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Electrocatalytic reduction of some imino compounds on a glassy carbon electrode electrochemically modified with a new copper-salen complex
ELSEVTER
Journal of Electroanalytical
Chemistry
399 (1995) 121 - I25
Electrocatalytic reduction of some imino compounds on a glassy carbon electrode electrochemically modified with a new copper-salen complex M. Abdel Azzem a,*, Z.F. Mohamed b, H.M. Fahmy h ” Chemistry
Department,
h Chemistry
Faculty
Department,
of Scrence. El-Memu& Faculty
Received 6 October
of Science.
(ink crslty. Shihin El-Kom.
Cairo
IJnil,er\ity,
1094; in revised form
Cairo,
k,y\pt
E,yvpr
I Ma! 199.5
Abstract A copper complex of the half-unit 2,6-diacetylpyridine-moncAethylenediamine) was prepared and characterized using analytical, spectral and magnetic measurements. The electropolymerization of this complex on glassy carbon electrode was performed. A new electroactive polymeric film was prepared by potential sweep electrolysis. Electroanalytical studies were carried out using the cyclic voltammetric technique to examine the indirect electroreduction of 2-aryl-4-cyano-6-mercapto-5-phenyl-3-(2H)-pyridia~ininline derivatives (compounds II,-II,) modified with the Cu complex. In the absence of compounds II,-II, the film showed a single pair of peaks ccrresponding to the Cu(II)/Cu(I) redox system. However, in the presence of the imino compounds, a catalytic effect was observed and thz anodic peak for Cu(I) oxidation disappeared at low scan rates. The imino group was reduced to give the corresponding amino cc+mpounds. Direct electrochemical reduction of compounds II,-II, was performed for comparison. The proposed mechanism has been ccinfirmed by cyclic voltammetry and the product of controlled-potential electrolysis was identified via elemental analysis. IR spectroscopy and ‘H NMR spectroscopy. K~vYw~\: Electrocatalysis,
Modified carbon electrodes,
Electroreduction
1. Introduction
The ultimate approach to a pollution-free chemical industry is electrochemistry. For this reason and others, attention has been directed in recent years to reactions involving mediators. The main disadvantage of mediated e,ectrochemical reactions is that the mediator is still present in the electrolyte solution at the end of the reaction and must be dealt with in some way. In the idea1 system the mediator is directly attached to the electrode surface. hlodified electrodes based on a polymeric film of a metal complex are promising electrode materials with many applications [l-8]. Recently, metal-salen complexes have been used to prepare conducting poly(metal-salen) by direct electro-oxidation [9- 151. During the last few years our group has been involved in a program aimed at developing new routes for the electro-organic synthesis of heterocyclic derivatives which are valuable intermediates in the production of drugs and
agrochemicals [l&20]. In this work, we report a new copper complex of the half-unit 2,6-diacetylpyridine-mono (ethylenediamine) (I). The resulting modified electrode, based on a polymeric film of this complex, was used to examine the indirect electrochemical reduction of a series namely 2-aryl-4cyano-6-mercapto-5of compounds, phenyl-3-(2H)-pyridiazinimine derivatives (11,-H,) of anticipated biological activity to be used as potential biodegradable agrochemicals [21-2-L].
Ph
N-N :\I
C’O,,
1/ M
ClO,’
T
H,O * Corresponding
IM=Cu
author.
OO22-072X/YC/ 509.50 61 1995 Elwwer s.w/ 0022 0728(9531~4131-1
Science S.A. All rights reserved
CN
>,N
N
3
IIa Ar - C,H, IIb Ar = C,H4-CH,
IIc Ar = C,H,-NO, IId Ar = C,H,-Cl
of Electroanalytical
M.A. Auem et al./Journal
122
2. Experimental 2.1. Synthesis
of the complex
2,6_Diacetylpyridine (1 mmol) and Cu(ClO,), 6H ,O (1 mmol) was added to ethanol (20 ml) and the resulting solution was stirred for 20 min at approx. 70°C. Ethylenediamine (1 mmol) in ethanol (IO ml) was then added and the resulting precipitate was heated under reflux for 10 min. The precipitated product was collected, washed several times with ethanol and dried under vacuum over phosphorus pentoxide. 2.2. Synthesis of compounds Compounds 11,-H, literature [25]. 2.3. Apparatus
II,-II,
were prepared as described
in the
and solvents
The elemental analyses (C, H, N, Cl) were carried out at the microanalytical unit of Cairo University, and metal analyses were performed using a Perkin-Elmer 2380 atomic absorption spectrophotometer. IR spectra were measured with a Perkin-Elmer 1430 spectrophotometer using KBr discs. Molar conductivity measurements were performed in lop3 M dimethylformamide (DMF) solution using a Tacussel type CD 6N conductimeter. The electronic spectra were recorded in DMF using a Perkin-Elmer Lambda 4B spectrophotometer. Magnetic susceptibilities were measured at 27°C by the modified Gouy method using a Johnson Matthey magnetic susceptibility balance. Diamagnetic corrections were performed using Pascal’s constants [26]. The magnetic moment was calculated from the equation U& = (corr.7)
“2
Cyclic voltammetry was carried out using a Wenking POS73 potentioscan in conjunction with an x-y recorder (JJPL3). A conventional Tacussel RM04 cell was used in which the working electrode was a Tacussel platinum disk electrode with a diameter of 2.0 mm and the reference
Ph
CN
H5
NH
-e=\ N-N I AC
Ph
Chemistry 399 (1995) 121-125
electrode was a saturated calomel electrode (SCE). Lithium chloride was obtained from Cambrian Chemicals and was used without further purification. Dimethylformamide (Merck) and ethyl alcohol (ADWIC) were distilled under reduced pressure.
3. Results and discussion 3.1. Characterization
of the copper complex
Complex I has the general formula Cu L (H,O) (ClO,), where L is the half-unit 2,6-diacetylpyridine-mono(ethylenediamine). Analytical data for Cu,C9H,Cl,N,0,, are as follows. Calculated: C, 27.2; H, 3.5; Cl, 14.6; N, 8.7; Cu, 13.1. Found: C, 27.6; H, 3.8; Cl, 13.8; N, 9.2; Cu, 12.9. This air-stable complex is non-hygroscopic and is soluble in DMF and acetonitrile. The value of the molar conductivity (28 0.. ’ cm2 mall ‘) indicates its non-electrolytic nature and the coordination of the perchlorate groups. The IR spectrum of complex I shows two bands at 3260-320 and 3130 cm- ’ assigned to coordinated NH,. The spectrum also shows two other bands at 1690 and 1590 cm-’ assigned to an uncoordinated carbonyl group and a coordinated azomethine group respectively. The appearance of the last two bands indicates that the ligand is a 1 + 1 condensation product, i.e. 1 mol 2,6-diacetylpyridine is condensed with 1 mol ethylenediamine. The spectrum of the complex shows three bands near 1580,625 and 450 cm-’ assigned to pyridine ring deformation bands. These bands are at higher frequencies than those of the free 2,6-diacetylpyridine, indicating coordination of the pyridine nitrogen. The mode of the perchlorate groups gives rise to several strong resolved absorptions in the range 1105-1035 cm- ’ which are typical of coordinated monodentate perchlorate [27-291. The spectrum exhibits two bands at 3420 and 520 cm-’ which is assigned to water as indicated by microanalytical data and Cu-N respectively. In addition to these spectral bands, the complex exhibits a band at 870 cm-’ which is assigned to the coordinated water [30]. On the basis of this evidence, it is concluded that the ligand is monobasic tridentate. The electronic spectrum of complex I in DMF shows a broad band at 600 nm. The position and shape of this band suggests a six-coordinate pseudo-octahedral structure [3 11. The room temperature magnetic moment of copper(B) chelate is 1.74 Bohr magnetons (= 1.61 X 10m2’ J T- ‘>, which is equal to the spin-only value of one unpaired electron. 3.2. Electra-oxidatiue polymerization plex and film formation
CN
of the copper com-
NH2
IiS -E+\ N-N Ar
‘The potential cycling method was used to form the film on a glassy carbon electrode in 10e2 M LiCl + DMF solution. Different scan rates (5-150 mV so ’>, potential
M.A. Azzem et al. / Journal
of Elecrroanalyrical
Chemisrry 39Y (I 995) I21 - 125
123
cycling limits and numbers of cycles (5-30) were investigated. It was found that the best response of the polymeric film was obtained if the electrode potential was swept continuously at a rate of 100 mV s-’ between - 1.O and 1.7 V for 25 cycles [32]. 3.3. Cyclic rtoltammetric
response of the polymeric film
After 25 scans, the electrode was rinsed thoroughly before being transferred to fresh DMF containing 10m2 LiCl as the supporting electrolyte. In the 0.0 to - 1.4 V region the film exhibited a single pair of peaks (E,,, = - 0.4 V and EPL = -0.48 V at 2.5 mV s-l), with approximately equal oxidation and reduction currents (Fig. 1). Plots of peak currents Ipa and against scan rate and the square root of the scan rate were linear in the range 16-500 mV s--l which is characteristic of thin polymer films. The linear I,--scan rate relation at low scan rates is characteristic of a surface-attached redox species [33,34]. 3.4. Direct electrochemical
reduction of compounds II,-II,
The cyclic voltammograms of 10m3 M solutions of compounds II .-II d in DMF containing 0.1 M LiCl as supporting electrolyte at a glassy carbon electrode showed two cathodic peaks at -0.77 to -0.93 V and at - 1.17 to - 1.36 V/SCE (Fig. 2). The nitro derivative (II,) showed an additional peak at - 1.65 V corresponding to the reduction of the nitro group in the well-known classical reduc-
Fig. 1. Cyclic voltammetry of a glassy carbon electrode coated with a film of complex I in DMF + LiCl at various scan rates: (1) 16 mV s- ’; (2) 25 mV s ’; (3) 36 mV s- ’; (4) SO mV s- ’
Fig. 2. Cyclic voltammograms of IO-’ M II, in DMF containing 0.1 M LiCl at a glassy carbon electrode at various scan rates: (1) 100 mV s ’; (2) 200 mV s-‘; (3) 300 mV s-‘: (4) 400 mV s ‘; (5) 500 mV s ‘.
tion step [35]. The cathodic peaks shifted towards the negative potential with increasing scan rate. The relation between I,_ and the square root of the scan rate was linear, indicating the diffusion-controlled nature of the process 133,341. Controlled-potential electrolysis (CPE) of compound II,, as a representative example of the series studied, on a carbon sheet electrode of surface area 25 cm2 was carried out in DMF containing 500 mg of II, in 10 -’ M LiCl. CPE was performed twice. First, the potential was held at - 0.8 V and the electrolysis was continued until a constant current was reached after 5 h. The progress of the electrolysis was monitored by recording the decay of current with time, which started with a maximum recorded current of 38 mA and dropped to 31 mA after 30 min. Finally, when the current reached 4 mA, the cell was disconnected from the circuit and the solvent was evaporated to dryness. The product gave a positive reaction to the azo dye test [36] (orange precipitate), indicating the formation of an -NH, group. The yield of the brown substance (m.p. 220°C) was 400 mg. The IR spectra of the compound showed a broad band at ~3440 cm-’ assigned to the NH, .group and a sharp band at ~2220 cm- ’ assigned to the CN group. Analytical data for C,,H ,4N4S were as follows. Calculated: C,66; H, 4.5. Found: C, 52.5; H, 5.4. The NMR spectra of the compound showed bands at 6.31, (s, IH, SH), 7.5 (m, 9H, arom. H), 8.3 (s, 2H, NH2) and 6.OS(S, lH, CH).
124
M.A. Auem et aL/Journal
of Electroanalytical
In the second CPE the potential was held at - 1.4 V for 5 h. When the current reached a constant value of 7 mA the solvent was evaporated to dryness and the azo-dye test [36] was performed on the product. An orange precipitate was formed, indicating the presence of an -NH, group. An element test carried out to identify the presence of sulfur was negative, indicating the absence of the SH group. It is important to note that H,S gas was detected during the electrolysis by testing with a lead acetate paper which became black. On the basis of the preceding results, the following reaction scheme can be proposed for the electroreduction of compounds 11,-B,: first wave Ph
CN
Ph NH +2e-
HS -z=\ N-N
+2H+-HS
-8-
1
Ar
Ar
second wave Ph
CN
CN -
NH2 +2e-
HS z-k \
+2H+
-
1 fi N-N
N-N Ar
NH2 +H,S
a,
The protons involved in the reduction processes could come from water present in the DMF [37,38]. However, sulfides are generally cleaved at the DME according to the processes R-S-R
+ 2e- = 2H+ -
R-S-R
+ 2e-
-RS-+
Ph
CN
Ph
CN NH2+SH’LHzS
‘Ar
II,-II,
at the
NH?
N-N
Ph
second wave
3.5. Indirect electroreduction ofcompounds modijed glassy carbon electrode
CN -
-
Chemistry 399 (19951 121-125
RSH + R’H
(A)
R’-
(B)
In the case of indirect electrolysis the cathodic peak current Ir, for the electron transfer agent (mediator) in the presence of the substrate is increased compared with the peak current Ipd of the mediator in the absence of the substrate. The difference between I, and Zpd is equal to the catalytic current [47]. The size of the catalytic current depends on the potential scan rate and the ratio of the concentrations of the substrate and the mediator (sometimes called the excess factor y). To determine the size of the catalytic effect, cyclic voltammograms were recorded at different potential scan rates. The effect of substrate concentrations has not been verified because in the case of
in protic and aprotic media respectively to give the same electrolysis products [39,40]. To the best of our knowledge only a small number of papers dealing with the electroreductive cleavage of car bon-sulfur bonds exist in the literature [41-441. The mechanism usually proceeds as shown in processes (A) and (B) and the reduction potentials depend on the substituents R and R’. It should be noted that the presence of the heterocyclic ring leads to additional stabilization of the product anion through spreading of the negative charge over the ring. Finally, it is well documented that the reduction of CS in organic molecules usually proceeds catalytically [45,46]. first wave Ph
CN
Ph
-
CN -
HS E= \ N-N Ar
NH+2e-+2H+-HS
fi
1
NH2
N-N Ar
Fig. 3. Cyclic voltammetry of a glassy carbon electrode coated with a film of complex I in DMF+LiCl +compound II, at various scan rates: ~1~16mVs~‘;~2~25mVs~‘;~3~36mVs~‘;(4)50mVs~’.
MA.
Azwn
et al./JournalofElectrounal~rrcal Chemistrv 3W
modified electrode it is difficult to determine the concentration of complex I (mediator) incorporated in the film. Therefore it is difficult to determine whether the electron transfer or the chemical follow-up reaction is the rate-determining step [48]. In the presence of loo-’ M of the imino compounds 11,-U, the cyclic voltammetry of the metal complex incorporated in the film coating the electrode showed an elzctrocatalytic effect [19] illustrated by an increase in the cathodic peaks and the absence of the anodic peaks of the oxidation of Cu(1) at low scan rates, and considerable diamination was clearly observed at higher scan rates (Fig. 3‘1. CPE was carried out with this modified electrode, with the potential fixed at the peak potential of Cu(II)/Cu(I). The positive azo dye spot test at the end of experiment confirmed the formation of the amino group. Based on the above results. the following schematic representation can be proposed: These results have shown that it is possible to carry out indirect electrochemical reduction on a glassy carbon electrode coated with a film of polymeric copper complex. C’u(II) units present in this film act as simple electrode relays, reducing the imino moiety to the corresponding amino group at a lower potential, matching the classical electrochemical process. a
References [II B.R. Shaw in J.T. Stock and M.V. Ome (E?ds.), Electrochemistry. Past and Present, ACS Symp. Ser. 390, American Chemical Society, Washington, DC, 1989, p. 318. 121L. Cache and J.C. Moutet. J. Am. Chem. Sot., 108 (1987) 6887. [31 C.R. Cabrcra and H.D. Abruna, J. Electroanal. Chem., 209 (1986) 101. 141 S. Cosmer, A. Deronzier and J.C. Moutet, J. Electroanal. Chem., 207 (19x6) 315. [51 F. Garmer, G. Tourillon, M. Gazard and J.C. Dubois, J. Electroanal. Chem., 14X ( 1983) 299. [61 J.P. Collin. A. Jouaiti and J.P. Sauvage, J. Electroanal. Chem., 286 (1990) 75. [71 C.P. Andrleux, 0. Haas and J.M. Saveant, J. Am. Chem. Sot., 108 (19X6) X175. [81 G. Bidan. B. Divisia-Blohorn, J.M. Kern and J.P. Sauvage, J. Chem. Sot. Chern. Commun., (1988) 723. [91 P. Audebert, P. Hapiot, P. Capdevielle and M. Maumy, J. Electroanal. Chem.. 338 (1992) 269. and J. Devynck, J. I101 F. Bedioui, E. Labbe. S. Gutierrez-Granados Electroanal. Chem.. 301 (1991) 267. 1Ill P. Audebcrt. P. Capdevielle and M. Maumy, New J. Chem., IS (1991) 235. .121 P. Audebert. P. Capdevielle and M. Maumy, Synth. Met.. 41-43 (iY91) 3049. i131 L.A. Hoferkamp and K.A. Goldsby, Chem. Mater., 1 (1989) 348. Polyhedron, 8 [I41 J.K. Blaho, K.A. Goldsby and L.A. Hoferkamp, (1988) 113. [151 C.P. Horwitz and R.W. Murray, Mol. Cryst. Liq. Cry%, 160 (1988) 389.
(lYY51 III- 125
13
[I61 M. Abdel Alzem, G.ES.H. Elgemeie. M.A. Neguid. H.M. Fdhmy and Z.M. Elmasbry, J. Chem. Sot. Perkin Trans. II (1990) 545. [I71 H.M. Fahmy, M.E. Sharaf, M.A. Aboutabl. M.M.M. Ramlz, M. Abdel Azzem and H. Abd El-Rahman, J. Chem. Sot. Perkin Tran\. II, (1990) 1607. 31 (I 1) (1990) iI81 M. Abdel Azzem and E. Steckhan. Hcterocycle\. 1959. [191 M. Abdel Alzem, M. Zahran and E. Hagagg, Bull. Chem. Sot. Jpn.. 67 (51 (1994) 1390. L201M. Abdel Azzem and M. Zahran, Bull. C‘hern. Sot. Jpn.. 67 17) (1994) 1880.
[211 M.H. Elnagdi,
F.M. Abdelrazek. N.S. Ihrahlm and A.W. Enan, Tetrahedron. 45 (1989) 3597. (221 F.M. Abdelrazek. Prakt. Chem., 332 ( 1990) 479. [23] G.A.M. Nawwar. F.M. Abdelrazek and R.11. Swellam. ‘Arch. Pharm. (Weinheim. Ger.), 324 (199 I ) 875 A.M. Salah. and %.E. lilbar?a. Arch. Pharm. [241 F.M. Abdelrazek, (Weinhelm, Ger.), 325 (1992) 301. [251 F.M. Abdellazek. Bull. Chern. Sot. Jpn., 66(6) (1993) 1727. [261 J. Lewis and R.G. Wilkins, Modem Coordination Chemistry. Interscience, New York, 1960. p. 403. [271 S. Buffagm, L.M. Vallarino and J.V. Quagllno. J. Inorg. Chem.. 3 (1964) 671. [281 A.P. Wiskgndgo and K.A. Krause, J. Inorg. Chem.. 4 (1964) 40.1. [291 L.E. Moore. R.B. Gayhart and W.E. Bull. J. Inorg. Nucl. Chem.. 26 (1964) 896 [301 K.N. Thimmalah, W.D. Lloyd and Ci.1‘. Chadrappa, Inorg. Chem. Acta. 106 (1985) 81. [311 R.J. Fereday, J. Chem. Sot. A, (1972) 3075. [321 M. Abdel Azrem and Z.F. Mohamed. unpuhliqhed result\. [331 H.A. Abd ElLRahman and H.H. Rehan, J. Appl. Electrochern. 22 (1992) 1. Science and L-i41 A.R. Willman, in R.G. Linford (Ed.). Electrochemical Technolog! of Polymers, Elsevier Applied Science. I.ondon, 1087. Ch. 6. [3S] H. Lund in M.M. Baizer (Ed.), Cathodic Reduction of Nitro Cornpounds in Organic Electrochemistry. Dekker, New York. 1973, Ch. 7, p. 31s. (361 F. Feigl. Spot Tests in Organic Analysis, El\evter. New York. 1983, p. 243. [37] S.H. El-Hamouly, M. Abdel Azzem and U.S You\ei, Eur. Polym. J., 29 (1993) 1271. [38] T.F. Otero and A. Marijuan. J. Electroanal. Chem.. 304 (199 I) 153. [39] H.M. Fahmy. M.A.F. Sharaf and M.F. Aboulghar. Ann. Chim. 7X (1988) 703. [40] G. Capobldnco, G. Fania, M.G. Seveim and E. Vlanello, J. Electroanal. Chem.. 165 (1984) 251. [41] P.O. Kane. J. Electroanal. Chem., 2 (1961) 152: L. Meites and P. Zuman, Electrochemical Data. Part I, Vol. A. Wiley. New York. 1974, p. s92. [42] H.M. Fahmy, M. .4bdul Wahab and H. Abdcl Reheem. J. Electroanal. Chem.. 184 (1985) 135. [43] C. Dreux, M.L. Gerard and P Soudrdy, C.R. Acad. Sci.. Sect. C., 262 (1966) 1565; in M.M. Baizer and H. Lund (Eds.), Organic Electrochemistry, Dekker, New York. 1983. p. 576. [44] N. Schultz-von Itter and E. Steckhan. Tetrahedron, 43 (1987) 2484. [4S] P. &man. Organic Polarographic Analysis. Pergamon Press. New York, 1964, p. 20. [46] B. Kuznetsov, Experientia, 18 (SuppI.) (1971) 3.81. [47] C.P. Andrieux, C. Blocman, J.M. Dumas-Bouchlat, 1’. M‘Halla and J.M. Saveant, J. Electroanal. Chem.. 113 (1980) 19. [48] M. Platen and E. Steckhan. Chem. Ber.. II7 (1984) 1679. [49] E.R. Brown and R.N. Adams in A. Weissberger (Ed.). Techniques of Chemistry, Vol. 1, Physical Methods of Chemistry, Part Ila. Electrochemical Methods, Wiley, New York, 1971. p. 48X.
Journal of Electroanalytical
Chemistry
399 (1995) 121 - I25
Electrocatalytic reduction of some imino compounds on a glassy carbon electrode electrochemically modified with a new copper-salen complex M. Abdel Azzem a,*, Z.F. Mohamed b, H.M. Fahmy h ” Chemistry
Department,
h Chemistry
Faculty
Department,
of Scrence. El-Memu& Faculty
Received 6 October
of Science.
(ink crslty. Shihin El-Kom.
Cairo
IJnil,er\ity,
1094; in revised form
Cairo,
k,y\pt
E,yvpr
I Ma! 199.5
Abstract A copper complex of the half-unit 2,6-diacetylpyridine-moncAethylenediamine) was prepared and characterized using analytical, spectral and magnetic measurements. The electropolymerization of this complex on glassy carbon electrode was performed. A new electroactive polymeric film was prepared by potential sweep electrolysis. Electroanalytical studies were carried out using the cyclic voltammetric technique to examine the indirect electroreduction of 2-aryl-4-cyano-6-mercapto-5-phenyl-3-(2H)-pyridia~ininline derivatives (compounds II,-II,) modified with the Cu complex. In the absence of compounds II,-II, the film showed a single pair of peaks ccrresponding to the Cu(II)/Cu(I) redox system. However, in the presence of the imino compounds, a catalytic effect was observed and thz anodic peak for Cu(I) oxidation disappeared at low scan rates. The imino group was reduced to give the corresponding amino cc+mpounds. Direct electrochemical reduction of compounds II,-II, was performed for comparison. The proposed mechanism has been ccinfirmed by cyclic voltammetry and the product of controlled-potential electrolysis was identified via elemental analysis. IR spectroscopy and ‘H NMR spectroscopy. K~vYw~\: Electrocatalysis,
Modified carbon electrodes,
Electroreduction
1. Introduction
The ultimate approach to a pollution-free chemical industry is electrochemistry. For this reason and others, attention has been directed in recent years to reactions involving mediators. The main disadvantage of mediated e,ectrochemical reactions is that the mediator is still present in the electrolyte solution at the end of the reaction and must be dealt with in some way. In the idea1 system the mediator is directly attached to the electrode surface. hlodified electrodes based on a polymeric film of a metal complex are promising electrode materials with many applications [l-8]. Recently, metal-salen complexes have been used to prepare conducting poly(metal-salen) by direct electro-oxidation [9- 151. During the last few years our group has been involved in a program aimed at developing new routes for the electro-organic synthesis of heterocyclic derivatives which are valuable intermediates in the production of drugs and
agrochemicals [l&20]. In this work, we report a new copper complex of the half-unit 2,6-diacetylpyridine-mono (ethylenediamine) (I). The resulting modified electrode, based on a polymeric film of this complex, was used to examine the indirect electrochemical reduction of a series namely 2-aryl-4cyano-6-mercapto-5of compounds, phenyl-3-(2H)-pyridiazinimine derivatives (11,-H,) of anticipated biological activity to be used as potential biodegradable agrochemicals [21-2-L].
Ph
N-N :\I
C’O,,
1/ M
ClO,’
T
H,O * Corresponding
IM=Cu
author.
OO22-072X/YC/ 509.50 61 1995 Elwwer s.w/ 0022 0728(9531~4131-1
Science S.A. All rights reserved
CN
>,N
N
3
IIa Ar - C,H, IIb Ar = C,H4-CH,
IIc Ar = C,H,-NO, IId Ar = C,H,-Cl
of Electroanalytical
M.A. Auem et al./Journal
122
2. Experimental 2.1. Synthesis
of the complex
2,6_Diacetylpyridine (1 mmol) and Cu(ClO,), 6H ,O (1 mmol) was added to ethanol (20 ml) and the resulting solution was stirred for 20 min at approx. 70°C. Ethylenediamine (1 mmol) in ethanol (IO ml) was then added and the resulting precipitate was heated under reflux for 10 min. The precipitated product was collected, washed several times with ethanol and dried under vacuum over phosphorus pentoxide. 2.2. Synthesis of compounds Compounds 11,-H, literature [25]. 2.3. Apparatus
II,-II,
were prepared as described
in the
and solvents
The elemental analyses (C, H, N, Cl) were carried out at the microanalytical unit of Cairo University, and metal analyses were performed using a Perkin-Elmer 2380 atomic absorption spectrophotometer. IR spectra were measured with a Perkin-Elmer 1430 spectrophotometer using KBr discs. Molar conductivity measurements were performed in lop3 M dimethylformamide (DMF) solution using a Tacussel type CD 6N conductimeter. The electronic spectra were recorded in DMF using a Perkin-Elmer Lambda 4B spectrophotometer. Magnetic susceptibilities were measured at 27°C by the modified Gouy method using a Johnson Matthey magnetic susceptibility balance. Diamagnetic corrections were performed using Pascal’s constants [26]. The magnetic moment was calculated from the equation U& = (corr.7)
“2
Cyclic voltammetry was carried out using a Wenking POS73 potentioscan in conjunction with an x-y recorder (JJPL3). A conventional Tacussel RM04 cell was used in which the working electrode was a Tacussel platinum disk electrode with a diameter of 2.0 mm and the reference
Ph
CN
H5
NH
-e=\ N-N I AC
Ph
Chemistry 399 (1995) 121-125
electrode was a saturated calomel electrode (SCE). Lithium chloride was obtained from Cambrian Chemicals and was used without further purification. Dimethylformamide (Merck) and ethyl alcohol (ADWIC) were distilled under reduced pressure.
3. Results and discussion 3.1. Characterization
of the copper complex
Complex I has the general formula Cu L (H,O) (ClO,), where L is the half-unit 2,6-diacetylpyridine-mono(ethylenediamine). Analytical data for Cu,C9H,Cl,N,0,, are as follows. Calculated: C, 27.2; H, 3.5; Cl, 14.6; N, 8.7; Cu, 13.1. Found: C, 27.6; H, 3.8; Cl, 13.8; N, 9.2; Cu, 12.9. This air-stable complex is non-hygroscopic and is soluble in DMF and acetonitrile. The value of the molar conductivity (28 0.. ’ cm2 mall ‘) indicates its non-electrolytic nature and the coordination of the perchlorate groups. The IR spectrum of complex I shows two bands at 3260-320 and 3130 cm- ’ assigned to coordinated NH,. The spectrum also shows two other bands at 1690 and 1590 cm-’ assigned to an uncoordinated carbonyl group and a coordinated azomethine group respectively. The appearance of the last two bands indicates that the ligand is a 1 + 1 condensation product, i.e. 1 mol 2,6-diacetylpyridine is condensed with 1 mol ethylenediamine. The spectrum of the complex shows three bands near 1580,625 and 450 cm-’ assigned to pyridine ring deformation bands. These bands are at higher frequencies than those of the free 2,6-diacetylpyridine, indicating coordination of the pyridine nitrogen. The mode of the perchlorate groups gives rise to several strong resolved absorptions in the range 1105-1035 cm- ’ which are typical of coordinated monodentate perchlorate [27-291. The spectrum exhibits two bands at 3420 and 520 cm-’ which is assigned to water as indicated by microanalytical data and Cu-N respectively. In addition to these spectral bands, the complex exhibits a band at 870 cm-’ which is assigned to the coordinated water [30]. On the basis of this evidence, it is concluded that the ligand is monobasic tridentate. The electronic spectrum of complex I in DMF shows a broad band at 600 nm. The position and shape of this band suggests a six-coordinate pseudo-octahedral structure [3 11. The room temperature magnetic moment of copper(B) chelate is 1.74 Bohr magnetons (= 1.61 X 10m2’ J T- ‘>, which is equal to the spin-only value of one unpaired electron. 3.2. Electra-oxidatiue polymerization plex and film formation
CN
of the copper com-
NH2
IiS -E+\ N-N Ar
‘The potential cycling method was used to form the film on a glassy carbon electrode in 10e2 M LiCl + DMF solution. Different scan rates (5-150 mV so ’>, potential
M.A. Azzem et al. / Journal
of Elecrroanalyrical
Chemisrry 39Y (I 995) I21 - 125
123
cycling limits and numbers of cycles (5-30) were investigated. It was found that the best response of the polymeric film was obtained if the electrode potential was swept continuously at a rate of 100 mV s-’ between - 1.O and 1.7 V for 25 cycles [32]. 3.3. Cyclic rtoltammetric
response of the polymeric film
After 25 scans, the electrode was rinsed thoroughly before being transferred to fresh DMF containing 10m2 LiCl as the supporting electrolyte. In the 0.0 to - 1.4 V region the film exhibited a single pair of peaks (E,,, = - 0.4 V and EPL = -0.48 V at 2.5 mV s-l), with approximately equal oxidation and reduction currents (Fig. 1). Plots of peak currents Ipa and against scan rate and the square root of the scan rate were linear in the range 16-500 mV s--l which is characteristic of thin polymer films. The linear I,--scan rate relation at low scan rates is characteristic of a surface-attached redox species [33,34]. 3.4. Direct electrochemical
reduction of compounds II,-II,
The cyclic voltammograms of 10m3 M solutions of compounds II .-II d in DMF containing 0.1 M LiCl as supporting electrolyte at a glassy carbon electrode showed two cathodic peaks at -0.77 to -0.93 V and at - 1.17 to - 1.36 V/SCE (Fig. 2). The nitro derivative (II,) showed an additional peak at - 1.65 V corresponding to the reduction of the nitro group in the well-known classical reduc-
Fig. 1. Cyclic voltammetry of a glassy carbon electrode coated with a film of complex I in DMF + LiCl at various scan rates: (1) 16 mV s- ’; (2) 25 mV s ’; (3) 36 mV s- ’; (4) SO mV s- ’
Fig. 2. Cyclic voltammograms of IO-’ M II, in DMF containing 0.1 M LiCl at a glassy carbon electrode at various scan rates: (1) 100 mV s ’; (2) 200 mV s-‘; (3) 300 mV s-‘: (4) 400 mV s ‘; (5) 500 mV s ‘.
tion step [35]. The cathodic peaks shifted towards the negative potential with increasing scan rate. The relation between I,_ and the square root of the scan rate was linear, indicating the diffusion-controlled nature of the process 133,341. Controlled-potential electrolysis (CPE) of compound II,, as a representative example of the series studied, on a carbon sheet electrode of surface area 25 cm2 was carried out in DMF containing 500 mg of II, in 10 -’ M LiCl. CPE was performed twice. First, the potential was held at - 0.8 V and the electrolysis was continued until a constant current was reached after 5 h. The progress of the electrolysis was monitored by recording the decay of current with time, which started with a maximum recorded current of 38 mA and dropped to 31 mA after 30 min. Finally, when the current reached 4 mA, the cell was disconnected from the circuit and the solvent was evaporated to dryness. The product gave a positive reaction to the azo dye test [36] (orange precipitate), indicating the formation of an -NH, group. The yield of the brown substance (m.p. 220°C) was 400 mg. The IR spectra of the compound showed a broad band at ~3440 cm-’ assigned to the NH, .group and a sharp band at ~2220 cm- ’ assigned to the CN group. Analytical data for C,,H ,4N4S were as follows. Calculated: C,66; H, 4.5. Found: C, 52.5; H, 5.4. The NMR spectra of the compound showed bands at 6.31, (s, IH, SH), 7.5 (m, 9H, arom. H), 8.3 (s, 2H, NH2) and 6.OS(S, lH, CH).
124
M.A. Auem et aL/Journal
of Electroanalytical
In the second CPE the potential was held at - 1.4 V for 5 h. When the current reached a constant value of 7 mA the solvent was evaporated to dryness and the azo-dye test [36] was performed on the product. An orange precipitate was formed, indicating the presence of an -NH, group. An element test carried out to identify the presence of sulfur was negative, indicating the absence of the SH group. It is important to note that H,S gas was detected during the electrolysis by testing with a lead acetate paper which became black. On the basis of the preceding results, the following reaction scheme can be proposed for the electroreduction of compounds 11,-B,: first wave Ph
CN
Ph NH +2e-
HS -z=\ N-N
+2H+-HS
-8-
1
Ar
Ar
second wave Ph
CN
CN -
NH2 +2e-
HS z-k \
+2H+
-
1 fi N-N
N-N Ar
NH2 +H,S
a,
The protons involved in the reduction processes could come from water present in the DMF [37,38]. However, sulfides are generally cleaved at the DME according to the processes R-S-R
+ 2e- = 2H+ -
R-S-R
+ 2e-
-RS-+
Ph
CN
Ph
CN NH2+SH’LHzS
‘Ar
II,-II,
at the
NH?
N-N
Ph
second wave
3.5. Indirect electroreduction ofcompounds modijed glassy carbon electrode
CN -
-
Chemistry 399 (19951 121-125
RSH + R’H
(A)
R’-
(B)
In the case of indirect electrolysis the cathodic peak current Ir, for the electron transfer agent (mediator) in the presence of the substrate is increased compared with the peak current Ipd of the mediator in the absence of the substrate. The difference between I, and Zpd is equal to the catalytic current [47]. The size of the catalytic current depends on the potential scan rate and the ratio of the concentrations of the substrate and the mediator (sometimes called the excess factor y). To determine the size of the catalytic effect, cyclic voltammograms were recorded at different potential scan rates. The effect of substrate concentrations has not been verified because in the case of
in protic and aprotic media respectively to give the same electrolysis products [39,40]. To the best of our knowledge only a small number of papers dealing with the electroreductive cleavage of car bon-sulfur bonds exist in the literature [41-441. The mechanism usually proceeds as shown in processes (A) and (B) and the reduction potentials depend on the substituents R and R’. It should be noted that the presence of the heterocyclic ring leads to additional stabilization of the product anion through spreading of the negative charge over the ring. Finally, it is well documented that the reduction of CS in organic molecules usually proceeds catalytically [45,46]. first wave Ph
CN
Ph
-
CN -
HS E= \ N-N Ar
NH+2e-+2H+-HS
fi
1
NH2
N-N Ar
Fig. 3. Cyclic voltammetry of a glassy carbon electrode coated with a film of complex I in DMF+LiCl +compound II, at various scan rates: ~1~16mVs~‘;~2~25mVs~‘;~3~36mVs~‘;(4)50mVs~’.
MA.
Azwn
et al./JournalofElectrounal~rrcal Chemistrv 3W
modified electrode it is difficult to determine the concentration of complex I (mediator) incorporated in the film. Therefore it is difficult to determine whether the electron transfer or the chemical follow-up reaction is the rate-determining step [48]. In the presence of loo-’ M of the imino compounds 11,-U, the cyclic voltammetry of the metal complex incorporated in the film coating the electrode showed an elzctrocatalytic effect [19] illustrated by an increase in the cathodic peaks and the absence of the anodic peaks of the oxidation of Cu(1) at low scan rates, and considerable diamination was clearly observed at higher scan rates (Fig. 3‘1. CPE was carried out with this modified electrode, with the potential fixed at the peak potential of Cu(II)/Cu(I). The positive azo dye spot test at the end of experiment confirmed the formation of the amino group. Based on the above results. the following schematic representation can be proposed: These results have shown that it is possible to carry out indirect electrochemical reduction on a glassy carbon electrode coated with a film of polymeric copper complex. C’u(II) units present in this film act as simple electrode relays, reducing the imino moiety to the corresponding amino group at a lower potential, matching the classical electrochemical process. a
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