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

Reaction of [Ni­(NO)­(bipy)]­[PF<sub>6</sub>] (<b>2</b>) with AgPF<sub>6</sub> or [NO]­[PF<sub>6</sub>] in MeCN results in formation of [Ni­(bipy)<sub><i>x</i></sub>(MeCN)<sub><i>y</i></sub>]<sup>2+</sup> and release of NO gas in moderate yields. In contrast, the addition of the inner sphere oxidant Ph<sub>2</sub>S<sub>2</sub> to <b>2</b> does not result in denitrosylation. Instead, the diphenyldisulfide adduct [{(bipy)­(NO)­Ni}<sub>2</sub>(μ-S<sub>2</sub>Ph<sub>2</sub>)]­[PF<sub>6</sub>]<sub>2</sub> (<b>3</b>) is formed in good yield. However, oxidation of <b>2</b> 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(η<sup>2</sup>-TEMPO)]­[PF<sub>6</sub>] (<b>4</b>), can be isolated as an orange-brown solid in excellent yields. In the solid state, complex <b>4</b> contains a side-on bound TEMPO<sup>–</sup> ligand, which is characterized by a long N–O bond length [1.383(2) Å]. The contrasting reactivity of Ph<sub>2</sub>S<sub>2</sub> and TEMPO likely relates to their different redox potentials, as Ph<sub>2</sub>S<sub>2</sub> is a relatively weak oxidant. Finally, the addition of pyridine-<i>N</i>-oxide to <b>2</b> results in the formation of the adduct, [(bipy)­Ni­(NO)­(ONC<sub>5</sub>H<sub>5</sub>)]­[PF<sub>6</sub>] (<b>5</b>). No evidence of NO release is observed in this reaction, probably because of the low one-electron (1e<sup>–</sup>) reduction potential of pyridine-<i>N</i>-oxide.