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 [(CeCl5)-Cl-IV(OR)]2(-) [Ce-IV, cerium(IV); OR, -OCH3 or -OCCl2CH3] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO center dot) formed by Ce-IV-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 [NEt4](2)[CeCl6] (NEt4+, tetraethylammonium), and RO center dot are not intermediates. Spectroscopic analyses and kinetics were investigated for C-H activation to identify chlorine radical (Cl center dot) generation as the rate-limiting step. Density functional theory calculations support the formation of [Cl center dot][alcohol] adducts when alcohols are present, which can manifest a masked RO center dot character. This result serves as an important cautionary note for interpretation of radical trapping experiments.

Automated summary

The reported carbon-hydrogen (C-H) activation of alkanes is actually mediated by the photocatalyst [NEt4](2)[CeCl6], and RO center dot are not intermediates. The generation of chlorine radical (Cl center dot) in the photocatalytic system is found to be the rate-limiting step. Density functional theory calculations support the formation of [Cl center dot][alcohol] adducts, which can manifest a masked RO center dot character.