Synthesis of Ni(II) Complexes for Allylic C(sp3)-H Bond Activation for Aerobic Oxidation Catalysis
Abstract
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.