Light-Initiated C-H Activation via Net Hydrogen Atom Transfer to a Molybdenum(VI) Dioxo.

MoOCl(bpy-Bu) () is shown to be a potent one-electron oxidant upon irradiation with 365 nm light in various solvents, while being a weak two-electron oxidant in the dark. Complex is characterized to activate various types of C-H bonds photochemically, including allylic and benzylic positions as well as alkanes and aldehydes. In all of these oxidations, ultimately forms a bimetallic Mo(V)/Mo(V) species with a μ-oxo ligand (). Depending on the substrate, the major organic product is identified as either an oxygenated or a C-C coupled (homodimerized) compound along with a minor chlorinated species. The product selectivity is proposed to be dependent upon the relative values between the bond dissociation enthalpy (BDE) of a potentially new C-OH bond within the product versus the BDE of a Mo-OH motif within a Mo(V)O(OH) intermediate. Based on this, we can estimate the BDE for Mo-OH to be 83-93 kcal/mol. Mechanistic studies suggest that the C-H activation occurs via a net hydrogen atom transfer (HAT) from occurring either asynchronously or via a stepwise electron-proton transfer (ET-PT) process. Complex is further demonstrated to reform dioxo in the presence of chemical oxidants.