Cobalt-dioxygen model systems

Complexes of other amino acids or their derivatives with cobalt(II) that have been investigated include dipeptides (120) these complexes have long been known to absorb dioxygen. For example, the mononuclear cobalt(II) complex of N, N,N", N "-diglycylethylenediaminete-traacetic acid (121) absorbs one mole of dioxygen per two moles of complex. This system has been proposed as a simple, convenient model system for the study of dioxygen complexes of cobalt(II) peptides in solution because of its relatively slow conversion to the irreversibly formed cobalt(III) dioxygen complex. 

Any adequate theoretical treatment must also explain how iron-porphyrin systems can bind not only O2, but also CO, NO, alkyl isocyanides, and alkyl-nitroso moieties. A simple qualitative model presented by Wayland and coworkersconveniently summarizes ligand-binding geometries of cobalt and iron porphyrins. Although a reasonable quantitative theoretical consensus exists for 1 1 cobalt-dioxygen species, the same cannot be said yet for iron-dioxygen systems. 

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 dioxygen ligand is said to be stabilized by the presence of strong (T-donors in the trans position. Such complexes are of specific interest as models of the active myoglobin center (473). Preliminary reports of a related copper(II) system have appeared (286). Studies on the effect of extraplanar Me SO coordination on phthalocyanine derivatives of cobalt (109) and iron (315-317) have been performed and show that, for example, the low-spin d iron(II) phthalocyanine complex dissolves in Me SO to give a diamagnetic blue solution which may be reacted further with donors such as imidazole, effecting replacement at the axial sites (62). 

In the naturally occurring systems as well as in the large number of model compounds which have been proposed for the active cotter for both types of function (in both cases, most often macrocyclic complexes of iron or cobalt), the factors influencing in the right way the character of the metal - dioxygen interaction ate still not completely understood [4]. 

Photolysis of [Co(CH2R)(L)(Hdmg)2] under oxygen proceeds by insertion of dioxygen into the cobalt carbon bond to provide a solution species for which nmr spectroscopic data is rq)orted. Reduction of this intermediate produces primary alcohols whereas thermolysis produces aldehydes and alcohols. Treatment of [Co oep)] with simple aldehydes and rm-butylhydroperoxide in the presence of sodium borohydride produces cobalt(III) acyls in 65-98% yields. In the absence of the borohydride the yield is reduced. The reaction is proposed to proceed by acyl radical trapping by the Co(n) centre. Methyl transfer in a protein free model of vitamin B12 dependent methyl transf enzymes has been studied. These systems convert homocysteine to methionine in nature. Trimethyl-phenylammonium icm reacts with the CoG) centre in cobalamin producing methylcobalamin. ... 

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