Spin Crossover Behavior in a Series of Iron(III) Alkoxide Complexes

The synthesis, crystal structures, magnetic behavior, and electron paramagnetic resonance studies of five new Fe^III spin crossover (SCO) complexes are reported. The [Fe^IIIN₅O] coordination core is constituted of the pentadentate ligand bztpen (N₅) and a series of alkoxide anions (ethoxide, propoxide, <i>n</i>-butoxide, isobutoxide, and ethylene glycoxide). The methoxide derivative previously reported by us is also reinvestigated. The six complexes crystallize in the orthorhombic <i>Pbca</i> space group and show similar molecular structures and crystal packing. The coordination octahedron is strongly distorted in both the high- and low-temperature structures. The structural changes upon spin conversion are consistent with those previously observed for [Fe^IIIN₄O₂] SCO complexes of the Schiff base type, except for the Fe–O­(alkoxide) bond distance, which shortens significantly in the high-spin state. Application of the Slichter–Drickamer thermodynamic model to the experimental SCO curves afforded reasonably good simulations with typical enthalpy and entropy variations ranging in the intervals Δ<i>H</i> = 6–13 kJ mol^–1 and Δ<i>S</i> = 40–50 J mol^–1 K^–1, respectively. The estimated values of the cooperativity parameter Γ, found in the interval 0–2.2 kJ mol^–1, were consistent with the nature of the SCO. Electron paramagnetic resonance spectroscopy confirmed the transformation between the high-spin and low-spin states, characterized by signals at <i>g</i> ≈ 4.47 and 2.10, respectively. Electrochemical analysis demonstrated the instability of the Fe­(II) alkoxide derivatives in solution