J. Phys. Chem. 1980, 84, 2557-2560
2557
Reactions of Cation Radicals of EE Systems' 10. The Influence of Ion Association on Cation Radical Disproportionation' Eric E. Bancroft, Jeanne E. Pemberton, and Henry N. Blount* Brown Chemical Laboratory. The University of Delaware, Newark, Delaware 19711 (Received: January IS, 1980)
Disproportionation equilibria of the cation radicals of 10-phenylphenothiazine,thianthrene, and 9,lO-diphenylanthracene have been voltammetrically characterized in acetonitrile, butyronitrile, nitrobenzene, and methylene chloride containing hexafluorophosphate, tetrafluoroborate, and perchlorate supporting electrolytes. Disproportionation was observed to be significantly influenced by associative interactions between the oxidized forms of the substrates and the anions of the supporting electrolytes. Disproportionationwas found to be favored by a:nions of greater ionic potential. This anion effect in associated systems was observed to be "leveled" by solvents of increased polarity. In the absence of ion association, disproportionation was found to be favored by more polar solvents.
Characteristic of oxidative EE systems2 are two successive monoelectronictransfers occurring at two distinctly different formal potentials A id A+. + e$10' (1)
A+. 6 A2+ + e-
E2"'
(2)
where
Ez"' > El"' (3) Native to such systems is the thermodynamically dictated disproportionation process
+
21A+A A2+ (4) whose characteristic disproportionation equilibrium constant, KD,may be expressed in terms of the reversible formal potentials as KD = exp(-(F/RT)(E,O' -E,"')} (5) Detailed kinetic investigations have established the halfregeneration mechanism (HRM) as a tenable mechanistic description for both electron transfer and addition reactions of cation radicals derived from various EE subs t r a t e ~ . ~ "However, in reaction environments notably different from those (examinedto date, the dominant mode of reaction of the oxidized forms of EE systems with nucleophilic species may involve direct reaction of the dicationic form of the substrate (the disproportionation r n e ~ h a n i s m ~rather ~ ~ ) than direct reaction of the cation radical (the HRM1-'l). In particular, in vivo reactions of the oxidized forms of phenothiazine-based neuroleptic agents are believed to proceed via the dication which arises from disproportionation.6 That the reaction medium might alter the domiinance of one of these two mechanisms (HRM vis-2i-vis disproportionation) prompted an examination of the influence of the reaction environment (counterion and solvent) on the disproportionation equilibria of representative EE systems. This report details the effects of hexafluorophosphate (PF6-), tetrafluoroborate (BF,), and perchlorate (C104-) on disproportionation of the cation radicals of 10phenylphenothiazine (pH+.), 9,lO-diphenylanthracene (DPA'.), and thianthrene (TH+.) in acetonitrile (AN), butyronitrile (BN), nitrobenzene (NB), and methylene chloride (MC).
Experimental Sect,ion Materials. Sources and purification procedures for TH, AN, BN, and tetra-n-butylammonium perchlorate have 0022-3654/80/2084-2557$01 .OO/O
already been summarized1* as have those for DPA.3a Preparation and purification of P H was as previously reported.3b Tetra-n-butylammonium hexafluorophosphate (Fluka/Tridom) was twice recrystallized from absolute ethanol, crushed, and dried in vacuo (60 "C, 24 h). Tetra-n-butylammonium tetrafluoroborate (Fluka/Tridom) was crystallized three times from ethyl acetate by the addition of n-pentane, crushed, and dried in vacuo (60 "C, 24 h). Methylene chloride (Burdick and Jackson, UV grade) was purified by repetitive percolation through an 18 X 1in. column of freshly activated (500 "C, 48 h) alumina. Nitrobenzene (Fisher) was twice passed through an 18 X 1in. column of freshly activated (500 "C, 48 h) alumina and then refluxed with a mixture of potassium permanganate (25 g/L) and potassium carbonate (45 g/L) for 6 h under reduced pressure (bp 50-60 "C,5 mm). This was followed by vacuum distillation through an 18-in. Vigreux column, discarding the first and last 10% of the distillate. The retained fraction was then similarly refluxed over P205(5 g/L) for 4 h and distilled. The final fraction was further purified by percolation through a 12411. column of activated alumina immediately prior to use. All other chemicals were reagent grade or equivalent. Apparatus. All electrode potentials were controlled by a three-electrode potentiostat of conventional d e ~ i g n . ~ Cyclic voltammetric characterizations were carried out at platinum foil electrodes fitted to cells of a previously reported design.8 All solution preparations and electrochemical experimentation were executed in a nitrogenfilled drybox maintained at 22 f 1 OC. The aqueous saturated calomel reference electrodes employed in this work were routinely calibrated by using the quinhydrone sy~tem.~
Results and Discussion Equilibria. Association of a cation radical with anions, X-, in solution (e.g., the supporting electrolyte) to yield a single dominant complex (A+ X,-) e...
A+* + p X -
(A+....X,-)
(6)
is described by the equilibrium constant K1, where K1 =
[(A+*...X,-)]
[A+-][X-]P
(7)
In the presence of such associative equilibria, the formal potential observed for the precursor/associated cation 0 1980 American Chemical Society
2558
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980
Bancroft et al.
TABLE I: Disproportionation Equilibrium Constants for Cation Radicals of 10-Phenylphenothiazine, Thianthrene, and 9,lO-Diphenylanthracene in Solvent-Electrolyte Systems Employed in ,this Studya
PH solvent
anionb
PF,-
acetonitrile
BF,(210,-
?F,BF,-
butyronitrile
c10,-
PF,BF;
nitrobenzene
c10,PF,-
methylene chloride
BF,c10,-
mV
TH
DPA
-log
2D
mV
-logkD
A , $ o ~ , c mV
746d 751e 72gd 723e 69 tid 704e 694 678 545 735 688 625 796 682 64 1
12.75d 12.85e 12.46d 12.3Se 11.88d 12.03e 11.85 11.59 9.31 12.57 11.76 10.68 13.60 11.66 10.96
448
7.66
470
8.03
411
7.02
436
7.46
399
6.82
409
7.00
411 336 292 485 434 372 546 430 358
7.02 5.75 4.99 8.29 7.41 6.35 9.33 7.35 6.12
436 386 307 510 462 393 5 56 473 400
7.45 6.60 5.25 8.72 7.90 6.72 9.51 8.09 6.83
A ~ Q ‘ , C
-log
Iz,
A ~ ~ ( , C
a Measured at 22 -L. 1 “C; substrate concentration = 1.0 mM. As the tetra-n-butylammonium salt, 0.10 M. Mean of thLee deiermin@ons; measurement uncertainty (standard deviation) = 8 mV. Scan rate = 100 mV/s except as noted. A E ” ’ = E,”’ - E,”’. Scan rate = 10.0 V/s, dication kinetically stable on time scale of experiment. e Scan rate = 100 mV/s, dication not kinetically stable on time scale of experiment.
(elof)
radical couple is shifted cathodically relative to khat observed in the absence of association (El0’,eq 1). Elo’ is related to El0’bylo
Elo’= Elo’- ( R T / F ) In K1 -(RT/F)(p)In [X-]
(8)
Similarly, association of the dication with X- anions to produce a single dominant aggregate may be written as
A2+
+ qX-
(A2+...X;)
(9)
for which the characteristic equilibrium constant, K2, is given by K2 =
...
[ (A2+ X,-)] [A2+][X-] q
....
Oxidation of (A+ X,-) in the presence of free X- to give the (A2+ X;) species occurs with a shift in the formal potential for the A2+/A+. couple relative to that noted under conditions where neither A+. nor A2+are associated. The formal potentiaj of the A2+/A+-couple in the presence of ion association, E20f,is related to the “free ion” value, -&Of, by E20’ = E20’ - (RT/F) In (K2/K1) ( R T / F ) ( Q- P)In [X-I (11)
...
In the presence of ion association, the disproportionation equilibrium which is characteristic of these EE systems
2(A+....X);
& A + (A2+...X,-) + (2p - q)X- (12)
may be described in terms of the free ion disproportionation equilibrium constant, KD (eq 4), and the ion association equilibrium constants, K1 and K2, by’’
RD= KdK2/Ki2)
(13)
KDcan be determined directly from the formal potentials observed in the presence of association by K D = e x p ( - ( ~ / ~ ~-) @1(0’)]e ~ ~ ~ (14) The individual ion association equilibria involving cation radical and dication can be characterized electrochemically if the formal potentials of the two redox couples can be ascertained in the absence of ion association (i-e.,Elofand E20‘). For the substrates examined in this work, these data
are not yet in hand.12 However, an examination of the disproportionation equilibria of these EE systems provides insight into those parameters which influence the association processes involving the oxidized forms of these EE substrates with anions of the supporting electrolytes. PH, TH, and DPA Systems. Typically for these substrates, the dication is sufficientlyreactive that, on the time scale of the experiment, no voltammetric wave for the reduction of the dication is evident.13 The influence of consumptive kinetic processes involving one form of the redox couple on the observed potential of that couple is well-known.14 In the context of the present work, kinetic inst_abilityof the dication could result in a cathodic shift in E2”, thereby causing a positive bias in values of KD calculated by eq 14. To assess the influence of coupled homogeneous kinetic processes on the voltammetrically determined values of KD, one of the substrates of interest to this work (PHI was examined both under conditions where the dication was completely stable on the time scale of the experiment and under conditions where the dication was totally consumed on the time scale of t h t experimental observation. A comparison of the values of KD determined under these two sets of experimental conditions shows QO perceptible influence of coupled dication reactions on KD (Table I). However, in all cases where the dication is kinetically unstable, the disproportionation equilibrium constant evaluated in this manner (eq,14) must be regarded as an upper limit. Further inspestion of the PH data in Table I shows that in all solvents KD increases as the anion is changed from PF, to BF4; to C104-. This effect is most prominent in MC where KD is altered by nearly three orders of magnitude. From the variations of KD with anion in these solvents, the extent to which anion solvation affects the ion association equilibria (eq 6 and 9) and, in turn, the disproportionation equilibrium (eq 12) is not perceptible. However, anion solvation by aprotic solvents, even dipolar ones, is ~ma1l.l~ The effects of solvent and anion on l?D observed for PH are also found in the T H and DPA systems as evidenced by the data in Table I. Combined solvent and anion effects can result in a four order of magnitude alteration of the disproportionation equilibrium constant for all structures. These alterations are indicative of the associative interactions between the oxidized forms of the EE substrates and the anions examined in this work. Analogous results
The Journal of Physical Chemistry, Vol. 84, NO. 20, 1980 2559
Reactions of Cation Radlicais of EE Systems
TABLE 11: Calculated Ionic Potentials of Perchlorate, Tetrafluoroborate. and HexafluoroDhosDhatea c10,-
BF,-
PF,a
0.5385 0.4438 0.4100
1.42 1.40 1.66
-0.319 -0.317 - 0.241
See text for method of calculation.
have been reported for anion radical systems, Szwarc and co-workers have shown that both the equilibrium position and the kinetics of anion radical disproportionation can be markedly altered by aggregation of the anionic forms of the substrate with various alkali metal cations.lGlg In the case of tetracene anion radical, for example, the disproportionation equilibrium constant was caused to vary over some seventeen orders of magnitude by appropriate choice of cation and s01vent.l~ Several spectroscopic characterizations of ion pairing processes involving cation radicals similar to those examined in this work have recently been reported. Sorensen and Bruning20 examined electron-transfer kinetics of phenothiazine cation radical in acetonitrile-chloroform mixtures. Their work provided evidence for ion pairing between the phenothiazine cation radical and both C10, and 13-. Recently, Ocasio and Sullivan21 reported ion pairing between phenothiazine cation radical and 13-in nitromethane. Szwairc and co-workers22have also examined ion association in cation radical systems and have noted a dimeric THS./C1O[ species in propionitrile at low temperature. There are, however, no previous reports which consider the impact of such associative equilibria on cation radical dirrproporti~nation.~~ In studies of anion radical pairing with alkali and alkaline earth metal ioiis, Kalinowski24-26 has shown that the equilibrium constant for ion pairing in a given solvent, K;, can be related to the ionic potentialz7of the counterion, 9, by eq 15. In this formulation, 9 is the charge-to-radius In Kp9 = ms@
I
1
I
I
1
0.300
0.250
I
I
1
0,350
-I Figure 1. Dependence of log kDs for IO-phenylphenothiazineon ionic BN; (-0-)AN; + () NB; (..-O--) MC. Regression potential: parameters are given in Table 111. ( 4 J - s )
5.0
1
6.0 m n 'X
7.0
a 0 J
8.0
0.300
0.250
0.350
-P
(15)
ratio of the spherical counterion and mSreflects the sensitivity of the pairing process to ionic potential in that solvent. Extension of this simple model to the present work affords corresponding expressions for the dependenqies of K1 (eq 6 and 7) and K2 (eq 9 and 10) on 9 in a given solvent, namely In Kls = mls@
14.ot-
Figure 2. Dependence of log kDSfor thianthrene on ionic potential: (0) BN; (0)AN: (M) NB; (0) MC. Regression parameters are given in Table 111. I
I
I
n
(16)
In K," = m2s9
(17) The dependence of KDon @ follows from consideration of eq 16 and 17 together with eq 13: In l?Ds = In KDs + (m2s- 2mIs)9
(18) where &DS is the displroportionation equilibrium constant in solvent S in the presence of associative interaction and KDs is the corresponding parameter in that sokvent in the absence of ion association. An increase in KD with increasing ionic potential of the anion would be expected if the extent of associative interaction between the dication and the anion were more than twice that of the cation radical with the anion. Assignment of ionic potential values to the polyatomic anions examined in this work is less straightforward than in the case of spherical monoatomic species. The polyatomic PF6-,BF,, and C10, anions are of the general form MZn- and are considered as a central atom, M, surrounded by n Z atoms. Each Z atom bears a net negative charge, 6z-. The ionic potentials of these anions are then defined
0.250
-*
0.300
0.350
Figure 3. Dependence of iog kD3for 9,lOdiphenylanthraceneon ionic potentials: (0)BN; (0)AN; (M) NB; (0) MC. Regression parameters are given in Table 111.
as the ratio of 62- to the internuclear separation distance, rMz, between M and Z. For PF6-, BF4-, and C104-, the values of aZ- determined from molecular orbital calculgtions28and rMzvalues taken from crystallographic data29
2500
The Journal of Physical Chemistry, Vol. 84,
No. 20,
1980
Bancroft et 81.
TABLE 111: Estimated Values of KDS and mzs - 2m,s for Cation Radicals of 10-Phenylphenothiazine,Thianthrene,
sociation alters the reactivities of cation radicals is presently being investigated. Acknowledgment. Stimulating interactions with M. C. Zerner are gratefully acknowledged.
and 9,lO-Diphenylanthracenea substrate solvent
PH
TH
DPA
AN NB BN MC AN NB BN WIG AN NB BN MC
log KDS
mzs 2m,S
rd
14.37 (k0.03)e 16.15 (k0.05) 16.9 (k0.5) 18.4 (20.3) 9.19 (kO.09) 11.96 (20.07) 10.77 (kO.09) 15.3 ( t O . 1 ) 9.95 (20.01) 12.5 (kO.1) 11.6 ( i O . 1 ) 14.52 (kO.01)
6.2 14.3 18.9 20.2 6.4 14.7 15.4 24.4 7.8 15.1 16.6 20.3
0.996 0.993 0.894 0.974 0.967 0.996 0.994 0.995 0.999 0.990 0.986 0,999
a From linear re ression analyses of data shown in Figures 1, 2, and 3. Intercept of regression line (@ = 0, eq 18). Slope of regression line (eq 18). Coefficient of correlation. e Parentheses contain one standard deviation.
are given in Table I1 together with the calculated ionic potentials. The dependencies of RDon for PH, TH, and DPA are shown in Figures 1, 2 , and 3, respectively. Although only three anions have been found to be experimentally compatible with these substrate systems,12the linear dependencies noted for all substrates in all solvents speak to the qualitative validity of eq 18. The regression parameters for these linear relationships are summarized in Table I11 and provide insight into the influence of both anion and solvent on the disproportjonation process. The trend to increased KD with increasing ionic potential of the anion observed for all substrates in all solvents suggests greater associative interaction between the dication and anion than between cationradical and anion (eq 18). The relative slopes of the log KDvs. @ relationships (mi- 2mls, eq 18) should afford a measure of the effect of solvent on the sensitivity of the disproportionation process to ionic potential of the anion. For all substrate/solvent combinations examined, these slopes are seen to increase with solvent in the order AN, NB, BN, MC. This order, which corresponds to decreasing solvent polarity (e.g., Dimroth-Reichardt ET parameters30), suggests greater sensitivity of the disproportionation process to the ionic potential of the anion in solvents of lower polarity as would be expected in associating systems.16PvW4 Extrapolation of the linear RD-@relationships to zero ionic potential should afford estimates of the disproportionation equilibrium constants in the absence of ion association (KD’, eq 18). These values, summarized in Table 111, suggest that, in the absence of ion association, solvents of greater polarity30 are more conducive to disproportionation. However, further investigation of the influence of solvent on both disproportionation and ion association processes involving these cation radicals is in order. The work reported here indicates that disproportionation equilibria of cation radicals of EE systems are significantly altered by reaction environment. Disproportionation has been found to be favored by anions of greater ionic potential. This anion effect, however, is “leveled” by more polar solvents. In the absence of ion association, cation radical disproportionation appears to be favored by solvents of greater polarity. The extent to which ion as-
References and Notes (1) (a) Part 9: Pemberton, J. E.; McIntlre, G. L.; Blount, H. N.;Evans, J. F. J. phys. Chem. 1979, 83, 2696. (b) Part 8: Evans, J. F.; Blount, H. N. Ibid. 1979, 83, 1970. (2) Blount, H. N.; Evans, J. F. In “Characterization of Solutes In Nonaqueous Solvents”; Mamantov, G., Ed.; Plenum Press: New York, 1978; pp 105-29. (3) (a) Evans, J. F.; Blount, H. N. J. Am. Chem. SOC. 1978, 700, 4191. (b) Evans, J. F.; Lenhard, J. R.; Blount, H. N. J. Org. Chem., 1977, 42, 983. (c) Evans, J. F.; Blount, H. N. Ibid. 1977, 42, 976. (d) Evans, J. F.; Bbunt, H. N. J. phys. Chem. 1976, 80, 1011. (e) Evans, J. F.; Blount, H. N. J. Org. Chem. 1976, 41, 516. (f) Blount, H. N. J . Electroanal. Chem. 1973, 42, 271. (4) Cheng, H. Y.; Sackett, P. H.; McCreery, R. L. J. Am. Chem. SOC. 1978, 700, 962. (5) Shine, H. J. ACS Symp. Ser. 1978, No. 6 9 , pp 359-375. (6) Forrest, I. S.; Usdin, E. In “Psychotherapeutic Drugs”; Usdin, E., Forrest, 1. S., Eds.; Marcel Dekker: New York, 1977; pp 699-753. (7) Pilla, A. A. J. Electrochem. SOC. 1971, 718, 702. (8) Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1976, 9,241. (9) Albertson, D. E.; Blount, H. N.; Hawkridge, F. M. Anal. Chem. 1979, 57,556. (10) Galus, Z. “Fundamentals of Electrochemical Analysis”; Horwood: Chlchester, 1976; Chapter 14. Equation 8 is a simplified form of the more general equation. The simplification is based upon the assumption that K,[X-]’>> 1. 11) I n this treatment, p and 9 are not constrained. I f the assumption of a single dominant complex for each oxidized species is Invalid, then the approach must be expanded to encompass a series of complexes: DeFord, D. D.; Hume, D. N. J. Am. Chem. SOC.1951, 73, 5321. 12) The only anions which have been found to be suitable for use with these substrate systems are PF8-, BF,-, and C10,- because of their low nucleophilicities and electroinactivitles. As this report Indicates, the oxidized forms of PH, TH, and DPA all associate with these anions. Consequently, no formal potentials in the absence of Ion association could be determined. (13) At sufficiently high rates of potential sweep or in the presence of addilves such as trifluoroacetlc acid or trlfluoroacetic anhydride, PH and TH exhibit fully developed reduction waves for the d i ~ a t l o n . ~ ~ ~ ~ DPA does not give rise to a stable dication in these ~olvents.*~~ (14) Nicholson, R. S.; Shah, I. Anal. Chem. 1964, 36, 706. (15) Hormadaly, J.; Marcus, Y. J. Phys. Chem. 1979, 83, 2843. (16) Ralnis, A.; Szwarc, M. J. Am. Chem. SOC. 1974, 9 6 , 3008. (17) Levln, G.; Holloway, B. E.; Szwarc, M. J. Am. Chem. SOC.1976, 98, 5706. (18) Wang, H. C.; Levln, G.; Szwarc, M. J. Am. Chem. SOC. 1977, 9 9 , 5056. (19) Levln, G.; Szwarc, M. J . Am. Chem. SOC.1976, 9 8 , 4211. (20) Sorensen, S. P.; Bruning, W. H. J. Am. Chem. Soc. 1973, 95,2445. (21) Ocaslo, I. J.; Sullivan, P. D. J. Am. Chem. SOC. 1979, 101, 295. (22) de Sorgo, M.; Wasserman, B.; Szwarc, M. J. Phys. Chem. 1972, 76, 3468. (23) Solvent effects on cation radical disproportionatlo? in analogous systems have been reported where alterations In K, comparable to those observed in the present work were noted: Hammerich, 0.; Parker, V. D. Electrochim.Acta 1973, 78,537. These workers report disproportionation equllibrlum constants for EE systems in the presence of BF-, In solvents containing either suspended alumina, trlfluoroacetlc acid, or trifluoroacetlc anhydride. (24) Kallnowskl, M. K. Chem. Phys. Letf. 1970, 7 , 55. (25) Kalinowski, M. K. Chem. Phys. Lett. 1971, 8 , 378. (26) Kallnowskl, M. K.; Tenderende-Gumlnska, B. J. Electroanal. Chem. 1974, 55,277. (27) Cartledge, G. H. J. Am. Chem. SOC. 1928, 50,2855. (28) INDO calculations were performed after the manner of the following: (a) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111. (b) Ridley, J. E.; Zerner, M. C. Ibid. 1978, 42, 223. (29) Sutton, L. E., Ed. Spec. Pub/.-Chem. SOC. 1965, No. 18. (30) Dlmroth-Relchardt solvent polarity (ET) parameters31v32 are the following: AN, 46.0; NB, 42.0; EN, 41.2; MC, 41.1.” (31) Dlmroth, K.; Relchardt, C.; Slepmann, T.; Bohlmann, F. Justus Lleblgs Ann. Chem. 1963, 667, 1. (32) Relchardt, C. Angew. Chem., Int. Ed. Engl. 1965, 4 , 29. (33) Garst, J. F. In “Solute-Solvent Interactions”; Coetzee, J. F., Rlchle, C. D., Eds.; Marcel Dekker: New York, 1969; pp 539-605. (34) Szwarc, M. In “Characterization of Solutes In Nonaqueous Solvents”; Mamantov, G., Ed.; Plenum Press: New York, 1978; pp 79-104.
