Tuning of Spin Transition in Radical-Containing Iron(III) Complexes by Remote Ligand Substituents

Two new iron(III) complexes, FeIII(LF•)3 (1) and FeIII(Lt-Bu•)3 (2), of remote substituted o-aminophenol-based ligands
are reported; complexes 1 and 2 contain three O,N-coordinated o-iminobenzosemiquinonate(1−) radical anions
with ferric centers in high-spin and low-spin configurations. The crystal structures of 1 and 2 were determined by
X-ray diffraction at 100 and 293 K, and the electronic structures were established by various physical methods
including Mössbauer (4−290 K) and variable-temperature (2−290 K) susceptibility measurements. Electrochemical
measurements (cyclic and square-wave voltammetry) indicate primarily ligand-centered redox processes. Complex
1, with the more electron-withdrawing fluoro substituents, retains the high-spin character of the ferric ion throughout
the temperature range studied (2−290 K) and exhibits, as expected, strong antiferromagnetic coupling operating
between three radicals (SR = 1/2) and the high-spin Fe(III) center (SFe = 5/2) yielding an St = 1 as the ground
state. In contrast, the occurrence of a thermally induced spin crossover process (SFe = 5/2 ↔ SFe = 1/2) is
observed for complex 2 FeIII(Lt-Bu•)3, in which more electron donating tert-butyl substituents in the ligand are present.
A rationale for the control of the electronic state of ferric ions in 2 together with spin-coupling schemes for 1 and
2 are provided.