Stabilizing Terminal Ni(III)-Hydroxide Complex Using NNN-Pincer Ligands: Synthesis and Characterization

Area of Science:

• Organometallic Chemistry
• Nickel Catalysis
• Dioxygen Activation

Background:

• The reactivity of low-valent nickel complexes with molecular oxygen is crucial for catalysis.
• Understanding the intermediates and oxidation states involved is key to designing efficient catalysts.

Purpose of the Study:

• To investigate the reaction of [Ni(COD)₂] with a bis(imino)pyridyl ligand (L₁) under oxidative conditions.
• To characterize the resulting nickel complex and elucidate its electronic structure and magnetic properties.

Main Methods:

• Synthesis and crystallographic characterization of the nickel complex.
• Isotopic labeling experiments (¹⁶O/¹⁸O) to trace oxygen origin.
• Spectroscopic techniques (NMR, IR, XPS, EPR) for structural and electronic analysis.
• Density Functional Theory (DFT) calculations for electronic structure prediction.
• Variable temperature magnetic susceptibility measurements.

Main Results:

• Formation of a transient Ni:dioxygen adduct that abstracts a hydrogen atom from THF.
• Isolation and characterization of an uncommon Ni(III) complex, [Ni^III(L₁)^2-(OH)] (1).
• Confirmation of dioxygen activation and Ni(III) oxidation state via isotopic labeling, XPS, and spectroscopy.
• Evidence for an S = 1/2 ground state with dominant antiferromagnetic interactions, supported by magnetic and EPR data.
• DFT calculations align with experimental electronic structure predictions.

Conclusions:

• The NNN-pincer ligand facilitates the stabilization of a Ni(III) complex through dioxygen activation.
• The Ni(III) complex is catalytically active in hydrogen atom transfer reactions.
• The study provides insights into the electronic and magnetic properties of Ni(III) complexes relevant to catalysis.