STRATEGIES IN LIGAND DESIGN FOR CARBON DIOXIDE REDUCTION CATALYSTS

There is a growing concern over emission of carbon dioxide into the atmosphere and its implication in global climate change. Natural sinks for carbon dioxide, such as oceans and vegetative growth, cannot compete with anthropogenic source emissions. Efforts to counteract carbon dioxide emission involve photochemical and electrochemical reduction of carbon dioxide into value increased feedstock chemicals. The high bond strength of carbon-oxygen bonds and energy needed for structural rearrangement from a linear to bent conformation after reduction result in high overpotentials, thus making the process inefficient. Transition metal catalysts and proximal protons sources have been shown to stabilize the bent carbon dioxide intermediates in carbon dioxide reduction and facilitate the formation of reductive products at energy inputs closer to thermodynamic values. The development of earth-abundant transition metal catalysts which can function efficiently and robustly at an economically feasible level using energy supplied from renewable sources including solar light remains a challenge in the field of renewable energy. The first section involves the design and synthesis of macrocyclic cobalt complexes of carbon-functionalized cyclams which include linkers of various length and rigidity, as well as different anchoring groups, to allow attachment on semiconductor surfaces for photochemical carbon dioxide reduction. Two methods were utilized to form the carbon-functionalized cyclam precursor; one involved synthesis of functionalized bis-electrophiles and the second method involved synthesis of carbon-functionalized tetraamines. Photocatalysis was performed by surface attachment of the ligands/complexes on titanium dioxide; pre-coordination of cobalt was found to be essential for carbon dioxide reduction activity. The second section involves design and synthesis of derivatives of the macrocycle 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene, or HMD. Two methods were used to form HMD derivatives; 1,2-diamines were condensed with ketones or α,β-unsaturated carbonyl compounds. Each route formed tetrasubstituted HMD of one moiety or two moieties, respectively. Coordination of cobalt was difficult and unsuccessful in some cases. Further work is needed to form macrocycles with bulky substitution. The third section has two parts, and involves electropolymerization of alkene modified bipyridyl rhenium complexes on electrodes, and the design and synthesis of ferrocene functionalized with bipyridyl ligands for manganese and rhenium complexes. Two methods were used to electropolymerize rhenium catalysts on gold and fluorine-doped tin oxide electrode surfaces: multi-cycle cyclic voltammetry, and chronoamperometry. A vinyl-bipyridyl rhenium catalyst was successfully electropolymerized, while a dodecene-bipyridyl rhenium catalyst was unreactive. In the second part, ferrocene was functionalized on either one or two rings with bipyridine and used as ligands for manganese and rhenium complexes. Electrochemical studies of these complexes have demonstrated interesting activity in electrocatalytic carbon dioxide reduction. Infrared and optical characterization of the complexes suggest successful formation of the stated complexes