Selective transfer hydrogenation coupling of nitroaromatics to azoxy/azo compounds by electron-enriched single Ni-N4 sites on mesoporous N-doped carbon

Abstract

Developing an active and selective non-precious metal catalyst to boost transfer hydrogenation coupling (THC) of nitroaromatics to their corresponding aromatic azoxy/azo compounds is desirable, but remains a great challenge. Herein, for the first time, we report a facile microwave-assisted pyrolysis strategy to successfully prepare the unique Ni-N4 sites on open-mouth quasi-stellate mesoporous N-doped carbon (NC) with enlarged electronic density led by the shortening Ni-N coordination distance (h-Ni1N4 /NC) by microwave heating the 1,10-phenanthroline-Ni complex coated stellate mesoporous SiO2 followed by alkali-leaching, distinct from the single-atom Ni-N5 sites on NC (l-Ni1N5/NC) prepared by a conventional pyrolysis method, unambiguously confirmed by diverse characterization technologies and DFT calculations. More importantly, h-Ni1N4/NC efficiently boosts the selective THC of nitrobenzene to azoxy under mild conditions using isopropyl alcohol (IPA) as hydrogen donor, showing much higher turnover frequency (33.4 h−1) and selectivity (95.6%) than traditional l-Ni1N5/NC catalyst (13.5 h -1°/85.3%) and supported Ni nanoparticles on NC (10.1 h -1, no detectable azoxy). Kinetic experiments and theoretical calculations reveal that the enriched electron density of Ni atoms in single Ni-N 4 sites efficiently lowers activation barrier for IPA/nitrobenzene activation and azoxy formation over h-Ni1N4/NC, which contributes to the outstanding catalytic performance for azoxybenzene production. Moreover, the approach was extended to the transformation of diverse nitroaromatics to corresponding azoxy compounds with high yield. In addition, by varying reaction parameters, h-Ni1N4/NC catalyst enables highly active and selective production of azo compounds via THC reaction of nitroaromatics.