Orchestrating the Semiquinone Stability for Catalytic Proton-Coupled Electron Transfer

Area of Science:

• Synthetic Chemistry
• Biochemistry
• Materials Science

Background:

• Semiquinone radicals are vital intermediates in biological proton-coupled electron transfer (PCET).
• Their inherent instability makes them challenging to study and utilize in synthetic chemistry.
• Existing synthetic systems struggle to replicate the stability and catalytic activity of biological semiquinones.

Purpose of the Study:

• To engineer stable semiquinone radicals for catalytic applications in PCET.
• To develop a method for accessing, manipulating, and recycling short-lived semiquinone intermediates.
• To bridge the gap between biological PCET mechanisms and synthetic chemical systems.

Main Methods:

• Utilizing a high-valent titanium(IV) center to weaken the hydroquinone O-H bond and stabilize the semiquinone via radical delocalization.
• Employing reversible covalent masking with a persistent carboradical to create an isolable semiquinone synthon.
• Investigating the reversibility of the masking process under reductive conditions, exploring negative hyperconjugation effects.

Main Results:

• Achieved engineered stability of semiquinone radicals through dual stabilization strategies.
• Developed an isolable and air-stable semiquinone synthon.
• Demonstrated reversible interconversion between hydroquinone and semiquinone states, enabling a recyclable PCET mediator.
• Successfully rendered semiquinones catalytically competent for PCET in synthetic systems.

Conclusions:

• The developed dual stabilization strategy overcomes the instability of semiquinone radicals.
• This work provides a recyclable PCET mediator with access to a key one-electron redox intermediate.
• Opens new avenues for designing synthetic systems that mimic biological proton-coupled electron transfer processes.