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Synthetic Models for Metalloenzyme Active Sites: Accessing High-Valent Bimetallic Complexes with [M–(μ-O)–M’] Cores

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Abstract

Metal ions are ubiquitous in biology and aid in facilitating many enzymatic transformations. For instance, high–valent iron–oxido and –hydroxido species are proposed to be key intermediates in oxidative processes catalyzed by non-heme iron proteins. A subclass of these proteins contains bimetallic sites that house either FeFe or FeM cores. To investigate the properties of both mono- and bimetallic cores, the Borovik Lab has developed a symmetric tripodal ligand framework containing phosphinic amido groups ([poat]3–), which can stabilize high-valent metal centers, provide an auxiliary binding-site for a second metal ion, and contains a pendant arm that can provide hydrogen-bond stabilization as well as facilitate proton shuttling. The work presented in this dissertation describes my efforts to access high-valent homo-/heterobimetallic oxido and hydroxido species by a variety of synthetic routes.

Chapter 1 provides an overview of bimetallic proteins, and efforts by chemists to synthetically model the structure and function of these active sites. Previous work performed in the Borovik Lab to approach these synthetic challenges is discussed.

Chapter 2 explores the oxidation chemistry of the mononuclear complexes FeII– and FeIII–hydroxido complexes in the [poat]3– framework, which serve as important synthons in the stepwise assembly of subsequent bimetallic complexes. While previous members in the Borovik Lab have studied these complexes, the electrochemical properties of these compounds are reassessed in my effort to access high-valent Fe–hydroxido species.

Chapters 3 and 4 introduce the synthesis and characterization of discrete heterobimetallic complexes, with an MII–OH–FeIII or MII–O–FeIV core (M = Zn, Cu, Ni). This stepwise assembly approach allows for the formation of dinuclear compounds in an unsymmetric framework. The Fe center is supported by the [poat]3– ligand, while MII is supported by an N-based macrocycle. These species are compared with bimetallic complexes in similar frameworks.

Chapters 5 describes the preparation of a series of di-Fe compounds with an Fe–O–Fe or Fe–OH–Fe core. These complexes are housed within the same unsymmetric scaffold, and span across four oxidation levels, from FeIIFeII to FeIIIFeIV. This study illustrates the multifunctional nature of the [poat]3- framework, and provides useful insight into the thermodynamic parameters of proton and electron transfers in bimetallic systems. These findings have significant implications for di-Fe containing enzymes, such as ribonucleotide reductase and soluble methane monooxygenase.

Using the synthetic strategies developed in the previous chapters, Chapters 6 and 7 describe the preparation and characterization of a library of FeMn and MnFe complexes to model their biological counterparts. Their structural, electrochemical, magnetic, and reactivity properties are discussed. An FeIII–O–MnIV species was trapped and studied spectroscopically, and its relevance to similar high valent FeMn active sites is biology is discussed.

Chapter 8 discusses the observation of a rare high-spin CoIII center in a bimetallic system via different spectroscopic techniques. This is an unusual finding, as most six-coordinated CoIII complexes adopt a low-spin configuration due to significant ligand field stabilization. A series of CoFe and CoGa complexes were prepared to probe the factors for this phenomenon, including the P=O groups in [poat]3- being weak field ligands, and modulation of spin exchange interactions.

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This item is under embargo until December 10, 2023.