MODELS OF THE CYTOCHROMES-B - LOW-SPIN BIS-LIGATED (PORPHINATO)IRON(III) COMPLEXES WITH UNUSUAL MOLECULAR-STRUCTURES AND NMR, EPR, AND MOSSBAUER-SPECTRA

The synthesis and characterization of the following bis-pyridine and bis-imidazole complexes of (tetramesityl-porphinato)iron(III) are reported: [Fe(TMP)(4-NH2PY)2]ClO4, 1, [Fe(TMP)(3-EtPy)2]ClO4, 2, [Fe(TMP)(3-ClPY)2]ClO4, 3, [Fe(TMP)(2-MeHIm)2]ClO4,4, [Fe(TMP)(4-CNPY)2]ClO4, 5, and [Fe(TMP)(3-CNPY)2]ClO4,6. The crystal structures of complexes 2, 3, and 5 are reported. All three complexes have relative perpendicular alignment of the axial ligands with bond lengths consistent with low-spin iron(III). The porphinato cores are strongly S4 ruffled; the Fe-N(p) bonds show a commensurate shortening. EPR investigations reveal "large g(max)" type EPR spectra for complexes 1-4, with complexes 2, 3, and 4 having unusually low g values (<3.2). Complex 1 has a "normal" "large g(max)" g value of 3.40. Complexes 5 and 6 have axial EPR spectra with g(perpendicular-to) = 2.53 and g(parallel-to) = 1.56 or g(perpendicular-to) = 2.62 and g(parallel-to) unresolved, which result from a substantial change in the usual ground state of low-spin (porphinato)iron(III) complexes, (d(xy))2(d(xz),d(yz))3, to that having a predominantly (d(x2),d(yz))4(d(xy))1 ground state. Complexes 2, 3, 4, and 5 have been characterized by Mossbauer spectroscopy. The Mossbauer isomer shifts (delta) range from 0. 1 8 to 0.20 mm/s and the quadrupole splittings (DELTA-E(Q)) from 0.97 to 1.48 mm/s. The quadrupole splittings, like the EPR g values of these complexes, are unusually low, compared with DELTA-E(Q) values of approximately 1.7-1.8 mm/s expected for imidazole complexes with perpendicular axial ligand orientation. The H-1 NMR isotropic shifts at -80-degrees-C (193 K) of the pyrrole protons of all complexes except 1 and 2, but including additional low-spin bis-pyridine complexes with 4-NMe2, 3,4-Me2, 3,5-Me2, 4-Me, and 3-Me substituents and pyridine itself as axial ligands, varied from -39.5 ppm (4-NMe2PY) to -6.5 ppm (4-CNPy) through this series. This result indicates a smooth change from an electronic ground state which is largely (d(xy))2(d(xz),d(yz))3 to one which is at least 50% (d(xz),d(yz))4(d(xy))1 in nature at 193 K for the (4-CNPy)2 complex, supporting the conclusions reached on the basis of the variation of EPR and Mossbauer parameters observed at 77 K and below.