Basicity-Controlled C-H Bond Activation by a Structurally Characterized Ni(III)-Hydroxo Complex.

Area of Science:

• Organometallic Chemistry
• Catalysis
• Synthetic Chemistry

Background:

• Selective oxidation of strong C-H bonds is a significant challenge in synthetic chemistry.
• The mechanisms and active oxidants for C-H bond activation are not fully understood.
• Developing stable and reactive metal-oxo or metal-hydroxo complexes is crucial.

Purpose of the Study:

• To isolate and characterize a novel mononuclear Ni(III)-hydroxo complex.
• To investigate the C-H bond activation capabilities of the Ni(III)-hydroxo complex.
• To elucidate the mechanism of hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET).

Main Methods:

• Isolation and complete characterization of the Ni(III)-hydroxo complex using X-ray crystallography.
• Hydrogen atom transfer (HAT) reactivity studies with various C-H substrates, including cyclohexane.
• Kinetic studies to correlate reaction rates with substrate properties (pKa, BDE).
• Semiempirical free energy analysis to determine the degree of proton transfer (PT) character.

Main Results:

• A room-temperature-stable mononuclear Ni(III)-hydroxo complex, [Na(15c5)][Ni(PS3″)(OH)] (2), was successfully synthesized and characterized.
• Complex 2 demonstrated hydrogen atom transfer (HAT) reactivity towards strong C-H bonds.
• Kinetic studies indicated an asynchronous PCET pathway, predominantly governed by proton transfer (PT), with a best-fit x value of 0.18.
• The O-H bond dissociation free energy of the resulting Ni(II)-aqua species was determined to be 96.6-100.3 kcal mol⁻¹.

Conclusions:

• The Ni(III)-hydroxo complex 2 is a rare, well-defined oxidant capable of activating strong C-H bonds.
• Basicity of the substrate plays a critical role in modulating the PCET reactivity.
• The findings provide valuable mechanistic insights into C-H bond oxidation mediated by metal-hydroxo species.