Date of Award
2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Rory Waterman
Abstract
Understanding the reactivity of phosphorus using abundant and sustainable metals has become a major topic of research. With the limited understanding of how light enhances the formation of carbon–phosphorus bonds in catalytic hydrophosphination, further exploration in its generality with respect to the metal catalyst needs to be probed. Titanium, as the second most abundant transition metal, as well as being in the same group as the highly active zirconium, stands out as an attractive metal to explore for hydrophosphination.Initial investigations using triamidoamine-supported titanium compounds for hydrophosphination catalysts were undertaken. While the simple alkyl titanium (N3N)TiMe shows modest hydrophosphination activity, the terminal phosphido (N3N)TiPHPh was inactive. Analysis by EPR spectroscopy and radical trap experiments give evidence that the terminal Ti–C bond undergoes homolytic cleavage to initiate radical hydrophosphination. The phosphido compound does not undergo homolysis under irradiation, although the Ti–P bond undergoes insertion reactions with polar C–O and C–N multiple bonds. Computational analysis of the titanium phosphorus bond in (N3N)TiPHPh indicates that irradiation populates a molecular orbital with d character centered on titanium. This contrasts with previously explored zirconium systems, in which irradiation populates a molecular orbital with significant σ* character. This unproductive pathway may partially explain why the titanium complex is unable to be activated by light and could be general to other titanium-based systems. Because titanium–alkyl species can undergo homolysis under irradiation, explorations of other titanium precatalysts that do not contain Ti–C bonds was undertaken. This was done in order to impede radical generation and allow for the probing of insertion-based hydrophosphination. Titanium supported by amidates were synthesized to allow for titanium precursors with different ligand geometries, as these might allow for enhanced hydrophosphination. Unfortunately, the difficulty of synthesis of these complexes, combined with the meager activity of the complexes described herein, warrants pause for the future of titanium catalyzed hydrophosphination. To compare reactivity of group 4 complexes with terminally bound phosphido ligands and their photocatalysis, a structural and spectroscopic study of several compounds containing hafnium-pnictogen bonds were undertaken. Structural differences between the zirconium and hafnium analogues mostly follow from the more diffuse 5d orbitals of hafnium. Accordingly, the complex (N3N)HfPHPh is a competent hydrophosphination catalyst under UV irradiation. Overall, this work provides a better understanding of the broader effects of light on the metal–phosphorus bond. While titanium may not be active for hydrophosphination due to its unproductive excited state, other metals may be more amenable to this process.
Language
en
Number of Pages
144 p.
Recommended Citation
Seth, Dennis Michael, "Probing The Photochemistry Of Group Iv Phosphidos Towards Efficient Hydrophosphination" (2024). Graduate College Dissertations and Theses. 1865.
https://scholarworks.uvm.edu/graddis/1865