Stepwise Reduction of Redox Noninnocent Nitrosobenzene to Aniline via a Rare Phenylhydroxylamino Intermediate on a Thiolate-Bridged Dicobalt Scaffold

Abstract

Nitrosobenzene (PhNO) and phenylhydroxylamine (PhNHOH) are of paramount importance because of their involvement as crucial intermediates in the biological metabolism and catalytic transformation of nitrobenzene (PhNO 2 ) to aniline (PhNH 2 ). However, a complete reductive transformation cycle of PhNO to PhNH 2 via the PhNHOH intermediate has not been reported yet. In this context, we design and construct a new thiolate-bridged dicobalt scaffold that can accomplish coordination activation and reductive transformation of PhNO. Notably, an unprecedented reversible ligand-based redox sequence PhNO 0 ↔ PhNO •− ↔ PhNO 2− can be achieved on this well-defined {Co III (μ-SPh) 2 Co III } functional platform. Further detailed reactivity investigations demonstrate that the PhNO 0 and PhNO •− complexes cannot react with the usual hydrogen and hydride donors to afford the corresponding phenylhydroxylamino (PhNHO − ) species. However, the double reduced PhNO 2− complex can readily undergo N-protonation with an uncommon weak proton donor PhSH to selectively yield a stable dicobalt PhNHO − bridged complex with a high pK a value of 13−16. Cyclic voltammetry shows that there are two successive reduction events at E 1/2 = −0.075 V and E p = −1.08 V for the PhNO 0 complex, which allows us to determine both bond dissociation energy (BDE N−H ) of 59−63 kcal•mol −1 and thermodynamic hydricity (ΔG H − ) of 71−75 kcal•mol −1 of the PhNHO − complex. Both values indicate that the PhNO •− complex is not a potent hydrogen abstractor and the PhNO 0 complex is not an efficient hydride acceptor. In the presence of BH 3 as a combination of protons and electrons, facile N−O bond cleavage of the coordinated PhNHO − group can be realized to generate PhNH 2 and a dicobalt hydroxide-bridged complex. Overall, we present the first stepwise reductive sequence, PhNO