Spin-crossover ferric complexes: Curiosities observed for unperturbed solids

Article abstract of DOI:10.1021/ic50215a028

The preparation and characterization of a series of spin-crossover ferric complexes with the composition [Fe(X-SalEen)₂] Y, where X-SalEen results from the Schiff-base condensation of X-substituted salicylaldehyde and N-ethylethylenediamine, are reported. Several of the complexes exhibit a gradual, but complete, spin-crossover transition from high spin to low spin in the solid state, whereas certain complexes exhibit an incomplete transition and there is a plateau in the μ_eff/Fe vs. temperature curve. One compound, [Fe(3-OCH₃-SalEen)₂]PF₆, experiences a sudden spin-crossover phase transition. The conversion from high spin to low spin occurs within a 2° range around the temperature 159 K. A sensitivity of physical properties to exact compound preparation is demonstrated. In spite of the variation in the series, it is shown by variable-temperature ⁵⁷Fe Mo?ssbauer and EPR techniques that only two electronic states, the high-spin ⁶A₁ excited state and one low-spin Kramers doublet ground state, are thermally populated for any of the compounds. There is no evidence of an intermediate-spin state. The ground-state Kramers doublet in these complexes has one unpaired electron in an orbital which is largely d_xy in composition.