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

Abstract

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(η(2)-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.

Nickel-mediated N–N bond formation and N2O liberation via nitrogen oxyanion reduction

“…There is precedence for nitric oxide liberation upon one electron oxidation of [Ni(NO)(bipy)][PF 6 ] with either AgPF 6 or [NO][PF 6 ] as the oxidant. 48 …”

“…The Ni1-N3-O1 angle is linear (175.5 ), and the complex has a strong nitrosyl stretching frequency at 1785 cm À1 , similar to other LNi(NO)(X) complexes. [46][47][48][49] The nitrito linkage in (DIM)Ni(NO)(ONO) contrasts the N-bound nitrite linkage in (Me 2 -ortho-phenanthroline)Ni(NO)(NO 2 ). 49…”

“…Compound (DIM)Ni(NO)(ONO) shows two oxygen 1s binding energies with relative intensities of 1 : 2, indicating resolution [PF 6 ] as the oxidant. 48 Even an oxidant with a more weakly nucleophilic triate anion succeeds in this oxidative process. The reaction of [(DIM) Ni(NO)] 2 with AgOTf in THF results in a color change to green and homogeneity of the solution.…”

Simultaneous nitrosylation and N-nitrosation of a Ni-thiolate model complex of Ni-containing SOD

“…In accord with other tetrahedral {NiNO} 10 complexes,28 the Ni–N(O) distance is short (1.659 Å), the N–O bond (1.182 Å) is intermediate between free NO˙ (1.15 Å) and 1 HNO (1.21 Å),10 and the Ni–N–O bond angle is close to linear albeit slightly bent (167.8°) (see Table 1). Complex 2 is analogous to the limited number of four-coordinate/S-bound Ni-nitrosyls,30–34 fewer of which contain Ni–S thiolate bonds31,33 which display Ni–N(O) (1.663–1.683 Å), N–O (1.131–1.173 Å), and Ni–N–O (156.6–173.9°) distances/angles in similar ranges. Even neutral/cationic P-35–40 and N-bound34,41–43 L 3 Ni–NO/L 2 XNi–NO complexes exhibit similar metric parameters.…”

“…Complex 2 is analogous to the limited number of four-coordinate/S-bound Ni-nitrosyls,30–34 fewer of which contain Ni–S thiolate bonds31,33 which display Ni–N(O) (1.663–1.683 Å), N–O (1.131–1.173 Å), and Ni–N–O (156.6–173.9°) distances/angles in similar ranges. Even neutral/cationic P-35–40 and N-bound34,41–43 L 3 Ni–NO/L 2 XNi–NO complexes exhibit similar metric parameters. The coordinated nitrosamine is bent (N–N–O: 115.4°), i.e.…”