ELECTROCHEMICALLY

GENERATED

FREE

RADICALS

COMMUNICATION 8. EPR SPECTRA OF ANION RADICALS OF CERTAIN ETHYLENIC DICARBONYL COMPOUNDS A. N. B.

V. N. V.

II'yasov, Sotnikova, Mel'nikov,

Y u . M. K a r g i u , V. Z. K o n d r a n i n a , a n d A. A. V a f i n a

UDC 541.124:541.138.3:541.515:547.442.8

In the e l e c t r o c h e m i c a l reduction of dialkyl f u m a r a t e s and m a l e a t e s in dimethyl sulfoxide, the f o r m a tion of free radicals has been noted by the EPR method [1]. The EPR s p e c t r u m that appears in the first m o m e n t and rapidly d i s a p p e a r s has been a s c r i b e d to a mixture of s h o r t - l i v e d c i s - and t r a n s - a n i o n r a d i c a l s with a concentration ratio ~ 1 : 3. Then a s p e c t r u m consisting of (2 • 2 • 4) • 3 lines was observed, which was assigned [1] to the m o r e stable dianion radical, f o r m e d on account of the elimination of an alkyl cation. The m e c h a n i s m of the e l e c t r o c h e m i c a l reduction and the conditions of generation of r a d i c a l s are not cited in [1]. In this work, on the basis of a study of the m e c h a n i s m and peculiarities of the e l e c t r o c h e m i c a l r e d u c tion of ethylenie dicarbonyl compounds in d i m e t h y l f o r m a m i d e (DMFA) [2], we attempted to r e c o r d and study the E PR s p e c t r a of the p a r a m a g n e t i c products formed. Considering the complex nature of the f i r s t step of the reduction of ethylenic dicarbonyl compounds, we paid special attention to control of the electrode potential and the creation of conditions for the r e c o r d i n g of the EPR s p e c t r a of s h o r t - l i v e d free radicals. EXPERIMENTA

Fig. 1. Mierocell for the e l e c t r o c h e m i c a l generation of free radicals; 1, 2) m e r c u r y electrode; 3) s i l v e r wire; 4) platinum electrode; 5) outlet to vacuum pump.

L

The synthesis and c h a r a c t e r i s t i c s of the compounds studied, purification and quality control of DMFA were d e s c r i b e d e a r l i e r [3]. Solutions in DMFA with (5-8) 9 10 -3 M d e p o l a r i z e r and 8 : 10-2 M Et4NI were studied. To r e c o r d the cyclic p o l a r o g r a m s we used an L P - 6 0 polarograph with a s t a t i o n a r y m e r cury electrode [4]. The EPR s p e c t r a were r e c o r d e d on a R~-1301 r a d i o s p e c t r o m e t e r with a frequency of 9320 MHz. In the case of e l e c t r o c h e m i c a l generation of unstable anion radicals with a s m a l l poteatiaI difference between s u c c e s s i v e stages of charge t r a n s f e r , the use of two-electrode ceils and the previously proposed t h r e e - e l e c t r o d e cell was difficult [5]. The vacuum m i c r o cell with a t h r e e - e l e c t r o d e s y s t e m that we have designed (Fig. 1) permitted e l e c t r o c h e m i c a l generation of free radicals directly in the r e s o n a t o r of the s p e c t r o m e t e r with simultaneous monitoring of the electrode potential. The r e f e r e n c e electrode was the a m a l g a m a t e d end of a s i l v e r wire, situated close to the m e r c u r y electrode (1-2 ram). The a c c u r a c y of the m e a s u r e m e n t of the electrode potential during e l e c t r o l y s i s was no p o o r e r than 9 0.05 V. Reaching the potentials of the limiting c u r r e n t r e q u i r e d an external voltage of ~.10-40 V, depending on the concentration of the depolarizer. Diffusion p r o c e s s e s e n c o m passed a small volume of the solution between the cathode and the r e f e r e n c e electrode, which p e r m i t t e d complete and rapid reduction of the d e p o l a r i z e r and avoidance of broadening of the lines of the hyperfine s t r u c t u r e (ttFS) on account of exchange interactions between the anions radicals and the initial molecules.

A. E. Arbuzov Ins~ilute of Organic and Physical Chemistry, Academy of Sciences of the USSR. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 932-936, May, 1971. Original article submitted July 21, 1969. O 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 V/eel 17th Street, New York~ N. Y. 10011_ All rights reserved. This article cannot be reproduced /or any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00. ~J

855

Thus, the cell designed satisfies all the basic r e q u i r e ments set for cells for e l e c t r o c h e m i c a l generation of free radicals for their investigation by the EFR method [51.

a

o

0,2

44

46

o,8

1,o

(-E,V)

Fig. 2. Cyclic p o l a r o g r a m s at 25~ a) t r a n s dibenzoylethylene; b) dimethyl fumarate; c) dimethyl maleate; d) diphenyl fumarate. DISCUSSION

The EPR s p e c t r a of p r i m a r y anion r a d i c a l s * of the ethylenic dicarboayl compounds studied could be r e c o r d e d only during reduction in the interval of potentials of the limiting c u r r e n t of the first wave. At other potentials the EPR spectra could not be r e c o r d e d on account of the insufficient rate of formation of unstable anion radicals o r subsequent chemical t r a n s f o r m a t i o n s . OF RESULTS

F o r a qualitative evaluation of the stability of anion radicals in a time c o m m e n s u r a t e with the d u r a tion of e l e c t r o l y s i s in the m i c r o c e l l and r e c o r d i n g of the EPR spectra, we used the method of cyclic p o l a r o graphy with slow polarization [5] (Fig. 2). The anion radical of trans-dibenzoylethylene proved to be the most stable. Even at r o o m t e m p e r a t u r e , an anodic peak equal in height to the cathodic peak is distinctly visible. The anion radical of dimethyl f u m a r a t e is considerably less stable: at 25 ~ the anodic peak is weakly e x p r e s s e d . Lowering the t e m p e r a t u r e reduces the rate of the subsequent chemical reaction and i n c r e a s e s the stability of the anion radicals. The anion radical of diphenyl fumarate is even less stable, and the anodic peak of its oxidation cannot be observed even at reduced t e m p e r a t u r e . F o r c i s - d i c a r b o n y l c o m pounds (dimethyl maleate and dibenzoylethylene), only a peak c o r r e s p o n d i n g to the oxidation of the t r a n s anion radical is detected on cyclic p o l a r o g r a m s , which a g r e e s with the results of the switched method [2]. In all c a s e s , no other peaks appear on the cyclic p o l a r o g r a m s , which might c o r r e s p o n d to c o m p a r a b l e concentrations of s e c o n d a r y p a r a m a g n e t i c p a r t i c l e s . F r o m the aforementioned it can be concluded that the EPR s p e c t r a obtained in e l e c t r o l y s i s at the f i r s t reduction wave of the t r a n s - d i c a r b o n y l compounds studied belong to the p r i m a r y products of o n e - e l e c t r o n reduction. F o r c i s - c o m p o u n d s we should expect a rapid t r a n s - i s o m e r i z a t i o n of the p r i m a r y c i s - a a i o n radical. In the e l e c t r o c h e m i c a l reduction of dimethyl e s t e r s of fumaric and maleic acids at the potentials of the limiting c u r r e n t of the f i r s t wave, identical E P R s p e c t r a with g = 2.033 are observed. The coincidence of these spectra at various t e m p e r a t u r e s confirms the conclusion of a rapid and p r a c t i c a l l y complete t r a n s i s o m e r i z a t i o n of the c i s - a n i o n radicals [2]. At the same time, it is not v e r y probable that the EPR spectra of c i s - and t r a n s - a n i o n radicals were entirely identical. The basic triplet of the EPR s p e c t r u m (Fig. 3) with line intensity ratio 1 : 2 : 1 and HFS constant a H = 6,6 Oe is due to the interaction of the unpaired elect r o n with two equivalent protons. The splitting of the lines of the triplet into seven hyperfine components with intensities 1 : 6 : 1 5 : 2 0 : 1 5 : 6 : 1 and constant a H3 1.2 O e i s due to the six equivalent protons of the methyl groups. Prolonged e l e c t r o l y s i s of solutions of dimethyl fumarate and dimethyl maleate at the potentials of the limiting c u r r e n t of the first wave leads to a further splitting of 3 x 7 lines of the hyperfine s t r u c t u r e , which b e c o m e s m o r e distinct with i n c r e a s i n g time of e l e c t r o l y s i s (Fig. 3c). We believe that this splitting is caused by p a r a m a g n e t i c p r o c e s s e s , which appear as a r e s u l t of subsequent conversions of the anion radicals. It s e e m s most probable that such impurities are formed by stripping of a methoxy anion, followed by dimerization, electron t r a n s f e r , and the formation of [CHaOOC--CH----CH--C--C--CH~CH--COOCHs] ~" II II 0 0 The E P R spectrum of the anion radical of dimethyl fumarate is also complicated by the contribution of the aaisotropic HFS, which is manifested even at r o o m temperature. The H F S of the anion radicals of

dibenzoylethylenes, d e s c r i b e d below, is almost entirely isotropic. The i n c r e a s e in the time of c o r r e l a t i o n of the brownian motion of the anion radical of dimethyl fumarate is evidently due to the influence of p o l y m e r products, formed in the interaction of the initial molecules with anion radicals [2]. In the case of prolonged e l e c t r o l y s i s of solutions of dimethyl e s t e r s of maleic and fumaric acids at the potentials of the third wave, an EPR s p e c t r u m was r e c o r d e d (Fig. 4) with g = 2.0089, consisting of 2 • 2 • 4 lines of the HFS. The o b s e r v a b l e hyperfine s t r u c t u r e can be a s c r i b e d to the interaction of an unpaired *The peculiarities of the c i s - i s o m e r s a r e d i s c u s s e d below.

856

20e

F i g . 3. EPR s p e c t r a of anion r a d i c a l s p r o d u c e d by r e d u c t i o n : a) d i m e t h y i f u m a r a t e at E = - 1 . 0 to - 1 . 3 V; b) d i m e t h y i m a l e a t e at E = - 1 . 1 to - 1 . 2 V; e) change in the s p e c t r a a and b during p r o longed e l e c t r o l y s i s (-20~ 4.10e

<_-._

I

Fig. 4. E PH s p e c t r u m of the f r e e r a d i c a l p r o d u c e d by the r e d u c t i o n of d i m e t h y l fumar a t e at the p o t e n t i a l s of the t h i r d wave (-20~

2oe

Fig. 5. EPR s p e c t r u m of the anion r a d i c a l of t r a n s d i b e n z o y l e t h y l e n e (-20~

e l e c t r o n with two nonequivalent (a H = 19.5, a H = 6 . 5 0 e ) and t h r e e equivalent p r o t o n s (a HH oo : 2 2 0 e ) . In [1] such a s p e c t r u m was a s s i g n e d to the dianion r a d i c a l f o r m e d on account of s t r i p p i n g of a m e t h y l cation. However, the m e c h a n i s m of this r e a c t i o n is not e x p l a i n e d by the author, and it s e e m s r a t h e r inaprobable to us. More l i k e l y the EPR s p e c t r u m (see Fig. 4), belongs to the f r e e r a d i c a l f o r m e d in the t r a n s f e r of an e l e c t r o n to the r e a c t i o n p r o d u c t of the dianion with a n e u t r a l m o l e c u l e [2] of the type :~

[

o-

4"-

U.!
857

The compounds c o n s i d e r e d are a graphic illustration of the necessity of careful control of the conditions of e l e c t r o c h e m i c a l generation of free radicals and -~consideration of the peculiarities of the e l e c t r o chemical p r o c e s s . CONCLUSIONS i. A vacuum microcell with a three-electrode system for the electrochemical free radicals in the resonator of the spectrometer was proposed.

generation

of unstable

2. Anion radicals of dimethyl esters of maleic and fumaric acids, cis- and traus-dibenzoylethylene were produced, and their EPR spectra were interpreted. The rapid and practically complete trans-isomerization of the cis-anion radicals was demonstrated. 3. Electrolysis at the potentials of the steps of reduction following the first leads to the formation of free radicals that are conversion products of the primary anion radicals. The structure of these products is discussed. LITERATURE 1.

2. 3. 4. 5. 6.

858

CITED

S. F. Nelsen, T e t r a h e d r o n L e t t e r s , 3795 (1967). A. V. II'yasov, Yu. M. Kargin, and V. Z. Kondranina, Izv. Akad. Nauk SSSR, Ser. Khim., 927 (1971). A. V. II'yasov, Yu. M. Eargin, Ya. L. Levin, I. D. Morozova, N. N. Sotnikova, V. Kh. Ivanova, and R. T. Satin, Izv. Akad. Nauk SSSR, Ser. Khim., 736 (1968). Yu. M. Kargin and E. A. Berdnikov, Zavod. Lab., 26, 1078 (1960). A. V. II'yas0v, Yu. M. Kargin, Ya. A. Levin, I. D. Morozova, V. Kh. Ivanova, and N. I. Bessolitsyna, Izv. Akad. Nauk SSSR, Ser. Khim., 740 (1968). V. Kh. Ivanova, A. V. II'yasov, Yu. M. Kargin, Ya. A. Levin, and I. D. Morozova, in: Materials of Reports at the Scientific Conference (Yearbook) [in Russian], Izd-vo IOFKh im. A. E. Arbuzov, AN SSSR, Kazan' (1969).