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

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 (1)H 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 lambda = 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.