Electrophilic high-valent metal ions are potent intermediates for the catalytic functionalization of methane, but in many cases, their high redox potentials make these intermediates difficult or impossible to access using mild stoichiometric oxidants derived from O2. Herein, we establish electrochemical oxidation as a versatile new strategy for accessing high-valent methane monofunctionalization catalysts. We provide evidence for the electrochemical oxidation of simple PdSO4 in concentrated sulfuric acid electrolytes to generate a putative Pd2III,III species in an all-oxidic ligand field. This electrogenerated high-valent Pd complex rapidly activates methane with a low barrier of 25.9 (±2.6) kcal/mol, generating methanol precursors methyl bisulfate (CH3OSO3H) and methanesulfonic acid (CH3SO3H) via concurrent faradaic and nonfaradaic reaction pathways. This work enables new electrochemical approaches for promoting rapid methane monofunctionalization.

The direct conversion of methane to methanol would enable better utilization of abundant natural gas resources. In the presence of stoichiometric Pt IV oxidants, Pt II ions are capable of catalyzing this reaction in aqueous solutions at modest temperatures. Practical implementation of this chemistry requires a viable strategy for replacing or regenerating the expensive Pt IV oxidant. Herein, we establish an electrochemical strategy for continuous regeneration of the Pt IV oxidant to furnish overall electrochemical methane oxidation. We show that Cl-adsorbed Pt electrodes catalyze facile oxidation of Pt II to Pt IV at low overpotential without concomitant methanol oxidation. Exploiting this facile electrochemistry, we maintain the Pt II/IV ratio during Pt II -catalyzed methane oxidation via in situ monitoring of the solution potential coupled with dynamic modulation of the electric current. This approach leads to sustained methane oxidation catalysis with 70% selectivity for methanol.

This tutorial review provides a general account of the electrochemical behavior of quinones and their various applications. Quinone electrochemistry has been investigated for a long time due to its complexity. A simple point of view is developed that considers the relative stability of the reduced quinone species and the values of the first and second reduction potentials. The 9-membered square scheme in buffered aqueous solutions is explained and semiquinone radical stability is discussed in this context. Quinone redox reaction has also been employed in various studies. Diverse examples are presented under three broad categories defined by the roles of quinone: molecular tool for physical chemistry, versatile electron mediator, and charge storage for energy conversion devices.

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 <−4.5 kcal/mol for Pd2 II,III and <−9.1 kcal/mol for PdIII 2. The structural and thermochemical data suggest that the aggregate effect of metal–metal and axial metal–ligand bond formation drives the critical Pd dimerization reaction in between electrochemical oxidation steps. This work establishes a structural basis for the facile electrochemical oxidation of PdII to a M–M bonded PdIII dimer and provides a foundation for understanding its rapid methane functionalization reactivity.

An electrogenerated Pd III 2 species in fuming sulfuric acid is competent for rapid and concurrent methane monohydroxylation to methyl bisulfate (CH 3 OSO 3 H) and methane sulfonation to methanesulfonic acid (CH 3 SO 3 H). In situ NMR at 50 °C is used to track the selective transformation of methane to CH 3 OSO 3 H and CH 3 SO 3 H at high conversions. Integrating a set of kinetic and computational studies, the mechanism of methane monofunctionalization by Pd III 2 is examined. Experimental rate laws and common kinetic isotope effects for CH 3 OSO 3 H and CH 3 SO 3 H formation suggest that both transformations proceed via a common rate-limiting C−H activation step. Introduction of O 2 or Pd 2 II,III suppresses CH 3 SO 3 H generation, indicating a radical chain sequence. Although the metal−metal bonded Pd III 2 complex is a net two-electron oxidant, our aggregate kinetic data point to a mechanistic model that features rate-limiting H atom abstraction by the Pd III 2 complex to generate a methyl radical intermediate. The CH 3 • intermediate then recombines with Pd 2 II,III to furnish a CH 3 Pd III 2 intermediate that reductively eliminates CH 3 OSO 3 H. Alternatively, the CH 3 • intermediate can enter a chain reaction with SO 3 to generate CH 3 SO 3 H. DFT computations support the radical-based C−H activation by Pd III2 and delineate H atom abstraction pathways with computed reaction barriers and kinetic isotope effects (KIEs) that are consistent with experimental data. These mechanistic investigations challenge the paradigm of electrophilic C−H activation and highlight H atom abstraction as a potent pathway for selective methane C−H oxidative functionalization at high reaction rates.

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