Voltammetric studies of the electrochemical behaviour of salicylidene-2-aminopyridine at a hanging-mercury-drop-electrode

REFAT ABDEL-HAMID Department of Chemistry, Faculty of Science, Sohag, Egypt

Received August 9, 1985

REFAT ABDEL-HAMID. Can. J. Chem. 64, 702 (1986). Electrochemical behaviour of salicylidene-2-aminopyridine has been investigated in 0.1 M tetraethylammonium perchlorate -

dimethylformamide solutions by cyclic voltammetric and convolution potential sweep voltammetric methods. It was found that the depolarizer exhibits two well-defined diffusion-controlled irreversible one-electron waves. The cyclic voltammetric charac- teristics and the convolution, deconvolution, and logarithmic convolution analyses reveal that salicylidene-2-aminopyridine in such conditions follows a set of two one-electron transfer reactions each followed by an irreversible chemical reaction. The values of the first-order rate constant of the irreversible chemical reaction and E I l 2 were computed.

REFAT ABDEL-HAMID. Can. J. Chem. 64, 702 (1986)

Faisant appel i des mtthodes de voltamttrie cyclique et de voltamttrie a balayage de potentiel avec convolution et optrant dans des solutions A 0 , l M de perchlorate de tttratthylammonium dans le dimtthylformamide, on a ttudit le comportement tlectrochimique de la base de Schiff formte par l'aldthyde salicylique et l'amino-2 pyridine. On a trouvt que le dtpolarisant prtsente deux vagues monotlectroniques, bien dtfinies, irrtversibles et contrBltes par la diffusion. Les caracttristiques de la voltamttrie cyclique, de la convolution, de la dtconvolution et des analyses logarithmiques de la convolution suggkrent que, dans les conditions utilistes, la base de Schiff suit un mtcanisme tlectronique-chimique-tlectronique-chimique. On a calcult les valeurs des constantes de vitesse du premier ordre de la r6action chimique irreversible ainsi que de El12 .

[Traduit par la revue]

Introduction The electrochemical reduction mechanism of numerous aro-

matic imines or Schiff bases at a mercury electrode has been investigated in aprotic media (1-6). Relatively few studies, on the other hand, have concerned the electrochemical behaviour of heteroaromatic Schiff bases (7). In view of the considerable success achieved by others in clarifying the mechanism of reduction of aromatic hydrocarbons and carbonyl compounds through investigations in aprotic solvents, a study of the electrochemical reduction behaviour of heteroaromatic Schiff bases in N,N-dimethylforrnamide was undertaken.

This communication is concerned with electrochemical re- duction of salicylidene-2-aminopyridine as investigated by cyclic voltammetry and convolution and deconvolution poten- tial sweep voltammetry. The reduction mechanism at the hanging-mercury-drop-electrode (HMDE) is elucidated and discussed.

Experimental Salicylidene-2-aminopyridine was synthesized by the condensation

of salicylaldehyde with 2-aminopyridine in a 1:l ratio in an ethanolic solution as reported earlier (8). The solid product obtained was recrystallized from ethanol. Tetraethylammonium perchlorate (Fluka) was recrystallized from methanol and dried in a vacuum oven at 60°C. N,N-Dimethylformamide (A.R. grade BDH) was purified by passing it through active neutral alumina as described elsewhere (9).

based upon twin chanel 12-bit analogue-to-digital converter (50 ps conversion time) and a Gemini Galaxy 2 microcomputer. Data capture was written in Macro 80 assembler language which allowed a minimum acquisition time of 100 ks per point. In all experiments 500 data points were routinely captured, equally spaced in time, with a time interval appropriate to the time-scale of the particular experiment. Background data were also stored, and were subtracted from the experimental data set, minimizing effects such as double-layer charging currents.

Solutions were purged with nitrogen before each experiment and an atmosphere of nitrogen was maintained above the working solution. ~nternal resistance ohmic drop distortions were minimized by applying positive feedback compensation.

Convolution and deconvolution procedures were carried out accord- ing to the method described earlier (10). The convolution logarithmic analysis was carried out on the basis of the following general equations '

and

~ r e s h stock solution of tetraethylammonium perchloia;e (TEAP) (0.1 mol dm-3) in dimethylformamide (DMF) was prepared. Fresh where 5 = ( E - @ ) ~ F / R T , a is the symmetry factor, Il and I 2 are the stock solution of the Schiff base (10 mM) was prepared from a fresh convolution currents given in eqS. [3] and [4], respectively, Ilim is the

electrolyte solution on the same day that measurements were carried limit of 11 and E approaches infinity and i, is the exchange current for nut. the electron-transfer process at E = @.

The experiments were performed using 0.1 M TEAPIDMF as supporting electrolyte. The measurements were done using a three i ( ~ ) d u

[3] I; = I,(t) = 5,- l I 2 - electrode cell. It consists of a hanging mercury electrode with surface ,,(t - u)? areaof 1.05 x cm2as working electrode, Ag/AgN03 (0.01 M) in 0.1 M TEAPIDMF as reference electrode and 1-cmZ platinum sheet as

I ' 1: i ( ~ ) exp (- ki ( t - u))

auxiliary electrode. [4] I; = 12(k1 ,t) = 5,- ' I2 du Cyclic voltammetry was performed using an E.G. & G. PAR model (t - n ) ?

363 scanning potentiostat-galvanostat. The current response and potentials were stored on magnetic disk via a fast data-capture system IT. Boddington, I. D. Dobson, and N. Taylor. Unpublished results.

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ABDEL-HAMID

TABLE 1. First wave characteristics for salicylidene-2-aminopyridine in 0.1 M TEAP/DMF at 291 K

Sweep rate - Ep ip Ep - Ep12 ip/v1I2 - E a kcb -E1/2= (mV s-l) (V) (PA) (mV) ( P A V - " ~ ) (V) (s-') (V)

"Deconvoluted peak current potential bk, ? 3%.

? 0.5%.

Results and discussion It has been found that salicylidene-2-aminopyridine was

readily hydrolysed in aqueous and aqueous-organic solvent media into their parent constituents (8, 12). Thus, N,N- dimethylformamide is a convenient medium for this electro- chemical investigation. Furthermore, studies in DMF give a more direct insight into the eIectrochemica1 properties of the azomethine group since, under the usual conditions, a prepro- tonation step can be excluded in this solvent.

Cyclic voltammograms of salicylidene-2-aminopyridine in anhydrous N,N-dimethylformamide containing 0.1 mol dmp3 tetraethylarnrnonium perchlorate as supporting electrolyte show two well-defined irreversible waves. The reversal peaks of the two waves are almost nonexistent, indicating that the two waves correspond to two slow one-electron transfers or a fast one- electron transfer coupled with a rapid chemical reaction. The cathodic peak currents, i,, of the two waves correlated with the square root of the sweep rate, v1I2, with correlation coefficients of 0.9997 and 0.9987 for the first and second waves, respec- tively. They are also proportional to the concentration of the Schiff base in the range 0.49-1.19 x lop3 M (r = 0.9967 and 0.9970, respectively). These are consistent with diffusion as the rate-limiting step (13). A number of electrochemical criteria are applied to establish the nature of the two waves.

The widths of the first cyclic voltammetric wave (E, - Ep12) at different sweep rates are observed to be insignificantly greater than the theoretical value expected for a reversible charge-transfer step (Table 1) (13). The peak potential, E,, does shift in a negative direction with increasing scan rate. The plot of E, against log v yields a straight line with a slope less than 28.8 mV per decade (-6.9 k 1 mV). Both observations are indicative of reversible electron transfer coupled with a follow- up chemical reaction (13, 14). This reaction could be regarded as a protonation of the Schiff base radical anion (2).

The variation of current function, iP/v1l2, with scan rate v is an important diagnostic criterion for establishing the type of mechanism by cyclic voltammetry. For a reversible electronic process followed by an irreversible chemical process (E,,,Ci,) the current function should slightly increase with scan rate (15). The ratio slowly increases with scan rate (Table 1) revealing that the observed behaviour is consistent with a reversible electron- transfer process followed by a first-order irreversible chemical reaction.

Convolution of the cyclic voltammetric data of salicylidene- 2-aminopyridine is carried out according to eqs. [3] and [4]. Figure 1 illustrates convoluted currents I , and l2 cyclic voltammograms for the first wave of 1.19 X lop3 M solution. It is clear that the convoluted current I , does not reach the same plateau value, regardless of the scan rate, and does not return

FIG. 1. Cyclic convoluted currents I l and I2 voltammograms for the first wave of 1.19 X M salicylidene-2-aminopyridine in 0.1 M TEAP/DMF solution at v = 500 mV s-I.

..,f: .. a*'

I I 1.675 1 .875 2.075

-E/V vs. Ag/Ag

FIG. 2. Curves of (Ili, - II ) for the correct treatment of kc , 12, and for the first wave of salicylidene-2-aminopyridine in 0.1 M

TEAP/DMF solution at v = 500 mV s-I.

during the reverse half of the sweep to its initial (zero) value. This shows that the starting material is not conserved within the course of the cycle. This behaviour can be ascribed to irreversible electron-transfer or reversible electron-transfer coupled with a rapid chemical reaction.

Deconvolution cyclic voltammetric results reveal that the

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CAN. I. CHEM. VOL. 64. 1986

TABLE 2. Second wave characteristics for salicylidene-2-aminopyridine in 0.1 M TEAP/DMF at 29 1 K

Sweep rate - Ep ip Ep - EpI2 ip/v1'2 - Epa k,b (mV s-I) (V) (1-w (mV) ( p , ~ v-'I2) (V) (s-l)

"Deconvoluted peak current potential. bk,' 4%.

anodic to cathodic deconvoluted peak-current ratio, dllp,/dll ,, , is almost less than unity (0.28) through the scan rate range used. This is consistent with the reversible electron-transfer process followed by an irreversible first-order chemical reaction (&,,Ci, mechanism), because the anion radical formed is removed from the vicinity of the electrode surface by reaction as well as by diffusion.

The first-order rate constant of the following chemical reaction, Kc, the convoluted current 12, and EI l2 of the first wave are computed using the digital simulation and experi- mental data according to the method described earlier1 (Table 1). The method relies on the equality of (Ilim - 11) and I2 on both the forward and reverse halves of the sweep at E = EI l2 , where EI l2 = @ + (RT/nF) In ( D A / ~ B ) 1 1 2 . This is done by iterating different values of k;. If the k: value used to compute 12 is large the crossing point of the (Ilim - 1,) and 12 - E curves on the reverse sweep lies negative of the crossing on the forward sweep and I, rapidly changes sign after this point. For the low value of k: used, the crossing point on the return lies positive of the crossing on the forward scan and I, fails to return to zero at the end of the sweep. The correct treatment of data, the correct convoluted I2 data, and the k; and E l I 2 values show zero displacement of the crossings which both lie at correct E'". Also I2 reaches zero at the end of the sweep. The crossing points of (Ilim - 11) and I2 as a function of potential are demonstrated in Fig. 2 for the correct treatment of data as a representative

for the second wave shows that II during the reverse of the sweep, whatever the scan rate, does not coincide with II on the forward scan and also it does not reach its initial (zero) value at the end of the sweep. Moreover, the anodic-to-cathodic deconvoluted peak-current ratio is always less than unity (0.37) through the scan rate range used. These facts indicate very good consistency of the data and provide a further verification of the ErevCi, mechanism. The first-order rate constants of the follow-up reaction are computed as mentioned above and the values obtained are collected in Table 2.

Logarithmic analysis of the convoluted results is tested for various mechanisms and it is found that the data follow satisfactorily the ErevCi, mechanism.

Taking into account the acid-base properties of the sali- cylidene-2-aminopyridine (HSAP) (pK, = 8.25 in aqueous solutions (16)), the radical anions should be rapidly protonated by the starting compound through the "father-son" reaction (17) (reactions ii and iv).

Thus, the whole electrochemical reduction of HSAP in anhydrous N,N-dimethylformamide can be represented as follows:

[ i ] HSAF' + e e HSAP;

K, 1st wave [ii] HSAP; + HSAP + HSAPH' + SAP-

[iii] HSAPH' + e eHSAPH- 1 example. 1 The convolution logarithmic analysis is tested for various civ] H S A P H + HSAP HSAPH2 + SAP- 2nd wave reaction schemes (Ere,, EC, CE, Ei,, ECE, ... ) according to eqs. [ l ] and [2]. In the sweep-rate range used, it is found that the Acknowledgement data of the first wave fit satisfactorily the following equation

This work has been ~erformed at The Universitv of Leeds.

It gives rise to straight lines (correlation coefficients nearly one) with slopes close to 39.88 V-' (at 18°C) expected for an E,,Ci, mechanism. This gives further support to the above conclusion that the first wave is ascribed to a reversible one-electron transfer followed by irreversible first-order chemical reaction.

The cyclic voltammetric results in the study of the second wave are recorded in Table 2. It can be seen that (E, - Ep12) is slightly greater than the value expected for a reversible process and E, does shift towards more negative potentials, to the extent of -24.5 mV per log v unit. Furthermore, the ratio iP/vu2 decreases on increasing the scan rate. All the above evidence suggests that the irreversibility of the second wave is due to a moderately fast first-order reaction involving the product of the second electron-transfer.

Examination of the convoluted cyclic voltammetric data (I,)

England. The author &hes to thank Dr. N. ailo or for the provision of facilities.

1. V. N. DM~TRIEVA, N. A. ROZANEL'SKAYA, L. V. KONONENKO, B. I. STEPANOV, and V. D. BEZUGLYL. Zh. Obshch. Khim. 41, 60 (1971).

2. C. P. ANDRIEUX and J. M. SAVEANT. J. Electroanal. Chem. Interfacial Electrochem. 33, 453 (1971); and references therein.

3. M. USHARA and J-I. NAKAYA. Bull. Chem. Soc. Jpn. 43, 3136 (1970).

4. V. D. BEZUGLYL, L. V. KONONENKO, A. F. KORUNORA, V. N. DMITRIEVA, and B. L. TIMAN. Kh. Obshch. Khim. 39, 1680 (1969).

5. A. J. FRY and R. C. REED. J. Am. Chem. Soc. 91,6448 (1969). 6. J. M. W. SCOTT and W. H. JURA. Can. J. Chem. 45,2375 (1969). 7. F. MARTINEZ-ORTIZ, J. VERA, and P. MOLINA. J. Electroanal.

Chem. Interfacial Electrochem. 154, 193 (1983). 8. M. R. MAHMOUD, A. EL-NADY, R. ABDEL-HAMID, and A. A.

EL-SAMAHY. Chem. Scr. 19, 154 (1982).

Can

. J. C

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. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

FLO

RID

A S

TA

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UN

IVE

RSI

TY

on

11/1

2/14

For

pers

onal

use

onl

y.

9. 0. HAMMERICH and V. D. PARKER. Electrochim. Acta, 18, 537 (1 973).

10. A. BLAGG, S. W. CARR, G. R. COOPER, I. D. DOBSON, J. B. GILL, D. C. GOODALL, B. L. SHAW, N. TAYLOR, and T. BODDINGTON. J. Chem. Soc. Dalton Trans. 1213 (1985).

11. J . C. IMBEAUX and J. M. SAVEANT. J. Electroanal. Chem. Interfacial Electrochem. 44, 169 (1973).

12. A. C. DASH, B. DASH, and P. K. MAHAPATRA. J. Chem. Soc. Dalton Trans. 1503 (1983).

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13. R. S. NICHOLSON and I. SHAIN. Anal. Chem. 36,706 (1964). 14. D. PLETCHER. Chem. Soc. Rev. 4, 471 (1975). 15. A. J. BARD and L. R. FAULKNER. Electrochemical methods -

fundamentals and applications. J. Wiley and Sons, New York. 1980.

16. M. T. EL-HATY. Ph.D. Thesis. Aswan, Egypt. 1978. 17. S. ROFFIA and C. GOTTARADI. J. Electroanal. Chem. Interfacial

Electrochem. 142, 263 (1982).

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