The Electronic Structure of Highly Anisotropic Low-Spin Ferric Porphyrin Complexes Based on Single-Crystal EPR Measurements

Article abstract of DOI:10.1021/ja00225a010

Single-crystal electron spin resonance g tensor determinations are reported for three low-spin ferric porphyrin complexes, ClO4*2THF, K*2acetone, and Fe(TPP)(CN)(py)*H2O, all of which exhibit "highly anisotropic low-spin" EPR spectra.These single-crystal measurements allow accurate extraction of the complete g tensor, which in turn provides estimates of the wave functions and relative energies of the three highest occupied molecular orbitals.It is the near-degeracy of the "d_xz" and "d_yz" orbitals in these complexes that is responsible for the large g value anisotropy; this near-degeneracy has a number of additional consequences.Analysis of the crystal field parameters, derived from the EPR measurements, indicates that for the (py)2 and (CN)2 complexes the product of the three principal g values is negative, whereas for "normal" ferric porphyrin complexes this product is positive.The spin in these complexes is delocalized over the two out-of-plane real d orbitals, whereas in "normal" complexes it is localized in a single real d orbital.For ferric porphyrin complexes with planar axial ligands, this delocalization is associated with a spin-orbit stabilization of the perpendicular ligand conformation.In the case of weak ? donors, this stabilization can exceed the crystal field stabilization of the parallel geometry.A crystal structure determination for ClO4*2THF at 128 K reveals that the axial pyridine ligands adopt a perpendicular conformation that is staggered with respect to the equatorial Fe-N(pyrrole) vectors.The idealized S₄ symmetry about the iron center is consistent with the spectroscopic observations.Crystal data: ClO4*2THF, space group A2/a, Z = 8, a = 22.814 (2) Angstroem, b = 17.010 (2) Angstroem, c = 27.423 (2) Angstroem, β = 102.51 (1) deg, at 128 K.