Fluoride-Ion Donor Properties of AsF₅

Area of Science:

• Inorganic Chemistry
• Fluorine Chemistry
• Coordination Chemistry

Background:

• Arsenic pentafluoride (AsF5) is a powerful Lewis acid.
• Ligand-induced autoionization is a known phenomenon in Lewis acid chemistry.
• Donor-stabilized cations offer unique reactivity and structural motifs.

Purpose of the Study:

• To investigate the reaction of arsenic pentafluoride with nitrogen-containing ligands.
• To synthesize and characterize novel donor-stabilized arsenic cations.
• To explore the electronic properties and Lewis acidity of these new species.

Main Methods:

• Ligand-induced autoionization reactions in SO2ClF solution.
• Synthesis and crystallographic characterization of arsenic salts.
• Density functional theory (DFT) calculations and natural bond orbital (NBO) analysis.
• Fluoride-ion affinity calculations.

Main Results:

• Formation of the donor-stabilized salt [AsF4(phen)][AsF6] via autoionization of AsF5 with 1,10-phenanthroline.
• Synthesis of the mixed arsenic-antimony salt [AsF4(phen)][Sb2F11].
• These represent the first crystallographically characterized donor-stabilized [AsF4]+ cations.
• The reaction with 2,2'-bipyridine yielded a neutral adduct, not autoionization.
• Calculated fluoride-ion affinities indicate [AsF4]+ and [SbF4]+ are strong Lewis acids.
• DFT and NBO analyses confirm significant electron donation from phen to As in [AsF4(phen)]+, quenching its electrophilicity.

Conclusions:

• Ligand choice dictates autoionization versus adduct formation with AsF5.
• Donor-stabilized [AsF4]+ cations are accessible and structurally characterized.
• Electron donation from ligands effectively moderates the high Lewis acidity of the [AsF4]+ core.