6350
J. Phys. Chem. B 1997, 101, 6350-6358
Solvent Effects on the Electrochemistry of C60. Thermodynamics of Solvation of C60 and Fullerides Indra Noviandri,† Robert D. Bolskar,‡ Peter A. Lay,*,† and Christopher A. Reed‡ School of Chemistry, UniVersity of Sydney, NSW, 2006 Australia, and Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089-0744 ReceiVed: February 27, 1997X
The previously published solvent dependence of the electrochemistry of C60 has been extended and reanalyzed. The potentials have been measured against the decamethylferrocenium/decamethylferrocene (Me10Fc+/0) couple, which is much less solvent-dependent than the ferrocenium/ferrocene (Fc+/0) couple, and hence gives a more accurate reflection of the solvent dependence of the C60n-/(n+1)- couples. The formal redox potentials of the C60n-/(n+1)- couples exhibit substantial dependences on the solvent and become more solvent-dependent as the charge increases. This dependence arises from two main interactions with the solvents: hydrogen-bonding interactions with acidic functionality and electrostatic interactions with solvent dipoles. The strength of the hydrogen-bonding interaction increases as the square of the solute charge due to the increase in basicity of the fullerides. In addition to the contribution of the hydrogen-bonding acidity and the solvent dipoles, the potentials of the C600/1- couple are influenced by the Lewis basicities of the solvents. The latter is likely to be due to ion-pair formation between the fullerides and the Bu4N+ ion and/or interactions between the Lewis basicity of the solvent and the Me10Fc+ ion of the reference couple. This interaction is much smaller compared to the solvent/solute interaction. π-Stacking interactions between C60 and aromatic solvents results in a significant contribution of polarizability to the first reduction. The solvent dependences of the fullerenes can be used to model electrode/solvent interactions in which the surface (albeit a distorted one) goes from the point-of-zero-charge (C60) to a variety of discrete negative surface charges (fullerides).
Introduction C60 is an isotropic molecule and hence has no dipole or multipole moment. Nevertheless, it is sufficiently soluble in a range of polar solvents1 to perform detailed studies of the solvent dependence of the electrochemistry.2 The interaction of solvent molecules with fullerenes and fullerides is an interesting problem since the large size of the solute compared to the solvent molecules is more akin to a solvent/surface (albeit a curved one) interaction than to a typical solvent/solute interaction. The solvent dependence of the electrochemistry of C60 is also interesting from the point of view of fulleride chemistry. Fullerides exhibit a variety of interesting spectroscopic properties3-8 and are the charge carriers in superconducting salts.9 All of these properties are influenced by molecular level contacts. The electrochemistry of C60 normally exhibits three to six chemically-reversible one-electron reductions corresponding to the C600/1-, C601-/2-, C602-/3-, C603-/4-, C604-/5-, and C605-/6couples in which the three-fold degenerate lowest unoccupied molecular orbital (LUMO) of C60 is populated.2,6-8,10-15 By contrast, the oxidation of C60 is generally irreversible in most solvents.10 Somewhat reversible oxidations have been observed at low temperatures in CH2Cl2/(CH3)2NH mixtures11 and a reversible C60+/0 oxidation has been reported to occur in 1,1,2,2tetrachloroethane (TCE).16,17 Solvent/fullerene interactions are expected to be quite significant since the surfaces of the fullerenes expose the solvents to a strained π-orbital system that consists of sp2 orbitals with enhanced s character. Solid state interactions between aliphatic18 * Address correspondence to this author. Fax: 61-2-9351 3329. E-mail:
[email protected]. † University of Sydney. ‡ University of Southern California. X Abstract published in AdVance ACS Abstracts, July 1, 1997.
S1089-5647(97)00740-2 CCC: $14.00
and aromatic19 solvents and neutral C60 have been reported. Solvent interactions are expected to be even larger for the charged fullerides because of solvent-dependent Jahn-Teller distortions3,4,20 and large quadrupole moments.21 Both C60 and C601- exhibit substantial solvatochromism in their electronic absorption spectra,22-25 and their resonance Raman spectra in solution also indicate strong solvent/solute interactions.26 Previously, the solvent dependence of the formal potentials of the C60 reductions have been measured Vs the ferrocenium/ ferrocene (Fc+/0) couple and have been correlated with the donor (DN) and acceptor (AN) numbers of the solvents.2,15 These authors also examined ion-dipole solvent/solute interactions (Ei-d), but the results were inconclusive because, as they acknowledged, the effects were small and they may have been dominated by solvation of the Fc+/0 redox couple.15 When multiple linear regressions were performed, they concluded that the DN and Ei-d solvent parameters were the most important in determining the C600/- redox potential, but the R2 values were poor in such correlations.15 In the present work, we have investigated the solvent dependence of the redox chemistry using the decamethylferrocenium/decamethylferrocene (Me10Fc+/0) redox standard27 as an internal reference, since it is less solventdependent than the IUPAC-recommended28 Fc+/0 couple. Correlations have also been investigated with a more extensive series of solvent parameters. The reexamination has provided valuable insights into the nature of the solvent/solute interactions and into the solvation thermodynamics of C60 and individual fulleride ions. A previously developed model29 that correlates the formal reduction potential of a redox couple with empirical solvent parameters is used to elucidate the nature of solvent/solute interactions of C60 and its fullerides with the solvents. In this model, the formal potential of a redox couple, Ef, is considered in terms of four contributions: © 1997 American Chemical Society
Solvent Effects on the Electrochemistry of C60
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6351
Ef ) Ef° + ∆Ep + ∆EH + ∆Es
(1)
Ef° is the formal reduction potential in the absence of all solvent interactions, ∆Ep is the contribution of nonspecific terms to the formal reduction potential due to differences in electrostatic interactions between the solvent dipoles and the charge of the members of the couple, ∆EH is an interaction associated with changes in internal solvent order caused by the change in oxidation state,30 and ∆Es represents the specific solvent interactions, such as solvent/solute hydrogen bonding, Lewis acid/base interactions and solvent/solute π-stacking of ring systems. The multiparameter equation of Kamlet, Abboud, and Taft for correlating a physicochemical quantity with solvent properties has been validated extensively with a large number of systems.31,32 Here it is used within this model to delineate the solvent effects on the formal potential giving the following expression for Ef.29,33
Ef ) Ef° + aR + bβ + c(π* + dδ) + hδH
2
(2)
where R and β are empirical parameters for solvent hydrogenbonding acidity and basicity, respectively, π* is an empirical parameter that measures the solvent dipolarity, δ is a polarizability correction term, and δH is the Hildebrand solubility parameter, which measures the solvent-solvent interactions that are interrupted in creating a cavity for the solute.30 The coefficients a, b, c, d, and h are extracted from the best fit of the experimental data. Another model, developed recently by Fawcett,34 is also used to analyze interactions between the solvents and the C60n-/(n+1)couples. This model correlates the formal reduction potential of a redox system with acceptor number, AN,35 donor number, DN,36,37 a polarity term, Y ) (s - 1)/(s + 2),38 and a polarizability term, P ) (n2 - 1)/(n2 + 2),38 where s and n are the dielectric constants and the refractive indexes of the solvents, respectively. Experimental Section Chemicals. Chloroform (Merck, AR grade), bromobenzene (Merck, AR grade), 1,2-dichlorobenzene (Merck, AR Grade), and benzyl alcohol (Ajax Chemical, AR Grade) were purified by literature methods.39 Aniline (Aldrich, 99.5+%), N-methylaniline (Aldrich, 99+%), and N,N-dimethylaniline (EGACHEMIE, GC 99%) were used as received. C60 was prepared and purified as previously described.40 Decamethylferrocene was prepared using a literature method.41 Tetrabutylammonium perchlorate (Fluka, electrochemical grade) was used as received. Instrumentation and Procedures. All electrochemical experiments reported here were performed at room temperature (22 ( 1 °C) on BAS-100 or BAS-100B/W electrochemical analyzers as reported previously.27 The scan rate used in the cyclic voltammetry (CV) experiments was 100 mV s-1. In the solvents where the solubility of C60 is too low for cyclic voltammetry experiments, the formal reduction potentials were obtained with differential pulse voltammetry (DPV). The experimental conditions used in the DPV experiments were the following: scan rate ) 4 mV s-1, pulse amplitude ) 50 mV, sample width ) 17 ms, pulse width ) 60 ms, pulse period ) 1000 ms. The data were reported as the difference of the formal reduction potentials of the C60n-/(n+1) - and Me10Fc+/0 couples calculated from experimental results. Other electrochemical data were obtained from ref 2 and were corrected to the Me10Fc+/0 couple using previously developed correction factors.27 This involved adding 505 mV to the values obtained for acetonitrile,
TABLE 1: Formal Reduction Potentials of C60n-/(n+1)Couples Referenced to the Reduction Potential of the Me10Fc+/0 Couple in 0.1 M TBAClO4a solvent
C600/1-
acetonitrile dimethylformamide pyridine anilineb benzonitrile nitrobenzene N-methylanilineb 1,2-dichloroethane benzyl alcoholb dichloromethane tetrahydrofuran 1,2-dichlorobenzeneb N,N-dimethylanilineb chloroformb chlorobenzene bromobenzeneb
-312 -343 -396c -397 -406 -442d -448 -443c -468 -473 -535c -547c -554d -573 -587d
C601-/2-
C602-/3-
C603-/4-
-735 -772 -763 -693c -817
-1225 -1362 -1283 -1158c -1297
-1685 -1902 -1813 -1626d -1807
-782d -848 -817c -858 -1063 -907c
-1308 -1633 -1360c
-908d -953
-1438
-1298 -1758 -2133 -1841d
a From ref 2. Data recorrected to the Me10Fc+/0 couple. b Determined in the present study. c CV data. d DPV data.
458 mV for N,N-dimethylformamide, 517 mV for pyridine, 523 mV for benzonitrile, 514 mV for nitrobenzene, 532 mV for 1,2dichloroethane, 532 mV for dichloromethane, 427 mV for tetrahydrofuran, and 497 mV for chlorobenzene. The statistical analyses were performed using the MINITAB42 program and methodology described previously.27 Only the statistically significant parameters (t ratio g 2) were retained (unless otherwise specified during the analysis). The t ratio is the ratio between the value of the coefficient and the value of the standard deviation of the coefficient. Results C60 is slightly soluble in chloroalkanes and appreciably soluble in aromatic solvents,1 but the very low solubility in the polar and hydrogen-bonding solvents that are normally used in electrochemical measurements limits the range of solvents that can be used in these studies. Thus, most of the solvents used in this study are chloroalkanes and aromatic solvents. To study the effects of the hydrogen-bonding acidity of the solvents, protonic haloalkanes such as chloroform and aromatic solvents such as benzyl alcohol and anilines were used. To avoid complications due to ill-defined liquid junction potentials, the formal redox potentials, Ef, of the C60n-/(n+1)couples were referenced to the Ef value of the Me10Fc+/0 couple, which is much less solvent-dependent than the Fc+/0 couple.27 Thus, the formal reduction potentials, Ef, of the C60n-/(n+1)couples reported here are the potentials of the following reaction.
C60n-(solvent) + Me10Fc(solvent) h C60(n+1)-(solvent) + Me10Fc+(solvent) (3) The formal reduction potentials of the C60n-/(n+1)- couples in 16 solvents are compiled in Table 1. They are very solventdependent, indicating a large change of solvation with the addition of each electron during the reduction. Over the range of solvents studied, the variation of the formal reduction potentials of the first four reductions of C60 are 275, 328, 408, and 448 mV, respectively. The shifts increase as the charges involved in the reduction processes increase. All of the couples were reversible or quasireversible. To evaluate the nature of the interactions between the solvents and the fullerene/fullerides, statistical analyses of the dependences of the formal reduction potential of the C60n-/(n+1)- couples were performed using known solvent parameters30,43 and the model
6352 J. Phys. Chem. B, Vol. 101, No. 33, 1997
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TABLE 2: Statistical Analyses of the Contributions of Solvent Parameters to the Formal Reduction Potentials of the C60n-/(n+1)- Couples Using Eq 2 β
R Ef°
a
0/1-
C60
C601-/2-
C602-/3-
C603-/4-
-864 -877 -550 -881 -815 -464 -550 -799 -1549 -1507 -1569 -1528 -1121 -1429 -879 -1288 -1113 -2141 -2113 -2140 -2115 -1384 -1903 -2559 -2562 -2584 -2577 -1899 -2188
a
b
243 238 10 308 221 19 618 716 617 745 399 469 923 1002 927 997 1165 1271 1212 1279 1076
t
c
1.96 2.03 0.05 1.53 4.27 0.07 4.55 4.79 4.42 4.62 1.62
d
b
π* t
c
215 209 275
4.19 4.59 3.72
418
3.93
275
3.89
67.2 101
1.18 1.57
c
e
t
516 498
4.37 4.94
625 -62.5
δH2
δ c
3.72 -1.86
f
c
t
h
-80.1 -75.8
-2.36 -2.52
-64
-0.35
367
1.95
435
2.37
577 750
2.17 2.43
633 241
2.22 0.91
200
0.86
255
1.10
174
0.84
d
-122
-2.48
530 583 745 605 830
3.20 3.71 4.70 3.77 5.11
-108 45.2 1.5 41.1 -18.7
-2.11 1.18 0.04 1.04 -0.50
265
1.03
97.9
1.45
578
2.09
0.5
0.01
783 885 783 878
3.33 4.37 3.62 4.80
75.6 49.1 78.9 52.7
1.41 1.11 1.61 1.40
682 654 800 691 794
1.59 2.91 4.25 3.22 4.68
52.5 91.9 57.6 91.0 63.9
0.59 1.85 1.43 1.88 1.85
0.63
98
0.74
g
tc
1.64 4.39 5.32 4.79 5.80 818 6.13 7.48 6.80 8.37 2.71
-41.1 -17.1
-0.49 -0.22
2.25 -72.8 -38.8
-0.91 -0.51
395
a
b
R2 (%)h
R2adj (%)i
88.3 88.1 55.7 60.3 82.7 0.0 55.7 51.0 92.0 86.9 90.1 82.3 48.1 62.6 21.1 35.0 33.1 91.9 90.2 91.4 90.1 38.7 34.8 97.7 96.2 96.7 95.9 55.1 24.2
81.0 82.9* 47.7 48.4 77.5 0.0 52.0 42.1 85.3 79.4 84.5* 75.6 36.6 48.5 13.2 20.5 26.4 81.8 82.4 84.5 85.2* 31.0 16.1 91.8 91.2 92.3 92.8* 47.6 0.0
c
Hypothetical formal reduction potential in non-interacting solvent. Coefficient for R. t ratio for the parameter: a measure of the significance of the contribution. d Coefficient for β. e Coefficient for π*. f Coefficient for δ. g Coefficient for δH2. h Correlation coefficient, unadjusted. i Correlation coefficient, adjusted for degrees of freedom. The asterisk indicates the most significant correlation.
described previously.29 The data for benzyl alcohol were excluded from the statistical analyses of the solvent dependence of the first two reductions of C60 because it is likely that protonation of the C60 anions occurs in benzyl alcohol (see Discussion).44-47 The results of the statistical analyses are summarized in Table 2. The most significant correlations between the formal reduction potentials of the C60n-/(n+1)- couples and solvent parameters for the first four reductions of C60 are given in eqs 4-7
Ef(C600/1-) ) -877 + 238R + 209β + 498(π* - 0.15δ) (mV) (4) Ef(C601-/2-) ) -1569 + 617R + 605(π* + 0.07δ) + 435δH2 (mV) (5) Ef(C602-/3-) ) -2115 + 997R + 878(π* + 0.06δ) (mV) (6) Ef(C603-/4-) ) -2577 + 1279R + 794(π* + 0.08δ) (mV) (7) with the correlation coefficients, R2adj, 82.9, 84.5, 85.2, and 92.8%, respectively. The correlations between the experimental formal reduction potentials and those calculated using eqs 4-7 are shown in Figure 1. As expected, the hydrogen-bonding acidity and polarity/ polarizability of the solvents exhibit significant contributions to the formal reduction potentials of all couples. The contributions to the Ef values are positive as shown by the signs of the coefficients of these parameters. The value of the coefficient
of the R parameter, a, increases with an increase of the charge of the C60n-/(n+1)- couples. The t ratio of this parameter also increases with increasing charge of the couple, the ratios being 2.03, 4.42, 5.80, and 8.37, respectively. This means that the hydrogen-bonding acidities of the solvents become more statistically important as the charges increase. The effect of solvent hydrogen-bonding acidity is well illustrated by examining the data for the anilines. As the N-H groups are progressively replaced by N-CH3 groups, the formal redox potentials move to much more negative values (by a total of 150 mV in the case of C600/-). On the other hand, an increase of the coefficients of the π* and δ parameters, c, with increasing charge occur only for the first three reductions. The value of the c coefficient of the C603-/4- couple decreases slightly from that of C602-/3-, but is still larger than those of the first two reductions of C60. In contrast with the t ratios of the coefficients of the R parameter, the t ratios of those of the π* parameter (4.94, 2.84, 4.80, and 4.68) are relatively constant except for the second reduction. The effect of the polarizability correction term, δ, is only significant for the C600/1- couple. The contribution of this parameter to the formal reduction potential of C600/1- is negative, as shown by the sign of this parameter in eq 4, while for the other three reductions it exhibits a small positive contribution. Consistent with previous results,2,15 the formal reduction potential of the C600/1- couple is also influenced by the Lewis basicities of the solvents (t ratio ) 4.59 for the β parameter, which has a linear relationship with DN31). As shown by the sign of the parameters, the contribution of this parameter to the formal reduction potential is positive. It is insignificant for the C601-/2- couple. The contribution of the Hildebrand parameter
Solvent Effects on the Electrochemistry of C60
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6353
Figure 1. Correlation between the formal reduction potential of the C60n-/(n+1)- couples obtained from the experiment and the formal reduction potential calculated using the best equation extracted from statistical analyses for (a) C600/1-, (b) C601-/2-, (c) C602-/3-, and (d) C603-/4- couples.
is significant (with a t ratio of 2.37) for the C601-/2- couple but not for any of the other couples. Similar trends were found from the statistical analyses using the Fawcett model34 (Table 3), where the most significant correlations between the formal reduction potentials of C60n-/(n+1)couples with the solvent parameters are given below.
Ef(C600/1-) ) -937 + 3.37AN + 3.83DN + 512Y (mV) (8) Ef(C601-/2-) ) -1804 + 14.4AN + 2.84DN + 525Y + 1048P (mV) (9) Ef(C602-/3-) ) -2103 + 33.7AN + 962P (mV) (10) Ef(C603-/4-) ) -2390 + 37.6AN (mV)
(11)
The correlation coefficients, R2adj, of the equations are 94.0, 75.9, 90.8, and 80.2%, respectively. The contribution of the solvent acceptor number, AN, to the formal reduction potentials of the C60n-/(n+1)- couples exhibits a similar trend to that of the R parameter in the earlier model. The contribution of AN is positive and increases as the charge increases giving t ratios of 2.22, 3.90, 8.71, and 5.03, respectively. On the other hand, the contribution of the solvent donor number, DN, decreases with increasing charge and is only significant for the first two reductions, with the t ratios being 7.82 and 2.11, respectively. The polarity term only contributes significantly to the first two reductions of C60. The significance of the contribution decreases as the charge increases (t ratio ) 9.65 and 3.45, respectively). The polarizability term only exhibits significant
contributions to the second and the third reductions, with t ratios of 2.05 and 2.63, respectively. Discussion The large shifts in the formal redox potentials of the C60n-/(n+1)- couples as a function of solvent indicate that a large difference of solvation occurs between the two oxidation states involved. The differences in the free energies of solvation (∆∆Gsolv) of the C600/1- couples, calculated using the method described previously,48 support this conclusion. Using the Me10Fc+/0 couple as the reference and the data of the formal reduction potential of this couple in water from our previous study,49 the ∆∆Gsolv values of the C600/1- couples in all of the solvents studied are estimated to range between -126 and -152 kJ mol-1, indicating a large solvent stabilization of the reduced state. Statistical analyses indicate that the major contributions to the formal reduction potentials of the C60n-/(n+1)- couples come from the hydrogen-bonding acidities and polarities of the solvents. Fullerides are Brønsted bases that are relatively easy to protonate.44-47 Thus, it is not surprising that the contributions of the hydrogen-bonding acidity of the solvent, as expressed by the coefficients of the R parameter obtained from the statistical analysis, are important. The sign of the contribution of the hydrogen-bonding acidity of the solvents to the formal reduction potential of the C60n-/(n+1)- couples is positive. The same result was found for the contribution of AN obtained using the Fawcett model.34 This shows that the solvent/solute hydrogen-bonding interactions are stronger for the reduced state of the couples and hence the basicity of the fullerides increases as the charge increases. This increase is further substantiated by the value of the t ratio of the R parameter, which increases
6354 J. Phys. Chem. B, Vol. 101, No. 33, 1997
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TABLE 3: Statistical Analyses of the Contributions of Solvent Parameters to the Formal Reduction Potentials of the C60n-/(n+1)- Couples Using the Fawcett Model31 AN Ef°
a
0/1-
C60
C601-/2-
C602-/3-
C603-/4-
-842 -937 -779 -940 -876 -500 -520 -843 -1804 -1397 -1166 -1931 -1103 -1374 -1044 -913 -2161 -1730 -2107 -2153 -1826 -1731 -2103 -1826 -2315 -2057 -2628 -2495 -2378 -2626 -2390
DN
b
t
3.35 3.37
2.35 2.22
5.14
1.23
2.96
0.47
a
c
14.4 13.3 12.3 15.2 12.2 14.0 12.8
3.90 3.03 2.12 3.39 2.24 2.49 1.98
33.4 36.0 34.2 32.9 33.5 35.8 33.7 33.3 43.4 45.9 38.8 38.4 39.2 36.8 37.6
5.99 5.21 7.88 6.64 5.68 5.86 8.71 6.35 4.50 5.61 5.31 4.38 4.66 5.44 5.03
d
Y c
b
t
3.92 3.83 4.08
8.47 7.82 7.13
3.99
6.88
4.38
2.84
2.84 3.77 3.95
2.11 2.47 1.89
4.12 4.34 -0.55 -0.22 -0.53 -0.19
-3.22 -3.54 -2.19 -1.73
e
c
P t
c
f
d
tc
503 512 485 555 494
10.06 9.65 7.84 3.80 7.87
-302
-1.46
-307
-1.19
531 525 379
3.58 3.45 2.34
1048
2.05
602
3.34
223 1414
0.31 2.40
419
2.03
56 -170
0.27 -0.77
1057
2.03
52
0.28
980 1033
2.48 2.19
-170
-0.84 962
2.63
570
0.70
948 711
1.52 0.86
881
1.47
2.17 1.91 -0.40 -0.13 -0.43 -0.11
-1.10 -1.34 -0.88
-317 -476
-0.81 -1.63
-132
-0.36
-0.91
R2 (%)g
R2adj (%)h
96.7 95.7 94.0 62.4 93.0 2.1 44.7 56.1 85.5 75.4 56.8 74.8 56.2 54.0 30.3 28.8 93.5 86.8 93.4 93.2 85.2 86.8 93.1 85.2 93.6 92.0 91.5 89.7 84.9 89.3 83.5
94.8 94.0* 91.8 54.1 91.4 0.0 39.2 51.7 75.9* 64.9 38.2 64.0 45.2 42.5 22.6 20.9 87.0 78.9 89.4 89.2 80.3 82.4 90.8* 83.1 80.7 84.0 82.9 79.4 77.3 83.9 80.2*
a
Hypothetical formal reduction potential in noninteracting solvent. b Coefficient for the acceptor number (AN). c t ratio for the parameter. d Coefficient for the donor number (DN). e Coefficient for polarity parameter (Y). f Coefficient for the polarizability parameter (P). g Correlation coefficient, unadjusted. h Correlation coefficient, adjusted for degrees of freedom. The asterisk indicates the most significant correlation.
as the charges involved in the redox processes increase. Nevertheless, the range of hydrogen-bonding acid solvents that could be studied was limited by both the solubilities of the fullerene/fullerides and the ability of the solvent to act as a Brønsted acid. The Brønsted acidity of benzyl alcohol is apparently too high for it to be considered as a normal solvent because the first and second formal reduction potentials calculated with eqs 4 and 5 deviate substantially from the experimental results. The deviation of the calculated second formal reduction potential is larger than that for the first, which indicates the greater extent of protonation of the more basic, more highly charged species.44-47 For the first reduction, our results are in contradiction to those obtained by Kadish and co-workers.2,15 They found that the contribution of AN was negative, indicating that the oxidized species is preferentially stabilized relative to the reduced one. This difference is likely to come from two main factors. First is the use of the Fc+/0 couple as an internal redox standard. We have shown that the formal reduction potential of the Fc+/0 couple is affected by interactions of the reduced species with the acidic functionality of the solvent.27 This interaction will shift the formal reduction potential of the Fc+/0 couple in the same direction as the shifts of the formal reduction potential of the C600/1- couple. Second, the negative slopes were obtained in single-parameter correlations involving AN. Using the published data,2,15 we have found that these correlations are not statistically significant (t ratios < 2). In addition, the AN parameter was not important in multiparameter correlations for data referenced to the Fc+/0 couple.15 The use of the Fc+/0 redox standard and single-parameter correlations with AN, combined with the lack of statistical analyses, led to the assertion that
correlations had negative slopes, which are not chemically reasonable. Not incidentally, the results reported here reinforce the postulate27 that the Me10Fc+/0 couple is a superior redox standard compared to the Fc+/0 couple. As expected, the contributions of solvent polarity (π*) to the formal reduction potential of the C60n-/(n+1)- couples are positive, indicating stabilization of the lower oxidation states. Due to the similar size of C60 and its fullerides, successive additions of electrons increase the charge density of the lower oxidation states, resulting in stronger solvent dipole interactions with the negative charge density of the fullerides than those with the oxidized state of the couple. This rationalizes the positive contribution to the formal redox potentials. Similar conclusions were made by Kadish and co-workers2,15 about positive contributions of solvent polarity terms, but they were made with data standardized to the Fc+/0 couple. In this respect, we have found that their single-parameter correlations were just statistically significant for the correlation between ∆Ei-d and the formal reduction potentials of the C600/1- couple (t ) 2.0) and that they were not significant for correlations involving Ef values of the other couples. Born50 developed a dielectric continuum model for the interaction of a hard spherical ion with a solvent. Because C60 and its fullerides are spherical and relatively rigid, it might be expected that this model would be applicable. The Gibbs free energy of electron transfer (∆Grc°) derived from this model is given by the following equation.51,52
∆Grc° ) (NA e2/32 π 0)(1 - 1/) (zox2/rox - zred2/rred) (14) where NA is Avogadro constant, e is the charge of the electron,
Solvent Effects on the Electrochemistry of C60
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6355 TABLE 4: Summary of the Correlation between the Formal Reduction Potentials of C60n-/(n+1)- Couples and zred2 - zox2 solvents dimethylformamide pyridine aniline benzonitrile dichloromethane tetrahydrofuran 1,2-dichlorobenzene
slope/mV slope/mVa Ef(C600/1-)/ Ef(C600/1-)/ (expt) (calcn) mV (expt) mVb (extrapln) 283 263 233 248 225 268 234
350 331 308 346 320 313 324
-312 -343 -396 -397 -468 -473 -535
-215 -236 -226 -316 -408 -539 -435
a Slope of the line calculated with eqs 4 and 5 using Van der Waals radii of C60.53 b The value of the formal reduction potential of the C600/1couple estimated using the linear regression equation obtained using the Ef data of the last three couples.
Figure 2. Correlation between the formal reduction potentials of the C60n-/(n+1)- couples and zred2 - zox2 in aniline.
0 is the permittivity of a vacuum, is the dielectric constant of the solvent, zox is the charge of the oxidized species, zred is the charge of the reduced species, rox and rred are the radii of the oxidized and the reduced species, respectively. In the case of C60 and its fullerides, the radius of the oxidized and the reduced species are similar, so that the value of ∆Grc° in a solvent will only depend on the difference of the square of the charge involved in the redox process (zox2 - zred2). Because ∆Grc° is related to the formal redox potential, Ef, by the following equation
∆Grc° ) -nFEf°
(13)
the same dependences with zred2 - zox2 are expected for the formal redox potential. The correlation between the formal redox potentials of the C60n-/(n+1)- couples and zred2 - zox2 are only linear for the last three couples (e.g., for aniline, Figure 2). The deviation of the first couple from linearity indicates a substantial difference in charge distribution between the reduced and oxidized species of the first couple. This result is in accord with calculations21 that suggest that the electron gained by the C60 is localized mainly in the vicinity of the equator, thereby producing a quadrupole. The linear correlation found for the last three couples indicates that the successive addition of electrons just increases the charge density of the fullerides and does not significantly perturb the distribution. This is consistent with the observation that all fullerides, C60n-, with n ) 1, 2, 4, and 5, are subject to Jahn-Teller distortions4 and may, therefore, all have comparable charge distributions. Even though C603should not be subject to Jahn-Teller distortions in the gas phase, in solution and the solid state the ground state is a doublet instead of the expected quartet.4 Therefore, this ion will also have charge localization in solution. Experimental evidence for a quadrupolar charge distribution in a fulleride came first from anisotropy in the EPR spectrum of C601-.3 Distortions from Ih symmetry have also been found in the X-ray crystal structures of C601- and C602-.4b,53 The extent of the distortion in both structures is quite similar to that calculated21 for C601-. Because the electron is localized in an equatorial band around the molecule, the charge density in this area will be larger than that when the charge is spread homogeneously over the entire ion.54 This concentration of charge will increase solvent/solute electrostatic interactions and, hence, will shift the formal redox potentials of the couples anodically. The positive shifts of the formal redox potential compared to the gas phase are larger for the couples having higher charges due to the stronger electrostatic interaction with the solvents. For strongly interacting
Figure 3. The dependences of the values of the (a) c coefficient and (b) a coefficient on zred2 - zox2.
solvents (solvents of high polarity and/or hydrogen-bonding acidity) this would lead to a decrease in the slope of the correlation between Ef and zred2 - zox2 compared to the macroscopic dielectric continuum model. This is consistent with the results in Table 4. A saturation effect from the charge being concentrated in a small area of the compound explains the lack of a linear correlation between the c coefficient and zred2 - zox2 (Figure 3a). Our results on the importance of the charge contrasts with the results obtained previously15 where a decrease in the importance of this contribution was found for the Ef values of the C601-/2- couple compared to those of the C600/1- couple. The differences between the measured formal reduction potential of the first couple and that estimated by the linear equation obtained from the last three couples range from smallest in dichloromethane (60 mV) to greatest in aniline (170 mV, Table 4). The large deviations in the more protonic solvents suggest that besides the electrostatic interactions, solvent/solute hydrogen-bonding interactions contribute a significant shift to the formal reduction potential of the C60n-/(n+1)- couples. These
6356 J. Phys. Chem. B, Vol. 101, No. 33, 1997 results are in accord with those obtained from the statistical analyses. Because of the strong contribution of the hydrogenbonding acidity of the solvents to the formal reduction potentials of the C60n-/(n+1)- couples, a linear correlation occurs between the a coefficient and zred2 - zox2 for the first three couples (Figure 3b). Again, the value of a for the fourth couple deviates from the linear plot, which may be due to either the saturation effects or the smaller range of solvents for which this data is available due to interferences from solvent reductions. The positive contribution of the β solvent parameter to the first reduction of C60 indicates that hydrogen-bonding (Lewis) basicities of the solvents stabilize the reduced species more than the oxidized species. This result is surprising because the reduced species has a higher negative charge than the oxidized species but is consistent with previous results.2,15 The statistical significance of the contribution of the β parameter to the first couple (t ratio ) 4.94) is much higher than that of the R parameter (t ratio ) 2.03), and the magnitudes of their coefficients are comparable. Since similar results are found from statistical analyses using the Fawcett model,34 where the statistical significance of the contribution of DN is much higher (t ratio ) 8.47) than that of AN (t ratio ) 2.35) for the first reduction, the presence of this term does not appear to be an artifact of the parameters chosen for the analysis. One possible explanation for this phenomenon may reside in the formation of surface double layers between C601- and the ions of the background electrolyte on reduction of C60, which has zero surface charge. For solvents that are good Lewis bases, the negative end of the dipole of the solvents would interact with the positive charge of the surface bound Bu4N+ ions, leading to stabilization of the positive charges around the reduced species. By contrast, in solvents that are weak Lewis bases, the positive charge of the surface Bu4N+ ions would be neutralized by the anions of the background electrolyte. The reduction in electrostatic attraction between the surface of the fulleride and the Bu4N+ ions by the formation of the double layer involving the cation and anion of the electrolyte would tend to destabilize the formation of the fulleride anions in comparison to solvents that are stronger Lewis bases. The lack of the contribution of the β parameter in the other couples indicates that the difference of the solvation caused by this double-layer interaction between the two members of the other couples is not significant. The formation of such double layers where the cation is concentrated around the negative charge in the equatorial belt is observed in crystal structures of C601- and C602-.4b,53 It has also been observed that the formal reduction potential of the couple is somewhat sensitive to the concentration and the nature of the cation of the background electrolyte.2,55 Ion-pairing constants have been estimated, but as pointed out by the authors, these measurements have been complicated by ion pairing with the Fc+/0 redox standard used in these measurements.55 Due to such interactions between C60- and Bu4N+, we would expect a much larger anodic shift of the formal reduction potential of the first couple compared to those of the other couples if this contribution dominated contributions from solvent/solute (fulleride) interactions. The observation that the shift of the formal reduction potential of the first couple is much smaller than those of the next three couples (Figure 2) indicates that the variation in the formal reduction potentials of the C60n-/(n+1)- couples are mainly due to solvent/solute (fulleride) interactions. While the surface/electrolyte interactions may explain the small effects of the background electrolyte on the redox behavior,2 they are not as significant as the solvent effects. An alternative explanation for the statistical significance of
Noviandri et al. the β parameter arises from interactions of the Lewis basicity of the solvent with the Me10Fc+ ion. This would result in a solvent dependence of the Ef values of the Me10Fc+/0 couple, which are involved in calculations of the formal reduction potentials according to the equilibrium in eq 3. Such an interaction would move the potential of the redox reference couple negative, which is the equivalent of a positive shift in the Ef value of the C600/1- couple. Although it has been shown that the values of Ef for the Me10Fc+/0 couple are less sensitive to the Lewis basicities of the solvents than those of the Fc+/0 couple,27 this does preclude a small residual dependence of the values of Ef for the Me10Fc+/0 couple on the Lewis basicities. Evidence against this possibility is that the solvent basicity is only statistically significant for the first couple using eq 2 and only the first two couples for the Fawcett model. If the redox reference was the reason for the statistical significance of the basicity parameters, it should contribute equally to the solvent dependences of the shifts of all of the couples that have been analyzed. The lack of correlations with these parameters for the more highly charged species may, however, be due to the dominance of the other solvent parameters on the redox potentials. The only way that definitive conclusions as to the origin of this effect can be made is to measure the potentials against a redox standard that has even less interactions with the solvent than the Me10Fc+/0 redox standard. As yet, no such redox standard has been characterized. If the positive contribution of the β parameter to the first formal reduction potential is due to the electric double-layer effect, many of the solvent molecules interacting with the double layer will have the negative ends of their dipoles oriented toward the C601- ion. The large contribution of the hydrogen-bonding acidity of the solvents in the second reduction (t ratio ) 4.42) compared to the first reduction (t ratio ) 2.03) reflects the large increase of the basicity of the species involved in the second reduction and suggests that the reduced state of this couple interacts preferentially with the positive end of the solvent dipoles. If this interaction reorients the solvent molecules, it may explain the statistical significance of the δH2 parameter. This solvent adjustment reduces the statistical significance of the interaction of solvent dipole with the couples as shown by the relatively low value of the t ratio (2.84) of the π* parameter. Alternatively, the statistical significance of the δH2 parameter to the correlation may not be chemically significant. However, the salient point is that the inclusion or exclusion of this parameter in the multiparameter correlation does not affect the interpretation of the contributions of the other parameters. The contribution of the polarizability correction term is only significant for the first reduction, but even here its magnitude is only 15% of that involving the π* parameter. The negative value of the d coefficient shows that the neutral molecule interacts more strongly than the reduced species. This difference is likely to arise from differences in solvent/solute π stacking. C60 is an electron acceptor and interacts quite strongly with π-donor solvents such as aromatics.19c This interaction is much weaker for the electron-rich fullerides. Summary and Conclusions. Even though C60 and its fullerides have a much larger molecular size than the solvent molecules with which they are interacting, the formal redox potentials of the C60n-/(n+1)- couples exhibit large dependences on the properties of the solvent. The fairly large solvent/solute interactions appear to arise from the sum of many relatively weak solvent/surface interactions. This is of importance in modeling solvent/electrode (surface) and electrode/electrolyte (double layer) interactions since the surface charge can be
Solvent Effects on the Electrochemistry of C60 controlled in going from the point-of-zero-charge (C60) to welldefined negative surface charges (fullerides). As expected, the couples become more solvent-dependent with increasing charge. This dependence mainly arises from specific hydrogen-bonding interactions with acidic functionalities of the solvents and from electrostatic interactions with solvent dipoles. The hydrogenbonding interaction increases with the square of the charge, reflecting the increase in basicity as the charge increases. The formation of ion pairs between the fullerides and Bu4N+ contributes to the complex solvent/solute interaction for the first reduction of C60. Due to this surface/electrolyte interaction (and/ or effects on the reference redox couple), an additional specific interaction with the Lewis basicities of the solvents occurs in the first reduction. Nevertheless, the contribution of this interaction is much smaller than other solvent/solute interactions. Because neutral C60 is considered an electron-deficient molecule, substantial π-stacking interactions with aromatic solvents occur leading to a significant contribution of the polarizability correction term in the first reduction. Another solvent reorganization may occur following the second reduction of C60. Expanding the analysis of solvent effects on the electrochemistry with solvents having acidic functionality to account for the effect of hydrogen-bonding interactions and analyzing the variation of formal reduction potentials with a better internal redox standard and more solvent parameters has resulted in a better and more comprehensive understanding of the electrochemical results. The internal consistency of the correlations obtained from eq 2 compared to those obtained from the Fawcett multiparameter approach indicates that the former is a better model for these redox couples. We conclude with a comment on the effects of cations. In their original work, Kadish and co-workers2 found that the electrochemistry of C60 was sensitive to the size of tetraalkylammonium ion, these effects being small compared to the effects of the solvent. In more recent work,15 an attempt has been made to compensate for the effects of the counterion by obtaining formal redox potentials with microelectrodes in the absence of added electrolyte. The validity of this approach is problematic, however, because the nature of the electrolyte is unknown and in addition, the precision of the measurement is inferior. Due to these and various other experimental difficulties alluded to in the paper (see ref 15 and Appendix A), the uncertainty in the values of the formal reduction potentials obtained in this manner may be greater than the effects of the electrolytes themselves. Acknowledgment. We are grateful to the AusAID for the Ph.D. scholarship of I.N. This work was supported by NIH Grant GM23851 (C.A.R.) and the Australian Research Council (P.A.L.). Appendix A In the present results, the Ef values in the presence of electrolyte were determined directly from the midpoint of the anodic and cathodic peaks in the non-steady-state linear diffusion limit. Thus, even when the response is distorted by iR drop terms, these are essentially canceled and the errors in Ef values over the range of solvents are small (several millivolts). By contrast, much of the data obtained from the microelectrode work required one or more corrections to obtain Ef values from the inflection points of the sigmoidal responses obtained from steady-state hemispherical diffusion.15 While Kadish et al. made a thorough attempt at making the appropriate corrections, each
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6357 correction necessarily introduces errors into the reported Ef values and hence, increases the uncertainty in the values with respect to those obtained in the presence of electrolyte. The errors that are introduced in the analysis of the literature data for each type of correction are summarized in the following paragraphs. Distortion due to iR drop was very apparent in the microelectrode results reported in some solvents, Viz., “Voltammetric steps recorded in THF and PhCl ..., i.e., solvents of low polarity, were markedly drawn out”.15 For instance, the first reduction in PhCl is spread out over ∼500 mV, overlaps with the second reduction, and is on a sloping background (Figure 1, ref 15). Responses in some other solvents were also distorted and were corrected assuming all of the distortion was due to iR drop. This correction increased the uncertainty in the values of Ef obtained by this method, but there are other problems in attaining such data. The first is that the overlap of the sigmoidal responses for the first two reductions (even when they are reversible) is compounded where the iR drop is greatest (see Figure 1 of ref 15). The necessarily arbitrary deconvolution of these distorted responses leads to additional subjectivity in choosing the Ef values. This is further complicated by the fact that much of the data was obtained in solvents with a sloping background current, leading to other uncertainties in the corrected Ef values. Even assuming that the sloping backgrounds have been subtracted, this leads to a reduction in the precision of the Ef values, since the background current is often affected by the presence of the electroactive species. This is especially the case in the absence of added background electrolyte because the negative charges produced on reduction of the neutral C60 to anions at the electrode surface drastically affect charging currents. Not only do these multiple uncertainties apply to the reduction of C60, but many also apply to the oxidation of Fc, which was also distorted in a similar way.15 Even if the authors took similar care in making corrections for the Ef value of Fc+/0, the combined errors in measuring the C60 formal reduction potentials Vs Fc+/0 are still unlikely to be less than (30 mV in these solvents. Other sets of data (those in pyridine and PhCN) were not obtained directly because of the extent to which the solvent reductions interfered with the C60 redox chemistry and were obtained by extrapolating data in the presence of electrolyte, again introducing greater uncertainties than values obtained directly. Finally, the data for the first two reductions in MeCN were extrapolated from data in py/MeCN solvent mixtures; again, this reduces the reliabilities of these values. Since 33% of the data (which can be calibrated using the Fc+/0 standard) contained in Table 215 are extrapolated or corrected and all lie near the extremes of the data range, correlations using this data set are going to lead to much greater errors than those obtained using conventional electrochemistry in the presence of electrolyte. All of the problems listed above do not take into account the effects of adventitious electrolyte due to a combination of impurities, solvent self-ionization, and ionization of the solvent induced under the influence of the electric field. One of the features of microelectrode chemistry, especially in the absence of added electrolyte, is the relatively long quiet time required before starting a scan in order for the electric double layer to reach equilibrium. The amount and nature of the electrolyte produced, especially in the vicinity of the electrode double layer will vary greatly from solvent to solvent and from one purification of the solvent to another. The long quiet time required for the microelectrodes to reach equilibrium under these conditions is probably the time required
6358 J. Phys. Chem. B, Vol. 101, No. 33, 1997 to attract electrolyte (or generate electrolyte) at the surface of the electrodes to help neutralize the charges on the electrodes. This electrolyte will be in relatively high concentration at the electrode surface and will be able to ion pair with the C60 anions as they are produced. Therefore, the notion that the Ef values produced in the absence of added electrolyte are free from ionpairing effects is probably wrong and there is likely to be a considerable variation in the degree and extent of ion-pairing over the range of solvents that have been used. This uncertainty is removed when known amounts of electrolyte are added. There are also other uncertainties in the data obtained in the absence of added electrolyte, e.g., is there a contribution of adsorption to the distortion of some of the responses? This is easy to ascertain by conventional cyclic voltammetry, but is not so easily evaluated with microelectrode electrochemistry in the absence of added electrolyte. References and Notes (1) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993, 97, 3379-3383. (2) (a) Dubois, D.; Moninot, G.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Phys. Chem. 1992, 96, 7137-7145. (b) Krishnan, V.; Moninot, G.; Dubois, D.; Kutner, W.; Kadish, K. M. J. Electroanal. Chem. Interfacial Electrochem. 1993, 356, 93-107. (3) Stinchcombe, J.; Penicaud, A.; Bhyrappa, P.; Boyd, P. D. W.; Reed, C. A. J. Am. Chem. Soc. 1993, 115, 5212-5217. (4) (a) Bhyrappa, P.; Paul, P.; Stinchcombe, J.; Boyd, P. W. D.; Reed, C. A. J. Am. Chem. Soc. 1993, 115, 11004-11005. (b) Paul, P.; Xie, Z.; Bau, R.; Boyd, P. W. D.; Reed, C. A. J. Am. Chem. Soc. 1994, 116, 41454146. (c) Boyd, P. D. W.; Bhyrappa, P.; Paul, P.; Stinchcombe, J.; Bolskar, R.; Sun, Y.; Reed, C. A. J. Am. Chem. Soc. 1995, 117, 2907-2914. (5) Kato, T.; Kodama, T.; Oyama, M.; Okazaki, S.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromaru, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991, 186, 35-39. (6) Dubois, D.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante, L. P. F.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 4364-4366. (7) Lawson, D. R.; Feldheim, D. L.; Foss, C. A.; Dorhout, P. K.; Elliott, C. M.; Martin, C. R.; Parkinson, B. J. Electrochem. Soc. 1992, 139, L68L71. (8) (a) Heath, G. A.; McGrady, J. E.; Martin, R. L. J. Chem. Soc., Chem. Commun. 1992, 1272-1274. (b) Fullagar, W. K.; Gentle, I. R.; Heath, G. A.; White, J. W. J. Chem. Soc., Chem. Commun. 1993, 525-527. (9) Haddon, R. C. Acc. Chem. Res. 1992, 25, 127-133. (10) Dubois, D.; Kadish, K. M.; Flanagan, S.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 7773-7774. (11) Meerholz, K.; Tschuncky, P.; Heinze, J. J. Electroanal. Chem. Interfacial Electrochem. 1993, 347, 425-433. (12) Xie, Q. S.; Pe´rez-Cordero, E.; Echegoyen, L. J. Am. Chem. Soc. 1992, 114, 3978-3980. (13) Fawcett, W. R.; Opallo, M.; Fedurco, M.; Lee, J. W. J. Am. Chem. Soc. 1993, 115, 196-200. (14) Mirkin, M. V.; Bulho˜es, L. O. S.; Bard, A. J. J. Am. Chem. Soc. 1993, 115, 201-204. (15) Soucaze-Guillous, B.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Electrochem. Soc. 1996, 143, 550-556. (16) Xie, Q.; Arias, F.; Echegoyen, L. J. Am. Chem. Soc. 1993, 115, 9818-9819. (17) Yang, Y. F.; Arias, F.; Echegoyen, L.; Chibante, L. P. F.; Flanagan, S.; Robertson, A.; Wilson, L. J. J. Am. Chem. Soc. 1995, 117, 7801-7804. (18) (a) Gorun, S. M.; Creegan, K. M.; Sherwood, R. D.; Cox, D. M.; Day, V. W.; Day, C. S.; Upton, R. M.; Briant, C. E. J. Chem. Soc., Chem. Commun. 1991, 1556-1558. (b) Nagano, Y.; Tamura, T.; Kiyobayashi, T. Chem. Phys. Lett. 1994, 228, 125-130. (19) (a) Meidine, M. F.; Hitchcock, P. B.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. J. Chem. Soc., Chem. Commun. 1992, 1534-1537; (b) Balch, A. L.; Lee, J. W.; Noll, B. C.; Olmstead, M. M. J. Chem. Soc., Chem. Commun. 1993, 56-58. (c) Rao, C. N. R.; Seshadri, R.; Govindaraj, A.; Mittal, J. P.; Pal, H.; Mukherjee, T. J. Mol. Struct. 1993, 300, 289301. (d) Scurlock, R. D.; Ogilby, P. R. J. Photochem. Photobiol. A: Chem. 1995, 91, 21-25. (20) Bolskar, R. D.; Gallagher, S. H.; Armstrong, R. S.; Lay, P. A.; Reed, C. A. Chem. Phys. Lett. 1995, 247, 57-62.
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