The End of the Rhodium: Synthesis, Characterization, and Redox Chemistry of Manganese and Rhodium Complexes Supported by Unorthodox Diimine Ligands

Abstract

Economical storage of the large quantities of energy that could be generated by intermittent renewable sources, such as solar and wind, remains one of the largest unmet technical challenges hindering the widespread adoption of such technologies. Storing renewable energy in chemical bonds by means of molecular electrocatalysis represents a promising potential solution to this pressing issue. The 2,2′-bipyridyl (bpy) moiety is ubiquitous among the current generation of fuel production molecular electrocatalysts, yet a fundamental understanding of the properties which make it exceptionally attractive for enabling redox catalysis is lacking. In this thesis, model systems supported by unconventional analogues of bpy were investigated in order to elucidate the influence of steric and electronic effects on catalyst candidates supported by bpy-type ligands. In Chapter 2, the synthesis and characterization of manganese and ruthenium complexes supported by 4,5-diazafluorene and 9,9′-dimethyl-4,5-diazafluorene are discussed. These ligands were found to enforce wider bite angles and act as weaker sigma-donors in the complexes than bpy in analogous species. In Chapter 3, our work focusing on half-sandwich rhodium complexes supported by non-symmetrically mono-substituted bpy ligands bearing either a chloro or nitro substituent is detailed. Non-symmetric mono-substitution of the bpy ligand system was found to be an effective strategy for engendering new redox behavior at the metal center. Furthermore, the redox properties of the complexes supported by the mono-substituted bpy ligands were found to be intermediate between those of the parent complex supported by bpy and the symmetrically di-substituted complex supported by 4,4′-dinitro-2,2′-bipyridyl. Taken together, the findings reported in this thesis highlight the power of seemingly subtle ligand modifications in facilitating new redox chemistry with transition metal complexes.