Reaction of cobalt(II) macrocyclic tetra-amine complexes with dioxygen

Abstract

Low-pH equilibrium and kinetic studies on the oxygenation of [CoL]²⁺ complexes containing 12–(L²), 13–(L³), and 14-membered (isomeric, L⁴ and L⁵) fully saturated macrocyclic tetra-amines are reported. The ring size strongly influences the stability of the oxygenated products. The relative stability is connected with the stoicheiometry of the oxygenated complex, [(CoL)₂(O₂)(OH)]³⁺ for L² and L³ and [(CoL)₂(O₂)]⁴⁺ for L⁵, which has been elucidated by potentiometric titration and polarographic measurements. The results for the macrocyclic systems are compared with a linear tetra-amine system (L¹). The cyclic nature of the macrocycles, while serving to raise the [CoL]²⁺ stability, markedly lowers the O₂ affinity of the complexes. Rates for the reaction of Co²⁺, L, and O₂ in acetate buffers (yielding the dioxygen adducts) are first order in [Co²⁺] and also in [L], but are independent of [O₂], indicating that the formation of [CoL]²⁺ is the slowest step in the O₂ uptake. The separate reaction of [CoL]²⁺ with O₂ is first order in [CoL²⁺] and also in [O₂], with the second-order rate constants being ca. 10⁴ times faster than those for the formation of [CoL]²⁺ under comparable conditions. The presence of the macrocyclic ligand slows down (by ca. 10² times) the overall rate of O₂ uptake compared with linear totra-amine systems.