The selective and efficient functionalization of C–H bonds remains a critical challenge to developing sustainable methods for petrochemical and bio-renewable feedstock utilization. Due to the thermodynamically strong and kinetically inert nature of C–H bonds, industrial methods for functionalizing alkanes relies on energy-intensive processes involving high pressures, high temperatures, and environmentally hazardous reagents. Biologically, late transition metal oxo cofactors can activate C–H bonds efficiently and under benign conditions. Motivated by understanding and replicating this reactivity, many synthetic late transition metal oxo systems have been reported, but the underlying product selectivity is dominated by oxygenation or desaturation of the substrate. This is a result of emphasizing high valent late transition metal platforms that are sufficiently oxidizing that, upon C–H activation, afford intermediate metal hydroxyls that remain reactive and capable of follow-up transformations with the activated substrate. We have been investigating metal oxos based on earlier transition metals that contain stronger bonds between the metal and oxygen atom to disfavor this follow-up reactivity. This allows for the exploitation of the unique C–H activation capacity of metal oxos to be redirected to a more diverse array of functionalized products. To achieve the requisite oxidizing potential to perform the initial C–H activation from a stable early transition metal oxo, we employ a photochemical strategy in which the oxo complex is destabilized in the excited state to initiate the activation. Using complexes of the form MO<sub>2</sub>X<sub>2</sub>(NN), M=Mo or W, X = a monodentate anionic ligand, and NN = a bidentate neutral nitrogen donor, we have characterized the light-initiated functionalization of various hydrocarbon substrates, which are capable of avoiding oxygenated products and homodimerized to yield new C–C bond. We have observed that more electron deficient and oxophilic metal centers afford greater selectivity of the non-oxygenated products. The anionic X ligand is found to be critical to supporting a NN-to-metal LMCT excitation that is required for tractable photochemistry. Additionally, photocatalysis can be obtained through the inclusion of oxygen atom donors that are compatible to the photochemical conditions. The characterization of these processes, and the underlying synthetic and mechanistic considerations, will be discussed within this presentation.

Diversifying hydrocarbon functionalization selectivity mediated by metal oxo complexes

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