Mechanistic Insights into the Formation of N<sub>2</sub>O by a Nickel Nitrosyl Complex
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Abstract
Reaction
of [Ni(NO)(bipy)(Me<sub>2</sub>phen)][PF<sub>6</sub>] with 1 equiv
of nitric oxide (NO) in CH<sub>2</sub>Cl<sub>2</sub> results in the
formation of N<sub>2</sub>O and [{(Me<sub>2</sub>phen)Ni(NO)}<sub>2</sub>(μ–η<sup>1</sup>-<i>N</i>:η<sup>1</sup>-<i>O</i>)-NO<sub>2</sub>)][PF<sub>6</sub>] (<b>3</b>), along with the known complex, [Ni(bipy)<sub>3</sub>][PF<sub>6</sub>]<sub>2</sub> (<b>4</b>). The isolation of complex <b>3</b>, which contains a nitrite ligand, demonstrates that the
reaction of [Ni(NO)(bipy)(Me<sub>2</sub>phen)][PF<sub>6</sub>] with
exogenous NO results in NO disproportionation and not NO reduction.
Complex <b>3</b> could also be accessed by reaction of [Ni(NO)(Me<sub>2</sub>phen)][PF<sub>6</sub>] (<b>5</b>) with (Me<sub>2</sub>phen)Ni(NO)(NO<sub>2</sub>) (<b>7</b>) in good yield. Complexes <b>3</b>, <b>5</b>, and <b>7</b> were fully characterized,
including analysis by X-ray crystallography in the case of <b>3</b> and <b>7</b>. Furthermore, addition of 0.5 equiv of bipy to
[Ni(NO)(bipy)][PF<sub>6</sub>] results in formation of the hyponitrite
complex, [{(bipy)Ni(κ<sup>2</sup>-O<sub>2</sub>N<sub>2</sub>)}η<sup>1</sup>:η<sup>1</sup>-<i>N</i>,<i>N</i>-{Ni(NO)(bipy)}<sub>2</sub>][PF<sub>6</sub>]<sub>2</sub> (<b>9</b>), in modest yield. Importantly, the hyponitrite
ligand in <b>9</b> is thought to form via coupling of two NO<sup>–</sup> ligands and not by coupling of a nucleophilic nitrosyl
ligand (NO<sup>–</sup>) with an electrophilic nitrosyl ligand
(NO<sup>+</sup>). X-ray crystallography reveals that complex <b>9</b> features a new binding mode of the <i>cis</i>-hyponitrite
ligand
