Formation, structure, and EPR detection of a high spin Fe^IV-Oxo Species Derived from Either an Fe^III-Oxo or Fe^III-OH Complex

High spin oxoiron(IV) complexes have been proposed to be a key intermediate in numerous nonheme metalloenzymes. The successful detection of similar complexes has been reported for only two synthetic systems. A new synthetic high spin oxoiron(IV) complex is now reported that can be prepared from a well-characterized oxoiron(III) species. This new oxoiron(IV) complex can also be prepared from a hydroxoiron(III) species via a proton-coupled electron transfer process-a first in synthetic chemistry. The oxoiron(IV) complex has been characterized with a variety of spectroscopic methods: FTIR studies showed a feature associated with the Fe-O bond at ν(Fe¹⁶O) = 798 cm ^-1 that shifted to 765 cm^-1 in the ¹⁸O complex; Mössbauer experiments show a signal with an Δ = 0.02 mm/s and |ΔE_Q| = 0.43 mm/s, electronic parameters consistent with an Fe(IV) center, and optical spectra had visible bands at λ_max = 440 (ε_M = 3100), 550 (ε_M = 1900), and 808 (ε_M = 280) nm. In addition, the oxoiron(IV) complex gave the first observable EPR features in the parallel-mode EPR spectrum with g-values at 8.19 and 4.06. A simulation for an S = 2 species with D = 4.0(5) cm ^-1, E/D = 0.03, σ_E/D = 0.014, and g_z = 2.04 generates a fit that accurately predicted the intensity, line shape, and position of the observed signals. These results showed that EPR spectroscopy can be a useful method for determining the properties of high spin oxoiron(IV) complexes. The oxoiron(IV) complex was crystallized at -35 °C, and its structure was determined by X-ray diffraction methods. The complex has a trigonal bipyramidal coordination geometry with the Fe-O unit positioned within a hydrogen bonding cavity. The Fe^IV-O unit bond length is 1.680(1) Å, which is the longest distance yet reported for a monomeric oxoiron(IV) complex.