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, n-butoxide, isobutoxide, and ethylene glycoxide). The methoxide derivative previously reported by us is also reinvestigated. The six complexes crystallize in the orthorhombic Pbca 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 ΔH = 6–13 kJ mol^–1 and ΔS = 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 g ≈ 4.47 and 2.10, respectively. Electrochemical analysis demonstrated the instability of the Fe­(II) alkoxide derivatives in solution.