Tuning of spin transition in radical-containing iron(III) complexes by remote ligand substituents

Two new iron(lll) complexes, Fe-lll(L-F center dot)(3) (1) and Fe-lll(Lt-Bu center dot)(3) (2), of remote substituted o-aminophenol-based ligands are reported; complexes 1 and 2 contain three O,N-coordinated o-iminobenzosemiquinonate(l-) 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 Mossbauer (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 (S-R = 1/2) and the high-spin Fe(lll) center (S-Fe, = 5/2) yielding an S-t = 1 as the ground state. In contrast, the occurrence of a thermally induced spin crossover process (S-Fe = 5/2 <-> S-Fe = 1/2) is observed for complex 2 Fe-lll(Lt-Bu center dot)(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.