MODELS OF THE CYTOCHROMES. AXIAL LIGAND ORIENTATION AND COMPLEX STABILITY IN IRON(II) PORPHYRINATES : THE CASE OF THE NONINTERACTING DPI ORBITALS

The synthesis and characterization of seven bis-pyridine and bis-imidazole complexes of iron(II) tetramesitylporphyrinate are reported. X-ray crystal structures of three of the complexes, [Fe(TMP)(4-CNPy)2], [Fe(TMP)(3-CNPy)2], and [Fe(TMP)(4-MePy)2], have been solved and all show parallel axial ligand orientations with nearly planar porphyrinato cores. The Mossbauer spectra of six of the complexes, having pyridine ligands with pKa(PyH+) ranging from ∼1.1 (4-CNPy) to 9.7 (4-NMe2Py), have been determined. The Mossbauer isomer shifts at 120 K are in the range of 0.36−0.45 mm/s, and the quadrupole splittings (ΔEQ) are in the range of 1.11−1.27 mm/s. Thus, unlike the corresponding Fe(III) complexes, the X-ray structures and Mossbauer spectroscopic parameters of these (tetramesitylporphyrinato)iron(II)−bis(pyridine) complexes are shown to be essentially independent of the basicity and π donor/acceptor properties of the axial pyridine ligands. These solid-state structural and spectroscopic properties are compar...