Development of New Zwitterionic Cyclopentadienyl Ligands and Their Sandwich Complexes

Abstract

Since the discovery of ferrocene, the development of various sandwich complexes and cyclopentadienyl (Cp) ligands has long attracted attention in organometallic chemistry due to their valuable applications in fields such as organocatalysis, pharmaceutical, novel materials for energy and data storage. N-heterocyclic carbenes (NHCs) are one of the most significant ligands in organometallic chemistry, along with Cp ligands. They have shown significant interactions with transition metals and main group elements through their strong σ-donating and π-accepting abilities and have been utilized in a variety of fields. In this thesis, I describe the development of new Cp ligand and their sandwich complexes by incorporating NHCs with unique electronic and steric properties. In Chapter I, the background on Cp and NHC ligands essential for designing new zwitterionic Cp (ZCp) ligands is introduced. In Chapter II, I present two novel ZCp ligands: imidazolium- and pyrrolinium-substituted zwitterionic Cps (IZCp and PZCp), whose key utility is redox non-innocence – ability to participate cooperatively with the metal center in redox reactions. Simple metalation of ZCps yielded Cr(0) and Mo(0) half-sandwich complexes (ZCp)M(CO)3, as well as Ru(II) sandwich complexes [(ZCp)RuCp]PF6. These complexes were fully characterized and showed a reversible one-electron reduction by cyclic voltammetry, which is typically known to be highly unstable. Furthermore, through chemical reduction, EPR studies, and DFT calculations, it has been demonstrated that redox non-innocent IZCp and PZCp ligands could stabilize these low- valent sandwich complexes by acting as electron reservoirs. In Chapter III, the application of our ZCp system to various scopes of sandwich complexes is explored. First, HAT catalytic reactions using (PZCp)Cr(CO)3 and synthesis of new Dy(III) sandwich complexes for single-molecule magnets were attempted. Second, the synthetic approaches for stabilized rhodocene and iridocene derivatives with group 9 heavy transition metals (Rh and Ir) were investigated. Metalation of IZCp successfully yielded Rh(I) and Ir(I) sandwich complexes [(IZCp)M(COD)]BF4; however, these complexes showed greater ligand lability in IZCp compared to COD in reactions with the Cp anion. Lastly, (IZCp)MCp metallocenes were applied to mesoionic carbenes (MICs). The metallocene-substituted MIC Au complexes were synthesized and fully characterized, suggesting their potential as redox- active Au self-assembled monolayers and nanoparticles.