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

## TL;DR

A stable Ni(III)-hydroxo complex is shown to activate strong C–H bonds through a proton-coupled electron transfer mechanism influenced by basicity.

## Contribution

A well-defined Ni(III)–hydroxo complex is characterized and shown to selectively activate strong C–H bonds with low redox potential.

## Key findings

- The Ni(III)–hydroxo complex activates cyclohexane C–H bonds (BDE = 99.5 kcal mol–1) via hydrogen atom transfer.
- Kinetic studies show a strong correlation between reaction rates and substrate pKa, indicating a proton-transfer-dominated PCET pathway.
- Thermodynamic analysis estimates the O–H bond dissociation free energy of the resulting Ni(II)–aqua species as 96.6–100.3 kcal mol–1.

## Abstract

The selective oxidation of strong C–H bonds remains
a central
challenge in synthetic chemistry, in part due to the elusive nature
of active oxidants and their underlying mechanisms. Herein, we report
the isolation and complete characterization of a room–temperature–stable
mononuclear Ni­(III)–hydroxo complex, [Na­(15c5)]­[Ni­(PS3″)­(OH)]
([Na­(15c5)]­[2]), supported by a tris(benzenethiolato)­phosphine ligand derivative. The X-ray crystallographic
structure of 2 reveals a trigonal bipyramidal Ni­(III)
center, in which the coordinated hydroxo ligand is stabilized by secondary
coordination sphere interactions. Complex 2 displays
hydrogen atom transfer (HAT) reactivity toward strong C–H bonds,
including that in cyclohexane (BDE = 99.5 kcal mol–1). Kinetic studies with various C–H substrates reveal a strong
linear correlation between log­(k
2) and
substrate pK
a, but a poor correlation
with C–H bond dissociation energies, indicating an asynchronous
PCET pathway with a transition state predominantly governed by proton
transfer (PT). The O–H bond dissociation free energy of a resulting
Ni­(II)–aqua species was estimated to be 96.6–100.3 kcal
mol–1 based on thermodynamic data. A semiempirical
free energy analysis following the approach of Barman et al. (Proc. Natl. Acad. Sci. U.S.A.
2021, 118, e2108648118)
gives a best-fit x value of 0.18 (R
2 = 0.99), where x = 1 indicates synchronous
PCET and lower values reflect greater PT character in the transition
state. These findings underscore the critical role of basicity in
modulating PCET reactivity and establish complex 2 as
a rare, well–defined Ni­(III)–OH oxidant capable of strong
C–H bond activation at low redox potential.