Theoretical mechanistic insights into dinitrogen cleavage by a dititanium hydride complex bearing PNP-pincer ligands

The mechanism of dinitrogen cleavage by a PNP-coordinated dititanium polyhydride complex has been computationally investigated. A “multi-state reactivity” scenario has been disclosed for the whole process of N2 coordination and...

Dinitrogen Cleavage and Hydrogenation to Ammonia with a Uranium Complex

The Haber–Bosch process produces ammonia (NH3) from dinitrogen (N2) and dihydrogen (H2), but requires high temperature and pressure. Before iron-based catalysts were exploited in the current industrial Haber–Bosch process, uranium-based materials were used as effective catalysts for production of NH3 from N2. Although some molecular uranium complexes are capable of combining and even reducing N2, however, further hydrogenation with H2 to NH3 has not yet been reported. Here, we report the first example of N2 cleavage and hydrogenation with H2 to NH3 with a molecular uranium complex. The N2 cleavage product contains three uranium centers that are bridged by three imido μ2-NH ligands and one nitrido μ3-N ligand. Labeling experiments with 15N 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. A synthetic cycle has been established by the reaction of the N2-cleaved complex with TMSCl. The isolation of this trinuclear imido-nitrido product implies that a multimetallic uranium assembly plays an important role in the activation of N2.

DFT Mechanistic Study on the Reaction of Benzenesulfonyl Azides with Oxabicyclic Alkenes

The reaction of benzenesulfonyl azides with oxabicyclic alkenes to form aziridines, reported by Chen et al (J. Org. Chem. 2019, 84, 18, 11863-11872), could proceed via initial [3+2] cycloaddition to form triazoline intermediates followed by dinitrogen cleavage or via initial dinitrogen cleavage of the benzenesulfonyl azide to afford a nitrene intermediate followed by insertion of this species into the olefinic bond of the oxabicyclic alkene. Calculations at the DFT M06-2X/6-311G+(d,p) level show that the initial [3+2] cycloaddition has barriers of 17.3 kcal/mol (endo) and 10.2 kcal/mol (exo) while the initial nitrogen extrusion step has a barrier of 38.9 kcal/mol. The rate-determining step along the former pathway is the dinitrogen cleavage from triazoline cycloadducts which has barriers of 32.3 kcal/mol (endo) and 38.6 kcal/mol (exo) and that along the latter pathway is dinitrogen cleavage from benzenesulfonyl azide with an activation of barrier of 38.9 kcal/mol. The [3+2] addition of benzenesulfonyl azide with oxabicyclic alkene to afford endo and exo triazoline intermediates is kinetically favored over the dinitrogen cleavage from benzenesulfonyl azide by 21.6 and 28.1 kcal/mol for endo and exo pathway respectively. Thus, the preferred pathway for the reaction of oxabicyclic alkene with benzenesulfonyl azide is via initial [3+2] addition followed by dinitrogen cleavage, contrary to the proposal by Chen et al. The lower activation barrier for the dinitrogen extrusion step leading to endo aziridine compared to exo isomer means that the endo product will be formed as the major product, confirming the experimental observation. The position of substituents on the benzene group of the benzenesulfonyl azide greatly affects the endo / exo diastereoselectivity.