Models of the Cytochromes. Axial Ligand Orientation and Complex Stability in Iron(II) Porphyrinates:  The Case of the Noninteracting dπ Orbitals

• Published in: Journal of the American Chemical Society 1997-10, Vol.119 (40), p.9438-9448
• Main Authors: Safo, Martin K, Nesset, Marlys J. M, Walker, F. Ann, Debrunner, Peter G, Scheidt, W. Robert
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja9715657

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 Mössbauer spectra of six of the complexes, having pyridine ligands with pK a(PyH+) ranging from ∼1.1 (4-CNPy) to 9.7 (4-NMe2Py), have been determined. The Mössbauer isomer shifts at 120 K are in the range of 0.36−0.45 mm/s, and the quadrupole splittings (ΔE Q) are in the range of 1.11−1.27 mm/s. Thus, unlike the corresponding Fe(III) complexes, the X-ray structures and Mössbauer 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 compared to the thermodynamic properties of the same series of complexes in solution (Nesset, M. J. M.; Shokhirev, N. V.; Enemark, P. D.; Jacobson, S. E.; Walker, F. A. Inorg. Chem. 1996, 35, 5188):  The equilibrium constants, β2 II, for binding two ligands to [FeII(TMP)(DMF)] are also nearly independent of the basicity of the axial pyridine ligand, although the FeIII/FeII reduction potentials vary strongly with ligand basicity due to the large variation in β2 III, the equilibrium constant for binding two ligands to the Fe(III) complex. Hence, it appears that low-spin d6 metalloporphyrins have a marked preference for parallel orientation of planar axial ligands, and that the charge asymmetry at the iron nucleus (deduced from Mössbauer quadrupole splittings) and the thermodynamics of ligand binding are unaffected by the electronic properties of the axial ligand. The major reason for the marked preference for parallel ligand orientation for iron(II) porphyrinates appears to be lack of a means of energy stabilization of the ruffled core of the perpendicular orientation.