Multisite Proton-Coupled Electron Transfer at a Keggin-Type Polyoxotungstate

Area of Science:

• Inorganic Chemistry
• Electrochemistry
• Catalysis

Background:

• Proton-coupled electron transfer (PCET) is crucial for redox reactions but faces thermodynamic limitations in single-site mechanisms.
• Developing efficient PCET systems is key to advancing catalytic transformations.

Purpose of the Study:

• Introduce a novel multisite PCET (MS-PCET) platform based on the Keggin-type polyoxotungstate [VW12O40]3− (VW12).
• Demonstrate the platform's ability to enable tunable oxidative and reductive hydrogen atom transfer reactions.
• Explore the mechanistic pathways and substrate scope of the MS-PCET system.

Main Methods:

• Synthesis and characterization of the VW12 polyoxotungstate.
• Pairing VW12 with Brønsted acids/bases to create reagent pairs with tunable properties.
• Kinetic studies using model hydrogen atom donors (e.g., 2,4,6-tBu3PhOH) to elucidate reaction mechanisms.
• Investigating H atom transfer to various substrates (N-H, O-H, C-H bonds).

Main Results:

• The VW12-based MS-PCET platform allows for tuning of effective bond dissociation free energies (BDFEeff) over a 15 kcal mol−1 range.
• Oxidative H atom transfer proceeds via an entropy-dominated concerted proton-electron transfer (CPET) pathway from a preorganized hydrogen-bonded complex.
• Reductive H atom transfer exhibits characteristics of a synchronous CPET mechanism, including measurable kinetic isotope effects.
• The platform demonstrates versatility for tunable (de)hydrogenation of N-H, O-H, and C-H bonds.

Conclusions:

• The novel VW12 MS-PCET platform offers a versatile and tunable approach for controlling H atom transfer reactions.
• This system overcomes thermodynamic constraints of single-site PCET, opening new avenues in redox catalysis.
• The findings have broad implications for designing catalysts for various chemical transformations.