium hexafluorophosphate, 1-(1-adamantyl)-pyridinium bromide, deuterium chloride, d3-sodium acetate, ferrocenecarboxaldehyde, Nmethylamine, and sodium borohydride were all commercially available. Cyclic voltammetric experiments were performed on a BAS-100W electrochemical workstation at room temperature. A single-compartment glass cell was fitted with a glassy carbon working electrode, Pt counter electrode, and Ag/AgCl reference electrode. The glassy carbon working electrode was polished with 5-μm alumina powder (0.05-μm, Buehler MicropolishII) on a felt surface before every measurement. Received: August 11, 2012 Revised: September 23, 2012
Figure 1. Guest and host structures. © XXXX American Chemical Society
EXPERIMENTAL SECTION
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1 H NMR spectra were recorded using a 500 MHz Bruker Avance NMR spectrometer. The pH measurements in H2O and D2O were done using a PHR-146 Micro Combination pH Electrode with an Accumet model 50 pH/ion/conductivity meter from Fisher Scientific, calibrated using standard buffers, pH 4, 7, and 10. Experimentally determined pH values were converted to pD values using the equation pD = 0.4 + pH.12
and shows the relevant CB7 complexation equilibria for the basic and acidic forms of the guest, 1 and 1H+. Scheme 2. Relevant Equilibria between the Acidic and Basic Forms of Guest 1 and the CB7 Hosta
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RESULTS AND DISCUSSION Initially, we verified that the acidic and basic forms of the guest (1H+ and 1) give rise to stable inclusion complexes with CB7 using 1H NMR spectroscopic data. Addition of CB7 to D2O solutions containing either form of the guest leads to upfield shifts for the guest proton resonances (see the Supporting Information, Figures S1 and S2), signaling the formation of inclusion complexes. At the millimolar concentration levels used in NMR spectroscopy, both complexes are formed quantitatively upon addition of 1.0 equiv of the host to an aqueous solution containing 1 and/or 1H+. These NMR spectroscopic data are similar to those obtained with other ferrocene derivatives already reported by our group13 and confirm that both forms of the guest are bound by CB7. We have also verified the complexation of 1H+ by CB7 using mass spectrometric data (Figure S3). In order to address the thermodynamic stabilities of both complexes we must take into account that the guest can undergo not only proton transfer but also electron transfer reactions. The full interplay between CB7 complexation of 1 and its proton and electron transfer reactions gives rise to a “cubic” scheme (Scheme 1).
This scheme corresponds to the front “face” of the cube in Scheme 1. Ka,f and Ka,c are the acid dissociation constants of the protonated guest and its CB7 complex, respectively. K+ and K represent the equilibrium constants for association with CB7 of the acidic and basic forms of the guest, respectively. a
Initially, we tried to obtain the ratio between the acid dissociation constants of free 1H+ and its CB7 complex, CB7·1H+. The four equilibrium constants in Scheme 1 are related by K+K a,c = KK a,f (1)
Scheme 1. Complete Schematic Representation of the Relevant Equilibria between All Acid/Base and Redox Species Associated with Guest 1 and Their Corresponding CB7 Complexesa
so measuring the ratio between Ka,c and Ka,f (or the pKa shift upon complexation) is the same as measuring the ratio between the association equilibrium constants K and K+. The pH dependence of the anodic electrochemical behavior of 1 may provide a simple way to accomplish this. Figure 2A shows how the set of waves, centered at a half-wave potential (E1/2) of +0.41 V vs Ag/AgCl at low pH, shifts to less positive values as the pH of the solution increases. The corresponding plot of E1/2 vs pH (Figure 2B) is sigmoidal and exhibits a clear inflection point at pH ∼9.0, which represents the pKa of compound 1H+ (or Ka,f ≈ 1 × 10−9 M). At low pH the acidic form of the guest, 1H+, exhibits a more positive E1/2 value because protonation of the amine nitrogen withdraws electron density from the ferrocene nucleus, making its oxidation thermodynamically less favorable than that of the basic form, free amine 1. In the presence of 1.0 equiv of CB7, a comparable pH dependence is observed (Figure 2C), corresponding to the transition from the protonated form of the inclusion complex, CB7·1H+, to its basic form, CB7·1. The half-wave potentials for the reversible oxidation of the acidic and basic forms of the CB7 complex are different from those recorded for the free ferrocene derivative, reflecting the effect of ferrocene inclusion inside the CB7 cavity (vide infra). The plot of E1/2 vs pH (Figure 2D) shows a gradual conversion from CB7·1H+ to CB7·1 and suggests that the presence of CB7 differentially stabilizes the protonated form relative to the free base, leading to a substantial increase (ca. 2 units) on the pKa of the CB7·1H+ complex relative to the value recorded for free 1H+. This finding is in general agreement with other reports on pKa
a
On the upper corners of the cubic scheme, 1 and 1H+ represent the basic and acidic forms of the guest, whereas 1+ and 1H2+ are the basic and acidic forms of the oxidized guest. On the lower corners of the scheme the same four species are shown as the corresponding CB7 complexes.
In this scheme proton transfer processes connect the left and right faces of the cube, electron transfer reactions link the front and back faces, and CB7 complexation processes relate the top and bottom faces of the cube. The scheme is also useful to provide definitions for the symbols representing the thermodynamic parameters that we will use here. In this work we will focus primarily on two “faces” of this cubic scheme, the front and the right faces. The first one is represented in Scheme 2 B
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which a limited amount of CB7 binds competitively to 1H+ (or 1) and a reference guest,7 with a previously measured and accepted association equilibrium constant. We selected 1-(1adamantyl)-pyridinium and cobaltocenium as the reference guests for competition with 1H+ and 1, respectively. Details of these competitive binding experiments are given in the Supporting Information. The equilibrium association constants with CB7 that we obtained in these binding competition experiments are 3.4 × 109 M−1 for 1 and 1.3 × 1012 M−1 for 1H+ in 50 mM sodium acetate solution. These values correspond to the symbols K and K+ in Scheme 2, respectively. Clearly, deprotonation of the CB7·1H+ complex leads to a substantial decrease in its overall stability; our measured values for K+ and K represent a decrease of 14.8 kJ mol−1 in the overall stability of the complex. Using the thermodynamic cycle in Scheme 2 and eq 1, we can now determine the acid dissociation constant for CB7·1H+. Therefore, from the estimated Ka,f value of 1 × 10−9 (see Figure 2B), we obtain Ka,c = 3 × 10−12 M, which represents a pKa increase of 2.6 units upon complexation by CB7, in reasonable agreement with the approximate values estimated directly from the pH titrations using voltammetric (Figure 2) and NMR spectroscopic data (Figures S4 and S5). We should note here that the electrochemical experiments of Figure 2 were done in 0.1 M NaCl, whereas the binding constant determinations were done in 50 mM sodium acetate. The latter medium was chosen for consistency with the work of Isaacs and co-workers,7 which has effectively established the standard for binding investigations involving cucurbituril hosts. The use of slightly different media in the electrochemical experiments, may introduce small inaccuracies in our calculations, but it is required by the nature of the experiments. The next issue at this stage is whether the binding affinity can be further affected using the redox chemistry of the ferrocene residue. Figure 3 shows the cyclic voltammetric behavior of guest 1 at several pH values in the absence and in the presence of the CB7 host. Under acidic conditions (Figure 3A), addition of 1.0 equiv of CB7 leads to a substantial anodic shift (ΔE1/2 = +0.112 V) on the half-wave potential for oxidation of 1H+. Furthermore, the current levels decrease substantially upon CB7 addition. Both effects are similar to those previously reported by us with (ferrocenylmethyl)trimethylammonium and other structurally related, cationic ferrocene guests.13 The observed anodic shift in the half-wave potential reflects the poorer solvation of the oxidized, positively charged ferrocenium form inside the hydrophobic cavity of CB7, whereas the reduced current levels result from the decreased diffusivity of the inclusion complex relative to that of the free guest. Notice that addition of 0.5 equiv of CB7 gives rise to the simultaneous observation of two redox couples corresponding to the reversible oxidations of free 1H+ and its complex, CB7·1H+. This is expected in light of the high binding constant between CB7 and 1H+ and the sizable difference in half-wave potentials for the oxidation of the free guest and its CB7 complex.17 The CB7-induced changes on the electrochemical behavior are similar at higher pH values, although the CB7-induced shift in the E1/2 value is slightly larger. The half-wave potentials measured in the cyclic voltammetric experiments allow the estimation of the association equilibrium constants between CB7 and the oxidized forms of the guest. For instance, using data obtained at pH 12 (Figure 3C, ΔE1/2 = +0.127 V), the equilibrium constant for CB7 complexation of 1+ (KOX) can be determined as follows:
Figure 2. Solution pH dependence of the cyclic voltammetric behavior on glassy carbon (0.07 cm2) of a 1 mM solution of guest 1 in 0.1 M NaCl (A) in the absence and (C) in the presence of 1.0 equiv CB7. Scan rate: 0.100 V s−1. Plots B and D show the pH variation of the measured E1/2 values in the absence and in the presence of 1.0 equiv CB7, respectively. The red dotted rectangle in (B) indicates a region in which two sets of CV waves were observed.The solution pH values were adjusted by addition of HCl or NaOH.
shifts measured upon CBn complexation of various classes of dyes.14−16 Because of the poorly defined shape of the plot in Figure 2D, we carried out similar NMR spectroscopic experiments in an attempt to measure more accurately the pKa shift from compound 1 to its CB7 complex. However, the pH driven changes of the chemical shifts for the CB7·1 protons were very small and the determination of the pKa for the complex is affected by large errors (see Figures S4 and S5). Since these attempts to determine the CB7-induced pKa shift did not lead to very accurate values, we decided to measure directly the equilibrium association constants between the CB7 host and the guests 1H+ and 1. This requires experiments in C
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Scheme 3. Relevant Equilibria between the Redox Forms of Guest 1 and the CB7 Hosta
This scheme corresponds to the right “face” of the cube in Scheme 1. E1/2 and E1/2,c are the half-wave potentials for the one-electron oxidation of the free guest and its CB7 complex, respectively. K and KOX represent the equilibrium constants for the association with CB7 of the guest and its oxidized form, respectively. a
be achieved by addition of higher salt concentrations18 or organic solvents.19 Overall, these results point to the possibility of developing a synthetic host−guest system that may reach binding affinities close to those available with avidin−biotin, while providing accessible mechanisms to facilitate dissociation on demand under mild conditions.
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Figure 3. Cyclic voltammetric behavior on glassy carbon (0.07 cm ) of a 1.0 mM solution of 1 in 0.1 M NaCl in the absence (black) or in the presence of 0.5 equiv (red) or 1.0 equiv (green) of CB7. Scan rate: 0.100 V s−1. (A) pH 5, (B) pH 9, and (C) pH 12. The solution pH values were adjusted by addition of HCl or NaOH.
K OX = K exp[−F(E1/2,c − E1/2)/RT ]
ASSOCIATED CONTENT
S Supporting Information *
2
Additional NMR spectroscopic data and experimental details on the competition binding experiments as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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(2)
AUTHOR INFORMATION
Corresponding Author
where all of the symbols are defined in Scheme 3. Using eq 2 we obtain Kox = 2.4 × 107 M−1 from the already determined K value (3.4 × 109 M−1) for CB7 complexation of the free base form of guest 1. This represents a net loss of 12.3 kJ mol−1 in the stability of the inclusion complex upon oneelectron oxidation of the ferrocene residue. Overall, the CB7·1H+ complex losses ca. 27.1 kJ mol−1 as a result of deprotonation and one-electron oxidation, which is equivalent to a decrease of more than 4 orders of magnitude in the corresponding association equilibrium constant. As discussed already, the full complement of binding interactions between the CB7 host and the various forms of guest 1 resulting from their redox and acid/base properties are complicated and give rise to the cubic scheme shown in Scheme 1. We have been able to estimate all of the relevant thermodynamic parameters (see the Supporting Information). However, the work described here shows unambiguously that simple chemical transformations accessible under relatively mild conditions can be combined to substantially decrease the very high binding affinity between CB7 and the protonated form of the guest. Further decreases on the binding affinity can
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the NSF for the generous support of this research (to A.E.K., CHE-0848637) and for an instrumentation grant (CHE-0946858) that supported the purchase of the high resolution ESI mass spectrometer.
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REFERENCES
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