Voltammetric studies on substituted 2-arylazoanthraquinones in non-aqueous medium

Electrochimica Acta, Vol. 36, No. S/6, pp. 921-933, 1991 Printed in Great Britain.

0013-4686/91 $3.00 + 0.00

0 1991.Pergamon Press pk.

VOLTAMMETRIC STUDIES ON SUBSTITUTED 2-ARYLAZOANTHRAQUINONES IN NON-AQUEOUS MEDIUM G. M. ABOU-ELENIEN,* N. A. Isr+4.4ILtand A. A. EL-MAGHRABY Chemistry Department, Faculty of Science, Cairo University, Giza, A.R. Egypt (Received 4 June 1990; in revised form 30 July 1990) Abstract-Substituted 2-arylazo-anthraquinones are electrochemically studied in benzonitrile at an rak These compounds are oxidized in a single quasi-reversible two-electron wave or in a two quasi-reversible one-electron processes depending on the nature of substituent, leading to the formation of the dication followed by proton removal to form the monocation which can be stabilized through its combination with the perchlorate anion from the supporting electrolyte. The reduction process occurs in three successive electron transfer processes. The coulometric measurements indicated two-electrons for the first step and one-electron for each of the second and third steps. The azo moiety is reduced in the first step to the dianion which accepts two protons from the solvent to give the corresponding hydrazo. The second and third electron transfers were assigned to the reduction of the quinone moiety to form the corresponding radical-anion and dianion. Key words: 2-arylazoanthraquinones, redox characteristics, dc-voltammetry, coulometry, controlled potential electrolysis, non-aqueous medium.

cyclic voltammetry

(cr),

EXPERIMENTAL

INTRODUCTION Reagents

A great variety of naphthoquinones are found in nature. They have several interesting applications in medicine as antibiotics[l], as vitamins[2] and also as penicilliopsin[3]. In spite of the great importance of such a class of compounds, little attention has been paid to their electrochemical behaviour. However, many recent attempts were made to investigate the electrochemical reduction of similar systems using polarographic techniques for some aminated naphthoquinone[4]. Also, dc polarography and cyclic voltammetric studies on strongly chelating anthraquinone derivatives[S, 61 in protic medium were conducted. On the other hand, the elucidation of electrode processes of the electroreduction of aromatic azo compounds at the dropping mercury electrode (dme) in aqueous media have been extensively studied[7-161. Recently attempts to study the electroreduction of aromatic azo moiety in proton free benzonitrile at the rotating disc electrode (rde) were detected in the literature[l7, 181. Unfortunately, it is rare to find new attempts for studying azo-quinones on solid electrodes, especially in non-aqueous systems, which is the aim of the present work. In the following paper it was thought worthwhile to investigate the electrochemical redox characteristics of anthraquinone (I) and 2-arylazo-anthraquinone (IIa-f) in benzonitrile using dc, cyclic voltammetry (co), coulometry and controlled potential electrolysis at a platinum electrode.

Benzonitrile (BN) (Rotitainerm, F.R.G.) was purified by first distillation under vacuum from orthophosphoric acid, followed by repeated distillations from phosphorous pentoxide and final fractional distillation from potassium metal. The supporting electrolyte, t&a-n -butyl-ammonium pet-chlorate (TBAP) (Fluka, Switzerland), was recrystallized several times from ethanol/water (9: 1 v/v) mixture and dried in vacuum before use. Apparatus and instruments

The voltammetric measurements on the following apparatus: potential source

&p

were carried out

potential Scan Generator VSG 72;

eN=NRq3 II

I Hl

B2

H

H

H CH3 CH3 H

t OH

H

-L H OH

*To whom all correspondence should be addressed. TPresent address: Chemistry Department, Faculty of Science, Zagazig University, Egypt. 921

Cl H

- R3

%

OH

H

OH OH

H H

t-l

CH3 H

OH H

Cl

G. M. ABOU-ELENIENet al.

928

potentiostate

X-Y recorder digital multimeter coulometer

potential control amplifier PCA 72C (Bank Electronic, Giittingen, F.R.G.); Servogor XY (Metrowatt, Nurnberg, F.R.G.); T 2201 (Hartman & Braum, Frankfurt, F.R.G.); (Bank Electronic, Giittingen, F.R.G.).

The rotating platinum disc electrode (rde) was connected to a rotating motor (750 rpm) (Metrohm, Switzerland). Melting points are uncorrected. The ir spectra (potassium bromide) were recorded on a Pye Unicam SP-1100 spectrophotometer, I H NMR spectra were measured in DMSO-dB on a Varian EM-360 90 MHz spectrometer, using TMS as internal standard and chemical shifts are expressed in (a ppm) units, elementary analysis were performed by the Microanalytical Centre, Cairo University. Measurements

The electrochemical measurements were carried out under a purified nitrogen stream in dry benzonitrile (BN) containing 0.1 M tetra-n -butyl-ammonium perchlorate (TBAP) as supporting electrolyte. The voltammetric study involved the employment of dc voltammetry at the rde, and cu at the stationary platinum disc electrode. The number of electrons participating in each electrode process was obtained using controlled potential coulometry (cpc) in the same electrolyte at a platinum sheet electrode. The electrode potentials are expressed us the saturated Ag/AgCl/Cl- (BN) electrode, which was from time to time calibrated against the redox potential of the cobaltocinium/cobaltocen system[l9]. The standard potential of the Ag/AgCl/Cl- (BN) electrode against the normal hydrogen electrode (he) is - 176 mV[20,21]. Controlled potential electrolysis (CPE) cpe Experiments were carried out under a purified N, atmosphere at a Pt net electrode in both dry benzonitrile and acetonitrile containing 0.1 mol dme3 lithium perchlorate as supporting electrolyte. The potential was controlled at -0.6 V and +2.3 V us Ag/AgCl/Cl- (BN) for both reduction (first reduction wave) and oxidation, respectively (ie on the limiting current plateau of redox waves of IIa taken as a typical representative example of the series studied [IIa-f]). The mixture was stirred with a magnetic stirrer. The progress of electrolysis in both directions was followed by recording periodically the decrease in current with time. In each case, after complete electrolysis, the cell was disconnected from the circuit and the solvent was evaporated in uacuo. The residue was shaken with dry ether and the supporting electrolyte was filtered off. The ethereal layer was in turn evaporated and the crude residue obtained was crystallized from absolute ethanol. Reduction product of IIa. This compound was identified as compound III. Analytically calculated/%: C-72.73; H4.24; N-8.48. Found/%: C-72.51; H4.32; N-8.23. ir In KBr v/cm-‘:

3210-3330 (two NH); 3500 (OH); 1690, 1680 (two C = 0). uu/nm: A,,, 220 (C = 0); ‘H NMR/G ppm: 6.9-7.8 (m. llH, aromatic protons); 11.5, 10.8, 9.5 (3s, 3H, OH, 2NH, exchangeable with D,O). Oxidation product of Ila. This compound was identified as compound IV. Analytically calculated/%: C-56.27; H-2.58; Cl-8.32; N-6.57. Found/%: C-56.09; H-2.62; Cl-8.24; N-6.49. ir/cm-‘: 1710, 1670 (three C =O), 1690, 163O(C = N), 1120 (ClO;). uv/nm: 1,, 360 (-N = N-); 220 (C = 0); ’ H NMR/G ppm DMSOd,, 7.1-8.0 (m, 1lH, aromatic protons). RESULTS

AND DISCUSSION

The electrochemical redox data of compounds I and IIa-f are listed in Table 1, whereas the pertinent cyclic voltammetric parameters are given in Table 2. Figures la and 2a represent the dc-voltammograms of compounds I and IIa, while Figs lb, 2b and c show the corresponding reduction and oxidation cyclic voltammograms, respectively. Anthraquinone (compound I) undergoes two quasi-reversible oneelectron reduction processes; but, the electron transfer is not very fast so that values of AEp/n are higher than 60 mV. These two processes may be assigned to the reduction of the quinone moiety to form the corresponding radical anion and the dianion where the negative charges are likely to be located on the oxygen atoms. This assignment seems to be compatible with the previously reported mechanisms on related compounds[22,23]. As shown from the cyclic voltammograms, compound I gave a stable monoand dianion during the reduction processes, which is also confirmed by the cyclic voltammetric parameters presented in Table 2. On the dc-voltammograms (Fig. 2a) of 2-aryl-azoanthraquinones (IIa-f) three successive reduction waves are displayed. These are referred to as RI, R,, and R,,,, starting from the less negative potential side. Coulometric measurements indicate that the first wave (R,) corresponds to a twoelectron uptake while the second and third waves (R,, and R,,,) each correspond to a one-electron process. Both the data of logarithmic analysis[24] and the E3,4 -_I?,,, values (Table 1) indicate that all the three reduction waves are quasi-reversible in nature. The cyclic voltammograms (Fig. 2b) showed no reverse peak corresponding to the first reduction process. Consequently, one can conclude that the first reduction step of these compounds proceeds according to the well known EC_mechanism[25]. Also, the cyclic voltammograms exhibited a small peak current corresponding to the first reduction step. Accordingly, it can be concluded that the product of this first reduction step undergoes a very rapid follow-up chemical reaction. For the understanding of the course of electroreduction processes of 2-arylazo-anthraquinones IIa-f, it was necessary to assign first the waves observed to the various electroactive centers in the molecule. Comparison of the half-wave potentials and peak shapes of arylazo-anthraquinones (IIa-f) with those of anthraquinone I revealed the identity of the second and third reduction steps of IIa-f; and the two one-electron reduction processes of I. Based on such

105 130

145 150 155

155 160

160 160

160

30 40 50

60 70

80 90

100

I”,&

165

160 165

140 150

105 120 130

80 95

I;/$

0.11 0.13 0.15 0.16 0.16 0.17 0.17 0.17 0.17 0.17

80 85 85 85 85 90 90 90 100 100

I+

R,,

AEJmV

90 90 90 95 95 95 100 100 105 105

S

0.55 0.54 0.56 0.54 0.53 0.53 0.53 0.53 0.52 0.52

Ii/I;

0.725

0.755 0.755

0.755 0.745

0.788 0.737 0.753

R,, S 131.93 207.86 100.2 114.59 90.79 81.23 158.31

-850 -365 -390 -400 -410 -240 -250

E,,JmV 140 200 110 170 90 130 110

E3,4- El,4

R* S 131.53 188.54 97.08 154.24 84.5 110.54 95.60

2130 1730 1760 1790 1895 2030

E,,JmV

R,,,

I%,

70 70 80 80 90 90 95 95 100 100

AEJmV

265

260 265

140 250

190 200 230

150 160

0.15 0.21 0.26 0.25 0.24 0.22 0.23 0.22 0.25 0.25

I;/$

I%,

350

320 340

295 310

230 245 260

220 205

80 80 85 85 90 95 95 95 100 105

AEJmV

I%

AE,/mV

Compound IIe

0.13

0.14 0.13

0.15 0.14

0.20 0.19 0.19

0.06 0.14

I; /I;

Compound IIa

0.62 0.57 0.58 0.59 0.55 0.55 0.55 0.55 0.58 0.58

I”,&

0.54

0.55 0.54

0.56 0.57

0.59 0.59 0.58

0.61 0.56

Ii/I;

I%,

90 90 95 95 100 105 110 113 120 125

AE,/mV

95

95 95

90 90

80 80 80

75 80

AE,/mV

I%,,

0.13 0.17 0.16 0.16 0.15 0.14 0.13 0.13 0.13 0.12

q/I;

0,

I%,

100

95 100

90 95

80 80 90

60 70

I+, AE, /mV 85 85 85 90 95 100 105 110 120 120

240 180 130 150 150 100

0.63 0.56 0.53 0.52 0.52 0.52 0.52 0.52 0.51 0.51

q/I’,

0.63

0.62 0.62

0.62 0.60

0.59 0.59 0.60

0.55 0.56

1; /I;

E,,4 - Elj4

AE,/mV

Compound IIf

0.24

0.23 0.23

0.22 0.22

0.22 0.22 0.22

0.16 0.20

Ii/I;

Compound IIb

Table 2. Cyclic voltammetric data of compounds I and IIa-f

155 220 100 80 100 80 170

Es,, - Ella

AEJmV

- 1670 -880 -740 -680 -690 -635 -565

E&mV

0.816 0.782

$/I’,

207.86 87.51 183.55 122.98 64.95 153.86

AEJmV

Compound IId

0.465

0.495 0.458

0.469 0.477

0.517 0.485 0.493

0.462 0.520

AE,/mV

I%,

AEJmV

10 20

I%,

VI mV s-’

S = dE/d (log!+

IId IIe IIf

220 70 200 60 50 135

EJla- EnId

Compound I

-1375 -1365 1420 -1360 -1215 - 1080

I IIa IIb

IIC

E&mV

Compound

I&,,

Table 1. de-Voltammetric data of compounds I and Ha-f in benzonitrile

S

I%,,

95

95 95

90 90

85 85 85

80 85

140 140 180 -

Ejj4 - El14

0.25

0.24 0.24

0.23 0.22

0.24 0.23 0.21

0.16 0.24

$/I’,

95

95 95

85 90

75 80 80

75 75

S

0.59

0.58 0.59

0.56 0.58

0.58 0.56 0.56

0.50 0.54

I;&,

133.05 146.63 170.57 -

I% AEJmV

Compound IIc

2235 2240 2200 -

E&mV

AE,/mV

97.79 190.48 229.30 160.26 143.06 99.95

01,

g

!

H

$ 6

;

g $ 5’

s F 3 3 % a. 2

G.

930

M. ABOU-ELENIENet al.

identity it was concluded that the second and third reduction processes of 2-arylazo-anthraquinones correspond to the reduction of the quinone moiety to form the radical anion and the dianion. Consequently, the first two-electron reaction step of these compounds should thus correspond to the reduction of the azo group. The reduction of the azo-group of compounds IIa-f, proceeds via a one irreversible two-electron process to form the corresponding dianion, which undergoes a very rapid follow-up chemical reaction[26] accepting two protons from the solvent to form a stable hydrazine derivative as a final product[ 15-l 8,27-321. The disappearance of the azo group and the appearance of the hydrazino group was confirmed by spectroscopic measurements before and after cpe of compound IIa at the potential corresponding to the first reduction wave. In addition, the formation of the hydrazino derivative was proved by carrying out a standard spot test[33]. Based on the above assignments and observations, the electrochemical reduction mechanism of compounds IIa-f is thought to be explained by the sequence of the reactions presented in Scheme 1. For benzenoid molecules bearing substituents in the meta- or para-position relative to an electroactive group, it has been shown that[34]

where pI is the reaction constant characterizing the susceptibility of the electroactive group to the effect of substituents and cr, is the Hammet substituent constant. The validity of this equation involves the condition that the values of a, (transition coefficient) must remain practically constant throughout the studied reaction series[35]. Therefore, it seemed of interest to study the effect of substituents on the redox processes in order to gain further insight into the proposed redox mechanism of 2-arylazoanthraquinones IIa-f. The values of the transition coefficient (a,) of these compounds were computed 0

from the slopes of logarithmic analysis and were found to have constant values which are fairly reasonable for studying the structure-free energy relationships. As can be seen from the E,,, values (Table I), the nature of substituents affects the first reduction process but has no significant effect on the two more negative processes. Such a fact seems to be compatible with our previous conclusion. The electroactive group (the azo group) is reduced firstly in the a-position, where the electroactive site reduced in the two more negative steps (quinone), is in the [-position with respect to the phenyl group bearing the substituents.

As expected from the electron-donating properties of the methyl group, the methyl derivatives (IIb, IIc and IId) are more difficult to reduce than the parent compound IIa. On the other hand, the chloroderivatives (IIe and IIf) are more easily reduced. As the azo group has an electron-withdrawing character, its reduction would be facilitated when it is in conjugation with the electron-withdrawing chlorine atom. On the other hand, the electron-donating substituents interact with such reducible groups in such a way as to stabilize the reactant and render it less reactive. The E,,, values of the first reduction step of the isomeric 2-aryl-azoanthraquinone (IIb, IIc and IId or IIe and IIf) indicate also that the relative positions of substituents affect such reduction step only slightly. Accordingly, neither the steric hindrance nor the hydrogen bonding of o -substituents has an important role in the reduction of the azo function. The oxidation of the investigated compounds has been also extensively studied. Compound I showed no oxidation waves, while the azo compounds IIa-f

n

@Jy=y&

+2e

&qJx b

0

Dianion 2; from the solvent

Radical

anion

+e

.

Scheme 1.

Voltammetric studies in non-aqueous medium

ElmVl

Q L

Fig. 1. (a) dc-voltammogram

of compound I in BN. (b) Reduction cyclic voltammogram of compound I in BN scan rate = IOOmV s-‘.

Fig. 2(a).

931

G. M. ABOU-ELENIEN et al.

932

E ImVl

Fig. 2(b).

ic

E

Fig. 2(c). Fig. 2. (a) dc-Voltammogram of compound IIa in BN. (b) Reduction cyclic voltammogram of compound IIa in EN scan rate = 100 mV s-l. (c) Oxidation cyclic voltammogram of compound IIa in BN scan rate = 100 mV s-‘.

were found to be electrochemically oxidized. Coulometric measurements have shown that the oxidation of these compounds involve the loss of two electrons. As revealed from the dc-voltammograms, compounds IIa, IIe and IIf are oxidized in a single two-electron process while the methyl derivatives 1Ib-d are oxidized in two successive one-electron waves. The oxidation of some simple phenols namely phenol, p-cresol and p-chlorophenol was studied under similar conditions. The half-wave potentials and the shape of the oxidation cyclic voltammograms of such simple phenols were found to be more or less similar to those obtained for the oxidation of com-

pounds IIa-f. Furthermore, a rough comparison of the ,r+values of the oxidation waves of compounds IIa-f with previously reported data for phenolic compounds[3643] in aprotic media at a platinum electrode has shown that they are comparable. Accordingly, the oxidation processes displayed for compounds IIa-f can be fairly ascribed to the phenolic moiety. In support of this assignment is the fact that an authentic sample of azobenzene was found to be non-oxidizable under identical conditions. Based on the separation and identification of the electrolysis product, the oxidation mechanism of these compounds can be represented schematically in Scheme 2.

Voltammetric studies in non-aqueous medium

0

N=N

-& /

_e

l

933

[eNzN@j 0

0

cot&-radical

Scheme 2.

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