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 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.
Lacy
David C
DC
Department of Chemistry, University of California-Irvine, 1102 Natural Sciences II, Irvine, California 92697-2025, USA.
Gupta
Rupal
R
Stone
Kari L
KL
Greaves
John
J
Ziller
Joseph W
JW
Hendrich
Michael P
MP
Borovik
A S
AS
D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D015394:Molecular Structure; D010084:Oxidation-Reduction
D005290:Ferric Compounds; C066317:ferryl iron; D007501:Iron