Nitric Oxide Release from a Nickel Nitrosyl Complex Induced by One-Electron Oxidation

Reaction of [Ni­(NO)­(bipy)]­[PF₆] (2) with AgPF₆ or [NO]­[PF₆] in MeCN results in formation of [Ni­(bipy)_x(MeCN)_y]²⁺ and release of NO gas in moderate yields. In contrast, the addition of the inner sphere oxidant Ph₂S₂ to 2 does not result in denitrosylation. Instead, the diphenyldisulfide adduct [{(bipy)­(NO)­Ni}₂(μ-S₂Ph₂)]­[PF₆]₂ (3) is formed in good yield. However, oxidation of 2 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) does results in cleavage of the Ni–NO bond and generation of NO. The metal-containing product, [(bipy)­Ni­(η²-TEMPO)]­[PF₆] (4), can be isolated as an orange-brown solid in excellent yields. In the solid state, complex 4 contains a side-on bound TEMPO^– ligand, which is characterized by a long N–O bond length [1.383(2) Å]. The contrasting reactivity of Ph₂S₂ and TEMPO likely relates to their different redox potentials, as Ph₂S₂ is a relatively weak oxidant. Finally, the addition of pyridine-N-oxide to 2 results in the formation of the adduct, [(bipy)­Ni­(NO)­(ONC₅H₅)]­[PF₆] (5). No evidence of NO release is observed in this reaction, probably because of the low one-electron (1e^–) reduction potential of pyridine-N-oxide.