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Date of Award
Cláudio N. Verani
Light-driven water splitting for hydrogen generation represents a compelling pathway for carbon-neutral energy conversion. We have paid considerable attention to the catalysts that are Earth-abundant and water-soluble, to make the water splitting process economically viable. In this dissertation, we have studied polypyridine metal complexes of nickel and iron for photo/electrocatalytic water reduction. In addition, we have paid strong attention to the mechanisms of hydrogen generation by these complexes in order to design affordable, efficient, and robust water-splitting catalysts in the future. We studied three nickel complexes with increasing electron donating functionalities for photo/electrocatalytic water reduction in order to understand the NiIII-H- formation step during catalysis. After a thorough investigation of electronic, redox, and catalytic properties of these three nickel complexes, we found that these complexes do not form a NiIII-H- intermediate species during hydrogen generation as mentioned by most of the published work on water splitting by nickel complexes. Rather, we observed that these complexes are not stable for long catalytic cycles under the tested conditions for water reduction.
To improve the stability, we designed another nickel complex by changing the denticity of the polypyridine ligand from tetradentate to pentadentate. The newly synthesized nickel [NiIILN2PY3]2+ complex was catalytic and showed better stability during the electro/photocatalytic water reduction, compared to the first three nickel complexes. Mechanistic studies on this complex presented further evidence to disregard the NiIII-H- formation during catalytic cycle. Furthermore, the mechanism of this nickel complex revealed that instead of forming NiIII-H-, the complex forms a NiII-H-. In this mechanism not only the metal, but also the ligand is redox involved. The ligand involvement during catalysis has a deleterious effect on the catalyst, which deactivates the catalyst during the hydrogen evolution reaction.
From recent studies in our group, we have observed that 3d metals exhibit different mechanisms of hydride formation when bound to the same polypyridine ligand environment. To compare the hydrogen evolution mechanism of iron in polypyridine ligand environment with respect to cobalt, nickel, and copper analogue complexes, we synthesized an iron polypyridine complex [FeIILN2PY3]2+. The iron polypyridine complex was catalytic towards hydrogen generation from water. The mechanistic studies on the iron complex revealed that the electrocatalytic mechanism of hydrogen evolution does not involve any reduction of the iron center as in cobalt, nickel, and copper analogues. Instead, the ligand gets reduced. The iron center only gets oxidized. Intermediate species of the catalytic cycle involve [FeIIL•] intermediate species. This is the first time that experimental data has been gathered to support the formation of [FeIIL•] radical species in iron catalysts for water reduction.
The work presented in this dissertation foregrounds ligand redox involvement during the hydrogen evolution reaction by nickel and iron polypyridine complexes. After considering all the results, I propose to modify polypyridine ligands by introducing electron-withdrawing groups such as nitro or cyano groups to its skeleton to stabilize ligand radical formation during the catalysis. Also, the ligand radical forms can be stabilized during the catalysis process by introducing quinoline instead of pyridine in the ligand architectures.
H. A. Kankanamalage, Pavithra, "Pyridine Platforms For Catalytic Water Reduction Based On Nickel And Iron Complexes" (2018). Wayne State University Dissertations. 2101.