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The electrochemistry of organotellurium compounds—III. The electrochemistry of triphenyltellurium chloride
THE ELECTROCHEMISTRY OF ORGANOTELLURIUM COMPOUNDS-III. THE ELECTROCHEMISTRY OF TRIPHENYLTELLURIUM CHLORIDE Y. LIFTMAN* Department
of Chemistry.
and
M. ALBECK
Bar-Ilan University,
(Received
Ramat-Gan 52100, Israel
19 May 1983)
Abstract-The electrochemical behaviour of Ph,TeCl in methylene chloride, containing tetrabutylammonium perchlorate (TBAP), at a platinum electrode has been studied. It is shown that Ph,TeCI is oxidized to Ph,(Cl)TeTePh,(CI) and reduced either in a one electron reaction to Ph3TeTePh, or in a two electron reaction to Ph,TeH. The electrochemical reactions depend on the concentration of Ph,TeCl in the solution, on the rate of rotation of the electrode and on the rate of the potential scan.
INTRODUCTION
RESULTS
In our previous work on the electrochemistry of tellurium compounds in aprotic media[l+ it has been shown that the replacement of chloride ions in TeCI,[i, 21 by phenyl groups[3, 4] shifts the halfwave potential of the first cathodic wave toward a more negative value. Matsuo(5, 61 observed two reduction waves of Ph,TeCl in aqueous solution of KCl. The half wave potential of the first wave (-0.77 V) was independent of pH, while that of the second one ( - 1.10 V) shifted toward more negative potentials with increasing pH. It was reportedC71 that the electroreduction of triphenylsulfonium nitrate in dimethylformam ide or in water results in formation of dipheaylsulfide and benzene either in a one or in a two electron reaction. However in the first step of reduction of organometalJic compounds like Ph,GeCl[8] or Ph,SnCI[9%11 J the hexaphenyldigermane and hexaphenylditin were formed. In the second step the formation of the triphenylgermane[8] and triphenyltin anion[ 10, 1 l] or diphenyltinC9, lo] were suggested. It is shown here that in analogy to Ph,GeCl (and different from Ph,SNO,) triphenyltellurium chloride is reduced to hexaphenylditellurane or triphenyltellurane.
The uoltammograms
AND
DISCUSSIONS
of Ph,TeCl
The cyclic voltammograms of Ph,TeCl in methylene chloride containing TPAB as supporting electrolyte are shown in Fig. 1. The cathodic and anodic waves shown in curve (a) are due respectively to the reduction and oxidation of PhsTeCl. At higher rates of potential scan (1 V s - ‘) the other two cathodic waves, shown in curve (b), which follow the oxidation wave of Ph,TeCl are due to the reduction of the material formed in the oxidation of PhsTeCl. while the anodic wave shown in curve (c) which follows the reduction wave of Ph,TeCl is due to the oxidation of the material formed in the reduction of Ph,TeCl. As shown in Fig. 2, the limiting currents of both the oxidation (E,,, = 1.23 kO.02 V) and the reduction (E,,, = - 1.23 f 0.02 V) waves, at rde, are linearly proportional to the concentration of Ph,TeCl (pp to 0.6 mmol) and to fl” (up to 20revK’).
The dimerization
reactions
The lower values of il/[PhJTeCl] than in Fig. 2, which were obtained at high concentrations of Ph,TeCl, are ascribed to dimerization reactions (Equations 5 and 7). The following supports the assumption that the dimerization reactions are favoured at high rates of rotation of the electrode: (a) it was found by coulometric measurements performed as described eisewhere[3] that n = 1 when the electrode was rotating at a high rate and n = 2 when it was stationary, (b) it can be seen from curves (c) and (d) in Fig. 2 that i,/f - ‘I2 decreases wh& f in&&es, and from curve (g) that the decrease is more than expected when there is a linear dependence of i;’ onf-‘12 and 31 for the (cl employing the Levich equation[l2, limiting currents of the waves using the value of n = 1 gave a value of 4.2 x fOT6 cm’s_’ ( f 10 %) for the diffusion coe@cient of Ph3TeCI, which is in reasonable agreement with those
EXPERIMENTAL The description of the materials and instruments used was given elsewhere[l, 33. Reagent grade triphenyltelluronium chloride (PhsTeCl) was supplied by Organometallic Inc. (USA). The surface area of the rde electrode was 0.196 cm’. This same electrode was used in C.V. experiments with a stationary electrode. The volume of the solution was 50 ml. The surface of the electrode was cleaned by a method given in[ 11. The temperature in all experiments was 11 f 1°C unless otherwise indicated. . .” * Part of a Ph.D. thesis.
91
Y. LIFTMAN AND M. ALBECK
92
c
4OpA
L
Fig. 1. Cyclic voltammograms
of Ph,TeCl in CH,CI, (0.1 M TBAP). (b) and (c) 1.0 x 10m3 M; 1000 mV s-l.
(a) 2.0 x 10m3M; 2OOmVs-‘,
(a)
k&A
500
/
f
I 0.2
0.4
[Ph. Te Cl] /m 2
4
6
8
t-0
mol
-1, L,
rPs 0.6
112
1-l
f 112 r Ps-‘/’
Fig. 2. The dependence of the limiting currents of the waves on the concentration of PhaTeCl (a), (b), (e), on the rate of rotation of the electrode (c), (d),(f) and the i; ’ vsf- ‘a curve (g). Half-wave potentials ( f 0.02 V): (a) and (d) anodic 1.23 V; (b), (c), (e) and (g) cathodic - 1.23 V and (f) anodic 0.50 V. Conditions: (f) After a passage of 9.42 cb cathodic at -0.80 V. Concentration of Ph,TeCk (c), (d) and (g) 1.0 x 1O-’ M, (f) 2.3 x 10e3 M. Concentration of TBAP 0.1 M. Rotation rate: (a) and (b) 10 revs-’ and (e) 40 rev s-l.
Electrochemistry
of organotellurium
found for Ph,TeCI, (4.8 x 10m6 cm2 s-‘[4]) and PhTeCl, (5.5 x 10m6 cm* s-‘[3]). Coulometric
measurements
compounds-111
formed (identified Reaction (6).
The electrode
Controlled potential coulometry experiments were carried out with a platinum gauze electrode and a platinum rde, as described elsewhere[3]. The changes in the concentrations of Ph,TeCl during the coulometry experiments were determined by measuring the limiting currents at the rde electrode. The dependence of the limiting currents of the waves on the amount of charge passed during coulometry experiments are shown in Fig. 3. During reduction of Ph,TeCl at - 0.80 V (Fig. 3a) a small amount of solid was precipitated at a total Faradaic efficiency of a few per cent which was identified (mass spectra) as a mixture of Ph,TeTePh,, (Bu,N)(Ph,Te), (Bu,N)(Ph,TeCl,) and (Ph,Te) (Ph,TeCl). It was also found that after cathodic electrolysis at - 0.80 V the solution contained Ph,TeTePh, (vpc-mass spectra) in measurable amount. Moreover, during cathodic coulometry at - 0.80 V a material was formed whose rate of diffusion controls the limiting current of its oxidation wave (E , 12 = 0.50 f 0.02 V), as the limiting current of the wave is linearly proportional to the amount of cathodic charge passed at - 0.80 V (Fig. 3b) and to the square root of the rate of rotation of the electrode (Fig. 2f). During oxidation of Ph,TeCl at 1.20 V (Fig 3ck the solution became saturated with Ph,(CI)TeTePh,(Cl) which precipitated and was identified by its mass spectra. It was also found that during the anodic electrolysis at 1.20 V benzene was
93
by vpc-mass
c
t
_b L 2
6
4
6
IO
q Icb
Fig. 3. The dependence of the limiting currents of the waves (at rde electrode) on the amount of charge 4 passed at constant potentials (at gauze electrode). Half-wave potentials ( + 0.02 V) and the constant potentials during mulornetry: (a) cathodic E, ,7 = - 1.23 V. u, cathodic at -0.80 V. (b) anodic E 1,2 = 0.50 V, q< cathodic at -0.80 V and (c) an&k ,E, ,2 = 1.23 V, qr anodic at 1.20 V. Conditions: conceniration of Ph,TeCk (a) and (bj 2.3 x lo-’ M, and (c) 4.2 x 10e3 M. Cokentration of TBAP 0.1 M. Rotation rate: 10 rev s-l. _I-
oia
The reduction of Ph,TeCL duction reaction of PhsTeC1 the following equations:
The two electrons may be summarized
rein
Ph,TeCl + e- + Ph,Te’ + Cl
(11
Ph,Te’+Bu,N++e-+ Ph,TeH
+ NBu, + C4H,.
(2)
Similar elimination of the quatemary ammonium cation (Bu,N+) in the presence of radicals formed in the reduction of organotellurium compounds were reported[3, 4]. While two reduction waves (E,,, = -0.77; - 1.10 V) were observed by Matsuo in aqueous solution[5], only one reduction wave was observed by us in aprotic solvents. The second cathodic wave found by Matsuo (El,t = - 1.10 V) whose half-wave potential shifted toward more negative potentials with increase in pH[S] may be due to reactions similar to those in which the triphenylgermane was formed[8] Ph,Te-
+ H * + Ph3TeH+
(31
+ e- + Ph,TeH.
(41
Ph,TeH+
However, similarly to the case of Ph,GeCl[S] and Ph,SnC1[9-11) the first electron transfer to Ph,TeCl in aprotic solvent can be followed by a dimerization reaction
---, Ph,TeTePh,
(5)
and as a result of this the reduction of Ph,TeCI shows a one electron transfer reaction. It was found that the dimedzation reaction is favoured by a high concentration of the Ph,Te’ radical in the vicinity of the electrode as was obtained at high concentration of Ph,TeCI in solution ( > 6 x 10e4 M) and at a high rate of rotation of the electrode. The two electron process following Equations (1) and (2) is favoured by a high rate of potential scan as in a cyclic voltammogram recorded at a high rate of potential scan the oxidation wave (Ep = 1.00 kO.02 V) of the tellurane appeared (Fig. Ic). Generally, the reduction of Tetv compounds like PhTeCl,[3] or Ph,TeC1,[4] results in the formation of diphenyltelluride. Similarly diphenylsulfide is formed in the electroreduction of triphenylsulfonium nitrate[7] and diphenyltin in the electroreduction of triphenyltin chloride[9, lo]. However Ph,Te was not formed in the reduction of triphenyltellurium chloride (even at - 1.40 V).
&,A
-
probably
reactions
ZPh,Te’
300
spectra),
The oxidation oJ Ph,TeCL The oxidation of of Ph,TeCl (El,, = 1.23 & 0.02 V) and the formation the ditellurlde may be summarized in the following equations: Ph,TeCl
+ Ph,TeCl
+ Ph
l
+ e-
(6)
__
2PhsTeCl’
4 Ph,(Cl)TeTePh,(Cl).
(7)
Thus when the potential was cycled at high rate of scan the cathodic waves of Ph+ (Ep = 0.70 + 0.02 V) and
94
Y. LIFTMANAND
of the ditelluride (E,,, = - 0.04 kO.02 V) appeared (Fig. lb). It was shown[3] that Ph,Te is formed and tellurium is precipitated in the reduction of C1,(Ph)TeTeCl,(Ph) (which is formed in the one electron reduction of PhTeCI,). However tellurium was not precipitated in the reduction of Ph,(Cl)TeTePh,(Cl).
M. ALBECK
4. Y. Liftraan and M. Albeck, Ekcrrorhim. 5. ‘. 7. 8.
REFERENCES 1. Y. Liftman. M. Albeck. J. M. E. Goldschmidt and Ch. Yarnit&, to be published. 2. Y. Liftman, M. Albeck and Ch. Yamitzky, to be published. 3. Y. Liftman and M. Albeck, Ekctrochim Acto 28, 1835 (1983).
9. 10.
11. 12.
Aera 28, 1841 (1983). H. Matsuo. J. Sci. Hiroshima Univ. A, 22.51 (1958); Chem. Abstr. 53, 9857b. H. Matsuo, J. Sci. Hiro,shimn Univ. A, 22, 281 (1958): Chem. Abstr. S4,4206h. M. Finkelstein, R. C. Peterson and S. D. Ross, J. electrodwm. Sot. 111, 422 (1963). B. Fleet and N. B. Fouzder, J. electroanal. Chem. 101,375 ( 1979). R. B. Allen, Diss. Abstr. 20, 897 (1959). C. K. Mann and K. K. Barnes, Electrochemical Reactions in Nonaqueous Systems, Marcel Dekker, New York (1970). 8. Fleet and N. B. Fouzder, J. elecrronnaL Chem. 63, 59-78 (1975). F. Opekar and P. Beran, J. elecrroanal. Chem. 69, 42 (1976).
of Chemistry.
and
M. ALBECK
Bar-Ilan University,
(Received
Ramat-Gan 52100, Israel
19 May 1983)
Abstract-The electrochemical behaviour of Ph,TeCl in methylene chloride, containing tetrabutylammonium perchlorate (TBAP), at a platinum electrode has been studied. It is shown that Ph,TeCI is oxidized to Ph,(Cl)TeTePh,(CI) and reduced either in a one electron reaction to Ph3TeTePh, or in a two electron reaction to Ph,TeH. The electrochemical reactions depend on the concentration of Ph,TeCl in the solution, on the rate of rotation of the electrode and on the rate of the potential scan.
INTRODUCTION
RESULTS
In our previous work on the electrochemistry of tellurium compounds in aprotic media[l+ it has been shown that the replacement of chloride ions in TeCI,[i, 21 by phenyl groups[3, 4] shifts the halfwave potential of the first cathodic wave toward a more negative value. Matsuo(5, 61 observed two reduction waves of Ph,TeCl in aqueous solution of KCl. The half wave potential of the first wave (-0.77 V) was independent of pH, while that of the second one ( - 1.10 V) shifted toward more negative potentials with increasing pH. It was reportedC71 that the electroreduction of triphenylsulfonium nitrate in dimethylformam ide or in water results in formation of dipheaylsulfide and benzene either in a one or in a two electron reaction. However in the first step of reduction of organometalJic compounds like Ph,GeCl[8] or Ph,SnCI[9%11 J the hexaphenyldigermane and hexaphenylditin were formed. In the second step the formation of the triphenylgermane[8] and triphenyltin anion[ 10, 1 l] or diphenyltinC9, lo] were suggested. It is shown here that in analogy to Ph,GeCl (and different from Ph,SNO,) triphenyltellurium chloride is reduced to hexaphenylditellurane or triphenyltellurane.
The uoltammograms
AND
DISCUSSIONS
of Ph,TeCl
The cyclic voltammograms of Ph,TeCl in methylene chloride containing TPAB as supporting electrolyte are shown in Fig. 1. The cathodic and anodic waves shown in curve (a) are due respectively to the reduction and oxidation of PhsTeCl. At higher rates of potential scan (1 V s - ‘) the other two cathodic waves, shown in curve (b), which follow the oxidation wave of Ph,TeCl are due to the reduction of the material formed in the oxidation of PhsTeCl. while the anodic wave shown in curve (c) which follows the reduction wave of Ph,TeCl is due to the oxidation of the material formed in the reduction of Ph,TeCl. As shown in Fig. 2, the limiting currents of both the oxidation (E,,, = 1.23 kO.02 V) and the reduction (E,,, = - 1.23 f 0.02 V) waves, at rde, are linearly proportional to the concentration of Ph,TeCl (pp to 0.6 mmol) and to fl” (up to 20revK’).
The dimerization
reactions
The lower values of il/[PhJTeCl] than in Fig. 2, which were obtained at high concentrations of Ph,TeCl, are ascribed to dimerization reactions (Equations 5 and 7). The following supports the assumption that the dimerization reactions are favoured at high rates of rotation of the electrode: (a) it was found by coulometric measurements performed as described eisewhere[3] that n = 1 when the electrode was rotating at a high rate and n = 2 when it was stationary, (b) it can be seen from curves (c) and (d) in Fig. 2 that i,/f - ‘I2 decreases wh& f in&&es, and from curve (g) that the decrease is more than expected when there is a linear dependence of i;’ onf-‘12 and 31 for the (cl employing the Levich equation[l2, limiting currents of the waves using the value of n = 1 gave a value of 4.2 x fOT6 cm’s_’ ( f 10 %) for the diffusion coe@cient of Ph3TeCI, which is in reasonable agreement with those
EXPERIMENTAL The description of the materials and instruments used was given elsewhere[l, 33. Reagent grade triphenyltelluronium chloride (PhsTeCl) was supplied by Organometallic Inc. (USA). The surface area of the rde electrode was 0.196 cm’. This same electrode was used in C.V. experiments with a stationary electrode. The volume of the solution was 50 ml. The surface of the electrode was cleaned by a method given in[ 11. The temperature in all experiments was 11 f 1°C unless otherwise indicated. . .” * Part of a Ph.D. thesis.
91
Y. LIFTMAN AND M. ALBECK
92
c
4OpA
L
Fig. 1. Cyclic voltammograms
of Ph,TeCl in CH,CI, (0.1 M TBAP). (b) and (c) 1.0 x 10m3 M; 1000 mV s-l.
(a) 2.0 x 10m3M; 2OOmVs-‘,
(a)
k&A
500
/
f
I 0.2
0.4
[Ph. Te Cl] /m 2
4
6
8
t-0
mol
-1, L,
rPs 0.6
112
1-l
f 112 r Ps-‘/’
Fig. 2. The dependence of the limiting currents of the waves on the concentration of PhaTeCl (a), (b), (e), on the rate of rotation of the electrode (c), (d),(f) and the i; ’ vsf- ‘a curve (g). Half-wave potentials ( f 0.02 V): (a) and (d) anodic 1.23 V; (b), (c), (e) and (g) cathodic - 1.23 V and (f) anodic 0.50 V. Conditions: (f) After a passage of 9.42 cb cathodic at -0.80 V. Concentration of Ph,TeCk (c), (d) and (g) 1.0 x 1O-’ M, (f) 2.3 x 10e3 M. Concentration of TBAP 0.1 M. Rotation rate: (a) and (b) 10 revs-’ and (e) 40 rev s-l.
Electrochemistry
of organotellurium
found for Ph,TeCI, (4.8 x 10m6 cm2 s-‘[4]) and PhTeCl, (5.5 x 10m6 cm* s-‘[3]). Coulometric
measurements
compounds-111
formed (identified Reaction (6).
The electrode
Controlled potential coulometry experiments were carried out with a platinum gauze electrode and a platinum rde, as described elsewhere[3]. The changes in the concentrations of Ph,TeCl during the coulometry experiments were determined by measuring the limiting currents at the rde electrode. The dependence of the limiting currents of the waves on the amount of charge passed during coulometry experiments are shown in Fig. 3. During reduction of Ph,TeCl at - 0.80 V (Fig. 3a) a small amount of solid was precipitated at a total Faradaic efficiency of a few per cent which was identified (mass spectra) as a mixture of Ph,TeTePh,, (Bu,N)(Ph,Te), (Bu,N)(Ph,TeCl,) and (Ph,Te) (Ph,TeCl). It was also found that after cathodic electrolysis at - 0.80 V the solution contained Ph,TeTePh, (vpc-mass spectra) in measurable amount. Moreover, during cathodic coulometry at - 0.80 V a material was formed whose rate of diffusion controls the limiting current of its oxidation wave (E , 12 = 0.50 f 0.02 V), as the limiting current of the wave is linearly proportional to the amount of cathodic charge passed at - 0.80 V (Fig. 3b) and to the square root of the rate of rotation of the electrode (Fig. 2f). During oxidation of Ph,TeCl at 1.20 V (Fig 3ck the solution became saturated with Ph,(CI)TeTePh,(Cl) which precipitated and was identified by its mass spectra. It was also found that during the anodic electrolysis at 1.20 V benzene was
93
by vpc-mass
c
t
_b L 2
6
4
6
IO
q Icb
Fig. 3. The dependence of the limiting currents of the waves (at rde electrode) on the amount of charge 4 passed at constant potentials (at gauze electrode). Half-wave potentials ( + 0.02 V) and the constant potentials during mulornetry: (a) cathodic E, ,7 = - 1.23 V. u, cathodic at -0.80 V. (b) anodic E 1,2 = 0.50 V, q< cathodic at -0.80 V and (c) an&k ,E, ,2 = 1.23 V, qr anodic at 1.20 V. Conditions: conceniration of Ph,TeCk (a) and (bj 2.3 x lo-’ M, and (c) 4.2 x 10e3 M. Cokentration of TBAP 0.1 M. Rotation rate: 10 rev s-l. _I-
oia
The reduction of Ph,TeCL duction reaction of PhsTeC1 the following equations:
The two electrons may be summarized
rein
Ph,TeCl + e- + Ph,Te’ + Cl
(11
Ph,Te’+Bu,N++e-+ Ph,TeH
+ NBu, + C4H,.
(2)
Similar elimination of the quatemary ammonium cation (Bu,N+) in the presence of radicals formed in the reduction of organotellurium compounds were reported[3, 4]. While two reduction waves (E,,, = -0.77; - 1.10 V) were observed by Matsuo in aqueous solution[5], only one reduction wave was observed by us in aprotic solvents. The second cathodic wave found by Matsuo (El,t = - 1.10 V) whose half-wave potential shifted toward more negative potentials with increase in pH[S] may be due to reactions similar to those in which the triphenylgermane was formed[8] Ph,Te-
+ H * + Ph3TeH+
(31
+ e- + Ph,TeH.
(41
Ph,TeH+
However, similarly to the case of Ph,GeCl[S] and Ph,SnC1[9-11) the first electron transfer to Ph,TeCl in aprotic solvent can be followed by a dimerization reaction
---, Ph,TeTePh,
(5)
and as a result of this the reduction of Ph,TeCI shows a one electron transfer reaction. It was found that the dimedzation reaction is favoured by a high concentration of the Ph,Te’ radical in the vicinity of the electrode as was obtained at high concentration of Ph,TeCI in solution ( > 6 x 10e4 M) and at a high rate of rotation of the electrode. The two electron process following Equations (1) and (2) is favoured by a high rate of potential scan as in a cyclic voltammogram recorded at a high rate of potential scan the oxidation wave (Ep = 1.00 kO.02 V) of the tellurane appeared (Fig. Ic). Generally, the reduction of Tetv compounds like PhTeCl,[3] or Ph,TeC1,[4] results in the formation of diphenyltelluride. Similarly diphenylsulfide is formed in the electroreduction of triphenylsulfonium nitrate[7] and diphenyltin in the electroreduction of triphenyltin chloride[9, lo]. However Ph,Te was not formed in the reduction of triphenyltellurium chloride (even at - 1.40 V).
&,A
-
probably
reactions
ZPh,Te’
300
spectra),
The oxidation oJ Ph,TeCL The oxidation of of Ph,TeCl (El,, = 1.23 & 0.02 V) and the formation the ditellurlde may be summarized in the following equations: Ph,TeCl
+ Ph,TeCl
+ Ph
l
+ e-
(6)
__
2PhsTeCl’
4 Ph,(Cl)TeTePh,(Cl).
(7)
Thus when the potential was cycled at high rate of scan the cathodic waves of Ph+ (Ep = 0.70 + 0.02 V) and
94
Y. LIFTMANAND
of the ditelluride (E,,, = - 0.04 kO.02 V) appeared (Fig. lb). It was shown[3] that Ph,Te is formed and tellurium is precipitated in the reduction of C1,(Ph)TeTeCl,(Ph) (which is formed in the one electron reduction of PhTeCI,). However tellurium was not precipitated in the reduction of Ph,(Cl)TeTePh,(Cl).
M. ALBECK
4. Y. Liftraan and M. Albeck, Ekcrrorhim. 5. ‘. 7. 8.
REFERENCES 1. Y. Liftman. M. Albeck. J. M. E. Goldschmidt and Ch. Yarnit&, to be published. 2. Y. Liftman, M. Albeck and Ch. Yamitzky, to be published. 3. Y. Liftman and M. Albeck, Ekctrochim Acto 28, 1835 (1983).
9. 10.
11. 12.
Aera 28, 1841 (1983). H. Matsuo. J. Sci. Hiroshima Univ. A, 22.51 (1958); Chem. Abstr. 53, 9857b. H. Matsuo, J. Sci. Hiro,shimn Univ. A, 22, 281 (1958): Chem. Abstr. S4,4206h. M. Finkelstein, R. C. Peterson and S. D. Ross, J. electrodwm. Sot. 111, 422 (1963). B. Fleet and N. B. Fouzder, J. electroanal. Chem. 101,375 ( 1979). R. B. Allen, Diss. Abstr. 20, 897 (1959). C. K. Mann and K. K. Barnes, Electrochemical Reactions in Nonaqueous Systems, Marcel Dekker, New York (1970). 8. Fleet and N. B. Fouzder, J. elecrronnaL Chem. 63, 59-78 (1975). F. Opekar and P. Beran, J. elecrroanal. Chem. 69, 42 (1976).