Anodic voltammetry of some aromatic amines in chloroform

the electro-oxidation of N,N,N', N'-tetramethyl-p-phenylenediamine (TMPD) in acetonitrile (ACN) by chronocoulometry and other electrochemical tech...
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energy level diagram with lines assigned according to energy levels and/or connectivity. UEAITR contained only a stick plot option. GINA contains the usual Calcomp plot option of the full spectrum drawn on a sheet of paper the exact size (50 cm) of the normal NMR spectrum chart paper. In addition, a plot option has been added that allows the user to plot that portion of the spectrum that is on the display screen. This latter plot is 25.9 cm by 25.9 cm. To get a display plot which can be directly overlapped with an experimentally obtained spectrum (Varian NMR instrument), the factor 1.93 (50.0 cml25.88 cm) must be inserted for plotting purposes. The 100-Hz sweep width spectra reproduced in this paper were obtained via the graphic display plot, the 250-Hz sweep width spectral figures via Calcomp plots. While both contain identical information, and the latter are, perhaps, “prettier,” these Calcomp plots require recomputing all the points on the graph, whereas the

display plot uses the points already computed and displayed and is thus a faster plotting routine. The program, including the plotting, requires 48000 words of core to run. The display option, when used, requires 4000-8000 more words of core, depending on how much of the spectrum (how many points of the full curve) is being displayed on the screen. The entire program is written in FORTRAN IV and requests for further information should be addressed to S. R. H. ACKNOWLEDGMENT

We would like to thank H. J. C. Yeh for the 100-MHz spectrum of codeine and J. A. Ferretti and M. McNeel for valuable discussions about the computer program. RECEIVED for review May 2, 1972. Accepted July 31, 1972.

Anodic Voltammetry of Some Aromatic Amines in Chloroform Absorpti,on Spectra and Association Phenomena of the Corresponding Cation Radicals Georges Cauquis and Denis Serve Laboratoire d‘Electrochimie Organique et Analytique du Centre &Etudes Nucliaires de Grenoble, €3. P. 85, 38041 Grenoble-Cedex, France

Two RECENT PAPERS describe the formation of aromatic amine cation radicals in chloroform, the process used being either amine oxidation ( I ) through bromine coulometrically generated in situ from the bromide ions of the electrolyte 0.1M Et4NBr or photooxidation at room temperature (2). The study is devoted to N,N‘-diphenyl p-phenylenediamine (DPPD), N,N’-dimethyl N,N’-diphenyl-p-phenylenediamine (DMDPPD), N,N,N’,N’-tetraphenyl-p-phenylenediamine (TPPD), N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) and 4-hydroxydiphenylamine (HDPA). Cation radicals are characterized only through the position of the higher wave length absorption band. After an independent study of the oxidation of some N phenyl-p-phenylenediamines(3) and diphenylamines (4-6) in acetonitrile 0.1M Et4NC104,we noticed that in the case of DMDPPD . +, TPPD +,and HDPA. + cation radicals, there exists important disagreements between reported maxima of absorption ( I ) and those we observe in solutions resulting from the controlled potential oxidation of these amines in acetonitrile at the level of the first wave of their voltammetric curve (Table I). These discrepancies cannot be explained by either change of the solvent from acetonitrile to chloroform 9

(1) P. Wuelfing, E. A. Fitzgerald, and H. H. Richtol, ANAL. CHEM.,42,299 (1970). ( 2 ) E. A. Fitzgerald, P. Wuelfing, and H. H. Richtol, J . Phys. Cliem.. 75, 2737 (1971). (3) G. Cauquis, H. Delhomme, and D. Serve, Tetrahedron Letf., 1972, 1965. (4) Zbid.,1971, 4113. ( 5 ) G. Cauquis, J. Cognard, and D. Serve, ibid., p 4645. (6) G. Cauquis, H. Delhornme, and D. Serve, BUN.SOC.Chim. Fr.,

in press. 2222

or the intervention of the anion probably associated to the cation radical. It thus seemed interesting to complete our study by a survey of the electrochemical oxidation of the above mentionned amines in chloroform 0.25M n-Bu4NC104. We also consider the case of N-(p-methoxypheny1)-p-phenylenediamine (MPPD) which was not studied in papers ( I , 2) but which enables us to observe a striking radical association phenomenon. EXPERIMENTAL

Prolabo RP chloroform is made free from ethanol by washing with water and subsequent drying on calcium chloride, then on sodium carbonate. After distillation from sodium carboGate, the solvent is collected on molecular sieves Linde 4 A, kept in the dark, and used within three days. Tetrabutylammonium perchlorate, polarographic grade (Southwestern Analytical Chemicals) was dried under vacuum. DPPD and TPPD are Eastman Kodak products recrystallized from benzene. DMDPPD has been prepared in the same way as indicated by the authors in ( 2 ) . TMPD and MPPD are precipitated from the acidic aqueous solution of their respective chlorhydrate (Fluka puriss and Merck for analysis) by adding sodium hydroxide, the last precipitating fraction only being re-collected. Schuchardt HDPA is purified by successive precipitation from a benzene solution by addition of hexane. All products are dried under vacuum. Voltammetric curves are obtained with a Tacussel potentiostat (Model PRT 500). The reference electrode, Agl10-M Ag+ in 0.25M n-BuaNC104,is made by anodic dissolution of a siIver wire and proves to be reproducible in many experiments. The half-wave potential of ferrocene is -0.35 V us. this electrode.

ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972

Absorbance measurements are made with a Beckman DK-2A recording spectrophotometer and I-mm cells which enable us to follow the absorption change during the process of oxidation of the amine performed with the aid of a Tacussel potentiostat (Model ASA 100).

Table I. Electrochemical and Spectral Data for Aromatic Amines and Corresponding Cation Radicals in Chloroform 0.25M n-BudNClOc , , , ,A

RESULTS AND DISCUSSION

To the best of our knowledge, the only papers devoted to the electrochemistry of organic compounds in chloroform are concerned with reductions on mercury electrode (7, 8). On bright platinum electrodes, the electroactivity range of the medium falls between -2.0 V and 1.5 V L'S. AgI10-3MAg+. As in acetonitrile, the different N-phenyl-p-phenylenediamines, at the 5.10-4Mconcentration level, give rise on the platinum rotating disk electrode to a voltammetric curve presenting two well separated waves whose potentials are gathered in the table. The first wave corresponds to a reversible oneelectron transfer as shown through cyclic voltammetry and controlled potential coulometry. In the course of the latter. the exchange of 1 faraday for a mole of amine is observed and the voltammetric curve after electrolysis indicates the formation of cation radicals. These radicals are quite stable at room temperature and the quantitative amine regeneration is observed through the reverse coulometry except in the case of MPPD (see below). This simple transformation is confirmed by the modification of the UV and visible absorption spectrum during the electrolysis (Table I). In fact, in every case, the cation radical formation is characterized by the existence of an isobestic point. The EPR spectra were obtained after a complete electrolysis in chloroform. Though they are not as well resolved as in acetonitrile ( 6 ) , their structure is indicative of coupling with equal nitrogen atoms and there is no appreciable difference in the u,, values between the two solvents. The results reported here indicate that the chloroform 0.25M n-Bu4NC104system is a potentially useful electroanalytical medium with such oxidable products as the Nphenyl-p-phenylenediamines. Nevertheless, two sorts of small limitations can arise independently of its poor electrical properties. The first one is the possible existence of complexes between the more basic compounds and the solvent. In fact, one can observe an inversion of the Ell2values of the first wave of the DMDPPD and DPPD by solvent change from acetonitrile to chloroform. In addition, the observed E112 value for ferrocene in this last medium is 70 mV more positive than the expected value on the basis of TPPD taken as a reference compound. This tertiary amine gives a very stable cation radical and a dication in a great variety of solvents and is more useful than ferrocene for the purpose of comparison of redox scales in media where the ferricinium cation had a poor stability (9). The second limitation arises from the fact that quantitative radical formation cannot be deduced from the ratio R of limiting currents of the cathodic wave of the radical and the corresponding wave of the amine before the electrolysis. This ratio is always smaller than unity even in the case of very stable radicals. This is, for example, the case of TPPD.+ and DMDPPD. whose quantitative formation is indicated by the result of the reverse coulometry and for which R is equal to unity in acetonitrile (3). In chloroform, R is only 0.83 for TPPD.- and 0.60for DMDPPD.+ radicals. +

(7) M. E. Peover, Trans. Furuduy SOC.,60,417 (1964). (8) R. D. Holm, W. R. Carper, and J. A. Blancher, J. Phys. Clzem., 71, 3960 (1967). (9) D. Serve, unpublished work.

Parent amine A TMPD DMDPPD DPPD TPPD HDPA MPPD

Ell2 cs. Agl 10-3MAg-0.64 -0.43 -0.43 -0.25 -0.26 -0.51

nrn for A . in chloroform

The present workc

-O.ll' 262,328,566,615 0.065 317, 356,623 0.11 307,391,714 0.17 256,347,404,853 * 355,62O(sh),690 0.02 280, 361,615(~h), 680,761

+

Amax.

From (I) and (2) 620 675 700 725 450

nm for A,'in CH,CN (3) 606 611 700 825 690 750

Wave with a maximum. Two other waves with poor resolution near -0.10 and 0.20 V. The absorptivities of the cation radicals are very near the values observed in acetonitrile (3) and in every case higher than reported in ( I ) . lL

A value of the ratio R near 0.85 has also been observed after the oxidation of other nitrogen compounds in chloroform 0.25M n-Bu4NC104. This is the case, for example, of 5,10-di-p-anisyl-2,7-dimethoxy-5,lO-dihydrophenazine ( 4 ) and 2,3,4,5-tetra-p-anisylpyrrole(9) which also give stable cation radicals in this medium. Such a value of R suggests that the diffusion coefficient of oxidized species and of reduced species are in a ratio near 0.80. This result is probably due to formation of low mobility ion pairs between the cation radicals and the anions Clod- in great excess in the low dielectric solvent '(ECHCl, = 4.8). It is possible that smaller values of R , as that of DMDPPD.' or that of MPPD.+ which is equal to 0.55:are .the result of another type of association, namely, that of two or more cation radicals in aggregates bonded with CIOc;anions. This type of radical association, already observed :for many cation radicals ( I O ) , is apparent in the case of MPPD.+ at room temperature. The value of the ratio R decreases when the initial concentration of MPPD increases, :and one can observe a modification of the morphology of the broad absorption band of the radical cation compared with its spectrum in acetonitrile. In this latter solvent, the absorptivity at 606 nm is greater than the one at 750 nm but in chloroform media the higher value is at 680 nm with only a :;boulder near 615 nm. These associations in chloroform are perhaps the source of the particular stability of the studied cation radicals and this fact explains that a species such as MPPD +,for example, can be obtained in this medium with a high yield. This is equal to 92 in CHC13, as indicated by the result of reverse coulometry, whereas in acetonitrile 0.1M Et4NC104,it is difficult to obtain a value higher than 70 %. Let us come back to the observed discrepancies between spectroscopic results in the literature ( I , 2 ) and ours in acetonitrile medium. It first seems that they could originate from the interactions that have just been described. However our own results in chloroform (Table I) suggest that previous workers ( I , 2 ) have not observed the spectrum of the primary cation radicals, at least in the case of the p-phenylenediamines DMDPPD and TPPD and that of the diphenyla(10) H. N. Blount and T. Kuwana, J . Amer. Chem. SOC.,92, 5773 (1970).

ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972

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mine HDPA. Possibly, the spectra obtained by these authors belong to secondary species resulting from a substitution of bromine in the case of electrochemical oxidation ( I ) and secondary reactions with chloroform degradation products in the case of photo-oxidation ( 2 ) . It could also be that the electrochemical oxidation went to the dication formation favored by the presence of bromide ions. In such conditions, these ions acting as nucleophiles can promote brominated amine derivatives which lead themselves to the corresponding cation radicals. A preliminary experiment showed that in fact DMDPPD oxidation in acetonitrile and in the presence of bromide ions gives rise to a new species absorbing at 675 nm, which is just the wavelength attributed to DMDPPD * + in chloroform 0.1M Et4NBr. In the latter medium, the anodic limit of which begins at -0.30 V us. AgllOdaMAg+; anodic oxidation of DMDPPD at the controlled potential of -0.35 V allows the formation of the true cation radical whose maximum absorption appears at 630 nm. This value is very near the one observed in perchlorate medium (Table I) and this result supports the fact that the nature of the anion associated with the cation radical has only a small influence on the wavelength maximum of this species. The true nature of the 450 nm absorption band attributed to HDPA . + can be indicated with still better precision. In chloroform as well as in acetonitrile ( 6 ) , a solution 10-3M in HDPA gives a complex voltammetric curve whose aspect is sensitive to residual water concentration, state of purity of the solvent, and the nature of the electrode. In chloroform 0.25M n-Bu4NC104the first wave appearing on bright platinum is the more intense one, but does not correspond to a single one-electron transfer. At the beginning of the controlled potential electrolysis at -0.10 V there appears a yellow color, Amax = 450 nm, which must be attributed to the N-phenyl-p-benzoquinone imine resulting from loss of two electrons and two protons in the relatively basic medium which is a solution of HDPA. It seems that this is the spectrum observed in ( I ) . The N-phenyl-p-benzoquinoneimine may be obtained with a nearly quantitative yield through a complete oxidation at 0.5 V with exchange of two faradays for one amine mole. The quinoneimine is then obtained as the monoprotonated form [C6H5-NH=C6H1= 0]+(in,,, = 269 and 406 nm) but the addition of a base such as acetate ion = 264, 291, and 452 nm) produces the neutral form (A, identified with the corresponding spectrum of an authentic sample prepared following (11). 11) I. Bhatnagar and M. V. George, J. Org. Clzcm., 33,2407 (1968).

2224

0

The HDPA radical may nevertheless be observed in chloroform through partial oxidation of the amine solution at 0.1 V that is interrupted after one faraday per mole. The absorption spectrum then possesses the bands of the partially protonated amine, the 406 nm band and two new bands at 355 and 690 nm which belong to the species giving an EPR spectrum whose structure is similar to the one observed on a = solution of the same radical in trifluoroacetic acid (A, 688 nm) (6). The absorption at 690 nm must be related to the band at 680-690 nm attributed to diphenylamine cation radical (12, 13) and to the fact that other mono and disubstituted diphenylamine cation radicals that we studied all give wavelength maximum between 700 and 770 nm ( 4 , 6 ) . The mechanism of formation of cation radicals proposed to explain amine oxidation by constant current electrolysis in chloroform 0.1M Et,NBr calls for some comments. Even though the authors do not indicate the amine concentration ( I ) , one may think that it is very low as compared to that of the bromide ion in order to study radical formation through absorption spectrometry. I n such conditions, it seems reasonable to accept, as suggested (I), that most of the amine is oxidized by mean of a species coming from the oxidation of Br- ions though these anions are electroactive at a potential higher than the amine. This species is probably Br3- and not bromine. In fact, Br- ion at the 10-3M level in chloroform 0.25M n-Bu4NC10, gives two anodic waves at -0.06 and 0.21 V corresponding, respectively, to the exchange of 'laand IfJof a faraday per gram-ion. During an oxidation at 0.40 V, there appears an absorption band at 273 nm, whose intensity is maximum when * / 3 faraday have been consumed, which must be attributed to the complex Brd- (u = 4.85 x 10' I. M-1 cm-1) which is only very slightly dissociated. When partially oxidizing a solution 0.1M in bromide at constant current, it is possible that the authors formed the Bra- ion which oxidizes the amine. In any case, the previously mentioned remarks about the role of bromide ion during the process lead us to think that the best way of obtaining an aromatic amine cation radical remains the direct anodic oxidation at controlled potential in a weakly nucleophilic medium. 9

+

RECEIVED for review Feburary 24, 1972. Accepted July 3, 1972. (12) V. E. Kholmogorov and E. V. Baranov, Opt. Specfrosc., 14, 440 (1963). (13) 0. D. Dmitrievskii, ibid., 19, 460 (1965).

ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972