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Electrochemistry of some substituted pyrroles
J. Eleetroanal. Chem., 130 (1981) 181-187 Elsevier Sequoia S.A., Lausanne--Printed in The Netherlands
181
ELECTROCHEMISTRY OF SOME SUBSTITUTED PYRROLES
A.F. DIAZ, A. MARTINEZ * and K.K. KANAZAWA IBM Research Laboratory, San Jose, CA 95193 (U.S.A.)
M. SALMON Instituto de Quimiea, Universidad Nacional A utbnoma de Mexico, Mexico 20 D.F. (Mexico) (Received 16th February 1981; in revised form 19th June 1981)
ABSTRACT A brief study of the electrochemical oxidation reaction of some a,a'-disubstituted pyrroles is presented. The oxidation reactions of these compounds are irreversible and produce a variety of soluble products which do not alter the electrode as is the case with N-substituted pyrrole derivatives which form conducting polymeric films on the electrode surface. Changes in the Epa values produced by the substituents are found to correlate with changes in the dipole moment of the molecule.
INTRODUCTION
The derivatives of pyrrole are readily oxidized [1] and they could be of interest for electrochemical preparations if their reactions could be controlled. We have, in fact, made use of this property for the preparation conducting polymeric films of pyrrole [2,3]. However, very little information exists in the literature on the electrochemical oxidation of substituted pyrroles [ 1], and this situation may be due, in part, to the difficulty in quantifying these rather complicated reactions. The reactions of these compounds are usually irreversible and are complicated by the inherent instability of the radical cation intermediate which reacts rapidly and produces a variety of reaction products. Furthermore, these reactions often produce polymeric products which can alter the electrode surface [2,3]. This characteristic alone makes it difficult to obtain analytical electrochemical data. The instability of the pyrrole radical cations also influences the mass spectrometry of a-arylpyrrole derivatives [4]. For example, the m-I peak is totally absent in the spectra of these compounds where this observation is most certainly produced by the rapid fragmentation of the radical cation species. In order to familiarize ourselves with the electrooxidation reactions of pyrrole derivatives we proceeded to study the influence of substituents on these reactions. * Universidad de Guanajuato, Guanajuato, Mexico. 0022-0728/81/0000-0000/$02.75 © 1981 Elsevier Sequoia S.A.
182 EXPERIMENTAL
The pyrrole derivatives (I-IV) were available from a previous s t u d y [4]. the monosubstituted pyrrole derivatives with N-alkyl groups were prepared by the ~COOC2H5 C6H5"/ \N /
-CH3
L~'~.COOCH3 I
~COOC2H5 C6H5'f \N /
L"--...COOH II
/•.•COOH
/•/C00C2H5 III
~CH3
IV
reaction of the pyrrole anion-potassium salt with the appropriate alkyl halide. The cyclic voltammetry and chronocoulometry measurements were performed in distilled acetonitrile solutions containing 0.1 M tetraethylammonium tetrafluoroborate and 10 -3 M pyrrole derivative. A one-compartment cell equipped with a platinum button (0.2 cm2) working electrode, gold wire counter electrode and a sodium chloride calomel reference electrode was used. All of these measurements were performed with iR compensation. The time spans used in the chronocoulometric measurements were 5 and 10 s. The electrolysis experiments for the evaluation of n values were performed in a two-compartment divided cell using a platinum gauze working electrode and a gold wire counter electrode as previously described [5,6]. The appropriate derivative (40 mg of I and 50 mg of II) was electrolyzed in a divided cell at + 1.4 V for 30 min during which time the current decreased to 1% of the original value. The voltammogram of the resulting solution shows only one peak at + 1.7 V. TLC analysis of this mixture in the original electrolyte solution or after recovering the organic components by extraction between ether and aqueous salt solution showed the complete disappearance of the starting material. RESULTS A N D DISCUSSION
Cyclic voltammetric data for the various compounds are listed in Table 1. The cyclic voltammograms of these compounds display two general characteristics. They show several oxidation peaks and each peak corresponds to an irreversible reaction. The radical cation produced in the first oxidation reaction of these compounds must be extremely unstable since no hint of a cathodic peak is seen in the reverse sweep even up to l0 V s - ~. This is the case even with the highly substituted derivatives (III and IV) which contain the electron-rich 3,4-dimethoxyphenyl substituent. Thus, the
183 TABLE 1 Summary of cyclic voltammetric data a Compound
Eva/V
Pyrrole
1.20 (1.2) c 1.54 1.14 (1.19) ¢ 1.22 1.24 1.80 0.84 0.82 1.15 0.85 1.23 1.33 0.72 1.22 1.32 1.71
l-Methylpyrrole l-Ethylpyrrole 1-( n-Propyl)pyrrole 1-Phenylpyrrole 2,5-Dimethylpyrrole 1,2,5-Trimethylpyrrole 2-Methyl-5-phenylpyrrole
1,2-Dimethyl-5-(3',4'dimethoxyphenyl)pyrrole I II III
1.30 1.69 1.01 1.34 1.50
IV
0.98 1.30 1.53
Rel. ipa
#/D ~' 1.80 a
Shoulder 1.92 1.96
1.0 0.3 1.0 0.1 0.1 1.0 0.4 1.0 0.3
1.32 2.08 2.07
1.0 0.2 1.0 0.2
0.4 1.0 0.2 0.3
a Measured in CH3CN containing 0.1 M Et4NBF4 using Pt vs. NaCE electrodes, and using 0.7-2.0 m M pyrrole concentration. b Ref. 10. 1D=3.34× 10-3° C m. c Ref. 12. d Ref. 9.
irreversible n a t u r e of these reactions m u s t result from the a c c o m p a n y i n g chemical reactions of the very reactive cation i n t e r m e d i a t e (eqn. 1):
©
~N
E P ~ ~
H~,~ Polymer ~'~) Film phile (b)~,,,..-Soluble Product
T h e anodic peak potentials (Epa) listed were m e a s u r e d at 50 m V s - 1 sweep rate,, a n d the relative peak heights were estimated b y m e a s u r i n g the difference b e t w e e n the foot a n d the m a x i m u m of each peak. The ipa (anodic peak current) values for the peak appearing at the lowest potential show a linear fit with v ~/2 i n the sweep range 1 0 - 1 0 0 m V s - I for every case a n d indicates that these reactions involve dissolved
184
pyrrole molecules which are not adsorbed on the electrode [7]. The oxidation reactions of the N-substituted pyrroles proceed with the formation of insoluble polymeric film on the electrode (eqn. la) as was previously observed with pyrrole and N-methylpyrrole [3]. Because these films are conducting they will continue to grow when the pyrrole derivative is oxidized by continuous scanning over the potential region near 1.2 V. However, for the analytical measurements, the electrode surface had to be cleaned after each scan because the voltammograms are altered by the presence of the film on the electrode. The N-alkyl substituents have very little effect on the Epa value of pyrrole which is reminiscent of the results found with carbazole where N-alkyl substituents produce variations in the Epa value of < 60 mV [8]. The n values for the oxidation of pyrrole and the N-substituted pyrrole derivatives were not estimated because the reactions are complicated by the formation of a polymeric film on the electrode surface. However, it is known from previous studies [3] that the polymer-forming reaction of pyrrole and the Nsubstituted derivatives also has electrochemical stoichiometry. The apparent n values for these reactions are in the range 2.2-2.4, where the n value for the polymerization reaction is 2.0 (eqn. 2), when x is very large, and the excess charge, 0.2-0.4, is used to oxidize the polymer (eqn, 3) [3].
(3) The oxidation products from the a-arylpyrroles were all soluble with no evidence for the formation of insoluble material, either on the electrode surface which could alter its behavior or anywhere else in the cell (eqn. 1a). In this regard these reactions are different from the N-substituted pyrroles which produce polymeric films which adhere to the electrode surface and are conducting [3]. The value of n and D were also estimated for the first oxidation reaction by combining the results from cyclic voltammetry and chronocoulometry, and by making use of the Nicholson and Shain treatment for a totally irreversible electron-transfer process which provides n3/2D~/2, and the Cottrell equation which provides nD 1/2 [7]. The chronocoulometric experiments were performed by stepping the potential from zero to 0.1.V past the peak. These results are listed in Table 2 and they show that even the reactions which produce soluble products are not simple. The non-integral n values could indicate the presence of two separate reaction routes leading to the formation of monomeric (n = 2) and dimeric (n = 1) products. Because we were concerned about the nonintegral n values, the value was measured using 10-4, 10 -3 and 1 0 - 2 M solutions of the 5-phenylpyrrole derivatives, I and II respectively, in order to probe for the
185
presence of competitive unimolecular and bimolecular reactions. N o difference was detected in the values, outside of the experimental variation. Thus, to the extent that the concentration of these compounds at the electrode surface reflects the bulk concentration, this result argues against the presence of two competitive reactions with different molecularities (with respect to the pyrrole derivative) and different n values. Thus, it may be that only dimeric product is formed in the initial reaction and the value of n in excess of unity (0.1-0.4) results from further oxidation of the initial product (Epa at 1.7 V). The only noticeable difference in these cyclic voltammograms involves the peak at 1.7 V which is not present in the 1 0 - a M solutions (of I or II) and increases in size relative to the peak at 1.3 V at the higher concentrations. The peak at 1.7 V is due to the oxidation of a product from the first oxidation reaction whose concentration is increased at the higher concentrations of I or II. The n values for two of the 5-phenylpyrrole, derivatives, I and II, were also evaluated from the complete electrolysis of a known amount of material (ca. 1 0 - 2 M ) . The experimental conditions for these measurements are different from t h e above because these experiments were performed with stirring and a controlled potential of + 1.4 V. These experiments provided n values of 1.8 for both compounds which suggests the formation of mononieric product primarily. Thus, it is clear that these oxidation reactions are quite sensitive to changes in the electrolysis conditions. A fairly linear correlation is found between the Epa values for the five methyl- and phenyl-substituted pyrroles and their dipole moments measured in benzene for which the data are available [9,10]. While it is clear that E ° values should be used for plots of this type, they are not available for these irreversible reactions. However, the AEpa values must be linearly related to the relative ease of oxidation for this limited number of closely related structures. This relationship is seen in Fig. 1, and it shows TABLE 2 Summary of electrochemical data measured in CH3CN containing 0.1 M Et4NBF4 using Pt vs. NaCE electrodes; 1,=50 mV s t. Reactant concentration 0.7 to 2.0 ~mol c m - 3
(i/AC~,I/2)/ (q/dCtl/2)/ 103 nDl/2/ 103n3/2Dl/2/ n
106D/
A c m m o l -~
Gem
cm2s -I
sl/2V -~/2
tool -~ s ~/2
2,5-Dimethylpyrrole 1,2,5-Trimethylpyl:role 2-Methyl-5-phenylpyrrole 1,2-Dimethyl-5-(3',4'dimethoxyphenylpyrrole I
1430 1500 1155
Compound
cms -1/2
cms -t/2
760 755 530
7.0 7. I 4.9
7.0 7.0 5.5
1.0 1.0 1.3
49 50 15
1650 1614
725 795
6.7 6.4
7.8 7.7
23 21
II
1415
615
5.7
6.7
III IV
1475 1340
670 670
6.2 6.2
7.0 6.4
1.2 1.4 1.8 ~ 1.4 1.8 ,~ 1.3 1.1
Value from complete electrolysis at constant potential.
16 23 32
186 2.0
\~.~Phenyl >
1.5
~
tu~
Pyrrol~-~l-Methyl 1.0
~2,5-Dimethyl
1,2,5-Trimethyl*+~ 0.5
i
1.4
L
1.6
i
1.8
I
2.0
,
2.2
O/O Fig. 1. Plot of Epa (50 mV s - t ) measured in acetonitrile against the dipole moment measured in benzene for a series of substituted pyrroles.
that the compounds with the largest dipole moments are more easily oxidized. Since the dipole moment of pyrrole (located along the symmetry axis with the positive pole at the nitrogen atom) is calculated to be predominantly due to the 7r-moment [1] the empirical relationship observed between the Epa values and the dipole moments suggest that the ease of oxidation of these pyrrole derivatives are determined primarily by the nature of the ~r-system. The point for 1-methylpyrrole is off the Ep-/Z line and may be due to the value of the dipole moment which has been described as abnormally high and not adequately explained (ref. 1, p. 453). Not included in the plot is the Epa value for the 1-ethyl derivative which does not fit the correlation since these values are insensitive to changes in the 1-alkyl structure. While other considerations such as solvation effects, molecular orientations in the double layer, etc. are very important in these reactions and should not be neglected in attempting to interpret free energy relationships, these effects must remain fairly constant or vary in a linear manner for the compounds being considered here. It must be stressed that considerations of this type are at best applicable to a series of compounds having very similar substituents. Thus, it would be dangerous to generalize this relationship to include substituents with widely different structural variations. This fact is clear from the different effect produced by the phenyl group when substituted in the 2-position rather than the 1-position. Even in the 1-position the influence of the phenyl group is changed by the presence of methyl groups in the 2- and 5-positions. For example, the Ep -/~ relationship predicts a n Epa vahle of 1.0 V for 2,5-dimethyl-l-phenylpyrrole (2.00D) [9] which is, correspondingly, 800 mV more negative than the measured value for 1-phenylpyrrole. The conclusion that the oxidation reaction depends primarily on the nature of the ~r-system is supported further by the fact that the relative Epa values for the electrooxidation of pyrrole, 1-methyl, 2,5-dimethyl and 1,2,5-trimethylpyrrole also plot linearly with the relative pK~ values for a-protonation of these compounds in
187
aqueous sulfuric acid [11]. This relationship between these two reactions is expected since the former reaction involves ~r-electron transfer from the pyrrole ring to the electrode surface (eqn. 4) while in the latter reaction the transfer is to the proton (eqn. 5). RE H
H .~ G E ~
H30+÷
~ H
R
-nF~Epa
~.K~
~. H ~
R + H20
(5)
.~GH30* = RT,,dpKa
CONCLUSIONS
In conclusion, the electrochemical oxidation of a,a-disubstituted pyrrole derivatives is not a simple reaction and the reaction produces soluble products which do not alter the electrode surface. This latter observation is in complete contrast to that found with pyrroles which are not substituted in the a-positions and which lead to the formation of insoluble polymeric films on the electrode surface [3]. The reactions display non-integral n values which lie between one and two, and are again different from the reactions of the N-substituted pyrroles which undergo a polymeric reaction where the value is 2.2 [3]. The oxidation potential is sensitive to the presence of the substituents where the changes in the Epa values seem to correlate with changes in the ~r-dipole moment of the molecule. ACKNOWLEDGEMENT
The authors wish to thank J. Castillo for his technical assistance in various aspects of this study. REFERENCES 1 R.A. Jones and G.P. Bean, The Chemistry of Pyrroles, Academic Press, New York, 1977. 2 A. Dall'alio, G. Dascola, V. Varacca and V. Bocchi, C.R. Acad. Sci. (Paris), 267C (1968) 433. 3 (a) A. Diaz, K. Kanazawa and G.P. Gardini, J. Chem. Soc., Chem. Commun., 635 (1970); (b) A. Diaz, Electrochemical preparation and characterization of conducting polymers, Proceedings to the International Conference on Low Dimensional Synthetic Metals, Chemica Scripta, 1981. 4 M. Salmbn, O. Villarino, A. Jimenez and R. Zawadzki, Rev. Latinoam. Quim., 4 (1973) 59. 5 A. Diaz, J. Org. Chem., 42 (1977) 3949. 6 The instrumenfation used in these studies was constructed in these laboratories. K.K. Kanazawa and R. Galwey, J. Electrochem. Soc., 124 (1977) 1385. 7 R.N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, New York, 1969. 8 J.F. Ambrose and R.F. Nelson, J. Electrochem. Soc., 115 (1968) 1159. 9 H. Kofod, L.E. Sutton and J. Jackson, J. Chem. Soc., (1952) 1467. 10 See Table 11.8, p. 545 in ref. 1. I I (a) Y. Chiang and E.B. Wipple, J. Am. Chem. Soc., 85, (1963) 2736; (b) Y. Chiang, R.L. Himman, S. Theodoropulos and E.B. Wipple, Tetrahedron, 23, (1967) 745. 12 N.L. Weinberg (Ed.), Technique of Electroorganic Synthesis, Vol. V, Part II 1974, p. 1032.
181
ELECTROCHEMISTRY OF SOME SUBSTITUTED PYRROLES
A.F. DIAZ, A. MARTINEZ * and K.K. KANAZAWA IBM Research Laboratory, San Jose, CA 95193 (U.S.A.)
M. SALMON Instituto de Quimiea, Universidad Nacional A utbnoma de Mexico, Mexico 20 D.F. (Mexico) (Received 16th February 1981; in revised form 19th June 1981)
ABSTRACT A brief study of the electrochemical oxidation reaction of some a,a'-disubstituted pyrroles is presented. The oxidation reactions of these compounds are irreversible and produce a variety of soluble products which do not alter the electrode as is the case with N-substituted pyrrole derivatives which form conducting polymeric films on the electrode surface. Changes in the Epa values produced by the substituents are found to correlate with changes in the dipole moment of the molecule.
INTRODUCTION
The derivatives of pyrrole are readily oxidized [1] and they could be of interest for electrochemical preparations if their reactions could be controlled. We have, in fact, made use of this property for the preparation conducting polymeric films of pyrrole [2,3]. However, very little information exists in the literature on the electrochemical oxidation of substituted pyrroles [ 1], and this situation may be due, in part, to the difficulty in quantifying these rather complicated reactions. The reactions of these compounds are usually irreversible and are complicated by the inherent instability of the radical cation intermediate which reacts rapidly and produces a variety of reaction products. Furthermore, these reactions often produce polymeric products which can alter the electrode surface [2,3]. This characteristic alone makes it difficult to obtain analytical electrochemical data. The instability of the pyrrole radical cations also influences the mass spectrometry of a-arylpyrrole derivatives [4]. For example, the m-I peak is totally absent in the spectra of these compounds where this observation is most certainly produced by the rapid fragmentation of the radical cation species. In order to familiarize ourselves with the electrooxidation reactions of pyrrole derivatives we proceeded to study the influence of substituents on these reactions. * Universidad de Guanajuato, Guanajuato, Mexico. 0022-0728/81/0000-0000/$02.75 © 1981 Elsevier Sequoia S.A.
182 EXPERIMENTAL
The pyrrole derivatives (I-IV) were available from a previous s t u d y [4]. the monosubstituted pyrrole derivatives with N-alkyl groups were prepared by the ~COOC2H5 C6H5"/ \N /
-CH3
L~'~.COOCH3 I
~COOC2H5 C6H5'f \N /
L"--...COOH II
/•.•COOH
/•/C00C2H5 III
~CH3
IV
reaction of the pyrrole anion-potassium salt with the appropriate alkyl halide. The cyclic voltammetry and chronocoulometry measurements were performed in distilled acetonitrile solutions containing 0.1 M tetraethylammonium tetrafluoroborate and 10 -3 M pyrrole derivative. A one-compartment cell equipped with a platinum button (0.2 cm2) working electrode, gold wire counter electrode and a sodium chloride calomel reference electrode was used. All of these measurements were performed with iR compensation. The time spans used in the chronocoulometric measurements were 5 and 10 s. The electrolysis experiments for the evaluation of n values were performed in a two-compartment divided cell using a platinum gauze working electrode and a gold wire counter electrode as previously described [5,6]. The appropriate derivative (40 mg of I and 50 mg of II) was electrolyzed in a divided cell at + 1.4 V for 30 min during which time the current decreased to 1% of the original value. The voltammogram of the resulting solution shows only one peak at + 1.7 V. TLC analysis of this mixture in the original electrolyte solution or after recovering the organic components by extraction between ether and aqueous salt solution showed the complete disappearance of the starting material. RESULTS A N D DISCUSSION
Cyclic voltammetric data for the various compounds are listed in Table 1. The cyclic voltammograms of these compounds display two general characteristics. They show several oxidation peaks and each peak corresponds to an irreversible reaction. The radical cation produced in the first oxidation reaction of these compounds must be extremely unstable since no hint of a cathodic peak is seen in the reverse sweep even up to l0 V s - ~. This is the case even with the highly substituted derivatives (III and IV) which contain the electron-rich 3,4-dimethoxyphenyl substituent. Thus, the
183 TABLE 1 Summary of cyclic voltammetric data a Compound
Eva/V
Pyrrole
1.20 (1.2) c 1.54 1.14 (1.19) ¢ 1.22 1.24 1.80 0.84 0.82 1.15 0.85 1.23 1.33 0.72 1.22 1.32 1.71
l-Methylpyrrole l-Ethylpyrrole 1-( n-Propyl)pyrrole 1-Phenylpyrrole 2,5-Dimethylpyrrole 1,2,5-Trimethylpyrrole 2-Methyl-5-phenylpyrrole
1,2-Dimethyl-5-(3',4'dimethoxyphenyl)pyrrole I II III
1.30 1.69 1.01 1.34 1.50
IV
0.98 1.30 1.53
Rel. ipa
#/D ~' 1.80 a
Shoulder 1.92 1.96
1.0 0.3 1.0 0.1 0.1 1.0 0.4 1.0 0.3
1.32 2.08 2.07
1.0 0.2 1.0 0.2
0.4 1.0 0.2 0.3
a Measured in CH3CN containing 0.1 M Et4NBF4 using Pt vs. NaCE electrodes, and using 0.7-2.0 m M pyrrole concentration. b Ref. 10. 1D=3.34× 10-3° C m. c Ref. 12. d Ref. 9.
irreversible n a t u r e of these reactions m u s t result from the a c c o m p a n y i n g chemical reactions of the very reactive cation i n t e r m e d i a t e (eqn. 1):
©
~N
E P ~ ~
H~,~ Polymer ~'~) Film phile (b)~,,,..-Soluble Product
T h e anodic peak potentials (Epa) listed were m e a s u r e d at 50 m V s - 1 sweep rate,, a n d the relative peak heights were estimated b y m e a s u r i n g the difference b e t w e e n the foot a n d the m a x i m u m of each peak. The ipa (anodic peak current) values for the peak appearing at the lowest potential show a linear fit with v ~/2 i n the sweep range 1 0 - 1 0 0 m V s - I for every case a n d indicates that these reactions involve dissolved
184
pyrrole molecules which are not adsorbed on the electrode [7]. The oxidation reactions of the N-substituted pyrroles proceed with the formation of insoluble polymeric film on the electrode (eqn. la) as was previously observed with pyrrole and N-methylpyrrole [3]. Because these films are conducting they will continue to grow when the pyrrole derivative is oxidized by continuous scanning over the potential region near 1.2 V. However, for the analytical measurements, the electrode surface had to be cleaned after each scan because the voltammograms are altered by the presence of the film on the electrode. The N-alkyl substituents have very little effect on the Epa value of pyrrole which is reminiscent of the results found with carbazole where N-alkyl substituents produce variations in the Epa value of < 60 mV [8]. The n values for the oxidation of pyrrole and the N-substituted pyrrole derivatives were not estimated because the reactions are complicated by the formation of a polymeric film on the electrode surface. However, it is known from previous studies [3] that the polymer-forming reaction of pyrrole and the Nsubstituted derivatives also has electrochemical stoichiometry. The apparent n values for these reactions are in the range 2.2-2.4, where the n value for the polymerization reaction is 2.0 (eqn. 2), when x is very large, and the excess charge, 0.2-0.4, is used to oxidize the polymer (eqn, 3) [3].
(3) The oxidation products from the a-arylpyrroles were all soluble with no evidence for the formation of insoluble material, either on the electrode surface which could alter its behavior or anywhere else in the cell (eqn. 1a). In this regard these reactions are different from the N-substituted pyrroles which produce polymeric films which adhere to the electrode surface and are conducting [3]. The value of n and D were also estimated for the first oxidation reaction by combining the results from cyclic voltammetry and chronocoulometry, and by making use of the Nicholson and Shain treatment for a totally irreversible electron-transfer process which provides n3/2D~/2, and the Cottrell equation which provides nD 1/2 [7]. The chronocoulometric experiments were performed by stepping the potential from zero to 0.1.V past the peak. These results are listed in Table 2 and they show that even the reactions which produce soluble products are not simple. The non-integral n values could indicate the presence of two separate reaction routes leading to the formation of monomeric (n = 2) and dimeric (n = 1) products. Because we were concerned about the nonintegral n values, the value was measured using 10-4, 10 -3 and 1 0 - 2 M solutions of the 5-phenylpyrrole derivatives, I and II respectively, in order to probe for the
185
presence of competitive unimolecular and bimolecular reactions. N o difference was detected in the values, outside of the experimental variation. Thus, to the extent that the concentration of these compounds at the electrode surface reflects the bulk concentration, this result argues against the presence of two competitive reactions with different molecularities (with respect to the pyrrole derivative) and different n values. Thus, it may be that only dimeric product is formed in the initial reaction and the value of n in excess of unity (0.1-0.4) results from further oxidation of the initial product (Epa at 1.7 V). The only noticeable difference in these cyclic voltammograms involves the peak at 1.7 V which is not present in the 1 0 - a M solutions (of I or II) and increases in size relative to the peak at 1.3 V at the higher concentrations. The peak at 1.7 V is due to the oxidation of a product from the first oxidation reaction whose concentration is increased at the higher concentrations of I or II. The n values for two of the 5-phenylpyrrole, derivatives, I and II, were also evaluated from the complete electrolysis of a known amount of material (ca. 1 0 - 2 M ) . The experimental conditions for these measurements are different from t h e above because these experiments were performed with stirring and a controlled potential of + 1.4 V. These experiments provided n values of 1.8 for both compounds which suggests the formation of mononieric product primarily. Thus, it is clear that these oxidation reactions are quite sensitive to changes in the electrolysis conditions. A fairly linear correlation is found between the Epa values for the five methyl- and phenyl-substituted pyrroles and their dipole moments measured in benzene for which the data are available [9,10]. While it is clear that E ° values should be used for plots of this type, they are not available for these irreversible reactions. However, the AEpa values must be linearly related to the relative ease of oxidation for this limited number of closely related structures. This relationship is seen in Fig. 1, and it shows TABLE 2 Summary of electrochemical data measured in CH3CN containing 0.1 M Et4NBF4 using Pt vs. NaCE electrodes; 1,=50 mV s t. Reactant concentration 0.7 to 2.0 ~mol c m - 3
(i/AC~,I/2)/ (q/dCtl/2)/ 103 nDl/2/ 103n3/2Dl/2/ n
106D/
A c m m o l -~
Gem
cm2s -I
sl/2V -~/2
tool -~ s ~/2
2,5-Dimethylpyrrole 1,2,5-Trimethylpyl:role 2-Methyl-5-phenylpyrrole 1,2-Dimethyl-5-(3',4'dimethoxyphenylpyrrole I
1430 1500 1155
Compound
cms -1/2
cms -t/2
760 755 530
7.0 7. I 4.9
7.0 7.0 5.5
1.0 1.0 1.3
49 50 15
1650 1614
725 795
6.7 6.4
7.8 7.7
23 21
II
1415
615
5.7
6.7
III IV
1475 1340
670 670
6.2 6.2
7.0 6.4
1.2 1.4 1.8 ~ 1.4 1.8 ,~ 1.3 1.1
Value from complete electrolysis at constant potential.
16 23 32
186 2.0
\~.~Phenyl >
1.5
~
tu~
Pyrrol~-~l-Methyl 1.0
~2,5-Dimethyl
1,2,5-Trimethyl*+~ 0.5
i
1.4
L
1.6
i
1.8
I
2.0
,
2.2
O/O Fig. 1. Plot of Epa (50 mV s - t ) measured in acetonitrile against the dipole moment measured in benzene for a series of substituted pyrroles.
that the compounds with the largest dipole moments are more easily oxidized. Since the dipole moment of pyrrole (located along the symmetry axis with the positive pole at the nitrogen atom) is calculated to be predominantly due to the 7r-moment [1] the empirical relationship observed between the Epa values and the dipole moments suggest that the ease of oxidation of these pyrrole derivatives are determined primarily by the nature of the ~r-system. The point for 1-methylpyrrole is off the Ep-/Z line and may be due to the value of the dipole moment which has been described as abnormally high and not adequately explained (ref. 1, p. 453). Not included in the plot is the Epa value for the 1-ethyl derivative which does not fit the correlation since these values are insensitive to changes in the 1-alkyl structure. While other considerations such as solvation effects, molecular orientations in the double layer, etc. are very important in these reactions and should not be neglected in attempting to interpret free energy relationships, these effects must remain fairly constant or vary in a linear manner for the compounds being considered here. It must be stressed that considerations of this type are at best applicable to a series of compounds having very similar substituents. Thus, it would be dangerous to generalize this relationship to include substituents with widely different structural variations. This fact is clear from the different effect produced by the phenyl group when substituted in the 2-position rather than the 1-position. Even in the 1-position the influence of the phenyl group is changed by the presence of methyl groups in the 2- and 5-positions. For example, the Ep -/~ relationship predicts a n Epa vahle of 1.0 V for 2,5-dimethyl-l-phenylpyrrole (2.00D) [9] which is, correspondingly, 800 mV more negative than the measured value for 1-phenylpyrrole. The conclusion that the oxidation reaction depends primarily on the nature of the ~r-system is supported further by the fact that the relative Epa values for the electrooxidation of pyrrole, 1-methyl, 2,5-dimethyl and 1,2,5-trimethylpyrrole also plot linearly with the relative pK~ values for a-protonation of these compounds in
187
aqueous sulfuric acid [11]. This relationship between these two reactions is expected since the former reaction involves ~r-electron transfer from the pyrrole ring to the electrode surface (eqn. 4) while in the latter reaction the transfer is to the proton (eqn. 5). RE H
H .~ G E ~
H30+÷
~ H
R
-nF~Epa
~.K~
~. H ~
R + H20
(5)
.~GH30* = RT,,dpKa
CONCLUSIONS
In conclusion, the electrochemical oxidation of a,a-disubstituted pyrrole derivatives is not a simple reaction and the reaction produces soluble products which do not alter the electrode surface. This latter observation is in complete contrast to that found with pyrroles which are not substituted in the a-positions and which lead to the formation of insoluble polymeric films on the electrode surface [3]. The reactions display non-integral n values which lie between one and two, and are again different from the reactions of the N-substituted pyrroles which undergo a polymeric reaction where the value is 2.2 [3]. The oxidation potential is sensitive to the presence of the substituents where the changes in the Epa values seem to correlate with changes in the ~r-dipole moment of the molecule. ACKNOWLEDGEMENT
The authors wish to thank J. Castillo for his technical assistance in various aspects of this study. REFERENCES 1 R.A. Jones and G.P. Bean, The Chemistry of Pyrroles, Academic Press, New York, 1977. 2 A. Dall'alio, G. Dascola, V. Varacca and V. Bocchi, C.R. Acad. Sci. (Paris), 267C (1968) 433. 3 (a) A. Diaz, K. Kanazawa and G.P. Gardini, J. Chem. Soc., Chem. Commun., 635 (1970); (b) A. Diaz, Electrochemical preparation and characterization of conducting polymers, Proceedings to the International Conference on Low Dimensional Synthetic Metals, Chemica Scripta, 1981. 4 M. Salmbn, O. Villarino, A. Jimenez and R. Zawadzki, Rev. Latinoam. Quim., 4 (1973) 59. 5 A. Diaz, J. Org. Chem., 42 (1977) 3949. 6 The instrumenfation used in these studies was constructed in these laboratories. K.K. Kanazawa and R. Galwey, J. Electrochem. Soc., 124 (1977) 1385. 7 R.N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, New York, 1969. 8 J.F. Ambrose and R.F. Nelson, J. Electrochem. Soc., 115 (1968) 1159. 9 H. Kofod, L.E. Sutton and J. Jackson, J. Chem. Soc., (1952) 1467. 10 See Table 11.8, p. 545 in ref. 1. I I (a) Y. Chiang and E.B. Wipple, J. Am. Chem. Soc., 85, (1963) 2736; (b) Y. Chiang, R.L. Himman, S. Theodoropulos and E.B. Wipple, Tetrahedron, 23, (1967) 745. 12 N.L. Weinberg (Ed.), Technique of Electroorganic Synthesis, Vol. V, Part II 1974, p. 1032.