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

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.