Proton-mediated redox control in a nickel(II)-bisimidazolate complex: spectroscopic characterisation and density...

Area of Science:

• Coordination Chemistry
• Inorganic Chemistry
• Spectroscopy

Background:

• The synthesis and characterization of novel metal complexes are crucial for understanding electronic structures and reactivity.
• Nickel(II) complexes with imidazole-containing ligands offer diverse coordination environments and electronic properties.

Purpose of the Study:

• To synthesize and characterize nickel(II) complexes of the o-phenylenebisamide-2-imidazole ligand (L).
• To investigate the impact of imidazole deprotonation on the electronic structure of these nickel(II) complexes.
• To elucidate the electronic structure of the oxidized nickel(II) complex, particularly its radical character.

Main Methods:

• X-ray crystallography for structural determination of protonated and deprotonated complexes.
• Spectroscopic techniques including 1H NMR, UV/Vis, IR, and cyclic voltammetry.
• Electrochemical methods for generating and studying the oxidized species.
• Electron Paramagnetic Resonance (EPR) spectroscopy (X-band and Q-band).
• Computational methods: Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT).

Main Results:

• The X-ray structures of both protonated ([NiLH2]) and deprotonated ([NiL]) nickel(II) complexes were determined.
• Spectroscopic and electrochemical analyses revealed significant electronic structure changes upon imidazole deprotonation.
• Preparative electrolysis generated a singly oxidized form of [NiL], characterized by spectroelectrochemistry and EPR.
• EPR spectroscopy indicated a nickel(II)-radical species, with spin density localized on the phenylene and amidate groups of the ligand, distinct from phenolate-based systems.

Conclusions:

• Imidazole deprotonation profoundly influences the electronic properties of nickel(II) complexes.
• The oxidized nickel(II) complex exhibits a unique nickel(II)-radical character, with delocalized spin density on the ligand framework.
• This finding contrasts with the behavior of related nickel(II) complexes featuring phenolate donors, highlighting the ligand's role in electronic structure determination.