X-ray spectroscopic studies of highly covalent iron centers : contributions to bonding and reactivity
Keywords: Chemistry.

The main focus of this thesis is to apply various x-ray spectroscopic techniques in conjunction with different computational methods to analyze the electronic structure of highly covalent Fe systems. The complexes of interest in this thesis are Fe-NO and Fe-oxygen systems, which have delocalized bonding and whose electronic structures have been controversial. The Appendix chapters provide an overview of x-ray spectroscopy and geometric studies using K-edge EXAFS and XANES analysis. Chapter 1 provides an introduction to the systems of interest and the techniques used. It gives an overview of Fe in biology, and Fe-NO and Fe-oxygen systems. It also provides an overview of x-ray spectroscopy and the techniques used in the studies presented in the thesis. Furthermore, there is an overview of the computational methods used. Chapter 2 utilizes iron L-edge X-ray absorption spectroscopy (XAS), interpreted using a valence bond conﬁguration interaction multiplet model, to directly experimentally probe the electronic structure of an S = 0 Fe-NO compound and the S = 0 Fe-CO reference compound. This method allows separation of the σ-donation and π-acceptor interactions of the ligand through ligand-to-metal and metal-to-ligand charge-transfer mixing pathways. The analysis shows that the Fe-NO electronic structure is best described as FeIII−NO(neutral), with no localized electron in an NO π* orbital or electron hole in an Fe dπ orbital. This delocalization comes from the large energy gap between the Fe−NO π-bonding and antibonding molecular orbitals relative to the exchange interactions between electrons in these orbitals. This chapter demonstrates the utility of L-edge XAS in experimentally deﬁning highly delocalized electronic structures. Chapter 3 reports the synthesis and structural and spectroscopic characterization of mononuclear nonheme Fe-NO and iron(III)-nitrito complexes bearing a tetraamido macrocyclic ligand (TAML). The spectroscopic data, such as 1H nuclear magnetic resonance and XAS, combined with computational study suggest the neutral nature of nitric oxide with a diamagnetic Fe center (S = 0). To the best of our knowledge, the chapter reports the first photochemical nitrite reduction in nonheme iron models. Chapter 4 utilizes K-edge XAS and 1s2p resonant inelastic x-ray scattering (RIXS) to study oxyhemoglobin and a related heme model compound, which was previously analyzed using L-edge XAS. The K-edge XAS and RIXS data of the model compound and oxyhemoglobin are compared with the data for low-spin FeII and FeIII heme reference compounds. The x-ray data show that the model compound is similar to FeII, while oxyhemoglobin is qualitatively similar to FeIII, with some quantitative differences. DFT calculations show that the difference between the model compound and oxyhemoglobin is due to distal histidine H-bond to oxygen and less hydrophobic environment in the protein, which lead to more backbonding into the oxygen. A valence bond configuration interaction multiplet model is used to analyze the RIXS data and show that the model is dominantly FeII with 6-8% FeIII character, while oxyhemoglobin has a very mixed wavefunction that is 50-77% FeIII character and a polarized Fe-oxygen π bond. Chapter A1 gives an overview of the application of metal K-edge and L-edge XAS, as well as Kα RIXS, to the study of electronic structure in transition metal sites with emphasis on experimentally quantifying 3d orbital covalency. The specific sensitivities of K-edge XAS, L-edge XAS, and RIXS are discussed emphasizing the complementary nature of the methods. L-edge XAS and RIXS are sensitive to mixing between 3d orbitals and ligand valence orbitals, and to the differential orbital covalency (DOC), that is, the difference in the covalencies for different symmetry sets of the d orbitals. The application of RIXS as a probe of frontier molecular orbitals in a heme enzyme demonstrates the potential of this method for the study of metal sites in highly covalent coordination sites in bioinorganic chemistry. Chapter A2 reports the first example of a mononuclear nonheme iron(V)-imido complex bearing a tetraamido macrocyclic ligand (TAML). The spectroscopic characterization revealed an S = 1/2 Fe(V) oxidation state, an Fe-N bond length of 1.65(4) Å, and an Fe-N vibration at 817 cm--1. The reactivity was demonstrated in C--H bond functionalization and nitrene transfer reactions. Chapter A3 uses nuclear resonance vibrational spectroscopy coupled to XAS, to probe the bonding interaction between the iron center, its zeolite lattice-derived ligands, and the reactive oxygen. α-O is found to contain an unusually strong Fe(IV)=O bond resulting from a constrained coordination geometry enforced by the zeolite lattice. Density functional theory calculations clarify how the experimentally determined geometric structure of the active site leads to an electronic structure that is highly activated to perform H-atom abstraction. Chapter A4 reports that a MnIV-bis(hydroxo) complex, which was fully characterized by various physicochemical methods such as UV-vis, ESI-MS, EPR, X-ray and XAS, shows the naphthalene oxidation in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the MnIV-bis(hydroxo) complex exhibits one electron reduction potential of 1.09 V, which is the most positive potential for the previously reported nonheme MnIV-bis(hydroxo) species as well as MnIV-oxo analogues. Kinetic studies including kinetic isotope effect suggest that the naphthalene oxidation by the MnIV-bis(hydroxo) complex in the acid-promoted reaction occurs via a rate-determining electron transfer process.