Formation, Structure, and EPR Detection of a High Spin FeIV—Oxo Species Derived from Either an FeIII—Oxo or FeIII—OH Complex

Abstract

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 nu(Fe(16)O) = 798 cm(-1) that shifted to 765 cm(-1) in the (18)O complex; Mossbauer experiments show a signal with an delta = 0.02 mm/s and |DeltaE(Q)| = 0.43 mm/s, electronic parameters consistent with an Fe(IV) center, and optical spectra had visible bands at lambda(max) = 440 (epsilon(M) = 3100), 550 (epsilon(M) = 1900), and 808 (epsilon(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, sigma(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 degrees 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) A, which is the longest distance yet reported for a monomeric oxoiron(IV) complex.