# Spectroscopically Deciphering the Formation and Reactivity of a High-Valent Ni(IV)Cl2 Species

## TL;DR

Scientists created a nickel complex that forms a high-valent Ni(IV) species, which can transfer electrons and oxygen atoms to other molecules.

## Contribution

A new nickel complex was developed that allows the formation and characterization of a high-valent Ni(IV) species.

## Key findings

- The Ni(IV) species was formed from a Ni(II) precursor and shown to undergo electron and oxygen atom transfer reactions.
- X-ray absorption spectroscopy confirmed the formation of the Ni(IV) species.
- The reaction rate depends on the electronic properties of substituents.

## Abstract

Recent investigations have demonstrated the appeal of
using Ni­(II)
complexes with redox-active ligands in fields like catalysis, electrochemistry,
or materials sciences. Ni­(salen) complexes have particularly been
shown to exhibit temperature-dependent equilibrium based on the localization
of the unpaired spin. However, the usage of salen as a ligand has
always restricted the characterization of a Ni­(IV) species with Ni
bearing both the oxidizing equivalents. Hence, the current work aims
to develop the biologically relevant pseudopeptide-based Ni complex
that enables the formation and trapping of a high-valent Ni­(IV) species
from its Ni­(II) precursor. The synthesized [LNiII] (2) (L = N,N’-(4,5-dimethyl-1,2-phenylene)­bis­(pyrrolidine-2-carboxamide))
was shown to form a high-valent [LNiIVCl2] (4Cl) species, depending on the axial coordination, upon the
addition of excess ceric ammonium nitrate, in the presence of chloride
ions as an exogenous ligand, as supported by X-ray absorption spectroscopic
analysis. Favorably, the formed Ni­(IV) species has also demonstrated
electron transfer and oxygen atom transfer (OAT) reactions toward
thioanisoles. Computational analysis of the mechanism revealed that
the oxidation of thioanisoles proceeds via a stepwise pathway involving
a single electron transfer from thioanisole, followed by OAT to the
subsequent radical cation. The rate of these reactions demonstrated
a strong dependence on the electronics of the substituents.