Spin relaxation in high-spin iron(III) complexes of tetraphenylporphine

Abstract

The Mössbauer spectra of Fe(por)X (por = tetraphenylporphinato, TPP, and tetrakis( p-chlorophenyl)porphinato, T p-ClPP, dianions; X = Cl, Br and I) have been measured. They show varying quadrupolar pattern depending on the temperature: asymmetric broadening of the high-velocity line at higher temperatures, a symmetric doublet at a temperature T r, and a reversal in asymmetry below T r. The temperature dependence results from temperature-dependent spin-spin relaxation in high-spin iron(III) and the off-diagonal terms of the hyperfine operator, and reflects the ionic zero-field splitting. For a given X, T r is higher and the quadrupole splitting, Δ e Q, is larger for T p-ClPP derivatives than for TPP derivatives. The result is explained on the basis of the resonance effect of the p-chloro substituents of the meso-phenyl groups. When por is fixed, both the values of T r and Δ E Q are in the order Cl < Br < I. This is interpreted as due to the predominantly ionic character of the bond between the halide and iron ions.