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N2 Cleaved, Functionalized

Synthesis: hafnium complex enables reaction with carbon monoxide

by Sophie L. Rovner
December 14, 2009 | A version of this story appeared in Volume 87, Issue 50

Cornell University chemists have devised a new method to cleave and functionalize dinitrogen at room temperature and pressure.

N2 is cheap and abundant. But it’s tough to use as a reactant because it is virtually inert, due largely to its very strong triple bond and nonpolarity, Cornell’s Paul J. Chirik notes. Instead, most nitrogen compounds are derived from ammonia produced via the Haber-Bosch process, which uses N2 as a feedstock and requires energy-intensive conditions of high temperature and pressure.

Chirik and Donald J. Knobloch avoided these problems by forming a complex between nitrogen and a hafnium reagent with an ansa-bis(cyclopentadienyl) ligand. Formation of the complex lengthens and weakens the N2 bond enough that it can be cleaved under mild conditions when carbon monoxide, another abundant feedstock, is added. At the same time, the reaction creates new nitrogen-carbon and carbon-carbon bonds. Subsequently adding an acid produces the fertilizer oxamide, “which demonstrates that an important agrochemical can be synthesized directly from N2 and CO,” the researchers write (Nat. Chem., DOI: 10.1038.nchem.477).

“This work represents a whole new kind of reactivity for coordinated dinitrogen,” comments University of British Columbia chemist Michael D. Fryzuk, who also studies coordination complexes for activating N2. “Not only are N–C bonds being formed, but the cleavage of the N–N bond by addition of carbon monoxide is really exciting. Anytime someone finds a new chemical transformation for dinitrogen, we get closer to solving one of chemistry’s long-standing grand challenges, that of utilization of N2 as a feedstock for producing organonitrogen compounds.” In addition to fertilizers, such compounds are used in products ranging from pharmaceuticals to nylon.

Chirik previously used a similar hafnium reagent to react N2 with CO2 to form a substituted hydrazine (C&EN, Jan. 15, 2007, page 45) and a zirconium complex to cleave and hydrogenate N2 to form ammonia.

Chirik is now evaluating alternative metal and ligand combinations to determine whether the latest reaction can be extended to the synthesis of more elaborate organic molecules.



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