Redox-Active Ligands Permit Multielectron O₂ Homolysis and O-Atom Transfer at Exceptionally High-Valent Vanadyl...

Area of Science:

• Inorganic Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• Vanadium complexes are explored for catalytic applications, particularly in oxidation reactions.
• Ligand noninnocence plays a crucial role in modulating metal center reactivity.
• Understanding redox-active ligands is key to designing efficient catalysts.

Purpose of the Study:

• To synthesize and characterize a five-coordinate chlorovanadium species with redox-active N-phenyl aminophenol ligands.
• To investigate the O2 activation mechanism and oxygen atom transfer capabilities of the complex.
• To determine the vanadium-oxygen bond dissociation energy (BDE) and compare it to conventional vanadyl complexes.

Main Methods:

• Synthesis of a vanadium complex supported by two redox-active N-phenyl aminophenol ligands.
• Experimental characterization including structural and spectroscopic analysis.
• Computational studies (e.g., DFT) to elucidate electronic structure and reaction mechanisms.
• Kinetic studies involving reactions with O2, 9,10-dihydroanthracene, and main-group nucleophiles.

Main Results:

• A stable five-coordinate vanadium complex, [(Phap)(Phisq)VIVCl], was prepared, featuring dianionic amidophenolate and monoanionic iminosemiquinonate radical ligands.
• Exposure to O2 led to ligand-centered 2e- oxidation, forming [(Phisq)(Phibq)VIV(O)Cl] with a weak V≡O bond (73 ± 14 kcal mol-1).
• The complex demonstrated O2 activation and O-atom transfer, regenerating the reduced ligand form upon reduction.

Conclusions:

• The redox noninnocence and covalency of the vanadium-aminophenol ligands enable efficient O2 activation and O-atom transfer.
• The exceptionally weak V≡O bond in the vanadyl species is attributed to the stabilization of the reduced product.
• This work presents an alternative strategy for generating potent oxidants, distinct from strong outer-sphere electron acceptors, with implications for designing early metal catalysts for aerobic oxidations.