This undergraduate thesis was submitted to the Faculty of the Department of Chemistry in Partial Fulfillment of the Requirements for Departmental Honors in ChemistryThis thesis details a two-part investigation into the [Cp*Rh(bpy)] framework (where Cp* = pentamethylcyclopentadienyl and bpy = 2,2'-bipyridyl), a known platform for electrocatalytically generating hydrogen. Chapter 1 of this thesis describes an in-depth investigation into the characteristics of a [Cp*Rh] complex (1) bearing the 4,4´-dinitro-2,2´-bipyridyl (4,4´-NO2bpy) ligand and multiple reduced forms of 1. Isolated 1 was characterized by several forms of spectroscopy including nuclear magnetic resonance, electronic absorption, and mass spectrometry. Moreover, single crystals suitable for X-ray diffraction analysis were grown, and the structure of 1 solved with the help of Dr. Victor W. Day. Electrochemical studies reveal that 1 is the first example of a monometallic [Cp*Rh] complex that exhibits three quasi-reversible one-electron reductions and a fourth irreversible reduction. In these studies, a rather large spacing between the redox events (~ 0.5 V) suggested the possibility of isolating one or more of the reduced species. In accord with this theory, the singly reduced species (2) could be chemically prepared and isolated. UV-visible absorption spectra display new peaks that correspond to the readily observed color change from red/orange to green upon reduction of 1 to 2. X-band electron paramagnetic resonance spectroscopy confirms the paramagnetic nature of 2, and reveals a narrow signal at g = 2.006, consistent with the majority of the unpaired electron density being localized on the 4,4´-NO2bpy ligand. Cyclic voltammetry and spectroelectrochemistry further confirm that 2 is produced by both electrochemical and chemical reduction of 1, and that the second reduction of 1 is primarily metal-centered. Electrocatalytic studies reveal that the extremely electron-withdrawing nature of the nitro substituents effectively eliminates catalytic function, providing insight into the features that govern catalysis in [Cp*Rh] complexes.
Chapter 2 describes investigations of a second less-studied aspect of the [Cp*Rh(bpy)] framework: namely, the role of less symmetric substitution of the bipyridyl ligand in modulating reduction potentials and catalytic competence. In this work, a new divergent synthetic route was developed, in which known synthetic steps were strategically assembled to provide straightforward access to a small family of [Cp*Rh] complexes bearing a single substituent at the 4-position of the bpy ligand. The method thus developed enabled preparation of three new C1-symmetric complexes with different substitutions at the 4 position of one of the pyridine rings of the bpy ligand: –NO2 (3), –Cl (4), or –NH2 (6). NMR spectroscopic characterization supports successful formation of the new diamagnetic compounds. Hammett analysis reveals a dependence of intraligand charge transfer (ILCT) energy and metal-to-ligand or ligand-to-metal charge transfer (MLCT or LMCT) energy on the bpy ligand substituents, as reported by use of the σp+ parameter. Electrochemical studies also confirm a Hammett parameter dependence of the reduction potentials of the new compounds, confirming an important role for singly substituted bipyridyl-type ligands in influencing the electrochemical behavior of [Cp*Rh(bpy)]-type complexes. Specifically, the more electron-donating substituents (as judged via the σp– parameter) are associated with complexes displaying more negative reduction potentials. Building on the findings from Chapter 1, cyclic voltammograms collected with complex 3, bearing the 4-nitro-2,2´-bipyridyl ligand, reveal slow chloride ligand loss upon one-electron reduction, implicating significant stabilization of the singly-reduced form by the single electron-withdrawing nitro group. As the chemical and electrochemical properties of these complexes are readily modulated by substituent effects, the strategy of ligand modification can thus be realistically anticipated to afford fine-grained control of electrocatalysis in future studies
