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Electrochemical characteristics of some N-(hydroxyalkyl) and N-(tosylalkyl) pyrroles; electrochemical behaviour of the corresponding polymers
Synthetic Metals, 15 (1986) 49 - 58
49
ELECTROCHEMICAL CHARACTERISTICS OF SOME N-(HYDROXYALKYL) AND N-(TOSYLALKYL) PYRROLES; ELECTROCHEMICAL BEHAVIOUR OF THE CORRESPONDING POLYMERS G. BIDAN* and M. GUGLIELMIt Ddpartement de Recherche Fondamentale, Centre d'Etudes Nuclgaires de Grenoble, 85X, F-38041 Grenoble Cddex (France)
(Accepted January 16, 1986)
Abstract We present an electrochemical study of a series of thirteen N-alkyl derived pyrrole monomers, where the substituent of the alkyl chain is a h y d r o x y l or a tosyl group. The anodic peak potentials are mainly influenced by the steric hindrance of the substituent. Electropolymerization of hydroxylated pyrrole monomers is blocked for short alkyl chains (C2 or C3). This may be due to the internal attack of the pyrrole cation radical. The best electropolymerizability of pyrrolic monomers, as well as the cycling stability of the corresponding polymers, is observed for compounds with a linear C6 alkyl chain or with a C3 branched alkyl chain bearing two pyrrole groups for one substituent.
Introduction The use of polymers as a matrix for anchoring redox centres [1, a, b] in a three-dimensional zone appears to be an attractive way of modifying electrode surfaces. The advantages of conducting polymers include their behaviour as both electron carriers and reservoirs. Its convenient synthesis via anodic electrodeposition [2], the chemical stability and the electroconductivity of poly(pyrrole) make it a convenient material for fabricating polymer-modified electrodes containing active centres. For the inclusion of electroactive centres in poly(pyrrole) films, one approach consists of electrochemical incorporation of an anionic redox species [3] (acting as the dopant) such as Co 2+, RuO42-, Co porphyrin or Fe phthalocyanine. We recently
*Laboratoire d'Electrochimie, Laboratoires de Chimie Mol~culaire. tGroupe Dynamique de Spin et Propri~t~s Electroniques, Service de Physique-ER CNRS n° 216. 0379-6779/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
50
tested a new method of obtaining poly(derived pyrrole) in which the active centre R is covalently b o u n d to the pyrrole nitrogen via an alkyl chain (symbolized by a wavy line) by subjecting the derived pyrrole m o n o m e r to electro-oxidation:
X
I
-2xH - x(2+6)e ~____ anodic oxidation
R
~+mdoping level R
Using this approach, we have already reported the electrochemical preparation and behaviour of poly(pyrrole) films containing the polypyridinyl Ru(II) complex [4] or the viologen system [5]. In order to obtain a general procedure for preparing such poly(derived pyrrole) films, we first perfected a m e t h o d of binding an active centre to a pyrrole group via an alkyl chain [6]. We n o w want to correlate the structure of the m o n o m e r with its polymerizability and the electrochemical stability of the corresponding polymer. Therefore, we present here an electrochemical study of thirteen of the following compounds:
N--(CH2)~-R
©© ft.2 H
n/R
OH
OTs
2 3 6 11
1 2 3 4
5 6 7 8
P-2-OH P-3-OH P-6-OH P-11-OH
P-2-OTs P-3-OTs P-6-OTs P-11-OTs
R
© /
H2Cx HC / --R 1 R~'-CH 2
9: R ~ OH; P2OH 1 1 : R 1 -= OH, R2 ~ OH; P(OH)2 10: R =- OTs; P2OTs 1 2 : R 1 ~ OH, R2 ~- OTs; P(OH)(OTs) 1 3 : R 1 ~ OTs, R : -= OTs; P(OTs)2
These c o m p o u n d s were chosen as models in order to study the respective influences of the chain length (2, 3, 6 or 11 CH2), structure (linear or branched), the ratio of substituents to the number of pyrrole cycles (one R/ one pyrrole cycle, one R / t w o pyrrole cycles or two R/one pyrrole cycle), and the nature of the substituent (OH: nucleophilic b u t small volume, OTs: bulky group).
51
Experimental
(1) Chemical synthesis The syntheses of the N-(hydroxyalkyl) pyrroles 1, 2, 3, 4, 9 and 11 have been previously reported [6]. The tosylates 5, 6, 7, 8, 10, 12 and 13 were prepared according to the classical reaction of TsC1 on the corresponding N-(hydroxyalkyl) pyrrole in a pyridine medium*. A typical procedure was the following: to a solution of 2 X 10 -2 mole of an N-(hydroxyalkyl) pyrrole in 6.5 cm 3 (8 X 10 -2 mole) of pyridine was slowly added 4.2 g (2.2 X 10-2 mole) of TsC1¢. The reaction was driven under nitrogen at a temperature less than 10 °C. The solution became orange and was left for one night at T = 5 °C (in a refrigerator). Then the solution was poured into water, neutralized with HC1 (in a slight deficiency) and the product extracted with diethyl ether. Elimination of solvent under vacuum resulted in a crude oil, which was submitted to chromatography on silica gel (Merck; 0.063 - 0.2 mm) using benzene as eluent. Recrystallization in a mixture of diethyl ether/cyclohexane yielded the tosylates 5 (F = 60 °C), 6 (oil), 7 ( F = 5 7 °C), 8 ( F = 3 5 °C), 10 ( F = 1 1 7 °C), 12 ( F = 7 2 °C) and 13 ( F = 101 °C), respectively, in 55%, 60%, 66%, 78%, 67%, 57% and 29% yields. All compounds had correct chemical analyses (elemental or high resolution mass spectroscopy). Infrared spectra (KBr) of these compounds are similar, and differ only in the ratio of the intensity of the following bands (cm-1): 3100 {pyrrolic H), 3040 - 3070 (aromatic H), 2930 - 2960 (alkyl CH and CH:), 1355- 1373 (sulphonyl). IH-NMR (CDCla) data are as follows: 5, 5 2.33 (s, 3H), 4.03 (m, 4H), 6 (t, 2.2 Hz, 2H), 6.47 (t, 2.2 Hz, 2H), 7.17 (d, 8 Hz, 2H), 7.57 (d, 8 Hz, 2H); 6, 5 1.93 (q, 6 Hz, 2H), 2.43 (s, 3H), 3.83 (t, 6.6 Hz, 2H), 3.87 (t, 5.4 Hz, 2H), 6 (t, 2.2 Hz, 2H), 6.43 (t, 2.2 Hz, 2H), 7.25 (d, 8 Hz, 2H), 7.72 (d, 8 Hz, 2H); 7, 5 1.25 - 1.63 (br, 8H), 2.4 (s, 3H), 3.75 (t, 6.6 Hz, 2H), 3.95 (t, 5.4 Hz, 2H), 6.05 (t, 2.2 Hz, 2H), 6.54 (t, 2.2 Hz, 2H), 7.27 (d, 8 Hz, 2H), 7.73 (d, 8 Hz, 2H); 8, 5 1.23 - 1.67 (s-br, 18H), 2.4 (s, 3H), 3.81 (t, 6.6 Hz, 2H), 3.97 (t, 5.4 Hz, 2H), 6.07 (t, 2.2 Hz, 2H), 6.58 (t, 2.2 Hz, 2H), 7.27 (d, 8 Hz, 2H), 7.75 (d, 8 Hz, 2H); 10, 5 2.37 (s, 2H), 3.92 (m, 4H), 4.77 (q, 5.5 Hz, 1H), 6.07 (t, 2.2 Hz, 4H), 6.5 (t, 2.2 Hz, 4H), 7.15 (d, 8 Hz, 2H), 7.5 (d, 8 Hz, 2H); 12 5 2.41 (s, 3H), 3.91 (s-br, 5H), 6.03 (t, 2.2 Hz, 2H), 6.52 (t, 2.2 Hz, 2H), 7.28 (d, 8 Hz, 2H), 7.75 (d, 2 Hz, 2H); 13, 5 2.43 (s, 6 H), 3.92 (d, 4.8 Hz, 2H), 4.05 (d, 5.1 Hz, 2H), 4.69 (q, 5.2 Hz, 1H), 5.97 (t, 2.2 Hz, 2H), 6.4 (t, 2.2 Hz, 2H), 7.27 (dd, 8 Hz, 4H), 7.63 (dd, 8 Hz, 4H).
*Our r e c e n t e x p e r i m e n t s o n t h e c o n d e n s a t i o n of s u l p h o n y l or c a r b o n y l c h l o r i d e o n t h e s e a l c o h o l s s h o w t h a t t h e use o f t h e a l c o h o l a t e in T H F gives b e t t e r yields t h a n t h e pyridine medium. t T h e s y n t h e s i s o f 13 n e e d s t w o e q u i v a l e n t s o f TsC1. T h e t o s y l a t i o n is i n c o m p l e t e a n d t h e m o n o t o s y l a t e d c o m p o u n d 12 is s e p a r a t e d d u r i n g t h e c h r o m a t o g r a p h y .
52
(2) Electrochemical experiments Analytical studies were made using a 3 m m 2 Pt disk electrode in a twoc o m p a r t m e n t cell. The concentrations in the substrate were 4 × 10 -3 M in acetonitrile and 0.1 M in LiC104. Potentials were referenced versus an Ag/10 -2 M Ag + electrode. The electrochemical units were PAR models 173D, 175 and 179 with a SEFRAM TGM 164 recorder. All experiments were performed under argon in a dry box. The acetonitrile was distilled three times, first over H2SO4, then over Call2 and the last time in the dry box over P2Os. LiC104 was melted under vacuum before use.
Results and discussion
(1) Electroanalytical study o f pyrrole m o n o m e r s I to 13 Cyclic voltammetry data for the N-(hydroxyalkyl) and N-(tosylalkyl) pyrrole monomers are summarized in Table 1. They are plotted in Fig. 1 for visualization of the influence on the oxidation potential of the substitutent and the alkyl chain respectively. Compounds 9 and 10 are classified as C2 (considering the N--C2--R chaining), while compounds 11, 12 and 13 are C3. The oxidation potential values of these monomers are in agreement with the range of oxidation potentials of N-alkylpyrroles [7]. For the tosylated linear compounds 5 to 8, the substituent effect on the oxidation potential values decreases when increasing the chain length and therefore becomes negligible for the Cll chain (see Fig. 1). This chain length effect is almost absent for the small OH group, indicating that neither electro-inductive nor steric effects are perceptible, even for the C2 chain length. A factor of approximately 2.8 for the reduction of the inductive effect of a substituent resulting from the interposition of a methylene group is obtained for --CH2C1, --CH2CH2C1 and CH3C(O)-- , CH3C(O)CH 2 - [8]. In the C6Hs(CH2) n - series, the ratios on/o,+, * * of the Taft constants are 2.8, 2.7 and 4 for n = 0, 1 and 2 respectively [8]. Thus, inductive effects are fairly equal for n-alkyl groups of two or more carbons [9]. In our study, the numbers of methylene groups TABLE 1 Oxidation peak potential values Epa (vs. Ag/10 - 2 M Ag +) of monomers 1 to 13 (4 × 10 -3 M) in LiC104 (0.1 M)-MeCN, sweep rate: 200 mV s-1 Alcohols
Epa (V)
Tosylates
Epa (V)
P-2-OH P-3-OH P-6-OH P-11-OH P2-OH P(OH)(OTs) P(OH)2
0.950 0.960 0.950 0.950 0.930 1.040 1.060
P-2-OTs P-3-OTs P-6-OTs P-11-OTs P2-OTs P(OH)(OTs) P(OTs)2
1.100 1.025 0.990 0.960 1.000 1.040 1.100
53 Number of carbon atoms between the pyrrole cycle and the OH or OTs substituent
C11
~ O'~" 0 P-n.OH
x
~ x ~ - x P_n-OTs
C6
o
x
~
P(OH)2 \ P(OH){OTs) /
P(OTs)
L
~
t
1.00
1.05
C3
/
C2 P2 OH
, P2OTs Epa
i
0.90
0,95
b
1.10 (V)
Fig. 1. O x i d a t i o n peak p o t e n t i a l s ( r e f e r r e d to A g / 1 0 - 2 M Ag +) v s . c h a i n l e n g t h curves for N - ( h y d r o x y a l k y l ) (©) a n d N - ( t o s y l a l k y l ) (×) p y r r o l e m o n o m e r s . The m i x e d c o m p o u n d P ( O H ) ( O T s ) is i n d i c a t e d b y (m).
of the aliphatic chains take the values 2, 3, 6 and 11. So, whatever may be the difference in inductive effects between the OH and OTs groups*, the inductive effects of the --(CH2)nOH and --(CH2)nOTS groups are negligible. Consequently, the tosylate effect is mainly due to its bulkiness. This steric *We have n o t b e e n able t o find t h e Taft c o n s t a n t o f t h e OTs group. T h e H a m m e t t a l i p h a t i c c o n s t a n t s o I [ 1 0 ] a n d m e t a - s u b s t i t u e n t c o n s t a n t s Om [ 1 1 ] for d i f f e r e n t groups are as follows:
Substituent
OI
O"m
--CH3 --COCH3 --SO2CH3 --OH --OCH3 --OCOCH 3 --OSO~CHa
0. 0.28 0.60 0.25 0.25 0.39 --
--0.07 0.38 0.60 0.12 0.12 0.39 0.39
In t h e m e t a p o s i t i o n , t h e r e s o n a n c e e f f e c t is o f t e n weak, so o m c o m p a r e s well w i t h o I. This a! for t h e --OTs g r o u p (similar t o t h e --O--SO2CH 3 g r o u p ) m a y be e s t i m a t e d as b e i n g close to t h e o n e of t h e - - O C O C H 3 group.
54
effect also appears with the OH substituent, as seen in c o m p o u n d P(OH)2 where t w o OH groups are b o u n d to the C3 N-alkyl chain. This leads to an increase in the potential value of 110 mY. In a similar way, c o m p o u n d P(OTs)2 exhibits an oxidation potential value 75 mV higher than that of P-3-OTs. As expected the oxidation potential value of the mixed P(OH)(OTs) is lower than that of the P(OTs)2 and also, more surprisingly, lower than that of the P(OH)2 (see Fig. 1). We propose that the high oxidation potential value of P(OH)2 is explained not only in terms of steric hindrance, but also by considering an additional effect due to spatial interaction between OH groups and the pyrrolic cycle. OH-~ complexes between pyrrole and phenols or alcohols have been reported [12]. Such an effect is favoured in the case of P(OH)2, and is in accordance with the favoured internal cyclization further involved to explain the poor polymerizability of compounds 1, 2, 11 and 12. In contrast with these latter results issued from monopyrrolic monomers, the use of two pyrrolic cycles for one substituent greatly reduced the steric effect, lowering the potentials by 100 mV and 20 mV for P2OTs and P2OH respectively, as compared to P-2-OTs and P-2-OH. The relatively small decrease of the potential value from P-2-OH to P2OH is explained by the fact that the steric hindrance in P-2-OH is inherently weak compared with that of P-2-OTs.
(2) Electrochemical syntheses and behaviours of poly(1 to 13) Films of polymers were grown on the Pt electrode by controlled potential oxidation at 0.9 V of the corresponding monomers. Voltamperometric data for these polymers, when they exist, are summarized in Table 2. All the tosylated monomers give rise to electropolymerization and the corresponding polymers show well-shaped voltammograms comparable with that of the P-6-OH m o n o m e r of the hydroxyalkyl series (see Fig. 2). TABLE 2 O x i d a t i o n potential values ((Epa + Epc)/2 ) (vs. A g / 1 0 - 2 M Ag +) of polymers, w h e n existing, o b t a i n e d f r o m e l e c t r o p o l y m e r i z a t i o n of m o n o m e r s 1 to 13. Peak potentials are determ i n e d f r o m cyclic v o l t a m m o g r a m s of 10 - 3 C deposited on a 3 m m 2 Pt electrode, sweep rate 100 (mV s) -1. Thicker deposits (10 - 2 C) give the same (Epa + Epc)/2 values w h e n cycled at 5 m V s- 1 Alcohols
(Epa + Epc)/2 (V)
Tosylates
(Epa + Epc)/2 (V)
P-2-OH P-3-OH P-6-OH P-11-OH P2-OH
0.10 - 0.26 0.26 0.48" 0.30
P-2-OTs P-3-OTs P-6-OTs P-11-OTs P2-OWs
0.40 0.32 0.28 0.34" 0.22
P(OH)(OTs)
--
P(OH)(OTs)
--
P(OH)2
--
P(OTs)2
0.58
* Unstable.
55
/f~\
'A\b ,c},,, ~/~ \ ~ /d
j.-
OH
\~'
-
E (V)
Fig. 2. Cyclic v o l t a m m o g r a m s at a Pt e l e c t r o d e (diam. 2 m m ) o f P-6-OH (4 × 10 - 3 M) in LiC104 (0.1 M ) - M e C N ; v = 100 m V s- 1 (a) first s w e e p ; (b), (c), (d), (e), (f), (g) a n d (h) are successive sweeps a f t e r o x i d a t i o n at 0.9 V, increasing t h e a m o u n t of charge by 2 × 10 - 4 C each time.
In contrast, N-(hydroxyalkyl) pyrroles bearing a short alkyl chain (n = 2, 3) are poorly polymerizable or unstable on cycling. The intermediate voltammogram of poly(1 or 2) rapidly vanishes, accounting for the loss of redox properties and the passivation of electrode. Moreover, compounds P(OH): and P{OH)(OTs) do not polymerize. We assume that an internal cycling reaction would block the polymerization process. As a matter of fact, it is known that electro-oxidation of pyrrole in the presence of nucleophiles such as cyanide [13] or methoxide [14 a, b] ions yields cyanated or methoxylated pyrroles. Molecular models show that internal alkoxylation of pyrrole would be favoured in c o m p o u n d s 1, 2, 11 and 12 since they can provide five- and six-membered rings*. By analogy with previous studies [14a], we assume that the first step of this hypothetical cyclo-alkoxylation would be the nucleophilic attack of the pyrrole cation radical 14 by the h y d r o x y l group, followed by the classical electrochemical competitive steps
*We have n o t t r i e d t o isolate s u c h h y p o t h e t i c a l c o m p o u n d s as 15 a n d 16, w h i c h must undergo further electro-oxidation.
56 (dimerization or further oxidation) in nitrogen heterocyclic compounds [ 15]. We propose the following hypothetical process exemplified with 2:
~ ~k~,H(+,, - e(-)- H(÷) -e(-) ~ ](,+) cyclization~/ ~N/ ~'0 oxidatni~n ~"~I~I'~"0 ~0: ~0:-" k ii ar°matizati°no'1~~6 ~ 1"-5
~
,
-\
The pyrrole cation radical 14, similar to the one involved in the mechanism of polymerization of pyrrole [16], is trapped and the polymerization blocked. For tosylated polymers, the dependence of potential v e r s u s alkyl chain nature (see Table 2) is similar to that in the monomers, i.e., an increase in potential with steric hindrance. However, the poly(P-11-OTs) compound behaves abnormally as its potential appears higher than that of the poly(P-3-OTs). For monomers, steric hindrance is associated with the electronic accessibility of the electrophoric site of the molecule. In polymers, steric hindrance is related to electronic transfers between polypyrrolic chains. The poor stability upon cycling, as well as the comparatively high oxidation potential of poly(P-11-OTs) may be explained by considering a poor percolation of electrons through the polypyrrolic chains. Oxidation potential values (see Table 2) for hydroxylated polymers are more difficult to interpret. The poly(P-6~)H) and poly(P2OH ) appear as the only stable polymers. Abnormal behaviour is also observed for hydroxylated polymers, in the case of the Cll poly(4). Its lack of stability is explained in a manner similar to that for the poly(P-11-OTs). Chronocoulomeric data give access to the doping level x defined by the amount of perchlorate anion per pyrrolic cycle. Assuming a quantitative synthesis of the polymer, n(2 + x) represents the quantity of electricity, Qs, required for the synthesis of the polymer film if n is the number of pyrrolic units polymerized, n x represents the charge returned, Qr, during complete reduction of the polymer at low potentials (--0.2 V). We can extract x from the equation Qs/Qr = n(2 + x ) / n x . Hydroxylated poly(3), poly(4) and poly(9), and tosylated poly(5), poly(6), poly(7), poly(8) and poly(10) show doping levels in the usual range 0.25 to 0.32. There is a lack of correlation between the doping level value and the nature of the polymer. In contrast to our previous study on polyhalopyrroles [17], we have not been able to compare these chronocoulomeric data with elemental analysis data. As a matter of fact, these polymers are difficult to synthesize in thick films and consequently a suitable amount could not be produced for elemental analysis. This difficulty probably arises from the low conductivity of these poly(N-substituted pyrroles), as expected by analogy with the reported conductivity of poly(N-alkyl pyrrole) [7].
57 N--(CH2)~-R
Doping level
%
P-n-R
a) P-6-OH b) P-11-OH c) P-3-OTs d) P_2_OTs
10
/ / ~o=\ . . . , 0//0"%
100
50
~
•
\~\
d) \ \ 8 o ,
I
,
o.s
,
,
,
[
1.o
,
,
,
I
Es ,
1,s (v)
Fig. 3. V a r i a t i o n o f t h e d o p i n g level w i t h t h e m a x i m u m s w i t c h i n g p o t e n t i a l E s w h e n t h e p o l y m e r is s w i t c h e d b e t w e e n - - 0 . 2 V a n d E s.
Switching the polymer between --0.2 V and a positive potential E s leads to an increase in the doping level up to Es = 0.9 - 1 V (see Fig. 3). If the polymer is switched b e y o n d these potentials, the apparent doping level decreases, indicating that the degradation process becomes predominant. Conclusion Steric hindrance of the substituent appears to be the main factor determining the polymerizability of a derived pyrrole m o n o m e r as well as the cycling stability of the corresponding polymer. To diminish greatly this steric influence, the use of two pyrrole groups with a short alkyl chain (i.e., c o m p o u n d s 9 and 10) or of a single pyrrole group with a long C 6 alkyl chain (i.e., 3 and 7) leads to comparable effects. The longer Cll chain c o m p o u n d s do n o t behave better than the C6 ones, as the stability of their polymers is decreased by inherent alkyl chain steric hindrance. Taking these results into account, we have recently investigated the P2-R and P-6-R electrochemical behaviour where, among others, R is a ferrocene or a nitroxide group [18]. Our preliminary results on these c o m p o u n d s confirm our assumptions concerning their polymerizability and the stability of the corresponding polymers. References 1 (a) F o r a general review o n m o d i f i e d e l e c t r o d e s see L. R. F a u l k n e r , C h e m i c a l micros t r u c t u r e o n e l e c t r o d e s , Chem. Eng. News, ( F e b . 2 7 ) ( 1 9 8 4 ) 28. (b) R. N o w a k , F. A. S c h u l t z , M. U m a n a , H. A b r u n a a n d R. W. Murray, J. Electroanal.
58 Chem. Interfacial Electrochem., 94 (1978) 219; A. Merz and A. J. Bard, J. A m . Chem. Soc., 100 (1978) 3222; M. S. Wrighton, M. C. Palazzoto, A. N. Bocarsly, J. M. Bolts, A. B. Fisher and L. Nadjo, J. A m . Chem. Soc., 100 (1978) 7264. 2 K. K. Kanazawa, A. F. Diaz, W. D. Gill, P. M. Grant, G. B. Street, G. P. Gardini and J. F. Kwak, Synth. Met., 1 (1979/80) 329. 3 0 . Ikeda, K. Okabayashi and H. Tamura, Chem. Lett. (1983) 1821; R. Noufi, J. Electrochem. Soc., 130 (1983) 2126; K. Okabayashi, O. Ikeda and H. Tamura, J. Chem. Soc., Chem. Commun, (1983) 684; R. A. Bull, F.-R. Fan and A. J. Bard, J. Electrochem. Soc., 130 ( 1 9 8 3 ) 1 6 3 6 . 4 G. Bidan, A. Deronzier and J.-C. Moutet, Nouv. J. Chim., 8 (1984) 501. 5 G. Bidan, A. Deronzier and J.-C. Moutet, J. Chem. Soc., Chem. Commun., (1984) 1185. 6 G. Bidan, Tetrahedron Lett., 26 (1985) 735. 7 A. Diaz, J. Castillo, K. K. Kanazawa, J. A. Logan, M. Salmon and O. Fajardo, J. Electroanal. Chem. Interfacial Electrochem., 133 (1982) 233. 8 See F. H. Westheimer, in M. S. Newman (ed.), Steric Effects in Organic Chemistry, John Wiley, New York, 1956, pp. 592 - 619. 9 J. March, Advanced Organic Chemistry: Reactions, Mechanisms and Structure, McGraw-Hill, New York, 1968, p. 19. 10 E. M. Kosower, An Introduction to Physical Organic Chemistry, John Wiley, New York, 1968, p. 49. 11 J. Hine, Structural Effects on Equilibria in Organic Chemistry, Wiley-Interscience, New York, 1975, p. 67. 12 V.M. Zezyulinskii, Zhur. Fiz. Khim., 24 (1950) 1442; Chem. Abstr., 45 (1951) 4558. R. Giavarini, B. Castagna and M. Gomel, Compt. Rend., 268 (1969) 1936. 13 K. Yoshida, J. A m . Chem. Soc., 101 (1979) 2116. 14 (a) J. M. Bobbit, C. L. Kulkarni and J. P. Willis, Heterocycles, 15 (1981) 495. (b) N. L. Weinberg and E. A. Brown, J. Org. Chem., 31 (1966) 4054. 15 G. Cauquis, M. Genies and E. Vieil, Nouv. J. Chim., 1 (1977) 307. 16 E. M. Genies, G. Bidan and A. F. Diaz, J. Electroanal. Chem. Interracial Electrochem., 149 (1983) 101. 17 P. Audebert and G. Bidan, J. Electroanal. Chem. Interfacial Electrochem., 190 (1985) 129. 18 G. Bidan and D. Limosin,Ann. Phys. (Paris), 11 (C1) (1986) 5.
49
ELECTROCHEMICAL CHARACTERISTICS OF SOME N-(HYDROXYALKYL) AND N-(TOSYLALKYL) PYRROLES; ELECTROCHEMICAL BEHAVIOUR OF THE CORRESPONDING POLYMERS G. BIDAN* and M. GUGLIELMIt Ddpartement de Recherche Fondamentale, Centre d'Etudes Nuclgaires de Grenoble, 85X, F-38041 Grenoble Cddex (France)
(Accepted January 16, 1986)
Abstract We present an electrochemical study of a series of thirteen N-alkyl derived pyrrole monomers, where the substituent of the alkyl chain is a h y d r o x y l or a tosyl group. The anodic peak potentials are mainly influenced by the steric hindrance of the substituent. Electropolymerization of hydroxylated pyrrole monomers is blocked for short alkyl chains (C2 or C3). This may be due to the internal attack of the pyrrole cation radical. The best electropolymerizability of pyrrolic monomers, as well as the cycling stability of the corresponding polymers, is observed for compounds with a linear C6 alkyl chain or with a C3 branched alkyl chain bearing two pyrrole groups for one substituent.
Introduction The use of polymers as a matrix for anchoring redox centres [1, a, b] in a three-dimensional zone appears to be an attractive way of modifying electrode surfaces. The advantages of conducting polymers include their behaviour as both electron carriers and reservoirs. Its convenient synthesis via anodic electrodeposition [2], the chemical stability and the electroconductivity of poly(pyrrole) make it a convenient material for fabricating polymer-modified electrodes containing active centres. For the inclusion of electroactive centres in poly(pyrrole) films, one approach consists of electrochemical incorporation of an anionic redox species [3] (acting as the dopant) such as Co 2+, RuO42-, Co porphyrin or Fe phthalocyanine. We recently
*Laboratoire d'Electrochimie, Laboratoires de Chimie Mol~culaire. tGroupe Dynamique de Spin et Propri~t~s Electroniques, Service de Physique-ER CNRS n° 216. 0379-6779/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
50
tested a new method of obtaining poly(derived pyrrole) in which the active centre R is covalently b o u n d to the pyrrole nitrogen via an alkyl chain (symbolized by a wavy line) by subjecting the derived pyrrole m o n o m e r to electro-oxidation:
X
I
-2xH - x(2+6)e ~____ anodic oxidation
R
~+mdoping level R
Using this approach, we have already reported the electrochemical preparation and behaviour of poly(pyrrole) films containing the polypyridinyl Ru(II) complex [4] or the viologen system [5]. In order to obtain a general procedure for preparing such poly(derived pyrrole) films, we first perfected a m e t h o d of binding an active centre to a pyrrole group via an alkyl chain [6]. We n o w want to correlate the structure of the m o n o m e r with its polymerizability and the electrochemical stability of the corresponding polymer. Therefore, we present here an electrochemical study of thirteen of the following compounds:
N--(CH2)~-R
©© ft.2 H
n/R
OH
OTs
2 3 6 11
1 2 3 4
5 6 7 8
P-2-OH P-3-OH P-6-OH P-11-OH
P-2-OTs P-3-OTs P-6-OTs P-11-OTs
R
© /
H2Cx HC / --R 1 R~'-CH 2
9: R ~ OH; P2OH 1 1 : R 1 -= OH, R2 ~ OH; P(OH)2 10: R =- OTs; P2OTs 1 2 : R 1 ~ OH, R2 ~- OTs; P(OH)(OTs) 1 3 : R 1 ~ OTs, R : -= OTs; P(OTs)2
These c o m p o u n d s were chosen as models in order to study the respective influences of the chain length (2, 3, 6 or 11 CH2), structure (linear or branched), the ratio of substituents to the number of pyrrole cycles (one R/ one pyrrole cycle, one R / t w o pyrrole cycles or two R/one pyrrole cycle), and the nature of the substituent (OH: nucleophilic b u t small volume, OTs: bulky group).
51
Experimental
(1) Chemical synthesis The syntheses of the N-(hydroxyalkyl) pyrroles 1, 2, 3, 4, 9 and 11 have been previously reported [6]. The tosylates 5, 6, 7, 8, 10, 12 and 13 were prepared according to the classical reaction of TsC1 on the corresponding N-(hydroxyalkyl) pyrrole in a pyridine medium*. A typical procedure was the following: to a solution of 2 X 10 -2 mole of an N-(hydroxyalkyl) pyrrole in 6.5 cm 3 (8 X 10 -2 mole) of pyridine was slowly added 4.2 g (2.2 X 10-2 mole) of TsC1¢. The reaction was driven under nitrogen at a temperature less than 10 °C. The solution became orange and was left for one night at T = 5 °C (in a refrigerator). Then the solution was poured into water, neutralized with HC1 (in a slight deficiency) and the product extracted with diethyl ether. Elimination of solvent under vacuum resulted in a crude oil, which was submitted to chromatography on silica gel (Merck; 0.063 - 0.2 mm) using benzene as eluent. Recrystallization in a mixture of diethyl ether/cyclohexane yielded the tosylates 5 (F = 60 °C), 6 (oil), 7 ( F = 5 7 °C), 8 ( F = 3 5 °C), 10 ( F = 1 1 7 °C), 12 ( F = 7 2 °C) and 13 ( F = 101 °C), respectively, in 55%, 60%, 66%, 78%, 67%, 57% and 29% yields. All compounds had correct chemical analyses (elemental or high resolution mass spectroscopy). Infrared spectra (KBr) of these compounds are similar, and differ only in the ratio of the intensity of the following bands (cm-1): 3100 {pyrrolic H), 3040 - 3070 (aromatic H), 2930 - 2960 (alkyl CH and CH:), 1355- 1373 (sulphonyl). IH-NMR (CDCla) data are as follows: 5, 5 2.33 (s, 3H), 4.03 (m, 4H), 6 (t, 2.2 Hz, 2H), 6.47 (t, 2.2 Hz, 2H), 7.17 (d, 8 Hz, 2H), 7.57 (d, 8 Hz, 2H); 6, 5 1.93 (q, 6 Hz, 2H), 2.43 (s, 3H), 3.83 (t, 6.6 Hz, 2H), 3.87 (t, 5.4 Hz, 2H), 6 (t, 2.2 Hz, 2H), 6.43 (t, 2.2 Hz, 2H), 7.25 (d, 8 Hz, 2H), 7.72 (d, 8 Hz, 2H); 7, 5 1.25 - 1.63 (br, 8H), 2.4 (s, 3H), 3.75 (t, 6.6 Hz, 2H), 3.95 (t, 5.4 Hz, 2H), 6.05 (t, 2.2 Hz, 2H), 6.54 (t, 2.2 Hz, 2H), 7.27 (d, 8 Hz, 2H), 7.73 (d, 8 Hz, 2H); 8, 5 1.23 - 1.67 (s-br, 18H), 2.4 (s, 3H), 3.81 (t, 6.6 Hz, 2H), 3.97 (t, 5.4 Hz, 2H), 6.07 (t, 2.2 Hz, 2H), 6.58 (t, 2.2 Hz, 2H), 7.27 (d, 8 Hz, 2H), 7.75 (d, 8 Hz, 2H); 10, 5 2.37 (s, 2H), 3.92 (m, 4H), 4.77 (q, 5.5 Hz, 1H), 6.07 (t, 2.2 Hz, 4H), 6.5 (t, 2.2 Hz, 4H), 7.15 (d, 8 Hz, 2H), 7.5 (d, 8 Hz, 2H); 12 5 2.41 (s, 3H), 3.91 (s-br, 5H), 6.03 (t, 2.2 Hz, 2H), 6.52 (t, 2.2 Hz, 2H), 7.28 (d, 8 Hz, 2H), 7.75 (d, 2 Hz, 2H); 13, 5 2.43 (s, 6 H), 3.92 (d, 4.8 Hz, 2H), 4.05 (d, 5.1 Hz, 2H), 4.69 (q, 5.2 Hz, 1H), 5.97 (t, 2.2 Hz, 2H), 6.4 (t, 2.2 Hz, 2H), 7.27 (dd, 8 Hz, 4H), 7.63 (dd, 8 Hz, 4H).
*Our r e c e n t e x p e r i m e n t s o n t h e c o n d e n s a t i o n of s u l p h o n y l or c a r b o n y l c h l o r i d e o n t h e s e a l c o h o l s s h o w t h a t t h e use o f t h e a l c o h o l a t e in T H F gives b e t t e r yields t h a n t h e pyridine medium. t T h e s y n t h e s i s o f 13 n e e d s t w o e q u i v a l e n t s o f TsC1. T h e t o s y l a t i o n is i n c o m p l e t e a n d t h e m o n o t o s y l a t e d c o m p o u n d 12 is s e p a r a t e d d u r i n g t h e c h r o m a t o g r a p h y .
52
(2) Electrochemical experiments Analytical studies were made using a 3 m m 2 Pt disk electrode in a twoc o m p a r t m e n t cell. The concentrations in the substrate were 4 × 10 -3 M in acetonitrile and 0.1 M in LiC104. Potentials were referenced versus an Ag/10 -2 M Ag + electrode. The electrochemical units were PAR models 173D, 175 and 179 with a SEFRAM TGM 164 recorder. All experiments were performed under argon in a dry box. The acetonitrile was distilled three times, first over H2SO4, then over Call2 and the last time in the dry box over P2Os. LiC104 was melted under vacuum before use.
Results and discussion
(1) Electroanalytical study o f pyrrole m o n o m e r s I to 13 Cyclic voltammetry data for the N-(hydroxyalkyl) and N-(tosylalkyl) pyrrole monomers are summarized in Table 1. They are plotted in Fig. 1 for visualization of the influence on the oxidation potential of the substitutent and the alkyl chain respectively. Compounds 9 and 10 are classified as C2 (considering the N--C2--R chaining), while compounds 11, 12 and 13 are C3. The oxidation potential values of these monomers are in agreement with the range of oxidation potentials of N-alkylpyrroles [7]. For the tosylated linear compounds 5 to 8, the substituent effect on the oxidation potential values decreases when increasing the chain length and therefore becomes negligible for the Cll chain (see Fig. 1). This chain length effect is almost absent for the small OH group, indicating that neither electro-inductive nor steric effects are perceptible, even for the C2 chain length. A factor of approximately 2.8 for the reduction of the inductive effect of a substituent resulting from the interposition of a methylene group is obtained for --CH2C1, --CH2CH2C1 and CH3C(O)-- , CH3C(O)CH 2 - [8]. In the C6Hs(CH2) n - series, the ratios on/o,+, * * of the Taft constants are 2.8, 2.7 and 4 for n = 0, 1 and 2 respectively [8]. Thus, inductive effects are fairly equal for n-alkyl groups of two or more carbons [9]. In our study, the numbers of methylene groups TABLE 1 Oxidation peak potential values Epa (vs. Ag/10 - 2 M Ag +) of monomers 1 to 13 (4 × 10 -3 M) in LiC104 (0.1 M)-MeCN, sweep rate: 200 mV s-1 Alcohols
Epa (V)
Tosylates
Epa (V)
P-2-OH P-3-OH P-6-OH P-11-OH P2-OH P(OH)(OTs) P(OH)2
0.950 0.960 0.950 0.950 0.930 1.040 1.060
P-2-OTs P-3-OTs P-6-OTs P-11-OTs P2-OTs P(OH)(OTs) P(OTs)2
1.100 1.025 0.990 0.960 1.000 1.040 1.100
53 Number of carbon atoms between the pyrrole cycle and the OH or OTs substituent
C11
~ O'~" 0 P-n.OH
x
~ x ~ - x P_n-OTs
C6
o
x
~
P(OH)2 \ P(OH){OTs) /
P(OTs)
L
~
t
1.00
1.05
C3
/
C2 P2 OH
, P2OTs Epa
i
0.90
0,95
b
1.10 (V)
Fig. 1. O x i d a t i o n peak p o t e n t i a l s ( r e f e r r e d to A g / 1 0 - 2 M Ag +) v s . c h a i n l e n g t h curves for N - ( h y d r o x y a l k y l ) (©) a n d N - ( t o s y l a l k y l ) (×) p y r r o l e m o n o m e r s . The m i x e d c o m p o u n d P ( O H ) ( O T s ) is i n d i c a t e d b y (m).
of the aliphatic chains take the values 2, 3, 6 and 11. So, whatever may be the difference in inductive effects between the OH and OTs groups*, the inductive effects of the --(CH2)nOH and --(CH2)nOTS groups are negligible. Consequently, the tosylate effect is mainly due to its bulkiness. This steric *We have n o t b e e n able t o find t h e Taft c o n s t a n t o f t h e OTs group. T h e H a m m e t t a l i p h a t i c c o n s t a n t s o I [ 1 0 ] a n d m e t a - s u b s t i t u e n t c o n s t a n t s Om [ 1 1 ] for d i f f e r e n t groups are as follows:
Substituent
OI
O"m
--CH3 --COCH3 --SO2CH3 --OH --OCH3 --OCOCH 3 --OSO~CHa
0. 0.28 0.60 0.25 0.25 0.39 --
--0.07 0.38 0.60 0.12 0.12 0.39 0.39
In t h e m e t a p o s i t i o n , t h e r e s o n a n c e e f f e c t is o f t e n weak, so o m c o m p a r e s well w i t h o I. This a! for t h e --OTs g r o u p (similar t o t h e --O--SO2CH 3 g r o u p ) m a y be e s t i m a t e d as b e i n g close to t h e o n e of t h e - - O C O C H 3 group.
54
effect also appears with the OH substituent, as seen in c o m p o u n d P(OH)2 where t w o OH groups are b o u n d to the C3 N-alkyl chain. This leads to an increase in the potential value of 110 mY. In a similar way, c o m p o u n d P(OTs)2 exhibits an oxidation potential value 75 mV higher than that of P-3-OTs. As expected the oxidation potential value of the mixed P(OH)(OTs) is lower than that of the P(OTs)2 and also, more surprisingly, lower than that of the P(OH)2 (see Fig. 1). We propose that the high oxidation potential value of P(OH)2 is explained not only in terms of steric hindrance, but also by considering an additional effect due to spatial interaction between OH groups and the pyrrolic cycle. OH-~ complexes between pyrrole and phenols or alcohols have been reported [12]. Such an effect is favoured in the case of P(OH)2, and is in accordance with the favoured internal cyclization further involved to explain the poor polymerizability of compounds 1, 2, 11 and 12. In contrast with these latter results issued from monopyrrolic monomers, the use of two pyrrolic cycles for one substituent greatly reduced the steric effect, lowering the potentials by 100 mV and 20 mV for P2OTs and P2OH respectively, as compared to P-2-OTs and P-2-OH. The relatively small decrease of the potential value from P-2-OH to P2OH is explained by the fact that the steric hindrance in P-2-OH is inherently weak compared with that of P-2-OTs.
(2) Electrochemical syntheses and behaviours of poly(1 to 13) Films of polymers were grown on the Pt electrode by controlled potential oxidation at 0.9 V of the corresponding monomers. Voltamperometric data for these polymers, when they exist, are summarized in Table 2. All the tosylated monomers give rise to electropolymerization and the corresponding polymers show well-shaped voltammograms comparable with that of the P-6-OH m o n o m e r of the hydroxyalkyl series (see Fig. 2). TABLE 2 O x i d a t i o n potential values ((Epa + Epc)/2 ) (vs. A g / 1 0 - 2 M Ag +) of polymers, w h e n existing, o b t a i n e d f r o m e l e c t r o p o l y m e r i z a t i o n of m o n o m e r s 1 to 13. Peak potentials are determ i n e d f r o m cyclic v o l t a m m o g r a m s of 10 - 3 C deposited on a 3 m m 2 Pt electrode, sweep rate 100 (mV s) -1. Thicker deposits (10 - 2 C) give the same (Epa + Epc)/2 values w h e n cycled at 5 m V s- 1 Alcohols
(Epa + Epc)/2 (V)
Tosylates
(Epa + Epc)/2 (V)
P-2-OH P-3-OH P-6-OH P-11-OH P2-OH
0.10 - 0.26 0.26 0.48" 0.30
P-2-OTs P-3-OTs P-6-OTs P-11-OTs P2-OWs
0.40 0.32 0.28 0.34" 0.22
P(OH)(OTs)
--
P(OH)(OTs)
--
P(OH)2
--
P(OTs)2
0.58
* Unstable.
55
/f~\
'A\b ,c},,, ~/~ \ ~ /d
j.-
OH
\~'
-
E (V)
Fig. 2. Cyclic v o l t a m m o g r a m s at a Pt e l e c t r o d e (diam. 2 m m ) o f P-6-OH (4 × 10 - 3 M) in LiC104 (0.1 M ) - M e C N ; v = 100 m V s- 1 (a) first s w e e p ; (b), (c), (d), (e), (f), (g) a n d (h) are successive sweeps a f t e r o x i d a t i o n at 0.9 V, increasing t h e a m o u n t of charge by 2 × 10 - 4 C each time.
In contrast, N-(hydroxyalkyl) pyrroles bearing a short alkyl chain (n = 2, 3) are poorly polymerizable or unstable on cycling. The intermediate voltammogram of poly(1 or 2) rapidly vanishes, accounting for the loss of redox properties and the passivation of electrode. Moreover, compounds P(OH): and P{OH)(OTs) do not polymerize. We assume that an internal cycling reaction would block the polymerization process. As a matter of fact, it is known that electro-oxidation of pyrrole in the presence of nucleophiles such as cyanide [13] or methoxide [14 a, b] ions yields cyanated or methoxylated pyrroles. Molecular models show that internal alkoxylation of pyrrole would be favoured in c o m p o u n d s 1, 2, 11 and 12 since they can provide five- and six-membered rings*. By analogy with previous studies [14a], we assume that the first step of this hypothetical cyclo-alkoxylation would be the nucleophilic attack of the pyrrole cation radical 14 by the h y d r o x y l group, followed by the classical electrochemical competitive steps
*We have n o t t r i e d t o isolate s u c h h y p o t h e t i c a l c o m p o u n d s as 15 a n d 16, w h i c h must undergo further electro-oxidation.
56 (dimerization or further oxidation) in nitrogen heterocyclic compounds [ 15]. We propose the following hypothetical process exemplified with 2:
~ ~k~,H(+,, - e(-)- H(÷) -e(-) ~ ](,+) cyclization~/ ~N/ ~'0 oxidatni~n ~"~I~I'~"0 ~0: ~0:-" k ii ar°matizati°no'1~~6 ~ 1"-5
~
,
-\
The pyrrole cation radical 14, similar to the one involved in the mechanism of polymerization of pyrrole [16], is trapped and the polymerization blocked. For tosylated polymers, the dependence of potential v e r s u s alkyl chain nature (see Table 2) is similar to that in the monomers, i.e., an increase in potential with steric hindrance. However, the poly(P-11-OTs) compound behaves abnormally as its potential appears higher than that of the poly(P-3-OTs). For monomers, steric hindrance is associated with the electronic accessibility of the electrophoric site of the molecule. In polymers, steric hindrance is related to electronic transfers between polypyrrolic chains. The poor stability upon cycling, as well as the comparatively high oxidation potential of poly(P-11-OTs) may be explained by considering a poor percolation of electrons through the polypyrrolic chains. Oxidation potential values (see Table 2) for hydroxylated polymers are more difficult to interpret. The poly(P-6~)H) and poly(P2OH ) appear as the only stable polymers. Abnormal behaviour is also observed for hydroxylated polymers, in the case of the Cll poly(4). Its lack of stability is explained in a manner similar to that for the poly(P-11-OTs). Chronocoulomeric data give access to the doping level x defined by the amount of perchlorate anion per pyrrolic cycle. Assuming a quantitative synthesis of the polymer, n(2 + x) represents the quantity of electricity, Qs, required for the synthesis of the polymer film if n is the number of pyrrolic units polymerized, n x represents the charge returned, Qr, during complete reduction of the polymer at low potentials (--0.2 V). We can extract x from the equation Qs/Qr = n(2 + x ) / n x . Hydroxylated poly(3), poly(4) and poly(9), and tosylated poly(5), poly(6), poly(7), poly(8) and poly(10) show doping levels in the usual range 0.25 to 0.32. There is a lack of correlation between the doping level value and the nature of the polymer. In contrast to our previous study on polyhalopyrroles [17], we have not been able to compare these chronocoulomeric data with elemental analysis data. As a matter of fact, these polymers are difficult to synthesize in thick films and consequently a suitable amount could not be produced for elemental analysis. This difficulty probably arises from the low conductivity of these poly(N-substituted pyrroles), as expected by analogy with the reported conductivity of poly(N-alkyl pyrrole) [7].
57 N--(CH2)~-R
Doping level
%
P-n-R
a) P-6-OH b) P-11-OH c) P-3-OTs d) P_2_OTs
10
/ / ~o=\ . . . , 0//0"%
100
50
~
•
\~\
d) \ \ 8 o ,
I
,
o.s
,
,
,
[
1.o
,
,
,
I
Es ,
1,s (v)
Fig. 3. V a r i a t i o n o f t h e d o p i n g level w i t h t h e m a x i m u m s w i t c h i n g p o t e n t i a l E s w h e n t h e p o l y m e r is s w i t c h e d b e t w e e n - - 0 . 2 V a n d E s.
Switching the polymer between --0.2 V and a positive potential E s leads to an increase in the doping level up to Es = 0.9 - 1 V (see Fig. 3). If the polymer is switched b e y o n d these potentials, the apparent doping level decreases, indicating that the degradation process becomes predominant. Conclusion Steric hindrance of the substituent appears to be the main factor determining the polymerizability of a derived pyrrole m o n o m e r as well as the cycling stability of the corresponding polymer. To diminish greatly this steric influence, the use of two pyrrole groups with a short alkyl chain (i.e., c o m p o u n d s 9 and 10) or of a single pyrrole group with a long C 6 alkyl chain (i.e., 3 and 7) leads to comparable effects. The longer Cll chain c o m p o u n d s do n o t behave better than the C6 ones, as the stability of their polymers is decreased by inherent alkyl chain steric hindrance. Taking these results into account, we have recently investigated the P2-R and P-6-R electrochemical behaviour where, among others, R is a ferrocene or a nitroxide group [18]. Our preliminary results on these c o m p o u n d s confirm our assumptions concerning their polymerizability and the stability of the corresponding polymers. References 1 (a) F o r a general review o n m o d i f i e d e l e c t r o d e s see L. R. F a u l k n e r , C h e m i c a l micros t r u c t u r e o n e l e c t r o d e s , Chem. Eng. News, ( F e b . 2 7 ) ( 1 9 8 4 ) 28. (b) R. N o w a k , F. A. S c h u l t z , M. U m a n a , H. A b r u n a a n d R. W. Murray, J. Electroanal.
58 Chem. Interfacial Electrochem., 94 (1978) 219; A. Merz and A. J. Bard, J. A m . Chem. Soc., 100 (1978) 3222; M. S. Wrighton, M. C. Palazzoto, A. N. Bocarsly, J. M. Bolts, A. B. Fisher and L. Nadjo, J. A m . Chem. Soc., 100 (1978) 7264. 2 K. K. Kanazawa, A. F. Diaz, W. D. Gill, P. M. Grant, G. B. Street, G. P. Gardini and J. F. Kwak, Synth. Met., 1 (1979/80) 329. 3 0 . Ikeda, K. Okabayashi and H. Tamura, Chem. Lett. (1983) 1821; R. Noufi, J. Electrochem. Soc., 130 (1983) 2126; K. Okabayashi, O. Ikeda and H. Tamura, J. Chem. Soc., Chem. Commun, (1983) 684; R. A. Bull, F.-R. Fan and A. J. Bard, J. Electrochem. Soc., 130 ( 1 9 8 3 ) 1 6 3 6 . 4 G. Bidan, A. Deronzier and J.-C. Moutet, Nouv. J. Chim., 8 (1984) 501. 5 G. Bidan, A. Deronzier and J.-C. Moutet, J. Chem. Soc., Chem. Commun., (1984) 1185. 6 G. Bidan, Tetrahedron Lett., 26 (1985) 735. 7 A. Diaz, J. Castillo, K. K. Kanazawa, J. A. Logan, M. Salmon and O. Fajardo, J. Electroanal. Chem. Interfacial Electrochem., 133 (1982) 233. 8 See F. H. Westheimer, in M. S. Newman (ed.), Steric Effects in Organic Chemistry, John Wiley, New York, 1956, pp. 592 - 619. 9 J. March, Advanced Organic Chemistry: Reactions, Mechanisms and Structure, McGraw-Hill, New York, 1968, p. 19. 10 E. M. Kosower, An Introduction to Physical Organic Chemistry, John Wiley, New York, 1968, p. 49. 11 J. Hine, Structural Effects on Equilibria in Organic Chemistry, Wiley-Interscience, New York, 1975, p. 67. 12 V.M. Zezyulinskii, Zhur. Fiz. Khim., 24 (1950) 1442; Chem. Abstr., 45 (1951) 4558. R. Giavarini, B. Castagna and M. Gomel, Compt. Rend., 268 (1969) 1936. 13 K. Yoshida, J. A m . Chem. Soc., 101 (1979) 2116. 14 (a) J. M. Bobbit, C. L. Kulkarni and J. P. Willis, Heterocycles, 15 (1981) 495. (b) N. L. Weinberg and E. A. Brown, J. Org. Chem., 31 (1966) 4054. 15 G. Cauquis, M. Genies and E. Vieil, Nouv. J. Chim., 1 (1977) 307. 16 E. M. Genies, G. Bidan and A. F. Diaz, J. Electroanal. Chem. Interracial Electrochem., 149 (1983) 101. 17 P. Audebert and G. Bidan, J. Electroanal. Chem. Interfacial Electrochem., 190 (1985) 129. 18 G. Bidan and D. Limosin,Ann. Phys. (Paris), 11 (C1) (1986) 5.