Structural, magnetic, and dynamic characterization of the (d(xz),d(yz))⁴(d(xy))¹ ground-state low-spin iron(III) tetraphenylporphyrinate complex [(p-TTP)Fe(2,6-xylylNC)₂]CF₃SO₃

The synthesis and characterization of the trifluoromethanesulfonate salt of bis(2,6-xylyl isocyanide)tetrakis(p-tolyl)porphyrinatoiron(III), [(p- TTP)Fe(2,6-xylylNC)₂]CF₃SO₃ (1), is reported. The crystal structure shows that the porphyrinate ring is strongly ruffled. The equatorial Fe-N bond distances average to 1.961(7) Å for 1, which is quite short for low-spin iron(III) porphyrinate derivatives. Two additional complexes, having TPP (2) and m-TTP (3) as the porphyrinates, were also synthesized and studied by NMR, EPR, and Mossbauer spectroscopy. All physical properties are consistent with a low-spin iron(III) porphyrinate with the less-common ground-state configuration (d(xz),d(yz))⁴(d(xy))¹. The ¹H NMR chemical shift of the pyrrole protons at 297 K is +10.7 ppm for 1. The EPR spectrum of 1 in solution is axial, with g(is perpendicular to) = 2.15 and g(is parallel to) = 1.94, Σg² = 13.0, while in the solid state g(is perpendicular to) = 2.2 and g(is parallel to) = 1.94, Σg² = 13.4. The Mossbauer spectrum of 1 at 190 K has an isomer shift of 0.14 mm/s and quadrupole splitting of 1.81 mm/s. Magnetic Mossbauer spectra analyzed in the intermediate spin-spin relaxation regime by the dynamic line-shape formalism of Blume and Clauser confirm this electron configuration and yield large negative quadrupole splittings, ΔE(Q) = -1.8 to -2.0 mm/s for the three complexes. To our knowledge, this is the first case in which the Mossbauer spectra of low-spin ferriheme systems have been analyzed in terms of the effect of intermediate rates of spin fluctuations on the appearance of the spectra. Analysis of the temperature dependence of the quadrupole splitting, ΔE(Q), for 2 yielded a different estimate of the energy separation between the (d(xz),d(yz))⁴(d(xy))¹ ground state and an excited state than did the temperature dependence of the NMR isotropic shifts. It is postulated that the excited state is actually the planar transition state between the two ruffled conformations of the porphyrinate that are related by 'inversion'. To explain the temperature dependence of both NMR isotropic shifts and Mossbauer quadrupole splittings, the planar transition state must have the (d(xy))²(d(xz),d(yz))³ electron configuration. The energy barrier appears to be smaller in homogeneous solution than in the solid state and is considerably lower than that predicted for the (d(xz),d(yz))³(d(xy))² excited electronic state of the ruffled conformation on the basis of the EPR g values.