Iron Tetraanthracenotetraazaporphyrins: Synthesis, Structural Characterization, Ligand Binding Properties, and Unexpected Selectivity of a Bis-“Bowl” Tetraazaporphyrin

The synthesis and characterization, by optical spectroscopy, mass spectrometry, superconducting quantum interference device (SQUID) magnetometry, and single-crystal X-ray diffraction, of six iron complexes of tetraanthracenotetraazaporphyrin (TATAP) are reported. Eight benzo groups, flanking the macrocycle periphery, form a nonpolar “bowl” on each face of the porphyrazine and prevent μ-oxo dimer formation. Fe(TATAP) readily binds THF, a variety of neutral nitrogenous axial ligands, and carbon monoxide. The equilibrium binding constants for the first two are higher than those of analogous porphyrins while those of the latter are smaller. We attribute these differences to the higher π acidity of the porphyrazine ligand. Fe(TATAP) also shows different relative magnitudes of the successive equilibrium binding constants, K 1 and K 2, for hindered nitrogenous ligands when compared to those of porphyrin analogues. Surprisingly, Fe(TATAP), in toluene solution, shows no affinity for O2 when exposed to 1 atm partial pressure of O2 at 25 °C. These results are explained in terms of an unusually positive iron(III/II) redox potential when coordinated by the TATAP ligand.