Wilson, Samuel A., Kroll, Thomas, Decreau, Richard A., Hocking, Rosalie K., Lundberg, Marcus, Hedman, Britt, Hodgson, Keith O., and Solomon, Edward I. (2013) Iron L-Edge X-ray absorption spectroscopy of oxy-picket fence porphyrin: experimental insight into Fe-O2 bonding. Journal of the American Chemical Society, 135 (3). pp. 1124-1136.

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Abstract

The electronic structure of the Fe-O₂ center in oxy-hemoglobin and oxy-myoglobin is a long-standing issue in the field of bioinorganic chemistry. Spectroscopic studies have been complicated by the highly delocalized nature of the porphyrin, and calculations require interpretation of multi-determinant wave functions for a highly covalent metal site. Here, iron L-edge X-ray absorption spectroscopy, interpreted using a valence bond configuration interaction multiplet model, is applied to directly probe the electronic structure of the iron in the biomimetic Fe-O₂ heme complex [Fe(pfp)-(1-Melm)O₂] (pfp ("picket fence porphyrin") = meso-tetra(α,α,α,α-o-pivalamidophenyl)porphyrin or TpivPP). This method allows separate estimates of σ-donor, π-donor, and π-acceptor interactions through ligand-to-metal charge transfer and metal-to-ligand charge transfer mixing pathways. The L-edge spectrum of [Fe(pfp)(1-MeIm)O₂] is further compared to those of [Fe-II(pfp)(1-MeIm)₂], [Fe-II(pfp)], and [Fe-III(tpp)(ImH)₂]Cl (tpp = meso-tetraphenylporphyrin) which have Fe-II S = 0, Fe-II S = 1, and Fe-III S = 1/2 ground states, respectively. These serve as references for the three possible contributions to the ground state of oxy-pfp. The Fe-O₂ pfp site is experimentally determined to have both significant σ-donation and a strong π-interaction of the O₂ with the iron, with the latter having implications with respect to the spin polarization of the ground state.

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