UC Irvine

Abstract of the DissertationPreparation and Properties of Mono- and Bimetallic Paramagnetic Co Complexes Supported by Tripodal Ligand Frameworks by Meghen E. Goulet Doctor of Philosophy in Chemistry University of California, Irvine, 2022 Distinguished Professor Andrew S. Borovik, Chair Many important and synthetically challenging chemical reactions are carried out by metalloenzymes, including water oxidation, nitrogen fixation, O2 reduction, and oxidation of C–H bonds in substrates such as methane, fatty acids, and pharmaceutical products. Metalloenzymes are able to catalyze these reactions in part due to their ability to access reactive intermediates which is facilitated by the protein host. To replicate these effects synthetically, the Borovik lab designs organic ligands that incorporate both primary and secondary coordination sphere interactions to control the local environment within metal complexes. This dissertation presents research that employs these ligand frameworks in both mono- and bimetallic small molecule Co complexes. Chapter 1 provides a brief introduction to key features of metalloenzyme active sites that support their reactivity. A concise overview of relevant synthetic ligand designs is given, in addition to summaries of the following chapters. Chapter 2 describes the preparation of mononuclear CoII– and CoIII–OH complexes within an unsymmetric, hybrid tripodal ligand framework: [H2pout]3–. The characterization of these complexes is discussed, including the molecular structure of K[CoIIIH2pout(OH)]. xx K[CoIIIH2pout(OH)] is paramagnetic, an unusual trait for a CoIII center. The reactivity of K[CoIIIH2pout(OH)] towards substrates with an O–H bonds is also examined. This work is the first reactivity study of a CoIII–OH complex. Chapter 3 describes the preparation of a trigonal monopyramidal, four-coordinate Co complex supported by the C3 symmetric [poat]3– ligand framework. This complex, K[CoIIpoat] is used in Chapter 3 – 5 to prepare unsymmetric bimetallic species. Chapter 3 focuses specifically on the formation of a [poatCoIII–μ-OH–ZnIIMe3tacn]+ complex via a proposed CoII–oxidant adduct that reacts in the presence of [ZnIIMe3tacn(OTf)2], where Me3tacn is a heterocyclic N-based ligand. In Chapter 4, the preparation of a [poatCoII–μ-OH–FeIIIMe3tacn]+ complex from K[CoIIpoat] and O2 in the presence of [FeIIMe3tacn(OTf)2] is discussed. The [poatCoII–μ-OH–FeIIIMe3tacn]+ complex is subsequently oxidized to [poatCoIII–μ-OH–FeIIIMe3tacn]2+ and the changes in the magnetic coupling between the Co and Fe centers are examined. Mössbauer and EPR spectroscopies support antiferromagnetic coupling in [poatCoII–μ-OH–FeIIIMe3tacn]+, but ferromagnetic coupling in [poatCoIII–μ-OH–FeIIIMe3tacn]2+. Finally, Chapter 5 describes the development of a new series of bimetallic species, [Mx IIpoat⋅⋅⋅My IIMe3tacn]+, (Mx = Mn, Co, Zn; My = Mn, Fe, Co, Cu, Zn). The metal ion coordinated in the [poat]3– ligand backbone is in a trigonal monopyramidal geometry, which is shielded by the binding of a [My IIMe3tacn]2+ solely through the P=O groups. The two metal ions interact through three, triple-atom N–P=O bridging units and are highly symmetric, crystallizing in the R3 space group. The metal ions were found to couple ferromagnetically to one another through the N–P=O bridges and the magnetism of these complexes was studied to determine if the complexes exhibit single molecule magnetic behavior. While the [CoIIpoat⋅⋅⋅My IIMe3tacn]+ complexes were most thoroughly investigated, the [Mx IIpoat⋅⋅⋅My IIMe3tacn]+ have also been prepared where Mx = Mn, Zn and My = Mn, Zn, suggesting that these complexes could be expanded to a wide array of metal ion.