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Date of Award
Claudio N. Verani
The storage of visible light energy in the form of chemical bond energy within molecules remains a scientific challenge, yet it is an efficient, as well as a sustainable, approach to employ renewable resources such as water and generate a clean and energy-dense source, like dihydrogen. Efforts have been directed towards the development of molecular metal complexes that would accelerate this conversion with high activity. However, our focus was to develop a profound understanding of the structural as well as the electronic alterations that the molecular catalyst endure in solution for the greater good of generating dihydrogen during a catalytic cycle. In this dissertation, we have investigated the electro/photocatalytic, mechanistic pathways and the variations in the electronic/chemical environment of late, first-row transition metals when chelated to nitrogen-rich, tetradentate ligands. Two CoIII complexes, each with an N4-aminopyridine ligand framework and two monodentate ligands (NO2-/Cl-), are examined for their electro/photocatalytic dihydrogen generation from water. The presence of the good leaving group (Cl-) has resulted in an enhanced catalytic performance, as its labile elimination facilitates the formation of the catalytic CoI state with a vacant orbital for the incoming proton.
In order to amplify the robustness of these N4-tetradentate CoIII systems especially under electrocatalytic conditions, we have switched from the flexible N4-aminopyridine to the rigid, planar N4-oxime ligand. Four CoIII N4-oxime complexes were synthesized and bound axially to either a Cl- or SCN- monodentate ligands. Low overpotential values of less than 500 mV were measured and distinct electrocatalytic activities amongst these complexes were detected; the chlorido complexes have shown turnover numbers of less than 1, and therefore are deemed non-catalytic, whereas the thiocyanato counterparts have shown a catalytic activity with TONs of ca. 20. Elaborate mechanistic insights into the catalytic intermediates were gained through spectroelectrochemistry, Infrared/UV-visible spectroscopic techniques coupled with bulk electrocatalysis, and single X-ray crystallography of the chemically reduced species, as well as via DFT calculations. We established the cornucopia of trans-effect in CoIII hydrides, amongst its axial ligands whereby installing good electron donors trans to the hydride renders the latter more labile towards dihydrogen generation, following its protonation.
The adjustable architecture of N4-oxime frameworks, which presents various sites of attachment to a Ru photosensitizer moiety, and the molecular orbital topology of the NiII/CuII oxime complexes have urged us to envisage three new multinuclear, supramolecular species: [RuNi], [RuCu], and [RuNiRu]. The axial coordination of the Ru moiety to the empty dx2-y2 molecular orbital of the NiII ion facilitated the intramolecular transfer from the Ru photosensitizer to the NiII catalyst, as catalysis was detected for the [RuNiRu] complex in the absence of an external photosensitizer with a TON24h of 49. However, two Ru centers were quintessential for providing the NiII ion with the two electrons, required for generating dihydrogen, as no catalysis was observed for the [RuNi] complex in the absence of a separate photosensitizer.
The work detailed herein highlights the significance of: the positional and electronic influence of monodentate groups in electro/photocatalytic systems containing 3d metals with N4-tetradentate ligand frameworks, the rigidity of the tetradentate ligand, and the molecular topology through which the chelating ligands are bound through to the metal ion in activating the electro/photocatalyst towards dihydrogen generation. These findings are pivotal for charting electrocatalytic mechanisms of similar systems and envisioning future designs.
El Harakeh, Nour, "Investigation Of Mechanisms In 3d-Containing Electro/photocatalysts For Proton/water Reduction" (2020). Wayne State University Dissertations. 2350.