Stepwise 6H⁺/6e^- Electron-Coupled Proton Buffers Based on Fe and Redox-Active Ligands

Area of Science:

• Inorganic Chemistry
• Electrochemistry
• Materials Science

Background:

• Electron-coupled-proton transfer (ECPT) is crucial for many chemical and biological processes.
• Developing molecular systems that mimic these functions requires precise control over redox and protonation states.
• Iron complexes with redox-active ligands offer a promising platform for exploring complex ECPT phenomena.

Purpose of the Study:

• To synthesize and characterize novel iron-based electron-coupled-proton buffers (ECPBs).
• To investigate the stepwise and reversible 6-electron, 6-proton transformations in these Fe-ECPBs.
• To explore the thermochemistry and redox properties, including redox unleveling and decompensation, of these systems.

Main Methods:

• Synthesis of Fe complexes featuring redox-active ortho-phenylenediamine (opda) ligands.
• Structural and spectroscopic characterization of key Fe complexes.
• Electrochemical studies including open-circuit potential measurements and comproportionation reactions to determine thermochemistry.
• Evaluation of H-atom transfer capabilities and dehydrogenation of organic substrates.

Main Results:

• Reported Fe-based ECPBs capable of stepwise and reversible 6H+/6e- transformations.
• Characterized four Fe complexes (X62+, X8H22+, X10H42+, X12H62+) involved in the proton-coupled electron transfer (PCET) process.
• Observed redox unleveling, where the initial 2H+/2e- steps had higher average bond dissociation free energies (BDFEavg) than the overall 6H+/6e- transformation.
• Demonstrated redox decompensation, with unsubstituted opda ligands leading to higher BDFEavg compared to substituted opda ligands.
• Showcased the ability of these Fe-ECPBs to accept/donate H-atom equivalents and dehydrogenate organic substrates using O2.

Conclusions:

• Fe-based ECPBs with opda ligands provide a tunable platform for complex redox and protonation chemistry.
• The observed redox unleveling and decompensation highlight unique electronic properties influencing PCET pathways.
• These findings contribute to the design of advanced molecular systems for catalysis and energy conversion.