Synthesis and characterization of a new series of iron(II) single-face hindered porphyrins. Influence of central steric hindrance upon carbon monoxide and oxygen binding

Abstract

A series of single-face hindered porphyrins has been synthesized from 5,10,15,20-tetrakis-(o-aminophenyl)porphyrin having one ‘handle’ and two pivaloyl ‘pickets’ as substituents in order both to vary the cavity size and to hold the handle in a central position. They have been characterized from the analysis of the ¹H n.m.r. spectra of their four-co-ordinate iron(II) derivatives. The five-co-ordinate complexes obtained by addition of 1-methylimidazole in toluene solution form stable and reversible oxygen adducts. The carbon monoxide and oxygen affinities of these complexes have been determined and are compared with similar data for other model compounds. The systematic decrease of the available space for the iron-bound CO produces a decrease of affinity constants for this ligand which is due to decreased association rates. In contrast, O₂ affinities are only slightly affected. I.r. spectroscopy confirms the presence of a hydrogen bond between O₂ and the NH amide of the handle anchoring groups. No such interaction is observed with the pivaloyl pickets. These compounds display a ratio of CO and O₂ affinity constants from 5000–7. These results are discussed in terms of the distal-side steric effects.