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.