Dynamics of dioxygen binding to vacant cobalt(II) sites in lacunar cyclidene complexes: barrier-free oxygenation

Abstract

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

Kinetics and Equilibria of Dioxygen Binding to a Vacant Site in Cobalt(II) Complexes with Pentadentate Ligands

“…In our laboratories, we are studying the dynamics of dioxygen binding to cobalt(II) complexes which were designed to have vacant coordination sites . A recent publication, reported our observations with lacunar macrobicyclic ligand complexes of cobalt(II). The lacunar structures of the ligands prevents the coordination of solvent or axial base at the sixth position while accommodating the small O 2 molecule.…”

Deciphering the mechanism of O2 reduction with electronically tunable non-heme iron enzyme model complexes

“…Iron centers that bind O 2 often have small enthalpy of activation, reflecting the fact that O 2 is a poor ligand. 38 , 39 Our lack of a clear relationship between temperature and rate may also reflect a small entropic contribution. This is only speculative however and there might be other factors such as competing pathways with relatively similar barriers.…”

Kinetics and Thermodynamics of Copper(I)/Dioxygen Interaction

“…This may be especially significant for the case of Co(III), which is always low-spin (d 6 ). Strong M(III)-O 2 -bonding is supported by examination of ∆H°values (for M(II) + O 2 a M(III)-O 2 -), commonly found in the range ∆H°f ormation ) -40 to -80 kJ mol -1 for natural or synthetic hemes, cobalt porphyrins, and cobalt complexes, 15,26,33 compared to the ∆H 1 °) -32 to -35 kJ mol -1 range for formation of [(L)Cu II (O 2 -)] + . Compared to our copper complexes, similar or even far more unfavorable negative ∆S°values are found in Fe(II)-O 2 and Co(II)-O 2 reactions, except for certain cases found in nature.…”