MoO2Cl2(bpy- t Bu) (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.

The photoreactivity of d 0 metal dioxo complexes in activating CÀ H bonds has been recently studied. [1][2][3] We have previously reported that MoO 2 Cl 2 (bpy-t Bu) is an effective platform for light initiated CÀ H activation with unique product selectivity for the overall functionalization. [1] Herein we expand on these studies and report the synthesis and photoreactivity of several new Mo(VI) dioxo complexes with the general formula MoO 2 (X) 2 (NN); where X=F À , Cl À , Br À , CH 3 À , PhO À , t BuO À and NN = 2,2'-bipyridine (bpy) or 4,4'-tert-butyl-2,2'bipyridine (bpy-t Bu). Among these compounds, MoO 2 Cl 2 (bpy-t Bu) and MoO 2 Br 2 (bpy-t Bu) are able to participate in bimolecular photoreactivity with several substrates containing CÀ H bonds of various types such as allyls, benzyls, aldehydes (RCHO) and alkanes. MoO 2 (CH 3 ) 2 bpy and MoO 2 (PhO) 2 bpy do not participate in bimolecular photoreactions and instead they undergo photodecompositions. Computational studies indicate that the nature of the HOMO and LUMO is critical in supporting photoreactivity, with access to an LMCT (bpy!Mo) being necessary for tractable hydrocarbon functionalization.