Ligand Modification of Mo(VI) Dioxo Complexes Toward Investigation of Photocatalytic C-H Activation
Doctor of Philosophy (PhD)
The activation of C–H bonds is an important chemical process in utilizing petrochemical feedstocks but is limited due to thermodynamically inert, non-polar bonds. Industrial processes overcome this inertness by using high temperatures and pressures, making this an energy intensive process. Biological enzymes utilize late transition metal oxos to mediate C–H bond activation under mild conditions. Synthetic work to mimic these conditions use late transition metal oxos that are difficult to isolate, making mechanistic studies difficult. Alternatively, earlier transition metals are less reactive and easier to study but require more activation energy. Using a light-driven pathway to excite earlier transition metal oxo can generate novel reactivity and provide a path to using solar energy for C–H activation.
Previous studies show that a MoO2Cl2(bpy-tBu) complex can mediate C–H bond activation upon light irradiation. The formation of a bridging oxo, a bimetallic product, limits the photoreactivity of this complex. Adding pyridine N-oxide to the bimetallic complex causes reoxidation that promotes photocatalysis of MoO2Cl2(bpy-tBu). A TON of 12 is achieved with cyclohexene but is limited due to a buildup of water produced during photocatalysis. Water leads to catalyst decomposition.
The properties of a MoO2Cl2(bpy-tBu) complex were investigated further to determine the ability to optimize its photochemical reactivity. The bipyridine ligand was functionalized with various electron-donating and withdrawing groups at the para position. This functionalization showed that the photochemical properties and the product selectivity remained relatively stable. The stable photochemical properties are due to an orbital node at the para position. The thermodynamic energies of these complexes can be favorably tuned with more electron-withdrawing groups. Despite more favorable thermodynamics, kinetic properties are limited by lifetimes and self-quenching pathways.
Meta functionalized complexes were also synthesized and studied to assess their properties due to greater electron density. More electron-donating groups at the meta position demonstrated a red shift in absorption properties and a greater variability in product formation. The thermodynamic and kinetic studies demonstrated similar trends to that of the 4,4'functionalized complexes. The synthetic modification of a Mo(VI) dioxo complex showed that meta functionalization provides more tunability of the photochemical properties.
Baumberger, Courtney Linanne, "Ligand Modification of Mo(VI) Dioxo Complexes Toward Investigation of Photocatalytic C-H Activation" (2023). LSU Doctoral Dissertations. 6111.
Available for download on Wednesday, April 03, 2030