1. Department of Chemistry and Chemical Biology, Baker Laboratory Cornell University, Ithaca, New York 14853, USA

Correspondence to: Paul J. Chirik¹ Email: pc92@cornell.edu
X-ray crystallographic coordinates for 1 and 2 have been deposited at the Cambridge Crystallographic Database under numbers 223980 and 223981, respectively.

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.

1. Department of Chemistry and Chemical Biology, Baker Laboratory Cornell University, Ithaca, New York 14853, USA

Correspondence to: Paul J. Chirik¹ Email: pc92@cornell.edu
X-ray crystallographic coordinates for 1 and 2 have been deposited at the Cambridge Crystallographic Database under numbers 223980 and 223981, respectively.

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