Electron Transfer Dynamics in Molten Salts of Mono- and Dinuclear

Oct 27, 2001 - Benjamin Yancey , Jonathan Jones , and Jason E. Ritchie ... Wei Wang, Srikanth Ranganathan, Mary Elizabeth Williams, and Royce W. Murra...
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J. Phys. Chem. B 2001, 105, 11523-11528

11523

Electron Transfer Dynamics in Molten Salts of Mono- and Dinuclear Ruthenium Complexes Jason E. Ritchie† and Royce W. Murray* Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599 ReceiVed: March 22, 2001; In Final Form: August 14, 2001

We have synthesized highly viscous, room-temperature, molten salts by associating various Ru(NH3)5L cations (L ) 4,4′-bipyridine, pyrazine, pyridine, 3-chloropyridine, benzonitrile) with polyether-tailed sulfonate anions. Microelectrode voltammetry in the undiluted melts yields, on the basis of charge transport occurring by electron hopping, electron self-exchange rate constants (kEX) for the various Ru3+/2+ couples. The rate constant (and activation barrier) for the pyrazine bridged binuclear pentaamineruthenium melt (Creutz-Taube ion) is similar to those obtained for mononuclear pentaammine[ligand]ruthenium melts, meaning that charge transport in the former is dominated by the rate of intermolecular, not intramolecular, electron transfer. The data at 35 °C are (Creutz-Taube ion) kEX ) 3.7 × 104 M-1 s-1, (pyridine) 1.1 × 104 M-1 s-1, (3-chloropyridine) 2 × 104 M-1 s-1, and (benzonitrile) 3 × 104 M-1 s-1. All kEX values are smaller than those for [Ru2+/3+(bpy)3] in semisolid melts having equivalent MePEG tail contents.

It has thus far proven general (e.g., metal poly-pyridine and cyano complexes,1-5 metalloporphyrins and metallophthalocyanines,6 perylene,7 DNA,8 and viologens9) that the disorder provided to a redox (or other) moiety by attaching polyether tails either to the counterion of an ionic moiety or to the moiety (neutral or ionic) itself disrupts crystallinity and provokes formation of a highly viscous (semisolid) molten salt or molecular melt.10 This paper describes electron-transfer dynamics within five semisolid, room-temperature molten salts based on cationic pentaammineruthenium complexessthree mononuclear and two binuclearsthat have counterions composed of methyl and sulfonate-terminated poly(ethylene glycol) oligomers. Their structures and the tailed counterion are shown in Chart 1. Three of the melt materials have shown spectral evidence of intermolecular optical electron transfers in their mixed valent forms.11 Here, we describe the synthesis and physical properties of the new pentaammineruthenium complex melts, and report microelectrode voltammetry in the undiluted molten salts that yields electron self-exchange rate constants and associated thermal activation barrier energies. The rate constant measurements are based on the acceleration of charge transport through a melt to a working electrode by the electron self-exchange reactions (i.e., electron hopping) that occur in the mixed valent diffusion layer created around the working electrode by the electrochemical reaction. This acceleration is described by the so-called Dahms-Ruff relation,12 which is based on a fictitious cubic lattice model of electron-transfer sites

kEXδ2C DAPP ) DPHYS + DE ) DPHYS + 6

(1)

where DAPP is the experimentally observed charge transport diffusion coefficient, DPHYS is the actual physical diffusion coefficient, DE is the electron diffusion coefficient generated * To whom correspondence should be addressed. E-mail: rwm@ email.unc.edu. † Current address: Department of Chemistry and Biochemistry, The University of Mississippi, University, MS 38677.

CHART 1

by the electron hopping, kEX is the electron self-exchange rate constant between the oxidized and reduced forms of the melt species, δ is the charge displacement distance (equilibrium center-center) accomplished by an electron hop, and C is the redox site concentration. To obtain the electron self-exchange rate constant, DPHYS must be measured or estimated to correct the experimental DAPP. The present complexes include the first examples of binuclear complexes, which include the famous Creutz-Taube ion (the pyrazine complex above), that we have investigated in the melt phase. Experimental Section Synthesis of the Tailed Counterion (MePEG-SO3-)(H+) (see Chart 1). The procedure is analogous to that reported by Ito and Ohno.13 Specifically, 50 g of dry monomethylpoly(ethylene glycol) [CH3(OCH2CH2)nOH] (MW ) 350, n ) 7.2, Aldrich, dried in a vacuum oven at 50 °C immediately prior to use) was refluxed in a mixture of 50 mL thionyl chloride and 5 mL pyridine, producing a chlorinated product, [CH3(OCH2CH2)nCl] that was isolated by evaporating the thionyl chloride and pyridine. The clear, brownish liquid was slurried with a large molar excess of sodium sulfite (Na2SO3) in ∼250 mL of 3:1 ethanol/water; after stirring for 3 h at room temperature, the mixture was brought to reflux for 18 h. After cooling and filtering off undissolved Na2SO3, the mixture was dissolved in

10.1021/jp011097u CCC: $20.00 © 2001 American Chemical Society Published on Web 10/27/2001

11524 J. Phys. Chem. B, Vol. 105, No. 46, 2001 progressively less polar solvents (i.e., methanol, acetone, dichloromethane), filtering off undissolved salts in each solution. An aqueous (Barnstead NANOpure System Model 4754) solution of the material was run through base (OH-) and acid (H+) form ion exchange columns to remove any remaining salt and to produce the acid form of the sulfonate. The synthesized (MePEG-SO3-)(H+) typically had >95% acidity as assayed by adding tetrabutylammonium hydroxide (Bu4N+OH-) until the solution pH ) 7.0, and then measuring (nmr) the ratio of MePEG-SO3- and Bu4N+ methyl groups in the solution. An earlier version14 of the MePEG-tailed anion was based on a sulfonated benzoate grouping. This material was more difficult to purify and less stable than the sulfonic acid, which avoids use of the ester linkage. Synthesis of [Ru2+(NH3)5py](MePEG-SO3-)2. [Ru2+(NH3)5(py)][PF6]2 was synthesized according to the literature.15 Briefly, 100 mg [Ru(NH3)5Cl][Cl2] (Strem) was reduced in the presence of ∼1 g of pyridine by freshly prepared zinc amalgam in 15 mL of degassed (Ar) water (the analogous preparation of the benzonitrile complex was performed by reduction then addition of zinc, because benzonitrile is subject to reduction). The Zn pieces were filtered and a saturated solution of 2 g NH4PF6 added, precipitating the ruthenium(II) complex. An aqueous solution of this product was passed through a (OH-) form ion exchange column, producing a [Ru2+(NH3)5(py)][OH-]2 solution that was collected in a Ar purged flask in an ice bath and immediately titrated with (MePEG-SO3-)(H+) to neutral pH. Due to this complex’s known sensitivity to base, we attempted to minimize the interval between basic exchange and neutralization (typically 3-5 min). Ar-degassed ethanol was added and the solvent evaporated on a rotovap at 30 °C (the complex displays sensitivity to heat and vacuum), producing a deep orange, viscous melt. The other melt complexes were prepared in basically the same manner. We examined the extinction coefficient of the main visible band and found that the melt displayed no significant change in  from the [Ru2+(NH3)5(py)][PF6]2 starting material. In addition, the electrochemical potentials of the resulting melts were similar (within (100 mV due to use of Ag QRE) to literature values. Synthesis of the Mixed Valent Melt [Ru2.5+(NH3)5py](MePEG-SO3-)2.5. A degassed aqueous solution of [Ru2+(NH3)5(py)][PF6]2 was oxidized to the Ru(III) state by bubbling Cl2 gas (generated by the reaction of concentrated HCl with MnO2) through the solution, which changed from bright yellow to nearly clear. The Ru(III) solution was placed under vacuum in a rotovap (at RT) to remove excess Cl2, and then combined with an equi-molar quantity of the [Ru2+(NH3)5(py)][PF6]2 complex. This 1:1 mixed valent solution was then run through a (OH-) form ion exchange column, titrated with (MePEGSO3-)(H+), and evaporated to the melt state, as above. Measurements. Neutralizations were monitored with a Corning PS-30 pH Meter; proton NMR spectra taken with a Bruker AC-200 MHz NMR, and electronic spectra (checking for spectroscopic impurities) with a Unicam UV-4 spectrometer. Differential scanning calorimetry (Seiko DSC220-CU) was done on ca. 10 mg of a mixed valent melt [Ru2.5+(NH3)5(3chloropyridine)][MePEG-SO3]2.5 dried at 50 °C under vacuum for ca. 12 h. There was no evidence of melting transitions, and a vague glassing transition, TG, appears at ca. -65 °C (Figure S-1), that is significantly lower than the experimental temperatures in this report. Thus, the pentaammineruthenium materials remain as molten salts in all experiments described. The Ru complex concentrations in the melts were calculated based on ideal stoichometric formulas and measured densities.

Ritchie and Murray

Figure 1. Temperature-dependent microelectrode voltammetry of the Creutz-Taube [Ru(NH3)5(pyrazine)Ru(NH3)5]5+ (MePEG-SO3-)5 melt at a 12.5 µm microdisk electrode.

Densities were measured from the mass of dry melt that could be pulled into a preweighed 1 µL volumetric pipet. Temperature-dependent melt viscosities were evaluated on ca. 1 g of dried [Ru2+(NH3)5(pyridine)](MePEG-SO3-)2, under flowing N2, using a cone-plate rheometer (Brookfield Model DV-III, CP52 cone) and a constant temperature circulator. Cyclic voltammetry (CV) and potential step chronoamperometry (CA) of undiluted melts were performed as described before,14,16 with a locally built low-current potentiostat and using a three-electrode platform onto which thin films of the melts were cast. The working microelectrode is a ca. 10 µm diameter Pt wire tip. Temperature control of the jacketed cell was maintained with a circulating bath and chiller. The film for each experiment was typically maintained under vacuum for several hours prior to and during measurements. To avoid possible diffusion-plasticizing effects of residual small molecules (such as the casting solvent or water), it has been our practice14 to heat the melt film on the electrode assembly under active vacuum for a prolonged period (24 h). The pentaammineruthenium melts involving the pyridine, 4-chloropyridine, and benzonitrile ligands were however not tolerant of prolonged heating or active vacuum. These melts when heated under vacuum for extended periods displayed depressed apparent diffusion coefficients (DAPP), suggesting some form of decomposition. A less aggressive approach was taken, in which melt films cast onto the microelectrode assembly from solution were dried at 50 °C under Ar flow for ca. 60 min. Vacuum pulses of ca. 60 s duration were then applied (maintaining the cell at 50 °C), followed by backfilling with dry Ar (dried with a dry ice/acetone foreline trap). Because water acts as a strong diffusion-plasticizer, the melt drying process could be followed by monitoring the decrease in currents in the melt voltammetry, halting the process when the voltammetric signal became stable. Results and Discussion Creutz-Taube Melt. Voltammetry and DAPP. Although ionic conductivities were not directly measured, all of the pentaammineruthenium molten salts were sufficiently ionically conductive to support microelectrode voltammetry in the neat, undiluted melts. Voltammetry of the melt based on the mixed valent binuclear Creutz-Taube ion17 is shown as a function of temperature in Figure 1. The electrode reaction in the [Ru2+(NH3)5(pyrazine)Ru3+(NH3)5] (MePEG-SO3-)5 melt corresponds to oxidation of the ruthenium [23] mixed valent form to the [33] form. The reduction to the [22] form is not shown; it is more negative than the Figure 1 oxidation wave by ca. 400 mV. This separation is comparable to previous observations17 of

Electron Transfer Dynamics in Molten Salt

J. Phys. Chem. B, Vol. 105, No. 46, 2001 11525 from the Ru(III/II) reaction of the [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 host, which causes the rising currents at more positive potentials. The general appearance of the wave indicates a linear diffusion geometry, and the diffusion coefficient of the [Co(bpy)3]2+ complex was estimated using the Randles-Sevcik linear potential sweep equation21

iPEAK ) 0.446nFAC

Figure 2. Physical diffusion-controlled voltammetry of ∼10% (140mM) [Co(bipy)3]2+ in [Ru2+(NH3)5(benzonitrile)] (MePEG-SO3-)2 melt, potential scan rate 1 mV/s, 12.5 µm microdisk electrode. EA ) 70 kJ/ mol.

voltammetry of the Creutz-Taube ion in dilute solutions. The [23] f [22] reduction is poorly defined in this melt due to a large iRUNC which we attribute to the electroneutrality-based expulsion of anions, with their solvating PEG tails, from the electrode interface upon reduction, diminishing the ionic conductivity in the region near the electrode. The same effect should decrease iRUNC in the oxidation reaction, as electroneutrality demands that anions (and their PEG tails) be brought to the electrode, thereby decreasing the viscosity, and the ionic resistance. The increase in the limiting current with increased temperature (Figure 1) means that the rate of charge transport (DAPP, Equation 1) increases and that charge transport is an activated process. The well-formed voltammetric wave does not exhibit serious uncompensated resistance distortion and its shape is characteristic of radial diffusion conditions, for which the relevant equation is18

iLIM,SS ) 4nrFDAPPC where iLIM,SS is the steady-state limiting current, r the microelectrode radius, F the Faraday, and C the concentration of the Creutz-Taube ion. On the basis of this relation, DAPP for the [23] f [33] reaction of the [Ru2+(NH3)5(pyrazine)Ru3+(NH3)5] (MePEG-SO3-)5 melt is 6.9 × 10-11 cm2/s at 35 °C. DAPP results at other temperatures are given in Table S-I. Physical Diffusion. Obtaining the electron-transfer rate constant kEX requires correcting the apparent diffusion rate (DAPP) for the contribution of physical diffusion (DPHYS, eq 1). DPHYS can be measured using a surrogate diffusant added to the pentaammineruthenium molten salt that does not undergo electron hopping transport. The requirements for a suitable diffusant are as follows: (a) a negligibly slow electron selfexchange rate, and/or (b) a concentration low enough to depress its DE (see eq 1) or lower than its percolation threshold4 for neighbor-neighbor electron hopping. We chose the metal complex melt [Co2+(bpy)3](MePEG-SO3-)2 with these factors in mind; it has a kinetically slow Co(III/II) electron-transfer reaction, and was dissolved at low concentration (∼10% w:w, or 140 mM) in the pentaammineruthenium molten salt [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 as the host melt example. DAPP for this Ru melt is nearly identical to that of the CreutzTaube melt, vide infra. The Co complex has been used previously3,19 as a surrogate20 diffusant in redox melts. Cyclic voltammetry of the solution of [Co2+(bpy)3](MePEGSO3-)2 in the [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 melt is shown in Figure 2 at a series of temperatures. The Co(III/II) oxidation wave shape is not well-defined and is quasi-reversible, as is often the case for Co complexes. It is adequately separated

nF (RT )

1/2

/ Co

ν1/2DPHYS1/2

(2)

producing DPHYS ) 5 × 10-13 cm2/s in the [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 melt at 35 °C. This rough value is over 102-fold smaller than the DAPP of the host [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 melt, and the DAPP of three of the remaining four pentaammineruthenium molten salts. Additionally, eq 3 is expected to over-estimate DPHYS because the Co(III/II) reaction is quasi-reversible, not reversible. According to eq 1, the large difference between DAPP and DPHYS values means that effectively DAPP ≈ DE, meaning kEX can be calculated directly from DAPP results. This has been a common result1-7 in semisolid, electroactive melts. Results for DPHYS at a series of temperatures (data in Table S-II) led to a diffusion activation barrier plot (Figure S-2) with a slope corresponding to an activation energy of 70 kJ/mol. The analogous barrier for physical self-diffusion of [Co(bpy)3]2+ in its own undiluted melt is a similar 79 kJ/mol. Electron Self-Exchange in the Creutz-Taube Melt. We return to the analysis of electron self-exchange dynamics of the mixed valent [Ru2+(NH3)5(pyrazine)Ru3+(NH3)5] (MePEGSO3-)5 melt knowing now that DAPP ) DE, i.e., {kEXδ2C/6} ) 6.9 × 10-11 cm2/s at 35 °C. Charge transport in the [23] Creutz-Taube ion melt is potentially more complicated than in previous examples of mixed valent semisolids, because it is binuclear in electron donor/acceptor sites. It is also not spherical. Potential pathways for electron transport include both intermolecular and intramolecular electron transfers. Thus, following generation of the ruthenium [33] ion at the electron surface (abbreviating the structure)

3spzs2 - e-1 f 3spzs3

(3)

electron hopping transport can conceivably occur both by the bimolecular intermolecular reaction

3spzs3 + 2spzs3 f 3spzs2 + 3spzs3

(4)

and by the unimolecular intramolecular reaction

3spzs2 f 2spzs3

(5)

reaction 5 is the thermal analogue of the optically driven intervalence charge-transfer reaction, observations17,22,23 of which have brought considerable attention to this and related binuclear complexes. It is a step that has to be generally considered for complexes in which the charge is electronically localized on the metal centers. However, it is now widely viewed that the particular case of the Creutz-Taube ion belongs in Robin and Day’s class III (delocalized charge) and thus, assuming that ion-pairing in the melt does not alter the delocalization, the barrier to charge transfer in reaction 5 is expected to be essentially zero. Rotational diffusion is the equivalent of reaction 5, in the charge transport sense, but an estimation of its time constant compared to that to electron hopping leads us to believe that it is not important in charge transport in this or other binuclear complexes.24

11526 J. Phys. Chem. B, Vol. 105, No. 46, 2001

Ritchie and Murray

TABLE 1: Electron Transfer Dynamics in Pentaammine[Ligand]Ruthenium Melts melt

δ, Å

concen, M

DAPP, cm2/s 35 °C

EA, kJ/mol

kEX, M-1 s-1 35 °C

kEX0, M-1 s-1

[Ru2+(NH3)5(py)](MePEG-SO3-)2 [Ru2+(NH3)5(3-Cl py)](MePEG-SO3-)2 [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 [Ru2+(NH3)5(pyrazine)Ru3+ (NH3)5](MePEG-SO3-)5 [Ru2+(NH3)5(4,4′-bipy)Ru3+ (NH3)5](MePEG-SO3-)5 0.14M [Co2+(bipy)3] in [Ru2+ (NH3)5(benzonitrile)](MePEG-SO3-)2

11.3 11.4 11.3 14.7

1.17 1.14 1.15 0.52

2.9 × 10-11 4.8 × 10-11 7.1 × 10-11 6.9 × 10-11

49 42 45 47

1.1 × 104 2 × 104 3 × 104 3.7 × 104

2.2 × 1012 3.0 × 1011 1.8 × 1012 1.0 × 1012

15.0

0.49

2.6 × 10-12

71

(1.4 × 103)a

22.8

0.14

5.2 × 10-13 b

70

N/Ab

N/A

δ is the charge displacement distance (equilibrium center-center) accomplished by an electron hop, DAPP is the experimentally observed charge transport diffusion coefficient, EA is the activational barrier of DAPP, kEX is the electron self-exchange rate, and kEX0 is the thermal activation prefactor a Problematical because of similarity of DAPP and EA to physical diffusion results. b In this melt, DPP ) DPHYS.

Figure 3. Arrhenius plots of DAPP results for mono- and binuclear [Ru2+(NH3)5L](MePEG-SO3-)2 melts: b L) benzonitrile; 9 L) pyrazine (binuclear melt); O L)3-chloropyridine; 1 L) pyridine; 3 L)4,4′-bipy (binuclear melt).

Whether rotational diffusion occurs or not, charge transport over multiples of the Creutz-Taube ion’s dimension cannot be accomplished by the delocalization in reaction 5 alone, given that physical diffusion is very slow. Charge transport in the mixed valent [33]/[23] melt formed around the working electrode must thus occur either by serial reaction 4 steps. Using the aVerage center-center separation (1.47 nm, derived from the measured 0.52 M Creutz-Taube melt concentration) as the average electron hopping distance (δ) in eq 1, we obtain a value of kEX ) 3.7 × 104 M-1 s-1 from the measured DE ) 6.9 × 10-11 cm2/s at 35 °C. We will compare this rate constant to those for mono-nuclear pentaammineruthenium complex melts, below. The comparison will confirm that the measured kEX is dominated by the rate of reaction 4, i.e., the charge displacement of reaction 5 is not kinetically limiting. Figure 3 (9) shows an Arrhenius plot of DAPP (i.e., DE) values measured in the [Ru2+(NH3)5(pyrazine)Ru3+(NH3)5] (MePEGSO3-)5 melt. The plot is linear, consonant with that of others3,19,25 in redox polyether hybrid melts in which DAPP was established to be dominated by electron self-exchange. The activation barrier energy for charge transport is 47 kJ/mol (Table 1), and the intercept of Figure 3 yields a thermal activation prefactor kEX0 ≈ 1 × 1012 M-1 s-1. Coulombic work terms are absent (the melt is undiluted and the complexes are forced into intimate contact) and the reaction 4 precursor complex formation constant KA is (like similar melts) near unity),3,19,25 so the prefactor can be identified roughly with the product of electronic and frequency factors κν (s-1). Typical adiabatic electron transfers have κν values of 1012-1013 s-1, whereas nonadiabatic ones have smaller κν values.26 The intercept of Figure 3 (9) thus signals that the electron-transfer reactions in the [Ru2+(NH3)5(pyrazine)Ru3+(NH3)5] (MePEG-SO3-)5 melt are adiabatic or nearly so.

Figure 4. Microdisk (r)12.5µm) electrode voltammetry of mononuclear [Ru2+(NH3)5L](MePEG-SO3-)2 melts: (s) L ) pyridine; (‚‚‚) L ) benzonitrile; (- - -) L ) 3-chloropyridine. For the benzonitrile and pyridine complexes, 35 °C at 5 mV/s, for the 3-chloropyridine complex 40 °C and 10 mV/s.

Mononuclear Pentaammineruthenium Melts. Charge transport was studied in three mono-nuclear pentaammineruthenium molten salts in which the sixth ligand was pyridine, benzonitrile, or 3-chloropyridine. These three melts have physical properties qualitatively similar to the bridged pyrazine (above) and 2,2′bipyridine (later) melts. The Ru electron donor/acceptor site concentrations (Table 1) are also similar to the bridged melts, when the site multiplicity in the latter is accounted for. The shapes of the microelectrode voltammograms in these melts (Figure 4) show that they are under linear diffusion control in the benzonitrile and 3-chloropyridine complex melts and under mixed linear-radial control for the pyridine complex melt. The DAPP values were measured using potential steps across the Ru(IIfIII) wave (chronoamperometry), recording and plotting the diffusion-controlled current-time response as current vs. t-1/2 at times short enough that linear diffusion prevails. Analyzing the slopes21,27 of the Cottrell plots gives results for DAPP as in Table 1. These DAPP results are uniformly . the DPHYS ) 5 × 10-13 cm2/s at 35 °C estimated above (Figure 2); i.e., DAPP is dominated by the rate of electron hopping in the mononuclear complex melts and not by physical diffusion. Table 1 gives the 35 °C electron self-exchange rate constants calculated from eq 1 and the activation barrier energies and preexponential factors derived from their temperature dependence (Figure 3; numerical values are found in Table S-2). Electron hopping transport in the mononuclear melts is intrinsically inter-molecular, and simpler than in binuclear melts in that intramolecular steps (hopping or delocalization) are absent. An important result in Table 1 is that both the DAPP (and thus kEX) and EA results for the mononuclear melts are similar to those for the Creutz-Taube ion. The similarity of the DAPP and activation results for the three mono-nuclear

Electron Transfer Dynamics in Molten Salt

J. Phys. Chem. B, Vol. 105, No. 46, 2001 11527

pentaammineruthenium based molten salts to those of the [Ru2+(NH3)5(pyrazine)Ru3+(NH3)5] (MePEG-SO3-)5 melt confirms that electron transport in the Creutz-Taube melt is dominated by the rate of reaction 4 as opposed to reaction 5, as discussed above. Additionally, we note that kEX for the mononuclear melts is not particularly sensitive to the ligand. The thermodynamics of the Ru(III/II) couple are in contrast quite sensitive to the sixth ligand; the redox potentials for the Ru(III/II) couples vary by over 0.4 V (Figure 4). The Ru(III/II) electron self-exchange rate constants kEX for the above complexes are substantially smaller (and the energy barriers larger) than those for Ru(III/II) electron self-exchange in semisolid melts of ruthenium tris-bipyridine complexes having equivalent MePEG tail contents. The latter results are kEX ) 5 × 105 and 2 × 105 M-1 s-1 and EA ) 26 and 34 kJ/mol when each counterion is MePEG-tailed or one of the bipyridine ligands has two affixed MePEG tails, respectively.19 Further, the kEX rate constants for the ruthenium tris-bipyridine complexes in the semisolid state are themselves considerably smaller than those of their nontailed analogue (i.e., [Ru(bpy)3]3+/2+ in dilute, fluid solution kEX ) 4.2 × 108 M-1 s-1 at 25 °C in 0.1M HClO4).28,29 Similarly, a kEX self-exchange rate constant for dilute [Ru(NH3)5py]3+/2+ at 25 °C in 0.1M CF3SO3H has been measured as 1.1 × 105 M-1 s-1.30,31 Thus, similarly to the above case, the kEX value of 1.1 × 104 M-1 s-1 measured at 35 °C in the semisolid melt appears to be at least 1 order of magnitude slower than the dilute solution case. Binuclear Ru Melt Based on 4,4′-bipyridine Bridging Ligand. Ru centers connected by 4,4′-bipyridine ligands are expected17,22,23 to have significantly smaller electronic coupling values, compared with the pyrazine bridged species. In fact, in the [Ru2+(NH3)5(4,4′-bipy)Ru3+(NH3)5](MePEG-SO3-)5 melt, the coupling through the 4,4′-bipyridine is so weak that the voltammetric waves for the [22]/[23] and [23]/[33] couples are not separately distinguishable. The observed voltammetry (Figure S-3) is thus a net two electron, and consequently more complicated process, ([33]f[22]), in which electron hopping charge transport occurs by some combination of intermolecular steps

3sbpys3 + 2sbpys3 f 3sbpys2 + 3sbpys3 (6) and

2sbpys2 + 2sbpys3 f 2sbpys3 + 2sbpys2 (7) and the intramolecular analogue of reaction 5

3sbpys2 f 2sbpys3

(8)

The experimental DAPP (Table 1) for the [33]f[22] reaction in the bipyridine bridged melt is considerably smaller (and the activation barrier energy larger) than those in the other pentaammineruthenium melts. The bipyridine bridged melt’s DAPP is in fact only about 5-fold larger than that for the physical diffusion of [Co(bpy)3]2+ complex through the [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 melt, and it’s activation barrier energy (71 kJ/mol) is nearly the same (70 kJ/mol for physical diffusion). We conclude that physical diffusion plays a much larger, and probably dominant, role in charge transport through the [Ru2+(NH3)5(4,4′-bipy)Ru3+(NH3)5](MePEG-SO3-)5 melt. One might infer that the slowness of charge transport in the bipyridine bridged melt could arise from poor electronic coupling by the bipyridine bridge; thereby inferring that the intramolecular reaction 8 is in fact a key step in charge transport in binuclear melts. The corollary of this inference is that in the

Figure 5. Temperature dependence of fluidity of [Ru2+(NH3)5(pyridine)](MePEG-SO3-)2 melt.

Creutz-Taube melt, charge transport does not occur solely by reaction 4, but by a serial combination of reaction 4 and the delocalization of 5. An important (and unresolved) caveat of the bipyridine-bridged results however, is the complication of the two-electron character of the DAPP measurement in this case. Temperature Dependence of Viscosity. The temperature dependence of the viscosity of the [Ru2+(NH3)5(py)](MePEGSO3-)2 melt is shown in Figure 5. This measurement was done to compare the fluidity activation barrier (fluidity ) viscosity-1) to that of physical diffusion. Fluid flow, like physical diffusion, involves structural motions that include chain segmental motion in the polyether tails of the counterions. As was the case19 for a counterion-tailed melt of the [Co(bpy)3]2+ complex, the fluidity activation plot is curved. The slope in the 50-70 °C temperature range gives a barrier of 81 kJ/mol, which is near that (70 kJ/ mol) for the physical diffusion of [Co(bpy)3]2+ complex through the [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 melt. Additionally, the viscosity of the [Ru2+(NH3)5(py)](MePEG-SO3-)2 melt at 25 °C (ca. 1 × 106 cP) is similar to that of an undiluted [Co(bpy)3]2+ melt that has a similar polyether-tailed counterion (4 × 106 cP). These additional data support our view that the DPHYS measured (Figure 2) for a solution of the [Co(bpy)3]2+ complex in a [Ru2+(NH3)5(benzonitrile)](MePEG-SO3-)2 melt adequately represents the physical diffusion of the ruthenium complexes there as well. Acknowledgment. This research was supported in part by grants from the National Science Foundation and the Department of Energy. Supporting Information Available: Two tables of temperature-dependent DAPP and DPHYS data, figures for differential scanning calorimetry, DPHYS activation, and voltammetry of the bipyridine bridged melt. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Williams, M. E.; Lyons, L. J.; Long, J. W.; Murray, R. W. J. Phys. Chem. B 1997, 101, 7584-7591. (b) Williams, M. E.; Crooker, J. C.; Pyati, R.; Lyons, L. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 10 249-10 250. (2) Kulesza, P. J.; Dickinson, E.; Williams, M. E.; Hendrickson, S.; Murray, R. W. J. Phys. Chem. B 2001, 105, 5833-5838. (3) Masui, H.; Murray, R. W. Inorg. Chem. 1997, 36, 5118-5126. (4) Long, J. W.; Velazquez, C. S.; Murray, R. W. J. Phys. Chem. 1996, 100, 5492-5499. (5) Emmenegger, F.; Williams, M. E.; Murray, R. W. Inorg. Chem. 1997, 36, 3146-3151. (6) Long, J. W.; Kim, I. K.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 11 510-11 515.

11528 J. Phys. Chem. B, Vol. 105, No. 46, 2001 (7) Williams, M. E.; Murray, R. W. Chem. Mater 1998, 10, 36033610. (8) Leone, A. M.; Weatherly, S. C.; Williams, M. E.; Thorpe, H. H.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 218-222. (9) Hatazawa, T.; Terrill, R. H.; Murray, R. W. Anal. Chem. 1996, 68, 597-603. (10) The soft material state is essential for quantitative voltammetry by forming a reproducible and complete contact with a solid electrode. (11) Ritchie, J. E.; Murray, R. W. J. Am. Chem. Soc 2000, 122, 29642965. (12) Majda, M., Chapter in Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; John Wiley and Sons: New York, 1992, pp 159. (13) Ito, K.; Ohno, H. Electrochem. Acta 1998, 43, 1247-1252. (14) Dickinson, E.; Williams, M. E.; Hendrickson, S. M.; Masui, H.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 613-616. (15) Curtis, J. C.; Meyer, T. J. Inorg. Chem. 1982, 21, 1562-1571. (16) Wooster, T. T.; Longmire, M. L.; Zhang, H.; Watanabe, M.; Murray, R. W. Anal. Chem. 1992, 64, 1132-1140. (17) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1973, 95, 1086-1094. (18) Wightman, R. M.; Wipf, D. O. Voltammetry at Ultramicroelectrodes; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15. (19) Dickinson, E.; Masui, H.; Williams, M. E.; Murray, R. W. J. Phys. Chem. B. 1999, 103, 11 028-11 035. (20) It was not possible to prepare a metal complex melt that is isostructural with the [Ru(NH3)5L]2+ melts, i.e., [Co(NH3)5L]2+.

Ritchie and Murray (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, second edition; John Wiley:, 2001. (22) Richardson, D. E.; Taube, H. Coord. Chem. ReV. 1984, 60, 107129. (23) Sutton, J. E.; Sutton, P. M.; Taube, H. Inorg. Chem. 1979, 18, 1017-1021. (24) (a) In the melt, the direction of the charge displacement in reaction 5 is measured relative to the electrode, making rotational diffusion equivalent to reaction 5. Rotational correlation times tROT in the pentaamineruthenium melts are however much longer (in the range of 20-200 ms, based on viscosities of 105 to 106 cP at 25 to 35 °C) than the electron hopping time scale that can be inferred from DAPP. (b) tROT ) 4π(radius)3(viscosity)/ 3kBT. (c) see Masui et al. Inorg. Chem. 1997, 36, 5118-5126 for a complete discussion. (25) Williams, M. E.; Masui, H.; Long, J. W.; Malik, J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 1997-2005. (26) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 2001. (27) Cottrell, F. G. Z. Physik. Chem. 1902, 42, 385. (28) Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1978, 82, 2542. (29) Young, R. C.; Keene, F. R.; Meyer, T. J. J. Am. Chem. Soc. 1977, 99, 2468-2473. (30) Brown, G. M.; Sutin, N. J. Am. Chem. Soc. 1979, 101, 883-892. (31) Brown, G. M.; Krentzien, H. J.; Abe, M.; Taube, H. Inorg. Chem. 1979, 18, 3374-3379.