Date of Award

2019

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

Keywords

Arsenic, Carbene, Cyanide, Phosphorus, Pnictogen, Synthesis

Supervisor

Charles Macdonald

Rights

info:eu-repo/semantics/openAccess

Abstract

The overarching theme of this dissertation is the use of the conveniently prepared, air- and moisture-stable phosphorus(I) and arsenic(I) sources [Pndppe][X] (Pn = P and As; dppe = 1,2-bis(diphenylphosphino)ethane ; X = non-specific anion) as reagents for the synthesis and subsequent study of new or underexplored carbon-donor-stabilized phosphorus and arsenic compounds. Chapter 1 gives a historical account of the discovery and development of low-coordinate phosphorus- and arsenic-centred molecules. It provides insight into the nature of the bonding in these systems and recent successes in the field are highlighted. In Chapter 2, a convenient one-pot synthetic approach to chelating bis-N-heterocyclic carbene-ligated phosphorus(I) salts is described. The solid-state structures of these remarkably stable phosphamethine cyanine dyes with various N-alkyl groups and counterions were determined using single crystal X-ray diffraction. Initial reactivity results reveal that they may act as ligands towards gold(I). In Chapter 3, the synthesis and solid-state structures of various thiazolium iodide salts are reported. Their use as S,N-heterocyclic carbene synthons for the generation of classical phosphamethine cyanine dyes is expounded. Oxidation of the phosphamethine cyanines with elemental sulfur results in cationic dithiophosphinates that are air- and moisture-stable. In Chapter 4, a convenient, reliable, and high-yielding synthesis of N-heterocyclic carbene-stabilized phosphorus(I) cations is reported. Characterization of the materials by nuclear magnetic resonance spectroscopy, ultraviolet-visible spectroscopy, single crystal X-ray diffraction, and quantum chemical calculations was done. They reveal that increasing N-alkyl group size causes twisting of the carbene fragments from the C−P−C plane, which decreases the magnitude of negative hyperconjugation between the π-type lone pair on phosphorus and the carbene fragments. Furthermore, increasing the N-alkyl group size increases the stability of the dyes towards air and moisture. The reactivity profile of these compounds was assessed and include oxidation by elemental sulfur to give cationic dithiophosphinates; coordination chemistry with gold(I) to yield mono- and bi-metallic complexes; and protonation/alkylation using triflate-based electrophiles, to afford dicationic phosphines. In Chapter 5, general synthetic approaches for accessing asymmetrically substituted phosphorus(I) cations are reported. The first method grants access to acyclic derivatives and is accomplished by sequential substitution of bis(diphenylphosphino)ethane. The second method grants access to cyclic derivatives and utilizes hybrid phosphine/N-heterocyclic carbene ligands. In this case, modifying the stoichiometries or ligand type can also generate homoleptic derivatives with pendant phosphines. In Chapter 6, the synthesis, isolation and full characterization of a series of cationic metal-carbonyl complexes bearing an N‐heterocyclic carbene-stabilized phosphorus(I) ligand are reported. The donor ability of the ligand was assessed by infrared spectroscopy of its metal-carbonyl complexes and by quantum chemical calculations. The results indicate that the new PI ligands are weak π‐acceptors with moderate σ-donor strength. In Chapter 7, the synthesis of dicyanopnictides using [Pndppe][BPh4] is reported. The protocol requires three synthetic steps from commercially available starting materials. As the heavy homologues of dicyanamide – a long-known and industrially-important anion – these materials should find use in a variety of synthetic applications. Chapter 8 summarizes the conclusions drawn from this research and provides initial results to guide future endeavours.

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