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A voltammetric study of several oxygen, sulfur and selenium heterocyclic carbonium ions in acetonitrile
J. Electroanal. Chem., 79 ( 1 9 7 7 ) 3 8 1 - - 3 9 0
381
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
A VOLTAMMETRIC STUDY OF SEVERAL OXYGEN, SULFUR AND SELENIUM HETEROCYCLIC CARBONIUM IONS IN ACETONITRILE
R O B E R T D. B R A U N
Department o f Chemistry, Vassar College, Poughkeepsie, N.Y. 12601 (U.S.A.) DENNIS C. G R E E N
IBM Thomas J. Watson Research Center, Yorktown Heights, N.Y. 10598 (U.S.A.) (Received 3rd May 1976; in revised form 8th July 1976)
ABSTRACT
Cyclic voltammetric data are reported on seven new oxygen, sulfur and selenium heterocyclic carbonium ions. Each c o m p o u n d undergoes electrochemical reduction to form a dimer. The reaction mechanisms are discussed. The compounds with selenoethoxy substituents formed heterofulvalenes at room temperature.
INTRODUCTION
Considerable interest has recently been shown [ 1--5] in several multi-sulfur heterocycles which are potentially useful in the electrochemical synthesis of substituted tetrathiafulvalenes. Tetrathiafulvalenes have been shown to have importance as cations of potential highly electrically-conducting organic salts [6]. This study was undertaken to test the feasibility o f electrochemically synthesizing new cations for use in potential new organic conductors. The results clearly indicate the usefulness of the electrochemical preparation of several new compounds. Moses et al. have examined 18 trithiocarbonium ions [5] and have concluded that these compounds can be electrochemically reduced to form dimers through a simple radical-radical coupling (DIM1 mechanism). The present cyclic voltammetric study of compounds I--VII is an extension of their work to several different heterocyclic compounds containing oxygen, sulfur and selenium. A cursory s
J~+~-SEtP6F ~Se I
Se
6
se
s
Se Tr
s "m
J~+~SEtPE~ :
seEtPFg
~
s~S-
,
o
0
~S--
sEtP~g
382 examination of I and VII has been reported [5]. C o m p o u n d VIII has been studied by Moses et al. [5,7] and was used in this study to estimate experimental diffusion coefficients of I--VI as well as for comparison purposes. EXPERIMENTAL Instrumentation Cyclic voltammetric experiments were carried out with a conventional threeelectrode assembly in conjunction with a Tacussel PRT 100--IX potentiostat and a Tacussel GSTP function generator. Voltammograms were recorded on a Hewlett-Packard/Mosely XY recorder for sweep rates up to 0.5 V s- 1 , and on a Hewlett-Packard 141A storage oscilloscope equipped with 14D2A dual-trace amplifiers and a Polaroid camera for faster sweep rates. All voltammetric experiments were carried out in a Vacuum Atmospheres model HE243 Dri-Lab dry box equipped with a MO 40-1 Dri-Train inert gas purifying unit. The n.m.r, spectra were recorded using a Varian T60 n.m.r, spectrometer. Cell The cell was a 150 ml beaker fitted with a Teflon cap containing holes for the electrodes. The electrodes were rigidly held a fixed distance apart by the cell cover. The vertical positions of the electrodes were adjustable. Stirring was accomplished with a magnetic stirrer and treflon-coated stirring bar. Electrodes The working electrode was a Beckman, platinum-inlay electrode. The area of the electrode was found to be 0.228 cm 2 by cyclic voltammetry using a 1 mM potassium ferrocyanide in 2 M KC1 solution. The counter electrode was constructed of flat platinum gauze. The reference electrode was a silver wire in a 0.01 M silver nitrate--0.1 M t e t r a e t h y l a m m o n i u m perchlorate (TEAP) solution in acetonitrile. The reference solution was contained in a glass tube with a 10 mm, fine-porosity, glass frit in the lower end, This reference electrode had a potential of 0.263 + 0.025 V when measured against an aqueous saturated calomel electrode (SCE). The potential of the reference electrode was compared to that of the SCE after the completion of each experiment. Fresh reference solution was prepared daily. Chemicals Acetonitrile (Aldrich Chemical Co.) was purified using the procedure of Moses [7]. Diethyl ether (J.T. Baker Chemical Co.) was distilled over sodium under a nitrogen atmosphere. Argon (Matheson Co., prepurified grade) was used to provide an inert atmosphere. Polarographic grade t e t r a e t h y l a m m o n i u m perchlorate (Southwestern Analytical Chemicals) was dried for three days prior to use at 65°C under vacuum.
383 Compounds I--VI and VIII were prepared from the corresponding thiones or selones (IX--XV) b y established procedures [4,7,8]. C o m p o u n d VII was prepared from XVI in 95% yield by the same general procedure used to prepare compounds I--VI and VIII. C o m p o u n d VII showed n.m.r, peaks at 8.5, 3.6 and 1.5 ppm (CDsCN, TMS) and had a melting point of 100--101°C. The salt was
se
s~___
extremely hygroscopic and deliquesced in minutes after removal from the inert atmosphere. The 1,3-dioxole-2-thione (XVI) was formed in 10% yield b y the reaction of P4S10 in refluxing toluene with 1,3-dioxole-2-one [9]. The 1,3-dithiol-2-thione (XV) was prepared from dimethylacetylene-dicarboxylate (Aldrich) and 1,3dithiolane-2-thione (Aldrich) b y the method of Melby et al. [10]. Compounds IX--XIV were the generous gifts of E.M. Engler and V.V. Patel. Procedures
All materials brought into the dry box were placed in the ante-chamber which was subsequently evacuated and filled with argon prior to moving them into the main compartment. Solvent distillation receivers were flushed with argon in the ante-chamber after six or seven partial evacuations and prior to solvent introduction. During electrochemical measurements the purification unit was turned off in order to prevent vibration and poisoning of the purification catalyst with sulfur compounds. Prior to a trial or a series of trials, an accurately weighed sample (10--50 mg) was placed in a clean cell and diluted to the desired concentration with solventsupporting electrolyte. When desired, the concentration of the sample was varied by in-cell dilution with solvent-supporting electrolyte. Before each voltammetric scan the platinum working electrode was polished with 0.03 pm Buehler Gamma micro-polish. Peak currents were measured from the base line obtained by extrapolating the background current before each cathodic or anodic peak. Voltammetric scan rates were varied from a minimum of 0.050 V s- 1 to a maximum of 20 V s- 1 . The apparent resistance of the cell was calculated with the aid of benzoquinone in TEAP-acetonitrile solutions. The first cyclic voltammetric reduction and reverse-scan reoxidation peaks of this system have been shown to be reversible at the platinum electrode [ 11 ]. The theoretical difference between the cathodic and anodic peak potentials for a reversible one-electron transfer is 58 mV [12]. The larger difference in potential observed while using benzoquinone in the cell used for the present study was attributed to iR drop through the cell. The apparent cell resistance was calculated using Ohm's law from the benzoquinone peak currents and the increase in peak potential separation above 58
384 inV. The calculated value at scan rates between 0.1 and 20.0 V s- 1 was f o u n d to be 64 + 3 ~ . This value was used in conjunction with peak and half-peak currents to correct the potentials reported in this paper for iR drop. RESULTS AND DISCUSSION The 1,3-dioxolium cation (compound VII) exhibited behavior unlike that of compounds I--VI. Initially a single cathodic peak was observed at about --1.3 V. Reversal of the scan showed no anodic peak. Further scans revealed t h a t a second cathodic peak at about --1.0 V appeared and grew with time while the original peak became smaller. Eventually the --1.0 V peak completely obscured the original peak. The appearance and growth of the second peak made meaningful measurements on the original peak impossible. Compounds I--VI exhibited single, well-defined, cathodic peaks (Fig. 1, Table 1) in acetonitrile during the initial scan toward negative potentials. During the reverse scan no anodic peak complementary to the cathodic peak was observed even at the highest scan rates employed in this study for compounds I, II and VI. The reverse scan of compounds III--V revealed two reversible oxidation peaks (Fig. 2) that correspond to the respective tetrathiafulvalene or selenium substituted analog mono-cation and di-cation oxidation waves. Neither of these peak pairs was present when the initial scan was toward positive rather than negative potentials, i.e., the peaks are apparently due to products of the initial reduction. Compounds III, IV and V exhibited two apparently reversible anodic peaks at 0.32 and 0.69, 0.40 and 0.72, and 0.47 and 0.75 V vs. SCE, respectively. These compare to oxidation waves observed [13] at 0.33 and 0.70, 0.40 and 0.72, and 0.48 and 0.76 V vs. SCE, respectively for tetrathiafulvalene (TTF), diselenadithiafulvalene (DSeDTF), and tetraselenofulvalene (TSeF). The cathodic peak heights (ip) of compounds I--VI observed on initial scan
+I00
-IO0 -I0
E/V
+I0
-I
E/V
Fig. 1. A cathodic peak typical of tho~e observed for compounds I--VI. The voltammogram of 1.0 rnM VI at a scan rate of 0.351 V s-1 Fig. 2. A cyclic voltammogram of 4.62 mM IV at a scan rate of 1 V s-1. The scan starts at 0 V and initially moves toward more negative potentials.
385 TABLE 1 Voltammetric peak potentials and half-peak potentials of several heterocyclic carbonium ions at a scan rate of 0.5 V s- 1 Compound
Concentration/raM
--Ep/V a
__Epl2/V a
I II III IV V VI VIII
3,33 1,00 0,84 0.93 0 91 1.00 1.00
0.800 0.809 0.857 0.821 0.802 0.562 0.811
0.753 0.766 0.775 0.765 0.751 0.500 0.769
a Potentials are relative to the silver-silver nitrate (0.01 M) in acetonitrile reference electrode.
toward negative potentials varied linearly with the square root of scan r a t e (01/2 ) (Fig. 3 ) . Compound VIII reportedly follows a radical-radical coupling dimerization mechanism (D]M1 mechanism) and has a coulometric n of 1.00 [5]. At 25°C for a DIM1 mechanism voltammetric peaks obey the expression given in eqn. ( 1 ) ip = (3.17 X 1 0 5 n a l 2 A D l l 2 c ) v 112
(1)
where ip is the peak current (pA), n is the n u m b e r of electrons transferred in the faradaic process, A is the electrode area (cm2), D is the diffusion coefficient (cm 2 s - l ) , c is the bulk solution concentration (raM), and v is the scan rate (V
1500
A 700
A 1000
500
•
<
§
V
.< =1.
500
300
100
v V2/V'/~ s-'/~
5
O
2.5
c / rnM
5
Fig. 3. The variation of peak current, ip, with the square root of scan rate, v1/2, for (A) 0.91 mM V, and (B) 0.84 mM III. Fig. 4. The variation of peak current, ip, with concentration, c, for IV at scan rates of (A) 5.7 V s- 1 , (B) 1.0 V s- 1 and (C) 0.2 V s- 1 .
386 TABLE 2 Electrochemical parameters for c o m p o u n d s I--VI Compound
c/mM
( Aip/ Avl/2)/
nb
(/2A-s1/2 v--1/2) a I II III IV V VI
1.00 1.00 0.835 0.925 0.914 1.00
220 176 162 165 168 170
0.99 0.94 0.91 0.86 0.88 0.83
a Tbe slopes of ip--v 1/2 plots. b Calculated assuming a diffusion coefficient o f 9.61 × 10 - 6 cm 2 s"-1 f r o m the slopes of ip--V112 plots. These values are calculated assuming a DIM1 reaction mechanism.
s - l ) . By using the coulometric n of 1.00 and the measured values of A and c, the diffusion coefficient for c o m p o u n d VIII can be calculated by setting the slope of the ip--V 1]2 plot for this c o m p o u n d equal to the bracketed terms in eqn. (1). Completion of this procedure resulted in a calculated D for c o m p o u n d VIII of 9.61 × 1 0 - 6 c m 2 s - 1 . Since compounds I--VI are similar in structure, size, molecular weight, and ionic charge to c o m p o u n d VIII, it is safe to conclude that at least to a first approximation the diffusion coefficients of c o m p o u n d s I--VI are equal to that of c o m p o u n d VIII. By setting the'slope of t h e ip--V 1/2 plot for each c o m p o u n d equal to the bracketed terms in eqn. (1) while using the D for c o m p o u n d VIII, an estimate of n can be obtained for compounds I--VI. These values are listed in Table 2. It should be noted that although the estimated values of n assume a DIM1 reaction mechanism (see later discussion of reaction mechanisms), the assumption of other reaction mechanisms yiels similar values. The peak current for compounds I--VI varied linearly with concentration at the lower scan rates (0.05--0.5 V s - l ) ; however, the ip--C plots became increasingly nonlinear at higher scan rates (Fig. 4). At the slower scan rates, single sweep voltammetry may prove to be useful for the quantitative analysis of c o m p o u n d s I--VI. Unfortunately, with the exception of c o m p o u n d VI which has a significantly lower Ep than c o m p o u n d s I--V, it would be difficult to differentiate between these c o m p o u n d s in a mixture. APPLICATION OF DIAGNOSTIC CRITERIA
In an effort to determine the mechanistic pathway followed during the electrochemical reactions, t h e results obtained in this study were compared with those predicted from theory for different types of electrochemical reactions. Nicholson and Shain [14] have calculated theoretical curves for the variation of ip/v 1/2 with log v and AEp/2/A log v with log v for eight cases including reversible and irreversible charge transfers, a chemical reaction preceding a reversible or irreversible charge transfer, a reversible or irreversible chemical reaction following a,reversible charge transfer, and a catalytic reaction following a reversible or irreversible charge transfer. Plots of i,/vl12--1og v obtained in this study (Fig. 5)
387
20O
>
2>
\
O,OL
>
<150
P
-
O.OL
uJ
I
-10
I--
log (v/V S-~)
0.0~ I
110
-1.0
i
log (v/V 5-'1
110
Fig. 5. The variation, characteristic of compounds I--VI, of ip/V1/2 with log v for 1.0 mM II. Fig. 6. The variation, characteristic of compounds I--VI, of AEpl2[ A log v for 1.0 mM II.
were similar in shape to those plots predicted for a chemical reaction preceding a reversible or irreversible charge transfer followed by an irreversible chemical reaction. All three of these possibilities were eliminated however when the shapes of the AEp/2/Alog v--log v plots (Fig. 6) were compared with the theoretical plots. None of the theoretical plots had a shape similar to those observed for compounds I--VI. Moses et al. [3,5] in their studies of c o m p o u n d s structurally similar to those examined in this study, concluded that the charge transfer was followed by a dimerization reaction. In order to check this possibility, the results of this study were compared with the results which have been theoretically predicted by Andrieux et al. [15] for various types of dimerization reaction mechanisms following a charge transfer. Plots of Ep as a function of log v (Fig. 7) for the cathodic waves resulted in nearly straight lines for compounds V and VI. Compounds II, III and IV appeared to have two straight line portions, one from scan rates of 0.05 to 1 V s - 1 and the second more steep portion from scan rates of I to 20 V s- 1 . C o m p o u n d I also exhibited two straight line portions, the first for scan rates up to about
80C " 550
UJ p
90C 65O -lO
10 log ( v / V s-D
Fig. 7. The variation of Ep with log v for (A) 1.0 mM VI and (B) 0.93 mM IV.
388 TABLE 3 Diagnostic voltammetric parameters for compounds I--VI Compound
I II III IV V VI
--(A~,p]A log v)/
"-( AEpl2/A
(mV]deeade)
log (mV/decade)
15 20 22 15 18 33
28 29 30 29 32 40
v)]
Ep --Ep/2lmV a 46.5 + 6.1 46.6 -+ 12.2 89.8 + 7.2 52.9 -+ 11.1 51.2-+ 7.3 71.8 -+ 11.0
(13) (16) (15) (18) (20) (17)
a U.ncertainties are expressed as standard deviations. The numbers in parentheses are the number of trials upon which the figures are based.
0.33 V s- 1 and the second, more steep portion at higher scan rates. It is difficult to explain those plots with two portions; however, it is possible that the increased negative slopes at the higher scan rates are due to a shift in peak potentials toward more negative values as a result of uncompensated iR drop in the cell. Plots of Ep]2--1og v were either straight lines throughout or lines with only a slight curvature toward more negative Ep/2 values. The slopes of the lines obtained for each of the six compounds from both the Ep--log v and Ep/2--1og v plots are listed in Table 3. For compounds I--IV the slopes listed for the Ep--log v plots are for the lower scan rates. At higher scan rates the slopes varied between --41 and --46 mV/decade. Values of E , -- Ep/2 for all six compounds were calculated at varying concentrations and scan rates. The results including uncertainties expressed as standard deviations are listed in Table 3. Another diagnostic criterion which may be applied to possible electrochemical dimerization reactions is the rate in change of Ep with log c. The values of Ep are plotted as a function of log c and the slope of the resulting straight line is compared with theoretically calculated values [15]. The present data were too scattered to permit the accurate determination of a value of the slope of the Ep--log c plot. REACTION MECHANISMS
The application of the diagnostic criteria of Nicholson and Shain [14] to compounds I--VI revealed that these compounds do n o t obey the reaction mechanisms for reversible or irreversible electron transfers uncoupled with a chemical reaction, a chemical reaction preceding a reversible or irreversible electron transfer, a reversible or irreversible chemical reaction following a reversible electron transfer, or a catalytic reaction following a reversible or irreversible electron transfer. Consequently these reaction mechanisms need no longer be considered possibilities for compounds I--VI. Both the calculated values of n (Table 2} and the application of the diagnostic criteria of Andrieux et al. [ 15] i n ~ c a t e that compounds I--VI electrochemically form dimers. Unfortunately comparison of the Ep -- Ep/2 values (Table 3) with theoretical values does n o t permit the limiting of each dimerization reaction to one mechanism.
389 The products of the electrochemical dimerization reactions for compounds III--V are probably tetrathiafulvalene (XVII), diselenadithiafulvalene (XVIII and XIX), and tetraselenafulvalene (XX). These conclusions are based on the appearance of cyclic voltammetric, anodic peaks with the expected Ep values (cf. earlier 'discussion). Similar behavior was not observed with the analogs previously
Z]zlI
XIX
XX
reported [ 5] and with compounds I, II, VI, VII and VIII. The latter compounds apparently form stable dimers analogous to the dimer (XXI) of compound VIII [ 5]. Thermolytic cleavage of the thioethoxy groups of these dimers to yield
X~
dimers analogous to compounds XVII--XX occurs at about 100°C [13,16]. Apparently the selenoethoxy group is cleaved at room temperature rather than at 100°C. The results obtained with compound VII are inconclusive since the rapid growth of a second cathodic peak masked the original peak. Data on this compound have been reported [5,17]. That work, however, was carried out in the atmosphere as opposed to the present study which was done under a dry, inert gas. Since compound VII is extremely hygroscopic and deliquesces in minutes in moist air, the previously reported data are not likely to be accurate. The position of the peak reported by Moses et al. makes it seem likely that those workers did not observe the original cathodic peak. It is not apparent whether compound VII undergoes a decomposition reaction in the solvent prior to electron transfer, or a decomposition or polymerization reaction after electron transfer. REFERENCES 1 D.L. Coffen, J.Q. Chambers, D.R. Wflhams, P.E. Garrett and M.D. Canfield, J. Amer. Chem. Soc., 93 (1971) 2258. 2 J.Q. Chambers, P.S. Moses and R.N. Shelton, J. Eleetroanal. Chem., 38 (1972) 245. 3 P.I~. Moses and J.Q. Chambers, J. Electroanal. Chem., 49 (1974) 105. 4 P.R. Moses and J.Q. Chambers, J. Amer. Chem. Soe., 95 (1973) 948. 5 P.R. Moses, J.Q. Chambers, J.O. Sutherland and D.R. Williams, J. Electrochem. Soc., 122 (1975) 608. 6 W.D. Metz, Semnce, 180 (1973) 1041. 7 P.R. Moses, Ph.D. Dissertation, University of Tennessee, Knoxville, 1974. 8 E.M. Engler and V.V. Patel, private correspondence. 9 H.M. Fishler and W. Hartmann, Chem. Ber., 105 (1972) 2769. 10 L.R. Melby, H.D. Hartzler a n d W . A . Sheppard, J. Org. Chem., 39 (1974) 2456. 11 B.R. Eggins, Chem. Commun., (1967) 1267. 12 R.N. Adams, Electro chemistry at Solid Electrodes, Marcel Dekker, New York, 1969, p. 145.
390 13 E.M. Engler, F.B. Kaufman, D.C. Green, C.E. Klots and R.N. C ompt on, J. Amer. Chem. Soc., 97 (1975) 2921. 14 R.S. Nlcholson and I. Shain, Anal. Chem., 36 (1964) 706. 15 C.P. Andrieux, L. NadJo and J.M. Saveant, J. Electroanal. Chem., 26 (1970) 147. 16 P.R. Moses and J.Q. Chambers, J. Amer. Chem. Soc., 96 (1974) 945. 17 These data were o btained by D.C. Green and are p u b h s h e d with acknowledgement.
381
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
A VOLTAMMETRIC STUDY OF SEVERAL OXYGEN, SULFUR AND SELENIUM HETEROCYCLIC CARBONIUM IONS IN ACETONITRILE
R O B E R T D. B R A U N
Department o f Chemistry, Vassar College, Poughkeepsie, N.Y. 12601 (U.S.A.) DENNIS C. G R E E N
IBM Thomas J. Watson Research Center, Yorktown Heights, N.Y. 10598 (U.S.A.) (Received 3rd May 1976; in revised form 8th July 1976)
ABSTRACT
Cyclic voltammetric data are reported on seven new oxygen, sulfur and selenium heterocyclic carbonium ions. Each c o m p o u n d undergoes electrochemical reduction to form a dimer. The reaction mechanisms are discussed. The compounds with selenoethoxy substituents formed heterofulvalenes at room temperature.
INTRODUCTION
Considerable interest has recently been shown [ 1--5] in several multi-sulfur heterocycles which are potentially useful in the electrochemical synthesis of substituted tetrathiafulvalenes. Tetrathiafulvalenes have been shown to have importance as cations of potential highly electrically-conducting organic salts [6]. This study was undertaken to test the feasibility o f electrochemically synthesizing new cations for use in potential new organic conductors. The results clearly indicate the usefulness of the electrochemical preparation of several new compounds. Moses et al. have examined 18 trithiocarbonium ions [5] and have concluded that these compounds can be electrochemically reduced to form dimers through a simple radical-radical coupling (DIM1 mechanism). The present cyclic voltammetric study of compounds I--VII is an extension of their work to several different heterocyclic compounds containing oxygen, sulfur and selenium. A cursory s
J~+~-SEtP6F ~Se I
Se
6
se
s
Se Tr
s "m
J~+~SEtPE~ :
seEtPFg
~
s~S-
,
o
0
~S--
sEtP~g
382 examination of I and VII has been reported [5]. C o m p o u n d VIII has been studied by Moses et al. [5,7] and was used in this study to estimate experimental diffusion coefficients of I--VI as well as for comparison purposes. EXPERIMENTAL Instrumentation Cyclic voltammetric experiments were carried out with a conventional threeelectrode assembly in conjunction with a Tacussel PRT 100--IX potentiostat and a Tacussel GSTP function generator. Voltammograms were recorded on a Hewlett-Packard/Mosely XY recorder for sweep rates up to 0.5 V s- 1 , and on a Hewlett-Packard 141A storage oscilloscope equipped with 14D2A dual-trace amplifiers and a Polaroid camera for faster sweep rates. All voltammetric experiments were carried out in a Vacuum Atmospheres model HE243 Dri-Lab dry box equipped with a MO 40-1 Dri-Train inert gas purifying unit. The n.m.r, spectra were recorded using a Varian T60 n.m.r, spectrometer. Cell The cell was a 150 ml beaker fitted with a Teflon cap containing holes for the electrodes. The electrodes were rigidly held a fixed distance apart by the cell cover. The vertical positions of the electrodes were adjustable. Stirring was accomplished with a magnetic stirrer and treflon-coated stirring bar. Electrodes The working electrode was a Beckman, platinum-inlay electrode. The area of the electrode was found to be 0.228 cm 2 by cyclic voltammetry using a 1 mM potassium ferrocyanide in 2 M KC1 solution. The counter electrode was constructed of flat platinum gauze. The reference electrode was a silver wire in a 0.01 M silver nitrate--0.1 M t e t r a e t h y l a m m o n i u m perchlorate (TEAP) solution in acetonitrile. The reference solution was contained in a glass tube with a 10 mm, fine-porosity, glass frit in the lower end, This reference electrode had a potential of 0.263 + 0.025 V when measured against an aqueous saturated calomel electrode (SCE). The potential of the reference electrode was compared to that of the SCE after the completion of each experiment. Fresh reference solution was prepared daily. Chemicals Acetonitrile (Aldrich Chemical Co.) was purified using the procedure of Moses [7]. Diethyl ether (J.T. Baker Chemical Co.) was distilled over sodium under a nitrogen atmosphere. Argon (Matheson Co., prepurified grade) was used to provide an inert atmosphere. Polarographic grade t e t r a e t h y l a m m o n i u m perchlorate (Southwestern Analytical Chemicals) was dried for three days prior to use at 65°C under vacuum.
383 Compounds I--VI and VIII were prepared from the corresponding thiones or selones (IX--XV) b y established procedures [4,7,8]. C o m p o u n d VII was prepared from XVI in 95% yield by the same general procedure used to prepare compounds I--VI and VIII. C o m p o u n d VII showed n.m.r, peaks at 8.5, 3.6 and 1.5 ppm (CDsCN, TMS) and had a melting point of 100--101°C. The salt was
se
s~___
extremely hygroscopic and deliquesced in minutes after removal from the inert atmosphere. The 1,3-dioxole-2-thione (XVI) was formed in 10% yield b y the reaction of P4S10 in refluxing toluene with 1,3-dioxole-2-one [9]. The 1,3-dithiol-2-thione (XV) was prepared from dimethylacetylene-dicarboxylate (Aldrich) and 1,3dithiolane-2-thione (Aldrich) b y the method of Melby et al. [10]. Compounds IX--XIV were the generous gifts of E.M. Engler and V.V. Patel. Procedures
All materials brought into the dry box were placed in the ante-chamber which was subsequently evacuated and filled with argon prior to moving them into the main compartment. Solvent distillation receivers were flushed with argon in the ante-chamber after six or seven partial evacuations and prior to solvent introduction. During electrochemical measurements the purification unit was turned off in order to prevent vibration and poisoning of the purification catalyst with sulfur compounds. Prior to a trial or a series of trials, an accurately weighed sample (10--50 mg) was placed in a clean cell and diluted to the desired concentration with solventsupporting electrolyte. When desired, the concentration of the sample was varied by in-cell dilution with solvent-supporting electrolyte. Before each voltammetric scan the platinum working electrode was polished with 0.03 pm Buehler Gamma micro-polish. Peak currents were measured from the base line obtained by extrapolating the background current before each cathodic or anodic peak. Voltammetric scan rates were varied from a minimum of 0.050 V s- 1 to a maximum of 20 V s- 1 . The apparent resistance of the cell was calculated with the aid of benzoquinone in TEAP-acetonitrile solutions. The first cyclic voltammetric reduction and reverse-scan reoxidation peaks of this system have been shown to be reversible at the platinum electrode [ 11 ]. The theoretical difference between the cathodic and anodic peak potentials for a reversible one-electron transfer is 58 mV [12]. The larger difference in potential observed while using benzoquinone in the cell used for the present study was attributed to iR drop through the cell. The apparent cell resistance was calculated using Ohm's law from the benzoquinone peak currents and the increase in peak potential separation above 58
384 inV. The calculated value at scan rates between 0.1 and 20.0 V s- 1 was f o u n d to be 64 + 3 ~ . This value was used in conjunction with peak and half-peak currents to correct the potentials reported in this paper for iR drop. RESULTS AND DISCUSSION The 1,3-dioxolium cation (compound VII) exhibited behavior unlike that of compounds I--VI. Initially a single cathodic peak was observed at about --1.3 V. Reversal of the scan showed no anodic peak. Further scans revealed t h a t a second cathodic peak at about --1.0 V appeared and grew with time while the original peak became smaller. Eventually the --1.0 V peak completely obscured the original peak. The appearance and growth of the second peak made meaningful measurements on the original peak impossible. Compounds I--VI exhibited single, well-defined, cathodic peaks (Fig. 1, Table 1) in acetonitrile during the initial scan toward negative potentials. During the reverse scan no anodic peak complementary to the cathodic peak was observed even at the highest scan rates employed in this study for compounds I, II and VI. The reverse scan of compounds III--V revealed two reversible oxidation peaks (Fig. 2) that correspond to the respective tetrathiafulvalene or selenium substituted analog mono-cation and di-cation oxidation waves. Neither of these peak pairs was present when the initial scan was toward positive rather than negative potentials, i.e., the peaks are apparently due to products of the initial reduction. Compounds III, IV and V exhibited two apparently reversible anodic peaks at 0.32 and 0.69, 0.40 and 0.72, and 0.47 and 0.75 V vs. SCE, respectively. These compare to oxidation waves observed [13] at 0.33 and 0.70, 0.40 and 0.72, and 0.48 and 0.76 V vs. SCE, respectively for tetrathiafulvalene (TTF), diselenadithiafulvalene (DSeDTF), and tetraselenofulvalene (TSeF). The cathodic peak heights (ip) of compounds I--VI observed on initial scan
+I00
-IO0 -I0
E/V
+I0
-I
E/V
Fig. 1. A cathodic peak typical of tho~e observed for compounds I--VI. The voltammogram of 1.0 rnM VI at a scan rate of 0.351 V s-1 Fig. 2. A cyclic voltammogram of 4.62 mM IV at a scan rate of 1 V s-1. The scan starts at 0 V and initially moves toward more negative potentials.
385 TABLE 1 Voltammetric peak potentials and half-peak potentials of several heterocyclic carbonium ions at a scan rate of 0.5 V s- 1 Compound
Concentration/raM
--Ep/V a
__Epl2/V a
I II III IV V VI VIII
3,33 1,00 0,84 0.93 0 91 1.00 1.00
0.800 0.809 0.857 0.821 0.802 0.562 0.811
0.753 0.766 0.775 0.765 0.751 0.500 0.769
a Potentials are relative to the silver-silver nitrate (0.01 M) in acetonitrile reference electrode.
toward negative potentials varied linearly with the square root of scan r a t e (01/2 ) (Fig. 3 ) . Compound VIII reportedly follows a radical-radical coupling dimerization mechanism (D]M1 mechanism) and has a coulometric n of 1.00 [5]. At 25°C for a DIM1 mechanism voltammetric peaks obey the expression given in eqn. ( 1 ) ip = (3.17 X 1 0 5 n a l 2 A D l l 2 c ) v 112
(1)
where ip is the peak current (pA), n is the n u m b e r of electrons transferred in the faradaic process, A is the electrode area (cm2), D is the diffusion coefficient (cm 2 s - l ) , c is the bulk solution concentration (raM), and v is the scan rate (V
1500
A 700
A 1000
500
•
<
§
V
.< =1.
500
300
100
v V2/V'/~ s-'/~
5
O
2.5
c / rnM
5
Fig. 3. The variation of peak current, ip, with the square root of scan rate, v1/2, for (A) 0.91 mM V, and (B) 0.84 mM III. Fig. 4. The variation of peak current, ip, with concentration, c, for IV at scan rates of (A) 5.7 V s- 1 , (B) 1.0 V s- 1 and (C) 0.2 V s- 1 .
386 TABLE 2 Electrochemical parameters for c o m p o u n d s I--VI Compound
c/mM
( Aip/ Avl/2)/
nb
(/2A-s1/2 v--1/2) a I II III IV V VI
1.00 1.00 0.835 0.925 0.914 1.00
220 176 162 165 168 170
0.99 0.94 0.91 0.86 0.88 0.83
a Tbe slopes of ip--v 1/2 plots. b Calculated assuming a diffusion coefficient o f 9.61 × 10 - 6 cm 2 s"-1 f r o m the slopes of ip--V112 plots. These values are calculated assuming a DIM1 reaction mechanism.
s - l ) . By using the coulometric n of 1.00 and the measured values of A and c, the diffusion coefficient for c o m p o u n d VIII can be calculated by setting the slope of the ip--V 1]2 plot for this c o m p o u n d equal to the bracketed terms in eqn. (1). Completion of this procedure resulted in a calculated D for c o m p o u n d VIII of 9.61 × 1 0 - 6 c m 2 s - 1 . Since compounds I--VI are similar in structure, size, molecular weight, and ionic charge to c o m p o u n d VIII, it is safe to conclude that at least to a first approximation the diffusion coefficients of c o m p o u n d s I--VI are equal to that of c o m p o u n d VIII. By setting the'slope of t h e ip--V 1/2 plot for each c o m p o u n d equal to the bracketed terms in eqn. (1) while using the D for c o m p o u n d VIII, an estimate of n can be obtained for compounds I--VI. These values are listed in Table 2. It should be noted that although the estimated values of n assume a DIM1 reaction mechanism (see later discussion of reaction mechanisms), the assumption of other reaction mechanisms yiels similar values. The peak current for compounds I--VI varied linearly with concentration at the lower scan rates (0.05--0.5 V s - l ) ; however, the ip--C plots became increasingly nonlinear at higher scan rates (Fig. 4). At the slower scan rates, single sweep voltammetry may prove to be useful for the quantitative analysis of c o m p o u n d s I--VI. Unfortunately, with the exception of c o m p o u n d VI which has a significantly lower Ep than c o m p o u n d s I--V, it would be difficult to differentiate between these c o m p o u n d s in a mixture. APPLICATION OF DIAGNOSTIC CRITERIA
In an effort to determine the mechanistic pathway followed during the electrochemical reactions, t h e results obtained in this study were compared with those predicted from theory for different types of electrochemical reactions. Nicholson and Shain [14] have calculated theoretical curves for the variation of ip/v 1/2 with log v and AEp/2/A log v with log v for eight cases including reversible and irreversible charge transfers, a chemical reaction preceding a reversible or irreversible charge transfer, a reversible or irreversible chemical reaction following a,reversible charge transfer, and a catalytic reaction following a reversible or irreversible charge transfer. Plots of i,/vl12--1og v obtained in this study (Fig. 5)
387
20O
>
2>
\
O,OL
>
<150
P
-
O.OL
uJ
I
-10
I--
log (v/V S-~)
0.0~ I
110
-1.0
i
log (v/V 5-'1
110
Fig. 5. The variation, characteristic of compounds I--VI, of ip/V1/2 with log v for 1.0 mM II. Fig. 6. The variation, characteristic of compounds I--VI, of AEpl2[ A log v for 1.0 mM II.
were similar in shape to those plots predicted for a chemical reaction preceding a reversible or irreversible charge transfer followed by an irreversible chemical reaction. All three of these possibilities were eliminated however when the shapes of the AEp/2/Alog v--log v plots (Fig. 6) were compared with the theoretical plots. None of the theoretical plots had a shape similar to those observed for compounds I--VI. Moses et al. [3,5] in their studies of c o m p o u n d s structurally similar to those examined in this study, concluded that the charge transfer was followed by a dimerization reaction. In order to check this possibility, the results of this study were compared with the results which have been theoretically predicted by Andrieux et al. [15] for various types of dimerization reaction mechanisms following a charge transfer. Plots of Ep as a function of log v (Fig. 7) for the cathodic waves resulted in nearly straight lines for compounds V and VI. Compounds II, III and IV appeared to have two straight line portions, one from scan rates of 0.05 to 1 V s - 1 and the second more steep portion from scan rates of I to 20 V s- 1 . C o m p o u n d I also exhibited two straight line portions, the first for scan rates up to about
80C " 550
UJ p
90C 65O -lO
10 log ( v / V s-D
Fig. 7. The variation of Ep with log v for (A) 1.0 mM VI and (B) 0.93 mM IV.
388 TABLE 3 Diagnostic voltammetric parameters for compounds I--VI Compound
I II III IV V VI
--(A~,p]A log v)/
"-( AEpl2/A
(mV]deeade)
log (mV/decade)
15 20 22 15 18 33
28 29 30 29 32 40
v)]
Ep --Ep/2lmV a 46.5 + 6.1 46.6 -+ 12.2 89.8 + 7.2 52.9 -+ 11.1 51.2-+ 7.3 71.8 -+ 11.0
(13) (16) (15) (18) (20) (17)
a U.ncertainties are expressed as standard deviations. The numbers in parentheses are the number of trials upon which the figures are based.
0.33 V s- 1 and the second, more steep portion at higher scan rates. It is difficult to explain those plots with two portions; however, it is possible that the increased negative slopes at the higher scan rates are due to a shift in peak potentials toward more negative values as a result of uncompensated iR drop in the cell. Plots of Ep]2--1og v were either straight lines throughout or lines with only a slight curvature toward more negative Ep/2 values. The slopes of the lines obtained for each of the six compounds from both the Ep--log v and Ep/2--1og v plots are listed in Table 3. For compounds I--IV the slopes listed for the Ep--log v plots are for the lower scan rates. At higher scan rates the slopes varied between --41 and --46 mV/decade. Values of E , -- Ep/2 for all six compounds were calculated at varying concentrations and scan rates. The results including uncertainties expressed as standard deviations are listed in Table 3. Another diagnostic criterion which may be applied to possible electrochemical dimerization reactions is the rate in change of Ep with log c. The values of Ep are plotted as a function of log c and the slope of the resulting straight line is compared with theoretically calculated values [15]. The present data were too scattered to permit the accurate determination of a value of the slope of the Ep--log c plot. REACTION MECHANISMS
The application of the diagnostic criteria of Nicholson and Shain [14] to compounds I--VI revealed that these compounds do n o t obey the reaction mechanisms for reversible or irreversible electron transfers uncoupled with a chemical reaction, a chemical reaction preceding a reversible or irreversible electron transfer, a reversible or irreversible chemical reaction following a reversible electron transfer, or a catalytic reaction following a reversible or irreversible electron transfer. Consequently these reaction mechanisms need no longer be considered possibilities for compounds I--VI. Both the calculated values of n (Table 2} and the application of the diagnostic criteria of Andrieux et al. [ 15] i n ~ c a t e that compounds I--VI electrochemically form dimers. Unfortunately comparison of the Ep -- Ep/2 values (Table 3) with theoretical values does n o t permit the limiting of each dimerization reaction to one mechanism.
389 The products of the electrochemical dimerization reactions for compounds III--V are probably tetrathiafulvalene (XVII), diselenadithiafulvalene (XVIII and XIX), and tetraselenafulvalene (XX). These conclusions are based on the appearance of cyclic voltammetric, anodic peaks with the expected Ep values (cf. earlier 'discussion). Similar behavior was not observed with the analogs previously
Z]zlI
XIX
XX
reported [ 5] and with compounds I, II, VI, VII and VIII. The latter compounds apparently form stable dimers analogous to the dimer (XXI) of compound VIII [ 5]. Thermolytic cleavage of the thioethoxy groups of these dimers to yield
X~
dimers analogous to compounds XVII--XX occurs at about 100°C [13,16]. Apparently the selenoethoxy group is cleaved at room temperature rather than at 100°C. The results obtained with compound VII are inconclusive since the rapid growth of a second cathodic peak masked the original peak. Data on this compound have been reported [5,17]. That work, however, was carried out in the atmosphere as opposed to the present study which was done under a dry, inert gas. Since compound VII is extremely hygroscopic and deliquesces in minutes in moist air, the previously reported data are not likely to be accurate. The position of the peak reported by Moses et al. makes it seem likely that those workers did not observe the original cathodic peak. It is not apparent whether compound VII undergoes a decomposition reaction in the solvent prior to electron transfer, or a decomposition or polymerization reaction after electron transfer. REFERENCES 1 D.L. Coffen, J.Q. Chambers, D.R. Wflhams, P.E. Garrett and M.D. Canfield, J. Amer. Chem. Soc., 93 (1971) 2258. 2 J.Q. Chambers, P.S. Moses and R.N. Shelton, J. Eleetroanal. Chem., 38 (1972) 245. 3 P.I~. Moses and J.Q. Chambers, J. Electroanal. Chem., 49 (1974) 105. 4 P.R. Moses and J.Q. Chambers, J. Amer. Chem. Soe., 95 (1973) 948. 5 P.R. Moses, J.Q. Chambers, J.O. Sutherland and D.R. Williams, J. Electrochem. Soc., 122 (1975) 608. 6 W.D. Metz, Semnce, 180 (1973) 1041. 7 P.R. Moses, Ph.D. Dissertation, University of Tennessee, Knoxville, 1974. 8 E.M. Engler and V.V. Patel, private correspondence. 9 H.M. Fishler and W. Hartmann, Chem. Ber., 105 (1972) 2769. 10 L.R. Melby, H.D. Hartzler a n d W . A . Sheppard, J. Org. Chem., 39 (1974) 2456. 11 B.R. Eggins, Chem. Commun., (1967) 1267. 12 R.N. Adams, Electro chemistry at Solid Electrodes, Marcel Dekker, New York, 1969, p. 145.
390 13 E.M. Engler, F.B. Kaufman, D.C. Green, C.E. Klots and R.N. C ompt on, J. Amer. Chem. Soc., 97 (1975) 2921. 14 R.S. Nlcholson and I. Shain, Anal. Chem., 36 (1964) 706. 15 C.P. Andrieux, L. NadJo and J.M. Saveant, J. Electroanal. Chem., 26 (1970) 147. 16 P.R. Moses and J.Q. Chambers, J. Amer. Chem. Soc., 96 (1974) 945. 17 These data were o btained by D.C. Green and are p u b h s h e d with acknowledgement.