THE DYNAMICS OF FORMATION OF THE O-2-CO-II BOND IN THE COBALT(II) CYCLIDENE COMPLEXES

The kinetics of O-2 binding to a vacant coordination site on the cobal t(II) ion have been determined, revealing a radical-like character for the reaction. Reversible oxygenation of Co(II) cyclidenes (C4, C5, C6 , C8, C12-bridged and unbridged) was studied by a cryogenic stopped-fl ow method. In the presence of axial base, kinetic parameters are insen sitive to the nature of the solvent, and negative entropies of activat ion suggest that dissociation of a solvent molecule is not the rate-de termining step for the dioxygen binding process. This is in contrast t o the behavior of previously studied Co(II) complexes. A very low acti vation energy (1-4 kcal mol(-1)), typical of diffusion controlled proc esses, was found for dioxygen binding. The binding rate constants for the highest affinity complexes (10(8) M-1 s(-1)) are comparable to the values for natural dioxygen carriers. The size of the lacuna primaril y affects the dioxygen binding rates, while the axial bases influence the dioxygen dissociation rates. (C) 1998 Elsevier Science S.A.