Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex

Abstract

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 ligands^8,9 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.