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Decomposition of 1-phenylethyl hydroperoxide by methyl(pyridine)-bis(dimethylglyoximato)-cobalt(III)
Journal of Molecular Catalysis, 45 (1988)
17
17 - 23
DECOMPOSITION OF l-PHENYLETHYL HYDROPEROXIDE METHYL(PYRIDINE)--BIS(DIMETHYLGLYOXIMATO)--COBALT(III) I. P. HAJDU, V. N. VETCHINKINA*,
BY
J. LUKACS and D. GAL
International Laboratory, Central Research Znstitute for Chemistry of the Hungarian Academy of Sciences, H-1525, P.O. Box 17, Budapest (Hungary) (Received June 1, 1987; accepted October 5, 1987)
Summary The decomposition of 1-phenylethyl hydroperoxide (HROOH) catalyzed by methylcobaloxime (Co(II1)) at 70 “C has been investigated in oxygen-free chlorobenzene-acetonitrile solvent. The rate of hydroperoxide decomposition increases with increasing conversion until it reaches a maximum, followed by a decrease. The maximum rate coincides with the maximum of the absorbance of the reaction mixture. A simple mechanism has been suggested which describes the kinetics of the overall reaction and explains (i) the linear dependence of the maximal decomposition rate of the hydroperoxide on the initial substrate concentration, (ii) the relation between d[ Co(III)]/dt and [HROOHlO, as well as and [ Co(III)] ,,, observed experimentally. between (d [ HROOH] /dt),,, Deactivation of the catalytically active species takes place with participation of the hydroperoxide.
Introduction The decomposition of secondary ‘hydroperoxides in the liquid phase catalyzed by transition metal complexes has been widely studied 111. As a result of such investigations, it is generally assumed that initially a complex is formed between the catalyst and hydroperoxide molecules in an equilibrium [2 - 41 and that the overall process is, at least partially, a radical reaction [ 51. Experimental data indicate that the decomposition proceeds via two different mechanisms. According to the first mechanism, decomposition starts with a maximum rate and therefore it is assumed that the catalyst introduced into the system is in its active form, or the active form is generated immediately after introducing the catalyst. As an example of this *Permanent address: Institute of Chemical Physics, Moscow, U.S.S.R. @ Elsevier Sequoia/Printed
in The Netherlands
18
mechanism, the decomposition of n-decylhydroperoxide in the presence of cobalt(I1) stearate [6] as well as the decomposition of 1-phenylethyl hydroperoxide in the presence of molybdenum naphthenate [7] can be mentioned. Detailed kinetic analysis of this type of decomposition has been described earlier by us [ 81. In the case of the second type of decomposition, the overall rate increases initially and, after achieving a maximum, decreases, as is typical for autocatalytic processes [9]. Therefore it is assumed that the catalyst introduced into the system becomes activated (transformed) in a relatively slow process., It $hould be mentioned that deactivation of the catalyst takes place in both types of decomposition. Here we intend to give a kinetic analysis of this latter type of decomposition, describing the whole region of conversion.
Experimental section 1-Phenylethyl hydroperoxide (HROOH) was synthesized by airoxidation of ethylbenzene at 136 “C with small amounts of HROOH as initiator. After an oxidation of 8 h, the accumulated amount of HROOH was 0.6 M, which was then precipitated in the form of sodium salt and subsequently freed by HCl. This procedure was repeated until an HROOH solution of 90% was achieved. The purity of this hydroperoxide was better than that of hydroperoxide obtained by oxidation of ethylbenzene with Co”‘-acetylacetonate at 120 Y!. Methyl(pyridine)bis(dimethylglyoximato)cobalt(III) was synthesized according to Schrauzer [lo] and its purity found to be satisfactory by elemental analysis and thin layer chromatography. Hydroperoxide was determined iodometrically and the concentration of the catalyst measured by HPLC using a silica column modified by CN groups. Ethanol/hexane (1:6) was used as eluent, and detection was carried out spectrophotometrically at 215 nm. Reactions were carried out in a glass reactor with a solvent of acetonitrile/chlorobenzene 1:4 (v/v). Prior to each run, oxygen-free argon was bubbled through the mixture of the hydroperoxide and solvent for 15 min at the temperature used for experiments. The moment of the addition of catalyst was taken as the start of the reaction. Experimental results The changes in the concentrations of HROOH and Co(III), as well as in the absorbance of the reaction mixture measured at 370 nm, are represented in Fig. 1.
19
At the temperature of the experiments, T = 70 “C, no decomposition can be detected in the absence of catalyst. Figure 1 shows that both decomposition and change in the absorbance proceed with increasing rates in the early stages, and that the maxima of the decomposi~on rate and the absorbance coincide. Furthermore, at these reaction times the majority of the catalyst (about 70 - 80%) has undergone chemical transformation. These findings indicate that the catalyst introduced yields a new compound whose activity exceeds the activity of the original catalyst. Simultaneously, the color of the mixture varies during the reaction: the initial light yellow changes first into dark brown, which later becomes lighter until the original color is restored. It was assumed that the more active compound was the transformation product of the complex formed between HROOH and the catalyst. This assumption is subported by the fact that its formation rate increases with increasing [HROOH] 0, as shown in Fig. 2. The effect of the initial concentration of HROOH on its decomposition rate at fixed concentrations of Co(II1) is shown in Fig. 3. According to Fig. 3a, (i) the maximal rate of the decomposition increases and (ii) it is shifted toward shorter reaction times with increasing [HROOH],. Furthermore, the rate maximum of the decomposition depends linearly on [ HROOH],, as plotted in Fig. 3b. The corresponding changes in the concentration of the catalyst are summarized in Fig. 4, showing a relation between zuOcOon~ and [ HROOH], tending toward but not achieving saturation. In the next series, the initial concentration of the catalyst was varied at fixed [ HROOH Jo. The results are plotted in Fig. 5.
50 tlmin
I 100
Fig. 1. Change in the concentration of hydroperoxide bance of the solution (0) at & = 370 nm; T = 70 “C.
tlmin (*)
and Co(II1) (0) as well as absor-
Fig. 2. Change in absorbance at h = 370 nm; T = 60 “C, [Co(III)]o [HROON]o = 0.11 M; (2) [HROOHlo = 1.14 M.
= 5
X
lo-* M. (1)
20
s g
E
.\ ‘\,
100
.
a
\
50
\
I
tlmin
Fig. 3(a) Change in hydroperoxide concentration (points are for experimental, curves for calculated values). (b) The maximum decomposition rate of hydroperoxide as a function of [HROOH],,; T = 70 “C, [Co(III)]c = 1 x 10-3M. Fig. 4. The initial rate of Co(II1) consumption as function of [HROOH]e; T = 70 “C, [Co(III)]c = 1 x 10e3 M; (0) experimental values, (0) values calculated using the equation: ([Co(III)]e - [Co(III)]le)/10 min.
1.0
2.0
I,~'lo.103/M
Fig. 5. The maximum rate of hydroperoxide decomposition as a function of [Co(III)]e; [HROOH]e = 5.5 x 10m2 M, T = 70 “C; (0) experimental values, (0) calculated values.
21
Based on the experimental suggested: Firstly,
the catalyst
Co(II1) + HROOH -
results following
and HROOH form a complex
mechanism
has been
reversibly:
X
(1)
/I Co(II1) + HROOH X\, Y + Pl
(2) (3)
where X is the complex, Y corresponds to a more active form of the catalyst produced in a monomolecular transformation from X (eqn. 3), and Pl is a fragment without any catalytic property. Catalysis actually occurs during the interaction between HROOH and Y: Y+HROOH-Y+P2
(4)
where P2 refers to decomposition products of HROOH. Interaction of HROOH and Y, however, can lead to the deactivation of the active form Y: Y + HROOH __f
stable species
(5)
The existence of process (5) can be supported by the character of the [HROOHI-t relation. Namely, following the maximum of the absorbance, a rapid decrease and a marked slow-down in the consumption of [HROOH] is observed. It was found that this slow-down was stronger at higher [HROOH] W Therefore a deactivation step with participation of the hydroperoxide was necessary in order to explain this fact. From the mechanism (1 - 5) we obtain: wco(III)
= _
d[co(III)l = kl dt
[Co(III)][HROOH]
- k,[X]
If it is assumed that the equilibrium is shifted toward X, the second term of eqn. (6) can be neglected. Thus, the initial rate of the comsumption of Co(II1) yields the value of k,: Co(II1)
kl=
wQ [HROOH],
1 (7)
[Co(III)],
and from the experiments: k, = 9
X 1O-3 M-’ s-l
(8)
It seems very likely that X is transformed and therefore:
d[Xl = k, [Co(III)] [HROOH] dt
- k3[X]
= 0
into Y in a very fast process
(9)
and ‘s
= (k,[Co(III)]
- k,[Y])[HROOH]
(10)
22
After rearrangement: 1
iHROOHl where value.
d[Yl dt max= ~,[Co(WI,,x
subscript
- kdJ%nax = 0
max refers to the time at which [Y] reaches
Finally, it can be deduced
(11) its maximum
that:
It has already been mentioned that at the maximal concentration of Y about 70 - 80% of the catalyst has been transformed either into Y or into deactivated catalyst (eqn. (5)). Consequently, only the minimum value of k, can be determined by assuming that all of Co(II1) consumed can be found exclusively as Y, that is (k,),, = (0.25/0.75) 9 X 1O-3 W’ s-l and thus: = 3 X 1O-3 M-' s-’
(k,),,
(13)
Actually, k, exceeds the above value since, at least partially, it is transformed during the overall process into inactive product, and therefore:
[nn*x G [WIII)I, Furthermore WHROOH
= -
- [Co(III)I,,,
(14)
:
- d[HROOH]
= k,[Co(III)][HROOH] dt + k,[Y] [HROOH] + k,[Y] [HROOH] From eqn. (11) it follows that
kJWIWI,,x
= MVmax
(15)
(16)
and eqn. (15) can be transformed: w max HRooH = (2 k,[Co(III)],,,
+ k,[Y],,,)[HROOH],,,
(17)
From this equation, the minimum value of k4 can be calculated, taking into consideration the same assumptions which were used to obtain eqn. (13). Thus (k&i,,
= 1 M-’ s-i
(13)
In order to compare the mechanism (1 - 5) and experimental data, beside the above calculations we have solved the differential equations obtained from the mechanism using the Gear method [ 111 and varying sets of rate coefficients. The best fit was achieved using the following values: kl = 1 X lo-’
M-’ s-l
k2 < 1 X1O-4 s-l k3 > 1 X lo-* M-i s-’
23
k4 = 1.2 M-l s-l hs = 1 X lo-‘M--i
s-l
The curves calculated using this set of rate coefficients described satisfactorily the experimental results as shown in Fig. 3. It should be mentioned that the rate of eqn. (2) does not affect the calculations if k2 < 1 X 1O-4 M--i s-i Discussion Mechanism (1 - 5) fairly represents the decomposition of HROOH in the presence of the catalyst applied, and explains the linear dependence of the maximal rate of the hydroperoxide decomposition on [HROOH]a as shown in Fig. 3b. This means that the rate of process (1) can be neglected around the rate maximum, compared to the rate of process (4); that is, hydroperoxide is consumed actually in the latter process. Therefore the maximal rate of consumption of HROOH shows a first-order dependence upon [ HROOH] ,,. Simultaneously, the consumption rate of Co(II1) can be described (Fig. 4). The agreement between calculated and measured values is less satisfactory if the values with varying catalyst concentrations are compared (Fig. 5). It is worth mentioning that calculations carried out by the following set of equations: Co(II1) + HROOH Y + HROOH
Y + Pl
(19)
pY+P2 ,, stable species
(4)
(5) led to identical results with respect to the consumption of both HROOH and Co(II1). However, they were not able to describe the increasing rate of formation of Y, and therefore process (19) had to be separated into steps (l), (2) and (3). References 1 R. R. Hiatt, T. Mill and F. R. Mayo, J. Org. Chem., 33 (1968) 1416. 2 G. L. Banks, A. J. Chalk, J. E. Dawson and J. F. Smith, Nature, 174 (1954) 274. 3 I. V. Zakharov and V. Y. Slyapintokh, Kinet. K&d., 2 (1961) 165; ibid., 4 (1963) 239, 382. 4 W. H. Richardson, J. Am. Chem. Sot., 87 (1965) 247,1096. 5 F. Haber and J. Weiss, Proc. Roy. Sot., A, 147 (1934) 332. 6 G. M. Bulgakova, Z. K. Maizus and I. P. Skibida, Kinet. KutaZ., 7 (1966) 332. 7 L. Siimegi, I. Nemes, I. P. Hajdu and D. G&l, Oxid. Commun., 1 (1979) 23. 8 I. P. Hajdu, I. Nemes, L. Siimegi, T. Vidoczy and D. Gal, ht. J. Chem. Kinet., 13 (1981) 1191. 9 G. Vasvari and D. Gal, J. Chem. Sot. Faraday Trans. l., 73 (1977) 399. 10 G. N. Schrauzer, Znorg. Synth., 11 (1968) 61. 11 C. W. Gear: Commun. ACM, 14 (1971) 176.
17
17 - 23
DECOMPOSITION OF l-PHENYLETHYL HYDROPEROXIDE METHYL(PYRIDINE)--BIS(DIMETHYLGLYOXIMATO)--COBALT(III) I. P. HAJDU, V. N. VETCHINKINA*,
BY
J. LUKACS and D. GAL
International Laboratory, Central Research Znstitute for Chemistry of the Hungarian Academy of Sciences, H-1525, P.O. Box 17, Budapest (Hungary) (Received June 1, 1987; accepted October 5, 1987)
Summary The decomposition of 1-phenylethyl hydroperoxide (HROOH) catalyzed by methylcobaloxime (Co(II1)) at 70 “C has been investigated in oxygen-free chlorobenzene-acetonitrile solvent. The rate of hydroperoxide decomposition increases with increasing conversion until it reaches a maximum, followed by a decrease. The maximum rate coincides with the maximum of the absorbance of the reaction mixture. A simple mechanism has been suggested which describes the kinetics of the overall reaction and explains (i) the linear dependence of the maximal decomposition rate of the hydroperoxide on the initial substrate concentration, (ii) the relation between d[ Co(III)]/dt and [HROOHlO, as well as and [ Co(III)] ,,, observed experimentally. between (d [ HROOH] /dt),,, Deactivation of the catalytically active species takes place with participation of the hydroperoxide.
Introduction The decomposition of secondary ‘hydroperoxides in the liquid phase catalyzed by transition metal complexes has been widely studied 111. As a result of such investigations, it is generally assumed that initially a complex is formed between the catalyst and hydroperoxide molecules in an equilibrium [2 - 41 and that the overall process is, at least partially, a radical reaction [ 51. Experimental data indicate that the decomposition proceeds via two different mechanisms. According to the first mechanism, decomposition starts with a maximum rate and therefore it is assumed that the catalyst introduced into the system is in its active form, or the active form is generated immediately after introducing the catalyst. As an example of this *Permanent address: Institute of Chemical Physics, Moscow, U.S.S.R. @ Elsevier Sequoia/Printed
in The Netherlands
18
mechanism, the decomposition of n-decylhydroperoxide in the presence of cobalt(I1) stearate [6] as well as the decomposition of 1-phenylethyl hydroperoxide in the presence of molybdenum naphthenate [7] can be mentioned. Detailed kinetic analysis of this type of decomposition has been described earlier by us [ 81. In the case of the second type of decomposition, the overall rate increases initially and, after achieving a maximum, decreases, as is typical for autocatalytic processes [9]. Therefore it is assumed that the catalyst introduced into the system becomes activated (transformed) in a relatively slow process., It $hould be mentioned that deactivation of the catalyst takes place in both types of decomposition. Here we intend to give a kinetic analysis of this latter type of decomposition, describing the whole region of conversion.
Experimental section 1-Phenylethyl hydroperoxide (HROOH) was synthesized by airoxidation of ethylbenzene at 136 “C with small amounts of HROOH as initiator. After an oxidation of 8 h, the accumulated amount of HROOH was 0.6 M, which was then precipitated in the form of sodium salt and subsequently freed by HCl. This procedure was repeated until an HROOH solution of 90% was achieved. The purity of this hydroperoxide was better than that of hydroperoxide obtained by oxidation of ethylbenzene with Co”‘-acetylacetonate at 120 Y!. Methyl(pyridine)bis(dimethylglyoximato)cobalt(III) was synthesized according to Schrauzer [lo] and its purity found to be satisfactory by elemental analysis and thin layer chromatography. Hydroperoxide was determined iodometrically and the concentration of the catalyst measured by HPLC using a silica column modified by CN groups. Ethanol/hexane (1:6) was used as eluent, and detection was carried out spectrophotometrically at 215 nm. Reactions were carried out in a glass reactor with a solvent of acetonitrile/chlorobenzene 1:4 (v/v). Prior to each run, oxygen-free argon was bubbled through the mixture of the hydroperoxide and solvent for 15 min at the temperature used for experiments. The moment of the addition of catalyst was taken as the start of the reaction. Experimental results The changes in the concentrations of HROOH and Co(III), as well as in the absorbance of the reaction mixture measured at 370 nm, are represented in Fig. 1.
19
At the temperature of the experiments, T = 70 “C, no decomposition can be detected in the absence of catalyst. Figure 1 shows that both decomposition and change in the absorbance proceed with increasing rates in the early stages, and that the maxima of the decomposi~on rate and the absorbance coincide. Furthermore, at these reaction times the majority of the catalyst (about 70 - 80%) has undergone chemical transformation. These findings indicate that the catalyst introduced yields a new compound whose activity exceeds the activity of the original catalyst. Simultaneously, the color of the mixture varies during the reaction: the initial light yellow changes first into dark brown, which later becomes lighter until the original color is restored. It was assumed that the more active compound was the transformation product of the complex formed between HROOH and the catalyst. This assumption is subported by the fact that its formation rate increases with increasing [HROOH] 0, as shown in Fig. 2. The effect of the initial concentration of HROOH on its decomposition rate at fixed concentrations of Co(II1) is shown in Fig. 3. According to Fig. 3a, (i) the maximal rate of the decomposition increases and (ii) it is shifted toward shorter reaction times with increasing [HROOH],. Furthermore, the rate maximum of the decomposition depends linearly on [ HROOH],, as plotted in Fig. 3b. The corresponding changes in the concentration of the catalyst are summarized in Fig. 4, showing a relation between zuOcOon~ and [ HROOH], tending toward but not achieving saturation. In the next series, the initial concentration of the catalyst was varied at fixed [ HROOH Jo. The results are plotted in Fig. 5.
50 tlmin
I 100
Fig. 1. Change in the concentration of hydroperoxide bance of the solution (0) at & = 370 nm; T = 70 “C.
tlmin (*)
and Co(II1) (0) as well as absor-
Fig. 2. Change in absorbance at h = 370 nm; T = 60 “C, [Co(III)]o [HROON]o = 0.11 M; (2) [HROOHlo = 1.14 M.
= 5
X
lo-* M. (1)
20
s g
E
.\ ‘\,
100
.
a
\
50
\
I
tlmin
Fig. 3(a) Change in hydroperoxide concentration (points are for experimental, curves for calculated values). (b) The maximum decomposition rate of hydroperoxide as a function of [HROOH],,; T = 70 “C, [Co(III)]c = 1 x 10-3M. Fig. 4. The initial rate of Co(II1) consumption as function of [HROOH]e; T = 70 “C, [Co(III)]c = 1 x 10e3 M; (0) experimental values, (0) values calculated using the equation: ([Co(III)]e - [Co(III)]le)/10 min.
1.0
2.0
I,~'lo.103/M
Fig. 5. The maximum rate of hydroperoxide decomposition as a function of [Co(III)]e; [HROOH]e = 5.5 x 10m2 M, T = 70 “C; (0) experimental values, (0) calculated values.
21
Based on the experimental suggested: Firstly,
the catalyst
Co(II1) + HROOH -
results following
and HROOH form a complex
mechanism
has been
reversibly:
X
(1)
/I Co(II1) + HROOH X\, Y + Pl
(2) (3)
where X is the complex, Y corresponds to a more active form of the catalyst produced in a monomolecular transformation from X (eqn. 3), and Pl is a fragment without any catalytic property. Catalysis actually occurs during the interaction between HROOH and Y: Y+HROOH-Y+P2
(4)
where P2 refers to decomposition products of HROOH. Interaction of HROOH and Y, however, can lead to the deactivation of the active form Y: Y + HROOH __f
stable species
(5)
The existence of process (5) can be supported by the character of the [HROOHI-t relation. Namely, following the maximum of the absorbance, a rapid decrease and a marked slow-down in the consumption of [HROOH] is observed. It was found that this slow-down was stronger at higher [HROOH] W Therefore a deactivation step with participation of the hydroperoxide was necessary in order to explain this fact. From the mechanism (1 - 5) we obtain: wco(III)
= _
d[co(III)l = kl dt
[Co(III)][HROOH]
- k,[X]
If it is assumed that the equilibrium is shifted toward X, the second term of eqn. (6) can be neglected. Thus, the initial rate of the comsumption of Co(II1) yields the value of k,: Co(II1)
kl=
wQ [HROOH],
1 (7)
[Co(III)],
and from the experiments: k, = 9
X 1O-3 M-’ s-l
(8)
It seems very likely that X is transformed and therefore:
d[Xl = k, [Co(III)] [HROOH] dt
- k3[X]
= 0
into Y in a very fast process
(9)
and ‘s
= (k,[Co(III)]
- k,[Y])[HROOH]
(10)
22
After rearrangement: 1
iHROOHl where value.
d[Yl dt max= ~,[Co(WI,,x
subscript
- kdJ%nax = 0
max refers to the time at which [Y] reaches
Finally, it can be deduced
(11) its maximum
that:
It has already been mentioned that at the maximal concentration of Y about 70 - 80% of the catalyst has been transformed either into Y or into deactivated catalyst (eqn. (5)). Consequently, only the minimum value of k, can be determined by assuming that all of Co(II1) consumed can be found exclusively as Y, that is (k,),, = (0.25/0.75) 9 X 1O-3 W’ s-l and thus: = 3 X 1O-3 M-' s-’
(k,),,
(13)
Actually, k, exceeds the above value since, at least partially, it is transformed during the overall process into inactive product, and therefore:
[nn*x G [WIII)I, Furthermore WHROOH
= -
- [Co(III)I,,,
(14)
:
- d[HROOH]
= k,[Co(III)][HROOH] dt + k,[Y] [HROOH] + k,[Y] [HROOH] From eqn. (11) it follows that
kJWIWI,,x
= MVmax
(15)
(16)
and eqn. (15) can be transformed: w max HRooH = (2 k,[Co(III)],,,
+ k,[Y],,,)[HROOH],,,
(17)
From this equation, the minimum value of k4 can be calculated, taking into consideration the same assumptions which were used to obtain eqn. (13). Thus (k&i,,
= 1 M-’ s-i
(13)
In order to compare the mechanism (1 - 5) and experimental data, beside the above calculations we have solved the differential equations obtained from the mechanism using the Gear method [ 111 and varying sets of rate coefficients. The best fit was achieved using the following values: kl = 1 X lo-’
M-’ s-l
k2 < 1 X1O-4 s-l k3 > 1 X lo-* M-i s-’
23
k4 = 1.2 M-l s-l hs = 1 X lo-‘M--i
s-l
The curves calculated using this set of rate coefficients described satisfactorily the experimental results as shown in Fig. 3. It should be mentioned that the rate of eqn. (2) does not affect the calculations if k2 < 1 X 1O-4 M--i s-i Discussion Mechanism (1 - 5) fairly represents the decomposition of HROOH in the presence of the catalyst applied, and explains the linear dependence of the maximal rate of the hydroperoxide decomposition on [HROOH]a as shown in Fig. 3b. This means that the rate of process (1) can be neglected around the rate maximum, compared to the rate of process (4); that is, hydroperoxide is consumed actually in the latter process. Therefore the maximal rate of consumption of HROOH shows a first-order dependence upon [ HROOH] ,,. Simultaneously, the consumption rate of Co(II1) can be described (Fig. 4). The agreement between calculated and measured values is less satisfactory if the values with varying catalyst concentrations are compared (Fig. 5). It is worth mentioning that calculations carried out by the following set of equations: Co(II1) + HROOH Y + HROOH
Y + Pl
(19)
pY+P2 ,, stable species
(4)
(5) led to identical results with respect to the consumption of both HROOH and Co(II1). However, they were not able to describe the increasing rate of formation of Y, and therefore process (19) had to be separated into steps (l), (2) and (3). References 1 R. R. Hiatt, T. Mill and F. R. Mayo, J. Org. Chem., 33 (1968) 1416. 2 G. L. Banks, A. J. Chalk, J. E. Dawson and J. F. Smith, Nature, 174 (1954) 274. 3 I. V. Zakharov and V. Y. Slyapintokh, Kinet. K&d., 2 (1961) 165; ibid., 4 (1963) 239, 382. 4 W. H. Richardson, J. Am. Chem. Sot., 87 (1965) 247,1096. 5 F. Haber and J. Weiss, Proc. Roy. Sot., A, 147 (1934) 332. 6 G. M. Bulgakova, Z. K. Maizus and I. P. Skibida, Kinet. KutaZ., 7 (1966) 332. 7 L. Siimegi, I. Nemes, I. P. Hajdu and D. G&l, Oxid. Commun., 1 (1979) 23. 8 I. P. Hajdu, I. Nemes, L. Siimegi, T. Vidoczy and D. Gal, ht. J. Chem. Kinet., 13 (1981) 1191. 9 G. Vasvari and D. Gal, J. Chem. Sot. Faraday Trans. l., 73 (1977) 399. 10 G. N. Schrauzer, Znorg. Synth., 11 (1968) 61. 11 C. W. Gear: Commun. ACM, 14 (1971) 176.