Photocatalytic C-H activation and the subtle role of chlorine radical complexation in reactivity.

The functionalization of methane, ethane, and other alkanes derived from fossil fuels is a central goal in the chemical enterprise. Recently, a photocatalytic system comprising [CeCl(OR)] [Ce, cerium(IV); OR, -OCH or -OCClCH] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by Ce-OR bond photolysis. In this work, we present evidence that the reported carbon-hydrogen (C-H) activation of alkanes is instead mediated by the photocatalyst [NEt][CeCl] (NEt , tetraethylammonium), and RO• are not intermediates. Spectroscopic analyses and kinetics were investigated for C-H activation to identify chlorine radical (Cl•) generation as the rate-limiting step. Density functional theory calculations support the formation of [Cl•][alcohol] adducts when alcohols are present, which can manifest a masked RO• character. This result serves as an important cautionary note for interpretation of radical trapping experiments.