A PdIII Sulfate Dimer Initiates Rapid Methane Monofunctionalization by H Atom Abstraction

Palladium-Catalyzed Dual Csp2─Csp3 Bond Formation: A Versatile Platform for the Synthesis of Benzo-Fused Heterocycles

Transition-metal-catalyzed transformations offer a powerful approach to rapidly synthesize complex benzo-fused heterocycles, crucial for drug and material development. However, existing synthetic strategies face challenges such as limited functional group compatibility, reliance on complex ligands, and difficulties in controlling chemoselectivity with prefunctionalized substrates. Herein, a ligand-free Pd(II)/Cu(I) catalytic system is presented that facilitates reactions between arylsulfonyl chlorides and unactivated olefins under mild conditions, enabling the efficient synthesis of saturated benzo-fused six-membered heterocycles. This streamlined strategy employs dual Csp2─Csp3 bond formation, producing diverse N/O-polyheterocycles and allowing late-stage functionalization of bioactive molecules with excellent yields and high chemoselectivity. The key to the success of this reaction is the formation of high-valent Ar-Pd(III) intermediate, which drives the reaction through 1,2-Pd migration and electrophilic C─H arylation. This unique reactivity pathway facilitates the formation of benzo-fused heterocycles while effectively avoiding the β-H elimination typically associated with Heck-type reactions.

Electrothermal Conversion of Methane to Methanol at Room Temperature with Phosphotungstic Acid

Traditional methods for the aerobic oxidation of methane to methanol frequently require the use of noble metal catalysts or flammable H2-O2 mixtures. While electrochemical methods enhance safety and may avoid the use of noble metals, these processes suffer from low yields due to limited current density and/or low selectivity. Here, we design an electrothermal process to conduct aerobic oxidation of methane to methanol at room temperature using phosphotungstic acid (PTA) as a redox mediator. When electrochemically reduced, PTA activates methane with O2 to produce methanol selectively. The optimum productivity reaches 29.45μ m o l g P T A - 1 h - 1 ${\mu mol\ {g}_{PTA}^{-1}{h}^{-1}}$ with approximately 20.3 % overall electron yield. Under continuous operation, we achieved 19.90μ m o l g P T A - 1 h - 1 ${\mu mol\ {g}_{PTA}^{-1}{h}^{-1}}$ catalytic activity, over 74.3 % methanol selectivity, and 10 hours durability. This approach leverages reduced PTA to initiate thermal catalysis in solution phase, addressing slow methane oxidation kinetics and preventing overoxidations on electrode surfaces. The current density towards methanol production increased over 40 times compared with direct electrochemical processes. The in situ generated hydroxyl radical, from the reaction of reduced PTA and oxygen, plays an important role in the methane conversion. This study demonstrates reduced polyoxotungstate as a viable platform to integrate thermo- and electrochemical methane oxidation at ambient conditions.

Methane Oxidation to Methanol

The direct transformation of methane to methanol remains a significant challenge for operation at a larger scale. Central to this challenge is the low reactivity of methane at conditions that can facilitate product recovery. This review discusses the issue through examination of several promising routes to methanol and an evaluation of performance targets that are required to develop the process at scale. We explore the methods currently used, the emergence of active heterogeneous catalysts and their design and reaction mechanisms and provide a critical perspective on future operation. Initial experiments are discussed where identification of gas phase radical chemistry limited further development by this approach. Subsequently, a new class of catalytic materials based on natural systems such as iron or copper containing zeolites were explored at milder conditions. The key issues of these technologies are low methane conversion and often significant overoxidation of products. Despite this, interest remains high in this reaction and the wider appeal of an effective route to key products from C-H activation, particularly with the need to transition to net carbon zero with new routes from renewable methane sources is exciting.

Bisulfate as a redox-active ligand in vanadium-based electrocatalysis for CH4 functionalization

The roles of unforgiving H₂SO₄ solvent in CH₄ activation with molecular catalysts have not been experimentally well-illustrated despite computational predictions. Here, we provide experimental evidence that metal-bound bisulfate ligand introduced by H₂SO₄ solvent is redox-active in vanadium-based electrocatalytic CH₄ activation discovered recently. Replacing one of the two terminal bisulfate ligands with redox-inert dihydrogen phosphate in the pre-catalyst vanadium (V)-oxo dimer completely quenches its activity towards CH₄, which may inspire environmentally benign catalysis with minimal use of H₂SO₄.

Electrocatalytic Methane Functionalization with d0 Early Transition Metals Under Ambient Conditions

The undesirable loss of methane (CH₄ ) at remote locations welcomes approaches that ambiently functionalize CH₄ on-site without intense infrastructure investment. Recently, we found that electrochemical oxidation of vanadium(V)-oxo with bisulfate ligand leads to CH₄ activation at ambient conditions. The key question is whether such an observation is a one-off coincidence or a general strategy for electrocatalyst design. Here, a general scheme of electrocatalytic CH₄ activation with d⁰ early transition metals is established. The pre-catalysts' molecular structure, electrocatalytic kinetics, and mechanism were detailed for titanium (IV), vanadium (V), and chromium (VI) species as model systems. After a turnover-limiting one-electron electrochemical oxidation, the yielded ligand-centered cation radicals activate CH₄ with low activation energy and high selectivity. The reactivities are universal among early transition metals from Period 4 to 6, and the reactivities trend for different early transition metals correlate with their d orbital energies across periodic table. Our results offer new chemical insights towards developing advanced ambient electrocatalysts of natural gas.

Theory and Experiment Demonstrate that Sb(V)-Promoted Methane C-H Activation and Functionalization Outcompete Superacid Protonolysis in Sulfuric Acid

Sb(V) in strong Brønsted acid solvents is traditionally assumed to react with light alkanes through superacid protonolysis, which results in carbocation intermediates, H₂, and carbon oligomerization. In contrast to this general assumption, our density functional theory (DFT) calculations revealed an accessible barrier for C-H activation between methane and Sb(V) in sulfuric acid that could potentially outcompete superacid protonolysis. This prompted us to experimentally examine this reaction in sulfuric acid with oleum, which has never been reported because of presumed superacid reactivity. Reaction of methane at 180 °C for 3 h resulted in very high yields of methyl bisulfate without significant overoxidation. Our DFT calculations show that a C-H activation and Sb-Me bond functionalization mechanism to give methyl bisulfate outcompetes methane protonolysis and many other possible reaction mechanisms, such as electron transfer, proton-coupled electron transfer, and hydride abstraction. Our DFT calculations also explain experimental hydrogen-deuterium exchange studies and the absence of methane carbo-functionalization/oligomerization products. Overall, this work demonstrates that in very strong Brønsted acid solvent, Sb(V) can induce innersphere reaction mechanisms akin to transition metals and outcompete superacid reactivity.

AgII -Mediated Electrocatalytic Ambient CH4 Functionalization Inspired by HSAB Theory

Although most class (b) transition metals have been studied in regard to CH₄ activation, divalent silver (Ag^II ), possibly owing to its reactive nature, is the only class (b) high-valent transition metal center that is not yet reported to exhibit reactivities towards CH₄ activation. We now report that electrochemically generated Ag^II metalloradical readily functionalizes CH₄ into methyl bisulfate (CH₃ OSO₃ H) at ambient conditions in 98 % H₂ SO₄ . Mechanistic investigation experimentally unveils a low activation energy of 13.1 kcal mol^-1 , a high pseudo-first-order rate constant of CH₄ activation up to 2.8×10³  h^-1 at room temperature and a CH₄ pressure of 85 psi, and two competing reaction pathways preferable towards CH₄ activation over solvent oxidation. Reaction kinetic data suggest a Faradaic efficiency exceeding 99 % beyond 180 psi CH₄ at room temperature for potential chemical production from widely distributed natural gas resources with minimal infrastructure reliance.

Towards atomic precision in HMF and methane oxidation electrocatalysts

With the increasing emphasis on transitioning to a sustainable society, electrosynthetic routes to generate fuels and chemicals are rapidly gaining traction. While the electrolysis of water and CO2 has been heavily investigated over the last decade, electrocatalysis of other abundant resources such as biomass and methane is now increasingly coming into focus. As this area is relatively less mature, much work remains to be done. In particular, efforts to decipher reaction mechanisms and extract the fundamental insights are necessary to develop economically competitive electrosynthetic routes using biomass and methane. Against this backdrop, this feature article focuses on the recent developments within the community using atomically precise catalysts, both homogeneous and heterogeneous, as model systems to understand these reactions.