Insights into Dioxygen Bond Activation and Formation by Small Biomimetic Complexes

Abstract

Formation of an O–O bond is a crucial step in the biological oxidation of water to produce molecular O2. Its microscopic reverse, namely the activation of an O–O bond, is an essential step in key oxidative transformations during metabolism. Both of the reactions are employed by Nature to garner energy, synthesize, and degrade biomolecules in order to sustain life. A common feature of these two seemingly opposite, yet intimately interlinked, reactions in biological systems is that they are catalyzed by first-row transition metals such as manganese, iron, and copper. Both types of reactions are proposed to proceed through several reactive intermediates. Among them, metal-(hydro)peroxo and high-valent metal-oxo are common to both an O–O bond activation and formation step. Iron-containing metalloenzymes that utilize O2 as an oxidant perform a wide variety of biologically important chemical transformations. Cysteine-ligated iron metalloenzymes are of particular interest due to the unique properties that thiolates impart on transition metal ions. Thiolate (RS–) ligands form highly-covalent metal-ligand bonds, lower the activation energy barrier to O2 binding and one-electron reduction, facilitate an O–O bond activation. This dissertation describes a cis-thiolate ligated [FeII(SMeN4(tren))]+ complex that reacts with O2 to afford a series of metastable intermediates such as FeIII-superoxo, µ-peroxo FeIII dimer, FeIV-oxo, and FeIII-hydroxo at low temperature, (Chapters 2 – 3). Remarkably, addition of oxo-atom donors (OAD) to [FeII(SMeN4(tren))]+ affords an identical metastable µ-peroxo FeIII dimeric species, suggesting that a reversible O–O bond formation can occur, (Chapter 4). Chapter 5 describes the structure and properties of a O2-derived thiolate-ligated, µ-peroxo-bridged CoIII dimer and provides a comparison with structurally analogous µ-peroxo-bridged MnIII dimers. And lastly, Chapter 6 provides a detailed description of a successfully developed, versatile synthetic procedure to the Schiff-based precursors en route to heterobimetallic Robson-type target complexes. Moreover, a heterobimetallic NaMn3O4 cubane complex relevant to the oxygen-evolving complex (OEC) has been synthesized and crystallographically characterized.