Photocatalytic C-H activation and the subtle role of chlorine radical complexation in reactivity.
basic_science · Level V
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- Record sourced from PubMed, PMID 34016778.
- Also identified by DOI 10.1126/science.abd8408.
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Abstract
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 [Ce<sup>IV</sup>Cl<sub>5</sub>(OR)]<sup>2-</sup> [Ce<sup>IV</sup>, cerium(IV); OR, -OCH<sub>3</sub> or -OCCl<sub>2</sub>CH<sub>3</sub>] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by Ce<sup>IV</sup>-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<sub>4</sub>]<sub>2</sub>[CeCl<sub>6</sub>] (NEt<sub>4</sub> <sup>+</sup>, 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.