title: Tuning of spin transition in radical-containing iron(III) complexes by remote ligand substituents.
abstract: Two new iron(III) complexes, Fe(III)(LF*)3 (1) and FeIII(L(t-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(L(t-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.
authors.affiliation: Max-Planck-Institute for Bioinorganic Chemistry, Mülheim an der Ruhr, Germany.
authors.forename: Soumen
authors.initials: S
authors.lastname: Mukherjee
authors.forename: Thomas
authors.initials: T
authors.lastname: Weyhermüller
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Karl
authors.initials: K
authors.lastname: Wieghardt
authors.forename: Phalguni
authors.initials: P
authors.lastname: Chaudhuri