The functionalization of C–H bonds is challenging due its thermodynamic strength and non-polar nature, yet this process is foundational to the utilization of petrochemical feedstocks. Biologically, late transition metal oxo complexes can selectively functionalize hydrocarbons under benign reaction conditions. Synthetically, these late transition metal platforms are short-lived, requires highly oxidizing conditions to generate, are challenging to study mechanistically, and have a strong preference for producing hydroxylated products. An alternative approach is to utilize stable early transition metal oxos which can be activated by light to initiate a wider array of reactivity under more benign conditions. Such a light-driven pathway also provides a strategy to directly utilize solar energy to initiate chemical reactivity. Towards these goals, the photoreactivity of molybdenum dioxo complexes bearing a 2,2’- bipyridine ligands has been investigated. This work investigates the photochemical kinetic behavior of MoO2Cl2(bpy-tBu) and hydrocarbon substrates. The photoreaction of MoO2Cl2(bpy-tBu) typically affords C–H activation of the substrate followed by C–C oxidative coupling as major product (upon substrate radical diffusion away from the oxo), with minor chlorinated and oxygenated products. To increase the selectivity towards C–C coupling, the more oxophilic analogous tungsten dioxo complex was prepared. Relative to the Mo counterpart, the W species is significantly more stable in the ground state, as evidenced by minimal observed O-atom transfer reactivity with phosphines. In the excited state, rapid C–H activation reactivity occurs. However, due to the more oxophilic W–O bond, the reactivity results in selective for non-oxygenated products. Quantum yield studies and emission studies suggests that while the excited states of the Mo and W are similar, the W(V/VI) ground state reduction is the critical component that alters the thermodynamics of the excited state reactivity. This talk will compare the photochemical behavior of MoO2Cl2(bpy-tBu) and WO2Cl2(bpy-tBu) complexes.

Comparison of photochemical C–H functionalization by Mo(VI) and W(VI) dioxo complexes

ACS Fall 2021

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