This research is led by Prof. Congqing Zhu (State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, China) and Prof. Laurent Maron (LPCNO, CNRS & INSA, Université Paul Sabatier, Toulouse, France). The Haber–Bosch process produces ammonia (NH3) from dinitrogen (N2) and dihydrogen (H2), but requires high temperature and pressure. Although uranium-based materials have been reported as effective catalysts in Haber–Bosch process, the direct hydrogenation with H2 forming NH3 activated by a uranium complex has not been accomplished. “The study of N2 activation and hydrogenation by uranium molecular complex could help us to further understand the detail mechanism for Haber–Bosch process at the molecular level and also may help us to design more effective catalysts for ammonia formation.” Zhu says.
The research team initially synthesized a uranium azide complex 2 from uranium precursor 1 and NaN3. Then 2 was reduced with an excess of KC8 affording a N2-cleaved product 3, which contains three uranium centers that are bridged by three imido μ2-NH ligands and one nitrido μ3-N ligand. The 15N-labeling experiments demonstrate that the nitrido ligand in the product originates from N2. Reaction of the N2-cleaved complex with H2 or H+ forms NH3 under mild conditions. “The isolation of this trinuclear imido-nitrido product suggests that multimetallic uranium assembly plays an important role in the activation of N2 in the cleavage of the N≡N triple bond in N2 and that multiple uranium can cooperate is a promising strategy for activation and conversion of small molecules.” Zhu says. A synthetic cycle has been established by the reaction of the N2-cleaved complex with trimethylsilyl chloride (TMSCl) to generate N(SiMe3)3, HN(SiMe3)2 and 1. The detailed mechanism of N2 activation by multimetallic synergy was revealed by DFT calculations.
See the article:
Dinitrogen cleavage and hydrogenation to ammonia with a uranium complex
National Science Review