Pyridyl Phosphine Complexes in the Design of Hydration Catalysts
Recent advances in homogeneous catalysis have identified the importance of ligands able to participate in the catalytic cycle. Particularly relevant to making chemistry “greener” are those ligands that solubilise the catalyst in aqueous solution, and those that are able to activate water molecules towards reaction with the metal complex or substrate. This thesis describes the synthesis and coordination chemistry of a novel ligand bearing 2-pyridylphosphine substituents attached to a 2,6-pyridyl backbone (²⁻pyrPNP, [(C₅H₄N)₂PCH₂]₂C₅H₃N). These components were selected for their abilities to interact with water through dearomatisation processes, hydrogen bonding, and the basic pyridyl nitrogen atoms. The synthesis of pure ²⁻pyrPNP described here represents a much improved method for the synthesis of pyridylphosphines compared to those published in the literature. This is demonstrated by comparison with the original synthetic route, which produced many intractable impurities, as well as by the ability of the new method to provide PhPNP from an economical and air-stable starting material. Reactions of ²⁻pyrPNP with rhodium precursors show complicated reactivity, including the potential formation of paramagnetic species. Investigation into the reactivity of ²⁻pyrPNP with analogous iridium precursors resulted in the synthesis of [(²⁻pyrPNP)Ir(cod)]Cl. This is the first crystallographically characterised complex containing a facially coordinated PNP ligand. The cod ligand can be removed with ethene and hydrogen to form bis(ethene) and chloroiridium(III) bis(hydride) complexes [(²⁻pyrPNP)Ir(C₂H₄)₂]Cl and [(²⁻pyrPNP)Ir(H)₂Cl], respectively. Both complexes contain meridionally-coordinated ²⁻pyrPNP. Preliminary investigations reveal that the iridium complexes are fairly successful nitrile hydration catalysts under aqueous conditions. In addition, the cod and bis(ethene) complexes bearing ²⁻pyrPNP are more active than the cod complex of the pyridyl-free PhPNP ligand.