Reductive Binding and Ligand-Based Redox Transformations of Nitrosobenzene at a Dinickel(II) Core.

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)] (1) 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(PhNO)] (2). The addition of [2,2,2]cryptand leads to the ionic complex [K(crypt)][LNi(PhNO)] (3). Structural and spectroscopic analyses evidence that interaction with the Lewis acidic K in 2 causes significant elongation and weakening of the substrate's N-O bond [ d = 1.487(12) Å in 2 vs 1.374(4) Å in 3]. Complex 2 (or 3) reacts with [FeCp*][PF] to give LNi(PhNO) (4), which is shown by electron paramagnetic resonance and IR spectroscopies and density functional theory calculations to feature two low-spin d nickel(II) ions and a bridging (PhNO) 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 4 and the anion of 3 can be reversibly interconverted at very low potential ( E = -1.53 V vs Fc/Fc). Protonation of 2 leads to the N-phenylhydroxylamine complex [LNi(ONHPh)] (5) with a long N-O bond of 1.464(2) Å, and titration studies suggest a p K of around 23-25 in tetrahydrofuran. This allows one to derive a bond dissociation energy of 62-65 kcal mol for the N-H bond of 5. Accordingly, 5 readily reacts with the phenoxy radical 2,4,6- BuCHO to yield 4. 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.