Rapid Electrochemical Methane Functionalization Involves Pd-Pd Bonded Intermediates.

Area of Science:

• Organometallic Chemistry
• Electrocatalysis
• Spectroscopy

Background:

• High-valent palladium complexes enable C-H bond functionalization.
• Electrocatalytic methane monofunctionalization involves Pd(II) oxidation to a Pd(III) intermediate in sulfuric acid.
• The structure and formation mechanism of this reactive intermediate were previously unknown.

Purpose of the Study:

• To determine the structure of the reactive, unisolable Pd(III) intermediate in methane functionalization.
• To elucidate the structural basis for the electrochemical formation of this intermediate.
• To understand the mechanism of electrocatalytic methane activation by high-valent palladium.

Main Methods:

• X-ray absorption spectroscopy (XAS) to probe electronic and structural properties.
• Raman spectroscopy to identify vibrational modes and coordination environments.
• Electron paramagnetic resonance (EPR) spectroscopy to detect transient radical species.
• Electrochemical methods to study oxidation potentials and reaction thermodynamics.

Main Results:

• A structural model of the methane-activating intermediate as a Pd(III) dimer with a Pd-Pd bond was assembled.
• Each palladium center exhibits 5-fold O-atom coordination by sulfate ligands.
• A mixed-valent Pd2(II,III) species with a metal-metal bond was identified as a key intermediate during oxidation.
• Thermodynamic data indicate significant driving forces for Pd dimerization (<-4.5 kcal/mol for Pd2(II,III) and <-9.1 kcal/mol for Pd2(III)).

Conclusions:

• The study establishes a structural basis for the electrochemical oxidation of Pd(II) to a metal-metal bonded Pd(III) dimer.
• Metal-metal and axial metal-ligand bond formation are key drivers for Pd dimerization during electrochemical oxidation.
• This work provides a foundation for understanding the rapid methane functionalization reactivity of these palladium complexes.