Light hydrocarbons are the main component of petroleum and natural gas reserves and are common industrial byproducts but have minimal commercial value. They are seldom employed directly as commodity precursors since they are mainly alkanes with inert C–H bonds. Industrially, C–H bond functionalization requires high temperatures and pressures, but enzymes are able to catalyze and selectively activate C–H bonds under mild conditions using a metal-oxo motif. Towards understanding and replicating these mild conditions, many synthetic high valent metal-oxo complexes have been developed for C–H activation. However, late transition metal-oxos are highly reactive and difficult to isolate and study. Using earlier transition metals would allow for readily isolable oxos, but the stability of these complexes makes them less reactive or unreactive toward C–H functionalization. However, energy input in the form of light can be used to generate a more oxidizing excited state that can directly mediate C–H activation. Recently, traditionally inert MoO2Cl2(bpy-tBu), bpy-tBu= 4,4'-ditertbutyl-2,2'-bipyridine, has been shown to perform C–H activation upon light irradiation. This work further develops this photoreactivity via systematically altering the electronic properties of the parent MoO2Cl2(bpy) through substitutions of the bpy ligand. UV-vis spectroscopy supported by TD-DFT calculations show that variations of the 4,4'-position has a relatively small impact on the absorption properties of the system compared to similar variations at the 5,5'-position. The ground state electronic properties were interrogated through cyclic voltammetry (CV), which in combination with luminescence spectroscopy is used to determine the excited state potentials for each complex. These photochemical and thermodynamic measurements are related to the scope, selectivity and efficiency (quantum yields and Stern Volmer kinetics) of the C–H activation photoreactivity across the family of dioxo complexes. Trends are presented in the context of mechanistic insight into light driven C–H functionalization and ligand-field critical design criteria for engendering photoactivity within metal oxo complexes.

Thermodynamic and photochemical characterization of light-initiated C–H activation by synthetically modified Mo(VI) dioxo complexes

ACS Fall 2021

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