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

Abstract

The spin transition of Fe2+ ions in the mononuclear compoundcis-bis(thiocyanato)-bis(N-2'-pyridylmethylene)-4-(aminobiphenyl)-iron(II) isstudied by electron paramagnetic resonance (EPR) of Mn2+. Incooling the compound down to the temperature range 174-168 K, the Fe2+ ions undergo a complete transition from the high spin (HS, S = 2) tothe low spin (LS, S = 0) state, occurring with a narrow, ~5 K andunusually sharp hysteresis loop. The temperature dependence of the unit cellparameters is almost linear on both sides of the spin transition; thevariation of the unit cell parameters at the spin transition is veryanisotropic. The EPR spectra, typical of the Mn2+ ion, onlygradually change with temperature in the two spin states of Fe2+but undergo a striking transformation in the spin transition range. This showsthat a considerable cooperativity exists between the metal ions. Computersimulations using a laboratory-developed simulation program indicatesignificant changes in the zero-field splitting (zfs) parameters in the courseof the spin transition. Lower-than-axial symmetry of the environment of Mn2+ persists in both spin states of Fe2+; however, a strongeraxial distortion arises in the HS state.The temperature variations of the zfs parameter D are related totransformations of the crystal structure using the Newman superposition modelamended for contributions of thermal expansion of the crystal lattice andlattice vibrations. Computer fits show a reduction of the modelparameter - power law exponent - in the LS state, t2 = 4, in comparison withthe HS state, t2 = 8. Such a tendency is consistent with the decrease of theFe-N bond lengths in the HS to LS transition.