A Continuum of Proton-Coupled Electron Transfer Reactivity

Area of Science:

• Physical Chemistry
• Chemical Kinetics
• Biochemistry

Background:

• Proton-coupled electron transfer (PCET) encompasses reactions involving electron and proton transfer.
• Hydrogen atom transfer (HAT) is a well-studied PCET reaction, but distinctions blur with multiple-site concerted proton-electron transfer (MS-CPET).
• MS-CPET involves spatially separated electron and proton transfer, increasingly relevant in biological and synthetic chemistry.

Purpose of the Study:

• To demonstrate that HAT and MS-CPET reactions exist on a single reactivity continuum.
• To establish a unified thermodynamic and kinetic framework for understanding diverse PCET reactions.
• To highlight the common governing parameters for reactions involving proton and electron transfer.

Main Methods:

• Analysis of thermochemical frameworks, including bond dissociation free energies (BDFEs) and effective BDFEs (BDFE_eff).
• Application of kinetic models, such as linear free energy relationships (Brønsted catalysis law) and Marcus theory.
• Examination of experimental studies ranging from metal-mediated HAT to MS-CPET involving C-H bonds.

Main Results:

• BDFEs and BDFE_eff, derived from pKa and E° values, provide a unified thermochemical description for both HAT and MS-CPET.
• Linear free energy relationships and Marcus theory-type approaches effectively predict and explain rate constants for reactions across the PCET continuum.
• Studies reveal similarities in reactions previously considered distinct, emphasizing the role of hydrogen bonding and preorganization in MS-CPET.

Conclusions:

• HAT and MS-CPET are not fundamentally different but represent points along a single PCET reactivity continuum.
• A unified thermodynamic and kinetic framework simplifies the understanding and prediction of diverse PCET reactions.
• Recognizing the common features of PCET reactions facilitates exploration of new chemical reactivity.