THE DYNAMICS OF FORMATION OF THE O2-COII BOND IN THE COBALT(II) CYCLIDENE COMPLEXES

Abstract The kinetics of O 2 binding to a vacant coordination site on the cobalt(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-flow method. In the presence of axial base, kinetic parameters are insensitive to the nature of the solvent, and negative entropies of activation suggest that dissociation of a solvent molecule is not the rate-determining step for the dioxygen binding process. This is in contrast to the behavior of previously studied Co(II) complexes. A very low activation energy (1–4 kcal mol −1 ), typical of diffusion controlled processes, 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 primarily affects the dioxygen binding rates, while the axial bases influence the dioxygen dissociation rates.