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Homogeneous redox catalysis of the reduction of bianthrone via quinone redox couples
andXIENAIXIAN
DENNIS&EVANW
Deparfmqtt of Chek&y,
University of Wisconsin-Madison, Madison, WI 53706 -(U.S.A.)
(Received93rd I+&nber 1981).-
In-homogeneous redox catalysis the rate of an irreversible electrode reaction is increased by:the action of a catalyst redox couple which accepts electrons~ from the electrode (ii the case of a reduction) and transfers thein to the substrate. The phenomenon has been thoroughly. studied by Saveant and coworkers: [l-8] who have used it to determine otherwise inaccessible standard potentials, standard heterogeneous electron transfer rate constants, and rate con&&@ of fohowing chemical reactions. Numerous examples of homogeneous redox catalysis of orgzkic electrode reactions have been listed [5] and others continue to be published ]9-151. A commou feature of the systems studied to date is that the overall reaction of the substrate involves irreversible bond breaking as a key step in the sequence. Reduction of aromatic halogen compounds is a typical example 1’71. The redu&d form of the catalyst couple-(P/Q) transfers an electron to ArX f0rming.a sho+.ived anion radical (reaction 2). . P+e
=+
Q
QtArX
.=
P+Arx;-
(2)
Al-XT
-b
Ar
(3)
AIY +Q
+
P+Ar-
(4)
Ar’+A&
+
Ar+Arx
(5)
A.r-+H+
+
+x-
.ArH.
_. is,-
elec_Gon transfer reaction- i+_not energetically favored but it is driven _by the rapid decomposition of. Arx’ . ‘Reactions (2) and (4) recycle the- catalyst and.the-.:tisult is~mpid+I@ct~r~u$ion of-A.6 at potentials nearI& .where the -t _electrom+@on .ofArX is often immeasurably slow.. .‘.
This
*&&&
*&&;&gm&.;&;ul;l .-. _.
.-. .~~~&tY+
i::: 1. ::_ : ;.-
-. _,:.‘1, _,._ ,__‘y;,
-y -lL;;i). _y;=,: ,_;;:
368
The present communication reports redox catalysis in the reduction of a sub&mce which does not suffer bond-breaking in its reduction but instead experiences a significant change in conformation which drives the overall c&&tic reaction. Another significant feature of the reaction is that the. standard potential for the overall reduction of the substrate is known so that the. catalysis can be analyzed in terms of the relative free energies of all species the reaction scheme.
in
EXPERIMENTAL
Dimethylformamide (DMF, Burdick and Jackson) was vacuum distilled.and stored over Woelm 200 neutral alumina (activated at 300°C under nitrogen for 24 h). The supporting electrolyte was 0.10 M tetraethylammonium perchlorate [lS]. Bianthrone was obtained from Aldrich Chemical Company. All solutions were protected from exposure to atmospheric moisture and solutions containing bianthrone were kept in the dark. Voltammetry experiments used a Princeton Applied Research Model 170 electrochemistry system and a cell of the type previously described [17]. The working electrode was a hanging mercury drop [18] of approximately 0.016 cm2 area. The reference electrode was a silver wire in contact with O.@lO M AgNOs, 0.10 M tetraethylammonium perch&ate in DMF 1191 whose potential was found to be +0.423 V vs. aq. SCE. Solutions were purged with purified nitrogen and the solution temperature was maintained at 25.O”C. RESULTS AND DISCUSSION
An unusual example of redox catalysis has been discovered in the reduction of bianthrone, I. A brief review of the electrochemical behavior of bianthrone will aid in understanding of the catalytic process.
At room temperature, I exists as the ‘yellow A form (Fig. 1) ._inwhich .the .. steric interactions between hydrogen atoms at positions 1 and 8’ (and .l’ --&id 8) are relieved by folding each anthrone system away from the other without
.369 .
B
A
Fig. 1. A and B forms of bianthrone (I).
twisting the 9,9’ double bond. When a solution of I is heated, a significant fraction is converted to the green B form. The most widely accepted structure of the B form (Fig. 1) reduces the steric crowding by twisting about the 9,9’ bond and flattening the two anthrone systems [20]. The electrochemical reduction of I [16,21,22] induces structural changes similar to the A+B thermochromicconversion. The anion radical of I has reduced double bond character in the 9,9’ bond which lowers the energy required to twist that bond. T_hu-:it is not surprising that the radical anion adopts a B type structure, B' , with considerable twisting about the 9,9’ bond [16,21]. The dianion undoubtedIy exists in a B structure, B2-, with the angle between the two anthranolate systems probably being close to 90”. Scheme 1 summariies the electrochemical behavior of bianthrone. Reduction of A proceeds in an ECE process in which the initially formed A-Like anion’ radical, A’, rapidly .twists to BT which iri turn is rapidly reduced to B2- because the potential required to reduce A is more negative than EzB_ In DMF/ 0.10 M tetraethylammonium perchlorate, Km = 2.2 X lo-‘, LAB = = -0.50 V VS. aq. 8.1 X 10-j s-l, &A = 3.7 S+, a?& = -0.20 V- and &
SCHEME1
SC% [ 161. -Recently it. h& been ~deterrnined by &se mdiolysis that -k-m = 7 X 104 S'l 'iti,&k&e' ~o~rop~oI/t&r&ydrofuran/I&C -[23]. Assuming the same.y$uelpf k-h &- QMF ~leads'~ti, @t;imates -of ETA. =. ::_1;14 V vs_ SCE land
ry
E 2x
-1(p3;_
: ,- (_ .., _;,_:y-._.: ._-I
-
.'
-;,_,-_:
~:
:
-"y ;_
The’ above vaI&s of the stand& ‘$+ni+s -(aIong ..&ith_&r '&&&ily _ .: chosen &A) ‘may be used .d construct a~the&nodyx&ic Ability -&one dia;
370
3 -0”
I
I
I
-2o-
I
I I I
I I EP. I I
-3o-
I 0.0
i I I I
G-d I I I
I_
I -0.4
-~ J
-0.8 _
-1.2
-1.6
Fig. 2. Thermodynamic stability zone diagram. See scheme 1 for defmition of terms. (a) System with log KAB > Q; (b) system with log Km << 0; (c) bianthrone, log KAB = -2.7. Schematic voltammogram “rev”: expected behavior of bianthrone under reversible conditions; schematic vokammogram “ohs”: actual behavior of bianthrone.
gram (Fig. 2). This diagrampertainsto a family of systems havingthe indicated E” values and various values of HAB (once the E” values have been fixed, KG and K=m are determined by the choice of Km )_ For a system in which log Km > 0 (line a, Fig. 21 the B form predominates at equilibrkm and it would be reduced first to B’ and then to-B2- as the potential was made more negative_ On the other hand, if log Km << 0 (line b, Fig. 2), a single reduction process, A + 2e + B2-, would be predicted for a system showing thermodynamic reversibility. For bianthrone log Km = -2.7 (line c, Fig. 2) and regions of stability of A, BT and B*- are encountered .as the_ potential is moved in the negative direction. At -0.36 V Eo for the A/B’ couple occurs, followed shortly by & at -0.50 V. A schematic cyclic voltammogram (“rev”) expected for the case of reversible bianthrone reduction is shown in the top part of Fig. 2. However, the actual behavior of bianthrone is far removed from equilibrium conditions. The reduction of A does not occur at an appreciable rate until the potential is over 0.4 V negativeof the standardpotential 1161 (schematic
voltammogram“ohs”, Fig. 2). Thus, it is concluded that the r&duct& of bianthrone is potentially subject to homogeneous redox catalysisby couples. with standardpotentials in the r+nge of -0.5 to -1.0 V vs. SC?. .,
Y
I
I
-0.2
-0.4
-0.6
E vs
sap
-0:s
Fig.3. Catalyticreduction ofbiithrone in DMFvia Z-chloro-S,lO-anthraquinone. &an rate:O.lOOV/s.(A)l_OO miW'Ga&bro~e;(B)l.OO mMbiantbrone and 1.00 dquinone; (C)blank;(D)!.OOmiWqui+one.
A number of quinones were investigatedas possible catalysts. Results for 2-@loro-9,10-anthraquinone -a& ihustratedin Fig. 3. Bianthrone in the- absence of quhione shows no significant reduction current priot to -0.8 V (curve A, Fig. 3). Quinone in the absence of bianthrone is reduced in a reversible, diffusion-controlled reaction to the quinone anion radical (curve D, Fig. 3). An equimolg m@ure of quinofie 'andbianthrone (curve B, Fig. 3) indicates the occurnmce of homogeneous redox catalytic reduction of bianthroneat the potential of the quinone couple: the cathodic peak current has increased,the anodic peak current hasdiminished.and, most importantly, oxidation peaks for the product, B2-, are evident on the return sweep at -0.48 V and -0.18 V (the initial potential). When the potential was held at -0.8 V vs, 8kX -for 30 s following the first sweep, the B2- + BT and BT + B peaks on the return sweep were of a height correspondingto complete ;conversion of A to B2- m the reaction-layer[lS]. The postulated reaction scheme is given by reactions (‘7~(10). P-+-e . .-+
-Q
(7)
372 TABLE
1
Catalytic peak current function for reduction of bianthrone catalyzed by various quinones” Quinone
E,,,IV
1,4-Naphthoquinone 2,5-Di-tert-butyl-1,Cbenzoquinone 2,6-Dimethoxy-1,4-benzoquinone 2-Methyl-1,4-naphthoquinone 2-Chloro-9,10-anthraquiuone Tetramethyl-1,4-benzoquinone
-0.622 -0.633 -6.665 -0.687 -0.740 -0.754
b
ip /ipa c 1.00 1.01 1.05 1.14 1.29 1.56
a 1.00 x 10B3 M bianthrone; 1.00 X lo-” M quinone; 0.100 V/s scan rate. bv~. aq. SCE. c ip is observed peak current and iPd is quinone reduction peak current in absence of bianthrone.
a pre-equilibrium [ 51. In either case the rate will increase as quinones with more negative standard potentials are used as catalysts. This was found to be the case for the quinones listed in Table 1 for which the indicated values of the catalytic current function, i&,d, were found. The observed peak or plateau current is expressed by ip and ipd is the peak current for qu‘mone reduction in the absence of bianthrone. Clearly, the expected trend is observed. Catalysis is impossible with couples whose standard potential is positive of Ei B-, u-O.36 V. For observable catalysis, the standard potential of the catalyst must be negative of about -0.63 V. It was previously noted that B2- is not an effective redox catalyst (autocatalyst) for A reduction [ 161. This was deduced from the observed rate of reaction of B2- with A (B2- + A + 2B:). This reaction occurs by reactions (11) and (12) with (11) being rate-limiting. There was no significant contribution A
k AB U
B
(11)
kBA
B + B2-
*
2B=
A + B2-
+
A7 + BT
A’
(fast)
(12) (13) (14)
from the (13~(14) sequence which must occur if the BF/B2- couple wemeto serve as redox catalyst. The data in Table 1 provide an explanation, viz., f?& (-0.50 V) is not sufficiently negative for effective catalysis. Another interesting feature of the redox cataI$zed reduction of- bianthrone is that there must be a lower limit for the apparent rate of catalysis .due to the intervention of the alternative sequence of reaction (11) folJowed by reactions (15) and (10):
373
km *
-
B
(11)
kBA
Q+B
t
P + B;-
Q+B’
+
P + B2-
(fast)
(15)
(fast)
(19)
However, this sequence is apparently too slow to provide observable apparent catalysis with 1,4-naphthoquinone (see Table 1) although it might become effective at higher temperatures. Preliminary .&inetic analysis suggests that the process falls in the region of mixed control by reactions (8) and (9) [S]. Complete characterization of the system including evaluation of the rate constants and ETA requires measurement of catalysis at a variety of concentrations of quinone and bianthrone. Such studies are under way with bianthrone and several quinones as well as with some derivatives of bianthrone. ACKNOWLEDGMENT This research was supported by the National Science Foundation (Grant CHE81-11421).
REFERENCES 1 2 3 4 5
C.P. Andrieux. J.M. Dumas-Bouchiat and J.M. Saw&t. J. Electroanal. Chem.. 87 (1978) 39. C.P. Andrieux. J.M. Dumas- Bouchiat and J.M. Saw&t. J. Electroan& Chem.. 87 (1978) 56. C.P. Andrieux. J.M. Dumas-Bouchiat and J.M. Sav&nt. J. RlectroanaL Chem.. 88 (1978) 43. C.P. Andrieux. J.M. Dumss-Bouchiat and J.M. Saveant. J. ElectxoanaL Cheq.. 113 (1980) 1. C.P. Andrieux, C. Blocman. J.M_ D umas-Bouchiat. F. M’Hdla and J.M. Savesnt, J. FJectroanaL Chem.. 113 (1980) 19. 6 C.P. Andrieux. C. Bloc-. J.M. Dumas- Bouchiat and J.M. Saw&t. J. Am. Chem. Sot.. 101 (1979) 3431. . 7 C.P. Andxieus, C. Blocman. J.M. D umas-Bouchiat. F. h¶‘HaUa end AM. Saveant, J. Am. Chem. Sot.. 102 (198p) 3806. 8 J.M. Saveant. Ace. Chem. Res.. 13 (1960) 323. 9 K. Bouilel and J. Simonet. Ekctmchim. Acta, 24 (1979) 481. 10 P. Martigny and J. Simonet. J. Electmanal. Chem, 111 (1980) 133. 11 K. Boujlel, J. Simonet. J.-P_ Bamier, C. Girard and J.-M. Conia, J. EkctroanaL Chem.. 117 (1981) 161. 12 W.E. Bxitton and A.J. Fry. Anal. Chem.. 47 (1975) 95. J. Volke and J. Kuthan. J. Electroanal. Chem.. 119 (1981) 301. 13 F. Pragat, B. Kaltofen. 14 P. Miwtigny. G. Gabon. J. Simonet and G. Mouawt. J. Electroam& Chem., 121 (1981) 349. 15 M. Genies. J.-C. Moutet and G. Reverdy. Electrochhn., A+, 26 (1981) 931. 16 B.A. Oken and D.H. Evans, J. Am. Chem. Sot., 103 (1981) 839. 17 R-C. Buchta and D.H. Evans, AnaL Chem., 40 (1968) 2181. l&W. VandenBom and D.H. Evans, Anal Chera, 45 (1973) 1298. 18 P.E. Wbitson, 19 M.D. Ryan and D.R. Evans, J. Electrcanai. Chein, 67 (1976) 333. 20 B. Komnsteiri. K.A. Muszkat and S. Sharafy43zeri. J- Am. C&em. Sot.. 95 (1973) 6177. 21 0. Hammerich and V-D.-Parker. Acta Chem. Scsnd.. B36 (1981) 395. B.A. Olsen. D.H. Evans and I. Agrenat. J. Bkctrosnal. Chem_. in press. 2”3 P. Neta and D.H- Evans, J. Am. Chem. Sot.. 103 (1981) 7041.
DENNIS&EVANW
Deparfmqtt of Chek&y,
University of Wisconsin-Madison, Madison, WI 53706 -(U.S.A.)
(Received93rd I+&nber 1981).-
In-homogeneous redox catalysis the rate of an irreversible electrode reaction is increased by:the action of a catalyst redox couple which accepts electrons~ from the electrode (ii the case of a reduction) and transfers thein to the substrate. The phenomenon has been thoroughly. studied by Saveant and coworkers: [l-8] who have used it to determine otherwise inaccessible standard potentials, standard heterogeneous electron transfer rate constants, and rate con&&@ of fohowing chemical reactions. Numerous examples of homogeneous redox catalysis of orgzkic electrode reactions have been listed [5] and others continue to be published ]9-151. A commou feature of the systems studied to date is that the overall reaction of the substrate involves irreversible bond breaking as a key step in the sequence. Reduction of aromatic halogen compounds is a typical example 1’71. The redu&d form of the catalyst couple-(P/Q) transfers an electron to ArX f0rming.a sho+.ived anion radical (reaction 2). . P+e
=+
Q
QtArX
.=
P+Arx;-
(2)
Al-XT
-b
Ar
(3)
AIY +Q
+
P+Ar-
(4)
Ar’+A&
+
Ar+Arx
(5)
A.r-+H+
+
+x-
.ArH.
_. is,-
elec_Gon transfer reaction- i+_not energetically favored but it is driven _by the rapid decomposition of. Arx’ . ‘Reactions (2) and (4) recycle the- catalyst and.the-.:tisult is~mpid+I@ct~r~u$ion of-A.6 at potentials nearI& .where the -t _electrom+@on .ofArX is often immeasurably slow.. .‘.
This
*&&&
*&&;&gm&.;&;ul;l .-. _.
.-. .~~~&tY+
i::: 1. ::_ : ;.-
-. _,:.‘1, _,._ ,__‘y;,
-y -lL;;i). _y;=,: ,_;;:
368
The present communication reports redox catalysis in the reduction of a sub&mce which does not suffer bond-breaking in its reduction but instead experiences a significant change in conformation which drives the overall c&&tic reaction. Another significant feature of the reaction is that the. standard potential for the overall reduction of the substrate is known so that the. catalysis can be analyzed in terms of the relative free energies of all species the reaction scheme.
in
EXPERIMENTAL
Dimethylformamide (DMF, Burdick and Jackson) was vacuum distilled.and stored over Woelm 200 neutral alumina (activated at 300°C under nitrogen for 24 h). The supporting electrolyte was 0.10 M tetraethylammonium perchlorate [lS]. Bianthrone was obtained from Aldrich Chemical Company. All solutions were protected from exposure to atmospheric moisture and solutions containing bianthrone were kept in the dark. Voltammetry experiments used a Princeton Applied Research Model 170 electrochemistry system and a cell of the type previously described [17]. The working electrode was a hanging mercury drop [18] of approximately 0.016 cm2 area. The reference electrode was a silver wire in contact with O.@lO M AgNOs, 0.10 M tetraethylammonium perch&ate in DMF 1191 whose potential was found to be +0.423 V vs. aq. SCE. Solutions were purged with purified nitrogen and the solution temperature was maintained at 25.O”C. RESULTS AND DISCUSSION
An unusual example of redox catalysis has been discovered in the reduction of bianthrone, I. A brief review of the electrochemical behavior of bianthrone will aid in understanding of the catalytic process.
At room temperature, I exists as the ‘yellow A form (Fig. 1) ._inwhich .the .. steric interactions between hydrogen atoms at positions 1 and 8’ (and .l’ --&id 8) are relieved by folding each anthrone system away from the other without
.369 .
B
A
Fig. 1. A and B forms of bianthrone (I).
twisting the 9,9’ double bond. When a solution of I is heated, a significant fraction is converted to the green B form. The most widely accepted structure of the B form (Fig. 1) reduces the steric crowding by twisting about the 9,9’ bond and flattening the two anthrone systems [20]. The electrochemical reduction of I [16,21,22] induces structural changes similar to the A+B thermochromicconversion. The anion radical of I has reduced double bond character in the 9,9’ bond which lowers the energy required to twist that bond. T_hu-:it is not surprising that the radical anion adopts a B type structure, B' , with considerable twisting about the 9,9’ bond [16,21]. The dianion undoubtedIy exists in a B structure, B2-, with the angle between the two anthranolate systems probably being close to 90”. Scheme 1 summariies the electrochemical behavior of bianthrone. Reduction of A proceeds in an ECE process in which the initially formed A-Like anion’ radical, A’, rapidly .twists to BT which iri turn is rapidly reduced to B2- because the potential required to reduce A is more negative than EzB_ In DMF/ 0.10 M tetraethylammonium perchlorate, Km = 2.2 X lo-‘, LAB = = -0.50 V VS. aq. 8.1 X 10-j s-l, &A = 3.7 S+, a?& = -0.20 V- and &
SCHEME1
SC% [ 161. -Recently it. h& been ~deterrnined by &se mdiolysis that -k-m = 7 X 104 S'l 'iti,&k&e' ~o~rop~oI/t&r&ydrofuran/I&C -[23]. Assuming the same.y$uelpf k-h &- QMF ~leads'~ti, @t;imates -of ETA. =. ::_1;14 V vs_ SCE land
ry
E 2x
-1(p3;_
: ,- (_ .., _;,_:y-._.: ._-I
-
.'
-;,_,-_:
~:
:
-"y ;_
The’ above vaI&s of the stand& ‘$+ni+s -(aIong ..&ith_&r '&&&ily _ .: chosen &A) ‘may be used .d construct a~the&nodyx&ic Ability -&one dia;
370
3 -0”
I
I
I
-2o-
I
I I I
I I EP. I I
-3o-
I 0.0
i I I I
G-d I I I
I_
I -0.4
-~ J
-0.8 _
-1.2
-1.6
Fig. 2. Thermodynamic stability zone diagram. See scheme 1 for defmition of terms. (a) System with log KAB > Q; (b) system with log Km << 0; (c) bianthrone, log KAB = -2.7. Schematic voltammogram “rev”: expected behavior of bianthrone under reversible conditions; schematic vokammogram “ohs”: actual behavior of bianthrone.
gram (Fig. 2). This diagrampertainsto a family of systems havingthe indicated E” values and various values of HAB (once the E” values have been fixed, KG and K=m are determined by the choice of Km )_ For a system in which log Km > 0 (line a, Fig. 21 the B form predominates at equilibrkm and it would be reduced first to B’ and then to-B2- as the potential was made more negative_ On the other hand, if log Km << 0 (line b, Fig. 2), a single reduction process, A + 2e + B2-, would be predicted for a system showing thermodynamic reversibility. For bianthrone log Km = -2.7 (line c, Fig. 2) and regions of stability of A, BT and B*- are encountered .as the_ potential is moved in the negative direction. At -0.36 V Eo for the A/B’ couple occurs, followed shortly by & at -0.50 V. A schematic cyclic voltammogram (“rev”) expected for the case of reversible bianthrone reduction is shown in the top part of Fig. 2. However, the actual behavior of bianthrone is far removed from equilibrium conditions. The reduction of A does not occur at an appreciable rate until the potential is over 0.4 V negativeof the standardpotential 1161 (schematic
voltammogram“ohs”, Fig. 2). Thus, it is concluded that the r&duct& of bianthrone is potentially subject to homogeneous redox catalysisby couples. with standardpotentials in the r+nge of -0.5 to -1.0 V vs. SC?. .,
Y
I
I
-0.2
-0.4
-0.6
E vs
sap
-0:s
Fig.3. Catalyticreduction ofbiithrone in DMFvia Z-chloro-S,lO-anthraquinone. &an rate:O.lOOV/s.(A)l_OO miW'Ga&bro~e;(B)l.OO mMbiantbrone and 1.00 dquinone; (C)blank;(D)!.OOmiWqui+one.
A number of quinones were investigatedas possible catalysts. Results for 2-@loro-9,10-anthraquinone -a& ihustratedin Fig. 3. Bianthrone in the- absence of quhione shows no significant reduction current priot to -0.8 V (curve A, Fig. 3). Quinone in the absence of bianthrone is reduced in a reversible, diffusion-controlled reaction to the quinone anion radical (curve D, Fig. 3). An equimolg m@ure of quinofie 'andbianthrone (curve B, Fig. 3) indicates the occurnmce of homogeneous redox catalytic reduction of bianthroneat the potential of the quinone couple: the cathodic peak current has increased,the anodic peak current hasdiminished.and, most importantly, oxidation peaks for the product, B2-, are evident on the return sweep at -0.48 V and -0.18 V (the initial potential). When the potential was held at -0.8 V vs, 8kX -for 30 s following the first sweep, the B2- + BT and BT + B peaks on the return sweep were of a height correspondingto complete ;conversion of A to B2- m the reaction-layer[lS]. The postulated reaction scheme is given by reactions (‘7~(10). P-+-e . .-+
-Q
(7)
372 TABLE
1
Catalytic peak current function for reduction of bianthrone catalyzed by various quinones” Quinone
E,,,IV
1,4-Naphthoquinone 2,5-Di-tert-butyl-1,Cbenzoquinone 2,6-Dimethoxy-1,4-benzoquinone 2-Methyl-1,4-naphthoquinone 2-Chloro-9,10-anthraquiuone Tetramethyl-1,4-benzoquinone
-0.622 -0.633 -6.665 -0.687 -0.740 -0.754
b
ip /ipa c 1.00 1.01 1.05 1.14 1.29 1.56
a 1.00 x 10B3 M bianthrone; 1.00 X lo-” M quinone; 0.100 V/s scan rate. bv~. aq. SCE. c ip is observed peak current and iPd is quinone reduction peak current in absence of bianthrone.
a pre-equilibrium [ 51. In either case the rate will increase as quinones with more negative standard potentials are used as catalysts. This was found to be the case for the quinones listed in Table 1 for which the indicated values of the catalytic current function, i&,d, were found. The observed peak or plateau current is expressed by ip and ipd is the peak current for qu‘mone reduction in the absence of bianthrone. Clearly, the expected trend is observed. Catalysis is impossible with couples whose standard potential is positive of Ei B-, u-O.36 V. For observable catalysis, the standard potential of the catalyst must be negative of about -0.63 V. It was previously noted that B2- is not an effective redox catalyst (autocatalyst) for A reduction [ 161. This was deduced from the observed rate of reaction of B2- with A (B2- + A + 2B:). This reaction occurs by reactions (11) and (12) with (11) being rate-limiting. There was no significant contribution A
k AB U
B
(11)
kBA
B + B2-
*
2B=
A + B2-
+
A7 + BT
A’
(fast)
(12) (13) (14)
from the (13~(14) sequence which must occur if the BF/B2- couple wemeto serve as redox catalyst. The data in Table 1 provide an explanation, viz., f?& (-0.50 V) is not sufficiently negative for effective catalysis. Another interesting feature of the redox cataI$zed reduction of- bianthrone is that there must be a lower limit for the apparent rate of catalysis .due to the intervention of the alternative sequence of reaction (11) folJowed by reactions (15) and (10):
373
km *
-
B
(11)
kBA
Q+B
t
P + B;-
Q+B’
+
P + B2-
(fast)
(15)
(fast)
(19)
However, this sequence is apparently too slow to provide observable apparent catalysis with 1,4-naphthoquinone (see Table 1) although it might become effective at higher temperatures. Preliminary .&inetic analysis suggests that the process falls in the region of mixed control by reactions (8) and (9) [S]. Complete characterization of the system including evaluation of the rate constants and ETA requires measurement of catalysis at a variety of concentrations of quinone and bianthrone. Such studies are under way with bianthrone and several quinones as well as with some derivatives of bianthrone. ACKNOWLEDGMENT This research was supported by the National Science Foundation (Grant CHE81-11421).
REFERENCES 1 2 3 4 5
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