Metal oxo intermediates are central to the activation of C–H bonds in both biological and industrially relevant transformations. Prevailing strategies using molecular systems lean on late transition metal centers supported by highly-tailored ligands to finely tune both the primary and secondary coordination spheres. These platforms often require laborious syntheses and harsh oxidants, while also being difficult to control due to the instability of high valent late transition metal oxos. The transient nature of high valent late transition metal oxos also presents challenges to mechanistic studies and optimizations. Our alternative approach uses stable early transition metal oxos that are irradiated with light to generate unstable intermediates and unique electronic configurations with the aim of unlocking novel reactivity patterns for C–H activation.<br/>Towards these goals, a family of molybdenum and tungsten dioxo complexes supported by α-diimine ligands have been prepared. In the dark, these complexes are stable and capable of performing two-electron oxygen atom transfer reactions to phosphines and alkenes. Upon irradiation, these complexes rapidly mediate one-electron hydrogen atom abstraction from C–H bonds. When photoreactions are performed in the presence of alkenes, GC-MS analyses indicate that the major product is substrate dimerization at the allylic position (with a new C–C bond). Kinetic studies indicate the reaction is first order in both dioxo complex and substrate, with a primary kinetic isotope effect for C–H(D) activation. A comparative study between the reactivity of Mo and W systems is discussed. Methods for tuning the light absorption properties through ligand modifications is also presented. In the presence of alkanes, these photosystems show enhanced selectivity for hydroxylated products.

Using light to control light hydrocarbon functionalization with Group VI dioxo complexes

ACS Spring Meeting 2021

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