Rapid Electrochemical Methane Functionalization Involves Pd–Pd Bonded Intermediates

High-valent Pd complexes are potent agents for the oxidative functionalization of inert C–H bonds, and it was previously shown that rapid electrocatalytic methane monofunctionalization could be achieved by electro-oxidation of PdII to a critical dinuclear PdIII intermediate in concentrated or fuming sulfuric acid. However, the structure of this highly reactive, unisolable intermediate, as well as the structural basis for its mechanism of electrochemical formation, remained elusive. Herein, we use X-ray absorption and Raman spectroscopies to assemble a structural model of the potent methane-activating intermediate as a PdIII dimer with a Pd–Pd bond and a 5-fold O atom coordination by HxSO4 (x–2) ligands at each Pd center. We further use EPR spectroscopy to identify a mixed-valent M–M bonded Pd2 II,III species as a key intermediate during the PdII-to-PdIII 2 oxidation. Combining EPR and electrochemical data, we quantify the free energy of Pd dimerization as