Mechanistic studies on photochemical reactions of metal carbon bonds with oxygen
Creators: name: Ho, Kwok Yan
Publisher: name: Imperial College London
Year: 2020

description: The selective oxidation of alkanes has been investigated using three different approaches; namely heterogeneous, biomimetic and homogenous catalytic oxidations. Chapter 1 focuses on the selective oxidation of alkanes using homogenous transition metal catalysts. The first example was reported in the early 1970s, which is known as the Shilov system. Several drawbacks to this classic reaction include poor selectivity and the use of an expensive oxidant source. Therefore, alternative metal catalysts and oxidants have been developed to study the selective oxidation of alkanes. This thesis focuses on using oxygen as the oxidant and late transition metals, in particular platinum or palladium, as the metal catalyst. The reactivity of a range of novel platinum and palladium alkyl or aryl complexes with oxygen are discussed. Chapter 2 describes the synthesis of cationic palladium(II) chloro and palladium(II) methyl complexes bearing 6,6”-dimethylterpyridine ligand (dmterpy). The methyl substituents on the ligand are sterically bulky, which cause one arm of the ligand to flip away from the metal centre of [PdCl(dmterpy)]Cl (2.26a and 2.26b). Similar behaviour involving [PdMe(dmterpy)]SbF6 (2.25) was observed in acetone, but the complex is stable in acetonitrile and shows reactivity with oxygen. Chapter 3 focuses on the synthesis of a series of neutral platinum(II) and palladium(II) alkyl or aryl complexes containing the 1,3-bis(2-pyridylimino)isoindole (BPI) ligand. The structure of some of these complexes were characterised by X-ray crystallography. Chapter 4 illustrates the reactivity of oxygen with these neutral platinum(II) and palladium(II) alkyl or aryl complexes bearing BPI ligands in various chlorinated and non-chlorinated solvents. These oxygen insertion reactions were monitored upon UV light exposure and in the dark. The reaction time of [Pt(BPI)Me] (3.31), [Pd(BPI)Me] (3.32), [Pt(BPI)Et] (3.37) and [Pt(BPI)Ph] (3.38) with oxygen is fastest in chloroform. The chloro complex [M(BPI)Cl] (3.14 when M = Pt(II); 3.43 when M = Pd(II)) was formed whenever chlorinated solvents were used. The mechanistic studies on the reactions between these platinum(II) and palladium(II) alkyl complexes and chloroform was investigated. The studies carried out in this Chapter also showed that radicals and singlet oxygen are unlikely to be involved in oxygen insertion reactions. The decomposition of these neutral platinum(II) and palladium(II) alkylperoxo complexes were investigated in Chapter 5. The methylperoxo complex [M(BPI)OOMe] (4.18 when M = Pt(II); 4.20 when M = Pd(II)) is unstable and dissociates to methanol and formaldehyde. These compounds react further with the solvent or water to give a mixture of side products. Similarly, the ethylperoxo complex [Pt(BPI)OOEt] (4.24) decomposes to ethanol and acetaldehyde. In addition, deuterium labelling studies was carried out to understand the fate of the methyl ligands in [Pt(BPI)CD3] (5.8). Studies carried out here also showed that methylperoxo ligand exchange occurs between [Pt(BPI)CD3] (5.8) and [Pd(BPI)CH3] (3.32) upon exposure to UV light in the presence of oxygen. Finally, the oxygen insertion reaction involving [Pt(BPI)Me] (3.31) and [Pd(BPI)Me] (3.32) complexes were studied by in-situ NMR spectroscopy.