Beyond the classical thermodynamic contributions to hydrogen atom abstraction reactivity

Area of Science:

• Catalysis
• Physical Chemistry
• Organic Chemistry

Background:

• Hydrogen atom abstraction (HAA) reactions are fundamental in chemistry, with natural and synthetic catalysts developed for their efficiency.
• Understanding the factors governing HAA catalytic efficiency remains an ongoing challenge.

Purpose of the Study:

• To define a simple thermodynamic factor, η, that quantifies the synchronicity of concerted proton-electron transfers in HAA reactions.
• To explore the relationship between η, reorganization energy (λ), and activation energies in HAA processes.
• To demonstrate the utility of this approach by correlating properties of nonheme FeIVO complexes with HAA activation energies.

Main Methods:

• Utilized two thermodynamic cycles to define the factor η for oxidants and substrates.
• Employed computational methods to investigate the link between η and Marcus theory's reorganization energy (λ).
• Correlated experimental redox and acidobasic properties of nonheme FeIVO complexes with calculated activation free energies for HAA.

Main Results:

• The thermodynamic factor η quantifies the propensity for (a)synchronicity in concerted H+/e- transfers during HAA.
• η significantly contributes to HAA activation energies, complementing the Bell-Evans-Polanyi effect.
• A direct correlation was found between η and reorganization energy (λ), with maximum λ occurring at minimum |η|, indicating the most synchronous HAA mechanism.
• Among HAA reactions with similar free energy changes, the most synchronous proton-coupled electron transfer exhibits the highest activation barrier.
• Redox and acidobasic properties of nonheme FeIVO complexes were successfully correlated with HAA activation energies.

Conclusions:

• The thermodynamic factor η provides a new perspective on HAA reaction mechanisms and catalytic efficiency.
• The interplay between synchronicity (η) and reorganization energy (λ) is crucial for understanding HAA activation barriers.
• The findings offer a powerful conceptual framework for the rational design of advanced HAA catalysts.