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 FeIII spin crossover
(SCO) complexes are reported. The [FeIIIN5O]
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 [FeIIIN4O2] 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.