Studies of Electrocatalytic Carbon Dioxide Reduction by Biomimetic Bipyridyl-based Rhenium Tricarbonyl Complexes: Introducing Secondary Structure to Molecular Catalysts
The use of CO2 as a chemical feedstock has been the focus of much research in recent years due to the promise for carbon neutral fuel storage in chemical bonds. Specifically, complexes of the type Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine and analogues thereof) have been studied for their ability to electrocatalytically reduce CO2 to CO. These catalysts are among the most active, selective, and robust homogeneous catalysts for CO2 reduction in the literature.
Previous work has focused on mechanistic studies and determining the inductive effect of bipyridine functional groups on catalysis. The work presented in this dissertation focuses on the structural and biomimetic modification of these catalysts.
In order to determine the optimal point of modification, the CO2 reduction capabilities of a series of Re(n,n′–dimethyl–bpy)(CO)3Cl (n = 3, 4, and 5) catalysts with a bpy modified at the 3, 4, and 5 positions with methyl substituents were assessed. A decreased catalytic current response in the n = 3 case can be explained by steric hindrance in the 3,3′- substituted catalyst disfavoring optimal charge transfer in the catalytic cycle.
A series of rhenium catalysts were synthesized with bpy substituents amenable to common surface¬- and bio-conjugation techniques. Specifically, aminomethyl, groups were found to contribute to competent catalysts and were interrogated further. Interestingly, before the complex was coupled to amino acids, simple acylated amines (mimicking peptide bonds) on the 4- and 4,4′- positions of the bpy ligand were found to alter the mechanism by the rhenium catalyst operated electrochemically. Instead of a single metal site catalyzing the proton-dependent reduction of CO2 to CO and H2O, the bimetallic site, templated by hydrogen-bonding of the peptide bonds, reduces CO2 to CO and CO32- at potentials up to 240 mV more positive than previously studied catalysts of this type. The catalysts were then incorporated into amino acids and short peptides to investigate the advantageous effects of adding proton sources (tyrosine) and readily modifiable platform (peptides) on catalysis.
In addition to proton sources, Lewis-acids can serve to increase the rates of catalysis; however insoluble carbonates prevent the reaction from being catalytic with respect to the Lewis acid. Macromolecules were used to bind the metal Lewis acids to prevent metal-carbonate formation over the course of the reaction. Furthermore, hydrogen-bonding could be utilized to template these heterobimetallic interactions and the preliminary work is presented.