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Selective Oxidations with Short-lived Manganese(V)
V. CortCs Corberh and S. Vic Bell6n (Editors), New Developments in Selective Oxidation II 0 1994 Elscvier Science B.V. All rights reserved.
623
Selective Oxidations with Short-lived Manganese(V) Eva Zahonyi-Bud6 and Laszlo I. Simandi Central Research Institute for Chemistry of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary 1. INTRODUCTION
Manganese(V) intermediates have long been assumed to be involved in the oxidation of organic and inorganic compounds by permanganate ion [l-51, but little is known about the nature of their reactions. Hypomanganate ion has a fair degree of stability only in concentrated alkali [6] and could be detected in moderately alkaline solutions by stopped-flow rapid scan spectrophotometry as a short-lived intermediate during the oxidation of sulfite ion [7,8]. In the neutral or acidic solutions in which oxidations by permanganate ion are usually carried out Mn(V) escapes detection by both analytical and kinetic methods. A possible way of detecting short-lived manganese(V) in a given reaction would be the addition of a suitable reactant to the starting solution which would react with this intermediate, thus competing with the main reaction. In other words, the induced oxidation of an added substrate by Mn(V) should be accomplished. The term "induced oxidation" refers to a situation where a compound, which cannot be oxidized by the oxidant used, undergoes oxidation when another reducing agent (the inductor) is added to the system. Induced oxidations involving chromate ion have been widely studied [9], but only a few examples are available in which permanganate ion is the oxidant [ 101. In an attempt to examine the reactions of the putative h4n(V) intermediate, we have designed systems in which it is generated via the very fast reduction of permanganate ion with arsenite(II1) in the presence of various reducing substrates, In these systems arsenite(II1) plays the role of an inductor, leading to the oxidation of otherwise unreactive or only moderately reactive substrates.
2. EXPERIMENTAL Measurements were carried out by recording the successive UV-Vis spectra of the reacting solution on a Hewlett-Packard 8452 Diode Array Spectrophotometer combined with a HiTech Scientific stopped-flow type accessory. Reactions were run in buffered solutions, in the presence of pyrophosphate in a 20-fold excess over permanganate to avoid disproportionation of the Mn(II1) formed. The ionic strength was kept constant. Permanganate ion was added in excess over A@), ensuring that the latter should be fUy consumed in all cases. Substrates were added to the As(II1) solution before mixing with permanganate. Owing to the fast rate of
624
the Mn(VI1) - As(II1) reaction, hnetic measurements could not be made as the reactions to be discussed are complete within the mixing time of the stopped-flow instrument available to US. Measurements are based on analyzing the spectra of the product solutions. Residual permanganate and the stable manganese products, Mn(II1) and soluble Mn(IV), were measured by comparison with the known spectra of the individual species. Absorption coefficients of Mn(IV) were calculated from the absorbance of Mn(II1) and Mn(IV) formed in a known concentration ratio in the reaction of Mn(VI1) with As(II1) in the absence of other substrates. 3. RESULTS AND DISCUSSION We have found that the oxidation of phosphorous acid (H3PO3) as well as various organic compounds (alcohols, glycols, hydroxyacids, carboxylic acids, etc.) can be induced in the permanganate - arsenous acid system. This is demonstrated by the experimental finding that permanganate is consumed in excess of the amount used up by As(II4 alone, with no other reactant present. According to kinetic measurement in the absence of As(III), direct oxidation of each of these compounds by Mn(VI1) is negligible during the time intervals involved in our studies. Induced reactions can be characterized by the induction factor IF, which is the ratio of oxidation equivalents consumed by the acceptor (added substrate) and the inductor, As(II1) . It depends on the ratio of reactant concentrations. At sufficiently high acceptor concentrations IF reaches a limiting value, which may provide usehl information on the valence changes occumng in the redox process involved. In our system the limiting value of IF was found to be unity. The typical dependence of IF on the acceptor concentration is illustrated in Figure 1 in the case of ethanol as acceptor.
c
1.0 IF
I
I
1
I
625
In the absence of other reductants arsenic(II1) reduces permanganate to a mixture of Mn(II1) and Mn(1V). With an efficient acceptor present, mainly Mn(II1) is formed (Figure 2) which cannot be reduced hrther in the presence of pyrophosphate. We have observed
’t Figure 2. Permanganate consumption and stable manganese products as a function of acceptor concentration in the Mn(VI1) - As(II1) - Acceptor system (Acceptor: ethanol; conditions the same as in Figure 1) that not more than one half of the oxidizing capacity of permanganate is used up in the oxidation of the acceptor, which confirms that the intermediate formed in the reaction of Mn(VI1) with As (111) is indeed manganese(V): Mn(VI1) + As(II1)
d
Mn(V) + As(V)
(1)
The manganese(V) is hrther reduced to Mn(JI1) either by As(II1) or the acceptor molecule S, but may also undergo disproportionation:
Mn(V)
+ As(II1)
Mn(V) + S 3Mn(V)
A
d
+
Mn(II1)
+
h(V)
Mn(II1) + P
2Mn(IV) + Mn(VI1)
(2) (3) (4)
With increasing concentration of the acceptor (S), reaction (3) gradually becomes predominant. In the limiting case the overall reaction can be described by equations (1) and (3). It is important to emphasize that direct reduction of Mn(1V) by the acceptor and the involvement of Mn(II1) in the induced oxidation can be ruled out. Control experiments show
626
that if the acceptor is added to the solution after complete oxidation of As(III), no changes in the spectra can be detected. The data obtained in the induced oxidation of various acceptors are shown in Table 1. Table 1 Induction factors (IF) in the Mn(VI1) - As(II1) - Acceptor system ~ [pyrophosphate],= 8 . 0 ~ 1 0 M -~ [Mn(VII)],= 4 . 0 ~ 1 0-4 M; [As(III)IO= 4 . 0 ~ 1 0 -M; IF = oxidation equivs consumed by S/ oxidation equivs consumed by As(II1)
IF Acceptor (S)
S[MI
PKa
None
0.0
H3P03 HPO( OEt)( OH) HPO(OEt)2
0.02 0.02 0.02
Ethanol
0.02
1.0; 6.0 0.8
i-Propanol Ethylene glycol Glycolic acid Tartaric acid Formic acid Oxalic acid
0.10 0.02
Formaldehyde Acetaldehyde
0.02 0.02
0.82 (pH=O) 0.73 0.74 (pH=O) 0.0 0.0 (pH=O) 0.0
0.0
0.15
0.73 0.58 0.46 0.79
0.35 0.23 0.30 0.63
0.20
3.60 3.0; 4.3
0.52 0.62
0.32
1.2; 4.2
0.88 0.39
0.76 0.64 0.68 0.76
0.02 0.10
0.02
0.0
pH=6.8
0.16 0.40 0.66 0.93
0.10 0.10
0.02 0.02
pHz3.8
0.40 0.75 0.91
0.10
0.34 1 .O
n-pro pano
pHZ1.04
3.7
0.48
0.36 0.33
0.30
0.34 0.14
In general, the oxidation reactions of the acceptors used are typically not very rapid. In the oxidation of alcohols and aldehydes the rate-limiting step is usually hydride ion or H-atom transfer from a C-H bond to the oxidant, which requires a considerable activation energy. Especially surprising is the high reactivity of phosphorous acid, because its oxidation requires
621
the cleavage of the strong P-H bond, which is a slow process in all known cases (see e.g. [ 11131). We have concluded that the fast oxidation of phosphorous acid by Mn(V) can be interpreted by 2-electron transfer followed by fast deprotonation of the product, rather than by hydride or H-atom transfer. We proposed a mechanism involving the formation of a mixed anhydride of hypomanganous and phosphorous acids, in which the bridging oxygen atom offers a convenient path for inner-sphere electron transfer [ 141, equation ( 5 ) .
OH I
[O3Mn-OW2- + HO-P=O I
H
slow
--+
-H20
OH [03Mn-O-P=Ol2-
+
Mn(II1) + P(V) ( 5 )
H
For anhydride formation to take place, at least one OH-group should be available on P(II1). Deprotonation indeed suppresses the induced oxidation and at higher pH, where only HPO32- is present, it cannot be observed at all. We have also found that diethyl phosphonate and the anion of monoethyl phosphonate have no effect on the oxidation of As(II1). In contrast to this, induced oxidation of the protonated monoester does readily take place [ 131. These results are in accordance with the proposed mechanism: if the acidic protons of phosphorous acid are removed or replaced by ethyl groups, anhydride formation is not possible and oxidation by Mn(V) is inhibited. The reactivity of the organic compounds listed in Table 1 may also be attributed to their ability to form 0-bridged compounds with hypomanganous acid due to the presence of OH groups (alcohols, glycols, hydroxy acids). These species can be regarded as mixed anhydrides or as manganate(V) esters, depending on the nature of the reacting OH groups. Aldehydes may react in their hydrated form when OH groups are available. The oxidation of formic and oxalic acid may involve the hydroxy groups but an outer-sphere reaction is also conceivable. The reaction of simple alcohols with Mn(V) is also influenced by the pH, although in the pH range studied there are no protonation equilibria involving either the alcohols or As(II1). Protonation of hypomanganous acid may be responsible €or this effect , which seems to favour ester formation with alcohols over the competing reactions. In many respects the behaviour of manganese(V) resembles that of chromic acid. Fast formation of Cr(V1) esters in the oxidation of alcohols and aliphatic aldehydes, as well as that of a mixed anhydride in the oxidation of phosphorous acid is well established [ 1,3,9,12]. The overall oxidation rate is determined by the cleavage of the C-H or P-H bond, respectively, in contrast to similar reactions with Mn(V), where the rate-limiting step is the formation of an 0bridged intermediate or electron transfer through the bridge. 4. CONCLUSIONS
The results of this work have shown that short-lived manganese(V) shows a reactivity pattern toward some organic substrates which is distinctly different from that of permanganate
62 8
ion. In that sense it can be regarded as a selwtive oxidant and further studies may lead to evidence that it certainly is a short-lived intermediate in oxidations by permanganate ion. Acknowledgments. This work was supported by the Hungarian Research Fund (Grant No. 4074). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
R, Stewart, in Oxidation in Organic Chemistry, Part A, K.B. Wiberg (ed.), Academic Press, New York, 1965, Chapter 1. F. Freeman, Rev. React. Species Chem. React., 1 (1976) 179. D.G. Lee, in Oxidation of Organic Compounds by Permanganate Ion and Hexavalent Chromium, Open Court, La Salle, IL, 1980. D.G. Lee, in Oxidation in Organic Chemistry, Part D, W.S. Trahanovsky (ed.), Academic Press, New York, 1982, p. 147 L.I. Simindi, in The Chemistry of the Functional Groups, Supplement C, S. Patai, Z. Rappoport (eds), Wiley, Chichester, New York, Brisbane, Toronto, Singapore, 1983, Chapter 13. J.S. Pode, W.A. Waters, J. Chem. Soc. (1956) 717. L.I. Simindi, M. J&y, Z.A. Schelly, J. Am. Chem. Soc., 106 (1984) 6866. L.I. Simindi, M. J&y, C.R. Savage, Z.A. Schelly, J. Am. Chem. Soc., 107 (1985) 4220. L.J. Csinyi, Induced Reactions, in Comprehensive Chemical Kinetics, C.H. Bamford, C.F.H. Tipper (eds), Vol. 7, Elsevier, Amsterdam, London, New York, 1972, Chapter 5. J.H. Merz, G. Stafford, W.A. Waters, J. Chem. Soc. (1951) 638. A. Viste, D.A. Holm, P.L. Wang, G.D. Veith, Inorg. Chem., 10 (1971) 631. G.P. Haight, Jr., M. Rose, J. Preer, J. Am. Chem. Soc., 90 (1968) 4809. 8. Z5honyi-Bud6, L.I. Simindi, Inorg. Chim. Acta, 205 (1993) 207. 8. Zihonyi-Bud6, L.I. Simindi, Inorg. Chim. Acta, 191 (1992) 1.
DISCUSSION CONTRIBUTION
J.-M. BRkGEAULT (Universitk P. et M. Curie, Paris, France): You are considering on your slides (schemes) several redox processes. As the E" values depend on the pH, ligands, etc., are the redox potentials in manganese chemistry directly relevant to your system? L.I. SIMANDI (Central Res. Inst. Chem., Budapest, Hungary): Yes, they certainly are or would be if they were available. The reactions discussed take place in acidic solutions, where both manganese (V) and manganese (VI) are thermodynamically unstable and very reactive. It might be possible to calculate standard redox potentials for these species but at the moment no experimental technique for this purpose seems to be accessible.
623
Selective Oxidations with Short-lived Manganese(V) Eva Zahonyi-Bud6 and Laszlo I. Simandi Central Research Institute for Chemistry of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary 1. INTRODUCTION
Manganese(V) intermediates have long been assumed to be involved in the oxidation of organic and inorganic compounds by permanganate ion [l-51, but little is known about the nature of their reactions. Hypomanganate ion has a fair degree of stability only in concentrated alkali [6] and could be detected in moderately alkaline solutions by stopped-flow rapid scan spectrophotometry as a short-lived intermediate during the oxidation of sulfite ion [7,8]. In the neutral or acidic solutions in which oxidations by permanganate ion are usually carried out Mn(V) escapes detection by both analytical and kinetic methods. A possible way of detecting short-lived manganese(V) in a given reaction would be the addition of a suitable reactant to the starting solution which would react with this intermediate, thus competing with the main reaction. In other words, the induced oxidation of an added substrate by Mn(V) should be accomplished. The term "induced oxidation" refers to a situation where a compound, which cannot be oxidized by the oxidant used, undergoes oxidation when another reducing agent (the inductor) is added to the system. Induced oxidations involving chromate ion have been widely studied [9], but only a few examples are available in which permanganate ion is the oxidant [ 101. In an attempt to examine the reactions of the putative h4n(V) intermediate, we have designed systems in which it is generated via the very fast reduction of permanganate ion with arsenite(II1) in the presence of various reducing substrates, In these systems arsenite(II1) plays the role of an inductor, leading to the oxidation of otherwise unreactive or only moderately reactive substrates.
2. EXPERIMENTAL Measurements were carried out by recording the successive UV-Vis spectra of the reacting solution on a Hewlett-Packard 8452 Diode Array Spectrophotometer combined with a HiTech Scientific stopped-flow type accessory. Reactions were run in buffered solutions, in the presence of pyrophosphate in a 20-fold excess over permanganate to avoid disproportionation of the Mn(II1) formed. The ionic strength was kept constant. Permanganate ion was added in excess over A@), ensuring that the latter should be fUy consumed in all cases. Substrates were added to the As(II1) solution before mixing with permanganate. Owing to the fast rate of
624
the Mn(VI1) - As(II1) reaction, hnetic measurements could not be made as the reactions to be discussed are complete within the mixing time of the stopped-flow instrument available to US. Measurements are based on analyzing the spectra of the product solutions. Residual permanganate and the stable manganese products, Mn(II1) and soluble Mn(IV), were measured by comparison with the known spectra of the individual species. Absorption coefficients of Mn(IV) were calculated from the absorbance of Mn(II1) and Mn(IV) formed in a known concentration ratio in the reaction of Mn(VI1) with As(II1) in the absence of other substrates. 3. RESULTS AND DISCUSSION We have found that the oxidation of phosphorous acid (H3PO3) as well as various organic compounds (alcohols, glycols, hydroxyacids, carboxylic acids, etc.) can be induced in the permanganate - arsenous acid system. This is demonstrated by the experimental finding that permanganate is consumed in excess of the amount used up by As(II4 alone, with no other reactant present. According to kinetic measurement in the absence of As(III), direct oxidation of each of these compounds by Mn(VI1) is negligible during the time intervals involved in our studies. Induced reactions can be characterized by the induction factor IF, which is the ratio of oxidation equivalents consumed by the acceptor (added substrate) and the inductor, As(II1) . It depends on the ratio of reactant concentrations. At sufficiently high acceptor concentrations IF reaches a limiting value, which may provide usehl information on the valence changes occumng in the redox process involved. In our system the limiting value of IF was found to be unity. The typical dependence of IF on the acceptor concentration is illustrated in Figure 1 in the case of ethanol as acceptor.
c
1.0 IF
I
I
1
I
625
In the absence of other reductants arsenic(II1) reduces permanganate to a mixture of Mn(II1) and Mn(1V). With an efficient acceptor present, mainly Mn(II1) is formed (Figure 2) which cannot be reduced hrther in the presence of pyrophosphate. We have observed
’t Figure 2. Permanganate consumption and stable manganese products as a function of acceptor concentration in the Mn(VI1) - As(II1) - Acceptor system (Acceptor: ethanol; conditions the same as in Figure 1) that not more than one half of the oxidizing capacity of permanganate is used up in the oxidation of the acceptor, which confirms that the intermediate formed in the reaction of Mn(VI1) with As (111) is indeed manganese(V): Mn(VI1) + As(II1)
d
Mn(V) + As(V)
(1)
The manganese(V) is hrther reduced to Mn(JI1) either by As(II1) or the acceptor molecule S, but may also undergo disproportionation:
Mn(V)
+ As(II1)
Mn(V) + S 3Mn(V)
A
d
+
Mn(II1)
+
h(V)
Mn(II1) + P
2Mn(IV) + Mn(VI1)
(2) (3) (4)
With increasing concentration of the acceptor (S), reaction (3) gradually becomes predominant. In the limiting case the overall reaction can be described by equations (1) and (3). It is important to emphasize that direct reduction of Mn(1V) by the acceptor and the involvement of Mn(II1) in the induced oxidation can be ruled out. Control experiments show
626
that if the acceptor is added to the solution after complete oxidation of As(III), no changes in the spectra can be detected. The data obtained in the induced oxidation of various acceptors are shown in Table 1. Table 1 Induction factors (IF) in the Mn(VI1) - As(II1) - Acceptor system ~ [pyrophosphate],= 8 . 0 ~ 1 0 M -~ [Mn(VII)],= 4 . 0 ~ 1 0-4 M; [As(III)IO= 4 . 0 ~ 1 0 -M; IF = oxidation equivs consumed by S/ oxidation equivs consumed by As(II1)
IF Acceptor (S)
S[MI
PKa
None
0.0
H3P03 HPO( OEt)( OH) HPO(OEt)2
0.02 0.02 0.02
Ethanol
0.02
1.0; 6.0 0.8
i-Propanol Ethylene glycol Glycolic acid Tartaric acid Formic acid Oxalic acid
0.10 0.02
Formaldehyde Acetaldehyde
0.02 0.02
0.82 (pH=O) 0.73 0.74 (pH=O) 0.0 0.0 (pH=O) 0.0
0.0
0.15
0.73 0.58 0.46 0.79
0.35 0.23 0.30 0.63
0.20
3.60 3.0; 4.3
0.52 0.62
0.32
1.2; 4.2
0.88 0.39
0.76 0.64 0.68 0.76
0.02 0.10
0.02
0.0
pH=6.8
0.16 0.40 0.66 0.93
0.10 0.10
0.02 0.02
pHz3.8
0.40 0.75 0.91
0.10
0.34 1 .O
n-pro pano
pHZ1.04
3.7
0.48
0.36 0.33
0.30
0.34 0.14
In general, the oxidation reactions of the acceptors used are typically not very rapid. In the oxidation of alcohols and aldehydes the rate-limiting step is usually hydride ion or H-atom transfer from a C-H bond to the oxidant, which requires a considerable activation energy. Especially surprising is the high reactivity of phosphorous acid, because its oxidation requires
621
the cleavage of the strong P-H bond, which is a slow process in all known cases (see e.g. [ 11131). We have concluded that the fast oxidation of phosphorous acid by Mn(V) can be interpreted by 2-electron transfer followed by fast deprotonation of the product, rather than by hydride or H-atom transfer. We proposed a mechanism involving the formation of a mixed anhydride of hypomanganous and phosphorous acids, in which the bridging oxygen atom offers a convenient path for inner-sphere electron transfer [ 141, equation ( 5 ) .
OH I
[O3Mn-OW2- + HO-P=O I
H
slow
--+
-H20
OH [03Mn-O-P=Ol2-
+
Mn(II1) + P(V) ( 5 )
H
For anhydride formation to take place, at least one OH-group should be available on P(II1). Deprotonation indeed suppresses the induced oxidation and at higher pH, where only HPO32- is present, it cannot be observed at all. We have also found that diethyl phosphonate and the anion of monoethyl phosphonate have no effect on the oxidation of As(II1). In contrast to this, induced oxidation of the protonated monoester does readily take place [ 131. These results are in accordance with the proposed mechanism: if the acidic protons of phosphorous acid are removed or replaced by ethyl groups, anhydride formation is not possible and oxidation by Mn(V) is inhibited. The reactivity of the organic compounds listed in Table 1 may also be attributed to their ability to form 0-bridged compounds with hypomanganous acid due to the presence of OH groups (alcohols, glycols, hydroxy acids). These species can be regarded as mixed anhydrides or as manganate(V) esters, depending on the nature of the reacting OH groups. Aldehydes may react in their hydrated form when OH groups are available. The oxidation of formic and oxalic acid may involve the hydroxy groups but an outer-sphere reaction is also conceivable. The reaction of simple alcohols with Mn(V) is also influenced by the pH, although in the pH range studied there are no protonation equilibria involving either the alcohols or As(II1). Protonation of hypomanganous acid may be responsible €or this effect , which seems to favour ester formation with alcohols over the competing reactions. In many respects the behaviour of manganese(V) resembles that of chromic acid. Fast formation of Cr(V1) esters in the oxidation of alcohols and aliphatic aldehydes, as well as that of a mixed anhydride in the oxidation of phosphorous acid is well established [ 1,3,9,12]. The overall oxidation rate is determined by the cleavage of the C-H or P-H bond, respectively, in contrast to similar reactions with Mn(V), where the rate-limiting step is the formation of an 0bridged intermediate or electron transfer through the bridge. 4. CONCLUSIONS
The results of this work have shown that short-lived manganese(V) shows a reactivity pattern toward some organic substrates which is distinctly different from that of permanganate
62 8
ion. In that sense it can be regarded as a selwtive oxidant and further studies may lead to evidence that it certainly is a short-lived intermediate in oxidations by permanganate ion. Acknowledgments. This work was supported by the Hungarian Research Fund (Grant No. 4074). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
R, Stewart, in Oxidation in Organic Chemistry, Part A, K.B. Wiberg (ed.), Academic Press, New York, 1965, Chapter 1. F. Freeman, Rev. React. Species Chem. React., 1 (1976) 179. D.G. Lee, in Oxidation of Organic Compounds by Permanganate Ion and Hexavalent Chromium, Open Court, La Salle, IL, 1980. D.G. Lee, in Oxidation in Organic Chemistry, Part D, W.S. Trahanovsky (ed.), Academic Press, New York, 1982, p. 147 L.I. Simindi, in The Chemistry of the Functional Groups, Supplement C, S. Patai, Z. Rappoport (eds), Wiley, Chichester, New York, Brisbane, Toronto, Singapore, 1983, Chapter 13. J.S. Pode, W.A. Waters, J. Chem. Soc. (1956) 717. L.I. Simindi, M. J&y, Z.A. Schelly, J. Am. Chem. Soc., 106 (1984) 6866. L.I. Simindi, M. J&y, C.R. Savage, Z.A. Schelly, J. Am. Chem. Soc., 107 (1985) 4220. L.J. Csinyi, Induced Reactions, in Comprehensive Chemical Kinetics, C.H. Bamford, C.F.H. Tipper (eds), Vol. 7, Elsevier, Amsterdam, London, New York, 1972, Chapter 5. J.H. Merz, G. Stafford, W.A. Waters, J. Chem. Soc. (1951) 638. A. Viste, D.A. Holm, P.L. Wang, G.D. Veith, Inorg. Chem., 10 (1971) 631. G.P. Haight, Jr., M. Rose, J. Preer, J. Am. Chem. Soc., 90 (1968) 4809. 8. Z5honyi-Bud6, L.I. Simindi, Inorg. Chim. Acta, 205 (1993) 207. 8. Zihonyi-Bud6, L.I. Simindi, Inorg. Chim. Acta, 191 (1992) 1.
DISCUSSION CONTRIBUTION
J.-M. BRkGEAULT (Universitk P. et M. Curie, Paris, France): You are considering on your slides (schemes) several redox processes. As the E" values depend on the pH, ligands, etc., are the redox potentials in manganese chemistry directly relevant to your system? L.I. SIMANDI (Central Res. Inst. Chem., Budapest, Hungary): Yes, they certainly are or would be if they were available. The reactions discussed take place in acidic solutions, where both manganese (V) and manganese (VI) are thermodynamically unstable and very reactive. It might be possible to calculate standard redox potentials for these species but at the moment no experimental technique for this purpose seems to be accessible.