ABSTRACT
Direct photocatalytic C-H activation mediated by MoO2Cl2(bpy-tBu), a unique photoactive metal OXO, is presented. The limiting step, reoxidation to the Mo dioxo, is evaluated and proposed to occur via a key Cl- loss event. Photocatalyst degradation occurs upon substitution of bpy-tBu with H2O generated during catalysis.
ABSTRACT
MoO2Cl2(bpy-tBu) (1) 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 1 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, 1 ultimately forms a bimetallic Mo(V)/Mo(V) species with a µ-oxo ligand (2). 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 1* occurring either asynchronously or via a stepwise electron-proton transfer (ET-PT) process. Complex 2 is further demonstrated to reform dioxo 1 in the presence of chemical oxidants.
