Characterization of a six-coordinate ferrous high-spin heme with both intramolecular axial carboxylic acid and pyridine.

Synthesis of a bis-strapped porphyrin with a pyridyl residue on one side and a malonic acid on the other side gives after iron(II) insertion a six-coordinate complex in which both apical groups are the two axial ligands of the iron atom. Unexpectedly, this six-coordinate iron(II) complex proves to be high-spin, likely due to some stabilization of the axial metal-ligand antibonding orbitals.