Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex

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 annually1 for the chemical industry and agriculture2, it requires high temperature and pressure, in addition to a catalyst3, to induce the combination of hydrogen (H2) and nitrogen (N2). Coordination of molecular nitrogen to transition metal complexes can activate and even rupture the strong N–N bond4 under mild conditions, with protonation yielding ammonia in stoichiometric5 and even catalytic yields6. But the assembly of N–H bonds directly from H2 and N2 remains challenging: adding H2 to a metal–N2 complex results in the formation of N2 and metal–hydrogen bonds or, in the case of one zirconium complex7, in formation of one N–H bond and a bridging hydride. Here we extend our work on zirconium complexes containing cyclopentadienyl ligands8,9 and show that adjustment of the ligands allows direct observation of N–H bond formation from N2 and H2. Subsequent warming of the complex cleaves the N–N bond at 45 °C, and continued hydrogenation at 85 °C results in complete fixation to ammonia.