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 annually(1) for the chemical industry and agriculture(2), it requires high temperature and pressure, in addition to a catalyst(3), 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(4) under mild conditions, with protonation yielding ammonia in stoichiometric(5) and even catalytic yields(6). 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(7), in formation of one N-H bond and a bridging hydride. Here we extend our work on zirconium complexes containing cyclopentadienyl ligands(8,9) 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 45degreesC, and continued hydrogenation at 85degreesC results in complete fixation to ammonia.