Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex

摘要

Molecular nitrogen is relatively inert owing to the strength of its triple bond, nonpolarity and high ionization potential. As a result, the fixation of atmospheric nitrogen to ammonia under mild conditions has remained a challenge to chemists for more than a century. Although the Haber-Bosch process produces over 100 million tons of ammonia annually for the chemical industry and agriculture, it requires high temperature and pressure, in addition to a catalyst, to induce the combination of hydrogen (H_2) and nitrogen (N_2). Coordination of molecular nitrogen to transition metal complexes can activate and even rupture the strong N-N bond under mild conditions, with protonation yielding ammonia in stoichiometric and even catalytic yields. But the assembly of N-H bonds directly from H_2 and N_2 remains challenging: adding H_2 to a metal-N_2 complex results in the formation of N_2 and metal-hydrogen bonds or, in the case of one zirconium complex, in formation of one N-H bond and a bridging hydride. Here we extend our work on zirconium complexes containing cyclopentadienyl ligands and show that adjustment of the ligands allows direct observation of N-H bond formation from N_2 and H_2. Subsequent warming of the complex cleaves the N-N bond at 45℃, and continued hydro-genation at 85℃ results in complete fixation to ammonia.