Octahedral (cis-cyclam)iron(III) complexes with O,N-coordinated o-iminosemiquinonate(l-) π radicals and o-imidophenolate(2-) anions

Three octahedral complexes containing a (cis-cyclam)iron(III) moiety and an QN-coordinated o-iminobenzosemiquinonate pi radical anion have been synthesized and characterized by X-ray crystallography at 100 K: [Fe(ciscyclam)(L-1-3(ISQ))](PF6)(2) (1-3), where (L-1-3(ISQ)) represents the monoanionic pi radicals derived from one-electron oxidations of the respective dianion of o-imidophenolate(2-), L-1, 2-imido-4,6-di-tert-butylphenolate(2-), L-2, and N-phenyl-2-imido-4,6-di-tert-butylphenolate(2-), L-3. Compounds 1-3 possess an S-1 = 0 ground state, which is attained via strong intramolecular antiferromagnetic exchange coupling between a low-spin central ferric ion (SF, = 1/2) and an o-imino-benzosemiquinonate(1-) pi radical (S-rad = 1/2). Zero-field Mossbauer spectra of 1-3 at 80 K confirm the low-spin ferric electron configuration: isomer shift delta = 0.26 mm s(-1) and quadrupole splitting DeltaE(Q) = 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 [ Fe-III(cis-cyclam) (L-1-3(IP))](+) 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 (S-Fe = 5/2) as is deduced from its Mossbauer and EPR spectra.