Proton-Mediated Redox Control in a Nickel(II)-Bisimidazolate Complex: Spectroscopic Characterisation and Density Functional Analysis

• Published in: Chemistry : a European journal 2008-05, Vol.14 (14), p.4307-4317
• Main Authors: Lassalle-Kaiser, Benedikt, Guillot, Régis, Sainton, Joëlle, Charlot, Marie-France, Aukauloo, Ally
• Format: Article
• Language: English
• Subjects: amides; Crystallography, X-Ray; density functional calculations; electrochemistry; Hydrogen Bonding; imidazole; Imidazoles - chemical synthesis; Imidazoles - chemistry; Ligands; Magnetic Resonance Spectroscopy; Models, Molecular; nickel; Nickel - chemistry; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Oxidation-Reduction; protonation; Protons; Spectrophotometry, Infrared; Spectrophotometry, Ultraviolet; Thermodynamics
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200701662

The synthesis and characterisation of the pro‐ligand LH4, in which L is the o‐phenylenebisamide‐2‐imidazole and its nickel(II) complexes are reported. The X‐ray structures of the fully protonated [NiLH2] and deprotonated [NiL] complexes are presented. The effects of the deprotonation of the imidazole functions on the electronic structure of the complexes are analysed by 1H NMR, UV/Vis and IR spectroscopy and cyclic voltammetry. Density functional theory (DFT) and time‐dependent density functional theory (TDDFT) calculations support the analysis based on experimental data. The singly oxidised form of the deprotonated complex [NiL] was generated by preparative electrolysis and its electronic structure was investigated. Spectroelectrochemistry shows the appearance of intense transitions in the region λ=600–900 nm with several isosbestic points. X‐band EPR spectroscopy presents an isotropic signal at g=2.03, whereas the Q‐band EPR reveals a weak anisotropic signal characteristic of a metalloradical species. DFT and TDDFT data support the description of the species as a nickel(II)‐radical form, with a major contribution of the spin density on the phenylene ring and the amidate functions with a negligible participation of the imidazolate groups. This finding is in sharp contrast with those obtained from the one‐electron‐oxidised form of nickel(II) complexes containing phenolate groups.1–3 Innocent imidazole: An imidazole function is found to act as a redox potential tuner through its different protonic states in a square‐planar nickel(II) complex, but is innocent in the electronic description of the one‐electron‐oxidised form (see graphic).