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.