Comparative study of Mössbauer spectra of iron(III) complexes of para-substituted tetraphenylporphyrins. Electronic effects of substituents and axial ligands

Abstract

The Mössbauer spectra of high-spin iron(III) complexes of tetra-para-substituted derivatives of 5,10,15,20-tetraphenylporphyrin [FeL(X)](R = OMe, H, F or Cl; X = Cl, Br or I), have been compared with each other to clarify the electronic effects of the phenyl substituents and the axial ligands. They show a variety of patterns depending on the temperature: asymmetric broadening of the high-velocity line at higher temperatures, a symmetric doublet at a temperature T_r, and reversal in asymmetry below T_r. The temperature dependence results from the temperature-dependent spin–spin relaxation of high-spin iron(III) ion and the off-diagonal terms of the hyperfine operator, and reflects the ionic zero-field splitting. The reversal temperature, T_r, and the quadrupole splitting, ΔE_Q, show two kinds of dependence on R. This is explained by considering the σ- and π-bonding characteristics of the porphyrins under the influence of the substituents. With a given R, the values of both T_r and ΔE_Q increase in the order X = Cl < Br < I. This is interpreted as due to the predominantly ionic character of the bond between the halide and iron ions, except for R = OMe.