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Electrochemistry in a low dielectric medium in the presence of a crown ether. Suppression of anodic dissolution of metal electrodes
J. Electroanal. Chem., 111 (1980) 391--395 © Elsevier Sequoia S.A., Lausanne --Printed in The Netherlands
391
Preliminary note ELECTROCHEMISTRY IN A LOW DIELECTRIC MEDIUM IN THE PRESENCE OF A CROWN ETHER. SUPPRESSION OF ANODIC DISSOLUTION OF METAL ELECTRODES
SEIICHIRO NAKABAYASHI, AKIRA FUJISHIMA and KENICHI HONDA Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hondo, Bunkyo, Tokyo 113 (Japan) (Received l l t h June 1980)
The macrocyclic polyethers exhibit a pronounced selectivity for the complexation of certain cations [1--3]. They make very stable complexes with some alkali or alkaline earth metal cations [4,5]. The results lead to the conclusion that by the aid of crown ethers, some kinds of salts become soluble in low dielectric solvents such as benzene [3,6,7]. Because of this characteristic, crown ethers consequently have wide application in chemistry [8--11], but electrochemically they have been less used except for ion-selective electrodes [12--16]. In this report, we have f o u n d t h a t in the presence of a crown ether an electrochemical system can be designed in a very low dielectric media such as benzene, and we show here that in this solution, some metal electrode behaviors are very different from those in an ordinarily used electrochemical organic solution. The crown ether used, from a commercial source (Nippon Soda Co., Ltd.) 15-crown-5, is designed by its molecular radius to capture a sodium cation selectively. The dielectric constants of the benzene--(15-crown-15) mixture were measured as a function of composition by the lock-in capacitance bridge method. Sodium tetraphenylborate (NaBph4), whose anion is bulky and whose charge density is extremely low, was used as a supporting electrolyte. The conductivity of the crown--benzene--NaBh4 solution was measured as a function of the concentration relation of NaBph4 by the resistance bridge method. Before electrochemical measurements the solutions were deaerated by the repeated freeze--pump--thaw technique. The potential of the working electrode was controlled potentiostatically against Ag-wire reference electrode. A P t wire was used as a counter electrode. All measurements were conducted at room temperature (20 -+ 3°C). Figure 1 shows the changes of the relative dielectric constant (er) of the crownbenzene solution with respect to its composition. The molar additive nature of er is clear. All of the electrochemical experiments were carried o u t with a crown-benzene mixture with a volumetric ratio between 15-crown-5 and benzene of 2 : 3 (mole ratio about 2 : 7). The relative dielectric constant of that solution was about 5.72. This constant is extremely low when compared with t h a t of a common electrochemical solvent such as acetonitrile (er = 37.5). Figure 2 shows the conductivity change as a function of the concentration of sodium tetraphenylborate added as supporting electrolyte in this mixed solvent. Conductivity K of the mixed solution including 0.1 M sodium tetraphenylborate was 2.3 × 10 -4
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0,25 0 50 0.75 1.00 MOLAR RATIO OF BENZENE
0.05
MOLAR
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0.15
0.20
CONCENTRATION OF NaBph 4
Fig. 1. C h a n g e o f t h e relative dielectric c o n s t a n t as a f u n c t i o n o f t h e m i x i n g r a t i o b e t w e e n 15-crown-5 a n d b e n z e n e . T h e m e a s u r e m e n t was m a d e b y using the c a p a c i t a n c e bridge balancing t e c h n i q u e w i t h a 1 kHz sinusoidal i n p u t . T h e b a l a n c i n g d e t e c t i o n was m a d e b y t h e lockin a m p l i f i e r t e c h n i q u e . T h i s relative dielectric c o n s t a n t m e a s u r e m e n t was carried o u t b y using t h e cell w h i c h was e q u i p p e d w i t h t h e guard electrode. In this Figure, t w o p o i n t s o n t h e side o f a high c o n c e n t r a t i o n o f c r o w n e t h e r were m e a s u r e d b y t w o e l e c t r o d e s s y s t e m w i t h o u t t h e guard electrode. This was t h e r e a s o n w h y these t w o p o i n t s h a d relatively large e r r o r deviation. Fig. 2. C o n d u c t i v i t y c h a n g e as a f u n c t i o n o f t h e c o n c e n t r a t i o n o f s o d i u m t e t r a p h e n y l b o r a t e . T h e s o l v e n t was a m i x t u r e o f 15-crown-5 a n d b e n z e n e , h a v i n g the m o l e r a t i o o f 2 : 7. T h i s m e a s u r e m e n t was carried o u t using t h e c o n d u c t a n c e bridge b a l a n c i n g t e c h n i q u e w i t h a I kHz sinusoidal i n p u t .
~ - ~ cm -1 which would be enough to carry out electrolysis. In the electrolyte solution cyclic voltammograms of 10 -3 M ferrocene were measured with a Pt working electrode. Figure 3 shows the result, which indicates a good reproducibi] ity with an accuracy of -+ 30 inV. Thus, we see that in the low dielectric crown-benzene solution an electrochemical process can be carried out, and that a Ag reference electrode may be considered stable enough for at least qualitative discussion. Figure 4b shows a cyclic voltamogram of a Cu working electrode in a solution containing benzene--15-crown-5 (mole ratio 7 : 2) and 0.1 M sodium tetraphenylborate. For comparison with the usual electrode behavior in a nonaqueous solvent, Fig. 4a shows a cyclic voltammogram of a Cu working electrode in an aceto nitrile solution, in which 0.1 M sodium tetraphenylborate was added as a supporting electrolyte. It is clear that in the former solution the anodic dissolution of the Cu electrode is very much suppressed compared with t h a t in the latter. These results allow us to assume that in the crown--benzene solution only a sodium cat, on is complexed with a 15-crown-5. However, because 15-crown-5 cannot capture the copper cation which is distinguishable from the sodium cation, the selectivity of this crown ether makes the copper electrode insoluble even at such anodic polarization. Figures 5a and b show the similar behavior of a Zn working electrode.
393 I
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I 0.75
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Fig. 3. Cyclic v o l t a m m o g r a m of f e r r o c e n e (10 - 3 M ) in 15-crown-5 a n d b e n z e n e m i x t u r e ( m o l e r a t i o 2 : 7 , h a v i n g a relative dielectric c o n s t a n t o f 5 . 7 2 ) w i t h 0.1 M s o d m m t e t r a p h e n y l b o r a t e as s u p p o r t i n g e l e c t r o l y t e . T h e w o r k i n g e l e c t r o d e a n d t h e c o u n t e r e l e c t r o d e were Pt wires, a n d t h e r e f e r e n c e e l e c t r o d e was a Ag wire. (a) I4
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-0.5
I I 0.5 1.0 POTENTIAL/V vs. Ag
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Fig. 4. Cyclic v o l t a m m o g r a m o f a Cu w o r k i n g e l e c t r o d e w i t h o u t a n y r e d o x reagent. (a) I n t h e a c e t o n i t r i l e s o l u t i o n w i t h 0.1 M s o d i u m t e t r a p h e n y l b o r a t e as s u p p o r t i n g e l e c t r o l y t e . (b) In t h e c r o w n - b e n z e n e - N a B p h 4 s o l u t i o n having t h e s a m e c o m p o s i t i o n c h a r a c t e r as in Fig. 3. I n b o t h cases, t h e c o u n t e r e l e c t r o d e was a Pt wire, a n d t h e r e f e r e n c e e l e c t r o d e was a Ag wire. Fig. 5. Cyclic v o l t a m m o g r a m o f a Z n w o r k i n g e l e c t r o d e w i t h o u t a n y r e d o x r e a g e n t . (a) In the a c e t o n i t r i l e s o l u t i o n . (b) In t h e c r o w n - - b e n z e n e - - N a B p h 4 s o l u t i o n . O t h e r c o n d i t i o n s were t h e same as in Fig. 4.
394 15C
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solution
,/
50
/ -I.0
-0.5
-*
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I.S
-50
Fig. 6. Cyclic v o l t a m m o ~ a m o f a A g w o r k i n g e l e c t r o d e in t h e c r o w n - - b e n z e n e - - N a B p h 4 s o l u t i o n w i t h o u t a n y r e d o x r e a g e n t . O t h e r c o n d i t i o n s w e r e t h e s a m e as in Fig. 4.
Contrary to the above results, in the case of a Ag working electrode with the Ag reference electrode, an anodic dissolution current was observed at the potential region more positive than the rest potential 0.0 V of the Ag working electrode. Supposing that the crown ether can interact with a Ag cation n o t only by i o n - dipole interaction but also by a covalent one, a larger stability constant of a Ag + complex than that anticipated w o u l d be given [17]. So, this result may be reflected by the selectivity property of the crown ether. It is an interesting fact that under the cathodic polarization, the current corresponding to the Ag deposi tion was less than that anticipated. Therefore, the cyclic voltammogram might be shown as asymmetric. This p h e n o m e n o n has n o t been clearly explained up to now, b u t we are trying to interpret it through further study, for instance, by using the temperature-controlled rotating ring-disc electrode technique. Though a detailed reaction scheme remains to be clarified, this is the first case in which this type of electrochemistry has been carried o u t in such a low dielectric medium (er = 5.72), and an anodic dissolution of a metal working electrode can be suppressed by the action o f the selective solubilization property of the crown ether mixed solvent. As the crown ether is optically transparent over a wide visible wavelength region [4], this t y p e of technique is expected to be effec tive for the suppression of the self degradation of some n-type semiconductor photoanodes [18,19] for the electrochemical photocell.
REFERENCES 1 J.J. Christenzen, D.J. Eatough and R.M. Izatt, Chem. Rev., 74 (1974) 351. 2 R.M. Izatt, D.P. Nelson, J.H. Ryttmg, B.L. Haymore and J.J. Chnstenzen, J. Am. Chem. Soc., 93 (1971) 1619.
395
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
18 19
J . J . C h r l s t e n z e n , J . O . Hill a n d R . M . I z a t t , S c i e n c e , 1 7 4 ( 1 9 7 1 ) 4 5 9 . C.J. P e d e r s e n , J. A m . C h e m . S o e . , 8 9 ( 1 9 6 7 ) 7 0 1 7 . C.J. P e d e r s e n , J. A m . C h e m . S o c . , 9 2 ( 1 9 7 0 ) 3 8 6 . D . J . S a m a n d H . E . S l m o n s , J. A m . C h e m . S o c . , 9 4 ( 1 9 7 2 ) 4 0 2 4 . C.L. L l o f f a , H.P. H a r r i s , M. M c D e r m o t t , T o m G o n z a l e z , a n d K. S m i t h , T e t r a h e d r o n L e t t . , ( 1 9 7 4 ) 2 4 1 7 . J . H . F e n d l e r , a n d E.J. F e n d l e r , C a t a l y s i s m Mlcellar a n d M a c r o m o l e c u l a r S y s t e m s , A c a d e m i c Press, New York, 1975. J . M . L e h n , M . R . T r u t e r , W. S i m o n , W.E. M o r f , P . C h . Meier, R . M . I z a t t , D.J. E a t o u g h a n d J . J . C h r i s t e n z e n , S t r u c t . B o n d i n g , B e r h n , 16 ( 1 9 7 3 ) . R . M . I z a t t , J.J. C h r i s t e n s e n , S y n t h e t i c M u l t l d e n t a t e M a c r o c y c h c C o m p o u n d s , A c a d e m i c Press, New York, 1978. I.M. K o l t h o f f , A n a l . C h e m . , 51 ( 1 9 7 9 ) 1 R . G . A . R e c h n i t z a n d E. E y a l , A n a l . C h e m . , 4 4 ( 1 9 7 2 ) 3 7 0 . J. P e t r a n c k a n d O. R y b e , A n a l . C h i m . A c t a . , 72 ( 1 9 7 4 ) 3 7 5 . M. M a s c i n i a n d F. Pallozzi, A n a l . C l n m . A c t a , 7 3 ( 1 9 7 4 ) 3 7 5 . O . R y b a a n d J. P e t r a n e k , J. E l e c t r o a n a l . C h e m . , 4 4 ( 1 9 7 3 ) 4 2 5 . O. R y b a a n d J. P e t r a n e k , J. E l e c t r o a n a l . C h e m . , 6 7 ( 1 9 7 6 ) 3 2 1 . H . K . F r e n s d r o f f , J. A m . C h e m . S o t . , 9 3 ( 1 9 7 1 ) 6 0 0 . He s h o w e d m t h i s p a p e r t h a t f o r t h e silver c o m p l e x e s o f m t r o g e n - a n d s u l f u r - c o n t a i n i n g c y c h c p o l y e t h e r s , t h e r e are c o m e b o n d i n g c h a r a c t c n s t l c s o f c o m p l e x i n g o t h e r t h a n p u r e l y e l e c t r o s t a t i c f o r c e s , e.g., c o v a l e n t b o n d i n g . Stall, t h e b o n d i n g c h a r a c t e r o f silver a n d s t r a i g h t p o l y e t h e r r e m a i n e d t o h e c l a r i f i e d . T h e c o m p l e x stabLhty o f silver a n d c r o w n e t h e r m a y h e i n t e r p r e t e d b y t h e e n t r o p y f a c t o r , see r e f . 9, p. 1 6 8 . A. F u j l s h i m a , E. S u g : y a m a a n d K . H o n d a , Bull. C h e m . S o c . J a p a n , 4 4 ( 1 9 7 1 ) 3 0 4 . T. I n o u e , T. W a t a n a b e , A. F u h s h i m a , K . H o n d a a n d K . K o h a y a k a w a , J. E l e c t r o c h e m . S o c . , 1 2 4 ( 1 9 7 7 ) 719.
391
Preliminary note ELECTROCHEMISTRY IN A LOW DIELECTRIC MEDIUM IN THE PRESENCE OF A CROWN ETHER. SUPPRESSION OF ANODIC DISSOLUTION OF METAL ELECTRODES
SEIICHIRO NAKABAYASHI, AKIRA FUJISHIMA and KENICHI HONDA Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hondo, Bunkyo, Tokyo 113 (Japan) (Received l l t h June 1980)
The macrocyclic polyethers exhibit a pronounced selectivity for the complexation of certain cations [1--3]. They make very stable complexes with some alkali or alkaline earth metal cations [4,5]. The results lead to the conclusion that by the aid of crown ethers, some kinds of salts become soluble in low dielectric solvents such as benzene [3,6,7]. Because of this characteristic, crown ethers consequently have wide application in chemistry [8--11], but electrochemically they have been less used except for ion-selective electrodes [12--16]. In this report, we have f o u n d t h a t in the presence of a crown ether an electrochemical system can be designed in a very low dielectric media such as benzene, and we show here that in this solution, some metal electrode behaviors are very different from those in an ordinarily used electrochemical organic solution. The crown ether used, from a commercial source (Nippon Soda Co., Ltd.) 15-crown-5, is designed by its molecular radius to capture a sodium cation selectively. The dielectric constants of the benzene--(15-crown-15) mixture were measured as a function of composition by the lock-in capacitance bridge method. Sodium tetraphenylborate (NaBph4), whose anion is bulky and whose charge density is extremely low, was used as a supporting electrolyte. The conductivity of the crown--benzene--NaBh4 solution was measured as a function of the concentration relation of NaBph4 by the resistance bridge method. Before electrochemical measurements the solutions were deaerated by the repeated freeze--pump--thaw technique. The potential of the working electrode was controlled potentiostatically against Ag-wire reference electrode. A P t wire was used as a counter electrode. All measurements were conducted at room temperature (20 -+ 3°C). Figure 1 shows the changes of the relative dielectric constant (er) of the crownbenzene solution with respect to its composition. The molar additive nature of er is clear. All of the electrochemical experiments were carried o u t with a crown-benzene mixture with a volumetric ratio between 15-crown-5 and benzene of 2 : 3 (mole ratio about 2 : 7). The relative dielectric constant of that solution was about 5.72. This constant is extremely low when compared with t h a t of a common electrochemical solvent such as acetonitrile (er = 37.5). Figure 2 shows the conductivity change as a function of the concentration of sodium tetraphenylborate added as supporting electrolyte in this mixed solvent. Conductivity K of the mixed solution including 0.1 M sodium tetraphenylborate was 2.3 × 10 -4
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0,25 0 50 0.75 1.00 MOLAR RATIO OF BENZENE
0.05
MOLAR
0.10
0.15
0.20
CONCENTRATION OF NaBph 4
Fig. 1. C h a n g e o f t h e relative dielectric c o n s t a n t as a f u n c t i o n o f t h e m i x i n g r a t i o b e t w e e n 15-crown-5 a n d b e n z e n e . T h e m e a s u r e m e n t was m a d e b y using the c a p a c i t a n c e bridge balancing t e c h n i q u e w i t h a 1 kHz sinusoidal i n p u t . T h e b a l a n c i n g d e t e c t i o n was m a d e b y t h e lockin a m p l i f i e r t e c h n i q u e . T h i s relative dielectric c o n s t a n t m e a s u r e m e n t was carried o u t b y using t h e cell w h i c h was e q u i p p e d w i t h t h e guard electrode. In this Figure, t w o p o i n t s o n t h e side o f a high c o n c e n t r a t i o n o f c r o w n e t h e r were m e a s u r e d b y t w o e l e c t r o d e s s y s t e m w i t h o u t t h e guard electrode. This was t h e r e a s o n w h y these t w o p o i n t s h a d relatively large e r r o r deviation. Fig. 2. C o n d u c t i v i t y c h a n g e as a f u n c t i o n o f t h e c o n c e n t r a t i o n o f s o d i u m t e t r a p h e n y l b o r a t e . T h e s o l v e n t was a m i x t u r e o f 15-crown-5 a n d b e n z e n e , h a v i n g the m o l e r a t i o o f 2 : 7. T h i s m e a s u r e m e n t was carried o u t using t h e c o n d u c t a n c e bridge b a l a n c i n g t e c h n i q u e w i t h a I kHz sinusoidal i n p u t .
~ - ~ cm -1 which would be enough to carry out electrolysis. In the electrolyte solution cyclic voltammograms of 10 -3 M ferrocene were measured with a Pt working electrode. Figure 3 shows the result, which indicates a good reproducibi] ity with an accuracy of -+ 30 inV. Thus, we see that in the low dielectric crown-benzene solution an electrochemical process can be carried out, and that a Ag reference electrode may be considered stable enough for at least qualitative discussion. Figure 4b shows a cyclic voltamogram of a Cu working electrode in a solution containing benzene--15-crown-5 (mole ratio 7 : 2) and 0.1 M sodium tetraphenylborate. For comparison with the usual electrode behavior in a nonaqueous solvent, Fig. 4a shows a cyclic voltammogram of a Cu working electrode in an aceto nitrile solution, in which 0.1 M sodium tetraphenylborate was added as a supporting electrolyte. It is clear that in the former solution the anodic dissolution of the Cu electrode is very much suppressed compared with t h a t in the latter. These results allow us to assume that in the crown--benzene solution only a sodium cat, on is complexed with a 15-crown-5. However, because 15-crown-5 cannot capture the copper cation which is distinguishable from the sodium cation, the selectivity of this crown ether makes the copper electrode insoluble even at such anodic polarization. Figures 5a and b show the similar behavior of a Zn working electrode.
393 I
I
I
|
I
I O 50 V vs Ag
I 0.75
[
30
¢~20
1._l° z u.i IE ~o
-10 I -0.25
I I 0 0.25 POTENTIAL//
I 1.00
Fig. 3. Cyclic v o l t a m m o g r a m of f e r r o c e n e (10 - 3 M ) in 15-crown-5 a n d b e n z e n e m i x t u r e ( m o l e r a t i o 2 : 7 , h a v i n g a relative dielectric c o n s t a n t o f 5 . 7 2 ) w i t h 0.1 M s o d m m t e t r a p h e n y l b o r a t e as s u p p o r t i n g e l e c t r o l y t e . T h e w o r k i n g e l e c t r o d e a n d t h e c o u n t e r e l e c t r o d e were Pt wires, a n d t h e r e f e r e n c e e l e c t r o d e was a Ag wire. (a) I4
(a)
~A
4n, A
•=
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I 0 0.5 POTENTIAL/V vs Ag
Iig IIg
I 1.0
o!s
il '4 I -0.5
,b,/
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(b)
u
!
o 0.5 POTENTIAL/V vs Ag
11o
POTENTIAL/V vs A9
-0.5
I I 0.5 1.0 POTENTIAL/V vs. Ag
I !.5
Fig. 4. Cyclic v o l t a m m o g r a m o f a Cu w o r k i n g e l e c t r o d e w i t h o u t a n y r e d o x reagent. (a) I n t h e a c e t o n i t r i l e s o l u t i o n w i t h 0.1 M s o d i u m t e t r a p h e n y l b o r a t e as s u p p o r t i n g e l e c t r o l y t e . (b) In t h e c r o w n - b e n z e n e - N a B p h 4 s o l u t i o n having t h e s a m e c o m p o s i t i o n c h a r a c t e r as in Fig. 3. I n b o t h cases, t h e c o u n t e r e l e c t r o d e was a Pt wire, a n d t h e r e f e r e n c e e l e c t r o d e was a Ag wire. Fig. 5. Cyclic v o l t a m m o g r a m o f a Z n w o r k i n g e l e c t r o d e w i t h o u t a n y r e d o x r e a g e n t . (a) In the a c e t o n i t r i l e s o l u t i o n . (b) In t h e c r o w n - - b e n z e n e - - N a B p h 4 s o l u t i o n . O t h e r c o n d i t i o n s were t h e same as in Fig. 4.
394 15C
~10C III III U
stirring /*/ the T/ • olution/~ / / not/ .stirrin£ • the
solution
,/
50
/ -I.0
-0.5
-*
~'- - ' ~ . 5 !.O POTENTIAL / V vs Ag
I.S
-50
Fig. 6. Cyclic v o l t a m m o ~ a m o f a A g w o r k i n g e l e c t r o d e in t h e c r o w n - - b e n z e n e - - N a B p h 4 s o l u t i o n w i t h o u t a n y r e d o x r e a g e n t . O t h e r c o n d i t i o n s w e r e t h e s a m e as in Fig. 4.
Contrary to the above results, in the case of a Ag working electrode with the Ag reference electrode, an anodic dissolution current was observed at the potential region more positive than the rest potential 0.0 V of the Ag working electrode. Supposing that the crown ether can interact with a Ag cation n o t only by i o n - dipole interaction but also by a covalent one, a larger stability constant of a Ag + complex than that anticipated w o u l d be given [17]. So, this result may be reflected by the selectivity property of the crown ether. It is an interesting fact that under the cathodic polarization, the current corresponding to the Ag deposi tion was less than that anticipated. Therefore, the cyclic voltammogram might be shown as asymmetric. This p h e n o m e n o n has n o t been clearly explained up to now, b u t we are trying to interpret it through further study, for instance, by using the temperature-controlled rotating ring-disc electrode technique. Though a detailed reaction scheme remains to be clarified, this is the first case in which this type of electrochemistry has been carried o u t in such a low dielectric medium (er = 5.72), and an anodic dissolution of a metal working electrode can be suppressed by the action o f the selective solubilization property of the crown ether mixed solvent. As the crown ether is optically transparent over a wide visible wavelength region [4], this t y p e of technique is expected to be effec tive for the suppression of the self degradation of some n-type semiconductor photoanodes [18,19] for the electrochemical photocell.
REFERENCES 1 J.J. Christenzen, D.J. Eatough and R.M. Izatt, Chem. Rev., 74 (1974) 351. 2 R.M. Izatt, D.P. Nelson, J.H. Ryttmg, B.L. Haymore and J.J. Chnstenzen, J. Am. Chem. Soc., 93 (1971) 1619.
395
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
18 19
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