High-valent iron
species have been implicated as key intermediates
in catalytic oxidation reactions, both in biological and synthetic
systems. Many heteroleptic Fe(IV) complexes have now been prepared
and characterized, especially using strongly π-donating oxo,
imido, or nitrido ligands. On the other hand, homoleptic examples
are scarce. Herein, we investigate the redox chemistry of iron complexes
of the dianonic tris-skatylmethylphosphonium (TSMP2–) scorpionate ligand. One-electron oxidation of the tetrahedral,
bis-ligated [(TSMP)2FeII]2–leads to the octahedral [(TSMP)2FeIII]−. The latter undergoes thermal spin-cross-over both
in the solid state and solution, which we characterize using superconducting
quantum inference device (SQUID), Evans method, and paramagnetic nuclear
magnetic resonance spectroscopy. Furthermore, [(TSMP)2FeIII]−can be reversibly oxidized to the stable
high-valent [(TSMP)2FeIV]0complex.
We use a variety of electrochemical, spectroscopic, and computational
techniques as well as SQUID magnetometry to establish a triplet (S= 1) ground state with a metal-centered oxidation and
little spin delocalization on the ligand. The complex also has a fairly
isotropicg-tensor (giso= 1.97) combined with a positive zero-field splitting (ZFS) parameterD(+19.1 cm–1) and very low rhombicity,
in agreement with quantum chemical calculations. This thorough spectroscopic
characterization contributes to a general understanding of octahedral
Fe(IV) complexes.