Picket fence iron complexes

Figure 4 Molecular structure of O2-bound picket fence iron porphyrin complex. The iron porphyrin has four pivalamide groups forming a hydrophobic cavity for the ligand O2. The axial ligand is 1,2-dimethylimidazole. X-ray crystal structure shows the end-on and bent coordination of O2 to the heme-iron...

The conversion of the coordinated nitrosyl of the picket fence porphyrin complex [Fe(tetra(a,a,a,a-u-pivalamidophenyl)porphyrinato)(NO)] to nitrite provides the first unambiguous demonstration of a nitrosyl-to-nitrite conversion at an iron porphyrin center." The reaction of Equation (3) occurs in air in the presence of pyridine, and is biomimetic since the starting complex is formed on reduction with triphenylphosphine. The picket fence pocket appears to be crucial for the stabilization of the NO product in air, since nonpicket fence NO porphyrins were shown to form oxo dimers readily ... 

In order to prepare and isolate solid-state, crystalline, oxygenated iron-heme model complexes, chemists learned to synthesize (by self-assembly methods) and oxygenate many types of hindered porphyrins. For instance, capped porphyrins were synthesized by direct condensation of a suitable tetraaldehyde with four pyrrole molecules.37 Picket-fence porphyrins such as I e(TPP)((V-MeIm) (where TPP = meso-tetraphenylporphyrin and /V-Melm = (V-methylimidazole)... 

Support for the vtew that the globin portion of the molecule produces a constraint upon the iron atom (which would otherwise move into the heme pocket) comes from the behavior of myoglobin and model compounds (such as the picket-fence corn-pounds with I- or 2-mcthylimidazole mimicking the porphyrin and histidine), which are easier to study than the more complex hemoglobin ... 

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