Oxygenation and oxidation of CoII-chelates

J. l a e t ~ NucL Chem,. 1960. Vol. 13, pp. 95 to 100. P e r p m ~ P r u m Ltd. Primed ia Nocthem Ireland

OXYGENATION

AND

OXIDATION

OF CoXI-CHELATES*

V . C A O L ~ r l , P. SH, VESTgONI a n d C . FUgLANX Department of General and Inorganic C~emistry of the University of Rome, and Departmentof Physical Chemistry of the University of Trieste (Received 29 June 1959)

AIBtraet---CoIXdihistidine and Condiglycylglycine are known to bind oxygen reversibly (oxygen carriers) and to yield compounds containing the grouping X=Co--Os--CoX=. It is now shown that the product of the irreversible oxidation does not contain peroxo groups, but corresponds to the general formulae [ComX=(OH)=]-, [ComXt(OH)].

SOMEchelates of Con known as oxygen carriers show the property of binding oxygen reversibly, m The mechanism of the reaction has been studied by polarographic,¢=~ spectrophotometric and other methods, in the case of cobaltodihistidine (Co~2) and cobalt odiglycylglycine (Co(GG)z).

/=0

B

C

D

f

FIG. 1. A--before oxidation (~t = --0.196 V) B, C--successive oxidation stages (el, = --0-198 V and --0.200 V) D, E--suceessive reduction stages (~t = --0.196 V and --0.195 V).

In the literature it. is generally agreed that an intermediate product is formed when these complexes react with oxygen; this in turn changes into a stable compound of the type RsCo--O--O--Co R s which has the above-mentioned property. The complexes of CoII with histidine and with glycylglycine are polarographically oxidizable in a reversible manner to Com~2+ and Com(GG)~+, at potentials of approximately --0.200 V and --0.500 V respectively. Fig. 1 shows the successive polarograms obtained from a solution of Co~,s buffered at pH 7.3 during coulometric oxidation (at --0.08 V) and reduction (at --0.45 V); similar results are obtained by the Kalouseck reversel. The polarographic behaviour changes markedly when oxygen is bound by Co4,2 to form the intermediate compound, in which oxygen is * Presented at the International Conference on Co-ordination Chemistry; London, 6--11 April 1959. (I) J. Z. HEARON,D. BURg and R. $1~DE, J'. Nat. Cancer Inst. 9, 337 (1949); M. CALVINet al., J. Amer. Chem. Sac. 68, 2254, 2257, 2263, 2267 and 2273 (1946); see also A. E. MAaTELL and M. C^LW~. Chemistry of the Metal Chelate CampoRnd8 p. 337. Prentice Hall, New York (1952). ~t~ p. $ILVESTRONIand G. ILLUMINA~, RIc. S¢I. 28, 1211 (1958); P. SILWSTRONJ, Ibid. 19,301 (1959l. 95

96

V. CAGLIOTI,P. SJLvesrgomand C. FtrgLAm

reversibly bound. Fig. 2 shows a series of polarograms for solutions of Co~s in the presence of increasing quantities of oxygen. Curve A corresponds to the initial solution of Co~2 in the absence of oxygen; curves B, C and D refer to successive stages of oxygenation, and curve E was obtained after bubbling nitrogen through the solution. In these polarograms, three waves can be observed. The first concerns the reversible process C o ~ 2 ~ Com~2 + ~- e-; it may be an oxidation wave if only C o ~ l is present in the solution besides the oxygenated complex, or a reduction wave if only

FIG. 2.

A--solution of Col~bI in the absence of air B, C, D---sameas A, after passing air for 3, 6 and 9 rain E--same as D, after passing Na for 1 hr. Com~2 + produced by oxidation of C o ~ is present; if both are present, it is a redox wave. In general, soon after partial oxygenation of the solution, the first wave is almost totally an oxidation wave (curve B); on standing it becomes a redox wave and at later stages it becomes merely a reduction wave. TABLE

Curve

A B

C D E

idI (lst wave) ~,A) 5.10 4.40 3.42 2.25 5.20

1 :'~s (2nd wave) (/,A)

0.68 1"80

2.95

~A) 5.10 5-08 5"22 5.20 5.20

The second wave, which increases with increasing absorption of oxygen, is produced by the addition compound. Table 1 shows that the sum of the diffusion currents of the first and second waves is constant, despite the increase in oxygenation. The third wave is rather indefinite; its potential ( ~ --1.0 V) and shape, and the increase in magnitude on adding hydrogen peroxide to the solution, suggest that it

O x y g e n a t i o n a n d oxidation o f Con-chelates

97

may be attributed to the reduction of HIO2. The polarographic behaviour of freshly oxygenated solutions is summarized in Table 2, This behaviour may readily be correlated with the property of the complex of "carrying" oxygen. This property depends on the presence of Cow'm,and the ability of a solution of Co~s to bind oxygen is therefore measured by the current of the anodic portion of the first wave. The oxygen reversibly bound in such a solution is measured, at any given moment, by the diffusion current of the second wave, corresponding to the reduction of the oxygenated complex. TABLE 2.--I~DUCI'ION ~

OF C O B X L T O D ~ I N E

COI~WLI~¢I~

Partially-oxygenatedsolutions Ist wave (--0.20 V) CoU~s ~ (Cored,0+ + e- (redox wave) 2nd wave (-0.45 V) (Co~l)2"Os + 2H+ + 2e- --* HIOi + 2Con~2 3rd wave (,~ -- 1.0 V) HaOI + 2H+ + 2e- ---, 2HaO 4th wave (-~ -- 1"4V) CoII~| "~ 2~- ~ CO° "-[-2~Totally-oxygenatedsolutions 1st wave (--0"20 V) (ColXI@l)+ + e- "-* ColI~s (reduction wave) 2rid, 3rd and 4th wave: same as above Irreversible final product (red complex) 1st wave (-0.20 V) (Com~J+ + e- -, CoXX@2 2nd wave (-~ - 1.4 V) CoXX~+ 2e- ---,Co° + 2~The loss of the "carrying" power of the Co~,~ solution is measured by the height of the cathodic portion of the first wave, which gives the amount of Co(II) oxidized to Co(III). The final stage is irreversible; both the C o ~ and Co(GG) a solutions contain stable red ComX~+ complexes, which are not of the type X~Co---O--O--CoX~ as may be seen from the literature. These solutions give two reduction waves; the first corresponds to the reduction of tervalent to bivalent cobalt and the second to the reduction of bivalent to zero-valent cobalt. Further evidence of the nature of the red complex is afforded by analysis of the final product. Using the method of GILBERTet al., (3) but with a slightly larger excess of H2SO 4, we have obtained by precipitation with alcohol a microcrystalline, solid red complex with an elementary composition which corresponds satisfactorily with the formula [Com(GG)~OH]fHzSO4 or [Com(GG)2(H~O)]2~+ SO~-. The compound behaves as an acid of equivalent weight about 390 (calc. 386), and has an electrical conductivity of the same order as that of sulphuric acid; its apparent molecular weight, measured cryoscopicaUy in aqueous solution, is about 170, or slightly less than one fourth of the formula weight. The infra-red spectrum of the red complex shows several strong absorption peaks at 3101, 3194, 3370 and 3570 crn-t (very broad) (Table 3); since not all of these frequencies can be due to the C - - H and N - - H stretching of the organic ligands (the frequency at 3570 cm -t is too high even for the free N - - H stretching of a primary amide), the presence of an OH-grouping in the eS) j. B. GILBFAtT,M. C. OT~V and V. E. PmcE, J. Biol. Chem. 190 337 (1951); C. TANI~3RD,D. C. KIRK and M. K. CHANTOONI,J. Amer. Chem. $oc. 76, 5325 (1954).

98

v. CAOUOT[,P. SILVF,ffrgONIand C. FUgLAN!

complex must be admitted. The simplest form of the final red product should then be [Com(GG)jOH] and our precipitate therefore must be its adduct with sulphuric acid. This formulation is not very different from that originally proposed by Gn~n~T et al., CSI apart from the fact that oxygen is present not as molecular 0 2 or in a peroxo bridge, but as OH-.* We have also measured the visible-UV absorption spectra of the initial product and of the successive stages of the reaction between the complex of Co ~ with giycylglycine (probably [Co~(GG)~(OH)]-) cs~ and oxygen (Figs. 3 and 4). The spectrum o f the initial substance (Fig. 3, A) has exactly the same shape as those of normal complexes

TABLE 3.--PROPERTIES AND COMPOSITION OF THE RF..D COMPLEX (AS PRECIPITATED FROM SULPHURIC SOLUTIONS)

Ultra-violet absorption spectrum v(crn-1)

~(mp)

eM

19600 25500

510 392

346 115

Analytical composition Found

C H

N Co SOt-

24"94 4"45 14"45 14.90 11-78

Calc.d. for [CoXU(GG),OH]l.HiSO, (formula-weight 772) 24"87 4.14 14'51 15"28 12.43

Specificconductivity Concentration (mol/l.) 4.17 x 10-s 1.83 x 10-s 1.67 x 10-s

177 246 434

Moi. weight (cryoscopic,in water): 170 in approx. 0.1 M solutions Equivalent weight as an acid: 390 (calcd. 386) IR absorption peaks in the p-region: 3101, 3194, 3370 and 3530 cm-1

of bivalent Co, with one main band at 510 m p , eM "~ 5"5; as the absorption of O~ proceeds, a second unresolved maximum is initially formed at about 590 m p indicating the presence of at least another intermediate in the oxidation reaction besides the brown form; in succession the brown form develops, the formula of which is probably [Co(GG)2]2"Os, with a spectrum which exhibits an intense chargetransfer band with a maximum frequency at 29,000 cm -1 and e~r,~3000. ~s~(Fig. 3, C). This form is capable of carrying 0 2 and can be reduced at the dropping mercury electrode forming successively H 2 0 2 and H20; furthermore, it can undergo an internal redox process which gives rise to the observed charge-transfer band; the brown form is probably diamagnetic, as has been ascertained by CALVIN and * Dr. M. T. B~cKhas communicatedto us that he has prepared under similarconditionsa bis-hydroxocomplexof the same type, with formula NH~+[Com(GG)s(OH)I]. (Privatecommunication).

Oxygenation and oxidation of Con-chelates

f

/ 1

99

m

v ,4

tO

15

20

25

30

X I0 3 cm -I

Fie. 3.--Absorption spectra of a solution containing 5 x 10-* M Co, 0.1 M glycylgly¢ine and 0.2 M NaOH: A: before oxypmttion B, C, D: successive oxygenation and oxidation stases E: after passing exceu Ot (brown form) F: same as Eafter 1 day G: same as E and F after two days (red form).

B ~ (t) for the oxygenated form of the cobaltous complexes with ligands of the salicylaldehyde-ethylenediiminetype. We are then of the opinion that this brown form is the only one which contains an O--O bridge, and its behaviour may possibly correspond to either of the two formulae: GG

GG

GG

GG

V

V

V

V

COH---Os---CoN

A

GG

A

G G

or

COH[---Ot=--CoHI

A

GG

A

GG

The spectrum of the final red product (Fig. 3, D and Fig. 4, G) strongly resembles those of other octahedral or nearly-octahedral complexes of Co(lII); the maxima at 520 and 390 rap, with molar extinctions of 340 and 115 respectively, indicate a rather high ligand-field strength (>H20). It is therefore proved once again that the red form contains tervalent cobalt; we cannot however deduce solely from the spectra which ligands are bound to cobalt in this complex, but the results of all our measuremerits support the idea that the final red product corresponds to the formula [Com(GG)s(OH)]; the irreversible reaction leading from the brown to the red form should then consist essentially in the elimination of the peroxo groups, so that the t4~ M. CALVINand C. H. BAgKEt~W,J. Amer. Chem. $oc. 68, 2267 (1946).

|00

V. CAOLIOTI,P. SILVESTRONIand C. FURLANI

5

J F

E S,0

i

0.5

I0

C

15

~

20

'

25

30

x 103cm" I

Fro. 4.--Molar absorption spectra of A - - C o =+ with excess glycylglycine before oxygenation B--same as A in the first oxygenation stages C---same as A and B with excess O= (brown form) D--final red complex (cobalti-bis-glycylglycine hydroxide).

binuclear oxygenated complex of cobalt with glycylglycine becomes mononuclear. Since it is experimentally found that all the oxygen in the final products is reduced to water and that no more peroxo bridges arc present, we must necessarily include in this stage a reaction leading to decomposition of the hydrogen peroxide, which is possibly present as an anion in the brown complex. Such a reaction could be catalysed by the cobalt ion (several metallic complexes have been reported to exhibit peroxydase activity/~)), and occur with an oxidation of the organic part of the ligand molecules, e.g. dehydrogenation of an amino to an imino group. In this respect it should also be mentioned that HeARON and BuR~:(e) report the evolution of CO= during the reactions following the formation of oxygen-carriers from cobaltous salts, O8, and aminoacids; quite recently BECK(~) has found that the cobaltic complex of histidine, prepared from Co ~- ions and histidine, has an absorption spectrum which is different from that of irreversibly-oxidized Co(II)-histidine solutions, and therefore he assumes that the organic ligand has been oxidized, as in the case with the Cu(II)histidine complex. ~s) In the case of cobalt(III) bis-glycylglycine, a metal-catalysed reaction leading to decomposition of the brown form could start with charge-transfer from the group to be oxidized to the cobalt, i.e. with the same internal process as that which probably causes the chargr-transfer band observed experimentally in the spectrum of the brown form. (=) See e.g.F. He[N, Chemische Koordinationslehre Chapter H, 2, (a), (b) and (c). S. Hirzel Vcrlag Zucrich (1950). ~6~j. g. Hv.~zON and D. BUaK, Arch. Biochem. 14, 337 (1947). qTJ M. T. BV..CK,Naturwissenschaften 45, 162 (1958). (a) j, CSA,SZ,~R, L. Kiss and M. T. BecK, Naturwissenschaften 45, 210 (1958).