# Controlling Redox and Photophysical Properties of First-Row Transition Metal Complexes via Ligand Perhalogenation

## TL;DR

This paper shows how fully halogenated ligands can control the redox and light properties of nickel complexes, useful for designing better photoredox catalysts and light-based technologies.

## Contribution

The study introduces perhalogenated isocyanide ligands as a novel design strategy to control redox and photophysical properties of first-row transition metal complexes.

## Key findings

- Perfluorinated complexes show a dramatic anodic shift in Ni(0)/Ni(I) redox potential due to strong π-acceptor ligands.
- MLCT absorption energies remain stable despite ligand changes, supported by DFT calculations.
- Ultrafast spectroscopy reveals 3MLCT excited states with lifetimes of 66–141 ps across all complexes.

## Abstract

Halogenation of ligands
intensely modulates the redox
and photophysical
properties of transition-metal complexes, yet fully halogenated systems
remain largely unexplored. Here we report the synthesis and structural
characterization of homoleptic Ni(0) complexes with perhalogenated
aryl isocyanide ligands [Ni­(CN-C6X5)4] (X = F, Cl). Comparative electrochemical studies reveal a dramatic
anodic shift of the Ni(0)/Ni­(I) couple from −0.60 V in [Ni­(CN–C6H5)4] to +0.03 V vs Fc+/0 for the perfluorinated species, reflecting the exceptional π-acceptor
strength resulting from the C–H/C–F persubstitution.
Surprisingly, metal-to-ligand charge-transfer (MLCT) absorption energies
remain largely unchanged, a result supported by DFT calculations showing
concurrent stabilization of both the Ni-centered HOMO and ligand-based
LUMO. In contrast, the perchlorinated complex exhibits a red-shifted
MLCT band due to asymmetric frontier-orbital tuning. Ultrafast transient
absorption spectroscopy demonstrates 3MLCT excited states
with lifetimes in the regime of 66–141 ps for all complexes.
These findings establish perhalogenated isocyanides as powerful ligands
for controlling excited-state redox potentials without altering excitation
energies, an attractive feature for the rational design of robust
Ni-based photoredox catalysts. More broadly, our findings establish
ligand perhalogenation as a design strategy for developing new photoactive
first-row transition metal complexes with potential applications in
luminescent devices, photocatalysis, and photodynamic therapy.