N-Phosphino-pyridyl Imines: Flexible,
Multi-functional Reagents

Abstract

Abstract

Chapter I includes a summary of the chemistry of phosphines and bidentate phosphorus-nitrogen ligands of relevance to this thesis, and highlights their application in transition metal-based catalysis. In addition, a review of the characteristic properties and reactivity of iminophosphoranes and phosphenium cations is presented in preparation for work outlined later in this manuscript.
The synthesis and coordination studies of a range of N-phosphino-pyridyl imines with the general structure (C5H5N)(Ar)C=NPR2 (1) are reported in chapter II. The molecular structures of their palladium dichloride complexes are presented, highlighting the effect of differing substituents on framework 1 upon the geometry of the complexes.
The chemistry surrounding a novel equilibrium between two valence tautomers prepared from reaction of N-lithio pyridyl imine with bis(diisopropylamino)chlorophosphine is presented in chapters III and IV. The tautomers 1,1-bis(diisopropylamino)-3-phenyl-1λ5-[1,3,2]diazaphospholo [1,5-a]pyridine (2c) and bis(diisopropylamino)phosphino(phenyl-pyridin-2-yl methylene)amine (2o) exist in a ratio of 95 : 5, respectively, at ambient temperature. Investigations of the thermodynamic properties, the rate of interconversion and the possible mechanistic pathways of interconversion for 2o/2c have been conducted. The synthesis and characterisation of another equilibrium mixture, that of the tautomers 1,1-bis(diisopropylamino)-3-anthracenyl-1λ5-[1,3,2]diazaphospholo [1,5-a]pyridine (3c) and bis(diisopropylamino)phosphino(anthracenyl-pyridin-2-yl methylene)amine (3o), which exist in a ratio of 30 : 70, respectively, at ambient temperature is reported.
Chapter V describes the reaction of bis(diisopropylamino)phosphenium triflate with 1,1-bis(diisopropylamino)-3-phenyl-1λ5-[1,3,2]diazaphospholo[1,5-a]pyridine, which affords 1,1,1',1'- tetrakis(diisopropylamino)-3,3'-diphenyl-1H,1'H-[5,5'-bi-1λ5-[1,3,2]diazaphospholo[1,5-a]pyridinylidene]-1,1'-diium bis(triflate) (4) via a homo-bimolecular coupling process. This apparent ‘oxidative’ coupling by a phosphenium cation has no literature precedent and hence a detailed exploration of the possible mechanism of the reaction is outlined. Comparative reactions involving established oxidants, for example ferrocenium triflate, are examined and also give the diphosphonium salt 4.
Chapter VI outlines an exploration of the behaviour of N-dialkyl- and diaryl-phosphino-pyridyl imines in the coordination sphere of low valent palladium species. The isolation and characterisation of a novel dimeric palladium(I) complex is reported, which is found to undergo reactions with chlorobenzene and triphenylphosphine, acting as a source of ‘masked’ palladium(0). These reactions are consistent with redox processes occurring to give palladium(0) or palladium(II) species.