Four-atom-linked capped porphyrins: Synthesis and characterization

Three new capped porphyrins, H₂(OC₂OPor) (7), H₂(OC(CO)NPor) (13), and H₂(OC₂NPor) (14), to be used as heme model compounds, have been synthesized and characterized. These compounds have the shortest linkages between cap and porphyrin plane of any four-arm capped porphyrins synthesized to date. The general synthetic procedure for H₂(OC₂OPor) involves the reaction of a tetraaldehyde "cap" with pyrrole to form the capped porphyrin. In the synthesis of the tetraamide-capped porphyrin, H₂(OC(CO)NPor), a tetraacid chloride cap is reacted with meso-α,α,α,α-tetra-(o-aminophenyl)porphyrin. The amide groups may be reduced to form H₂(OC₂NPor). With the use of FeBr₂, Fe²⁺ may be inserted in high yield into H₂(OC₂OPor) and H₂(OC(CO)NPor). The crystal structures of H₂(OC₂NPor), Fe(OC₂OPor)(OMe), and [Fe(OC₂OPor)]₂(μ-O) provide details on the types and amounts of cap expansion necessary to accommodate small ligands of biological interest, such as O₂ and CO.