The electrochemical oxidation of thioselenanthrene in acetonitrile at conventional electrodes and microelectrodes

ELSEVIER

Journal

of Elecboanalytical

Chemistry

417 (1996)

35-43

The electrochemical oxidation of thioselenanthrene in acetonitrile at conventional electrodes and microelectrodes R. Miller a Facultb

Universitaires

Norre-Dame b RhSne-Poulenc

a, L. Lamberts a,* , M. Evers b

de lu Paix. Laboratoire d’Electrochimie et de Chimie Analytique, Rorer SA., Centre de Recherche de Vitry, PO Box 14, F-94403 Received

25 March

1996: revised 9 May

61 rue de Brmelles. B-5000 Vitry SW Seine, Frunce

Namur,

Belgium

1996

Abstract

The electrochemical oxidation of thioselenanthrene (1~) in acetonitrilewas studiedby conventionaland microelectrodetechniques. Steady-stateand cyclic voltammebyproved the Occurrence of a DISP2 mechanismfor the first oxidation signal.Controlledpotential coulometryled to the formation of the correspondingselenoxide.The first electrontransferproceedswith a high heterogeneous rate constant(0.58 + O.lOcms-l), whereasa pseudo-first-order apparentrate constantof 8.87+ 1.11s- ’ wascomputedfor the chemical step, giving a half-lifetime of 78 f 1Omsfor the cation-radicalof lc. In order to comparetheir cation-radicalstabilities,similar measurements Keywords:

were

also realized

Thioselenanthrene;

for

phenoxathiine

Phenoxaselenine;

(ld)

Phenoxathiine;

and phenoxaselenine Selenanthrene;

1. Introduction

As part of our investigation on the electrochemical oxidation mechanism of organosulphur and organoselenium compounds,we recently published and compared the behaviour of thianthrene (la) and selenanthrene(lb) [ 1,2]. The high stability of the corresponding cation-radicals observed was attributed to an intramolecular interaction between the unpaired electron from the oxidized chalcogen with a free electron pair from the second chalcogen atom. In order to verify whether similar interactions can occur between a sulphur and a selenium atom, we investigated the electrochemical behaviour of thioselenanthrene(phenothiaselenine) (lc), a mixed sulphur-selenium heterocycle with a structure close to thianthrene (la) and selenanthrene (lb). Comparedwith these derivatives, lc is also unique in the presenceof two different oxidation sites: the selenium and the sulphur atom. To the best of our knowledge, no electrochemical study of the unsubstituted thioselenanthrene (1~) has been published so far. Only two of its

* Corresponding

author.

0022-0728/96/$15.00 Copyright PII SOO22-0728(96)04754-7

0 1996 Elsevier

Science

(le).

Electrochemical

oxidation;

Microelectrodes;

Cation-radicals

derivatives, bearing electro-donating groups, were previously investigated by electrochemical techniques: the 2,3,7&tetramethoxyphenothiaselenine (2a) [3-51 and the 2,3,7,8-bis(ethylenedioxy)phenothiaselenine (2b) [5] (Scheme 1). In dichloromethane or acetonitrile solutions(containing 1% trifluoroacetic anhydride) Engman et al. [51observed quasi-reversible electron transfers for the oxidations of 2a and 2b into their cation-radicals and dications, the sole exception being the second oxidation of 2b in dichloromethane, which remained irreversible. The sole oxidation products identified were the tetrafluoroarsenite and perchlorate salts of the cation-radical of 2b, obtained by electrocrystallization in dichloromethane [5]. The remarkable stability of 2a and 2b cation-radicals is essentially due to the presence of the methoxy groups, sinceelectro-donating substituentsare well known to stabilize cation-radicals [6]. In consequence, the absence of these groups in the unsubstituted thioselenanthrene (lc) should result in an increaseof the cation-radical reactivity towards nucleophiles present in the solvent (i.e. residual water). The present study was therefore realized not only under dry conditions, but also by two microelectrode (ME) techniques with short timescales: high-speed cyclic

S.A. All rights reserved.

R. Miller

a:x.y=s b:X,Y=Se c:X=Sc.Y=S d:X=S.Y=O e:X=Se,Y=O

et al./ Journal

of Electroanalytical

417 (1996)

35-43

to a solution of KSeCN (9.1 g, 63.2mmol) in H,O (50ml). After an additional stirring (30min), the reaction medium was extracted with CH,Cl, (6 X 80ml). The organic layer was separated, washed with water (lOOml), dried over MgSO, and evaporated under reduced pressure. The residue (14g) was chromatographed on silica gel (Merck 7734) using pentane + diethylether 9: 1 (v/v) as eluent to yield 2-phenylthiophenylselenocyanate (7.5 g, 40% from 4) as a reddish-brown viscous oil. “Se NMR (17.04 MHz, CDCl,): 278 ppm.

a:R=OCH b : R =IR(-d-CH&H+-,

Scheme

Chemistry

I.

voltammetry (CV) [7-91 and steady-state voltammetry (SSV) [8-121.

2.2. Thioselenanthrene

(1~)

2. Experimental

A mixture of 2-phenylthioselenocyanate (6, 7.5 g, 26 mmol) and sulphurylchloride (4Oml) was refluxed for 30min. The excess sulphurylchloride was then eliminated by distillation under reduced pressure. The residue obtained (7.6 g) was dissolved in anhydrous 1,‘t-dichlorobenzene (75 ml), and AlCl 3 (5.3 g, 40 mmol) was added in one portion under stirring. The resulting mixture was heated at 120°C for 30 min, then cooled to room temperature. The crude product was washed successively with pentane (50ml), 6 M HCl (4Oml) and H,O (20 ml) before being diluted with CHCl, (50ml). The CHCl, was then washed with 6 M HCl (@ml) and H,O (20ml), dried over MgSO, and evaporated under reduced pressure. Na,S .9H,O (40 g, 0.41 mol) was added in one portion to the crude 8 obtained in the preceding step. The mixture was refluxed for 15 min. After cooling, it was extracted with diethylether (5 X 4Oml), dried over MgSO, and evaporated under reduced pressure. The residue obtained (1.8 g) was chromatographed twice on silica gel (Merck 7734) using cyclohexane + benzene 95:5 (v/v) and pentane as eluent to yield thioselenanthrene (lc) (0.38g, 2% from 4). M.p. 163°C (lit. 165°C [16]). ‘H NMR (4OOMHz, CDCI,): 7.227.68ppm (m, 8H, H,,,); 77Se NMR (76.1 MHz, CDCl,): 428.3 ppm. Phenoxathiine (Id) and phenoxaselenine (le) were obtained by reaction of the corresponding chalcogen with phenoxatellurine [ 171. This latter compound was prepared according to Drew [18], the reduction step of the intermediary dichloride being realized with Na,S [15].

Solvents, reagents, apparatus, electrodes and experimental procedures are the same as described in a previous paper [ 11.Trifluoroacetic anhydride was from Fluka (puriss, > 99%). Thin layer chromatography (TLC) was realized on silica gel (Alugram@ SIL G/UV,,,, Machery-Nagel). Thioselenanthrene (1~) was synthesized following reaction Scheme 2 provided by Irgolic [13]. 2-Phenylthionitrobenzene (4), prepared according to Roberts and Turner [ 141, was reduced to 2-phenylthioaniline (5). Reaction of KSeCN on the diazonium salt derived from 5 led to 2-phenylthiophenylselenocyanate (6). The latter was reacted with sulphurylchloride to yield the corresponding selenotrichloride (7). The crude dichloride (8), obtained by heating the selenotrichloride (7) at 120°C in the presence of two equivalents of AlCl,, was finally reduced to thioselenanthrene (lc) according to the procedure described by Reichel and Kirschbaum [ 151. 2.1. 2-Phenylthiophenylselenocyanate

(6)

A solution of NaNO, (5.23g, 75.8mmol) in H,O (20ml) was added dropwise under stirring over 30min to a solution cooled at 5 to 10°C of 2-phenylthioaniline hydrochloride (5, 15 g, 75 mmol) and cont. HCl (150ml) in water (150ml). After addition was complete, the mixture was stirred at 5 to 10°C for 20min. The pH was adjusted to 5 by addition of solid NaOAc. The latter solution maintained at 5 to 10°C was added dropwise, over 30min,

I

S02CI,.

8

cl’

‘Cl

Scheme

2.

A

R. Miller

et al. /Journal

of Electroanalytical

Chemistry

417 (1996)

37

35-43

3. Results and discussion Typical voltammetric experiments carried out with a millimolar solution of thioselenanthrene (lc) in acetonitrile containing OSM Et,NBF, (Fig. 1) showed two oxidation peaks A, and A, ( + 1.23 and + 2.03 V vs. SCE), a small unidentified signal at +2.2OV vs. SCE, and two cathodic signals C, and C, (-0.17 and -0.9V vs. SCE) upon scan direction reversal. No sign of reduction of lc was detected up to - 1.5 V vs. SCE.

@I40 2

20o-20 I -40

3. I. Controlled potential coulometry and preparative trolysis at the first oxidation potential

elec-

Controlled potential coulometry and bulk electrolysis were carried out with sodium perchlorate as supporting electrolyte in order to facilitate electrolysis product extraction after solvent evaporation under reduced pressure. Since voltammetric peak potentials depend on the supporting electrolyte [19], the values measured in NaClO, medium differ slightly from those in Et,NBF, (Fig. 1). Therefore, the following electrolysis potentials are expressed as a function of the peak potentials recorded by cyclic voltammetry in the same medium. Controlled potential coulometry at potentials about 50mV more positive than the corresponding peak potentials of lc indicate an exchange of two electrons per molecule for the first oxidation peak A, (1.98 + 0.01). Bulk electrolysis was carried out starting at the half-peak potential E,/2 and slowly raising the potential up to about 50mV above the peak potential where electrolysis progressed, During electrolysis at the first oxidation potential A,, the initially colourless solution became first slightly blue and then deep blue-black. Finally, it was completely uncoloured after an exchange of about two electrons per molecule. A cyclic voltammogram realized after completion of this electrolysis shows only the peak A, as the sole anodic signal. On the reverse scan, two reduction peaks were observed: C I and C, (Fig. 2). The origin of the C, peak was the liberation of protons during electrolysis, as has been ascertained by addition of

6oL

I

50 40

-1.0

,

0.0

,

,

0.5 1.0 E I V vs. SCE

1.5

2.0

Fig. 2. Cyclic voltammogram after electrolysis (two electrons per molecule) of a 2.0mM solution of thioselenanthrene (lc) in dry CH,CN (0.1 M NaClO,). Pt electrode, r = 1.5 mm, Y = 50mV s- ’ , scan direction Eq + +2.OOV+ -0.8OV+ +2.OOV vs. SCE.

concentrated perchloric acid (70% aqueous). The C, peak is related to the reduction of the oxidation product 3a of lc: in fact, a second cycle in the positive direction showed the reappearance of the peak A, (Fig. 2) and thioselenanthrene (lc) was regenerated in quantitative yield after electrolysis at the C, potential. This behaviour is similar to that previously described for the electrochemical oxidation of selenanthrene [l], for which we have proved its oxidation into 5-selenoxide. Therefore, we also expected to form a selenoxide in the present case. Although the selenoxide of thioselenanthrene was not isolated in pure form after controlled potential electrolysis, its formation was confirmed by thin layer liquid chromatography (TLC) and “Se NMR. The R, values of thioselenanthrene (1~) and its oxidation product 3a are respectively similar to those of selenanthrene (lb) and its selenoxide. Furthermore, investigation of the unpurified oxidation product by “Se NMR (76.1 MHz, CDCl,) proved an oxidation of the selenium atom of lc: the S value for the selenium atom shifted from 428.3 ppm in lc to 841.6ppm in the oxidation product 3a; the latter value is very cIose to that for the selenium atom in the 5-selenoxide of selenanthrene (6 = 851 ppm [ 1,201). All these observations are consistent with the formation of a selenoxide. Therefore, a general reaction scheme (Scheme 3) can be proposed. 3.2. Investigation oxidation signal

30

c

-0.5

of the electrode process

at the first

-3 2 20

Rotating disk electrode experiments revealed a clear deviation from Levich’s law for the first oxidation signal

10 0 -10 ~~ -20 -2.0 E 1 V vs. SCE

Fig. 1. Cyclic voltammetry of 1.OmM thioselenanthrene (1~) in dry CH ,CN (0.5 M Et,NBF,). Pt electrode, r = I .5 mm, v = 50mV s- ’

Scheme 3.

R. Miller

Ed al./Journal

of Electroanalytical

Chemistry

417 (1996) 35-43

?

YE 3.0 10”

l

.

l

.

u 2 . 2.5 IO” -WY a 2.0 10-3

-l.o-‘-“L”““” 0.4

Fig. 3. Cyclic CH,CN (OSM

0.6

0.8

voltammetry Et,NBF,).

1.0 1.2 1.4 E I V vs. SCE

1.6

1.8

-

. .

.

T

2.0

of 1.OmM thioselenanthrene (1~) Pt electrode, r = 125pm. Y = IOVs-‘.

O.OIOO~~ -1.5

.

”

-1.0

in dry

A, when the rotation speed was increased from 500 to 3000 rev min- ’ : the normalized limiting current (I, w-‘/‘, 1.1 mM of lc) decreased continuously from 1.16 X 10m5 to 0.85 X 10-5As’/2 rad- ‘I2 . This indicates that the overall electrochemical reaction is under kinetic control. Since the timescale of RDE experiments is rather limited, further investigations were carried out by CV and SSV.

3.2.1. Cyclic voltammetry The first oxidation peak A, became completely reversible (Fig. 3) upon raising the potential scan rate from about 50mV s- ’ (Fig. 1) to IOV s-‘, and the corresponding cathodic peak C, appeared on the reverse scan. Moreover, a further anodic peak (A’, > appeared at + 1.75 V vs. SCE. The evolution of the A, peak current, normalized with the electrode surface (A) and the square root of the potential scan rate (v> (Z,/A&) as a function of log V, exhibits no sigmoidal shaped curve in dried acetonitrile, as expected for an ECE-type reaction scheme. In fact, the normalized current seems not to be limited by a diffusion process at low potential scan rates (about 100 mV s- ’ >. Having shown the intervention of residual water in the kinetics of the selenanthrene oxidation mechanism [l], the measurements were repeated with undried acetonitrile (0.02% nominal water content) in order to speed up the chemical step so that the overall reaction became diffusion controlled at low potential scan rates. The corresponding graph (Fig. 4) clearly exhibits diffusion controlled processes at both long and short timescales. The ratio 2.55 ? 0.08 is, however, higher than expected for a pure ECE and DISPI mechanism (2.22) 1211. According to Aoki’s theory [22], a 2.2% diminution of the corresponding peak current was made in order to take into account a slight contribution of non-planar diffusion for the measurements at low potential scan rate (50mV s- ’ ) at the 1.5 mm radius electrode. This led to a corrected ratio 2.49 + 0.08, which is closer to the theoretical value expected for a DISP2 mechanism (2.36) [21] compared with a pure ECE or DISPl mechanism (2.22). Since the characteristic potential differ-

”

1 ”

-0.5

1

’

”

0.0 0.5 log (v I VX’)

I

1.0

,1

1.5

Fig. 4. Evolution of the peak current normalized by the electrode surface (A) and the square root of the potential scan rate (v) as a function of log Y for the oxidation of 1.1 mM thioselenanthrene (Ic) in CH,CN (H,O= 0.02%, 0.5M Et,NBF,). Pt electrodes, r = 1.5mm (0) and 5OOpm (0). Error bars indicate the standard deviation from three measurements.

ence E, - Ep,2 (38.1 + 2.0mV, from nine measurements between 50 and 100mV s- ’ ) is also consistent with a DISPZ process (38.8 mV at 25°C) [21], the reaction Scheme 4 can be proposed. Compared with the electrochemical oxidation of selenanthrene (lb), which proceeds via an ECE or DISPI mechanism [l], it is logical to observe a DISP2 scheme for thioselenanthrene (1~). In fact, since measurements for lc were carried out in undried acetonitrile, the kinetics of the hydroxylation of the cation-radical are enhanced so that the rate determining step is no longer this reaction (ECE and DISPl scheme) but the disproportionation between the hydroxylated radical and the - cation-radical (DISP2 scheme).

1C

Ic

Scheme 4.

R. Miiller

et ul./Journal

of Electroandyticul

The diffusion coefficient D of thioselenanthrene (lc) was computed from the normalized peak currents (Fig. 4) measured at high potential scan rates, where the oxidation proceeds by a reversible monoelectronic transfer. Under these conditions, the following relationship [23]: Zp = 0.4463nFAc,

417 (1996)

35-43

39

reaction intermediates (like ion-radicals) can be investigated with SW. In the case of the oxidation of thioselenanthrene (lc) in dried acetonitrile, steady-state voltammograms recorded with disk MEs of various radii showed an increase of the A’, signal in relation to A, when the disk radius was diminished (Fig. 5(a)-(c)). Simultaneously, the enhancement of the mass transfer rate also leads to the formation of an insulation layer at the electrode surface (Fig. 5(b)(d)). The increase of the A, signal towards A, upon reduction of the experimental timescale can be reasoned by the proposed DISP reaction scheme (Scheme 2). By reducing the experimental timescale via the electrode radius, fewer cation-radicals formed undergo hydroxylation and disproportionation inside the diffusion layer and cannot be further oxidized at the potential of the A, signal. As a consequence, these cation-radicals are oxidized into their dication at the A, signal. This observation was confirmed quantitatively by the measurements of steady-state limiting diffusion currents of the A, signal at disk MEs of various radii:

nFv RT

0”’

Chemistry

gave a diffusion coefficient of 1.8 X lo-’ cm* s- ’ (at room temperature, 22 * 1°C). This value is quite close to those computed previously for its analogues thianthrene (la, 1.7 X 10e5 cm* s- ‘) and selenanthrene (lb, 1.8 X 10m5 cm2 SK’) [l]. From the limiting current of the RDE experiments at the highest rotation velocity (3000 rev min- ’ ), supposing a monoelectronic exchange and a kinematic viscosity of 4.4 X 1O-5 cm* s- ’ [24], a diffusion coefficient of 2.0 X 10e5 cm* s-’ was computed for thioselenanthrene (1~). Although the apparent electron number involved under these conditions is slightly higher than 1, and in consequence this last diffusion coefficient is somewhat overestimated, the computed value confirms the voltammetric measurements.

I, = 4nFc, Dr

3.2.2. Steady-state voltammetry In steady-state voltammetry at disk MEs the characteristic time of the experiment can be reduced by diminishing the electrode radius. Therefore. the formation of unstable

(2)

Since this limiting current is directly related to the electrode radius, which is only known within lo%, a standard compound was used for comparison in order to eliminate the uncertainty. Thianthrene (la) was chosen for this pur-

400

I 3

2

1

0

-100 0.5

1.0

1.5 2.0 E / V VS. SCE

2.5

3.0

0.5

(d)

1.0

1.5 2.0 E I V vs. SCE

2.5

:

2.5

:

5

E I V vs. SCE

Fig. 5. Effect of the electrode radius diminution (from (a) to (d)) on the appearance of the signals observed during the oxidation thioselenanthrene (1~) in dry CH,CN (0.1 M Et,NBF,). h MEs r = 25 (a), 5 (b), 2.5 (c) and 1 p,m cd). Potential scan rate 5 mV s-‘.

of 4.4mM

40

R. Miller

et al./Journal

of Electroanalytical

Chemistry

n =PP

%,s

%.s

-

y = 0.0279

cs .s

(3)

I I .s ,s

+ 8.87x R= 0.950

0.4 -

0.3 d Y 0.2 -

0.1

o.or”“t 0.00

“‘I 0.01

0.02

”

lla

Fig. 7. Determination

”

”

0.03 I s

of the pseudo-first-order

”

0.04

“”

rate constant

( I5

k from

Eq.

(4).

In this equation, the symbols Se,S and S,S respectively represent the seleno-sulphur (lc) and the dithio (la) compound. Measuring the diffusion currents of la and lc at electrodes of radii between 25 and 1 pm, and plotting napp as a function of log r (Fig. 6) revealed that the apparent number of electrons exchanged per molecule of lc tends to the one of la (one electron per molecule). This is further proof for the existence of the thioselenanthrene cation-radical. Similar measurements realized after addition of water ([H,O] = 0.14 M) showed an increase of napp at all MEs (Fig. 6). 3.2.3. Influence of the water content on the speed of the chemical reaction As already described above, the water content in the medium influences the speed of the chemical step of the electrode mechanism: when CV experiments were realized in dried acetonitrile, no diffusion limited peak current was observed at low potential scan rates (50mV s-l), in contrast to measurements in undried acetonitrile. A further stabilization of the thioselenanthrene cation-radical occurred when the measurements were realized in ‘super-

2.0,

35-43

-._

pose, since it undergoes an uncomplicated monoelectronic oxidation in its cation-radical and also has nearly the same geometry as lc [25]. For this latter reason one could also assume the equality of the corresponding diffusion coefficients. Linear sweep voltammetric measurements (see above) confirmed this hypothesis. Taking into account the close values of both diffusion coefficients and writing the limiting currents (Eq. (2)) for la and lc, a relationship is obtained which allows the apparent number of electrons (n,,) exchanged per molecule of thioselenanthrene to be computed (1~):

rises I 1,Se.S =-=-xx

417 (1996)

I

L

,

m

dried’ acetonitrile [26], by stirring the solution with activated neutral alumina, or by adding 1 vol.% trifluoroacetic anhydride (TFA). Under such experimental conditions, the electrochemical oxidation of lc into its cation-radical was reversible at low potential scan rates (about 100mV SK’ ), but the following second oxidation step into its dication stayed irreversible. 3.2.4. Stability of the cation-radical The apparent first-order rate constant of the chemical step of the DISP2 mechanism, i.e. solution electron transfer between the cation-radical and the hydroxylated radical, was estimated by a relationship given for a pure ECE mechanism, since no similar data treatment has been published for a DISP2 process. The following computation should, however, be valid since (a) the maximum peak current functions are close for a pure ECE and a DISP2 mechanism (0.992 and 1.054 respectively [21]) and (b) the data used were obtained at relatively short timescales, where the normalized kinetic current (I,) is less than 1.3 times higher than the corresponding diffusional current (I,). Considering that the chemical reaction is of a quasifirst-order, the following Nicholson-Shain formula [27], rigorously valid for a pure ECE mechanism with equal electronicity (n, = n2 = n), allowed an estimation of the pseudo-first-order rate constant k: I,

h

I,=

hz(r/vm) Fig. 6. Apparent number of electrons exchanged per molecule of thioselenanthrene (1~) as a function of the Pt ME radius (25, 5, 2.5 and I pm). Solution 4.4mM thioselenanthrene (Ic) in dry CH,CN (0.1 M Et,NBF,) (0) and after addition of water ([H,O]= 0.14M) (0). Error bars indicate the standard deviation from three measurements.

0.4 + k/a r

0.396 + 0.469k/a

nFu ’ a= F

(4)

A plot of k/a as a function of l/a (Fig. 7) using the data from Fig. 5 gave k = 8.87 * 1.11 SK’; from which a halflifetime of 78 k 10 ms was computed for the cation-radical of lc under our experimental conditions. Taking into account that these measurements were conducted in undried acetonitrile (in order to enhance the kinetics of the hydroxylation reaction), whereas previously determined apparent rate constants and cation-radical half-lifetimes for selenanthrene (10.5 f 0.03 s- ’ and 66 + Oms) were obtained under dry conditions, the stability of the thioselenanthrene

R. Miiller

et al./Journal

of’Electroandytica1

cation-radical is higher than that of selenanthrene. Since the electrochemical oxidation of thianthrene (la) into its cation-radical is already reversible at 50 mV s- ’ in undried acetonitrile, the corresponding cation-radical is even more stable. 3.3. Determination of the electron transfer rate k” of the first oxidation step When the radius of the ME is reduced from 25 to 1 km in SSV experiments, the slope of the rising part of the voltammogram of lc decreases, while the Tomes potential difference ( E3,4 - E,,4) [28] increases from 47 to 55mV. The measured Tomes potential difference of 55 mV nearly equals the theoretical value (55.9 mV) expected for a completely reversible monoelectronic couple [29]. Compared with the corresponding measurement realized for selenanthrene (lb), for which the Tomes potential difference (1 km radius ME) and k” are 66mV and 0.17 + 0.02 cm s- ’ respectively, the heterogeneous electron transfer rate constant should be even higher for thioselenanthrene (1~). This implies that the electron transfer rate in the case of lc is very high. The value of the heterogeneous electron transfer rate constant was measured by high-speed cyclic voltammetry at MEs. As the potential scan rate was raised from 80 to 300V s- ‘, the anodic and cathodic peaks of lc broadened and their potential separation (A E,) increased from 74 to 125 mV. Under such conditions, the electron transfer became slow compared with the mass transfer rate, giving a quasi-reversible shape to the cyclic voltammogram. Numerical values of the electron transfer rate constant k” were obtained by Nicholson’s method, consisting of the determination of a dimensionless parameter +G from AE, with the aid of a +-A EP working curve. Since no dissymetric behaviour was observed for the oxidation and reduction peaks in the present case, a value of 0.5 can be attributed to the transfer coefficient (Y [30], and the corresponding $-AZ?, working curve was computed by digital simulation. Adapting the Nicholson formula to an oxidation process and supposing identical diffusion coefficients of both oxidized and reduced species, the following relationship allows the computation of k”: k” = $,/w

Chemistry

417

(1996)

41

35-43

or the sulphur atom of thioselenanthrene (1~) is replaced by an oxygen atom were also investigated. The corresponding derivatives, phenoxathiine (Id) and phenoxaselenine (le,) have already been studied in electrochemistry, by Barry et al. [31] and Cauquis and Maurey-Mey [32] respectively. These compounds were included in this investigation in order to compare their homogeneous and heterogeneous rate constants with those of the dibenzodichalcogenins la-c. Steady-state and cyclic voltammetric experiments on phenoxathiine (Id), realized under identical conditions as for thioselenanthrene (lc), showed that its cation-radical undergoes very slow hydrolysis by residual water. Upon addition of water ([H,O] = O.l4M), the measured peak currents (CV: 50mV s- ’ ) and limiting currents (SSV: MEs, r = 1 to 25 km> stayed nearly unchanged; the corresponding experimental timescales are too short to evidence a hydroxylation of the phenoxathiine cation-radical. Therefore, the apparent first-order rate constant of this reaction could not be measured. Cyclic voltammetric experiments, however, allowed the half-lifetime to be estimated as superior or equal to 5 s for the cation-radical by comparing the minimum potential scan rate at which reversible behaviour is observed with that of selenanthrene, for which a half-lifetime of 66ms was established [2]. The heterogeneous rate constant of the electron transfer during the oxidation of Id was measured by high-speed cyclic voltammetry at a platinum disk ME (r = 25 pm). When the potential scan rate was raised from 170 to 490V s-l, AEp changed from 73 to 84mV. The corresponding rate constant was 2.7 t- 0.2 ems- ‘, a value of 0.5 being assumed for the transfer coefficient (Y and the diffusion coefficient D (2.0 X 10e5 cm* s- ‘) being obtained from cyclic voltammetric experiments. The electrochemical behaviour of phenoxaselenine (le) is similar to that of selenanthrene (lb) and thioselenanthrene (113. The oxidation led to a selenoxide which, in contrast to previous observations [32], could be reduced at

(5)

The value of 0.58 f O.lOcms-’ obtained is consistent with the prediction made by SSV (see above), and indicates a relatively high degree of reversibility for the oxidation of lc into its cation-radical. 3.4. Comparison of the electrochemical oxidation mechunism in the I ,4-dibenzodichalcogenin family In order to complete our study of this family of molecules, two related compounds in which the selenium

-1.0

-0.5

0.0

0.5 1.0 E I V vs. SCE

1.5

2.0

2.5

Fig. 8. Cyclic voltammogram after electrolysis (two electrons per molecule) of a 2.0mM solution of phenoxaselenine (le) in dry CH,CN (0.1 M Et,NBF,). Pt electrode, r = lSmm, Y = SOmVs- ‘, scan direction I& -+ + 2.OOV --) - 0.8OV -+ + 2.OOV vs. SCE.

42

R. Miillrr

=

et al./Journal

t

of Electroanalytical

I

-0.1

0.1

0.3

0.5

0.7 log(r

0.9

1.1

1.3

1.5

/pm)

Fig. 9. Apparent number of electrons exchanged per molecule of phenoxaselenine (le) as a function of the Pt ME radius (2.5, 5, 2.5 and 1 pm). Solution 4.1 mM phenoxaselenine (le) in dry CH,CN (0.1 M Et,NBFJ (0) and after addition of water ([Ha01 = 0.07 M) (0). Error bars indicate the standard deviation from three measurements.

platinum electrodes to its parent compound (le, Fig. 8). Although SSV experiments at MEs showed a similar variation of the apparent electron number exchanged (nap,) as a function of the electrode radius (Fig. 9) to that for thioselenanthrene (Fig. 61, the normalized peak current (ZpA-‘u‘I21 as a function of log v (Fig. 10) showed a peculiar horizontal region between log v - 0.4 and 0.1. Compared with the normalized peak current at high scan rates (8 V s- ’ 1, the corresponding values at the horizontal part are 1.17 times greater. This behaviour can be attributed to the dimerization equilibrium of the phenoxaselenine cation-radical already mentioned by Cauquis and Maurey-Mey [32]: 2(le)‘++

Chemistry

417 (1996)

35-43

constant of the reaction between the phenoxaselenine cation-radical and residual water could not be obtained by relationship (4). The scan velocities used in cyclic voltammetric experiments allowed, however, the half-lifetime of the cation-radical to be estimated as about 50ms. The heterogeneousrate constant of this electron transfer was determinatedby cyclic voltammetry at platinum disk electrodes (r = 125, 62.5 and 25 km>. ‘The peak potential separation (A E,) varies from 76 to 144mV when the potential scan rate is increased from 11.5 to 300V s- ‘. Knowing the diffusion coefficient of le from CV experiments (1.7 X lo-’ cm* s-l) and assuminga transfer coefficient of 0.5, a rate constant of 0.24 + 0.04cm s- ’ was computed. This result is in good agreement with the prediction of Cauquis and Maurey-Mey [32], who estimated a value of 1O- ’ cm s- ’ from RDE experiments. The half-lifetimes of the cation-radicals of the dibenzo(c,e)- 1,4dichalocogenins la-e studied showed large differences in stability. In general, the cation-radicals formed by electron abstraction from a sulphur atom are much more stabilized than their selenium analogues. So, the cation-radicals of thianthrene (lc) and phenoxathiine (Id) have half-lifetimes above 5 s, whereas those of selenanthrene (lb), thioselenanthrene(lc) and phenoxaselenine (le) are only about 10 to 100ms. The stability of the cation-radical must mostly rely upon an intramolecular coordination of a chalcogen lone-pair to the oxidized chalcogen, forming a three-electron bond [34-361. According to the measured half-lifetimes, the cation-radical stabilization evolved in the dibenzo(c,e)1,4dichalcogenin family according to the sequence

(leg’

(6) In fact, the theoretical maximum of the peak current function for a dimerization EC,,, (0.527) is 1.18 times higher than the corresponding value for a fast uncomplicated electron transfer (0.446) [33]. Owing to this additional complication, the apparentrate

= s.t.s ( 11.4

4. Conclusion

7 a .@. 1.1 10-s E m1.0 10-33 -1.5

-1.0

-0.5 0.0 log (v I V.S.‘)

0.5

1.0

Fig. 10. Evolution of the peak current normalized by the electrode surface (A) and the square root of the potential scan rate (Y) as a function of log Y for the oxidation of 0.99mM phenoxaselenine (le) in CH,CN (OSM Et,NBF,). Pt electrodes, r = 1Smm (0) and 5OOpm (0).

The electrochemical oxidation of thioselenanthrene(1~) in acetonitrile led to the formation of the corresponding selenoxideby a DISP2 electrode mechanism.The chemical step of this process consistsof the reaction of the cationradical with residual water, followed by a disproportionation. Analysis of this electrode process by cyclic voltammetry gave a pseudo-first-order rate constant of 8.87 k 1.l 1 s- ’ for this step, from which a half-lifetime of 78 f 1Oms was computed for the cation-radical. High-speed cyclic voltammetry allowed us to assign a heterogeneous rate constant of 0.58 f O.lOcm s-l to the first electron transfer. Similar measurementscarried out for two other dibenzo(c,e)- 1,Cdichalcogenins, phenoxathiine (Id) and phenoxaselenine(le), gave cation-radical half-lifetimes and heterogeneousrate constantsabove 5 s and 2.7 + 0.2 cm s- ’

R. Miiller

er ai./Journal

of Electroanalytical

(Id) and around 50ms and 0.24 + 0.04 cm s- ’ (le) respcctively. Comparing the cation-radical half-lifetimes of several dibenzo(c,e)- 1,Cdichalcogenins showed that the stabilization by intramolecular lone-pair sharing of a chalcogen to the oxidized chalcogen follows the sequence (Se.TSe),,4=

(Se.TO),c<

(Se.T.S)l,4e

(CO),,,

=(s.5)I

,4

Acknowledgements We thank Professor K.J. Irgolic (Institut fur Analytische Chemie, Karl-Franzens-Universitst Graz, Austria) for providing the reaction scheme of the synthesis of thioselenanthrene, and Professor A. Krief and Dr. W. Dumont (Laboratoire de Chimie Organique, F.U.N.D.P. Namur) for their assistance in preparing this compound and for NMR measurements. R.M. is grateful to the “Faculte’s Universitaires Notre-Dame de la Paix” (Namur, Belgium) and to the “Institut pour 1’Encouragement de la Recherche Scientifique dans 1’Industrie et 1’Agriculture” (IRSIA) for financial support in order to accomplish his Ph.D. thesis.

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