Iron L-Edge X-ray Absorption Spectroscopy of Oxy-Picket Fence Porphyrin: Experimental Insight into Fe–O2 Bonding
2013
Wilson, Samuel A. | Kroll, Thomas | Decreau, Richard A. | Hocking, Rosalie K. | Lundberg, Marcus | Hedman, Britt | Hodgson, Keith O. | Solomon, Edward I.
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 FeᴵᴵS = 0, FeᴵᴵS = 1, and FeᴵᴵᴵS = 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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