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)2] (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 [KLNi2(PhNO)] (2). The addition of [2,2,2]cryptand leads to the ionic complex [K(crypt)][LNi2(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 [ dN-O = 1.487(12) Å in 2 vs 1.374(4) Å in 3]. Complex 2 (or 3) reacts with [FeCp*2][PF6] to give LNi2(PhNO) (4), which is shown by electron paramagnetic resonance and IR spectroscopies and density functional theory calculations to feature two low-spin d8 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 ( E1/2 = -1.53 V vs Fc/Fc+). Protonation of 2 leads to the N-phenylhydroxylamine complex [LNi2(ONHPh)] (5) with a long N-O bond of 1.464(2) Å, and titration studies suggest a p Ka of around 23-25 in tetrahydrofuran. This allows one to derive a bond dissociation energy of 62-65 kcal mol-1 for the N-H bond of 5. Accordingly, 5 readily reacts with the phenoxy radical 2,4,6- tBu3C6H2O• 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.