Simple and straightforward method to prepare highly dispersed Ni sites for selective nitrobenzene coupling to Azo/Azoxy compounds

Abstract

Azo and azoxy compounds are essential substances for the chemical industry. The development of catalysts to produce these molecules directly from nitroarenes is highly desirable. Nevertheless, this reaction is extremely challenging since the catalyst should not only activate nitro-compounds but also avoids the complete reduction to amines. Beyond noble-metals, such as Au, nickel-containing catalysts have shown attention for nitroarenes coupling to produce azo/azoxy compounds. Recently, outstanding results were reported for the nitrobenzene coupling using Ni-N4 sites obtained via pyrolysis of Ni(phen). However, pyrolysis methods lack on the control of metal loading and morphology of the metal sites, clearly, a mix of nickel nanoparticles and atomic dispersed metal sites were obtained. Here, we present a simple and straightforward method to synthesize highly dispersed nickel sites stabilized on highly crystalline carbon nitrides with poly(heptazine imide) structure (PHI). This method allows fine control of the metal loading and leads to mostly highly dispersed nickel sites, enabling to investigate in depth the coordination sphere of the Ni-sites during the reaction. The synthesized catalysts (Ni-PHI) were applied to nitrobenzene coupling reactions, showing high yields towards azo/azoxybenzene (98 %) and the highest TOF (435 h−1) for this reaction in the literature under mild conditions. Raman spectroscopy analyses show that nickel sites are crucial to adsorb nitrobenzene molecules through N atoms, favoring the production of nitrosobenzene. Furthermore, Ni-PHI exhibit high stability under different cycles and can yield azo/azoxy molecules with a broad scope, revealing the wide versatility of this catalyst.