Molecular oxygen reduction in the adducts of cobalt chelates

INORG.

NUCL.

CHEM.

LETTERS

Vol.

6,

pp-

191-195,

1970.

Pergamon Press.

Printed

In

Great

Brltoin

MOLECULAR OXYGEN REDUCTION IN THE ADDUCTS OF COBALT CHELATES

G. Costa, A. Pu_xeddu, L. Bardin Stefani (Instituto di Chimica - Universit~ di Trieste,

Trieste,

Italy)

(Receded 8 December 1969)

Though Co(ll)-chelates

have been shown to yield mainly binuclear

]:2 adducts by reaction with oxygen in the solid state or in suspension

(i) we have foumd that the formation

of i:i adducts can

be followed by measuring the oxygen uptake in suitable homogeneous solutions and that the oxygenated electrochemical

tech~iques.

fluence of ligands of dissolved

We have tried to elucidate

oxygen under the same experimental

dianion)

the in-

on the reduction process as compared with that

When Co(ll)-salen iminato)

complex can be studied by

conditions.

(salen : N,N'-ethylenebis

is dissolved

(salicylidene-

under nitrogen in anhydrous pyridine

containing NEt4CIO @ 0.i F as supporting electrolyte,

and 02

is bubbled through the solution at O°C, the polargraphic wave due to Co(ll) + e * Co(1) reduction decreases,

(I)

step at -1.38 V gradually

being replaced by a wave (II) at about -0.6 V (see Fig.l).

The ratio of oxygen absorbed per cobalt atom in 0.8 to 2.0 x i0 -3 M solution at O°C either in the presence

or absence of supporting

electrolyte was found to be very close to 02/Co = i (0.93 to 1.08). The value reported by Calderazzo suspensions

of Co(II)-salen

in anhydrous

revealed in the polarographic the reversible reduction

(i), 02/Co = 0.5, was co~firmed pyridine.

Excess 02 is

solutions by the wave (III) due to

step 02 + e * 02 - at about -0.8 ~, as

191

for

ADDUCTS OF COBALT CHELATES

It2

in the absence of the cobalt complex (2).

Vol. 6, Ho. 2

By bubbling dry nitrogen

through the solutions, the same changes are observed in the polarograms in the opposite order, until eventually the polarogram of Co(II)-salen starting solution is again obtained, showing the reversibility of the complex-formation.

Both waves (I) and (II) have the same diffusion

current constant id/m2/3tl/6c;

the sum of diffusion current inten-

sities id(I) + id(II ) is constant (Fig.l).

The slope of the plot E vs.

log i/(id-i) is the same for both waves (I) and (II) (56 mV), consistent with a reversible one-electron reduction as unequivocally found from direct coulometric determination.

Wave (II) is thus

attributed to the process Co(salen).02 + e ~ [Co(salen).02]-. grams of solutions of Co(II)-salen.02

Polaro-

in pyridine subjected to controll~

potential reduction at about -0.6 V show an anodic wave with the same half potential E~ as the cathodic wave (II) showing that the anion [Co(salen).02]-

can be detected in non protogenic solvents.

TABLE

E½(vs.S.C.E.-0°C) pyridine-NEt4C104

02 +

e , 02-

E~(vs.Ag/AgCl, KC1 in DMF-0°C)

0.2F

DMF-NaCIO 4 0.1F

-0.836

Co(II) BAE.0 2

-

-0.810 -O.54O

Co(II) salen.O 2

-0.660

-0.420

Co(II)3CH30-salen.O 2

-0.660

-0.324

Co(II) saloph.02(*)

-0.656

-0.300

*) saloph = N,N'-o-phenylenbis

(salicylidene-imenato)

dianion.

Vol. 6, Ho. 2

ADDUCTS OF COBALT CHELATES

193

400 mV

wave

I

-0.8

/ FIG. i.

Polarograms of increasingly oxygenated (from top to bottom) Co(ll) salem (1.50 x 10 -3 M/I) in anhydrous pyridine - NEt4CIO 4 0.2 F.

Analogous results differing only in the actual values of E½ and stability of the anion, were obtained with the 3-methoxysubstituted salen complex (Co(ll)-3CH30-salen), the only salicylaldehyde Schiff base cobalt complex which has recently been shown to form an isolable solid i:i adduct (i).

We observed

194

ADDUCT$ OF COBALT CHELATES

Vol. 6, No. 2

the polarographic wave due to the reduction of l:l adduct in several cobalt chelates with tetradentate ligands which we are studying as model molecules of Vitamin B12.

While this work was in

progress the reversible oxygenation of Vitamin B12 r itself yielding l:l adduct was reported (3). (BAE

= N,N'-ethylenebis

Unstable l:l 02-adducts of Co(II)-BAE

(acetylacetone-iminato) dianion) have been

also i~dependently obtained (4) in the presence of N,N' -dimethylformamide (DMF), pyridine or 4-substituted pyridines.

The

displacement of E~ of the wave (II) was determined for various cobalt(II)-chelates whenever the two waves (II) and (III) are sufficiently separated.

The same determinations were made also

in DMF. The results show both the influence of the solvent and of the 'chelating ligand on the reduction potential of the adduct; suggesting that the solvent is coordinated in the axial position "trans" to the 02 group, and that the transmission of electronic effects through the cobalt atom to the Co,O 2 bond is operative both from the trans and cis (equatorial) ligands forming the proposed octahedral environment. The investigation (5) of the l:l oxygen adducts by electron paramagnetic resonance spectroscopy shows that they are all well described as cobalt (IIi) superoxides with 02 - acting as a ligand. The product of the electrochemical reduction of the l:l adduct can be described either as Co(III)-022-, Co(II)-02- , or Co(I)-O 2.

Of

these the last would appear the least likely description, but the suitability of the first two formulations as a description of the reduced species needs further consideration.

Vol, 6, No. 2

ADDUCTS OF COBALT CHELATES

'195

i.

C. FLORIANI and F. CALDERAZZO, J.Chem.Soc., (A), 946, (1969).

2.

D.S. MARICLE and W.G. HODGSON, AnalytoChem., 37, 1562, (1965)

3.

J.H. BAYSTON, N. KELSO KING, F.D. LOONEY and M.E. WINFIELD, J.Amer. Chem.Soc., 91, 2775, (1969).

4.

A.L. CRUMBLISS and F. BASOLO, Science, 164, 1168, (1969).

5.

S.A. COCKLE, H.A.O. HILL and R.J.P. WILLIAMS, Inorg.Nucl.Chem Letters.