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

The electronic structure of the Fe-O-2 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-O-2 heme complex [Fe(pfp)-(1-Melm)O-2] (pfp (picket fence porphyrin) = meso-tetra(alpha,alpha,alpha,alpha-o-pivalamidophenyl)porphyrin or TpivPP). This method allows separate estimates of sigma-donor, pi-donor, and pi-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)O-2] 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-O-2 pfp site is experimentally determined to have both significant sigma-donation and a strong pi-interaction of the O-2 with the iron, with the latter having implications with respect to the spin polarization of the ground state.