Comparison of electrochemical behaviour of cobalt and molybdenum as binding sites for dioxygen in model chelates for oxygen carriers and oxygenases

Abstract

Complexing to a transition metal ion seems to be one of the most general methods to circumvent the low kinetic reactivity of triplet O 2 in biological systems. Direct comparison of the behaviour of different metal centres in a model system and model reaction can be very useful for unravelling different mechanisms of activation, which are dependent on the nature of the metal and its coordination sphere. Following our previous work on the electrochemical behaviour of end-on co-ordinated molecular oxygen to cobalt atom in oxygen carriers, we have investigated with the same techniques the side-on co-ordination of O 2 to molybdenum in peroxo complexes. In both types of coordination the oxygen activation is measured by the displacement of the reduction potentials towards less cathodic values as compared with the free molecular oxygen in the same solvent. In the presence of suitable acceptors and in aprotic solvents some cobalt chelates are active in the electrocatalytic generation of O −• 2. In aqueous acid solutions the same type of cobalt chelates lead to the electrocatalytic generation of H 2O 2. On the other hand it does not appear that end-on coordination to the cobalt atom weakens the OO bond enough to catalyse oxidation reactions of biological interest. Investigations of the side-on co-ordination of O 2 in a series of molybdenum peroxide complexes show that again the reduction is displaced toward less cathodic potential as compared with free O 2 in the same solvent. As an important difference from the behaviour of cobalt-di-oxygen adducts, H 2O 2 is not generated by reaction of co-ordinated with H + but is reversibly released from the peroxide function in alkaline solution. Moreover, in the presence of suitable substrates the cleavage of OObond and the transfer of atomic oxygen is observed, and can be monitored by electrochemical techniques. The above differences are substantiated by identification of products of simple model reactions and by thermodynamic and kinetic parameters of chemical reactions coupled with electron transfer. The results may explain the different role of the metal ion and of the coordination sphere in oxygen carriers and monooxygenases.