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

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) to aniline (PhNH). However, a complete reductive transformation cycle of PhNO to PhNH 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 ↔ PhNO ↔ PhNO can be achieved on this well-defined {Co(μ-SPh)Co} functional platform. Further detailed reactivity investigations demonstrate that the PhNO and PhNO complexes cannot react with the usual hydrogen and hydride donors to afford the corresponding phenylhydroxylamino (PhNHO) species. However, the double reduced PhNO 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 p value of 13-16. Cyclic voltammetry shows that there are two successive reduction events at = -0.075 V and = -1.08 V for the PhNO complex, which allows us to determine both bond dissociation energy (BDE) of 59-63 kcal·mol and thermodynamic hydricity (Δ) of 71-75 kcal·mol of the PhNHO complex. Both values indicate that the PhNO complex is not a potent hydrogen abstractor and the PhNO complex is not an efficient hydride acceptor. In the presence of BH as a combination of protons and electrons, facile N-O bond cleavage of the coordinated PhNHO group can be realized to generate PhNH and a dicobalt hydroxide-bridged complex. Overall, we present the first stepwise reductive sequence, PhNO ↔ PhNO ↔ PhNO ↔ PhNHO → PhNH.