Anion-Induced Electron Transfer.

Area of Science:

• Physical Chemistry
• Supramolecular Chemistry
• Organic Chemistry

Background:

• Electron transfer (ET) from anions to π-acids is often counterintuitive, particularly for fluoride (F⁻).
• In aprotic media, strong Lewis basic anions (F⁻, OH⁻, CN⁻) can donate electrons to π-acids (πA).
• Thermodynamic feasibility of ET depends on the relative energies of the highest occupied molecular orbitals (HOMOs) of anions and lowest unoccupied molecular orbitals (LUMOs) of π-acids.

Purpose of the Study:

• To investigate the mechanism of electron transfer from Lewis basic anions to π-acids in aprotic solvents.
• To clarify the electron-donating ability of the fluoride anion (F⁻) and challenge common misconceptions.
• To establish the thermodynamic criteria for anion-induced electron transfer (AIET) pathways.

Main Methods:

• Spectroscopic studies including UV-vis, EPR, and NMR.
• Thermodynamic analysis of free energy changes (ΔG°_ET) for electron transfer.
• Evaluation of alternative reaction mechanisms (nucleophilic attack, deprotonation, complex formation).

Main Results:

• Spectroscopic evidence confirms the formation of paramagnetic πA^•− radical anions and πA²⁻ dianions.
• Anion-induced ET (AIET) is thermodynamically justified (ΔG°_ET < 0) when anion HOMOs are above π-acid LUMOs.
• Fluoride (F⁻) effectively reduces π-acids with LUMO levels ≤ -3.8 eV in aprotic environments, demonstrating strong electron-donating ability.
• Alternative proposed mechanisms were found to be thermodynamically unviable and lacked experimental support.

Conclusions:

• Anion-induced electron transfer (AIET) is the sole viable mechanism explaining π-acid reduction by Lewis basic anions in aprotic solvents.
• The electron-donating ability of F⁻ is a function of its Lewis basicity, not fluorine's electronegativity.
• AIET has significant implications, including anion detection, materials science (silver plating, nanoparticle synthesis), and energy applications.