Spectroscopically Deciphering the Formation and Reactivity of a High-Valent Ni(IV)Cl₂ Species

Recent investigations have demonstrated the appeal of using Ni-(II) complexes with redox-active ligands in fields like catalysis, electrochemistry, or materials sciences. Ni-(salen) complexes have particularly been shown to exhibit temperature-dependent equilibrium based on the localization of the unpaired spin. However, the usage of salen as a ligand has always restricted the characterization of a Ni-(IV) species with Ni bearing both the oxidizing equivalents. Hence, the current work aims to develop the biologically relevant pseudopeptide-based Ni complex that enables the formation and trapping of a high-valent Ni-(IV) species from its Ni-(II) precursor. The synthesized [LNi^II] (2) (L = N,N'-(4,5-dimethyl-1,2-phenylene)-bis-(pyrrolidine-2-carboxamide)) was shown to form a high-valent [LNi^IVCl₂] (4Cl) species, depending on the axial coordination, upon the addition of excess ceric ammonium nitrate, in the presence of chloride ions as an exogenous ligand, as supported by X-ray absorption spectroscopic analysis. Favorably, the formed Ni-(IV) species has also demonstrated electron transfer and oxygen atom transfer (OAT) reactions toward thioanisoles. Computational analysis of the mechanism revealed that the oxidation of thioanisoles proceeds via a stepwise pathway involving a single electron transfer from thioanisole, followed by OAT to the subsequent radical cation. The rate of these reactions demonstrated a strong dependence on the electronics of the substituents.