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

Abstract

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.

Chelating tris(amidate) ligands: versatile scaffolds for nickel(ii)

“…FTIR (KBr, cm -1 ) n: 3056, 3021, 2927, 2247 (CH 3 CN), 1596 (CO), 1584, 1552, 1473, 1442, 1357, 1109, 1041, 997, 914, 754, 723, 690, 527. m eff = 3.27 m B (Evans Method, DMSO-d 6, 298 K). l max (e, M -1 cm -1 ) (DMF): 707(16). Sample for elemental analysis was prepared by the diffusion of diethyl ether into a concentrated DMF solution of the complex.…”