Octahedral (<i>cis</i>-Cyclam)iron(III) Complexes with <i>O</i>,<i>N</i>-Coordinated <i>o</i>-Iminosemiquinonate(1−) π Radicals and <i>o</i>-Imidophenolate(2−) Anions

Three octahedral complexes containing a (cis-cyclam)iron(III) moiety and an O,N-coordinated o-iminobenzosemiquinonate π radical anion have been synthesized and characterized by X-ray crystallography at 100 K: [Fe(cis-cyclam)(L1-3ISQ)](PF6)2 (1−3), where (L1-3ISQ) represents the monoanionic π radicals derived from one-electron oxidations of the respective dianion of o-imidophenolate(2−), L1, 2-imido-4,6-di-tert-butylphenolate(2−), L2, and N-phenyl-2-imido-4,6-di-tert-butylphenolate(2−), L3. Compounds 1−3 possess an St = 0 ground state, which is attained via strong intramolecular antiferromagnetic exchange coupling between a low-spin central ferric ion (SFe = 1/2) and an o-imino-benzosemiquinonate(1−) π radical (Srad = 1/2). Zero-field Mössbauer spectra of 1−3 at 80 K confirm the low-spin ferric electron configuration: isomer shift δ = 0.26 mm s-1 and quadrupole splitting ΔEQ = 1.96 mm s-1 for 1, 0.28 and 1.93 for 2, and 0.33 and 1.88 for 3. All three complexes undergo a reversible, one-electron reduction of the coordinated o-imino-benzosemiquinonate ligand, yielding an [FeIII(cis-cyclam)(L1-3IP)]+ monocation. The monocations of 1 and 2 display very similar rhombic signals in the X-band EPR spectra (g = 2.15, 2.12, and 1.97), indicative of low-spin ferric species. In contast, the monocation of 3 contains a high-spin ferric center (SFe = 5/2) as is deduced from its Mössbauer and EPR spectra.