Low-Temperature EPR Studies of Highly Anisotropic Low-Spin (Protoporphyrinato)iron(III) Complexes

Article abstract of DOI:10.1021/ja00405a015

Low-spin complexes of ferric protoporphyrin IX showing highly anisotropic EPR spectra (HALS) have been investigated by systematically changing the axial bases.From EPR measurements at low temperatures, complete sets of principal g values were obtained for a series of these complexes.The cubic field strength and tetragonal splitting in these HALS complexes appear to be smaller than those of ordinary low-spin complexes (LS).Interestingly, the tetragonal splitting of HALS complexes decreases with an increase of the axial field strength, in contrast to that of LS complexes where the tetragonal splitting increases with an increase of the axial field strength.From an analysis of the ligand field parameters obtained from the HALS complexes it has been suggested that the ferric ions in the HALS complexes may be a little displaced from the porphyrin plane and also that the HALS state may be close to the spin-crossover point between the intermediate-spin and the high-spin states.