Impact of counter anions on structural and magnetic properties of iron(III) meso-tetraphenylporphyrin tetrahydrofurane solvates

Intensively researched iron meso-tetraphenylporphyrin (FeTPP) complexes are central to the functionalities of many biological systems while at the same time they possess intriguing magnetic properties. Here we report on the synthesis and structural characterization of FeTPP complexes with axial tetrahydrofuran (THF) ligand and different non-coordinating counter anions. Structurally, the most interesting feature of these complexes is the stretching of the distance between the iron(III) and the oxygen atom of the THF ligand for different counter anions. This parameter affects the magnetic anisotropy of FeTPP as studied here with magnetic susceptibility, χm, and X-band electron paramagnetic resonance (EPR). Both magnetic probes are consistent with the iron(III) in its high-field state S = 5/2. Simulations of temperature dependencies of χm and EPR spectra show that the zero-field splitting magnetic anisotropy parameter D decreases with the tetragonal elongation of the hexa-coordinated iron(III) geometry, which is qualitatively discussed within the ligand field theory.