Date of Award
Chemistry and Biochemistry
Charles L.B. Macdonald
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The Macdonald research group has been active in the synthesis of low-oxidation main group elements, particularly, compounds containing phosphorus in the (+I) oxidation state (PI). One of the routes the group uses to stabilize phosphorus(I) center is through the use of N-heterocyclic carbenes (NHCs). Generally, NHC-PI compounds are synthesized via ligand re-placement reactions with the precursor [(dppe)PI]+ which was previously developed by the re-search group. Chapter 1 provides a review of the current status of low oxidation state P(I) chemistry as it presents an overview of the stabilization of P(I) centre through a synthesis of a remarkable compound (triphosphenium cation) using different reported routes. In addition, NHCs show a great ability to stabilize phosphorus in low oxidation state via producing various phosphacynine compounds employing imidazoles by different preparation methods. Accordingly, the properties of the analogue triazole compounds are also presented in order to illustrate their electronic and structural properties. In chapter 2 various derivatives of 1,2,4-triazol-5-ylidene phosphorus(I) compounds were synthesized. This was performed by following these steps: (1) synthesizing 1,4-dialkyl-1,2,4-triazolium salts; (2) deprotonation of these salts using a suitable base and producing carbenes; (3) addition of [(dppe)PI]+ salt to the resultant carbene in-situ to give the target phosphorus(I) com-pound. Metathesis reactions were employed to 1,4-dialkyl-1,2,4-triazolium iodide salts and the precursor [(dppe)PI]+Br by addition of NaBPh4 in order to improve their solubilities and obtain pure materials of the desired products. The electronic properties of 1,2,4-triazol-5-yilidene phos-phorus(I) compounds were assessed and compared with those of the previously reported 1,3-imidazol-2-yilidene phosphorus(I) adducts using 31P NMR spectroscopy and X-Ray crystallog-raphy data in conjunction with theoretical studies. The results revealed that the triazolylidenes are superior π-acids than their imidazolyl analogues and that the resulting PI complexes are less basic. Chapter 3 provides a study of the reactivity of TAZ2PI compounds in order to investigate to what extend the two pairs of electrons on the phosphorus centre are available to coordinate to Lewis acids as the phosphorus centre displayed higher π-acceptor properties than the previously reported imidazoles. This was accomplished via reactions with a variety of oxidation reagents such as elemental sulfur, triflic acid and methyl triflate, and coordination to transition metals such as gold, iron and cobalt. The observed results indicated that the TAZ2PI complexes with both main group elements and transition metals are unstable in solution and leading to decompo-sition in some cases. Theoretical studies show that the snapping energy of metal-ligand bond for the TAZs complexes is much weaker than that of IMIDs complexes which agrees well with the experimental results. In chapter 4, we treated 1,4-dialkyl-1,2,4-triazolium ions with phosphinidenes and ele-mental selenium to explore the electronic nature of the carbenes associated with π-accepting and σ-donor properties of the carbenes using NMR spectroscopy. Accordingly, various selenone de-rivatives of TAZs were synthesized and fully characterized. Experimental results along with the-oretical studies revealed that TAZ carbenes have higher π-acceptor properties and lower donor properties than their analogues IMID carbenes. Among the TAZ carbenes, the diisopropyl substi-tuted TAZ carbene is the strongest donor and weakest π-acceptors, however; dimethyl substitut-ed TAZ carbene is the highest π-acceptor and the worst donor.
Elnajjar, Fawzia Omar, "Synthesis, Reactivity and Computational Studies of 1,4-Dialkyl-1,2,4-Triazol-5-ylidene Phosphorus(I) Cations" (2019). Electronic Theses and Dissertations. 7637.