Magnetic and high-frequency EPR studies of an octahedral Fe(iii) compound with unusual zero-field splitting parameters

Temperature-dependent magnetic susceptibility and multi-frequency EPR (9.4, 34.5, 94 and 188 GHz) spectroscopic measurements have been carried out together with an X-ray study at 100 K to study [Fe(DMSO)6](NO3)3. The iron(III) ion remains high-spin (S = 5/2) in the temperature range studied, therefore, the EPR data were interpreted using the conventional S = 5/2 spin Hamiltonian. A full analysis of EPR spectra at 95 GHz of a powdered sample at 290 K revealed that they are extremely sensitive to D and E values. The zfs parameters were precisely determined: D = + 0.1730 cm−1, E = 0.00 cm−1 and λ = |E/D| = 0.00. A sequence of the spectra neatly shows that the compound has a clear magnetic dependence on temperature. The study at 5 K, showed that the zfs parameters increase: D = + 0.1970 cm−1, E = 0.017 cm−1 and λ = |E/D| = 0.086. These data indicate that as the temperature decreases the D tensor increases slightly showing an increase in the rhombicity. These results confirm that |2D| ≅hν at X-band in this case. Additionally, it has been shown by X-ray crystal analysis of [Fe(DMSO)6](NO3)3 at 100 K that this is involved in a hydrogen bonding network, consisting of C–H⋯O interactions between the nitrate anions and the methyl groups of the coordinated DMSO molecules, thus suggesting that the differences found in the spectroscopic parameters D and E at different temperatures must be due to these supramolecular interactions.