The dynamics of formation of the O-2-Co-II bond in the cobalt(II) cyclidene complexes

摘要

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. (C) 1998 Elsevier Science S.A. [References: 67]