Spin transition in [Fe(PM-BiA)₂(NCS)₂] studied by the electron paramagnetic resonance of the Mn²⁺ ion

The spin transition of Fe²⁺ ions in the mononuclear compound cis-bis(thiocyanato)-bis(N-2'-pyridylmethylene)-4-(aminobiphenyl)-iron(II) is studied by electron paramagnetic resonance (EPR) of Mn²⁺. In cooling the compound down to the temperature range 174-168 K, the Fe²⁺ ions undergo a complete transition from the high spin (HS, S = 2) to the low spin (LS, S = 0) state, occurring with a narrow, ~5 K and unusually sharp hysteresis loop. The temperature dependence of the unit cell parameters is almost linear on both sides of the spin transition; the variation of the unit cell parameters at the spin transition is very anisotropic. The EPR spectra, typical of the Mn²⁺ ion, only gradually change with temperature in the two spin states of Fe²⁺ but undergo a striking transformation in the spin transition range. This shows that a considerable cooperativity exists between the metal ions. Computer simulations using a laboratory-developed simulation program indicate significant changes in the zero-field splitting (zfs) parameters in the course of the spin transition. Lower-than-axial symmetry of the environment of Mn²⁺ persists in both spin states of Fe²⁺; however, a stronger axial distortion arises in the HS state.

The temperature variations of the zfs parameter D are related to transformations of the crystal structure using the Newman superposition model amended for contributions of thermal expansion of the crystal lattice and lattice vibrations. Computer fits show a reduction of the model parameter - power law exponent - in the LS state, t₂ = 4, in comparison with the HS state, t₂ = 8. Such a tendency is consistent with the decrease of the Fe-N bond lengths in the HS to LS transition.