Mossbauer-Zeeman Spectra of Some Octaethyl¬porphyrinato- and Tetraphenylporphinato iron(III) Complexes
David DolphinH., Sams, J.R., Tsin, T.B., and Wong, K.L.
J. Am. Chem. Soc.
Moessbauer spectra at 4.2 oK in zero field and in applied magnetic fields of up to 5.0 T are reported for 3 pairs of high-spin (S = 5/2) Fe(III) complexes: Fe(P)Cl, Fe(P)OMe, and [Fe(P)]2O (P = dianion of octaethylporphyrin or tetraphenylporphin). These spectra were analyzed in the spin Hamiltonian formalism to provide estimates of the crystal field splittings, components of the magnetic hyperfine tensors, signs of the electric field gradients (efg), and magnitudes of the asymmetry parameters η. In each of the 4 monomeric complexes the internal hyperfine field is large (approximately 50 T) and the efg is positive with effectively axial symmetry. However, the zero-field splitting parameters for the methoxides appear to be substantially smaller than those for the chlorides. In the μ-oxo dimers the 2 high-spin Fe atoms are strongly coupled, via an antiferromagnetic exchange interaction, to produce a spin singlet (nonmagnetic) ground state. In both dimers the efg is negative, η is nonzero, and the principal magnetic and efg axes are parallel. Careful measurements of the temperature dependences (4.2-295 oK) of the intensities, widths, and areas of the 2 zero-field Moessbauer lines of the dimeric complexes indicate slight relaxation broadening at high temperatuures, but provide no evidence of any Gol'danskii-Karyagin asymmetry.