Influence of polar amide groups upon the kinetics of ligand binding with iron(II) 5,10,15,20-tetrakis[α,α,α,α-o-(N-tert-butylcarbamoyl)phenyl]porphyrin

Abstract

We have investigated the consequences upon ligand binding of reversing the amide dipoles from PhNHCO in the ‘picket-fence’ porphyrin 2 to PhCONH in the newly synthesized heme model 1, by using laser photolysis. 1-1,2-Me₂Im has been found to be the first example of photodissociation of an iron(II) five-coordinated base–porphyrin complex. Moreover, 1 binds base and gaseous ligands on either porphyrin side, thus displaying little steric discrimination in its five- and six-coordinated states, at variance with the ‘picket-fence’ porphyrin. The variations of the association and dissociation rate constants both contribute to lower the affinities for binding CO and O₂ on the encumbered side of porphyrin 1 as compared to 2 by two to three orders of magnitude. The main effect is due, however, to an increase of the dissociation rate constants, reflecting the destabilization of the complexes. Linear free energy relationships show that, in spite of these wide differences, both compounds (as well as several ‘basket-handle-’ and ‘picket-fence’ derivatives) behave as one reactive family which is thermodynamically homogeneous.