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

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 Ni-II 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 (S=0) and the 5- and 6-coordinate were intermediate spin (S=1). The Ni-II complexes could be oxidized by one electron to form a series of metastable Ni-III species. EPR analysis confirmed their description as S=1/2 Ni-III compounds with signal shape and hyperfine coupling dependent on the coordination environment. The oxidation of phenols by the Ni-III 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.