Cobalt. Oxygen Carriers

Molecular oxygen can form both 1 1 and 2 1 adducts (Co - O2 and Co - Oj - Co) with different Co(II) complexes. For the 1 1 adducts, the 0-0 stretching fi-equencies are similar to those of 0 and an unpaired electron is located mainly on the two oxygen atoms. On the basis of these observations, the adducts have been formulated as CofllfjOj, with the 0 forming a coordinate bond to low-spin Co(III), which has a (3d) configuration. Alternatively, we may formulate the electronic structure as Co(II)02, for which the O2 valence-bond stmcture (8) has spin-paired one of its two unpaired electrons with the unpaired electron of low-spin Co(II) (3d) . Thus, we may generate the increased-valence stmcture (42) by means of this reaction. 

The Co(II)02 stracture (42) accounts simply for the observations discussed above. The O2 retains one unpaired-electron, and because the oxygen atom bonded to the cobalt must have a different electronegativity from that of the terminal oxygen atom, the four O2 bonding electrons will not be shared equally by the two oxygen atoms. This reduction in the extent of covalent bonding for the O2 of stmcture (42) should be chiefly responsible for the decrease in 0-0 stretching frequency that is observed when O2 bonds to Co(ll). 

We shall now compare the Cof Oj and the Co(III)02 structures. The valence-bond structures for low-spin Co(lll) and 0 are shown in (43). On coordinating the 0 with Co(lll), we obtain the Co(IIl)02 valence-bond structure (44). We may also form the long-bond Co(III)02 structure (45), which for non-symmetrical coordination, should be less stable than structure (44). We now note that the Co — 0-0 bonding unit summarizes resonance between Co — 06 and c 0 6. Therefore, the CoflTlOj structure (42) is equivalent 

We may construct another Co(llI)02 stracture, namely the increased-valence stracture (47). 

To do this, we spin-pair the unpaired electron of O2 with one of the two unpaired electrons of the Co(III) configuration of stracture (46). Because structure (47) retains an unpaired electron on the cobalt, it cannot represent the ground-state for any of the adducts which have so far been studied. However, it must 

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Herron, N. A cobalt oxygen carrier in zeolite Y— a molecular ship in a bottle, Inorg. Chem., 1986, 25, 4714-4717. 

Fig. 10. Chemical structures of some oxygen carriers (A) five-coordinate cobalt Schiff-base Co(salPr), (B) four-coordinate cobalt Schiff-base Co(3-MeOsaltmen), (C) cobalt dry-cave.

It has long been known that, when bound to cobalt(II), the pyridine-based chelate ligands 2,2 -bipyridine (bipy), 1,10-phenanthroline (phen), and 2,2 6, 2"-terpyridine (terpy) form complexes that react with dioxygen in aqueous solution (32-34). The mixed-ligand complexes [Co(terpy)(bipy)]2+ and [Co(terpy)(phen)]2+ can act as oxygen carriers in aqueous solutions at pH values as low as 3.0 (34b), and the superoxo species thus formed are apparently dinuclear. In addition, the dinuclear bipyridine complex [(bipy)2Coin(/ 2-0 )(/ 2-02 )CoIn(bipy)2 ]3+ has been shown to catalyze the oxidation of 2,6-di-ter -butylphenol to the feri-butyl-substituted diphenoquinone and quinone (35). 

The resultant hydroxyl radicals are effective in initiating many chain reactions. The number of metal ions and complexes which are capable of activating hydrogen peroxide in this manner is quite large and is determined in part by the redox potentials of the activator. Related systems in which free radicals are generated by the intervention of suitable metallic catalysts include many in which oxygen is consumed in autoxidations. Cobalt(H) compounds which act as oxygen carriers can often activate radicals in such systems by reactions of the type ... 

Figure 11.27 Examples of cobalt-based facilitated transport oxygen carriers [66]...

McLendon G, MarteU AE (1976) Inorganic oxygen carriers as models for biological systems. Coord Chem Rev 19 1-8 McLendon G, Harris W, MarteU AE (1976) Dioxygen compl-exation by cobalt amino acid and peptide complexes. 1. Stoichiometry and equilibria. J Am Chem Soc 98 8379-8386... 

The chemistry of non-peroxo polynuclear cobalt(III) ammines is reviewed with particular emphasis on Werner s major contributions. Modern work in this area has shown that Werner s conclusions regarding the structures of these compounds are substantially correct in spite of the relatively primitive techniques he had available. There is much current interest in polynuclear cobalt(III) complexes because of their relationship to oxygen carriers and intermediates in electron transfer reactions. Modern techniques such as spectroscopy and x-ray diffraction have been used to determine the electronic and molecular structures of these compounds. 

In spite of the many modern techniques available to the chemist, the known chemistry of polynuclear cobalt (III) complexes is essentially that deduced by Werner 60 years ago. Since his work, no new polynuclear cobalt complexes have been prepared and characterized and no new reactions uncovered. Modem work in this area is being aimed at attaining a better understanding of the electronic structures inherent in polynuclear ions, which would be of value in a variety of active fields. The chemistry of polynuclear complexes is important in such new areas as synthetic oxygen carriers, electron transfer reactions, and transition metal catalysis. The fact that these new investigations are solidly based on Werner s pioneer investigations testifies to the genius with which he opened up a new area of coordination chemistry, with only the simple chemical techniques available to him. His work in the area of polynuclear cobalt(III) ammine complexes should continue to serve as a model of solid research for some time to come. 

It is noteworthy that related complexes containing a metal other than cobalt have not been found to be reversible oxygen carriers. 

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