Unexpected activity of MgO nanoclusters for the reductive-coupling synthesis of organonitrogen chemicals with C = N bonds.

Reductive-coupling of nitro compounds and alcohols is a sustainable route for constructing C = N bonds in organonitrogen chemicals, yet challenging due to the inertness of α-C(sp3)-H bond in alcohols and the vulnerability of C = N bonds towards hydrogenation. Here, we report the surprising catalytic activity of ultrafine alkaline-earth metal oxide MgO nanoclusters (0.9 ± 0.3 nm) that efficiently activate α-C(sp3)-H bonds, facilitating the transfer hydrogenation and synthesis of value-added chemicals bearing C = N bonds with high to excellent yields (86-99%). Controlled experiments and characterizations showed the crucial role of oxygen vacancies (O(v)) and local Mg environment (Mg-O bond) in MgO for substrate adsorption and activation via electronic interactions between substrate's negatively charged oxygen atoms and O(v) sites in MgO nanoclusters. Theoretical calculation further confirmed that O(v) significantly lowered the energy barrier of the hydrogen atom transfer from α-C(sp3)-H in ethanol to the nitro group in nitrobenzene (29.3 vs. 52.9 kcal/mol), which is the rate-determining step with the highest energy barrier in reductive-coupling reactions. Our method not only provides an efficient and sustainable pathway for synthesizing organonitrogen chemicals with C = N bonds but also inspires the exploration of main group element catalysts as alternatives to transition metal and noble metal catalysts for organic transformations.