Synthesis and reactivity of novel ONN- and ONO-palladium(II) pincer complexes; applications in CH-halogenation of aryl-pyridones

In this thesis, a library of novel unsymmetrical tridentate NNO- and ONO'-type ligands are synthesized and complexed with sources of palladium(II) with a view to exploring the reactivity of the so-formed pincer complexes and their role in CH-halogenation. Chapter 1 presents a general introduction to pincer chemistry with the emphasis being placed on pincer ligands in which a pyridine unit defines the central donor atom and metals drawn from the platinum group and in particular palladium. In chapter two the synthesis and characterization of four unsymmetrical ONN-type pincer pro-ligands is described, two of which possess donor units that are capable of inducing hydrogen-bonding interactions. The phenomenon of complex self-dimerization via hydrogen bonding is demonstrated as is its inhibition through suitably placed steric bulk. Ligand exchange reactions of an acetate group with a halide, pyridonate, phenolate, carboxylate are reported and the intra- and intermolecular NH···A interactions studied. Chapter 3 describes the synthesis of four examples of ONO'-type pro-ligands and their ability to act as dianionic ligands when bound to soft palladium(II) in the form of dimers and tetramers. Means of breaking the dimeric species to form well-defined ONO'-PdL (L = pyridines, pyridones) type complexes have been developed as has their amenability to protonation reactions and to form non-covalent interactions. Chapter 4 documents the rich stoichiometric reactivity of 6-phenyl-2-pyridone with Pd(OAc)2 yielding products the result of CH, OH or NH activation. Furthermore, the ability of Pd(OAc)2 to mediate the catalytic halogenation (bromination, chlorination, fluorination) of 6-phenyl-2-pyridone is probed. In addition, the capacity of ONO'- and ONN-palladium pincers to generate complexes with 6-phenyl-2-pyridone is studied. Finally, the screening of selected palladium pincer complexes prepared in chapters 2 and 3 as catalysts in CH-bromination of 6-phenyl-2-pyridone is disclosed and the results compared to that observed with Pd(OAc)2. Full experimental details are given in Chapter 5.