Ligand, Oxygen, and Carbon Monoxide Affinities of Iron(II) Modified Capped Porphyrins

Article abstract of DOI:10.1021/ja00372a003

Ligand, O2, and CO binding constants are presented for iron(II) modified capped porphyrins.Steric effects at the axial position outside the cap were altered by changing a tetraphenyl-type cap to a corresponding tetranaphthalene-type cap.The steric hindrance caused by the peri hydrogens of the naphthalene groups is believed to be responsible for the decrease in ligation in the order n-propylamine > isobutylamine > sec-butylamine > tert-butylamine.Inductive effects in the porphyrin plane were altered by introducing a nitro group.Such an electron-withdrawing group enhances ligation but retards oxygenation.The solvent effect on O2 binding by capped complexes is small, presumably because the environment of the O2 bound inside the cap is not greatly changed by changes in solvent.The CO affinities of iron(II) capped porphyrins and of corresponding iron(II) flat-open porphyrins are similar.In contrast, the O2 affinities of the capped systems are lower than their flat-open counterparts, which means the iron(II) capped porphyrin complexes discriminate against O2 but not CO.This is discussed in terms of distal steric effects on the stable structures of Fe-O-O (bent) and Fe-C-O (linear).