Ligand non-innocence enables HAT at dinuclear nickel complexes

Reported are the syntheses, characterizations, and reactivities of the dinuclear nickel(II) complexes [Ni₂(κ²-OOCR)₃PNNP^iPr]⁺ (R = Me or ^tBu, PNNP^iPr = 2,7-bis-(di-iso-propylphosphino-methyl)-1,8-naphthyridine), isolated as the BF₄^- salts. Notably, the [Ni₂(κ²-OOCR)₃PNNP^iPr]⁺ cations can be deprotonated reversibly at the methylene carbon of the ligand scaffold to form the neutral dinuclear Ni(II) complexes [Ni₂(κ²-OOCR)₃(*PNNP^iPr)], where the *PNNP^iPr anion is the deprotonated and dearomatized PNNP^iPr. The latter complexes were also formed by hydrogen atom transfer (HAT) upon reaction of the mixed valent Ni₂(I,II) species [Ni₂(κ²-OOCR)₃PNNP^iPr] with hydrogen atom acceptors 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 2,4,6-triterbutyl phenoxyl radical (tBu₃ArO˙) and 2,4-diethoxybenzoquinone (DEBQ) or with air. Based on the reduction potentials obtained via cyclic voltammetry and the pK_a values tracked via UV-Vis spectroscopy, the bond dissociation free energies of the methylene C-H bonds were found between 59 and 61 kcal mol^-1. The kinetics of the proton/electron abstraction from the mixed valent complexes by various equivalents of TEMPO were monitored by UV-Vis spectroscopy. This process is reversible upon reaction with 5,10-dihydrophenazine, serving as both an electron and proton donor. No reaction intermediates were detected.