Investigation of the structure and reactivity of a series of mono- and dicopper complexes supported by biomimetic ligands

Abstract

Currently, selective oxidation of C–H bonds is a primary challenge in the organic synthesis of essential chemicals, including alternative fuels and pharmaceuticals. Nature carries out selective oxidations of C–H bonds via the use of copper-based monooxygenase (MO) enzymes such as lytic polysaccharide monooxygenase (LPMO). Research efforts in the Tolman group aim to model the copper-oxygen reactive intermediates of MO enzymes via synthesis of synthetic small molecule mimics. In Chapter 1, I will outline the proposed copper-oxygen intermediates for MO enzymes and discuss, in detail, the spectroscopic features and reactivity observed for synthetic mimics of these cores. Chapter 2 describes my efforts to synthesize a monoanionic ligand, bis(quinolinylcarbonyl)amide (–L1), that closely models the LPMO active site. The organic synthesis of HL1 as well as isolation of structurally diverse mono- and dinuclear copper complexes supported by –L1 via X-ray crystallography will be discussed. In addition, the fate of this ligand as a synthetic model of enzymatic active sites is assessed. In Chapter 3, I present the investigation of a novel copper(III)-benzoate complex supported by a dianionic bis(arylcarboxamido)pyridine ligand, LCuIII(O2CC6H4(Cl)), which is shown to undergo proton-coupled electron transfer (PCET) with O–H and C–H substrates. Comparison of the PCET reactivity of LCuIII(O2CC6H4(Cl)) with previously reported LCuIII(OH) and LCuIII(OOR) reveals the effect of the fourth ligand on [LCuIII]+ reactivity. In addition, mechanistic conclusions for each copper(III) complex are discussed with significant insight provided from time dependent-density functional theory (TD-DFT) and intrinsic bond order (IBO) analysis completed by Mukunda Mandal, a graduate student in the group of Professor Christopher Cramer. Chapter 4 investigates the spectroscopic features and PCET reactivity of a series of formally copper(III)-carboxylates supported by L2–, LCuIII(O2C–R), with the aim of understanding how electronic modification of the carboxylate ligand effects the observed properties and reactivity of the complexes. The reactivity trends of LCuIII(O2C–R) complexes with O–H and C–H bonds will be detailed as well as the implications of such trends on the mechanism of the reaction of LCuIII(O2C–R) complexes with substrates. Lastly, in Chapter 5, I present the study of a water soluble CuIII(OH) complex, [SO3LCu(OH)]2–, to investigate its ability to act as a water oxidation catalyst. The results of pH-dependent spectroscopic and electrochemical studies are detailed and the aqueous reactivity of [SO3LCu(OH)]2– with C–H substrates is discussed.