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Reaction of the anion radical of phenazine with carbon dioxide
ElecrrochimicaAcra, Vol. 35, No. 9, pp. 1405-1410, 1990
Printed in Great
00134686/90 $3.00 + 0.00 0 1990. Pergamon Press plc.
Britain.
REACTION
OF THE ANION RADICAL OF PHENAZINE WITH CARBON DIOXIDE
THERESACOMEAUSIMPSON* and RICHARD R. DURAND JR~
Department of Chemistry, University of Rhode Island, Kingston, RI 02881, U.S.A. (Received 23 October 1989; in revised form 11 December
1989)
Ahstrati-Phenazine has been observed to react with carbon dioxide in non-aqueous electrolyte via an ECE type mechanism at much lower potentials than aromatic hydrocarbons such as anthracene and olefins. The reactivity appears to be similar to that of a proton. The second order rate constant has been determined to be 2.6 x lo4 M-’ s-‘. This manuscript reports an electrochemical analysis of the kinetics of the reaction of the phenazine anion radical in DMSO (TBAP, 0.1 M) with CO,. Key words:
phenazine, electrochemistry, carbon dioxide.
Phen + e- o Phen’-
INTRODUCTION Considerable research has been aimed at utilizing electrocatalysis in the reduction of carbon dioxide to single carbon products such as formic acid, oxalate, formaldehyde, and methanol as well as more complex hydrocarbons and carbohydrates[ 11.Previous studies in our laboratory indicate that in the transition metal catalysis of CO_,, ligand reduction could play an important role in determining this reduction catalysis[l, 21. To further assess the role of the reduced organic ligands in determining reactivity with carbon dioxide, phenazine was examined as a model. The selection of phenazine was based on two primary factors. Phenazine and analogous molecules are inherently nucleophilic. In contrast, for transition metal complexes, nucleophilicity must be generated at the electrode. An organic molecule which is nucleophilic prior to reduction may be capable of reactivity with CO, at much more reasonable potentials. The second reason for the choice of phenazine is its structure. Phenazine is structurally similar to some of the biochemical catalysts of methanogenesis[3]. Many of the organic catalysts for the fixation of carbon dioxide in methagenic bacteria contain a nitrogen functionality as an active site, for which phenazine can serve as a model[4]. An examination of the reactivity of phenazine with carbon dioxide is discussed in this manuscript. The electrochemistry of phenazine under aqueous and non-aqueous conditions has been previously examined[&l6]. Phenazine is known to undergo two one-electron reductions, a reversible reduction to the anion radical followed by an irreversible reduction to the dianion, in non-aqueous electrolyte, as outlined in reaction scheme (1) [4-91:
*Current address: Dept. of Mat. Sci. and Eng., The Johns Hopkins University, Baltimore, MD 21218, U.S.A. tCurrent address: Sun Chemical Co., 631 Central Ave., Carlstadt, NJ 07072, U.S.A.
Phen’- + e- 0 Phen*- (Phen = phenazine).
(1)
The addition of water (between 0.1% and 10%) gradually shifts the second reduction wave positively until a single two-electron process results at concentrations of 10% or greater[3]. At water levels below 0.1% no effect is witnessed. In the presence of acid, phenazine is readily protonated and reduced by a single two-electron process[4-91. The reduction of phenazine in the presence of acid is outlined in reaction scheme (2). Phen(H)+ + 2e- o Phen(H)Phen(H)-
+ H+ 0 Phen(H,)
Phen(H,) = dihydrophenazine.
(2)
EXPERIMENTAL Reagents Electrochemical measurements were performed in HPLC grade, glass distilled dimethyl sulfoxide (DMSO) using high purity tetra-butyl ammonium perchlorate (TBAP) or sodium perchlorate electrolytes. The DMSO was supplied by Burdick and Jackson or Aldrich Chemicals, the TBAP by GFC Chemicals, and the phenazine and NaClO,( > 99%) by Aldrich. All were used without further purification. Nitrogen, carbon dioxide, and carbon dioxidenitrogen gas mixtures (1, 5 and 15% CO,, Certified) were obtained from Liquid Air, Inc. Prior to all electrochemical measurements solutions were thoroughly degassed (2(MO min) using nitrogen. The solutions were then saturated with the appropriate gas (N2, or C02-NZ mixture) by bubbling the gas through the solution for 15-60 min, depending on the volume of sample used in the experiment. The saturated solutions were blanketed during electrochemical measurements by maintaining a flow of the saturant gas over the solution and were resaturated between experiments.
1405
T. C. SIMFWNand R. R. DIJRANDJR
1406 Equipment
Linear and cyclic voltammetric experiments were completed using either an EG&G Princeton Applied Research Potentiostat Model 273 or a Pine Instrument Company RDE4 Potentiostat/ Galvanostat. Rotating disk voltammetric experiments utilized the Pine Instrument Company RDE4 Potentiostat/Galvanostat coupled with the MSR Speed Control Module and Bi-Mode Torque Unit. A Houston Model 200 x-y Recorder was used to plot data. Electrochemical cells Electrochemical measurements were made using standard three-electrode cells. The working electrodes used for cyclic and linear voltammetry were either glassy carbon, or edge plane pyrolytic graphite. Rotating disk voltammetric experiments were performed using edge plane graphite electrodes polished to a smooth finish with 0.3 pm alumina on a microcloth. Glassy carbon electrodes were obtained from Bioanalytical Systems (BAS); graphite (Union Carbide) electrodes were made in-house. Glassy carbon and edge plane graphite electrodes were polished using 0.3pm alumina prior to each use. Platinum wire served as the counter electrode throughout the analyses; commercially available see, Ag/AgCl, or silver wire were used as reference electrodes. Two primary cell configurations were used for electrochemical measurements. Standard H cells, with a fine porosity fiit to isolate the reference electrode from the working and counter electrodes, were used to perform some of the cyclic, linear and all of the rotating disk voltammetric measurements. Additional cyclic and linear voltammetric measurements were completed using a single compartment mini-cell with a permanently attached platinum counter electrode and an Ag/AgCl reference electrode partially isolated from the solution oia a Vycor frit, commercially available from BAS. Kinetic methods Data for kinetic studies of phenazine were obtained by application of a constant potential to the working electrode, experimentally determined to correspond to a potential where a steady state current existed. The current response as a function of rotation rate was then measured. All kinetic studies were restricted to rotation rates corresponding to conditions of laminar flow. This boundary condition was assumed for both graphical methods of analysis and for digital simulations. Saturation of all solutions was completed prior to the onset of these studies, with resaturation between experiments or if a 5 min lag time occurred at any point during a run. This method was adopted to minimize the time involved in such studies, thus preventing oxygen contamination from affecting the results. RESULTS
AND DISCUSSION
Mechanistic studies The reactivity of phenazine with carbon dioxide has not been previously reported. Figure 1 shows the cyclic voltammetric behavior of phenazine under N,
L 0.0
I
-0.1
I
- 1.0
I -1.5
J -1.9
Volts vs see Fig. 1. Cyclic voltammetry of 1mM phenazine at a glassy carbon electrode in DMSO (0.1 M TBAP) saturated with (a) N, and (b) CO, at a scan rate of 100mV s-l.
and after saturation with CO,. Two one-electron reduction waves are observed under nitrogen, the second of which shifts positively in the presence of carbon dioxide resulting in a single irreversible twoelectron wave. This cyclic voltammetric behavior is indicative of an ECE mechanism[ 171.The mechanism of phenazine reduction is postulated to include a oneelectron reduction of phenazine (E) followed by reaction with CO, (C) and finally a second oneelectron reduction of the carboxylated phenazine (E), with subsequent chemical reaction still possible. Reaction of carbon dioxide with the reduced phenazine anion radical stabilizes the molecule to accept a second electron, enabling reduction to the dianion carboxylated species. In the presence of CO, the second phenazine wave, seen under nitrogen, is completely absent. The reactivity of phenazine with carbon dioxide is likely to be similar to that of a proton, if a Lewis acid-base reaction occurs at the electrophilic carbon atom of CO2 with the nucleophilic sites of the phenazine ring. Figure 2 compares and contrasts the cyclic voltammetric data for the reaction of phenazine with acid to that for reactivity with carbon dioxide. Figure 2A shows the behavior of phenazine in the presence of concentrated formic acid. Note the similarities to the reactivity observed with COZ. The reaction with a proton causes a positive shift in reduction potential of the second phenazine peak resulting in a single two electron wave. The effect of a strong acid (approx. 2.5% v/v) is similar to that of CO, (approx. 8% v/v), but the potential shift is much stronger. It is recognized that reaction of phenazine at the secondary amine site with the formate is also possible; however, this reaction is expected to be subject to steric hindrance and would be expected to occur at a slower rate than simple protonation. For these reasons it is likely that the predominant reaction is that of phenazine protonation. Figure 2B shows the observed reactivity of phenazine with a much weaker acid. In the presence of 1 mM acetic acid under nitrogen we observe a two step reduction of phenazine. The first peak corresponds to reduction of the protonated phenazine anion radical and the second reduction of phenazine to the anion radical. Upon addition of CO2 the second wave is completely
Reaction of Phen’- with CO,
(A) T
2
f/
1407
proton concentration at trace water levels in carbon dioxide have been determined to be very low, far below the levels necessary to be responsible for our observed reactivity. For these reasons we do not attribute. the reactivity to be a simple protonation effect. We rather feel it is a distinct effect, unique to CO,, which can be compared, but not equated, to protonation. In an attempt to further understand the effect on phenazine reduction in the presence of C02, a number of other electrochemical studies were conducted. Figure 3 shows a plot of the cathodic peak current obtained in cyclic voltammetry as a function of phenazine concentration at different CO, concentrations. The purpose of the study was to determine the number of moles of carbon dioxide which are bound per mole of phenazine. The basis of the technique is similar to the method of standard additions commonly used in analytical chemistry. The cathodic current is linearly related to the phenazine concentration. Similarly, the current is linearly related to the concentration of the phenazine-<=Or product and within experimental error the proportionality constant is the same for both species:
Combining these two equations results in the following expression: 1
1 0
I -0.5
I -1.0
Ccomplex
I I -1.5
Volts vs SC8
Fig. 2. Cyclic voltammetry of 1 mM phenazine in DMSO (0.1 M TBAP) at a scan rate of 100 mV s-r. (A) At a glassy carbon electrode: (1) saturated with N, and (2) saturated with N, with 0.25ml HCOOH added. (B) At a pyrolytic graphite electrode: (1) saturated with N, with 1 mM CH,COOH added and (2) saturated with CO, with 1 mM
CH,COOH added. (C) At a glassy carbon electrode with 10% H,O added: (1) saturated with N, and (2) saturated with CO,. absent and the first wave is shifted slightly positive of the phenazine: CO2 wave in the absence of acid (Fig. 1). Figure 2C details the observed reactivity of phenazine under N, and CO2 in the presence of 10% water. Little or no effect on the nitrogen wave is detected; however, careful examination of the voltammogram upon addition of CO2 indicates that two overlapping waves exist. The protonated phenazine appears to be reduced at a different potential than the carboxylated phenazine. Thus, each of the above examples indicates that the reactivity of phenazine with COZ is similar to, but not the same as, the reactivity of phenazine with a proton. The potential shifts of phenazine reduction waves with acids of similar concentration to CO2 are larger than CO,, at lesser concentrations they simply distort or shift the behavior normally witnessed with CO*. In addition, the presence of appreciable quantities of protons in highly purified DMSO is not anticipated. Attempts in our laboratory to determine the H+ acidity in DMSO using dyes show no evidence for CO* contribution toward shifting the CO,: H,O equilibrium, the only likely proton source. In fact,
=
Grcc
phaazine
Ucolnplex
/La,
-
Lmplcx
).
Extrapolation to the zero current axis, therefore, gives an intercept which is the negative of the number of moles of complex formed. Referring to Fig. 3, when 1% COr is present (0.82 mM), 0.4 mM complex is formed and when 5% CO* is present (4.1 mM), 1.5 mM of complex is formed. These data indicate that 2 moles of CO2 are bound per mole of phenazine complex formed. The error in the case of the 5% CO, (1.5 us 2.05 mM) is most likely due to the concentration region employed since very high phenazine con-
-1.5
-0.4
2
6
4
CPhenazineI
6
mM
Fig. 3. Plot of cyclic voltammetric cathodic peak current as a function of phenazine concentration for phenazine in DMSO (0.1 M TBAP) at a glassy carbon electrode with a scan rate of KJOmVs-’ after saturation with 1% and 5% co,.
T. C. SIMPSON and R. R. DURANDJR
1408
centrations were not practical due to solubility restrictions. If higher phenazine concentrations were used, the line corresponding to 5% CO2 would most likely be shifted upward and an x-intercept value closer to 2.05 would be obtained. Low phenazine currents were observed when phenazine concentrations were not sufficient to completely complex all of the CO2 present. It is expected that unless at least molar equivalents of CO1 and phenazine are present appreciable complexation will not occur. Thus, the current values at very low phenazine concentrations (relative to the CO2 concentrations) should not be used in calculating the number of moles of CO, bound per mole of phenazine. Even for greater excesses of CO2 present, equilibria constraints do not predict complete complexation. Thus, the data are consistent with 2 moles of CO* bound per mole of complex formed. Kinetic studies were also completed using rotating disk voltammetry. The current-potential curves for phenazine under nitrogen and in the presence of carbon dioxide are seen in Fig. 4. Figure 4A shows the typical rotating disk voltammetric behavior of phenazine under nitrogen. The rapid increase in current at the reduction potential and the well defined plateaus indicate a reversible reduction. Rotating disk voltammetric scans of 1 mM phenazine after saturation with lo!, So!, 15% and 100% CO, are shown in Fig. 4B. Current increases with increasing CO* concentration and is coupled to a positive shift in reduction potential. The current approximately doubles from the value under with nitrogen when saturated with 100% carbon dioxide. Such effects are expected if an ECE mechanism is operative[l7]. Although these data strongly suggest that the mechanism of phenazine reduction and reaction with carbon dioxide follows an ECE type scheme, the (A)
iG it!
0.025
5 v
mA
N2 I 1000
I 0
I -0.5
I -1.0
rpm
I -1.6
kinetics of the mechanism remain to be addressed. The first concern which needs to be addressed is whether reaction of phenazine with carbon dioxide prior to-reduction takes place to any extent, which would constitute a CE type mechanism. Similar cyclic and rotating disk voltammetric behavior would be observed in either case. Spectral examination of phenazine prior to reduction does not indicate that appreciable association of phenazine with carbon dioxide has occurred. An ECE mechanism without prior reaction of CO2 with phenazine is antispat& by analogy to phenazine protonation reactions. Phenazine protonation has been identified by a number of investigators as following an ECE mechanism[4-8, 181. In addition, the protonation of phenazine proceeds in non-aqueous electrolyte without appreciable protonation prior to the first reduction of phenazine[4]. We postulate that the mechanism of phenazine reaction with carbon dioxide involves a one-electron reduction of phenazine to the anion radical, subsequent reaction with carbon dioxide, a second reduction of this intermediate species, and finally reaction with a second CO* molecule generating the product. Attempted product analysis was unsuccessful using conventional spectroscopic methods and would be necessary to confirm the reaction mechanism. Kinetic studies Kinetic examination of phenazine reactivity was conducted using rotating disk voltammetry. Since rotating disk voltammetric methods are steady state methods they provide ideal conditions for determining reaction kinetics. To investigate reaction kinetics quantitatively a parameter, napp, the apparent number of electrons, must be introduced. As defined by the Levich equation[l7], the limiting current is directly proportional to the number of electroni invol.ved in the electrochemical process. For our experiment it is reasonable to equate changes in plateau current with changes in the number of electrons reacting, since these are the only two variables which will change to any significant degree. The variable napp, which is the apparent number of electrons, uses this relationship and is defined, therefore, as the ratio of the limiting currents in the presence and absence of one of the reacting species: n = i,(k),
Volts vs SC8
naPP
I -0.4
I - 1.0
I -1.6
Volts vs see
Fig. 4. Rotating disk voltammetry for 1mM phenazine in DMSO (0.1 M TBAP) at a scan rate of 10 mV SK’: (A) saturated with N, at a glassy carbon electrode and (B) saturated with I%, 5%, 15%, and 100% CO, at an edge plane graphite electrode.
=
nh
=
i,,A;
2 = both reacting species are present, 1 = one reacting species is absent. For our reaction, nnppis equal to the ratio of the limiting current under CO2 to that under N,. The way in which nappchanges as a function of rotation rate can give important information regarding reaction kinetics. A very fast reaction would have an napp which is independent of rotation rate. If, however, the reaction is kinetically impeded the nnm value will change with increased rotation rate. Under pseudo first order conditions a large excess of one reactant is present and the napp only has a rotation rate dependency at higher rotation rates. The overwhelming excess of reactant often serves to shift the equilibrium favorably and enable even a slower
Reaction of Phen’- with CO,
= 1.7 .D ' 1.6
0
co2
0.621rnM
+
cop
82.10rnM
1.5
1.3 1.2
5
7
9
11
13
15
W,,2/rad
17
19
21
23
25
5-l
Fig. 5. Plots of napp as a function of the square root of rotation rate for 1mM phenazine in DMSO (0.1 M TBAP) saturated with 1% (0) and 100% (+) CO,.
reaction to take place with characteristics resembling a faster one. If true second order conditions prevail, one does not expect to attain the maximum value of napp even at the lowest rotation rates and a much stronger dependence of napp on rotation rate is expected. The influence of kinetic control for the reaction of phenazine with CO, is demonstrated in Fig. 5. Under pseudo-l”’ order conditions a much smaller and more gradual decrease in nappis witnessed than under second order conditions. Note also the point at which each curve begins. A true two-electron process is only obtained in large excess of CO, (Fig. 5, 82 mM CO,), ie, under pseudo first order conditions. Again, qualitative evidence for some kinetic limitations in the reaction of carbon dioxide with phenazine is apparent. Although graphical methods can be used to determine approximate rate constants, a much more accurate method for rate constant determination involves digital simulation. The algorithm used to determine the rate of reaction of the anion radical of phenazine with carbon dioxide was based on that developed by Feldberg which was modified by Bowers and Anson for use on a mainframe computer[19,20]. The software was then modified in our laboratory to enable its use on an XT-type persona1 computer. The simulation consists of two programs, the first of which uses parameters including actual concentrations of the reacting species and dimensionless diffusion coefficients for reactant and electrogenerated species to generate simulated working curves. The working curves consist of a dimensionless parameter XKTC defined as: XKTC = (~C,U/D,)“~/~,
where k = rate constant, u = kinematic viscosity, coefficient of the D, = diffusion substrate (phenazine), and w = rotation rate plotted as a function of riapp. The second program of the simulation uses these working curves in combination with experimentally determined napp values as a function of rotation rate to calculate rate constants via interpolation. The simulations are restricted to the use of rate data obtained under conditions corresponding to laminar flow. The rate constant is estimated in the program at least 18 times and from that an average value can be determined. The simulation method enabled work under true second order conditions, a
1409
necessity for fairly rapid reactions. The 2”d order rate constant for phenazine using data obtained for 1 mM phenazine reacting with 0.821 mM CO, in DMSO (0.1 M TBAP) was determined to be of the order of 2.6 x lo4 M-i s-t. Similar analysis for 1 mM phenazine reacting with 80 mM phenol in DMSO (0.1 M TBAP) resulted in a rate constant of the order of 10’ M-’ s-t. This value can only be considered a qualitative estimate of the rate constant for the reactivity of phenazine with phenol, however, since the very weak dissociation of phenol under nonaqueous conditions does not enable sufficient proton availability to maintain this very fast reaction. In addition, the digital simulation using rotating disk voltammetry is limited to measurement of rate constants of the order of lo6 M-r s-r. Thus, the value for phenol is outside the limits of the method. We are confident that the rate constant for reactivity of phenazine with phenol (a proton) is more rapid than that for phenazine with CO, but were not able to arrive at a definitive value for this rate constant.
CONCLUSION Thus, the rate of reaction of the anion radical of phenazine with carbon dioxide has been determined oiu a variety of electrochemical methods. Reactivity with this organic nucleophile occurs at potentials far positive of those at which olefins and other organics have been shown to react. The reactivity observed is clearly not catalytic. The phenazine reaction occurs via an ECE type mechanism and the rate for the reaction of phenazine anion radical with carbon dioxide is calculated to be 2.6 x lo4 M-’ s-l, which is significantly slower than that for reaction of phenazine with a proton. The actual nature of the products needs to be established. Acknowledgemenrs-The authors wish to thank the Research Corporation for partial financial support of this research and Dr. Mark Bowers for supplying the programs required for digital simulations.
REFERENCES 1. T. C. Simoson and R. R. Durand Jr. Electrochim. Acta 4, 581 (1988). 2. T. C. Simpson, Kinetics and Mechanisms of the Electroreduction of Carbon Dioxide via Transition Metal Catalysts and Organic Nucleophiles, Ph.D. Dissertation, University of Rhode Island (1988). 3. R. S. Wolfe, Trends Biochem. Sci., 396 (1985). 4. M. Maruyama and K. Murakami, J. elecrroanal. Chem. 102, 221 (1979). 5. A. Bard (Editor), Encyclopedia of Electrochemistry Vol. XV-2, p. 263, Marcel Dekker, New York (1975). 6. D. Van Der Meer, Rec. Trau. Chim. 87, 747 (1968). 7. D. Van Der Meer, Rec. Trau. Chim. 88, 1361 (1969).
8. D. Van Der Meer, Rec. Trau. Chim. 89, 51 (1970). 9. D. T. Sawyer and R. Y. Komai, Anal. Chem. 44, 715 (1972). 10. D. N. Bailey, D. M. Hercules and D. K. Roe, J. elecrrochem. Sot. 116, 191 (1969). 11. B. J. Tabner and J. R. Yandle, J. them. Sot. (A), 381 (1968).
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T. C. SMPSON and R. R. DURANDJR
12. B. Paduszek and M. K. Kalinowski, Electrochim. Acre 28, 639 (1983).
13. J. Armand, C; Bellee, L. Boulares and J. Pinson, J. org. Chem. 48, 2847 (1983). 14. S. Millefiori, J. Heterocyc. Chem. 7, 145 (1970). 15. C. Russel and W. Jaenicke, Electrochim. Acta 27, 1745
(1982). 16. kI. Svaan and V. D. Parker, Acta Chem. &and. B36, 351 (1982).
17. A. J. Bard and L. R. Faulkner (Editors), ElectrochemiCal Method,
Funakamentals and Applications, Q. 429,
Wiley, New York (1980). _18. E. Laviron, J. electroanal. Chem. 146, 15 (1983). 19. S. W. Feldberg, M. L. Bowers and F. C. Anson, J. efectroanal. Chem. 215, 11 (1986). 20. M. L. Bowers. F. C. Anson and S. W. Feldbera._. J. electroanal. Chem. 216, 249 (1987).
Printed in Great
00134686/90 $3.00 + 0.00 0 1990. Pergamon Press plc.
Britain.
REACTION
OF THE ANION RADICAL OF PHENAZINE WITH CARBON DIOXIDE
THERESACOMEAUSIMPSON* and RICHARD R. DURAND JR~
Department of Chemistry, University of Rhode Island, Kingston, RI 02881, U.S.A. (Received 23 October 1989; in revised form 11 December
1989)
Ahstrati-Phenazine has been observed to react with carbon dioxide in non-aqueous electrolyte via an ECE type mechanism at much lower potentials than aromatic hydrocarbons such as anthracene and olefins. The reactivity appears to be similar to that of a proton. The second order rate constant has been determined to be 2.6 x lo4 M-’ s-‘. This manuscript reports an electrochemical analysis of the kinetics of the reaction of the phenazine anion radical in DMSO (TBAP, 0.1 M) with CO,. Key words:
phenazine, electrochemistry, carbon dioxide.
Phen + e- o Phen’-
INTRODUCTION Considerable research has been aimed at utilizing electrocatalysis in the reduction of carbon dioxide to single carbon products such as formic acid, oxalate, formaldehyde, and methanol as well as more complex hydrocarbons and carbohydrates[ 11.Previous studies in our laboratory indicate that in the transition metal catalysis of CO_,, ligand reduction could play an important role in determining this reduction catalysis[l, 21. To further assess the role of the reduced organic ligands in determining reactivity with carbon dioxide, phenazine was examined as a model. The selection of phenazine was based on two primary factors. Phenazine and analogous molecules are inherently nucleophilic. In contrast, for transition metal complexes, nucleophilicity must be generated at the electrode. An organic molecule which is nucleophilic prior to reduction may be capable of reactivity with CO, at much more reasonable potentials. The second reason for the choice of phenazine is its structure. Phenazine is structurally similar to some of the biochemical catalysts of methanogenesis[3]. Many of the organic catalysts for the fixation of carbon dioxide in methagenic bacteria contain a nitrogen functionality as an active site, for which phenazine can serve as a model[4]. An examination of the reactivity of phenazine with carbon dioxide is discussed in this manuscript. The electrochemistry of phenazine under aqueous and non-aqueous conditions has been previously examined[&l6]. Phenazine is known to undergo two one-electron reductions, a reversible reduction to the anion radical followed by an irreversible reduction to the dianion, in non-aqueous electrolyte, as outlined in reaction scheme (1) [4-91:
*Current address: Dept. of Mat. Sci. and Eng., The Johns Hopkins University, Baltimore, MD 21218, U.S.A. tCurrent address: Sun Chemical Co., 631 Central Ave., Carlstadt, NJ 07072, U.S.A.
Phen’- + e- 0 Phen*- (Phen = phenazine).
(1)
The addition of water (between 0.1% and 10%) gradually shifts the second reduction wave positively until a single two-electron process results at concentrations of 10% or greater[3]. At water levels below 0.1% no effect is witnessed. In the presence of acid, phenazine is readily protonated and reduced by a single two-electron process[4-91. The reduction of phenazine in the presence of acid is outlined in reaction scheme (2). Phen(H)+ + 2e- o Phen(H)Phen(H)-
+ H+ 0 Phen(H,)
Phen(H,) = dihydrophenazine.
(2)
EXPERIMENTAL Reagents Electrochemical measurements were performed in HPLC grade, glass distilled dimethyl sulfoxide (DMSO) using high purity tetra-butyl ammonium perchlorate (TBAP) or sodium perchlorate electrolytes. The DMSO was supplied by Burdick and Jackson or Aldrich Chemicals, the TBAP by GFC Chemicals, and the phenazine and NaClO,( > 99%) by Aldrich. All were used without further purification. Nitrogen, carbon dioxide, and carbon dioxidenitrogen gas mixtures (1, 5 and 15% CO,, Certified) were obtained from Liquid Air, Inc. Prior to all electrochemical measurements solutions were thoroughly degassed (2(MO min) using nitrogen. The solutions were then saturated with the appropriate gas (N2, or C02-NZ mixture) by bubbling the gas through the solution for 15-60 min, depending on the volume of sample used in the experiment. The saturated solutions were blanketed during electrochemical measurements by maintaining a flow of the saturant gas over the solution and were resaturated between experiments.
1405
T. C. SIMFWNand R. R. DIJRANDJR
1406 Equipment
Linear and cyclic voltammetric experiments were completed using either an EG&G Princeton Applied Research Potentiostat Model 273 or a Pine Instrument Company RDE4 Potentiostat/ Galvanostat. Rotating disk voltammetric experiments utilized the Pine Instrument Company RDE4 Potentiostat/Galvanostat coupled with the MSR Speed Control Module and Bi-Mode Torque Unit. A Houston Model 200 x-y Recorder was used to plot data. Electrochemical cells Electrochemical measurements were made using standard three-electrode cells. The working electrodes used for cyclic and linear voltammetry were either glassy carbon, or edge plane pyrolytic graphite. Rotating disk voltammetric experiments were performed using edge plane graphite electrodes polished to a smooth finish with 0.3 pm alumina on a microcloth. Glassy carbon electrodes were obtained from Bioanalytical Systems (BAS); graphite (Union Carbide) electrodes were made in-house. Glassy carbon and edge plane graphite electrodes were polished using 0.3pm alumina prior to each use. Platinum wire served as the counter electrode throughout the analyses; commercially available see, Ag/AgCl, or silver wire were used as reference electrodes. Two primary cell configurations were used for electrochemical measurements. Standard H cells, with a fine porosity fiit to isolate the reference electrode from the working and counter electrodes, were used to perform some of the cyclic, linear and all of the rotating disk voltammetric measurements. Additional cyclic and linear voltammetric measurements were completed using a single compartment mini-cell with a permanently attached platinum counter electrode and an Ag/AgCl reference electrode partially isolated from the solution oia a Vycor frit, commercially available from BAS. Kinetic methods Data for kinetic studies of phenazine were obtained by application of a constant potential to the working electrode, experimentally determined to correspond to a potential where a steady state current existed. The current response as a function of rotation rate was then measured. All kinetic studies were restricted to rotation rates corresponding to conditions of laminar flow. This boundary condition was assumed for both graphical methods of analysis and for digital simulations. Saturation of all solutions was completed prior to the onset of these studies, with resaturation between experiments or if a 5 min lag time occurred at any point during a run. This method was adopted to minimize the time involved in such studies, thus preventing oxygen contamination from affecting the results. RESULTS
AND DISCUSSION
Mechanistic studies The reactivity of phenazine with carbon dioxide has not been previously reported. Figure 1 shows the cyclic voltammetric behavior of phenazine under N,
L 0.0
I
-0.1
I
- 1.0
I -1.5
J -1.9
Volts vs see Fig. 1. Cyclic voltammetry of 1mM phenazine at a glassy carbon electrode in DMSO (0.1 M TBAP) saturated with (a) N, and (b) CO, at a scan rate of 100mV s-l.
and after saturation with CO,. Two one-electron reduction waves are observed under nitrogen, the second of which shifts positively in the presence of carbon dioxide resulting in a single irreversible twoelectron wave. This cyclic voltammetric behavior is indicative of an ECE mechanism[ 171.The mechanism of phenazine reduction is postulated to include a oneelectron reduction of phenazine (E) followed by reaction with CO, (C) and finally a second oneelectron reduction of the carboxylated phenazine (E), with subsequent chemical reaction still possible. Reaction of carbon dioxide with the reduced phenazine anion radical stabilizes the molecule to accept a second electron, enabling reduction to the dianion carboxylated species. In the presence of CO, the second phenazine wave, seen under nitrogen, is completely absent. The reactivity of phenazine with carbon dioxide is likely to be similar to that of a proton, if a Lewis acid-base reaction occurs at the electrophilic carbon atom of CO2 with the nucleophilic sites of the phenazine ring. Figure 2 compares and contrasts the cyclic voltammetric data for the reaction of phenazine with acid to that for reactivity with carbon dioxide. Figure 2A shows the behavior of phenazine in the presence of concentrated formic acid. Note the similarities to the reactivity observed with COZ. The reaction with a proton causes a positive shift in reduction potential of the second phenazine peak resulting in a single two electron wave. The effect of a strong acid (approx. 2.5% v/v) is similar to that of CO, (approx. 8% v/v), but the potential shift is much stronger. It is recognized that reaction of phenazine at the secondary amine site with the formate is also possible; however, this reaction is expected to be subject to steric hindrance and would be expected to occur at a slower rate than simple protonation. For these reasons it is likely that the predominant reaction is that of phenazine protonation. Figure 2B shows the observed reactivity of phenazine with a much weaker acid. In the presence of 1 mM acetic acid under nitrogen we observe a two step reduction of phenazine. The first peak corresponds to reduction of the protonated phenazine anion radical and the second reduction of phenazine to the anion radical. Upon addition of CO2 the second wave is completely
Reaction of Phen’- with CO,
(A) T
2
f/
1407
proton concentration at trace water levels in carbon dioxide have been determined to be very low, far below the levels necessary to be responsible for our observed reactivity. For these reasons we do not attribute. the reactivity to be a simple protonation effect. We rather feel it is a distinct effect, unique to CO,, which can be compared, but not equated, to protonation. In an attempt to further understand the effect on phenazine reduction in the presence of C02, a number of other electrochemical studies were conducted. Figure 3 shows a plot of the cathodic peak current obtained in cyclic voltammetry as a function of phenazine concentration at different CO, concentrations. The purpose of the study was to determine the number of moles of carbon dioxide which are bound per mole of phenazine. The basis of the technique is similar to the method of standard additions commonly used in analytical chemistry. The cathodic current is linearly related to the phenazine concentration. Similarly, the current is linearly related to the concentration of the phenazine-<=Or product and within experimental error the proportionality constant is the same for both species:
Combining these two equations results in the following expression: 1
1 0
I -0.5
I -1.0
Ccomplex
I I -1.5
Volts vs SC8
Fig. 2. Cyclic voltammetry of 1 mM phenazine in DMSO (0.1 M TBAP) at a scan rate of 100 mV s-r. (A) At a glassy carbon electrode: (1) saturated with N, and (2) saturated with N, with 0.25ml HCOOH added. (B) At a pyrolytic graphite electrode: (1) saturated with N, with 1 mM CH,COOH added and (2) saturated with CO, with 1 mM
CH,COOH added. (C) At a glassy carbon electrode with 10% H,O added: (1) saturated with N, and (2) saturated with CO,. absent and the first wave is shifted slightly positive of the phenazine: CO2 wave in the absence of acid (Fig. 1). Figure 2C details the observed reactivity of phenazine under N, and CO2 in the presence of 10% water. Little or no effect on the nitrogen wave is detected; however, careful examination of the voltammogram upon addition of CO2 indicates that two overlapping waves exist. The protonated phenazine appears to be reduced at a different potential than the carboxylated phenazine. Thus, each of the above examples indicates that the reactivity of phenazine with COZ is similar to, but not the same as, the reactivity of phenazine with a proton. The potential shifts of phenazine reduction waves with acids of similar concentration to CO2 are larger than CO,, at lesser concentrations they simply distort or shift the behavior normally witnessed with CO*. In addition, the presence of appreciable quantities of protons in highly purified DMSO is not anticipated. Attempts in our laboratory to determine the H+ acidity in DMSO using dyes show no evidence for CO* contribution toward shifting the CO,: H,O equilibrium, the only likely proton source. In fact,
=
Grcc
phaazine
Ucolnplex
/La,
-
Lmplcx
).
Extrapolation to the zero current axis, therefore, gives an intercept which is the negative of the number of moles of complex formed. Referring to Fig. 3, when 1% COr is present (0.82 mM), 0.4 mM complex is formed and when 5% CO* is present (4.1 mM), 1.5 mM of complex is formed. These data indicate that 2 moles of CO2 are bound per mole of phenazine complex formed. The error in the case of the 5% CO, (1.5 us 2.05 mM) is most likely due to the concentration region employed since very high phenazine con-
-1.5
-0.4
2
6
4
CPhenazineI
6
mM
Fig. 3. Plot of cyclic voltammetric cathodic peak current as a function of phenazine concentration for phenazine in DMSO (0.1 M TBAP) at a glassy carbon electrode with a scan rate of KJOmVs-’ after saturation with 1% and 5% co,.
T. C. SIMPSON and R. R. DURANDJR
1408
centrations were not practical due to solubility restrictions. If higher phenazine concentrations were used, the line corresponding to 5% CO2 would most likely be shifted upward and an x-intercept value closer to 2.05 would be obtained. Low phenazine currents were observed when phenazine concentrations were not sufficient to completely complex all of the CO2 present. It is expected that unless at least molar equivalents of CO1 and phenazine are present appreciable complexation will not occur. Thus, the current values at very low phenazine concentrations (relative to the CO2 concentrations) should not be used in calculating the number of moles of CO, bound per mole of phenazine. Even for greater excesses of CO2 present, equilibria constraints do not predict complete complexation. Thus, the data are consistent with 2 moles of CO* bound per mole of complex formed. Kinetic studies were also completed using rotating disk voltammetry. The current-potential curves for phenazine under nitrogen and in the presence of carbon dioxide are seen in Fig. 4. Figure 4A shows the typical rotating disk voltammetric behavior of phenazine under nitrogen. The rapid increase in current at the reduction potential and the well defined plateaus indicate a reversible reduction. Rotating disk voltammetric scans of 1 mM phenazine after saturation with lo!, So!, 15% and 100% CO, are shown in Fig. 4B. Current increases with increasing CO* concentration and is coupled to a positive shift in reduction potential. The current approximately doubles from the value under with nitrogen when saturated with 100% carbon dioxide. Such effects are expected if an ECE mechanism is operative[l7]. Although these data strongly suggest that the mechanism of phenazine reduction and reaction with carbon dioxide follows an ECE type scheme, the (A)
iG it!
0.025
5 v
mA
N2 I 1000
I 0
I -0.5
I -1.0
rpm
I -1.6
kinetics of the mechanism remain to be addressed. The first concern which needs to be addressed is whether reaction of phenazine with carbon dioxide prior to-reduction takes place to any extent, which would constitute a CE type mechanism. Similar cyclic and rotating disk voltammetric behavior would be observed in either case. Spectral examination of phenazine prior to reduction does not indicate that appreciable association of phenazine with carbon dioxide has occurred. An ECE mechanism without prior reaction of CO2 with phenazine is antispat& by analogy to phenazine protonation reactions. Phenazine protonation has been identified by a number of investigators as following an ECE mechanism[4-8, 181. In addition, the protonation of phenazine proceeds in non-aqueous electrolyte without appreciable protonation prior to the first reduction of phenazine[4]. We postulate that the mechanism of phenazine reaction with carbon dioxide involves a one-electron reduction of phenazine to the anion radical, subsequent reaction with carbon dioxide, a second reduction of this intermediate species, and finally reaction with a second CO* molecule generating the product. Attempted product analysis was unsuccessful using conventional spectroscopic methods and would be necessary to confirm the reaction mechanism. Kinetic studies Kinetic examination of phenazine reactivity was conducted using rotating disk voltammetry. Since rotating disk voltammetric methods are steady state methods they provide ideal conditions for determining reaction kinetics. To investigate reaction kinetics quantitatively a parameter, napp, the apparent number of electrons, must be introduced. As defined by the Levich equation[l7], the limiting current is directly proportional to the number of electroni invol.ved in the electrochemical process. For our experiment it is reasonable to equate changes in plateau current with changes in the number of electrons reacting, since these are the only two variables which will change to any significant degree. The variable napp, which is the apparent number of electrons, uses this relationship and is defined, therefore, as the ratio of the limiting currents in the presence and absence of one of the reacting species: n = i,(k),
Volts vs SC8
naPP
I -0.4
I - 1.0
I -1.6
Volts vs see
Fig. 4. Rotating disk voltammetry for 1mM phenazine in DMSO (0.1 M TBAP) at a scan rate of 10 mV SK’: (A) saturated with N, at a glassy carbon electrode and (B) saturated with I%, 5%, 15%, and 100% CO, at an edge plane graphite electrode.
=
nh
=
i,,A;
2 = both reacting species are present, 1 = one reacting species is absent. For our reaction, nnppis equal to the ratio of the limiting current under CO2 to that under N,. The way in which nappchanges as a function of rotation rate can give important information regarding reaction kinetics. A very fast reaction would have an napp which is independent of rotation rate. If, however, the reaction is kinetically impeded the nnm value will change with increased rotation rate. Under pseudo first order conditions a large excess of one reactant is present and the napp only has a rotation rate dependency at higher rotation rates. The overwhelming excess of reactant often serves to shift the equilibrium favorably and enable even a slower
Reaction of Phen’- with CO,
= 1.7 .D ' 1.6
0
co2
0.621rnM
+
cop
82.10rnM
1.5
1.3 1.2
5
7
9
11
13
15
W,,2/rad
17
19
21
23
25
5-l
Fig. 5. Plots of napp as a function of the square root of rotation rate for 1mM phenazine in DMSO (0.1 M TBAP) saturated with 1% (0) and 100% (+) CO,.
reaction to take place with characteristics resembling a faster one. If true second order conditions prevail, one does not expect to attain the maximum value of napp even at the lowest rotation rates and a much stronger dependence of napp on rotation rate is expected. The influence of kinetic control for the reaction of phenazine with CO, is demonstrated in Fig. 5. Under pseudo-l”’ order conditions a much smaller and more gradual decrease in nappis witnessed than under second order conditions. Note also the point at which each curve begins. A true two-electron process is only obtained in large excess of CO, (Fig. 5, 82 mM CO,), ie, under pseudo first order conditions. Again, qualitative evidence for some kinetic limitations in the reaction of carbon dioxide with phenazine is apparent. Although graphical methods can be used to determine approximate rate constants, a much more accurate method for rate constant determination involves digital simulation. The algorithm used to determine the rate of reaction of the anion radical of phenazine with carbon dioxide was based on that developed by Feldberg which was modified by Bowers and Anson for use on a mainframe computer[19,20]. The software was then modified in our laboratory to enable its use on an XT-type persona1 computer. The simulation consists of two programs, the first of which uses parameters including actual concentrations of the reacting species and dimensionless diffusion coefficients for reactant and electrogenerated species to generate simulated working curves. The working curves consist of a dimensionless parameter XKTC defined as: XKTC = (~C,U/D,)“~/~,
where k = rate constant, u = kinematic viscosity, coefficient of the D, = diffusion substrate (phenazine), and w = rotation rate plotted as a function of riapp. The second program of the simulation uses these working curves in combination with experimentally determined napp values as a function of rotation rate to calculate rate constants via interpolation. The simulations are restricted to the use of rate data obtained under conditions corresponding to laminar flow. The rate constant is estimated in the program at least 18 times and from that an average value can be determined. The simulation method enabled work under true second order conditions, a
1409
necessity for fairly rapid reactions. The 2”d order rate constant for phenazine using data obtained for 1 mM phenazine reacting with 0.821 mM CO, in DMSO (0.1 M TBAP) was determined to be of the order of 2.6 x lo4 M-i s-t. Similar analysis for 1 mM phenazine reacting with 80 mM phenol in DMSO (0.1 M TBAP) resulted in a rate constant of the order of 10’ M-’ s-t. This value can only be considered a qualitative estimate of the rate constant for the reactivity of phenazine with phenol, however, since the very weak dissociation of phenol under nonaqueous conditions does not enable sufficient proton availability to maintain this very fast reaction. In addition, the digital simulation using rotating disk voltammetry is limited to measurement of rate constants of the order of lo6 M-r s-r. Thus, the value for phenol is outside the limits of the method. We are confident that the rate constant for reactivity of phenazine with phenol (a proton) is more rapid than that for phenazine with CO, but were not able to arrive at a definitive value for this rate constant.
CONCLUSION Thus, the rate of reaction of the anion radical of phenazine with carbon dioxide has been determined oiu a variety of electrochemical methods. Reactivity with this organic nucleophile occurs at potentials far positive of those at which olefins and other organics have been shown to react. The reactivity observed is clearly not catalytic. The phenazine reaction occurs via an ECE type mechanism and the rate for the reaction of phenazine anion radical with carbon dioxide is calculated to be 2.6 x lo4 M-’ s-l, which is significantly slower than that for reaction of phenazine with a proton. The actual nature of the products needs to be established. Acknowledgemenrs-The authors wish to thank the Research Corporation for partial financial support of this research and Dr. Mark Bowers for supplying the programs required for digital simulations.
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8. D. Van Der Meer, Rec. Trau. Chim. 89, 51 (1970). 9. D. T. Sawyer and R. Y. Komai, Anal. Chem. 44, 715 (1972). 10. D. N. Bailey, D. M. Hercules and D. K. Roe, J. elecrrochem. Sot. 116, 191 (1969). 11. B. J. Tabner and J. R. Yandle, J. them. Sot. (A), 381 (1968).
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12. B. Paduszek and M. K. Kalinowski, Electrochim. Acre 28, 639 (1983).
13. J. Armand, C; Bellee, L. Boulares and J. Pinson, J. org. Chem. 48, 2847 (1983). 14. S. Millefiori, J. Heterocyc. Chem. 7, 145 (1970). 15. C. Russel and W. Jaenicke, Electrochim. Acta 27, 1745
(1982). 16. kI. Svaan and V. D. Parker, Acta Chem. &and. B36, 351 (1982).
17. A. J. Bard and L. R. Faulkner (Editors), ElectrochemiCal Method,
Funakamentals and Applications, Q. 429,
Wiley, New York (1980). _18. E. Laviron, J. electroanal. Chem. 146, 15 (1983). 19. S. W. Feldberg, M. L. Bowers and F. C. Anson, J. efectroanal. Chem. 215, 11 (1986). 20. M. L. Bowers. F. C. Anson and S. W. Feldbera._. J. electroanal. Chem. 216, 249 (1987).