J. Phys. Chem. B 2000, 104, 10699-10706
10699
Solid-State Voltammetry in a Polyether-Tailed Co Tris(Bipyridine) Molten Salt: Ion Pairing Effects Mary Elizabeth Williams,† Hitoshi Masui,‡ and Royce W. Murray* Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed: April 24, 2000; In Final Form: August 29, 2000
This paper describes experiments exploring the role of the redox-inactive counterion in hybrid redox polyether molten salts. Because these materials are undiluted, they intrinsically contain “solvent-separated ion pairs”, where the “solvent” is the polyether tail attached to the redox moiety. Measurements on molten salts with polyether-derivatized Co(II) tris(bipyridine) cations show that stronger forms of ionic association can occur between the complex and its counterion, leading in the case of triflate counterion to additional voltammetric waves. The rate of conversion between differently associated Co(II) complexes in the molten salt is sluggish. Ion-association equilibria exist between Co(II) and Fe(II) tris(bipyridine) cations and their perchlorate or triflate counterions even in dilute dimethoxyethane solutions, as shown by solubility and microelectrode voltammetry.
Introduction Our previous work1 has demonstrated that combinations of redox molecules and ions with attached ethylene and propylene oxide oligomers reliably lead to room-temperature molecular melts and molten salts. These “hybrid redox polyethers” - in their undiluted state - have value as model, amorphous semisolids in which to study the structure and energy dependencies of mass and charge transport. We used1 electrochemically determined apparent diffusion coefficients (DAPP) to determine homogeneous electron self-exchange rate constants (kEX) of redox couples in the undiluted melts. The DAPP values are larger than actual physical diffusion coefficients (DPHYS) because of the coupling between the rates of physical transport and of electron hopping (or self-exchange) in the mixed valent layer formed around the working electrode during an electrochemical reaction. The DAPP-DPHYS difference provides a measure of the value of kEX. A consistent result in kEX measurements1a-e on hybrid redox polyether molten salts containing metal bipyridine redox centers has been that rate constants are smaller and electron transfer barrier energies larger than values for the nominally “outer sphere” reactions of analogous metal complexes in dilute fluid solutions. This paper is part of a project seeking to understand why the solid-state electron transfers are slower. There is ample evidence2 in fluid solutions that ion pairing can have a profound effect on electron transfer rates, and theoretical aspects have been recently summarized.3 Given the high ionic concentrations in undiluted metal bipyridine molten salts, ion association was previously suspected, but its presence was only indirectly inferred.1c,g This paper establishes the existence of strong ion-association phenomena in the Co(II) and Fe(II) complex molten salts shown in the structure. * Corresponding author. † Present address: Department of Chemistry, Northwestern University, Evanston, IL 60208. ‡ Present address: Department of Chemistry, Kent State University, Kent, OH 44242-0001.
The counterions in the molten salts are either perchlorate (ClO4-) or triflate (CF3SO3-). The polyether “tails” are attached to the bipyridine ligands. (They could alternatively have been attached to the counterion.4) The Co(II) complex is a concentrated, highly viscous (η ) 1.6 × 104 cP at 25 °C) ionic liquid in which the metal center concentration and average metal center-to-center spacing are, respectively, 0.45 M and ∼1.5 nm. The volume fraction of “solvent” (the ether shell around each complex) is only 55%. These melts are thus de facto “solventseparated”2 ion pairs because the metal complex cation cannot “escape” its counterions by more than the local volume fraction of polyether tails. We will show that there are even stronger ionic associations, which are possibly “contact” ion pairs, higher ion association aggregates, or reflect counterion entry into the metal coordination sphere, all of which could lead to distortions
10.1021/jp001545i CCC: $19.00 © 2000 American Chemical Society Published on Web 10/25/2000
10700 J. Phys. Chem. B, Vol. 104, No. 45, 2000 of the metal complex structure. The energetic consequences of such strong interactions could affect the melt electron-transfer dynamics.3 We present a body of evidencesbased on microelectrode voltammetrysfor strong ion association in the aforementioned Co(II) complex melt with its counteranion, in particular for triflate counterions. The descriptor “strong ion association” is taken to include all forms of ion association (e.g., “contact”, aggregate, etc.) because no information was developed on the actual stoichiometry of the ion associates. It is particularly noteworthy that the equilibrium between differently triflate ionassociated Co(II) complexes is sluggish enough to be detected by split waves in the voltammetry. Ion association also occurs between Co and Fe bipyridine complexes in dilute solutions in dimethoxyethane (DME) solvent. This association is shown by the effects of added counterions on the solubility and voltammetric formal potentials of the complexes. Electronic spectral observations are also presented that are consistent with the multiple chemical states of an ion-association equilibrium. Our previous abbreviation1 of the Co complex molten salt [Co(bpy(CO2MePEG-350)2)3](ClO4)2 is more compactly abbreviated here as [Co(bpy350)3](ClO4)2. The notation used to represent strong ion-association equilibrium for the Co(II) complex is shown in eq 1:
[Co(bpy350)3](ClO4)2 S {Co(bpy350)3, ClO4}(ClO4) S {Co(bpy350)3, (ClO4)2} (1) where the strongly associated complexes are enclosed in { }. Experimental Section Chemicals. Ethylene oxide dimethyl ether (DME, 99.9%, Aldrich) was transferred under nitrogen syringe. Tetrabutylammonium perchlorate (Bu4NClO4, 99%, Fluka) was recrystallized twice from ethyl acetate. Poly(ethylene oxide) monomethyl ether (MW 350 g/mol, MePEG-350, Aldrich) was dried under vacuum at 70 °C for 24 h. All other chemicals were used as received. Synthesis of Melts. The polyether-tailed bipyridine ligand, 4,4′-di(CO2MePEG-350)-2,2′bipyridine (bpy350) and its Co and Fe chelates (as ClO4 salts) have been described previously.1a-e The trifluoromethanesulfonate (triflate, CF3SO3-) analogue of the Co complex was prepared similarly using the Co(CF3SO3)2 salt. Most experiments were conducted with pure, dried, undiluted perchlorate or triflate melts. In a few (90 °C dried), LiCF3SO3 electrolyte was added by co-dissolving in acetone with dried, weighed [Co(bpy350)3](ClO4)2 melt in a glovebox, and then removing the solvent by rotary evaporation. Electrochemistry. Voltammetry of undiluted hybrid redox polyether melts was conducted as before,1 using a locally designed low-current potentiostat with computer control and an insulating platform on which the tips of a 4.2-µm radius Pt working microelectrode, a 0.5-mm diameter Ag quasi-reference electrode, and a 22-gauge Pt counter electrode were exposed. Films of the melts were cast from acetone solutions onto the electrode platform and dried under vacuum at 65 °C for at least 24 h. Ionic conductivity of undiluted melts was measured by alternating current (AC) impedance spectroscopy using a Solartron Model SI1287 Electrochemical Interface and SI1260 Impedance/Gain Phase Analyzer, with an AC amplitude of 20 mV, a direct current (DC) bias of 0 V, and a frequency range of 1 MHz to 1 Hz. The melt was cast over the fingers of an interdigitated array electrode (IDA).
Williams et al. Cyclic voltammetry in dilute DME solutions (thoroughly degassed) of polyether-tailed [Fe(bpy350)3](ClO4)2 complex (1.5 mM), with Bu4NClO4 supporting electrolyte, was performed with a BioAnalytical Systems Model 130B Potentiostat. After initial potential scans, an internal potential reference probe, tetracyanoquinonedimethane (TCNQ), was added. Solubility Studies. The effect of added electrolyte on the saturation solubility of metal bipyridines (not polyether-tailed) in DME was measured spectrophotometrically. Dry [M(bpy)3](ClO4)2 (M ) Co or Fe) was added in excess to DME solutions of Bu4NCF3SO3 and Bu4NClO4 and allowed to attain solubility equilibrium overnight. Filtering excess solid complex with a 0.2-µm PTFE syringe filter, the ultraviolet-visible (UV-vis) absorbance spectrum was taken immediately, using a reference cell of equivalent DME/electrolyte solution. Results and Discussion Non-tailed Metal Bipyridines in DME: Saturation Solubility. Solubility experiments show that nontailed Co and Fe bipyridines form strong ion-association complexes with their counterions even in dilute solutions in the monomeric ether solvent DME, which is an approximation of the polyether environment in the pure melts. The results show that although the metal bipyridine complexes are sparingly soluble in pure DME, they become strongly solubilized in the presence of added Bu4NCF3SO3 or Bu4NClO4. Saturation concentrations of metal complex in DME with various added concentrations of added Bu4NCF3SO3 and Bu4NClO4 were determined spectrophotometrically. We assume that the molar absorptivities6 () for different ion-associated forms are roughly the same. The results (Figure 1) show that in the absence of added electrolyte, the saturation concentrations of the metal complexes are small. In the case of perchlorate, the saturation concentration is depressed further by adding small concentrations of electrolyte (common ion effect). Higher electrolyte concentrations, on the other hand, invoke substantial increases in solubility. The effect of Bu4NCF3SO3 is larger than that of Bu4NClO4 and commences at lower added concentrations. These results are clear evidence for some form of strong ion association between these metal complexes and the two anions in DME solutions. The changes in solubility with electrolyte concentration seen in Figure 1 can be rationalized by a simplified treatment of the dissolution - according to the solubility product constant KSP and successive ion association equilibria (KIP1, KIP2, KIP3)
KSP KSP KSP A ) - 2 + - 2 KIP1[X-] + - 2KIP1KIP2[X-]2 + b [A ] [A ] [A ] KSP K K K [X-]3 (2) - 2 IP1 IP2 IP3 [A ] where A is absorbance of total dissolved metal complex, b is the cell path length, [A-] is perchlorate (from dissolved complex), and [X-] is either perchlorate or triflate (from added electrolyte). Equation 2 is most correct when A- ) X- ) ClO4-, and rationalizes the shape of the solubility curves in Figure 1, an initial decrease, a minimum, and (for the RHT) a linear increase. The analysis indicates formation of a ternary ion-association complex at high electrolyte concentration, with stoichiometry {M(bpy)3,(X-)3}-. The formation constant of this complex (from the {M(bpy)3,(X)2} complex) was estimated as ∼4 M-1 for the Co(II) and Fe(II) perchlorate complexes, and ∼40 and
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Figure 2. Cyclic voltammetry of undiluted [Co(bpy350)3]2+ redox melts with (ClO4)1- (- - -) or (CF3SO3)1- (s) counterions, at 50 °C and a potential scan rate of 10 mV/s. Current scale is 10 nA for the (ClO4)1- melt (working electrode radius ) 12.5 µm) and 1 nA for the (CF3SO3)1- melt (radius ) 4.3 µm).
Figure 1. Linear and log plots of saturation concentrations (vertical axis) of metal tris(bipyridine) complexes in DME solutions containing added (concentrations on horizontal axis) Bu4NCF3SO3 (Fe (3) and Co (O)) or Bu4NClO4 (Fe (1) and Co (b)).
30 M-1 for the Co(II) and Fe(II) triflate complexes, respectively. KSP (no added electrolyte) was estimated as ∼6 × 10-17 and 1 × 10-16 M3 for the Co(II) and Fe(II) complexes, respectively. The significant results of Figure 2 are that (i) ion association between the metal bipyridine and perchlorate and triflate occurs even in a dilute DME solution, and (ii) association is stronger for the triflate anion. The results strongly imply that, in the more concentrated redox polyether hybrid melts, strong ion association is likely to be prevalent, and that triflate association will be more prominent. The DME ion association equilibria are accompanied by changes (red shifts of 5-20 nm) in the [M(bpy)3]2+ UV-vis spectra (Figure S1,Table SI), that are similar to those seen in the undiluted melts (vide infra). Solid-State Voltammetry. Previous solid-state voltammetry1a-e of the hybrid redox polyether [Co(bpy350)3](ClO4)2 shows that the Co(III/II) oxidation and Co(II/I) reduction of this complex occur in single well-defined waves, as shown in Figure 2 (dashed line). The formal potentials of these two reactions are separated by 1.0 V. The peak currents for the Co(II/I) reduction wave are larger than those of the Co(III/II) oxidation reaction, because of the rapid electron self-exchange dynamics of the Co(II/I) couple.1a
When the counterion in the melt is triflate rather than perchlorate (i.e., an undiluted [Co(bpy350)3](CF3SO3)2 melt), the voltammetry changes in that both the Co(II/I) reduction and Co(II/III) oxidation show evidence of two waves (Figure 2, solid line). The Co(II/I) currents again exceed those of the Co(III/II) reaction. The Co (II/I) reduction wave exhibits a shoulder at -0.80 and a peak at -0.90 V; the Co(III/II) oxidation wave displays two peaks, at 0.22 and 0.38V. The two innermost waves (least negative and least positive potentials) are separated by a potential of 1.0 V, which is the same as the separation of the two waves in the perchlorate melt (Figure 2, dashed line). The outermost two waves are separated by 1.3 V. In contrast, there is no difference between the voltammetry (not shown) of [Co(bpy350)3](ClO4)2 and of [Co(bpy350)3](CF3SO3)2 in dilute, fluid DME solutions. In both solutions, the Co(III/II) and Co(II/I) reactions occur in single waves at the same formal potentials, separated by 1.0 V, and display equal currents (the rate of charge transport by physical diffusion exceeds that of electron hopping in a dilute fluid solution). The wave splitting in the voltammetry of the undiluted [Co(bpy350)3](CF3SO3)2 melt depends strongly on the temperature and potential scan rate, as shown in Figures 3 and 4. In both Figures, the relative magnitudes of the innermost peaks of the split Co(III/II) and Co(II/I) waves (the less positive Co(III/II) oxidation wave and less negative Co(II/I) reduction wave, respectively) increase with increased temperature and with decreased potential scan rate. This behavior is strongly suggestive of two differently associated states of the Co(II) complex. Although initially at equilibrium, as one of the two states begins to be consumed electrochemically, the equilibrium shifts slowlyson a time scale less than that of the electrochemical potential scan. One of the two initial states is reduced and oxidized at potentials less negative and positive, respectively (the innermost peaks), than the other. The behavior of the relative sizes of the innermost and outermost current peaks with changing temperature (Figure 3) and potential scan rate (Figure 4) is clearly consistent with slow kinetics of interconversion between the two Co(II) complex states, plus possibly a temperature-dependent equilibrium constant. In this light, the innermost peaks correspond classically7 to “CREVE” reactions, in which (for example), currents for reaction of the more easily
10702 J. Phys. Chem. B, Vol. 104, No. 45, 2000
Figure 3. Cyclic voltammetry of the undiluted [Co(bpy350)3](CF3SO3)2 melt at indicated temperatures, using a 4.3-µm radius Pt electrode and a potential scan rate of 10 mV/s. Current scale is 4 nA. The melt was thoroughly equilibrated at each temperature.
reducible Co(II) state become relatively larger by the more facile shifting of the equilibrium (as that state is electrochemically consumed) that is favored at higher temperature and slow experimental time-scales. The introductory discussion pointed out that the strong ionassociation equilibrium detected in the [Co(bpy350)3](CF3SO3)2 melt cannot be ascribed to ordinary “solvent-separated” ion pair formation, because all of the complexes in the melt are de facto separated from their counterions by at most their polyether “shells”. The outermost reduction and oxidation peaks (most negative and most positive, respectively) in Figures 2-4 are thus reasonably ascribed to Co(II) complexes that, in the initially equilibrated melt, are more strongly associated with their triflate counterions than are the Co(II) complexes that react in the innermost oxidation and reduction peaks. The chemical character of this stronger form of ion association may be classical “contact” ion pairing,2 or some other form of ion aggregation. It is unclear which of the reaction steps in eq 1 gives rise to the kinetic behavior seen in Figures 3 and 4; that is, the kinetically sluggish equilibrium may be that between nonassociated [Co-
Williams et al.
Figure 4. Cyclic voltammetry of the undiluted [Co(bpy350)3](CF3SO3)2 melt at (A) 58 °C and (B) 25 °C, using a 4.3-µm radius Pt working electrode and the indicated potential scan rates. Currents are normalized to scan rate (V1/2) to compare wave shapes.
(bpy350)3](CF3SO3)2 and singly strongly associated {Co(bpy350)3,CF3SO3}(CF3SO3), or between the latter and doubly strongly associated {Co(bpy350)3,(CF3SO3)2}, or involving some other, aggregated version of ion association. If we assume for the sake of illustration, that for the two Co(II/I) waves in Figures 3 and 4, the first step in eq 1 is kinetically slow, then a simple square scheme can be written where the CREVE
behavior of the less negative Co(II/I) wave is ascribed to the reduction at EoFIRST of CoII being coupled to the ion-association equilibrium for which KIP1 is the equilibrium constant. There is precedent for wave splitting by ion association, including an example8 involving the Ru(III/II) tris(bipyridine) reaction. Figures 2-4 comprise the first convincing evidence that strong
Ion Pairing Effects of Solid-State Voltammetry
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10703 TABLE 1: Electron and Mass Transport Data for [Co(bpy350)3](CF3SO3)2 [Co(bpy350)3](CF3SO3)2 [Co(bpy350)3](ClO4)2 parameter
pure
LiCF3SO3a
pure
LiClO4b
[ClO41-]c [CF3SO31-]c DPHYSd(25 °C) (cm2/s) kEXe(25 °C) (M-1 s-1) EAf (kJ/mol)
0 0.9 9.5 × 10-12 9.7 × 104 37 ( 3
0 1.8 1.0 × 10-12 1.1 × 105 36 ( 3
0.9 0 1.3 × 10-11 6.8 × 105 36 ( 3
1.75 0 1.8 × 10-12 2.7 × 105 48 ( 2
a [Co(bpy350) ](CF SO ) with 0.89 M LiCF SO (O:Li ) 22:1). 3 3 3 2 3 3 [Co(bpy350)3](ClO4)2 with 0.85 M LiClO4 (O:Li ) 21:1). c Total concentration of counterions in melt. d From chronoamperometry of Co3+/2+ wave at 25 °C. e Electron self-exchange rate constant for Co(II/ I) couple, δ ) 1.55 nm, C ) 0.48 M. f Activation energy barrier for kEX.
b
Figure 5. Cyclic voltammetry of pure [Co(bpy350)3](CF3SO3)2 melt (• • •) and melt containing added 0.9M LiCF3SO3 (-), at 60 °C using a 4.3-µm radius Pt microelectrode and potential scan rate of 10 mV/s. Currents have been normalized to Co(II/I) reduction peak current.
ion-association equilibria exist in the hybrid redox polyether materials and that such equilibria can display slow kinetics. It is worth emphasizing that eqs 3 and 4 are a greatly simplified picture of how ion association may affect the voltammetry in Figures 3 and 4. Ion-association equilibria will almost certainly exist for the Co(III) oxidation state, there may be multiple stages of association as in eq 1, and there will be differences in the kinetics of the ion-association steps as well as in the heterogeneous electron transfer kinetics of the variously associated complexes (especially for the Co(III/II) reaction). A complete analysis is an obviously complex problem, and is not attempted here. It seems reasonable to conclude, from the comparison in Figure 2, that strong association occurs to a substantially greater extent in the [Co(bpy350)3](CF3SO3)2 melt as compared with the [Co(bpy350)3](ClO4)2 melt; that is, triflate is more prone to strongly interact with the Co(II) complex than is perchlorate. The Co(II) strong ion-association equilibria can be shifted by adding a salt of the triflate counterion to the melt. Figure 5(s) shows that when two equivalents of LiCF3SO3 per [Co(bpy350)3](CF3SO3)2 complex are added to the melt (effectively doubling the anion concentration), the Co(III/II) reaction appears as a single wave at the outermost potential +0.38 V, and the outermost member of the split Co(II/I) wave becomes more prominent. Addition of LiCF3SO3 increases the initial equilibrium proportion of the more strongly associated form of the melt complex, either {Co(bpy350)3, CF3SO3}(CF3SO3) or {Co(bpy350)3,(CF3SO3)2}, so that the voltammetric features (the waves at -0.95 and +0.38 V) that represent reaction of these species become more prominent. We turn next to effects of the ion association on electrontransfer rates. Because Co(II/I) electron self-exchange is fast in the mixed valent layer around the working electrode, its apparent diffusion coefficient, DAPP, becomes larger than that for actual physical diffusion, DPHYS. The pertinent relation is9
DAPP ) DPHYS + kEXδ2C/6
(5)
where kEX is the Co(II/I) electron self-exchange rate constant and δ is the average center-to-center distance of charge displacement in the melt that is accomplished by the electron
transfer. The kEXδ2C/6 term in eq 6 is based9 on a hypothetical cubic lattice model of the amorphous melt. The kEX of the Co(III/II) couple is very small, so for the Co(III/II) wave, DAPP ) DPHYS. Thus, measuring DAPP for the Co(II/I) reaction allows calculation of kEX for that reaction. Potential step chronoamperometry was used to obtain the diffusion coefficients, DAPP and DPHYS, and their activation barrier energies in the pure [Co(bpy350)3](CF3SO3)2 and [Co(bpy350)3](ClO4)2 melts, and with added LiCF3SO3 and LiClO4, respectively. The diffusion and kEX results are given in Table 1. The rates of Co(II) complex physical diffusion are nearly identical in the two pure melts and are depressed by roughly equal extents when the respective electrolyte is added. (The physical diffusion-slowing effect of added electrolyte has been described before.1b) The electron self-exchange rate constant in the pure [Co(bpy350)3](CF3SO3)2 melt is, however, slower by ∼7-fold compared with the pure [Co(bpy350)3](CF3SO3)2 melt. It seems, thus, that the rate of Co(II/I) self-exchange is slower in the melt with the stronger ion association (triflate), with the obvious intimation that the slowed rate may arise from the more prominent ion association in the triflate melt. The parameter kEX is substantially unaltered in the [Co(bpy350)3](CF3SO3)2 melt by adding LiCF3SO3 electrolyte, and is depressed by a small factor in the analogous perchlorate melt. The activation barrier energies (Table 1, Figure S-2) mirror these effects. Interpretation of the Co(II/I) kEX results in the [Co(bpy350)3](CF3SO3)2 melt is slightly complicated by the fact that the chronoamperometric potential steps were taken over the combined reduction wave (ESTEP was to -1.05 V); the two reduction waves are too closely spaced to measure separately. The Co(II/I) DAPP thus represents an amalgam of the Co(II/I) selfexchange reactivity of the two differently associated forms of the Co(II) complex. On the other hand, currents were recorded over time scales (600 s) substantially longer than those estimated for shifting of the ion association equilibrium (e.g., Figures 3 and 4), so the effect should be relatively minor. Ionic Conductivity. At 25 °C, ionic conductivity (σIONIC) in the neat triflate and perchlorate melts is 2.6 × 10-7 and 6.4 × 10-7 S cm-1, respectively, further evidence that the extent of ion pairing is different in them. σIONIC is a function of the charge (z), diffusion coefficients, and concentrations of the constituent ions1a
σIONIC )
F2 2 (z D C + z2ANIONDANIONCANION + RT Co2+ Co2+ Co2+ z2IPDIPCIp) (6)
The subscripts ANION and IP refer, respectively, to the
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Williams et al. the Fe(III) form of the complex. Writing a Nernst relation (thus assuming fast electron transfer and ion association) for an electrode reaction in which an Fe(II) complex associated initially with q counterions is associated with p more on oxidation leads to
E1/2 ) E° -
Figure 6. Microelectrode voltammograms of 1.1 mM solutions of [Fe(bpy350)3](ClO4)2 in DME containing Bu4NClO4 at various concentrations (0 to 0.1 M). The working electrode was a 12-µm Pt microdisk, the reference electrode was an Ag wire, the counter was a 1-cm diameter Pt disk, and the potential scan rate was 10 mV/s. The potential was referenced to the TCNQ0/1- reduction as an internal reference couple; the TCNQ0/1- wave did not significantly shift (< ( 10 mV) relative to the Ag QRE as a function of the electrolyte concentration. The inset is a plot of the half wave potential, E1/2, versus concentration of Bu4NClO4, with linear regression over the three right-hand points.
(CF3SO3)1- and (ClO4)1- counterions and the ion pair {Co2+, X-}. The neutral ion pair {Co2+,(X-)2} does not contribute to σIONIC, but does reduce the overall ion population and thereby lowers σIONIC. That the triflate melt has a slightly lower σIONIC implies more extensive ion pairing with this counterion. The essentially equal activation energy barriers (Figure S-3) imply that the charge carriers have similar environments, consistent with the DPHYS measurements (Table 1). Based on eq 7 and the assumptions that DPHYS is the same (1 × 10-11cm2/s) for all Co complexes and that DCF3SO3 ) DClO4, the σIONIC difference between the two melts can be reconciled by a difference of 2to 3-fold in the extent of ion pairing in the two melts. The ionic conductivity data additionally serve to emphasize that the strong ion-association equilibria of eq 1 do not lie entirely to the right; that is, an abundance of ionic species remain in the melt to provide the ionic migration by which microelectrode voltammetry becomes indeed possible. Ion Association of Polyether-Tailed Metal Bipyridines in DME. We return to dilute DME solutions to vary the relative concentrations of the metal complex and associating counterion over a wider range than is possible in the pure melts. The [Fe(bpy350)3]2+complex (isostructural with the Co(II) complex) was employed for these experiments; the Fe(III/II) couple is more electrochemically reversible. Figure 6 shows Fe(III/II) microelectrode voltammograms of 1 mM [Fe(bpy350)3](ClO4)2 solutions in which the Bu4NClO4 electrolyte concentration is varied from zero to 10-4 to 10-1 M. The concentration of Fe(II) is too low, and DPHYS too large, for electron self-exchange reactions to be important in these solutions.9 In Figure 6, the half-wave potential of the Fe(III/II) wave clearly shifts to less positive potentials at higher Bu4NClO4 concentrations, an effect indicating favored formation of ionically associated states with
- p [Fe2+,(ClO2.3RT 4 )q][ClO4 ] log nF [Fe3+,(ClO4 )q+p]
(7)
The inset of Figure 6 shows a plot of E1/2 of the Fe(III/II) reaction versus electrolyte concentration. The point at lowest electrolyte concentration is actually the 1 mM [Fe(bpy350)3](ClO4)2 solution, with no added electrolyte; here and for the 10-4 M electrolyte concentration, the solution is unbuffered in ClO4- and the plot is not valid. At higher concentrations, the plot becomes linear with a slope of -30 mV. From eq 7, p ) 0.5 which could mean that one additional counterion becomes associated with (and presumably bridges) two Fe(III) cations. The value of the initial state of association (q) is undetermined by the analysis. Again we find strong ionic association even in a dilute ether solvent; that it should occur in a concentrated melt is even more likely. The solution resistance can be expected to increase at the low electrolyte concentrations in Figure 6. Microelectrode voltammetry is, however, tolerant of solution resistance effects and has been previously used at low electrolyte concentrations to study ionic association.10 Also, constancy of the TCNQ internal reference probe’s potential (vide supra) argues against any resistance effect. Thin Film UV-Visible Spectra. Electronic spectra of thin films of pure [Co(bpy350)3](X)2 melts are shown in Figure 7A for X ) perchlorate, triflate, and tosylate. In the case of underivatized metal tris(bipyridine) complexes, a ligand-centered π-π* transition would appear at ∼280 nm; the ester groups on the bipyridine ligands shift this band to lower energy. In the melts, the absorption band is slightly split; the lower energy peak is more predominant in the perchlorate melt and least so in the tosylate melt. Figure 7B shows that diluting the [Co(bpy350)3](ClO4)2 melt with MePEG-350 (so that the concentration of the Co(II) complex changes from 0.45 M to 1 mM) causes a 25-nm shift to higher energy. Dilution with unattached polyether is expected to decrease ion association. The 10002500 cm-1 (12-25 kJ/mol) energy shift caused by Co complex dilution is plotted in the Figure 7B inset; the large shift at lower Co(II) complex concentrations levels off at higher concentrations. One can infer that an ionic association equilibrium is being driven toward a more strongly associated state at the higher concentrations. The multiple bands and their shifts can be interpreted as either two different states of association or as ionic association driven splitting of electronic states. How Ion Pairing May Be Important in Melt ElectronTransfer Properties. The electron-transfer activation barriers observed for the Co(II/I) complex, and other redox centers in polyether-based melts, are much larger1 than would be expected based on the properties of the analogous redox centers in fluid ether solvents. Ionic association is one of the possible contributing factors to the barrier difference, which is why firm identification of its presence and characteristics in the present work is a significant step. We conclude this paper with some observations on ways in which ion pairing may influence electron self-exchange rates in the semisolid melts. The plausible effects are multiple. The results of Figures 2-4 and 6 show that redox potentials can be shifted by ionic associations. These potential shifts reflect
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Figure 8. Potential energy surface diagrams for self-exchange reactions without (top) and with asymmetrical ion pairing (bottom). The symbols O and R refer to the oxidized and reduced species in the reactant pair, respectively, and ∆G* and ∆G° are the free energy and standard free energy for the reaction, respectively. In the bottom cartoon, I represents a counterion ion paired with one of the two reactants.
Figure 7. The UV-vis spectra of thin films (∼100-300 nm thick) of [(Co(bpy350)3]2+ melts on quartz slides. (A) Undiluted complex with indicated counterion; absorbance normalized to one. (B) [Co(bpy350)3](ClO4)2 diluted with MePEG-350 to indicated concentrations; thicker films are used at higher dilutions. Inset: energy of π-π* band maximum versus metal complex concentration.
various changes in the extent of association and differences between the oxidized and reduced members of the metal complex couple, and are, ideally, equilibrium values. In the melts’ mixed valent layers where homogeneous self-exchange occurs (e.g., Co(II/I)), the reactions are not necessarily at equilibrium in regards to the ionic association. In particular, in the melts, where ionic motions are retarded, electron transfer may precede rearrangement or relaxation of specific ionic associations with the electron donor-acceptor pair. This leads to a change4 in the free energy, ∆G°, of the electron transfer, as well as possible appearance of an entropy term1a,11 that is absent for the symmetrical reaction. Figure 8 illustrates the change in reaction coordinates for a self-exchange reaction in which ionic relaxation is fast or comparable to the thermally activated electron-transfer time scale (top) and for one in which it is slow ((bottom, asymmetrical ion pairing). The latter circumstance has been detected in fluid solutions in optical electron-transfer reactions12 where ionic relaxation is “frozen” on the optical electron-transfer time-scale. In the melts, it may also be “frozen” on the thermal electron-transfer time scale. Strong ion association in the Co complex melt may also provoke structural distortions, and thereby introduce “inner sphere” components to the reorganizational energy barrier; the spectra of Figure 7 may well reflect this phenomenon. Ad-
ditionally, the dynamics of solvent dipolar relaxation (“solvent dynamics”, the “solvent” is the polyether tails) may be altered by changes in solvent-counterion interactions that accompany strong counterion-Co interactions. The heterogeneous kinetics of the Co(II/III) couple is known13 to be solvent dynamics controlled in these melts. Finally, we consider nonspecific interactions of the ionic atmosphere with the redox center as a possible contributor to the reorganizational energy. This contribution can occur in the presence or absence of ion pairing association. For an ion in a Debye solvent, the inverse screening length (κD, m-1) is given by:
κD )
(
)
8π103F2I SRT
1/2
(8)
where I is the ionic strength (M) and S is the solvent permittivity (C2 J-1m-1).14 When the separation between the two reactants is twice their molecular radii (a), then the contribution to the thermal barrier due to ionic atmosphere rearrangement, ∆G*ia (in kJ/mol), can be calculated from:15
∆G*ia )
(
)
NAe2 exp(-κDa) + κDa - 1 8aS 1 + κDa
(10)
where κD ) 1.1 × 1010 m-1 and ∆G*ia ) 2kJ/mol, results for the [Co(bpy350)3]2+ complex. The calculated κD is smaller than the [M(bpy)3]2+ cation radius (0.7 nm), so eq 9 has some limitations for the present situation. Calculation of ∆Gia* gives 2 kJ/mol, which, irrespective of the eq 9 limitation, is much smaller than the differences in activation barrier energies between solid state and fluid solution electron transfers. Weaver has also made comparisons between experimentally determined barrier energies and values calculated using eq 10 and found that the calculations underestimate the observed barriers by 3to 5-fold.15 Acknowledgment. This work was funded by grants from the National Science Foundation and the U.S. Department of
10706 J. Phys. Chem. B, Vol. 104, No. 45, 2000 Energy. MEW gratefully acknowledges a Graduate Student Fellowship from the ACS Division of Analytical Chemistry, sponsored by Eastman Chemical Company. Supporting Information Available: Arrhenius plots of DPHYS, DAPP, and σIONIC for the neat [Co(bpy350)3](CF3SO3)2 melt, and supplementary data on transport and spectra. This material is available free of charge via the Internet at http:// www.pubs.acs.org. References and Notes (1) (a) Williams, M. E.; Masui, H.; Long, J. W.; Malik, J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 1997. (b) Williams, M. E.; Lyons, L. J.; Long, J. W.; Murray, R. M. J. Phys. Chem. 1997, 101, 7584. (c) Williams, M. E.; Crooker, J. C.; Pyati, R.; Lyons, L. J.; Murray, R. M. J. Am. Chem. Soc. 1997, 119, 10249. (d) Masui, H.; Murray, R. M. Inorg. Chem. 1997, 36, 5118. (e) Long, J. W.; Velasquez, C. S.; Murray, R. W. J. Phys. Chem. 1996, 100, 5492. (f) Long, J. W.; Kim, I. K.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 11510. (g) Ritchie, J. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 1964. (2) Szwarc, M. Ions and Ion Pairs in Organic Reactions; WileyInterscience, John Wiley & Sons: New York, 1974; Vols 1 & 2. (3) Dickinson, E. V.; Williams, M. E.; Hendrickson, S. M.; Masui, H.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 613. (4) (a) Marcus, R. A. J. Phys. Chem. B. 1998, 102, 10071. (b) See especially refs 1-27 in ref 4a. (5) The IDA contained 50 pairs of electrodes, each 3 µm wide, 1 mm long, 0.2 µm high, and separated by 2 µm. The electrode was generously
Williams et al. donated by O. Niwa of Nippon Telephone and Telegraph. (6) In DME, [Co(bpy)3]2+ ) 2.63 × 104 at 312 nm; for [Fe(bpy)3]2+, ) 3.90 × 104 at 282 nm, based on known, unsaturated, solutions of metal tris(bipyridine) complex. is assumed to not change significantly with ion pairing. (7) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, John Wiley & Sons: New York, 1980. (8) Majda, M.; Faulkner, L. R. J. Electroanal. Chem. Int. Electrochem. 1984, 169, 77. (9) Majda, M. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; John Wiley & Sons: New York, 1992; p.159. (10) (a) Pletcher, D.; Thompson, H. J. Chem. Soc., Faraday Trans. 1998, 94, 3445. (b) Pletcher, D.; Thompson, H. J. Chem. Soc., Faraday Trans. 1997, 93, 3669. (11) Marcus, R. A.; Siddarth, P. In Photoprocesses in Transition Metal Complexes, Biosystems and Other Molecules, Experiment and Theory; Kochanski, E., Ed.; Kluwer Academic Publishers: Netherlands, 1992. (12) (a) Blackbourn, R. L.; Dong, Y.; Lyon, L. A.; Hupp, J. T. Inorg. Chem. 1994, 33, 4446. (b) Blackbourn, R. L.; Hupp, J. T. J. Phys. Chem. 1990, 94, 1788. (13) Williams, M. E.; Crooker, J. C.; Pyati, R.; Lyons, L. J.; Murray, R. W. J. Am. Chem. Soc., 1997, 119, 10249. (14) I is calculated from I - (1/2)(zi2CI).18 is calculated from the relative dielectric constant, , and the permittivity of vacuum, o, using s ) 4πo. For the hybrid redox-polyether melts, is 9.16, and s is 1.02 × 10-9C2J-1m-1. (15) Kuznetsov, A. M.; Phelps, D. K.; Weaver, M. J. Int. J. Chem. Kinetics 1990, 22, 815.
