MODELS OF THE CYTOCHROMES - AXIAL LIGAND ORIENTATION AND COMPLEX STABILITY IN IRON(II) PORPHYRINATES - THE CASE OF THE NONINTERACTING D(PI)ORBITALS

The synthesis and characterization of seven bis-pyridine and bis-imida zole complexes of iron(II) teoramesitylporphyrinate are reported. X-ra y 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 a ll show parallel axial ligand orientations with nearly planar porphyri nato cores. The Mossbauer spectra of six of the complexes, having pyri dine ligands with pK(a)(PyH+) ranging from similar to 1.1 (4-CNPy) to 9.7 (4-NMe2Py), have been determined. The Mossbauer isomer shifts at 1 20 K are in the range of 0.36-0.45 mm/s, and the quadrupole splittings (Delta E-Q) are in the range of 1.11-1.27 mm/s. Thus, unlike the corr esponding Fe(III) complexes, the X-ray structures and Mossbauer spectr oscopic parameters of these (tetramesitylporphyrinato)iron(II)-bis (py ridine) complexes are shown to be essentially independent of the basic ity and pi donor/acceptor properties of the axial pyridine ligands. Th ese solid-state structural and spectroscopic properties are compared t o the thermodynamic properties of the same series of complexes in solu tion (Nesset, M. J. M.; Shokhirev, N. V.; Enemark, P. D.; Jacobson, S. E.; Walker, F. A. Inorg. Chem. 1996, 35, 5188): The equilibrium const ants, beta(2)(II), for binding two ligands to [Fe-II(TMP)(DMF)] are al so nearly independent of the basicity of the axial pyridine ligand, al though the Fe-III/Fe-II reduction potentials vary strongly with ligand basicity due to the large variation in beta(2)(III), the equilibrium constant for binding two ligands to the Fe(III) complex. Hence, it app ears that low-spin d(6) metalloporphyrins have a marked preference for parallel orientation of planar axial ligands, and that the charge asy mmetry at the iron nucleus (deduced from Mossbauer quadrupole splittin gs) and the thermodynamics of ligand binding are unaffected by the ele ctronic properties of the axial ligand. The major reason for the marke d preference for parallel ligand orientation for iron(II) porphyrinate s appears to be lack of a means of energy stabilization of the ruffled core of the perpendicular orientation.