The kinetics of O2 binding to a demonstrably vacant coordination site on a cobalt(I1) ion are determined, revealing a radical-like character for the reaction, with a very low activation barrier (ca. 1-2 kcal mol-1) and large entropically controlled rate constants, whose values approach those for myoglobin and haemoglobin (up to 108 dm3 mol-1 s-1).CoII complexes constitute the most popular and successful class of synthetic 0 2 carriers.' In addition to the fundamental importance of understanding the dynamics of oxygen binding to CoII complexes, rapid binding and/or dissociation rates are critical to many applications of 0 2 carriers. In the limited number of kinetic studies reported,2-8 the oxygenation rates of CoI1 complexes were found to be slow compared to those for O2 binding to natural dioxygen carriers. Second-order rate constants are in the range 107-108 dm3 mol-1 s-1 for natural compounds,9 and, with a few exceptions,6,8 2-3 orders of magnitude smaller for synthetic Co complexes2. Bond formation between the doublet state of low-spin d7 CoI1 ion and the triplet O2 molecule is expected to be much faster than has been observed. The probable source of this behaviour is shown in Scheme l(a). The dissociation of a Cosolvent or Co-ligand bond may be involved in the rate-determining step in 1 : 1 Co-0 2 adduct This is strongly supported by recently determined activation volumes for reaction ( 1).8 Consequently, the reported kinetic parameters do not correspond directly to the process of forming the Co-02 bond. Instead, they characterize the rather trivial process of axial ligand dissociation from CoII. Additional complications arise because of the rapid formation of dinuclear complexes by Co-02 adducts [eqn. ( 2)].2~,5,6a COL + 0 2 & CoL(02) (1) k-1 CoL(02) + COL 2 CoL(O2)CoL (2) k-2
The cobalt(II) cyclidene complexes are known to bind dioxygen reversibly under ambient conditions; however, the lifetime of the dioxygen adduct is limited by autoxidation. This paper describes a study of the autoxidation reactions of the cobalt(II) cyclidenes in nonaqueous media. The kinetics of the autoxidation reaction of the cobalt(II) complex are dependent on the nature of the ligand substituents and dioxygen concentration and on the nature and concentration of added base. Mechanistic details have been explored through isotopic substitution of the ligand. The proposed mechanism involves preequilibrium deprotonation of the ligand in the dioxygen complex, followed by irreversible
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