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Role of proton transfer in the electrochemical reduction mechanism of salicylideneaniline
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ELSEVIER
Journal of Electroanalytical Chemistry431 (1997) 249-255
Role of proton transfer in the electrochemical reduction mechanism of salicylideneaniline Abdirisak Ahmed Isse, Ahmed Maye Abdurahman, Elio Vianello
*
Dipartimento di Chimica Fisica, via Loredan 2. 35131 Padova, Italy
Received 14 January 1997;revised 12 March 1997
Abstract
The electrochemical reduction mechanism of salicylideneaniline has been investigated by cyclic voltammetry, controlled potential electrolysis and coulometry. The main reduction product, characterised by HPLC, IR, IH NMR and X-ray diffractometry, is an anionic dimer, present in two diastereoisomeric forms, together with the conjugate base of the substrate. The latter stems from an intermolecular proton transfer from the substrate to a basic reduction intermediate. Kinetic analysis of the voitammetric results has allowed the electrode reaction mechanism to be fully characterised, showing in particular that the rate-determining step is the coupling between two anionic radicals, promoted by intramolecular H-bridging. © 1997 Elsevier Science S.A. Keywords: Electrodimerisation; Self-protonation; Salicylideneaniline
1. Introduction Much work has been devoted in the last few years to the physicochemical characterisation of OH-substituted aromatic Schiff bases, because these compounds show remarkable photochromic properties, mainly in the solid state [1]. The prototypical and most extensively investigated compound of this class is salieylideneaniline 1, which is known to exist in the enol form, E (see Scheme 1), in the crystal state [2]. The quinoid tautomer Q [3], the zwitterionic form Z [4], or a hybrid between them [5] have been considered as the possible structures of the metastable coloured species, which is formed upon UV irradiation of the solid, followed by intramolecular proton transfer and geometrical rearrangement in the excited electronic state. As to the ground state structure of 1 in solution, both Q and Z have been invoked as possible partners of the prototropic equilibrium undergone by E. Such an equilibrium has been viewed in fact as a keto-enol tautomerism involving E and Q [3] or as a proton transfer between the hydrogen-bonded E and Z [6,7]. The formation of Z appears to be favoured in solvents such as trifluoroethanol and trifluoroacetic acid, promoting proton transfer from the oxygen to the nitrogen atom [6]. In less acidic solvents the
proton Js :,nvolved in a strong intramolecular H-bond between the proton donor OH group and the proton acceptor N atom, as has been proven by IR and NMR measurements [7-11 ]. The stability of the H-bond derives from the hydroxyl group being conjugated through an aromatic ring with the azomethine linkage. The H-bonded enol appears to be the form of I exclusively present in several solvents, such as methanol [7], ethanol [8,9], chloroform [8,10] and dichloromethane [11]. In a dipolar aprotic solvent such as acetonitrile the molecule also takes the enol form, as it does in the solid state [5].
O
E
Z
1
'©© 2
* Corresponding author. Tel.: +39 49 8275135. 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0022-0728(97)00222-2
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Scheme 1.
A.A. lsse et aL / Journal of Eiectroanalytical Chemistry 431 (1997) 249-255
250
(1) (2) (3) (4) (s)
ROH ROH ~
+ +
~
=
ROH ~
ROH
RHOH ° +
~
RHOH-+
ROH
3ROH + 2e-
~---- RHOH" ~--~--~
~
+
RO-
RHOHRH2OH
+ RO-
an aqueous saturated calomel electrode (SCE) to which all potentials are referred. HPLC analyses were performed on a Perkin Elmer Series 4 liquid chromatograph, equipped with a UV detector and a reverse phase ODS2 c(~;umn. The eluent was a water + acetonitrile mixture.
RH2OH + 2RO-
Scheme 2.
The lack of evidence for a tautomeric equilibrium is relevant for the interpretation of the electrochemical reduction pattern in non-protogenic solvents. We have recently shown [12] that the two cathodic voltammetric peaks displayed in DMF by the para OH-substituted isomer of 1 are not attributable either to the separate one-electron reduction of a quinoid and a benzenoid form of the substrate or to the stepwise reduction of the iminic moiety, as previously indicated. Such peaks are due, instead, to the separate two-electron reduction of the undissociated p-hydroxyimine ROH and of its conjugate base RO-, to the p-hydroxyamine RH2OH. RO- derives from an intermolecular proton transfer from the substrate to the basic reduction intermediates (Scheme 2), resulting in the overall reaction (5). The same type of self-protonation sequence was expected to hold in the reduction of 1 as well. In this case, however, we observed some peculiar aspects of the reaction mechanism that seemed to deserve further consideration.
2. Experimental
2.1. Samples Dimethylformamide (DMF) (Carlo Erba, RPE) was vacuum-distilled and stored in a dark bottle under a nitrogen atmosphere. Tetrabutylammonium perchlorate (TBAP) (Fluka) was recrystaUised twice from ethanol + water and dried at 60"C under vacuum. Salicylideneaniline was prepared by adding equimolar amounts of salicylaldehyde and aniline, and then refluxing in alcohol on a water bath for 2 h. After cooling, the precipitated product was collected and recrystallised twice from ethanol. All other chemical substances used were reagent grade commercial products.
2.2. Apparatus Electrochemical measurements were performed with an EG & G-PAR apparatus composed of a 273A potentiostat and 175 universal programmer, and equipped with 310 Nicolet digital oscilloscope and an Amel 862A X-Y recorder. Cyclic voltammetry (CV) was performed using a Hg sphere as working electrode and a Pt wire as auxiliary electrode. The reference electrode was AglAgCII0.1 M Ci- in DMF, whose potential was always measured against
2.3. Electrolyses and product analysis Bulk electrolyses were performed in a divided cell with a mercury-pool cathode. We describe here a preparative scale experiment aimed at isolating and identifying the reduction products: 0.51 g of salicylideneaniline and 0.3 ml of CH3CO2H were dissolved in the cathodic compartment (40 ml DMF + 0.1 M TBAP) and the potential was set at -1.65 V. At the end of the electrolysis, which terminated after the passage of 249.6 C (1 e-/molecule of imine), the catholyte was poured into water (200 ml) and the reduction products extracted with toluene. The toluene extract was dried over MgSO4, and concentrated by rotary evaporation of the solvent and charged on a silica gel column which was eluted with toluene + petroleum ether + ethanol + triethylamine (1/0.5/0.05/0.05) mixture. Two products were isolated, recrystallised from ethyl ether + petroleum ether and characterised by IR, NMR and X-ray diffractometry as the diastereoisomers meso and dl of the dimeric product 2. meso-2: m.p. 158-159°C; IR (KBr) 3380, 3327, 3060, 1600, 1500, 1475, 1460, 1415, 1360, 1265, 1225, 1110, 890, 775, 750 cm- ~; ~H NMR (200 MHz) 6 4.76 (s, 2H), 5.16 (br s, 2H), 6.69-6.87 (m, 12H), 7.06-7.26 (m, 6H), 8.50 (br s, 2H). d l - 2: m.p. 167-168°C; IR (KBr) 3530, 3400, 3330, 3050, 2880, 1598, 1570, 1495, 1484, 1455, 1420, 1305, 1240, 750 cm-I; IH NMR (200 MHz) 8 4.33 (br s, 2H), 4.70 (s, 2H), 6.60-6.64 (m, 4H), 6.84-6.97 (m, 6H), 7.06-7.13 (m, 6H), 7.21-7.29 (m, 2H), 9.17 (br s, 2H).
3. Results and discussion
The voltammetric reduction of 1 in DMF + 0.1 M TBAP gives rise to two peaks of almost equal height (Fig. l), their potentials at a sweep rate u = 0.2 V s -m are reported in Table I. Both peaks are irreversible at low v values but they acquire partial reversibility with increasing v. The standard potential, E °, for the first peak, evaluated as the mid-point between the cathodic and anodic peak potentials, is also reported in the table. Progressive addition of a strong base such as tetrabutylammonium hydroxide causes a decrease of the first peak and an increase of the second one, the first disappearing in the presence of I equivalent of base (Fig. 2, curve b). This provides a first indication that the second peak is attributable to the reduction of the conjugate base of 1, which is the only form of the substrate present in solution after addition of an equimolar amount of the strong base. Formation of the conjugate base during reduction of I at
251
A.A. lsse et al./ Journal of Electroaaalytical Chemistry 431 (1907) 249-255
:t
t': slt | t . ! t i
a
s s s
IV
t\
'~ :
i
~. o v "
i
60
II
40
"~_a
]', i
20 I
-1.5
/,' 0
I -1.5
I -2
I -2.5
-3
E I V v$ SCE Fig. 1. Cyclic voltammogramsof 0.51 mmol dm -3 1 in DMF+0.1 tool dm -3 TBAP at a Hg electrode at a sweep rate of: a = 0.2 V s- n; b = 100 Vs -n"
the first peak potentials is likely to be the consequence of a proton transfer from the phenolic moiety o f the substrate to some basic reduction intermediate. This falls in line with the effect of addition of an acid stronger than the substrate being the opposite to that brought about by the base, as illustrated in Fig. 2. One equivalent of CH3CO2H is sufficient for providing all the protons required, thus quenching self-protonation. As a consequence, the second peak disappears, the first peak becomes 1.5 times higher and does not increase any more upon further acid addition (Fig. 2, curve c). Occurrence of a self-protonation reaction in the reduction of 1 is further supported by the voltammetric pattern of its methoxy derivative 3, where the phenolic proton is missing (Fig. 3). In this case the first peak, leading to the radical anion, is reversible even at the lowest sweep rates. Further reduction at the second peak yields a very basic dianion, which undergoes proton transfer from the medium. Oxidation of the ensuing monoanion in the following positive-going scan gives rise to the anodic peak at - 0 . 8 7
1 Ia 3 4c
" (0E~/ Epc #log c)
- E p'a
-E °
-
(mM)
(V)
(V)
(V)
(V)
0.58 0.48 0.96 0.93
1.564 1.490 1.883 1.939
1.602 2.340 , "~3
(mV)
- 20 - 30 1.853 2.377 0 2.511 - 30
aln the presence of e x c e s s C H 3 C O 2 H. bOEip/Ologcacid = 29.7 mV. c4 stands for the para OH isomer of 1.
(0E~/ Ologc)
-3
SCE
Fig. 2. Cyclic voltammogramsof 1.35 mmol dm-3 1 in DMF+0.1 mol dm -3 TBAP at a Hg electrode at v = 0.2 V s- =; a = 1 alone; b = after addition of 1 equivalentof Bu4NOH; c = after additionof I equivalentof CH 3CO2H.
V. This is just the behaviour expected for an aromatic imine bearing no acidic functions [13-16]. The potentials of the two cathodic peaks are reported in Table 1, together with the standard potential of the redox couple involved in the first reversible process. It should be noticed that the latter is considerably more negative than the corresponding E ° value of 1. A difference of 250 mV could be hardly justified in terms of the substituent effect alone, considering that the Hammett tr values are not much different for -OH and -OCH 3 [! 7]. The existence of a strong H-bond in ! is likely to be, at least in part, responsible for such difference. The molecular structure of the H-bonded substrate, featuring an additional six-membered ring, could contribute to lowering the LUMO energy, thus making reduction easier. Further information as to the stoichiometry of the selfprotonation process of 1 can be gained by controlled potential coulometry and product analysis. Reduction car-
j
Table ! Voltammetric data in DMF + 0.1 M TBAP -' - Epc
I
-2.5
E I V vs
-20
lmine c
I
-2
---
0-
(mV)
20 0b 0 32
-2
-
%1
i -0.5
-1
V I I i -1.5 -2 -2.5 E I V v$ SGE
-3
Fig. 3. Cyclic voltammograms of 1.23 mmol din-3 3 in DMF + 0.1 mol
dm- 3 TBAP at a Hg electrode at v = 0.2 V s- i.
A.A. Isse et al. / Journal of Electroanalytical Chemistry 431 (1997) 249-255
~2
!
1:0
2.0 t Imin 3'.0
4'0
1.'0
2:0 t/rain
4.0
reduction products responsible for the two new peaks ! observed (Fig. 4), ca. ~ of the substrate is still present as its conjugate base. Electrolysis carded out in the presence of an equimolar amount of CH3CO2H, requires le-/molecule up to total disappearance of the single peak initially present in these conditions. HPLC of the ~,;¢ctrolysed solution now shows only two peaks having the same retention time (Fig. 4c) and UV spectra like those of the products obtained from the electrolysis without added acid. Separation of such products as reported above (see Section 2) and analysis by IR, I H NMR and X-ray diffractometry allowed their characterisation as the diastereoisomers m e s o and d l of the dimer 2. The crystal structure of the m e s o isomer, obtained by X-ray diffractometry, is illustrated in Fig. 5. On the basis of the voltammetric and coulometric results and of the analysis of the electrolysis products, it can be concluded that the stoichiometry of the overall process taking place at the potentials of the fi~st peak, in the absence of any added proton donor, is consistent with the equation:
m
i
:3.0
o.~ .......
i-
2:0 t Imin 3:0
1.0
(6)
,t'.0
3
N
,© oi,"©© + °CC-
+ 2e-
Fig. 4. HPLC chromatogramsof 1 recorded before and after controlled potential elcctrolysescan~edout at - 1.65 V. (a) 3.37 mmol tim -3 l; (b) 3.37 mmol dm-3 1 after exhaustiveelectrolysis;(c) 6.50 mmol dm-3 1
after exhaustiveelectrolysisin the presenceof 1 equivalentof CH3CO2H. fled o u t at - 1.65 V, up to the disappearance of the first peak requires 0.67 e-/molecule, while the second peak remains practically unaltered. HPLC analysis of the electrolysed solution shows, however, that, together with the C2,,1.
C23
C25 ~ (~C22
C19
oz'-" N1 N2(
C
C18
-'-c17 if5 " cz6
featuring a dimerisation coupled with self-protonation. The dimer is present as monoanion, most likely because Hbridging entails a limited proton-donor ability of the substrate, leading to napp = 0.67. This is at variance with the value napp = 0.5 reported for the reduction of dibenzoylmethane [18] and 4,6-dimethyi-2-thiopyrimidine [19] in dipolar aprotic solvents leading to the undissociated dimers. However, as mentioned above, the reduction of 1 in the presence of an efficient proton-donor such as CH3CO 2H also yields the fully protonated dimer, but with nap p --'-- 1, since the substrate is no longer involved in the proton transfer processes. Useful information as to the mechanism of the overall
<,,
Cf°3;©
(9)
2Cro-
+ o-
b
~
© (10) C12 Fig. 5. Molecularstructureof meso-2.
+ O_H
O Scheme3.
.
.,0
A.A. lsse et al. /Journal of Electroanalyticai Chemistry 431 (1997) 249-255
reaction (6, Scheme 3) is provided by the sweep rate and concentration dependence of the first peak potential. The latter is shifted to negative values by increasing v, with a slope of - 20 mV of the Ep vs. log v linear dependence, in the range 0.1 < v / V s-~ < !0, For higher v values the slope increases to 37 mV, indicating that electron transfer is becoming the rate-determining step (rds) of the overall electrode process. The effect of an increase in the substrate concentration is to shift Ep in the positive sense, with a slope OEp/Ologc of 20 mV in the concentration range 0.5 < c/mM < 5. These results point out that the electron transfer to the substrate (7, Scheme 3) is fast, whereby the first peak would be fully reversible, at least for v < 10 V s-! (as is the first peak of the methoxy derivative 3) was it not for the fact that some follow up chemical reaction is undergone by the one-electron reduction intermediate, preventing its re-oxidation. Such a reaction could be an intermolecular prc~,ton transfer from the acidic substrate to the more basic radical anion, as we have shown to occur in the reduction of the para isomer of I [12] (see reaction (2) of Scheme 2). However, considering the well documented existence of a H-bridge between the oxygen and nitrogen atoms of 1 [8-11], proton transfer is likely to be intrarather than inter-molecular, as indicated by reaction (8) (Scheme 3). The latter cma be viewed as a strengthening of the N - H bond and a weakening of the O-H bond with respect to the H-bonding situation existing in the molecule before electron transfer. Such a process is likely to correspond to a rapidly established equilibrium. The values of the slopes reported above indicate, in fact, that the rds of the overall electrode process is a second-order reaction undergone by the electron transfer product [20,21], whereas proton transfer, either inter- or intra-molecular, would be first-order with respect to the radical anion. Considering the nature of the electrolysis product, it is conceivable that the second-order rate-determining step is the radical coupling (9, Scheme 3). The partial delocalisation of the negative charge in the Hbonded radical anions is likely to overcome, at least in part, the coulombic repulsion, making their coupling a relatively fast reaction. The ensuing dimeric dianion is expected to be a base stronger than the radical anion, capable of abstracting a proton from the substrate to give the dimeric anion and the conjugate base of 1, according to reaction (10). The latter, together with reactions (7), (8) and (9), make up the electrode reaction mechanism corresponding to the overall stoichiometry (6). It is worth noting that the potential of the single peak observable in the presence of CH3COzH also varies linearly with log v, but with a slope of - 3 0 mV for a ten-fold increase in the scan rate; while it becomes practically independent of the substrate concentration, provided that the acid is slightly in excess. Furthermore, an increase in the acid concentration in the range 1-10 mM (for a substrate concentration 0.56 mM) causes a shift in Ep
253
towards more positive potentials, with a slope 0Ep/01og Cacid= 29.7 mV. These results indicate that addition of an acid stronger than the substrate entails a variation, if not of the mechanism, at least of the rds, with respect to the situation observed in the aprotic solvent. The overall process is now kinetieally limited by a reaction which is first-order with respect to both the one-electron reduction product and CH3CO2H. Since the dimeric product obtained in these conditions does not differ, except for the charge, from that formed in the absence of the strong acid, the rds is likely to be the proton tra~asfer from CH3CO2 H to the H-bonded radical anion. The basicity of the latter is probably too low to promote an intermolecular proton traa~sfer from the H-bonded substrate, so that coupling (9) is the leading way for the radical anion decay in the aprotic solvent. However, in the presence of a stronger acid such as CH 3CO 2H, the pK a of which in DMF is at least 4 units lower than that of the phenolic substrate [22], proton transfer to the radical anion may become fast enough to overcome coupling (9): (11)
The ensuing neutral radical can now interact with a radical partner, either neutral or anionic, to yield the dimeric product. The only difference with respect to the situation in the aprotic solvent is that charge neutralisation should speed up radical-radical coupling so much that reaction (I 1), although very fast, as witnessed by irreversibility of the peak in the presence of the strong acid up to the highest explored sweep rates, becomes the rds. It should be noted, however, that other hydrodimerisation mechanisms leading to the same final product and having proton w,msfer (11) as the rds, would be compatible with the voltammetric results reported above [23]. These involve: (1) coupling of the neutral radical with the substrate followed by further electron and proton transfer to the ensuing dimer; (2) electron transfer to the neutral radical and coupling of the resulting anion with the substrate followed by final proton transfer. We have no means of distinguishing which of these mechanisms is prevailing in the presence of a strong acid. The one based on radical-radical coupling seems preferable since it involves the same reactions operating in the aprotic solvent, with only a change of the rds. Let us now consider the nature of the electrode process featuring the second voltammetric peak, where reduction of the conjugate base of 1 is expected to occur. Periodic monitoring, by cyclic voltammetry, of macroscale electrolysis carded out directly at -2.45 V, reveals that the second peak remains unaltered while it is the first peak to decrease and eventually to fade out after passage of 0.67 e-/molecule. Analysis of the solution electrolysed up to
254
A.A. lsse et al. / Journal of Electroanalytical Chemist~ 431 (1997) 249-255
the disappearance of the first peak indicates formation of the diastereoisomers of the dimer and of the conjugate base of the substrate, in the same proportions observed upon reduction at the first peak potentials. As a matter of fact, the substrate will undergo reduction at the second peak potentials with the same mechanism featuring the first peak. However, the conjugate base formed according to reaction (6) can now also undergo electron transfer, yielding a radical dianion which can act both as electron donor and proton acceptor. It is easy to verify [24] that, whatever role it plays, interaction of the radical dianion with the incoming substrate entails the formation of a radical anion, which will start a reaction sequence leading to the overall reaction (6). In other words, as long as the undissociated substrate is still present, reduction either at the first or at the second peak leads to the same products. The results of electrolysis continued after disappearance of the first peak are less straightforward. First of all, the amount of dimer in the electrolysed solution does not increase further. This may be not suqarising considering that, owing to the dearth of protons ensuing exhaustion of the undissociated substrate, the negative charge on the radical dianion formed by reduction of the conjugate base is not neutralised. Strong coulombic repulsion is then likely to prevent formation of the dimer through radicalradical coupling. A second intriguing aspect of the electrolysis at the second peak potentials, continued after disappearance of the first peak, is that charge consumption is much higher than expected. In fact, an almost total fading out of the second peak can be achieved only after controlled potential electrolysis has been carded out over long periods, the current density soon becoming rather low and practically constant. This indicates occurrence of a catalytic reaction, presumably involving some component of the medium. A possibility, as already observed in the reduction of 4nitroimidazole [25], is that Bu4N + undergoes electron transfer from the radical dianion, followed by decay of the resulting free radical. Apart from complications due to the above catalytic process, it can be concluded that the two voltammetric peaks displayed by 1 correspond to the reduction of the undissociated substrate and of its conjugate base, essentially with the same mechanism, involving radical-radical coupling assisted by intra- and inter-molecular proton transfer. This is quite different from the electrode process undergone by the para isomer of 1, leading to the corresponding amine [ 12]. The reaction sequence illustrated by Scheme 2 corresponds to an ECE self-protonation kinetic regime [26], having the intermolecular proton transfer (2) as rds. Such a considerable difference between the electrode reaction mechanisms undergone by the two isomers is to be mainly attributed to the effect of intramolecular H-bridghag, which is possible only in the ortho isomer. The radical anion stemming from electron transfer (1) of the
para isomer is a relatively strong base, whereby intermolecular proton transfer (2) from the acidic substrate is faster than radical-radical coupling, the latter being hampered by coulombic repulsion. The ensuing easily reducible neutral radical undergoes electron transfer (3) at the working potential to give an anion which is rapidly protonated by the substrate to the final amine. With the ortho isomer, the presence of an intramolecular H-bridge both in the substrate and in the reduction intermediates has a multifold effect. Not only does H-bonding displace the reduction potential of I towards more positive values (compare Epc values of the two hydroxyimines reported in Table 1) but it will partially screen the negative charge in the radical anion, thus helping to overcome the coulombic repulsion. Radical-radical coupling can then efficiently compete with intermolecular prot,~n transfer from the substrate, also because H-bridging has the effect of lowering both the proton-donor ability of the substrate and the basicity of the radical anion.
Acknowledgements Financial support by CNR and MURST is gratefully acknowledged. This work has been carded out in the framework of the Progetto Strategico Tecnologie Chimiche Innovative. We thank prof. G. Valle for X-ray diffractometric analysis of the electrolysis products.
References [1] H. Hadjoudis, in: H. Duff, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 1990. [2] R. Destro, A. Gavezzotti, M. Simoneua, Acta Cryst. B34 (1978) 2867. [3] J.W. Ledbeuer Jr., J. Phys. Chem. 81 (1977) 54. [4] J.W. Lewis, C. Sandorfy, Can. J. Chem. 60 (1982) 1738. [5] T. Yuzawa, H. Takahashi, H. Hamaguchi, Chem. Phys. Lett. 202 (1993) 221. [6] L.N. Kurkovskaya, R.N. Nurmukhametov, D.N. Shigorin, Zh. Strukt. Khim. 21 (1980)61. [7] W. Turbeville, P.K. Dutta, J. Phys. Chem. 94 (1990) 4060. [8] H.H. Freedman, J. Am. Chem. Soc. 83 (1961) 2900. [9] G.O. Dudek, E.P. Dudek, J. Am. Chem. Soc 88 (1966) 2407. [10] G.C. Percy, D.A. Thornton, J. Inorg. Nucl. Chem. 34 (1972) 3357. [11] G.O. Dudek, E.P. Dudek, Chem. Comm. (1965) 464. [12] A.A. lsse, A.M. Abdurahman, E. Yianello, J. Chem. Soc., Perkin Trans. 2 (1996) 597 and references therein. [13] J.M.W. Scott, W.H. Jura, Can. J. Chem. 45 (1967) 2375. [14] C.P. Andrieux, J.M. Saveant, J. Electroanal. Chem. 33 (1971) 453. [15] A. Loma~, R. Hirashawa, A.J. Bard, J. Electrochem. Soc. 119 (1972) 1679. [16] J.H. Barnes, F.M. Triebe, M.D. Hawley, J. Electroanal. Chem. 139 (1982) 393. [17] P. Zuman, Substituent Effects in Polarography, Plenum Press, New York, 1976. [18] R.C. Buchta, D.H. Evans, J. Electrochem. Soc. 117 (1970) 1494.
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255
[24] S. Roffia, V. Concialini, C. Paradisi, F. Maran, E. Vianello, J. Electroanal. Chem. 302 (1991) 115. [25] S. Roffia, C. Gottardi, E. Vianello, J. Electroanal. Chem. 142 (1982) 263. [26] C. Amatore, G. Capobianco, G. Farnia, G. Sandona, J.M. Saveant, M.G. Severin, E. Vianello, J. Am. Chem. Soc. 107 (1985) 1815.
ELSEVIER
Journal of Electroanalytical Chemistry431 (1997) 249-255
Role of proton transfer in the electrochemical reduction mechanism of salicylideneaniline Abdirisak Ahmed Isse, Ahmed Maye Abdurahman, Elio Vianello
*
Dipartimento di Chimica Fisica, via Loredan 2. 35131 Padova, Italy
Received 14 January 1997;revised 12 March 1997
Abstract
The electrochemical reduction mechanism of salicylideneaniline has been investigated by cyclic voltammetry, controlled potential electrolysis and coulometry. The main reduction product, characterised by HPLC, IR, IH NMR and X-ray diffractometry, is an anionic dimer, present in two diastereoisomeric forms, together with the conjugate base of the substrate. The latter stems from an intermolecular proton transfer from the substrate to a basic reduction intermediate. Kinetic analysis of the voitammetric results has allowed the electrode reaction mechanism to be fully characterised, showing in particular that the rate-determining step is the coupling between two anionic radicals, promoted by intramolecular H-bridging. © 1997 Elsevier Science S.A. Keywords: Electrodimerisation; Self-protonation; Salicylideneaniline
1. Introduction Much work has been devoted in the last few years to the physicochemical characterisation of OH-substituted aromatic Schiff bases, because these compounds show remarkable photochromic properties, mainly in the solid state [1]. The prototypical and most extensively investigated compound of this class is salieylideneaniline 1, which is known to exist in the enol form, E (see Scheme 1), in the crystal state [2]. The quinoid tautomer Q [3], the zwitterionic form Z [4], or a hybrid between them [5] have been considered as the possible structures of the metastable coloured species, which is formed upon UV irradiation of the solid, followed by intramolecular proton transfer and geometrical rearrangement in the excited electronic state. As to the ground state structure of 1 in solution, both Q and Z have been invoked as possible partners of the prototropic equilibrium undergone by E. Such an equilibrium has been viewed in fact as a keto-enol tautomerism involving E and Q [3] or as a proton transfer between the hydrogen-bonded E and Z [6,7]. The formation of Z appears to be favoured in solvents such as trifluoroethanol and trifluoroacetic acid, promoting proton transfer from the oxygen to the nitrogen atom [6]. In less acidic solvents the
proton Js :,nvolved in a strong intramolecular H-bond between the proton donor OH group and the proton acceptor N atom, as has been proven by IR and NMR measurements [7-11 ]. The stability of the H-bond derives from the hydroxyl group being conjugated through an aromatic ring with the azomethine linkage. The H-bonded enol appears to be the form of I exclusively present in several solvents, such as methanol [7], ethanol [8,9], chloroform [8,10] and dichloromethane [11]. In a dipolar aprotic solvent such as acetonitrile the molecule also takes the enol form, as it does in the solid state [5].
O
E
Z
1
'©© 2
* Corresponding author. Tel.: +39 49 8275135. 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0022-0728(97)00222-2
fY " 3
Scheme 1.
A.A. lsse et aL / Journal of Eiectroanalytical Chemistry 431 (1997) 249-255
250
(1) (2) (3) (4) (s)
ROH ROH ~
+ +
~
=
ROH ~
ROH
RHOH ° +
~
RHOH-+
ROH
3ROH + 2e-
~---- RHOH" ~--~--~
~
+
RO-
RHOHRH2OH
+ RO-
an aqueous saturated calomel electrode (SCE) to which all potentials are referred. HPLC analyses were performed on a Perkin Elmer Series 4 liquid chromatograph, equipped with a UV detector and a reverse phase ODS2 c(~;umn. The eluent was a water + acetonitrile mixture.
RH2OH + 2RO-
Scheme 2.
The lack of evidence for a tautomeric equilibrium is relevant for the interpretation of the electrochemical reduction pattern in non-protogenic solvents. We have recently shown [12] that the two cathodic voltammetric peaks displayed in DMF by the para OH-substituted isomer of 1 are not attributable either to the separate one-electron reduction of a quinoid and a benzenoid form of the substrate or to the stepwise reduction of the iminic moiety, as previously indicated. Such peaks are due, instead, to the separate two-electron reduction of the undissociated p-hydroxyimine ROH and of its conjugate base RO-, to the p-hydroxyamine RH2OH. RO- derives from an intermolecular proton transfer from the substrate to the basic reduction intermediates (Scheme 2), resulting in the overall reaction (5). The same type of self-protonation sequence was expected to hold in the reduction of 1 as well. In this case, however, we observed some peculiar aspects of the reaction mechanism that seemed to deserve further consideration.
2. Experimental
2.1. Samples Dimethylformamide (DMF) (Carlo Erba, RPE) was vacuum-distilled and stored in a dark bottle under a nitrogen atmosphere. Tetrabutylammonium perchlorate (TBAP) (Fluka) was recrystaUised twice from ethanol + water and dried at 60"C under vacuum. Salicylideneaniline was prepared by adding equimolar amounts of salicylaldehyde and aniline, and then refluxing in alcohol on a water bath for 2 h. After cooling, the precipitated product was collected and recrystallised twice from ethanol. All other chemical substances used were reagent grade commercial products.
2.2. Apparatus Electrochemical measurements were performed with an EG & G-PAR apparatus composed of a 273A potentiostat and 175 universal programmer, and equipped with 310 Nicolet digital oscilloscope and an Amel 862A X-Y recorder. Cyclic voltammetry (CV) was performed using a Hg sphere as working electrode and a Pt wire as auxiliary electrode. The reference electrode was AglAgCII0.1 M Ci- in DMF, whose potential was always measured against
2.3. Electrolyses and product analysis Bulk electrolyses were performed in a divided cell with a mercury-pool cathode. We describe here a preparative scale experiment aimed at isolating and identifying the reduction products: 0.51 g of salicylideneaniline and 0.3 ml of CH3CO2H were dissolved in the cathodic compartment (40 ml DMF + 0.1 M TBAP) and the potential was set at -1.65 V. At the end of the electrolysis, which terminated after the passage of 249.6 C (1 e-/molecule of imine), the catholyte was poured into water (200 ml) and the reduction products extracted with toluene. The toluene extract was dried over MgSO4, and concentrated by rotary evaporation of the solvent and charged on a silica gel column which was eluted with toluene + petroleum ether + ethanol + triethylamine (1/0.5/0.05/0.05) mixture. Two products were isolated, recrystallised from ethyl ether + petroleum ether and characterised by IR, NMR and X-ray diffractometry as the diastereoisomers meso and dl of the dimeric product 2. meso-2: m.p. 158-159°C; IR (KBr) 3380, 3327, 3060, 1600, 1500, 1475, 1460, 1415, 1360, 1265, 1225, 1110, 890, 775, 750 cm- ~; ~H NMR (200 MHz) 6 4.76 (s, 2H), 5.16 (br s, 2H), 6.69-6.87 (m, 12H), 7.06-7.26 (m, 6H), 8.50 (br s, 2H). d l - 2: m.p. 167-168°C; IR (KBr) 3530, 3400, 3330, 3050, 2880, 1598, 1570, 1495, 1484, 1455, 1420, 1305, 1240, 750 cm-I; IH NMR (200 MHz) 8 4.33 (br s, 2H), 4.70 (s, 2H), 6.60-6.64 (m, 4H), 6.84-6.97 (m, 6H), 7.06-7.13 (m, 6H), 7.21-7.29 (m, 2H), 9.17 (br s, 2H).
3. Results and discussion
The voltammetric reduction of 1 in DMF + 0.1 M TBAP gives rise to two peaks of almost equal height (Fig. l), their potentials at a sweep rate u = 0.2 V s -m are reported in Table I. Both peaks are irreversible at low v values but they acquire partial reversibility with increasing v. The standard potential, E °, for the first peak, evaluated as the mid-point between the cathodic and anodic peak potentials, is also reported in the table. Progressive addition of a strong base such as tetrabutylammonium hydroxide causes a decrease of the first peak and an increase of the second one, the first disappearing in the presence of I equivalent of base (Fig. 2, curve b). This provides a first indication that the second peak is attributable to the reduction of the conjugate base of 1, which is the only form of the substrate present in solution after addition of an equimolar amount of the strong base. Formation of the conjugate base during reduction of I at
251
A.A. lsse et al./ Journal of Electroaaalytical Chemistry 431 (1907) 249-255
:t
t': slt | t . ! t i
a
s s s
IV
t\
'~ :
i
~. o v "
i
60
II
40
"~_a
]', i
20 I
-1.5
/,' 0
I -1.5
I -2
I -2.5
-3
E I V v$ SCE Fig. 1. Cyclic voltammogramsof 0.51 mmol dm -3 1 in DMF+0.1 tool dm -3 TBAP at a Hg electrode at a sweep rate of: a = 0.2 V s- n; b = 100 Vs -n"
the first peak potentials is likely to be the consequence of a proton transfer from the phenolic moiety o f the substrate to some basic reduction intermediate. This falls in line with the effect of addition of an acid stronger than the substrate being the opposite to that brought about by the base, as illustrated in Fig. 2. One equivalent of CH3CO2H is sufficient for providing all the protons required, thus quenching self-protonation. As a consequence, the second peak disappears, the first peak becomes 1.5 times higher and does not increase any more upon further acid addition (Fig. 2, curve c). Occurrence of a self-protonation reaction in the reduction of 1 is further supported by the voltammetric pattern of its methoxy derivative 3, where the phenolic proton is missing (Fig. 3). In this case the first peak, leading to the radical anion, is reversible even at the lowest sweep rates. Further reduction at the second peak yields a very basic dianion, which undergoes proton transfer from the medium. Oxidation of the ensuing monoanion in the following positive-going scan gives rise to the anodic peak at - 0 . 8 7
1 Ia 3 4c
" (0E~/ Epc #log c)
- E p'a
-E °
-
(mM)
(V)
(V)
(V)
(V)
0.58 0.48 0.96 0.93
1.564 1.490 1.883 1.939
1.602 2.340 , "~3
(mV)
- 20 - 30 1.853 2.377 0 2.511 - 30
aln the presence of e x c e s s C H 3 C O 2 H. bOEip/Ologcacid = 29.7 mV. c4 stands for the para OH isomer of 1.
(0E~/ Ologc)
-3
SCE
Fig. 2. Cyclic voltammogramsof 1.35 mmol dm-3 1 in DMF+0.1 mol dm -3 TBAP at a Hg electrode at v = 0.2 V s- =; a = 1 alone; b = after addition of 1 equivalentof Bu4NOH; c = after additionof I equivalentof CH 3CO2H.
V. This is just the behaviour expected for an aromatic imine bearing no acidic functions [13-16]. The potentials of the two cathodic peaks are reported in Table 1, together with the standard potential of the redox couple involved in the first reversible process. It should be noticed that the latter is considerably more negative than the corresponding E ° value of 1. A difference of 250 mV could be hardly justified in terms of the substituent effect alone, considering that the Hammett tr values are not much different for -OH and -OCH 3 [! 7]. The existence of a strong H-bond in ! is likely to be, at least in part, responsible for such difference. The molecular structure of the H-bonded substrate, featuring an additional six-membered ring, could contribute to lowering the LUMO energy, thus making reduction easier. Further information as to the stoichiometry of the selfprotonation process of 1 can be gained by controlled potential coulometry and product analysis. Reduction car-
j
Table ! Voltammetric data in DMF + 0.1 M TBAP -' - Epc
I
-2.5
E I V vs
-20
lmine c
I
-2
---
0-
(mV)
20 0b 0 32
-2
-
%1
i -0.5
-1
V I I i -1.5 -2 -2.5 E I V v$ SGE
-3
Fig. 3. Cyclic voltammograms of 1.23 mmol din-3 3 in DMF + 0.1 mol
dm- 3 TBAP at a Hg electrode at v = 0.2 V s- i.
A.A. Isse et al. / Journal of Electroanalytical Chemistry 431 (1997) 249-255
~2
!
1:0
2.0 t Imin 3'.0
4'0
1.'0
2:0 t/rain
4.0
reduction products responsible for the two new peaks ! observed (Fig. 4), ca. ~ of the substrate is still present as its conjugate base. Electrolysis carded out in the presence of an equimolar amount of CH3CO2H, requires le-/molecule up to total disappearance of the single peak initially present in these conditions. HPLC of the ~,;¢ctrolysed solution now shows only two peaks having the same retention time (Fig. 4c) and UV spectra like those of the products obtained from the electrolysis without added acid. Separation of such products as reported above (see Section 2) and analysis by IR, I H NMR and X-ray diffractometry allowed their characterisation as the diastereoisomers m e s o and d l of the dimer 2. The crystal structure of the m e s o isomer, obtained by X-ray diffractometry, is illustrated in Fig. 5. On the basis of the voltammetric and coulometric results and of the analysis of the electrolysis products, it can be concluded that the stoichiometry of the overall process taking place at the potentials of the fi~st peak, in the absence of any added proton donor, is consistent with the equation:
m
i
:3.0
o.~ .......
i-
2:0 t Imin 3:0
1.0
(6)
,t'.0
3
N
,© oi,"©© + °CC-
+ 2e-
Fig. 4. HPLC chromatogramsof 1 recorded before and after controlled potential elcctrolysescan~edout at - 1.65 V. (a) 3.37 mmol tim -3 l; (b) 3.37 mmol dm-3 1 after exhaustiveelectrolysis;(c) 6.50 mmol dm-3 1
after exhaustiveelectrolysisin the presenceof 1 equivalentof CH3CO2H. fled o u t at - 1.65 V, up to the disappearance of the first peak requires 0.67 e-/molecule, while the second peak remains practically unaltered. HPLC analysis of the electrolysed solution shows, however, that, together with the C2,,1.
C23
C25 ~ (~C22
C19
oz'-" N1 N2(
C
C18
-'-c17 if5 " cz6
featuring a dimerisation coupled with self-protonation. The dimer is present as monoanion, most likely because Hbridging entails a limited proton-donor ability of the substrate, leading to napp = 0.67. This is at variance with the value napp = 0.5 reported for the reduction of dibenzoylmethane [18] and 4,6-dimethyi-2-thiopyrimidine [19] in dipolar aprotic solvents leading to the undissociated dimers. However, as mentioned above, the reduction of 1 in the presence of an efficient proton-donor such as CH3CO 2H also yields the fully protonated dimer, but with nap p --'-- 1, since the substrate is no longer involved in the proton transfer processes. Useful information as to the mechanism of the overall
<,,
Cf°3;©
(9)
2Cro-
+ o-
b
~
© (10) C12 Fig. 5. Molecularstructureof meso-2.
+ O_H
O Scheme3.
.
.,0
A.A. lsse et al. /Journal of Electroanalyticai Chemistry 431 (1997) 249-255
reaction (6, Scheme 3) is provided by the sweep rate and concentration dependence of the first peak potential. The latter is shifted to negative values by increasing v, with a slope of - 20 mV of the Ep vs. log v linear dependence, in the range 0.1 < v / V s-~ < !0, For higher v values the slope increases to 37 mV, indicating that electron transfer is becoming the rate-determining step (rds) of the overall electrode process. The effect of an increase in the substrate concentration is to shift Ep in the positive sense, with a slope OEp/Ologc of 20 mV in the concentration range 0.5 < c/mM < 5. These results point out that the electron transfer to the substrate (7, Scheme 3) is fast, whereby the first peak would be fully reversible, at least for v < 10 V s-! (as is the first peak of the methoxy derivative 3) was it not for the fact that some follow up chemical reaction is undergone by the one-electron reduction intermediate, preventing its re-oxidation. Such a reaction could be an intermolecular prc~,ton transfer from the acidic substrate to the more basic radical anion, as we have shown to occur in the reduction of the para isomer of I [12] (see reaction (2) of Scheme 2). However, considering the well documented existence of a H-bridge between the oxygen and nitrogen atoms of 1 [8-11], proton transfer is likely to be intrarather than inter-molecular, as indicated by reaction (8) (Scheme 3). The latter cma be viewed as a strengthening of the N - H bond and a weakening of the O-H bond with respect to the H-bonding situation existing in the molecule before electron transfer. Such a process is likely to correspond to a rapidly established equilibrium. The values of the slopes reported above indicate, in fact, that the rds of the overall electrode process is a second-order reaction undergone by the electron transfer product [20,21], whereas proton transfer, either inter- or intra-molecular, would be first-order with respect to the radical anion. Considering the nature of the electrolysis product, it is conceivable that the second-order rate-determining step is the radical coupling (9, Scheme 3). The partial delocalisation of the negative charge in the Hbonded radical anions is likely to overcome, at least in part, the coulombic repulsion, making their coupling a relatively fast reaction. The ensuing dimeric dianion is expected to be a base stronger than the radical anion, capable of abstracting a proton from the substrate to give the dimeric anion and the conjugate base of 1, according to reaction (10). The latter, together with reactions (7), (8) and (9), make up the electrode reaction mechanism corresponding to the overall stoichiometry (6). It is worth noting that the potential of the single peak observable in the presence of CH3COzH also varies linearly with log v, but with a slope of - 3 0 mV for a ten-fold increase in the scan rate; while it becomes practically independent of the substrate concentration, provided that the acid is slightly in excess. Furthermore, an increase in the acid concentration in the range 1-10 mM (for a substrate concentration 0.56 mM) causes a shift in Ep
253
towards more positive potentials, with a slope 0Ep/01og Cacid= 29.7 mV. These results indicate that addition of an acid stronger than the substrate entails a variation, if not of the mechanism, at least of the rds, with respect to the situation observed in the aprotic solvent. The overall process is now kinetieally limited by a reaction which is first-order with respect to both the one-electron reduction product and CH3CO2H. Since the dimeric product obtained in these conditions does not differ, except for the charge, from that formed in the absence of the strong acid, the rds is likely to be the proton tra~asfer from CH3CO2 H to the H-bonded radical anion. The basicity of the latter is probably too low to promote an intermolecular proton traa~sfer from the H-bonded substrate, so that coupling (9) is the leading way for the radical anion decay in the aprotic solvent. However, in the presence of a stronger acid such as CH 3CO 2H, the pK a of which in DMF is at least 4 units lower than that of the phenolic substrate [22], proton transfer to the radical anion may become fast enough to overcome coupling (9): (11)
The ensuing neutral radical can now interact with a radical partner, either neutral or anionic, to yield the dimeric product. The only difference with respect to the situation in the aprotic solvent is that charge neutralisation should speed up radical-radical coupling so much that reaction (I 1), although very fast, as witnessed by irreversibility of the peak in the presence of the strong acid up to the highest explored sweep rates, becomes the rds. It should be noted, however, that other hydrodimerisation mechanisms leading to the same final product and having proton w,msfer (11) as the rds, would be compatible with the voltammetric results reported above [23]. These involve: (1) coupling of the neutral radical with the substrate followed by further electron and proton transfer to the ensuing dimer; (2) electron transfer to the neutral radical and coupling of the resulting anion with the substrate followed by final proton transfer. We have no means of distinguishing which of these mechanisms is prevailing in the presence of a strong acid. The one based on radical-radical coupling seems preferable since it involves the same reactions operating in the aprotic solvent, with only a change of the rds. Let us now consider the nature of the electrode process featuring the second voltammetric peak, where reduction of the conjugate base of 1 is expected to occur. Periodic monitoring, by cyclic voltammetry, of macroscale electrolysis carded out directly at -2.45 V, reveals that the second peak remains unaltered while it is the first peak to decrease and eventually to fade out after passage of 0.67 e-/molecule. Analysis of the solution electrolysed up to
254
A.A. lsse et al. / Journal of Electroanalytical Chemist~ 431 (1997) 249-255
the disappearance of the first peak indicates formation of the diastereoisomers of the dimer and of the conjugate base of the substrate, in the same proportions observed upon reduction at the first peak potentials. As a matter of fact, the substrate will undergo reduction at the second peak potentials with the same mechanism featuring the first peak. However, the conjugate base formed according to reaction (6) can now also undergo electron transfer, yielding a radical dianion which can act both as electron donor and proton acceptor. It is easy to verify [24] that, whatever role it plays, interaction of the radical dianion with the incoming substrate entails the formation of a radical anion, which will start a reaction sequence leading to the overall reaction (6). In other words, as long as the undissociated substrate is still present, reduction either at the first or at the second peak leads to the same products. The results of electrolysis continued after disappearance of the first peak are less straightforward. First of all, the amount of dimer in the electrolysed solution does not increase further. This may be not suqarising considering that, owing to the dearth of protons ensuing exhaustion of the undissociated substrate, the negative charge on the radical dianion formed by reduction of the conjugate base is not neutralised. Strong coulombic repulsion is then likely to prevent formation of the dimer through radicalradical coupling. A second intriguing aspect of the electrolysis at the second peak potentials, continued after disappearance of the first peak, is that charge consumption is much higher than expected. In fact, an almost total fading out of the second peak can be achieved only after controlled potential electrolysis has been carded out over long periods, the current density soon becoming rather low and practically constant. This indicates occurrence of a catalytic reaction, presumably involving some component of the medium. A possibility, as already observed in the reduction of 4nitroimidazole [25], is that Bu4N + undergoes electron transfer from the radical dianion, followed by decay of the resulting free radical. Apart from complications due to the above catalytic process, it can be concluded that the two voltammetric peaks displayed by 1 correspond to the reduction of the undissociated substrate and of its conjugate base, essentially with the same mechanism, involving radical-radical coupling assisted by intra- and inter-molecular proton transfer. This is quite different from the electrode process undergone by the para isomer of 1, leading to the corresponding amine [ 12]. The reaction sequence illustrated by Scheme 2 corresponds to an ECE self-protonation kinetic regime [26], having the intermolecular proton transfer (2) as rds. Such a considerable difference between the electrode reaction mechanisms undergone by the two isomers is to be mainly attributed to the effect of intramolecular H-bridghag, which is possible only in the ortho isomer. The radical anion stemming from electron transfer (1) of the
para isomer is a relatively strong base, whereby intermolecular proton transfer (2) from the acidic substrate is faster than radical-radical coupling, the latter being hampered by coulombic repulsion. The ensuing easily reducible neutral radical undergoes electron transfer (3) at the working potential to give an anion which is rapidly protonated by the substrate to the final amine. With the ortho isomer, the presence of an intramolecular H-bridge both in the substrate and in the reduction intermediates has a multifold effect. Not only does H-bonding displace the reduction potential of I towards more positive values (compare Epc values of the two hydroxyimines reported in Table 1) but it will partially screen the negative charge in the radical anion, thus helping to overcome the coulombic repulsion. Radical-radical coupling can then efficiently compete with intermolecular prot,~n transfer from the substrate, also because H-bridging has the effect of lowering both the proton-donor ability of the substrate and the basicity of the radical anion.
Acknowledgements Financial support by CNR and MURST is gratefully acknowledged. This work has been carded out in the framework of the Progetto Strategico Tecnologie Chimiche Innovative. We thank prof. G. Valle for X-ray diffractometric analysis of the electrolysis products.
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[24] S. Roffia, V. Concialini, C. Paradisi, F. Maran, E. Vianello, J. Electroanal. Chem. 302 (1991) 115. [25] S. Roffia, C. Gottardi, E. Vianello, J. Electroanal. Chem. 142 (1982) 263. [26] C. Amatore, G. Capobianco, G. Farnia, G. Sandona, J.M. Saveant, M.G. Severin, E. Vianello, J. Am. Chem. Soc. 107 (1985) 1815.