Modulation of Nickel Pyridinedicarboxamidate Complexes to Explore the Properties of High‐valent Oxidants.

Abstract: High‐valent Ni oxidants have been implicated in hydrocarbon oxidation catalysis, however, little is understood about the properties of these oxidants. Herein, a family of NiII complexes supported by a pyridinedicarboxamidate ligand and different ancillary ligands was synthesized. The series spans coordination numbers 4, 5, and 6, and contains neutral, mono‐ and di‐anionic donor types. X‐ray crystallography and magnetic measurements showed that the 4‐coordinate complexes were square planar and low spin (<italic>S=</italic>0) and the 5‐ and 6‐coordinate were intermediate spin (<italic>S=</italic>1). The NiII complexes could be oxidized by one electron to form a series of metastable NiIII species. EPR analysis confirmed their description as <italic>S</italic>= 1 / 2 NiIII compounds with signal shape and hyperfine coupling dependent on the coordination environment. The oxidation of phenols by the NiIII species was probed, providing evidence for a correlation between oxidizing power and electron‐donating properties of the supporting ligands. Critically, we found that the pyridinedicarboxamidate ligand may be a non‐innocent proton acceptor in the oxidation reactions. [ABSTRACT FROM AUTHOR]