Enhanced Proton-Coupled Electron-Transfer Reactivity by a Mononuclear Nickel(II) Hydroxide Radical Complex

Area of Science:

• Inorganic Chemistry
• Organometallic Chemistry
• Redox Chemistry

Background:

• Development of novel metal complexes with tunable redox properties is crucial for catalysis.
• Nickel hydroxide complexes are of interest due to their potential in various chemical transformations.

Purpose of the Study:

• To synthesize and characterize a NiOH core with a tridentate redox-active ligand.
• To investigate the electrochemical behavior and reactivity of the NiOH complex in different oxidation states.
• To explore its potential in hydrogen atom abstraction reactions.

Main Methods:

• Single-crystal X-ray diffraction for structural analysis.
• Cyclic voltammetry to determine redox potentials.
• Spectroscopic and computational methods for characterization.
• Reactivity studies involving hydrogen atom abstraction from organic substrates.

Main Results:

• A square-planar NiOH core stabilized by intramolecular H-bonding was synthesized and characterized.
• The complex exhibits reversible oxidation to three distinct molecular states ([LNiOH]2-, [LNiOH]-, and [LNiOH]).
• All species are Ni(II) with the ligand adopting catecholate-like, semiquinone-like, and quinone-like forms, respectively.
• The NiOH species facilitate H-atom abstraction, with [LNiOH]- acting as a 1H+/1e- oxidant and [LNiOH] as a 2H+/2e- oxidant.
• [LNiOH]- demonstrates faster proton-coupled electron transfer (PCET) kinetics than [LNiOH] despite lower thermochemical driving force.

Conclusions:

• The synthesized NiOH complex with a redox-active ligand offers multiple accessible oxidation states.
• The complex's ability to promote H-atom abstraction highlights its catalytic potential.
• The enhanced PCET reactivity of the [LNiOH]- intermediate is attributed to its unique stereoelectronic structure, combining radical character with a basic NiOH core.