Synthesis of Ni(II) Complexes for Allylic C(sp3)-H Bond Activation for Aerobic Oxidation Catalysis
Master of Science
Blacquiere M. Johanna
Nickel is a widely abundant and inexpensive metal. Catalysts are substances that remain unconsumed and increase the rate of reactions by reducing the activation energy barrier, often by providing an alternative route. The main goal of this study is to prepare Ni(II) complexes that can promote a catalytic aerobic oxidation reaction by breaking allylic C-H bonds. Based on previous studies, allylnickel N-heterocyclic carbene complexes react with molecular oxygen to give useful carbonyl compounds with a Ni(II)-OH by-product, which undergoes dimerization. In order to achieve catalysis, dimerization of the Ni(II)-OH should be prevented and C-H activation has to be achieved to complete the cycle. This project attempts to promote C-H activation via a Ni-OH moiety utilizing two different strategies that are described in Chapters 2 and 3, respectively. A known tridentate ligand structure proposed to inhibit dimer formation was targeted, which includes an imidazolium salt precursor that contains a hemilabile picolyl group. Studies on synthesizing a Ni(II)-OH monomer were conducted to test its ability toward C-H activation. No success was seen due to the instability of the intermediate leading to the formation of a bis-ligated Ni complex. A new allyl ligand N-8-quinolinyl-4-pentenamide with a bidentate directing group was chosen as an alternative approach. Preliminary studies indicated the formation of a dynamic paramagnetic complex with an unknown structure and a presumed bis-ligated by-product. Catalytic studies were done to test the ability of the unknown complex to activate allylic C-H bonds, but no success was achieved due to a competing reaction of the base with the ligand.
Summary for Lay Audience
Catalysts are substances that make reactions faster without being used up. Aerobic oxidation is a chemical process that utilizes molecular oxygen from the air to form oxygen-containing organic molecules. Although oxygen is naturally available, the main challenge of using it in transition metal-catalyzed reactions to form useful compounds is its potential to form unwanted products after activation. The focus of this thesis is to carry out aerobic oxidation catalysis using nickel. Previous studies have shown successful results of Ni aerobically oxidizing molecules with no side reactions. The unstable Ni by-product formed in these reactions is the major challenge to making it catalytic. This project focuses on forming a stable Ni product through different strategies for aerobic oxidation catalysis. A successful catalytic reaction would be a cheaper alternative to other precious metals since Ni is a widely available metal.
Kukendran, Shagana, "Synthesis of Ni(II) Complexes for Allylic C(sp3)-H Bond Activation for Aerobic Oxidation Catalysis" (2023). Electronic Thesis and Dissertation Repository. 9107.
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