Doctor of Philosophy (PhD)
The functionalization of C–H bonds is challenging due to its thermodynamic strength and non-polar nature, yet this process is foundational to utilizing petrochemical feedstocks. Biologically, late transition metal oxo cofactors can selectively functionalize hydrocarbons under benign reaction conditions. Synthetically, late transition metal oxos that are more electron-rich are typically short-lived, difficult to generate, and challenging to study mechanistically. An alternative approach is to utilize stable early transition metal oxos that can be activated by light to initiate novel reactivity. Furthermore, a light-driven pathway also provides a strategy to utilize solar energy to drive chemical reactivity directly.
Towards these goals, the photo-reactivity of a molybdenum dioxo complex bearing a bipyridine ligand has been investigated. The photoreaction of MoO2Cl2(bpy-tBu) typically affords C–H activation via a stepwise pathway of the substrate followed by C–C homocoupling as a major product, and chlorinated and oxygenated products are observed as minor species. To increase the selectivity towards C–C homocoupling, the analogous and more oxophilic tungsten dioxo complex was prepared. Relative to the Mo dioxo, the W species is significantly more stable, as evidenced by minimal observed O-atom transfer reactivity with phosphines in the ground state. The excited state of WO2Cl2(bpy-tBu) displays light induced C–H activation but produces no oxygenated organic products. Nevertheless, quantum yields are low with this complex. This work further developed to identify the limiting factor of the photoreactivity via systematically altering the electronic properties of the parent WO2Cl2(bpy) through substitutions of the bpy ligand. By comparing the quantum yields of these complexes with cathodic peak potential, lifetime, and excited state energies, there is a linear trend with quantum yield versus cathodic peak potential, indicating that reduction potential is the limiting factor for photoreactivity. Irradiation of WO2Cl2(bpy-CF3) produces no oxygenated products but reacts with substantially higher quantum yields than WO2Cl2(bpy-tBu). Compared to the Mo analog, despite having a more negative reduction potential, WO2Cl2(bpy-CF3) still displayed lower quantum yields than the MoO2Cl2(bpy-tBu). This suggests that the photochemical electron transfer step is not efficiency limiting within these systems. Instead, a post-excited-state-quenching proton transfer is likely efficiency limiting, which agrees with kinetic isotope studies.
Siddhiaratchi, Siddhiaratchige Dinushini Meriam, "Mechanistic Investigations of Direct Photochemical C–H Bond Functionalization Mediated by Mo(VI) and W(VI) Dioxo Complexes" (2022). LSU Doctoral Dissertations. 5974.
Chambers, Matthew B.
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