Radiat..Phys. Chem. Vol. 23, No. 6, pp. 703-709, 1984 0146-5724/84 $3.00+.00 Printed in ~he U.S.A. Pergamon Press Ltd.

REACTIVITY OF SUPEROXIDE ANION TOWARDS C o II AND CoIIIedta COMPLEXES

A. LECHEHEB, M. TAKAKUBO, J. FAURE and J. BELLONI Laboratoire de Physico-Chimie des Rayonnements, Associ6 au CNRS, Universit6 de Paris-Sud,

91405 Orsay, France

(Received 29 March 1983; in revised form 10 August 1983)

Abstract--The reactions of 0 2 - with CoHedta = and C o l n e d t a - have been investigated at pH 9 using the pulse-radiolysis technique. From spectrophotometric observations in the UV, it is concluded that the reaction with Comedta - yields a long-lived transient (ko2-+co,l.at, - = 5.6 x 107dm3mo1-1 s -1) which slowly decays by a second-order law to yield Conledta - . The transient is identified with a superoxo complex (2m~x = 260 nm). If this complex is mononuclear, the extinction coefficient is ~max = 104 dm 3 mol - 1 cm - l, and the rate c o n s t a n t k2co.n~lta- 02 - = 6.8 x t 02 dm 3 mol - i s - '. With CoUedta ", the rate constant of the reaction with O 2 - , followed at 330 nm is ko~- + co..at. = = 2.5 x 105 dm 3 tool - 1 s - ~. The intermediate produced absorbs strongly also around 300 nm (2,~ = 290 nm), and is identified as (CoH edta =, O 2-). By a bimolecular reaction (k2co.~t~=,02- = 4 x 104 dm3mol - l s - t ) it yields a second intermediate which in turn decays by a first-order reaction (3 x 10 -3 s-l); the initial solution is finally oxidized to Comedta - . The second transient spectrum is assigned to a peroxo complex, mono or binuclear, able to undergo an internal redox reaction. Unlike many other ions, bi- and trivalent cobalt (edta) ions do not accelerate O2- dismutation, but delay the reaction by producing long-lived complexes involving O 2- as ligand.

I N T R O D U C T I O N

DUE TO its h igh electron affinity, molecular oxygen plays a de te rmining role whenever electron t ransfer processes are involved. A m o n g redox react ions fol- lowing p h o t o exci ta t ion of sensitizers in the presence of electron acceptors or donors , oxygen may also be reduced efficiently to the superoxide an ion O2- . The quest ion o f the possible react ions between O 2- and the reversible redox couple is raised. A l though some da ta concern ing react ions with ionic complexes are already avai lable in the l i terature, °'21 the in terac t ion of O2- with Co"ed ta = or co lned t a - which were proposed as a relay in solar energy s torage systems (31 have not been invest igated yet.

The s t andard redox potent ia l of the couple C o " ' e d t a - / C o " e d t a = is 0 .37V (3a) in water so tha t oxidat ion 0 (Eo2-/n2o2 = 0.89 V) (2) as well as reduct ion by 0 2 - 0 ( E o 2 / o 2 _ = - 0 . 1 6 V ) would be the rmo- dynamical ly al lowed at p H 7.

The a im of this work is to s tudy by pulse radiolysis the rate cons tan t s and the p roduc ts o f the react ions init iated by O2- + Conedta = or 0 2 - + co lned t a - .

E X P E R I M E N T A L Materials

Sodium formate, disodium tetraborate, 10 H20, nitroblue tetrazolium (NBT 2+) chloride, disodium ethylene di- aminotetraacetate (edta) and cobaltous sulfate were pur- chased from Merck. Solutions were prepared with doubly-

distilled water. Preparation ofCoIledta = and Comedta- has been described previously, o)

Apparatus and techniques Details of the pulse-radiolysis facilities (Febetron 706

accelerator delivering 3 ns pulses of 600 keV electrons) have been described, t4,s) The dose per pulse is 1.5 to 4 x 10 ~s eVml '. The quartz irradiation cell (61 is designed with a window 0.2 mm thick for entry of the electron beam. In the area of irradiation, the cell is 2 mm deep and from pulse to pulse a new fraction of the solution is allowed to flow from a reservoir to the irradiated region. Depending on the time range required, the time-dependent signal arising from the optical detection line is transferred either to a storage oscilloscope or to a transient digitizer and processed on-line by a computing system, tT)

A few irradiation experiments have also been performed with a ),6°Co source (dose rate: 10tgeVml -l h-l).

Method The superoxide anion radical is obtained classicaUy (s,91 as

the product of the reactions between the radiolytic species of water and the solutes, formate (10-1moldm -3) and oxygen (1.4 x l0 -3 tool dm -3 at saturation). Edta (2 x 10-4moldm -3) is also added as complexing agent of impurities.

(0) H20 ~ H, OH, e~, H30 +, H202, H2

(l) HCOO- + H--.H 2 + CO 2-

(2) HCOO- + O H ~ H 2 0 + CO 2-

(3) 02 + e~ ~ O 2 -

703

704 A. LECHEHEB et al.

(4) 02 + CO 2- ~02 - + CO 2.

Buffeting at pH 9 (Na2B407 10-2mol dm -3) ensures that the acid-base equilibrium

(5) HO2, ~ 02- + H + pK = 4.69 O°)

is displaced to the tight. The absorption of O2- (2m, = 245 nm, ~r~x = 2300 dm 3 mol-~ cm-~) was observed in the range 230-300 nm. Under these conditions 02- is known to be long-lived. "°) Therefore, before any addition of scavenger, we checked first that the 02- yield obtained within 0.1 kts was equal to 6.2G units and second that the decay of O 2- lasted for seconds; the decay obeys second- order kinetics with a rate constant k = 5 . 8 x l 0 3 d m 3 m o l ~s -~ to be compared with the value "°) at pH 9 of 5 x 103 dm 3 mol-t s-L

H20 (6) O2- "{- 0 2 - ) 02 + H202 + 2OH-.

pH 9

Depending on the dose per pulse (controlled by the required fraction of the total power of the beam), the 0 2- concentration lies initially between 1.5 and 4 x 10-4moldm -3.

R E S U L T S A N D D I S C U S S I O N

Unlike the stopped flow pulse radiolysis tech- nique, °t) our set-up does not allow differentiation between the production of O2- and its reaction steps. On the other hand, the successive transient species can be studied without any upper limit in time (see below) provided the conditions of detection are fulfilled.

In a preliminary run the rate constant o f the reaction of O2- with the nitroblue tetrazolium cation (NBT2+), ° ° was measured. Due to the presence of both molecular oxygen and N B T 2+ during the prod- uction of e£q (a precursor of O2-), the concentrat ion of the latter was chosen so as to keep below a few percent direct reduction to the radical N B T + (reac- tion 7)

(7) e~ + N B T 2+ , NBT +.

We have measured the rate constant k7 in the range 3 × 10 -5 to 5 × 1 0 - 4 m o l d m -3 NBT 2+ in deoxy- genated water. The decay of e~q was followed at 800 nm, i.e. in a region where NBT + does not absorb, and was correlated with the increase of NBT + at 405 nm. ° ° At longer times (50 ms), the dismutation of NBT + is second order and monoformazan M F + is produced (2m~ = 530 rim). The rate constants of the pseudo-first order decays depend linearly on [NBT 2+] and the bimolecular rate constant k 7 is 7 .3× 10~°dm3mol- ts -~ in water (ionic strength

= 0) and 4 × 10 ~° dm 3 tool s-~ in the presence of the buffer and formate salt (~ = 0.13). These values seem rather high in comparison with diffusion-controlled

reactions between small ions but the encounter radius with NBT 2+ is certainly much larger.

The rate of reduction of NBT 2+ to NBT + by 0 2 - was measured in O2-sat'urated solutions ([NBT 2+] = 3 x 10 -5 mol dm -3) either at 405 nm or at the isobestie point, 2 = 460 nm (2ENaT+ = ~MV+). <N) Due to the imposed low concentrat ion o f NBT 2+ and the self dismutation of O2-, it is not surprising that the value of ko2- +Nat2+ ~- 1.5 X 105 dm 3 mo1-1 s - j so obtained is much less accurate than the value, (5.8 + 0.12)× l & d m 3 mol -~ s - t , given by Bielski et

al. ° ° in the same pH region. However it indicates the lowest limit of rate constants o f reactions o f 0 2 - which we were able to measure in competi t ion with O2- dismutation.

Comedta - solutions

In the presence of the solute Comedta - , direct reduction of the ion by e~, which is known to be very fast (ke~+co,,edt~- = 2.9 x 10 I° dm 3 tool -I s-I) 02) must again be minimized. For this reason, the solution was saturated with 02 and the initial concentrat ion of C o " ' e d t a - was : ~ 6 x 1 0 - S m o l d m -3. As a con- sequence, the visible bands of the Comed ta - spec- trum are too weak and the only region available for detection is the UV range where the 02 - and Comedta - bands are also located (Fig. 1).

The change of optical density 0.5 ms after the pulse is shown in Fig. 2 together with the transient spec-

7 - 6 E

f\ I I i I I ,, I I

Col~ed to) -

t ÷ ,, +

÷ +

L I ...07) ~ ~ ~ ~ ~

, : L . . - ' . . . . . . . : . : : . . . . . . . . . . . . 2 0 0 300 tOO 5 0 0 6 0 0

-6 E

- o

r

FiG. 1. Optical absorption spectra at pH 9 of: . . . . ,02- ; . . . . , C o n e d t a = ; - - - - - - , Conledta-; + + + ,Comedta - (right hand scale); - - , transient species produced at 0.5ms after the pulse in 5 x 10-Smoldm -3 Comedta -

solution.

E r,i 0.2 0

0.1

0 200

Reactivity of superoxide anion towards Co It and Comedta complexes

I I I I I )

/

i t /

s , t " ~%% %

"5 o - ~

300 k ( n m ) 4 0 0

FIG. 2. Absorbance at 0.5 ms after the pulse: . . . . , O 2- spectrum (in absence of comedta-); , transient spec-

trum in presence of 5 x 10 -5 tool dm -3 Comedta -.

'° I 32c

I I I

t(s)

FIG. 3. Second-order plot of the absorbance decay at 330nm (5 x 10-Smoldm -3 Come&a-).

trum of 02- without Comedta. The new spectrum extends further to 400 nm, and is less intense around 250 nm. The absorbance is the same at 270 nm with or without Collledta-. However, at any wavelength the decay is much slower than that of 02-. At 330 nm, where the 02- absorbance does not interfere, the decay obeys a second-order law (Fig. 3). At times longer than 400 s the initial spectrum of the solution is recovered. A similar result is obtained in the range 200-700 nm with a y-irradiated solution Comedta - (5x 10-Smoldm-3), using doses from 0.39 to 2.1 x 10t9 eV ml-L The new band in Fig. 2 cannot be assigned to 02- , nor to the reduced form Colledta = which could not account for the enhancement of the absorbance in the range 300--400 rim. The known relative inertness of the complex ion Co"ledta -03) raises the question of the nature of the transient

705

species and, in any case, of the mechanism of its formation (see below). The final reversibility to Collledta- together with the obvious influence of the ions on the transient spectrum suggest that the ob- served transient does indeed result from the 02-+CoIl ledta - reaction and may again yield Co"ledta- by bimolecular reaction. The most likely product is a superoxo complex of conledta- which would explain a slight shift of the absorption band relative to Colliedta - without irreversible reduction or oxidation of the ion. As a ligand, 02- undergoes the same dismutation to 02 + H202 as free ions and finally Collledta - is liberated, as in negative catalysis. Various examples of similar complexation have been reported, (1.2) involving metal ions as different as Fe II, Mn ll, Co tl, Ba H, Ca t~, iron porphyrins and, with Co m as central ion, vitamin B12~ in dimethylformamide. (14) Some of these complexes were also obtained by reaction of dioxygen with the ion of immediate lower valency (see vitamin Bi2r + O2(14)),

Under the present conditions, 02- is initially more concentrated than Collledta-. An increase of [02-] does not cause an increased absorbance of the tran- sient so that we may assume that all cobalt ions were involved in oxocomplexes of mono or binuclear structure, the latter being frequent (1,2) in the case of Co m. The difference spectrum in Fig. 4 thus repre- sents, for a mononuclear complex, the wavelength dependence of the expression

/I O.D. =/[complex] ( • c o m p l - £O2- - - EColned ta - )"

With 1 = I em and [complex] = 5.6 x 10-s moldm -3 we can derive the complex spectrum given in Fig. l (2m~x = 260 rim, and Emax = (1 + 0.1) x 104 dm 3 tool -I cm -~, E330 = (1 + 0.1) x 103 dm 3 mo1-1 cm-I).

The rate constant, measured at 330 nm (Fig. 3), of

0.2

~ 0 <1

0.2

i i t i

X(nm)

FIG. 4. Difference spectrum between absorbanees with and without conledta - (Fig. 2). It is used (see text) to derive the

spectrum of the intermediate.

706 A. LECHEFEB et al.

the decay of the complex

(8) (Comedta-, 02-) + (Comedta-, 0 2 - ) H20

2 Comedta- + 02- + H202 + 2OH-

is then ks = 6.8 × 102dm3 mol -I s -I. At 2 < 300 nm, the absorbance in the presence of

Comedta- (Fig. 2) is the sum of two components, the first one due to the complex replacing Comedta- and 02- (the above expression), with a slow decay similar to that at 330 nm (reaction 8) and the second one due to excess 02-. The decay of the absorbance of excess O2- has been calculated at 270 nm by subtracting the time-dependent contribution of the complex from the total absorbance decay. It is found that decay of the free 02- is neither accelerated nor slowed down by the longer-lived complex, as if the species react by independent processes.

The formation of the above transient was studied over 300#s. Actually, from 0.1/~s (when 02- is produced) to 300 #s, the development of a spectrum different from that of 02- with an enhancement between 270 and 400 nm is observed. The growth of absorbance at 330 nm seems to occur in a single step and obeys pseudo first-order kinetics in agreement with an almost constant 02- concentration in this time range despite the reaction with Comedta (the dismutation (6) is slower)

(9) 02- + Comedta- --+ (Comedta-, 02-).

The value of the rate constant is then k 9 = (5.6 _ 1) x 107 dm3 mol -l s -I (an average figure from some different runs with initial [02-] ~-2 to 3 x 10-4moldm-3).

The simple formalism of reactions (9) and (8) corresponds to the balance and to the orders of the observed kinetics. Concerning the structure and the mechanism of formation of the postulated super- oxocomplex, a direct substitution in the inner sphere of a carboxylate ligand by 02- would certainly require a longer time than observed. Indeed, the initial CoIHedta - complex may be equated °3) as a quinquedentate chelate with a free carboxylate group and the sixth position occupied by H20 (or OH- at high pH as under our conditions). Substitution of this coordinate by 02 would then be easier and would account for the rate constant measured. Also, the formation could occur through a transient addition of O2- to form a heptadentate complex as postulated in the course of the hydrolysis of Comedta by OH-."5)

It is worth noting that the overall mechanism of reactions (9) and (8) leads to the same products of

02 dismutation as in the absence of Comedta-. The ion is only transitorily implicated, as in suggested mechanisms of the catalyzed dismutation of 02- by natural or synthetic dismutases (1,2) and different metal complexes. However, the marked difference in the case of Comedta- lies in that the complex is longer lived than most analogous complexes and even than 02 - itself. The excess 02 ions decay as slowly as in absence of Comedta - , ruling out a noticeable catalyzed dismutation pathway between (Comedta , 02-) and 02 .(2) On the contrary, the behaviour of the superoxo complex means that the 02- lifetime as ligand is extended. Very few examples of such long-lived complexes are reported in the literature: the bimolecular rate constant of the decay of [Coil(1.3.8.10 tetraene N4) 2+, 02- ] with superoxo or peroxo structure (Co m, O2 =) is k ~ 2.1 x 103 dm3mol -~ s 1.06) The decay of (Mn 2+, O 2 ) is -~ l0 times slower than that ofO2-. (t7) Complexation of O2- with the alkaline earth cations Ca 2+ and Ba 2+ also seems to yield compounds more stable than 02-. (is) The most stable com- pounds are binuclear superoxo complexes of Com. °9) The reactions of 02- with the /~- amido p-superoxo bis(bisethylenediamine cobalt m)

([(en)2Co./'O-O"".Co(en)2]4 +), and with the ~ N H 2 /

#-superoxo bis(pentacyano) cobalt m ([(CN)sCo- O-O-Co(CN)5] 5 have been studied. (19) It is inter- esting to note that the first one undergoes a reduction in the presence of O2- (5.8 x 107 dm 3 mol -I s -I) while the reaction with the second, which is negatively charged, is too slow relatively to other 02- processes to be detected, as indeed observed for (Comedta -, 02-).

Co"edta = solut ions Direct reduction of Colledta = by e~q

(kea~ +Colledta = 5 X 108 dm 3 mol - t s-1,02) can be easily neglected in oxygen-saturated solutions, provided [CoIledta =] ,~ 10-3moldm -3. Within a few ms an absorption band, more intense than that of O2-, develops between 230 and 400 nm together with a weaker absorbance near 540 nm (Fig. 5 shows the spectrum at [CoUedta =] = 5 x 10 -5 mol dm-3). Ki- netics of the growth of absorption at 300-330 nm are pseudo-first order. In solutions contain- ing 5 x 10 -5 mol dm -3 of CoHedta = and 4 x 10 -4moldm -3 of 02 it was found that tu2 = 6.5 × 10-3s leading to a bimolecular rate con- stant ko2 +Co.odta- of 2.5 X 105dm3mo1-1 s -I.

The absorbance at the end of this reaction in- creases with [Co"edta =] but then only slightly above 2 × 10 4mol dm -3. The shape of the band (Fig. 5) rules out the assignment of Comedta - as the product

Reactivity of superoxide anion towards C o Il and Comedta complexes 707

I i I D ' . .

." "'. r d" q.

~. ".

•

......... 0 200 300 400 500 600

X(nrn)

F[o. 5. Absorbance at 50 ms after the pulse: , 02- spectrum (in the absence of Co"edta=); . . . . . transient spectrum in the presence of 5 x 10-Smol dm -3 Co"edta = (O, experimental values); - - , spectrum of the inter- mediate (right hand scale), obtained by substracting the free

0 2 - band from the transient spectrum.

(X) of the reaction although the oxidation of Co" into Comedta - would be thermodynamically al- lowed. Moreover, if produced, Co"tedta - ions should readily react, as discussed above, with excess O 2- to yield the complex, Comedta - , 02-. The overall quan- titative oxidation of Co"edta = into Co"ledta- by 02- (see below) seems also to exclude any reductive role of 02- leading to Co I. Besides, the reverse reaction between Co x and molecular oxygen present in the system is more probable, t~6)

Whether or not the intermediate X could be the above superoxo complex may be concluded from the shape of the band and from the decay kinetics. The shape is not markedly different, except for a slight shift of the maximum to 290 nm (instead of 260 nm) although the extinction coefficient near 330nm is much higher for X(E330(X) = 9 x 103 dm 3 mo1-1 cm -]) than for the Co m superoxo complex. The kinetics of the decay of X are quite different. Figure 6, insert,

.~ 0

g

E 5C

O

& 0100

I I I I I

i

I , I ~ I i I , I 0 100 200 300 400

Hs)

FIG. 6. Solutions o f Co"edta = (2.5 x 10 -4moldm-3) . Os- cillogram at 330nm (100sdiv-~). Insert: oscillogram at

200 ms div- i (2 = 330 nm).

shows that the absorbance decay exhibits two com- ponents, one within 2 s which concerns only 10% of O.D.50m, and is second order, and another one which is much slower, lasting for 500 s, and which is first order. If the first component resulted from a reaction of X with excess O2-, still existing in this time range, the rate and the fraction of O.D. decrease would be also dependent on the initial 02- concentration, which is not shown by experiments. Likewise the 02- decay does not seem to be affected by the presence of X, although the contribution in excess 02- to the absorbance in the UV is less important and thus the uncertainty is higher than in the Co m experiments. Recently, (2°) analogous observations--a reaction be- tween Coltedta = and 02- without enhancing the decay of excess 02- - -have been reported. Therefore, we account for the decay shown in Fig. 6 by two successive steps. Within 2 s X yields another com- pound, Y, which is neither Co"edta = nor Comedta -. The absorbance of Y is 10% lower than that of X, so that the extinction coefficient per cobalt atom is 8 x 103 dm 3 mol - t c m -~. The intermediate X yields Y by a second order reaction with k 2 x ~ r = 4 x 105 dm 3 mol -t s -t (if E330(X ) - - •330(Y) = 103dm 3mol- lcm-t) . Finally Y decays by a first-order law (tt/2 - 225 s, k = 3 x 10 -3 s -l) and the stable product is Cot"edta - which corresponds stoi- chiometrically to the initial concentration of Co"edta=. 7 radiolysis of similar solutions also yields at ~ 2 min after irradiation a new spectrum which differs from that of Comedta - by a more intense absorbance near 320 nm and a less intense one near 540nm but the spectrum slowly becomes that of Comedta - with specific bands at 380 and 540 nm (Fig. 1).

Compound X, produced through the reaction of 02- with Co"edta = could well be identified with the complex (Co"edta =, O2-) homologue of the corre- sponding superoxocomplex (Fe"edta =, O2-) t2~) re- ported by Ilan and Czapski, with an extinction coefficient of a similar magnitude. The rate constant for the formation of the latter was found to be 2 x 10 6 dm 3 mol -t s -t compared to 2.5 x 105 dm 3 mol -t s -~ for that of Coltedta =. Again, the reaction is not believed to proceed by an inner sphere process. The second intermediate Y yields the ox- idized ion Comedta - by a very slow first order process lasting for minutes. In this time range, no free 02- ions are any longer present. The first-order reaction with liberation of Col"edta - suggests an internal electron transfer-process from the central cobalt ion to a ligand of Y. In addition, the formation of Y through a bimolecular reaction of the Co" superoxo complex may explain that Y contains as a ligand the required oxidizing agent. These arguments

708 A. LECHEHEB et al.

support the idea that Y is a peroxocomplex. The mechanism of its formation and decay would be

(10) 2(Colledta =, 02-) ---* (Conedta =, 02 =)

+ Coaedta = + O2

slow~H ÷

Comedta- + OH" + H20.

The OH radical would readily undergo reaction (2) with excess formate and the corresponding 02- pro- duced would be again complexed by Colledta =. This mechanism accounts for the experimental obser- vations, the number of steps and the orders of the reactions, although spectral properties alone do not allow unequivocal assignments.

We must rule out a mechanism of oxidation in- volving free 02 = (or H202). Indeed, we checked that H202 is present as a radiolytic product in a solution l0 -4 mol dm -3 Conedta = irradiated with a sufficiently low dose of ~ rays, so that not all of the ions are directly oxidized. Further oxidation of the remaining ions occurs very slowly and is accelerated by increasing the temperature. However, the mea- surement of the growth of Comedta - at 540 nm in an equimolar solution (10-4 mol dm - 3) of CoHedta = and H202 shows that it is first-order, with tt/2 = 9 x l03 s (k = 7 x l0 -s s-I), i.e. much slower than the direct radiolytic oxidation. Furthermore, shortly after mix- ing Co'edta = and H202, the solution also contains a long-lived compound 22 with a spectrum more intense than that of Conedta = but again the extinction coefficient does not correspond to that of the inter- mediate observed after pulse radiolysis.

The above interpretations imply that the peroxo as well as the superoxo complex absorbs in the UV up to 380 nm. The extinction coefficients of the proposed oxo-complexes are two orders of magnitude higher than that of bivalent cobalt ion Conedta =. It might be that these complexes already exhibit Co m charac- ter if formation of a ligand with O2- induces a partial transfer of an electron from the central ion to the ligand.

It has been shown t23) that the homologue (Fe"edta =, 02-) has mainly the structure of Femedta -, O2=), the peroxo ferricomplex, which di- rectly relaxes to Felnedta - and H20 2 (k = 1 s at pH 10~24)), a mechanism more simple than the two steps oxidation of Cotledta = by 02-. Another difference between mechanisms involving Co and Fe edta complexes is that Femedta - is reduced by O2 12~) (10 s dm 3 mol ~ s- t at pH 9), so initiating the catalytic dismutation of O2- t24) alternatively acting as

reducing and oxidizing species towards the couple Fem-Fe'edta =. Such conditions are not fulfilled with the Co edta couple, but this "ping-pong" mechanism has been recently postulated t2°) for the observed cata- lytic dismutation of O2- by (Co-Zn) and (Co-Co) proteins, even if the substitution by Co II of Cu and Zn ions in the native enzyme results in a decrease of its efficiency.

In conclusion, the marked feature of O2- reactions with the redox couple Colledta - or Comedta - is to yield new complexes within times earlier than that of the 02- dismutation and thus to indirectly stabilize O2-, since as ligand it lasts for a much longer time. The ability of certain metal complexes to catalyse the 02 dismutation could result <2> from convenient val- ues of reduction and oxidation potentials of the lower or the higher valency of the metal ion (or of the superoxo complex ion) thus allowing a faster dis- mutation of 02- as ligand. It is obvious here that ligand formation with Comedta - inhibits any ox- idation of 02 (contrarily to other analogous ions ~2) such as Femedta -, Mnmedta - , Fem(CN)63-), and that with CoHedta = the reduction is markedly slowed down. Such information on new long-lived ox- ocomplexes must provide a better understanding of the behaviour of O2- as a ligand and of the structure of these complexes. It may contribute to select redox couples used for solar energy conversion.

Acknowledgements~ne of us (M. Takakubo) is indebted to Electricite de France for a grant.

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