The free-radical divalent silver species (Ag^II) is expected to be a powerful stoichiometric oxidant for the activation of kinetically inert systems, such as the direct oxidation of methane (CH₄). However, the efficient catalysis with Ag^II suffers from thermodynamic instability because the Ag^II species self-degrades to the much more stable monovalent species (Ag^I), which is difficult to chemically reoxidize to Ag^II with strong acidic solvents, thus hampering a full construction of the catalytic cycle. Herein we design an electrocatalytic cycle for promoting homogeneous Ag-catalyzed CH₄ functionalization at room temperature. Continuous regeneration of the active Ag^II radicals from the precatalyst Ag^I is achieved via anode oxidation. Additionally, we discovered that the electrogenerated Ag^II monomer subsequently induces the formation of methyl radicals from CH₄, enabling facile access to form methyl bisulfate (CH₃OSO₃H) as the functionalized product. A low activation energy of 13.2 kcal mol⁻¹for CH₄ oxidation is observed along with a high turnover frequency at low pressure range of CH₄ (~2800 hr⁻¹ at 85 psi CH₄). The catalytic CH₄ oxidation process is observed to be much faster than the thermal decay of the catalytic species, which gives great promise to higher selectivity of CH₄ functionalization if operated at higher pressure of CH₄ (Faradaic efficiency theoretically approaching 100% at >100 psi CH₄). Furthermore, the idea to electrochemically replenish the active Ag^II species should spark the discovery of high-valent Agredox chemistry to broader reaction scope and offer the potentiality to the compelling yet challenging activation of other inert systems at ambient conditions.

Xun Guan

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