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Iron L‑Edge X‑ray Absorption Spectroscopy of Oxy-Picket Fence Porphyrin: Experimental Insight into Fe–O2 Bonding

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journal contribution
posted on 20.02.2016, 00:50 by Samuel A. Wilson, Thomas Kroll, Richard A. Decreau, Rosalie K. Hocking, Marcus Lundberg, Britt Hedman, Keith O. Hodgson, Edward I. Solomon
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 multideterminant 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‑MeIm)­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 [FeII(pfp)­(1‑MeIm)2], [FeII(pfp)], and [FeIII(tpp)­(ImH)2]Cl (tpp = meso-tetraphenylporphyrin) which have FeII S = 0, FeII S = 1, and FeIII 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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