Design and synthesis of new ligand saffolds and transition metal complexes for small molecule activation

Abstract

The objective of this thesis is to explore the synthesis of new ligand scaffolds designed to support late first row transition metals and to study their potential for small molecule activation. Many researchers have utilized sterically encumbered ligands to protect reactive metal centers. In the second chapter, the design and synthesis of novel cage ligands featuring a hydrophobic cavity to provide protection to reactive metal centers will be explored. Ideally, the cavity will effectively prevent bimolecular decomposition reactions that often plague small molecule activation. The synthesis and characterization of two cage ligands, a trianionic tri(amido)amine and a neutral tri(amino)amine variant, will be presented. Additionally, the preparation and characterization of the zinc(II) complex of the tri(amido)amine cage ligand will be discussed and its uptake of small molecules explored. In addition to employing protection of reactive metal centers in ligand design, a bio-inspired ligand design will be explored. The biological cofactor dihydronicotinamide adenine dinucleotide (NADH) can efficiently reduce and oxidize substrates through the transfer of a hydride (or a proton and two electrons). The third chapter explores the design and synthesis of a NADH-type ligand scaffold. The systems presented herein have three NADH-like moieties built into the ligand, designed to reduce substrates via either hydride transfer or proton coupled electron transfer. Having three NADH moieties allows for the possibility of multi-electron redox chemistry. The synthesis and characterization of zinc(II) and cobalt(II) complexes will be discussed. Additionally, the reaction of both the NADH ligand and the zinc(II) complex with known hydride acceptors will be explored. Finally, in chapter four, the synthesis and characterization of a family of diiron and iron cobalt bimetallic complexes with a third type of ligand design will be presented. Previous work in the Connie Lu group has shown reactivity toward small molecules using heterobimetallic complexes supported by a tripodal phosphine amide ligand scaffold. Herein, the same ligand scaffold is applied to late transition metals to explore their synthesis, reactivity toward small molecules, and electronic and magnetic properties to allow for a better understanding of metal-metal bonding interactions.