Reductive Binding and Ligand-Based Redox Transformations of Nitrosobenzene at a Dinickel(II) Core
2019-03-25T00:00:00Z (GMT) by
The metal-mediated activation of PhNO represents an important starting point for understanding the reactivity patterns of nitrosoarenes in biological systems and catalysis. Here we report that the pyrazole-based dinickel(II) dihydride complex [KL(NiH)<sub>2</sub>] (<b>1</b>) reacts with PhNO to eliminate dihydrogen concomitant with binding of the doubly reduced substrate in μ-κ(O):κ(N) mode in the bimetallic pocket of [KLNi<sub>2</sub>(PhNO)] (<b>2</b>). The addition of [2,2,2]cryptand leads to the ionic complex [K(crypt)][LNi<sub>2</sub>(PhNO)] (<b>3</b>). Structural and spectroscopic analyses evidence that interaction with the Lewis acidic K<sup>+</sup> in <b>2</b> causes significant elongation and weakening of the substrate’s N–O bond [<i>d</i><sub>N–O</sub> = 1.487(12) Å in <b>2</b> vs 1.374(4) Å in <b>3</b>]. Complex <b>2</b> (or <b>3</b>) reacts with [FeCp*<sub>2</sub>][PF<sub>6</sub>] to give LNi<sub>2</sub>(PhNO) (<b>4</b>), which is shown by electron paramagnetic resonance and IR spectroscopies and density functional theory calculations to feature two low-spin d<sup>8</sup> nickel(II) ions and a bridging (PhNO)<sup>•–</sup> radical anion ligand, with the out-of-plane π*(NO) being the singly occupied molecular orbital. Cyclic voltammetry and UV–vis spectroelectrochemical experiments show that <b>4</b> and the anion of <b>3</b> can be reversibly interconverted at very low potential (<i>E</i><sub>1/2</sub> = −1.53 V vs Fc/Fc<sup>+</sup>). Protonation of <b>2</b> leads to the <i>N</i>-phenylhydroxylamine complex [LNi<sub>2</sub>(ONHPh)] (<b>5</b>) with a long N–O bond of 1.464(2) Å, and titration studies suggest a p<i>K</i><sub>a</sub> of around 23–25 in tetrahydrofuran. This allows one to derive a bond dissociation energy of 62–65 kcal mol<sup>–1</sup> for the N–H bond of <b>5</b>. Accordingly, <b>5</b> readily reacts with the phenoxy radical 2,4,6-<sup><i>t</i></sup>Bu<sub>3</sub>C<sub>6</sub>H<sub>2</sub>O<sup>•</sup> to yield <b>4</b>. This work demonstrates the reductive binding of PhNO without prior formation of unstable nickel(I) species and the redox noninnocence of the PhNO ligand in the less common μ-κ(O):κ(N) bridging mode. Thermodynamic data for H-atom-abstraction chemistry at the activated PhNO may be valuable for understanding the reactivity patterns of the transient but biologically relevant nitroxyl (HNO) ligand.