O2 and CO Binding to Iron(II) Porphyrins: A Comparison of the Picket Fence and Pocket Porphyrins

Article abstract of DOI:10.1021/ja00348a019

Kinetic and equilibrium O2 and CO binding behavior for various iron(II) porphyrin systems are reported.Compared to the picket fence complexes, unprotected iron(II) porphyrins have both lowered affinities and lowered ligand association rates.This is explained in terms of a solvation effect which stabilizes the five-coordinate form in the unprotected iron(II) porphyrins.Five-coordinate iron(II) complexes of the sterically encumbered medium and small pocket porphyrins show substantially reduced CO affinities as compared to the analogous open-cavity picket fence complexes.Kinetic data indicate that the lowered CO affinities in the pocket complexes are primarily reflected in decreased assosiation rates.By contrast, the pocket models show both decrease O2 dissociation and association rates as compared to the picket fence analogues.As such, the O2 affinities are virtually identical throughout the series of picket and picket fence complexes.These results are discussed in terms of distal-side steric interactions.