The synthesis, crystal
structures, magnetic behavior, and electron
paramagnetic resonance studies of five new Fe<sup>III</sup> spin crossover
(SCO) complexes are reported. The [Fe<sup>III</sup>N<sub>5</sub>O]
coordination core is constituted of the pentadentate ligand bztpen
(N<sub>5</sub>) 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<sup>III</sup>N<sub>4</sub>O<sub>2</sub>] 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<sup>–1</sup> and Δ<i>S</i> = 40–50 J mol<sup>–1</sup> K<sup>–1</sup>, respectively. The estimated
values of the cooperativity parameter Γ, found in the interval
0–2.2 kJ mol<sup>–1</sup>, 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