J. Phys. Chem. B 2007, 111, 6949-6954
6949
Kinetics of the Electron Self-Exchange and Electron-Transfer Reactions of the (Trimethylammonio)methylferrocene Host-Guest Complex with Cucurbit[7]uril in Aqueous Solution† Lina Yuan and Donal H. Macartney* Department of Chemistry, Queen’s UniVersity, Kingston, ON K7L 3N6, Canada ReceiVed: January 7, 2007; In Final Form: February 20, 2007
The electron self-exchange rate constants for the (trimethylammonio)methylferrocene(+/2+) couple (FcTMA+/2+) have been measured in the absence and presence of the cucurbit[7]uril (CB[7]) host molecule in aqueous solution, using 1H NMR line-broadening experiments. The very strong binding of the ferrocene to CB[7] results in slow exchange of the guest on the NMR time scale, such that resonances for both the free and bound forms of the reduced ferrocene can be observed. The extents of line broadening in the resonances of the two forms of the guest in the presence of the FcTMA2+ species can be monitored independently, allowing for the determination of the rate constants for the possible self-exchange pathways involving the bound and free forms of both the oxidized and reduced members of the redox couple. The encapsulation of both the reduced and oxidized forms of the ferrocene increases the rate constant (25 °C) from (2.1 ( 0.1) × 106 M-1 s-1 (for FcTMA+/2+) to (6.7 ( 0.7) × 106 M-1 s-1 (for {FcTMA‚CB[7]}+/2+), whereas inclusion of the reduced form only decreases the rate constant to (6 ( 1) × 105 M-1 s-1. The changes in the exchange rate constants upon inclusion of the reactants are related to the effects of CB[7] acting as an outer, secondcoordination sphere and are compared to those observed previously for the electron-exchange process in the presence of β-cyclodextrin and p-sulfonated calix[6]arene hosts. The binding of FcTMA+ and hydroxymethylferrocene to CB[7] significantly reduces the rate constants for their oxidations by the bis(2,6pyridinedicarboxylato)cobaltate(III) ion (which does not bind to CB[7]) as a result of reduced thermodynamic driving forces and steric hindrance to close approach of the oxidant to the encapsulated ferrocenes.
Introduction The cucurbit[n]urils (CB[n], where n ) 5-8, 10) are a family of cyclic host molecules consisting of n glycoluril units, linked by 2n methylene bridges, containing a hydrophobic cavity and ureido carbonyl lined portals.1-4 Although cucurbit[6]uril was first prepared in 1905,5 it was not until the early 1980s that the structure of the molecule was determined6 and its use as a host molecule was first explored.1 Since the reporting of improved methods for the preparation and isolation of CB[7] and CB[8] (and other members of the CB[n] family),7 normally minor products in the synthesis of CB[6],6 there has been considerable interest in their use as host molecules for a large variety of guests.8-16 The host-guest chemistry of the cucurbit[n]urils can be compared to those of other families of water-soluble macrocyclic host molecules, such as the cyclodextrins (CD)17-18 and the p-sulfonated calix[n]arenes (p-SO3CX[n], where n ) 4-8).19 The cavity size of CB[7] is comparable to that of β-cyclodextrin (β-CD), and guests of similar sizes and shapes bind to the two hosts.2,4 The more restrictive and polar carbonyl portals of the CB[7] (Chart 1), however, have led to the observance of significantly stronger binding interactions with certain guests, such as protonated organic amines and diamines,8,9 metal ions3 and platinum amine complexes,10 and other organic cations,11-13 compared to β-CD. This is achieved by combining hydrophobic, ion-dipole, and hydrogen-bonding noncovalent interactions between the guest and the cavity and †
Part of the special issue “Norman Sutin Festschrift”. * To whom correspondence should be addressed. E-mail: donal@ chem.queensu.ca.
CHART 1: Structures of the (Trimethylammonio)methylferrocene (FcTMA+/2+) Complex and Cucurbit[n]uril (CB[n])
portals of the cucurbituril host. Ferrocene and substituted ferrocenes, in particular, have recently been shown to exhibit remarkably strong binding to CB[7],14-16 with stability constants for the host-guest complexes in the range of 109-1013 M-1 in aqueous solution,15,16 compared to 103-104 M-1 for the same guests with β-CD20-23 and 104-105 M-1 with p-SO3CX[6].24-26 We have recently shown that encapsulation of the (E)-1ferrocenyl-2-(1-methyl-4-pyridinium)ethylene cation [(E)-FcMPE+] by CB[7] [KCB[7] ) (1.3 ( 0.5) × 1012 M-1]16 prevents indefinitely the normally observed (E) f (Z) photoisomerization (t1/2 ≈ 10 min) in aqueous solution. Although the encapsulation of the ferrocene portion of the guest increases the reduction potential of the (E)-FcMPE2+/+ couple by only 30 mV, the rate constant for its outer-sphere chemical oxidation of (E)-FcMPE+ by [Co(dipic)2]- (dipic2- is 2,6-pyridinedicarboxylate) in aqueous solution decreases significantly (from k0 ) 2.1 × 104
10.1021/jp0701284 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/04/2007
6950 J. Phys. Chem. B, Vol. 111, No. 24, 2007 M-1 s-1 to kCB[7] ) 1.6 × 102 M-1 s-1) upon binding of ferrocene to CB[7]. We have also reported that the rate constants for the oxidation of the reduced forms of other substituted ferrocenes, such as (trimethylammonio)methylferrocene and ferrocene carboxylic acid, decrease in the presence of cyclodextrin27,28 and p-sulfonated calixarene26 host molecules. The electron self-exchange reactions of ferrocene/ferricinium couples have long been of interest in terms of the solvent29-30 and other medium effects on the kinetics of these processes.23-25 Weaver and co-workers found that the self-exchange rate constants for the (trimethylammonio)methylferrocene couple (FcTMA+/2+), determined by 1H NMR line-broadening experiments, varied somewhat with the nature of the solvent, with kex ranging from 2.1 × 106 M-1 s-1 in acetonitrile to 2.3 × 107 M-1 s-1 in propylene carbonate, compared to a value of 9 × 106 M-1 s-1 measured in aqueous solution.30 Hupp and co-workers have reported that, in the presence of the host molecule β-CD, the rate constant for the electron self-exchange reaction of the FcTMA+/2+ couple is decreased 20-50-fold upon inclusion of the reduced form of the compound.23 The very much weaker binding of β-CD to the oxidized form of the ferrocene prevented the determination of the rate constant for the symmetrical electron self-exchange between {FcTMA‚β-CD}+ and {FcTMA‚ β-CD}2+. In a related study, they reported that inclusion of the FcTMA+/2+ couple by the anionic para-sulfonated calix[6]arene host (p-SO3CX[6]) also affected the rate of electron exchange.25 In this case, the rate constants were similar when the reduced and oxidized forms were either both unbound (1.2 × 107 M-1 s-1) or bound (1.6 × 107 M-1 s-1), whereas the transfer of an electron from the unbound reductant to the bound oxidant (followed by rapid host transfer) was considerably faster (1.3 × 108 M-1 s-1) because of the favorable electrostatics in the formation of the precursor complex. With both of the hosts, electron transfer in the asymmetrical pathway is accompanied by fast host transfer (from the oxidized to the reduced form with β-CD and the reverse with p-SO3CX[6]). In this article, we report the results of a kinetic study of the electron self-exchange rate constants for the (trimethylammonio)methylferrocene couple (FcTMA+/2+, Chart 1) in the absence and presence of CB[7] in aqueous solution. It has recently been determined, by isothermal calorimetry, that the inclusion stability constant for the reduced form of FcTMA+ with CB[7] is (4 ( 2) × 1012 M-1 in pure water15 [a value of (3.31 ( 0.62) × 1011 M-1 was determined in D2O containing 50 mM NaO2CCD3 using a 1H NMR competitive binding method8] and that the reduction potential for the FcTMA2+/+ couple is ∼110 mV more positive when complexed to CB[7].15 In addition to measurements of the electron self-exchange rate constants for the FcTMA+/2+ couple, experiments on the kinetics of the oxidation of FcTMA+ and hydroxymethylferrocene (FcCH2OH) by Co(dipic)2- have been carried out in the presence of CB[7]. The effect of CB[7] encapsulation of the ferrocene on the electron-transfer rate constants are compared to those observed previously for the (E)-FcMPE+ complex16 and for electron-transfer reactions of substituted ferrocenes in the presence of the CD and p-sulfonated calixarene hosts in aqueous solution. Experimental Section Materials. Cucurbit[7]uril was prepared and characterized by the method of Day and co-workers.7b (Trimethylammonio)methylferrocene tetrafluoroborate ([FcTMA]BF4) was prepared by methylation of dimethylaminomethylferrocene (Aldrich) using a reported method,31 and the oxidized form, [FcTMA]-
Yuan and Macartney
Figure 1. 1H NMR spectra of FcTMA+ [1.0 mM total, (9) FcTMA+ and (b) {FcTMA‚CB[7]}+] in the presence of 0.21 mM CB[7] ([) with increasing concentrations of total FcTMA2+: (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 µM.
(BF4)2 (λmax ) 628 nm, ) 198 M-1 cm-1), was prepared by the method of Hupp et al.23 Hydroxymethylferrocene were used as received (Strem Chemicals). The NH4[Co(dipic)2] (dipic2) 2,6-pyridinedicarboxylate) compound was synthesized according to literature methods.32 Methods. 1H NMR spectra were obtained on Bruker AV400 (compound characterization) and AV-500 (line-broadening experiments) spectrometers in D2O at 25 °C, with the chemical shifts referenced to the HOD signal internally. Cyclic voltamograms were collected using a Bioanalytical Systems CV-1B cyclic voltammeter, with a glassy carbon working electrode, a platinum wire auxiliary electrode, and a Ag/AgCl reference electrode. Kinetics experiments were performed on an Applied Photophysics SV-MX-17 stopped-flow spectrophotometer in aqueous solution containing 0.10 M NaCl at 25.0 ( 0.1 °C. The second-order rate constants were determined from five to six replicate traces at each concentration of CB[7]. UV-visible spectra were measured using a Hewlett-Packard 8452A diodearray spectrophotometer. Results and Discussion Electron Self-Exchange Kinetics. The electron self-exchange reaction of the FcTMA+/2+ couple has previously been studied in the presence of the β-CD and p-SO3CX[6] host molecules.23,25 With these hosts, the encapsulated FcTMA+ guest exhibits fast exchange on the NMR time scale, producing proton resonances whose chemical shifts represent the average of the bound {FcTMA‚host}n+ and unbound FcTMA+ species. In the 1H NMR spectrum of FcTMA+ in the presence of CB[7] (Figure 1a), the unbound and bound (resonances shifted upfield, with the exception of the methyl resonance) guest molecules exhibit separate resonances, indicating that guest exchange is slow on the NMR time scale. (A value of 2 s-1, estimated from EXSY spectroscopy, has been reported15 for the dissociation of bound FcTMA+ from {FcTMA‚CB[7]}+.) Consequently, the line broadening resulting from electron self-exchange between FcTMA+ (1.0 mM, no added electrolyte) and added FcTMA2+ (10-80 µM) can be simultaneously monitored for both the bound {FcTMA‚CB[7]}+ [using the unsubstituted cyclopentadienyl proton resonance at 3.52 ppm (at lower CB[7] concentrations) or the methyl proton resonance at 2.92 ppm (at higher CB[7] concentrations)] and unbound FcTMA+ (using the resonance at 4.48 ppm for the cyclopentadienyl protons adjacent
(Trimethylammonio)methylferrocene Electron Self-Exchange
Figure 2. Dependences of WDP - WD on [FcTMA2+]total for the Cp resonance of FcTMA+ (4.24 ppm) or {FcTMA‚CB[7]}+ (3.52 ppm) at various concentrations of CB[7] at 25 °C: (b) 0, (9) 0.33, (O) 0.54, (0) 0.77, and (2) 0.86 mM.
to the substituent) species in the presence of a deficiency of CB[7]. The observed rate constant for an electron self-exchange reaction, which is in the slow-exchange region (broadening of the resonances in the presence of the paramagnetic species, but no change in their chemical shifts, were observed at 500 MHz), is given by eq 1, where WDP and WD are the line widths of the resonances at half-maximum height, in the presence and absence of the paramagnetic iron(III) species, respectively.
kexobs ) π(WDP - WD)/[FcTMA2+]total
(1)
Figure 2 shows the linear dependences of WDP - WD on [FcTMA2+]total for the FcTMA+ and {FcTMA‚CB[7]}+ species at various concentrations of CB[7] at 25 °C. The electronexchange processes that might be expected for the FcTMA+/2+ couple in the absence and presence of CB[7] involve symmetric (no thermodynamic driving force) self-exchanges (eq 2 and 4) and an asymmetric exchange (eq 3), in which one of the two species is bound to CB[7]. k11
FcTMA+ + FcTMA2+ y\z FcTMA2+ + FcTMA+ (2) +
{FcTMA‚CB[7]} + FcTMA
2+
Figure 3. Plots of kex against CB[7] concentration derived from the observed line broadening in the resonances for the (O) FcTMA+ and (b) {FcTMA‚CB[7]}+ species. The solid line represents the fit to eq 6 using the rate and equilibrium constants in Table 1. The dashed line represents the fit to eq 5 using the rate and equilibrium constants in Table 1.
bound and unbound couples [∆Eo ) (RT/F)ln(Kred/Kox)]. Assuming that the electron-exchange processes are fast compared to the decomplexation rates of the {FcTMA‚CB[7]}n+ species, the line broadening observed in the resonances for FcTMA+ is related to the values of k11 and k12, whereas the line broadening in the resonances for {FcTMA‚CB[7]}+ depends on the values of k21 and k22. With the very high stability constant for the {FcTMA‚CB[7]}+ complex, it can be assumed that the equilibrium concentration of {FcTMA‚CB[7]}+ and the total concentration of CB[7] are approximately equal up to the 1:1 equivalence point (1 mM). The change in the observed kex values with total CB[7] concentration (Figure 3) is therefore due to the increasing amounts of the oxidized {FcTMA‚CB[7]7}2+ as the CB[7] concentration is raised. The expressions for the observed exchange rate constants involving FcTMA+ (kexunbound) and {FcTMA‚CB[7]}+ (kexbound) are given in eqs 5 and 6, respectively, where [CB[7]]eq is the concentration of free cucurbit[7]uril in solution.33
kexunbound )
k12
y\ z k 12
{FcTMA‚CB[7]}2+ + FcTMA+ (3) +
J. Phys. Chem. B, Vol. 111, No. 24, 2007 6951
k22
{FcTMA‚CB[7]} + {FcTMA‚CB[7]} y\z {FcTMA‚CB[7]}2+ + {FcTMA‚CB[7]}+ (4) 2+
The reduction potential of the {FcTMA‚CB[7]}2+/+ pair is reported to be ∼110 mV more positive than the reduction potential for the FcTMA2+/+ in a solution containing 0.1 M NaCl as an added electrolyte. With no added electrolyte (1.0 mM FcTMA+), we measure a potential difference of ∼85 mV, with the difference attributable to differential ionic strength effects on the binding constants for the {FcTMA‚CB[7]}+/2+ species, as observed previously for other CB[7] host-guest complexes.13 As a result, the forward process in eq 3 would be considerably disfavored thermodynamically, whereas the reverse process would be thermodynamically favorable. Using the stability constant of 4 × 1012 M-1 for {FcTMA‚CB[7]}+,15 a stability constant of 1.5 × 1011 M-1 is calculated for {FcTMA‚CB[7]}2+ based on the 85 mV difference in the reduction potentials of
kexbound )
k11 + k12Kox[CB[7]]eq 1 + Kox[CB[7]]eq
k21 + k22Kox[CB[7]]eq 1 + Kox[CB[7]]eq
(5)
(6)
The directly measured self-exchange rate constant k11 for the FcTMA+/2+ couple, in the absence of CB[7], is (2.1 ( 0.1) × 106 M-1 s-1 at I ≈ 0.002 M (no added electrolyte). As might be expected for a reaction between complexes with charges of the same sign (+/2+), the self-exchange rate constants reported previously for the FcTMA+/2+ couple are dependent on ionic strength. Weaver reported a rate constant of 9 × 106 M-1 s-1 at I ) 0.03 M and indicated that it increased to 2.3 × 107 M-1 s-1 at I ) 0.5 M.30a Hupp et al. reported directly measured values of 1.2 × 107 M-1 s-1 with no added electrolyte (but using slightly higher concentrations of the ferrocene cations than in the present study) and 2.8 × 107 M-1 s-1 at I ) 0.1 M.23 The value determined in the present study is therefore in line with the previously measured rate constants. The plots of the observed electron-exchange rate constants, kexunbound and kexbound, as a function of CB[7] concentration are
6952 J. Phys. Chem. B, Vol. 111, No. 24, 2007
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TABLE 1: Rate Constants (25 °C) for the Self-Exchange Reactions of the FcTMA+/2+ Couple (k11, k12, k21, and k22) and the Electron-Transfer Reaction of FcTMA+ with Co(dipic)2- (kCo0 and kCohost) in the Presence of CB[7], β-CD, or p-SO3CX[6] in Aqueous Solution parameter
cucurbit[7]uril
k11 (M-1 s-1)
2.1 × 106 a
k12 (M-1 s-1) k21 (M-1 s-1) k22 (M-1 s-1) Kred (M-1)
2 × 107 d 6 × 105 a 6.7 × 106 a 4 × 1012 e 3.3 × 1011 f 1.5 × 1011 j
Kox (M-1) ∆Eo (mV) kCo0 (M-1 s-1) kCohost (M-1 s-1) kCo0/kCohost
+110e +85a 7.50 × 102 l 3.4 × 10-1 l 2.2 × 103
β-CD 1.2 × 107 b (2.5 × 106)b not observed 4.3 × 104 b not observed 4.8 × 103 g 4.9 × 103 b 1.5 × 102 g
