Synthetic, Reactivity and Structural Studies of Pyridyl-N-Phosphinoimines

This thesis reports an extension of the investigations into the factors, both internal and external, which impact upon the equilibrium position of a series of structurally analogous, interconverting pyridyl-N-phosphinoimine (“open”) - diazaphosphazole (“closed”) tautomeric systems. Chapter 1 presents the basic aspects of the chemistry relevant to the work discussed in this thesis: the structure and chemistry of pyridine, phosphines, phosphorus-nitrogen species, non-innocent ligands and frustrated Lewis pairs. Chapter 2 describes a study into the position of the dynamic “open”-“closed” equilibrium as a function of solvent polarity, it was found that polar solvents favour the more polar “closed” tautomer. The successful preparation and characterisation of a series of structurally analogous pyridyl-N-phosphinoimine – diazaphosphazole tautomeric systems, with varying substituents bound to the central pyridine and phosphinoimine motifs, is reported. The stereoelectronic impact of these substituents upon the position of the reversible “open”-“closed” equilibrium is discussed. Chapter 3 reports the unprecedented products of the reaction between 6-methyl-2-cyanopyridine, phenyllithium and phosphorus trichloride: a macrocyclic P,N-species and an intramolecularly base-stabilised phosphenium salt which were identified by a combination of experimental and computational techniques. The reactivity of these novel P,N-compounds towards Se, B(C6F5)3, DMF and trans-[Pt(PPh2Me)Cl(μ-Cl)]2 is investigated. Chapter 4 outlines a study of the behaviour of the pyridyl-N-phosphinoimine – diazaphosphazole tautomers towards small molecules e.g. MeNO2 and elemental Se. The P,N-species were found to demonstrate some of the characteristic reactivity of their various individual functional components, i.e. PIII, imine, P=N and dihydropyridine. Chapter 5 summarises the work reported in chapters 2-4 and discusses the future outlook of work with these interconverting pyridyl-N-phosphinoimine – diazaphosphazole tautomers.