Dioxygen affinity

Cyclodene Complexes Relationships between Macrocyclic Ring Size, Lacunar Cavity Shape, and Dioxygen Affinity of the Cobalt-Cyclidene Dioxygen Carriers. 

The capped porphyrins prepared by Baldwin et al. [56, 57] are other model systems designed to test the consequences of steric hindrance on CO binding (Scheme 4). These compounds were reported to discriminate against dioxygen in favor of carbon monoxide [62, 119-121]. The CO affinity of the capped porphyrins differs by less than a factor of three from that of unprotected iron(II) tetraphenylporphyrin, while the dioxygen affinity is more than a factor of 100 lower. Kinetic studies of CO binding show that the CO dissociation rate constants are very similar to those of unprotected hemes. Recently, the X-ray crystal structure of a carbonylated complex of the smallest capped porphyrin was obtained [122]. The CO ligand is reported to deviate 7° and 4° from the heme normal, respectively, for each independent molecule present in the unit cell. 

Dioxygen affinity of the Co(II) and Fe(II) cyclidene complexes is governed by both electronic and steric factors. Coordination of a fifth donor ligand, such as pyridine or imidazole, is required for efficient 02 binding and is usually accomplished by adding the corresponding base to a solution of the macrocyclic complex. The sixth coordination site of the metal should remain vacant or occupied by a labile ligand to allow... 

Dioxygen affinity of the Co(II) complexes parallels the cavity width (Figure 4.11), indicating that there are significant steric interactions between the walls of the... 

Dioxygen affinity of iron(II) cyclidenes depends on steric constraints for 02 binding, resulting in van der Waals repulsions in dioxygen adducts. This effect is evident from comparison of the complexes with aliphatic bridges of varying length... 

The dioxygen affinity is composed of enthalpic (AH) and entropic (A5) components, with... 

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